Electrochemical detection of cocaine metabolites (benzoylecgonine and ecgonine) at the miniaturized electrified liquid-liquid interface

Abstract

The main urinary cocaine metabolites are benzoylecgonine and ecgonine methyl ester. These molecules possess two chemical functional groups that can be charged within the conventional pH range, this is the tertiary amine group (both) and the carboxylic group (benzoylecgonine). We have found that in the pH range from 2 to 6 (for benzoylecgonine) and pH from 2 to 4 (for ecgonine methyl ester) both molecules can be detected at the electrified liquid-liquid interface (eLLI) by following the interfacial ion transfer reactions triggered in the presence of the concerned metabolites in the aqueous phase. The presence of the benzene ring in the benzoylecgonine structure increases its hydrophobicity (on the formal Galvani potential difference scale) by around 200mV as compared with ecgonine methyl ester which transfer is overlaid with the inorganic cations crossing the LLI at the positive end of the potential window. Macroscopic eLLI was used during physicochemical ecgonine methyl ester and benzoylecgonine characterization as we have defined their diffusion coefficients, ion partition, and concentration fraction diagrams (only for the latter). The miniaturized platform was used during electroanalytical studies focused on benzoylecgonine giving a few µM limit of detection. For this purpose, we have used the fused silica capillary as the eLLI support. All our findings allowed us to use this system to first develop the electroanalytical protocol for benzoylecgonine quantification to finally detect it in the spiked urine sample with nearly 100% recovery.