Ultrasensitive and Ultraselective Impedimetric Detection of Cr(VI

Jan 12, 2015 - Nanomaterials and Environmental Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei. 230031 ...
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Ultrasensitive and Ultraselective Impedimetric Detection of Cr(VI) Using Crown Ethers as High-Affinity Targeting Receptors Juan Wei,†,‡,§ Zheng Guo,† Xing Chen,† Dong-Dong Han,† Xiang-Ke Wang,*,‡,§ and Xing-Jiu Huang*,†,‡ †

Nanomaterials and Environmental Detection Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China § Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China S Supporting Information *

ABSTRACT: Detection of Cr(VI) by electrochemical methods generally focuses on noble-metal-modified electrodes in strong acid solution using voltammetric techniques. In this work, we report a new strategy to detect Cr(VI) as HCrO4− at pH 5.0 in drinking water using electrochemical impedance spectroscopy. The strategy is based on the high-affinity and specific binding of crown ethers (i.e., azacrown) to HCrO4−, which forms sandwich complexes between them via hydrogen bonds and moiety interactions with K+ captured by azacrown on its self-assembled Au electrode surface. This then blocks the access of redox probes (Fe(CN)63−/4−) to the self-assembled Au electrode, further resulting in an increase in the electron transfer resistance. This method offers a detection limit of 0.0014 ppb Cr(VI) with a sensitivity of 4575.28 kΩ [log c (ppb)]−1 over the linear range of 1−100 ppb (R2 = 0.994) at pH 5.0. In addition, the azacrown self-assembled Au electrode has good selectivity for Cr(VI) with good stability and low interferences. This approach can be performed on spiked Cr(VI) as well as real samples. To the best of our knowledge, this is the first example of electrochemical impedimetric sensing that allows ultrasensitive and ultraselective detection of Cr(VI).

C

however, they are currently being replaced with noble-metal electrodes, including Au, Ag, etc. Using LSV (linear sweep voltammetry), Banks et al.18 found a limit of detection of 228.8 ppb with a sensitivity of 1.04 × 10−3 μA ppb−1 for Cr(VI) in 0.05 M H2SO4 solution on a gold screen-printed macroelectrode. Wang et al.19 determined Cr(VI) on a Au microchip using DPCSV (differential pulse cathodic stripping voltammetry) and achieved a limit of detection (LOD) of 46.8 ppb with a sensitivity of 1.4 × 10−3 μA ppb−1. Lin et al.7 measured Cr(VI) in an acetate buffer electrolyte (pH 4.6) by SWV (square wave voltammetry) and obtained an LOD of 5 ppb with a sensitivity of 0.3 × 10−3 μA ppb−1 on a Au nanoparticle electrode. Compton’s group20 investigated Cr(VI) detection by LSV in 0.3 M HNO3 media on a gold-plated carbon-based composite electrode giving an LOD of 4.4 ppb with a sensitivity of 3.7 μA ppb−1. Zi et al.2 used a Au nanoparticle-modified glass carbon electrode to determine Cr(VI) by SWV in 0.1 M HCl media and obtained an LOD of 0.01 ppb with a sensitivity of 0.115 μA ppb−1. Xu and Liu21 prepared a Ag nanoparticle-coated Au nanoporous film for Cr(VI) determination in 0.1 M HNO3 by

hromium ion is a classic and dangerous water pollutant that has received great attention.1−3 Aqueous chromium is most commonly found in two main stable statesCr(III) and Cr(VI). Cr(III) is necessary for biological processes and mostly harmless.2,3 Cr(VI) is a carcinogen and is extremely harmful to the biosphere; even traces amounts pose a detrimental risk to human health. World Health Organization (WHO) guidelines limit Cr(VI) to 50 ppb in groundwater.4 Thus, the detection of trace levels of Cr(VI) in contaminated groundwater is an important topic. Currently, the most accurate detection methods include ultraviolet spectrometry, atomic absorption spectrometry, X-ray fluorescence spectrometry, spectrofluorimetry, high-pressure liquid chromatography, and inductively coupled plasma mass spectrometry, etc.4−13 In general, these methods suffer from an expensive laboratory infrastructure, high operating costs, and time-consuming protocols. Electrochemical methods are important alternatives that have been widely used for the analysis of heavy metal ions. These techniques offer rapid turnaround times, low costs, and portability. They can be used for routine in-field monitoring of many analytes. A well-known electrochemical technique for the determination of Cr(VI) is voltammetry using mercury14,15 or bismuth film16,17 electrodes. Considering their potential toxicity and operational limitations, © 2015 American Chemical Society

Received: November 29, 2014 Accepted: January 12, 2015 Published: January 12, 2015 1991

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

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Aldrich. N,N′-Dicyclohexylcarbodiimide (DCC; 99%) and 4(dimethylamino)pyridine (DMAP; 99%) were purchased from Aladdin. All other reagents were commercially available from Sinopharm Chemical Reagent Co. Ltd. (China) and were of analytical grade. The ultrapure water used to prepare all solutions was purified from a Millipore water purification system (Milli-Q, specific resistivity >18 MΩ cm, S.A., Molsheim, France). Stock solutions of Cr(VI) were prepared with potassium dichromate, with the pH of the HCrO4− or CrO42− solutions controlled by dilute HCl or KOH.30,31 Apparatus. The electrochemical experiments were performed with a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China) with a standard three-electrode system. A bare 2 mm Au electrode or modified Au electrode served as the working electrode; a Pt wire was the counter electrode versus a saturated Ag/AgCl electrode (ChenHua Instruments Co., Shanghai, China), completing the cell assembly. The solutions were deoxygenated with nitrogen bubbling for 10 min before each determination. All experiments were performed under N2 at room temperature. AFM images were taken using a Nanoscope III (Digital Instruments, Veeco Metrology). XPS was performed on a Thermo ESCALAB 250 spectrometer using an Al Kα X-ray source (1486.6 eV, 150 W). Mass spectrometry was measured on an Agilent Q-TOF 6540 mass spectrometer employing a regular ESI source setup. Silica gel 60 (230−400 mesh) was the solid phase for column chromatography. Synthesis of Azacrown. Azacrown (11-mercapto-N,N(1,4,7,10,13-pentaoxa-16-azacyclooctadecyl)undecanamide) was synthesized according to a previous report.32 First, 0.207 g of 11-mercaptoundecanoic acid (0.95 mM), 0.25 g of 1-aza-18crown-6 (0.95 mM), and 0.116 g of 4-(dimethylamino)pyridine (0.95 mM) were added to a 10 mL round-bottomed flask containing 3.125 mL of anhydrous dichloromethane and stirred until they were completely dissolved. Then 0.196 g of N,N′dicyclohexylcarbodiimide (0.95 mM) in 1.875 mL of anhydrous dichloromethane was added. The reaction was stirred overnight under N2 at room temperature. After the reaction was finished, the white solid byproduct was removed by vacuum filtration, and the filtrate was concentrated under reduced vacuum. The yellow oily crude product was dissolved in an appropriate amount of dichloromethane and purified by silica gel chromatography using dichloromethane−methanol (50:1, v/ v) as the eluent. Self-Assembly of the Azacrown Monolayer on a Au Electrode. Self-assembly of the azacrown monolayer on a Au electrode (azacrown monolayer/Au electrode) was performed as follows: First, the 2 mm Au electrode was polished with a 0.3 and 0.05 μm alumina−water slurry (Buehler) on a polishing cloth (Microcloth, Buehler). The electrode was then treated with successive rounds of sonication with ultrapure water and anhydrous ethanol. The electrode was further activated with a 0.5 M H2SO4 solution by a CV (cyclic voltammetry) scan in the range of −0.1 to +1.7 V at a scan rate of 50 mV s−1, until the reduction peak current of the Au electrode achieved a steady state. Second, the pretreated Au electrode was immersed in 10 mM azacrown in ethanol for 12 h at room temperature and then rinsed with ethanol and water and dried under a stream of N2. Electrochemical Measurements. The electrochemical characterization, including cyclic voltammograms and EIS, was performed in a degassed solution with high-purity N2 (Nanjing Special Gases Factory Co., Ltd.) containing 5 mM

amperometry with an LOD of 0.65 ppb and a sensitivity of 0.15 × 10−3 μA ppb−1. Jin et al.22 detected Cr(VI) in 0.1 M HNO3 media on a Ag nanoparticle-modified electrode by an amperometric method and obtained an LOD of 0.67 ppb with a sensitivity of 1.1 × 10−3 μA ppb−1. Other approaches include that of Chen and coauthors,23 who detected Cr(VI) in 0.1 M HCl media on Au nanoparticle-decorated titania nanotube electrodes with an LOD of 1.56 ppb and a sensitivity of 0.133 μA ppb−1. Xue et al.24 used flowerlike self-assembly of Au nanoparticles on a glass carbon electrode to determine Cr(VI) in fluoride buffer (pH 4.5) media by CSSWV (cathodic stripping square wave voltammetry) and obtained an LOD of 0.15 ppb. Jena and Raj25 reported a nanosized Au particle for the amperometric determination of Cr(VI) in 0.1 M HCl media and obtained an LOD of 0.1 ppb with a sensitivity of 0.03 μA ppb−1. Mandler et al.26 could detect 1.2 ppt Cr(VI) on the basis of a self-assembled monolayer of 4-(mercapto-n-alkyl)pyridinium on a gold surface in fluoride buffer (pH 4.5) media by SWV. However, despite this great success, Cr(VI) detection using noble-metal electrodes still largely relies on harsh conditions, including strong acids, the electrode morphology after modification, etc. Recently, crown ethers have been employed to adsorb anionic metal ion complexes from aqueous solution. For instance, Mohammad Reza et al.27 removed Cr(VI) from an acid solution with an oxonium ion−crown ether complex via the ion pair of a positively charged lipophilic complex (HCE+, where CE is crown ether). The counterion was HCrO4− or CrO3Cl−. Burgard et al.28,29 employed dicyclohexano-18crown-6 (L) as an extractant for Cr(VI) via the formation of the complex ion pair L(H3O)+CrO3Cl−. Similarly, Yilmaz et al.30 synthesized the novel compound p-tert-butylcalix[4]azacrown ionophore for the removal of Cr(VI) from aqueous solutions through the protonated effect of nitrogen atoms on an azacrown moiety. This scheme used hydrogen bonding to the host molecule at pH 1.5−3.5 or the complexation of the azacrown moiety with Na+ and hydrogen bonding to the host molecule at 4.5 < pH < 6.0. Inspired by this adsorption mechanism, we here report the first results of ultrasensitive and ultraselective detection of Cr(VI) by EIS (electrochemical impedance spectroscopy) using a self-assembled monolayer of crown ethers (i.e., azacrown). This approach may guarantee the success of the selection of aptamers with high affinity and specificity. The 2 mm diameter Au electodes were used as a solid matrix to immobilize the azacrown. Cr(VI) was mainly in the HCrO4− form (pH 5.0) and could bind specifically and tightly to the azacrown monolayer. This hinders the electron transfer from the electrode to the redox probe (Fe(CN)63−/4−). First, the synthesized azacrown was investiaged by ESI-MS (electrospray ionization mass spectrometry); the monolayer of azacrown before and after Cr(VI) adsorption was examined using AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy). Next, the responses of Cr(VI) on the azacrown monolayer were studied by EIS and compared to those of previous studies. The EIS was found to produce the most sensitive signal. Other parameters such as antiinterference, selectivity, and stability of the azacrown monolayer were studied in addition to the analysis of spiked Cr(VI) in real samples.

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EXPERIMENTAL SECTION Chemical Reagents. 11-Mercaptoundecanoic acid (95%) and 1-aza-18-crown-6 (98%) were obtained from Sigma1992

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

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form of HCrO4− ion. It is CrO42− above pH 7.0.35At pH 5.0, one azacrown could complex with K+ according to host−guest interactions. Thus, after introduction of Cr(VI), a sandwich complex is formed via N···H−[ HCrO4−]−K+ via hydrogenbonding and electrostatic interactions. This is responsible for the inhibition of electron transfer. Examination of Synthesized Azacrown and Azacrown Monolayers Self-Assembled on the Au Electrode. The amide reaction between the COOH groups of 11-mercaptoundecanoic acid and the amino group of 1-aza-18-crown-6 was confirmed by ESI-MS. Figure 1 presents ESI-MS data of the

Fe(CN)63−/4− and 0.1 M KCl (pH 5.0). The desired pH value of the solution was adjusted with HCl or KOH. The frequency range of EIS was from 0.1 to 100000 Hz or from 1 to 100000 Hz with a signal amplitude of 5 mV; the CV scan rate was 0.1 V s−1. Before detection of Cr(VI), the azacrown monolayer/Au electrode was immersed into the desired pH solution (pH 5.0) and preconcentrated for 1 h in open circuit potential conditions with stirring. After the electrode was rinsed with ultrapure water, EIS was performed in 10 mL of 0.1 M KCl solution containing 5 mM Fe(CN)63−/4− (pH 5.0). This process was repeated several times until the electron transfer resistance from the redox probe (Fe(CN)63−/4−) to the electrode surface (Ret) became stable. The azacrown monolayer/Au electrode was then used to detect Cr(VI) in water.

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RESULTS AND DISCUSSION Detection Strategy of Cr(VI). Scheme 1 illustrates the detection strategy of Cr(VI) based on special inhibition of Scheme 1. Schematic of Cr(VI) Detection via an Azacrown Monolayer Self-Assembled on a Au Electrodea

Figure 1. ESI-MS spectrum of the synthesized azacrown. The impurities were separated for the ideal product via silica gel column chromatography.

complex structures in anhydrous ethanol solution. The peaks of the complex structure at m/z = 464.31 and the complex structure + Na at m/z = 486.29 are clearly detected. According to W. Grant McGimpsey’s report, the m/z (fragment) peaks at 464.5 (M + 1) and 486.5 (M + Na)32 confirm that 11mercaptoundecanoic acid and 1-aza-18-crown-6 were successfully combined through a dehydration condensation reaction. Figure 2 shows AFM characteristics of the bare Au and azacrown monolayer/Au before and after adsorption of Cr(VI). For bare Au surfaces, the surface is nearly smooth and the surface root mean square (rms) is 0.151 nm (Figure 2a,b). This is accompanied by layer-by-layer assembly of azacrown in which the roughness gradually increases. The root mean square increases to 0.572 nm, and some large pinholes and inhomogeneities in the film surface are clearly seen (Figure 2c,d).36 This demonstrates that the azacrown molecules were assembled on the smooth Au electrode surface through the S− Au bond. The height is ca. 2 nm, as judged from Figure 2d, which is consistent with that theoretically estimated in Scheme 1. As shown in Figure 2e,f, after adsorption of Cr(VI), a dotlike morphology is observed, and the roughness of the surface markedly increases to 2.53 nm. The thickness reaches 4.788 nm, which indicates that Cr(VI) has been successfully adsorbed by the azacrown monolayer. To further illustrate the azacrown monolayer/Au before and after adsorption of Cr(VI), we next examined the surface using XPS. Figure 3 shows the typical XPS spectra for azacrown monolayer/Au after adsorption with Cr(VI). The peaks at

a Cr(VI) (i.e., HCrO4− in pH 5.0) could be selectively adsorbed by the azacrown monolayer due to the formation of azacrown−HCrO4− complexes via the electrostatic forces and hydrogen bonding. As such, a layer of azacrown−HCrO4− complexes was formed on the Au electrode surface, which can be a barrier to the access of the redox probe (Fe(CN)63−/4−) to the electrode surface, resulting in an increase in the electron tranfer resistance (Ret).

electronic transport on azacrown-modified Au electrodes. It is basically due to the formation of a sandwich complex after Cr(VI) adsorption that can be a barrier to the access of the redox probe (Fe(CN)63−/4−) to the electrode surface. Because of the long chain of azacrowns, the self-assembly process of azacrown increases the roughness of the surface of the gold electrode.33,34 The thickness is about 2.2 nm for the selfassembled monolayer, which is estimated from teh onedimensional size of as-synthesized azacrown along its long carbon chain (theoretically predicted by the DFT method using Gaussian 03). Below pH 5.0, Cr(VI) is mainly present in the 1993

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

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Figure 3. High-resolution XPS spectra for azacrown monolayer/Au before and after adsorption with Cr(VI): (a) C 1s, (b) N 1s, (c) O 1s, (d) S 2p, (e) Au 4f, and (f) Cr 2p.

azacrown monolayer, the current peak for the redox probe Fe(CN)63−/4− has a drastic decrease, and the oxidation peak potential shifts positively to 0.75 V. When the azacrown monolayer continues to adsorb Cr(VI), the faradic current almost disappears versus the bare Au electrode. This suggests that electron transfer between the Au substrate and probes in solution is nearly completely hindered by the film of sandwich complexes. This implies that the azacrown monolayer has been successfully assembled on the Au substrate and adsorbed with Cr(VI). The corresponding EIS diagram could also demonstrate this result. For bare Au electrodes, a very small semicircle domain (Ret) about 120 Ω was observed, suggesting that the electron transfer is good on the bare Au electrode. However, Ret increases to 299.1 kΩ after assembly of the azacrown monolayer. When the azacrown monolayer is adsorbed with Cr(VI), Ret greatly increases to 650.6 kΩ with only a semicircle portion shown in the EIS diagram. This is because the azacrown monolayer−Cr(VI) (azacrown monolayer adsorbed with Cr(VI)) inhibits the electron transfer to the electrode surface, in agreement with the results shown in Figure S1a. Impedimetric Detection of Cr(VI). Figure 4a shows electrochemical impedance spectra of the azacrown monolayer/ Au electrode toward Cr(VI) at different concentrations in the desired pH solution (pH 5.0) containing 5 mM Fe(CN)63−/4− and 0.1 M KCl. The addition of Cr(VI) produced a significantly large electron transfer resistance (Ret). The impedance data were fitted with commercial software Zview2. A modified Randle’s equivalent circuit and the fitting of each measured spectrum to the equivalent circuit (solid line) are both shown in Figure 4a and agree well with the circuit model. The equivalent circuit contains the following five elements (inset in Figure 4a): Rs, the resistance of the electrolyte solution; Qmono,

Figure 2. Tapping model AFM characteristics of the azacrown layer self-assembled on the Au electrode surface before and after adsorption with Cr(VI). (a, c, e) Phase images of bare Au, azacrown/Au, and azacrown/Au after absorption with Cr(VI), respectively. Scale: 2 μm × 2 μm. (b, d, f) Cross-sectional analysis of the AFM images corresponding to panels a, c, and e, respectively.

284.72, 399.7, 531.27, and 163.48 eV are for C 1s, N 1s, O 1s, and S 2p, respectively. They originate from the azacrown monolayer (Figure 3a−d). A shoulder at 532.33 eV in the O 1s spectrum may be associated with the hydroxyl group or water on the surface of the electrode.37 In Figure 3d, one peak at 163.3 eV can be assigned to the sulfur atom in the C−S−Au group, and the other peak at 164.6 eV is for the sulfur atom in the C−S−H group. The Au 4f peaks are located at 87.8 and 84.1 eV and obviously came from the Au electrode (Figure 3e). As seen from Figure 3f, the Cr 2p peak contains two spin−orbit split peaks: 2p3/2 and 2p1/2. For 2p3/2, two peaks at 579.08 and 576.28 eV are observed, which are attributed to Cr(VI) and Cr(III).38 The appearance of Cr(III) may be due to the reduction of Cr(VI) during the measurements. The amount of Cr is 1.39 atom % (Figure 3f). It is worthwhile to point out that the amount of Au dramatically decreases to 1.53 atom % before adsorption of Cr(VI) (29.61 atom %, Figure 3e). This indicates that the influence of adsorption of Cr(VI) on the Au substrate was much more severe. Electrochemical Characterization of the Azacrown Monolayer Self-Assembled on the Au Electrode. Electrochemical characterization of the azacrown monolayer/Au electrode was performed by CV (Figure S1a, Supporting Information) and EIS (Figure S1b). After assembly of the 1994

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

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concentrations, resulting in an increased difficulty of electron transfer. In the above linear equation, ΔRet is calculated by the following equation: ΔRet (kΩ) = Ret(azacrown monolayer− Cr(VI)) − Ret(azacrown monolayer), where Ret(azacrown monoalyer) and Ret(azacrown monolayer−Cr(VI)) represent the electron transfer resistance before and after adsorption of Cr(VI), respectively. To further understand the impedance response of the azacrown monolayer/Au electrode toward Cr(VI), the relationship between the response (ΔRet/(Ret)0) and the Cr(VI) adsorption of the azacrown monolayer can be investigated by considering a Langmuir isotherm.39,40 Here, we assume that the response (ΔRet) is approximately proportional to the surface density of Cr(VI) bound to the azacrown monolayer on a Au electrode. This directly depends on the concentration of Cr(VI) in the bulk solution. The Langmuir isotherm can be transformed as a linear equation as follows: [Cr(VI)] [Cr(VI)] 1 = + ΔR et /(R et)0 (ΔR et)max /(R et)0 K (ΔR et)max /(R et)0

where (ΔRet)max is the maximun response of all the azacrown on the electrode interacting with Cr(VI), [Cr(VI)] is the concentration of Cr(VI) in the bulk solution, and K is the binding constant between Cr(VI) and azacrown. In the range from 1 to 100 ppb, K is 1.113 × 107 M−1. Compared to previous reports (Table 1), the present method shows obvious advantages, including the neutral electrolyte, LOD (0.0014 ppb), and sensitivity (4575.28 kΩ [log c (ppb)]−1). This follows the United States Environmental Protection Agency’s (U.S. EPA’s) guideline value of 100 ppb. In the comparison with results obtained in the broader range of low frequency, the tendency obtained within the frequency ranging from 1 to 100000 Hz is almost similar. And thus, the following impedance detections were carried out within this frequency range. Furthermore, the impedance data within this frequency range toward Cr(VI) with different concentrations are presented in Figure S4 (Supporting Information). To further confirm that electron transfer is inhibited by the azacrown−Cr(VI) complex below pH 5.0, the impedance behavior was investigated in that pH regime (Figure S2, Supporting Information). We see that ΔRet changes irregularly and slowly with no linear relationship between ΔRet and the Cr(VI) concentration. This is attributed to the change in Cr(VI) species. At pH 7.0, Cr(VI) mainly exists as CrO42− ion in solution,35 and CrO42− ion cannot complex with azacrown. The changes in ΔRet are due to the physical adsorption of Cr(VI) on the azacrown monolayer. As such, Cr(VI) cannot be tightly attached to the monolayer and can easily peel from the substrate. At pH 2.0 (data not shown) the data are different from those at pH 5.0, because Cr(VI) also exists as HCrO4− ion in solution. At the beginning of the experiments, ΔRet regularly increases, but it is noteworthy that it gradually decreases after a few repetitions. This phenomenon may be because Cr(VI) has strong oxidation properties in a strong acid solution. This destroys the S−Au bond and leads to azacrown leaching off the Au electrode surface. Specificity, Interference, and Stability Studies. Selectively detecting Cr(VI) in real samples without interference is challenging. To evaluate the selectivity of the azacrown monolayer, other heavy-metal ions or inorganic nonmetallic ions such as Cr(III), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II), As(III), SO42−, and NO3− were tested. Their concentrations were designed more than 10 times that of Cr(VI). Figure 5

Figure 4. (a) Electrochemical impedance spectra of the azacrown/Au electrode for the determination of Cr(VI) in different concentrations (0, 1, 5, 10, 50, 100, 500, and 1000 ppb) with preconcentration for 1 h in the desired pH solution (pH 5.0). This was then dipped in solution containing 5 mM Fe(CN)63−/4− and 0.1 M KCl (pH 5.0). The frequency range of EIS was from 0.1 to 100000 Hz. The inset shows the equivalent circuit. Rs is the resistance of the electrolyte solution, Qmono is a constant-phase element (used here as a nonideal monolayer capacitance of the azacrown film on the Au electrode), Ret is the electron transfer resistance, Rx and Q are the resistance and nonlinear capacitor accounting for possible pinholes in the film’s structure. (b) ΔRet vs the logarithmic value of Cr(VI) concentrations (log c (ppb)).

a constant-phase element (used here as a nonideal monolayer capacitance of the azacrown film on the Au electrode); Ret, the electron transfer resistance; Rx and Q, defects in the monolayer resistance and nonlinear capacitor accounting for possible pinholes in the film’s structure. Judging from the significance of the physical chemistry of these elements, Ret was suitable for sensing the interfacial properties of the azacrown EIS sensor. The values of each electrical element in the equivalent circuit are shown in Table S1 (Supporting Information). The changes in Ret were much larger than those in other impedance components. Figure 4b shows a linear relationship between the electron transfer resistance and logarithmic value of the Cr(VI) concentrations. Linear increases in Ret were observed from 1 to 100 ppb and from 100 to 1000 ppb. At the low concentration range, the sensitivity was obtained from the slope of the calibration plot and was 4575.28 kΩ [log c (ppb)]−1 (R2 = 0.994); the lowest detection limit of Cr(VI) concentration was 0.0014 ppb. From 100 to 1000 ppb, the linear regression equation is ΔRet (kΩ) = −29717.97 + 20306.71 log c (ppb) with a correlation coefficient (R2) of 0.999. We suggest that Cr(VI) ions could partly be physically adsorbed on the azacrown monolayer at these higher 1995

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

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Analytical Chemistry Table 1. Comparison of the Performances of Different Modified Electrodes for the Determination of Cr(VI)a electrode

electrolyte

method

linear range (ppb)

LOD (ppb)

Au/SPE graphite/SPE Au microchip graphite/SPE BFE AuNPS/SPE poly-L-histidine/SPCE gold-plated carbon Ti/TiO2NT/Au GCE/Nf/Agnano AgGNF/SGE PB/GCE PET/AuNPs/MPTs/AuNPs/ GCE GNE Aunano/GCE Bi/SWNTs/GCE 4-(2-mercaptoethyl)pyridinium/ Au azacrown monolayer

0.05 M H2SO4 0.1 M H2SO4 0.1 M HCl H2SO4 (pH 1.0) acetate buffer (pH 4.5) acetate buffer (pH 4.6) acetate buffer (pH 4.0) 0.3 M HNO3 0.1 M HCl 0.1 M HNO3 0.1 M HNO3 0.1 M HCl fluoride buffer (pH 4.5)

LSV AM DPCSV LSV SWASV SWV LSV LSV AM AM AM AM CSSWV

520−83200 156−520000 104−10400 100−1000 10−70 10−50000 5.2−7800 20−100 5.2−5460 2−230 2−370 0.5−200 0.52−62.4

0.1 M HCl 0.1 M HCl acetate buffer (pH 6.0) fluoride buffer (pH 4.5)

AM SWV AdCSV SWV

1.0 × 10−5 M HCl, 0.1 M KCl (pH 5.0)

EIS

sensitivity (μA ppb−1)

ref

228.8 52 46.8 19 5.27 5 3.328 4.4 1.56 0.67 0.65 0.15 0.15

1.04 × 10−3 1.54 × 10−3 1.4 × 10−3 0.79 × 10−3 0.8 0.3 × 10−3 0.022 3.7 0.133 1.1 × 10−3 0.15 × 10−3 0.015

18 41 19 42 16 7 43 20 23 22 21 44 24

0.2−3 0.13−45 0−1.3 0.0022−0.028

0.1 0.01 0.002 0.0012

0.03 0.115 25

25 2 17 26

1−100

0.0014

4575.28 kΩ [log c (ppb)]−1

this work

a

Abbreviations: SPE, screen-printed electrode; Nf, nafion; Agnano, Ag nanoparticles; SGE, solid gold electrode; AgGNF, Ag nanoparticle-coated gold nanoporous film; BFE, bismuth film electrode; TiO2NTs, titania nanotubes; SPCE, screen-printed carbon electrode; GNE, nanosized Au particles grown on a conducting substrate modified with a sol−gel-derived thiol-functionalized silicate network; LSV, linear sweep voltammetry; DPCSV, differential pulse cathodic stripping voltammetry; AM, amperometry; ASV, anodic stripping voltammetry; SWV, square wave voltammetry; AdCSV, catalytic adsorptive cathodic stripping voltammetry; EIS, electrochemical impedance spectroscopy; CSSWV, cathodic stripping square wave voltammetry.

A drawback of Cr(VI) detection using voltammetry is that it is susceptible to interferences from various metals, including competition for deposition sites in the metal ions. In this work, interference experiments were performed in the desired pH solution (pH 5.0) containing a mixture of 10 ppb Cr(VI) and100 ppb interfering ions (i.e., As(III), Cr(III), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II), SO42−, and NO3−). As shown in Figure 6, versus bare azacrown monolayer, ΔRet increases to 53.1 kΩ upon addition of 10 ppb Cr(VI) ions to the solution. Consequently, ΔRet is nearly unchanged after addition of 100 Figure 5. Specificity of the azacrown monolayer/Au electrode toward Cr(VI). The electrochemical detection conditions are the same as in Figure 4.

compares ΔRet for each ion in the azacrown monolayer/Au electrode. ΔRet obtained toward Cr(VI) is nearly 6−18 times that of As(III), Cr(III), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II), SO42−, and NO3−. Furthermore, the calibration plots (ΔRet vs log c) for each ion were studied and are shown in Figure S3 (Supporting Information). In the range of 1−100 ppb, the signals almost remain unchanged and no linear calibration plots are obtained among the other tested ions. Particularly, although the size of Pb(II) matches the cavity of azacrown,45 the EIS signal changes little in the experiement. A slight increase in ΔRet likely comes from the inhibition of mass transfer by the self-assembled monolayer. The above results indicate that the passages of the redox probe (Fe(CN)63−/4−) to the electrode surface are not blocked in the presence of the investigated heavy-metal ions or inorganic nonmetallic ions. Analogous to other high-affinity targeting receptors,46 azacrown shows highaffinity and specific binding toward HCrO4−.

Figure 6. Interference studies of the azacrown monolayer/Au electrode in the desired pH solution (pH 5.0) containing 10 ppb Cr(VI) in the presence of 100 ppb interfering ions (i.e., As(III), Cr(III), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II), SO42−, and NO3−). The electrochemical detection conditions are the same as in Figure 4. 1996

DOI: 10.1021/ac504449v Anal. Chem. 2015, 87, 1991−1998

Article

Analytical Chemistry

formation of complexes between azacrown and HCrO4−. The detection limit of 0.0014 ppb is much lower than the drinking water safe limit of 2−20 ppb prescribed by the U.S. EPA. The practicality of this method has been validated by the analyses of spiked water samples. This strategy may offer additional applications of electrochemical analysis of Cr(VI) in the environmental samples.

ppb As(III), Cr(III), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II), SO42−, and NO3−. No other interfering metals were found. To evaluate the stability of the azacrown monolayer/Au electrode, we made repetitive measurements. The azacrown monolayer/Au electrode was first immersed in a stirred solution (pH 5.0) without metal ions for 1 h. This was then soaked in 5 mM Fe(CN)63−/4− and 0.1 M KCl (pH 5.0). As shown in Figure 7, we found that, after 7 days, the impedance

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ASSOCIATED CONTENT

S Supporting Information *

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

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-551-6559-1167. Fax: 86-551-6559-2420. *Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, People's Republic of China. E-mail: [email protected]. Phone: 86-551-6559-2788. Fax: 86-551-6559-1310. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research P r o g r a m o f C h i n a ( G r a n t s 2 0 11 C B 9 3 3 70 0 a n d 2013CB934300), the National High Technology Research and Development Program of China (863 Program) (Grant 2013AA065602), and the National Natural Science Foundation of China (Grants 61106012, 21475133, 21277146, 61102013, 21377131, and 61474122). X.C. thanks the Opening Project of the State Key Laboratory of High Performance Ceramics and Superfine Microstructure (Grant SKL201312SIC) for financial support.

Figure 7. Stability of the azacrown monolayer/Au electrode. EIS diagrams were collected within 1 week in 5 mM Fe(CN)63−/4− and 0.1 M KCl in the absence of Cr(VI) (pH 5.0). The inset shows Rs/R1 vs measurement days. R1 is the first measured impedance and Rs the following impedance measurement.

of the electrode was nearly unchanged. This demonstrated that the azacrown monolayer on the Au electrode is stable for the EIS method. Cr(VI) EIS data are thus reliable. Analysis of Real Samples. This method was then used to detect Cr(VI) in spiked river water from the Dongpu Reservoir in Hefei City, Anhui Province, China. Standard additions of Cr(VI) were performed in the diluted sample. After addition of 100 μL of sample water to 9.9 mL of the desired pH solution (pH 5.0), the azacrown monolayer/Au electrode was put into it and preconcentrated for 1 h under open circuit potential conditions. The EIS measurements show that no obvious Ret change could be found in the curve of the reservoir water sample itself. Additions of Cr(VI) spiked the diluted samples, and the recovery was further studied by standard additions of Cr(VI) into the real samples to demonstrate the practicality of the proposed EIS method (Table 2). The recovery was calculated to be 99.6 ± 3.3%, which indicated that the modified electrode has a good practical application potential.

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CONCLUSIONS In conclusion, we demonstrated that an azacrown monolayer can specifically interact with HCrO4− as a new electrochemical impedance sensing interface for the ultrasensitive and ultraselective detection of Cr(VI). Changes in electron transfer resistance allowed us to readily detect Cr(VI) through the Table 2. Determination of Cr(VI) in Real Samples (Three Samples Assayed) sample

[Cr(VI)] added (ppb)

[Cr(VI)] found (ppb)

recovery (%)

Dongpu water

20

19.9 ± 0.67

99.6 ± 3.3

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