Ultrasensitive photoelectrochemical aptasensor for diclofenac sodium based on surface-modified TiO₂-FeVO₄ composite

Herein, a photoelectrochemical (PEC) aptasensing platform was designed by integrating surface oxygen vacancy (OV) defects, Ti³⁺ self-doping, the heterojunction, and resonance energy transfer (RET) effect into one platform for the detection of diclofenac sodium (DCF). Briefly, OV defects were introduced on TiO₂ nanospheres with simultaneous Ti³⁺ self-doping, followed by a well-separated deposition of FeVO₄ nanoparticles on TiO₂ to obtain a Ti³⁺-O-TiO₂/FeVO₄ heterojunction. The surface modification of OVs, Ti³⁺ doping, and deposition of FeVO₄ were confirmed by SEM, XPS, EPR, DRS, and PEC measurements. The surface OVs and doping of Ti³⁺ species created a new donor (defect) energy level under the conduction band of TiO₂, which minimized the bandgap and thereby improved the visible light absorption of TiO₂. Moreover, the capture of photo-excited electrons by surface OVs could hinder the electron-hole recombination. Due to the intimate surface contact and perfect energy matching between TiO₂ and FeVO₄, the formation of heterojunction decreased the bandgap and facilitated the electron-hole separation of TiO₂. All these above events contributed to the enhancement of the PEC signals, which were then quenched by the RET effect between Ti³⁺-O-TiO₂/FeVO₄ and Au nanoparticle (AuNP)-labeled cDNA that had been attached to its complementary DCF aptamer on Ti³⁺-O-TiO₂/FeVO₄|ITO. The addition of target-DCF detached AuNP-labeled cDNA from the electrode to recover the photocurrent, resulting in a "signal-on" PEC aptasensor that exhibited a 0.1-500-nM linear range and a detection limit of 0.069 nM for DCF, attributed to the excellent amplification of the proposed aptasensing platform.