Portable photoelectrochemical immunoassay for thyroid-stimulating hormone detection employing ZnO/Ag/g-C3N4 ternary heterojunction combined with enzyme-mediated ultrasonic signal amplification.

Serum thyrotropin (thyroid-stimulating hormone, TSH) is the most sensitive and specific biomarker for evaluating thyroid function, yet conventional immunoassay often suffer from limited sensitivity, complex instrumentation, and insufficient portability for point-of-care applications. Herein, we developed a portable photoelectrochemical (PEC) immunoassay based on a ZnO/Ag/g-C3N4 ternary Z-scheme heterojunction for the highly sensitive and selective detection of TSH. The hierarchical ZnO nanoflower scaffold was successively decorated with Ag nanoparticles and g-C3N4 nanosheets, forming intimate interfacial contacts that significantly promote charge separation and visible-light harvesting. Alkaline phosphatase (ALP)-labeled immunocomplexes catalytically convert L-ascorbic acid 2-phosphate (AAP) to ascorbic acid (AA), which served as an effective hole scavenger to amplify the photocurrent. Notably, the developed immunoassay exhibited a broad linear range spanning five orders of magnitude (R2 = 0.9939) with a detection limit (LOD) of 0.71 pg mL-1, with reliable interference immunity and accuracy. The integration of portable smartphone-based readout, ultrasound-enhanced piezo-phototronic effects, and the robust Z-scheme charge-transfer mechanism provides a versatile and reliable platform for point-of-care thyroid-function screening.