J. Phys. Chem. B 2010, 114, 9713–9719
9713
Electrooxidative Decarboxylation of Vanillylmandelic Acid: Voltammetric Differentiation between the Structurally Related Compounds Homovanillic Acid and Vanillylmandelic Acid Qian Li, Christopher Batchelor-McAuley, and Richard G. Compton* Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: June 19, 2010
Vanillylmandelic acid (VMA) and homovanillic acid (HVA) are the major end products of catecholamine metabolism. Abnoramally high levels in both plasma and urine may be indicative of a number of diseases including neuroblastoma and phaeochromocytoma. Commonly the VMA:HVA ratio is used as a disease marker, so that any measurement techniques need to be able to differentiate between these two structurally similar compounds. Electrochemistry is often limited in selectivity due to many organic molecules being oxidized or reduced at similar potentials. This work investigates the electrochemical oxidation mechanism of VMA at an edge-plane pyrolytic graphite electrode and highlights how, although structurally similar to HVA, their voltammetric responses may be differentiated through appropriate selection of the electrode material. The oxidation of VMA exhibits two clear peaks and the mechanism is shown to proceed through the decarboxylation of VMA to form vanillin, which is further oxidized resulting in the second peak. Modification of the electrode with a porous layer of multiwalled carbon nanotubes so as to change the mass transport to that of a thin layer system causes the voltammetric resolution between the two species to be enhanced. Differential pulse voltammetry is used to measure the limits of detection for VMA on an edge-plane pyrolytic graphite electrode and on commercially available multiwalled carbon nanotube screen printed electrode, with limits of detection of 1.7 and 1.0 µM, respectively. These limits of detection are well within the range of sensitivity required for clinical sample measurement. Introduction Although a number of biologically relevant molecules are found to be electroactive and upon first assessment the use of electroanalytical methods would appear to be highly appropriate for their detection and quantification, this is frequently found to not be the case due to inherent limitations arising from a lack of selectivity. This is in part due to species often being oxidized and reduced at similar potentials, as such serious errors may occur due to overlapping of electrochemical signals.1 A common method to overcome this is to modify the working electrode, with either an enzyme or chemical species.2,3 The former attempts to infer selectivity through harnessing the innate selectivity of the enzyme, whereas the latter often has the stated aim of changing the electrode kinetics of either the analyte and/ or interferent and thereby improving resolution. The modification of an electrode surface with a porous layer can cause the observed voltammetry to be altered with a lowering of the peak potential; in a number of cases this lowering in peak potential may be ascribed as being due to a change in the mass transport, from semi-infinite diffusion to a thin-layer diffusional regime. This altering of the mass transport regime might in principle allow the resolution of voltammetric peaks without altering the electrode kinetics.4 For the modern electrochemist there is a large number of possible carbon substrates available for use as electrode materials. Moreover in recent years the availability of more exotic forms, such as carbon nanotubes and fullerenes, has increased. Single-walled and multiwalled carbon nanotubes have been the * To whom correspondence should be addressed. Fax: +44 (0) 1865 275410. Phone: +44 (0) 1865 275413. E-mail: richard.compton@ chem.ox.ac.uk.
Figure 1. The structures of the related compounds, VMA, HVA, and vanillin.
focus of a large amount of attention primarily due to their high conductivity and large surface area making them highly suitable for a number of analytical systems. Although regularly used, the interpretation of their results and the physical reasoning behind them have often been misinterpreted; work by Banks et al. demonstrated that with multiwalled carbon nanotubes (MWCNTs) the electroactive sites are the edge-plane like defects, subsquently edge-plane pyrolytic graphite (EPPG) electrodes are good model substrates for MWCNTs.5 Even given their similarities there may nevertheless be significant differences between the response of an EPPG electrode and MWCNTs.6 Vanillylmandelic acid (VMA) and homovanillic acid (HVA), are structurally closely related (figure 1) and only differ through the presence of an alcohol group R to the aromatic ring on VMA; both compounds are the major end products of catecholamine metabolism.7 Abnormally high plasma and urinary levels of these metabolities can be indicative of the presence of a neuroblastoma, specifically a low VMA:HVA ratio (
