Ultrasensitive determination for flavin coenzyme by using a ZnO nanorod photoelectrode in a four-electrode system

A photoelectrochemical sensor for FAD and FMN was fabricated by incorporating the reduced flavin cofactor as an electron donor into a photoelectrochemical reaction in a ZnO nanorod photoelectrode (ZnO-NR-PhE). The assay is based on the pre-reduction of the flavin coenzyme on a second working electrode. The ZnO-NR-PhE was electrochemically preparated, and synthesis was optimized. The mechanism of the reaction of flavin coenzyme on the ZnO-NR-PhE was studied. Under optimized conditions (a pH value of 6.5; a pre-reduction potential of −0.40 V; illumination at 365 nm; a light energy of 1.6 mW∙cm−2), the photocurrent at a bias voltage of 0.1 V is proportional to the logarithm of the coenzyme concentration in the range from 10 nmol⋅L−1 to 1.0 μmol⋅L−1. The detection sensitivity is 193.5 nA/logC (μmol⋅L−1) for FMN, and 195.6 nA/logC (μmol⋅L−1) for FAD. The respective detection limits are 8.0 and 5.0 nmol⋅L−1 (at an S/N ratio of 3), respectively. Compared to other methods, the one presented here has a wide analytical range, high sensitivity, and convenient operation. Common biochemical substances do not interfere.