RSC Advances

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Inkpen-printed r

aDepartment of Chemistry and Center of

Faculty of Science, Mahasarakham Unive

E-mail: kchatthai@gmail.combDepartment of Chemistry, Faculty of E

Technology, Isan Khon Kaen Campus, KhoncSupramolecular Chemistry Research Unit

Science, Chulalongkorn University, Bangkok

† Electronic supplementary informa10.1039/c4ra07578a

Cite this: RSC Adv., 2014, 4, 46145

Received 24th July 2014Accepted 3rd September 2014

DOI: 10.1039/c4ra07578a

www.rsc.org/advances

This journal is © The Royal Society of C

eusable colorimetric sensors forthe detection of Hg(II)†

Chatthai Kaewtong,*a Yuwapon Uppa,*b Mangkorn Srisa-ard,a Buncha Pulpokac

and Thawatchai Tuntulanic

An inkpen-based reversible biodegradable sensor (IRBS) has been developed for the detection of mercuric

ions (Hg2+). The colour response can be tuned to allow detection with the naked eye based on Hg2+-

induced opening of the rhodamine spirocycle, leading to an increase in fluorescence and other colour

changes. After dipping in water, the colour change in the IRBS paper can be quantified by visual

comparison with standard images of IRBS papers, allowing the concentration of Hg2+ to be determined

over the concentration range 4.0 � 10�8 M to 1.0 � 10�6 M. Reusability has been established by

repeatedly dipping and rinsing the paper in aqueous Hg2+ and EDTA solutions. This approach provides a

sensitive and accurate method for the estimation of Hg2+ in environmental and biological applications.

1. Introduction

Pollution with heavy metal ions has become a worldwideissue in recent years due to its severe risk to human healthand to the environment.1 Mercury is one of the most toxicheavy metals found in aquatic systems. It persists for a longtime aer the pollutant source has been removed and it isable to pass into the human body through the skin and therespiratory and gastrointestinal tissues, causing damage tothe central nervous and endocrine systems.2 Many of thecurrent techniques for mercury detection, including anodicstripping voltammetry, X-ray uorescence spectrometry,neutron activation analysis and inductively coupled plasmamass spectrometry, require complex sample preparation andexpensive sophisticated instrumentation.3 New methods areneeded to provide simple, safe, effective and rapid detectionof Hg2+, paving the way for its removal.

Paper-based functional devices have recently been developedthat are readily available, lightweight, exible and disposable.4

The basic system in such paper-based methods consists of avariety of colorimetric indicators (pH indicators, sol-vatochromic dyes, chromogenic sensors, etc.), each sensitive toa specic range of compounds when spotted on a hydrophobicsubstrate, giving a specic colour change response.5

Excellence for Innovation in Chemistry,

rsity, Mahasarakham 44150, Thailand.

ngineering, Rajamangala University of

kaen 40000, Thailand

, Department of Chemistry, Faculty of

10330, Thailand

tion (ESI) available. See DOI:

hemistry 2014

A number of research groups have reported adaptations andvariations of this concept. For example, a paper-based colori-metric sensor array using polydiacetylene derivatives fordetecting the presence of volatile organic compounds has beendeveloped,6 as has a uorescent sensor array based on metalcomplexes for detecting 2,4,6-trinitrophenol.7 An ingeniouscolorimetric probe based on push–pull chromophores con-taining receptor sites for mercury(II)8 or copper(II)9 probes hasbeen developed.

A printing ink is a combination of colorants (pigments ordyestuffs) carried in a vehicle (resin, drying oil, solvent or othermaterial) to form a uid capable of being applied to paper orother substrates, either on a printing press or in some othermanner.10 The colorant can be a uorescent material such as arhodamine dye, which has a high absorption coefficient, broaduorescence throughout the visible spectrum, a high uores-cence quantum yield and photostability aer complexing with ametal ion. The mechanism is based on switching the spirocyclicmoiety of rhodamine off and on by guest compounds. Spi-rolactam derivatives of rhodamine are non-uorescent, whereasits analogous amide system, ring-opened by guest compounds,exhibits a strong pink uorescence.

Rhodamine-based sensors for cations and other analyteshave recently attracted signicant interest.11 In addition, earlierwork has reported that rhodamine-based sensors are highlyselective for the Hg2+ cation.12 Our own previous investigationshave shown that rhodamine connected to an azacrownmoiety isreadily synthesised and can selectively bind to Hg2+, with a 350-fold increase in emission compared with the unmodiedchemosensor.13

Most paper-based sensor systems using a two-componentsystem (chemosensor and solvent)6–9 have problems in boththeir physical properties and their stability, and require the

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incorporation of a binder. A wide choice of binders are availablefor paints and inks, including polyacrylic and polyurethaneresins.14–16 Although they show excellent properties in terms ofhardness, stability, chemical resistance and gloss, these exhibitpoor biocompatibility and biodegradability. Among the biode-gradable polymers, polylactide is a promising choice, bothenvironmentally and in terms of cost, and it is available fromrenewable resources.17 In order to improve its encapsulated–chemosensing efficiency, a poly(L-lactide)maltitol combinationhas been developed as binder. Acetone is a convenient solventfor both chemosensor and binder.

An inkpen reversible biodegradable sensor based on rhoda-mine derivatives and a branched poly(L-lactide)maltitol combi-nation (B-PLLA-M) dissolved in acetone has been developed forthe visual detection of mercury (Hg2+) without the need forinstrumentation. The method uses rhodamine derivativesencapsulated in a branched poly(L-lactide)maltitol binder on asolid substrate such as paper or glass. The binder providesprotection against hydrolysis of rhodamine derivatives, but oninteracting with Hg2+ the spirolactam ring structure of rhoda-mine is opened and a colour change takes place. A paper-basedfunctional device and a thin-lm sensor using an inkpenreversible colorimetric sensor have also been developed.

2. Results and discussion2.1 Synthesis and physical properties of the ligands

The sensors L1–L5 were synthesized by slight modication tothe methods described previously.13,18 Rhodamine B wasrespectively condensed with ethylenediamine, diethylenetri-amine, triethylenetetra-amine, tetraethylenepenta-amine andtris(2-aminoethyl)amine under reux for three days undernitrogen, to give chemosensors L1–L5 (Scheme 1). L1–L5 eachgive a colourless solution in dimethylsulfoxide (DMSO) and thesolutions do not exhibit uorescence, conrming that thesecompounds were predominantly in the spirolactam form. Thechange in uorescence intensity of each aer the addition of10 mM of a variety of cations was determined in order to assesstheir cation-binding ability. The results are summarised inFig. 1, and show that L2 gave the highest response. The effect ofother metal ions was determined by adding Hg2+ to a solution of

Scheme 1 Synthetic pathways to L1–L5.

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L2 in the presence of other metal ions. The results indicatedthat the sensing of Hg2+ by L2 was affected hardly at all by thepresence of commonly coexistent ions (Fig. 2).

2.2 Design and characterisation of a novel branched poly(L-lactide)maltitol (B-PLLA-M)

To improve the encapsulated chemosensor efficiency, thephysical properties and stability of the ink-based chemosensor,B-PLLA-M, were modied and assessed as a binder. Thesynthesis of B-PLLA-M at 140 �C using maltitol and stannousoctoate as activating system is described in ESI Scheme S1.† Thebranched polymer was characterized by FT-IR, 1H NMR, DSCand TGA. The 1H NMR spectrum (ESI Fig. S1†) of the branchedpolymer showed the CH2 of maltitol at 4.39 ppm and the CH2

and CH3 of poly(L-lactide) at 5.23 and 1.50 ppm, respectively.The FT-IR results (ESI Fig. S2†) showed that the OH stretchingband from maltitol and the C]O stretching band from thepoly(L-lactide) were located at 1745 and 3510 cm�1. TGA curves(ESI Fig. S3†) at 181–215 �C and 216–385 �C were attributable tothe weight loss of maltitol and poly(L-lactide), respectively.

2.3 Preparation of the inkpen reversible biodegradablesensor

The inkpen reversible biodegradable colorimetric sensor (IRBS)was developed using a three-component system. Variable ratiosof binder to chemosensor were examined and led to a binder:chemosensor weight ratio of 1 : 0.8, as demonstrated in ESIFig. S4.† Acetone was used as the solvent, due to its ability todissolve all the components and its high evaporation rate (ESITable S1†).

L2, a highly selective and sensitive uorescent chemosensorfor Hg2+, was entrapped within the B-PLLA-M microspheresusing a water–oil–water emulsication–solvent evaporationmethod. The formation of microparticles improved the waterresistance, in addition to the distribution of chemosensorwithin the binder, and increased the stacking of the micro-spheres and the amount of hydrophobic chemosensor in anaqueous dispersion,19 and also prevented the hydrolysis of thechemosensor.20 The inkpen sensor prepared by mixing thethree-component ink was able to write easily on paper, as

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Fluorescence responses of L1–L5 with 5 equiv. of various cations (10 mM of receptor in 0.01 mol L�1 of tetrabutylammonium hexa-fluorophosphate (TBAPF6) in MeCN).

Fig. 2 Increase in fluorescence response of L2 (10 mM in 0.01 mol L�1

of TBAPF6 in MeCN) to 5 equiv. of various cations (black bars) and to amixture of 0.01 M different metal ions with 67 mM of Hg2+ (grey bars);lex ¼ 520 nm, lex ¼ 580 nm.

Fig. 3 The response kinetics of IRBS papers with and without B-PLLA-M after dipping into a 8 � 10�8 M Hg2+ solution. Inset: the colourchanges of IRBS papers (a) without and (b) with binder.

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illustrated in ESI Fig. S5,† in exactly the same way as writing onordinary paper.21

To study the improvement in the physical properties andstability of the ink-based chemosensor, using B-PLLA-M asbinder and chemosensor protector, the response kinetics ofIRBS papers with and without B-PLLA-M were investigated bychecking the UV-Vis absorption of the aqueous solution using aHg2+ solution. Aer dipping into a 1 mM Hg2+ solution thecolour of both types of IRBS papers changed suddenly from paleyellow to pink, inducing ring-opening of the rhodamine spi-rolactam structure.21 In the case of IRBS papers with binder, noabsorption at 556 nm (ring-opened rhodamine peak) was foundover 10 min, indicating that the efficiency, physical propertiesand stability of the ink-based chemosensor were improved byusing B-PLLA-M as binder. On the other hand, IRBS paperswithout binder showed a statistically signicant increase inabsorbance at 584 nm, suggesting that water may have inducedhydrolysis of the rhodamine, conrmed by the change in thesolution from colourless to pink (Fig. 3).

The morphologies of IRBS in solution before and aeraddition of Hg2+ were characterized by SEM, as shown in

This journal is © The Royal Society of Chemistry 2014

Fig. 4. Shrinkage of the IRBS microparticles was observedaer adding Hg2+. This may have been caused byshrinkage due to solvent evaporation, or it might also beattributed to a reorganization of the chemosensor to form a2 : 1 complex,19 resulting in decreasing the space within themicrospheres.

In addition, to characterize the coatings on paper substrates,optical SEM images before and aer the addition of Hg2+ wereobtained. Fig. 5 shows optical prolometric images of the papercoating (Fig. 5(a) and (b)), and with IRBS in the presence of Hg2+

(Fig. 5(c) and (d)).The SEM graphs suggest that the chemosensor–binder-based

ink possessed excellent adhesion to the coated paper substrate,indicating that the compatibility between ink and the coatedpaper was functionally favourable. The complicated andaggregated nature of the surfaces present in both cases wasdemonstrated on the paper substrates. In addition, the SEMimage clearly changed upon addition of Hg2+. The cross-sectional SEM images (ESI Fig. S6†) suggested that the majorityof the sensing layer was due to the surface of the paper ratherthan its texture.

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Fig. 4 Typical SEM images of IRBS in solution (a) before and (b) afterthe addition of Hg2+.

Fig. 5 Typical SEM images of IRBS on paper (a and b) before and (c andd) after addition of Hg2+.

Fig. 6 Visual observation of the IRBS paper sensor before and aftertreatment with Hg2+ ions at concentrations between 4.0� 10�8 M and1.0 � 10�4 M.

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To test the performance of our system on paper, IRBSpapers were dipped in Hg2+ solutions at concentrationsbetween 4.0 � 10�8 M and 1.0 � 10�4 M. Images wereobtained aer 1 min with an LCD digital camera andappropriate lighting. The accuracy of determination of theHg2+ concentration using the colour intensity (utilizing theAdobe Photoshop programme) was conrmed by measuringthe cationic concentration of a sample prepared with tapwater. As shown in Fig. 6 and ESI Fig. S7,† clear visualdifferences were observed at concentrations ranging from 4.0� 10�8 M to 1.0 � 10�6 M. The colour of the images on IRBSpaper changed from pale yellow to pink, and the uorescentimages on IRBS paper showed a strong uorescence responseaer addition of Hg2+. The degree of change conrmed thehigh sensitivity of the IRBS paper sensor.

In addition, the plot of colour intensity versus Hg2+ concen-tration (ESI Fig. S8†) showed a bimodal response towards Hg2+,with two distinct linear ranges, 4.0 � 10�8 to 1.0 � 10�7 M and2.0 � 10�7 to 1.0 � 10�6 M. Higher concentrations gave lowersensitivity. The bimodal distribution of the linear range was aconsequence of the two sequential processes involved in thechemical binding of the reaction product to L2.22 As colour

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changes were compared to the blank, the detection limit wasseen to be 4.0 � 10�8 M.

We also evaluated the reversibility of the above Hg2+ detec-tion procedure in aqueous solutions treated with aqueous EDTAunder basic conditions.23 As expected, on dipping into the EDTAsolution the uorescence and colour intensity of IRBS papersplus Hg2+ were quenched. Aer washing the IRBS papers and re-exposure to Hg2+, both uorescence and colour intensity wererestored completely.

3. Conclusions

This study has demonstrated a new inkpen-based reversiblebiodegradable sensor, IRBS, for the detection of Hg2+ ion. Thesensor is portable, easy to use, inexpensive, and suitable for usein the eld. The sensing platform has a built-in detectionmechanism, with all the reagents needed for detectioncombined in a three-component ink (rhodamine derivativesand branched poly(L-lactide)maltitol (B-PLLA-M) in acetonesolution) for the detection of Hg2+ ion on the paper with thenaked eye. The colour change is concentration-dependent andprovides a semi-quantitative visual assessment of Hg2+ ionwithout additional instrumentation, together with the option ofusing more advanced colour quantication tools available fromAdobe Photoshop. The lower detection limit of the paper sensortowards Hg2+ (4.0 � 10�8 M) was less than that obtained fromL2 in solution (1.33 � 10�5 M), and the response time was lessthan 40 s. Reusability was evaluated by repeating the dippingand rinsing cycles in aqueous Hg2+ and EDTA solutions. Theapproach thus provides a simple but inherently sensitivemethod for the assessment of Hg2+ ion in environmental andbiological applications.

4. Experimental4.1 Chemicals and methods

All reagents were analytical grade. Rhodamine was purchasedfrom Aldrich, and ethylenediamine, diethylenetriamine, trie-thylenetetra-amine, tetraethylenepenta-amine, tris(2-amino-ethyl)amine and methanol were obtained from Merck and usedwithout further purication. Commercial grade solvents, suchas acetone, hexane, dichloromethane, methanol and ethyl

This journal is © The Royal Society of Chemistry 2014

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acetate, were distilled before use. Dimethyl formamide, aceto-nitrile and dichloromethane were dried over calcium hydrideand redistilled under nitrogen prior to use. Tetrahydrofuranwas dried using sodium benzophenoneketyl and distilled undernitrogen immediately before use.

4.2 Instrumentation

NMR spectra were recorded on a Varian 400 MHz spectrometerin deuterated chloroform and DMSO-d6. MALDI-TOF massspectra were recorded on a Biex Bruker mass spectrometerusing 2-cyano-4-hydroxycinnamic acid or 2,5-dihydroxybenzoicacid as matrix. UV-Vis absorption spectra were measured on aPerkin-Elmer Lambda 25 UV-Vis spectrophotometer. Fluores-cence spectra were recorded using a Perkin-Elmer luminescencespectrophotometer LS50B. Infrared spectra were obtained on aNicolet Impact 410 using KBr pellets. Column chromatographywas conducted using silica gel (Kieselgel 60, 0.063–0.200 mm,Merck).

The morphology and size of the hybrid material before andaer adding Hg2+ were observed by transmission electronmicroscopy (JEOL, JSM1230) at 20 kV. Prior to examination,composite samples were immersed in liquid nitrogen 30 minand then fractured. The specimens were sputter-coated withgold for enhanced surface conductivity.

4.3 Synthesis

4.3.1 Rhodamine derivatives. Rhodamine derivatives weresynthesized by adapting the procedure for similar compoundsreported in the literature.19 Rhodamine B (0.20 g, 0.42 mmol)was dissolved in 20 mL of ethanol, and ethylenediamine,diethylenetriamine, triethylenetetra-amine, tetraethylenepenta-amine or tris(2-aminoethyl)amine (0.22 mL, excess) were addeddropwise to the solution and reuxed overnight (24 h) until thesolution lost its red coloration. The solvent was evaporated offand water (20 mL) added to the residue, and the solutionextracted with methylene chloride (CH2Cl2; 20 mL � 2). Theorganic phase was combined and washed twice with water, thendried over anhydrous Na2SO4. The solvent was removed byevaporation, and the product dried in vacuo, giving pale yellowsolid compounds L1–L5.

L1: (yield 0.17 g, 84%). 1H NMR (400 MHz, CDCl3): d 7.86–7.81 (m, 1H, ArH), 7.45–7.32 (m, 2H, ArH), 7.08–7.03 (m, 1H,ArH), 6.42 (s, 1H, ArH), 6.39 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.38–6.21 (m, 2H, ArH), 3.32 (q, J ¼ 6.8 Hz, 8H, NCH2CH3), 3.12 (t, J¼6.8 Hz, 2H, NCH2CH2), 2.23 (t, J ¼ 6.8 Hz, 2H, NCH2CH2NH2),2.05 (s, 2H, CH2CH2NH2) and 1.16 (t, J ¼ 7.2 Hz, 12H,NCH2CH3).

MS (MALDI-TOF): calc. for [C30H36N4O2]+:m/z 484.28; found:

m/z 485.91 [M + H]+.L2: (yield 0.20 g, 91%). 1H NMR (400 MHz, CDCl3): d 7.90–

7.88 (m, 1H, ArH), 7.44–7.42 (m, 2H, ArH), 7.09–7.07 (m, 1H,ArH), 6.43 (s, 1H, ArH), 6.41 (s, 1H, ArH) 6.37 (s, 2H, ArH), 6.28–6.25 (m, 2H, ArH), 3.32 (q, J ¼ 6.8 Hz, 8H, NCH2CH3), 3.26 (t, J¼6.8 Hz, 2H, NCH2CH2), 2.59 (t, J ¼ 6.0 Hz, 2H, NCH2CH2NH),2.44–2.38 (m, 4H, NCH2CH2NH), 1.70 (s, 3H, NCH2CH2NH andCH2CH2NH2) and 1.16 (t, J ¼ 7.2 Hz, 12H, NCH2CH3).

This journal is © The Royal Society of Chemistry 2014

MS (MALDI-TOF): calc. for [C32H41N5O2]+:m/z 527.33; found:

m/z 528.95 [M + H]+.L3: (yield 0.23 g, 95%). 1H NMR (400 MHz, CDCl3): d 7.89–

7.88 (m, 1H, ArH), 7.48–7.43 (m, 2H, ArH), 7.08 (s, 1H, ArH),6.43–6.39 (m, 3H, ArH), 6.37 (s, 1H, ArH), 6.28–6.26 (m, 2H,ArH), 3.32 (q, J ¼ 6.8 Hz, 8H, NCH2CH3), 3.28–2.3 (m, 12H,NCH2CH2, NCH2CH2NH and NCH2CH2NH), 2.04 (s, 4H,NCH2CH2NH and CH2CH2NH2) and 1.16 (t, J ¼ 7.2 Hz, 12H,NCH2CH3).

MS (MALDI-TOF): calc. for [C34H46N6O2]+:m/z 570.37; found:

m/z 572.02 [M + H]+.L4: (yield 0.22 g, 87%). 1H NMR (400 MHz, CDCl3): d 7.82–

7.81 (m, 1H, ArH), 7.38–7.36 (m, 2H, ArH), 7.15 (s, 1H, ArH),6.37–6.34 (m, 2H, ArH), 6.30 (s, 2H, ArH), 6.21–6.19 (m, 2H,ArH), 3.25 (q, J ¼ 6.8 Hz, 8H, NCH2CH3), 3.20–2.1 (m, 21H,NCH2CH2, NCH2CH2NH, NCH2CH2NH, NCH2CH2NH andCH2CH2NH2) and 1.09 (t, J ¼ 7.2 Hz, 12H, NCH2CH3).

MS (MALDI-TOF): calc. for [C36H51N7O2]+:m/z 613.41; found:

m/z 615.86 [M + H]+.L5: (yield 0.19 g, 85%). 1H NMR (400MHz, CDCl3) d 7.85–7.80

(m, 1H, ArH), 7.40 (s, 2H, ArH), 7.05 (s, 1H, ArH), 6.33–6.24 (m,6H, ArH), 5.27 (bs, 2H, NCH2CH2N), 5.06–4.08 (bs, 4H, CH2-CH2NH2), 3.28 (bs, 8H, NCH2CH3), 3.11 (bs, 2H, NCH2CH2N),2.92–2.86 (m, 2H, NCH2CH2N), 2.61–2.55 (m, 2H, NCH2CH2N),2.31–2.29 (m, 2H, NCH2CH2N), 2.10–2.02 (m, 2H, NCH2CH2N),1.11 (bs, 12H, NCH2CH3).

MS (MALDI-TOF): calc. for [C34H46N6O2]+:m/z 570.37; found:

m/z 570.94 [M + H]+.4.3.2 Branched poly(L-lactide)maltitol (B-PLLA-M). B-PLLA-

M was obtained by direct melt ring-opening polycondensationof L-lactide on the hydroxyl groups of maltitol using a catalyticamount of Sn(II)2-ethylhexanoate (SnOct2). The poly-condensation was carried out under vacuum using L-lactide(10.00 g), maltitol (0.1722 g) and SnOct2 (0.0056 g). The mixtureof L-lactide and maltitol was placed in a round-bottomed askand magnetically stirred in an oil bath at 140 �C. The reactionwas continued for 24 h to obtain a viscous product. The crudeproduct was dissolved in chloroform, precipitated using coldhexane and washed several times with hexane before lteringand drying in vacuo at 60 �C for 48 h to obtain B-PLLA-M.

1H NMR (400 MHz, CDCl3): 5.16 (q, –OCH(CH3)C(O)– ofPLLA), 4.18 (d, –CH2 of maltitol), 1.55 (d, –OCH(CH3)C(O)– ofPLLA). FT-IR (KBr, cm�1): n 3510 and 3823 (O–H stretching),2947 and 2997 (C–H stretching), 1745 (C]O ester), 1452 (C–Hbending), 1136 and 1183 (C–O stretching), 756 (CH2 bending).

4.4 Complexation studies of chemosensors with cations

Studies of selectivity and sensitivity of the complexes of hybridmaterials towards cations, such as transition metals (Ni2+, Co2+,Cd2+, Hg2+, Zn2+, Ag+, Cu2+, Fe3+), alkali metals (Li+, Na+, K+) andalkaline earths (Ca2+, Mg2+), were carried out by FT-IR, UV, NMRor uorescence spectroscopy.

The complexation ability of hybrid sensors with cations wasinvestigated by spectrophotometric titration in DMSO at 25 �C.2 mL of the hybrid sensor solution were placed in a spectro-photometric cell (1 cm path length). The cation solutions of

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were added successively into the cell using a microburette. Themixture was stirred for 40 s aer each addition and the spec-trum recorded.

4.5 Competition experiments

Hg2+ was added to the solution containing chemosensors andother metal ions of interest. All test solutions were stirred for 1min and then allowed to stand at room temperature for 30 min.For uorescence measurements, the excitation wavelength was345 nm, and the emission spectrum was recorded between 355and 650 nm.

4.6 Preparation of IRBS

The chemosensor L2 0.0053 g and B-PLLA-M binder 0.0042 gwere dissolved in 10 mL acetone and the resulting ink solutionwas stored at 4 �C.

4.7 Preparation of inkpen-based sensor (IRBS)

The pen body was lled with the three-component ink to about6 mm from the top. The IRBS papers were stored undernitrogen.

4.8 Quantication of colour and calibration of the papersensor

A mobile Samsung Galaxy was used to take the digitalpictures of the IRBS paper sensor aer exposure to Hg2+

solution. The colour intensity was quantied using the eyedropper tool and the colour wheel chart in Adobe Photoshop(version CS). For example, the blue derivatives are thecomplementary colours (the negative images) of orange-brownish, while the greenish colour derivatives are comple-mentary colours of pinkish-red. The reaction of Hg2+ solutionwith the IRBS paper formed a pinkish-red colour on thesensor surface. The greenish colour intensity was thereforeused for quantication, as its complementary colourprovided greater sensitivity than the other colours in theAdobe Photoshop.

4.9 The reusable process of the IRBS paper sensor

An IRBS paper sensor was immersed in aqueous EDTA solutionunder basic conditions for 3 min. The IRBS paper was thenremoved from the solution, dried in a vacuum oven and storedunder nitrogen.

Acknowledgements

The authors gratefully acknowledge funding from Mahasarak-ham University and the Thailand Research Fund (RTA5380003),and the Center of Excellence for Innovation in Chemistry(PERCH-CIC), the Office of the Higher Education Commission,Ministry of Education. PSTAT mini was purchased using a loanfund from the Faculty of Science, MSU.

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RSC Adv., 2014, 4, 46145–46151 | 46151