Colorimetric detection, quantification and extraction of Fe(III) in water by acrylic polymers with pendant Kojic acid motifs

Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 1 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) Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 2 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 Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 3 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]. Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 4 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 Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 5 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 Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 6 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 Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 7 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 Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 8 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; Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 9 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) Senso s and Ac ua o s B: Chemical, 2016, 233, 120-126 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 16 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. Re e ences and no es [1] H. G. Seile , A. Sigel, H. Sigel, Handbook on Me als in Clinical and Analy ical Chemis y, Ma cel Dekke , Inc., NY, 1994, p.13. [2] V. K. Gup a, N. Me gu, L. K. 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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 .