Polydopamine with Tailorable Photoelectrochemical Activities for the Highly Sensitive Immunoassay of Tumor Markers

A simple and highly sensitive photoelectrochemical (PEC) immunoassay sensor was fabricated by using the two forms of polydopamine (PDA), the thin film and nanosphere, to serve as the photoelectrode-modified material and signal reporting label, respectively. The two forms of PDA show similar light absorption behavior but totally different PEC activities. The PDA film can extend the light absorption from the ultraviolet to near infrared light range, transfer a photoelectron to TiO2 nanoparticles and the underlying photoelectrode, and largely amplify the photocurrent response. However, the PDA nanospheres have insignificant photoelectron transport ability. When they are brought close to the PDA film and TiO2 nanocomposite-modified electrode via the sandwich immunoreaction, they function like a black hole to compete with the PDA film for light absorption, resist the access of the electron donor to regenerate the photoactive material, and capture the photoelectron generated from the PDA film. Besides, the heat generated from the PDA nanospheres also contributes to the photocurrent decrease. The PDA nanospheres with multiple quenching effects on the PDA film greatly decrease the photocurrent signal and lead to a highly sensitive PEC immunosensing strategy. Under optimal conditions, a wide linear range from 0.1 to 106 pg·mL-1 is obtained toward carcinoembryonic antigen, with a low limit of detection of 40 fg·mL-1. Besides, the PDA with excellent biosafety can be readily assembled with proteins, which thus simplifies the preparation procedures and decreases the costs. All these features indicate that the whole PDA-based PEC sensing strategy may have great application prospects for the point-of-care assay of various kinds of tumor markers.