Flow Injection Analysis with Diamond Thin-Film Detectors - American

hydrodynamic voltammetry, and flow injection analysis with electrochemical detection (FIA-EC) at films without any prior surface pretreatment. The dia...
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Anal. Chem. 1997, 69, 4099-4107

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Flow Injection Analysis with Diamond Thin-Film Detectors Sharlene Jolley,† Miles Koppang,‡ Tom Jackson, and Greg M. Swain*

Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300

The viability of using polycrystalline, boron-doped diamond thin-film electrodes for the amperometric detection of ferrocyanide, ferricyanide, ethylamine, and ethylenediamine has been investigated for the first time. These redox reactions were studied by cyclic voltammetry, hydrodynamic voltammetry, and flow injection analysis with electrochemical detection (FIA-EC) at films without any prior surface pretreatment. The diamond films, 1-5 µm thick, were grown on conducting Si substrates at a dopant level of ∼1019 boron atoms/cm3. The detector performance was evaluated as a function of the linear dynamic range, sensitivity, limit of detection, response variability, and response stability. The results indicate that diamond films produce analytically useful responses for all these analytes in both the scanning and constant potential modes. Noteworthy are the observations that boron-doped diamond (i) can be used as a substrate for the detection of both oxidation and reduction reactions (e.g., ferro- and ferricyanide) and (ii) can support the oxidation of ethylamine and ethylenediamine, mechanisms that involve the anodic transfer of oxygen from H2O. Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy film characterization data are discussed along with the electrochemical results. Our research group has been investigating the electrochemical behavior of polycrystalline, boron-doped diamond thin-film electrodes with the goal of developing applications for this material in electroanalysis, energy storage devices and electrosynthesis and as a corrosion-protective coating.1-9 The use of diamond in electrochemistry has only recently been demonstrated, so, unlike † Present address: Department of Chemistry, Kansas State University, Manhattan, KS 66506. ‡ Present address: Department of Chemistry, University of South Dakota, Vermillion, SD 57069. (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. (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.

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other carbon-based electrodes, little is known about the relationship between the physical, chemical, and electronic film properties and the observed electrochemical response.1-20 The knowledge amassed so far by our group1-9 and others10-20 suggests that diamond may become a useful new electrode material, particularly as more is learned regarding the structure-reactivity relationship. Electroanalysis is one field that could possibly benefit from the use of diamond because of the material’s interesting electrochemical properties, which have been found to include the following: (i) a low and stable voltammetric background current (about a factor of 10 less than that of polished glassy carbon for similar geometric areas),3,5,6 (ii) a wide working potential window in aqueous electrolyte solutions (2.5-3 V),19,21 (iii) reversible to quasireversible electron transfer kinetics for the inorganic redox analytes Fe(CN)63-/4-, IrCl62-/3-, and Ru(NH3)62+/3+ without any conventional surface pretreatment,4-6,9,20 (iv) enhanced S/B ratios for these analytes in voltammetric measurements because of the low background current,6 and (v) resistance to deactivation as some films have been observed to exhibit a relatively unchanging ∆Ep for Fe(CN)63-/4- over a 1 month period of exposure to the laboratory atmosphere. The latter would appear to be a unique property of diamond because, often, other carbon-based materials deactivate quickly upon exposure to the laboratory atmosphere and subsequently require some form of pretreatment in order to overcome sluggish electron transfer kinetics. This observation should not be interpreted to mean that diamond requires no pretreatment in order to exhibit rapid electron transfer kinetics. Rather, it means that some films, particularly those with no extensive electrochemical past history, can retain a high degree (10) Pleskov, Y.; Sakharova, A.; Krotova, M. D.; Bouilov, L. L.; Spitsyn, B. V. J. Electroanal. Chem. 1987, 228, 19. (11) Sakharova, A.; Sevast’yanov, A. E.; Pleskov, Y.; Templitskaya, G. L.; Surikov, V. V.; Voloshin, A. A. Electrokhimiya 1991, 27, 239. (12) Sakharova, A.; Nyikos, L.; Pleskov, Y. Electrochim. Acta 1992, 37, 973. (13) Patel, K.; Hashimoto, K.; Fujishima, A. Denki Kagaku 1992, 60, 659. (14) Natishan, P. M.; Morrish, A. Mater. Lett. 1989, 8, 269. (15) Tenne, R.; Patel, K.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1993, 347, 409. (16) Miller, B.; Kalish, R.; Feldman, L. C.; Katz, A.; Moriya, N.; Short, K.; White, A. E. J. Electrochem. Soc. 1994, 141, L41. (17) 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. (18) Sakharova, A. Y.; Pleskov, Y. V.; Di Quarto, F.; Piazza, S.; Sunseri, C.; Teremetskaya, I. G.; Varin, V. P. J. Electrochem. Soc. 1995, 142, 2704. (19) Martin, H. B.; Argoitia, A.; Landau, U.; Anderson, A. B.; Angus, J. C. J. Electrochem. Soc. 1996, 143, L133. (20) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996, 143, L238. (21) Chen, P.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 3115.

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of activity for Fe(CN)63-/4-, for example, even after long-term exposure to the laboratory atmosphere. These electrode properties could render this material extremely useful for electrochemical detection coupled with liquid chromatography (LC) and flow injection analysis (FIA). We report presently on the use of diamond thin-film electrodes for the detection of ferrocyanide, ferricyanide, ethylamine, and ethylenediamine in FIA. These analytes were chosen for evaluation on the basis of differences in the electrode reaction mechanisms of each. The Fe(CN)63-/4- redox couple is often considered to undergo simple “outer sphere” electron transfer. However, it is well established that k° at carbon-based electrodes (e.g., HOPG) can show orders of magnitude variability, depending on the fraction of exposed edge plane exposed and the surface defect density (ref 21 and references cited therein). This is reflective of a complex reaction mechanism influenced by the carbon electrode surface structure. Therefore, ferrocyanide and ferricyanide were chosen as probes to (i) evaluate the performance of diamond-based detectors because of the sensitivity of the electron transfer kinetics to the surface defect density on carbonbased electrodes and (ii) demonstrate that both oxidation and reduction reactions can be detected. The ethylamine and ethylenediamine oxidations can be classified as complex “inner sphere” electron transfer processes involving the transfer of oxygen from H2O in the solvent phase to the reactants (ref 22 and references cited therein). Electrochemical detection of many amines can be accomplished oxidatively, but success has been limited because these reactions require the transfer of oxygen from water, and conventional anode materials lack the ability to support and sustain such oxygen transfer mechanisms.23,24 Voltammetric investigations of aliphatic amine oxidation in aqueous and mostly nonaqueous media have been reported previously for platinum and carbon-based electrodes.25-30 Ge and Johnson23 have reported the successful oxidative detection of aliphatic amines using mixed Ag-Pb oxide film electrodes. The authors have suggested that several electrode characteristics must be met before a material might serve as an electrocatalytic electrode for anodic oxygen transfer reactions: (i) stable surface phases must exist under the pH conditions of the application, (ii) a low density of surface sites must exist at which anodic discharge of water (i.e., evolution of oxygen) occurs at a lower overpotential than is characteristic of the majority of the surface matrix, and (iii) surface sites must exist that are effective for adsorption of the nonbonded electron pair of the N atoms in the amine compounds.23 Boron-doped diamond thin films appear to meet these three criteria. First, we have found that high-quality diamond films are stable and undergo little if any corrosion in strongly alkaline,9 acidic,3 and acidic chloride media.31 Second, given the fact that the films often contain small (22) He, L.; Anderson, J. R.; Franzen, H. F.; Johnson, D. C. Chem. Mater. 1997, 9, 715. (23) Ge, J.; Johnson, D. C. J. Electrochem. Soc. 1995, 142, 1525. (24) Ge, J.; Johnson, D. C. J. Electrochem. Soc. 1995, 142, 3420. (25) Masui, M.; Sayo, H.; Tsuda, Y. J. Chem. Soc. (B) 1968, 973. (26) Mann, C. K. Anal. Chem. 1964, 13, 2424. (27) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757. (28) Deinhammer, R. S.; Ho, M.; Aneregg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306. (29) Surmann, P.; Peter, B. Electroanalysis 1996, 8, 685. (30) Gorski, W.; Cox, J. A. Anal. Chem. 1994, 66, 2771. (31) Chen, Q.; Granger, M. C.; Lister, T.; Swain, G. M. Submitted to J. Electrochem. Soc.

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levels of nondiamond carbon impurities, some of these can exist at the surface, and such impurities have been observed to lead to a lower overpotential for oxygen evolution than impurity-free diamond films.9,19,20 Third, boron dopant atoms located at the surface could serve as adsorption/coordination sites for the amine compounds. Based of these favorable features, it was decided to explore the detection of these two aliphatic amines at diamond thin-film electrodes. Cyclic voltammetry, hydrodynamic voltammetry, and FIA-EC (amperometric detection mode) results are presented herein for the oxidation of ferrocyanide, ethylamine, and ethylenediamine and the reduction of ferricyanide. The results clearly reveal that boron-doped diamond thin-film electrodes produce analytically useful responses for these analytes both in the scanning and constant applied potential modes. In addition to the electrochemical results, film characterization data obtained by scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) are discussed. EXPERIMENTAL SECTION The diamond films were grown on conducting p-Si(111 or 100) substrates (17 MΩ, E-pure Barnstead). RESULTS AND DISCUSSION Film Characterization. Selected diamond films were examined by SEM to obtain information regarding the film morphology, grain size, and coverage. In all cases, the films were continuous over the entire substrate surface and possessed a polycrystalline, microfaceted morphology, very similar to the images reported by our group previously.1-7,9 The films were composed of sharp, wellfaceted triangular, cubic, and/or cubo-octahedral microcrystallites ranging in diameter from 0.5 to 3 µm. There were numerous twinned crystallites in all of the films, and, in some cases, secondary growths were observed. Regardless of the electrochemical past history, all of the diamond films examined had surface morphologies nearly identical to those observed for freshly grown films, suggesting that the imposed electrochemical conditions had little effect on the morphology. Very stable surface morphologies have been reported previously by our group for diamond films exposed to extensive anodic polarization in acidic fluoride media,3 15 w/o KOH,9 and acidic chloride media.31 XRD analysis was performed on selected films, and, in all cases, the Analytical Chemistry, Vol. 69, No. 20, October 15, 1997

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Table 1. Summary of Raman Spectral Features for Diamond Filmsa film

I1332/I1550

I1332/I1450

fwhm (cm-1)

crystallite size (µm)

D72696(1) D120795(1,2,3) D022497(1)

24 30 5

5 5 2

11 9 9

0.65 ( 0.14 2.1 ( 0.63 1.2 ( 0.58

a See text for a discussion of the intensity values. The crystallite sizes were determined by analyzing atomic force microscope images of the film surface.

Figure 2. Raman spectrum of a diamond thin film (D72696). Excitation, 514. 5 nm. Integration time, 10 s.

films possessed preferential 〈111〉 crystallite orientation, as evidenced by an intense reflection at a 2θ of ∼43° using monochromatic Cu KR1 radiation. XPS analysis was applied to selected films to determine the surface elemental composition. Freshly grown films contained carbon with minor amounts of oxygen. The O/C ratio (atomic %) was between 0.03 and 0.06. For comparison, a freshly polished glassy carbon surface exhibits an O/C ratio of ∼0.20.32 Films with extensive electrochemical past histories generally had elevated O/C ratios of 0.10-0.15, indicating that, while the films show no tendency to corrode, carbon-oxygen functionalities can be formed electrochemically on the surface. Boron was not detected due to the low film concentration (∼0.01%). In summary, the films were (i) continuous over the substrate surface, (ii) polycrystalline with significant 〈111〉 crystallite orientation, and (iii) for the most part, relatively oxygenfree with the surface carbon atoms terminated by hydrogen.33 Raman spectroscopy was employed to evaluate the film microstructure. This technique has been used extensively to characterize the microstructure of various carbonaceous materials.34-38 A summary of the spectral data is presented in Table 1. All of the films used in this work exhibited a sharp, crystalline diamond band at 1332 ( 2 cm-1, as shown in Figure 2. The fwhm of this band ranged between 8 and 15 cm-1, depending on the film growth conditions. In comparison, a fwhm value of ∼8 cm-1 was observed for a piece of high-pressure, hightemperature grown diamond. It can be noticed from the data in Table 1 that the largest fwhm value is observed for the film with the smallest grain size, consistent with the results presented by (32) Fagan, D, T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 57, 2759. (33) Angus, J. C.; Hayman, C. C. Science 1988, 241, 913. (34) Dennison, J. R.; Holtz, M. W.; Swain, G. M. Spectroscopy 1996, 10, 1. (35) Knight, D. S.; White, W. B. J. Mater. Res. 1989, 4, 385. (36) Nemanich, R. J.; Solin, S. A. Phys. Rev. B 1979, 20 (2), 392. (37) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53 (3), 1126. (38) McCreery, R. L.; Packard, R. T. Anal. Chem. 1989, 61, 775A.

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Robins et al.39 Most films also exhibited minor scattering intensity in the region between 1500 and 1600 cm-1, attributable to incorporated sp2-bonded carbon impurities. 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 (ref 39 and references cited therein). The ratio of the diamond-tonondiamond scattering intensity (I1332/I1550) is a qualitative measure of the film quality. Increasing values of the I1332/I1550 ratio may reflect higher film quality (i.e., more diamond character). This is based on the fact that the Raman cross-sectional scattering coefficients for diamond and graphite (i.e., nondiamond carbon) are 9 × 10-7 and 500 × 10-7 cm-1/sr, respectively.35 The average ratios for the films used in this work (D72961, D120795, and D022497) were 24, 30, and 5, respectively. The unusually low ratio for the latter film is believed to have resulted from some observed instabilities in the plasma during growth. The majority of the films examined had ratios in excess of 15. Multiple regions of each film were analyzed, and the spectral features were generally very reproducible from region to region. These ratios, as well as the fwhm values, were very similar for films prepared using identical growth conditions both before and after electrochemical use. This observation is consistent with the SEM results and supports the notion that diamond is microstructurally stable under a variety of imposed electrochemical conditions. In fact, we have previously reported that the diamond microstructure is largely unaffected by severe anodic polarization in 15 w/o KOH9 or acidic chloride media.31 A very broad photoluminescence background, underlying the entire carbon region of the spectrum (1300-1600 cm-1), was also observed for most films. The photoluminescence magnitude was estimated from the spectral intensity at 1450 cm-1.39 Thus, the ratio of the diamond-tophotoluminescence scattering intensities (I1332/I1450) is another qualitative measure of the film quality. The films reported on presently (D729961, D120795, and D022497) had average ratios of ∼4, 5, and 2, respectively. The trends in this ratio are interrelated with those for the I1332/I1550 ratio. For example, the film with the largest graphitic content, based on the I1550 value, also exhibited the largest photoluminescence intensity, I1450. This correlation is consistent with the findings of Robins et al.,39 who showed that the I1550 and I1450 values track defect density and the defects consist of sp2-bonded carbon. In summary, most of the films tested possessed significant diamond character (i.e., sp3 bonding) with lesser amounts of incorporated nondiamond carbon impurities. Ferro-/Ferricyanide Response. Figure 3A shows a background cyclic voltammetric i-E curve for a diamond film in 0.1 M KCl. The response is rather featureless within this potential window and remained so even after multiple scans. On the forward sweep, the anodic current increases sharply at ∼1200 mV, presumably due to chlorine evolution. Just prior to this current, there is a poorly resolved peak near 750 mV. The origin of this peak has not yet been established but may be related to effects associated with electrochemically active grain boundary carbon (i.e., nondiamond carbon).19,40 Little cathodic current flows on the reverse sweep until about -500 mV. This current may result (39) Robins, L. H.; Farabaugh, E. N.; Feldman, A. J. Mater. Res. 1990, 5, 2456. (40) Martin, H. B.; Argoitia, A.; Angus, J. C.; Landau, U. Presented at the International Mini-Symposium on Diamond Electrochemistry and Related Topics, The University of Tokyo, March 17-18, 1997.

Figure 3. (A) Background cyclic voltammetric i-E curve for a diamond film (D022497) in 0.1 M KCl. Scan rate, 100 mV/s. (B) Cyclic voltammetric i-E curve (total current) for the diamond film in 1 mM Fe(CN)64- + 0.1 M KCl. Scan rate, 100 mV/s. Geometric area, 0.2 cm2.

from the reduction of residual chlorine remaining near the electrode surface formed during the forward sweep. Regardless, it is clear from the response that there is a large overpotential for the reduction of chlorine to chloride as little cathodic current flows until some 1700 mV negative of the potential, at which point the chlorine evolution commences. Similarly large overpotentials for the reduction of chlorine at diamond have been reported by Vinokur et al.20 and by our group.31 We define the potential window on the basis of the potentials at which the anodic and cathodic currents reach 10 µA. The potential window for this film is in excess of 2.6 V. A most interesting feature is that, overall, the background current for diamond is an order of magnitude lower than that observed for freshly polished glassy carbon of comparable size. Low voltammetric background currents and capacitance have been reported previously for diamond films in contact with aqueous electrolytes1,3,5,6,19,20 and would appear to be an attractive feature of this new electrode material. There are three possible, and not necessarily unrelated, explanations for the low background current and capacitance at diamond. First, the relative absence of electroactive carbonoxygen functionalities on the hydrogen-terminated diamond surface, as compared with polished glassy carbon, results in a lower current. For example, our group has recently reported that oxygen-free, hydrogenated glassy carbon exhibits a background voltammetric current response that is a factor of 3-6 lower than that of 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.32,41 Therefore, the absence of electroactive surface carbon-oxygen 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 due to the semimetal-semiconductor nature of boron-doped diamond.5 A lower surface charge carrier density at a given potential 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.42,43 It should be noted that the background voltammetric current and the capacitance for diamond are very similar in magnitude to that for the basal plane of highly oriented pyrolytic graphite.3,5 A third possible contributing factor could be that the diamond film surface is composed of an array of microelectrodes. In other words, perhaps the diamond surface has “electrochemically active” sites separated by 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.44 So far, our experimental results have not provided evidence to unequivocally distinguish between the second and third possible factors. Figure 3B shows a cyclic voltammetric i-E curve (total current) for a diamond film in 1 mM Fe(CN)63-/4- + 0.1 M KCl. This response was typical for diamond films with no extensive electrochemical past history. It should be noted, however, that this film was exposed to the laboratory atmosphere for several weeks prior to this measurement and yet good activity is still observed for this analyte without any pretreatment. This observation suggests that the hydrogen-terminated diamond film may be resistant to deactivation (i.e., adsorption of impurities). A welldefined, peak-shaped response is observed with ∆Ep and Ep/2 values of 100 and 230 mV, respectively, and an ipa/ipc ratio of 1.0. The peak currents varied linearly with the concentration between 1 and 0.01 mM (r > 0.998) and linearly with the square root of the scan rate, ν1/2 (shown in the Figure 3B inset). The latter trend indicates that the currents are limited by semi-infinite linear diffusion of reactant to the interfacial reaction zone. Commonly, ∆Ep values for this analyte ranged between 70 and 150 mV (100 mV/s) for diamond films with no extensive electrochemical past history.45 Films with extensive electrochemical past histories exhibited ∆Ep values greater than 250 mV (100 mV/s). Quasireversible electron transfer kinetics are evidenced by two observations: (i) the ∆Ep value increases with increasing scan rate (100500 mV/s) and (ii) the ipa versus ν1/2 plot is linear (r ) 0.998) but with a non-zero y-axis intercept (13 µA). Figure 4A shows a hydrodynamic i-E curve for a diamond film in 0.1 M KCl. These data were recorded after 30 min at each potential at a flow rate of 1.0 mL/min. This response was typical for most diamond films, regardless of the past history. These (41) Swain, G. M.; Kuwana, T. Anal. Chem. 1992, 64, 565. (42) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257. (43) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1975, 58, 313. (44) Vitt, J. E.; Johnson, D. C. J. Appl. Electrochem. 1994, 24, 107. (45) Granger, M. C.; Xu, J.; DeClements, R.; Hirsche, B. L.; Swain, G. M. In New Directions in Electroanalytical Chemistry; Leedy, J., Wightman, R. M. Eds.; The Electrochemical Society Proceedings Series PV 96-9; The Electrochemical Society: Pennington, NJ, 1996; p 138.

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Table 2. Summary of the Diamond FIA-EC Detector Performance Parameters for Ferrocyanide and Ferricyanide Ferrocyanide linear dynamic range 1 × 10-3-1 × 10-6 M (r > 0.995) limit of detection (S/B ) 3) 2.05 × 10-7 ( 0.651 M (n ) 7) sensitivity 7.04 ( 0.656 nA/µM (r > 0.995) peak height variability 13.3 ( 4.05% (n ) 7) Ferricyanide linear dynamic range 1 × 10-3-1 × 10-6 M (r > 0.995) limit of detection (S/B ) 3) 5.73 × 10-7 ( 2.95 M (n ) 3) sensitivity 11.9 ( 7.63 nA/µM (r > 0.998) peak height variability 11.1 ( 7.63% (n ) 3)

Figure 4. (A) Hydrodynamic background i-E curve for a diamond film (D120795) in 0.1 M KCl at a flow rate of 0.5 mL/min. The data were obtained at the end of a 30 min period at each potential. (B) Hydrodynamic i-E curve for the diamond film using 20 µL injections of 1 mM Fe(CN)64- in 0.1 M KCl. The mobile phase was 0.1 M KCl at a flow rate of 0.5 mL/min.

data reveal that the steady state background current is less than 4 nA at potentials from 0.1 to 0.5 V. In comparison, the background current for freshly polished glassy carbon was about a factor of 1-2 larger in this potential region. Between 0.5 and 0.6 V, the current for diamond increases to ∼16 nA, and it is in this potential region that the background current for glassy carbon is significantly larger. For example, at potentials positive of 0.5 V, the background current was as much as a factor of 7 larger. The excessive background current for glassy carbon, particularly at potentials positive of 0.5 V, is likely associated with the oxidation of surface carbon-oxygen functionalities (e.g., hydroquinones) existing at the edge plane sites. Such surface carbon-oxygen functionalities are not expected to exist to any appreciable extent on the hydrogen-terminated diamond surface at these potentials. Figure 4B shows a hydrodynamic i-E curve for a diamond film in 1 mM Fe(CN)64- + 0.1 M KCl. The data were obtained by averaging the peak heights of 10 analyte injections (20 µL) at each potential using a 0.1 M KCl mobile phase and a flow rate of 1.0 mL/min. Again, this hydrodynamic response was typical for films with no extensive electrochemical past history. A welldefined, sigmoidal shape is observed with a mass transport limited current response at potentials positive of 350 mV. The half-wave potential of ∼250 mV is nearly identical to that observed for freshly polished glassy carbon. Interestingly, this well-defined, sigmoidal hydrodynamic voltammogram was obtained without any pretreatment of the film. Films with extensive electrochemical past histories (e.g., chlorine electrolysis, anodic polarization studies, metal electrodepositions, etc.) rarely exhibited a sigmoidal shape, and the apparent E1/2 values for such films were often in excess of 500 mV. These observations indicate that electrochemical 4104 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997

usage can deleteriously affect the film response, but the freshly grown, hydrogen-terminated surfaces are inert enough to resist deactivation during exposure to the laboratory atmosphere. If one compares the voltammetric data in Figures 3B and 4B, an interesting observation can be made. The two films exhibited reasonably fast charge transfer kinetics, even though the apparent film qualities are very different. For example, the D022497 film has an I1332/I1550 ratio of 5 and an I1332/I1450 ratio of 2, both indicative of lower quality (i.e., more sp2 carbon impurities). On the other hand, the D72696 film has ratios of 24 and 5, respectively, both indicative of higher film quality. The trend that is beginning to emerge in our studies is that neither the ∆Ep nor the E1/2 values for the Fe(CN)63-/4- couple correlate with the film quality as determined by Raman spectroscopy. This would seem to suggest that the electron transfer is not mediated or does not occur exclusively through nondiamond carbon impurity sites at the surface, as, if this were the case, then a correlation between the electrochemistry and spectroscopy would be expected. The oxidative and reductive detection of ferrocyanide and ferricyanide, respectively, were used to evaluate the diamond FIAEC detector performance in terms of the linear dynamic range, limit of detection, sensitivity, response variability, and response stability. A summary of the results for three films (D120796) is presented in Table 2. The peak responses for both analytes were narrow and symmetric with no evidence of tailing. For the ferrocyanide detection, the linear dynamic range was 3 orders of magnitude, from 1 × 10-3 to 1 × 10-6 M (r > 0.999), with a theoretical limit of detection (S/B ) 3) of 2.05 × 10-7 ( 0.651 M (4.10 pmol). The sensitivity was 7.04 ( 0.656 nA/µM (r > 0.999). The peak height variability for multiple injections of a given concentration was sizeable, with a value of 13.3 ( 4.05%. Good response stability over time was observed, as some films exhibited peak height variations (1 mM injected concentration) of under 10% over a 16 h period of continuous use. For the ferricyanide detection, the linear dynamic range was also 3 orders of magnitude, from 1 × 10-3 to 1 × 10-6 M (r > 0.995), with a theoretical limit of detection (S/B ) 3) of 5.73 × 10-7 ( 2.95 M (11.5 pmol).The sensitivity was 11.9 ( 5.76 nA/ µM (r > 0.995). As was the case for the ferrocyanide results, the peak height variability was large, with a value of 11.1 ( 7.63% for these three films. Films with the most extensive electrochemical past histories usually exhibited higher detection limits (>1 × 10-6 M, S/B ) 3) and lower linear dynamic ranges (2.5 orders of magnitude) for both ferrocyanide and ferricyanide. This is not surprising, since most electrodes, particularly carbon-based materials, exhibit decreasing activity with increasing usage due to deactivation phenomena. Pretreatments are often required to

Figure 5. (A) Cyclic voltammetric i-E curves (background and total current) for a diamond film (D72696) in 1 mM ethylamine + 0.1 M NaClO4/0.01 M carbonate buffer, pH 10. Scan rate, 20 mV/s. (B) FIAEC results for a diamond film (D120795) using 20 µL injections of 0.25 mM ethylamine in 0.1 M carbonate buffer, pH 10. The mobile phase was 0.1 M carbonate buffer, pH 10, at a flow rate of 0.5 mL/ min.

reactivate the surface. The use of diamond in electrochemistry is relatively new, so little is known about how conventional surface pretreatments (e.g., polishing, anodic polarization, etc.) affect the electron transfer kinetics. Efforts are currently underway in our laboratory to learn what pretreatment protocols are most effective for reconditioning diamond electrodes. Because these films were used without any activation pretreatment, the detector performance characteristics should be considered as the lower limits of what can probably be achieved for pretreated diamond. The sizeable peak height variability would appear to be a problematic feature of diamond. Similar experiments with freshly polished glassy carbon yielded peak height variabilities of 2-4%, so the large values for diamond did not result from instrumental effects but rather resulted from influences of the film properties. Experiments using freshly grown films also revealed peak height variabilities of 2-5%. Based on these two observations, it is supposed that surface contamination and unknown effects from past electrochemical use are the cause for the large response variability. It is anticipated that improvements in linear dynamic range, sensitivity, limit of detection, and response variability can be achieved by using either freshly grown films or by pretreating “used” films adequately to recondition the surface. Ethylamine Oxidation. Figure 5A shows cyclic voltammetric i-E curves for a diamond film in 1 mM ethylamine + 0.1 NaClO4/ 0.01 M carbonate buffer, pH 10. Both the total and background current responses are shown for comparison. The oxidation of

this molecule has previously been investigated by Ge and Johnson,23,24 and their work provided the motivation for our efforts. It is clear from the data that there is a net faradaic signal above the background current for the oxidation of ethylamine. The most interesting feature is the shape of the response. There is a current crossover at 1.1 V, followed by a pseudo-steady-state oxidation current on the reverse sweep at ∼0.96 V rather than on the forward sweep. The limiting current is ∼9 µA. The product of the oxidation reaction is believed to be ethyl aldehyde, based on the results of Ge and Johnson,23,24 and is irreversible, as no reduction current is observed during the reverse scan. Similar voltammetric shapes were observed in 50:50 mixtures of acetonitrile and the pH 10 carbonate buffer, but the faradaic currents were somewhat attenuated. The pseudo-steady-state oxidation current on the reverse sweep, as well as the current crossover at 1.1 V, generally disappeared at scan rates above 50 mV/s, resulting in a poorly defined oxidation wave on the forward sweep superimposed on the rising background current. Also, the pseudo-steady-state current and the current crossover were generally observed only for analyte concentrations of 1 mM or greater. At concentrations below this level, a poorly defined oxidation wave on the forward sweep was observed superimposed on the rising background current. Most of the diamond films examined exhibited a net faradaic current for the oxidation of ethylamine at potentials greater than 0.9 V, regardless of the electrochemical past history. However, only a few exhibited the pseudo-steady-state oxidation current on the reverse sweep. Initial results indicated that the current increased in a linear fashion with the analyte concentration between 0.01 and 10 mM. This response is consistent with the reaction mechanism proposed by Johnson and co-workers22-24,44 involving the transfer of oxygen from reactive hydroxyl radicals produced initially in the water discharge reaction. The boron-doped diamond films contain small amounts of nondiamond carbon impurity at the surface, separated by larger regions of diamond carbon. The nondiamond carbon exhibits a lower overpotential for the water discharge reaction than does the surrounding diamond lattice. This has two important implications: (i) overall, the background current associated with the oxygen evolution reaction at diamond is relatively low at the amine oxidation potentials because only a limited amount of the surface is involved in the reaction and (ii) given the assumed localization of these impurities in the grain boundaries, good access of the amine molecules to the hydroxyl radical producing sites should be possible. There are also likely boron dopant atoms at the surface that provide favorable coordination sites for the lone pair of electrons on the amine functional group. The most ideal situation for the oxidation of the amines would be when surface boron dopant atoms are located nearby the nondiamond carbon impurity sites. The current crossover and pseudo-steady-state oxidation current observed on the reverse scan occur because, initially on the forward scan at ∼0.9 V, there is little hydroxyl radical produced. It is not until potentials exceed 1.0 V that significant current flows associated with the water discharge reaction. A sufficient concentration of hydroxyl radical is built-up during this period, perhaps stabilized in some fashion by the nondiamond carbon impurity sites, such that attack of nearby amine substrates commences, and a measureable oxidation current is observed. The higher the amine concentration nearby, the larger the anodic current is. The current crossover is consistent with the idea that a sufficient hydroxyl radical concenAnalytical Chemistry, Vol. 69, No. 20, October 15, 1997

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Figure 6. (A) Cyclic voltammetric i-E curves (background and total current) for a diamond film (D72696) in 1 mM ethylenediamine + 0.1 M NaClO4/0.01 M carbonate buffer, pH 10. Scan rate, 100 mV/s. (B) FIA-EC results for the diamond film using 20 µL injections of 1 mM ethylenediamine + 0.1 M NaClO4/0.01 M carbonate buffer, pH 10. The mobile phase was 0.1 M NaClO4/0.01 M carbonate buffer, pH 10, at a flow rate of 1.0 mL/min.

observed on the forward sweep at 0.960 V and is well-resolved from the onset current associated with oxygen evolution. For all of the films examined, the oxidation wave was observed on the forward sweep, in contrast to some of the film responses for ethylamine oxidation. The reason for this difference in the voltammetric response for these two analytes is not known. Similarly shaped cyclic voltammetric i-E curves were also observed in 50:50 mixtures of acetonitrile and carbonate buffer with slightly reduced current levels. The oxidation product is not known but may be the dialdehyde, glyoxal (HCOCHO). Whatever the oxidation product is, the process is irreversible, as no cathodic current is observed during the reverse scan. The limiting current varied little with the square root of the scan rate between 20 and 200 mV/s, consistent with active sites on the surface separated by distances larger than the diffusion layer thickness (e.g., microelectrode array). A systematic study of the concentration dependence of the limiting current was not performed; however, initial experiments indicated that quantifiable responses between 1 × 10-2 and 1 × 10-5 M (r > 0.995) level are possible. We suppose that an oxidation mechanism similar to that for ethylamine is operative for the diamine as well, except that the current per molecule is larger due to the fact that the diamine has two oxidizable amine groups. Figure 6B shows preliminary diamond FIA-EC results for four 20 µL injections of 1 mM ethylenediamine + 0.1 M NaClO4/0.01 M carbonate buffer, pH 10. The mobile phase was 0.1 M NaClO4/ 0.01 M carbonate buffer, pH 10, and the flow rate was 1.0 mL/ min. The detection potential was 0.750 V, which was a compromise between an optimum faradaic response and a low background current from water electrolysis. Good peak height reproducibility and minimal peak tailing are observed, even though this particular film received no pretreatment prior to use. The relative standard deviation of the peak height is ∼4%. Response linearity was observed between 1 × 10-3 and 1 × 10-5 M (r > 0.980). The theoretical limit of detection ranged between 5 × 10-6 and 1.0 × 10-5 M (S/B ) 3).

tration must be generated before a sizeable amine oxidation current can be measured. Figure 5B shows shows diamond FIA-EC results for eight 20 µL injections of 0.25 mM ethylamine + 0.1 M carbonate buffer, pH 10. The mobile phase was 0.1 M carbonate buffer, pH 10, at a flow rate of 1.0 mL/min. The detection potential was 0.925 V, a value chosen as a compromise between an optimum faradaic response and a low background current from water electrolysis. Good peak height reproducibility and minimal peak tailing are observed, even though this particular film received no pretreatment prior to use. The relative standard deviation of the peak heights is ∼2%, much less than what was observed for ferro- and ferricyanide. This suggests that the FIA-EC using diamond film electrodes can provide a reproducible analytical response for the oxidation of ethylamine. Generally, measurable signals could be obtained between 1 × 10-2 and 1 × 10-5 M; however, no systematic study of the response dependence on concentration was performed. Ethylenediamine Oxidation. Figures 6A shows cyclic voltammetric i-E curves for a diamond film in 1 mM ethylenediamine + 0.1 M NaClO4/0.01 M carbonate buffer, pH 10. Both the total and background current responses are shown for comparison. A pseudo-steady-state oxidation current of 31 µA is

CONCLUSIONS This work represents the first effort to use boron-doped diamond thin films in electroanalysis. A more complete evaluation of the film response toward these analytes is still needed, but these initial results clearly demonstrate that films, particularly those with no extensive electrochemical past history, exhibit analytically useful responses for both the oxidative detection of ferrocyanide and the reductive detection of ferricyanide. Responses obtained using untreated diamond films were often comparable to those obtained using freshly polished glassy carbon in terms of the linear dynamic range, sensitivity, and limit of detection. Three of the most important observations are the following: (i) diamond can be used as a substrate for the detection of both oxidation and reduction reactions (e.g., ferro- and ferricyanide), (ii) diamond films exhibit low and stable background currents with time, leading to improved response stability and S/B ratios as compared with those obtained with polished glassy carbon, and (iii) diamond films, particularly those without extensive electrochemical past histories, often exhibit activity comparable to that of freshly polished glassy carbon, and such films can retain a high degree of activity, even after long-term exposure to the laboratory atmosphere. The preliminary results also indicate that diamond can support the oxidation of aliphatic amines in aqueous media. This is a very

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important finding, as many pharmaceutically important compounds contain amines that are often difficult to detect at carbon-based electrodes because of passivation or excessively large background currents due to solvent oxidation at the amine oxidation potentials. The preliminary results suggest that one may be able to construct electrochemical detectors for HPLC or FIA using diamond that could make more of these compounds accessible to detection. Diamond appears to possess the requisite criteria, as outlined by Ge and Johnson,23 for the stable detection of the primary amines. The boron-doped diamond film surface contains small amounts of nondiamond carbon impurities that are separated by larger regions of diamond carbon. The nondiamond carbon exhibits a lower overpotential for the water discharge reaction, the most important step being the initial one-electron oxidation to form reactive hydroxyl radical [OH•]. It is from this species that the transferred oxygen comes as this species oxidizes the substrate. It is also likely that some of the boron dopant atoms at the surface provide suitable coordination sites for the lone pair of electrons on the amine functional group of the molecule. Boron coordination sites in close proximity to the nondiamond impurity sites where hydroxyl radical is being produced would provide an ideal situation for the oxidation of the primary amines. It is supposed that better quality (i.e., films with low levels of nondiamond carbon impurity) and higher doped films will lead to improvements in the oxidative detection of primary amines. Experiments are

currently underway to test this hypothesis. It is also anticipated that improvements in the flow cell design, electronic circuitry, and electrode surface pretreatment will allow us to increase the linear dynamic range and the sensitivity and lower the limit of detection for these and other analytes. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, as well as to the National Science Foundation, Analytical and Surface Chemistry Program (CHE-9505683), and the DoD DURIP Program (ONR/DAAH04-95-1-0513) for the financial support provided. Special thanks are extended to Blayne Hirsche for performing some of the FIA work with ethylamine, to Professor Peter Sherwood and students (Kansas State University) for performing the XRD analyses, to Dr. Arthur Heuer and co-workers (Case Western Reserve University) for performing the boron nuclear reaction analysis measurements, and to Dr. Ganesh Raikar (FOSSIL, University of Utah) for performing the XPS measurements.

Received for review December 13, 1996. Accepted July 14, 1997.X AC961269X X

Abstract published in Advance ACS Abstracts, September 1, 1997.

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