Oxidation of Azide Anion at Boron-Doped Diamond Thin-Film Electrodes

Mendenhall and co-workers at Morton/Thiokol and Autoliv were greatly appreciated. Received for review September 2, 1997. Accepted. January 30, 1998...
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Anal. Chem. 1998, 70, 1502-1510

Oxidation of Azide Anion at Boron-Doped Diamond Thin-Film Electrodes Jishou Xu and Greg M. Swain*

Department of Chemistry & Biochemistry, Utah State University, Logan, Utah 84322-3900

The oxidation of dissolved inorganic azide anion in aqueous media was investigated using high-quality, borondoped diamond thin-film electrodes. Linear sweep and differential pulse voltammetry, along with flow injection analysis in the amperometric detection mode, were used to study the reaction at neutral pH as a function of the potential sweep rate, analyte concentration, and electrolyte composition. Comparison experiments were performed using polished glassy carbon. Azide undergoes an irreversible oxidation (1 e-/equiv) at both of these carbon electrodes, presumably with nitrogen as the primary product. A linear dynamic range of 3-4 orders of magnitude and a detection limit as low as 0.1 µM (4.3 ppb) at a S/N ) 3 were observed for diamond in the voltammetric measurements. The flow injection analysis results for diamond indicated a linear dynamic range of 5 orders of magnitude and a detection limit of 8 nM (0.3 ppb) at a S/N ) 3. The diamond response was generally reproducible from film to film, and the background signal and signal-to-background ratio were extremely stable for up to 12 h of continuous use. The results demonstrate that this new electrode material serves as an analytically useful substrate for the detection of azide anion and exhibits superior performance characteristics compared with glassy carbon. Synthetic diamond possesses several technologically important properties such as extreme hardness, high thermal conductivity, variable electrical conductivity via doping, optical transparency, and chemical inertness. Advantageous utilization of these unique properties has been the motivation of our group’s efforts to investigate boron-doped diamond as a new electrode material for electroanalysis. Unlike the more extensively studied carbonaceous electrodes (e.g., highly oriented pyrolytic graphite, glassy carbon, and carbon fibers), the use of diamond in electrochemistry has only recently been reported.1-31 (1) Swain, G. M.; Ramesham, R. Anal. Chem. 1993, 65, 345. (2) Swain, G. M. Adv. Mater. 1994, 6, 388. (3) Swain, G. M. J. Electrochem. Soc. 1994, 141, 3382. (4) Awada, M.; Strojek, J. W.; Swain, G. M. J. Electrochem. Soc. 1995, 142, L42. (5) Alehashem, S.; Chambers, F.; Strojek, J. W.; Swain, G. M.; Ramesham, R. Anal. Chem. 1995, 67, 2812. (6) Strojek, J. W.; Granger, M. C.; Swain, G. M.; Dallas, T.; Holtz, M. W. Anal. Chem. 1996, 68, 2031. (7) DeClements, R.; Hirsche, B. L.; Granger, M. C.; Xu, J.; Swain, G. M. J. Electrochem. Soc. 1996, 143, L150.

1502 Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

Diamond is normally a superb insulator (>1012 Ω‚cm); however, with controlled boron doping, films can possess resistivities as low as 0.01 Ω‚cm rendering them useful for electrochemistry. High-quality, boron-doped diamond thin-film electrodes have been found to possess several interesting electrochemical properties as described elsewhere.10 These properties include the following: (i) low and stable voltammetric background current and double-layer capacitance; (ii) a working potential window of 3 V or more in aqueous media; (iii) negligible adsorption of polar organic molecules such as anthraquinone-2,6-disulfonate; (iv) (8) DeClements, R.; Swain, G. M.; Dallas, T.; Holtz, M. W.; Herrick, R., III; Stickney, J. L. Langmuir 1996, 12, 6578. (9) DeClements, R.; Swain, G. M. J. Electrochem. Soc. 1997, 144, 856. (10) Xu, J.; Granger, M. C.; Chen, Q.; Lister, T. E.; Strojek, J. W.; Swain, G. M. Anal. Chem. 1997, 69, 591A, and references therein. (11) Jolley, S.; Koppang, M.; Jackson, T. Swain, G. M. Anal. Chem. 1997, 69, 4041. (12) Chen, Q.; Granger, M. C.; Lister, T. E.; Swain, G. M. J. Electrochem. Soc. 1997, 144, 3086. (13) Iwaki, M.; Sato, S.; Takahashi, K.; Sakairi, H. Nucl. Instrum. Methods 1983, 209, 1129. (14) Pleskov, Y.; Sakharova, A.; Krotova, M. D.; Bouilov, L. L.; Spitsyn, B. V. J. Electroanal. Chem. 1987, 228, 19. (15) Patel, K.; Hashimoto, K.; Fujishima, A. Denki Kagaku 1992, 60, 659. (16) Natishan, P. M.; Morrish, A. Mater. Lett. 1989, 8, 269. (17) Sakharova, A.; Sevast’yanov, A. E.; Pleskov, Y.; Templitskaya, G. L.; Surikov, V. V.; Voloshin, A. A. Electrokhimiya 1991, 27, 239. (18) Sakharova, A.; Nyikos, L.; Pleskov, Y. Electrochim. Acta 1992, 37, 973. (19) Sakharova, A. Ya.; Pleskov, Yu. V.; Di Quarto, F.; Piazza, S.; Sunseri, C.; Teremetskaya, I. G.; Varin, V. P. J. Electrochem. Soc. 1995, 142, 2704. (20) Pleskov, Yu. V.; Mishuk, V. Ya.; Abaturov, M. A.; Elkin, V. V.; Krotova, M. D.; Varin, V. P.; Teremetskaya, I. G. J. Electroanal. Chem. 1995, 396, 227. (21) Pleskov, Yu. V.; Elkin, V. V.; Abaturov, M. A.; Krotova, M. D.; Mishuk, V. Ya.; V. P. Varun; Teremetskaya, I. G. J. Electroanal. Chem. 1996, 413, 105. (22) Modestov, A. D.; Pleskov, Yu. V.; Varnin, V. P.; Teremetskaya, I. G. Russ. J. Electrochem. 1997, 33, 60. (23) Tenne, R.; Patel, K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1993, 347, 409. (24) Reuben, C.; Galun, E.; Cohen, H.; Tenne, R.; Kalish, R.; Muraki, Y.; Hashimoto, K.; Fujishima, A.; Butler, J. M.;. Levy-Clement, C. J. J. Electroanal. Chem. 1995, 396, 233. (25) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143, L133. (26) Miller, B.; Kalish, R.; Feldman, L. C.; Katz, A.; Moriya, N.; Short, K.; White, A. E. J. Electrochem. Soc. 1994, 141, L41. (27) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996, 143, L238. (28) Zhang, X.; Wang, R.; Yao, Y.; Chen, C.; Zhu, J.; Liu, X.; Wu, J.; Zhang, G. In 2nd International Conference on the Applications of Diamond Films and Related Materials; Yoshikawa, M., Murakawa, M., Tzeng, Y., Yarbrough, W. A., Eds.; MYU: Tokyo, 1993; pp 65-70. (29) Peilio, Z.; Jianzhong, Z.; Shonzhong, Z.; Xikang, Z.; Guoxiong Z. Fresenius J. Anal. Chem. 1995, 353, 171. (30) Boonma, L.; Yano, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electrochem. Soc. 1997, 144, L142. (31) Li, L.-F.; Totir, D.; Miller, B.; Chottiner, G.; Argoitia, A.; Angus, J. C.; Scherson, D. A. J. Am. Chem. Soc. 1997, 119, 7875. S0003-2700(97)00959-1 CCC: $15.00

© 1998 American Chemical Society Published on Web 03/15/1998

resistance to morphological damage and severe surface oxidation during anodic polarization; (v) good activity toward some redox analytes without any conventional pretreatment; and (vi) longterm response stability. For example, the cyclic voltammetric ∆Ep values for redox analytes such as Fe(CN)63-/4-, Ru(NH3)62+/3+, and IrCl62-/3- at films exposed to the laboratory air for weeks are often comparable to the values observed at freshly polished glassy carbon.10 It is because of these interesting and important properties that we are investigating the response of diamond toward different classes of aqueous redox analytes in an effort to learn whether the material can be used to sensitively, stably, and reproducibly detect certain compounds. Sodium azide is reactive and widely used commercially, particularly as a propellant in automotive airbags. The azide anion is highly toxic and can present health hazards at relatively modest levels in the forms of headaches, cytochrome oxidase inhibition, and vasodilation. Therefore, industries producing or using azide generally have tight controls on the levels of the anion allowed in wastewater effluent. Azide is currently being phased out as a propellant for airbags in the United States and is not a normal toxin in water supplies. However, as current model cars containing azide-based airbags are retired to slavage yards, the likelihood of azide contamination of groundwater will increase significantly in future years. To the best of our knowledge, there exists no EPA-recommended protocol for the detection of dissolved azide in aqueous media nor is there a mandated concentration limit in water. The Occupational Safety and Health Administration has set the exposure limits to 0.1 ppm for gaseous HN3 and to 0.3 ppb for solid NaN3.32 Generally, gas chromatography (detection of HN3), ion chromatography, and capillary electrophoresis are used to detect azide anion with quantifiable limits in the highppb to low-ppm range. Improved detection methods are needed to reproducibly detect azide in the low-ppb range. Electrochemical methods of analysis are inexpensive and can provide high sensitivity, long- term response stability, and good assay reproducibility for analytes that are amenable to such detection. It turns out that azide anion is electrochemically active at carbon, platinum, and gold electrodes,32-37 but surprisingly, there have been only a few investigations of the electrochemistry of this anion. Roscoe and Conway37 reported that azide anion can be oxidized to nitrogen and/or nitric oxide, nitrous oxide, and nitrogen dioxide, and reduced to NH3, at platinum, depending on the applied potential. The oxidation product distribution at platimum and gold is influenced by the oxygen coverage as the reaction occurs at potentials where the metal surface oxides are formed. These surface oxides take part in the oxidation reaction. Most recently, Savinell and co-workers32-34 reported on azide oxidation and reduction at platinum, gold, and glassy carbon. The elegance of these studies was the combined use of electrochemistry and in situ mass spectrometry to monitor the gaseous (32) Dalmia, A.; Wasmus, S.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1996, 143, 556. (33) Dalmia, A.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1996, 143, 1827. (34) Dalmia, A.; Wasmus, S.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1995, 142, 3735. (35) Miyama, H.; Nosaka, Y.; Fukushima, T. J. Electrochem. Soc. 1986, 133, 336. (36) DeFelippis, M. R.; Faraggi, M.; Klaper, M. H. J. Phys. Chem. 1990, 94, 2420. (37) Roscoe, S. G.; Conway, B. E. J. Electroanal. Chem. 1988, 249, 217.

reaction products generated. Nitrogen gas and various nitrogen oxides were formed during azide oxidation at platinum and gold electrodes, but nitrogen was the only oxidation product formed at glassy carbon. Nitrogen is the primary product because of the relative low surface oxide coverage on glassy carbon, as compared to the metals. The overall oxidation reaction at carbon has been proposed to be32

2N3- f 3N2 + 2eWe report on the oxidation of azide anion in aqueous media using high-quality, boron-doped diamond thin-film electrodes. The reaction was investigated by linear sweep and differential pulse voltammetry, and by flow injection analysis in the amperometric detection mode, at neutral pH as a function of the potential sweep rate, analyte concentration, and electrolyte composition. Similar experiments were conducted at polished glassy carbon, for comparison. The results demonstrate that diamond provides a sensitive, reproducible, and stable response for azide oxidation leading to superior detector performance compared to glassy carbon. EXPERIMENTAL SECTION Diamond Film Growth. The diamond films were grown on conducting p-Si(100) substrates (10 µM). Under such conditions, usually the magnitude of the baseline noise superimposed on the analytical signal is significantly smaller than the signal itself. A more pertinent comparison for performance analysis is the magnitude of the analytical signal compared to the magnitude of the background signal. The S/N, defined as Ifaradaic/ Ibaseline noise, is used to describe the performance at low analyte concentrations (17 MΩ) from a Barnstead E-pure system were used to prepare all the solutions. All glassware was cleaned by thorough rinsing with ultrapure water after a short soak in a warm 1:1 nitric acid/ water bath. Potassium dibasic and monobasic phosphate salts (Mallinckrodt) were mixed equally at 0.1 M to prepare the pH 7.2 buffer solution which served as the supporting electrolyte. The sodium azide (Sigma) was used as supplied. The dissolved azide concentration ranged from 3.3 mM to 0.1 µM. In the experiments with added anions, 10 mM KF, 10 mM KCl, 1 mM KBr, and 100 mM KNO3 (Fisher) were mixed separately with the phosphate buffer. RESULTS AND DISCUSSION Film Characterization. AFM measurements confirmed that the diamond films were all well-faceted, polycrystalline, and continuous over the entire substrate. The image features re-

Figure 1. Raman spectrum for a diamond thin film (D803961).

sembled those reported by our group previously.6,12 Figure 1 shows a Raman spectrum from one of the diamond films (D0803961) used in these investigations. Raman spectroscopy has been used extensively to characterize the microstructure of various carbonaceous materials, including diamond.38-43 The spectrum shows one sharp peak at 1332 cm-1 characteristic of crystalline diamond. The fwhm is 11 cm-1. This compares with a 6-cm-1 bandwidth for a Type IIa single-crystal diamond (100 orientation, Harris Diamond). The bandwidth is inversely related to the phonon lifetime and is a measure of the crystalline perfection. The broader bandwidth for the diamond films is consistent with a small nominal crystallite size. The spectrum shows negligible scattering intensity in the region between 1500 and 1600 cm-1, intensity normally observed when significant amounts of sp2bonded carbon impurities are present. It has been suggested that sp2-bonded carbon is present in several dispersed forms within CVD-grown polycrystalline diamond films: as point defects within the diamond lattice; in association with extended defects such as twin planes and stacking faults; and at grain boundaries.43 The spectral features indicate that this particular film is good quality. This assessment is based on the fact that the Raman crosssectional scattering coefficients for diamond and graphite (i.e., nondiamond carbon) are 9 × 10-7 and 500 × 10-7 cm-1/sr, respectively.39 Spectra from the other two films used in this work (D0102971, D0305971) were qualitatively similar except that the intensity of the diamond peak was attenuated and the scattering intensity from sp2-bonded carbon impurities was slightly larger. These spectral features are attributed to a reduced film thickness and nominal crystallite size. Linear Sweep and Differential Pulse Voltammetry. Figure 2 shows linear sweep voltammetric i-E curves for glassy carbon and diamond (D0803961) in 1 mM NaN3/0.1 M phosphate buffer, pH 7.2. The total and background current profiles are presented. A peak-shaped oxidation response is observed for both electrodes, but the background currents, upon which the faradaic response is measured, are dramatically different. The background current for this particular glassy carbon is significantly larger than that for diamond with the difference at 1200 mV being a factor of 200. The double-layer capacitance also generally ranged from a factor of 10 to 100 lower for diamond in the potential region from 800 to (38) Dennison, J. R.; Holtz, M. W.; Swain, G. M. Spectroscopy 1996, 10, 1. (39) Knight, D. S.; White, W. B. J. Mater. Res. 1989, 4, 385. (40) Nemanich, R. J.; Solin, S. A. Phys. Rev. B 1979, 20, 392. (41) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (42) McCreery, R. L.; Packard, R. T. Anal. Chem. 1989, 61, 775A. (43) Robins, L. H.; Farabaugh, E. N.; Feldman, A. J. Mater. Res. 1990, 5, 2456, and references therein.

Figure 2. Linear sweep voltammetric i-E curves for freshly polished glassy carbon and a diamond thin film (D803961) in 1 mM NaN3/0.1 M phosphate buffer, pH 7.2. The total and background currents are shown for both electrodes. Sweep rate 100 mV/s.

1200 mV. Low voltammetric background current and capacitance have previously been reported for boron-doped diamond thin films in contact with several different aqueous electrolytes.1,3,5,10,25,27,30 The large background current for glassy carbon in the vicinity of azide oxidation results from a combination of oxygen evolution and surface oxidation. A variety of surface oxides can be formed at the exposed edge plane sites of glassy carbon and significant oxidation occurs at the azide oxidation potential.3,9,10 The magnitude of the background current difference showed some variability from glassy carbon sample to sample but, in all cases, was significantly larger than diamond. As a result, the azide oxidation current is superimposed on a large rising background signal while the faradaic current for diamond is recorded against a low and unchanging background. This allows for a rigorous and reproducible background correction when diamond is used. The oxidation peak potentials are 1045 and 1100 mV for glassy carbon and diamond, respectively. The peak potentials for diamond were consistently 25-100 mV more postive than those for glassy carbon. This difference is attributed to a slightly larger inherent overpotential for this reaction at diamond as resistance effects were determined to be minor at these currents. The reason for the overpotential is unknown at present. The backgroundcorrected peak currents are 57 and 88 µA for glassy carbon and diamond, respectively. In this example, the faradaic response is 35% larger for diamond, but this was generally not the case. Most often, the faradaic signals were within 5% for both electrodes while the background currents were always a factor of 5-100 larger for glassy carbon. Nitrogen is assumed to be the primary oxidation product.32 The lower background current for diamond at the azide oxidation potentials results because the electrode material has a higher overpotential for oxygen evolution and a resistance to severe surface oxidation.10,12 Table 2 summarizes some linear sweep voltammetric data for glassy carbon and three different diamond films. The films were grown at different times over a seven-month period and each had an unknown past history. It can be seen that the oxidation peak potentials for the diamond films are 22-90 mV more positive that those for glassy carbon while the peak currents and charges for all the electrodes are similar. The response for both electrodes increases by 1 order of magnitude with a comparable increase in the azide concentration, as expected. The most noteworthy data are the S/B ratios. The S/B ratios for the diamond films, even without pretreatment, are 38-50 times larger than for glassy carbon at the 0.1 mM azide concentration level. The enhanced Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

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Table 2. Linear Sweep Voltammetric Data for Azide Oxidation in 0.1 M Phosphate Buffer, pH 7.2, for Diamond and Glassy Carbon Electrodesa electrode

azide (mM)

Epox (mV)

Ipox (µA)

Qpox (µC)

S/B

GC D0803961 D0305971 D0102971 GC D0102971 D0102972 D0102973

0.1 0.1 0.1 0.1 1 1 1 1

999 1090 1021 1086 1004 1046 1064 1055

10.4 9.2 9.4 9.6 62 65 67 65

28.4 27.6 27.1 27.9 237 277 280 257

0.87 39 59 48 5.5 361 447 325

a GC, glassy carbon. Sweep rate 50 mV/s. S/B ) i total ibackground/ibackground.

S/B ratio and the low and stable background current for diamond lead to a lower detection limit and more reproducible results, as will be shown below. The low and stable background current and the large overpotential for oxygen evolution are clearly advantages of diamond for this particular assay. There are three possible, and not necessarily unrelated, explanations for the low background current and capacitance. First, the relative absence of electroactive carbon-oxygen functionalities on the hydrogen-terminated diamond surface, as compared with polished glassy carbon, leads to a lower current. Such surface groups can undergo redox chemistry in the potential range studied in this work and give rise to increased background currents. For example, our group has recently reported that oxygen-free, hydrogenated glassy carbon exhibits a background voltammetric current that is a factor of 3-6 less than the freshly polished surface.8 Vacuum heat treatment of glassy carbon and carbon fibers has also been shown to remove electroactive carbon-oxygen functionalities, thus lowering the voltammetric background current and capacitance.44,45 Therefore, the absence of electroactive surface carbonoxygen functionalities can explain some but not all of the decreased current and capacitance for diamond. A second contributing factor may be a lower density of surface electronic states near the fermi level and/or a lower charge carrier concentration due to the semimetal-semiconductor nature of boron-doped diamond.5,10 A lower density of states at a given potential, or a lower charge carrier concentration, would lead to a reduced accumulation of counterbalancing ions and water dipoles on the solution side of the interface, thereby lowering the background current and capacitance. Similar reasoning has been invoked to explain the anomalously low background current and capacitance for the basal plane of highly oriented pyrolytic graphite.46-49 It should be noted that the background voltammetric current and the capacitance for diamond are similar in magnitude to those values for the basal plane of highly oriented pyrolytic graphite.3,5 A third possible contributing factor could be that the diamond film surface is constructed like an array of (44) Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 57, 2759. (45) Swain, G. M.; Kuwana, T. Anal. Chem. 1992, 64, 565. (46) Randin, J.-P.; Yeager, E. J. Electrochem. Soc. 1971, 118, 711. (47) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257. (48) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1975, 58, 313. (49) Gerischer, H.; McIntyre, R.; Scherson, D.; Storck W. J. Phys. Chem. 1987, 91, 1930.

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microelectrodes. In other words, perhaps the diamond surface has “electrochemically active” sites separated by less reactive or more insulating regions much in the same way that composite electrodes have very reactive regions of carbon separated by insulating regions of the Kel-F polymer support.50 The latter is pure speculation at this point as we have no confirming experimental data. The data in Table 2 also demonstrate that the oxidation response is reproducible from film to film. All the films were grown at different times over a seven-month period, and all had vastly different past histories, and yet all had nearly identical peak currents and similar peak potentials. The oxidation peak potentials for two of the films (D0803961, D0102971) were similar at ∼1090 mV and the potential for the third film (D0305971) was shifted a little negative to 1020 mV. The slightly more negative potential is attributed to an increased reactivity for this particular film. AFM investigations revealed that the nominal crystallite size for this film was 246 nm (see Table 1); the smallest of the three films. The smaller crystallite size leads to a larger fractional coverage of grain boundaries compared with the other two films. It is possible that the grain boundary regions provide the most conductive pathways for the electron transfer, and if so, then this film would be expected to yield most negative oxidation peak potential. Recent results, however, have indicated that films with a nominal crystallite size in excess of 1 µM exhibit nearly identical voltammetric behavior. This would appear to suggest that the azide oxidation response does not depend on the crystallite size (i.e., fraction of grain boundaries). Further investigations are needed to fully evaluate the effect of film morphology and crystallinity on the azide oxidation response. The response was examined as a function of the potential sweep rate and the azide concentration. The oxidation peak current varied linearly with the square root of the sweep rate for diamond (D0803961) in 1 mM NaN3/0.1 M phosphate buffer, pH 7.2. Sweep rates from 1 to 50 mV/s were examined. Linear regression statistical analysis (y ) mx + b) yielded coefficients of m ) 39.0 µA‚s/V, b ) -0.23 µA, and r2 ) 0.997. Recent results have shown linearity to scan rates as large as 1 V/s. The linearity indicates that the current is limited by semiinfinite linear diffusion of azide to the interfacial reaction zone. The linearity and nearorigin intercept also suggest that there are no complicated, ratelimiting adsorption steps or specific surface interactions in the oxidation reaction mechanism at diamond. A linear response was also obtained for glassy carbon at scan rates between 1 and 50 mV/s. The oxidation peak current also varied linearly with the azide concentration between 3.3 mM and 1.0 µM. Linear regression analysis of the log-log calibration plots (background corrected) yielded coefficients of m ) 0.93 A/M (theoretical value 1.00) and r2 ) 0.990. At the lowest concentration measured, 1.0 µM, the S/N ratio was 8 leading to a theoretical detection limit of 400 nM (S/N ) 3). This corresponds to 17 ppb azide. In comparison, a linear response was observed for glassy carbon only from 3.3 mM to 33 µM. The S/N ratio at this concentration was 5 leading to a theoretical detection limit of 19 µM (S/N ) 3). This corresponds to 798 ppb azide which is over 1 order of magnitude higher than for diamond. The reduced dynamic range and higher detection limit for glassy carbon result from the large and (50) Vitt, J. E.; Johnson, D. C. J. Appl. Electrochem. 1994, 24, 107.

Table 3. Differential Pulse Voltammetric Data for 0.01 mM Azide in 0.1 M Phosphate Buffer, pH 7.2, for Diamond and Glassy Carbon Electrodesa electrode

Epox (mV)

Ipox (nA)

Qpox (µC)

S/N

GC D0803961 D0102971 D0305971

934 1012 994 950

405 403 352 405

8.0 6.4 4.9 5.8

8.1 56 70 81

a GC, glassy carbon. Pulse height 50 mV. Step height 2 mV. Pulse width 40 ms. Sample time 35 ms. S/N ) ifaradaic/ibaseline noise.

changing background current. The variable background signal for glassy carbon makes rigorous background correction impossible. Rigorous background correction requires a stable and unchanging background signal, like that for diamond. These voltammetric results demonstrate that diamond provides an analytically useful response over a dynamic range of at least 3 orders of magnitude with a detection limit over 1 order of magnitude lower than for glassy carbon. Differential pulse voltammetry was also used to investigate the azide oxidation reaction as this method better discriminates against background currents compared to linear sweep voltammetry. The benefit can be improved S/N ratios when the signal can be rigorously background corrected. Table 3 summarizes some data for glassy carbon and three diamond films in 0.01 mM NaN3/0.1 M phosphate buffer, pH 7.2. It can be seen that the oxidation peak potentials for diamond are 16-78 mV more positive than for glassy carbon while the peak currents are comparable in magnitude, the exception being the current response for film D0102971, which is ∼50 nA lower. Also included in the table are the S/N ratios for the measurements at this concentration. It can be seen that the S/N ratios for the diamond films range from a factor of 7 to 10 times larger than for glassy carbon. The differential pulse oxidation peak current (background corrected) for diamond (D0305971) varied linearly with the analyte concentration. A linear dynamic range of 4 orders of magnitude was observed from 3.3 mM to 0.1 µM. The linear regression analysis of the log-log calibration plots yielded coefficients of m ) 0.93 A/M and r2 ) 0.997. At the lowest concentration measured, 0.1 µM, the S/N ratio was 3, which corresponds to an actual detection limit of 4.3 ppb azide. Like the linear sweep voltammetric data, azide oxidation at concentrations less than 10 µM could not be reproducibly detected using glassy carbon because of the problem with background correction. The detection limit at glassy carbon actually showed some variability from sample to sample but was always at least 1 order of magnitude larger than for diamond. The influence of different coadded electrolyte anions on the azide oxidation response was also investigated. Linear sweep voltammetric i-E curves were obtained in solutions containing 1 mM NaN3/0.1 M phosphate buffer, pH 7.2, with added amounts of KF, KCl, or KBr. No influence on the azide oxidation peak potential or current for either glassy carbon or diamond was observed in the presence of 10 mm KF or 10 mM KCl. However, some influence was observed in the presence of 1 mM KBr. Figure 3A shows total and background current responses for a diamond film (D0305971) exposed to an azide solution with and

Figure 3. Linear sweep voltammetric i-E curves for (A) a diamond thin film and (B) freshly polished glassy carbon in 1 mM NaN3/0.1 M phosphate buffer, pH 7.2, with and without coadded 1 mM KBr. The total and background currents are shown for both electrodes. Sweep rate 50 mV/s. PB, 0.1 M phosphate buffer, pH 7.2.

without 1 mM KBr. It can be seen that a peak-shaped oxidation response is observed both with and without KBr. The oxidation peak potential is shifted slightly positive by ∼25 mV in the presence of the added electrolyte, and the peak current is attenuated slightly by ∼8%. It can also be observed that the background current is slightly larger in the presence of KBr. The increased background current and decreased azide oxidation current are caused by the competitive oxidation of bromide. The azide oxidation potential is near the potential at which bromide begins to oxidize at this electrode. Consequently, some of the bromine generated at the interfacial reaction zone could be oxidizing azide according to the following homogeneous chemical reaction:

2N3- + Br2 h 3N2 + 2Br-

The bromide oxidation current is not substantial at this potential so the azide oxidation current is attenuated only slightly. On the other hand, the effect of added KBr is more pronounced for glassy carbon as shown in Figure 3B. The azide oxidation peak potential is unaffected by the added KBr but the peak current is attenuated by ∼25%. On the basis of the large increase in the background current, as compared to the response in the absence of KBr, there is a significantly lower overpotential for the oxidation of bromide at glassy carbon compared to diamond. Therefore, the azide oxidation current is attenuated by a larger amount. Differential pulse voltammetric results showed that the addition of 0.1 M KNO3 also caused no change in the azide oxidation response for diamond. Nitrate anion is a common interference for azide detection in ion chromatography with conductivity detection as both anions have similar retention factors. Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

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Figure 4. Background hydrodynamic i-E curves for (9) glassy carbon, (b) diamond D0305971, and (2) diamond D0102971 electrodes in 0.1 M phosphate buffer, pH 7.2. Flow rate 1.0 mL/min. Each datum was recorded at the end of a 30-min period.

Figure 5. Hydrodynamic i-E curves for (9) glassy carbon, (b) diamond D0305971, and (2) diamond D0102971 electrodes exposed to 20-µL injections of 0.1 mM N3- in 0.1 M phosphate buffer, pH 7.2. Flow rate 1.0 mL/min. Each datum represents the average of 10 injections.

Flow Injection Analysis. Figure 4 shows background hydrodynamic i-E curves for glassy carbon and two diamond films (D0305971, D0102971) in 0.1 M phosphate buffer, pH 7.2. The mobile phase flow rate was 1.0 mL/min. Each datum was recorded at the end of a 30-min period at the applied potential. The diamond responses are comparable except at potentials between 1.1 and 1.3 V, where one of the films exhibits a 25 nA larger current. Over the entire potential range, the film responses range from 10 to 50 nA. The glassy carbon response, on the other hand, is much larger particularly at potentials positive of 1.2 V. For example, the background current is 25-50 nA larger at potentials between 0.8 and 1.2 V. The signal increases from 100 to 350 nA at potentials positive of 1.2 V such that, at 1.3 V, the glassy carbon response is a factor of 7 larger than that for diamond. The larger current is caused by a combination of surface oxidation and oxygen evolution. This result demonstrates one of the advantages of diamond in dc amperometric detection, compared to glassy carbon, and that is the low and stable background signal due to a larger overpotential for oxygen evolution and a higher resistance to severe surface oxidation.3,9,10 Also, the background response for glassy carbon is dynamic and evolving with time due to the surface oxidation processes. In fact, as will be shown below, the background current for glassy carbon increases significantly over time at the azide oxidation potential while the response for diamond remains low and unchanged. Figure 5 shows hydrodynamic i-E curves for glassy carbon and the two diamond films during 20-µL injections of 0.1 mM N3in 0.1 M phosphate buffer, pH 7.2. The mobile-phase flow rate was 1.0 mL/min. Each datum corresponds to the average 1508 Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

Figure 6. (A) Diamond D0305971 thin-film response to 10 20-µL injections of 0.1 mM N3- in 0.1 M phosphate buffer, pH 7.2. Flow rate 1.0 mL/min. Detection potential 1.25 V. (B) A diamond thin-film response to five 20-µL injections of 0.3 µM N3- in 0.1 M phosphate buffer, pH 7.2. Same flow rate and detection potential as in A.

response for 10 injections. Well-defined sigmoidal curves are seen for all three electrodes. The half-wave potential for glassy carbon is 1.13 V and is some 0.04 V more negative of the half-wave potential of 1.17 V observed for both diamond films. This difference is again attributed to a larger inherent overpotential for this reaction at diamond. The hydrodynamic i-E curves for all the electrodes are shifted positive from their position in the linear sweep voltammetry. This can be seen by comparing the half-wave potentials with the linear sweep Ep/2 values. This shift is caused by resistance effects within the flow cell. The limiting currents are 2500 and 2150 nA for glassy carbon and the two diamond thin films, respectively. The lower limiting current for diamond may be due to a larger channel thickness than is present when glassy carbon is used. In our thin-layer cell assembly, the hard glassy carbon electrode can be tightly pressed against the gasket, resulting in a reproducible channel height (i.e., cell volume). The diamond films are coated on fragile Si substrates that are easy to crack. Therefore, it is more difficult to reproduce the channel thickness. A larger channel thickness, or cell volume, would result in a lower analyte concentration (i.e., dilution effects), hence a smaller limiting current.51 The variation in limiting currents might also be due to different electrochemically active areas. Figure 6A shows the response of a diamond thin film (D0305971) to 10 injections of 0.1 mM N3- in 0.1 M phosphate buffer, pH 7.2. The mobile-phase flow rate was 1.0 mL/min, and the dc potential was 1.25 V. Excellent response reproducibility is observed as the coefficient of variation (RSD × 100) in the peak heights is 1%. The peaks are sharp and narrow with a fwhm of 2 s. Figure 6B shows the response of a diamond film to five (51) Wang, J. Analytical Electrochemistry; VCH: New York, 1994; p 61.

Table 4. Summary of Flow Injection Analysis Data for Azide Oxidation in 0.1 M Phosphate Buffer, pH 7.2, for Diamond and Glassy Carbon

electrode

linear range

sensitivity (nA/µM)

diamond glassy carbon

3.3 mM-0.30 µM 3.3 mM-1.0 µM

33 ( 5 36 ( 7

injections of 0.3 µM N3- in the same medium. Excellent response S/N, reproducibility, and stability are observed. A summary of the FIA results for several diamond films and glassy carbon are presented in Table 4. The data were obtained using two diamond films from batch D010297 and three films from batch D030597. The linear dynamic range for diamond is over 4 orders of magnitude from 3.3 mM to 0.3 µM. A more limited range is typical for glassy carbon from 3.3 mM to 1.0 µM. Again, the more limited range results from the large and unsteady background signal at the detection potential which makes detection of concentrations less than 1.0 µM problematic. The sensitivity for both electrodes is comparable with an average value of ∼34 nA/µM. The real advantages of diamond are seen in the detection limit and peak variabilty data. Diamond provides the lowest actual detection limits with an average value of 8 nM. This corresponds to 0.3 ppb azide (0.1 pmol). In most cases, the response for glassy carbon was not tested at injected concentrations less than 1.0 µM. The average S/N ratio at 1.0 µM was 60. This translates into a theoretical detection limit of 50 nM or 2 ppb (1 pmol), as reported in the table. In practice, this detection limit could never be achieved because of the large and unsteady background current. The response was recorded in analog form for both electrodes so there was no electronically corrected background signal. Interestingly, no correction was needed to obtain the reported detection limit for diamond. Of course, the actual detection limit for glassy carbon could be lowered by electronic data processing, but the limits achievable would not likely approach diamond because of the increasing and unsteady background signal that is characteristic of the material. The detection limits were all determined (diamond) or estimated (glassy carbon) at a S/N ) 3. The peak variability (coefficient of variation, RSD × 100) is superior for diamond, as well, ranging from 0.5 to 5%; in most cases less than 2%. This compares favorably with the 6-20% range observed for glassy carbon. These data demonstrate, in the dc amperometric detection mode, diamond films outperform glassy carbon for the detection of azide anion in terms of detection limit and response variability. Response stability is another key aspect of the diamond film performance. It turns out that the performance data shown in Table 4 for glassy carbon can only be obtained at short times at the detection potential. Progressive deterioration of the glassy carbon response occurs over time because of surface oxidation processes. The oxidation leads to a continuously increasing and variable background signal, thereby progressively decreasing the S/B and detection limit. Figure 7A shows that, over a 12-h period of continuous use, the background current for diamond changed by less than 5%, whereas the current for glassy carbon increased

detection limit (nM) (S/N ) 3)

peak variability (%)

8 ( 8 (0.3 ppb) 50 ( 20 (2.1 ppb)

0.5-5 6-20

Figure 7. (A) Background current versus time profiles for (b) diamond D0305971 at 1.25 V and (9) glassy carbon at 1.20 V in 0.1 M phosphate buffer, pH 7.2. Flow rate 1.0 mL/min. (B) S/B ratios versus time for (b) diamond D0305971 and (9) glassy carbon during exposure to 0.1 mM N3- in 0.1 M phosphate buffer, pH 7.2.

by over 300%. Figure 7B shows that the corresponding S/B ratios for injections of 0.1 mM N3- changed by less than 5% for diamond and decreased by more than 50% for glassy carbon during this time period. These data demonstrate the excellent response stability of diamond even at the anodic detection potentials. CONCLUSIONS The oxidation of dissolved inorganic azide anion in aqueous media has been investigated using high-quality, boron-doped diamond thin-film electrodes. This new electrode material exhibits an analytically useful response for azide oxidation and yields superior performance characteristics for the detection of this toxic anion, as compared to glassy carbon. Diamond exhibits a larger overpotential for oxygen evolution and a resistance to morphological changes and severe surface oxidation during anodic polarization such that the voltammetric background currents are low and stable in the potential region where azide oxidation occurs. This is a key aspect of the diamond response. While there is a larger overpotential for oxygen evolution at diamond, there is not a comparable overpotential for the oxidation of azide. This results from the fact that this oxidation reaction does not appear to involve Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

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any specific surface interactions but rather proceeds throughsimple diffusion of the analyte to the interfacial reaction zone. Both linear sweep and differential pulse voltammetric measurements were used to study the reaction. The linear dynamic range is 3-4 orders of magnitude and the limit of detection is 0.1 µM (4.3 ppb). The flow injection analysis data indicated a linear dynamic range of 5 orders of magnitude with a detection limit of 8 nM (0.3 ppb). The diamond response and S/B ratios were stable for up to 12 h. Diamond clearly outperforms glassy carbon in terms of the linear dynamic range, detection limit, response variability, and response stability. The results indicate that the use of borondoped diamond thin-film electrodes offers the possibility of a novel electrochemical-based detection method for trace amounts of this environmentally important analyte.

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ACKNOWLEDGMENT This work was generously supported by grants from the National Science Foundation (CHE-9505683) and the Utah State University Faculty Research Grant Program. Special acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. The informative discussions with Dr. Ivan Mendenhall and co-workers at Morton/Thiokol and Autoliv were greatly appreciated.

Received for review September 2, 1997. January 30, 1998. AC970959D

Accepted