Detection of Trace Nitroaromatic Isomers Using Indium Tin Oxide

Sep 21, 2012 - School of Chemistry, University of New South Wales, Sydney 2052, Australia. Anal. Chem. , 2012, 84 (20), pp 8557–8563. DOI: 10.1021/a...
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Detection of Trace Nitroaromatic Isomers Using Indium Tin Oxide Electrodes Modified Using β‑Cyclodextrin and Silver Nanoparticles Xin Chen, Xiaoyu Cheng, and J. Justin Gooding* School of Chemistry, University of New South Wales, Sydney 2052, Australia S Supporting Information *

ABSTRACT: The determination of nitroaromatic compounds in aqueous solution was investigated at β-cyclodextrin (β-CD)/ silver nanoparticle (AgNPs) composite modified ITO electrodes. This method relies on the different reduction potentials for the various nitroaromatic isomers, the different binding strengths of the nitroaromatic isomer guests to the β-CD host, and excellent electron transfer ability of AgNPs. After incubation in a solution with different single nitroaromatic compounds, reduction peaks in the range from −550 to −913 mV were observed at the modified electrode, depending on the nitroaromatic compound present. The sensor exhibited selectivity for some isomers in a solution containing a mixture of nitroaromatic compounds. In particular, the sensor shows specificity for 4-nitroaniline and 1chloro-2-nitrobenzene over other nitroaniline isomers and nitrochlorobenzene isomers, respectively. The results show that all the nitroaromatic compounds, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, 1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, and 1chloro-4-nitrobenzene, could not only be detected but the electrode demonstrated a preference for the more strongly complexing species.

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low detection limit and high selectivity. However, the preparation of molecular imprinted polymers remains a challenge. Herein we focus on the second method: nanometer-scale materials. Briefly, we use an abundant, commercially available receptor for the detection of nitroaromatics, βcyclodextrin, and combine it with nanoparticles, as nanomaterials have had considerable success in improving the sensitivity of sensors.9,12,25−31 The properties of these nanomaterials are often different from that of their bulk counterparts, due to their extremely small size and large surface-to-bulk ratio as well as, in some instances, quantum size effects.32−34 Some nanometer-scale inorganic materials possess high sensitivity and selectivity and good stability to adsorbates and are becoming promising candidates for sensor fabrication. For example, zirconia nanoparticles were used as selective adsorbents in electrochemical sensors for organophosphate pesticides and nerve agents, 26 silicon nanowires for glucose and hydrogen peroxide,31 and gold clusters for dopamine.28 Among nanomaterials, noble metal nanoparticles (mainly AuNPs and AgNPs) possess the advantages of stability, conductivity, biocompatibility, low cytotoxicity, and size-related electronic and optical properties.35−37 Recent work has demonstrated that the immobilization of noble metal nanoparticles on bulk electrodes

itroaromatic compounds are environmental contaminants associated with anthropogenic activities, such as production and use of dyes, explosives, pesticides, and pharmaceuticals.1,2 Many of these substances, especially nitroaniline and chloronitrobenzene, which have been found in the wastewater of relevant industries, are considered highly toxic because of their purported mutagenicity and toxicity, even just in contact with skin.3 Moreover, because nitro-substituted aromatic compounds have strong electron-withdrawing groups, they are poorly biodegradable in the environment. Therefore, it is very important to develop simple and rapid analytical methods to identify and quantify these nitroaromatic isomers. At present, various methods have been employed for the determination of nitroaromatic compound, including fluorescence,4−8 luminescence,9,10 and electrochemistry.11−13 Among these methods, electrochemistry is one of the favored techniques in environmental and biological analysis, because of the potential for methods that are low-cost, highly sensitive, and simple to operate.11 Despite a variety of different electrochemical sensors being developed for nitroaromatic compound determination, there have been challenges with regard to sensitivity and moderate selectivity.14−18 In order to enhance the sensitivity and also selectivity, two main methods have been reported. One is to use molecularly imprinted polymers, which are synthetic polymeric materials with specific recognition sites complementary in shape, size, and functional groups to the template molecule, making ultratrace detection possible.19−24 These studies are interesting, particularly with respect to the reported © 2012 American Chemical Society

Received: May 29, 2012 Accepted: September 21, 2012 Published: September 21, 2012 8557

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contaminants. The ITO substrates were then rinsed with copious amounts of Milli-Q water, dried, and finally stored under nitrogen. Synthesis of β-CD Modified Silver Nanoparticles. The modified AgNPs were synthesized in analogy to the previous simple one-pot synthesis procedures.49 Briefly, AgNO3 aqueous solution (1 mL, 0.01 M) and trisodium citrate dehydrate aqueous solution (1 mL, 0.03 M) were added to Milli-Q water (97 mL), and sodium borohydride aqueous solution (1 mL, 1.79 mg/mL) was dropped into the solution with stirring. The formation of the AgNPs was confirmed by the solution changing to a yellow color after about 20 min, and this AgNPs aqueous dispersion was used without further purification in the following procedure. β-CD and AgNPs were mixed in a 2:1 molar ratio of β-CD to AgNO3, and the reaction mixture developed a buff color immediately. The reaction was allowed to proceed for 24 h with continuous stirring. At this point, a precipitate was collected by centrifugation. The obtained β-CDmodified AgNPs were washed three times with ethanol and deionized water separately and then were dried at 60 °C under vacuum for 4 h. Pretreatment of Self-Assembled Monolayers (SAMs). SAMs on ITO surfaces were prepared as described previously.52 Self-assembling molecules (phenylphosphonic acid) were adsorbed onto clean ITO surfaces from a 1 mM solution with THF as solvent for 24 h. The surfaces were rinsed with copious amounts of THF to remove loosely adsorbed species and dried in nitrogen. After that the modified substrates were annealed at 200 °C for 48 h under nitrogen to promote stable covalent bonding formation. The annealed surfaces were then rinsed with copious amounts of THF and Milli-Q water to remove possible multilayers and weakly bound molecules. Subsequently, the surfaces were dried under nitrogen. All the films were prepared immediately prior to use. Preparation of β-CD/AgNPs Composite Modified ITO Electrodes. Scheme 1 shows a simplified version of the

using self-assembled monolayers (SAMs) provides a simple, fast, and versatile approach for preparing biocompatible electrode surfaces with good electron transfer kinetics and low background signal.38,39 Compared with AuNPs, AgNPs possess larger scattering cross section and unique capabilities to amplify certain behaviors, such as Raman scattering40 and fluorescence.41 Moreover, AgNPs also have the excellent property of electrocatalytic reduction of many oxidative molecules, such as H 2 O 2 42,43 and nitroaromatic compounds.44,45 The latter studies highlight the potential of using AgNPs modified electrodes in electrochemical sensors to detect nitroaromatic compounds. Nonetheless, research on functional materials remains a challenge in the sensor development for detecting ultratrace nitroaromatic compounds, which motivated us to modify the AgNPs with cyclodextrin to give well-defined host−guest interactions on nanometer-scale electrodes. Cyclodextrins (CDs) are cyclic oligosaccharides that consist of six, seven, or eight glucopyranose units in α, β, and γ forms, respectively. They are well-known for forming an inclusion complex with various guest molecules because of their molecular structure containing a hydrophobic internal cavity and hydrophilic external surface.46,47 As reported in our previous work,48,49 CDs had been successfully used to identify the aromatic compounds isomers both spectroscopically and by the naked eye by exploiting the different association constants of the CDs with the different isomers. However, using the host−guest interaction between CDs and aromatic compounds to detect this series of pollutants by electrochemical methods has yet to be investigated to a significant extent. The purpose of the present paper is to explore the potential of an electrochemical sensor for the detection of nitroaromatic compounds formed using silver nanoparticles (AgNPs) modified with β-cyclodextrin (β-CD). These β-CD/AgNPs were complexed to an indium tin oxide (ITO) electrode modified with phenylphosphonic acid by exploiting the host− guest interaction between the β-CD and the phenyl ring of the phenylphosphonic. The resultant modified electrode, referred to as ITO/SAM/AgNPs/β-CD electrode, was explored for the detection of nitroaromatic compounds. AgNPs were used because previous studies have already established that silver nanoparticles can be used as an electrocatalyst for reduction of nitroaromatic compounds.50,51 The electrochemical behaviors of nitroaniline and chloronitrobenzene isomers on the modified electrode have been studied by square wave voltammetry (SWV), to provide and sensitive approach for determination of all the nitro aromatic compounds.

Scheme 1. Schematic of the Steps in the Preparation of ITO/ SAM/AgNPs/β-CD Electrode for Nitroaromatic Compound Isomers and the Expected Electrochemical Responses

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EXPERIMENTAL METHODS Reagents and Materials. 2-Nitroaniline, 3-nitroaniline, 4nitroaniline, 1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, 1-chloro-4-nitrobenzene, phenylphosphonic acid, potassium carbonate, potassium chloride, potassium ferricyanide, methanol, tetrahydrofuran, silver nitrate, sodium borohydride, trisodium citrate dehydrate, and β-cyclodextrin (β-CD) were obtained from Sigma-Aldrich Chem. Co. (Sydney, Australia). All reagents were used as received, and aqueous solutions were prepared with purified water (18 MΩ cm−1, Millipore, Sydney, Australia). Pretreatment of Electrode. Commercial ITO substrates obtained from SPI USA (6471-AB, 15−30 Ω) were first cleaned in an ultrasonicator with methanol for 10 min, followed by treatment with 0.5 M K2CO3 in 2:1 methanol:Milli-Q water mixture for 20 min to remove any residual organic

preparation procedure for ITO/SAM/AgNPs/β-CD electrode. SAMs-modified ITO surface was immerged into the β-CDmodified AgNPs solution (1.5 mg/L) and incubated for 24 h. The obtained surfaces were then rinsed with copious amounts of Milli-Q water to remove the possibility of physical absorption. Subsequently, the surfaces were dried under nitrogen. All the films were prepared immediately prior to use. 8558

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Characterization. All electrochemical measurements were performed with a BAS-100B electrochemical analyzer (Bioanalytical Systems Inc.) and a conventional three-electrode system. ITO substrates were obtained from SPI USA and have the surface characteristics we described previously that give high-quality monolayers by self-assembly of phosphonates.52,53 Platinum foil and a Ag/AgCl (3.0 M NaCl) electrode were used as the counter and reference electrodes, respectively. All potentials are reported versus the Ag/AgCl reference electrode at room temperature. All cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements were carried out in 0.1 M KCl solutions after 30 min of degassing with argon. For the SWV measurements, the current versus potential profile was measured in the negative direction, the parameters used were step potential = 4 mV, frequency = 15 Hz, amplitude = 25 mV, and quiet time = 2 s. All the scanning electron microscopy (SEM) images reported here were taken with an FEI Nova NanoSEM 230 FESEM instrument. Transmission electron microscopy (TEM) images were recorded on a Philips CM200 transmission electron microscope operated at 200 kV. For the TEM observation, samples were obtained by dropping 5 μL of solution onto carbon-coated copper grids. All the TEM images were visualized without staining. The infrared (IR) spectra were measured by AVATAR 320 FT-IR using KBr pellets. The ultraviolet−visible (UV−vis) spectra were measured with dilute aqueous solution in a 2 mm thick quartz cell using a Shimadzu UV-2401 PC spectrophotometer.

Figure 1. SEM images of the ITO/SAM/AgNPs/β-CD electrode.

molecules to the ITO surfaces, which is good evidence for the SAM passivating the electrode surface. In contrast, after attaching the AgNPs/β-CD, obvious redox peaks of ferricyanide appeared due to the excellent electron transfer ability of AgNPs. Thse observations are consistent with work by us and others, where it was shown by comparing electron transfer at the corresponding modified electrode with and without metal nanoparticles absorption that attaching AuNPs onto the distal ends of otherwise passivating organic layers allows electron transfer to proceed efficiently through the organic layer as though the organic layer is not even present.54−57 A theoretical description of the mechanism of the nanoparticle-mediated electron transfer was also reported by Chazalviel and Allongue very recently.58 The theory suggested that the effective nanoparticle-mediated electron transfer is due to a high exchange current density between the underlying electrode and the nanoparticles because of the high density of states available in the nanoparticle. The consequence of this high exchange current density is that the potential applied to the underlying electrode is effectively transferred to the nanoparticle. These results not only showed that β-CD-modified AgNPs could bind to the surface by host−guest interaction but indicated that the attachment of AgNPs offsets the passivating ability of the organic modification layer (phenylphosphonic acid and β-CD) in the application of electrochemical analysis and, hence, give the resultant sensing surface depicted in Scheme 1. Efficiency of the ITO/SAM/AgNPs/β-CD Sensor. To evaluate the ability of the modified electrode to detect nitroaromatic compounds, square wave voltammetry (SWV) was performed in solutions containing nitrobenzene over the potential range from 0 to −650 mV. Figure 2 shows SWV of the ITO electrode (Figure 2a) and ITO/SAM/AgNPs/β-CD electrode (Figure 2b) in an unstirred 0.1 M KCl solution without and with different concentrations of nitrobenzene. It was observed that with the addition of nitrobenzene, the reduction peak at −555 mV appeared and increased gradually over time. The reduction peaks of nitrobenzene at ITO/SAM/ AgNPs/β-CD electrode have much larger current than that at a bare ITO electrode, and the reduction peaks were shifted cathodically, indicating that the nitrobenzene complexed by the β-CD was harder to reduce. That is, the reduction peak was located at −537 mV with the peak height of about 3 μA (after baseline adaptation) for bare ITO electrode and at −555 mV with the peak height of about 12 μA (after baseline adaptation) for ITO/SAM/AgNPs/β-CD electrode when the concentration of nitrobenzene was 0.1 mM (as shown in figure S-4, Supporting Information). ITO surfaces modified with AgNPs without the β-CD showed a similar peak height to the ITO/ SAM/AgNPs/β-CD electrode, but again its position was more positive and similar to that on bare ITO. The peak shift when

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RESULTS AND DISCUSSION Characterization of the ITO/SAM/AgNPs/β-CD Electrode. The preparation of β-CD-modified AgNPs was monitored by UV−vis absorption, TEM, and IR spectroscopy (as shown in Figures S-1 and S-2 in the Supporting Information). The only absorption band at about 400 nm in the UV−visible spectrum represents the localized surface plasmon resonance (LSPR) for isolated nanoparticles, ascribed to the collective oscillation of the free electrons of the particles, which means that the attachment of β-CD did not have an obvious effect on the dispersion of AgNPs in solution. Further evidence regarding the structure of these modified AgNPs was given by transmission electron microscopy (TEM). As shown in Figure S-1 (Supporting Information), although the β-CDmodified AgNPs had a range of sizes between 10 and 30 nm, they were well-dispersed without any apparent aggregation. The interaction between the AgNPs and β-CD was also investigated by the IR spectrum (Figure S-2, Supporting Information). As can be seen in the spectra of β-CD compared with β-CD-modified AgNPs, obvious shifts in absorbance bands associated with β-CD (C−OH stretching vibration absorption from 1155 to 1111 cm−1 and the O−H stretch shifting from 3389 to 3357 cm−1) were observed after mixing with AgNPs. All bands are ascribed to the adsorption of β-CD on AgNPs surface via OH moieties. The modified electrode was characterized using SEM and the electrochemistry method. As shown in the SEM image in Figure 1, the AgNPs on the surface were well-dispersed on the surface, again with little or no aggregation. Figure S-3 (Supporting Information) displays the cyclic voltammograms (CV) for ITO/SAMs (black curve) and ITO/SAM/AgNPs/βCD (red curve) in 0.1 mM ferricyanide in a 0.1 M KCl solution (pH 7.0) at a scan rate of 100 mV s−1. The SAM-modified surfaces almost totally restricted access of the ferricyanide 8559

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0.1 mM in 0.1 M KCl solution. The 4-nitroaniline, 2nitroaniline, 3-nitroaniline, 1-chloro-2-nitrobenzene, 1-chloro4-nitrobenzene and 1-chloro-3-nitrobenzene show distinct current peaks situated respectively at −911, −856, −813, −802, −727, and −692 mV vs the Ag/AgCl reference electrode at pH 7.0. The electrochemical peak is attributed to the reduction of the nitro moiety to a nitroso group, which has been further investigated by corresponding cyclic voltammetry (CV) experiments (Figures S-10 and S-11, Supporting Information). As can be seen from Figure 3, the reduction peak positions of all the nitroaniline isomers are more negative than those of the chloronitrobenzene isomers, with the overall order of the reduction peak for the different nitroaromatic compounds simplistically being progressively more negative with electron-donating groups.62,63 However, this simple trend is complicated by the fact that steric effects from the orthosubstituent group make the reduction potential of orthosubstituted nitrobenzene shift to more negative than that of the other compounds,62 and also complicated by the binding strength inclusion complex between the nitroaromatic isomer guests and β-CD host, which is dramatically dependent on the structure of the molecules and the position of any substituents.63 Previous studies show that the stronger the interaction between β-CD and a nitroaromatic isomer is, the more energy needed to reduce the nitro group and hence the more negative the reduction potential.62 Although the specific process of the substituent position effect on the reduction potential of nitroaromatic compounds is not fully understood, the results clearly show that the sensor can identify nitroaniline and chloronitrobenzene isomers. So far, the identification of different nitroaromatic isomers from single-component solutions has been achieved. However, samples for analysis in the real world, such as industrial waste, are significantly more complicated and could contain several different nitroaromatic compounds. As discussed above, the sensor is selective to the molecules most strongly captured by β-CD. Hence, the selectivity of the sensor to various nitroaniline isomers and nitrochlorobenzene isomers was investigated by running SWV in solutions containing mixtures of nitroaromatic compounds. Figure 4 compares the SWV of the ITO/SAM/AgNPs/β-CD electrode in a mixed solution with three different nitroaniline isomers (4-nitroaniline:3nitroaniline:1-nitroaniline in a 1:1:1 ratio) where each is at a concentration of 1 × 10−4 M to SWVs for the three individual nitroanilines at the same concentration (1 × 10−4 M). For the mixture a reduction peak at about −911 mV appeared in the

Figure 2. Square wave voltammetry for different concentrations of nitrobenzene at bare ITO electrode (a) and ITO/SAM/AgNPs/β-CD electrode (b) in 0.1 M KCl solution (pH 7.0), and the corresponding calibration lines after background subtraction and baseline correction (c).

β-CD was present is attributed to the change in environment of the nitro group upon insertion into the β-CD cavity, as has been suggested previously,59,60 stabilizing the nitro moiety and making it harder to reduce. Further evidence that the nitrobenezene is complexed with the β-CD is presented in the Supporting Information (Figures S-5−S-9). All these results indicated that the sensor preferentially detects the determinand molecules captured by β-CD, instead of the determinand molecules surrounding AgNPs in solution. This not only leads to greater efficiency but gives the sensor an ability to identify different nitroaromatic compounds according to their host− guest interaction with β-CD.61 Identification of Different Nitroaromatic Compound Isomers. Figure 3 shows SWV of different isomers of nitroaniline and chloronitrobenzene with the concentration of

Figure 3. Square wave voltammetry of 1 × 10−4 M nitroaniline (a) and chloronitrobenzene (b) isomers at ITO/SAM/AgNPs/β-CD electrode in 0.1 M KCl solution (pH 7.0) and the corresponding calibration lines of molecular structure effect to reduction peak position (c). Part c represents 4-NA (4-nitroaniline), 2-NA (2-nitroaniline), 3-NA (3nitroaniline), 2-NC (1-chloro-2-nitrobenzene), 4-NC (1-chloro-4nitrobenzene), 3-NC (1-chloro-3-nitrobenzene), and NB (nitrobenzene).

Figure 4. (a) Square wave voltammetry of 1 × 10−4 M nitroaniline isomers mixed system (4-nitroaniline:3-nitroaniline:2-nitroaniline = 1:1:1, black line), 4-nitroaniline (red line), 2-nitroaniline (green line), and 3-nitroaniline (blue line). (b) The corresponding reduction peak position versus different samples. The mixed system, 4-nitroaniline, 3nitroaniline, and 2-nitroaniline are presented as mixed, 4-NA, 2-NA, and 3-NA, respectively. 8560

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SWV, which is almost identical to that for the 4-nitroaniline. This result indicates that β-CD has a preference for the 4nitroaniline over the other isomers in the mixture and hence the sensor can distinguish 4-nitroaniline from the other nitroaniline isomers. Similar results were observed in the experiment involving nitrochlorobenzene isomers; however, this time the 1-chloro-2-nitrobenzene is the selected isomer (as shown in Figure S-12, Supporting Information). The selectivity for 4-nitroaniline isomers and 1-chloro-2-nitrobenzene isomers over the others in a mixture is attributed to the shape selectivity of the β-CD cavity and the different inclusion binding strength of the various enantiomer guests to β-CD host. For nitroaniline isomers, the inclusion binding strength order is 4-nitroaniline > 2-nitroaniline > 3-nitroaniline.64 However the order is 1-chloro2-nitrobenzene > 1-chloro-4-nitrobenzene > 1-chloro-3-nitrobenzene for nitrochlorobenzene isomers. 65 Hence, the dominance of 4-nitroaniline and 1-chloro-2-nitrobenzene in their respective mixed solutions tests is consistent with the previously reported relative binding strengths. The selectivity of the ITO/SAM/AgNPs/β-CD electrode for one nitroaromatic compound over the others does not preclude all from being detected. If a high surface area electrode was employed, the sensor could not only be used to specifically detect the captured nitroaromatic compound but also remove it from the sample. Hence, in a series of repeated steps, each nitroaromatic compound in a sample could be identified and quantified. This is illustrated to some degree using solutions containing mixtures of the three nitrochlorobenzene isomers plus either 4-nitroaniline or 2-nitroaniline (the ratio of the nitroaniline:1-chloro-2-nitrobenzene:1-chloro-4-nitrobenzene:1-chloro-3-nitrobenzene = 1:1:1:1, 1 × 10−4 M). As shown in Figure S-12 (Supporting Information), nitroaniline will be selected in a mixed solution over 2-nitroanoline. In Figure S-13 (Supporting Information) it can be seen that 4nitroaniline is selected over any of the nitrochlorobenzene isomers. However if 4-nitroaniline is absent but 2-nitroaniline is present, it too can be selected by the β-CD over any of the nitrochlorobenzene isomers61,66 in a mixture (see Figure S-13, Supporting Information). Sensitivity to Different Nitroaromatic Compound Isomers. In order to check the potential use of the nitroaromatic compounds sensor for practical applications, the dynamic detection concentration range was evaluated. Quantitative analysis results of the nitroaromatic compound isomers are shown in Figures 5, 6, S-14, and S-15 (Supporting Information), which display the square wave voltammetry as a function of the concentration of nitroaniline (Figures 5 and Figure S-14, Supporting Information) and chloronitrobenzene

Figure 6. (a) Square wave voltammetry for different concentrations of 1-chloro-2-nitrobenzene at ITO/SAM/AgNPs/β-CD electrode in 0.1 M KCl solution (pH 7.0) and (b) the corresponding calibration lines after background subtraction and baseline correction.

(Figures 6 and Figure S-15, Supporting Information) isomers. At lower concentrations (0.1−10 μM), the reduction peak height is particularly sensitive to increases in concentration; however, the trend indicates a saturation of surface binding sites after the concentration exceeds 10 μM. All of these above results clearly indicated that the reduction peaks are undoubtedly associated with the electrochemical reduction of nitro group from the nitroaromatic compound isomers, and the lowest detected concentration for the sensor to nitroaromatic compound isomers was as low as 5 × 10−8 M (see Table S1 of the Supporting Information for the lowest detected concentration for each nitroaromatic compound). According to the integrated wastewater discharge standards of Australia, the concentration of nitroaromatic compounds in discharged wastewater should be less than 6 μM. Hence, the detection limit of our sensor to trace amounts of different nitroaromatic compounds is sufficient to evaluate the total toxicity of an environmental water sample. Determination of Nitroaromatic Compound in Complex Matrices. To explore the detection ability of our β-CDAgNP/SAM/ITO electrode to nitroaromatic compound in complex matrices, we chose 1-chloro-2-nitrobenzene as a test analyte to be detected in both groundwater and tap water. The results show very strong square wave voltammetry signals and high recoveries for both groundwater and tap water samples, which means our β-CD-AgNP/SAM/ITO electrode can also be used successfully for nitroaromatic compounds determination (as can be seen from Figures S-16 and S-17 and is further described in the Supporting Information). The percentage recovery, compared to laboratory solutions, was 90% for 1chloro-2-nitrobenzene determinations from groundwater and 95% from tap water samples. Note that these natural groundwater and tap water samples did have some impact on the lowest detected concentration, though, as this rose from 5 × 10−8 M for 1-chloro-2-nitrobenzene in buffer to 1 × 10−7 M in the natural water samples (see Table S1, Supporting Information), but these lowest detect concentrations were still well below the required detection limits.

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CONCLUSION In this work, we fabricated a β-CD/AgNPs/SAMs modified ITO electrode to detect nitroaniline and chloronitrobenzene isomers by square wave voltammetry (SWV). Under the same concentration, various isomers of nitroaromatic compounds show characteristic reduction peaks, which make them easily distinguished from each other. The lowest detection concentrations were 1 × 10−7 mol/L or lower for all the isomers. Moreover, the sensor can also selectively detect 4-nitroaniline

Figure 5. (a) Square wave voltammetry for different concentrations of 2-nitroaniline at ITO/SAM/AgNPs/β-CD electrode in 0.1 M KCl solution (pH 7.0) and (b) the corresponding calibration lines after background subtraction and baseline correction. 8561

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from other nitroanilines and 1-chloro-2-nitrobenzene from other nitrochlorobenzene isomers. The selectivity was determined by the strength of the host−guest interaction between β-CD and the respective isomers, which gives us an ability to detect the composition one-by-one from a complicated sample containing multiple nitroaromatic compounds and isomers in a series of detect and remove cycles. Moreover, the proposed sensor was also used successfully for determination of nitroaromatic compounds in natural water samples. This proposed method exhibited many excellent properties, such as high sensitivity and selectivity, low detection limit, and wide detection range, which makes it very useful in applications for monitoring nitroaniline and chloronitrobenzene isomer pollutants in environmental and biological samples.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council (LP100200593) and the University of New South Wales for funding.

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REFERENCES

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