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Highly Sensitive Detection of Arsenite Based on its Affinity towards Ruthenium Nanoparticles Decorated on Glassy Carbon Electrode Ruma Gupta, Jayashree S. Gamare, Ashok Kumar Pandey, Deepak Tyagi, and Jayshree V Kamat Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04625 • Publication Date (Web): 18 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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Analytical Chemistry

Highly Sensitive Detection of Arsenite Based on its Affinity towards Ruthenium Nanoparticles Decorated on Glassy Carbon Electrode Ruma Gupta,†*Jayashree S. Gamare,† Ashok K. Pandey,‡* Deepak Tyagi,¥ and Jayshree V. Kamat† †

Fuel Chemistry Division, ‡Radiochemistry Division, ¥Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India

ABSTRACT. Metallic ruthenium nanoparticles (Ru NPs) are formed on the glassy carbon electrode (GC) at electrodeposition potential of -0.75 V as observed from X-ray photoelectron spectroscopy. Thus formed Ru NPs have the arsenite selective surface and conducting core that is ideally suited for designing a highly sensitive and reproducible response generating matrix for the arsenite detection at ultratrace concentration in aqueous matrices. Contrary to this, arsenate ions sorb via chemical interactions on the ruthenium oxides (RuO2 and RuO3) NPs formed at 0.25 V but not on the Ru NPs. For exploring a possibility of the quantification of arsenite in ultratrace concentration range, the Ru NPs have been deposited on the GC by a potentiostatic pulse method of electrodeposition at optimized -0.75V for 1000 s. Arsenite preconcentrates onto the Ru surface just by dipping the RuNPs/GC into the arsenite solution as it interacts chemically with Ru NPs. Electrochemical impedance spectroscopy of As(III) loaded RuNPs/GC shows a

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linear increase in the charge transfer resistance with increase in As(III) conc. Using differential pulse voltammetric technique, arsenite is oxidized to arsenate leading to its quantitative determination without any interference of Cu2+ ions that are normally encountered in the water systems. Thus, the use of RuNPs/GC eliminates a need for preconcentration step in stripping voltammetry, which requires optimization of the parameters like preconcentration potential, time, stirring, inferences etc. The RuNPs/GC based differential pulse voltammetric (DPV) technique can determine the concentration of arsenite in a few min with a detection limit of 0.1 ppb and 5.4 % reproducibility. The sensitivity of 2.38 nA ppb-1 obtained in the present work for As(III) quantification is considerably better than that reported in the literature with similar detection limit and mild conditions (pH=2).The RuNPs/GC based DPV has been evaluated for its analytical performance using the lake water, ground water and seawater samples spiked with known amounts of As(III).

INTRODUCTION Ru nanopartcles (NPs) have strong affinity towards carbene π bonds, and have been functionalized by the unique olefin metathesis reactions of the carbene-stabilized nanoparticles with vinyl derivatives for different applications.1-5 For example, Ru NPs have been functionalized with 1-vinylpyrene and 1-allylpyrene using a olefin metathesis for the detection of nitroaromatic compounds by fluorescence quenching.6 The intraparticle charge delocalization has been observed due to strong Ru-carbene π bonds and conducting Ru metal core.7-8 However, it is not known whether Ru NPs have similar affinity towards lone pair of electrons bearing moiety such as arsenite. The arsenic in natural waters occurs normally in the inorganic forms such as arsenate (As(V)) and arsenite (As(III)) having chemical structures shown in Scheme 1. As(III) is highly toxic in


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the biological system among the inorganic and organic arsenic species existing in the natural waters.9-10 All three pKa values for As(V) are in a pH range (2.19, 6.94 and 11.5) and, therefore, it exits as arsenate anions (H2AsO4−and HAsO42−) in the natural waters.11-12 Unlike As(V), As(III) has only one pKa value at a high pH (9.1), and thus found mostly in undissociated form H3AsO3 in the natural waters. Because of suitable pKa value, it is easier to scavenge As(V) ions using a variety of anion-exchange materials, metal complexes, and metal oxides such as iron oxides etc from the natural waters.13-25 H O H O

O

As

O H

O

As

H O

O H

As(V)

H

As(III)

Scheme 1. Chemical structures of inorganic arsenic ions commonly occur in waters. The removal of undissociated arsenite is quite tricky and require either conversion of As(III) to As(V) or adjusting higher pH to dissociate arsenite.11,15,26-29 It is interesting to note that the arsenite adsorption may occur by redox transformation on the iron oxide and other oxide surfaces.30-33 Sharma et al. have given a account of ferrate as the greener oxidant for toxic elements including As(III) in the water treatment technologies.34 However, a direct sorption of arsenite on the sensing matrix/electrode is desirable for developing a matrix with a reproducible response. It has been reported in the literature that the thiol groups have a good affinity towards undissociated arsenite.35 A number of electrochemical, turn-on fluorescent and surface enhanced Raman scattering (SERS) methods have developed for the detection of arsenite in ultra trace


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concentration in drinking water.36-39 These methods utilize the thiol based reagents, selfassembled monolayer and SERS platforms. In general, thiols are non-selective and could absorb a variety of ions such as Hg2+, Cd2+, Pb2+ etc. The mesoporous MnFe2O4 nanocrystals have been used for arsenite detection using square wave stripping voltammetry with a detection limit of 1.95 ppb.40 MnFe2O4 exhibits good affinity towards arsenite, but would not have the desirable electrical properties. To improve electrochemical detection, a room temperature ionic liquid has been used along with Fe3O4 microsphere.41 This resulted to 8×10−4 ppb As(III) detection limit. However, Au, Pd and Pt NPs are better suited for the electrochemical detection due to their good conducting core. The Au NPs have been extensively used for the electrochemical detection of As(III).42-45 Chen and Huang have used EDTA as masking agent to suppress the interference in detection of As(III) using Au NPs modified glassy carbon electrodes.44 An anodic stripping voltammetric determination of arsenite with a detection limit of 0.5 ppb has been reported using a glassy carbon electrode modified with gold-palladium bimetallic NPs.46 Similar detection limit for arsenite has been observed using a Pt NPs modified boron doped diamond microelectrode.47 Moghimi et al. have observed that the bimetallic FePt NPs decorated anode in stripping voltammetry can quantify As(III) at natural pH with a limit of detection of 0.8 ppb.48 This seems to suggest that the detection limit and sensitivity for the As(III) quantification would be further improved if metal NPs anchored on the electrode have chemical (covalent) interactions with arsenite i.e. selective adsorption on surface of metal NPs having conducting core. In the present work, Ru NPs decorated on the glassy carbon have been studied to explore such a possibility. Ru NPs have been never subjected to study their affinity towards ligating ions/molecules except for carbene π bonds. However, it has been reported that the arsenate


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adsorption on RuOx NPs occurs by surface complexation, and arsenite also adsorbs due to its oxidation on hydrous ruthenium oxide surface to arsenate.49-50 RuOx has been also used as an electrocatalyst for oxidation of arsenite to arsenate.51 To study affinity As(III), the Ru NPs have been formed on the glassy carbon electrode (Ru NPs/GC) by controlled electrodeposition. The Ru NPs have been characterized by X-ray Photoelectron Spectroscopy (XPS) and field emission scanning electron microscopy (FE‐SEM).The Ru NPs/GC were equilibrated with separate solutions containing As(III) or As(V) and subjected to energy dispersive X‐ray fluorescence (EDXRF) and XPS analyses for understanding the affinity of Ru NPs towards As(III) and As(V). These studies show that Ru NPs have a high selectivity towards As(III). Electrochemical impedance spectroscopy has been carried out to study the change in charge transfer resistance of Ru NPs/GC on As(III) sorption. As Cu2+ ions are most likely interfering ions,52 the differential pulse voltammetry of RuNPs/GC electrode in Clark-Lubs buffer (pH=2) solution containing 1 mM As(III) and 0.1 M Cu2+ ions has been examined in a potential range -0.2V to 1 V. Finally, RuNps/GC based differential pulse voltammetric (DPV) technique have been used to quantify As(III) at sub-ppb concentrations spiked in the lake water, ground water and seawater samples. EXPERIMENTAL SECTION All chemicals, namely, ruthenium trichloride hydrate (RuCl3.xH2O) (Aldrich), AsNaO2 (Aldrich), HCl (Merck), KCl (Merck) and HNO3 (Merck) were used as received. All the solutions were prepared in Millipore Milli-Q water (~ 18 MΩ cm-1). Cyclic voltammetry and differential pulse voltammetry were performed using CHI 760 D electrochemical workstation with a three electrode voltammetric cell having a glassy carbon disk working electrode (geometrical area, A = 0.07 cm2), platinum wire counter electrode and Ag/AgCl reference electrode. All the potentials were quoted with respect to Ag/AgCl (3 M KCl) reference electrode.


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The choice of glassy carbon electrode was based on its wide potential range accessibility. As RuNPs had to be formed at -0.75 V, a wide potential window was required that was not possible with the noble metal electrodes such as Pt and Au electrodes because of hydrogen evolution at 0.2 V on these electrodes. The electrochemically active surface areas of the GC and RuNPs/GC electrodes were calculated using cyclic voltammogram of 5 mM potassium ferricyanide as a redox probe. The electrochemically active surface areas of GC and RuNPs/GC were calculated to be 0.015 cm2 and 0.15 cm2, respectively, as described in Supporting Information (S.I.). The experiments were carried out at room temperature (25 ± 1oC) and the solutions were deoxygenated using high purity nitrogen prior to electrochemical experiments. Each measurement was repeated thrice and the average numerical value of each parameter was quoted for discussion (relative error < ± 0.1%).The morphology of RuNPs was recorded by JEOL make field emission gun-scanning electron microscope (FEG-SEM).The oxidation states of the elements present on the electrodes were studied using X-Ray Photoelectron Spectroscopy (XPS). XPS experiments were done in the SPECS instrument with a PHOBIOS 100/150 Delay Line Detector (DLD), and employed non-monochromatic Al Kα radiation (1486.6 eV) with operating power of 380 W as a source. All these measurements were carried out in a constant analyzer energy mode with pass energy 50 eV. All XPS spectra were first analyzed using the Casa XPS software and background from each spectrum was subtracted using a Shirley-type background. Ru NPs were deposited on the glassy carbon electrode by applying a potentiostatic pulse of 0.75 V for 1000 s. The potential was optimized by studying the nucleation and growth phenomena by potentiostatic transient measurements. Current transients were obtained at different electrode potentials in the range -0.9≤E≤-0.4V. The experimental current transient at 0.75V fits relatively well with the theoretical curve for instantaneous nucleation model proposed


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by Scharifker and Hills.53 Hence, for getting homogeneous and monodisperse RuNPs, the deposition potential was fixed to -0.75 V for further experiments. The time of deposition was optimized as 1000 s based on maximum electrocatalytic activity obtained. When the potential of working electrode vs. Ag/AgCl exceeds to -0.4 V, Ru3+ ions were oxidized to RuO2 and RuO3. Hence, RuOx NPs were prepared on GC electrode by using potentiostatic pulse of -0.25V. As(III) stock solution (1 mM) was prepared from AsNaO2, by dissolving in Clark-Lubs (pH=2) buffer (0.1 M KCl + x mL 0.1M HCI). The selectivity of As(III) ion on RuNPs electrode surface was confirmed with EDXRF spectrum recorded by Jordan Valley EX-3600-TEC spectrometer having Rh target and Ge secondary target operated at 40 kV and 500 µA. The electrochemical impedance spectroscopy (EIS) measurements were done at open circuit potential using ac perturbation amplitude of 5 mV for 48 different frequencies ranging between 0.01 Hz to 106 Hz. The impedance spectra were analyzed with help of the Frequency Response Analyzer software (Eco Chemie B.V., Utrecht, Netherlands). The equivalent circuit of the experiment is shown in the Figure S1 (S.I.). The runs were analyzed assuming that the RuNPs/GC electrode behaved as a parallel RC circuit in the frequency range used. The RuNP/GC electrode was first equilibrated with solution containing As (III) in pH 2 buffer and then subjected to impedance measurements. The analytical performance of Ru NPs/GC was evaluated by differential pulse voltammetry (DPV) using solutions having known concentrations of As(III) in Clark-Lubs buffer. The interference studies were carried out for the As (III) detection using a mixture of 1mM As (III) with 0.1M CuSO4.The developed methodology was tested for its efficacy for the determinations of As(III) in the real water samples collected from lake of Raja Ramanna Centre for Advanced Technology (RRCAT), Indore , ground water from west Bengal and seawater from Trombay


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area, Mumbai. Since As(III) in these samples were not detected, a precise measured quantity of As(III) was spiked in these samples and buffer was added to each sample of known volume for adjusting pH to 2. The Clark-Lubs buffer having pH=2 was taken as a blank, and subtracted from the sample data. DPV of each sample were recorded and the concentrations of As(III) were evaluated from the calibration curve. RESULTS AND DISCUSSION To study inorganic arsenic affinity, the Ru or RuOx NPs were deposited on the glassy carbon electrode by a potentiostatic pulse method of electrodeposition ranging 100-1000 s at -0.75 V or -0.25 V, respectively. The FE-SEM images of electrodeposited RuNPs on a glassy carbon electrode are shown in Figure 1. The FE-SEM images indicated that the homogeneously dispersed flower shaped Ru NPs were formed at the 1000 s deposition time. However, nonuniform distributed small Ru NPs were formed during shorter (100 s) deposition time.

(a)

(b)

Figure 1. FE-SEM images under different magnification showing Ru NPs formed on the electrode after electrodepositions of (a) 100 s and (b) 1000 s by potentiostatic pulse method. The XPS measurements were carried out for the RuNPs deposited on GC electrode. The initial survey of binding energy spectrum shown in Figure S2a (S.I.) indicated the presence of Ru0


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peaks (3d,3p, 4d) and O peak (1s). The narrow scan spectra in the O 1s and Ru 3d and 3p regions for the Ru NPs are presented in Figures S2b, S2c & S2d (S.I.), respectively. The peaks of the O 1s, Ru3d5/2 and Ru3d3/2 were observed at 530 eV and between 280-285 eV, respectively. The Ru 3d5/2 peak at BE = 280eV could be attributed to metallic ruthenium [Ru0] that was in agreement with the literature.54 The second 3d3/2 peak, occurring at higher BE values (BE = 284 eV), was due to Ru atoms with lower charge densities. As C1s had same binding energy as Ru3d3/2; therefore it was difficult to separate Ru3d3/2 from C 1s. The metallic nature of Ru was also confirmed by XPS spectra of Ru 3p region which has 3p3/2 and 3p1/2 components at 462.2 and 484.6 eV (Figure S2d (S.I.)), respectively. Thus, the XPS characterizations of RuNPs suggested that the RuNPs synthesized by the potentiostatic deposition were in a metallic form with the presence of chemisorbed oxygen on its surface. The adsorptions of arsenite and arsenate on the Ru and RuOx NPs decorated electrodes were studied by dipping these in the respective solutions for overnight, and removing physically adsorbed species by washing with an excess of water, drying in air, and finally subjecting to EDXRF analyses. As can be seen from Figure 2a, the arsenite and arsenate were adsorbed preferentially on Ru NPs and RuOx decorated electrodes, respectively. Arsenite was also adsorbed on RuOx NPs decorated electrode to some extent as expected from redox conversion to As(V) reported in the literature.49-50 The XPS spectra given in Figures 2b & S3 (S.I.) also confirmed the arsenite sorption on Ru NPs by chemical interactions. The mechanism involved in the sorption of As(V) on RuOx surface was studied by Luxton et al. using extended X-ray absorption fine structure spectroscopy (EXAFS).50 This study seems to suggest two possible mechanisms of the sorption of As(V) on the RuOx surface. The first mechanism is a single step involving a ligand exchange of arsenate ions with a Ru-oxide hydroxyl group resulting in a


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formation of monodentate inner-sphere complex. The second mechanism occurs via two steps reaction where arsenate first undergoes a ligand exchange reaction with a surface hydroxyl group to form a monodentate complex, followed by a second ligand exchange reaction resulting in the formation of a bidentate inner-sphere complex. There is no possibility of these mechanisms in the case of Ru NPs hence arsenate ions are not sorbed on the Ru NPs. In case of As(III), a coordination bond would be formed involving a lone pair of electrons on arsenite and vacant d orbital of Ru0. As can be seen from Scheme 1, arsenate ion does not have a lone pair of electrons. The selective sorption of arsenite on the Ru NPs offers numerous possibilities of designing the chemical sensor based on a change in fluorescence of a suitable signal generating functionality attached to Ru NPs by carbene π bond, the change in electrical properties such as charge transfer resistance of Ru NPs, or voltammtery etc.

(a)

(b)

Figure 2. (a) EDXRF spectra showing adsorption of arsenite preferentially on Ru NPs and arsenate on RuOx NPs decorated glassy carbon electrodes, and (b) deconvoluted core level As3p XPS spectrum of RuNPs/GC after equilibration with a solution containing arsenite. Differential pulse voltammetry was used to study the efficacy of the Ru NPs decorated glassy carbon electrode (RuNPs/GC) for the detection and quantification of arsenite in water. Arsenite


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interacts chemically with Ru NPs, and preconcentrates onto the Ru surface just by dipping the RuNPs/GC into the arsenite solution. Using differential pulse voltammetric technique, arsenite preconcentrated on RuNPs/GC was oxidized to arsenate leading to its quantitative determination. It is interesting to note that the use of RuNPs/GC eliminates a need for preconcentration step in stripping voltammetry, which requires optimization of the parameters like preconcentration potential, time, stirring, interferences etc. The RuNPs/GC based differential pulse voltammetric technique could determine the concentration of arsenite in a few min. The electrochemical response of RuNPs/GC and bare GC electrodes have been studied by cyclic voltammetry to verify the electrocatalytic activity of modified electrode towards As(III) oxidation. The cyclic voltammograms of RuNPs/GC and bare GC electrodes recorded for 1 Mm As(III) in pH-2 ClarkLubs buffer are shown in Figure 3. As can be seen from the curve (i) of Figure 3, the voltammograms of bare GC exhibit a very broad peak at around 1.05 V with a current density of 10.2 µAcm-2. In (ii) curve of Figure 3, an intense oxidation peak at around 0.7 V was observed corresponding to the oxidation of As(III) to As(V) with a current density of around 625 µA cm-2. The peak current density on RuNPs/GC electrode was about 60 times greater than that of bare GC electrode under similar conditions.


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Figure 3. Comparison of cyclic voltammograms of bare GC (i) and RuNPs/GC (ii) electrodes recorded for 1 mM As(III) in pH 2 Clark-Lubs buffer. Cyclic voltammograms of the RuNPs/GC electrode recorded at different scan rates using 1mM As(III) in pH 2 Clark-Lubs buffer solution are shown in Figure S4 (S.I.). It was observed that the peak current density (Jp) for oxidation increased with an increase in scan rate (ν). The oxidation peak potential shifted towards more positive direction with increasing scan rate. The redox reaction at the electrode surface could be controlled by the slowest process either diffusion or rate of adsorption of As(III). The rate determining process can be identified by observing the slope of the straight line of ln Jp versus ln (ν) plot. Theoretically, the slopes of 0.5 or 1 should be observed for the rate of pure diffusional or adsorption controlled process, respectively.55-56 The plots of ln Jp versus ln (ν) for the anodic peaks yielded the straight lines of gradients 0.51 (R2 = 0.994), see Figure S5 (S.I.). This suggested clearly that the electrochemical reaction of As(III) to As(V) is governed by the diffusion controlled process, and rate of adsorption of As(III) on the electrode surface might be instantaneous. The electrochemical impedance spectroscopy of As(III) was performed using As(III) loaded RuNPs/GC, and Nyquist curves thus obtained for different added concentration of As(III) in the range 2-12 µM in pH 2 buffer are shown in Figure 4. The diameters of the semicircles on the xaxis of the plot correspond to the interfacial electron transfer resistances (Rct). It is evident from Figure 4 that the charge transfer resistance of RuNPs/GC electrode was increased linearly with an increase in concentration of As(III). This could be attributed to the chemical interactions of As(III) with RuNPs leading to blocking of the active sites on the surface that hinders the electron-transfer process at the modified electrode.


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320

2 µM As(III) 12µM As(III)

280 240 200

Charge Transfer resistance (Rct)

105

100

160

Rct / Ohm

-Z" / Ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 80

95

90

85

80

40

2

4

6

8

10

12

Concentration of As(III) / µM

0 0

40

80

120

160

200

240

280

320

Z' / Ohm Figure 4. Nyquist plots obtained from impedance measurement for As(III) loaded RuNPs/GC electrode. Z and Z″are the real and imaginary components, respectively, of the impedance as a function of frequency ranging 106 Hz–10-2 Hz. Inset shows the plot of charge transfer resistance values obtained as a function of As(III) concentration. For determination of ultratrace concentration of As(III), differential pulse voltammetry (DPV) was employed to oxidize As(III) to As(V). For DPV experiment, the pulse amplitude of 50 mV, pulse width 0.05 s, and potential increment of 4 mV were employed. A reproducible peak current appeared at 0.6 V and such peak current was absent in the pH 2 solution without As(III), see Figure 5. There was a linear increase in DPV current with an increase in concentration of As(III) with a detection limit of 1.42 nM (0.1 ppb), at a signal to noise ratio of 3 as shown in inset of Figure 5. The value of correlation coefficient was 0.999 indicating an excellent linear fit of the experimental data. The slope of the linear plot (sensitivity) obtained for RuNPs/GC was 2.38nA ppb-1. The reproducibility of the electrochemical signal of the RuNPs/GC electrode was checked


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by carrying out a series of repetitive experiments for a fixed concentration of As (III) of 1.6 µM. Precise peak currents was obtained with a relative standard deviation (RSD) of 5.4% (n= 10). The same electrode was used for the determination of 1.6 µM of As(III) for five consecutive days and reproducible peak currents were obtained. No fouling of electrode was observed during the repeated uses. This may be due to a fact that As(III) is oxidized to As(V) which is not retained by RuNPs/GC electrode. Thus, the RuNPs/GC electrode exhibited a highly precise, reproducible and stable response for the quantification of ultratrace concentration of As(III).

2.0

1.6 1.4 1.2 1.0 0.8 0.6 0.4

0.14 0.12

Sensitivity=2.38 nA ppb-1 R2 = 0.9976

0.10

Current/ µ A

1.8

Increasing concentration of As(III)

Current density, J / µAcm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.08 0.06 0.04 0.02 0.00 0

10

20

30

40

50

60

Concentration of As(III) / ppb

0.2 0.0 0.4

0.5

0.6

0.7

0.8

0.9

1.0

E / V vs. Ag/AgCl

Figure 5. Differential pulse voltammetry curves as a function of As(III) conc. Inset shows the corresponding variation of current density as a function of conc. of As(III) in pH 2 Clark-Lubs buffer.


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The comparison of analytical performances of different NPs decorated electrodes for As(III) quantification in terms of chemical conditioning of the sample, detection limit (LOD) and sensitivity are given in Table 1. It is evident from Table 1 that RuNPs/GC based voltammetry could be used for As(III) in the pH range while most of the other NPs decorated electrode require high concentration of acid. In some cases, pH condition could be used but their LOD and sensitivity for As(III) quantification is not better than that obtained using the RuNPs/GC electrode. The sensitivity 2.38 nA ppb-1 obtained in the present work is highest among the different NPs decorated electrode based voltammetry reported in the literature. The sensitivity is a important parameter for the quantification of analyte and dependent on the response generating matrix.


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Table 1. Comparison of the analytical performances of the NPs decorated electrodes based electrochemical quantifications of As(III) reported in the literature. Analytical Methods

Electrode

Electrolyte

LOD(p pb)

Sensitivity

Ref.

SWV

MnFe2O4NPs on gold electrode

0.1 M acetate buffer pH=4.5

1.95

0.295 µA ppb-1

40

ASV

Au-Pd NPs /GC

0.1 M acetate buffer, pH=4.5

0.5

-

46

Potentiometry

As(III) imprinted membrane electrode

pH=6

37.5

-

57

LSV

Pt NPs on boron doped diamond electrode

0.1 H2SO4

0.5

-

47

ASV

FePt NPs on Si(100) substrate

0.1 MNa2HPO4−NaH2PO4 −KCl, pH=7.2

0.8

0.42 µA ppb−1

48

LSV/SWV

Au NPs on GC electrode

1 M HCl

0.0096 (LSV)

18 (LSV)/95 (SWV) µA µM-

42

1

SWV

Fe3O4-RTIL

0.1 M acetate buffer pH = 5

8 × 10-4

4.91 µA ppb-1

41

LSV

Au NPs/GC microspheres, bound together and “wiredup” using MWCNTs

1 M H2SO4

2.5

6 µA µM-1

43

SWV

Au NPs/GC

0.1 M PB buffer (pH = 5.0) solution containing 0.01 M EDTA

0.0025

16.15

44

µA µM-1

ASV

Au NPs/GC

1 M HNO3

0.25

0.400 µA .V ppb-1

45

DPV

Ru NPs/GC

Clark-Lubs buffer, pH=2

0.1

2.38 nA ppb-1

Present work

SWV: Square wave stripping voltammetry, ASV: Anodic stripping voltammetry, LSV: Linear sweep voltammetry, DPV: Differential pulse voltammetry.


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Dai et al. reported that the gold and platinum electrodes based determinations of As(III) by anodic stripping voltammetry suffer the problem of interference by Cu2+ ions, the most common and ubiquitous in water systems, but not Pt NPs modified glassy carbon electrode using the As(III) oxidation peak.51 In the present work, the possibility of Cu(II) ions interference in As(III) determination was examined using a solution containing 1 mM As(III) and 0.1M Cu(II) ions in Clark-Lubs buffer (pH 2). It is seen from Figure S6 (S.I.) that the As(III) oxidation peak is well separated from Cu(II) ions. Thus, there is no possibility of interference of Cu(II) ions present in a large amount in the determination of As(III). The analytical application of RuNPs/GC based DPV for As(III) quantifications was studied by using the real water samples such as lake water, ground water and seawater after adjusting pH to 2. The NaCl content of seawater is 0.55 M and, therefore, the As(III) was also determined in the spiked samples having varying concentrations of NaCl. In the real water samples, As(III) was not detected by either DPV or total X-ray reflection fluorescence (TXRF) analyses. Therefore, the known quantities of As(III) were spiked in these samples. It is seen from Table 2 that there is a constant deviation of 4.5 % in the values determined by RuNPs/GC based DPV with respect to As(III) spiked in all the samples except seawater where it is more. It appears that there is a constant efficiency factor of 95.5±1.7% for As(III) determinations in the NaCl solutions, lake water and ground water samples. The origin of this efficiency factor is not clear but appears to be related to As(III) speciation in these samples. This efficiency factor was further decreased to 83.0±0.5 % in the seawater samples. This also could be attributed to a change in As(III) speciation with a constant factor in seawater. However, this problem would not be encountered if As(III) is determined using a calibration plot obtained under same chemical conditions. As such, the As(III) concentration determined by using RuNPs/GC based DPV is highly reproducible


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(±0.5% at 10 ppb conc.) and have efficiency factor that does not depend on As(III) conc. but dependent upon the chemical composition of the aqueous sample. Table 2. Comparison of determination of As(III) spiked in the NaCl solutions and real water samples. Efficiencyb

(ppb)

Concentration determineda (ppb)

0.1 M NaCl

10

9.46

94.6

0.5 M NaCl

10

9.39

93.9

1.0 M

10

9.62

96.2

Lake water

7.6

7.2

94.7

Ground water

10

10

98.2

Seawater

7.6

6.3

82.9

Seawater

4

3.3

82.5

Seawater

2

1.67

83.5

Sample

As(III) spiked

(%)

a: average of three experiments, b: (Conc. determined/Conc. spiked)×100.

CONCLUSIONS Ru NPs have a remarkable capability for the inorganic arsenic speciation in waters by distinguishing arsenite from arsenate. In addition to chemisorption of As(III), the Ru NPs decorated glassy carbon electrode provides three important functions such as improved mass transport, sensitivity and catalytic effects. The work carried in the present paper indicates that the analytical performance of Ru NPs decorated glassy carbon electrode for the quantification of arsenite (LOD=0.1 ppb, sensitivity= 2.38 nA ppb-1, pH=2) is better than other NPs decorated electrodes based voltammetry. The 0.1 ppb detection limit obtained for arsenite is 100 times lower than arsenic WHO and EPA permissible limit of 10 ppb.58-59 However, there was a


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constant efficiency factors of 95.5±1.7% for ground water and lake water, and 83.0±0.5 % for seawater for As(III) determinations that should be taken care by using the calibration plot obtained under same chemical conditions. The Ru NPs would be amenable to anchor on the different chemical platform such as graphene, electrodes, Fe3O4 etc for designing the robust and reproducible chemical sensor for the arsenite detection at ultratrace concentration in a variety of aqueous matrices, and also its remediation. It is possible to further bring down arsenite detection limit by first preconcentrating arsenite on the RuNPs/GC electrode from a large volume sample using flow cell, and then subjecting to the electroanalysis. Supporting Information. Determination of electrochemical active surface area of electrodes, equivalent circuit used for fitting the impedance, XPS and voltammetry experiments figures are given. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Tel: +91-22-25594598; E-mail: [email protected] (RG), and Fax: +91-22-25505151; Tel: +91-22-25594566; E-mail: [email protected] (AKP). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT Authors thanks Mrs. Bharti Patra of IIT Powai and Dr. C.B. Basak, BARC, for carrying out the FEG-SEM experiments, and Dr. Sangita D. Lenka, Fuel Chemistry Division, BARC, for performing the EDXRF experiments. REFERENCES (1) Kang, X.; Zuckerman, N.B.; Konopelski, J.P.; Chen, S. J. Am. Chem. Soc. 2012, 134, 14121415. (2) Chen, W.; Pradhan, S.; Chen, S. Nanoscale 2011, 3, 2294-2300. (3) Nelson, D.J.; Manzini, S.; Urbina-Blanco, C.A.; Nolan, S.P. Chem. Commun. 2014, 50, 10355-10375. (4) Ren, F.; Feldman, A.K.; Carnes, M.; Steigerwald, M.; Nuckolls, C. Macromolecules 2007, 40, 8151-8155. (5) Chen, W.; Zuckerman, N.B.; Kang, X.; Ghosh, D.; Konopelski, J.P.; Chen, S. J. Phys. Chem. C 2010, 114, 18146-18152. (6) Chen, W.; Zuckerman, N.B.; Konopelski, J.P.; Chen, S. Anal. Chem. 2010, 82, 461-465. (7) Kang, X.; Chen, W.; Zuckerman, N.B.; Konopelski, J.P.; Chen, S. Langmuir 2011, 27, 12636-12641. (8) Chen, W.; Zuckerman, N.B.; Lewis, J.W.; Konopelski, J.P.; Chen, S. J. Phys. Chem. C 2009, 113, 16988-16995.


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As(III)

As(V)

2.0

Increasing concentration of As(III)

Current density, J / µAcm-2

TOC

RuNPs/GC disc electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.4

0.5

0.6

0.7

0.8

0.9

1.0

E / V vs. Ag/AgCl


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Highly Sensitive Detection of Arsenite Based on Its ... - ACS Publications

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