Voltammetric detection with a comprehensive electrochemistry study of minoxidil using nuclear magnetic resonance and infrared analyses: Applications in the forensic and pharmaceutical fields.

Minoxidil (MN) is a vasodilator used to treat hair loss and severe hypertension. However, its illegal use in cosmetics and pharmaceutical formulations has been reported in several countries. An efficient method for MN detection is of great interest for forensic and pharmaceutical applications. Electrochemical sensors have been reported as an interesting alternative for MN detection in various samples. However, a more in-depth study of the redox processes and a more selective electrochemical detection for MN are still required. In this context, we present, for the first time, the use of nuclear magnetic resonance and Fourier transform infrared spectroscopy analyses for understanding the electrochemical behaviour of MN after electrolysis procedures on a boron-doped diamond electrode (BDDE). Using these combined techniques, we have proposed and confirmed an electrochemical mechanism for all redox processes of MN on a BDDE, where in phosphate buffer (pH 6.0) two oxidation processes at +0.72 V and +0.97 V vs (Ag/AgCl/ sat. KCl) are presented. The last generated product by MN oxidation is reduced on the BDDE surface at -0.01 V with a quasi-reversible redox process. The use of this redox process is the strategy for a selective and sensitive detection of MN on the BDDE. This innovative approach was successfully applied to determine MN in adulterated cosmetics and pharmaceutical formulations, showing a low limit of detection (5.7 µmol. L^-1) and high stability of electrochemical responses (RSD < 1.5 %, n = 6) using the same BDDE. Therefore, the proposed method provides a simple, fast and selective method for the identification and quantification of MN in pharmaceutical and forensic samples.