Boron-doped diamond electrodes: The role of surface termination in the oxidation of dopamine and ascorbic acid

https://doi.org/10.1016/j.diamond.2007.02.002Get rights and content

Abstract

Due to the lack of strong adsorption of reactants, intermediates or products on the boron-doped diamond electrode surface, the electrochemical behavior can be simulated based on a model involving only electron transfer, chemical reactions and solution mass transport (DigiSim). The electrochemical behavior of several different biogenic redox-active species, dopamine, ascorbic acid, uric acid and 3,4-dihydroxyphenylacetic acid, was compared experimentally at as-deposited (hydrogen-terminated) and anodically oxidized (oxygen-terminated) boron-doped diamond surfaces, and molecular orbital theory was used to help explain the results. Using semi-empirical calculations, we find that, for dopamine, which is protonated in acid solution, the interaction of the positively charged quaternary ammonium group with the surface is relatively strong and nearly equal for both hydrogen- and oxygen-terminated surfaces, which explains the lack of sensitivity of the cyclic voltammetry to the surface termination. For ascorbic acid, the interaction of the neutral compound with the hydrogen-terminated surface is weaker, while that with the oxygen-terminated surface is very weak, consistent with the highly inhibited electron transfer.

Introduction

Boron-doped diamond (BDD) is a near-ideal electrode material for analytical chemistry, because it interferes so little with the electrochemistry of the species being measured [1], [2], [3], [4], [5], [6], [7], [8], [9]. The typically used microcrystalline BDD films that are prepared via chemical vapor deposition or hot filament deposition are hydrogen-terminated. This surface provides relatively high electron transfer (ET) rates to many redox couples involving a single electron transfer. Either purposefully or inadvertently, the surface can become oxidized during electrochemical usage, particularly if used extensively for oxidation processes at potentials of 1 V or higher [10], [11], [12], [13]. The anodic oxidation process tends to produce a surface on which oxygen coverage approaches a full monolayer. The electrochemically generated surface groups are thought to be similar to those that are formed via gentle oxidation (i.e., without the use of high-energy oxygen atoms, which can damage the surface). The groups found to exist on (100) facets are ethers (C–O–C) or carbonyls (C=O) [14]. On the (111) facets, hydroxyls (–OH) can exist [15]. The electrochemical behavior of the anodically oxidized (AO) surface is significantly different from that of the as-deposited (AD) surface. For example, for transition metal complexes, anionic complexes appear to be repelled from the oxidized surface, and thus they are generally more difficult to oxidize or reduce than they are on the hydrogen-terminated surface AD [16]. In contrast, cationic complexes appear to be attracted to the surface, and in some cases are easier to oxidize or reduce than on the H-terminated surface. We have proposed that these effects are due to interactions of the solution-phase ionic species with either the carbon–hydrogen surface groups, which have a dipole with the positive end pointing away from the electrode surface, or with the carbon–oxygen groups, which have oppositely oriented dipoles [17]. Thus, the interactions can be considered to be of the ion–dipole type.

For organic reactants in aqueous solution, interesting effects have been observed, for example, for the oxidation of dopamine and other catecholamines in the presence of ascorbic acid at BDD electrodes [18], [19]. The detection of dopamine in the presence of ascorbic acid remains an important problem in the treatment of Parkinson's disease. Ascorbic acid (AA) is neutral in acid, while dopamine (DA) is protonated and thus possesses a single positive charge (Fig. 1). In acid solution, Popa et al. found that the AD BDD surface was not able to electrochemically resolve a mixture of DA and AA, while the AO surface was able to do so [18]. The precise origin of these effects has never been completely clarified. We have speculated that ion–dipole and dipole–dipole interactions might be involved. In the present work, we have examined this hypothesis in detail using molecular orbital (MO) calculations.

Specifically, we have used density functional theory (DFT B3LYP, Gaussian 03) to calculate the dipole moments of AA, DA, UA, as well as 3,4-dihydroxyphenylacetic acid (DOPAC) (see Fig. 1 for structures). We have found that a simple hypothesis, in which a strong dipole causes a molecule to be oriented either favorably or unfavorably for ET, is inadequate. This initial negative finding has prompted us to examine the ET process in greater detail. We have initiated such a study, using a semi-empirical method, to calculate the interactions of AA and the protonated form of DA (DAH+) with both the H-terminated and O-terminated surfaces. We find that it is possible to explain the interactions of these molecules with both types of BDD surfaces, specifically the ease of oxidation, in terms of the optimized orientations and distances from the BDD surface, together with the stabilization energies.

Section snippets

Experimental

Boron-doped polycrystalline diamond films were deposited with a 5-W microwave plasma-assisted chemical vapor deposition system (Seki Technotron Corp., Tokyo, Japan (www.sekitech.co.jp), formerly ASTeX), as previously described [20]. The boron source used was B2O3 dissolved into the methanol–acetone feed. The B/C atomic ratio was 1:100, and the typical boron concentration was 1–2 × 1021 cm 3, as determined by nuclear reaction analysis [21].

The electrochemical measurements were made in a

Results

A most striking effect of the electrochemical oxidation (AO) oxidation treatment was observed for ascorbic acid (AA) oxidation (Fig. 2). Here, the oxidation peak potential at the H-terminated surface was 0.813 V vs. SCE, whereas it was 1.333 V at the O-terminated surface, yielding a shift of ca. 0.520 V. This is an extreme inhibition in the ET rate for the overall two-electron process. For DOPAC, the shift was smaller (ca. 0.25 V). For UA, it was still smaller (0.15 V), and for DA, the shifts

Discussion

It is expected that the present approach may be a fruitful one in trying to understand the interactions between the diamond electrode surface and solution-phase organic molecules. The results are probably better than expected from such a simple model, in which the effect of the boron dopant is neglected. This may indicate that an important factor in controlling the ET for the diamond electrode is the highly localized surface interaction. This kind of conclusion would hardly be surprising for a

Conclusions

The present results represent a preliminary first step in trying to understand the local interactions of solution-phase reactants with hydrogen- and oxygen-terminated BDD surfaces that could be important in controlling the electrochemical behavior. From these results, we conclude that interaction between the ammonium group of protonated DA is relatively strong with both hydrogen- and oxygen-terminated BDD, so that the equilibrium distances are quite short, and the ET rates are both moderate and

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