Ci a ion: Cepeda, J.; Blasco-Pascual, I.;
Rojas, S.; Choquesillo-Laza e, D.;
Gue e o-A oyo, F.J.; Mo ales, D.P.;
Ga cía, J.Á.; Rod íguez-Diéguez, A.;
Salinas-Cas illo, A. Sensing Capaci y
in Dysp osium Me al–O ganic
F amewo ks Based on
5-Aminoisoph halic Acid Ligand.
Senso s 2022,22, 3392. h ps://
doi.o g/10.3390/s22093392
Academic Edi o : An onie a Tau ino
Recei ed: 17 Janua y 2022
Accep ed: 25 Ap il 2022
Published: 28 Ap il 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
senso s
A icle
Sensing Capaci y in Dysp osium Me al–O ganic F amewo ks
Based on 5-Aminoisoph halic Acid Ligand
Ja ie Cepeda 1, Isabel Blasco-Pascual 2, Sa a Rojas 3, Duane Choquesillo-Laza e 4,
F ancisco J. Gue e o-A oyo 3, Diego P. Mo ales 5, Jose Ángel Ga cía6, An onio Rod íguez-Diéguez 3,*
and Al onso Salinas-Cas illo 2,*
1Depa men o Applied Chemis y, Facul y o Chemis y, Uni e si y o he Basque Coun y (UPV/EHU),
20018 Donos ia-San Sebas ián, Spain; ja ie [email p o ec ed]
2Depa men o Analy ical Chemis y, Facul y o Science, Uni e si y o G anada, 18071 G anada, Spain;
isablasco@ug .es
3Depa men o Ino ganic Chemis y, Facul y o Science, Uni e si y o G anada, 18071 G anada, Spain;
s ojas@ug .es (S.R.); [email p o ec ed] (F.J.G.-A.)
4Labo a o io de Es udios C is alog á icos, IACT, CSIC-UGR, A . Las Palme as nº4, 18100 G anada, Spain;
[email p o ec ed]
5
Pe asi e Elec onics Ad anced Resea ch Labo a o y (PEARL), Depa men Elec onics and Compu e Technology,
Uni e si y o G anada, 18071 G anada, Spain; diegopm@ug .es
6Depa amen o de Física Aplicada II, Facul ad de Ciencia y Tecnología, Uni e sidad del País Vasco/Euskal
He iko Unibe si a ea (UPV/EHU), 48940 Leioa, Spain; [email p o ec ed]
*Co espondence: an onio5@ug .es (A.R.-D.); al onsos@ug .es (A.S.-C.)
Abs ac :
Two no el me al-o ganic amewo ks (MOFs), based on dysp osium as he me al and
he 5-aminoisoph halic acid (5aip) ligand, ha e been sol o he mally syn hesized, wi h he aim o
s udying and modula ing hei luminescence p ope ies acco ding o he a ia ion o sol en in he
s uc u e. These ma e ials display in ense pho o-luminescence p ope ies in he solid s a e a oom
empe a u e. In e es ingly, one ascina ing senso y capaci y o compound
2
ega ds ob aining a
a ia ion o he signal, depending on he sol en o which i is exposed. These esul s pa e he way
o a new gene a ion o sensi i e chemical senso s.
Keywo ds: MOF; dysp osium; 5-aminoisoph halic; sensing; luminescence
1. In oduc ion
Dysp osium ions ha e been widely employed as ascina ing magne ic and luminescen
cen e s o cons uc ing coo dina ion compounds, due o hei la ge magne ic momen and
signi ican single-ion aniso opy, de i ed om spin
−
o bi coupling and he c ys al ield
e ec and he cha ac e is ic na ow luo escence emission, in such a way ha i is he op i-
mal ion o cons uc in e es ing ma e ials o exhibi ing i s sensing capabili ies [
1
]. This is
pa icula ly ue o me al–o ganic amewo ks (MOFs), which a e c ys alline coo dina ion
polyme s wi h a la ge s uc u al e sa ili y ha ha e a wide ange o applica ions [
2
]. These
ma e ials a e syn hesized by mixing me al ions (ac ing as he nodes o he amewo k) wi h
o ganic molecules (which play he ole o links) o o m po en ially po ous a chi ec u es
ha a e o en ex ended o e wo o h ee dimensions. Among hem, dysp osium-based
MOFs (Dy-MOFs) ha e been less widely s udied, owing o he la ge size o he Dy(III)
ion, because he usual high coo dina ion numbe s i p esen s impose dis o ed symme-
ies ha in oduce oo much coo dina ion lexibili y in o he c ys al-building, a ac ha
somehow p e en s he p edic ion o he esul ing opology o he amewo k. The s udy o
lan hanide-based MOFs has been adi ionally linked o ields such as luminescence [
3
], gas
adso p ion [
4
], op ical s o age [
5
], and magne ism [
6
]. In pa icula , ou esea ch g oup and
o he s a e ocusing on his poin in o de o u he analyze he luminescen p ope ies o
hese ma e ials [
7
]. In his ega d, in ecen yea s, se e al Dy-MOFs based on dica boxylic
Senso s 2022,22, 3392. h ps://doi.o g/10.3390/s22093392 h ps://www.mdpi.com/jou nal/senso s
Senso s 2022,22, 3392 2 o 11
ligands, showing in e es ing magne ic and luminescen p ope ies, ha e been designed and
syn hesized by ou g oup [
8
]. Wi h he aim o con inuing his ask, we decided o explo e
and p epa e dysp osium-based MOFs by in oducing small s uc u al modi ica ions in o
he ligands o compa e hei luminescen p ope ies. The e o e, in he p esen wo k, we
a e making use o 5-aminoisoph halic (5aip) acid as he ligand due o i s g ea a ie y o
coo dina ion modes, a cha ac e is ic ha makes i e y use ul o ob aining a di e si y o
sys ems [9].
The main challenge in senso esea ch can be summa ized by he ollowing expec a-
ions: (a) he sea ch and selec ion o he app op ia e ma e ials, as imp o emen and no el y
in he ecogni ion mechanisms necessa y o he ins an aneous iden i ica ion o analy es,
and he imp o emen o he mechanism o c ea e he signal o be ob ained om he senso ;
(b) he de elopmen o new ma e ials as ma ices ha e ec i ely immobilize he ecep o
molecules and, in his way, achie e s abili y and ep oducibili y in he senso s; (c) he
design o sys ems o de elop au oma ion in he use o senso s and hei implemen a ion
in new echnologies. The con en ional analysis p ocedu es used in many cases a e expen-
si e, slow, complex, and dependen on high-cos in as uc u es and ained pe sonnel o
hei use.
Pape -based analy ical de ices (PADs) o e powe ul pla o ms o he design o
chemical analysis pla o ms due o hei ad an ages, such as a lowe cos pe es , he
educ ion o he assay ime and olume consump ion, eco- iendly suppo , and, in gene al,
simple s a egies in e ms o ood and en i onmen al moni o ing [
10
,
11
]. On accoun o
hese me i s, PADs a e capable o in eg a ing wi h a sensing sys em based on an op ical
signal (luminescence o colo ) [
12
], Along wi h hese a ac i e ea u es and p omising
p ospec s, he de elopmen and implemen a ion o nanoma e ials in hese sys ems a e
necessa y o hei u u e applica ion [13].
Bea ing in mind he la e poin , we epo on he syn hesis and s uc u al cha ac e -
iza ion o wo new MOFs ha exhibi luminescen p ope ies; speci ically, one o hem
shows in e es ing sensing capaci ies, depending on he sol en o which i is exposed.
2. Ma e ials and Me hods
2.1. Chemicals
All chemicals we e o eagen g ade and we e used as comme cially ob ained wi hou
any u he pu i ica ion. N,N-dime hyl o mamide anhyd ous (HCON(CH
3
)
2
,
≥
99.8%),
dysp osium(III) chlo ide hexahyd a e (DyCl
6·
6H
2
O,
≥
99.99%), 5-aminoisoph halic acid
(H
2
NC
6
H
3
-1,3-(CO
2
H)
2
, 94%), me hanol (CH
3
OH, 99.8%), e hanol (C
2
H
5
OH, 99.8%),
dime hyl sul oxide anhyd ous (C
2
H
6
OS,
≥
99.9%), and ace ic acid (CH
3
COOH, 99–100%)
we e pu chased om Sigma Ald ich (Da ms ad , Ge many). All he aqueous solu ions
we e p epa ed wi h ul apu e wa e (0.22
µ
S/cm, 25
◦
C, MilliQ
©
, Millipo e, Bu ling on,
CA, USA).
Cellulose il e pape (Re . 1240, shee 203
×
254 mm, base weigh 85 g/m
2
; hickness
200 µm; e en ion 14–18 µm) was ob ained om Fil e -Lab (Ba celona, Spain).
2.2. Syn hesis o Compound 1
A DMF:H
2
O mix u e (1 mL, 1:1) con aining DyCl
3·
6H
2
O (0.035 mmol, 13.9 mg) was
slowly added o ano he DMF:H
2
O solu ion (1 mL, 1:1) o he 5aip ligand (0.055 mmol,
10 mg
). The esul ing mix u e was pou ed in o a glass essel o 8 mL capaci y and sonica ed
o 2 min o ob ain a homogeneous solu ion, hen i was closed wi h a sc ew cap and placed
in an o en a 95
◦
C. So pink c ys als, o med om compound
1
, we e g own a e 1 day.
They we e il e ed, washed wi h wa e , and d ied. Yield: 15%, based on he me al.
2.3. Syn hesis o Compound 2
The p ocedu e is simila o he p e ious one bu i in ol ed doubling he concen a ion
o he ini ial eagen s. The essel was closed wi h a sc ew cap and placed in an o en a
95 ◦C
.
So pink c ys als co esponding o compound
2
we e g own a e 1 day o sol o he mal
Senso s 2022,22, 3392 3 o 11
eac ion o he eagen mix u e in an o en a 95
◦
C. The c ys als we e also il e ed, washed
wi h wa e , and d ied. Yield: 21%, based on he me al.
2.4. X- ay Di ac ion Da a Collec ion and S uc u e De e mina ion
C ys als o he compounds we e moun ed on glass ibe and we e used o he
da a collec ion on a B uke D8 Ven u e, wi h a pho on de ec o equipped wi h g aphi e
monoch oma ed MoK
α
adia ion (
λ
= 0.71073 Å). The da a educ ion was pe o med wi h
he APEX2 [
14
] so wa e and was co ec ed o abso p ion using SADABS [
15
]. The c ys al
s uc u es we e es ablished by di ec me hods using he SIR97 p og am [
16
] and e ined by
ull-ma ix leas -squa es on F
2
, including all e lec ions, using aniso opic displacemen
pa ame e s by means o he WINGX c ys allog aphic package [
17
,
18
]. Gene ally, aniso opic
empe a u e ac o s we e assigned o all a oms excep o hyd ogen a oms, which we e
iding hei pa en a oms wi h an iso opic empe a u e ac o a bi a ily chosen o be
1.2 imes ha o he espec i e pa en . The inal R(F), wR(F2), goodness-o - i ag eemen
ac o s, de ails on he da a collec ion, and analysis can be ound in Table 1. C ys allog aphic
da a o he s uc u es epo ed in his pape ha e been deposi ed wi h he Camb idge
C ys allog aphic Da a Cen e unde he supplemen a y publica ion nos. CCDC 2103353 and
2103354 o compounds. Copies o he da a can be ob ained ee o cha ge on applica ion
o he Di ec o , CCDC, 12 Union Road, Camb idge, CB2 1EZ, UK ( ax: +44-1223-335033;
e-mail: [email p o ec ed]).
Table 1. C ys allog aphic da a and s uc u e e inemen de ails o compounds.
Compound 1 Compound 2
CCDC 2103354 2103353
Fo mula C57H75Dy4N9O39 C54H70Dy4N8O39
M 2160.26 2105.18
C ys al Sys em T iclinic T iclinic
Space g oup P-1 P-1
T(K) 100 100
a(Å) 12.0914 (16) 13.5278 (7)
b(Å) 13.4186 (17) 14.2508 (8)
c(Å) 24.493 (3) 19.8040 (10)
α(deg) 76.929 (4) 77.679 (2)
β(deg) 85.627 (4) 70.471 (2)
γ(deg) 87.332 (5) 83.739 (2)
V(Å3)3858.0 (8) 3512.4 (3)
Z 2 2
Densi y (g cm−3)1.86 1.991
µ(mm−1)3.927 4.31
Obse ed e lec ions 10617 56588
Rin 0.092 0.0435
R1b/wR2c(I > 2σ(I)) 0.0553/0.1279 0.0260/0.0513
R1b/wR2c(all da a) 0.0783/0.1409 0.0366/0.0544
GoF (S) a1.029 1.048
La ges di . pk and hole (eÅ
−3
)
3.355 and −3.289 0.872 and −0.555
a
S = [
Σ
w(F
02−
F
c2
)
2
/(N
obs −
N
pa a
m)]
1/2
.
b
R
1
=
Σ
||F
0
|–|F
c
||/
Σ
|F
0
|.
c
wR
2
= [
Σ
w(F
02−
F
c2
)
2
/
Σ
wF
02
]
1/2
w = 1/[σ2(F02) + (aP)2+ bP] whe e P = (max(F02,0) + 2Fc2)/3.
2.5. Pho oluminescence Measu emen s
Pho oluminescence spec a we e eco ded on an Edinbu gh Ins umen s FLS920
(
Edinbu gh
, UK) spec ome e equipped wi h a closed-cycle helium c yos a . All mea-
su emen s we e pe o med unde a high acuum (o ca. 10
−
9 mba ) o a oid he p esence
o oxygen o wa e in he sample holde . Fo s eady-s a e measu emen s, an IK3552R-G
HeCd (GoPho onics, Zu ich, Swi ze land) con inuous lase (325 nm) was used as an exci-
a ion sou ce, whe eas a Mülle -Elek onik-Op ik SVX1450 (Ald idge, UK) Xe lamp was
employed o collec he exci a ion spec a. The luminescence collec ed in he UV- is egion
Senso s 2022,22, 3392 4 o 11
was analyzed wi h a pho omul iplie ube (PMT) coupled o he spec ome e . Howe e ,
he decay cu es we e measu ed using a
µ
F900 mic osecond pulsed lamp (Edinbu gh,
UK) o a pulsed lase diode LDH-P-C-375 (PicoQuan , Be lin, Ge many) (
λ
= 375 nm),
depending on he emission longe i y.
2.6. P epa a ion o he Tes Pape
The PAD was ab ica ed using a c a -cu ing echnique as a cos -e icien , simple and
ep oducible p ocess; he pa e n o he
µ
PAD was i s designed using Illus a o so wa e
(Adobe Sys ems, San Jose, CA, USA) and he design was cu wi h a CO
2
lase eng a e
(Rayje , Ba celona, Spain). To be speci ic, disks (8 mm in diame e ) o cellulose-g ade il e s
we e i s p oduced using a cu ing echnique ha was op imized using a shee o pape .
The de ice was p epa ed by d op-cas ing he necessa y eagen s in each sensing a ea; hen,
15
µ
L o MOFs was dispensed on o he de ice, hen d ied a oom empe a u e o 5 min,
and s o ed in he da k un il use unde ambien a mosphe ic condi ions. Then, 15
µ
L sol en
solu ion in wa e a di e en concen a ions was d opped on o he manu ac u ed es pape
and hen, we p oceeded o measu e he luo escence spec a.
The luo escence spec a o PAD we e ob ained using a Ca y Eclipse UV-Vis luo-
escence spec opho ome e in he ange o 400–550 nm, wi h a xenon lamp as he ex-
ci a ion sou ce. Luminescence in ensi y was measu ed using he ollowing condi ions:
λex = 325 nm;
he pho omul iplie ol age was se a 700 V; exci a ion and emission sli s o
5 nm we e used.
3. Resul s and Discussion
The hyd o he mal eac ions o he 5-aminoisoph halic acid wi h dysp osium chlo ide
a di e en concen a ions (10 mL) a 95
◦
C o 48 h p oduced p isma ic c ys als o di e en
MOFs. The c ys al s uc u es o he compounds we e de e mined using single-c ys al
X- ay di ac ion.
3.1. Desc ip ion o he S uc u es
Compound
1
c ys allized in he iclinic space g oup P-1. I consis s o a 2 D-laye ed
s uc u e composed o Dy(III) dime ic uni s, es ablished by he coo dina ion o ca boxyla e
g oups pe aining o di e en 5aip ligands (Figu e 1). The asymme ic uni o his ma e ial
is composed o ou dysp osium a oms, six dianionic ligands, h ee coo dina ion wa e
molecules, wo coo dina ion DMF molecules, six la ice wa e molecules, and one la ice
DMF molecule. The dysp osium ions a e b idged by di e en linke s, o ini ially gene a e
dime ic en i ies ha a e u he linked o one ano he by means o bo h o he o he
ca boxyla e g oups pe aining o he 5aip ligands, o ming he dime and he es o he
5aip ligands coo dina ing h ough he edges o he dime . The esul ing wo-dimensional
ne wo k possesses ca i ies ha a e occupied by DMF and wa e molecules wi hin i .
The ou dysp osium a oms a e c ys allog aphically independen , whe e all show DyO
8
coo dina ion polyhed a excep o he Dy2 a om, which exhibi s a DyO7polyhed on.
Figu e 1.
The wo-dimensional s uc u e o compound
1
. C ys alliza ion molecules and hyd ogen
a oms a e omi ed o cla i y. Colo code: C = g ay, Dy = g een, N = blue, O = ed.
Senso s 2022,22, 3392 5 o 11
The Dy1 a om p esen s a coo dina ion polyhed on, es ablished by eigh oxygen a oms
belonging o six di e en 5aip linke s. Ins ead, he DyO
7
coo dina ion en i onmen o
he Dy2 a om is buil by i e oxygen a oms o i e di e en dianionic 5aips, one wa e
molecule (O12), and one DMF molecule. The main di e ence be ween Dy3 and Dy4 a oms
wi h espec o Dy1 is he p esence o sol en molecules in hei coo dina ion en i onmen .
Tha is o say, he Dy3 a om con ains one oxygen a om (O50) o a coo dina ion wa e
molecule, while he Dy4 a om possesses one wa e (O20) and one DMF e minal ligand.
The Dy–O
ca b
bond dis ances a e in he ange o 2.234(7)–2.596(6) Å, among which he
Dy–O
wa e
dis ances a e a ound 2.3 Å. Wi hin he dime s, i mus be highligh ed ha
he in adinuclea Dy
· · ·
Dy dis ance, which in ol es Dy1 and Dy4 a oms, is 3.9120(7) Å.
Finally, he laye s a e packed close o one ano he by means o hyd ogen bonds, es ablished
be ween he amino g oups o 5aip ligands and sol en molecules, among which he o me
ac as hyd ogen-bonding dono s and he la e as accep o s. In spi e o he p esence o
oids among he laye s, he 2D-laye ed s uc u e did no emain s able enough o allow he
emo al o sol en molecules du ing i s he mal ac i a ion, which led o an amo phous
p oduc ha could no be u he s udied (see Supplemen a y In o ma ion).
Compound
2
c ys allized in he iclinic space g oup o P-1. The 2D-MOF s uc u e
is composed simila ly o Dy(III) dime s connec ed h ough he ca boxyla e g oups o di -
e en 5-amino-1,3-benzenedica boxylic ligands, gene a ing a bidimensional coo dina ion
polyme (Figu e 2). The asymme ic uni o his ma e ial is composed o ou dysp osium
a oms, six dianionic ligands, i e coo dina ion wa e molecules, nine c ys alliza ion wa e
molecules, and wo c ys alliza ion DMF molecules. These dysp osium a oms a e b idged by
di e en linke s, gene a ing a wo-dimensional ne wo k wi h big ca i ies ha a e occupied
by c ys alliza ion DMF molecules and wa e molecules. The ou dysp osium a oms a e
c ys allog aphically independen , bo h ha ing DyO
8
coo dina ion polyhed a excep in he
case o dysp osium 1, which has a DyO7polyhed on.
Figu e 2.
Bidimensional s uc u e o compound
2
. C ys alliza ion molecules and hyd ogen a oms
a e omi ed o cla i y. Colo coding: g ey = C, g een = Dy, blue = ni ogen, ed = oxygen.
The coo dina ion en i onmen o he Dy1 a om is buil by i e oxygen a oms, pe -
aining o i e di e en 5aip linke s, and comple ed by wo oxygen a oms (O37 and O40)
o wa e molecules. The Dy2 a om has a DyO
8
ch omopho e, wi h eigh oxygen a oms
o six di e en dianionic linke s and wo sol en molecules. On he o he hand, he Dy3
and Dy4 a oms show simila DyO
8
en i onmen s ha a e dis inguished om Dy2 by he
ac ha hey possess one (O31) and wo (O32 and O41) coo dina ion wa e molecules,
espec i ely. The Dy–O
ca b
bond dis ances a e o he same o de as in compound
1
, whe e
he in adinuclea Dy
· · ·
Dy dis ance is 3.9089(3) Å o he Dy2
· · ·
Dy4 b idge. Finally, he
laye s a e packed close o one ano he by means o hyd ogen bonding in e ac ions among
he amino g oups o ligands, c ys alliza ion DMF molecules, and wa e molecules. The
p incipal di e ence be ween he wo compounds is ha in compound
2
, he DMF molecules
coo dina e wi h dysp osium a oms, whe eas in compound
1,
he e a e no coo dina ed DMF
molecules. The po en ially po ous s uc u e o compound
2
p esen s a con inuous elease
o he molecules occupying he po es, mixed wi h he coo dina ion sol en molecules (see
Supplemen a y Ma e ials), a ac ha p ecludes i s adso p ion capaci y in a simila way o
ha men ioned o compound 1.
Senso s 2022,22, 3392 6 o 11
3.2. Luminescence P ope ies
The luminescence p ope ies o compounds
1
and
2
we e i s s udied in he solid s a e
by measu ing he exci a ion and emission spec a a oom empe a u e o e he polyc ys-
alline samples. The emission spec um o compound
1,
measu ed unde monoch oma ed
lase i adia ion (
λex
= 325 nm), is domina ed by a wide band co e ing he 400–500 nm
ange (Figu e 3), which may be a ibu ed o he ligand luo escence, in iew o he simila
p o ile shown by he ee ligand, as desc ibed in a p e ious s udy [
19
]. In pa icula , he
band may be assigned o he
π←π
* ansi ions occu ing a he a oma ic ing o he ligand,
as p e iously shown o o he compounds based on he 5aip ligand [
20
,
21
]. In any case,
his band also p esen s a mino shoulde a a ound 483 nm, in addi ion o ano he band o
simila in ensi y si ed a 576 nm. Acco ding o epo s ega ding o he p e ious Dy-based
compounds [
22
], hese wo signals co espond o he main in aionic ansi ions o he ion,
namely, he
6
H
15/2 ←4
F
9/2
and
6
H
13/2 ←4
F
9/2
, espec i ely, o
λem
= 483 and 576 nm.
The exci a ion spec um, measu ed by moni o ing he la e , shows a wide band whe e no
na ow maxima a ibu ed o he in aionic exci a ions o Dy(III) a e obse ed, meaning
ha he Dy(III)-cen e ed emission is p e e en ially occu ing h ough ligand abso p ion,
i.e., he an enna e ec .
Figu e 3.
The (
a
) exci a ion (
λem
= 576 nm) and (
b
) emission (
λex
= 325 nm) spec a o compound
1
,
acqui ed a oom empe a u e.
Al hough consis ing o he same ligand, when he ligand is coo dina ed o Dy(III)
ions o o m he s uc u e o compound
2
, he Dy(III)-cen e ed emission is signi ican ly
imp o ed wi h ega d o
1
, which inding may be co obo a ed by he ela i e inc ease in
he in ensi y o he bands co esponding o he in aionic ansi ions peaking a λem = 483
and 576 nm (Figu e 4). Mo eo e , he s onge luminescence o his compound also allows
he obse a ion o a hi d mul iple si ed a
λem
= 665 nm, which can be a ibu ed o he
6
H
11/2 ←4
F
9/2
ansi ion. Among he h ee Dy(III)-cen e ed bands, he second one is
he mos in ense, as usually happens o mos o he coo dina ion polyme s analyzed in
p e ious s udies [23–25].
In good ag eemen wi h he p e ious s a emen s, he exci a ion spec um o
2
also
p esen s nume ous na ow bands co esponding o he in aionic bands o Dy(III) ion,
h ough which exci a ion akes place om he g ound
4
F
9/2
e m o he exci ed s a es.
Paying a en ion o he c ys al s uc u e o bo h compounds, he lowe in ensi y shown by
compound
1
may be a ibu ed o he p esence o he DMF molecules coo dina ed o he
me al cen e . As is well known, he occu ence o ce ain molecules possessing C–H, O–H,
and N–H bonds, which beha e as oscilla o s ha a e coupled o he ene gy gaps exis ing
be ween he exci ed and g ound e ms o he lan hanide, may ac as e ec i e quenche s
o he lan hanide-cen e ed luminescence [
26
]. In his way, pa o he cha ge popula ing
he exci ed le els (
4
F
9/2
, in his case) may be ans e ed o hese ib a ions ac i a ing he
subs an ial non- adia i e pa hways esponsible o he weak luminescence o compound
1
,
a ac ha is no so ma ked in compound
2
. Inspi ed by he be e emission p ope ies o
compound
2
, he decay cu es we e measu ed unde he same wa eleng hs (
λex
= 325 and
Senso s 2022,22, 3392 7 o 11
λem
= 576 nm). The i ing o he cu e (Figu e 5) by means o a con en ional exponen ial
equa ion [I
= A
0
+ A
1
exp(
−
/
τ1
)] ga e a sho li e ime alue o 10
µ
s, which may be
conside ed a simila alue o o he Dy(III)-based compounds [7,27].
Figu e 4.
The (
a
) exci a ion (
λem
= 576 nm) and (
b
) emission (
λex
= 325 nm) spec a o compound
2,
acqui ed a oom empe a u e.
Figu e 5. Decay cu e wi h he bes i ing o compound 2.
The good emissi e p ope ies and po en ial po osi y o compound
2,
gi en he p es-
ence o la ice sol en molecules enclosed in he ca i ies o he s uc u e, encou aged us
o seek deepe insigh s in o he sensing capaci y o he ma e ial. To ha end, se e al
suspensions, s a ing om 3 mg o he compound in a ious sol en s ((H
2
O, MeOH,
E OH, DMF, DMSO, and ace ic acid (AcOH)) we e analyzed by measu ing he emis-
sion spec a, keeping he a o emen ioned expe imen al se up (
λex
= 325 nm). I is im-
po an o no e ha he compound e ains i s s uc u al in eg i y a e being dispe sed
in H
2
O and AcOH, as co obo a ed by he PXRD collec ed o samples il e ed om
hese expe imen s (see
Figu e S2 in he Supplemen a y Ma e ials
). As can be in e ed
om Figu e 6, he dispe sion o he solid in he sol en s does no p omo e signi ican
changes in he emission p o ile (compa ed o he solid s a e), al hough i does cause
a d op in he o e all emission capaci y. The signal u ns o in he ollowing o de :
H2O > MeOH > E OH > DMSO > DMF > AcOH,
which ollows he same end o he po-
la i y o he sol en s. This end is in good ag eemen wi h he lowe capaci y o he sol en
o c ea e and keep he exci ons ac i e upon UV ligh abso p ion as he pola i y o he
sol en dec eases. This ac has been p e iously obse ed o many o he lan hanide-based
complexes [
28
,
29
]. Taking in o accoun he ac ha his compound does no beha e as
a pe manen ly po ous amewo k, in iew o i s he mal beha io , i may be specula ed
ha he quenching mechanism migh in ol e he in e ac ions aking place be ween he
quenche s and he ex e nal ace o he c ys alline pa icles.
Senso s 2022,22, 3392 8 o 11
Figu e 6. The emission spec um o compound 2. Inse : colo coding.
3.3. Tes Pape
To be speci ic, disks (8 mm in diame e ) o cellulose il e pape we e i s p oduced
using a cu ing echnique ha was op imized using a shee o pape (203
×
254 mm) o
he p oduc ion o 396 eplicas (see Figu e S5 in he Supplemen a y Ma e ials).
As men ioned, he luo escence in ensi y o PAD pape is quenched when he sol en
is p esen ed. The dec ease in luo escence can be quan i ied by co ela ing he a io o
ini ial in ensi y (Io) and he in ensi y (I) o he pape wi h he sol en p esen in he sample
(see Figu e 7and Figu e S6 in he Supplemen a y Ma e ials). The expe imen al dependence
be ween he Io/I alue and he sol en (%) is in linea dependence. The eg ession equa ion
is Io/I = 0.0197C + 1.03, wi h R
2
= 0.9862 o ace ic acid and Io/I = 0.0068C + 1.0087, wi h
R
2
= 0.9921. The de ec ion limi o he sol en was calcula ed, in ligh o he 3
σ
/slope laws,
o be as low as 0.30% o ace ic acid and 0.88% o e hanol. In addi ion, he quan i ica ion
limi was calcula ed wi h he 10
σ
/slope equa ion, esul ing in 1.02% and 2.94% o ace ic
acid and e hanol, espec i ely. The epea abili y as a ela i e s anda d de ia ion, ob ained
using 10 di e en PADs a 10% ( / ) o ace ic acid, was 4.2%, an accep able p ecision
conside ing he measu ing sys em used. The li e ime o he PAD was s udied o 20 days in
wo cycles o he p epa ed de ice, egula ly checking hei esponses o a 10% ( / ) ace ic
acid. Du ing he s udy pe iod, i was obse ed ha he e was no dec easing signal; he
esponse o he de ice was s able.
Figu e 7.
Calib a ion cu e o ace ic acid (o ange iangle) and e hanol (blue do ) using a es pape
wi h compound 2.
Senso s 2022,22, 3392 9 o 11
Conside ing ha he concen a ion-dependen e olu ion o bo h quenche s, in he o m
o S e n-Volme plo s, i ed well o a linea equa ion, o he p ocesses, such as in e sys em
c ossing, he o ma ion o cha ge ans e complexes, and s a ic/dynamic quenching can
be disca ded, while he majo con ibu ion uling he quenching mechanism seems o be
he pola i y o he sol en s.
4. Conclusions
Two no el me al–o ganic- amewo ks (MOFs), based on dysp osium as he me al and
he 5-aminoisoph halic acid ligand, ha e been sol o he mally syn hesized, wi h he aim o
s udying and modula ing hei luminescence p ope ies acco ding o he a ia ion o he
sol en in he s uc u e. These ma e ials display in ense pho o-luminescence p ope ies
in hei solid s a e a oom empe a u e. Bo h compounds exhibi he main cha ac e is ic
emission o he ligand, whe eas he Dy(III)-based luminescence a oom empe a u e is com-
pa a i ely weak, gi en ha he ene gy ans e om he ligand o he lan hanide ion is no
e icien . Howe e , compound
2
p esen s be e p ope ies, al hough bo h compounds sha e
he same ligand, a ac ha may be ela ed o he absence o coo dina ed DMF molecules.
The dispe sion o he solid in he sol en s causes a d op in he o al emission capaci y,
ollowing he same end o he pola i y o he sol en s. In he same way, his sys em
wo ks in pape -based de ices, opening he doo s o u u e uses and he implemen a ion o
hese compounds in po able, cheap, and en i onmen ally sus ainable sys ems.
Supplemen a y Ma e ials:
The ollowing suppo ing in o ma ion can be downloaded a : h ps:
//www.mdpi.com/a icle/10.3390/s22093392/s1, Figu e S1: Compa ison o he PXRD da a o
compound 1
; Figu e S2: Compa ison o he PXRD da a o compound
2
; Figu e S3: The mog a i-
me ic p o iles o compounds
1
and
2
; Figu e S4: View along a (le ), b (middle) and c ( igh )
axis o
compound 1
(abo e) and compound
2
(below); Figu e S5: Pape analy ical de ice de el-
opmen ;
Figu e S6:
Spec um emission using he es pape , wi h compound
2
o ace ic acid (A)
and
e hanol (B).
Condi ions o luo escence exci a ion a 325 nm, wi h in ensi y based (Io/I) a a
maximum peak emission o 410 nm (C).
Au ho Con ibu ions:
J.C.: luminescence measu emen s; I.B.-P.: luminescence measu emen s and
es pape de elopmen ; S.R.: chemical cha ac e iza ion; F.J.G.-A.: syn hesis o wo MOFs; D.C.-L.: XR
di ac ion; D.P.M.: sensing cha ac e iza ion; J.Á.G.: da a p ocessing; A.R.-D.: w i ing he manusc ip
and di ec ion; A.S.-C.: w i ing he manusc ip and di ec ion. All au ho s ha e ead and ag eed o he
published e sion o he manusc ip .
Funding:
This wo k was suppo ed by he Spanish Minis y o Science, Inno a ion and Uni e si ies
(MCIU/AEI/FEDER,UE) (PGC2018-102052-A-C22, PGC2018-102052-B-C21, PID2020-117344RB-I00
and PID2019-108028GB-C21), Jun a de Andalucía (FQM-394, B.FQM.734.UGR20 and P20_00265),
Gobie no Vasco/Eusko Jau la i za (IT1005-16, IT1291-19), he Uni e si y o he Basque Coun y
(UPV/EHU) (GIU 17/13). S.R. acknowledges he Juan de la Cie a Fellowship (IJC2019-038894-I).
The au ho s acknowledge he echnical and human suppo p o ided by SGIke o UPV/EHU and
Eu opean unding (ERDF and ESF).
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Re e ences
1.
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o dysp osium hyd ogen-bonded o ganic amewo ks wi h high s abili y and acid s imulus- esponse luminescence p ope ies.
J. Ma e . Chem. C 2021,9, 9319–9330. [C ossRe ]
2.
Qiu, S.; Zhu, G. Molecula enginee ing o syn hesizing no el s uc u es o me al-o ganic amewo ks wi h mul i unc ional
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Cepeda, J.; Sebas ian, E.S.; Pad o, D.; Rod íguez-Diéguez, A.; Ga cia, J.A.; Ugalde, J.M.; Seco, J.M. A Zn based coo dina ion
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