Simultaneous detection of three drugs labeled by cationic metal complexes at a nafion-loaded carbon paste electrode

The synthesis of the pentaammine ruthenium(II) complex of N-isonicotinoyl-nortriptyline (NORPy-Ru(2+)) was performed and its electrochemical properties at a nafion-loaded carbon paste electrode were examined. The anodic oxidation of the positively charged labeled antidepressant proceeded at -0.06 V (versus Ag/AgCl, 0.05 M Cl(-)). A detection limit of 0.075 muM (S/N=3) was achieved at physiological pH by square-wave voltammetry after a 5-min preconcentration step, with a linear response over the range 0.075-5.0 muM. With a view to a future triple-analyte immunoassay, the detection of NORPy-Ru(2+) was also examined in the presence of two labeled antiepileptics previously synthesised, i.e. phenytoin labeled by a ferroceneammonium salt (oxidation potential at 0.26 V) and phenobarbital labeled by a cobaltocenium salt (reduction potential at -1.05 V). The simultaneous detection of the three labeled drugs proceeded with analytical performances similar to those corresponding to the separate accumulation of each tracer. However it was observed that the pentaammine ruthenium(II) complexes of pyridine and its derivatives were not stable in the presence of serum, which does not allow for their use as redox cationic labels in a multi-analyte immunoassay to be envisaged.