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Ci e his: Chem. Soc. Re ., 2024,
53, 2530
Po ous ma e ials as effec i e chemi esis i e
gas senso s
Akashdeep Sha ma,
a
Sunil Babu Eadi,
b
Heman h Noo halapa i,
c
Michal O yepka,
de
Hi-Deok Lee *
b
and Kolleboyina Jaya amulu *
a
Chemi esis i e gas senso s (CGSs) ha e e olu ionized he ield o gas sensing by p o iding a low-
powe , low-cos , and highly sensi i e means o de ec ing ha m ul gases. This echnology wo ks by
measu ing changes in he conduc i i y o ma e ials when hey in e ac wi h a es ing gas. While
semiconduc ing me al oxides and wo-dimensional (2D) ma e ials ha e been used o CGSs, hey suffe
om poo selec i i y o speci ic analy es in he p esence o in e e ing gases and equi e high ope a ing
empe a u es, esul ing in high signal- o-noise a ios. Howe e , nanopo ous ma e ials ha e eme ged as
a p omising al e na i e o CGSs due o hei high speci ic su ace a ea, unsa u a ed me al ac i es, and
densi y o h ee-dimensional in e -connec ed conduc i e and pendan unc ional g oups. Po ous
ma e ials ha e demons a ed excellen esponse and eco e y imes, ema kable selec i i y, and he
abili y o de ec gases a ex emely low concen a ions. He ein, ou cen al emphasis is on all aspec s o
CGSs, wi h a p ima y ocus on he use o po ous ma e ials. Fu he , we discuss he basic sensing
mechanisms and pa ame e s, diffe en ypes o popula sensing ma e ials, and he c i ical explana ions
o a ious mechanisms in ol ed h oughou he sensing p ocess. We ha e p o ided examples o
ema kable pe o mance demons a ed by senso s using hese ma e ials. In addi ion o his, we compa e
he pe o mance o po ous ma e ials wi h adi ional me al-oxide semiconduc o s (MOSs) and 2D
ma e ials. Finally, we discussed u u e aspec s, sho comings, and scope o imp o emen in sensing
pe o mance, including he use o me al–o ganic amewo ks (MOFs), co alen -o ganic amewo ks
(COFs), and po ous o ganic polyme s (POPs), as well as hei hyb id coun e pa s. O e all, CGSs using
po ous ma e ials ha e he po en ial o add ess a wide ange o applica ions, including moni o ing wa e
quali y, de ec ing ha m ul chemicals, imp o ing su eillance, p e en ing na u al disas e s, and imp o ing
heal hca e.
1. In oduc ion
The de imen al impac o g owing ai pollu ion on ou plane
and human well-being is undeniable. The apid su ge in
indus ializa ion and globaliza ion has led o he widesp ead
elease o ha m ul gases and ola ile compounds in o ou
en i onmen and homes. These emissions ha e a - eaching
consequences, p o oundly affec ing bo h human heal h and he
delica e ecological balance o Ea h.
1
Acco ding o he Uni ed
Na ions En i onmen al Agency, app oxima ely 7 million p e-
ma u e dea hs e e y yea a e due o ai pollu ion. The deadlies
illnesses linked o PM 2.5 ai pollu ion a e s oke, hea disease,
lung disease, lowe espi a o y diseases (such as pneumonia),
and cance . High le els o ine pa icles also con ibu e o o he
illnesses, like diabe es, can hinde cogni i e de elopmen in
child en and also cause men al heal h p oblems.
2
Fu he mo e,
as emphasized by he Uni ed Na ions, ai pollu ion has exac ed
a de as a ing oll on he biodi e si y o ou plane ’s plan and
animal ispecies. The p esence o sul u and ni ogen oxides in
ou a mosphe e has gi en ise o acid ain and smog, causing
ex ensi e ha m o plan li e and ma ine ecosys ems. To comba
hese p essing p oblems o ai pollu ion, inno a i e solu ions
a e u gen ly needed. One p omising app oach in ol es he use
o chemical senso s capable o de ec ing and ala ming us o he
p esence o ha m ul pollu an s. These senso s can se e as
a
Hyb id Po ous Ma e ials Labo a o y, Depa men o Chemis y, Indian Ins i u e o
Technology Jammu, Jammu & Kashmi , 181221, India.
E-mail: jaya [email protected], jaya amulu.kolleboyin[email p o ec ed]
b
Depa men o Elec onics Enginee ing, Chungnam Na ional Uni e si y, Daejeon,
Sou h Ko ea. E-mail: [email p o ec ed]
c
Facul y o Li e and En i onmen al Sciences, Shimane Uni e si y, Ma sue, 690-
8504, Japan
d
Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced
Technology and Resea ch Ins i u e (CATRIN), Palacky
´Uni e si y Olomouc,
S
ˇlech i elu
˚27, 783 71 Olomouc, Czech Republic
e
IT4Inno a ions, VSB–Technical Uni e si y o Os a a, 17. lis opadu 2172/15, 708
00 Os a a-Po uba, Czech Republic
Ko ea Senso Lab, Depa men o Elec onics Enginee ing, Chungnam Na ional
Uni e si y, Daejeon, Sou h Ko ea
Recei ed 19 h Sep embe 2023
DOI: 10.1039/d2cs00761d
sc.li/chem-soc- e
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ea ly wa ning sys ems, allowing us o ake p oac i e
measu es o educe pollu ion and p o ec ou heal h and he
en i onmen . Chemi esis i e gas senso s (CGSs) ha e eme ged
as a p omising echnology o he accu a e and simple de ec-
ion o ha m ul gases and ola ile o ganic compounds (VOCs)
in a ious applica ions. These senso s a e based on he p inci-
ple ha ce ain gases and VOCs can induce changes in he
elec ical esis ance o a sensing ma e ial.
3–5
This p ope y
allows CGSs o de ec and quan i y he p esence o speci ic
subs ances, p o iding aluable in o ma ion o assessing
ai quali y, ensu ing sa e y, and moni o ing en i onmen al
condi ions.
CGSs ha e ound applica ions in di e se ields. Ad ances in
nano echnology, ma e ials science, and ab ica ion echniques
con inue o expand he ange o sensing ma e ials and imp o e
he o e all unc ionali y o hese senso s. CGSs play a i al ole
in en i onmen al moni o ing, pa icula ly in de ec ing and
quan i ying pollu an s. Fo example, in u ban a eas wi h hea y
affic, hese senso s a e deployed in ai quali y moni o ing
s a ions o measu e concen a ions o gases like ca bon mon-
oxide (CO), ni ogen dioxide (NO
2
), and ola ile o ganic com-
pounds (VOCs). Such senso s p o ide eal- ime da a, allowing
au ho i ies o assess ai quali y and implemen measu es o
mi iga e he ad e se effec s o pollu ion, such as he educ ion
Akashdeep Sha ma
Akashdeep Sha ma, a doc o al
candida e in he Hyb id Po ous
Ma e ials Labo a o y a he
Depa men o Chemis y o he
Indian Ins i u e o Technology,
Jammu (India) unde P o .
Kolleboyina Jaya amulu. He
ob ained his mas e ’s deg ee
om he Uni e si y o Jammu
specialized in o ganic chemis y.
His cu en esea ch ocuses on
de eloping o ganic po ous
ma e ials o senso based
applica ions. Sunil Babu Eadi
D Sunil Babu Eadi holds a BS
deg ee in Chemis y om Andh a
Uni e si y, India, and MS deg ee
in Chemis y om he Uni e si y
o Hyde abad, India. He ea ned
his PhD deg ee in Ad anced
Ma e ials Enginee ing om
Chungnam Na ional Uni e si y
in 2015. Following his doc o al
s udies, D Eadi se ed as a
pos doc o al esea che a he
Kumoh Na ional Ins i u e o
Technology in Gumi, Ko ea,
om 2015 o 2018.
Subsequen ly, om 2018 o 2023, he held he posi ion o
Resea ch P o esso a Chungnam Na ional Uni e si y, Republic o
Ko ea. Cu en ly, a Scien i ic Office a he Indian Ins i u e o
Technology Jodhpu , his esea ch ocuses on chemical senso
de elopmen , Con ac Resis ance Reduc ion Technology and
Silicide Technology.
Heman h Noo halapa i
D Heman h Noo halapa i, an
Assis an P o esso a Facul y o
Li e and En i onmen al Sciences,
Shimane Uni e si y, Japan, ea ned
his PhD in Applied Chemis y om
Na ional Chiao Tung Uni e si y
(NCTU), Taiwan, h ough a
p es igious Taiwan Schola ship in
2013. Following a pos doc o al
ellowship a NCTU’s Ul ima e
Spec oscopy and Imaging
labo a o y, he s a ed his
independen esea ch a Shimane
Uni e si y. Specializing in label-
ee molecula spec oscopy and imaging, D Noo halapa i employs
chemome ics, machine lea ning and a i icial in elligence o explo e
di e se applica ions in biology, medicine, ma e ials, and he
en i onmen . His inno a i e app oach con ibu es o ad ancing
scien i ic unde s anding and echnological applica ions in hese ields.
Michal O yepka
Michal O yepka is Head o CATRIN-
RCPTM, a esea ch di ision a
Palacky
´Uni e si y in Olomouc. He
is a membe o he Scien i ic Boa d
o he Czech G an Agency and he
LUMI Supe compu e (Finland). His
esea ch in e es s co e physical–
chemical p ope ies and eac i i y
o g aphene de i a i es and 2D
ma e ials, nonco alen in e ac ions
o 2D ma e ials, and pho olumines-
cence p ope ies o ca bon do s
(CDs). He has been de eloping he
chemis y o luo og aphene (2D
chemis y) owa d g aphene de i a i es, which can be applied in
(bio)sensing, ca alysis, and ene gy s o age. He specializes also in
modeling o biomolecules, nanoma e ials, and complex molecula
sys ems and he de elopmen o o ce ields, mul iscale me hods, and
hei applica ions.
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o CO emissions om ehicles.
4,6,7
In heal hca e, hey offe he
po en ial o non-in asi e disease diagnosis by de ec ing spe-
ci ic bioma ke s in exhaled b ea h. They enable he de ec ion o
bioma ke s associa ed wi h a ious diseases. Fo ins ance, in
diabe es managemen , senso s can de ec glucose le els in
blood o in e s i ial luid, p o iding c ucial in o ma ion o
insulin dosing. Addi ionally, b ea h analysis using CGSs has
shown p omise o non-in asi e disease diagnosis and mon-
i o ing. A p ime example is he de ec ion o ace one in he
b ea h o indi iduals wi h diabe es as an indica o o hei
blood glucose le els.
8–10
They also play a c ucial ole in indus-
ial sa e y, whe e he de ec ion o haza dous gases is i al o
p o ec wo ke s and p e en acciden s. CGSs a e indispensable
in indus ial se ings o p ocess con ol and sa e y. They
moni o and egula e pa ame e s like gas concen a ions,
humidi y, and sol en le els, ensu ing op imal ope a ing con-
di ions. These senso s a e i al o leak de ec ion as well,
p e en ing po en ially haza dous si ua ions in indus ies deal-
ing wi h ola ile subs ances. Fo example, in he chemical
indus y, hese senso s can de ec leaks o oxic o lammable
gases, allowing o immedia e esponse and con ainmen .
11,12
The ood indus y elies on CGSs o assess ood quali y and
sa e y. They play a c ucial ole in moni o ing ood eshness
and p e en ing was e. Fo example, hese senso s can de ec
spoilage gases, such as ammonia and e hylene, emi ed by
ui s, and ege ables, enabling imely in e en ions o main-
ain ood quali y. Addi ionally, CGSs a e used in ood packaging
o ensu e in eg i y, p e en con amina ion and ensu e p oduc
sa e y.
12–14
CGSs a e a c ucial componen o chemical and
biological wa a e agen de ec ion sys ems. They can iden i y
oxic subs ances and p o ide ea ly wa ning in de ence and
secu i y applica ions. Fo ins ance, in mili a y applica ions,
hese senso s a e used o de ec chemical agen s and p o ec
pe sonnel om exposu e o ha m ul subs ances. Addi ionally,
hey a e employed in explosi es de ec ion, enhancing secu i y
measu es a ai po s and public spaces by de ec ing ace
amoun s o explosi e ma e ials.
15–17
Recen ad ancemen s ha e expanded he u ili y o CGSs o
eme ging applica ions. These include he de ec ion o ola ile
o ganic compounds (VOCs) in indoo ai quali y moni o ing.
Fo example, in sma buildings, hese senso s can de ec VOCs
eleased om building ma e ials o cleaning p oduc s, ensu -
ing heal hy indoo ai quali y. Wea able de ices inco po a ing
CGSs ha e been de eloped o pe sonal heal h moni o ing,
measu ing pa ame e s like swea elec oly e le els. In he ood
and pe ume indus ies, elec onic noses equipped wi h hese
senso s a e used o la ou and ag ance analysis, ensu ing
p oduc consis ency and quali y. Effo s a e ongoing o enhance
he pe o mance o CGSs by imp o ing hei selec i i y, sensi-
i i y, and esponse ime. In he medical ield, CGSs a e
ex ensi ely used o diagnos ic pu poses. Fig. 1 shows he
diffe en ypes o gas de ec ion echniques in gene al use.
The signi icance o CGSs lies in hei po en ial o add ess
he d awbacks associa ed wi h adi ional de ec ion echnolo-
gies, While elec ochemical, colo ime ic, luminescen , sol–gel,
in a ed (IR), and pa amagne ic IR op ical senso s a e e ec i e,
hey o en su e om complexi ies such as high cos s, limi ed
scalabili y, and ene gy consump ion issues. In con as , CGSs
o e ad an ages such as simplici y, cos -e ec i eness, com-
pac ness, and low powe equi emen s, making hem a ac i e
candida es o widesp ead adop ion. These senso s can be
designed using a a ie y o sensing ma e ials, including me al
oxides, polyme s, nanoma e ials, and ca bon-based ma e ials
like g aphene and ca bon nano ubes. The sensing ma e ial’s
su ace in e ac s wi h he a ge gases o VOCs, causing
changes in i s elec ical conduc i i y o esis ance. These
Hi-Deok Lee
P o . Hi-Deok LEE, wi h BS, MS,
and PhD deg ees in elec ical
enginee ing om he Ko ea
Ad anced Ins i u e o Science and
Technology (1990, 1992, 1996),
joined LG Semicon Company L d.
in 1993, con ibu ing o CMOS
echnology de elopmen . Since
2001, he is a P o esso a
Chungnam Na ional Uni e si y,
specializing in nanoscale CMOS
echnology, eliabili y physics,
and senso enhancemen . A
ecipien o he Excellen P o esso
Awa d (2001, 2003, 2014), P o . LEE was a Visi ing Schola a he
Uni e si y o Texas a Aus in (2006–2008). A membe o he Ins i u e
o Elec onics, he cu en ly heads he In elligen ICT Educa ion &
Resea ch P og am o Fu u e De ense Technology. His esea ch ocuses
on nanoscale CMOS echnology, eliabili y physics, silicide echnol-
ogy, es elemen g oup design, and senso de elopmen .
Kolleboyina Jaya amulu
Kolleboyina Jaya amulu (Ram) is
an Assis an P o esso in he
Depa men o Chemis y Indian
Ins i u e o Technology Jammu,
India. He ea ned a PhD in
Ma e ials Chemis y a
Jawaha lal Neh u Cen e o
Ad anced Scien i ic Resea ch,
Bangalo e, India. His schola ly
pu sui s ha e been u he
en iched by in e na ional
expe iences, ha ing been hono ed
wi h an Alexande on Humbold
Pos doc o al Fellowship in
Ge many, an ICMS Pos doc o al Fellowship and Saku a Science
Exchange P og am Japan. Ram is indeed a dis inguished membe o
he p es igious Indian Na ional Young Academy o Sciences (INYAS)
(2023–2027). His esea ch expe ise is in he design and de elopmen
o he s uc u e–p ope y ela ionship o hyb id (2D) po ous ma e ials
o indus ially ele an condi ions.
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changes a e hen ansla ed in o measu able signals ha can be
analyzed o iden i y and quan i y he concen a ion o he a ge
subs ances. T adi ional chemi esis i e ma e ials and po ous
ma e ials o gas sensing ha e dis inc cha ac e is ics and
ad an ages, and compa ing hem can help us unde s and he
di e ences and po en ial bene i s o using po ous ma e ials in
gas sensing applica ions.
T adi ional chemi esis i e gas senso s a e o en based on
me al oxides e.g., SnO
2
, ZnO, WO
3
e c. These ma e ials change
hei elec ical esis ance in he p esence o speci ic gases due
o chemical eac ions on hei su ace. The apid expansion o
me al-oxide-based senso s can be aced back o he pionee ing
wo k o Seyama e al., employing a ZnO hin ilm as he sensing
laye , success ully showcasing he easibili y o gas sensing
h ough uncomplica ed elec ical de ices.
18
Since hen, he e
ha e been emendous epo s on he applica ions o semicon-
duc ing me al oxides as gas senso s such as TiO
2
, SnO
2
,WO
3
,
V
2
O
5
,Fe
2
O
3
, NiO, CeO
2
, CuO, In
2
O
3
,Nb
2
O
5
,e c. Tin dioxide
(SnO
2
) senso s a e widely used o de ec ing gases like me hane
and ca bon monoxide.
19,20
SMO gas senso de ices ha e se e al
unique ad an ages such as low cos , small size, measu emen
simplici y, du abili y, ease o ab ica ion, and low de ec ion
limi s (oppm le els). In addi ion, mos SMO-based senso s
end o be long-li ed and somewha esis an o poisoning. Fo
hese easons, hey ha e apidly g own in popula i y, becoming
he mos widely used gas senso s a ailable hese days using
a ious conduc ing polyme s, ca bonaceous ma e ials and a -
ious me al/me al oxide nanopa icles. Howe e , he selec i i y
o speci ic analy es such as ace one, me hanol, e hanol, iso-
p ene e c., by a ious me al oxide ma e ials is s ill a big
p oblem.
Howe e , po ous ma e ials possess ce ain equisi e ea u es
ha make hem po en ial ma e ials o gas sensing. In o de o
ge de ec ed by he sensing de ice, adso p ion and deso p ion
o gas molecules is he p ima y equi emen .
21–30
Po ous
ma e ials being cus odians o excep ional su ace a ea and
bea e s o app op ia e in e ac i e unc ional si es ul il he
need o hos –analy e in e ac ion. CGSs ea u e he ansduc ion
o chemical in e ac ions in o elec ical ou pu s such as con-
duc ance o esis ance in low-cos , high pe o mance, less-
Fig. 1 The schema ic illus a ion po ays a ange o ad anced po ous ma e ials such as MOFs, COFs, and POPs, designed o applica ions in
chemi esis i e gas senso s ac oss he indus ial, en i onmen al, heal hca e, and ood quali y moni o ing sec o s.
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ene gy consump ion and po able de ices. In o de o b ing
chemi esis i i y in he sensing ma e ials, conduc ion o
cha ges in he ma e ial is he o emos equi emen . Po ous
ma e ials p o ide as op ions o selec ing such app op ia e
ma e ials owing o hei ex ended amewo ks ha wo k as
conduc ion highways o he mo emen o cha ges.
Po ous ma e ials a e an in e es ing class o ma e ials ha
ha e de eloped a g ea ca ee in e ms o s o age, molecula
le el sei ing, ca alysis, wa e ea men , sensing and so on. The
use o mode n c ys alline po ous ma e ials such as MOFs, COFs
and POPs was well-es ablished as a sensing pla o m p ima ily
due o hei po ous na u e.
21,23,31–49
An una oidable ea u e o
an e icien chemi esis i e gas sensing ma e ial is he elec ical
esponse owa ds he analy e gas which no all po ous ma e ials
usually o e . Thus, his sec ion will deal wi h only hose
ma e ials me iculously which a e usually conduc i e in na u e
o which show an elec ical esponse owa ds he a ge gases
based on hei band-gap ene gies.
Me al o ganic amewo ks (MOFs) p o ide excep ionally
high su ace a ea, igid and o de ed amewo k, and unc ional
g oup e sa ili y ha p o ide op imal hos –analy e in e ac ion
and high selec i i y. MOFs can be 2D o 3D based on he
geome y o he o ganic linke being used.
29,50–57
Al hough
3D MOFs p o ide acile hos –gues in e ac ions, hei non-
conduc i e na u e limi s hei use in chemi esis i e sensing.
Con e sely, 2D MOFs se e as highly conduc i e ma e ials
owing o he plana and conjuga ed o ganic monome ic uni s
such as po phy in, iphenylene, ph halocyanine, e c. and hus
o e an excellen op ion as a chemi esis i e gas sensing
ma e ial.
58
COFs can p o ide highly unable s uc u es and
as unc ionaliza ion oppo uni ies which emain bene icial
o e icien gues –hos in e ac ions. Al hough hei elec ical
conduc i i ies a e no much highe , hei c ys alline na u e
enables hem o be excellen chemi esis i e sensing ma e ials
e en wi h hei minu e conduc i i ies, hough me al and con-
duc i e ca bon doping based conduc i i y inc emen s we e
epo ed p e iously. G aphene-based ma e ials a e conduc i e
in na u e and hus hei unc ionalized coun e pa s a e ea-
lized o show chemi esis i e na u e o gas sensing pu poses.
G aphene has ex ensi ely gone h ough a ious modi ica-
ions and hyb idiza ions o p oduce ma e ials such as g aphene
oxide (GO), educed g aphene oxide ( GO), me al/me al oxide
deco a ed g aphene nanocomposi es, G aphene–polyme com-
posi es and so on.
59
These modi ica ions ha e been used o
acili a ing hos –analy e in e ac ions ha a e bene icial o
sensing a ious analy es including gases. In his sec ion, MOFs
and COFs will be ocused ex ensi ely as po ous ma e ials o
chemi esis i e gas sensing, while o he p ominen po ous
Fig. 2 Publica ion his o y o chemi esis i e gas sensing (a) adi ional ma e ial (b) ad anced po ous MOFs, COFs and POPs o e he yea s. Da a a e
ob ained om he web o science by sea ching he keywo ds ‘‘chemi esis i e gas sensing’’ and ‘‘po ous ma e ial’’ (up o Sep embe 15, 2023). The numbe
o publica ions on chemi esis i e gas sensing ma e ials using adi ional ma e ials has almos doubled e e y yea since 2000, and a simila end has been
ollowed by ad anced po ous ma e ials since 2014. Da a a e ob ained (c) imeline cha showing he majo de elopmen s o and p og ess in ad anced
po ous ma e ials owa ds chemi esis i e gas sensing in e ms o syn hesis, design p is ine, hyb ids and de i a i es o MOFs, COFs and POPs o a ious
gas sensing applica ions.
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ma e ials ha ha e c ea ed miles ones in his ield will also be
discussed. The ema kable expansion o chemi esis i e gas
sensing using ad anced po ous ma e ials is clea ly e iden , as
e lec ed in he s eadily ising numbe o publica ions om
2014 o 2023 (Fig. 2a and b). Howe e , a di e se a ay o po ous
ma e ials ha e ound applica ion in chemi esis i e gas senso s,
wi h key miles ones highligh ed in Fig. 2c. In summa y, he
choice be ween adi ional chemi esis i e ma e ials and po ous
ma e ials o gas sensing depends on he speci ic equi emen s
o he applica ion. T adi ional ma e ials a e o en p e e ed o
hei apid esponse, obus ness, and cos -e ec i eness, while
po ous ma e ials a e gaining a en ion o hei excep ional
sensi i i y and selec i i y, especially in applica ions whe e p e-
cise gas de ec ion is c i ical. Resea che s con inue o explo e
ways o ha ness he ad an ages o bo h ypes o ma e ials o
enhanced gas sensing capabili ies.
2. De ini ion and ope a ing p inciple o
chemi esis i e senso s
CGSs a e de ices ha de ec and quan i y he p esence o gases
and ola ile o ganic compounds (VOCs) based on changes in
elec ical esis ance. The ope a ing p inciple o CGSs elies on
he in e ac ion be ween a sensing ma e ial and he a ge
analy e. In gene al, a gas senso comp ises wo main elemen s:
a ecep o and a ansduce , as depic ed in Fig. 3 The sensing
ma e ial ( ecep o ) used in CGSs is ca e ully chosen o exhibi a
change in i s elec ical conduc i i y o esis ance ( ansduce )
when exposed o speci ic gases o VOCs (analy es).
This ma e ial can be a me al oxide, a conduc ing polyme , a
nanoma e ial, o a combina ion o hese. The su ace o he
sensing ma e ial is designed o ha e a high su ace- o- olume
a io o maximize he in e ac ion wi h he a ge analy e. When
he a ge gas o VOC molecules come in o con ac wi h
he sensing ma e ial’s su ace, hey adso b on o i , causing
a change in he elec ical conduc i i y o esis ance o he
ma e ial. This change in esis ance is p opo ional o he
concen a ion o he a ge analy e in he su ounding en i on-
men . The elec ical esis ance o he sensing ma e ial is
ypically measu ed using a se up ha includes elec odes con-
nec ed o a measu emen ci cui . The esis ance measu emen
can be pe o med using a ious echniques, including ou -
e minal measu emen s, wo- e minal measu emen s, o impe-
dance spec oscopy.
The measu ed esis ance alue is hen co ela ed wi h he
concen a ion o he a ge gas o VOCs using calib a ion cu es
o ma hema ical models. The selec i i y and sensi i i y o CGSs
can be enhanced by unc ionalizing he sensing ma e ial’s
su ace. This in ol es modi ying he su ace wi h speci ic coa -
ings, ca alys s, o ecep o s ha selec i ely in e ac wi h he
a ge analy e, inc easing he senso ’s esponse o he desi ed
gas while minimizing in e e ence om o he subs ances.
O e all, he ope a ing p inciple o CGSs elies on he change
in elec ical esis ance o a sensing ma e ial when exposed o
a ge gases o VOCs, allowing o he de ec ion and quan i ica-
ion o hese subs ances as shown in Fig. 4. The simplici y,
sensi i i y, and selec i i y o CGS make hem a ac i e o a
wide ange o applica ions, including en i onmen al moni o -
ing, indus ial sa e y, heal hca e, and mo e.
2.1. De ice s uc u e
De ice s uc u e, in he con ex o gas senso s, e e s o he
physical a chi ec u e and composi ion o he senso ’s co e
componen s. I encompasses he a angemen o ma e ials,
elec odes, and o he in eg al elemen s wi hin he senso
amewo k. This seemingly echnical aspec ca ies immense
signi icance as i undamen ally shapes he senso ’s pe o -
mance cha ac e is ics. One c i ical ace in luenced by he
de ice s uc u e is sensi i i y. The me iculous selec ion o
ma e ials and hei spa ial o ganiza ion can de e mine how
esponsi e he senso is o he p esence o speci ic gases. Fo
example, semiconduc o gas senso s employ ca e ully designed
sensing laye s, whe e he a angemen o semiconduc o
Fig. 3 Schema ic illus a ion depic s gas senso pa s and ypical measu emen cha ac e is ics o chemi esis i e senso s.
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ma e ials di ec ly impac s he senso ’s abili y o de ec and
espond o pa icula gases. The design and con igu a ion o
hese laye s a e ailo ed o op imize sensi i i y. Selec i i y is
ano he i al aspec in luenced by he de ice s uc u e. I
de e mines he senso ’s capaci y o dis inguish be ween diffe -
en gases in complex en i onmen s. By enginee ing he sen-
so ’s s uc u e o in e ac selec i ely wi h speci ic gas
molecules, in e e ence om un ela ed gases can be mini-
mized. This selec i i y is c ucial in applica ions whe e accu a e
iden i ica ion o a ge gases is pa amoun , such as en i on-
men al moni o ing o sa e y sys ems.
Fig. 5 shows he diffe en de ice s uc u es used o gas
de ec ion. Response ime, a c i ical me ic o gas senso s, is
in ima ely ied o he de ice s uc u e. The a angemen o
senso componen s can affec he ime i akes o he senso o
de ec and egis e changes in gas concen a ion. An op imized
s uc u e ensu es apid esponse, enabling imely ac ions in
c i ical si ua ions like gas leak de ec ion o ai quali y moni o -
ing. Fu he mo e, he o e all pe o mance and eliabili y o a
gas senso a e hea ily in luenced by i s s uc u al design. A
obus s uc u e can wi hs and en i onmen al a ia ions,
ensu ing s able and consis en ope a ion o e ex ended pe i-
ods. This du abili y is c ucial, pa icula ly in indus ial se ings
whe e senso s may be exposed o ha sh condi ions. In essence,
he de ice s uc u e in gas senso s is no me ely a echnical
de ail bu a ounda ional elemen ha de e mines he senso ’s
abili y o ul il i s in ended pu pose. I is h ough ca e ul
conside a ion and enginee ing o his s uc u e ha gas
Fig. 4 A schema ic illus a ion p o ides he po en ial chemi esis i e sensing mechanism, including: (a) he s ep-by-s ep syn hesis o a chemi esis i e gas
senso se up, (b) he in e ac ion o he effec o gas molecules on inducing a posi i e change in esis ance. No ably, upon he emo al o gas molecules,
he sys em e u ns o a s eady-s a e baseline, indica ing he e e sibili y o gas molecules in e ac ion.
Fig. 5 Chemi esis i e gas senso s a e commonly buil using a ious s uc u es, including (a) sin e ed blocks, (b) hin alumina ube coa ings, (c) sc een-
p in ed hick ilms, (d) small beads wi h coil and needle elec odes, (e) small beads wi h a single coil (hea e and elec ode), and ( ) p ac ical senso
elemen s assembled wi h me al caps and il e s. Red aw he pic u e wi h pe mission om e . 60 Copy igh 2023 Else ie .
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senso s can exhibi enhanced sensi i i y, selec i i y, esponse
ime, and eliabili y, making hem in aluable ools ac oss a
spec um o applica ions, om ensu ing wo kplace sa e y o
sa egua ding he en i onmen and ad ancing heal hca e diag-
nos ics. Looking o wa d o he u u e o gas senso echnology,
he exis ing guidelines o de ice ab ica ion a e poised o se e
as a co ne s one o u he ad ancemen s. These p inciples a e
expec ed o guide he de elopmen o nex -gene a ion gas
senso s wi h enhanced capabili ies and b oade applica ions.
An icipa ed ad ancemen s include he explo a ion o nanos-
uc u ed oxide semiconduc o s wi h e en smalle c ys alli e
sizes, po en ially e olu ionizing gas sensing by signi ican ly
boos ing sensi i i y. Fu u e echnologies may also ocus on
ad anced me hods o he p ecise dispe sion o sensi ize s
wi hin semiconduc o ma e ials, leading o senso s wi h
ema kable selec i i y and esponsi eness. The op imiza ion
o sensing laye pa ame e s, enabled by ad anced ma e ials
enginee ing and compu a ional modelling, p omises o p o-
duce senso s ha a e bo h highly selec i e and excep ionally
du able. Addi ionally, hin ilm- ype gas senso s, o en o e -
looked bu showing p omise, may gain enewed a en ion wi h
he ma u a ion o ad anced ab ica ion echniques like Spu e -
ing, PECVD, ALD e c. In his ision o he u u e, gas senso s a e
poised o play a pi o al ole in add essing mul i ace ed chal-
lenges, om en i onmen al moni o ing o heal hca e diagnos-
ics, se ing new s anda ds in senso pe o mance and u ili y.
2.2. Elec ical and gas sensing cha ac e iza ion
measu emen s
The elec ical and gas sensing p ope ies o ab ica ed gas
senso s wi h a comp ehensi e analysis o hei pe o mance
conduc ed h ough a sys ema ic measu emen app oach. The
e alua ion can be ca ied ou using a mul i-me e (Kei hley
2400) wi h wo conduc i e elec odes, as illus a ed in he
schema ic diag am and digi al pho og aph p esen ed in
Fig. 6. The expe imen al se up in ol es placing he senso s
wi hin a sealed chambe on a manually con olled hea e
equipped wi h one inle and one ou le . Ini ial e acua ion o
he chambe should be pe o med using a high acuum pump,
achie ing a p essu e ange om 760 o o 10
3
o . Subse-
quen ly, he ca ie gas, ei he d y ai o N
2
gas, can be
in oduced in o he sensing chambe , wi h p ecise con ol o e
he gas amoun acili a ed by ex e nal mass low con olle s
(MFCs). Ensu ing he o ma ion o an Ohmic con ac be ween
he ac i e laye and elec odes is a c ucial s ep be o e each
measu emen . This con i ma ion can be achie ed h ough
cu en – ol age (I–V) measu emen s, whe ein he applied ol-
age a ies om 5 V o +5 V in inc emen s in applied bias V.
To enhance s abili y, he senso is p ehea ed be o e he ac ual
sensing measu emen s. The gas dynamics o he senso a e
assessed by passing he sensing gas (Ta ge ), along wi h he
ca ie gas, and measu ing he esul ing cu en and esis ance.
Fu he mo e, he sensing esponse can be examined a
Fig. 6 Schema ic illus a ion o elec ical and gas sensing measu emen s.
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di e en empe a u es by adjus ing he hea e empe a u e.
The ca ie gas low a e is main ained in he chambe , while
he sensing gas concen a ion is a ied by con olling he low
a e. This sys ema ic and con olled app oach allows o a
comp ehensi e examina ion o he gas senso s’ pe o mance,
shedding ligh on hei elec ical and gas sensing capabili ies
unde a ying condi ions.
2.3. Fab ica ion echniques
Va ious me hods ha e been employed o ab ica e nanos uc-
u ed me al oxide ma e ials o gas sensing applica ions, each
impa ing a wide ange o senso cha ac e is ics. The p ope ies
o hese ma e ials hea ily ely on hei composi ion and s uc-
u e, which, in u n, a e dic a ed by he building blocks o
nanopa icles. Key ac o s go e ning he o e all gas senso
pe o mance include pa icle size, mo phology, and c ys al
s uc u e.
61
Syn hesis echniques o nanoma e ials can be
b oadly ca ego ized as ei he ‘‘ op-down’’ o ‘‘bo om-up’’
app oaches.
62–77
Top-down app oaches ypically ini ia e wi h
a bulk ma e ial, om which nanoscale s uc u es a e c ea ed by
subsequen ma e ial emo al p ocesses. Common op-down
me hods encompass e-beam li hog aphy, pho oli hog aphy,
milling, and d y o ion/plasma e ching. While op-down p o-
cesses gene ally o e high manu ac u ing h oughpu , hey
ha e limi ed con ol o e su ace mo phology. Addi ionally,
hese me hods o en in ol e complex ab ica ion echniques
ha a e less sui able o cos -e ec i e and la ge-scale indus ial
p oduc ion, pa icula ly in applica ions like gas sensing.
2.3.1. Top-down app oaches. Top-down app oaches
in ol e s a ing wi h a bulk ma e ial and hen c ea ing nano-
scale s uc u es by emo ing o e ching away ma e ial. Com-
mon op-down me hods include:
E-beam li hog aphy. This echnique uses a ocused elec on
beam o pa e n a su ace, allowing o he p ecise c ea ion o
nanos uc u es. Howe e , i is limi ed in i s scalabili y and is
mo e sui ed o esea ch and small-scale ab ica ion.
Pho oli hog aphy. Pho oli hog aphy uses ligh o ans e a
pa e n on o a subs a e coa ed wi h a pho osensi i e ma e ial.
I is widely used in he semiconduc o indus y bu may no be
sui able o gas senso ma e ials ha equi e speci ic
mo phologies.
Milling. Mechanical milling in ol es g inding and educing
bulk ma e ial in o ine nanopa icles. I is a e sa ile echnique
bu can lead o agglome a ion and limi ed con ol o e pa icle
size and shape.
D y o ion/plasma e ching. These me hods in ol e emo ing
ma e ial om a subs a e using chemical eac ions o ion
bomba dmen . While hey offe high h oughpu , hey may
no p o ide p ecise con ol o e su ace mo phology and o en
equi e sophis ica ed equipmen .
Top-down p ocesses a e known o hei po en ial o high
manu ac u ing h oughpu , making hem a ac i e o ce ain
indus ies. Howe e , hey may no offe he le el o con ol o e
su ace mo phology and composi ion needed o gas-sensing
ma e ials. Addi ionally, he complex ab ica ion echniques
in ol ed in some o hese me hods can be cos -p ohibi i e o
la ge-scale p oduc ion.
78,79
2.3.2. Bo om-up app oaches. On he o he hand, bo om-
up app oaches in ol e he assembly o nanoma e ials a om by
a om o block by block. Nanopa icle building blocks a e
gene a ed on su aces by deposi ing apo molecules (in he
gas phase) o ions (in he liquid phase). These a oms/ions a e
hen assembled o o m c ys al planes o a omic clus e s, which
can subsequen ly g ow in o la ge pa icles and ma e ial s uc-
u es. These c ys al planes and clus e s ul ima ely gi e ise o
he nanos uc u e o he sensing ma e ial. Al e na i ely, one
can u ilize nanopa icles syn hesized in ei he he gas phase
(ae osol) o he liquid phase (colloid) and hen u he manip-
ula e hese building blocks o desi ed senso ma e ial
p ope ies.
Vapo phase deposi ion. In his me hod, apo molecules a e
deposi ed on o a subs a e, whe e hey assemble a om by a om
o block by block o o m nanos uc u es. Techniques like
chemical apo deposi ion (CVD) and me al o ganic chemical
apo deposi ion (MOCVD) a omic laye deposi ion, and Spu -
e ing all unde his ca ego y. Solu ion-based me hods: hese
me hods in ol e c ea ing nanopa icle building blocks in he
gas phase (ae osol) o he liquid phase (colloid). These building
blocks can hen be assembled in o desi ed s uc u es. Solu ion-
based echniques include sol–gel syn hesis and hyd o he mal/
sol o he mal g ow h. Bo om-up app oaches o e p ecise con-
ol o e c ys al s uc u e, pa icle size, and mo phology. This
le el o con ol is essen ial o op imizing gas sensing ma e ials
o enhance sensi i i y and selec i i y. These me hods a e mo e
sui able o esea ch and de elopmen gea ed owa d ailo ing
ma e ials o speci ic gas sensing applica ions.
80
In summa y,
he choice o syn hesis me hod signi ican ly impac s he cha -
ac e is ics o me al oxide nanoma e ials o gas sensing
applica ions. While op-down app oaches o e high h ough-
pu , hey o en lack su ace mo phology con ol and cos -
e ec i eness. Bo om-up me hods, on he o he hand, enable
p ecise con ol o e ma e ial s uc u e and a e mo e sui able
o ailo ing p ope ies o speci ic gas sensing equi emen s.
2.4. Gas sensing cha ac e is ics
2.4.1. Response and esponse ansien s. The ‘‘ esponse’’
o a gas senso is a undamen al pa ame e ha desc ibes how
he senso eac s when exposed o a pa icula gas o a change
in gas concen a ion. I essen ially quan i ies he senso ’s
abili y o de ec and espond o he p esence o a speci ic gas.
This esponse is ypically exp essed as a change in an elec ical
p ope y (e.g., esis ance, capaci ance, ol age) o ano he mea-
su able ou pu o he senso when exposed o he a ge gas.
The esponse is o en cha ac e ized by me ics such as sensi-
i i y, which quan i ies how much he senso ’s ou pu changes
in esponse o a gi en change in gas concen a ion. Fo
ins ance, in a semiconduc o gas senso , an inc ease in he
concen a ion o a speci ic gas like ca bon monoxide (CO) may
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a ge analy es, imp o ing sensi i i y and selec i i y. Su ace
unc ionaliza ion can in ol e he deposi ion o speci ic coa -
ings, ca alys s, o ecep o s ha selec i ely bind o he a ge
analy e. This p ocess enhances he adso p ion and de ec ion o
he desi ed gas, while minimizing in e e ence om o he
subs ances, hus op imizing he senso ’s pe o mance.
T ansduc ion echniques. CGSs employ a ious ansduc-
ion echniques o con e he changes in elec ical esis ance
in o measu able signals. The choice o ansduc ion echnique
depends on he speci ic applica ion equi emen s and he
desi ed sensi i i y. Common ansduc ion me hods include
di ec esis ance measu emen , impedance spec oscopy,
and equency-dependen measu emen s. These echniques
allow o he accu a e quan i ica ion o he a ge gas o VOC
concen a ion.
Signal p ocessing and analysis. Signal p ocessing and ana-
lysis echniques play a i al ole in ex ac ing meaning ul
in o ma ion om he senso ’s esponse. This in ol es il e ing
and ampli ying he senso ’s elec ical signal, ollowed by da a
analysis and in e p e a ion. Signal p ocessing echniques can
enhance he senso ’s signal- o-noise a io and imp o e he
de ec ion limi . Ad anced da a analysis me hods, such as
pa e n ecogni ion algo i hms and machine lea ning echni-
ques, can be employed o iden i y and classi y diffe en gases o
VOCs based on hei unique senso esponses.
Calib a ion and alida ion. Calib a ion is essen ial o es ab-
lishing a ela ionship be ween he senso ’s elec ical esponse
and he concen a ion o he a ge analy e. Calib a ion
cu es o ma hema ical models a e de eloped using known
concen a ions o he a ge gas o VOC. Regula calib a ion
and alida ion ensu e he senso ’s accu acy and eliabili y
o e ime.
S abili y and d i compensa ion. Long- e m s abili y o
CGSs is c ucial o con inuous and eliable gas de ec ion.
Senso d i , caused by en i onmen al ac o s o aging effec s,
can lead o alse eadings and educed accu acy. D i compen-
sa ion echniques, such as baseline co ec ion, empe a u e
compensa ion, and pe iodic ecalib a ion, help main ain he
senso ’s s abili y and compensa e o any d i s, ensu ing
consis en and accu a e measu emen s.
O e all, op imizing he pe o mance o CGSs in ol es ca e-
ul selec ion o sensing ma e ials, su ace unc ionaliza ion,
app op ia e ansduc ion echniques, signal p ocessing, cali-
b a ion, and add essing s abili y and d i issues. By conside -
ing hese ac o s and employing sui able echniques, CGSs can
achie e high sensi i i y, selec i i y, and accu acy, enabling
eliable gas de ec ion o a ious applica ions in heal hca e,
sa e y, en i onmen al moni o ing, and indus ial p ocesses.
4.2. S a egies o imp o e selec i i y and sensi i i y
Func ionaliza ion echniques a e used o enhancing he in e -
ac ion be ween sensing ma e ials and a ge analy es. T ans-
duc ion me hods include esis ance measu emen , impedance
spec oscopy, and o he signal ansduc ion app oaches. This
comp ehensi e e iew p o ides aluable insigh s in o he
ad ancemen s made in chemi esis i e senso echnology,
highligh ing i s po en ial o e olu ionize gas de ec ion ac oss
a ious sec o s. The knowledge and unde s anding p esen ed
in his e iew will aid esea che s, enginee s, and s akeholde s
in ha nessing he ull po en ial o CGS and d i ing u he
ad ancemen s in his ield.
5. Chemi esis i e gas sensing o
po ous ma e ials
To highligh he impo ance and ad ancemen o his ield,
some e y good e iews ha e al eady been published on po ous
ma e ials o sensing pu poses.
172
We ha e also ied o sum-
ma ise all o he p ominen po ous ma e ials such as MOFs,
COFs, POPs and hei hyb id examples which a e also abula ed
as gi en in he espec i e sec ions based on p is ine and
hyb id sensing ma e ials. Me al–o ganic amewo ks (MOFs)
ha e eme ged as an in e es ing class o po ous ma e ials
ha possess highly c ys alline s uc u es, obus amewo ks,
i s - a e high su ace a ea, high conduc i i ies and unable
po e sizes. These p ope ies o MOFs make hem p ac ically
use ul and highly unc ional in many a enas such as ene gy
s o age,
173–175
gas s o age
176–178
and sepa a ion,
179–181
sensing,
182–184
biomedical ield
185–187
and o he scien i ic
a eas. Looking in o a wo ld whe e sensing is one o he
ine i ably equi ed hings, MOFs a e one o hose ma e ials
ha ha e p o ed hemsel es o wo k excellen ly in his ield
owing o he equisi e p ope ies ha a e equi ed o he
pu pose o sensing.
188–190
5.1. CGS sensing me al–o ganic amewo k based ma e ials
5.1.1. P is ine MOFs. The e y i s wo k o using a p is ine
MOF o chemi esis i e gas sensing was epo ed in 2014, when
Zhang e al.
191
s udied he chemi esis i e na u e o he cobal -
based zeoli ic imidazole amewo k (Co-ZIF-67); al hough gas
sensing wi h ZIFs has al eady been s udied be o e,
192
he
‘chemi esis i e’ sensing echnique o gas sensing by using a
MOF was ne e been used be o e. In his wo k, he g oup
s udied and demons a ed he sensing o a ious gases such
as ace one, o maldehyde, me hanol, and ie hylamine
(Fig. 8a–d). The band Gap o a ma e ial is a c ucial p ope y
o a ma e ial ha plays a d i ing ole in i s chemi esis i e
na u e since he op imum band gap is di ec ly ela ed o he
conduc i e p ope ies o a ma e ial. MOFs gene ally ha e a high
band gap o B4–8 eV which is no sui able o hei applica ion
in ol ing elec ical p ope ies. In his wo k, Co-ZIF-67 was used
which was ound o ha e a low band gap o 1.98 eV which is
adequa e o he elec ical conduc i i y equi ed o chemi e-
sis i e ope a ion. The po ous sodali e-like s uc u e o Co-ZIF-
67 possesses high speci ic su ace a ea (1832.2 m
2
g
1
), p o id-
ing a highly accessible su ace o he in e ac ion o gases
(Fig. 8a). Among he a ge ed gases, o maldehyde was ound
o gi e he highes sensing esponse (Fig. 8b). Response and
eco e y pe o mances (Fig. 8c) o his ma e ial we e ound o
be as bu no as e han ypical me al-oxide-based sensing
ma e ials. This can be due o he high su ace a ea o MOFs
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ha ake mo e ime o each he highes adso p ion–deso p ion
equilib ium ex en and hus shows he highes esis ance alue.
The selec i i y o a gas by a pa icula sensing ma e ial is due o
a ious ac o s (discussed in Sec ion 4). The same g oup
epo ed he use o a cobal imidazola e amewo k [Co(Im)
2
]
n
as a senso o ime hylamine (TMA) gas (Fig. 8d). The esul -
ing imidazole amewo k exhibi s excellen selec i i y, high gas
esponse, and a low de ec ion limi o 2 ppm, p ima ily due o
he weak in e ac ion be ween he TMA molecules and he
amewo k.
The ZIF-based ma e ials we e able o sense speci ic gases
wi h high selec i i y and good de ec ion limi s, bu when i
comes o sensing di e si y o gases, efficien sensing was
shown only o a ew analy e gases. Also, he band gap is low
o he ZIF-based MOFs bu s ill, highe conduc i i ies a e
needed o as chemi esis i e sensing ope a ions. Conduc i i y
in chemi esis i e gas sensing ma e ials is o u mos impo ance
in o de o b ing he bes pe o mance ou o hem. Al hough
high conduc i i ies in MOFs we e epo ed p e iously,
194–201
hei applica ions emain limi ed and had ne e been used
speci ically o chemi esis i e gas sensing be o e he e olu-
iona y wo k by Campbell e al.
202
(Fig. 9a). High elec ical
conduc i i ies o wo-dimensional s uc u es inspi ed he ab-
ica ion o 2D MOFs using such speci ic me als ha can o m
2D geome ies as well as 2D o ganic linke s such as
iphenylene, po phy in and ph halocyanine-based monome s.
In his cu en wo k, au ho s syn hesized a ious no el 2D
conduc ing Cu and Ni-MOFs (2D-cMOFs).
202,203
Response-
eco e y pe o mance o he Cu
3
(HITP)
2
based chemi esis i e
gas senso (Fig. 9b) e ealed a as chemi esis i e esponse o
he sensing ma e ial owa ds a ious NH
3
apou concen a-
ions. The swi esponse could be due o he 2D mo phologi-
cally p o ided highly accessible in e ac ion si es. Since Ni and
Cu a e in di e en elec onic con igu a ions (d
8
s. d
9
, espec-
i ely), he heo e ical s udies sugges ha he cha ge ans e
be ween he a ge gas and he sensing 2D MOF ma e ial
di ec ly impac s he elec onic esponse because me als wi h
di e en elec on coun s esul in o di e en Fe mi le els and
hus une he band gap alues.
204
Fo example, i is p edic ed
ha he use o highe elec on coun elemen s in place o Ni,
such as Cu, in M
3
HITP
2
ype MOFs aises he Fe mi le el o his
conduc ing ma e ial (Fig. 9d and e).
205
This was also ound o
be legi ima e in expe imen al e idence as pola analy es such as
me hanol, e hanol, and ace one show high a ia ions in sen-
sing esponses whe eas no much di e ence could be seen o
non-pola analy es, such as cyclohexane and pen ane, which
lack ee elec ons (Fig. 9c). These esul s p o ide us wi h he
oppo uni y o une he sensing pe o mance o chemi esis i e
gas senso s by examining he e ec o e sa ile me als in 2D
MOFs. To u he s udy he e ec o di e en me als on he
Fig. 8 (a) Sodali e opology o Co-ZIF-67. (b) Sensi i i y o he Co-ZIF-67 senso o a ious gases om 75–200 1C. (c) Chemi esis i e sensing
pe o mance o ZIF-67 a di e en gas concen a ions. Rep oduced wi h pe mission om e . 191 Copy igh 2014, Ame ican Chemical Socie y. (d)
Sensi i i y o he [Co(im)
2
]
n
senso o a ious gases om 50–175 1C. Rep oduced wi h pe mission om e . 193 Copy igh 2014, Ame ican Chemical
Socie y.
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sensing o gases in 2D MOFs, Dinca e al. wo ked ou h ee
iphenylene-based 2D MOFs wi h coppe and nickel o me al-
lic nodes. The high in insic conduc i i ies o hese MOFs come
h ough he plana ex ended s uc u e ha is s acked on one
ano he o o m 1D po es. The elec ons a e ee o mo e mo e
swi ly h ough he igid and plana iphenylene co es consis -
ing o conjuga ed double bonds and elec on- ich ni ogen and
oxygen a oms, in he case o HITP and HHTP, espec i ely.
Fig. 9 (a) Fi s epo on 2D conduc i e MOFs (wi h conduc i i y and po e size) (b) ela i e esponses o a Cu
3
(HITP)
2
senso o 0.5, 2, 5, and 10 ppm o
ammonia. Rep oduced wi h pe mission om e . 202 Copy igh 2015, Wiley-VCH. (c) PCA analysis o g oupings o a ious VOCs o sensing pu poses.
Rep oduced wi h pe mission om e . 203 Copy igh 2015, Ame ican Chemical Socie y. (d) and (e) Theo e ical band, DOS, cha ge densi y isosu ace and
Kagome bands and SOC gaps in 2D M
3
(HITP)
2
MOFs (d) Ni
3
(HITP)
2
and (e) Cu
3
(HITP)
2
espec i ely. Rep oduced wi h pe mission om e . 204 Copy igh
2015, Royal Socie y o Chemis y.
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Ca e ully looking a he sensing pe o mances o hese 2D
MOFs wi h a ious analy es, he Cu
3
(HITP)
2
,Co
3
(HHTP)
2
and
Ni
3
(HITP)
2
show high di e ences in sensing esponses. The
main di e ence is sugges ed o come majo ly om di e en
me al cen es (Cu s. Ni), al hough o he ac o s such as
di e en he e oa oms (NH and O) in HATP and HHTP linke s,
espec i ely, and sensing de ice na u e should also ha e sig-
ni ican e ec s.
Ano he essen ial pa ame e ha canno be o e looked
while s udying he sensing o an analy e is he limi o de ec ion
o LOD alues. The LOD o a sensing ma e ial can be desc ibed
by he abili y o he ma e ial o sense he lowes gas concen-
a ion and p oduce a eadable in ensi y o he chemi esis i e
signal. This pa ame e is di ec ly ela ed o he in e ac ion
be ween he analy e and he sensing ma e ial. The sensing
ma e ial would be able o sense he a ge analy e when he
analy e molecules ge ee access o he ma e ials’ in e ac ion
si es and he in e ac ion causes a change in he elec onic
pa ame e s o he ma e ial.
MOFs a e known o p o ide eco d-b eaking su ace a ea
om a ma e ials pe spec i e
172,206
and accessible unc ional
si es
207,208
o analy es owing o hei highly o de ed igid
po osi y due o excellen c ys allini y. Two-dimensional MOFs
possess o de ed long 1D channels wi h accessible me al and
linke unc ional si es ha induce sub-ppm le el de ec ion o
analy e gases. Fo ins ance, 2D conduc i e MOFs we e able o
sense lowe gas concen a ions. Conduc i e MOFs p o ide
eliable high c oss- eac i e de ec ion o a pa icula analy e
gas among a ious VOCs and high elec ical conduc i i ies
a ou he ease o sensing esponse o gas analy es. Howe e
2D MOFs wi h powde o bulk mo phology a e no much
a ou able o as cha ge ans e and also as gas diffusion.
In his ega d, Yao e al. came up wi h a laye -by-laye (LBL)
ab ica ion echnique o syn hesizing conduc i e MOFs, he e
Cu
3
(HHTP)
2
-xC, which can excellen ly con ol he 2D conduc-
i e MOF’s hin- ilm hickness down o an accu acy o 2 nm
(Fig. 10a–c).
209
The LBL hickness con olled syn hesis o 2D-
cMOFs was e ec ed by simul aneously sp aying me al sal and
o ganic ligand on he –OH unc ionalized subs a e such as
qua z, sapphi e e c (Fig. 10a). This imp o ed he sensi i i y o
al eady exis ing 2D conduc i e MOFs ha we e used in powde
o bulk o ms, as depic ed by he sensing o he ammonia gas
(DR
a g
= 129%) which is many olds as compa ed o he p e ious
s udies (Fig. 10b and c).
210
This hin laye mo phology o he
MOFs also helped in a as esponse ime due o he as
accessibili y o he a ailable su ace a ea o he a ge gases.
In e ms o he laye ed g ow h o 2D conduc i e MOFs, Smi h
e al.,
183
ha e been wo king ex ensi ely on ex ile-based ma e i-
als, came up wi h a 2D conduc i e MOF ab ica ion me hodol-
ogy on ex iles o lexible MOFs using Ni
3
(HITP)
2
and
Ni
3
(HHTP)
2
MOFs ia a bo om-up modula g ow h app oach
(Fig. 10d). This me hodology led o he expec ed sub-ppm le el
de ec ion o gases (NO and H
2
S), high gas esponses (49–98%)
and oom empe a u e sensing. MOFs can also be equipped
wi h a ious unc ionali ies ha can play as in e ac ion si es o
a ious gases and VOCs o p oduce chemi esis i e modula ion
in he sensing MOF ma e ial. MOFs can be uned e y easily in
e ms o unc ionali y which comes om he o ganic linke s.
Di e en gases in e ac wi h di e en unc ional g oups wi h
some pa icula o ce o in e ac ion and his ea u e can be
easily induced in o MOFs o imp o e he MOF–gas in e ac ion.
One o he such ini ial s udies was demons a ed by Wang
e al.
211
whe e hey exploi ed he ee ca boxyl unc ionali ies o
MOF linke s o hei s ong H-bonding abili y wi h ola ile
amine p o ons. MD Mello e al. played wi h he in e ac ion si es
in he o ganic linke agmen s o UiO-66 MOF whe e hey used
BDC, BDC–NH
2
and BDC–OH as o ganic linke s o he de ec-
ion o acidic gases such as NO
2
,SO
2
e c.
212
As expec ed om
common acid–base in e ac ions, in hese MOFs, basic unc-
ionali ies such as NH
2
,OHe c in e ac ed in a simila way wi h
he acidic gases such as NO
2
,SO
2
e c. Since he e is some
elec onic cha ge ans e om basic o acidic moie ies in acid–
base in e ac ions, he esul ing change in he elec on densi y
o he basic moie y can be seen e lec ed as a chemi esis i e
signal when employing he MOF ma e ial in chemi esis i e gas
sensing.
The de ec ion o he gases was ca ied ou a a empe a u e
o 150 1C, bu o an ideal gas senso , i should be ope able a
ambien empe a u e o widen he ope a ion window o gas
sensing in a ious ields. A ele a ed empe a u es, he mul iple
hea cycles can lead o abno mal c ys al g ow h which can
b eak he elec onic in e connec ions
213
and can also educe
he ab ica ed senso s’ li e ime. Thus, chemi esis i e gas sen-
sing is always sough and p e e ed a oom empe a u e o e
high- empe a u e gas sensing. Bu , high empe a u e is
equi ed o highe sensi i i y and e e sibili y in he case o
mos o he adi ional me al-oxide gas senso s. Since po ous
ma e ials p o ide a high deg ee o sensing ma e ial–gas analy e
in e ac ion owing o hei high su ace a ea and app oachable
in e ac i e si es due o hei highly c ys alline na u e (especially
in he case o MOFs and COFs), hus high- empe a u e equi e-
men s o gas sensing a e unneeded. Ex ensi e wo k on 2D
MOFs and he s udy o hei elec onic p ope ies o apply o
chemi esis i e gas sensing has been done. The gas sensing
pe o mance gi es excellen esul s a oom empe a u e
e ealing he s uc u al and unc ional supe io i y o 2D
po ous ma e ials such as 2D MOFs o e con en ional non-
po ous ma e ials. S assen e al. wo ked up on coppe
hexaiminobenzene(HIB)-based 2D elec onically conduc i e
MOFs o CO
2
sensing.
214
Cu
3
(HIB)
2
consis ing o imino-
semiquinona e (–NH–) g oups which a e elec on- ich and
hus in e ac wi h acidic gases such as CO
2
. –NH– moie ies
a e ound o be o signi ican ele ance since CO
2
sensing
wi h he hexaoxy iphenylene (HOTP)-based MOF,
Cu
3
(HOTP)
2
p o ided unmeasu able signals indica ing he
e icien elec on dona ing in luence o imino moie ies in
con as o oxo moie ies. Wo king in he di ec ion o 2D
conduc i e MOFs, Meng e al. ab ica ed ph halocyanine-
based 2D MOFs con aining Ni and Cu as me al cen es and
as ph halocyanine-ca i y me al – hus p oducing bime allic
2D MOFs.
215
These 2D MOFs p o ided h ee majo ad ances
in chemi esis i e gas sensing. Fi s , he p esence o wo-
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dimensional s uc u es o he MOFs due o plana , conju-
ga ed ph halocyanine linke s, hey p o ide a high su ace
a ea, excellen conduc i i y, and a highly o de ed dis ibu-
ion o ac i e si es in he sensing ma e ial. In insic conduc-
i i y o me al-ph halocyanine moie ies p o ides ul a
sensi i i y o he sensing ma e ial owa ds he a ge gases
(H
2
S, NO, NH
3
). Second, by a ying he o ganic linke (me al-
loph halocyanine and me allonaph halocyanine), sensi i i y
and selec i i y can be uned ia iso e icula modula ion
o he sensing MOF ma e ial. Thi d, due o high in insic
conduc i i ies (B10
2
Scm
1
), excellen sensing pe o -
mance can be seen e en a lowe sensing ol ages (B0.01 V
o 1.0 V).
Thus, he senso equi es a lesse amoun o ene gy o
ope a e. 2D c-MOFs p o ide highly elec oac i e chemi esis i e
sensing ma e ials bu as e sensing pe o mance and ai gas
selec i i y a e challenging ask. Imp o ising his condi ion,
Wang e al. imp o ed hese wo ea u es in a single 2D c-MOF
by uning i s su ace-pola i y.
216
Usually, 2D c-MOFs a e hyd o-
philic in na u e due o he p esence o me al nodes and pola
Fig. 10 (a) Illus a ion o he Cu
3
(HHTP)
2
c ys al s uc u e (b) he p epa a ion o Cu
3
(HHTP)
2
hin- ilm gas senso s. (c) Response o Cu
3
(HHTP)
2
owa ds
diffe en educing gases. (d) Response o Cu
3
(HHTP)
2
owa ds NH
3
analy es wi h diffe en concen a ions. Rep oduced wi h pe mission om e . 209
Copy igh 2017, Wiley-VCHGmbH. (e) Cus omized Te lon con aine wi h SOFT- ex ile-MOF based sequen ially s acked sensing ma e ial and gas
esponse cu es o he I-ma e ial (solid line) and II-ma e ial (dashed line) o Ni
3
(HHTP)
2
and Ni
3
(HITP)
2
. Rep oduced wi h pe mission om e . 183
Copy igh 2017, Ame ican Chemical Socie y.
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a oms such as N, O e c (Fig. 11a). G a ing wi h long alkyl
chains, hyd ophobici y can be in oduced in o he same ma e-
ial. Wi h he in usion o hyd ophobici y, he adso p ion/
deso p ion o wa e apou s becomes mo e equen , hus
dec easing esponse/ eco e y imes by a much g ea e ex en .
Fo ins ance, su ace modi ica ion s udies o 2D c-MOF
Ni
2
[MPc(NH)
8
] wi h o ganosilanes such as phenyl ichlo osi-
lane (PTCS), (3-aminop opyl) ime hoxysilane (APTMS), and
oc adecyl ime hoxysilane (OTMS) (Fig. 11b). Ou o hese,
Ni
2
[MPc(NH)
8
]-OTMS p oduced he highes le els o hyd opho-
bici y (wa e con ac angle = 1381) which leads o as e eco e y
om humidi y (Fig. 11c) and hus was used as a VOC sensing
ma e ial. Since he pola i y end o lowe alcohols ollows
me hanol 4e hanol 4isop opanol end, me hanol wi h he
highes pola i y showed he as es diffusion and esponse
owa ds he su ace modi ied sensing ma e ial (Fig. 11d– ).
Thus, su ace modi ica ion o 2D c-MOFs es ablished oppo u-
ni ies o de elop hese ma e ials in he ield o elec onics.
Discussing he ad an ages o 2D c-MOFs, hey p o ide a 2D
su ace o he analy es o he in e ac ion o sensing pu poses.
The su ace unde lying a ea hus emains unapp oached by he
analy e pa icles which was highligh ed by Lin e al. while
p oposing an imp o emen in his scena io by eplacing 2D c-
MOFs wi h LBL g own 3D c-MOFs.
217
Langmui –Blodge
deposi ion o one kind o ma e ial on he ele an subs a e
p o ides a hickness-con olled and highly o ien ed ma e ial
designing app oach. This app oach was used o ab ica e a Cu-
HHTP based 3D sensing ma e ial which o he wise p e iously
used o be syn hesized as 2D c-MOF ma e ials. LBL o e la
su aces yields only su ace-exposed ma e ials whe eas o e 3D
subs a es such as a nanowi e a ay, p o ides highe su ace
a ea o laye ed ma e ial wi h highe exposu e o ac i e si es.
This esul ed in an ul alow de ec ion o ammonia gas (5 ppb)
which is abou a housand imes lowe han epo ed o 2D Cu-
HHTP MOFs. Since he 3D Cu-HHTP ma e ial is g own on TiO
2
-
NWAs, which doesn’ show an obse able esponse owa ds he
ammonia gas, he sensing pe o mance o his ma e ial can be
conside ed solely due o he 3D-g own Cu-HHTP MOF.
Ca ying he excellen po en ial o 2D-based c-MOFs, Meng
e al. (Fig. 12a and b) and Aykana e al. syn hesized (Fig. 12c–e)
bime allic ph halocyanine and nap haloph halocyanine based
2D conduc ing MOFs in wo di e en s udies bu by he same
g oup o enligh en he sensing and di e en ia ing a ious
gases such as H
2
S, NH
3
and NO in he o me s udies by Meng
e al. and CO gas sensing unabili y and in ensi ying he
sensing pe o mance in he la e s udies by Aykana e al.
218
In he o me expe imen s, iso e icula syn hesis o obus
bime allic MOFs and hei chemi esis i e esponse owa ds
a ious gases e eals he e ec i e in e ac ion be ween analy e
and sensing ma e ial due o he p esence o uni o mly dis ib-
u ed me allic and linke si es, which a e able o e ec i ely
di e en ia e be ween a ious ypes o gases. Du ing he la e
expe imen s, Co and Ni-based MPc-O
8
-Cu MOFs we e gene -
a ed, s uc u ed as Co o Ni inse ed hyd oxy- unc ionalized
ph halocyanine linke s in e connec ed ia Cu nodes. The 2D
na u e and p esence o bi-me als in he s uc u e esul ed in o
he high conduc i i y, hus p o iding he ad an age o using a
low ol age (0.1 V) o powe he sensing de ice and ecei e
op imal esul s. The bime allic ph halocyanine pa e n o he
sensing ma e ial helped o wo pa icula easons – i s , he
Fig. 11 (a) Chemical s uc u e and Ni
2
[MPc(NH)
8
] (M = Cu & Ni). (b) Su ace modi ica ion o Ni
2
[MPc(NH)
8
] MOF by APTMS, PTCS and OTMS and (c)
esponse cu es (DG/G
o
alues s ime) o diffe en H
2
O apo s concen a ion ed o he sensing ma e ial. (d) and (e) Response cu es (DG/G
o
alues
s ime) o diffe en (d) me hanol and (e) e hanol concen a ions ed o he sensing ma e ial. ( ) Response cu es (DG/G
o
alues s. ime) o a ious
p o ic and ap o ic analy es in e ac wi h Ni
2
[MPc(NH)
8
]. Rep oduced wi h pe mission om e . 216 Copy igh 2021, Wiley-VCH.
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need o ac i e CO hos si es and he si es o elec onic
ansduc ion o he elec onic pe u ba ion due o CO in e -
ac ion wi h he me al-ph halocyanine MOF.
Second, he p esence o Cu induces edox ac i e cha ge
hopping h ough he 2D-cMOF ne wo k by ac ing as a node
o me alloph halocyanine ne wo k ex ension. The p esence o
hese ea u es in hese MOFs helped in he ul alow sub-ppm
le el de ec ion o CO. Fu he insigh in o he CO sensing
pe o mance was achie ed by compu a ional s udies using
DFT calcula ions. The in e ac ion o CO wi h Co-based MOFs
Fig. 12 (a) Illus a ion o and naph halocyanine- and ph halocyanine-based MOFs – NiNPc-M and NiPc-M espec i ely. (b) Chemi esis i e gas sensing
esponse o NiPc-M and NiNPc-M MOFs upon exposu e o 40 ppm o NH
3
and H
2
S, and 1 ppm o NO in d y ni ogen (solid ba ) and d enched in 5000
ppm H
2
O (ba wi h d ople symbols). PCA analysis o NiPc-Cu (blue), NiPc-Ni (g een), NiNPc-Cu (yellow) and NiNPc-Ni ( ed) senso a ays. Rep oduced
wi h pe mission om e . 183 Copy igh 2017, Ame ican Chemical Socie y. (c) Bonding op imiza ion o Co- and Ni-based Pc-MOFs. (d) Binding ene gy
calcula ions o CO in e ac ion wi h Co and Ni-Pc MOFs. (e) Highes sensing pe o mance o Co- and Ni-based Pc-MOFs as compa ed wi h o he
iphenylene linke -based 2D c-MOFs. Rep oduced wi h pe mission om e . 218 Copy igh 2015, Wiley-VCHGmbH.
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(Fig. 12c) was op imized and some cons an CO–Cu and CO–Co
bonding dis ances we e ealized o he bes sensing esul s.
The linea i y o hese bonds was ealized by he me al–CO back
bonding. Binding ene gy op imiza ion s udies e ealed he
incompa ibili y o he Ni based MOF o bind CO e icien ly
because o he posi i e ee ene gy o he Ni–CO binding
whe eas he nega i e Co–CO ee ene gy sugges ed he posi i e
esul s in he case o he Co-based MOF (Fig. 12d). Also, he
high sensi i i y o he Co-based MOF han he Ni-based MOF
can be explained by he same heo e ical easons. The supe io
esponse om ph halocyanine based sensing ma e ials as
compa ed wi h he iphenylene based 2D sensing ma e ials
could be because o he p esence o doped Co- and Ni- me allic
en i ies. Thus, he syn hesis o p is ine MOFs, hei inco po a-
ion in o he ab ica ed chemi esis i e gas sensing de ice and
uning o hei ea u es such as po osi y, unc ionaliza ion, and
mo phology esul s in he uning o sensing pe o mances o
he p is ine MOF-based gas senso s (Table 1).
5.1.2. MOF hyb ids/de i a i es. MOFs can be b oadly clas-
si ied in o wo ypes o ma e ials – one ype ha includes MOFs
as empla es o p ecu so s o hyb ids/composi es ha con ain
MOF as a cons i uen and o he ypes ha includes ma e ials
ha hemsel es don’ con ain MOF en i ies bu a e de i ed
om MOFs. Me al-oxide semiconduc o (MOS)-based chemi -
esis i e gas senso s ha e al eady been de eloped signi ican ly
by imp o ing hei sensing pe o mance by hyb idizing wi h
he e onanos uc u es,
219
doping o loading o me al oxides/
mixed me als
220
and making composi es wi h ca bon ma e ials
such as g aphene.
221
MOFs p o ide an excep ional su ace a ea
and a obus amewo k o g an ing chemical s abili y o he
ma e ials hey ge linked wi h. MOF–MOS composi es ha e
been de eloped wi h c i ical insigh in o he conduc i i y,
s abili y, selec i i y and sensing ime o he esul an ma e ial
in chemi esis i e gas sensing. These composi es ha e mos ly
se ed as an upg ade o he p is ine ma e ials. MOFs can ha e
ixed-size po es ha can be used o molecula sepa a ion.
222
D obek e al ecognized his ai o MOFs and used i o
c ea ing a me al-oxide/MOF composi e iz. ZIF-8 cloaked ZnO
nanowi es (NWs) (ZnO/ZIF-8 NWs) (Fig. 13a) o size-based
esponse selec i ely o H
2
while being aloo o la ge C
6
H
6
and C
7
H
8
molecules (c edi o ZIF-8 po osi y) while gi ing
excellen sensi i i y o H
2
a 300 1C (c edi o ZnO NWs)
(Fig. 13b).
113
Since MOF syn hesis equi es a me al sou ce,
Tian e al. ollowed an in e es ing s a egy whe e hey used ZnO
nano ods (NRs) as he sou ce o he zinc ions o he syn hesis
o ZIF-8 MOF shell a ound hemsel es.
223
This syn he ic ech-
nique no only p o ided he MOF–MOS co e–shell he e os uc-
u e, he selec i i y a io o o maldehyde sensing was also
imp o ed (Fig. 13c) wi h espec o o he in e e ing VOCs such
as e hanol, ace one, ammonia e c. in con as o he ba e ZnO
nano od senso s because o he molecula sie ing ia ZIF-8
windows.
224
Koo e al using a simila p o ocol, ab ica ed H
2
gas senso s by de eloping a ZIF-8 nano il a ion ne wo k on Pd
NWs.
225
This wo k imp o ed he hyd ogen gas sensing speeds
and selec i i y by as ening he adso p ion/deso p ion o H
2
molecules on/ om Pd-NWs. Molecula sie ing was exploi ed o
es ic he en y o la ge molecules o O
2
(0.345 nm) and N
2
(0.364 nm) han H
2
molecules (0.289 nm) wi h a ZIF-8 mic o-
po e size o 0.34 nm. Molecula sie ing p ope ies o p is ine
MOFs can be modula ed wi h he help o me al nanopa icles.
As me al NPs ha e he endency o be engul ed inside he MOF
po es, hei spacious occupa ion u he es ic s he la ge
molecules and hus smalle gas molecules can be sepa a ed
mo e speci ically.
Zhou e al., while conside ing his idea, pe o med H
2
sensing by using Ag NPs encapsula ed ZnO/ZIF-71 which
showed highe selec i i y owa ds H
2
and dec eased esponse
o ace one owing o hei size p e e ences (Fig. 14a).
226
This
wo k hus suppo ed he enhancemen o me al oxide sensing
pe o mance which o he wise would ha e been poo as he
no mal case. The a ia ion in he sie ing p ocess changes om
po e o po e due o he diffe en le els o Ag nanopa icles
illing inside he po es o ZIF-71. Tempe a u e p og ammed
deso p ion (TPD) s udies e ealed he obs uc ion o ace one
deso p ion while suppo ing he hyd ogen deso p ion by
inc easing he sil e nanopa icle concen a ion inside he
ZIF-71 po es (Fig. 14b). This e ealed he effec o po e size
uning on he molecula sie ing by embedding me al nano-
pa icles inside he ca i ies o he MOF and hus he subse-
quen sensing o singled ou gases. The sensing esponse o he
ZnO@ZIF-71*Ag NRA owa ds ace one and hyd ogen wi h
diffe en Ag-loading concen a ions also suppo he me al
nanopa icle based sie ing uning whe e wi h an inc ease in
he Ag-load, H
2
esponse inc eases and ace one esponse
dec eases (Fig. 14c).
Simila ly, ecen ly, he molecula sie ing p ope y o ZIF-8
was exploi ed o he H
2
sensing a lowe empe a u es o
100 1C as compa ed o he p e iously s udied ZnO-NPs/ZIF-8
composi es. In his wo k, Poschmann e al. poin ed ou he use
o ZnO nanopa icles in polyc ys alline powde o m and he
conduc i i y hind ance effec o he ZnO–MOF–ZnO sequenced
sensing ma e ial mo phology.
228
Thus, hey used he e apodal
single-c ys alline ZnO mic opa icles o a oid his undesi ed
conduc i i y educ ion. The ZIF-8 laye o e he single-
c ys alline ZnO mic opa icles p o ides excellen selec i i y
owa ds he H
2
molecules, e en in he p esence o op imum
me hane concen a ion. This is obse ed due o he pinhole-
ee mo phology o ZnO mic opa icles plus ZIF-8’s mic opo -
osi y. Ano he compounding eason is he inc eased igidi y o
he po e windows due o he composi e o ma ion wi h single
uni ZnO pa icles. He e, due o he single-c ys alline na u e o
he ZnO pa icles which a e su ounded by MOF laye s, he
change in conduc i i y canno possibly occu due o he lack o
adso bed a mosphe ic oxygen molecules. This migh be
because o he p esence o oxygen de ec s on he c ys al su ace
which p o ide he in e ac ing su ace o he hyd ogen mole-
cules and esul in o he change in he elec ical pa ame e s.
One impo an hing o no e he e is he op imum hickness o
he MOF laye s g own o e he me al/me al-oxide nanowi es.
Thin MOF laye s a e being used he e o p o ide molecula
sie ing and as e di usion, whe eas hicke MOF laye s
dec ease he a eling speed o gas molecules and hus inc ease
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Table 1 MOF and i s hyb ids/de i a i es o chemi esis i e gas senso s
S. no. Ma e ial VOCs Sensing mechanism Response (%) LOD (ppm)
es
–
ec
(s) Re .
1 ZIF-67 TMA 14.1 2 193
2 ZIF-67 FA 13.9 5 191
3Cu
3
(HITP)
2
NH
3
0.5 202
4M
3
(HHTP)
2
(M = Cu, Ni) Va ious VOCs CT 203
5Ni
3
(HHTP)
2
@ ex ile NO CT 49 10 1.4 183
6H
2
S989 0.23
7Ni
3
(HITP)
2
@ ex ile NO CCT 81 6 0.16
8H
2
S972 0.52
9Cu
3
(HHTP)
2
NH
3
DA 129 0.5 81.6–46.6 209
10 Cd(H
2
L)
2
EDA HD-DA o3–10 211
11 [Cd(TMA)(DPP)
0.5
H
2
O]
n
Humidi y P o on hopping 11–56 282
12 UiO-66-NH
2
SO
2
DA 21.6–2.7 1 26.8 5.4 212
NO
2
7.6–0.4 10
CO
2
11.4–2.2 500 35 2
13 Cu
3
HIB
2
CO
2
DA 0.62 420–660 214
14 Cu-TCPP@ Cu-HHTP NH
3
CCT 94 91.8–643.2 283
C
6
H
6
153
15 NiNPc NH
3
CT/Redox ac ion 43–45 0.31 215
H
2
S 64–98 0.019
NO 657–397 0.001
16 Cu-BHT NH
3
ET 7.88 0.23 58–102 284
17 Ni
2
[MPc(NH)
8
]CH
3
OH P o on ans e 4.7 10 36–13 216
18 Cu-HHTP NH
3
Cha ge/Mass anspo 42 0.005 35–900 217
19 Cu-HITP NH
3
CCT 7.1 0.5 285
20 NiPc@CoTAA NO
2
ET 37.6 300–3600 286
21 HITP@Cu-HHTP NH
3
CCT 80 0.024 o60–600 287
Benzene 75 0.096
22 HIB-Cu Humidi y CT 200 21–40 288
23 Zn-BDC-NH
2
H
2
2.93 289
24 CoPc-O
8
-Cu CO 27.4 0.8 0.5-3 218
NiPc-O
8
-Cu 18.9 0.8
25 CopyNDI NH
3
CT 46.7 0.00015 ppb 168–198 290
NipyNDI NH
3
426
ZnpyNDI 32.5
26 Cu
2
O/CuO E hanol DA 291
27 ZIF-CoZn/ZnO Ace one DA 27 0.0019 43.2–61.2 292
28 ZnO@ZIF-8 N H
2
DA 1.44 113
29 ZnO@ZIF-8 FA DA — 5.6 16–9 223
30 Pd@ZnO (ZIF-8)-WO
3
NFs Toluene DA S= 4.37 0.1 293
31 PEDOT@MIL101(C ) NO
2
DA 0.9 0.06 30–150 259
32 PdO@ZnO(ZIF-8)-SnO
2
Ace one DA 5.6 0.010 o20–64 294
33 PdO@Co
3
O
4
(ZIF-67) Ace one DA 2.51 0.1 295
34 Pd NWs@ZIF-8 H
2
DA 3. 0.6 7–10 225
35 ZnFe
2
O
4
@MOF-5 Ace one DA 64.4 296
36 PdO@Co
3
O
4
-nSnO
2
Ace one DA 22.8 1 90.8–108.4 297
37 ZnO@ZIF-8 Ace one e hanol DA 298
ZnO@ZIF-67
38 Pd-ZnO@ZnCo
2
O
4
Ace one DA 69 299
39 ZIF8/Pd/ZnO NWs H
2
DA 8.5 0.5 227
40 ZIF- Co
3
O
4
ods@ZnO Ace one DA 25 0.005 300
41 ZIF-67@Co
3
O
4
p-Xylene DA 78.6 63–86 301
Toluene 43.8
42 ZIF-67@WS
2
NO
2
DA 48.2 0.1 302
43 ZIF-67@SnO
2
CO
2
DA 48.2 18–25 229
44 POM@ZIF-8@ZnO FA DA 4.4 0.387 15.1–16.2 230
45 TiO
2
/Co
3
O
4
NFs by TBT@ZIF-67 E hanol DA 16.7 5 303
46 ZnO NSs@ZIF-L CO DA 3.2 0.134 30–1 304
VOCs 1.4 0.02
47 P @Cu
3
(HHTP)
2
NO
2
DA 62.11–57.38 828–840 233
48 Ag@ZnO@ZIF-71 H
2
DA 226
49 Au-ZnO@ZIFs Ace one DA 231 0.0001 305
50 In
2
O
3
/MoS
2
MIL-68(In) NO
2
DA 371.9 306
51 MIL-53(Al)/CNT CO
2
Po e ansi ion o MOF 15 o30 249
52 ZIF-8/ZnO NRs H
2
S DA 52.1 0.05 420 307
53 HKUST-1/MoS
2
H
2
O PT 8–14 0.38 182
54 Cu(BTC)@PDMS CO DA 0.46 264
MIL-160@PDMS Humidi y
55 Ln(acac)
3
@ZIF-8 NO
2
DA 187.9 0.0002 1050–1230 308
56 CoSnO
3
@MOF H
2
S DA 12.1 10 0.00018 234
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he esponse and eco e y imes. SnO
2
NPs/ZIF-67, due o he
uneable po osi y o he MOF coun e pa , was explo ed as a
no el gas sensing ma e ial by DMello e al.
229
The exclusi e and
s able sensing esponse o CO
2
was deli e ed because o he
sugges ed syne gis ic e ec be ween SnO
2
and ZIF-67. Syne gis-
ic in ol es he ans e o elec ons a he in e ace o ZIF-67
and SnO
2
om he o ganic linke (Imidazole) o he MOS me al
(Sn) and pa allelly om he oxide o he MOF me al (Co), which
s abilizes he ca bona e o ma ion om CO
2
a e in e ac ion
wi h oxides. Syne gis ic e ec s we e u he explo ed by Wang
e al. as hey s udied o maldehyde de ec ion ia highe pho o-
cu en eme gence alues o he POM@ZIF-8@ZnO (POM =
Polyoxome ala e) as compa ed o ZIF-8/ZnO nano ods.
230
ZIF-8
po es se ed as an analy e concen a ing medium so ha
o maldehyde molecules can ga he densely and p oduce an
equi alen ly highe pho ocu en . These obse a ions u he
co obo a ed he ad an ageous ole o syne gis ic e ec s o
me al-oxide-MOF combina ions. Ge ing some mechanis ic
insigh in o hese excellen ou comes o me al-oxide NWs/
MOFs hyb ids, inco po a ing me al nanopa icles g ea ly
ad ances he sensing pe o mance o he MO NWs/MOFs
ma e ials. Me al a oms can exis as agglome a ed nanopa icles
which a e ound o show a spill-o e e ec . Gaseous molecules
while in e ac ing wi h he me allic nanopa icles dissocia e in o
smalle agmen s and hus can in e ac mo e in ensely wi h
he su ounding hos ma e ial.
Hyd ogen spill-o e effec (HSPE), i s disco e ed in 1964, is
a peculia su ace phenomenon in which H
2
molecules dis-
socia e in o H-a oms a e being ‘‘cu ’’ by iny me al nano-
pa icles and subsequen ly ‘‘spill’’ o e he suppo ed
ma e ial.
231
Webe e al. ab ica ed a ZnO NWs/ZIF-8 hyb id
embedded wi h Pd nanopa icles (NPs) o chemi esis i e H
2
sensing and inc eased selec i i y excellen ly owa ds he H
2
sensing compa ed o ba e ZnO NWs obse ed because o he
molecula il e ing ou VOCs o he han H
2
on he basis o
size.
227
The imp o ed esul s we e seen because o he spill-o e
effec due o Pd-NPs combined wi h he nano il a ion due o
ZIF-8 po es. Al hough Pd@ZnO NWs/ZIF-8 exhibi s a lowe
esponse owa ds H
2
sensing, he po ous ZIF-8 cladding helped
his ma e ial o ou pe o m bo h Pd@ZnO NWs and ZnO NWs
in H
2
selec i i y. The compa ison o H
2
sensing mechanisms
o ZnO NWs, Pd/ZnO NWs and ZIF-8@Pd/ZnO NWs as gas
sensing ma e ials is schema ically shown in (Fig. 14d). H
2
being
a educing gas leads o educ ion o ZnO o Zn me al in ZnO
NWs and addi ional Pd NPs o PdH
x
in Pd/ZnO NWs and ZIF-
8@Pd/ZnO NWs, each o which causes conduc i i y inc emen s.
Due o he absence o a spill-o e effec o he educing gases
o he han H
2
esul s in no signi ican enhancemen in he
sensing pe o mance o hese VOCs, hus p oducing a be e
compa a i e s udy o H
2
sensing. Me al nanopa icles a e
ema kable ca alys s and due o hei nano-sized na u e, hey
can be in used in o nano, mic o and mac opo ous ma e ials.
Smalle size o me al NPs p o ides hem wi h la ge su ace
a ea which hus imp o es he ca aly ic pe o mance o hei
hyb ids.
232
Based on hese p ope ies o me al NPs, hei encapsula ion
in MOFs has also shown an enhancemen in hei ca aly ic
p ope ies. 2D conduc i e MOFs inco po a ed in o me al NPs
such as Pd, P and Au NPs, ha e been shown o imp o e bo h
hei ca aly ic as and conduc ing p ope ies. Koo e al. demon-
s a ed he e amping o Cu-HHTP based c-MOFs by using
hem wi h Pd and P NPs (Fig. 15a and b).
233
The ca aly ic effec
o me al NPs was obse ed o P @Cu
3
(HHTP)
2
and can be
suppo ed by he ac i a ion ene gy deple ion o NO
2
adso p ion
by he sensing ma e ial which esul ed in he highe dec ease
in he p- ype c-MOFs esis ance while in e ac ing wi h s ong
elec on accep o NO
2
gas (Fig. 15e). In he case o
Pd@Cu
3
(HHTP)
2
, he ac i a ion ene gy was only aised up bu
i s ill showed a dec ease in he esis ance o he sensing
ma e ial. This p o ided an insigh u he in o he elec onic
s uc u e change upon NO
2
adso p ion on he sensing ma e ial.
While in e ac ing wi h he Pd NPs, NO
2
molecules in e ac ed
ia he o ma ion o a ni i o-complex wi h he Pd NPs, abso b-
ing elec on densi y om hem, and hus educing he Scho ky
junc ion ba ie ha pe sis s be ween Pd NPs and c-MOF. This
inc eased he conduc i i y o he sensing ma e ial and hus
enhanced i s sensing pe o mance ia elec onic sensi iza ion
o NO
2
by Pd NPs (Fig. 15e). This p o ides he sensing ma e ial
wi h a mo e enhanced esponse owa ds he NO
2
gas molecules
(Fig. 15c) and hus ac s as a s a e-o - he-a NO
2
gas sensing
de ice (Fig. 15d). The s udy o eac ion kine ics o hese
e en ual eac ions was bene icial o unde s anding he effec
o me al nanopa icles’ ca aly ic p ope ies on he NO
2
gas
in e ac ion eac ions wi h he sensing ma e ial su ace. Adso p-
ion/deso p ion a es o NO
2
on/ om M@Cu
3
(HHTP)
2
we e
enhanced mo e p ominen ly du ing P NP doping as compa ed
o Pd NP doping. This end was obse ed because P NPs
dec ease he ac i a ion ene gy o NO
2
in e ac ion wi h he me al
Table 1 (con inued)
S. no. Ma e ial VOCs Sensing mechanism Response (%) LOD (ppm)
es
–
ec
(s) Re .
57 G@Cu-BTC CHCl
3
DA B0.15 41 309
G@ZIF-8 B0.10
G@UiO-66 B0.04
58 ZnO/CuO om Zn/Cu-BTC H
2
S DA 393.35 0.0003 173–3000 310
59 GA@UiO-66-NH
2
CO
2
DA 8.6 18 221
60 GO@PDDA@Co
3
(HITP)
2
NO
2
IC 9.05 0.0068 24–41 311
61 -ZnO@ZIF-8 H
2
DA 546 1–2 228
DA = Dono –accep o ; CT = Cha ge ans e ;Cha ge ca ie anspo = CCT;E hylenediamine = EDA;Fo maldehyde = FA;T ime hylamine =
TMA;Elec on ans e = ET;In ashee conduc i i y = IC;P o on ans e = PT;Hyd ogen bonding = HB.
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access o he amine in e ac ion si es ensu es as e esponse
and eco e y imes o CO
2
sensing (
es
= 18.5 s,
ec
= 18.7 s).
Consequen ly, he in oduc ion o amine unc ional g oups
inside he po ous pa hway o GA@UiO-66-NH
2
p o ides a high
esponse o 8.6% (Fig. 19b–d) compa ed o he p is ine GA
which shows a negligible esponse o CO
2
unde he same
sensing condi ions. In compa ison, g aphene has a low elec-
ical esis i i y (B10
6
ohms) and high ca ie elec on mobi-
li y. S ill, he lack o speci ic and s ong in e ac ion si es o he
adso p ion o gaseous analy es, ende s i unusable o chemi -
esis i e sensing in i s p is ine o m. These esul s a e cla i ied
by empe a u e-dependen in si u Raman spec oscopy du ing
he exposu e o he ma e ial o CO
2
. While up o 125 1C, he
so ening akes place which can be obse ed u he up o
200 1C, sugges ing he g adual up ake o CO
2
in o he po ous
GA-UiO-66-NH
2
ne wo k while s ongly in e ac ing wi h he
amide bonds. A u he inc ease in he empe a u e causes an
inc ease in he equencies o bo h –CO–NH linkages and Fe mi
modes o CO
2
sugges ing a e y weak in e ac ion o CO
2
wi h
he sensing ma e ial.
5.2. Co alen -o ganic amewo ks as chemi esis i e gas
senso s
Co alen -o ganic amewo ks (COFs) ha e been employed o
sensing pu poses due o hei highly o de ed s uc u e, unc-
ional g oup e sa ili y, and high he mal and chemical s abi-
li y. They ha e been p ima ily u ilized o explosi e sensing,
me al ion sensing, humidi y sensing, biosensing, gas sensing,
and mo e.
312
Howe e , co alen -o ganic amewo ks (COFs) a e
in hei nascen s age ega ding he applica ion o chemi esis-
i e gas sensing.
313
One majo eason o his is he challenging
ask o b inging in insic elec onic conduc i i y in hem in
addi ion o he obus amewo k ha is he undamen al
necessi y o a success ul gas sensing ma e ial wi h equisi e
esul s ha can ou shine o a leas compe e wi h he al eady
a ailable sensing ma e ials’ pe o mance (Table 2).
314–321
5.2.1. P is ine COFs. Despi e hei poo conduc i i y, some
esea che s ha e ac i ely pu sued using COFs as chemi esis i e
gas senso s, aiming o compa e hem di ec ly wi h o he
sensing ma e ials due o hei p omising po ous and e sa ile
unc ionali y. Singh e al. epo ed a T uxene-based COF o
humidi y sensing.
322
The high su ace a ea and o de ed s uc-
u al po osi y along wi h he bo ona e es e g oups in he COF
esul ed in o an excellen in e ac i e ma e ial o humidi y. The
s abili y o his COF in a high humid en i onmen is migh be
due o i s s uc u al obus ness. The conduc i i y equi ed o
he chemi esis i eness comes om he plana COF shee s and
he mul ilaye o ma ion o wa e apou s a highe RH% ha
ac s as a p o on conduc ion medium h oughou he sensing
COF ma e ial. Thus, he sensing can be a ibu ed much o he
humid medium a he han he in insic COF conduc i i y.
COFs mos o en a e e y low (B10
3
o 10
10
Scm
1
)o
non-conduc i e ma e ials. To ackle his p oblem o conduc i -
i y equi emen s, many esea che s ha e wo ked o p oduce
conduc i e COFs by modula ing hei spa ial s acking, elec o-
nic band gap and uning amewo k backbone unc ionali ies.
Meng e al. ha e epo ed a p is ine COF, named COF-DC-8,
wi h an in insic bulk elec onic conduc i i y o 2.51 
10
3
Sm
1
ha was he highes epo ed o a COF a ha
ime
323
(p esen ly, Huang e al. achie ed 12.7 S m
1
o a
pipe azine-linked COF).
324
The high conduc i i y can be a ib-
u ed o ull p–pconjuga ion sp ead o e he en i e in insic 2D
amewo k and he DFT-sugges ed ou -o -plane cha ge ans-
e ing due o he aniso opic band s uc u e (Fig. 20a and b)
Fig. 19 (a) Schema ic illus a ion o he syn hesis o GA@UiO-66-NH2 and GA@UiO-66-NH
2
po ous ne wo k wi h amide linkage. (b) Gas sensing
pe o mance o GA@UiO-66-NH
2
( om 100% o 5% CO
2)
a an ope a ing empe a u e o 200 1C. Gas sensing cha ac e is ics (% esponse, esponse ime
and eco e y ime) o he hyb id a diffe en concen a ions o CO
2
; Tempe a u e effec on he sensing cha ac e is ics o GA@UiO-66-NH
2
(c) on he
esponse % and (d) esponse and eco e y imes. Rep oduced wi h pe mission om e . 221 Copy igh 2021 Royal Socie y o Chemis y.
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(syn hesized by condensa ion o wo plana , igid and hem-
sel es ully conjuga ed me al-ph halocyanine and py ene co e
monome s). This COF showed chemi esis i e na u e owa ds
a ious gases wi h excellen esul s o NH
3
,NO
2
,H
2
S and NO
gases (Fig. 20c– ). The me al-analy e in e ac ion was exploi ed
in his case o chemi esis i eness since me al doped ph halo-
cyanine ings ha e imp o ed elec onic conduc i i ies in con-
as wi h he p is ine ph halocyanine ings. The cha ge
ans e in e ac ion o oxidizing (NO and NO
2
) and educing
(H
2
S and NH
3
) gases wi h he Ni-cen e co esponds o he
mechanis ic pe o mance o he sensing COF ma e ial. Ni
con aining ee alence elec ons has he po en ial o in e ac
weakly ia non-co alen in e ac ions as well as s onge co a-
len in e ac ions oo wi h he analy e gases. This esul s in o a
signi ican change in he elec onic pa ame e s suppo ing he
highly sensi i e chemi esis i e na u e o he COF owa ds he
a ge ed gases which migh also be a eason o he low and
ul alow ppb le el de ec ion limi s o his ma e ial. COFs a e
usually o med as a powde ed ma e ial by adi ional sol o he -
mal p ocesses bu o con enien p ac ical applica ions, obus
ma e ials play he equi ed ole. Fo an e icien , p ac ical gas
senso , he sensing COF ma e ial should ha e such p ope ies
so ha he senso can wo k o longe i y wi h he same
pe o mance. Fa homing his need o he COF-based chemi -
esis i e gas senso s, Mei e al. ab ica ed a lexible and obus
COF-5 ilm on a polyimide (PI) subs a e and employed i o
humidi y sensing.
325
The syn hesis o COF ilms was app oached ia a apou -
assis ed me hod. This me hod p o ides a sophis ica ed way o
con ol he ilm hickness o he COF ma e ial by changing he
monome concen a ion in he solu ion. A e achie ing he
equi ed hickness o he COF-5 ilm and senso ab ica ion,
he de ice was es ed om 11–98% RH ange which showed a
ema kable linea esponse be ween log o esis ance and
Fig. 20 (a) Schema ic ep esen a ion o a ph halocyanine-based 2D conduc ing COF (COF-DC-8). (b) DFT calcula ed aniso opic elec onic band
s uc u e and he co esponding i s B illouin zone. Response cu es o COF-DC-8 when in e ac ing wi h (c) NH
3
,(d)H
2
S, (e) NO, ( ) NO
2
gases.
Rep oduced wi h pe mission om e . 323 Copy igh 2019, Ame ican Chemical Socie y.
Table 2 COF and i s hyb ids o chemi esis i e gas senso s
S. no. Ma e ial VOCs Sensing mechanism Response (%) LOD (ppm)
es
/
ec
(s) Re .
1 COF-TXDBA Humidi y Lewis acid–base in e ac ion 2.3 37 322
42
2 COF-DC-8 NH
3
CT-DA 39 0.07 323
H
2
S 62 0.204
NO 3939 0.005
NO
2
6338 0.016
3 M-TPCOF NO
2
DA 2713 (Co) 0.0068 318 (Co) 329
2056 (Cu) 510 (Cu)
690 (Cu)
4 COF-5 Humidi y 90 26/16 325
5 COF@PANI NH
3
710
5
5
COF@Ppy
6 TiO
2
@COF-316 NO
2
DA 572 (NA) 1.41 8–1.1 330
7 Pd@TpPa-SO
3
H COF H
2
DA 60.1 10 0.2% 5.3–3.1 332
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ela i e humidi y. The COF-5 ilm showed be e pe o mance
in gas sensing han he COF-5 powde . The uni o m mo phol-
ogy and unin e up ed conduc i e con ac due o he 2D ilm
na u e p o ed o be supe io o he disc e e powde in e ms o
elec ical sensing equi emen s.
5.2.2. COF hyb ids. Syn hesizing in insic COFs wi h high
elec ical conduc i i ies is one emendous challenge ha
limi s hei applica ion as a p ac ically usable chemi esis i e
ma e ial. Polyme s, on he o he hand, such as polyaniline
326
and polypy ole,
327
ha e high in insic conduc i i ies – abou
600 S cm
1
and 380 S cm
1
, espec i ely. Conduc ing polyme s
a e known o hei composi ion in o a ious ma e ials such as
MOFs, COFs, Me al oxides, Ca bon ma e ials e c. o ei he
induce o inc ease hei elec ical conduc i i ies o he la e .
Thei in usion wi h hese ma e ials has been explo ed ex en-
si ely in he ield o chemi esis i e gas sensing. This s a egy
o COF-Conduc i e polyme composi es o chemi esis i e gas
sensing was e ec ed insigh ully by Sahine and g oup in 2019,
by using a mesopo ous COF de eloped om condensa ion
be ween melamine and dib omoalkane monome ic uni s, o
c ea e in si u o ma ion condi ions o conduc ing polyme s
such as polypy ole (PPy) and polyaniline (PANi) which esul ed
in o a semi-in e pene a ed ne wo k o COF and conduc i e
polyme s.
5
This in insic g ow h o conduc ing polyme s inside
he COF led o an inc ease in he elec ical conduc i i y by 3-
million olds o COF-PANi and 0.5-million olds o COF-PPy
ne wo ks, espec i ely. This high conduc i i y was u ilized o
gas sensing (NH
3
& HCl apou s) as well as dye sensing (Me hyl
O ange and Me hyl Blue) pu pose in a chemiconduc i e man-
ne . Bo h p is ine COF and COF-Polyme composi es we e
es ed o gas and dye sensing. Towa ds NH
3
and HCl apou s,
p is ine COF showed an inc ease in conduc ance while bo h
COF-Polyme composi es showed an opposi e e ec . This could
p ima ily be due o he p o ona ion o amine g oups which
esul ed in o he o ma ion o posi i ely cha ged ammonium
ions, hus causing an inc ease in he ionic conduc i i y o he
p is ine COF ma e ial. The opposi e e ec in he case o
composi es could be explained by he ac ha he educing
gases such as ammonia cause de-doping o PANi
328
by dep o-
ona ing he b idging amino g oups and o e -oxida ion o
chlo ine ions by PPy. Ma e ials in hei bulk phase canno able
o o e he en i e su ace a ea o in e ac ion wi h he analy e
pa icles. In con as , 2D laye ed ma e ials o bulk ma e ials
a e ex olia ion e eal hei maximum su ace a ea and hus
can in e ac wi h he lowe concen a ions o he analy e
pa icles. In his ega d, Liu e al. de eloped po phy in-based
2D COF nanoshee s (M-TPCOF, M = Co, Cu) whe e po phy in
moie ies can be used o coo dina e me al ac i e si es as a esul
o hei pos -me alliza ion (Fig. 21a).
329
The easily o med s able nanoshee s a e ex olia ion a e a
esul o he non-linea linke (2,6-py idinedica boxaldehyde)
used in syn hesizing his COF which hinde s he p–ps acking
o ex ended po phy in laye s due o he s ong de ia ion om
he plana COF laye s. Thus, i p oduced high su ace a ea
nanoshee s wi h exposed me al ac i e si es which inc eased he
ex en o NO
2
gas in e ac ion wi h he me al ac i e si es and
hus esul ed in he excellen sensi i i y. The highly speci ic
a ini y owa ds he NO
2
gas is p oposed o he p esence o
elec on- ich me al cen es (Co & Cu) due o he highly elec on
wi hd awing na u e o acidic NO
2
gas, which e en ually p o-
duces excellen selec i i y pe o mance as can be igu ed by he
adso p ion s udies o a ious analy es (Fig. 21b). DRIFT IR
spec oscopy has p o en o be a use ul echnique o analysing
gas-hos in e ac ions by ca e ully obse ing he changes in he
in ensi y o in e ac ion signals (Fig. 21c). The b oad peak a
1500–1290 cm
1
co esponds o he NQO s e ching o he
monoden a e ni i e o he asymme ic s e ching NO
2
ib a-
ion o he ni o g oup. The peak a 1398–1353 cm
1
implies o
Co–NO
2
species, indica ing Co as he ac i e si e o NO
2
.A
signi ican ly wide peak obse ed a 2310–2230 cm
1
migh be
due o he NQO s e ching in he NO
+
ions. Wi h an ul a-low
LOD o 6.8 ppb, he de ice showed a as esponse and s ong
binding o NO
2
wi h he me al cen e o M-TPCOF nanoshee s.
This le el o sensing pe o mance is a esul o ew-laye ed
s uc u es o he ma e ials, which p o ide as di usion access
pa hways o he gases. Thus, dis o ion-based ex olia ion o
me allopo phy in bulk COFs o COF nanoshee s p o ides scope
o he ex olia ed ma e ial-based sensing pe o mance imp o e-
men s. COFs can a une su ace p ope ies e ec i ely due o
hei e sa ile na u e owa ds unc ional modi ica ions. They
can be used o he easy uning o edox p ope ies o he
ma e ials based on hei elec onic in e ac ions wi h hem.
This na u e o COFs was beau i ully explo ed by Chen e al.
while wo king wi h he MOS-based chemi esis i e gas senso s.
The TiO
2
nanowi e a ay was coa ed wi h COF-316 (a dioxin-
linked COF), which esul ed in o he e e sing o he educ i e
o oxida i e na u e o TiO
2
(Fig. 22a).
330
This wo k ep esen s
he bene i s o 2D po ous ma e ials o enhancing he sensing
p ope ies o a p e-exis ing me al-oxide gas senso . The high
po osi y, acile edox in e change and abundan ac i e si es
offe ed by he COF-shea h o e TiO
2
nanowi es o NO
2
gas
sensing can be possible due o hese in insic p ope ies o
po ous COFs. Redox e e sing being he s iking ad an age o
his COF/TiO
2
he e ojunc ion can be explained by he Z-scheme
o cha ge sepa a ion.
331
Since he band gaps in COF-316 and
TiO
2
o e lap, he elec ons exci ed om he alence band o he
conduc ion band in TiO
2
elax by occupying he alesnce band
in COF-316 unde he in luence o an in insically e ol ed
elec ic ield be ween COF/TiO
2
he e ojunc ion. This e en
esul s in o he concealmen o oxida i e si es on TiO
2
and
e ela ion o he educ i e si es on he ou e COF-shea h o he
sensing e e sal om he educ i e gas sensing such as E OH
owa ds he oxidizing gas sensing such as NO
2
(Fig. 22b). Due
o he in insic hyd ophobic na u e o he TiO
2
/COF-316 hyb id
(con ac angle = 108.31) in con as wi h he hyd ophilic na u e
o he ba e TiO
2
(con ac angle = 7.11), he sensing de ice
in eg a ed wi h his hyb id leads o undis u bed sensing pe -
o mance o he de ice wi h consis en esul s o he NO
2
sensing (Fig. 22c).
The NO
2
sensing mechanism o he TiO
2
/COF-316
hyb id e eals he e e sal o he sensing o oxidizing NO
2
gas (Fig. 22d). This hyb id p omo es he educ i e sensing
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mechanism o NO
2
gas which o he wise p oduces an oxida i e
sensing mechanism o p is ine TiO
2
-based senso s. Hyd ogen
gas de ec ion is o c ucial conce n since i is a highly lammable
gas and i p o ides one o he cleanes uels, making i a
p ecious en i y. Va ious hyd ogen chemi esis o s a e al eady
in unc ion including me al based (Pd, P ) ma e ials, ansi ion
me al dichalcogenides (TMDs) and g aphene-based ma e ials.
Al hough g aphene-based ma e ials a e good in e ms o con-
duc i i y and s abili y, bu hey ha e poo in e ac ion wi h gases
due o he lack o any unc ionali y. Thus, inducing unc ion-
ali y o adding o he ma e ials such as me al nanopa icles (Pd/
P ) imp o es in e ac ion o g aphene ma e ials wi h hyd ogen
gas molecules. E en ually, K ishan eni e al. syn hesized Pd-
doped imine-COF which offe s a s able hyb id s uc u e and
highly efficien hyd ogen sensing pe o mance.
332
The p esence
o sulphonic unc ional g oups in he cons i uen COF mono-
me effec i ely s abilizes he Pd-nanopa icles. The in e ac ion
o H
2
molecules wi h he Pd-deco a ed COF pa icles was
excellen ly imp o ed as compa ed o ha o bulk COF by acid
ex olia ing he bulk COF ma e ial o COF-nanoshee s since
indi idual Pd-doped COF nanoshee s p o ide maximum su -
ace a ea o hyd ogen molecules o ge in e ac ed wi h he
ex olia ed nanoshee s. This p o ides highly app oachable Pd
nanopa icles o hyd ogen spillo e , i.e., dissocia ion o H
2
pa icles on o he Pd nanopa icles and he esul ing o ma ion
o PdH
x
species. This causes elec on en ichmen o he ma e-
ial and a subsequen dec ease in he esis ance which can be
e med as chemi esis i e change in he ma e ial.
Fig. 21 (a)–(d) Syn hesis, s uc u al cha ac e iza ion and sensing pe o mance o M-TPCOF (M = Co & Cu) owa ds NO
2
gas. (a) Syn hesis and s uc u al
ea u es o H
2
-TPCOF be o e me ala ion. (b) Adso p ion ene gy DFT based compa ison o in e ac ion o a ious gases wi h Co-TPCOF. (c) In si u DRIFT
spec a o Co-TPCOF du ing NO
2
adso p ion. (d) Schema ic demons a ion o speci ic chemi esis i e sensing o NO
2
by M-TPCOF. Rep oduced wi h
pe mission om e . 329 Copy igh 2022, Wiley-VCHGmbH.
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5.3. Po ous conjuga ed polyme s as chemi esis i e gas senso s
Po ous conjuga ed polyme s o conjuga ed po ous polyme s
(CPPs) a e a class o po ous ma e ials ha a e ealized by he
polyme iza ion o monome ic uni s ia a s ong bonding
be ween hem.
333–335
They a e di e en om o he po ous
ma e ials such as MOFs and COFs as hey do no ely on he
c ys allini y o he ma e ials. Thus, hey can be p epa ed wi h
obus condi ions keeping in mind only he s abili y o he
p oduc and no special condi ions o achie ing c ys allini y.
This makes CPPs a p omising choice o chemi esis i e gas
sensing ma e ials. Al hough hey a e e y s able ma e ials and
can be po en ially used in ha sh en i onmen s, no much wo k
in he di ec ion o chemi esis i e gas sensing is done (Table 3).
In his ega d, Wisse e al. pe o med a polyme isa ion o 4,4-
diace yl-2,2-diamino-biphenyl and 1,4-diace ylbenzene esul -
ing in o mic opo ous polyme s (DUT-92(NH
2
) and DUT-
92(NO
2
)).
336
These polyme ic ma e ials we e mixed wi h PTFE
binde and ca bon black o o m lexible senso ilms. The
po ous s uc u e o he polyme causes a huge up ake o he
a ge VOCs which causes expansion o he ma e ial, esul ing
Fig. 22 (a) Schema ic ep esen a ion o COF-316 s uc u e and ‘ il e ampli ie ’ and ‘ edox e e sal’ na u e o TiO
2
/COF-316 hyb id. (b) Sensing esponse
o TiO
2
/COF-316 hyb id owa ds NO
2
compa ed wi h o he gases. (c) Response analysis o ba e TiO
2
and TiO
2
/COF-316 hyb id owa ds RH%. (d)
Chemi esis i e esponse o NO
2
in e ac ion wi h TiO
2
/COF-316 hyb id in he p esence o ligh and co esponding illus a ed sensing mechanism.
Rep oduced wi h pe mission om e . 330 Copy igh 2023, Ame ican Chemical Socie y.
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in o an inc ease in he unnelling dis ance be ween he
embedded pa icles o conduc i e ca bon black and hus a
simul aneous inc ease in he esis i i y o he ma e ial. This
mechanism is in con as wi h he change in he esis i i y in
non-po ous polyme ic ma e ials. A high su ace a ea o abou
680 m
2
g
1
p o ides e ec i e in e ac ion wi h VOCs which
leads o he LOD alues o o1 ppm. The swelling beha iou
is obse ed o be e ec i e o e ec i e sensing pe o mances o
DUT- amily polyme s owa ds a ious VOCs.
5.3.1. P is ine po ous o ganic polyme s as chemi esis i e
gas senso s. Po ous o ganic polyme s ep esen he polyme ic
po ous ma e ials consis ing o only o ganic cons i u ing pa s.
The class consis s o – conjuga ed mic opo ous polyme s
(CMPs), po ous a oma ic amewo ks (PAFs), hype c oss-
linked polyme s (HCPs), polyme s o in insic-mic opo osi y
(PIMs), co alen o ganic polyme s (COPs) and co alen iazine
amewo ks (CTFs). Co alen o ganic amewo ks (COFs) a e
also a kind o POP ma e ial bu hei high supe io i y o e he
o he POPs in e ms o c ys allini y and po en ial conduc i e
na u e makes hem o be discussed c i ically as a sepa a e
sec ion. Owing o hei po ous na u e, we will be in e es ed in
POPs o check hei po en ial o applica ions which in ol e he
in e ac ion o analy e molecules while passing h ough he
po ous passage, like gas sensing. No all POPs ha e been
explo ed o he chemi esis i e gas sensing applica ion because
o he need o conduc i i y in he ma e ial, whe eas mos
POPs, excep COFs and CTFs, a e ei he non-conduc i e o
become conduc i e in hyb id o ms.
Thus, we would be highligh ing he e some impo an land-
ma ks ha p o e he po en ial o hese ma e ials in chemi -
esis i i y based sensing applica ions. Since CTFs a e conduc i e
in na u e, hey emain as he p o agonis in he cu en
scena io. Yang e al. syn hesized a 2D o ganic polyme de i ed
om a co alen iazine amewo k (CTFs), sho ened as T-2DP
(Fig. 23a).
337
The p esence o a po ous 2D pa hway p o ides he
acile gas anspo and he p esence o he high-densi y imine
unc ional g oups p o ides op imum adso p ion/deso p ion
which leads o highly imp o ed sensing pe o mances com-
pa ed wi h he CTF-based senso . This T-2DP-based senso hus
p oduces supe as esponse and eco e y imes o NO
2
sen-
sing. Response/ eco e y imes (35–47 s) a e as due o he
p esence o uni o mly dis ibu ed in e ac ing si es (–CQN–
si es in his ma e ial) (Fig. 23b). The dis ibu ion o in e ac ing
si es is usually ou -o -plane in he case o 2D ino ganic ma e i-
als such as MXenes, G aphene, GO and so on. In he case o
he p esen ma e ial, he in e ac ing si es a e in he 2D plane,
which con ibu es o he conduc ion o elec ons in his
ma e ial. Thus, he in e ac ion o a ge gas molecules (NO
2
)
wi h hese in-plane in e ac ing si es p oduces highly sensi i e
ou pu signals (Fig. 23c) wi h an excellen sensi i i y o
452.6 ppm
1
. The pe o mance compa ison wi h he co es-
ponding bulk CTF based sensing ma e ial gi es simila sensing
beha iou wi h diminished sensi i i y because o he slow
di usion o he analy e gas in o he bulky CTF assembly. Due
o he excellen eco e y p ope y o T-2DP, i p oduces highly
ep oducible esponse esul s wi h a me e 4% pe u ba ion in
esponse alues o e a se ies o 15 cycles (Fig. 23d). Compa a-
i e chemi esis i e s udies o CTF, RGO and MXene indica e
ha T-2DP is supe io o all o hem o as e esponse/ eco e y
alues by se e al magni udes (Fig. 23e–h).
Non-c ys alline POPs usually show poo elec onic conduc-
i i y due o he absence o an elec onic conduc ion channel in
con as o wha is usually ound in, say, 2D conduc i e COFs.
Mai i e al. syn hesized a ialdehydebenzene and phenyl-
dihyd azine combined hyd azide linkage based COP o he
oom empe a u e sensing o H
2
S gas.
338
Amine based mac o-
molecules show conduc i i y h ough p o on-conduc ion
mechanism. This sensing ma e ial also showed as sensing
beha iou due o i s high sensi i i y owa ds acidic gases. H
2
S
being an acidic gas, when in e ac s wi h his sensing ma e ial,
p oduces an ab up dec ease in he p o on conduc i i y o he
ma e ial which is seen as as esponse ime (9 s) o 200 ppm o
gas inpu . Thus, non-elec onic conduc i e ma e ials can be
used o gas sensing by exploi ing hei p o on conduc i i y,
simila o wha exis s in amine-based co alen o ganic poly-
me s. Me al-oxide semiconduc o s gi e excellen sensi i e pe -
o mance in he case o gas sensing bu hei selec i i y and
sensi i i y a e demolished o a g ea ex en when hey a e kep
unde humid condi ions because o he ac i e si e’s deac i a-
ion by he humidi y. O ganic-based ma e ials p o ide a supe -
io al e na i e o chemi esis i e gas sensing jobs when hey a e
s able unde humid condi ions, wi h accu a e and eliable
alues. Taking o wa d he concep , Ko e al. ab ica ed a NO
2
sensing de ice, powe ed by co alen o ganic nanoshee s which
a e wo-dimensional semiconduc i e in na u e, abb e ia ed
CON-10.
339
The 2D nanoshee mo phology o his ma e ial
endows i wi h a hyd ophobic na u e wi h a wa e con ac angle
o 135.41along wi h he he mal s abili y o his ma e ial up o
320 1C. The ex olia ed na u e o CON-10 nanoshee s p o ides a
highly exposed su ace a ea o he analy e gas molecules o
in e ac wi h he p-elec ons mo ing on hese nanoshee s su -
ace analogous o he g aphene elec onic conduc ion beha-
iou . This can be he eason o he lowe concen a ion
de ec ion limi o 2.242 ppb. The hyd ophobic beha iou o
Table 3 Po ous o ganic polyme s based chemi esis i e gas senso s
S. no. Ma e ial VOCs Sensing mechanism Response (%) LOD (ppm)
es
–
ec
(s) Re .
1 T-2DP NO
2
CT 452.6 0.0002 35–47, 56–140 337
2 Hep azine-based COP NH
3
DA 70 1 65–9 340
3 Hyd azide-based COP H
2
S P o on conduc ion 51 9–12 338
4 CONs NO
2
CT 73.7 0.00024 339
5P
2
O
5
-CTF NH
3
CT 8 54–200 341
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CON-10 bene i s he ma e ial o de ec NO
2
gas in he ambien
a mosphe e and wea he . Thus, he ma e ial is p oposed o be
sui able o ace-le el de ec ion o NO
2
. The inhe en hyd opho-
bici y in his ma e ial is supposed o be he esul o uni o mly
dis ibu ed co alen bonding and he p esence o N–S bonds.
The hyd ophobic na u e o he sensing ma e ial is an
impo an equi emen o he eal- ime applica ion o a che-
mi esis i e gas senso since i p o ec s he ma e ial om he
baseline esis ance change when epea ed cycles o he gas
sensing a e pe o med as is being obse ed in he p esen case.
Compa ing wi h he SnO
2
NW-based gas senso , he damping
effec o ela i e humidi y on he baseline esis ance alue o
he sensing ma e ial can be obse ed as highly impac ul. High
selec i i y owa ds he NO
2
gas sensing can be obse ed in his
ma e ial because o he hyd ophobic na u e o he CON
nanoshee s as hey p e en he in e ac ion o o he gases wi h
he sensing ma e ial, hus causing a negligible change in he
conduc i i y alues o he sensing ma e ial. CONs display
excellen sensing pe o mance a he eal-li e applica ion alue
and hus p o e he ex ensi e mo phological dependence o gas
sensing pe o mance.
Simila o o he po ous ma e ials, hyd ogen-bonded o ganic
amewo ks (HOFs) consis o he polyme iza ion o unc ional
o ganic moie ies h ough hyd ogen bonding. They exhibi high
pe manen po osi y, acile syn hesis, and show p omising
applica ions such as molecula /gas sepa a ion, op ical
and biomedical applica ions.
342
Wang e al. syn hesized a
po phy in-based hyd ogen-bonded o ganic amewo k (HOF)
o NO
2
sensing, le e aging abundan amine g oups ha con-
ibu e o excellen selec i i y and an ul a as esponse a oom
empe a u e.
343–345
The one-dimensional channels o HOF, he
senso demons a ed esponse and eco e y imes o 17.6 s and
15.4 s, espec i ely. The abundance o amine g oups in he
senso esul s in n- ype semiconduc o beha io , leading o an
ul a as esponse o acidic NO2 gas molecules. Simila ly, Lee
e al. demons a ed he signi icance o HOFs in he sepa a ion
o noble gases, such as isola ing xenon (Xe) om an Xe/K
mix u e.
346
Xenon (Xe) gas-based de ices ind di e se applica-
ions, anging om ligh ing, lase echnology, space explo a-
ion o medical de ices. Ne e heless, moni o ing selec i e Xe
gas and achie ing sensi i e sepa a ion using chemi esis i e gas
sensing will be an in e es ing s udy in he ield o senso s.
6. Conclusion and u u e pe spec i e
The ield o chemi esis i e gas sensing has apidly gained
p ominence due o i s ema kable esponsi eness o a ious
Fig. 23 (a) Diag amma ic illus a ion o a iazine-based 2D polyme . (b) Exclusi e chemi esis i e esponse owa ds NO
2
gas while compa ing wi h o he
ino ganic and o ganic analy es. (c) Response cu e o he T-2DP senso owa ds NO
2
sensing a ppb-le el. (d) Chemi esis i e esponse o T-2DP owa ds NO
2
(1 ppm) eed. Dynamic sensing esponse o (e) p is ine CTF o a ious NO
2
concen a ions (5 ppm o 200 ppb), ( ) RGO o diffe en NO
2
le els (5 ppm o
500 ppb). (g) MXene o diffe en NO
2
le els (5 ppm o 500 ppb). (h) Response s. concen a ion cu es o T-2DP compa ed o CTF, RGO, and MXene. (i)
Response o MXene, RGO and T-2DP o a ious a ge analy es. Rep oduced wi h pe mission om e . 337 Copy igh 2020, Ame ican Chemical Socie y.
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gases, d i en by swi elec onic changes in he sensing ma e-
ial. Achie ing sensi i i y in gas sensing demands an ideal
in e play be ween he gas sensing ma e ial and a ge gases, a
subs an ial su ace a ea p o iding accessible in e ac ion si es,
and apid esponses o analy es. Po ous ma e ials ha e
eme ged as highly p omising candida es in he ealm o
chemi esis i e gas sensing, as hey effec i ely mee hese essen-
ial equi emen s o mode n and efficien gas sensing
ma e ials.
This e iew offe s an up- o-da e o e iew o ad ancemen s in
chemi esis i e gas sensing, co e ing undamen al p inciples,
sensing pa ame e s, and he a ious ma e ials employed. I
places pa icula emphasis on po ous ma e ials, including
me al–o ganic amewo ks (MOFs) and hei hyb ids, co alen
o ganic amewo ks (COFs) and hei hyb ids, g aphene-based
ma e ials, and po ous conjuga ed polyme s. The u u e o
chemi esis i e gas senso s holds exci ing p ospec s d i en by
ad ancemen s in ma e ials, ab ica ion echniques, and senso
designs. Ad anced ma e ials, such as 2D wonde s like g aphene
and ansi ion me al dichalcogenides, hyb id nanoma e ials,
and unc ionalized polyme s a e a he o e on .
146,347–354
These ma e ials o e excep ional sensi i i y and selec i i y,
p omising he de ec ion o a b oade spec um o gases,
including elusi e ola ile o ganic compounds and indus ial
haza ds. The wo ld o nano echnology con inues o un old,
wi h nanowi es, quan um do s, and hyb id nanoma e ials
d i ing he de elopmen o highly e icien senso s capable o
swi esponses and as onishingly low de ec ion h esholds.
Addi ionally, he in eg a ion o machine lea ning and da a
analy ics p omises o make gas senso s sma e and mo e
esponsi e. These senso s will no only de ec gases bu also
in e p e da a in eal- ime, ecognizing pa e ns and iden i ying
mul iple gases simul aneously. Such in elligen senso s a e se
o e olu ionize en i onmen al moni o ing, heal hca e, and
sa e y sys ems ac oss a ious domains (Fig. 24). Fu he mo e,
we a e on he b ink o wi nessing gas senso s embedded in
wea able de ices and seamlessly in eg a ed in o he In e ne o
Things (IoT).
355–373
This ans o ma ion enables con inuous moni o ing o ai
quali y, pe sonal heal h, and sa e y in di e se en i onmen s,
om u ban landscapes o emo e indus ial si es. Flexible and
p in able senso s a e also eme ging, offe ing a cos -effec i e,
disposable solu ion o a ious applica ions. These senso s a e
used in ood packaging, heal hca e, and beyond. En i onmen-
al applica ions emain a key ocus, wi h gas senso s leading
he cha ge in moni o ing pollu ion, g eenhouse gases, and
indoo ai quali y. Heal hca e is ano he a ea whe e gas senso s
a e poised o make signi ican con ibu ions. F om disease
diagnosis h ough b ea h analysis o non-in asi e heal h
assessmen s ia skin emissions, hese senso s a e poised o
enable ea ly disease de ec ion and pe sonalized heal hca e.
Indus ial sa e y, pa icula ly in he ene gy sec o , is se o
bene i as gas senso s play pi o al oles in de ec ing haza dous
gases. Thei deploymen ensu es sa e oil and gas explo a ion
and p oduc ion. As he wo ld u ns i s gaze owa ds en i on-
men al sus ainabili y, gas senso s will emb ace eco- iendly
ma e ials and sus ainable manu ac u ing p ocesses. This shi
aligns wi h he g owing commi men o en i onmen al espon-
sibili y. Finally, global collabo a ions a e ca alysing hese
ad ancemen s. Scien is s, enginee s, and indus ies wo ldwide
Fig. 24 Schema ic illus a ion o cu en summa y, u u e pe spec i es o ad anced po ous ma e ials based CGS including c i ical challenges.
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a e uni ing, pooling knowledge, and esou ces o accele a e he
de elopmen and comme cializa ion o cu ing-edge senso
echnologies. In conclusion, he u u e o gas senso s is adi-
an , holding p omise o imp o ing ai quali y, sa e y, heal h-
ca e, and en i onmen al s ewa dship on a global scale. In his
pe spec i e, po ous ma e ials p o ide almos all he basic
equi emen s as an effec i e chemi esis i e gas sensing ma e-
ial. They p o ide high su ace a ea, unc ional g oup e sa i-
li y, an ex ended s uc u al amewo k wi h elec onic
conjuga ion and many mo e p ope ies o op imal hos –ana-
ly e in e ac ion and he a e ma h esul s consis ing o eliable
and accu a e sensing pe o mance and da a. Since he e is
always a scope o imp o emen in any wo k, his ield can
also be aken u he owa ds a mo e powe ul gas sensing le el
by conside ing some key u u e pe spec i e poin s.
1. MOFs ha e al eady p o en aluable in chemi esis i e gas
sensing due o hei ou s anding su ace a ea and uneable
unc ionali y, acili a ing close in e ac ion wi h a ge gases.
Addi ionally, ce ain 2D conduc i e MOFs like M-HHTP, M-
HITP, and M-TCPP exhibi conduc i i y, offe ing elec onic
conduc ion o chemi esis i e beha iou . To enhance hei
pe o mance, he e is oom o imp o ing MOFs by ansi ion-
ing owa d a mo e c ys alline na u e and de eloping high-
su ace-mo phology amewo ks. Con olling he shape o
MOF-based nanopa icles can also be op imized o maximize
he accessible su ace a ea o analy e gases. MOFs a e p ima -
ily used in powde o m, posing p ocessing challenges. Con-
side ing he ac ual ma e ial o gas sensing pu poses, hin-
ilms and memb anes like ma e ial a e o conside able impo -
ance. The e o e, i is essen ial o ocus on de eloping acile
syn he ic me hods o MOF memb anes o easily p ocessable
solu ions. Explo a ion in o MOF combina ions wi h ma e ials
like CNTs, me al oxides, and conduc ing polyme s has shown
p omise. Fu he in es iga ions in o highly conduc i e ma e i-
als like MXenes hold po en ial, al hough s abili y imp o e-
men s, especially unde ambien condi ions, a e necessa y.
2. COFs ace challenges due o hei ins abili y in humid o
ambien condi ions and limi ed conduc i i y. To make hem
iable o gas sensing, mo e obus and conduc i e COFs a e
needed. COFs offe adap able amewo ks and e sa ile unc-
ionali y, c ucial o ideal a ge gas sensing. They a e pa icu-
la ly sui able o gas sensing in li ing o ganisms, a oiding
conce ns associa ed wi h me al-based ma e ials like MOS and
MOFs. While examples using o he ma e ials like MOS and
MOFs al eady exis , pa icula ly o VOCs sensing in humans
and plan s, esea ch in ol ing COFs in his con ex emains
unexplo ed. Also, he equi emen o lexible and obus COF
ma e ials is a compelling need o a de ice le el de elopmen o
his wo k. Some COF memb anes and hin- ilms a e al eady
being explo ed,
374–376
bu no epo s on hei p is ine mem-
b anes o ilms we e ound o he bes o ou knowledge.
Fu he , in es iga ion in his di ec ion can be done by de el-
oping new uni e sal me hods o p ocessing COFs o hin- ilms
and memb anes.
3. G aphene, a 2D conduc i e ma e ial, has been success-
ully employed in chemi esis i e gas sensing applica ions.
G aphene, p ima ily in i s 2D monolaye o m, se es as an
excellen chemi esis i e ma e ial owing o i s supe io elec o-
nic p ope ies compa ed o mul ilaye g aphene. Consequen ly,
g aphene and i s de i a i es, as well as hyb ids, ha e eme ged
as op-no ch ma e ials o oom- empe a u e chemi esis i e gas
sensing. Howe e , despi e hei physical and chemical obus -
ness, g aphene-based ma e ials ace challenges such as limi ed
selec i i y and sensi i i y due o hei chemical ine ness, which
hampe s hos -gas analy e in e ac ions. To add ess his issue,
effo s ha e been made o in oduce me al/me al oxide si es
and ela ed unc ionali ies in o p is ine g aphene, esul ing in
imp o ed gas sensing pe o mance. Fu he enhancemen s can
be achie ed by de eloping hese hyb ids wi h mo e in iguing
mo phologies and addi ional unc ionali ies, building upon he
success o ma e ials like GO, GO, and G-COOH in gas sensing.
Inco po a ing dopan s like B, N, o -SH becomes c ucial o
ine- uning he edox p ope ies and elec onic s uc u e,
including band gaps and Scho ky ba ie s, o enable p ecise
sensing o in e ac ing gases. Syn hesis o g aphene-based
de ices is s ill es ic ed o monolaye s o hin-laye g aphene
de i a i es which a e difficul o achie e as i equi e c i ical
p ecision and sophis ica ed ools. Chemical ex olia ion me h-
ods o de elop la ge monolaye s need o be explo ed u he .
4. Po ous o ganic polyme s including amo phous po ous
ma e ials like po ous ca bon black, ionic liquids, hype -c oss-
linked polyme s (HCPs), and PIMs offe no able ad an ages in
e ms o s abili y, ease o syn hesis, and unc ionali y. Despi e
hese me i s, hei complex mic os uc u e and lack o long-
ange o de pose challenges, pa icula ly ega ding elec onic
conduc i i y. To o e come his hu dle, inducing conduc i i y
h ough space becomes a iable s a egy, add essing he need
o ex ended conjuga ion and h ough-bond o h ough-space
conduc ion. This can be accomplished by me iculously con ol-
ling he o ien a ion o indi idual moie ies, allowing hei emp y
o bi als o cha ges o a e se he en i e amo phous amewo k
and enhancing he u ili y o hese ma e ials in chemi esis i e
gas sensing. Powde s a e usually no p ocessable i hey a e
obus in na u e. Thus, solu ion p ocessable POPs o mem-
b anes a e he need o he hou . To syn hesize hese mem-
b anes o ilm me hods like doc o blade, Langmui –Blodge
nanos uc u ed ilm o ma ion kind o me hods a e equi ed o
be mo e in use o p oducing e ec i e chemi esis i e ilms.
Con lic s o in e es
The e a e no con lic s o in e es o decla e.
Acknowledgemen s
A. S. and S. E. equally con ibu ed o his wo k. K. J. R. acknowl-
edges suppo om he Indian Ins i u e o Technology Jammu
o p o iding UGC-DAE CSR p ojec CRS/2022-23/01/708, SERB
CRG/2023/004685,CSIR-HRDG 01/3101/23/EMR-II and DST/
INT/DAAD/P-06/2023. We acknowledge he suppo o he p o-
jec TECHSCALE (no. CZ.02.01.01/00/22_008/0004587) inanced
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om he ERDF and ESF. This wo k was suppo ed by he
Minis y o Educa ion, You h and Spo s o he Czech Republic
h ough he e-INFRA CZ (ID:90254). This wo k was suppo ed
by he Ma e ials and Componen s Technology De elopmen
P og am o Minis y o T ade, Indus y and Ene gy (MOTIE)/
Ko ea Ins i u e o Indus ial Economics and T ade
(KIET) (10080527). This wo k was also suppo ed by he Minis-
y o Educa ion o he Republic o Ko ea and he Na ional
Resea ch Founda ion o Ko ea (NRF-2022S1A5C2A0309-
3218). The inancial suppo o he Eu opean Union unde
he REFRESH – Resea ch Excellence Fo Region Sus ainabili y
and High- ech Indus ies p ojec numbe CZ.10.03.01/00/
22_003/0000048 ia he Ope a ional P og amme Jus T ansi ion
is also acknowledged
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