A dual electrochemical sensor for nitrite and nitric oxide Jyh-Myng Zen,* Annamalai Senthil Kumar and Hsu-Fang Wang Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan. E-mail: [email protected] Received 10th October 2000, Accepted 1st November 2000 First published as an Advance Article on the web 15th November 2000 Nafion/lead-ruthenate pyrochlore chemically modified electrode (NPyCME) showed a remarkable dual sensing activity toward NO22 oxidation and NO reduction as demonstrated by cyclic voltammetry (CV), ac-impedance spectroscopy and flow injection analysis (FIA). The mechanistic parameters of current function, charge transfer resistance and exchange current for the NPyCME, GCE and Nafion-coated GCE were evaluated and compared. The disproportionation reaction of NIIIO22 into NIVO32 + NIIO in acidic solution was used as a model system for testing the dual sensing ability of the NPyCME. The obtained crossover peak response for NO22 oxidation and NO reduction in pH 1.65 buffer solution gave the direct proof for the applicability of the NPyCME in the dual electrocatalytic action. By flow injection analysis, under optimized conditions, the calibration curve was linear in the range of 100 nM–100 µM and 800 nM–63.3 µM and the detection limit (S/N = 3) was 4.8 nM and 15.6 nM for NO22 and NO, respectively.

Introduction Nitrite (NO22) is reported to be a human health-hazard chemical the excess of which may cause poisoning and its derivatives are also major components in low-level radioactive waste solution.1,2 Nitric oxide (NO) acts as a vasodilatory messenger and does cellular communication in the central and peripheral nervous system and in host defense mechanisms of eukaryotes.3,4 NO exists in biological systems not only in the form of itself but also of its decomposed products, of which NO22 is the dominant species.5 In addition, it is known that the disproportionation of NO22 into nitrate (NO32) and NO is strongly pH-dependent with significant concentrations of NO and NO32 present in acidic environment.6 Sensitive sensing of NO22 and NO by a single detection system is an unrevealed problem in analytical chemistry. Conventional glassy carbon electrode (GCE), Pt and Hg electrodes suffer from overpotential (h) problem, not to mention the lack of dual catalytic properties. Based on the derivatives of bipyridines, phenanthrolines and porphyrin–transition metal complexes, several electrochemical systems for the individual determination of NO22 and NO were reported in the past.7–16 It is advantageous for continuous monitoring of both reactions in real-time application which can simplify the operation steps and enhance the precision of the working system. Until now, no such active single electrochemical system was developed to monitor the above species in different pHs. An alternative way for this aim, however, is to use bicatalyst either in solution or on a chemically modified ring-disk electrode.7,8 Nevertheless, it always requires two active mixtures to individually monitor the oxidation (NO22) and reduction (NO or NO32) reactions. We report here a unique Nafion/lead-ruthenate pyrochlore electrode (NPyCME) as a dual electrochemical sensor for both the NO22 oxidation and NO reduction reactions. The NPyCME was prepared by in-situ preparation technique in which the catalyst was formed directly inside the Nafion matrix.17,18 The chief advantage of using the NPyCME is that it contains a redox group with tunable oxidation states. Since the disproportionaDOI: 10.1039/b008176k

tion of NO22 is strongly pH-dependent, we took this as a model system to characterize the catalytic behavior of the NPyCME. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to further understand the electrocatalytic activity of the NPyCME. Finally, a dual electrochemical sensor for nitrite and nitric oxide was developed using the NPyCME by flow injection analysis (FIA).

Experimental Chemicals and reagents Nafion perfluorinated ion-exchange powder, 5 wt.% solution in a mixture of lower aliphatic alcohols and 10% water, was obtained from Aldrich (Milwaukee, WI, USA). Lead nitrate and ruthenium trichloride were also obtained from Aldrich. KNO2 was purchased from RDH (Frankfurt, Germany). Sealed highpurity NO cylinder was obtained from Aldrich. All the other compounds (ACS-certified reagent grade) were used without further purification. Aqueous solutions were prepared with doubly distilled deionized water with a resistivity of 18 MW cm21. Apparatus Cyclic voltammetry (CV) and FIA experiments were performed on a BAS 100B electrochemical analyzer and a BAS VC-2 electrochemical cell (West Lafayette, IN, USA). The threeelectrode system consisted of a NPyCME as the working electrode either from conventional or FIA’s base cell, an Ag/ AgCl reference electrode, and a platinum wire auxiliary electrode. Since oxygen did not interfere with the analysis, no deaeration was performed in this study. The EIS measurements were performed using an Autolab frequency response analyzer with FRA2 module that was controlled by an IBM-compatible PC. It was measured at 10 discrete frequencies per decade from 0.01 to 100 Hz with 5 mV amplitude at selected bias potentials. In-built FRA2 software program was used for fitting analysis. In flow injection analysis (FIA), a wall-jet type working system consists of a Cole–Parmer microprocessor pump drive, a Rheodyne Model 7125 sample injection valve (20 µl loop), interconnecting Teflon tubing and a BAS CC-5 electrochemical detector with a BAS MF-1015 GCE. Procedures The NPyCME was prepared as described previously.17,18 Shortly, the Nafion-coated GCE (designated as Nafion-GCE) was first prepared by spin-coating 4 µl of 4 wt.% Nafion solution at 3000 rpm. The Nafion-GCE was then ion-exchanged with Ru3+ and Pb2+ and further reacted in 1.1 M KOH at 53 °C for 24 h with purging of O2. The preparation procedure resulted in uniform distribution of the catalytically active microparticles throughout the Nafion matrix. The formation of leadruthenate pyrochlore (Py) inside the Nafion film was confirmed by X-ray diffraction pattern in our previous study.17 Analyst, 2000, 125, 2169–2172

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For disproportionation studies by CV, 5 mM NO22 dissolved in different KCl–HCl solutions were used. For FIA, NO samples were prepared by purging with high purity NO gas in a KCl–HCl solution of pH 1.65, prior degassed with Ar in order to expel the dissolved O2. The NO gas purging set up consists of three separate trap units of 6 M NaOH, 0.1 M KCl and distilled water to remove the contamination. In addition to the commercial NO gas, the solutions were also prepared from laboratory made NO gas samples by using the stoichiometric equation of 2KI + 2KNO2 + 2H2SO4 ? 2NO— + 2H2O + 2K2SO4 + I2–. The NO saturated solution at 25 °C is reported to be 1.9 mM.19 For the analytical assays, 100–fold diluted NO saturated solutions were used. Since the prime aim of the present work is to construct a dual electrochemical sensor for NO22 and NO, qualitative characterization and mechanistic aspects were first studied with CV and ac-impedance using 5 mM nitrite in a KCl–HCl solution of pH 1.65. Due to the disproportion reaction of nitrite, an appreciable amount of nitric oxide can be formed in solution. For the analytical studies using FIA, individual samples from NO22 and NO (from commercial and laboratory made samples) were used. A carrier solution of pH 1.65 (0.05 M HCl–0.1 M KCl) and 0.1 M KCl were used for the NO and NO22 detection, respectively, throughout the FIA experiments.

superiority of the NPyCME for the electrocatalytic purpose, a Nafion-coated GCE (NGCE) was also studied under identical conditions for comparison. Table 1 provides comparative data for NO22 oxidation and NO reduction in terms of current function (If) and Epa. Amongst all, the NPyCME showed the highest If and lowest overpotential. To further investigate the electrocatalytic features of NPyCME, ac-impedance analysis was done at fixed bias potentials of 820 mV and 2430 mV for NO22 oxidation and NO reduction, respectively. Figs. 1C and 1D show the complex plane plots for both reactions on GCE and the NPyCME, respectively. It is obvious that the working resistance values were considerably lowered in magnitude for the NPyCME than those for GCE. The charge transfer resistance (RCT) values were determined directly from the diameters of the high frequency semicircle by an FRA2 fitting program using the Boukamp equivalent (Randle’s) circuit of Rs(RCTQedl), where Rs is the solution resistance and Qedl is a generalised capacitance. The analysis showed tremendous decrease in RCT values for both NO22 oxidation ( ~ 45 times) and NO reduction ( ~ 40 times) on

Result and discussion Electrocatalytic behavior Fig. 1 shows the typical CV responses of 5 mM NO22 in a solution of pH 1.65 at GCE and the NPyCME. An anodic peak (Epa) at 1060 mV was observed on a bare GCE. On the other hand, two peaks at Epa = 850 mV and Epc = 2480 mV corresponding to NO22 oxidation and NO reduction, respectively, were noticed on the NPyCME. Note that the assignment will be discussed in detail later. This observation indicates the effective catalytic behavior of the NPyCME and the formation of NO from NO22 in acidic solution due to the disproportionation. The obtained oxidation and reduction potentials matched an earlier observation on a bi-metal complex electrode.7 Both the anodic and cathodic electrocatalytic reactions on the NPyCME are diffusion-controlled since the (∂logip/∂logv) were measured as ~ 0.5. To demonstrate the

Fig. 1 CV responses for 5 mM NO22 at GCE (A) and the NPyCME (B) under quasi-steady state condition (v = 5 mV s21). Complex plane acimpedance plots for NO22 oxidation (C) and NO reduction (D) at an applied potential of 820 mV and 2430 mV, respectively, in a KCl–HCl solution of pH 1.65.

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Fig. 2 (A) Proposed mechanism for NO22 oxidation and NO reduction on the NPyCME. (B) Plots of ipa and ipc vs. pH for 5 mM NO22 on the NPyCME at quasi-steady state condition (v = 5 mV s21).

Fig. 3 Hydrodynamic voltammograms in FIA for (A) 10 µM NO22 in 0.1 KCl and (B) 19 µM NO in a KCl–HCl solution of pH 1.65 on the NPyCME.

the NPyCME (Table 1). The decrease in RCT directly reflects the increase in the exchange current (Io) values for both reactions at the NPyCME and hence increases in the electron transfer rate constants. The Io and apparent heterogeneous rate constant (khapp) can be calculated from the following relations: Io = (RT/nF)(1/RCT); Io = nFAkhappC* using the RCT data at E1/2 of each corresponding reaction.20 In the above equations, C* is the analyte concentration, n is the number of electrons involved in the reaction and other symbols have their usual significance. The khapp values for NO22 were calculated by taking the analyte concentration in bulk with n = 2. Since the exact concentration of NO is unknown, we restricted our calculation only up to RCT for NO. Table 1 summarizes all kinetic parameters for both reactions on various electrodes. The fact that the highest khapp values were obtained on the NPyCME again reveals the effective catalytic performance. This enhancement in the electrocatalytic effect on the NPyCME originates from its specific redox species in the Py catalyst as reported earlier.21–23. It is reported that RuO2 is an efficient catalyst for oxidation reaction and not as efficient for reduction reaction.21,24 A unique property and obvious advantage of the NPyCME is its good efficiency even for reduction reaction due to the presence of active O’ species and in turn to high internal cathodic charge in the Py network.21–23 Thus, the characteristics of good ability towards both oxidation and reduction by the Py catalyst makes the NPyCME highly efficient for dual electrochemical sensing actions. The electrocatalytic mechanism for NO22 oxidation and NO reduction on the NPyCME based on higher oxidation states of ruthenium in the octahedral sites of Py network is illustrated in Fig. 2A.18,21 CV experiments at different pH values for 5 mM NO22 under quasi-steady state condition (v = 5 mV s21) yield different peak current ratios with a crossover pattern of ipa and ipc as shown in Fig. 2B. The inverse trend of [NO22] and [NO] with a crossover point at pH 1.72 is an obvious evidence of the disproportionation of NO22 in acidic solution. Note that a separate experiment by purging with pure NO gas on this working pH solution confirms that the reduction reaction is indeed from NO. These evidences are clear enough to construct an efficient electrochemical sensor for NO22 and NO using the NPyCME. The

analyses were further extended to an analytical characterization by FIA. Analytical applications by FIA FIA coupled with a wall-jet system has a clear advantage in high precision since the radial flow in the above system minimizes the diffusion layer thickness of the analyte during the electron transfer reaction at the electrode/electrolyte interface and thus amplifies the detection current signal considerably. Effects of poised potential (HE) and flow rate (HF) to the hydrodynamic voltammograms of NO22 oxidation and NO reduction were separately optimized as shown in Fig. 3. The optimized parameters for NO22 and NO detection are HE = 1100 mV and HF = 0.3 ml min21; and HE = 2800 mV and HF = 0.5 ml min21, respectively. Since these reactions are pH-dependent, the obtained HE and HF value will shift negatively upon increasing the solution pH. Thus, the obtained optimized potential (in FIA) was comparable to the reported electrocatalytic system. For example, Doherty et al. detected the nitrite at 925 mV vs. SCE under neutral condition (pH ~ 5) on the [Ru(bipy)2poly-(4-vinylpyridin)10Cl]Cl-modified electrode.25 By adjusting the potential with respect to the Nernstian case (∂Ep/∂pH = 260 mV) to pH 1.65, the obtained value is ~ 1100 mV. Moreover, the decrease in the overpotential by this electrode was clearly demonstrated by CV and ac-impedance studies (Table 1). Under the optimized conditions, well-defined signals for NO22 and NO were observed on the NPyCME as shown in Fig. 4. The working concentration range and detection limit (S/N = 3) for NO22 and NO are 100 nM–100 µM and 4.8 nM; and 800 nM–63.3 µM and 15.6 nM, respectively. The reproducibility of the NPyCME was checked by successive injections (n = 5) and the obtained RSD for both analytes are < 2%. This result indicates the good reproducibility of the present system. Furthermore, since the process is diffusion-controlled, the response time for the NPyCME is very fast. No change in responses regarding peak potential or peak current was observed after the same electrode was repeatedly used and stored in 1.1 M KOH solution for more than 2 months. It is

Table 1 Electrochemical features of NO22 oxidation and NO reduction on various electrodes in a KCl–HCl solution of pH 1.65. NO22 Electrode

If/Amol21cm3/V s2121/2 Epa/mV

NO RCT/kWa

Io/1026 A

khapp/1025 cm s21 Epc/mVb

RCT/kWc

NPyCME 117.3 850 2.68 ± 0.02 6.15 9.03 2480 19.11 ± 0.1 NGCE 27.9 1011 48.81 ± 11.1 0.26 0.38 — 820.85 ± 29 GCE 54.3 919 90.78 ± 0.26 0.14 0.21 — 771.49 ± 24 a Applied potential = 820 mV. b v = 5 mV s21. c Applied potential = 2430 mV, [NO 2] = 5 mM in a KCl–HCl solution of pH 1.65. 2

Fig. 4 Determination of NO22 and NO on the NPyCME by FIA at applied potentials of 1100 and 2800 mV and flow rate of 0.3 and 0.5 ml min21, respectively.

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possible to detect NO22 and NO simultaneously on the NPyCME if a bi-potentiostat was used. Further work along this line is currently in progress.

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Conclusions The NPyCME showed excellent electrocatalytic activity towards NO22 oxidation and NO reduction reactions. ACimpedance analysis of the above analytes shows a much lower charge transfer resistance (RCT) on the NPyCME. This also results in an increased exchange current and apparent heterogeneous rate constant. CV studies based on the NO22 disproportionation reaction at a pH of 1.65 clearly provide the explanation. The precision and stability of the NPyCME offers a good possibility for extending the analysis also to other pHs.

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Acknowledgement

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The authors gratefully acknowledge financial support from the National Science Council of the Republic of China.

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References

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