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Colo ime ic de ec ion, quan i ica ion and ex ac ion o Fe(III) in wa e by ac ylic
polyme s wi h pendan Kojic acid mo i s
Saúl Vallejos,1 Asunción Muñoz,1 Félix Clemen e Ga cía1, Robe a Colleoni,2 Ra aela
Biesuz,2 Gianca la Albe i,2,* José Miguel Ga cía1,*
1 Depa amen o de Química, Facul ad de Ciencias, Uni e sidad de Bu gos, Plaza de
Misael Bañuelos s/n, 09001 Bu gos, Spain. Fax: (+) 34 947 258 831, Tel: (+) 34 947
258 085. E-mail: [email protected]
2 Dipa imen o di Chimica, Uni e si à di Pa ia, ia Ta amelli 12, 27100 Pa ia, I aly.
Tel: (+) 39 0382 987347. E-mail: galbe i@unip .i
G aphical abs ac
Resea ch highligh s
• A colo ime ic senso y polyme o he de ec ion o Fe(III) was syn hesized.
• This polyme was p epa ed om a monome de i ed om Kojic acid, which is a
na u al p oduc .
• The ilm-shaped polyme was cu o ob ain manageable solid senso y ki s.
• Fe(III) was e icien ly ex ac ed, de ec ed, disc imina ed and quan i ied om
wa e .
• UV/ is and compu e ision-based echniques we e used o Fe(III) analysis.
ABSTRACT
We syn hesized a solid senso y ma e ial o he ex ac ion, de ec ion and quan i ica ion
o i on(III) in aqueous media. The ma e ial is a ilm-shaped colo less polyme
memb ane ha exhibi s gel beha io . The Fe(III) ex ac ion and sensing cha ac e is ics
1.65x10
-6
M
1.65x10
-1
M
Fe(III)
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a e impa ed by a new monome de i ed om a na u al p oduc (i.e., Kojic acid), which
exhibi s chela ing p ope ies owa d Fe(III). The so p ion o Fe(III) on he memb ane in
wa e has been ho oughly cha ac e ized, including he so p ion kine ics, so p ion
iso he ms and p o iles as a unc ion o he pH. Fe(III) so p ion ollowed pseudo i s -
o de kine ics and equi ed app oxima ely 30 min o each equilib ium. The maximum
so p ion capaci y was app oxima ely 0.04 mmol/g, and he so p ion iso he ms a e well
modeled by he Langmui equa ion. The complexes ha we e ound in he solid phase
a e in good ag eemen wi h hose p e iously iden i ied in he aqueous phase. Mo eo e ,
he so p ion is highly speci ic (i.e., a ecogni ion p ocess) and esul s om he o ma ion
o a colo ed complex (i on(III)-Kojic acid de i a i e moie ies). The e o e, he colo less
senso y memb ane u ns ed upon imme sion in aqueous solu ions con aining Fe(III).
The colo ou pu allows o bo h he quali a i e isual de e mina ion o he Fe(III)
concen a ion as well as also i a ion o Fe(III) using a) a UV/ is echnique (limi o
de ec ion o 3.6x10-5 M; dynamic ange o i e decades, lowe concen a ion = 1.65x10-
6 M) and b) a compu e ision-based analy ical chemis y app oach ia colo de ini ion
o he senso y memb ane (RGB pa ame e s) ob ained om an image eco ded wi h a
handy de ice (e.g., a sma phone) (limi o de ec ion o 2.0x10-5 M).
Keywo ds
Senso y polyme s, Kojic acid, isual de ec ion, i on de ec ion, i on ex ac ion
1. INTRODUCTION
The wide dis ibu ion o i on as ca ions in di e en oxida ion s a es in he en i onmen
has bo h na u al and an h opogenic o igins. The la e is caused by i s mul iple
applica ions in ag icul u e, indus y, cons uc ion, medicine, households, and ad anced
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echnological ields, which has led o inc easing conce ns o e he po en ial e ec s o
i on on he en i onmen as well as in li ing hings [1].
The e o e, he de ec ion and quan i ica ion o i on sal s is a undamen al ask in
chemis y due o i s c ucial ole in li ing o ganisms and being ex emely dange ous
when i s concen a ion exceeds a c i ical le el. I s quan i ica ion is s ill a undamen al
ask in clinical chemis y bu i is also o in e es in en i onmen al and ag i- ood ields.
Cu en ly, ou ine analyses a e pe o med using me hods ha equi e ained s a and
expensi e equipmen and echniques, such as a omic abso p ion spec oscopy (AAS) o
induc i ely coupled plasma mass spec ome y (ICP-MS). Howe e , chemical senso s
allow o con e sion o he i on sal concen a ion in o signals ha can be ead by
widely a ailable ins umen s o e en by an un ained obse e , especially i he signal is
a colo change (ch omogenic senso ). A senso is a sel -con ained analy ical de ice ha
is able o con e a physical quan i y, which is ela ed o a chemical species
concen a ion, in o a signal ha can be ead by an obse e o an ins umen [2-8].
Senso s a e used in e e yday objec s and ha e innume able applica ions ha a e o en
o e looked [9].
In ecen yea s, we ha e ocused on in es iga ing solid-phase senso s o
i on(III) analysis o de elop a quick, easy and cheap me hod o he quan i a i e
de e mina ion o me al ions [10-13]. In his s udy, we epo a no el polyme ic
memb ane o isual i on(III) sensing ha ea u es a de i a i e o Kojic acid as he
ecep o moie y. Kojic acid, which is a γ-py one de i a i e (5-hyd oxy-2-
(hyd oxyme hyl)-4-py one), is a na u al he e ocyclic chela ing ligand [14], and anions
o Kojic acid and i s de i a i es ac as biden a e ligands o s ongly coo dina e o
i on(III) ia ca bonyl and phenolic hyd oxyl g oups [15].
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The ilm-shaped memb ane is a manageable ma e ial ha can be cu in o solid
senso y ki s (e.g., small senso y discs). To de elop a solid-phase senso , he so p ion o
he selec ed a ge species as well as he kine ic and he modynamic p ope ies mus be
cha ac e ized. The e o e, a ull cha ac e iza ion o he ma e ial was pe o med o
de e mine he kine ics, iso he ms and so p ion p o iles o i on(III) as a unc ion o pH.
Fo he colo ime ic i on(III) sensing beha io o he memb ane, he memb ane is
ini ially colo less and anspa en bu a e imme sion in i on(III) solu ions, i u ns ed
wi hin minu es. I s pe o mance was e alua ed using ul a iole - isible spec oscopy
(UV/ is). In addi ion, a i a ion cu e was cons uc ed using he UV/ is da a om he
spec a eco ded om senso y discs placed in con ac wi h wa e solu ions con aining
di e en i on(III) concen a ions. Mo eo e , he i a ion was also pe o med using an
image o he senso y discs (i.e., colo digi al de ini ion (RGB pa ame e s) o he
p e iously men ioned senso y discs (compu e ision-based analy ical chemis y).
The e o e, he solid senso y discs a e manageable ma e ials ha can be handle wi hou
ca e, s o ed unde ambien condi ions o long pe iods o ime, and used o measu e he
concen a ion o Fe(III) e en by un ained pe sonnel using images eco ded using
widely a ailable de ices, such as mobile phones and able s.
2. EXPERIMENTAL SECTION
The ma e ials, syn he ic p ocedu es and measu emen s me hods and me hodologies a e
desc ibed in he Suppo ing In o ma ion (ESI) in Sec ion S1.
2.1. P epa a ion o senso y ma e ials
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The senso y monome de i ed om Kojic acid (3) was p epa ed acco ding wi h he
expe imen al p ocedu e desc ibed in he ESI (Sec ion S1) and shown schema ically in
Scheme 1.
The ilm-shaped senso y memb anes we e p epa ed ia bulk adical
polyme iza ion o hyd ophilic monome s including 1- inyl-2-py olidone (VP), 2-
hyd oxye hyl ac yla e (2HEA), and a monome de i ed om Kojic acid (3). E hylene
glycol dime hac yla e (EGDMA) was used as a c oss-linking agen (Scheme 2). The
VP/2HEA/(3)/EGDMA comonome mola a io was 75/24/1/10. AIBN (1 w %) was
employed as a he mal adical ini ia o . The bulk adical polyme iza ion eac ion was
ca ied ou in a 200 µm hick silanized glass mold in an oxygen- ee a mosphe e a 60ºC
o e nigh . A e demolding, he ilm was condi ioned a 60ºC o an addi ional nigh .
The solid senso y subs a es we e manu ac u ed om he ilm-shaped memb ane by
using a punche o cu ou senso y discs (8 mm diame e ).
Scheme 1.
Scheme 2
3. RESULTS AND DISCUSSION
Ou objec i e was o p epa e an easily handled solid ma e ial o he ex ac ion,
de ec ion and quan i ica ion o Fe(III) in pu e wa e . The ma e ial consis s a ilm-shaped
polyme ne wo k wi h de i a i e o Kojic acid in i s s uc u e, which ac s as a ecep o
and chemosenso y co e o Fe(III). The polyme has a c oss-linked s uc u e ha is
hyd ophilic and ac s as a memb ane wi h gel beha io , allowing Fe(III) ions in wa e o
en e in o he ma e ial as sol a ed species whe e hey in e ac wi h he ecep o s o gi e

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ise o he ex ac ion and sensing phenomena. Kojic acid was chosen as he ecep o
because i is a na u al p oduc and a well-known chela ing ligand ha s ongly
coo dina es o i on(III), which esul s in ed-colo ed complexes [14-16]. Mo eo e ,
Kojic acid has been used o many yea s o he spec opho ome ic de e mina ion o
i on in di e en con ex s (e.g., in o es) [17,18]. The e o e, we designed and p epa ed an
ac ylic monome (3) con aining a Kojic acid senso y mo i o syn hesize a memb ane
o use as a colo ime ic senso and solid-phase ex ac an (Scheme 1). The mechanism
o de ec ion and ex ac ion o Fe(III) in ol es he o ma ion o ed Fe(III)n:(Koja e
mo i s)m chela es wi h p ima y s oichiome ies (n:m) o 1:1, 1:2 and 1:3 (Scheme S1,
ESI). The s abili y cons an s o he in e ac ion be ween i on(III) and Kojic acid ha e
been epo ed by Mu aki and a e log K1 = 10.20, log K2 = 8.78, and log K3 = 7.53 [16].
The s oichiome y o he Fe(III)n:(Koja e mo i s)m species in he solid s a e (i.e., inside
he memb ane) and he ela i e exchange coe icien s ha a e desc ibed below o ou
sys em a e in ag eemen wi h hose o he species in solu ion [16].
3.1. Ma e ial p epa a ion and cha ac e iza ion
The me hac yla e senso y monome (3) could no be con en ionally p epa ed in a single
s ep om Kojic acid and me hac yloyl chlo ide bu was syn hesized using a wo-s ep
p ocedu e. Fi s , he ea men o Kojic acid wi h hionyl chlo ide led o he eac ion o
he p ima y alcohol wi h he o ma ion o a Kojic acid p ima y chlo o-de i a i e [19]
ha led o (3) by eac ion wi h po assium me hac yla e a 100ºC (no he mal ini ia ed
polyme iza ion was obse ed) [20]. The eac ion s eps a e schema ically shown in
Scheme 1. The 1H and 13C NMR as well as FTIR spec a o he in e media es and
monome s a e p o ided in he expe imen al sec ion and in he ESI, Sec ion S1. The
po en ial applicabili y o he senso y memb ane was con i med by he ac ha less han
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1.6% by weigh o he senso y syn he ic monome (3) was used in he p epa a ion o he
senso y ma e ial and >98.4% by weigh o comme cial and inexpensi e comonome s.
The memb ane o ilm exhibi s a good physical appea ance and was c easable
and easily handled. The he mal esis ance was e alua ed by he mog a ime ic analysis
(TGA). The deg ada ion empe a u es ha esul ed in a 5% and 10% weigh loss unde
ine and oxidizing a mosphe es (T5) we e ~280ºC and ~280ºC, espec i ely, which is in
ag eemen wi h he TGA da a o he VP and 2HEA copolyme s [10,21]. The he mal
deg ada ion pa e ns we e a ec ed by he so p ion o Fe(III) by he memb ane. The
imme sion o he memb ane in wa e con aining a ela i ely low concen a ion o Fe(III)
inc eased T5 o 355ºC due o he addi ional c osslinking caused by he o ma ion o
Fe(III)1:(Kojic acid moie ies)2 and Fe(III)1:(Kojic acid moie ies)3 (Figu e S5, ESI). In
con as , imme sion in wa e wi h a highe concen a ion o Fe(III) dec eased T5 o
300ºC due o pa ial displacemen o complexa ion o Fe(III)1:(Kojic acid moie ies)1
species wi h a concomi an dec ease in he c osslinking densi y (please see he so p ion
and sensing esul s below).
Gel beha io is ele an o a memb ane o sense in pu e wa e because he
a ge species en e in o he ma e ial as sol a ed species by di usion. Howe e , he
wa e up ake has o be modula ed o main ained good mechanical p ope ies in he
swelled s a e. The e o e, a mode a e wa e swelling pe cen age anging om 40% o
100% is desi able. The memb ane composi ion was designed o mee his c i e ion, and
i s wa e swelling pe cen age was 65%.
3.2. So p ion s udies
3.2.1. So p ion kine ics and iso he ms
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Fo example, he kine ic p o iles o i on(III) so p ion on he memb ane a e shown in
Figu e 1. Kine ic expe imen s a e use ul o de e mine he ime equi ed o each
equilib ium be ween he wo phases. The expe imen s we e pe o med unde acid
condi ions (pH 2.5) o a oid i on(III) hyd olysis eac ions.
I on(III) so p ion on he memb ane equi ed app oxima ely 30 min o achie e
equilib ium. The expe imen al da a a e i ed using a HPDM model (homogeneous
pa icle di usion model) whe e he di usion o ions om he solu ion o he so ben is
he slowes s ep. The ela ionship be ween he amoun o so bed me al ion in he solid
phase (q, mmol g-1) and ime ( , min) is o mally equal o he pseudo i s o de kine ic
equa ion [22], and he a e cons an (k) was de e mined o be 0.20(2) min-1 (R2=0.979;
SE(y) = 0.015).
Figu e 1.
The so p ion iso he m o i on(III) on he memb ane in a 0.1 M KNO3 solu ion a
25°C and pH 2.5 a e shown in Figu e 2.
Figu e 2.
The Langmui and F eundlich models a e ypically employed o desc ibe he
ela ionship be ween q (so bed quan i y in he solid phase, mmol g-1) and Ceq (solu e
concen a ion a equilib ium). Thei a ied pe o mances ha e been p e iously e iewed
[23].
The Langmui model p o ided he bes desc ip ion o he so p ion o Fe(III) on
he memb ane. Based on non-linea i ing o he da a in Figu e 2, he maximum
so p ion capaci y qmax was 0.041(2) mmol g-1, and KL= 6.5(9) × 103 M-1 (R2 = 0.984;
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M H L MH L H
pn
nq++

SE(y) = 0.002). The heo e ical o al numbe o ac i e si es calcula ed om he weigh
pe cen ages o (3) in he memb ane was 0.07 mmol g-1, which is in ag eemen wi h he
ob ained qmax.
3.2.2. So p ion p o iles
The he modynamic cha ac e iza ion o ion-exchange and chela ing ma e ials is
undamen al o desc ibing he sepa a ion p ocess and p edic ing he beha io o analy e
so p ion on solid phases in di e en sys ems. As p e iously men ioned, especially o
ion-exchange and complexing esins [23-25], a me al ion (M) can be so bed on o a
chela ing solid phase ia complexa ion eac ions, and he gene al equilib ium can be
exp essed as ollows:
(1)
whe e M is he me al ion, H L is he -p o ona ed o m o he ac i e si e o he solid
phase, H is he p o on, and he o e ba ep esen s species in he solid phase. Cha ges a e
omi ed o simplici y. This eac ion is s udied based on he so p ion p o iles as a
unc ion o pH. This me hod has been p e iously desc ibed in se e al pape s and
e iews [23-28]. This me hod enables us o desc ibe he so p ion cu e wi h an equa ion
ob ained using a combina ion o exchange coe icien s (
β
1npex) ha a e associa ed wi h
each possible eac ion be ween he me al ion and he ac i e si e and selec ed o
minimize he di e ence be ween he calcula ed and expe imen al so p ion p o iles.
The exchange coe icien (β1npex) can be exp essed as ollows:
n
q
np
np
]
LH[[M]
[H]]LMH[
=
ex1
⋅
⋅
β
(2)
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able s and sma phones, allow o ou -o -lab quan i ica ion o a ge species in an in
si u, apid and inexpensi e ashion.
Acknowledgmen s
We g a e ully acknowledge he inancial suppo p o ided by FAR (Fondi A eneo pe la
Rice ca) o he Uni e si y o Pa ia, by he Spanish Minis e io de Economía y
Compe i i idad-Fede (MAT2014-54137-R) and by he Conseje ía de Educación –
Jun a de Cas illa y León (BU232U13).
Suppo ing in o ma ion
Expe imen al (in e media es and memb ane cha ac e iza ion) and p incipal componen
analysis (PCA) da a.
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doi: 10.1016/j.snb.2016.04.040
Link o he published a icle:
h p://www.sciencedi ec .com/science/a icle/pii/S092540051630510X
18
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1
FIGURES AND SCHEMES
Cap ions
Scheme 1. Syn hesis o ac ylic monome (2).
Scheme 2. Monome s and chemical s uc u e o he senso y memb ane. The pic u e
shows he physical aspec o he memb ane on a no ebook.
Figu e 3. So p ion p o iles o Fe(III) on 26.5 mg o d y memb ane in 1 M KNO3, V =
10 mL, [Fe(III)] = 1.8 µM. Red ci cles = p o ile in absence o compe i i e ligand, blue
diamonds = p o ile in p esence o py idine dica boxylic acid (PDCA) 0.5 mM, g een
ci cles = p o ile in p esence o sodium oxala e 0.05 M.
Figu e 2. So p ion iso he m o Fe(III) on he memb ane (so bed me al ion in he solid
phase (q) as a unc ion o he solu e concen a ion when he equilib ium is eached
(Ceq)). Condi ions: olume = 10 mL 0.1 M KNO3, pH 2.5, empe a u e = 25ºC, 36.5 mg
o d y memb ane. G ay ci cles: expe imen al poin s; con inuous line: bes i ing by
Langmui model.
Figu e 1. Kine ic p o ile o Fe(III) up ake on he memb ane (so bed me al ion in he
solid phase (q) as a unc ion o ime ( )). Condi ions: olume = 10 mL 0.1 M KNO3, pH
= 2.5, empe a u e = 25ºC, [Fe(III)] = 1.34·10-4 M, 13.7 mg o d y memb ane. The lines
ep esen he i ob ained using a pseudo i s o de equa ion.
Figu e 4. Colo ime ic de e mina ion o he Fe(III) concen a ion in wa e using senso y
discs cu om he memb ane: a) UV/ is spec a (inse : pic u e o he discs, le disc is
con ol disc); b) UV/ is i a ion cu e; c) i a ion using he RGB pa ame e s om he
digi al image aken om he senso y ma e ials (see inse , Figu e 5a). The h ee
pa ame e s (R, G and B) de ining he colo o each disc we e educed o one p incipal
componen (PC1) by p incipal componen analysis. Condi ions: p io o each
measu emen , each disc was imme sed o e nigh in Milli-Q wa e ( empe a u e = 25ºC,
pH = 2 -bu e HCl/KCl-) con aining a Fe(III) concen a ion anging om 1.65x10-6 o
1.65 x10-1 M and hen emo ed om he medium and d ied a .
Figu e 5. Response ime. Selec ed UV/ is spec a as a unc ion o he ime ha a piece
o memb ane (discs, 8 mm diame e ) was imme sed in wa e (pH = 2, bu e : KCl-HCl,
2 mL) in a UV/ is qua z cu e e upon addi ion o Fe(III) (concen a ion = 5x10-3 M).
Inse = Abso bance (490 nm) as a unc ion o ime.
Figu e 6. In e e ence s udy. a) UV/ is abso bance a ia ion a 483 nm o (3) in
solu ion (DMAc/H2O, 50/50; pH = 2, bu e : KCl-HCl, 2 mL; [(3)] = 1.6x10-3 M) a e
addi ion o a ious ca ions. Each ca ion was added indi idually in a (3)/ca ion mola
a io o 1/10 ( he concen a ion o each ca ion was 1.6x10-2 M). Inse : sample UV/ is
spec a o solu ion (3) a e addi ion o a ious ca ions (i.e., Fe(III), Co(II) and Hg(II));
and b) pic u e o wo senso y discs a e imme sion o e nigh in wa e (pH = 2, bu e :
KCl-HCl, 2 mL) con aining Fe(III) ([Fe(III)] = 1 x 10-2 M) and wo mix u es o ca ions
( he concen a ion o each ca ion was 1 x 10-2 M).
2
Scheme 1. Syn hesis o ac ylic monome (3).
O
OH
O
HOO
OH
O
ClO
OH
O
O
O
SOCl
2
0ºC, 4h
OK
O, 2-pen anone
100ºC, 2h
(1)
(2)
(3)
3
Scheme 2. Monome s and chemical s uc u e o he senso y memb ane
. The pic u e
shows he physical aspec o he memb ane on a no ebook.
Monome s:
OH
O
O
2-hyd oxye hyl
ac yla e (2HEA)
O
O
O
O
C osslinke :
e hylene glycol
dime hac yla e
(EGDMA)
XZ
Y
Memb ane chemical s uc u e:
X:Y:Z:V = 75:24:1:10
O
O
O
HO
O
(5-hyd oxy-4-oxo-4H-py an-2-
yl)-me hyl me hac yla e
(3)
N
1- inyl-2-
py olidinone (VP)
O
O
O
O
O
OH
O
O
HO
O
OO
N
OV
O

4
Figu e 1. Kine ic p o ile o Fe(III) up ake on he memb ane (
so bed me al ion in he
solid phase (q) as a unc ion o ime ( )). Condi ions: olume = 10 mL 0.1 M KNO3, pH
= 2.5, empe a u e = 25ºC, [Fe(III)] = 1.34·10-4 M, 13.7 mg o d y memb ane. The lines
ep esen he i ob ained using a pseudo i s o de equa ion.
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
0,040
0,045
0,050
020 40 60 80 100
q, mmol g-1
, min
5
Figu e 2. So p ion iso he m o Fe(III) on he memb ane (
so bed me al ion in he solid
phase (q) as a unc ion o he solu e concen a ion when equilib ium is eached (Ceq)).
Condi ions: olume = 10 mL 0.1 M KNO3, pH 2.5, empe a u e = 25ºC, 36.5 mg o d y
memb ane.
G ay ci cles: expe imen al poin s; con inuous line: bes i ing by Langmui
model.
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
0,040
0,045
0,050
0,0E+00 5,0E-04 1,0E-03 1,5E-03 2,0E-03
q, mmol g-1
Ceq, M
6
Figu e 3. So p ion p o iles o Fe(III) on 26.5 mg o d y memb ane in 1 M KNO3, V =
10 mL, [Fe(III)] = 1.8 µM. Red ci cles = p o ile in absence o compe i i e ligand, blue
diamonds = p o ile in p esence o py idine dica boxylic acid (PDCA) 0.5 mM, g een
ci cles = p o ile in p esence o sodium oxala e 0.05 M.
0,00
0,20
0,40
0,60
0,80
1,00
1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0
pH
7
Figu e 4. Colo ime ic de e mina ion o he Fe(III) concen a ion in wa e using senso y
discs cu om he memb ane: a) UV/ is spec a (inse : pic u e o he discs, le disc is
con ol disc); b) UV/ is i a ion cu e; c) i a ion using he RGB pa ame e s om he
digi al image aken om he senso y ma e ials (see inse , Figu e 5a). The h ee
pa ame e s (R, G and B) de ining he colo o each disc we e educed o one p incipal
componen (PC1) by p incipal componen analysis. Condi ions: p io o each measu emen ,
each disc was imme sed o e nigh in Milli-Q wa e ( empe a u e = 25ºC, pH = 2 -bu e
HCl/KCl-) con aining a Fe(III) concen a ion anging om 1.65x10-6 o 1.65 x10-1 M and hen
emo ed om he medium and d ied a .