Electrochemically triggered michael addition on the self-assembly of 4-thiouracil: Generation of surface-confined redox mediator and electrocatalysis

Abstract

Generation of a surface-confined redox mediator (RM) by an electrochemically triggered Michael addition reaction and the electrocatalytic properties of the mediator are described. Electrogenerated o-quinone undergoes Michael addition reaction with the self-assembled monolayer (SAM) of 4-thiouracil (4-TU) on a gold (Au) electrode and yields a surface-confined RM, 1-(3,4-dihydroxyphenyl)-4-mercapto-1H-pyrimidin-2-one (DPTU). The Michael addition reaction depends on the electrolysis potential and time, solution pH, and concentration of catechol (CA) used in the reaction. The redox mediator, DPTU, exhibits reversible redox response, characterstic of a surface-confined species at ∼0.22 V in neutral pH. The anodic peak potential of DPTU shifts by 58 ± 2 mV while changing the solution pH by one unit, suggesting that protons and electrons taking part in the redox reaction are in the ratio of 1:1. The apparent rate constant (k^app_s) for the heterogeneous electron-transfer reaction of the RM was determined to be 114 ± 5 s^-1. The surface coverage (Γ) of DPTU on the electrode surface was 8.2 ± 0.1 × 10^-12 mol/cm². DPTU shows excellent electrocatalytic activity toward oxidation of reduced nicotinamide adenine dinucleotide (NADH) with activation overpotential, which is ∼600 mV lower than that observed at the unmodified Au electrode. The dipositive cations in the supporting electrolyte solution amplify the electrocatalytic activity of DPTU. A 2.5-fold enhancement in the catalytic current was observed in the presence of Ca²⁺ or Ba²⁺ ions. The sensitivity of the electrode toward NADH in the presence and absence of Ca²⁺ ions was 0.094 ± 0.011 and 0.04 ± 0.0071 nA cm^-2 nM^-1, respectively. A linear increase in the catalytic current was obtained up to the concentration of 0.8 mM, and the electrode can detect amperometrically as low as 25 nM of NADH in neutral pH.