Electrochemical Oxidation of Polyamines at Diamond Thin-Film

synthesizer (HP Model 8904A) and dialing in an offset voltage. The ethylenediamine (99% ... showed a progressive attenuation of the current. Similar a...
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Anal. Chem. 1999, 71, 1188-1195

Electrochemical Oxidation of Polyamines at Diamond Thin-Film Electrodes Miles D. Koppang,† Małgorzata Witek,‡ John Blau,‡ and Greg M. Swain*,‡

Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300, and Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069

The oxidation of five polyamines (ethylenediamine, putrescine, cadaverine, spermine, and spermidine) was investigated at polycrystalline, boron-doped, diamond thin-film electrodes using cyclic voltammetry and flow injection analysis (FIA) with amperometric detection. Cyclic voltammetry of the polyamines was conducted in pH 10 carbonate buffer. Well-resolved oxidation waves with respect to the background signal were observed, and the current-potential curves exhibited a scan rate dependence characteristic of slow desorption kinetics of the polyamine oxidation product. E1/2’s of ∼+0.88 V vs Ag/ AgCl were measured for all five polyamines. A mechanism is proposed whereby the polyamine oxidation occurs by oxygen transfer from reactive OH radicals. These radicals are produced during the initial stage of oxygen evolution at the nondiamond carbon impurity sites. These sites are believed to be located primarily at the grain boundaries, isolated from one another by the diamond microcrystallites, although the impurities could also exist as extended defects within the lattice. Stabilization of the polyamine prior to oxidation is achieved through adsorption/complexation of the amine functionality with surface boron dopant atoms, also clustered at the grain boundaries. In general, the FIA results demonstrated that the diamond can be used to effectively detect all five polyamines with a concentration limit of quantitation of ∼1 µM (S/N g 3) and a linear dynamic range from 10-3 to 10-6 M (r2 > 0.97). These detector figures of merit were achieved at constant potential without prior derivatization. Lower quality rather than higher quality diamond films are needed for this assay, and the requisite film properties can be introduced by judicious choice of the deposition conditions. A number of reports describing the usefulness of diamond as an electrode material have appeared recently (see refs 1, 2, and references therein). Our group has been investigating the basic electrochemical properties of polycrystalline, boron-doped, diamond thin-film electrodes in order to determine their utility for * Corresponding author. E-mail: [email protected]. † University of South Dakota. ‡ Utah State University. (1) Xu, J.; Granger, M. C.; Chen, Q.; Lister, T. E.; Strojek, J. W.; Swain, G. M. Anal. Chem. 1997, 69, 591A. (2) Swain, G. M.; Anderson, A.; Angus, J. C. Mater. Res. Bull. 1998, 23, 56. Tenne, R.; Levy-Clement, C. Isr. J. Chem. 1998, 38, 57.

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electroanalysis, energy storage devices, electrosynthesis, and corrosion protection (see refs 1, 2 and references therein). The films are moderately to highly doped, so electronically they behave as a semimetal over a wide potential range. Also, the properties of high-quality diamond are unique and clearly different from those of conventional sp2 carbon electrodes. Diamond electrodes seem to be well-suited for electroanalysis because of their properties:1,2 (i) low and stable voltammetric background current, (ii) wide working potential window in aqueous electrolyte solutions, (iii) reversible to quasi-reversible electron-transfer kinetics for various inorganic redox analytes without conventional pretreatment, and enhanced signal-to-background (S/B) ratios for these analytes due to the low background current, (iv) morphological and microstructural stability at extreme anodic and cathodic potentials and current densities, (v) long-term response stability, and (vi) low and weak adsorption of polar molecules such as anthraquinone2,6-disulfonate. Good-quality diamond thin films have been successfully used in amperometric detection schemes coupled with flow injection analysis (FIA) for several analytes, out-performing freshly polished glassy carbon for the detection of azide,3 nitrite,4 hydrazine, and aliphatic polyamines.5 Electroanalysis of the polyamines (e.g., ethylenediamine) is especially noteworthy, since it requires the unique structure/reactivity relationship the diamond surface provides. Aliphatic compounds, including alcohols and amines, are usually observed to be electroinactive at constant applied potential in aqueous media and have been classified as such historically.6 Direct electrochemical oxidation of aliphatic amines has been reported in nonaqueous media7-16 but has only been observed in aqueous media for secondary and tertiary amines.17 (3) Xu, J.; Swain, G. M. Anal. Chem. 1998, 70, 1502. (4) Granger, M. C.; Xu, J.; Strojek, J. W.; Swain, G. M. Anal. Chim. Acta. In press. (5) Jolley, S.; Koppang, M.; Jackson, T.; Swain, G. M. Anal. Chem. 1997, 69, 4041. (6) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969. (7) Mann, C. K. Anal. Chem. 1964, 13, 2424. (8) Barnes, K. K.; Mann, C. K. J. Org. Chem. 1967, 32, 1474. (9) Hull, L. A.; Davis, G. T.; Rosenblatt, D. H.; Mann, C. K. J. Phys. Chem. 1969, 75, 2142. (10) Portis, L. C.; Bhat, V. V.; Mann, C. K. J. Org. Chem. 1970, 35, 2175. (11) Matsui, Y.; Kurosaki, Y.; Date, Y. Bull. Chem. Soc. Jpn. 1973, 46, 147. (12) Matsui, Y.; Date, Y. Bull. Chem. Soc. Jpn. 1973, 46, 460. (13) Hojo, M.; Imai, Y. J. Electroanal. Chem. 1986, 209, 297. (14) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757. (15) Antoniadou, S.; Jannakoudakis, A. D.; Jannakoudakis, P. D.; Theodoridou, E. J. Appl. Electrochem. 1992, 22, 1060. 10.1021/ac980697v CCC: $18.00

© 1999 American Chemical Society Published on Web 02/10/1999

Despite favorable thermodynamics, the oxidation of aliphatic amines in aqueous media is kinetically limited because the reactions require transfer of oxygen from water, and many conventional anode materials lack the ability to support and sustain this reaction mechanism.18-20 The oxidation of aliphatic amines in aqueous media has been usually limited to anode materials such as copper oxide, nickel oxide, gold and platinum oxides, or composites of metal oxides, which are capable of transferring oxygen from the solvent to the analyte.21-28 However, surface adsorption and fouling of the electrodes by accumulated detection products has limited the application of amperometric detection at constant potential.29 Johnson and co-workers developed pulsed amperometric detection (PAD) with gold and platinum electrodes (i) to produce surfaces with the requisite oxides and (ii) to clean the surfaces fouled by the accumulation of reaction products.29 Ge and Johnson have recently reported successful oxidative detection of aliphatic amines at constant potential using mixed Ag-doped, Pb oxide composite electrodes.18,19 The authors proposed the following reaction mechanism:

S[ ] + H2O f S[OH] + H+ + e{OH radical generation at a surface site} S′[ ] + R a S′[R] {coordination of the amine at a Ag surface site} S[OH] + S′[R] f S[ ] + S′[ ] + RO + H+ + e{anodic oxygen-transfer reaction}

The authors have indicated that several requirements must be met before a material might serve as an electrocatalytic electrode for anodic oxygen transfer reactions:18-20 (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 (OH radical formation) occurs at lower overpotential than is characteristic of the majority of the surface matrix, and (iii) surface sites must exist that are effective for adsorption/complexation of the nonbonded electron pair on the N atoms in the amine compounds. Some aliphatic polyamines are important molecules physiologically. They are present in tumor cells,30 serve as markers for (16) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1307. (17) Masui, M.; Sayo, H.; Tsuda, Y. J. Chem. Soc. (B) 1968, 973. (18) Ge, J.; Johnson, D. C. J. Electrochem. Soc. 1995, 142, 1525. (19) Ge, J.; Johnson, D. C. J. Electrochem. Soc. 1995, 142, 3420. (20) He, L.; Anderson, J. R.; Franzen, H. F.; Johnson, D. C. Chem. Mater. 1997, 9, 715. (21) Fleischmann, M.; Korinek, K.; Pletcher, D. J. Chem. Soc., Perkin Trans. 2 1972, 1396. (22) Feldhues, U.; Schaefer, H. J. Synthesis 1982, 145. (23) Kok, W. Th.; Brinkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1983, 256, 17. (24) Kok, W. Th.; Hanekamp, H. B.; Box, P.; Frei, R. W. Anal. Chim. Acta 1982, 142, 31. (25) Cox, P.; Pletcher, D. J. Appl. Electrochem. 1991, 21, 11. (26) Hui, B. S.; Huber, C. O. Anal. Chim. Acta 1991, 243, 279. (27) Fried, I.; Meyerstein, D. J. Electroanal. Chem. 1971, 29, 429. (28) Pakalapati, S. N. R.; Popov, B. N.; White, R. E. J. Electrochem. Soc. 1996, 143, 1636. (29) Johnson, D. C.; LaCourse, W. R. Anal. Chem. 1990, 62, 589A and references therein. (30) Zhang, R.; Cooper, C. L.; Ma, Y. Anal. Chem. 1993, 65, 704.

monitoring the extent of tissue rejection in organ transplants,31 and are present at elevated urinary levels for cancer patients.32 Development of simple, rapid, and sensitive assays for individual polyamines in biological samples including urine, blood, and cerebrospinal fluid has been pursued by a number of investigators. Typically, separation of polyamines by liquid chromatography coupled with pre- or postcolumn derivatization has been used.33-35 Examples include postcolumn derivatization with o-phthalaldehyde (OPA)36 and postcolumn enzymatic reaction of the polyamine coupled with a suitable detector.37,38 Analysis by capillary zone electrophoresis coupled with indirect photometric detection avoided time-consuming derivatization of the polyamines.30 A recent report by Hoekstra and Johnson achieved sensitive electrochemical detection of biogenic polyamines without derivatization using pulsed waveforms at gold electrodes following liquid chromatographic separation.39 The fact that diamond can be used to detect polyamines at constant applied potential is important because the use of this material could lead to simpler electroanalytical assays for these analytes. Cyclic voltammetric studies of the oxidation of five polyamines (ethylenediamine, cadaverine, putrescine, spermine, and spermidine) at polycrystalline, boron-doped, diamond thin-film electrodes in pH 10 carbonate buffer are reported herein. In addition, the performance of diamond for the detection of these analytes was examined by flow injection analysis (FIA) with amperometric detection. The results show that polycrystalline diamond possesses the requisite surface structure and chemical composition necessary to support and sustain the polyamine oxidation reaction. A mechanism for polyamine oxidation at diamond is offered that is analogous to the model for anodic oxygen-transfer reactions put forth by Johnson and Ge.18-20 EXPERIMENTAL SECTION Diamond Thin Film Deposition and Characterization. The polycrystalline diamond thin films were grown using the following deposition protocol. This protocol produces good-quality films using our reactor and, most importantly, yields films with fairly reproducible electrochemical properties. The diamond was deposited on conducting p-Si (100) substrates (Virginia Semiconductor, Inc., Fredericksburg, VA) using an in-house microwaveassisted CVD reactor. The substrates (0.1 cm thick × 1 cm2 in area) were pretreated by solvent cleaning in toluene, methylene chloride, acetone, 2-propanol, and methanol. The substrates were then etched in concentrated HF for 60 s. After rinsing with ultrapure water and drying, the substrates were hand-polished for 5 min with 2 parts B2O3 powder (Aldrich) and 1 part 0.1-µm diamond powder (GE Superabrasives, Worthington, OH). The polishing “seeds” the surface with diamond particles which serve as nucleation centers during film growth. Polishing in the presence (31) Russel, D. H.; Durie, B. G. Polyamines as Biochemical Markers of Normal and Malignant Growth; Raven Press: New York, 1978. (32) Russell, D. H.; Levy, C. C.; Schimpff, S. C.; Hawk, I. H. Cancer Res. 1971, 31, 1555. (33) Minocha, S. C.; Minocha, R.; Robie, C. A. J. Chromatogr. 1990, 511, 177. (34) Seiler, N. Methods Enzymol. 1983, 94, 10. (35) Matsumoto, T.; Tsuda, T. Trends Anal. Chem. 1990, 9, 292. (36) Roth, M.; Hampal, A. J. Chromatogr. 1973, 83, 353. (37) Kamei, S.; Ohkubo, A.; Saito, S.; Takagi, S. Anal. Chem. 1989, 61, 1921. (38) Hiramatsu, K.; Kamei, S.; Sugimoto, M.; Kinoshita, K.; Iwasaki, K.; Kawakita, M. J. Biochem. 1994, 115, 584. (39) Hoekstra, J. C.; Johnson, D. C. Anal. Chem. 1998, 70, 83.

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of B2O3 appears to embed boron-containing particles in the surface and leads to a higher doping level than is normally achieved in the absence of the ingredient. The polished substrates were sonicated in acetone for 60 s and placed in the CVD reactor on top of a boron diffusion source (BoronPlus, GS 126, Techneglas Inc.). A piece of boron nitride (Goodfellow Metals) was also placed adjacent to the substrates. These solids, in addition to the embedded B2O3 particles, serve as sources for the incorporated boron dopant atoms during deposition. A nominal particle coverage of ∼1 × 108 cm-2 after sonication was determined by atomic force microscopy (AFM). Boron outgasses from the diffusion source and is incorporated into the depositing diamond at the growth temperature. Boron is apparently gasified from the boron nitride as B2H6 and also incorporated into the film. The reactor was evacuated overnight to a base pressure of ∼20 mTorr before initiating the deposition. The films were deposited from a methane/hydrogen (C/H) gas mixture with volumetric ratios of 0.33, 0.67 and 1.0%, a total gas flow of ∼200 sccm, a forward power of 1000 W (reflected power 17 MΩ, E-pure Barnstead). The glassware was cleaned either by gently refluxing in concentrated HNO3 or by washing in an alconox solution, followed by rinsing in a KOH/methanol bath. Both cleaning procedures were followed by copious rinsing with ultrapure water. RESULTS AND DISCUSSION Cyclic Voltammetry of Polyamines. Figure 1 shows typical cyclic voltammetric i-E curves for 1.0 mM cadaverine (CAD) (Figure 1A) and 1.1 mM putrescine (PUT) (Figure 1B), both in the CBpH10 electrolyte, at a film prepared with a 1.0% C/H ratio (D62797(2)). Very similarly shaped curves were observed for ethylenediamine (EDA), spermine (SPM), and spermidine (SPMD). The oxidation peak currents for both are clearly discernible from the background current (actual background current is shown only for PUT). There is a rise in the faradaic current on the reverse scan such that the forward and reverse scans are similar is shape. The Epox is 0.98 V, and the E1/2 is 0.88 V for both analytes. The ipox values are ∼41 and 50 µA, respectively. Repetitive potential

Figure 1. Cyclic voltammetric i-E curves (background and total current) for (A) 1.0 mM cadaverine (CAD) and (B) 1.1 mM putrescine (PUT) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, at D62797(2) diamond electrode. Scan rate, 100 mV/s. Table 1. Cyclic Voltammetric Data for Aliphatic Polyamine Oxidation Using a Diamond Film Electrode Deposited with a 1.0 % C/H Ratio polyamine

Epox (V)

E1/2 (V)

wave shape

ethylenediamine (EDA) putrescine (PUT) cadaverine (CAD) spermine (SPM) spermidine (SPMD)

+1.02 +0.99 +0.97 +1.08 +1.04

+0.88 +0.88 +0.88 +0.89 +0.91

peak peak peak pseudolimiting pseudolimiting

cycling in the SPM or SPMD solutions on this particular film showed a progressive attenuation of the current. Similar attenuation was generally not observed for EDA, CAD, or PUT. The voltammetric response could be regenerated by vigorous mixing and/or time delay intervals of up to 5 min between the cycles. This behavior suggests that the oxidation products for SPM and SPMD adsorb and block the electrode surface. Since vigorous stirring or time delays regenerated the initial voltammetric response, the electrode fouling is not permanent and can be reversed. A summary of the cyclic voltammetric Epox and E1/2 values for the five polyamines in CBpH10 is presented in Table 1. EDA, PUT, and CAD produced oxidation peaks, and the data for Epox correspond to the peak potentials for these analytes. SPM and SPMD produced more sigmoidally shaped waves with less welldefined peaks, and the Epox values correspond to the potential where the current reached a near-steady-state value. Whether or not a peak shape was observed appears to significantly depend on the physicochemical properties of the film surface. For all the polyamines, the E1/2 values represent potentials where the oxidation current is one-half the peak or steady-state current. It is clear that the oxidation potentials for the five polyamines were essentially identical. Finally, cyclic voltammograms of the polyamines in the CBpH10 were obtained at glassy carbon. No quantifiable oxidation signal was detected for any of the amines. The only observable change in the voltammetric response upon addition of the polyamine was a slight increase in the total current and a negative shift in the onset of oxygen evolution. Model for Polyamine Oxidation at Polycrystalline Diamond. A key finding is the fact that polyamines can be oxidized Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

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directly on diamond.5 The results from numerous measurements indicated that the response is extremely dependent on the physicochemical properties of the polycrystalline diamond surface. A mechanism for polyamine oxidation at diamond is offered below that is analogous to the model for anodic oxygen-transfer reactions put forth by Johnson and co-workers.18-20

SND[ ] + H2O f SND[OH] + H+ + e{OH radical generation at a nondiamond surface site} SB[ ] + R a SB[R] {coordination of the amine at a B surface site} SND[OH] + SB[R] f SND[ ] + SB[ ] + RO + H+ + e{anodic oxygen-transfer reaction} The subscripts ND and B refer to nondiamond and boron, respectively. Boron-doped diamond thin-film electrodes possess the requisite surface structure and chemical properties to support and sustain polyamine oxidation. First, high-quality diamond films are stable and resistant to corrosion in strongly acidic and alkaline media (see refs 1, 2 and references therein). Therefore, at the anodic potentials used to detect the polyamines, the electrode structure is stable. Second, films may contain nondiamond sp2 carbon impurities distributed very locally over the surface, and these impurities can exist at the grain boundaries and as extended defects within the diamond lattice. These surface impurities, which have have lower overpotential for oxygen evolution than does diamond, can be intentionally introduced into the films by adjustment of the deposition conditions.4 This means that reactive OH radical will be generated locally at these sites at low overpotential and not to any appreciable extent on the diamond lattice. Third, boron dopant atoms located at the surface can serve as adsorption/coordination sites for the lone pair of electrons on the N atoms of the polyamines. Boron atoms can, of course, insert directly into the growing diamond lattice, but they can also cluster or accumulate in the grain boundaries. The distribution of boron atoms in the diamond lattice is not homogeneous as preferential incorporation occurs along the (111) planes (see refs 40, 41 and references therein). The polyamine adsorption/coordination at the boron sites near the grain boundaries is important mechanistically as these are sites very near where OH radical is being generated at lower overpotential. Several cyclic voltammetric measurements were performed in order to test aspects of the mechanism, and these results are presented below. Effect of Scan Rate on the Voltammetric Response. The effect of scan rate variation on the cyclic voltammetric response of CAD in CBpH10 at a 1.0% C/H film (D62797(2)) is shown in Figure 2. The scan rate was varied from 50 (Figure 2A) to 500 mV/s (Figure 2D). Despite the 10-fold increase in scan rate, the oxidation peak current remained constant. Similar voltammetric behavior was observed for EDA and PUT. No scan-rate-dependent studies, however, were performed for SPM and SPMD. It can be seen that the oxidation current is nearly independent of changes in scan rate, particularly above 100 mV/s. However, examination of scan rates from 10 to 100 mV/s, using several different films, revealed that the oxidation peak current varied linearly with (scan rate)1/2. The unchanging peak current at higher scan rates is likely attributable, at least in part, to slow desorption kinetics of the 1192 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

Figure 2. Cyclic voltammetric i-E curves for 1.0 mM cadaverine (CAD) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, at D62797(2) diamond electrode. Scan rates are (A) 50, (B) 100, (C) 200, and (D) 500 mV/s.

reaction product molecule from the surface boron sites. At the slower scan rates, there is sufficient time for the desorption of the reaction product to occur, but as the scan rate is increased, the current becomes limited by the availability of open boron coordination sites. Once the rate of the desorption process is exceeded, the current reaches a maximum value and is constant in magnitude with further increases in scan rate. The rise of the current on the reverse scan is consistent with some of the sites becoming available because of product desorption. Effect of Amine Concentration on the Voltammetric Response. The oxidation peak current was investigated at a 1.0% C/H film (D62697(2)) using four EDA concentrations from 0.10 to 3.7 mM. Linear regression analysis of current (µA) versus concentration (mM) profiles showed reasonable linearity from 0.1 to 1.0 mM (slope ) 35 µA/mM, y-intercept ) 7.2 µA, and r2 ) 0.9785). A negative deviation from the expected linear trend was observed for the 3.7 mM concentration. Similar currentconcentration profiles were observed for CAD and PUT, but with higher correlation coefficients. Therefore, the analytical utility of the voltammetric measurements would appear to be limited to concentrations below 1 mM. More extensive concentration dependence studies are needed for all of the amines, but one possibility for the negative deviation from linearity at concentrations above 1 mM could be that the surface boron sites become saturated. High polyamine solution concentrations would lead to saturation of the surface boron sites, limiting the observed oxidation current. Importance of Nondiamond Carbon Impurities. Figure 3 shows cyclic voltammetric i-E curves for 1.0 mM CAD in CBpH10 at a 0.67% C/H film (D41798(1)). Figure 3A shows the curve for the “as-deposited” film. The oxidation response for CAD is well-resolved from the background, with an Epox of +990 mV, an E1/2 of +920 mV, and an ipox of 32 µA. If the film is acid etched (see Experimental Section) to remove the nondiamond carbon impurities and then hydrogen plasma treated to rehydrogenate the surface, then the voltammetric response in Figure 3B is observed. Clearly, the removal of the nondiamond carbon impurities causes an attenuation in the response such that there is almost no discernible oxidation current above the background. This result

Figure 3. Cyclic voltammetric i-E curves for 1.0 mM cadaverine (CAD) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, at D41798(1) diamond electrode (A) as deposited and (B) after acid washing and rehydrogenation. Scan rate, 10 mV/s.

provides compelling evidence for the necessity of nondiamond carbon impurities in the amine oxidation reaction mechanism on diamond. Effect of C/H Ratio on the Voltammetric Response. The above result demonstrates the importance of nondiamond carbon surface impurities. Nondiamond phases can be introduced by adjusting the C/H ratio used to deposit the films. Therefore, it should be possible to optimize the response by manipulation of the C/H ratio.4 Cyclic voltammetric i-E curves for 1.0 mM CAD in CBpH10 at diamond thin films deposited with different methaneto-hydrogen (C/H) ratios (0.33% (D41498(4)), 0.67% (D41798(1), and 1.0% (D42398(1))) are shown in Figure 4A. It is important to note that these films received no postdeposition atomic hydrogen annealing. In general, as the C/H ratio is increased, the film quality decreases, with the highest quality films being deposited with a 0.33% C/H ratio. Nondiamond phases and morphological defects tend to increase with increasing C/H ratio. Figure 4B shows the corresponding macroscale Raman spectrum for each film. The cyclic voltammetric results indicate that the oxidation response can be manipulated by making changes in the deposition conditions, with the 0.67% film providing the optimum performance in terms of the waveform shape and current magnitude. This film shows a well-defined oxidation peak with an Epox of 1050 mV, an E1/2 of 910 mV, and an ipox of 56 µA. The 0.33% film is next best in performance, with more of a limiting current response. The voltammetric parameters are an E1/2 of 880 mV and an ipox of 42 µA. The 1.0% film exhibits no quantifiable response for CAD, but the overpotential for oxygen evolution is less than that for the other two films. Clearly, this film does not have the requisite

Figure 4. (A) Cyclic voltammetric i-E curves for 1.0 mM cadaverine (CAD) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, at diamond electrodes deposited with different methane-to-hydrogen (C/H) ratios. Scan rate, 10 mV/s. (B) Macroscale Raman spectra for the films: 514.4-nm source radiation, 10-s integration time.

surface structure and chemical composition needed for the polyamine oxidation reaction (i.e., too much nondiamond carbon). The response of this 1.0% film is very different from that shown in Figure 1. Even though the films were deposited with the same C/H ratio, some of the other deposition parameters were different (i.e., pressure, temperature, total flow rate, growth time, and postdeposition annealing), resulting in different film properties. The macroscale Raman spectra shown in Figure 4B reveal decreasing film quality with increasing C/H ratio, as expected. The laser spot size was ∼10 µm, meaning that several grains and grain boundaries were probed in a measurement. An intense firstorder diamond phonon line is observed at 1332 cm-1 for the 0.33% C/H film. The full width at half-maximum (fwhm) of this band is 8 cm-1. The fwhm is a measure of the film quality, as the value is inversely related to the phonon lifetime. This fwhm compares reasonably well with the 3 cm-1 observed for a single-crystal diamond standard. There is minimal scattering intensity observed between 1500 and 1600 cm-1; such intensity normally is observed when amorphous nondiamond carbon impurities are present. The cross-sectional scattering (514.4-nm excitation) coefficients for diamond and graphite (i.e., nondiamond carbon) are 9 × 10-7 and 500 × 10-7 cm-1/sr, respectively.42-44 Therefore, the technique is quite sensitive to the presence of nondiamond phases. There is a very low photoluminescent background in the spectrum. As (42) Knight, D. S.; White, W. B. J. Mater. Res. 1989, 4, 385. (43) Robins, L. H.; Farabaugh, E. N.; Feldman, A. J. Mater. Res. 1990, 5, 2456. (44) Dennison, J. R.; Holtz, M.; Swain, G. M. Spectroscopy 1996, 11, 38.

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Figure 5. Cyclic voltammetric i-E curves for 1.0 mM cadaverine (CAD) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, at D41798(1) diamond electrode as a function of the cycle number. Scan rate, 10 mV/s.

Figure 7. FIA-EC results for the D62797(3) diamond electrode using 20-µL injections of 9.4 × 10-6 M ethylenediamine (EDA) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, with 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, as the carrier solution. The applied potential was +0.78 V, and the flow rate was 1.0 mL/min.

Figure 6. Hydrodynamic voltammetric i-E curves for 20-µL injections of 1.0 mM ethylenediamine (EDA) in 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, with 0.1 M NaClO4 + 0.01 M carbonate buffer, pH 10, as the carrier solution. The working electrode was D62797(3) diamond, and the flow rate was 1.0 mL/min. (A) Plot of the response (9) and the residual background current (b) versus the applied potential. (B) Plot of the signal/background ratio (from A) versus the applied potential.

the C/H ratio is increased, the diamond band decreases in intensity, and the fwhm of this band, the scattering intensity in the spectral region from 1500 to 1600 cm-1, and the photoluminescent background all increase, consistent with an increased level of defects and nondiamond carbon phases. The band at 520 cm-1 is due to scattering by the underlying Si substrate. The extent of any electrode fouling was examined by repetitive potential cycling through the oxidation peak. Figure 5 shows cyclic voltammetric i-E curves for 1.0 mM CAD in CBpH10 at the 0.67% C/H film (D41798(1)). There is a decrease in the peak current 1194 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

for each of the first three cycles. This decrease is due, in part, to the formation of a depletion layer at the interface, as the oxidation reaction is irreversible, but perhaps also to slow desorption of the product from the electrode surface. If a time delay of 3 min is allowed at an applied potential of 0 V, then the peak current for the subsequent scan (cycle 4) is nearly the same as the initial scan. This suggests that the electrode is not irreversibly fouled by reaction products, as is the case for other metallic electrodes, necessitating the need for pulsed waveforms. FIA-EC Detection of Polyamines. Figure 6A shows a hydrodynamic voltammetric i-E curve for 20-µL injections of 1.0 mM EDA + CBpH10, using CBpH10 as the carrier solution. The electrode was the same 1.0% C/H film used for the data presented in Figures 1 and 2. Each datum represents the average of at least three injections. The absolute magnitude of the background current at each potential is also shown for comparison. Although the cyclic voltammetric results indicate that polyamine oxidation was well-resolved from the background current, the hydrodynamic voltammetry did not produce the expected sigmoidal plot of the signal versus potential. This may be because of the high flow rate used. At 750 mV, the signal current increased to detectable levels when compared to the background signal. Between applied potentials of 750 and 825 mV, the signal current was larger than the background current. However, at potentials beyond 825 mV, the background current became larger than the abolute magnitude of the analytical signal. Detection potentials of 750-780 mV produced the optimal S/B ratio, as is shown in Figure 6B. All five polyamines gave similarly shaped hydrodynamic voltammograms and S/B profiles. Figure 7 represents a series of repetitive 20-µL injections of 10 µM EDA in CBpH10 using a detection potential of 780 mV. Well-defined but variable signals were recorded for this concentration. An injected concentration of 1 µM EDA yielded an analytically

Table 2. FIA-EC Data for Aliphatic Polyamine Detection Using a Diamond Film Electrode Deposited with a 0.67% C/H Ratio (D41798(1))a polyamine

linear dynamic range

sensitivity (µA/mM)

putrescine (PUT) cadaverine (CAD) spermine (SPM) spermidine (SPMD)

1 µM-1 mM (r2 ) 0.999) 1 µM-1 mM (r2 ) 0.999) 1 µM-1 mM (r2 ) 0.970) 1 µM-1 mM (r2 ) 0.988)

0.247 2.58 0.908 1.34

limit of quantitation (µM)

response variability (RSD, %)

1 (S/N ) 7.4) 1 (S/N ) 6.6) 1 (S/N ) 6.9) 1 (S/N ) 10)

20.8 (131 injections) 9.7 (50 injections) 15.9 (39 injections) na

a The calibration curves are based on 4-7 concentrations between the 1 µM and 1 mM levels. Injection volume, 20 µL; flow rate, 1 mL/min; detection potential, +825 mV (CAD), +850 mV (PUT), +800 mV (SPM), and +850 mV (SPMD).

useful signal (S/N g 3) but with a high relative standard deviation (RSD) of 27% for repeat injections. This corresponds to a mass detection limit of 1.2 ng for EDA. The peaks exhibit some tailing which is consistent with the slow desorption of the reaction product. It is unclear at this point how much of the response variability is related to the complex electrode reaction mechanism and how much is attributable to the instrumentation. Our view at present is that the excessive variability is due in large part to the fact that a sufficient amount of time was not allowed between injections (1 vs 3 min). This time is necessary to allow for the complete desorption of the reaction product (see Figure 5). Table 2 shows detector figures of merit for CAD, PUT, SPM, and SPMD at the 0.67% C/H film used to obtain the data presented in Figures 4 and 5. The linear dynamic range for all four analytes is 3 orders of magnitude, from 1 µM to 1 mM (r2 > 0.97), and the limit of quantitation (determined experimentally) is 1 µM, with a S/N from 6 to 10. The biggest differences for the four analytes are the sensitivity and response reproducibility. The sensitivity is lowest for CAD and highest for PUT, with the values varying by an order of magnitude. The response variability for PUT is lowest at 9.7% for 50 injections, and that for CAD is highest at 20.8% for 131 injections. These results are for only one diamond film, so it is unclear at this point if the sensitivity differences shown are statistically significant. Also, well-controlled delay times between injections were not used to obtain the present data. Further work is necessary to improve the detection figures of merit, but these preliminary results are certainly encouraging. CONCLUSIONS We have shown that aliphatic polyamines can be oxidized and quantitatively detected using boron-doped, polycrystalline diamond thin films. Polycrystalline diamond possesses the requisite surface structure and chemical composition to support and sustain the oxidation reaction. Because of the synthetic nature of the film

deposition, the appropriate structure and composition can be fabricated into a film. A model is proposed whereby the nondiamond carbon impurity sites, presumably located in the grain boundaries, generate reactive OH radicals at lower overpotential than the surrounding diamond. The radicals then attack the polyamine molecules adsorbed/coordinated at surface boron sites near the grain boundaries. Evidence was presented which clearly demonstrates the importance of nondiamond carbon phases in the reaction mechanism. While no direct proof for surface boron involvement is available at present, recent Raman microprobe imaging analysis has clearly revealed localized domains of highly concentrated boron dopant atoms that are of the dimensions of a micrometer or two. FIA results indicated that the aliphatic polyamines can be detected at constant potential without any prior derivatization or the use of pulsed waveforms. A linear dynamic range from 1 mM to 1 µM with a limit of quantitation of 1 µM (S/N ≈ 3-6) was observed for all five polyamines. The response variability with multiple injections was unacceptably high in these preliminary data ranging from 10 to 30%. Future efforts will focus on improving the precision. The results presented herein represent another electroanalytical application where diamond outperforms the more commonly used glassy carbon. Lower quality rather than high-quality diamond films are needed for this oxidation reaction mechanism. ACKNOWLEDGMENT The research was generously supported by the donors to the Petroleum Research Fund, administered by the American Chemical Society, and the Dow Chemical Co. The helpful insights provided by Professor Dennis C. Johnson are greatly appreciated. Received for review June 30, 1998. Accepted January 6, 1999. AC980697V

Analytical Chemistry, Vol. 71, No. 6, March 15, 1999

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