The emergence of photoelectrochemical (PEC) biosensors has opened the door to achieve biosensors with a low limit-of-detection as PEC technique offers reduced background signals ^1,2. As a result, researchers are trying to use PEC technique to achieve low-cost and ultrasensitive biosensors. In this work, we proposed a novel method for fabricating flexible, low-cost PEC sensor substrates. Exploiting wrinkled surfaces, we have shown an increase in the photocurrent density of more than 700%. We investigated photocurrent density of CdTe Quantum dots (QD) on top of wrinkled and planar ITO surface. The quality of the substrates has been also verified by measuring sheet resistance and electron mobility. Moreover, these substrates, with an enhanced PEC response, have been used to investigate the interplay between localized surface plasmonic effects (LSPR) and photoactive semiconductor materials using DNA as spacer and gold nanoparticle as plasmonic materials.

The wrinkled structure has been achieved by thermally shrinking (heating at 140°C for 5 minutes) pre-stretched PS substrates modified using various surface treatments. RF sputtering technique has been used to deposit ITO on planar and wrinkled PS. The thicknesses of the ITO film used for this study are 50nm and 100nm. Two methods have been used to prepare the wrinkled electrodes. In the first method, thermal shrinking has been done after putting ITO on PS, whereas for other method, PS substrates were cleaned using UV-Ozone (UVO) and then heated to shrink. ITO was sputtered on those shrunk substrates. CdTe QDs have been deposited using the layer-by-layer method. The photocurrent was measured by using chronoamperometry in a three-electrode setup illuminated using an LED excitation source. Scanning electron microscopy (SEM) has been used to visualize the structure of the electrodes. Cross-sectional Transmission electron microscopy (TEM) is used to probe the QD distribution in both planar and wrinkled ITO surface. To investigate the interplay between LSPR of metallic nanoparticles and the photo-activity of semiconductor quantum dots, a material architecture based on a DNA spacer was developed on the optimized wrinkled electrode. The length of DNA spacer was adjusted to tune the distance between the nanoparticles and the QDs.

Substrates created by ITO deposition following the shrinking process showed the highest photocurrent density (more than 700% enhancement) among the analyzed samples. SEM images showed that when the substrates were heated after depositing ITO, it tends to break in places, which creates more surface defects, whereas more uniform ITO is obtained if the thin film is applied after wrinkling. TEM images showed much higher QD density on the wrinkled substrate compared to the planar one. By controlling the distance of the QD and Au-NP, the photocurrent can be enhanced or quenched. Quenching of the photocurrent have been observed when longer DNA was used as probe, whereas photocurrent enhancement was achieved by using shorter oligonucleotide.

The substrates developed here are ideally suited for enhancing the limit of detection of biosensors. Moreover, this platform has enabled us to study the interaction between plasmonic nanoparticles and photoactive semi-conductive QDs to understand the underlying mechanisms for enhancing or quenching the photocurrent.

1. Devadoss, A., Sudhagar, P., Terashima, C., Nakata, K. & Fujishima, A. Photoelectrochemical biosensors: New insights into promising photoelectrodes and signal amplification strategies. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 24, 43–63 (2015).
2. Fan, G., Han, L., Zhang, J. & Zhu, J. Enhanced Photoelectrochemical Strategy for Ultrasensitive DNA Detection Based on Two Different Sizes of CdTe Quantum Dots Cosensitized TiO 2 /CdS:Mn Hybrid Structure. Analytical Chemistry (2014). doi:10.1021/ac503043w