Abstract
<title>Abstract</title> <p>Carbohydrate antigen 19 − 9 (CA19-9) serves as a pivotal serological indicator for pancreatic and gastrointestinal oncology. In this work, we engineered a sandwich-style colorimetric immunosensor employing a CdS/Ag₂S/g-C₃N₄ ternary heterojunction as a photo-responsive nanozyme label. Comprehensive physicochemical characterization including TEM, XRD, FTIR, and DRS confirmed the successful synthesis of the nanocomposite. Optical analysis revealed a narrowed bandgap of 2.25 eV, which facilitates broad visible-light harvesting and efficient suppression of photogenerated carrier recombination, thereby boosting catalytic kinetics. Mechanistic investigations identified photogenerated holes (h⁺) as the primary oxidizing agents mediating the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB). Under optimized assay conditions, the platform achieved a broad linear dynamic range of 2.5–45 U/mL and a low detection limit of 0.108 U/mL. Crucially, analysis of clinical serum specimens demonstrated high concordance with standard clinical methodologies, highlighting the sensor’s robustness for early-stage tumor screening and point-of-care testing.</p>