Modulating the photoelectrochemical response of titanium dioxide (TiO2) photoelectrodes using gold (Au) nanoparticles excited at different wavelengths

Gold (Au) nanoparticles (NPs) have been widely used to modulate the photoelectrochemical current of TiO2 photoelectrodes in energy conversion and sensing systems. Understanding the different mechanisms responsible for photocurrent modulation at different frequencies is important for building optimized photoelectrochemical systems. Herein, we investigated the photocurrent magnitude at different excitation wavelengths by varying Au NP concentration at the photoelectrode surface. Under UV illumination, increasing the surface loading of Au NPs initially increased the photocurrent, and above a threshold loading level, decreased the measured photocurrent. However, under visible light excitation, increasing the Au NP surface density resulted in a steady increase in photocurrent. Mott-Schottky measurements, incident photon to current conversion efficiency measurements, and electrochemical impedance spectroscopy were used to understand the mechanisms responsible for these different observations. It was found that both current loss – due to reduced light absorption by TiO2 – and gain – due to direct charge transfer between Au and TiO2 NPs were possible under UV light. Under visible light illumination, strong light absorption and localized surface plasmon resonance of Au NPs and negligible light absorption by TiO2 NPs led to signal gain at varying Au NP surface concentrations. This bimodal signal modulation was further demonstrated in the context of biosensing using Au NP-labeled DNA barcodes, optically excited at different wavelengths. This study allows photoelectrochemical systems to be engineered for programmable signal-off, signal-on, or ratiometric biosensing combining the former sensing modes.