Electrochemical oxidation of polyamines at diamond thin-film electrodes

Abstract

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. E( 1/2 )'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 ≥ 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.