Study of the effects of surface pKa and electron transfer kinetics of electroactive 4-nitrothiophenol/4-mercaptobenzoic acid binary SAM on the simultaneous determination of epinephrine and uric acid

In the present paper, the use of a gold electrode modified by 4-nitrothiophenol/4-mercaptobenzoic acid binary self-assembled monolayer (4NTP/4MBA SAM) for the simultaneous determination of epinephrine (EP) and uric acid (UA) is described. Electron transfer (ET) kinetics through 4NTP/4MBA SAM was studied using scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). The SECM strategy was used to measure the heterogeneous rate constant for activated 4NTP/4MBA SAM and non-activated 4NTP/4MBA SAM using [ Fe ( CN ) 6 ] 4 - / 3 - as probe. The apparent surface pKap value of the 4NTP/4MBA SAM modified electrode was estimated by EIS (pKap = 5.0). The electrochemical response of the modified electrode toward epinephrine (EP) and uric acid (UA) were investigated by differential pulse voltammetry (DPV). The results showed an efficient electrochemical response and surface activity of the electrode for the electro-oxidation of EP and UA, which leads to improvement of selectivity of the electrode response. Two well-defined voltammetric peaks, in a buffered solution at pH 5.5, were obtained by differential pulse voltammetry (DPV) at 0.120 and 0.475 V vs Ag/AgCl for EP and UA, respectively. Linear calibration plots were obtained in the range of 0.1–2.0 μmol L−1 for EP and 1–175 μmol L−1 for UA, with sensitivity of 1.2 μA L μmol−1 and 0.0117 μA L μmol−1, respectively.