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Cite this: NewJ.Chem., 2014,

38, 3831

Optical chemosensors for Hg2+ from terthiopheneappended rhodamine derivatives: FRET basedmolecular and in situ hybrid gold nanoparticlesensors†

Chatthai Kaewtong,*a Noi Niamsa,a Banchob Wanno,a Nongnit Morakot,a

Buncha Pulpokab and Thawatchai Tuntulanib

A multifunction sensor based on rhodamine B with a terthiophene substituent has been developed as a

highly sensitive chemosensor for Hg2+. This method is mainly based on Hg2+-induced spirocycle opening

leading to fluorescence and colorimetric enhancement. The binding properties of a terthiophene appended

rhodamine based fluorescence chemosensor (RhoT) was studied using the fluorescence resonance energy

transfer (FRET) process, and RhoT displayed highly selective fluorescence enhancement and a color change

in the presence of Hg2+. Moreover, the in situ formation of gold nanoparticle/conducting polymer

nanocomposite based-chemosensors (AuNPs-RhoT) was achieved to explore the sensitive and selective

detection of Hg2+ in aqueous solution over 12 other metal ions using a colorimetric technique. The hybrid

sensor became aggregated in solution in the presence of Hg2+ by an ion-templated chelation process,

which caused an easily measurable change in the emission spectrum of the particles and provided an

inherently sensitive method for Hg2+ detection in aqueous solution.

Introduction

Heavy metal ion pollution has been an important worldwideissue for years due to the severe risks posed to human healthand environment.1 Mercury is one of the most toxic heavymetals present in aquatic systems, remains for a long timeeven after the pollutant source is removed and causes seriousenvironmental and health problems because it can easily passthrough skin, respiratory, and gastrointestinal tissues into thehuman body, where it damages the central nervous and endo-crine systems.2 Many current techniques for mercury detection,such as anodic stripping voltammetry, X-ray fluorescencespectrometry, neutron activation analysis, inductively coupledplasma mass spectrometry, etc., usually require expansive andsophisticated instrumentation and/or complicated sample pre-paration processes.3 Therefore, it is important to explore new

methods with simple, safe, effective and rapid sensing to detectand remove Hg2+ in vitro and in vivo.

In the past few years, molecular sensor has become apowerful tool for sensing and imaging a trace amount of thesample because of its simplicity and sensitivity. The molecularsensors contain two basic functional units: a receptor unit and asignaling unit. The most common modes of signal transductiontypically involve electrochemical or optical changes in the sensorincurred by association of the analyte with the receptor. Amongthe various detection techniques, fluorescence sensors make thebest choice, since they are bestowed with high sensitivity, highselectivity, fast response, easy on-line detection and inexpensiveinstallations. Furthermore, remote sensing is possible by meansof optical fibers. Most fluorometric sensors have been developedto adopt photophysical changes produced upon guest complexa-tion, including photoinduced electron/energy transfer (PET),charge-transfer (CT) excited state, excimer/exciplex and fluores-cence resonance energy transfer (FRET).4 FRET is an active field insupramolecular chemistry due to its potential practical benefits incell physiology, optical therapy, as well as selective and sensitivesensing toward the targeted molecular or ionic species.5

Many recent developments have shown that rhodaminespirolactam is a promising structural scaffold for the design ofselective chemosensors. It can undergo a structural change from aspirolactam to an open ring amide by cations induced activationof a carbonyl group in the spirolactone or spirolactam moiety,

a Nanotechnology Research Unit and Supramolecular Chemistry Research Unit,

Department of Chemistry and Center of Excellence for Innovation in Chemistry,

Faculty of Science, Mahasarakham University, Mahasarakham, 44150, Thailand.

E-mail: kchatthai@gmail.com; Fax: +66 0437 54246; Tel: +66 0437 54246b Supramolecular Chemistry Research Unit, Department of Chemistry,

Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand.

Fax: +66 0221 87598; Tel: +66 0221 87643

† Electronic supplementary information (ESI) available: Spectroscopic data andcompound characterization data including proposed binding modes. See DOI:10.1039/c4nj00412d

Received (in Montpellier, France)19th March 2014,Accepted 27th May 2014

DOI: 10.1039/c4nj00412d

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resulting in a magenta-colored highly fluorescent compound.6

Rhodamine-based chemosensors have received ever-increasinginterest in the preparation of cations and other analytes such asPb2+, Cu2+, Hg2+, Fe3+, Cr3+, NO, and OCl�.7–14 In our previous work,we have successfully designed and synthesized ditopic receptorsand sensors for cations and anions, utilizing rhodamine B asthe fluorophore.15

Although many fluorescent chemosensors for Hg2+ have beenreported,16 some systems have exhibited limited features in theirpractical use such as poor aqueous solubility, low sensitivity andselectivity, short emission wavelengths, and/or weak fluorescenceintensities. Thus, the development of novel practical assays for Hg2+

remains a challenge. In recent years, nanostructured materials suchas carbon nanotubes, polymer nanotubes, metal nanotubes, andmetal nanoparticles have received a great deal of attention due totheir unique physical and chemical properties as well as theirenormous potential applications such as catalysts, nanoelectronicdevices, magnetic devices, sensors, and biomaterial separationmembranes.17 Specifically, nanoparticles of gold show distinctand well-defined plasmon absorption in the visible spectrum, anabsorption characterized by an extremely large molar absorptioncoefficient. Attention has recently focused on functionalizing colloi-dal nanoparticles with molecular recognition components forpotential sensing applications.18 Several methods for making goldnanoparticles by using chemical reduction of hydrogen tetrachloro-aurate(III) and gold(III) chloride have been developed.19 Recently,terthiophenes have been extensively investigated as the electroactivegroups which can be oxidized to prepare monodisperse goldnanoparticles and polymerized to conducting polymers.20 In 2006,rhodamine B (RB)-AuNP (RB-AuNP) was prepared via RB self-adsorption on the surface of AuNPs and used for detecting Hg(II)ions in aqueous solution. They improved the selectivity of the probefurther by modifying the AuNP surfaces with thiol ligands (MPA,MSA, and HCys) and adding PDAC to the sample solutions whichwas a complicated method of detecting Hg(II) ions.21

This paper focuses on the synthesis of a terthiophene function-alized rhodamine (RhoT) containing a terthiophene group (energydonor) and a rhodamine group (energy acceptor) as new compo-nents to generate FRET-based chemosensors and report a simplestep to produce gold nanoparticle/conducting polymer nanocom-posite based-chemosensors (AuNPs-RhoT) via simultaneousreduction of AuCl3 with oxidative polymerization of terthiophenefunctionalized rhodamine (RhoT), as illustrated in Scheme 1. Amultiple-site model of hybrid materials has been used to increasethe selectivity and sensitivity of the measurement. We believe thatthe relatively easy manipulation of rhodamine derivatives shouldinterest many organic chemists, analytical chemists and biochemistsinvolved in sensor technology for monitoring heavy metals.

Results and discussionSynthesis and characterization of fluorescence chemosensors

Fluorescence chemosensor (RhoT) was synthesized in a goodyield by an amidation reaction between 2-(2,5-di(thiophen-2-yl)-thiophen-3-yl)acetic acid (arylation of thiophene derivatives via

palladium-catalyzed coupling reactions, then hydrolyzationusing KOH in THF/MeOH) and rhodamine–ethylenediamine(condensation reaction of rhodamine B and with ethylenediamineunder N2 with refluxing for 3 days) in the presence of dicyclohexyl-carbodiimide/dimethylaminopyridine (DCC/DMAP) as a couplingreagent under N2 with refluxing for 3 days in THF to obtain RhoT(82%) as demonstrated in Scheme 2. The chemical structure andpurity of the fluorescence chemosensor were proven using mass,NMR and IR spectroscopy and designed to chelate with metal ionsvia its carbonyl O atoms of amide groups.

The characteristic FTIR of RhoT showed aromatic rhodamineand terthiophene peaks (1683, 1511 cm�1) and an amide peak(1789 cm�1). The MALDI-TOF mass spectrum (Fig. S8, ESI†)showed a mother peak at 773.388 m/z attributed to (M + H)+. The1H NMR spectrum of RhoT (Fig. S9, ESI†) showed the characteri-stic signals of –CH2 and –CH groups in the region of 1.0–2.1,2.2–2.6 ppm, respectively and aromatic proton at 7.8–5.55 ppm.Terthiophene without any substituents was synthesized througha Stille coupling reaction between commercial 2-(tributylstannyl)-thiophene and 2,5-dibromothiophene.20 The compound is usedto demonstrate that its emission (E500 nm) spectrum can overlap

Scheme 1 Fabrication of gold nanoparticles-based (AuNPs-RhoT) chemo-sensors from the FRET-based (RhoT) molecular sensor.

Scheme 2 Synthetic pathway of the FRET-based chemosensor (RhoT).

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with that of the ring-opened rhodamine B and can undergo theFRET phenomenon as shown in Fig. 1.

Complexation properties of RhoT

RhoT is colorless and fluorescence inactive in solution in eitherlow or high concentration, indicating that the spirolactam formpredominantly exists. The fluorescence intensity and absorptionspectra changes of chemosensors were investigated to determinetheir cation binding abilities. The fluorescence spectra of RhoTin the presence and absence of 10 mM of various cations wereinvestigated. A high-intensity fluorescence band at 578 nm andcolor change were observed upon addition of Hg2+ into solutionsof RhoT, no significant changes are promoted by other metalions (Fig. 2). The color and fluorescence changes of RhoT uponthe addition of Hg2+ are shown in Fig. S1 (ESI†). To furtherelucidate the binding ability, 1H NMR experiments were employedin DMSO-d6, and the spectra are depicted in Fig. S2 (ESI†). Thedownfield shift and broader peaks of all protons were observed,especially the aromatic protons, which may be due to Hg2+

complexation and conformational organization.UV-vis spectroscopy was also used to analyze binding abilities

of the complexes. A significant enhancement of the absorbanceintensity (at 555 nm) was observed after addition of Hg2+ to RhoTin DMSO as a result of the Hg2+-induced ring opening of thespirolactam form (Fig. S3, ESI†). Moreover, UV-vis data were

employed to determine the stoichiometry and stability constantsfor the complexes using the SIRKO program.22 According tothe observed absorption spectra changes (Fig. 3a), the resultsdemonstrated a 2 : 1 stoichiometry for the RhoT�Hg2+ complexwith an association constant (log Ka) of ca. 8.46 (0.01). The resultsare also consistent with those obtained from fluorescence experi-ments. Upon irradiation at 345 nm, the fluorescence spectrumof RhoT shifted to 578 nm, the region of the energy acceptor(FRET ON). A statistically significant increase (almost 300-foldenhancement of IF/I0) of fluorescence at 578 nm was observed,that is, Hg2+ ion induced the formation of ring-opened rhodaminewith a strong FRET process (Fig. 3a, Scheme 3).

Fig. 1 Spectral overlaps between terthiophene emission (black) and ring-opened rhodamine B absorption (red).

Fig. 2 Fluorescence enhancement response of RhoT (10 mM) in 0.01 mol L�1

of TBAPF6 in DMSO to 10 mM of various cations (the black bar portion) andto the mixture of 10 mM different metal ions with 10 mM of Hg2+ (the graybar portion).

Fig. 3 (a) Absorption spectra of RhoT (10 mM) in 0.01 mol L�1 of TBAPF6 inDMSO in the presence of different amounts of Hg2+. Inset: mol ratio plots byusing absorbance at 558 nm as a function of equivalent of Hg2+, indicating a1 : 2 metal–ligand ratio. (b) Fluorescence spectra of RhoT (10 mM) under thesame conditions. Excitation was performed at 345 nm. Inset: mol ratio plotsby using fluorescence at 575 nm, indicating a 1 : 2 metal–ligand ratio.

Scheme 3 Proposed binding mechanism of RhoT toHg2+.

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For more detailed studies of FRET occurring in RhoT + Hg2+,the titration experiment using UV-vis and fluorescence techniqueswere carried out. Upon portioned additions of 10 mM of Hg2+, thefluorescence intensity of the solution of RhoT undergoes adecrease of terthiophene emission (470 nm) and increase ofrhodamine emission (575 nm). In addition, UV-vis also showedthe characteristic peaks of the opened spirolactam ring of rhoda-mine moieties which strongly supported the FRET mechanism.To test the practical applicability of RhoT as a chemosensor forHg2+, competition experiments were carried out by adding variouscations to the aqueous solutions of RhoT in the presence of Hg2+

as shown in Fig. 2. RhoT exhibited a selective signaling behaviortoward Hg2+ in the presence of other physiologically metal ions.The fluorescence ratios (I578/I480) were obtained by treating RhoTwith 10 mM Hg2+ in the presence of 10 mM background metal ions.All background metal ions did not significantly interfere with thesensing of Hg2+. The competition experiments confirmed thatRhoT could be used as a potential Hg2+-selective FRET chemo-sensor in the presence of physiologically important metal ions.The RhoT based fluorescence and UV-vis absorption showed alimit of detection (LOD) of ca. 1.34 mM for Hg2+ detection.23

To explain the recognition abilities of RhoT towards Hg2+,the optimized geometries and the HOMO and LUMO energiesof the complexes were calculated using the density functionaltheory (DFT) at the B3LYP/LanL2DZ level.24 The optimizedstructures of RhoT complexes with Hg2+ are illustrated in Fig. 4and Fig. S4 (ESI†). The optimized location of Hg2+ was found

between the oxygen atoms of two acyclic lactam groups inrhodamine moieties, confirming that the complexation indeedrequires the participation of amide and opening of the spiro-lactam ring which would lead to an increase of the C�O bonddistance, which was supported by the previous reports.15 More-over, the negative and high values of calculated complexationenergies found in complex indicated that Hg2+ could formstable complexes with synthesized chemosensor. Interestingly,for the RhoT�Hg2+ complexation system (Table S1, ESI†), thecalculated complexation energy was lower for the 2 : 1 stoichio-metry (�350.47 kcal mol�1) than that of the 1 : 1 model(�276.28 kcal mol�1) which was in good agreement with earlierobservations (mole ratio plot and the SIRKO results). In addition,these structural changes were found to affect the HOMO/LUMOenergy levels of rhodamine chromophores as shown in Table S2and Fig. S4 (ESI†) (both HOMO and LUMO energy levels of theHg2+ complex became lower than those of the free chemosensor).Moreover, the energy gaps of the complex also decreased from thefree chemosensor. The calculation of the distance between thedonor and acceptor was estimated to be about 8.7 Å, whichstrongly supported the occurrence of the FRET process. The resultssuggested that RhoT formed stable complexes with Hg2+ through alarge number of cation–dipole and ion–ion interactions.

Preparation of the fluorescent and optical Au-NPs-basedsensors

As the minimum calculated complexation energy and moleratio plot method (Fig. 3) indicated a 2 : 1 stoichiometry, goldnanoparticles (AuNPs) were used to increase the preorganiza-tion of the system which resulted in higher binding propertiesand increased selectivity of the chemosensor. Several methodsfor making gold nanoparticles by using chemical reduction ofhydrogen tetrachloroaurate(III) and gold(III) chloride have beendeveloped. In this work, terthiophene was used as the electro-active groups which can be oxidized to prepare gold nano-particles and at the same time they can be polymerized toconducting polymers.20 Upon adding the AuCl3 solution intothe RhoT solution, the formation of hybrid sensor in particularcan be tracked by monitoring the change in UV-visible absor-bance (plasmon band at 440–490 nm) at various time intervals(Fig. S5, ESI†). The results showed that the complete fabricationof AuNP-based sensors was obtained after vigorously stirringthe reaction for 3 hours and showed a high stability in aqueoussolution at room temperature for at least one month.

The sensitivity of the hybrid sensor with cations was alsoinvestigated. The presence of Hg2+ let to a marked red shift andbroad distinct band, whereas the presence of other ions led tono significant changes even at concentrations as high as100 mM (Fig. S6, ESI†). In addition, the color of AuNPs-RhoTalso suddenly changed to the back color after adding Hg2+ ions(Fig. 5), which may stem from the aggregation of gold nano-particles induced by Hg2+ ions. The strong proof of this wasobserved from the TEM images of AuNPs-RhoT before and afteradding Hg2+ (Fig. 6), which demonstrated the clearly aggrega-tion of AuNPs-RhoT in the presence of Hg2+ ions. It has beennoted that at least two rhodamine units were bound to different

Fig. 4 The optimized structures of (a) L, (b) L–Hg2+ (1 : 1) and (c) 2L–Hg2+

(2 : 1) obtained at the B3LYP/LanL2DZ level of theory, the O–Hg bonddistances are in Å.

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gold nanoparticles which could have interacted with Hg2+

through chelation and/or electrostatic interactions. Moreover,the slight increase of fluorescence at 575 nm with increasingconcentration of Hg2+ was also detected (Fig. S6, ESI†), that is,Hg2+ ion induced the formation of ring-opened rhodamine(Scheme 4). Our results are consistent with the hypothesis thatrhodamine-functionlized gold nanoparticles (AuNPs-RhoT) canimprove the sensitivity toward Hg2+.

According to changes in UV-vis absorption and fluorescenceemission upon adding various Hg2+ concentrations, the limit ofdetection23 of AuNPs-RhoT for Hg2+ was 0.32 mM, which islower than that observed using RhoT (1.34 mM) and the detec-tion time was less than 30 seconds. As compared to RhoT,AuNPs-RhoT gave a lower detection limit and a higher sensi-tivity toward Hg2+.

Conclusion

Multifunction sensors based on rhodamine B with a terthiophenesubstituent have been developed as highly sensitive chemosensorsfor Hg2+: a fluorescence resonance energy transfer (FRET)rhodamine-based sensor (RhoT) and rhodamine-functionlizedgold nanoparticle (AuNPs-RhoT) sensors. A FRET sensor displayedhighly selective fluorescence enhancement and ‘‘naked-eye’’ inthe presence of Hg2+. Common co-existing metal ions displayedinsignificant interference to the detection of Hg2+. Theoreticalcalculations showed that a suitable number of O-donor atoms anddistance between rhodamine and terthiophene allowed the bestFRET sensor for Hg2+. In addition, the rhodamine-functionalizedgold nanoparticles (AuNPs-RhoT) were designed and synthesized.The Hg2+ chelation-induced aggregation of AuNPs resulted inTEM, color and fluorescence changes. The limit of detection ofAuNPs-RhoT for Hg2+ was 0.32 mM, lower than that observedusing RhoT (1.34 mM) and the detection time was less than30 seconds. We believe that this approach may provide an easilymeasurable and inherently sensitive method for Hg2+ detectionin environmental and biological applications.

Experimental sectionChemical and methods

All reagents were of standard analytical grade. AuCl3 andrhodamine were purchased from Aldrich. DCC, DMAP, ethylene-diamine, MeOH were from Merck and were used without furtherpurification. Commercial grade solvents, such as acetone, hexane,dichloromethane, methanol and ethyl acetate, were distilledbefore use. DMF was dried over CaH2 and freshly distilledunder nitrogen prior to use. Acetonitrile and dichloromethanefor the set-up reaction were dried over calcium hydride andfreshly distilled under a nitrogen atmosphere prior to use.Tetrahydrofuran was dried using sodium benzophenone ketyland immediately distilled under nitrogen before use.

Instrumentation

NMR spectra were recorded on a Varian 400 MHz spectrometerin deuterated chloroform and DMSO-d6. MALDI-TOF massspectra were recorded on a BiflexBruker mass spectrometerusing 2-cyano-4-hydroxycinnamic acid (CCA) or 2,5-dihydroxy-benzoic acid (DHB) as the matrix. UV-vis absorption measure-ments were performed on a Perkin Elmer Lambda 25 UV/VISspectrophotometer. Fluorescence spectra were recorded using aPerkin Elmer luminescence spectrophotometer LS50B. Infraredspectra were obtained on a Nicolet Impact 410 using KBrpellets. Column chromatography was carried out using silicagel (Kieselgel 60, 0.063–0.200 mm, Merck).

The morphology and size of the hybrid material before andafter added Hg2+ were observed under the TransmissionElectron Microscopy (TEM) (TEM; JEOL, JSM1230) operatedwith the voltage of 20 kV. Prior to examination, compositesamples were immersed in liquid nitrogen for 30 min and thenfractured. The specimens were sputter-coated with gold forenhanced surface conductivity.

Fig. 5 Color changes (A–C) of AuNPs-RhoT in the presence of 10 mM ofvarious metals. (A) RhoT only, (B) RhoT + Hg2+, (C) RhoT + other metalsin50 : 50, DMSO : H2O.

Fig. 6 TEM images of AuNPs-RhoT solution in the absence and presenceof Hg2+ ions in 50 : 50, DMSO : H2O.

Scheme 4 Proposed selective detection mechanism of Hg2+ by usingrhodamine-functionlized gold nanoparticles (AuNPs-RhoT).

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Synthesis

Ethyl-2,5-dibromothiophene-3-acetate (1)20. A solution of3.94 mL (77.00 mmol) of bromine was added to a solution of5.94 g (34.89 mmol) of ethyl thiophene-3-acetate in 100 mLof chloroform. The solution was stirred for 4 hours, and thenquenched with a 10% aqueous sodium hydroxide solution andwashed with distilled water, dried with anhydrous MgSO4, andevaporated. The resulting residue was distilled, under reducedpressure at 200 1C, yielding 1 (9.32 g, 82%).

Ethyl-2-(2,5-di(thiophen-2-yl)thiophen-3-yl)acetate (2). A solutionof 1 (3.20 g, 10.00 mmol) and 2-(tributylstannyl) thiophene (7.45 g,20.00 mmol) were added to a solution of dichlorobis-(triphenyl-phosphine) palladium (0.46 g, 0.4 mmol) in 5 mL of THF. Themixture was heated at 80 1C for 24 hours. The solvent was removedunder vacuum, and the residue was dissolved in CH2Cl2, washedwith water, and dried with anhydrous MgSO4. The crude productwas purified by chromatography on silica gel with toluene as theeluent to obtain 2 as a pale yellow solid (2.51 g, 77% yield).

2-(2,5-Di(thiophen-2-yl)thiophen-3-yl)acetic acid (3). A mixtureof 2 (3.40 g, 10.18 mmol) and KOH (5.71 g, 101.79 mmol) intetrahydrofuran/methanol (55 mL/110 mL) was refluxed withvigorous stirring overnight. After cooling to room temperature,the mixture was concentrated to dryness under reduced pressure.The residue was acidified to pH 2–3 with HCl, and then extractedwith ether. The combined organic layer was dried with anhydrousNa2SO4. After removal of the solvent, the residue was dried to give3 as a yellow solid (2.70 g, 81% yield). 1H NMR (400 MHz, CDCl3)d 7.40–7.03 (m, 7H, ArH), 3.82 (s, 2H, ArCH2COOH).

N-(Rhodamine B)lactam-ethylenediamine (Rho). RhodamineB (0.20 g, 0.42 mmol) was dissolved in 30 mL of ethanol andethylenediamine (0.22 mL, excess) was added dropwise to thesolution and refluxed overnight (24 hours) until the solution lostits red color. The solvent was removed using a rotary evaporator.Water (20 mL) was added to the resultant residue and thesolution extracted with CH2Cl2 (20 mL � 2). The combinedorganic phase was washed twice with water and dried overanhydrous Na2SO4. The solvent was removed by a rotary evapora-tor and dried in vacuo, affording a pale-yellow solid of 4 (0.17 g,yield 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 = 6.8 Hz, 8H, NCH2CH3), 3.12 (t, J = 6.8 Hz, 2H, NCH2CH2),2.23 (t, J = 6.8 Hz, 2H, NCH2CH2NH2), 2.05 (s, 2H, CH2CH2NH2)and 1.16 (t, J = 7.2 Hz, 12H, NCH2CH3). MS (MALDI-TOF) calcd for[C30H36N4O2]+: m/z 484.28. Found: m/z 485.91 [M + H]+.

N-(Rhodamine B)lactam-ethylenediamine-2-(2,5-di(thiophen-2-yl)thiophen-3-yl)acetate (RhoT). RhoT was synthesized from thereaction of compound Rho (0.20 g, 0.65 mmol) and 3 (0.15 g,0.82 mmol) in the presence of dicyclohexylcarbodiimide (DCC)(0.20 g, 0.98 mmol) and p-(dimethylamino)pyridine (DMAP)(0.12 g, 0.98 mmol) in 20 mL of anhydrous THF. After purificationby column chromatography on silica gel (CH2Cl2/n-hexane (1 : 1)),RhoT was obtained as an orange crystal (0.13 g, 82% yield). 1HNMR(400 MHz, CDCl3) 7.71 (d, J = 7.6 Hz, 1H, ArH), 7.35–7.26(m, 3H, ArH), 7.15–7.11 (m, 2H, ArH), 7.03–7.01 (m, 2H, ArH),

6.97–6.90 (m, 3H, ArH), 6.66 (bs, 1H, CH2NHCO), 6.28–6.26(m, 1H, ArH) 6.23 (s, 1H, ArH), 6.21 (s, 1H, ArH), 6.11–6.07 (m,2H, ArH), 3.98–3.92 (m, 2H, ArCH2CO), 3.52 (s, 1H, NCH2CH2),3.46–3.36 (m, 2H, NCH2CH2N), 3.20 (q, J = 7.6 Hz, 8H,NCH2CH3), 2.87–2.84 (m, 1H, NCH2CH2N), 1.05 (t, J = 7.2 Hz,12H, NCH2CH3). IR (KBr) n 3385, 3240, 2971, 1789, 1683, 1526,1511, 1312, 1277, 1202, 1103, 779 and 693 cm�1. MS (MALDI-TOF)calcd for [C44H44N4O3S3]+: m/z 772.26. Found: m/z 773.388 [M + H]+.

Preparation of AuNPs-RhoT. Au-NPs encapsulated RhoT wasprepared by adding an aliquot of 100 mL of AuCl3 (1 � 10�3 M)in water into a solution (1 � 10�3 M) of compounds RhoT intoluene. These pale yellow solutions were vigorously stirred for30 min to allow the reduction of Au3+ to zero valent Au-NPssimultaneously with the oxidative chemical polymerization ofterthiophene on the Au-NP surface.

Complexation studies of chemosensors with cations. Studiesof selectivity and sensitivity of the complexes of hybrid materialstowards cations such as transition metals (Ni2+, Co2+, Cd2+, Hg2+,Zn2+, Ag+, Cu2+, Fe3+), alkali metals (Li+, Na+, K+) and alkali earthmetals (Ca2+, Mg2+) were carried out by FTIR, UV, NMR, orfluorescence spectroscopy.

The complexation abilities of hybrid sensors with cationswere investigated by spectrophotometric titration in DMSO at25 1C. 2 mL of the hybrid sensors solution were placed in aspectrophotometric cell (1 cm path length). The solutions ofcations were added successively into the cell from a microburette.The mixture was stirred for 40 seconds after each addition and itsspectrum was recorded.

Competition experiments. Hg2+ was added to the solutioncontaining chemosensors and other metal ions of interest. Alltest solutions were stirred for 1 min and then allowed to standat room temperature for 30 min. For fluorescence measure-ments, the excitation wavelength was 345 nm, and the emissionspectrum was collected from 355 to 650 nm.

Acknowledgements

The authors gratefully acknowledge funding from theMahasarakham University and the Thailand Research Fund(RTA5380003) and the Center of Excellence for Innovation inChemistry (PERCH-CIC), Office of the Higher Education Com-mission, Ministry of Education. PSTAT mini was purchasedwith a loan fund from the Faculty of Science MSU.

Notes and references

1 (a) G. F. Nordberg, B. A. Fowler, M. Nordberg and L. Frigerg,Handbook on the Toxicology of Metals, Academic Press,Burlington, MA, 3rd edn, 2007; (b) T. Hosono, C. C. Su,F. Siringan and S. C. Onodera, Mar. Pollut. Bull., 2010, 60,780–785; (c) H. Ali, E. Khan and M. A. Sajad, Chemosphere,2013, 91(7), 869–1072.

2 (a) L. D. Hylander and M. E. Goodsite, Sci. Total Environ.,2006, 368, 352; (b) E. M. Nolan and S. J. Lippard, Chem. Rev.,2008, 108, 3443–3480; (c) D. P. Wojcik, M. E. Godfrey,

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3 (a) S. Anandhakumar, J. Mathiyarasua and K. L. N. Phania,Anal. Methods, 2012, 4, 2486–2489; (b) J.-F. Huang andB.-T. Lin, Analyst, 2009, 134, 2306–2313; (c) S. P. Wang,E. S. Forzani and N. J. Tao, Anal. Chem., 2007, 79,4427–4432; (d) O. T. Butler, J. M. Cook, C. F. Harrington,S. J. Hill, J. Rieuwerts and D. L. Miles, J. Anal. At. Spectrom.,2006, 21, 217–243; (e) Y. Li, C. Chen, B. Li, J. Sun, J. Wang,Y. Gao, Y. Zhao and Z. Chai, J. Anal. At. Spectrom., 2006, 21,94–96; ( f ) M. Leermakers, W. Baeyens, P. Quevauviller andM. Horvat, TrAC, Trends Anal. Chem., 2005, 24, 383–393;(g) M. Alcalde-Molina, J. Ruiz-Jimenez and M. D. Luque deCastro, Anal. Chim. Acta, 2009, 652, 148–153; (h) X. Yuan,R. L. Chapman and Z. Wu, Phytochem. Anal., 2011, 22(3),189–198.

4 (a) A. P. de Silva, H. Q. Gunaratne, T. Gunnlaugsson,A. J. M. Huxley, C. P. McCoy, J. T. Rademacher andT. E. Rice, Chem. Rev., 1997, 97, 1515–1566; (b) Z. Zhou,M. Yu, H. Yang, K. Huang, F. Li, T. Yi and C. Huang, Chem.Commun., 2008, 3387; (c) J. S. Kim and D. T. Quang, Chem.Rev., 2007, 107, 3780–3799; (d) A. K. Mahapatra, R. Maji,K. Maiti, S. S. Adhikari, C. D. Mukhopadhyay andD. Mandal, Analyst, 2014, 139, 309–317; (e) S. Sarkar,S. Roy, A. Sikdar, R. N. Saha and S. S. Panja, Analyst, 2013,138, 7119–7126; ( f ) A. Ganguly, B. K. Paul, S. Ghosh, S. Karand N. Guchhait, Analyst, 2013, 138, 6532–6541; (g) S.-Y.Jiao, L.-L. Peng, K. Li, Y.-M. Xie, M.-Z. Ao, X. Wang andX.-Q. Yu, Analyst, 2013, 138, 5762–5768.

5 (a) N. Wache, A. Scholten, T. Kluner, K.-W. Koch andJ. Christoffers, Eur. J. Org. Chem., 2012, 5712–5722;(b) Z. Liu, W. He and Z. Guo, Chem. Soc. Rev., 2013, 42,1568–1600; (c) J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang,Chem. Soc. Rev., 2011, 40, 3483–3495; (d) Y. H. Lee,M. H. Lee, J. F. Zhang and J. S. Kim, J. Org. Chem., 2010,75, 7159–7165; (e) Y. Kurishita, T. Kohira, A. Ojida andI. Hamachi, J. Am. Chem. Soc., 2010, 132, 13290–13299;( f ) H. J. Kim, S. Y. Park, S. Yoon and J. S. Kim, Tetrahedron,2008, 64, 1294–1300; (g) B. Liu, F. Zeng, G. Wu and S. Wu,Analyst, 2012, 137, 3717–3724.

6 (a) B. Jacobs, M. D. Allendorf and J. R. Long, Chem. Mater.,2010, 22, 4120; (b) E. Kim, H. J. Kim, D. R. Bae, S. J. Lee,E. J. Cho, M. R. Seo, J. S. Kim and J. H. Jung, New J. Chem.,2008, 32, 1003; (c) J. F. Zhang, Y. Zhou, J. Yoon and J. S. Kim,Chem. Soc. Rev., 2011, 40, 3416; (d) H. Son, H. Y. Lee,J. M. Lim, D. Kang, W. S. Han, S. S. Lee and J. H. Jung,Chem. – Eur. J., 2010, 16, 11549; (e) K. Saha, S. S. Agasti,C. Kim, X. Li and V. M. Rotello, Chem. Rev., 2012, 112, 2739;( f ) A. M. Breul, M. D. Hager and U. S. Schubert, Chem. Soc.

Rev., 2013, 42, 5366; (g) L. Yuan, W. Lin, K. Zheng, L. He andW. Huang, Chem. Soc. Rev., 2013, 42, 622.

7 For Pb2+: (a) J. Y. Kwon, Y. J. Jang, Y. J. Lee, K. M. Kim,M. S. Seo, W. Nam and J. Yoon, J. Am. Chem. Soc., 2005, 127,10107–10111; (b) Z.-Q. Hu, C.-S. Lin, X.-M. Wang, L. Ding,C.-L. Cui, S.-F. Liu and H. Y. Lu, Chem. Commun., 2010, 46,3765–3767; (c) C.-Y. Li, Y. Zhou, Y.-F. Li, X.-F. Kong, C.-X.Zou and C. Weng, Anal. Chim. Acta, 2013, 774, 79–84;(d) H. Ju, M. H. Lee, J. Kim, J. S. Kim and J. Kim, Talanta,2011, 83(5), 1359.

8 For Cu2+: (a) Z. Yang, M. She, J. Zhang, X. Chen, Y. Huang,H. Zhu, P. Liu, J. Li and Z. Shi, Sens. Actuators, B, 2013, 176,482–487; (b) L. Yuan, W. Lin, B. Chen and Y. Xie, Org. Lett.,2012, 14, 432–435; (c) M. Kumar, N. Kumar, V. Bhalla,P. R. Sharma and T. Kaur, Org. Lett., 2012, 14, 406–409;(d) J. F. Zhang, Y. Zhou, J. Yoon, Y. Kim, S. J. Kim andJ. S. Kim, Org. Lett., 2010, 12, 3852–3855; (e) C. Yu, J. Zhang,R. Wang and L. Chen, Org. Biomol. Chem., 2010, 8,5277–5279; ( f ) G. He, X. Zhang, C. He, X. Zhao andC. Duan, Tetrahedron, 2010, 66, 9762–9768; (g) Z. Jiang,S. Tian, C. Wei, T. Ni, Y. Li, L. Dai and D. Zhang, Sens.Actuators, B, 2013, 184, 106–112; (h) A. Sikdar, S. Roy,K. Haldar, S. Sarkar and S. S. Panja, J. Fluoresc., 2013,23(3), 495–501; (i) X. Qiu, S. Han and M. Gao, J. Mater.Chem. A, 2013, 1(4), 1319–1325; ( j ) H. She, F. Song, J. Xu,X. Xiong, G. Chen, J. Fan, S. Sun and X. Peng, Chem. – Asian J.,2013, 8, 2762–2767; (k) L. Feng, H. Li, Y. Lv and Y. Guan,Analyst, 2012, 137, 5829–5833.

9 For Hg2+ (irreversible sensor): (a) Z.-Q. Hu, W.-M. Zhuang,M. Li, M.-D. Liu, L.-R. Wen and C.-X. Li, Dyes Pigm., 2013,98(2), 286–289; (b) W. Shi and H. Ma, Chem. Commun., 2008,1856–1858; (c) X. Zhang, Y. Xiao and X. Qian, Angew. Chem.,Int. Ed., 2008, 47, 8025–8029; (d) J. J. Du, J. L. Fan, X. J. Peng,P. P. Sun, J. Y. Wang, H. L. Li and S. G. Sun, Org. Lett., 2010,12, 476–479; (e) M. G. Choi, D. H. Ryu, H. L. Jeon, S. Cha,J. Cho, H. H. Joo, K. S. Hong, C. Lee, S. Ahn and S. K. Chang,Org. Lett., 2008, 10, 3717–3720; ( f ) J. F. Zhang, C. S. Lim,B. R. Cho and J. S. Kim, Talanta, 2010, 83, 658–662;(g) H. Lu, S. Qi, J. MacK, Z. Li, J. Lei, N. Kobayashi andZ. Shen, J. Mater. Chem., 2011, 21(29), 10878–10882.

10 For Hg2+ (reversible sensor): (a) N. Zhang, G. Li, J. Zhao andT. Liu, New J. Chem., 2013, 37, 458–463; (b) X. Zhan, Z. Qian,H. Zheng, B. Su, Z. Lan and J. Xu, Chem. Commun., 2008,1859–1861; (c) M. Kumar, N. Kumar, V. Bhalla, H. Singh,P. R. Sharma and T. Kaur, Org. Lett., 2011, 13, 1422–1425;(d) J. Huang, Y. Xu and X. Qian, J. Org. Chem., 2009, 74,2167–2170; (e) V. Bhalla, M. R. Kumar, P. R. Sharma andT. Kaur, Inorg. Chem., 2012, 51, 2150–2156; ( f ) C. Wang andK. M.-C. Wong, Inorg. Chem., 2011, 50, 5333–5335; (g) Z. Jin,D.-X. Xie, X.-B. Zhang, Y.-J. Gong and W. Tan, Anal. Chem.,2012, 84, 4253–4257; (h) Y. Zhou, X.-Y. You, Y. Fang, J.-Y. Li,K. Liu and C. Yao, Org. Biomol. Chem., 2010, 8, 4819–4822;(i) Z. Dong, X. Tian, Y. Chen, J. Hou, Y. Guo, J. Sun andJ. Ma, Dyes Pigm., 2013, 97(2), 324–329.

11 For Fe3+: (a) N. R. Chereddy, S. Thennarasu and A. B.Mandal, Analyst, 2013, 138, 1334–1337; (b) M. H. Lee,

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T. V. Giap, S. H. Kim, Y. H. Lee, C. Kang and J. S. Kim, Chem.Commun., 2010, 46, 1407–1409; (c) A. J. Weerasinghe,C. Schmiesing, S. Varaganti, G. Ramakrishna and E. Sinn,J. Phys. Chem. B, 2010, 114, 9413–9419; (d) Z. Yang, M. She,B. Yin, J. Cui, Y. Zhang, W. Sun, J. Li and Z. Shi, J. Org.Chem., 2012, 77, 1143–1147; (e) L. Zhang, J. Fan and X. Peng,Spectrochim. Acta, Part A, 2009, 73A, 398–402; ( f ) L. Dong,C. Wu, X. Zeng, L. Mu, S.-F. Xue, Z. Tao and J.-X. Zhang,Sens. Actuators, B, 2010, 145, 433–437; (g) B. Ma, S. Wu,F. Zeng, Y. Luo, J. Zhao and Z. Tong, Nanotechnology, 2010,21, 195501–195508; (h) B. Bag and B. Biswal, Org. Biomol.Chem., 2012, 10(14), 2733–2738; (i) H. Ouyang, Y. Gao andY. Yuan, Tetrahedron Lett., 2013, 54(23), 2964–2966;( j ) R. Czoik, L. Zur, B. Szpikowska-Sroka, J. Połedniok,A. S. Swinarew and W. A. Pisarski, J. Lumin., 2013, 139,35–39.

12 For Cr3+: (a) Z. Zhou, M. Yu, H. Yang, K. Huang, F. Li, T. Yiand Ch. Huang, Chem. Commun., 2008, 3387–3389;(b) K. Huang, H. Yang, Z. Zhou, M. Yu, F. Li, X. Gao, T. Yiand Ch. Huang, Org. Lett., 2008, 10, 2557–2560;(c) A. J. Weerasinghe, C. Schmiesing and E. Sinn, Tetra-hedron Lett., 2009, 50, 6407–6410; (d) Y. Wan, Q. Guo,X. Wang and A. Xia, Anal. Chim. Acta, 2010, 665, 215–220;(e) H. Ouyang, Y. Gao and Y. Yuan, Tetrahedron Lett., 2013,54(23), 2964–2966; ( f ) N. R. Chereddy, S. Thennarasu andA. B. Mandal, Dalton Trans., 2012, 41(38), 11753–11759.

13 For Noble-Metal Ions (gold, silver, platinum, and palladium):(a) A. Chatterjee, M. Santra, N. Won, S. Kim, J. K. Kim,S. B. Kim and K. H. Ahn, J. Am. Chem. Soc., 2009, 131,2040–2041; (b) M. J. Jou, X. Chen, K. M. K. Swamy,H. N. Kim, H.-J. Kim, S.-G. Lee and J. Yoon, Chem. Commun.,2009, 7218–7220; (c) O. A. Egorova, H. Seo, A. Chatterjee andK. H. Ahn, Org. Lett., 2010, 12, 401–403; (d) H. Li, J. Fan, J. Du,K. Guo, S. Sun, X. Liu and X. Peng, Chem. Commun., 2010, 46,1079–1081; (e) M. E. Jun and K. H. Ahn, Org. Lett., 2010, 12,2790–2793; ( f ) H. Kim, K.-S. Moon, S. Shim and J. Tae, Chem.– Asian J., 2011, 6, 1987–1991.

14 For NO and OCl: (a) H. Zheng, G. Shang, S. Yang, X. Gao andJ. Xu, Org. Lett., 2008, 10, 2357–2360; (b) X. Chen, X. Wang,S. Wang, W. Shi, K. Wang and H. Ma, Chem. – Eur. J., 2008,14, 4719–4724; (c) X. Chen, K.-A. Lee, E.-M. Ha, K. M. Lee,Y. Y. Seo, H. K. Choi, H. N. Kim, M. J. Kim, C.-S. Cho,S. Y. Lee and W. J. Lee, Chem. Commun., 2011, 47, 4373–4375;(d) L. Long, D. Zhang, X. Li, J. Zhang, C. Zhang and L. Zhou, Anal.Chim. Acta, 2013, 775, 100–105; (e) F. Wei, Y. Lu, S. He, L. Zhaoand X. Zeng, Anal. Methods, 2012, 4, 616–618; ( f ) N. Kandoth,M. Malanga, A. Fraix, L. Jicsinszky, E. Fenyvesi, T. Parisi, I. Colao,M. T. Sciortino and S. Sortino, Chem. – Asian J., 2012, 7,2888–2894.

15 (a) C. Kaewtong, J. Noiseephum, Y. Uppa, N. Morakot,N. Morakot, B. Wanno, T. Tuntulani and B. Pulpoka, NewJ. Chem., 2010, 34, 1104–1108; (b) C. Kaewtong, B. Wanno,Y. Uppa, N. Morakot, B. Pulpoka and T. Tuntulani, DaltonTrans., 2011, 40, 12578–12583; (c) N. Niamsa, C. Kaewtong,V. Srinonmuang, B. Wanno, B. Pulpoka and T. Tuntulani,Polym. Chem., 2013, 4, 3039–3046.

16 (a) J. F. Zhang and J. S. Kim, Anal. Sci., 2009, 25(11),1271–1281; (b) J. M. Thomas, R. Ting and D. M. Perrin,Org. Biomol. Chem., 2004, 2, 307–312; (c) A. Ono andH. Togashi, Angew. Chem., Int. Ed., 2004, 43, 4300–4302;(d) I.-B. Kim and U. H. F. Bunz, J. Am. Chem. Soc., 2006, 128,2818–2819; (e) M. Likun, L. Xingfen, F. Quli and H. Wei,Prog. Chem., 2010, 22(12), 2338–2352.

17 (a) K. Singh, D. Sareen, P. Kaur, H. Miyake and H. Tsukube,Chem. – Eur. J., 2013, 19, 6914–6936; (b) N. R. Shiju andV. V. Guliants, Appl. Catal., A, 2009, 356, 1–17; (c) R. Yu,Q. Lin, S.-F. Leung and Z. Fan, Nano Energy, 2012, 1, 57–72;(d) C. Granata, E. Esposito, A. Vettoliere, L. Petti andM. Russo, Nanotechnology, 2008, 19, 275501–275506; (e) D. A.Bernards, K. D. Lance, N. A. Ciaccio and T. A. Desai, NanoLett., 2012, 12(10), 5355–5361; ( f ) R. S. Dey and C. R. Raj,Chem. – Asian J., 2012, 7, 417–424; (g) C. Wang, L. Xu,Y. Wang, D. Zhang, X. Shi, F. Dong, K. Yu, Q. Lin andB. Yang, Chem. – Asian J., 2012, 7, 1652–1656.

18 (a) E. Boisselier and D. Astruc, Chem. Soc. Rev., 2009, 38,1759–1782; (b) M. Hu, J. Chen, Z.-Y. Li, L. Au, G. V. Hartland,X. Li, M. Marquez and Y. Xia, Chem. Soc. Rev., 2006, 35,1084–1094; (c) K. G. Thomas and P. V. Kamat, Acc. Chem.Res., 2003, 36(12), 888–898; (d) D. A. Giljohann, D. S.Seferos, W. L. Daniel, M. D. Massich, P. C. Patel andC. A. Mirkin, Angew. Chem., Int. Ed., 2010, 49, 3280–3294.

19 (a) J. Turkevich, P. C. Stevenson and J. Hillier, Discuss.Faraday Soc., 1951, 11, 55–75; (b) S. T. Selvan, Chem. Com-mun., 1998, 351–352; (c) S. K. Pillalamarri, F. D. Blum,A. T. Tokuhiro and M. F. Bertino, Chem. Mater., 2005, 17,5941–5944; (d) Y. Tan, Y. Li and D. Zhu, Synth. Met., 2003,847, 135–136; (e) J. H. Youk, J. Locklin, C. Xia, M.-K. Parkand R. Advincula, Langmuir, 2001, 17, 4681–4683; ( f ) S. S.Kumar, C. S. Kumar, J. Mathiyarasu and K. L. Phani, Langmuir,2007, 23, 3401–3408.

20 C. Kaewtong, G. Jiang, R. Ponnapati, B. Pulpoka andR. Advincula, Soft Matter, 2010, 6, 5316–5319.

21 C.-C. Huang and H.-T. Chang, Anal. Chem., 2006, 78, 8332–8338.22 (a) V. I. Vetrogon, N. G. Lukyanenko, M. J. Schwing-well and

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23 (a) M. Shortreed, R. Kopelman, M. Kuhn and B. Hoyland,Anal. Chem., 1996, 68, 1414–1418; (b) H. Y. Chang,T. M. Hsiung, Y. F. Huang and C. C. Huang, Environ. Sci.Technol., 2011, 45(4), 1534–1539.

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