Molecularly Modified Diamond Electrodes

Molecular modification of the electrode surface is an important technique in various fields ranging from analytical chemistry to molecular electronics. For example, in the field of analytical chemistry, highly sensitive immunosensors have been reported where the electrode surfaces were modified with an antibody or a protein. In the field of molecular electronics, excellent rectifiers and organic light-emitting diodes have been achieved by immobilizing self-assembled monolayers on the electrode surface. Functional molecules can be modified via formation of C−C bonds when carbon materials are used as electrodes. A widely applied method is electrografting of an aryl diazonium salt or aryl halide. In this method, an aryl diazonium salt or aryl halide is electrochemically reduced to afford an aryl radical followed by covalent immobilization onto the electrode surface. The electrochemical process completes in a short reaction time and does not require an inert atmosphere. We have previously reported the surface functionalization of diamond electrodes by the electrochemical method: highly sensitive detection of influenza viruses utilizing the molecular layer of the sialic acid-mimic peptide and the controlled molecular decoration simply by tuning the reaction conditions. Herein, we report two applications of molecularly modified diamond electrodes; one is an electrochemical CO2 reduction reaction [1], and the other is an electrochemical sensor for drug detection [2]. Since amines are widely used for efficient separation and capturing of CO2, we immobilized the amine molecular layer on the surface of boron-doped diamond (BDD) to investigate an effect of the modification on products in the electrochemical CO2reduction reaction. An amine-modified BDD (NH2-BDD) electrode was prepared by electrografting of p-iodonitrobenzene using cyclic voltammetry. The intense N(1s) peak at around 400 eV was observed in X-ray photoelectron spectrum for NH2-BDD, confirming the existence of the amino group at the surface. Electrochemical CO2 reduction reactions were performed in a constant potential mode to investigate a potential dependence of the product selectivity. The selectivity of CO production was enhanced up to 8 times by amine modification. In situ attenuated total reflectance infrared spectroscopy showed that peak intensity of the stretching vibration of the carbonyl group at around 1640 cm-1 decreased as the applied potential became more negative, which strongly supports formation of a C-N bond. Therefore, it is assumed that capture of CO2 and anion radical of CO2 captured on the NH2-BDD surface enhances the selectivity of CO production. Detection of drugs in biological solutions is essential in pharmacokinetic studies. We developed an electrochemical sensor based on a BDD electrode modified with a molecularly imprinted polymer (MIP-BDD) to achieve a specific drug sensing. Doxorubicin, an anticancer drug, was chosen as a template molecule. When using an unmodified BDD electrode, the reduction peak of doxorubicin was observed at around -0.5 V (vs. Ag/AgCl), at which other drugs such as mitomycin C and clonazepam undergo a reduction reaction. On the other hand, MIP-BDD only provided a reduction current derived from doxorubicin even in the presence of mitomycin C and clonazepam. MIP-BDD realized the selective electrochemical detection of doxorubicin in human plasma, in which the respective limits of detection of doxorubicin in phosphate buffer saline and human plasma were 32.10 and 16.61 nM, respectively. [1] Tatsuhiko Mikami, Takashi Yamamoto, Mai Tomisaki, Yasuaki Einaga, ACS Sustainable Chem. Eng. 2022, 10(45), 14685–14692. [2] Kanako Ishii, Genki Ogata, Takashi Yamamoto, Shuyi Sun, Hiroshi Shiigi, Yasuaki Einaga, ACS Sens. 2024, 9(3), 1611–1619.