Chemical Engineering Journal 232 (2013) 364–371

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Chemical Engineering Journal

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Rhodamine based reusable and colorimetric naked-eye hydrogel sensorsfor Fe3+ ion

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.07.111

⇑ Corresponding author. Tel.: +90 286 522 61 04 1033; fax: +90 286 522 61 01.E-mail address: ozgurozay@comu.edu.tr (O. Ozay).

Hava Ozay a, Ozgur Ozay b,⇑a Canakkale Onsekiz Mart University, Art and Science Faculty, Department of Chemistry, 17100 Canakkale, Turkeyb Canakkale Onsekiz Mart University, Lapseki Vocational School, Department of Chemistry and Chemical Processing Technologies, 17800 Lapseki/Canakkale, Turkey

h i g h l i g h t s

� Rhodamine based monomer.� Naked-eye hydrogel sensor.� Reusable hydrogel sensor.� Colorimetric hydrogel.� Fe3+ ion separation.

g r a p h i c a l a b s t r a c t

3+Fe EDA

Reusable naked-eye hydrogel sensor

a r t i c l e i n f o

Article history:Received 15 April 2013Received in revised form 25 July 2013Accepted 29 July 2013Available online 9 August 2013

Keywords:HydrogelIon sensorMetalNaked-eyeAcrylamide2-Hydroxyethylmethacrylate

a b s t r a c t

In this study, N-(Rhodamine-6G)lactam-N0-acryloyl-ethylenediamine (RH6GAC) as co-monomer wassynthesized in two steps and characterized by FT-IR, 1H, 13C NMR and mass spectrometry. Metal ion(Na+, K+, Ca2+, Fe2+, Fe3+,Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Hg2+, Mn2+, Ba2+, Mg2+) selectivity of RH6GACwas investigated by UV–Vis and fluorescence spectrophotometry. Then, Fe3+ ion selective and reusablenaked-eye colorimetric copolymeric sensor hydrogels were synthesized using 2-hydroxyethylmethacry-late (HEMA) and acrylamide (AAm) as the primary monomer and N-(Rhodamine-6G)lactam-N0-acryloyl-ethylenediamine as comonomer by free radical polymerization. The sensor hydrogel was characterizedby FT-IR and swelling properties.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogels as smart materials are chemically crosslinked, hydro-philic polymeric networks [1–3]. They can swell up to hundreds oftimes or even thousands of times their mass in aqueous media dueto functional groups (AOH, ACOOH, ANH2, ACONH2, ASO3H) intheir structure [4–6]. These properties make them useful in theapplication of drug delivery systems, antimicrobial materials, sen-sors, agriculture, matrix materials for tissue engineering, catalyst

support material, separations, immobilization, and environmentaltechnologies. Hydrogels can respond in a very short time to varia-tions in pH, light, temperature, solvent, or electric field thanks totheir functional groups by swelling or deswelling [6–10].

Currently, studies on the development of colorimetric chemo-sensors for the visual detection and quantitative determinationof ionic species attract considerable interest, due to their veryimportant role in a large variety of chemical and biological pro-cesses [11,12]. The determination of ion contents in water andthe removal of toxic species from water are important for thewhole world. Increasing industrialization increases ionic pollutantsin water resources. For the determination of ion contents in water,

H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371 365

instrumental methods such as atomic absorption spectrometry(AAS), inductively coupled plasma (ICP) and ion chromatography(IC) are used. These devices have disadvantages such as requiringa laboratory and high cost [13–15]. For this reason, cheaper andmore effective methods are being investigated. The utilizing of sen-sors in this area is rapidly increasing due to their simple, fast andaccurate results [15]. A colorimetric chemosensor can be preparedthrough the design of molecules that change their color in solutiondue to an alteration in their molecular structure in the presence ofan ion [16–19].

Fe3+ exists in the structure of numerous enzymes which areused as catalysts for oxygen metabolism. However, a high concen-tration of Fe3+ ion has a toxic effect on living organisms [20–22].Therefore, various colorimetric chemosensors have been devel-oped for the visual detection and quantitative determination ofiron ion species. Amongst these chemosensors, rhodamine deriva-tives are the most common [11–22]. Rhodamine derivatives arecolorless with a tricyclic lactam structure and their color changesas a result of the opening of this lactam ring in the presence ofan ion [15,17]. These molecules are insoluble in water due to theirhydrophobic nature. Therefore, several hydrophilic copolymerscontaining rhodamine derivatives have been prepared in the liter-ature [23,24]. Although they are soluble in aqueous media, thesepolymeric sensors are not usually reusable. Crosslinked hydrogelsare not dissolved in aqueous media. Therefore, they are reusableand have been recently attracting the attention of researchers.Especially color changing hydrogels, which change color thanksto their functional groups in the presence of metal ions, are beingsynthesized for environmental applications. In the literature, thereare some studies of hydrogels which were synthesized as naked-eye metal ion sensors [25–29]. Baek et al. synthesized dye-basedPHEMA hydrogels. These hydrogels were used as pH, metal ionand humidity sensors [25]. Lin et al. synthesized a new type ofDNAzyme crosslinker and used this crosslinker in hydrogelproduction. They used these synthesized hydrogels as selectivehydrogel sensors for Cu2+ ions [26]. In addition, acrylamide, 2-acry-lamido-2-methyl-1-propanesulfonic acid, and allylamine basedaptamer or DNA functionalized hydrogels which can change colorin the presence of Hg2+ were synthesized and reported in the liter-ature [27–29]. These hydrogels were also used in removal of Hg2+

from aqueous media [29]. In these studies performed based onDNA, the visual detection of Hg2+ was achieved by a dye that canbe immobilized on DNA by electrostatic interactions. Differentlyfrom these studies, we attached the RH6GAC monomer showingsensor feature to hydrogel network by radical polymerization. Inaddition to that, the sensor hydrogels that we have designed canbe prepared relatively more simply and with low cost comparedto DNA based hydrogels mentioned above. The hydrogels preparedby us in this study are in macro size. Thanks to their size, they haveadvantage to be separated from aqueous media by simple filtering.The most important of all, thanks to this study, rapidly color chang-ing and reusable hydrogel sensors in presence of Fe3+ were addedto the literature apart from the Cu2+ and Hg2+ sensitive naked-eye sensors.

In this study, two different reusable rhodamine group-contain-ing polymeric sensors were synthesized. These new hydrogel sen-sors are insoluble in water but they show sensor features fordissolved Fe3+ ions in water due to their swelling features in aque-ous media. In the first step of the study, we synthesized and char-acterized N-(Rhodamine-6G)lactam-N0-acryloyl-ethylenediamine(RH6GAC) by FT-IR, 1H, 13C NMR and mass spectrometry. After-ward, the sensor behavior of RH6GAC was investigated for metalions in DMSO:H2O mixture by UV–Vis and fluorescence spectra.The obtained monomer (RH6GAC) was copolymerized with non-io-nic 2-hydroxyethylmethacrylate (HEMA) and acrylamide (AAm) atdifferent mole ratios (99.5/0.5; 99.0/1.0; 97.5/2.5; 95.0/5.0) using

N,N0-Methylenebisacrylamide (MBA) as crosslinker to synthesisnovel p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels.The obtained hydrogels were characterized by FT-IR, gel contentand the swelling characteristics of the hydrogels were investigated.Then, p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogelscontaining rhodamine derivative were used for the detection offerric iron (Fe3+) by eye. In order to investigate the reusability ofhydrogels, the ethylenediamine solution was used.

2. Experimental

2.1. Materials

Rhodamine 6G (95%, Aldrich), acryloyl chloride (97%, Aldrich),CH2Cl2 (99.9%, Aldrich) and DMSO (99.9%, Aldrich) were all usedas received. The monomers acrylamide (AAm) (Fluka, 98%) and 2-hydroxyethylmethacrylate (HEMA) (99%, Sigma–Aldrich), thecrosslinking agent N,N0-Methylenebisacrylamide (MBA) (99%, Sig-ma–Aldrich), initiator ammonium persulfate (APS) (98%, Sigma–Al-drich) and accelerator N,N,N0,N0-Tetramethylethylenediamine(TEMED) (99%, Acros Organics) were all used as received. All metalsalts such as perchlorate of Na+, K+, Ca2+, Fe2+, Fe3+,Co2+, Ni2+, Cu2+,Zn2+, Pb2+, Cd2+, Hg2+, Mn2+, Ba2+, and Mg2+ were purchased fromAldrich and used as received. 4-(2-Hydroxyethyl)piperazine-1-eth-anesulfonic acid (HEPES) as buffer was purchased from Aldrich. Allthe reagents were of analytical grade or the highest purity avail-able, and were used without further purification. All solutions(monomer and metal ions) were prepared using distilled water(0.055 lS cm�1). The pH measurements were carried out using aConsort C864 pH meter.

2.2. Instrumental characterization

1H and 13C NMR spectra were recorded on a Bruker-300 spec-troscope using DMSO-d6 as the solvent and tetramethylsiliane(TMS) as the internal reference. The Fourier transform infraredradiation (FT-IR) spectra of the monomer and hydrogels were re-corded with an FT-IR (Perkin Elmer Spectrum 100) instrumentusing an ATR apparatus with 4 cm�1 resolution between 4000and 650 cm�1. UV–Vis spectra were recorded with a PG T80 + spec-trophotometer. Fluorescence spectra were recorded on a VarianCary Eclipse spectrofluorometer using 1 cm path length cuvettesat room temperature.

2.3. Synthesis of N-(Rhodamine-6G)lactam-N0-acryloyl-ethylenediamine (RH6GAC)

N-(Rhodamine-6G)lactam-ethylenediamine [30] (4.56 g,10.00 mmol) and triethylamine (2.1 ml, 15.00 mmol) were dis-solved in 150 ml of CH2Cl2 and this solution was cooled to 0 �C.To the mixture was added dropwise acryloyl chloride (0.81 ml,10.00 mmol) via a syringe through a rubber septum. Afterwardsthe solution was stirred for 4 h at 0 �C and for 12 h at ambient tem-perature. At the end of this time, the reaction mixture was ex-tracted with water (3 � 150 ml), and the organic layer waswashed with saturated sodium bicarbonate solution (3 � 50 ml)and brine (3 � 50 ml). Afterwards the organic solutions were driedover magnesium sulfate and filtered. The solvent was removed byrotary evaporator. The light pink crude product was purified bychromatography on neutral alumina (Al2O3), eluting with ethylacetate to give 4.04 g of (white solid) with 79.2% yield. FTIR-ATR(mmax, cm�1): 3427 and 3255 (NAH), 3072 (Aromatic CAH),2962–2863 (Aliphatic CAH) 1698 and 1620 (C@O), 1514 (C@C).1H NMR (300 MHz, DMSO-D6, ppm): 7.91 (t, 1H), 7.81–7.78 (m,1H), 7.51–7.48(m, 2H), 6.99–6.94 (m, 1H), 6.27 (s, 1H), 6.14

366 H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371

(s, 1H), 5.48 (dd, 1H), 5.09 (t, 2H), 3.40 (s, 2H), 3.19–3.03 (m, 6H),2.95–2.95 (m, 2H), 1.87 (s, 6H), 1.21 (t, 6H). 13C NMR (300 MHz,DMSO-D6, ppm): 167.8, 164.8, 154.3, 151.4, 148.4, 133.2, 131.9,130.6, 128.1, 125.2, 124.0, 122.8, 118.7, 104.9, 96.1, 64.8, 60.2,37.9, 37.4, 21.2, 17.5, 14.6. HRMS m/z: calc. for C21H35N4O3 +(Na+), 533.2597. Found m/z: 533.2601.

2.4. Synthesis and swelling behavior of hydrogels

P(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels weresynthesized through free radical polymerization at different moleratios as shown in Table 1. RH6GAC monomer taken in amounts gi-ven in Table 1 was dissolved in DMSO. Then, AAm was dissolved indistilled water (treated with argon gas) while HEMA was used inwater without dissolving. All monomers (RH6GAC, AAm or HEMA)were mixed in a vial homogeneously. Then, MBA at mole ratio of0.25% (based on total monomer amount) was added to this mono-mer solution. 100 ll of TEMED and the initiator dissolved in 0.5 mldistilled water (APS, 1 mol% with respect to monomer) were addedto the reaction mixture. This mixture was stirred until a homoge-neous solution formed and was filled into plastic pipettes with adiameter of 5 mm using an injector. This mixture was maintainedat 30 �C for 24 h. Then, the hydrogels extracted from the pipetteswere cut at a length of approximately 10 mm and washed for48 h in total; first in DMSO for 24 h and later in distilled waterfor 24 h. At this stage, gel content in % of the synthesized hydrogelswas determined by gravimetric methods. Then p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels were dried in anoven at 40 �C.

Triplicate studies of water absorption kinetics of p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels were conducted indistilled water. To study swelling kinetics of the hydrogels versustime, approximately 100 mg of hydrogel was put into distilledwater and the increase in mass was measured at certain intervals.The swelling ratios of the hydrogels were calculated using the fol-lowing equation:

%S ¼ ½ðMS �MDÞ=MD� � 100 ð1Þ

where MS is the weight of swollen hydrogel, and MD is the weight ofdried hydrogel.

2.5. Spectral studies of RH6GAC as chemosensor for metal ions

The stock solutions of RH6GAC (1 � 10�4 M) and all the metalions (1 � 10�3 M) were prepared in DMSO and distilled water forthe spectra analysis, respectively. Each time a 3 ml solution ofDMSO:H2O (10:90, buffer of HEPES pH = 7.4) was placed in a quartzcell of 1 cm optical path length, and different stock solutions ofRH6GAC and all the metal ion stocks were added into the quartzcell gradually by a micro-pipette. The total volume of the added

Table 1The synthesis compositions of p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels.

Hydrogel Code AAm or HEMAamounta

RH6Am

p(AAm-co-RH6GAC) Gel 1 2.00 g 0.07p(AAm-co-RH6GAC) Gel 2 1.99 g 0.14p(AAm-co-RH6GAC) Gel 3 1.96 g 0.36p(AAm-co-RH6GAC) Gel 4 1.91 g 0.72p(HEMA-co-RH6GAC) Gel 5 3.38 ml 0.07p(HEMA-co-RH6GAC) Gel 6 3.36 ml 0.14p(HEMA-co-RH6GAC) Gel 7 3.31 ml 0.36p(HEMA-co-RH6GAC) Gel 8 3.22 ml 0.72p(AAm) – 2.00 g –p(HEMA) – 3.38 ml –

Total monomer amount (a + b): 0.277 mol, 1% APS (in 0.5 ml deiyonized water), 0.25% M

stock solutions was less than 100 ll with the purpose of keepingthe total volume of the testing solution roughly constant. Ion bind-ing properties were investigated by UV–Vis spectrophotometerand fluorescence spectrophotometer at 532 nm wavelength andthe excitation wavelength was 530 nm.

2.6. The utilization of hydrogels as simple naked-eye sensors

It was ensured that p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels retained water at maximum capacity (in dis-tilled water) before they were used as sensors. The swollen hydro-gels were cut to a thickness of approximately 3 mm(approximately 15 mg dry hydrogel). Then, they were put intosolutions of Na+, K+, Ca2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+,Cd2+, Hg2+, Mn2+, Ba2+, and Mg2+ with a concentration of 1 ppm(100 ml) to find selective ions. From this process; it was found thatthe hydrogels were selective for Fe3+ ion as naked-eye sensors. Inorder to determine whether the hydrogels had fluorescent proper-ties after the interaction with Fe3+ ions or not, UV light with365 nm wavelength was used. Then, to determine detection limitsof sensor hydrogels, they were put into Fe3+ solutions (100 ml)with different concentrations in the range of (100–0.01 ppm(100 ml)). To investigate the reusability of hydrogel sensors theywere transferred into 1 M ethylenediamine (EDA) (100 ml) solu-tion after the absorption process of iron (III) ion. Then, the decolor-ized hydrogel sensor was washed with distilled water until neutralpH.

3. Results and discussion

3.1. Synthesis and spectral characterization

As seen in Scheme 1, N-(Rhodamine-6G)lactam-N0-acryloyl-eth-ylenediamine (RH6GAC) was synthesized in two steps. Firstly, N-(Rhodamine-6G)lactam-ethylenediamine was synthesized fromthe reaction of rhodamine 6G with ethylenediamine according toliterature procedures [30]. Then, RH6GAC was synthesized as a re-sult of the reaction of N-(Rhodamine-6G)lactam-ethylenediaminewith acryloyl chloride in the presence of triethylamine with a yieldof 79.2%. RH6GAC was characterized by FT-IR, 1H, 13C NMR andmass spectrometry. The IR spectrum of RH6GAC is given inFig. 1S (see supporting information). The characteristic m(NAH)bands for the compound were observed at 3427 and 3255 cm�1.The C@O stretching frequencies of the compound were observedat 1698 and 1620 cm�1 for lactam and amide groups, respectively.The characteristic C@C stretching band for aromatic compoundswas observed at 1514 cm�1.

The 1H NMR for RH6GAC spectra is given in Fig. 1(a). The spec-tra confirm the structure of RH6GAC. According to the 13C NMRspectra, the compound has 22 signals. The carbonyl carbons were

GACountb (g)

Mol ratio Water (ml) DMSO (ml)

2 99.5/0.5 2 0.24 99.0/1.0 2 0.40 97.5/2.5 2 1.00 95.0/5.0 2 2.02 99.5/0.5 – 0.24 99.0/1.0 – 0.40 97.5/2.5 – 1.00 95.0/5.0 – 2.0

100/0 2 –100/0 – –

BA.

ONH

NH

N

O HN

ONH

NH

N

O

NH2

ONH

NH

O

O

Cl

C2H5OH

H2N NH2

O

Cl

O

CH2Cl2

N

RH6GAC

Fe3+

ONH

NH

N

ONH

O

Fe3+

Scheme 1. The synthesis route of RH6GAC.

4.54.55.05.05.55.56.06.06.56.57.07.07.57.58.08.08.58.59.09.0ppm

a b c,d e

f g

h ip

abc

de

f

g

hj

k

l

m

np

ONH

NH

N

O HN

O

(a)

9090100100110110120120130130140140150150160160170170ppm

1 2

(b)

ONH

N

O

23

45

6

7 891011

12 1314

1520

21

22

14

12,16

17

Fig. 1. (a) 1H NMR (b) 13C NMR sp

H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371 367

observed at 167.8 ppm and 164.8 ppm and the chemical shifts foraromatic and olephinic carbons were observed at 154.3, 151.4,148.4, 133.2, 131.9, 130.6, 128.1, 125.2, 124.0, 122.8, 118.7,104.9, and 96.1. The signals related to aliphatic carbons were ob-served at 64.8, 60.2, 37.9, 21.2, 17.5, and 14.6 (Fig. 1(b)). The signalrelated to M+ + Na+ for the compound in MS spectra (see support-ing information, Fig. 2S) confirmed the structure.

3.2. The metal ion binding properties of RH6GAC as chemosensor

RH6GAC monomer has chemosensor features only for Fe3+ ionsin visible light as seen in Fig. 2(a). In sensor studies, the ability tochange color is very important for naked-eye detection of anyion. This property is more important for speciation iron ions(Fe2+ and Fe3+) and their qualitative and quantitative analyses.RH6GAC monomer, which produces a color change with Fe3+ in vis-ible light, can also emit fluorescence. As seen in Fig. 2(b), RH6GACmay also be used in manufacturing a fluorescent sensor because ofits luminescent features under 365 nm light in presence of 100 lMof Fe3+ ion. Sensors must have the ability to determine the amount

0.50.51.01.01.51.52.02.02.52.53.03.03.53.54.04.0

j

k, l

m

ni

10102020303040405050606070708080

NH

HN

O

116

1718

19

22

2021

ectra of RH6GAC in DMSO-d6.

(b)

(a)

(c)Fig. 2. The color changes of RH6GAC in presence of different metal ions: (a) visiblelight (ligand:10 lM, metal:100 lM), (b) UV light at 365 nm (ligand:10 lM,metal:100 lM) and (c) in presence of different concentration of Fe3+

(ligand:10 lM).

1

Wavelength (nm)

Abs

orba

nce

2 3

(a)

1

2,3

Wavelength (nm)

Fluo

resc

ence

Int

ensi

ty (

a.u.

)

(b)

Fig. 3. (a) The UV–Vis absorption spectra of RH6GAC with different metal ions(ligand:10 lM, metal:100 lM) and (b) the fluorescence spectra of RH6GAC withdifferent metal ions (1: Fe3+; 2: other metals; 3: sensor).

368 H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371

of the cation as well as a detection feature for the cations. As seenin Fig. 2(c), the color intensity of the solution containing RH6GAC-Fe3+ increased with the increase in Fe3+ ion concentration(0–100 lM).

The metal ion binding property of RH6GAC was confirmed byUV–Vis spectroscopy. Upon the addition of Fe3+ (10 equiv) to thesolution of RH6GAC (10 lM in 10:90 DMSO:H2O buffer of HEPESpH = 7.4), new absorption maxima appeared around k = 532 nmas seen Fig. 3(a). These spectral changes could also be observedby the naked-eye as a result of the color change of the solutionof RH6GAC from colorless to pink–orange. The sensing phenomenawere also monitored by fluorescence spectroscopy. As Fe3+ (10equiv) was added to the solution of RH6GAC (10 lM in 10:90DMSO:H2O buffer of HEPES pH = 7.4), a dramatic increase was ob-served in the fluorescence intensity of the solution and the fluores-cent emission maximum is centered at about 555 nm (Fig. 3(b)).However, other cations such as Na+, K+, Ca2+, Fe2+, Fe3+,Co2+, Ni2+,Cu2+, Zn2+, Pb2+, Cd2+, Hg2+, Mn2+, Ba2+, and Mg2+ did not causeany significant changes in fluorescent emission intensities andabsorbance at 555 nm and 532 nm, respectively (Fig. 3(a and b)).

In order to estimate the specific concentration for Fe3+, the solu-tion of RH6GAC (10 lM in 10:90 DMSO:H2O buffer of HEPESpH = 7.4) was titrated by adding different equivalents of Fe3+ andmonitoring the UV–Vis absorption spectra (Fig. 4(a)). As clearlyseen from the figure, the absorption of RH6GAC solution at532 nm increased with the Fe3+ concentration.

In order to determine the stoichiometric ratio between RH6GAChost and Fe3+ guest, Job’s plot was used. The total concentration ofRH6GAC and Fe3+ was constant at 100 lM with continuous varia-tion of mole fraction of Fe3+. The Job’s plot is shown in Fig. 4(b).The results demonstrate that a 1:1 complex is formed for RH6GACwith Fe3+ ions.

3.3. Synthesis and characterization of the hydrogels and their use asnaked-eye sensors

Hydrogels have been used for environmental applications(adsorption, separation) in many studies in the literature. In thesensor studies, the solubility of the molecule designed as sensoris important in aqueous media. One of the most important prob-lems in these studies is the fact that anion or cation as pollutantexists in water, but the sensor cannot dissolve directly in water.Furthermore, the sensor dissolved in solution media is only suit-able for a single use. Researchers have preferred to polymerizethe molecules with sensor features to cope with these problems.Herein, it is required that the sensor becomes insoluble to be reus-able for further work and can be easily removed from the solutionmedia. The best example of a polymer, which can be removed fromthe solution with a filtering process, is hydrogel. They are uniquefor sensor applications because they can be synthesized with highyield, they have properties such as swelling in water, are reusableand rapidly respond to changes in the environment. Due to thesereasons, copolymeric hydrogels were produced from AAm andHEMA with RH6GAC monomer.

Abs

orba

nce

X (Fe3+)

(b)

Wavelength (nm)

Abs

orba

nce

120 µM 100 µM

80 µM 60 µM 40 µM 20 µM 10 µM 5 µM 0 µM

(a)

Fig. 4. (a) The UV–Vis absorption spectra of RH6GAC with different concentrationof Fe3+ (ligand:10 lM) and (b) job’s plot between RH6GAC and Fe3+ ions.

H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371 369

AAm and HEMA containing ANH2 and AOH functional groups,respectively, are neutral monomers. Sensor hydrogels were syn-thesized under the conditions given in Table 1 as seen in Scheme 2using a crosslinker (MBA). The IR spectra of p(AAm) and p(AAm-co-RH6GAC) (97.5/2.5) (Gel 3) are given as supplementaryinformation in Fig. 3S(a). On the IR spectra of p(AAm) hydrogelsthe characteristic NAH stretching bands are observed at 3332and 3186 cm�1. In the spectrum bands at 1649 and 1602 cm�1

are the characteristic C@O stretching and NAH bending bands forprimary amides. In the IR spectra of Gel 3, C@O stretching for thelactam group of RH6GAC is observed as a peak similarly

HN O NH

N

ONH

O

O

NH2

OO

HO

RH6GAC

or

+

HEMA or A

Scheme 2. The representation of the synthesis of hydrogel

shouldered at 1681 cm�1 due to overlapping with C@O bands ofacrylamide. In the spectrum bands observed at 1550 and1518 cm�1 are NAH bending and C@C stretching bands for second-ary amide or amine and aromatic carbons of RH6GAC, respectively.Thus, according to the FT-IR spectra of p(AAm) and Gel 3 given inFig. 3S(a), it may be said that the sensor hydrogel was successfullysynthesized.

Similarly, the FT-IR spectra of p(HEMA) and p(HEMA-co-RH6GAC) (97.5/2.5) (Gel 7) are given in Fig. 3S(b). According tothe FT-IR spectrum of p(HEMA) hydrogels, AOH and C@O stretch-ing bands were observed at 3375 and 1718 cm�1, respectively. Inthe FT-IR spectrum of Gel 7 was observed that C@O stretching ofthe lactam group of RH6GAC overlapped with the band C@Ostretching of the ester group of HEMA at 1718 cm�1. However,the C@O stretching and NAH bending bands of the amide groupof RH6GAC were observed at 1624 and 1550 cm�1, respectively.In addition, C@C stretching band of the aromatic carbons ofRH6GAC was observed at 1519 cm�1 in the spectrum. Thus, accord-ing to these FT-IR spectra assessments it may be said that p(HEMA-co-RH6GAC) hydrogels were successfully synthesized.

Synthesis yield of the hydrogels is directly related to the moleratios of the used monomers and crosslinker ratio. RH6GAC mono-mer could not be synthesized as a homopolymer hydrogel with theuse of crosslinker (MBA) at a ratio of 0.25–10%. Instead, it was pre-pared as a copolymer using AAm and HEMA monomers and cross-linker at a mole ratio of 0.25% (MBA, based on total monomeramount). Fig. 5(a) shows gelation and swelling ratios in distilledwater for p(AAm-co-RH6GAC) hydrogels. According to this, as theamount of RH6GAC increases (0.5%, 1%, 2.5% and 5%), gelation ra-tios decrease. Gelation ratio for Gel 1 synthesized using 99.5%AAm and 0.5% RH6GAC is 71.1 ± 2.4% while it is 40.2 ± 4.7% forGel 4 synthesized using 95% AAm and 5% RH6GAC. Maximumswelling capacity in distilled water for the same hydrogels in-creases directly proportional to RH6GAC ratio added to the poly-merization media. Accordingly, maximum swelling amounts forp(AAm-co-RH6GAC) hydrogels are 929 ± 89%, 2188 ± 176%,4112 ± 288% and 3545 ± 245% for Gel 1, Gel 2, Gel 3 and Gel 4respectively. The reason for the lower swelling capacity of Gel 4compared with Gel 3, may be the fact that polymerization hasnot occurred completely and crosslinking has been restricted.Fig. 5(b) shows gelation ratios and swelling degree in distilledwater for p(HEMA-co-RH6GAC) hydrogels. According to this, yieldin gelation for Gel 5, Gel 6, Gel 7 and Gel 8 are 74.3 ± 2.8%,66.7 ± 3.2%, 60.4 ± 3.7% and 35.8 ± 3.6% respectively. As seen inFig. 5(b), equilibrium swelling values in distilled water for Gel 5,Gel 6, Gel 7 and Gel 8 are 93.7 ± 9%, 102.9 ± 10%, 108.6 ± 16% and212 ± 23% respectively. Fig. 5(c and d) shows graphics for swellingof Gel 3 and Gel 7 in distilled water versus time. According to thegraphics, Gel 3 and Gel 7 achieve equilibrium swelling value in

Am

Fe3+

Naked-eye hydrogel sensor

sensor and their use for the reusable naked-eye sensor.

Time (min)

Swel

ling

(%)

(c)

Gel 3p(AAm)

Time (min)

Swel

ling

(%)

(d)

Gel 7

p(HEMA)

RH6GAC mol ratio

Swel

ling

(%)

Gel

Con

tent

(%

)

(b)

Gel content

Swelling

(a)

Gel

Con

tent

(%

)

RH6GAC mol ratio

Swel

ling

(%)

Gel content

Swelling

Fig. 5. Effect of RH6GAC mol ratio on swelling percent and gel content of (a) p(AAm-co-RH6GAC), (b) p(HEMA-co-RH6GAC), the swelling isotherms of the, (c) Gel 3 and (d)Gel 7 hydrogels with time in distilled water.

(a)

(b)

Fe (III) EDA

(c)

1 2

Wavelength (nm)

Abs

orba

nce

(d)

Fe3+ Sensor Fe2+ Hg2+ others

100 ppm 20 ppm 1 ppm 0,1 ppm

Fig. 6. The color changes of p(HEMA-co-RH6GAC) (a) in presence of different metalions (Metal ion concentration: 1 ppm, 100 ml), (b) in presence of differentconcentration of Fe3+ , (c) the reusability of sensor hydrogels and (d) UV–Visspectra of Gel 7 (1) and Gel 7-Fe3+ (2) hydrogels.

370 H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371

1080 and 600 min respectively. Gel 7 was selected to be used as asensor in aqueous media after the swelling characterization stageof the hydrogels. The reason for selecting Gel 7 is its high yieldin synthesis and its ability to swell in aqueous media.

The used RH6GAC as a monomer does not lose sensor ability forFe3+ when it is synthesized to form a hydrogel with AAm or HEMA.P(AAm) or p(HEMA) hydrogels cannot act as a naked-eye sensorsfor any metal ions. As seen in the Fig. 6(a), Gel 7 act as a naked-eye sensor for 1 ppm (100 ml) Fe3+ ion. Fe3+ solutions with variousconcentrations ranging between 100 and 0.01 ppm (100 ml) wereused in the determination of detection limits of the hydrogels forFe3+ ions. As can be seen in Fig. 6(b), color tone of the sensor hydro-gel increases as ion concentration in the media increases, the col-oring of Gel 7 being visible at a concentration of 0.1 ppm for Fe3+

ions in aqueous media. Response time of a sensor is also quiteimportant. Color change begins in 5 s for Gel 7 with a thicknessof approximately 3 mm in 100 ppm Fe3+ concentration. This isapproximately 5 min for Gel 3, which swells more. The color ofGel 7 changes in presence of Fe3+ ions at 20 ppm, 1 ppm and0.1 ppm in 10 s, 2 min and approximately 20 min respectively(the moment at which color change is detected by eye).

Soluble sensors in the solution media are single use only. There-fore, designing sensors in the form of insoluble polymers enablespreparation of reusable sensors. As seen in Fig. 6(c), Gel 3 andGel 7, which were used as naked-eye sensors for Fe3+ ions, returnto their original color and become clear after they are retained in1 M (100 ml) EDA solution. Thus, the hydrogels can be reused asnaked-eye sensors after they are washed until neutral pH. If amaterial can be prepared in the form of a film, which can changeits color in the presence of any ion, it can be used in manufacturinga device which works based on the color change principle and pro-duces an signal when color changes. As seen in Fig. 6(d), an in-crease in absorption in UV–Vis spectrum of Gel 7 and Gel 7-Fe3+

is observed at a wavelength of 660 nm. This indicates that the sen-sor hydrogel with a thickness of 1 mm can be used in manufactur-ing a device in addition to its use as a naked-eye sensor. RH6GAC,which is fluorescent-responsive as monomer, is

Sensor Sensor + Fe3+ Sensor Sensor+ Fe3+

(b) at 365 nm

(a) visible light

Fig. 7. Digital camera images of (a) Gel 7 and Gel 3 hydrogels under visible light and(b) Gel 7 and Gel 3 hydrogels under UV light at 365 nm.

H. Ozay, O. Ozay / Chemical Engineering Journal 232 (2013) 364–371 371

fluorescent-responsive in the polymer form also. As seen in Fig. 7,Gel 7 and Gel 3 can be used as naked-eye and fluorescent light-responsive sensors for Fe3+ (100 ppm, 100 ml) ions under visiblelight (Fig. 7 (a)) and light at 365 nm (Fig. 7 (b)).

4. Conclusions

In this study, RH6GAC monomer was synthesized in two stepsand it was characterized by the techniques of FT-IR, 1H NMR, 13CNMR and mass spectrometry. Then, the monomer was used as anaked-eye colorimetric sensor for detection of Fe3+ ions. Cross-linked hydrogel networks were synthesized by the polymerizationof RH6GAC monomer with AAm and HEMA at different mole ratios.The FT-IR, gel content and swelling characterization of p(AAm-co-RH6GAC) and p(HEMA-co-RH6GAC) hydrogels were conducted.They were used as naked-eye sensors for Fe3+ ions in aqueous med-ia. The minimum detection limit of Gel 7 was determined to be0.1 ppm (100 ml) Fe3+ concentration. Furthermore, it was observedthat color change of the hydrogels in 100 ppm Fe3+ solution beganin two seconds.

Acknowledgments

This work is supported by Canakkale Onsekiz Mart University(Project No.: COMU-BAP 2013/74) and partially supported by theScientific and Technological Research Council of Turkey (ProjectNo.: TUB_ITAK 112T278).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2013.07.111.

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