Probing cysteine self-assembled monolayers over gold nanoparticles--towards selective electrochemical sensors.

Cysteine forms self-assembled monolayers over gold nanoparticles. Based on this knowledge, a novel electrochemical sensor (Au-Au(nano)-Cys-SDS) has been constructed by the formation of self-assembly monolayer (SAM) of cysteine on gold-nanoparticles modified gold electrode (Au-Au(nano)-Cys) to be utilized for determination of dopamine in the presence of sodium dodecyl sulfate (SDS). Electrochemical investigation and characterization of the modified electrode sensor was achieved using cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron, and atomic force microscopies. Au-Au(nano)-Cys electrode in the presence of SDS gave comparable high current response to that of the gold nanoparticles modified gold electrode (Au-Au(nano)). The Au-Au(nano)-Cys-SDS electrode current signal was remarkably stable via repeated cycles and long term stability due to the strong AuS bond. Very small peak separation, almost zero or 15 mV peak separation was also obtained by repeated cycles indicating unusual high reversibility. The oxidation peak current was determined to be linearly dependent on the dopamine concentration. A resulting calibration curve using square wave voltammetry (SWV) was obtained over concentration range of 30-100 μmol L(-1) and 120-320 μmol L(-1) with correlation coefficients of 0.996 and 0.994 and a limit of detection of 16 and 57 nmol L(-1), respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of tertiary mixture of ascorbic acid AA, dopamine, and acetaminophen APAP was explored at this modified electrode. It has been demonstrated that Au-Au(nano)-Cys-SDS electrode can be used as a sensor with excellent reproducibility, sensitivity, and long term stability.