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Europium-Coordinated Gold Nanoparticles on Paper for the Colorimetric Detection of Arsenic (III, V) in Aqueous Solution Peuli Nath, Nivedita Priyadarshni, and Nripen Chanda ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00038 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Nano Materials

Europium-Coordinated Gold Nanoparticles on Paper for the Colorimetric Detection of Arsenic (III, V) in Aqueous Solution Peuli Nath,a,b Nivedita Priyadarshni,a,b and Nripen Chandaa,b* a

Micro System Technology Laboratory, CSIR-Central Mechanical Engineering Research Institute, MG Avenue, Durgapur 713209, India. b Academy of Scientific and Innovative Research (AcSIR), CSIR Campus, CSIR Road, Taramani, Chennai 600113, India *Corresponding author, E-mail- [email protected]; Fax: +91-(0343)2546745; Tel: +91-9933034370 KEYWORDS. Gold nanoparticles, europium, nanosensor, arsenic, detection, quantification. ABSTRACT: Europium-functionalized gold nanoparticle is developed as a sensor for highly sensitive and specific detection of the very low concentration of AsIII and AsV ions in water and using the paper strip. GNP-MMT@Eu nanosensor is synthesized by stepwise chemical conjugations of gold nanoparticle (GNP) with 2-mercapto-4-methyl-5-thiazoleacetic acid (MMT) followed by europium chloride (EuCl3) in DI water. GNP-MMT@Eu shows a visible color change in presence of both As III and AsV ions in an aqueous medium, because of arsenic mediated aggregation through electrostatic attraction and covalent type interaction that form innersphere arsenic complex between nanoparticles, which is proportional to the concentration of arsenic. The fluorometric properties of the nanosensor are not significant and thus, only colorimetric and spectroscopic methods that are very much selective for AsIII and AsV ions are used with a detection limit of ≤10.0 ppb. GNP-MMT@Eu also shows excellent capabilities for regeneration and quantitative estimation of total dissolved arsenic in real water sample, signifying the usefulness of the developed nanosensor for fieldtest applications such as arsenic level screening during the water quality monitoring process.

Gold nanoparticles (GNP), owing to unique optical properties, provide extraordinary opportunities for realistic color-based sensing applications. The size-dependent optical properties of spherical GNP are highly sensitive to the local dielectric environment and thus easily changed upon interaction with external analytes. This inherent sensitivity towards external species makes GNP ideal for the detection of various biologically and environmentally relevant species including metal toxins.1-7 Among various heavy metal toxins, arsenic is one of the most toxic element present in water for which WHO set a safe permissible limit of 10 ppb above which the water is unfit for drinking. In ground water, arsenic is predominant mainly in trivalent AsIII and pentavalent AsV states at pH~7.o. Among them, AsV (arsenate) is more dominant in upper oxygenated ground water as monovalent H2AsO4- and divalent HAsO42- species, though AsIII (arsenite) is more toxic form and present in water as uncharged H3AsO3 species in deep ground water.8 Pro-longed drinking of arsenic contaminated water can cause several health risks like liver and kidney damage; lung and skin cancer.9-10 Laboratory techniques like Atomic Absorption Spectrometry (AAS), Atomic Fluorescence Spectrometry (AFS), and Inductively Coupled Plasma with Mass Spectrometry (ICPMS) are accurate in determining trace level concentration of arsenic but are highly expensive, require sophisticated laboratory set up and trained personnel to operate these instruments.11-13 Moreover, these techniques cannot be implanted in portable devices for on-site applications. GNP based systems, on the other hand, can be used for quick detection of such toxic ions due to analyte induced aggregation property leading to distinct visible color change during analysis. Kalluri et al. developed gold nanoparticle functionalized with thiol-based compound glutathione (GSH), dithiothreitol (DTT) and cysteine (CYST) for colorimetric detection of AsIII ion in water. In their studies, the reported detection limit is 5 ppb for DTT capped GNP, 20 ppb for GSH capped GNP and 25 ppb for CYST conjugated GNP.14 Yu et al. developed aptamer-based arsenic detection method using gold nanoparticles. The aptamer functionalized GNP remains stabilized and red in color in aqueous solution but becomes unstable in presence of AsIII ion resulting in an aggregation based color change with a detection limit of 1.26 ppb.15 Xia and co-workers reported a novel detection system using unmodified GNP and a phytochelatin-like peptide (g-Glu-Cys)3-Gly-Arg (PC3R) for the detection of AsIII

ion. In their study, PC3R, an oligomer of glutathione forms a chemical complex in presence of AsIII ion as arsenic has a strong affinity towards thiol compound, thus preventing the aggregation of GNP and retains its red color in solution. In absence of AsIII, the peptide molecules bind to GNP surface forming aggregates and change in color from red to blue is observed.16 Recently our group designed a gold-thioguanine based nanosensor for effective detection of AsIII ion in aqueous sample up to a detection limit of 10 ppb in solution.17 Due to high extinction coefficient and unique size based optical properties; GNP shows visual color change upon aggregation with AsIII ion, demonstrating its outstanding ability for solution based analysis. In this way, researchers have developed various gold nanoparticle based sensors for the detection of AsIII containing species in water. However, to the best of our knowledge, gold nanoparticle based sensing technology has not received much attention on miniature scale screening of arsenic where both AsIII and AsV states are simultaneously present in water and would be user-friendly, low cost and safe-to-use. Motivated by these clear demands, we hypothesize EuIII functionalized gold nanoparticle for the detection of both oxidation states of arsenic (AsIII and AsV) in water sample. Designing of functional nanomaterials with monolayer of lanthanide complex is the current trend for detecting metal ions in environmental samples. Ipe et al. developed lanthanide capped gold nanoparticles for sensing alkaline earth (CaII, MgII) and transition metal ions based on substitution of the attached Eu(III)/Tb(III) ion from gold nanoparticle surface, that causes lowering of the emission of the sensor.18 More recently, lanthanide doped hybrid nanoparticles were used by Ghosh et al for the detection of arsenic in aqueous solution. They have synthesized EuIII doped YPO4 nanorod by coprecipitation method and observed the enhancement of luminescence property of EuIII ion in presence of arsenate and quenching of the same in presence of the arsenite. The drawback of this system is that the detection limit of this is ~6.0 ppm which is way higher than WHO safe limit.19 Below this concentration, the researcher did not find any change in the luminescence behaviour of EuIII doped YPO4 nanoparticles. Also, none of the lanthanide functionalized nanoparticle based sensors are useful for onsite detection of arsenic as fluorescence measurements is required during sensing process.

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sodium fluoride were obtained from Merck, Germany. Cadmium nitrate, chromium chloride were purchased from Himedia, India. Other metal salts e.g. lead chloride, ferric chloride, calcium chloride, mercury nitrate and solvents were obtained as analytical grades. Water was purified with a Milli-Q purification system Instrumentations. UV-vis absorption spectra were recorded on a Cary 60 Agilent technologies spectrophotometer at room temperature. Fluorescence spectra were taken on a Cary Eclipse fluorescence spectrometer. Size and charge were measured using NS500 (NanoSight) instrument. Transmission electron microscopy (TEM) was performed on a Technai G 20 (FEI) microscope operated at an accelerating voltage of 200 kV. Centrifugation was performed on a SORVALL RC 6+ centrifuge. Fourier Transform InfraRed spectroscopy was carried on Jasco 4700 FT/IR instrument. Detailed surface characterizations of nanosensor were performed using X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe II, FEI Inc.).

Figure 1. Schematic of fabrication of gold nanosensor, GNP-MMT@Eu and its interaction with arsenic ions (AsIII/AsV) in water.

Herein, we develop EuIII functionalized gold nanoparticle based sensor, GNP-MMT@Eu by bottom-up approach for the detection of AsIII and AsV states in the water sample. Characterization of the nanosensor is done by size and charge analysis, UV-vis, fluorescence, fourier transformed infra-red (FT-IR), x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. GNP-MMT@Eu shows colorimetric detection capabilities for both AsIII and AsV ions in water up to the limit of ≤10 ppb. The performance of the nanosensor is monitored by observing the characteristic change in color from red (λ max ~525 nm at pH~6.5) to blue (λmax ~650 nm) in presence of AsV and AsIII. AsV exhibits quicker response due to the effects of electrostatic and covalent type interactions in comparison to AsIII ion which is bounded only through covalent type interaction taking little bit more time during sensing process. GNP-MMT@Eu is highly selective for AsIII and AsV ions against other common ions present in water. Moreover, nanosensor has regeneration ability after treatment of GNP-MMT@Eu-AsIII/V complex with a strong chelating agent EDTA. The analytical performance of GNPMMT@Eu is checked with real arsenic water sample collected from Malda district, West Bengal, India. The nanosensor displays excellent recoveries of spiked samples up to a range of 92-100 %. Since fluorescent EuIII is present in the nanosensor, the detection ability of GNP-MMT@Eu is also checked through fluorometric approach but could not achieve a successful result as EuIII ion showed low-intensity luminescence in an aqueous environment due to the presence of –OH group vibration causing non-radiative decay. Another reason for quenching luminescence property of GNP-MMT@Eu may be due to efficient energy transfer from the excited state of EuIII to the excited state of MMT attached to gold nanoparticle.

EXPERIMENTAL SECTION Materials. Hydrogen tetracholoroaurate (III) dihydrate (HAuCl4.2H2O) was purchased from Himedia, India. 2-mercapto4-methyl-5-thiazoleacetic acid, arsenic (III/V) ICP standard solutions were purchased from Sigma Aldrich, USA. Sodium chloride, copper chloride, magnesium chloride, potassium chloride and

Synthesis of GNP-MMT@Eu. GNP-MMT@Eu nanosensor was prepared by a two-step chemical conjugation of gold nanoparticles (GNPs) with 2-mercapto-4-methyl-5-thiazoleacetic acid (MMT) followed by fluorescent europium chloride (EuIII) molecules. Briefly, 10.0 ml of citrate stabilized gold nanoparticles was prepared by adding 50.0 µl of 0.1 M HAuCl4 and 1.0 ml of sodium citrate (5.0 mg/mL) solutions in 10.0 ml DI water and stirred at 90°C. Color of the solution gradually changed to bright red indicating the formation of gold nanoparticles (GNPs). Next, 2.5 mg of 2-mercapto-4-methyl-5-thiazoleacetic acid (MMT) dissolved in 1.0 ml NaOH (0.01 M) was added to 10.0 ml of citrate stabilized gold nanoparticles. The mixture was stirred at room temperature overnight (8 hours). Unbound MMT ligand was removed by centrifuging the mixture at 7,000 rpm for 7 minutes. The obtained nanosensor precursor, i.e. MMT conjugated gold nanoparticle (GNP-MMT) pellet was dissolved in 5.0 ml water in order to perform second conjugation with europium (III) chloride. For this purpose, 1 mg/ml aqueous solution of europium (III) chloride was prepared and 30 micro-liters amount was added to the 5.0 ml GNP-MMT solution and stirred for 2 hour at room temperature to form the GNP-MMT@Eu. Characterization of GNP-MMT@Eu. GNP-MMT@Eu nanosensor was characterized by UV-vis spectroscopy, particle size analysis and transmission electron microscopy (TEM) techniques. Since fluorescence active europium is used in this nanosensor, a parallel characterization study was performed to determine the presence of Eu(III) on the surface of GNP-MMT. For this, emission spectra of GNP-MMT, GNP-MMT@Eu and only EuCl3 were recorded after exciting at 394 nm and compared. Further, XPS and IR spectra were recorded to determine the existence of europium (III) conjugation through the 2-mercapto-4-methyl-5thiazoleacetic acid (MMT) acid via carboxylic group. The raw XPS data were corrected using the binding energy of the C1s peak at 284.8 eV. The FT-IR spectrum was blank subtracted and baseline corrected using software. The charge and size of GNP-MMT and GNP-MMT@Eu were evaluated to determine the stability of the nanosensor in solution. The average diameter was measured by NS500 in aqueous medium at 25°C. The surface charge was determined by zeta potential measurement according to the manufacturer’s instructions for measurement in high ionic strength media at 25°C. All measurements were performed in triplicate following dilution of nanoparticles by dispersing in high grade HPLC water (1.0 mg/ml). Next, the well-characterized GNPMMT@Eu nanosensor was tested for AsIII and AsV binding affinity using UV-vis spectroscopy, hydrodynamic size-charge analysis and XPS techniques. The values of size and charge for GNPMMT, GNP-MMT@Eu before and after arsenic interactions are


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ACS Applied Nano Materials Analyses of arsenic in ground water. For the quantitative analysis of arsenic in real ground water sample, a standard addition method was employed. Natural arsenic containing ground water was collected from a place, called Amrity in Malda district, West Bengal, India. To estimate the total arsenic, 4.0 mL water was divided into four parts; one part (1.0 ml) was used as unspiked while other three parts were used as spiked samples. Different amounts of arsenic solutions were added to these water samples to obtain the spiked-water solutions with concentrations of 0.1, 0.2, and 0.3 mg/L. All spiked and un-spiked samples were analyzed by using UV-vis spectroscopic method as reported elsewhere.20 Figure 2. (A) UV-vis spectra of (1) GNP, (2) GNP-MMT and (3) GNP-MMT@Eu, before and after addition of AsIII (4) and AsV (5) ions showing red shift of the peak from 525 to ~650 nm. (B) TEM image of GNP-MMT@Eu and corresponding size distribution showing core size of 20.0 ± 4.0 nm.

shown in Table S1. The stability study of GNP-MMT@Eu was performed in different pH solutions: pH~4.0, pH~8.0 and in 1.0% NaCl solution and UV data were recorded by UV-vis spectrophotometer. AsIII/V ions detection by visual and UV-vis techniques. A stock solution of GNP-MMT@Eu (2.0 mg/ml), and arsenic ions (10, 1, 0.1, 0.01 and 0.001 ppm for each AsIII, AsV) were prepared in MilliQ water. Colorimetric detection of AsIII/V ions was performed to determine the selectivity and lowest concentration detection ability of the nanosensor. During colorimetric detection, 0.1 ml of different concentrations of each arsenic ion (10, 1, 0.1, 0.01 and 0.001 ppm) was added into GNP-MMT@Eu solution (0.1 ml) to estimate the lowest limit of detection by naked eye and characterized by UV-vis spectrophotometer. Selectivity of AsIII/V ions for the synthesized GNP-MMT@Eu nanosensor was tested against various ions other than arsenic ion. 0.1 ml of gold nanosensor in aqueous phase was mixed separately with 0.1 ml of various common ions (1.0 ppm each) NaI (NaCl), KI (KCl), CuII (CuCl2) PbII (PbCl2) CdII (Cd(NO3)2), MgII (MgCl2) CrIII (CrCl3), FeIII (FeCl3), HgII (Hg(NO3)2), CaII (CaCl2) and F- (NaF). Color changes of the solutions were observed visually by naked eye. The corresponding UV-vis absorption spectra were also recorded. Following this experiment, the sensitivity test was also performed on straight paper strips. For this purpose Whatman filter paper was cut using CO2 laser engraving system (VLS 2.30, Universal Laser Inc., USA) at 3 Watt. The engraved strip parameters were as follows: length (l) = 15.0 mm, width (w) = 5.0 mm and height (h) = 0.1 mm (100 μm). The filter paper strips were soaked in warm HPLC grade water for removing impurities, if any. GNPMMT@Eu was spotted and dried at one end of the strip and was kept vertically on petridishes having solution of mixture of AsIII and AsV ions with different concentrations and real ground water sample. As the solution moved up due to capillary action and reached the spotted red color zone, differences in color were observed and recorded using a digital camera. Reversibility of GNP-MMT@Eu. The reversibility study of GNP-MMT@Eu nanosensor was performed by UV-visible spectrometric method after addition of strong chelating agent EDTA to a mixture of GNP-MMT@Eu and arsenic (AsIII + AsV, 1:1 ratio) solution (10 ppm). Upon addition of 1.0 x 10-4 M EDTA, the visible absorption spectrum was changed back to the original color of the nanosensor. This reversibility of the nanosensor was monitored through UV-vis absorbance analysis before and after EDTA treatment.

RESULTS AND DISCUSSION Characteristics of GNP-MMT@Eu nanosensor. GNPMMT@Eu nanosensor synthesis and its working principle for sensing AsIII/V ions are shown in schematic presentation of Figure 1. Citrate stabilized gold nanoparticle of ca. 20 nm in diameter was used in the synthesis of the bright red color nanosensor in solution that changes to blue in presence of arsenic ions. The UV– vis absorption spectrum of GNP-MMT@Eu showed a broadening and a slight red shift (5 nm) of the plasmon resonance peak (~525 nm) as compared to that of the unmodified GNP (~520 nm), indicating attachment of europium (III) ions on the surface of GNP through 2-mercapto-4-methyl-5-thiazoleacetic acid (MMT) linker (Figure 2A). The average hydrodynamic diameter of GNPMMT@Eu was 101 nm, while GNP-MMT and bare gold nanoparticles were around 57 nm and 35 nm respectively (Table S1). TEM image displayed core size of the spherical nanosensor with a size distribution in the range of 20 ± 4.0 nm (Figure 2B). In case of charge of the nanosensor, GNP-MMT@Eu exhibited a negative zeta potential (-29 ± 6 mV), whereas GNP-MMT nanoparticle showed more negative zeta potential (−65 ± 3 mV), as shown in Table S1. The reason is probably the presence of positively charged EuIII ion, which partially neutralizes the net negative charge of the GNP-MMT. Since EuIII shows characteristic luminescence property, fluorescence spectra of bare europium and GNP-MMT@Eu were compared as shown in Figure S1. Under an identical experimental condition, same quantity of EuCl3 and GNP-MMT@Eu was excited at 394 nm and the emission spectra were recorded in the range of 400 to 650 nm. The uncoordinated EuIII showed a sharp peak at 594 nm for 5D0→7F1 magnetic-dipole transition and another peak at 615 nm for 5D0→7F2 electric-dipole transition.21 However, when coordinated with GNP-MMT, emissions intensity of EuIII ions decrease significantly at the same excitation wavelength, implying that luminescence property of europium (III) is quenched by the GNP-MMT of the nanosensor. One possible explanation of quenching EuIII emission in presence of GNP-MMT is that there would be an energy transfer from the long-lived excited sate of EuIII (donor) to the short-lived excited state of MMT (acceptor). The energy transfer between the excited states of the donor and acceptor is a competitive process in respect to the relaxation to the ground state of the donor. If the luminescence lifetime of the donor is long, for example in EuIII, and that of the acceptor is short, energy transfer can be very efficient at the excited levels that causes quenching of luminescence.22 This energy transfer is also evident in the emission profile of MMT at 416 nm where the peak intensity of MMT in GNP-MMT@Eu increased in compare to MMT in GNP-MMT. The MMT concentration was same in both GNP-MMT and GNP-MMT@Eu for quantitative comparison. The quenching effect of GNP in GNPMMT@Eu may be minimal due to very weak absorption of GNP in the ~600 nm region. This study indirectly proves the stable chemical conjugation of EuIII ion with GNP-MMT precursor which is responsible for quenching of the luminescence of GNPMMT@Eu nanosensor.



all –SH groups of MMT ligand are used to form covalent bond with GNP and leaving carboxylate group free for further conjugation with europium ions. It is interesting to note that FTIR spectrum of GNP-MMT showed the asymmetric (ᴠas) and symmetric stretching (ᴠs) of the deprotonated carboxylate group (-COO-) at 1587 cm-1 and 1388 cm-1 respectively. Upon complex formation with europium ions in GNP-MMT@Eu, the asymmetric and symmetric stretching vibrations of the carboxylate groups were shifted to higher frequency i.e. 1632 cm-1 and lower frequency i.e. 1366 cm-1 respectively. According to literature, the degree of spectral splitting of the ᴠs/as (COO) (Δᴠ) can be used to determine 28 the type of coordination of the carboxylate group. For GNPMMT@Eu, the spectral splitting νsym/assym (∆ν ~ 266 with EuIII) is greater than that of uncomplexed GNP-MMT (∆ν ~ 199) (Figure S2B), suggesting monodentate coordination between EuIII and – COOH group of MMT attached with gold nanoparticles. The peak at ~ 466 cm-1 also indicates that the GNP-MMT is indeed coordinated with EuIII ions through the Eu-O bond.29

Figure 3. Chemical composition analysis of GNP-MMT@Eu using XPS for (A) gold (Au4f), (B) sulphur (S2p) and (C) europium (Eu4d).

To elucidate the nature of chemical bonding environment and valence state of europium in the nanosensor, GNP-MMT@Eu was analyzed by X-ray photoelectron spectroscopy (XPS) and Fouriertransform infrared (FTIR) spectroscopy techniques. The Au 4f XPS spectrum in Figure 3A showed that the Au peaks are located at 83.2 eV and 86.9 eV corresponding to the electronic states of Au 4f7/2 and Au 4f5/2 respectively which indicates the presence of Au(0) in the nanosensor.23 Of note, we initially did not observe the signal for Au(I) oxidation state that was necessary for MMT conjugation. The Au 4f7/2 was then deconvulated which produces two distinct components located at the binding energies at 83.0 and 83.5 eV respectively. We assumed that the XPS peak at 83.5 eV is due to the presence of Au (I) on the surface of gold nanoparticles. The Au 4f7/2 binding energy difference between Au(0) and Au(I) is +0.4 eV which is in agreement with the reported value in literature.24 The XPS spectrum for S 2p in Figure 3B revealed a peak at 163.0 eV, which is assigned to sulphur atoms bound to gold surfaces as thiolate species.24 This study suggests that a fraction of Au(0) is oxidized to Au(I) in presence of MMT which are bonded through Au(I)-S covalent interactions. A peak centered at ≥168 eV can be assigned to oxidized sulfur species that may be present as impurities.25 This happens due to some oxidation of free-thiols/sulphur atoms during their storage under ambient condition. The high resolution Eu 4d core level spectrum is shown in Figure 3C. The Eu 4d core level is splitted into two signals due to the incomplete occupancy of 4f-subshells at final-state conFigureuration.26 The Eu 4d signals of GNP-MMT@Eu were assigned to the Eu 4d5/2 and 4d3/2 peaks at energy levels of 135.6 and 142.2 eV, which reveals the presence of Eu(III) state in the nanosensor. The spin orbital splitting for EuIII ion is 6.6 eV which is close to the reported value elsewhere.26 The FTIR spectra of GNP-MMT@Eu further provides evidences for stepwise conjugation of MMT followed by EuIII ions with GNP. The characteristic IR signals in Figure S2 corresponding to stretching vibration band at νC=O ~1720 cm-1 indicates the presence of –COOH groups of MMT ligand.27 As expected, the peak at νSH ~2555 cm-1 for free thiol stretching vibration became negligible after conjugation with MMT, suggesting the fact that almost

To determine the stoichiometric ratio between the GNP-MMT and EuIII ion, the absorption isotherm and Job’s method are followed by varying the concentrations of both EuIII and MMT attached to GNP. With successive additions of EuIII ion, the changes in absorbance were measured at λmax = 635 nm and plotted against the mole ratios of EuIII to GNP-MMT which reaches to a maximum absorbance at ~0.42 suggesting a 1:2 EuIII-to-MMT binding ratio (Figure S3). In order to confirm the stoichiometry, a different binding assay was performed using Job’s method of continuous concentration variations of EuIII and GNP-MMT. Figure S3 showed the corresponding Job’s plot with a maximum value for the absorbance at 635 nm when the mole fraction of EuIII ion was ~0.3, further supporting a 1:2 binding between EuIII ion and MMT attached to GNP. It is known that EuIII ion can increase coordination number from 6 to 11.30 In this study, the absorbance decreased with increasing EuIII ion concentration after reaching the maximum value of 0.3 indicating self-aggregation of GNPMMT@Eu due to the interaction of EuIII with MMT of neighboring gold nanoparticle. To avoid this aggregation, the molar ratio of EuIII: GNP-MMT was maintained as 1:2 to consider final composition of the nanosensor. The stability of GNP-MMT@Eu was checked in presence of 1.0% NaCl and different pH media and monitored by UV-vis spectroscopy as shown in Figure S4. Negligible change in UV-vis band at ~525 nm confirmed the excellent stability of the nanosensor. However, GNP-MMT@Eu became unstable in presence of AsIII and AsV ions by showing visual color change from red to blue due to inter-particle coupled plasmon resonance. This distinct visible color change encourages us for the present study towards the detection of both oxidation states of arsenic in water using the GNP-MMT@Eu nanosensor. Spectroscopic characteristics of GNP-MMT@Eu upon addition of AsIII and AsV ions. As GNP-MMT@Eu did not show any visible fluorescence signal in water, the detection ability towards AsIII and AsV ions in solution was established by UV-vis spectroscopic technique. Figure 2A demonstrated the absorption spectra of GNP-MMT@Eu before and after treatment with AsIII and AsV ions at pH ~ 6.0-7.0. GNP-MMT@Eu is highly stable due to electrostatic repulsion imposed by negative charge of the nanosensor (-29 mV, Table S1). However, the nanosensor showed a decrease in the intensity of plasmon band (λmax = 525 nm) with a concomitant growth of a new band at λmax = 650 nm in presence of both oxidation sates of arsenic, indicating aggregation of spherical nanoparticles due to AsIII/AsV ion binding over GNP-MMT@Eu surface. The hydrodynamic diameter of GNP-MMT@Eu was changed from 100.9 nm to 116.0 ± 3.0 nm after interaction with


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Figure 4. Sensing mechanism of GNP-MMT@Eu with arsenic ions (AsV /AsIII) through (A) covalent type and (B) electrostatic interactions.

arsenic which suggests that the increase in size occurred due to arsenic mediated aggregation of nanosensors. The GNPMMT@Eu contains coordinated Eu-OH groups at the surface that are mainly responsible for the arsenic ion binding in the detection process. Since, AsV and AsIII ions remain as H2AsO4−/HAsO42and H3AsO3 forms at neutral pH, we believe that the available AsOH/As-O- groups of arsenic species take part in the sensing reaction with –OH groups of Eu(III) centers and release H2O and OH− to form an inner-sphere arsenic complex31 between the nanoparticles in self-assembly mode as shown in Figure 4. Nevertheless, as there is a possibility of weak protonation of Eu-OH groups in the pH~6.0-7.0, surface of the nanosensor attains partial positive charge due to the conversion of some Eu-OH to Eu-OH2+. These positively charged centers provide the initial driving force for H2AsO4−/HAsO42- to bind with the nanosensor through electrostatic attractions. This could be a reason for fast response of the nanosensor towards AsV in comparison to AsIII ion. Indeed, GNPMMT@Eu showed a rapid color change for AsV compared to AsIII, which suggests that both electrostatic and covalent type interactions are happening between As-OH/As-O- groups of arsenic species and Eu-OH/Eu-OH2+ of the GNP-MMT@Eu. In order to understand the mechanism, we utilized XPS to characterize the surface status of GNP-MMT@Eu after arsenic addition. The full-scan XPS spectrum (Figure S5) of the aggregates clearly displayed the 3d peak of arsenic after interaction with the nanosensor. To further determine the binding nature of arsenic, the peaks of As 3d were explored for each AsV and AsIII ions bonded with GNP-MMT@Eu by high resolution XPS spectra. In case of AsV as shown in Figure 5A, 3d peak was observed at 45.2 eV which is assigned to AsV-O bonding while the 3d peaks at 44.0 and 46.7 eV in Figure 5B indicated the presence of AsIII ion. The appearance of a peak for AsV in case of GNP-MMT@Eu after interaction with only AsIII indicates partial oxidation of AsIII species occurred during the interaction process, that may be happening by dissolved oxygen in the experimental solution. Ghosh et al. reported similar results where they confirmed AsV binding with Eu(III) capped YPO4 nanoparticles by observing As 3d XPS peak at 46.5 eV.19 In another study, Chen at al. observed As 3d peaks at 44.3 and 45.5 eV for AsIII and AsV bonding on the surface of CeFe oxides/carbon nanotube composite materials.31 It is interesting to note that the peak intensity for EuIII ion decreased a little after interaction with AsIII/AsV. This slight decrease in intensity suggests a direct interaction between H2AsO4−/HAsO42-/H3AsO3 and Eu(III)-centers. However, the position of Eu 4d peak, as shown in Figure 5C, did not alter suggesting the fact that the oxidation state of europium remains the same after arsenic interaction. Thus,

Figure 5. XPS signals for 3d peaks of (A) AsV, (B) AsIII and 4d peak of (C) EuIII ion of GNP-MMT@Eu after interaction with arsenic ions.

sensing of AsV using GNP-MMT@Eu nanosensor involves a complex mechanism, which include both electrostatic attraction and covalent type interaction that leads to surface complexation between the nanosensor and specific ion. As there is a rare probability for obtaining any ionic species of AsIII at neutral pH, the interaction of the GNP-MMT@Eu and AsIII is happening only through covalent interaction to form an ion-mediated surface complexation. Details of the proposed mechanism for GNPMMT@Eu nanosensor based AsIII/V sensing are demonstrated in Figure 4. Sensitivity and selectivity towards AsIII and AsV ions. In order to define the sensitivity of GNP-MMT@Eu nanosensor, various concentrations (10, 1, 0.1, 0.01 and 0.001 ppm) of AsIII and AsV solutions were treated with GNP-MMT@Eu and the colorimetric responses were monitored through UV-vis technique (Figure 6). As mentioned above, the nanosensor showed an immediate response to AsV in comparison to AsIII. With the increase of AsV concentrations, the color of sensor solution gradually changed from red to blue, indicating an increase of the ion-based aggregation of GNP-MMT@Eu (Figure 6, inset). The nanosensor exhibited a gradual decrease in absorbance at concentrations 0.001, 0.01, 0.1, 1 and 10 ppm respectively. In case of AsIII, similar visible color change occurred, but was not significant when the arsenic concentration was 0.001 ppm (1.0 ppb). Thus, the visual sensitivity of GNP-MMT@Eu is significant up to the level of 0.001 ppm (1.0 ppb) when AsV is present, but it did not show a prominent colorimetric response at ~ 1.0 ppb in case of AsIII (Figure 6, inset). This reveals the fact that the GNP-MMT@Eu nanosensor is highly sensitive for AsV, and little less sensitive to AsIII ion (limit: ~0.01 ppm) when the colorimetric technique is employed. By comparing the degree of aggregation of GNP-MMT@Eu through UV-vis absorbance (A650) measurements at different concentrations as shown in (Figure S6), A650 of AsV was a little bit higher than AsIII indicating rapid ion-induced aggregation and therefore fast color change with AsV ion compared to AsIII ion. This is quite reasonable because AsV exist as negatively charged species, e.g. H2AsO4−/HAsO42- that are initially attracted electrostically by the protonated Eu-OH (Eu-OH2+) groups even at very lesser concentration of ions at pH~ 6.0-7.0. This driving



Figure 6. UV-visible spectra of GNP-MMT@Eu in presence of increasing concentrations of (A) AsV and (B) AsIII ions showing gradual decrease in absorbance. Inset: Image of various concentrations of AsV (left) and AsIII (right) solutions treated with GNP-MMT@Eu showing gradual change in color from red to blue due to aggregation of the gold nanosensor.

III

force is not present in neutral As species to be attracted by the nanosensor under the similar experimental conditions and thus, the observed sensitivity limit is restricted to ~0.01 ppm (~10 ppb, WHO limit) with a clear colorimetric signal. To further understand the mechanism of arsenic ions binding to EuIII ion, we have tested only GNP-MMT in presence of both AsIII and AsV ions and corresponding absorption was recorded in UV-vis spectroscopy. There was no change in the absorption spectrum of GNP-MMT at λmax ~ 520 nm even after addition of arsenic ions (Figure S7) which affirmed that AsIII/V binds only with EuIII centers present in GNP-MMT@Eu. From the above studies, it is evident that GNP-MMT@Eu is able to bind with both AsIII and AsV ions in water. Now it is worth to explore whether the nanosensor can selectively detect arsenic in presence of other ions that may interfere with the detection process. For this purpose, the selectivity of GNP-MMT@Eu was tested in presence of various metal ions; NaI, KI, CuII, PbII, CdII, MgII, CrIII, FeIII, HgII and CaII including a mixture of AsIII and AsV (Figure 7). Each metal ion (conc. 1.0 ppm) was individually added to the red solution of GNP-MMT@Eu and monitored through visual color change and UV-vis spectroscopy. The color of the nanosensor solution remained almost unchanged i.e. red against each interfering ions or the mixture of ions, but turns into blue color when AsIII and AsV were added to the mixture. In UV-vis spectra, negligible differences were observed in case of each ion as well as in their mixture as shown in Figure 7 and Figure S8. We also checked the selectivity in presence of fluoride ion (10.0 ppm, basic in nature like H2AsO4−/H2AsO3−) and found no interference during AsIII and AsV detection (Figure S9). All these studies revealed that GNP-MMT@Eu nanosensor can act as a highly selective probe for both oxidation states of arsenic with slight to negligible interferences from other metal ions on its performance. Reversibility of GNP-MMT@Eu. To check whether the arsenic binding is reversible and GNP-MMT@Eu nanosensor can be reused for sensing the same, a strong complexation agent (EDTA) was utilized after aggregation of GNP-MMT@Eu in presence of a mixture of AsIII and AsV ions (1:1). EDTA was used to explore the reversibility of the nanosensor by monitoring the change in color of the aggregated GNP-MMT@Eu to original red color of the pristine nanosensor. In this study, reversibility of both color

Figure 7. Aspect ratio A650/525 of gold nanosensor GNPMMT@Eu in presence of various metal ions and mixture of AsIII/V ions (at 1.0 ppm concentrations). (Error bars were obtained from three experiments). Inset image: UV-vis spectra of each experiment from where A650/525 were calculated. and UV-vis absorbance of the nanosensor were observed in presence of 10.0 ppm of total arsenic ions followed by the addition 1.0 x 10-4 M EDTA solution. It is evident from Figure 8 that the color and UV-vis absorbance of GNP-MMT@Eu in presence of AsIII + AsV mixture (graph 2) reached close to its original position (graph 1) after interaction with EDTA solution (graph 3). The retrieval of the red color of the nanosensor also indicated that there is no adverse effect of EDTA on the stability of GNP-MMT@Eu (Figure 8, inset). The probable mechanism by which this phenomenon occurs is due to the formation of stable EDTA-AsIII/AsV complex that displaces the bonded arsenic from the surface of GNPMMT@Eu. This study describes the fact that the GNP-MMT@Eu nanosensor can be reused for the detection of new arsenic ions in the aqueous sample. Analysis of total arsenic on paper strip using GNPMMT@Eu. In another approach, paper strip was considered to check whether GNP-MMT@Eu nanosensor can be used for onsite detection of arsenic ions. For that purpose concentrated drop of the nanosensor was spotted at one end of the filter paper strip and dried. The other end of the strip is slightly immersed in a solution of AsIII + AsV mixture (1:1 molar ratio) of different concentrations

Figure 8. (A) Retrieval of visible red color and UV-vis absorbance (at ~525 nm) of arsenic treated GNPMMT@Eu in presence of EDTA. (1) indicates UV-vis absorption profile and red color of GNP-MMT@Eu, (2) indicates the change in color and decrease in absorbance of GNP-MMT@Eu after interaction with arsenic ions, (3) indicates retrieval of the red color as well as UV-vis absorbance of GNP-MMT@Eu-arsenic complex after interaction with EDTA solution. (B) The change in absorbance from 1→ 2→ 3 (at ~650 nm) during course of the reversibility study with GNP-MMT@Eu.


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Table 1. Analytical data to quantify the arsenic in ground water sample using GNP-MMT@Eu.

Figure 9. Image showing color change of GNP-MMT@Eu upon interaction with various concentrations of a solution containing AsIII and AsV mixture on straight paper strips. Strips (a-f) exhibit colorimetric responses in presence of arsenic concentrations 0 ppm (control), 10 ppm, 0.1 ppm, 0.01 ppm, 0.001 ppm and arsenic contaminated real ground water sample respectively. (1.0 ppm, 0.1 ppm, 0.01 ppm, 0.001 ppm) and ground water sample, kept in petridishes. As soon as the solution touched the strip, the ions flow due to the capillary action of the paper strip. As the ion touched the red nanosensor spot, the color changed to blue indicating the presence of arsenic in aqueous solution (Figure 9). The intensity of the blue color varied with concentration gradient of arsenic species. Figure 9f (inset, extreme right) shows the study of the nanosensor with real ground water sample where no arsenic was added externally. From the color matching by naked eye, it can easily be assumed that ~ 0.1 ppm of total arsenic may be present in the unknown ground water sample. For quantitative measurement of arsenic, we performed AAS with same sample and observed that the result is very close to the AAS value (0.125 ppm). This study suggests that simple paper strip can easily be applied to develop a dipstick type device for onsite arsenic detection in a user-friendly way. GNP-MMT@Eu performance in real water sample monitoring. The analytical performance of the GNP-MMT@Eu nanosensor was explored by assessing the concentration of total arsenic in real samples. The standard addition method was employed to quantify the arsenic in ground water sample shown in Table 1. Three samples were spiked with analytical grade mixture of AsIII and AsV (1:1) and UV-vis absorbance was measured to plot against the concentration of arsenic added. From the linear regression produced by the standard addition curve (r2 = 0.991), it was found that the ground water sample contains on an average of 0.13 ppm (from AAS, arsenic concentration ~0.125 ppm, see Figure S10). From Table 1, it is clearly observed that the result is not significantly different from the AAS value for arsenic in the ground water sample. In addition, the recoveries from three arsenic spiked samples were in the range of 92.0-100.0 % for the water sample. The analysis suggests that the nanosensor could effectively be employed to quantitatively detect arsenic in any real water samples without involving any complicated operational procedure and any system error of detection.

CONCLUSION

Type of sample

Before spiking (mg/L)

Mixture of AsIII/AsV spiked (mg/L)

Total arsenic found (mg/L)

Recovery (%)

Ground water

0.13 (measured by AAS)

0

0.13

100.0

0.1

0.23

100.0

0.2

0.32

92.3

0.3

0.43

100.0

of ≤0.01 ppm (WHO recommended limit: 10 ppb) in aqueous medium. The well-characterized nanosensor displayed a distinct color variation from red to blue after mixing with arsenic ions. The arsenic-induced inter-particle aggregation of GNP-MMT@Eu through electrostatic attraction and covalent type interactions followed by inner-sphere complex formation made the AsV detection process rapid, and more sensitive (up to 1.0 ppb) in comparison to AsIII ions (limit ~ 10 ppb) which undergo only surface complexation with the nanosensor. In addition to sensing, GNPMMT@Eu showed the capability of quantitative estimation of total arsenic in ground water. In this aspect, analytical tests along with quantitative analysis of real ground water sample containing natural arsenic have been performed in order to reveal the potentiality of this sensing system. Other advantage of the developed nanosensor is that only small amount of GNP-MMT@Eu is required to sense low-level arsenic that makes the overall detection process low-cost, user-friendly and safe-to-use in field arsenic detection. A number of previously reported works demonstrated either AsIII or AsV sensing through nanotechnological pathways, but none of them used single gold nanoparticle based platform as a colorimetric sensor for monitoring both arsenic oxidation states in the water. For the first time, we used europium conjugated gold nanoparticle, GNP-MMT@Eu as sensitive nanostructured material to develop color based sensor for efficient monitoring of total arsenic in water.

ASSOCIATED CONTENT Supporting Information. Stability in different pH and 1.0% NaCl solutions, FTIR, fluorescence, full XPS spectra (in presence of arsenic), Job’s plot, UV-vis plot and the standard addition curve of GNP-MMT@Eu supplied as Supporting In formation. “This material is available free of charge via the Internet at http://pubs.acs.org.”

ACKNOWLEDGMENT

In conclusion, europium functionalized gold nanoparticle, GNPMMT@Eu is developed as a new sensor material for colorimetric detection of both AsIII and AsV ions at lower concentration level

The authors thank Dr. Harish Hirani, Director, and Dr. Nagahanumaiah, HOD, MST Lab, CSIR-CMERI, Durgapur for their en-



couragements. Support from DBT grant under project no. GAP101612 is gratefully acknowledged.

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