nanoma e ials
A icle
Coa ing o Conduc ing and Insula ing Th eads wi h Po ous
MOF Pa icles h ough Langmui -Blodge Technique
Sakanda Rau 1,† , Miguel A. And és2,3,† , Oli ie Roubeau 2, Ignacio Gascón2,3,* , Ch is ian Se e 4,
Mohamed Eddaoudi 5and Khaled N. Salama 1,*


Ci a ion: Rau , S.; And és, M.A.;
Roubeau, O.; Gascón, I.; Se e, C.;
Eddaoudi, M.; Salama, K.N. Coa ing
o Conduc ing and Insula ing
Th eads wi h Po ous MOF Pa icles
h ough Langmui -Blodge
Technique. Nanoma e ials 2021,11,
160. h ps://doi.o g/10.3390/
nano11010160
Recei ed: 4 Decembe 2020
Accep ed: 7 Janua y 2021
Published: 10 Janua y 2021
Publishe ’s No e: MDPI s ays neu-
al wi h ega d o ju isdic ional clai-
ms in published maps and ins i u io-
nal a ilia ions.
Copy igh : © 2021 by he au ho s. Li-
censee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and con-
di 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/).
1Senso s Lab, Ad anced Memb anes & Po ous Ma e ials Cen e (AMPMC), Compu e , Elec ical,
and Ma hema ical Sciences and Enginee ing (CEMSE) Di ision, King Abdullah Uni e si y o Science and
Technology (KAUST), Thuwal 23955-6900, Saudi A abia; sakanda [email p o ec ed]
2Ins i u o de Nanociencia y Ma e iales de A agón (INMA), CSIC and Uni e sidad de Za agoza,
50009 Za agoza, Spain; mand es@uniza .es (M.A.A.); oubeau@uniza .es (O.R.)
3Depa amen o de Química Física, Uni e sidad de Za agoza, 50009 Za agoza, Spain
4
Ins i u des Ma é iaux Po eux de Pa is, FRE 2000 CNRS Ecole No male Supé ieu e de Pa is, Ecole Supé ieu e
de Physique e de Chimie Indus ielles de Pa is, PSL Resea ch Uni e si y, 75005 Pa is, F ance;
[email p o ec ed]
5Func ional Ma e ials Design, Disco e y & De elopmen Resea ch G oup (FMD3), Ad anced Memb anes &
Po ous Ma e ials Cen e , Di ision o Physical Sciences and Enginee ing, King Abdullah Uni e si y o Science
and Technology (KAUST), Thuwal 23955-6900, Saudi A abia; [email p o ec ed]
*Co espondence: igascon@uniza .es (I.G.); [email p o ec ed] (K.N.S.)
† These au ho s con ibu ed equally.
Abs ac :
The Langmui -Blodge (LB) me hod is a well-known deposi ion echnique o he ab i-
ca ion o o de ed monolaye and mul ilaye hin ilms o nanoma e ials on o di e en subs a es
ha plays a c i ical ole in he de elopmen o unc ional de ices o a ious applica ions. This
pape desc ibes de ailed s udies abou he bes coa ing con igu a ion o nanopa icles o a po ous
me al-o ganic amewo k (MOF) on o bo h insula ing o conduc i e h eads and nylon ibe . We
design and ab ica e cus omized polyme hylme hac yla e shee s (PMMA) holde s o deposi MOF
laye s on o he h eads o ibe using he LB echnique. Two di e en o ien a ions, namely, ho izon al
and e ical, a e used o deposi MIL-96(Al) monolaye ilms on o i e di e en ypes o h eads and
nylon ibe . These s udies show ha LB ilm o ma ion s ongly depends on deposi ion o ien a ion
and he ype o h eads o ibe . Among all he samples es ed, co on h ead and nylon ibe wi h
e ical deposi ion show mo e homogenous monolaye co e age. In he case o conduc i e h eads,
he MOF pa icles end o agg ega e be ween he conduc i e h ead’s ibe s ins ead o o ming a
con inuous monolaye coa ing. Ou esul s show a signi ican con ibu ion in e ms o MOF mono-
laye deposi ion on o single ibe and h eads ha will con ibu e o he ab ica ion o single ibe o
h ead-based de ices in he u u e.
Keywo ds:
me al-o ganic amewo k (MOF); MIL-96(Al); Langmui -Blodge (LB) ilms; ibe ; h ead;
conduc i e h ead; hin ilms; ex ile coa ings; unc ional ex iles
1. In oduc ion
The in e es o he scien i ic communi y in he p epa a ion, cha ac e iza ion, and
s udy o me al-o ganic amewo ks (MOFs) has con inuously g own du ing he las o y
yea s [
1
–
3
]. MOFs possess excep ional chemical e sa ili y and unable po osi y ha make
hem p omising ma e ials o a wide ange o applica ions [
4
], including, among o he s,
sepa a ion science [
5
–
8
], heal h [
9
–
11
], and he en i onmen [
12
]. The de elopmen o
many MOF applica ions ne e heless equi es he o ma ion o uni o m MOF hin ilms
on o app op ia e su aces [
13
–
16
]. In pa icula , he deposi ion o MOF hin coa ings on o
he su ace o ex iles is a challenging a ge ha has been sca cely explo ed. P e ious
s udies ha e shown ha i is possible o in eg a e MOF and ab ics and he use o mixed
Nanoma e ials 2021,11, 160. h ps://doi.o g/10.3390/nano11010160 h ps://www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2021,11, 160 2 o 10
ma e ials in a a ie y o applica ions, such as de oxi ying il e s [
17
], p o ec ion agains
sola adia ion [
18
,
19
], and gas sensing [
20
]. Howe e , in gene al, complica ed p ocedu es
ha equi e mo e han wo s eps need o be used o ob ain he desi ed MOF/ ex ile
combina ions: ib oin p ocessing du ing se e al days, ollowed by elec ospinning and
cu ing [
17
]; ibe incuba ion a con olled empe a u e in o di e en solu ions du ing
se e al hou s ollowed by d ying and washing [
18
]; eac an s g inding ollowed by ho -
p essing, cleaning, and d ying [
19
]; ibe imme sion in o he eac ion mix u e, sonica ion,
hea ing a a con olled empe a u e, washing, and d ying [
20
]. Mo eo e , ollowing hese
p ocedu es, MOFs a e no loca ed speci ically on he su ace o he ibe s, bu also inside
hem, which is no always con enien o some applica ions and gene ally equi es he use
o la ge MOF amoun s.
In his con ex , he use o he Langmui -Blodge (LB) echnique [
21
,
22
] o MOF
nanopa icle (NP) deposi ion p esen s some signi ican ad an ages since homogeneous
and dense MOF monolaye s can be deposi ed on he su ace o di e en ma e ials wi hou
any p e- ea men o he subs a e, using only a minimal MOF amoun . P e ious s udies in
he li e a u e ha e deal wi h he deposi ion o colloidal NPs and me allic NPs on o di e en
subs a es using he LB echnique [
23
–
25
]. Recen ly, we ha e shown he ab ica ion o
monolaye ilms o NPs o MIL-101(C ) [
26
,
27
] and MIL-96(Al) [
28
–
30
], wi h sizes o ca.
50 and 200 nm espec i ely, on o subs a es o di e en na u e, including in e digi a ed
ex ile elec odes made o a sil e -coa ed conduc i e h ead on o di e en ab ics, and hei
use as a ex ile humidi y senso [
30
]. In his con ibu ion, we p esen a sys ema ic s udy
abou he di ec ab ica ion o monolaye s o MIL-96(Al) NPs on o di e en comme cially
a ailable ibe s and h eads o explo e wha kind o ex ile ab ics could be mo e sui able
o he de elopmen o MOF-coa ed ex iles. We demons a e ha i is possible o deposi
MOF LB ilms on o di e en ypes o ibe s o h eads, wi hou he need o a mo e o less
elabo a ed p e- ea men ha would modi y he ibe o h ead su ace o could imply
expensi e equipmen (e.g., plasma ea men ).
2. Ma e ials and Me hods
MIL-96(Al) nanopa icles o size ca. 200 nm we e ob ained ollowing a p ocedu e p e-
iously epo ed [
31
]. De ails on NPs cha ac e iza ion ha e al eady been epo ed by some
o us [
28
,
29
,
31
]. Fi e di e en conduc i e and non-conduc i e h eads and a nylon ibe
we e used in hese s udies o es he deposi ion o MIL-96(Al) NPs. The non-conduc i e
ibe and h eads used we e co on h ead (whi e colo , no b and), nylon ibe (in isible
h ead, 100% nylon om Hemline), and den al loss h ead (den al loss, O ex). Mo eo e ,
co on h ead ea ed wi h ace ic acid was also used in ou expe imen s, ollowing he
ea men p o ocol epo ed by Owyeung e al. [
32
]. Film deposi ion on conduc i e h eads
was pe o med on Kookye sil e conduc i e h ead (Kookye conduc i e h ead, esis ance
2
Ω
/ , Kookye
®
om Pine ee Elec onics L d., Canada), Libe a o 40 Vec an sil e -coa ed
conduc i e h ead (Libe a o 40
®
, esis ance 1
Ω
/ , Syscom Ad anced Ma e ials Inc., USA),
and BCP conduc i e sewing h ead (B and BCP, no esis ance in o ma ion). All o hese
h eads we e pu chased om Amazon USA and used as ecei ed. All he samples used in
hese s udies a e made ou o many single ibe s wis ed o bundled oge he o make a
h ead excep nylon ibe , as his is a single ibe . The e o e, we used he e minology nylon
ibe , and o all o he samples, we used he wo d h ead a he end o each ma e ial name.
The cus om-buil squa e holde s o anspa en polyme hylme hac yla e shee s (PMMA)
(Figu e 1) we e ab ica ed using a CO
2
lase (VLS 3.5 Desk op lase pla o m). These PMMA
holde s we e used o hold he ibe o h eads du ing he LB ilm deposi ion p ocess.
Nanoma e ials 2021,11, 160 3 o 10
Nanoma e ials 2021, 11, x FOR PEER REVIEW 3 o 10
Figu e 1. Schema ic depic ion and pho og aph o he se up used o Langmui -Blodge (LB) deposi ion in ho izon al (a)
and e ical (b) con igu a ions o he ibe o h eads. (c) Dimensions and pic u e o he holde showing e ical and
ho izon al g oo es o ixing he ibe o h eads.
LB ilms we e ab ica ed using a comme cially a ailable KSV-NIMA ough, model
2000-Sys em 3, wi h dimensions 775  ×  120 mm and a symme ical double-ba ie sys em.
Su ace p essu e was egis e ed by means o an elec obalance using he Wilhelmy pla e
me hod. P e ious s udies ha e shown ha dense and homogeneous MIL-96(Al)
monolaye s can be o med a he ai -wa e in e ace ollowing his p ocedu e (see mo e
de ails in p e ious publica ions) [28–30]: a e ough and wa e su ace cleaning, 8 mL o
a MOF dispe sion in chlo o o m (0.2 mg o MOF pe mL) p epa ed h ough p obe
ul asonica ion we e sp ead d op by d op using a Hamil on mic osy inge. Then, a e
sol en e apo a ion, Langmui ilms we e comp essed a 6 cm2/min o achie e he desi ed
ans e p essu e (30 mN/m).
LB ilms we e deposi ed on o samples using he cus om made PMMA holde s. Each
holde inco po a ed di e en ypes o ibe o h eads up o 3 samples (Figu e 1). These
holde s we e ini ially imme sed in he wa e and we e wi hd awn a 1 mm/min a e
eaching he a ge su ace p essu e (30 mN/m). All samples we e allowed o d y in he
ai o e nigh , and hen hey we e s o ed in a desicca o un il cha ac e iza ion was
pe o med. MOF deposi ion on o he ibe s was cha ac e ized using wo ield emission
scanning elec on mic oscope (FE-SEM) ins umen s: an FEI Quan a 3D FEG and an
Inspec F50 sys em, bo h ope a ed wi h an accele a ing ol age o 10 kV. A coa ing o 5
nm hickness o I o P was applied o he samples be o e SEM inspec ion. A leas h ee
di e en samples o each h ead o ibe and deposi ion o ien a ion we e analyzed.
Co e age analysis was pe o med using ImageJ so wa e.
Figu e 1.
Schema ic depic ion and pho og aph o he se up used o Langmui -Blodge (LB) deposi ion in ho izon al
(
a
) and e ical (
b
) con igu a ions o he ibe o h eads. (
c
) Dimensions and pic u e o he holde showing e ical and
ho izon al g oo es o ixing he ibe o h eads.
LB ilms we e ab ica ed using a comme cially a ailable KSV-NIMA ough, model
2000-Sys em 3, wi h dimensions 775
×
120 mm and a symme ical double-ba ie sys em.
Su ace p essu e was egis e ed by means o an elec obalance using he Wilhelmy pla e
me hod. P e ious s udies ha e shown ha dense and homogeneous MIL-96(Al) monolay-
e s can be o med a he ai -wa e in e ace ollowing his p ocedu e (see mo e de ails in
p e ious publica ions) [
28
–
30
]: a e ough and wa e su ace cleaning, 8 mL o a MOF
dispe sion in chlo o o m (0.2 mg o MOF pe mL) p epa ed h ough p obe ul asonica ion
we e sp ead d op by d op using a Hamil on mic osy inge. Then, a e sol en e apo a ion,
Langmui ilms we e comp essed a 6 cm
2
/min o achie e he desi ed ans e p essu e
(30 mN/m).
LB ilms we e deposi ed on o samples using he cus om made PMMA holde s. Each
holde inco po a ed di e en ypes o ibe o h eads up o 3 samples (Figu e 1). These
holde s we e ini ially imme sed in he wa e and we e wi hd awn a 1 mm/min a e
eaching he a ge su ace p essu e (30 mN/m). All samples we e allowed o d y in he ai
o e nigh , and hen hey we e s o ed in a desicca o un il cha ac e iza ion was pe o med.
MOF deposi ion on o he ibe s was cha ac e ized using wo ield emission scanning
elec on mic oscope (FE-SEM) ins umen s: an FEI Quan a 3D FEG and an Inspec F50
sys em, bo h ope a ed wi h an accele a ing ol age o 10 kV. A coa ing o 5 nm hickness o
I o P was applied o he samples be o e SEM inspec ion. A leas h ee di e en samples
o each h ead o ibe and deposi ion o ien a ion we e analyzed. Co e age analysis was
pe o med using ImageJ so wa e.
3. Resul s and Discussion
To allow an e ec i e deposi ion o MOF pa icles on o he ibe o h eads using he
LB me hod, he sample needs o be kep elonga ed du ing he ilm ans e . Fo his, he
Nanoma e ials 2021,11, 160 4 o 10
i s essen ial s ep was designing a cus om holde , shown in Figu e 1. MOF monolaye
ilms o 200 nm MIL-96(Al) pa icles wi h a homogeneous size we e hen deposi ed on o
nylon ibe (insula ing) and i e di e en ypes o h eads, namely wo insula ing (co on
h ead and den al loss h ead) and h ee conduc i e h eads (Kookye conduc i e h ead,
Libe a o 40 conduc i e h ead and BCP conduc i e h ead), all a 30 mN/m using he
LB me hod (see expe imen al sec ion). Two deposi ion o ien a ions, namely, ho izon al
and e ical deposi ion, we e used o he LB coa ing o MIL-96(Al) NPs on o he nylon
ibe and i e o he h eads. In he case o ho izon al deposi ion, he ibe o h eads we e
moun ed on o he ame in he ho izon al di ec ion, and in he case o e ical deposi ion,
he nylon ibe o h eads we e placed in a e ical di ec ion in e e ence o he wa e
su ace whe e he MOF dense monolaye was p e iously o med. The use o cus om made
holde s allows he co e age o mul iple samples in a single un, which is also an exci ing
ea u e o ou me hod in e ms o ge ing highe ou pu in one deposi ion cycle. LB ilms
deposi ed on o he samples we e cha ac e ized using scanning elec on mic oscopy (SEM).
In he case o non-conduc ing samples, single nylon ibe and wo h eads, namely,
co on h ead and den al loss h ead, we e used o LB ilm deposi ion. In he case o
co on h ead, he deposi ion was pe o med on bo h p is ine h eads and h eads ea ed
wi h ace ic acid (Figu e 2). The la e is a common p ocedu e o chemical pu i ica ion o
cellulose [
33
,
34
], ei he in c ude o m o p ocessed as ibe s. The e ec o ace ic acid is he
hyd olysis o he non-c ys alline egions o he cellulose [
35
], and we expec ed a possible
inc ease in he in e ac ion be ween he co on h ead and he MOF pa icles. We ollowed
he p o ocol op imized by Owyeung e al. whe e hey used comme cial co on h ead like
in ou case, and hey epo ed ha ace ic acid ea men inc eased he in e ac ion o a dye
wi h he co on h ead by emo ing any wax o non-cellulosic coa ing on o he ibe s [
32
].
Figu e 2depic s he esul ing coa ings, as obse ed h ough SEM. Addi ional SEM images
can be ound in Supplemen a y Ma e ial, Figu e S1. A e he ace ic acid ea men , a sligh
inc ease in he diame e o he co on h ead was obse ed ( om 352
±
41
µ
m o 392
±
50
µ
m,
Table S1) which is due o he pa ial opening o he h ead ibe s, and hese esul s a e in
ag eemen wi h he p e iously epo ed s udies abou he co on h ead modi ica ion wi h
ace ic acid [32].
I can be seen ha o he p is ine co on h ead, e ical deposi ion o ien a ion gi es
he bes monolaye co e age o MIL-96(Al) (70–99%), he h ead being only pa ially coa ed
in he case o ho izon al deposi ion (60–80%). Howe e , e en o he mo e a o able e ical
con igu a ion, ull co e age o he h eads was no obse ed. On he con a y, in he case
o ace ic acid- ea ed co on h ead, he ho izon al deposi ion gi es much be e esul s
han he e ical deposi ion (89–94% s. 30–45%). This di e ence may be asc ibed o he
di e en in e ac ions unde lying each ans e ype. While ho izon al deposi ion simply
occu s by he emo al o ma e ial om he ai -wa e in e ace du ing h ead wi hd awal
om inside he wa e , e ical deposi ion implies he o ma ion o a conca e meniscus. The
ace ic acid ea men pa ially opens he h ead ibe s inc easing he inhomogenei y on he
su ace o he h ead and he o uosi y o indi idual ibe s. This inhomogenei y p obably
dis up s he ilm in he meniscus zone leading o a poo e in e ac ion and co e age. The
obse ed coa ing o ho izon al deposi ion is also signi ican ly be e han on p is ine co on
h ead, wi h a mo e homogenous and comple e co e age o he h ead. This can easonably
be asc ibed o he emo al o species po en ially p esen on he su ace o he co on h ead
h ough he ace ic acid p e- ea men . The e o e no only he su ace oughness bu also he
physical and chemical homogenei y play a key ole in he deposi ion o LB ilms o NPs.
Nanoma e ials 2021,11, 160 5 o 10
Nanoma e ials 2021, 11, x FOR PEER REVIEW 5 o 10
Figu e 2. Rep esen a i e SEM images o ho izon al and e ical deposi ion o me al-o ganic amewo k (MOF)
nanopa icles (NPs) on o p is ine co on h ead (le , a) and co on h ead ea ed wi h ace ic acid ( igh , b). F om op o
bo om, each column shows he inc eased magni ica ion o he h eads.
The deposi ion o MIL-96(Al) NPs on o he nylon ibe and den al loss h ead is
depic ed in Figu e 3. Addi ional SEM images a e included in Supplemen a y Ma e ial,
Figu e S2. We used a comme cially a ailable den al loss h ead, consis ing o a bundle o
nylon ibe s winded oge he , ha ing a wax coa ing. O e all, a good coa ing o he ibe s
is ob ained in all cases, o bo h nylon ibe and den al loss h ead, and using ei he
ho izon al o e ical deposi ions. The highes su ace co e age is ob ained on nylon ibe
using e ical deposi ion (84–97%), wi h a dense and mo e homogeneous coa ing han
ho izon al deposi ion (80–96%). This high co e age is achie ed wi hou he need o any
special p e- ea men , such as he plasma ea men o PMMA op ical ibe s p io o silica
NPs deposi ion epo ed by Kohou ek e al. [25]. Mo eo e , he nylon ibe diame e (126
 8.3 µm, Table S1, Supplemen a y Ma e ial) is signi ican ly smalle han he alue
epo ed o hose PMMA op ical ibe s (ca. 3 mm). Conce ning he homogenei y o he
MOF deposi s, almos no agg ega es a e obse ed in nylon ibe samples. SEM images in
Supplemen a y Ma e ial, Figu e S3 illus a e his monolaye cha ac e o he MOF coa ing.
On he o he hand, some unco e ed a eas, as well as some egions wi h mo e han one
laye o MOF NPs, can be obse ed in he case o den al loss h ead, he la e especially
when he e ical deposi ion was used. The e o e al hough a good coa ing can be ob ained
on den al loss h ead (66–94% and 70–75% o e ical and ho izon al ans e ), g ea e
ex en s o deposi ion a e achie able on single nylon ibe s. This could be asc ibed o ei he
o bo h he bundled na u e and he wax coa ing o den al loss h ead.
Figu e 2.
Rep esen a i e SEM images o ho izon al and e ical deposi ion o me al-o ganic ame-
wo k (MOF) nanopa icles (NPs) on o p is ine co on h ead (le ,
a
) and co on h ead ea ed
wi h ace ic acid ( igh ,
b
). F om op o bo om, each column shows he inc eased magni ica ion o
he h eads.
The deposi ion o MIL-96(Al) NPs on o he nylon ibe and den al loss h ead is
depic ed in Figu e 3. Addi ional SEM images a e included in Supplemen a y Ma e ial,
Figu e S2. We used a comme cially a ailable den al loss h ead, consis ing o a bundle
o nylon ibe s winded oge he , ha ing a wax coa ing. O e all, a good coa ing o he
ibe s is ob ained in all cases, o bo h nylon ibe and den al loss h ead, and using ei he
ho izon al o e ical deposi ions. The highes su ace co e age is ob ained on nylon
ibe using e ical deposi ion (84–97%), wi h a dense and mo e homogeneous coa ing
han ho izon al deposi ion (80–96%). This high co e age is achie ed wi hou he need o
any special p e- ea men , such as he plasma ea men o PMMA op ical ibe s p io o
silica NPs deposi ion epo ed by Kohou ek e al. [
25
]. Mo eo e , he nylon ibe diame e
(126
±
8.3
µ
m, Table S1, Supplemen a y Ma e ial) is signi ican ly smalle han he alue
epo ed o hose PMMA op ical ibe s (ca. 3 mm). Conce ning he homogenei y o he
MOF deposi s, almos no agg ega es a e obse ed in nylon ibe samples. SEM images in
Supplemen a y Ma e ial, Figu e S3 illus a e his monolaye cha ac e o he MOF coa ing.
On he o he hand, some unco e ed a eas, as well as some egions wi h mo e han one
laye o MOF NPs, can be obse ed in he case o den al loss h ead, he la e especially
when he e ical deposi ion was used. The e o e al hough a good coa ing can be ob ained
on den al loss h ead (66–94% and 70–75% o e ical and ho izon al ans e ), g ea e
ex en s o deposi ion a e achie able on single nylon ibe s. This could be asc ibed o ei he
o bo h he bundled na u e and he wax coa ing o den al loss h ead.

Nanoma e ials 2021,11, 160 6 o 10
Nanoma e ials 2021, 11, x FOR PEER REVIEW 6 o 10
Figu e 3. Rep esen a i e SEM images o ho izon al and e ical deposi ion o MOF NPs on o nylon ibe (le , a) and
den al loss h ead ( igh , b). F om op o bo om, each column shows inc eased magni ica ion.
Thus, among he non-conduc i e samples, bo h co on h ead and nylon ibe wi h
e ical deposi ion ga e simila esul s in e ms o homogenei y and ilm quali y, wi h
nylon ibe pe o ming he bes in co e age deg ee. The ac ha ace ic acid ea men o
co on h ead esul s in an imp o ed coa ing, while ha on nylon ibe is be e han on
den al loss h ead in which a wax su ace coa ing is p esen , may be associa ed wi h he
necessi y o clean he su ace o he h ead. To summa ize hese esul s, Table S1 in
Supplemen a y Ma e ial includes all he co e age alues o he non-conduc i e ibe and
h eads deduced om he analysis o SEM images.
To u he e alua e he applica ion o ou coa ing s a egy, we coa ed MIL-96(Al)
on o h ee di e en comme cially a ailable conduc i e h eads wi h di e en diame e s
(Table S2, Supplemen a y Ma e ial), again using ho izon al and e ical deposi ion
o ien a ions. Figu e 4 shows he SEM mic og aphs o he conduc i e h eads be o e and
a e he deposi ion o he MOF NPs laye . Addi ional SEM images a e included in
Supplemen a y Ma e ial, Figu e S4. I is impo an o no e ha hese conduc i e h eads
al eady ha e a coa ing o me al nanopa icles (Ag), esul ing in a ough su ace. In some
cases, i was no easy o clea ly dis inguish MOF NPs om he conduc i e coa ing. Fo
ha pu pose, back-sca e ed elec on de ec o (BSE) p o ed o be use ul (Supplemen a y
Ma e ial, Figu e S5), whe e MOF coa ed a eas appea ed as da ke zones in he images due
o he smalle a omic numbe o he me allic cen e (Al, Z = 13) in compa ison o he Ag
NPs (Z = 47).
The BCP conduc i e h ead showed he poo es deposi ion o MOF pa icles, wi h
only a ew small a eas co e ed in he ho izon al and e ical deposi ion. Mo eo e ,
esis ance showed an inc ease compa ed o he uncoa ed h ead ( om 500 Ω o 2000 Ω),
which can be easoned by a pa ial loss o he conduc i e coa ing du ing Langmui ilm
ab ica ion. None heless, he conduc i i y o Kookye and Libe a o 40 conduc i e h eads
emained unchanged a e MOF LB deposi ion (3.0 Ω o Kookye conduc i e h ead and
0.5 Ω o conduc i e h ead), which demons a es he po en ial applica ions o LB ilms
di ec ly deposi ed on o conduc i e h eads o applica ion in chemical senso s (e.g., as
Figu e 3.
Rep esen a i e SEM images o ho izon al and e ical deposi ion o MOF NPs on o nylon ibe (le ,
a
) and den al
loss h ead ( igh , b). F om op o bo om, each column shows inc eased magni ica ion.
Thus, among he non-conduc i e samples, bo h co on h ead and nylon ibe wi h
e ical deposi ion ga e simila esul s in e ms o homogenei y and ilm quali y, wi h
nylon ibe pe o ming he bes in co e age deg ee. The ac ha ace ic acid ea men
o co on h ead esul s in an imp o ed coa ing, while ha on nylon ibe is be e han
on den al loss h ead in which a wax su ace coa ing is p esen , may be associa ed wi h
he necessi y o clean he su ace o he h ead. To summa ize hese esul s, Table S1 in
Supplemen a y Ma e ial includes all he co e age alues o he non-conduc i e ibe and
h eads deduced om he analysis o SEM images.
To u he e alua e he applica ion o ou coa ing s a egy, we coa ed MIL-96(Al)
on o h ee di e en comme cially a ailable conduc i e h eads wi h di e en diame e s
(Table S2, Supplemen a y Ma e ial), again using ho izon al and e ical deposi ion o ien a-
ions. Figu e 4shows he SEM mic og aphs o he conduc i e h eads be o e and a e he
deposi ion o he MOF NPs laye . Addi ional SEM images a e included in Supplemen a y
Ma e ial, Figu e S4. I is impo an o no e ha hese conduc i e h eads al eady ha e a
coa ing o me al nanopa icles (Ag), esul ing in a ough su ace. In some cases, i was no
easy o clea ly dis inguish MOF NPs om he conduc i e coa ing. Fo ha pu pose, back-
sca e ed elec on de ec o (BSE) p o ed o be use ul (Supplemen a y Ma e ial, Figu e S5),
whe e MOF coa ed a eas appea ed as da ke zones in he images due o he smalle a omic
numbe o he me allic cen e (Al, Z = 13) in compa ison o he Ag NPs (Z = 47).
Nanoma e ials 2021,11, 160 7 o 10
Nanoma e ials 2021, 11, x FOR PEER REVIEW 7 o 10
in e digi a ed elec odes in ex ile senso s [30]). The Kookye conduc i e h ead also
showed a a he poo MOF pa icle deposi ion in bo h deposi ion o ien a ions, al hough
much be e han he BCP conduc i e h ead. Indeed, a eas co e ed wi h a con inuous
ilm o MOF pa icles can be obse ed bu sepa a ed by la ge a eas wi h no coa ing
p esen . A mo e e icien deposi ion o MOF NPs was ob ained in he case o Libe a o 40
conduc i e h ead, he ilms being mo e con inuous and homogeneous in heigh in he
e ical con igu a ion as compa ed o he ho izon al deposi ion. Howe e , e en in his
mo e a o able case, he deposi ion does no occu as a monolaye , as ob ained in he case
o non-conduc i e co on h ead and nylon ibe . The ed a ow in Figu e 4 highligh s ha
he MOF pa icles deposi p e e en ially a he ibe junc ions o he g oo es be ween he
small conduc i e ibe s inside he ibe bundle o he conduc i e h ead. Con a y o he
insula ing ibe and h eads, deposi ion on he conduc i e h eads esul s in a mul ilaye
coa ing wi h he MOF pa icles. The ini ial oughness o he conduc i e h eads is one o
he mos likely pa ame e s a he o igin o his. The bundle na u e o he conduc i e
h eads also plays a ole in hei ela i ely poo coa ing, bu since a easonable coa ing
was ob ained o he bundle o nylon ibe s in den al loss h ead, his is likely only
compa a i ely less ele an .
Figu e 4. SEM images o he h ee conduc i e h eads s udied, as indica ed, p io o deposi ion (le ), a e LB deposi ion
o MOF NPs in ho izon al (middle) and e ical ( igh ) con igu a ions. The ed and whi e a ows highligh he MOF NPs
deposi s.
Figu e 4.
SEM images o he h ee conduc i e h eads s udied, as indica ed, p io o deposi ion (
le
), a e LB deposi ion
o MOF NPs in ho izon al (
middle
) and e ical (
igh
) con igu a ions. The ed and whi e a ows highligh he MOF
NPs deposi s.
The BCP conduc i e h ead showed he poo es deposi ion o MOF pa icles, wi h only
a ew small a eas co e ed in he ho izon al and e ical deposi ion. Mo eo e , esis ance
showed an inc ease compa ed o he uncoa ed h ead ( om 500
Ω
o 2000
Ω
), which can
be easoned by a pa ial loss o he conduc i e coa ing du ing Langmui ilm ab ica ion.
None heless, he conduc i i y o Kookye and Libe a o 40 conduc i e h eads emained
unchanged a e MOF LB deposi ion (3.0
Ω
o Kookye conduc i e h ead and 0.5
Ω
o
conduc i e h ead), which demons a es he po en ial applica ions o LB ilms di ec ly
deposi ed on o conduc i e h eads o applica ion in chemical senso s (e.g., as in e digi a ed
elec odes in ex ile senso s [
30
]). The Kookye conduc i e h ead also showed a a he poo
MOF pa icle deposi ion in bo h deposi ion o ien a ions, al hough much be e han he
BCP conduc i e h ead. Indeed, a eas co e ed wi h a con inuous ilm o MOF pa icles
can be obse ed bu sepa a ed by la ge a eas wi h no coa ing p esen . A mo e e icien
deposi ion o MOF NPs was ob ained in he case o Libe a o 40 conduc i e h ead, he
ilms being mo e con inuous and homogeneous in heigh in he e ical con igu a ion
as compa ed o he ho izon al deposi ion. Howe e , e en in his mo e a o able case,
he deposi ion does no occu as a monolaye , as ob ained in he case o non-conduc i e
co on h ead and nylon ibe . The ed a ow in Figu e 4highligh s ha he MOF pa icles
deposi p e e en ially a he ibe junc ions o he g oo es be ween he small conduc i e
ibe s inside he ibe bundle o he conduc i e h ead. Con a y o he insula ing ibe
and h eads, deposi ion on he conduc i e h eads esul s in a mul ilaye coa ing wi h he
Nanoma e ials 2021,11, 160 8 o 10
MOF pa icles. The ini ial oughness o he conduc i e h eads is one o he mos likely
pa ame e s a he o igin o his. The bundle na u e o he conduc i e h eads also plays a
ole in hei ela i ely poo coa ing, bu since a easonable coa ing was ob ained o he
bundle o nylon ibe s in den al loss h ead, his is likely only compa a i ely less ele an .
4. Conclusions
This wo k epo s he di ec deposi ion o MOF LB ilms on o con en ional and
conduc i e h eads wi h diame e s below 1 mm and o di e en na u e, also using di e en
o ien a ions in he deposi ion p ocess. O e all, he deposi ion p ocess we epo does no
equi e any special p e- ea men o he samples and is ep oducible, ei he o he nylon
ibe o a ious h eads o each deposi ion ba ch o om deposi ion ba ch o deposi ion
ba ch. I has been demons a ed ha he chemical na u e, su ace oughness, and ype o
ibe o h ead, ei he single- ibe o bundles, as well as deposi ion o ien a ion all appea
o in luence he abili y o ob ain a monolaye coa ing o MOF pa icles. Mo eo e , in he
case o conduc i e h eads, he na u e o bo h ibe and conduc i e coa ing plays a key
ole in LB deposi ion o a oid he leaching o he me allic NPs by imme sion in o he
wa e subphase. The deposi ion o small amoun s o MOF NPs using he LB echnique
did no al e he h ead conduc i i y, which is essen ial o implemen ing hese h eads
in o elec onic de ices. Mo eo e , he MOF ilms we e no emo ed by bending he nylon
ibe o h eads. Fu he s udies o hese MOF LB coa ed ibe s will pa e he way o he
design o single- ibe o h ead-based de ices ha could be in eg a ed in o chemical o gas
senso s. Pa icula ly, ab asion esis ance and imme sion s abili y, among o he p ope ies,
would ha e o be add essed o he design o hese de ices.
Supplemen a y Ma e ials:
The ollowing a e a ailable online a h ps://www.mdpi.com/2079-499
1/11/1/160/s1, Figu e S1 o Figu e S5: Addi ional SEM images o ho izon al and e ical deposi ion,
Table S1: Diame e o nylon ibe , co on h ead and den al loss h ead and co e age (in %) ob ained
om SEM images, Table S2: Diame e o conduc i e h eads.
Au ho Con ibu ions:
The manusc ip was w i en h ough he con ibu ions o all au ho s. S.R. and
K.N.S. concei ed and discussed he idea o he p ojec . S.R. and M.A.A. designed he expe imen al
me hodology, conduc ed he SEM analysis and analyzed and discussed he esul s. C.S. p o ided
he MOF NPs. M.A.A. pe o med he hin ilm deposi ion expe imen s. K.N.S., M.E., I.G. and O.R.
acqui ed unding and supe ised he p ojec . S.R., I.G., O.R. and M.A.A. w o e and e ised he
manusc ip wi h he help o K.N.S., M.E. and C.S. All au ho s ha e ead and ag eed o he published
e sion o he manusc ip .
Funding:
We acknowledge he inancial suppo om King Abdullah Uni e si y o Science and
Technology (KAUST), Saudi A abia. K.N. Salama would like o acknowledge he unding om
AMPM cen e unde he CCF g an . We hank he KAUST Senso Ini ia i e and KAUST isi ing
s uden p og am o suppo ing his wo k. Also, he esea ch leading o hese esul s has ecei ed
unding om Spanish MINECO and FEDER (p ojec s MAT2016-78257-R and MAT2017-86826-R),
and he A agón Go e nmen (DGA) and FEDER ( esea ch g oup E31_17R).
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen : Da a is con ained wi hin he a icle o supplemen a y ma e ial.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
Re e ences
1.
Hoskins, B.F.; Robson, R. In ini e Polyme ic F amewo ks Consis ing o 3 Dimensionally Linked Rod-Like Segmen s. J. Am. Chem.
Soc. 1989,111, 5962–5964. [C ossRe ]
2.
Fu ukawa, H.; Co do a, K.E.; O’Kee e, M.; Yaghi, O.M. The Chemis y and Applica ions o Me al-O ganic F amewo ks. Science
2013,341, 974. [C ossRe ] [PubMed]
3.
Cheng, P.; Wang, C.H.; Kane i, Y.V.; Eguchi, M.; Lin, J.J.; Yamauchi, Y.; Na, J. P ac ical MOF Nanoa chi ec onics: New S a egies
o Enhancing he P ocessabili y o MOFs o P ac ical Applica ions. Langmui 2020,36, 4231–4249. [C ossRe ] [PubMed]
Nanoma e ials 2021,11, 160 9 o 10
4.
Dals ein, O.; Gkania sou, E.; Sica d, C.; Sel, O.; Pe o , H.; Se e, C.; Boissie e, C.; Faus ini, M. E apo a ion-Di ec ed C ack-
Pa e ning o Me al-O ganic F amewo k Colloidal Films and Thei Applica ion as Pho onic Senso s. Angew. Chem. In . Ed.
2017
,
56, 14011–14015. [C ossRe ]
5. Li, J.R.; Sculley, J.; Zhou, H.C. Me al-O ganic F amewo ks o Sepa a ions. Chem. Re . 2012,112, 869–932. [C ossRe ]
6.
Wang, H.; Liu, Y.L.; Li, J. Designe Me al-O ganic F amewo ks o Size-Exclusion-Based Hyd oca bon Sepa a ions: P og ess and
Challenges. Ad . Ma e . 2020, 2002603. [C ossRe ]
7.
Peng, Y.; Yang, W.S. 2D Me al-O ganic F amewo k Ma e ials o Memb ane-Based Sepa a ion. Ad . Ma e . In e aces
2020
,7,
1901514. [C ossRe ]
8.
Couck, S.; Van Assche, T.R.; Liu, Y.Y.; Ba on, G.V.; Van de Voo , P.; Denaye , J.F. Adso p ion and Sepa a ion o Small Hyd oca -
bons on he Flexible, Vanadium-Con aining MOF, COMOC-2. Langmui 2015,31, 5063–5070. [C ossRe ]
9.
McKinlay, A.C.; Mo is, R.E.; Ho cajada, P.; Fe ey, G.; G e , R.; Cou eu , P.; Se e, C. BioMOFs: Me al-O ganic F amewo ks o
Biological and Medical Applica ions. Angew. Chem. In . Ed. 2010,49, 6260–6266. [C ossRe ]
10.
Zhang, W.; Jia, G.; Li, Z.S.; Yuan, C.W.; Bai, Y.F.; Fu, D.G. Selec i e Elec ochemical De ec ion o Dopamine on Polyoxome ala e-
Based Me al-O ganic F amewo k and I s Composi e wi h Reduced G aphene Oxide. Ad . Ma e . In e aces
2017
,4, 1601241.
[C ossRe ]
11.
Roy, K.; Jana, S.; Ghosh, S.K.; Mahan y, B.; Mallick, Z.; Sa ka , S.; Sinha, C.; Mandal, D. Th ee-Dimensional MOF-Assis ed Sel -
Pola ized Fe oelec e : An E ec i e Au opowe ed Remo e Heal hca e Moni o ing App oach. Langmui
2020
,36, 11477–11489.
[C ossRe ] [PubMed]
12.
Wang, Q.; Gao, Q.Y.; Al-Enizi, A.M.; Na ady, A.; Ma, S.Q. Recen ad ances in MOF-based pho oca alysis: En i onmen al
emedia ion unde isible ligh . Ino g. Chem. F on . 2020,7, 300–339. [C ossRe ]
13.
Zhuang, J.L.; Te o , A.; Woll, C. Fo ma ion o o ien ed and pa e ned ilms o me al-o ganic amewo ks by liquid phase epi axy:
A e iew. Coo din. Chem. Re . 2016,307, 391–424. [C ossRe ]
14.
Sanchez, E.P.V.; Knebel, A.; Sanchez, L.I.; Klumpp, M.; Woll, C.; Di meye , R. S udying ZIF-8 SURMOF Thin Films wi h a
Langa a e C ys al Mic obalance: Single-Componen Gas Adso p ion Iso he ms Measu ed a Ele a ed Tempe a u es and P essu es.
Langmui 2020,36, 8444–8450. [C ossRe ] [PubMed]
15.
Tanaka, S.; Sakamo o, K.; Inada, H.; Kawa a, M.; Takasaki, G.; Imawaka, K. Vapo -Phase Syn hesis o ZIF-8 MOF Thick Film by
Con e sion o ZnO Nano od A ay. Langmui 2018,34, 7028–7033. [C ossRe ] [PubMed]
16.
Ohnso g, M.L.; Beaudoin, C.K.; Ande son, M.E. Fundamen als o MOF Thin Film G ow h ia Liquid-Phase Epi axy: In es iga ing
he Ini ia ion o Deposi ion and he In luence o Tempe a u e. Langmui 2015,31, 6114–6121. [C ossRe ]
17.
Lopez-Maya, E.; Mon o o, C.; Rod iguez-Albelo, L.M.; Ce an es, S.D.A.; Lozano-Pe ez, A.A.; Cenis, J.L.; Ba ea, E.; Na a o, J.A.R.
Tex ile/Me al-O ganic-F amewo k Composi es as Sel -De oxi ying Fil e s o Chemical-Wa a e Agen s. Angew. Chem. In . Ed.
2015,54, 6790–6794. [C ossRe ]
18.
Emam, H.E.; Abdelhameed, R.M. An i-UV Radia ion Tex iles Designed by Emb acing wi h Nano-MIL (Ti, In)-Me al O ganic
F amewo k. ACS Appl. Ma e . In e aces 2017,9, 28034–28045. [C ossRe ]
19.
Li, G.P.; Cao, F.; Zhang, K.; Hou, L.; Gao, R.C.; Zhang, W.Y.; Wang, Y.Y. Design o An i-UV Radia ion Tex iles wi h Sel -Assembled
Me al-O ganic F amewo k Coa ing. Ad . Ma e . In e aces 2020,7, 1901525. [C ossRe ]
20.
Smi h, M.K.; Mi ica, K.A. Sel -O ganized F amewo ks on Tex iles (SOFT): Conduc i e Fab ics o Simul aneous Sensing, Cap u e,
and Fil a ion o Gases. J. Am. Chem. Soc. 2017,139, 16759–16767. [C ossRe ]
21.
Tahghighi, M.; Janne , D.; Ignes-Mullol, J. Op imizing Gold Nanopa icle Size and Shape o he Fab ica ion o SERS Subs a es
by Means o he Langmui -Blodge Technique. Nanoma e ials 2020,10, 2264. [C ossRe ] [PubMed]
22.
Sil a, P.; No a, D.; Teixei a, M.; Ca doso, V.; Mo gado, P.; Nunes, B.; Colaco, R.; Fau e, M.C.; Fon aine, P.; Goldmann, M.; e al.
Langmui Films o Pe luo ina ed Fa y Alcohols: E idence o Spon aneous Fo ma ion o Solid Agg ega es a Ze o Su ace
P essu e and Ve y Low Su ace Densi y. Nanoma e ials 2020,10, 2257. [C ossRe ] [PubMed]
23.
Paul, S.; Pea son, C.; Molloy, A.; Cousins, M.A.; G een, M.; Kolliopoulou, S.; Dimi akis, P.; No mand, P.; Tsoukalas, D.;
Pe y, M.C. Langmui -Blodge ilm deposi ion o me allic nanopa icles and hei applica ion o elec onic memo y s uc u es.
Nano Le . 2003,3, 533–536. [C ossRe ]
24.
Reculusa, S.; Ra aine, S. Syn hesis o colloidal c ys als o con ollable hickness h ough he Langmui -Blodge echnique. Chem.
Ma e . 2003,15, 598–605. [C ossRe ]
25.
Kohou ek, T.; Pa chine, M.; Ba doso a, M.; Pemble, M.E. Con olled sel -assembly o Langmui -Blodge colloidal c ys al ilms o
monodispe sed silica pa icles on non-plana subs a es. Colloids Su ace A 2020,593, 124625. [C ossRe ]
26.
Beni o, J.; So ibas, S.; Lucas, I.; Co onas, J.; Gascon, I. Langmui -Blodge Films o he Me al-O ganic F amewo k MIL-101(C ):
P epa a ion, Cha ac e iza ion, and CO
2
Adso p ion S udy Using a QCM-Based Se up. ACS Appl. Ma e . In e aces
2016
,8,
16486–16492. [C ossRe ]
27.
Na a o, M.; Beni o, J.; Pase a, L.; Gascon, I.; Co onas, J.; Tellez, C. Thin-Film Nanocomposi e Memb ane wi h he Minimum
Amoun o MOF by he Langmui -Schae e Technique o Nano il a ion. ACS Appl. Ma e . In e aces
2018
,10, 1278–1287.
[C ossRe ]
28.
And es, M.A.; Benzaqui, M.; Se e, C.; S eunou, N.; Gascon, I. Fab ica ion o ul a hin MIL-96(Al) ilms and s udy o CO
2
adso p ion/deso p ion p ocesses using qua z c ys al mic obalance. J. Colloid In e . Sci. 2018,519, 88–96. [C ossRe ]