Photoelectrochemical Biosensor for MicroRNA-21 Based on High Photocurrent of TiO₂/Two-Dimensional Coordination Polymer CuClₓ(MBA)y Photoelectrode

Conventional photosensitive materials such as TiO₂ suffer from restricted absorption in the ultraviolet region, fast recombination of photogenerated electron–hole pairs, and a lack of functional groups for biocoupling, which hinder their application in photoelectrochemical (PEC) biosensing. Herein, a new coordination polymer (CP) based on Cu(I), chloridion, and 4-mercaptobenzoic acid (MBA) has been designed and synthesized (called CuClₓ(MBA)y). The prepared p-type CuClₓ(MBA)y exhibits visible-light absorption due to its narrow optical band gap (2.59 eV), and its proper band edge position enables it to form a p–n junction with TiO₂. Through layer-by-layer assembling, the photocurrent intensity of the CuClₓ(MBA)y/TiO₂/FTO composite photoelectrode was 3.7-fold higher than that of a TiO₂/FTO electrode and 35-fold higher than a CuClₓ(MBA)y/FTO electrode. The potential enhancement mechanism was discussed, which lies in the contributions of CuClₓ(MBA)y in enhancing absorption in the visible-light region and boosting the separation of electron–hole pairs of TiO₂ by the p–n junction. Furthermore, CuClₓ(MBA)y nanosheets can realize bioconjugation directly, thanks to its abundant carboxyl groups. The CuClₓ(MBA)y/TiO₂/FTO composite photoelectrodes were applied to develop a sensitive PEC biosensor for microRNA-21 (model target). By subtly exploiting the energy transfer between CuClₓ(MBA)y and Au nanoparticles (AuNPs), AuNPs served as effective quenchers. In the presence of the target, AuNP-labeled sDNA1 connected to the electrode surface, and thus, a decreased photocurrent was obtained. The proposed biosensor has a low detection limit of 0.29 fM (S/N = 3), good selectivity, and reproducibility. The proposed system was applied to monitor microRNA in cancer cells with satisfying results.