RESEARCH ARTICLE
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Mul i-Sensing Pla o m Based on 2D Monoelemen
Ge manane
Jay aj V. Vaghasiya, Ca men C. Mayo ga-Ma inez, Ke al K. Soniga a, Pe Laza ,
and Ma in Pume a*
Co alen ly unc ionalized ge manane is a no el ype o fluo escen p obe ha
can be employed in ma e ial science and analy ical sensing. He e, a
fluo ome ic sensing pla o m based on me hyl- unc ionalized ge manane
(CH3Ge) is de eloped o gas (humidi y and ammonia) sensing, pH (1–9)
sensing, and an i-coun e ei ing. Luminescence ( ed–o ange) is seen when a
gas molecule in e cala es in o he in e laye space o CH3Ge and he
luminescence disappea s upon dein e cala ion. This allows o di ec
de ec ion o gas abso p ion ia fluo ome ic measu emen s o he CH3Ge.
S uc u al and op ical p ope ies o CH3Ge wi h in e cala ed gas molecules
a e in es iga ed by densi y unc ional heo y (DFT). To demons a e eal- ime
and on- he-spo es ing, abso bed gas molecules a e fi s p ecisely quan ified
by CH3Ge using a sma phone came a wi h an ins alled colo in ensi y
p ocessing applica ion (APP). Fu he , CH3Ge-pape -based senso is
in eg a ed in o eal ood packe s (e.g., fish and milk) o moni o he shel li e
o pe ishable oods. Finally, CH3Ge-based ew i able pape is applied in wa e
je p in ing o illus a e he po en ial o sec e communica ion wi h quick
colo a ion and good e e sibili y by wa e e apo a ion.
1. In oduc ion
In ecen yea s, 2D-based g aphene analogs ha e opened up
ascina ing oppo uni ies o basic and applied esea ch in di-
e se field applica ions such as biomedical, en i onmen al, and
elec onic de ices.[1–7] Ge manane has a ac ed a lo o in e -
es as a unique elec onic ma e ial because i can une he
J. V. Vaghasiya, K. K. Soniga a, M. Pume a
Fu u e Ene gy and Inno a ion Labo a o y
Cen al Eu opean Ins i u e o Technology
B no Uni e si y o Technology
Pu kyˇ
no a 123, B no 61200, Czech Republic
E-mail: [email p o ec ed].cz
J.V.Vaghasiya,C.C.Mayo ga-Ma inez,M.Pume a
Cen e o Ad ancedFunc ionalNano obo s
Depa men o Ino ganicChemis y
Facul y o ChemicalTechnology
Uni e si yo Chemis yandTechnologyP ague
Technická5,P ague16628,CzechRepublic
The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/adma.202304694
© 2023 The Au ho s. Ad anced Ma e ials published by Wiley-VCH
GmbH. This is an open access a icle unde he e ms o he C ea i e
Commons A ibu ion License, which pe mi s use, dis ibu ion and
ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed.
DOI: 10.1002/adma.202304694
mo phological and physical p ope ies by
changing he na u e o he co alen ly e -
mina ed ligand.[8–12] Typically, his ma e-
ial can be p oduced by ex olia ion o
CaGe2(also known as he Zin l phase) in
an acidic media. A e wa d, opochemical
dein e cala ion o CaGe2wi h alkyl halide
yields alkyl ge manane, ha is, hyd ogen
o me hyl ge manane.[13,14] Ge manane has
ecen ly been p esen ed as a no el ac i e
ma e ial wi h ailo ed p ope ies o en-
e gy s o age,[15,16] sensing (chemical and
biological),[17,18] op oelec onic,[19,20] hyd o-
gen s o age,[21,22] ene gy con e sion, and
pho o-elec oca alysis.[23,24] The fine- uning
o ge manane p ope ies (i.e., bandgap and
physical p ope ies) can be achie ed by he
co alen su ace e mina ion o ge manane.
Recen ly, ou esea ch g oup uned he pho-
oca aly ic p ope ies o ge manane by al-
e ing a ious o ganic unc ional g oups.[14]
When he me hyl unc ional g oup eplaced
hyd ogen on he su ace o ge manane, i
enhanced i s he mal s abili y and widened he bandgap.[25]
Mo eo e , CH3Ge has been employed as a biosenso o iden-
i y li e ci hosis bioma ke s and glucose.[17,18] Howe e ,
ge manane-based biosenso s a e s ill in hei in ancy and equi e
mo e esea ch.
CH3Ge also demons a es ou s anding band-edge fluo es-
cence and pho oluminescence (PL) p ope ies.[26–28] The e o e,
P. Laza
Regional Cen e o Ad anced Technologies and Ma e ials
Czech Ad anced Technology and Resea ch Ins i u e (CATRIN)
Palacký Uni e si y Olomouc
Šlech i el˚
u 27, Olomouc 779 00, Czechia
M. Pume a
Facul y o Elec ical Enginee ing and Compu e Science
VSB – Technical Uni e si y o Os a a
17. lis opadu 2172/15, Os a a 70800, Czech Republic
M. Pume a
Depa men o Medical Resea ch
China Medical Uni e si y Hospi al
China Medical Uni e si y
No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan
M. Pume a
Depa men o Chemical and Biomolecula Enginee ing
Yonsei Uni e si y
50 Yonsei- o, Seodaemun-gu, Seoul 03722, Sou h Ko ea
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CH3Ge has been documen ed as a fluo escen ma ke on a mi-
c o obo sys em o d ug deli e y.[29] On he o he hand, he
PL o CH3Ge is highly associa ed wi h he abso p ion o wa e
molecules in i s in e laye space. The s udy e eals ha when
CH3Ge is hyd a ed, i exhibi s eddish–o ange luminescence bu
when i is d y he luminescence is in isible. Bu owing o a lack o
ge manane-based senso enginee ing knowledge in he esea ch
communi y, i is ye o be in es iga ed as a eal-scale applica ion.
The mul i-s acked compac s uc u e o CH3Ge makes i chal-
lenging o in eg a e in sensing de ices wi h p ecise in e aces.
Hence, he e we demons a ed ma e ial enginee ing o de elop
a mul i unc ional pape -based senso om CH3Ge. CH3Ge’s
buckled honeycomb s uc u e wi h sp3hyb idiza ion could help
inc ease i s abso p ion o gas molecules (humidi y) and chemi-
cal eac i i y. Due o i s unique s uc u al and PL p ope ies, i is
concei able o exploi CH3Ge o de ec o he ola ile gases in he
a mosphe e such as wa e apo , and acidic and basic gases. This
would be a new di ec ion o he de elopmen o CH3Ge-based
gas senso s.
G aphene-analogous 2D ma e ials, such as ansi ion me al
dichalcogenides (e.g., MoS2,MoSe
2,andWS
2),[30–32] ansi ion
me al ca bides (e.g., MXene Ti3C2Tx),[33,34] hexagonal bo on
ni ide,[35] and black phospho ous[36] ha e become popula e-
cen yea s o gas sensing. Func ionalized ge manane also has
ou s anding capabili ies o sense a b oad ange o ha m ul gases
such as NH3,SO
2,andNO
2.[37] Un o una ely, epo ed s a e-o -
a de ices o gas sensing applica ions ely on elec ochemical
wo ks a ions ha a e ully dependen on powe sou ces and no
sui able o on-si e and eal- ime applica ions.[29–35,37] The e o e,
i is c i ically impo an o de elop quick, ins umen - ee, and
apidly sensi i e senso s o he effec i e de ec ion o a a ie y o
gases.
Addi ionally, ma e ials ha change colo in esponse o chem-
ical inpu s (gas, pH, and sol en ) ac as he p ima y compo-
nen s in biome ic secu i y and sec e communica ion applica-
ions. Recen ly de eloped ew i able imaging echnologies ha e
been unc ionalized o wa e je ew i able pape s, sel -e asing
images, and pho och omic pape .[38,39] The majo i y o ma e ials
used in his echnology a e based on o ganic ma e ials (polyme s,
dyes, and pigmen s) and ino ganic ma e ials (pe o ski e).[40–42]
Ne e heless, he echnology is no eco- iendly enough due o
he complex syn hesis p ocess o ma e ials and he e asing sol-
en s a e ei he flammable o only a ailable om specific chem-
ical endo s.[43,44] Thus, he de elopmen o ew i able pape us-
ing new ma e ials in an en i onmen ally iendly manne is es-
sen ial.
He e, we p esen a fi s - ime simple and s aigh o wa d
CH3Ge fluo ome ic sensing pla o m ha can be used o gas
and pH de ec ion and sec e communica ion. The abso bed gases
can be iden ified based on hei pa icula luminescence e-
sponse ( ed- o blueshi ) h ough he in e cala ion o gases in o
he CH3Ge an de Waals gap. The ex en o abso bed gases
affec s PL in ensi y, making i possible o di ec ly moni o gas
abso p ion by quan i ying he luminescence in ensi y o he
CH3Ge. Sma phone-based fluo ome ic senso s ha e ecei ed
a lo o in e es ecen ly due o easily quan ified colo changes
in he sample by using a sma phone applica ion.[45,46] Specifi-
cally, he amoun o gas abso bed by CH3Ge was measu ed us-
ing a sma phone came a enabled wi h he Colo G ab colo
in ensi y p ocessing applica ion o measu e changes o he R
( ed)/G (g een)/B (blue) colo componen s. This sma phone-
assis ed fluo ome ic pla o m could p o ide quick and on-si e
analysis wi hou he use o complica ed ins umen s. To demon-
s a e he applica ion, he CH3Ge-pape -based senso was used o
moni o he eshness o eal ood (i.e., fish and milk). Ano he
impo an applica ion has been illus a ed by p in ing confiden-
ial in o ma ion on CH3Ge-based pape using a wa e inkje -
p in ing echnique, whe e wa e is used as a sec e ink sou ce o
hide symbolic o ex ual in o ma ion on op-sec e documen s.
Mos impo an ly, once such a documen is ead, he in o ma-
ion can be easily emo ed by simple wa e e apo a ion. This is
he fi s ime demons a ing he use o CH3Ge o eal-wo ld
applica ions such as humidi y moni o ing, ood quali y analy-
sis, and sec e communica ion. To he bes o ou knowledge,
he mono-elemen 2D ma e ials-based wa e inkje ew i able
pape has ne e been epo ed o sec e communica ion
applica ions.
2. Resul and Discussion
In he amily o laye ed an de Waals 2D ma e ials, CH3Ge is
one o he ascina ing ma e ials wi h s ong PL p ope ies as-
socia ed wi h wa e in e ac ion. He e, we de elop o he fi s
imeaCH
3Ge-based fluo ome ic sensing pla o m ha can be
used o he de ec ion o gas, pH, and sec e communica ion.
The PL in ensi y o CH3Ge is highly dependen on he pH o
sol en ha is in e cala ed in o he in e laye spacing o CH3Ge
and emi s ligh acco dingly. The in ensi y o PL is igge ed by
he in e cala ed molecules owing o local s uc u al dis o ion o
he CH3Ge caused by he da i e in e ac ion o wa e molecules
wi h he Ge─C. Taking benefi o his p ope y, we ocused on
he de ec ion o diffe en gases (humidi y and ammonia) and pH
o wa e (1–9). The abso bed gases can be iden ified based on
he pa icula colo esponse, ha is, acidic gases emi ed ligh
and basic gases emi ed–o ange ligh .[27] Besides, luminescence
in ensi y is influenced by he abso bed gases. This colo change
abili y can be used o e alua e bo h gases ( ela i e humidi y and
ammonia) and he eshness o ood (Figu e 1a,b). Fo ins ance,
ammonia gas p oduced by spoiling fish and lac ic acid gene a ed
by spoiling milk[23] could be easily measu ed using a CH3Ge sen-
so . A e ha , he amoun o gases abso bed by CH3Ge was e al-
ua ed using a sma phone came a and colo in ensi y p ocess-
ing so wa e o quan i y he colo change wi h RGB colo compo-
nen s (Figu e 1d). Taking he benefi o hyd och omic p ope ies,
CH3Ge has been used o sec e communica ion as well. He e,
we show a apid and acile app oach o he p epa a ion o a com-
munica ion medium by applying CH3Ge ma e ials o he su ace
o o dina y p in ing pape ; a e ha , CH3Ge-coa ed pape could
be p in ed mul iple imes using wa e as an ink (Figu e 1c). In-
e es ingly, when he CH3Ge pape comes in con ac wi h wa-
e , he p in ed in o ma ion appea s upon exposu e o UV ligh
and hen disappea s when he wa e e apo a es. Conce ns abou
an inc ease in leaks o confiden ial in o ma ion and coun e ei -
ing indica e an oppo uni y o an i-coun e ei ing and enc yp ed
communica ions as illus a ed he ein. Be o e e alua ing CH3Ge
senso pe o mance in gases and pH de ec ion, i is c i ical o fi s
in es iga e he mo phology and s uc u al p ope ies o CH3Ge.
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Figu e 1. Sma phone-assis ed fluo ome ic pla o m using CH3Ge-based pape o sensing and sec e communica ion. a) CH3Ge-based gas senso s
o humidi y moni o ing and ood quali y analysis (by ammonia sensing). b) CH3Ge-based pH- esponsi e senso o ood quali y analysis (by lac ic
acid sensing). c) Schema ic illus a ion o CH3Ge-based ew i able pape wi h wa e je p in ing o an i-coun e ei ing and enc yp ed communica ions.
d) Colo in ensi y was de e mined using a sma phone module.
2.1. Mo phological, S uc u al, and Op ical P ope ies
Figu e 2a shows a schema ic ep esen a ion o he syn hesis o
CH3Ge and he ab ica ion o he CH3Ge-based senso . In a nu -
shell, CH3Ge was p epa ed by he opo ac ic dein e cala ion o
CaGe2wi h me hyl iodide in an o ganic sol en .[14,18] The ob-
ained mul ilaye CH3Ge was ex olia ed by he liquid phase ex o-
lia ion me hod[11] and CH3Ge flakes we e hen sp ay-coa ed on o
cellulose pape o u he use. A mo e de ailed desc ip ion o
he syn hesis and sp ay coa ing p ocedu e is gi en in he Expe -
imen al Sec ion. The mo phology o p epa ed samples was in-
es iga ed by scanning elec on mic oscopy (SEM) and ene gy-
dispe si e X- ay spec oscopy (EDS). The mul ilaye s uc u e
o CH3Ge is clea ly isible in he SEM image (Figu e 2b). The
magnified SEM image (Figu e 2c) e eals an enla ged lamella
s uc u e, indica ing ha calcium was success ully emo ed. A -
e ul asonic ea men , CH3Ge has a nanoflake-like mo phol-
ogy wi h la e al sizes in he mic ome e ange (Figu e 2d). The
scanning ansmission elec on mic oscopy (STEM) image also
depic s a anspa en flake-like s uc u e o CH3Ge, no ing good
delamina ion. A omic o ce mic oscopy opog aphy image shows
ha he delamina ed CH3Ge flakes hickness o 4.2 ±0.5 nm
(Figu e S2, Suppo ing In o ma ion), which is compa able
wi h p e iously published epo s.[47] EDS analysis (Figu e 2e, )
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Figu e 2. P epa a ion and mo phology s udy o CH3Ge. a) Syn hesis p ocedu e o CH3Ge and sp ay-coa ed CH3Ge-based pape . b,c) Low- and high-
esolu ion SEM images o bulk CH3Ge. d) STEM image o CH3Ge flakes. e, ) EDS images o CH3Ge flakes. g) SEM images o CH3Ge-coa ed pape .
e eals ha Ge and C a e dis ibu ed uni o mly h oughou he
flakes, confi ming he success o me hyl unc ionaliza ion on ge -
manane. Figu e 2g shows SEM images o CH3Ge-deco a ed cel-
lulose pape . The pape ab ic has a andomly o ien ed fib ous
s uc u e and a clean su ace as shown in Figu e S1a, Suppo -
ing In o ma ion. In con as , CH3Ge-deco a ed pape (Figu e 2g)
shows a ough su ace and CH3Ge flakes can be seen on he su -
ace o pape fibe and gaps. High- esolu ion SEM image (in-
se o Figu e 2g) and EDS mapping (Figu e S1b–e, Suppo ing
In o ma ion) indica e ha fibe s we e uni o mly co e ed by he
CH3Ge flakes.
Fu he CH3Ge op ical and s uc u al s udies in d ied and
hyd a ed condi ions we e conduc ed o in es iga e he PL colo
change mechanism. In humid condi ions, CH3Ge-deco a ed pa-
pe quickly changes PL colo om whi e o ed upon 365 nm
ligh i adia ion (Figu e 3a). Figu e 3b depic s PL spec a in bo h
d ied and hyd a ed s a es, demons a ing ha PL in ensi y in-
c eases when wa e molecules a e abso bed. P e ious esea ch
ound ha he in e cala ion o wa e molecules in o he in e -
laye space o CH3Ge causes abo e-bandgap luminescence.[27]
The in ensi y o he PL will also change depending on he el-
a i e humidi y (RH) le els. Fu he , he luminescence in ensi y
was con e ed o he ch oma ici y alues xand y.Figu e3cde-
pic s he colo change a e exposu e o humidi y. The Commis-
sion in e na ionale de l’éclai age (CIE) coo dina es change om
x=0.586, y=0.412 in d ied CH3Ge o x=0.626, y=0.372 in hy-
d a ed CH3Ge. The X- ay diff ac ion (XRD) pa e n (Figu e 3d)
in d ied-s a e CH3Ge exhibi s a highly c ys alline phase ha
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Figu e 3. S uc u al and pho ophysical p ope ies o CH3Ge. a) Pho og aph o CH3Ge-deco a ed pape o change colo in humid condi ions upon 𝜆ex
=365 nm. b) PL spec a (𝜆ex =365 nm) o CH3Ge in d ied and hyd a ed s a es. c) CH3Ge colo s in CIE 1931 colo space. d) XRD pa e n o d ied
and hyd a ed CH3Ge. e) ATR-FTIR spec a o CH3Ge eco ded a a diffe en ime in 30% humidi y. ) DFT calcula ions o CH3Ge in e ac ing wi h wa e
molecules (ge manane: ligh g ay; ca bon; da k g ay; hyd ogen: whi e; oxygen: ed).
is consis en wi h p e iously published epo s.[25] In con as ,
he high-in ensi y diff ac ion peak (002) changes om 9.661 o
9.850 Å upon exposed humidi y due o he ac ha wa e
molecules could en e he in e laye space and b oaden he lay-
e s. Fu he , FTIR spec oscopy was used o in es iga e CH3Ge
wa e abso p ion (30% humidi y) p ope ies as a unc ion o ime.
As shown in Figu e 3e, he CH3Ge abso p ion band a 3299 cm−1
inc eases as a unc ion o ime. This is due o he s e ching i-
b a ion peak c ea ed by ─OH when wa e is abso bed by CH3Ge.
Ul ima ely, long- ime humidi y exposu e aises he numbe o
─OH g oups by inc easing bound wa e molecules on CH3Ge.
The e o e, all humidi y expe imen s we e ca ied ou in cons an
ime in e als. The ypical abso p ion peaks a 2898 and 580 cm−1
a e esponsible o ─CH3and Ge─C s e ching ib a ions, e-
spec i ely. Peaks a 1403 and 1226 cm−1a e a ibu ed o ─CH3
bending ib a ion while peaks a 758 cm−1a e asc ibed o ─CH3
ocking ib a ion.[14,25] A sligh diffe ence obse ed a ≈700 cm−1,
indica ing enhanced hyd ogen bonding be ween wa e molecules
and CH3Ge.
We pe o med densi y unc ional heo y (DFT) calcula ions o
CH3Ge in e ac ing wi h wa e molecules o gain u he insigh
in o he cha ac e is ics o he s uc u al and op ical esponse. We
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conside ully unc ionalized bulk CH3Ge in he 1T p imi i e cell
as he compu a ional model (Figu e 3 ). The calcula ed la ice
pa ame e s o he ully elaxed p imi i e cell a e a=3.98 and
c=8.01 Ǻ. We no e ha he calcula ed alue o he cla ice pa-
ame e is smalle han ha ob ained om powde XRD due o
he in e laye u bos a ic diso de and cu a u e, which a e o -
en p esen in he syn hesized CH3Ge flakes. Nex , we add a wa e
molecule in e cala ed in be ween he laye s. No ice ha we in e -
cala e wa e in he laye s o he bulk p imi i e cell so he model
co esponds o he ully in e cala ed wa e laye because he wa-
e molecule is pe iodically epea ed in he xy (in-plane) di ec-
ion. The oxygen a om o a wa e molecule c ea es he hyd ogen
bond o he hyd ogen o he CH3g oup ( he O─H bond leng h
o 2.57 Ǻ), making he in e cala ion p ocess he modynamically
exo he mic, ha is, he o ma ion ene gy o in e cala ion is neg-
a i e (−0.03 eV). In e cala ion o wa e inc eases he cla ice con-
s an o 10.3 Ǻ. This alue should be conside ed as he uppe
bounda y o he la ice expansion because i is calcula ed o he
ully in e cala ed wa e laye . The s uc u e o CH3Ge does no
unde go any no able de o ma ion upon wa e in e cala ion (apa
om he inc ease o he in e laye spacing); he bond leng hs o
he C─Ge and C─H bonds a e unchanged and he me hyl g oup
nex o he wa e molecule is no de o med o il ed. We calcu-
la ed he ue op ical bandgap using he ime-dependen DFT
(TD-DFT) me hod wi h hyb id- unc ional HSE06 as he ke nel.
The op ical gap is he lowes ene gy dipole-allowed ansi ion ob-
se ed in he abso p ion spec a and is due o exci onic effec s
gene ally lowe han he elec onic gap, which is commonly cal-
cula ed as he ene gy diffe ence be ween he DFT ene gy le els.
The calcula ed op ical band gap o pu e bulk CH3Ge is 1.76 eV
while wa e in e cala ion inc eases he bandgap o 2.01 eV. No
mid-gap s a e eme ges upon wa e in e cala ion and he bandgap
emains as he di ec gap, loca ed a he Γpoin o he fi s B il-
louin zone. Thus, he in e cala ion blueshi s he op ical esponse
in o he ed pa o isible ligh in ag eemen wi h expe imen al
spec a. Asel e al. s udied he mechanism o wa e in e cala ion
in o CH3Ge and demons a ed ha he in ense abo e-gap pho o-
luminescence was caused by a local s uc u al dis o ion o he
Ge amewo k due o he da i e bonding o in e cala ed wa e
molecules.[27]
2.2. CH3Ge-Pape -Based Gas-Responsi e Senso
Based on he unique in e ac ion o CH3Ge wi h wa e molecules
and colo -changing p ope ies, CH3Ge-deco a ed pape can be
u ilized as a humidi y senso . Al hough he emission colo in
p esence o humidi y could be ecognized isually, i could no
achie e a o able accu acy by human eyes due o incapabili y o
dis inguish monoch oma ic in ensi y change. In gene al, such
de ec ion s ill depends on sophis ica ed and bulky labo a o y in-
s umen s ha a e ime-consuming, necessi a e qualified man-
powe o ope a e, and a e no app op ia e o on-si e o eal- ime
de ec ion. The e o e, he in ensi y o he CH3Ge-deco a ed pape
as a unc ion o RH was measu ed using a sma phone and an-
alyzed using he Colo G ab applica ion (Figu e 4a). The sma -
phone and CH3Ge pape we e placed ≈15 cm apa and pe pen-
dicula o one ano he , wi h he inciden 365 nm ligh p o iding
homogeneous illumina ion a 60°. To achie e a be e humid-
i y esponse o he CH3Ge-deco a ed pape , he amoun o ac-
i e ma e ials loaded on he subs a e mus be op imized. He e,
we sp ay-coa ed h ee diffe en amoun s o CH3Ge flakes on o
a pape subs a e: 0.3 mg cm−2(sample 1), 0.65 mg cm−2(sam-
ple 2), and 1 mg cm−2(sample 3) (Figu e 4b). The colo in en-
si y change o he CH3Ge-deco a ed pape du ing he 30% hu-
midi y exposu e can be seen isually in he inse o Figu e 4b.
We can also say he pape becomes edde as he loading o he
ac i e ma e ial inc eases. A sma phone can be u ilized o iew
he exac alue o colo in ensi y change. Especially in compa -
ison wi h spec ofluo ome e s, he use o a mobile de ice as a
colo in ensi y de ec o has been defined as one o he as es and
simples ools. In g ayscale pe cen ages, he mean in ensi y o
diffe en loading samples 1, 2, and 3 a e 37.66% (±1.45), 46%
(±2.3), and 55.3% (±1.45), espec i ely. When an excess amoun
o CH3Ge (>1mgcm
−2) is coa ed on pape causes c acking, in-
c eased su ace non-in o ma i i y, and nanoshee agg ega ion, all
o which educe sensing pe o mance. Thus, we decided o un-
de ake all subsequen humidi y and pH sensing s udies wi h op-
imum sample 3.
In o de o demons a e he CH3Ge-deco a ed pape ’s abil-
i y o change colo in ensi y upon a ious RH le els, he R/G/B
alues o CH3Ge pape s we e measu ed be ween RH 0% and
RH 100% using a sma phone module. Figu e 4d (g een colo
squa e) depic s o iginal pho og aphs o CH3Ge pape a a i-
ous humidi y le els. I can be challenging o dis inguish colo
changes wi h ou naked eyes. The ob ained diffe en colo in en-
si y shades p ocessed by he Colo G ab sma phone applica ion
we e eplica ed o c ea e a solid colo as depic ed in Figu e 4e
(black-colo ed squa e). Figu e 4c shows ha he R- alue ises
om 148 in RH 10% o 216 in RH 100% while a sub le diffe ence
is ound in G and B alues. A simila end was also obse ed in
he g ayscale pe cen age (Figu e 4 ). This in es iga ion success-
ully e ealed he hues and hei ela ed in ensi ies isually in he
o m o g ayscale pe cen age and RGB alue. The acqui ed end
as a unc ion o RH was u he alida ed by a spec opho ome e
(Figu e 4g). I clea ly depic s he inc ease in PL in ensi y a a -
ious RH le els. Simila ends o fluo escence de ec ion we e
also seen on he sma phone sensing pla o m wi hou he use
o addi ional ins umen s. Fu he , we demons a ed a p o o ype
mono colo change CH3Ge-based humidi y indica o ca d ha
can de ec he en i e ange (Figu e 4h). This ca d can be u ilized
in ood/medicine packaging applica ions o p o ide a isual cue
when he ela i e humidi y is inc easing. The e a e many diffe -
en kinds o comme cial humidi y indica o ca ds a ailable and
some p o ide benefi s such as low cos , ease o use, and long li es-
pan. Howe e , hei low sensi i i y and de ec ion ange emain
majo issues. Figu e 4h clea ly shows he colo change a ia ion
wi h diffe en RH. Un o una ely, his ca d only p o ided a ange
o humidi y le els. Fo hese easons, we used a sma phone pla -
o m o measu e accu a ely and p ecisely he humidi y in eal
ime.
To examine he sol en selec i i y o he de eloped CH3Ge-
pape -based senso , RGB alues we e eco ded a e he addi ion
o me hanol, ace one, e hanol, chlo o o m, dichlo ome hane, iso-
p opyl alcohol, and wa e . Figu e 5a depic s RGB alues o he
CH3Ge senso in diffe en sol en apo . The RGB alue o
CH3Ge senso s in wa e apo is he g ea es , e ealing high
specifici y in compa ison o all o he sol en s. Howe e , he
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Figu e 4. CH3Ge-based humidi y- esponsi e senso . a) Schema ic ep esen a ion o colo in ensi y quan ifica ion using a sma phone pla o m equipped
wi h he Colo G ab APP. b) Mean in ensi y o diffe en amoun s o CH3Ge loading on a pape subs a e. c) Mean RGB alue as a unc ion o humidi y.
d) Real images in diffe en humidi y le els cap u ed by a sma phone came a. e) Colo G ab APP-gene a ed duplica ed images. ) A e age g ayscale
pe cen age in diffe en humidi y le els. g) PL spec a as a unc ion o humidi y. h) Mono-colo -change o CH3Ge-based humidi y indica o .
sligh ly highe alue obse ed in e hanol compa ed o o he o -
ganicsol en sisdue oi s>1.0% wa e con en . The de ec ion
o wa e o ace le els o humidi y om comme cial o ganic sol-
en s will be made easie by his senso . Fu he mo e, he limi
o de ec ion (LOD) o he CH3Ge senso was calcula ed using he
inc easing luminescence in ensi y a diffe en RH. As shown in
Figu e 5b, he LOD o he CH3Ge-pape -based humidi y senso
was as low as 9.32% RH.
Fu he mo e, we use CH3Ge pape o de ec ammonia gas.
Figu e 5c inse depic s a no iceable a ia ion ollowing ammo-
nia gas exposu e on CH3Ge, which esul s in an emission ha
appea s mo e o ange. The ed colo alue ound a ound 189 us-
ing he colo in ensi y p ocessing so wa e Colo G ab. To un-
de s and his colo shi , we calcula ed he op ical bandgap o
CH3Ge in e cala ed by ammonia. The TD-DFT alue o he op-
ical bandgap is 2.11 eV, highe han he alue o in e cala ed
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Figu e 5. CH3Ge-based ammonia esponsi e senso . a) Sol en apo selec i i y wi h CH3Ge-deco a ed pape . b) Low limi o de ec ion (LOD) plo o
CH3Ge senso . c) R- alue as a unc ion o ammonia exposu e. d) RGB alues o CH3Ge in esh and spoiled fish. Inse : digi al images o CH3Ge pape s
in esh fish (g een squa e) and spoiled fish (black squa e). e) Schema ic ep esen a ion o colo change o CH3Ge pape in ela ion o fish spoilage and
quan ifica ion using a sma phone wi h colo in ensi y p ocessing so wa e, Colo G ab.
wa e calcula ed wi h he same compu a ional pa ame e s. Thus,
his inc ease explains he shi o mo e o ange colo . In e ca-
la ion o ammonia is also exo he mic: he o ma ion ene gy is
−0.038 eV, simila o ha o wa e .
We also in es iga ed exposu e o ammonia gas a a ious ime
in e als on CH3Ge and he esul s a e shown in Figu e 5d. Fol-
lowing ha , fish mea eshness and spoilage we e examined in
ela ion o he package en i onmen shi ing o an alkaline con-
di ion. A e 5 days o s o age a oom empe a u e, he colo o
CH3Ge pape changes om ed o ligh o ange (Figu e 5e). Us-
ing Colo G ab so wa e o p ocess colo in ensi y, he ed colo
alue inc eased om 161 o 186, indica ing a mo e alkaline pack-
age en i onmen (Figu e 5i). The ola ile basic gases gene a ed
om spoiled fish may ha e been a majo ac o in changing he
colo o CH3Ge pape . Ammonia compounds (e.g., di- and i-
me hylamine) a e a common by-p oduc o spoiled fish due o
p o ein decomposi ion by enzymes and bac e ia.[48] These e-
sul s indica e ha CH3Ge has a good esponse o ammonia gases
and can imply he quali y o oods in he beginning phases o
spoilage.
2.3. CH3Ge-Pape -Based pH-Responsi e Senso
A e success ully demons a ing he changing colo in ensi y o
CH3Ge in diffe en gas en i onmen s, we ound ha he emis-
sion ene gy o CH3Ge a ies depending on he pH. pH is an im-
po an pa ame e in many fields, including ood es ing, medical
diagnosis, and soil moni o ing.[49,50] The de elopmen o unique
isual and quan i a i e pH measu ing pla o ms emains c i i-
cal o some pa icula on-si e applica ion fields (e.g., ood anal-
ysis). The e o e, we ab ica ed a CH3Ge-pape -based pH senso .
The p o ona ed lone pai s on wa e molecules emo ed he na-
i e in e ac ion be ween he oxygen and ge manane, which could
cause colo changes a a ious pH.[27] Hence, he pe o mance
o a colo -changing CH3Ge-based pH senso was ho oughly
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Figu e 6. CH3Ge-based pH- esponsi e senso . a) Images o he pH colo change CH3Ge pape cap u ed wi h a sma phone came a. b) RGB alues a
diffe en pH. c) PL spec a as a unc ion o pH. d) DRS in pH 1, 7, and 11. e) RGB alues o CH3Ge in esh and spoiled milk. ) Schema ic ep esen a ion
colo change o CH3Ge pape in ela ion o milk spoilage and quan ifica ion using a sma phone wi h Colo G ab colo in ensi y p ocessing so wa e.
in es iga ed. As shown in Figu e 6a, he colo o CH3Ge g ad-
ually changes as pH inc eases. When pH le els a e in acidic and
basic anges, he CH3Ge changes om da k ed o o ange, e-
spec i ely. These isible colo changes aid use s in de e mining
he es ima ed pH o he sample using hei naked eyes and wi h-
ou any help om ins umen s. Howe e , he pe cep ion o colo
diffe ences a ies om pe son o pe son, and some iewe s may
ha e a colo ision deficiency. These obs acles could be o e come
by using a sma phone equipped wi h a colo in ensi y p ocess-
ing applica ion as we ha e demons a ed he e.
The pH ange was quan ified by measu ing he colo pa ame-
e (RGB alue) om images cap u ed wi h a sma phone came a
using he Colo G ab applica ion (Figu e 6b). The ed colo in-
ensi y was a unc ion o inc easing pH while sub le diffe ences
we e obse ed in he blue and g een colo alues. Figu e 6c de-
pic s he PL spec a o CH3Ge a diffe en pH le els. The ed ligh
emission in pH 1, 3, and 5 is ≈1.977 eV (627 nm) while 1.971 eV
(629 nm) was obse ed in pH 7. A pH 9 and 11, he blueshi o
he CH3Ge was ound a 1.990 (623 nm) and 1.996 eV (621 nm),
espec i ely. The obse ed blueshi is p obably a ibu able o
pa ial decomposi ion, which will gene a e Ge(OH)2.[27] Mul iple
pH sensing cycles o CH3Ge pape we e pe o med a pH 9 and
pH 11. The e is no significan change obse ed in sensing pe -
o mance a pH 9, whe eas a d ama ic change was obse ed a
pH 11 (Figu e S3, Suppo ing In o ma ion). This indica es ha a
CH3Ge-based senso can unc ion below pH 11 o eal-wo ld ap-
plica ions. Fu he mo e, he a ia ion in band edge as a unc ion
o pH was eco ded using diffuse eflec ance spec oscopy (DRS).
The bandgap was calcula ed om DRS using he Kubelka–Munk
unc ion. Figu e 6d shows a clea diffe ence in band edge wi h
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