Adsorptive stripping voltammetric determination of podophyllotoxin, an antitumour herbal drug, at multi-walled carbon nanotube paste electrode

Adsorption stripping voltammetry, a very sensitive electroanalytical method, was employed to determine podophyllotoxin, a kind of antitumour herbal drug at a multi-wall carbon nanotube (MWCNT)-modified carbon paste electrode (CPE) surface. In the following anodic sweep from 0.5 to 1.5 V, podophyllotoxin, adsorbed at the MWCNT-modified CPE surface, was oxidized and yielded a sensitive oxidation peak with E1/2/Ep approximately 1.16 V/1.18 V over the scan rates of 10–120 mV s−1. From CV and SWV studies of podophyllotoxin in the acetate buffers of various pH values, it was found that protons were involved in the oxidation of the drug at the H+/e− ratio of one (∆Ep/pH = 56 mV at 25 °C). Its electrochemical behaviour was irreversible. The experimental conditions, such as supporting electrolyte, pH value, accumulation time, ionic strength and scan rate, were optimized for the measurement of podophyllotoxin. The best results were obtained in 0.02 M acetate/acetic acid buffer (pH 4.6) containing 0.04 M KCl (1:49, v/v) for 60 s accumulation. The oxidation peak current varies linearly with the concentration of podophyllotoxin over the range of 199–1796 pg mL−1. The limits of detection and quantification of the pure drug are 4.5 and 14.96 pg mL−1, with the correlation coefficient, r = 0.998 and the relative standard deviation, RSD = 1.3% (n = 5). This new method was successfully applied to the determination of podophyllotoxin in a plant sample of the rhizome of Podophyllum hexandrum. Recoveries were 99.173–101.231%. The relative standard deviations of intraday and interday analyses for podophyllotoxin were 0.55 and 0.61%, respectively (n = 3).