Photochemically Engineered, DNA-Functionalized AgCl@AuAg Nanoprobes for Colorimetric Detection of Target DNA via In Situ Catalytic Amplification.

Colorimetric DNA detection is attractive for point-of-care testing, yet sensitivity often remains insufficient for clinically relevant applications/targets. We report a signal-amplification strategy that uses DNA-functionalized AgCl@AuAg nanoparticles (DNA-AgCl@AuAgNPs) as chemically transformable catalytic nanoprobes. To overcome the inherent challenge of functionalizing ionic AgCl surfaces, we employed a photochemical reduction method to decorate cubic AgCl nanoparticles (AgClNPs) with Au/Ag metallic domains (AgCl@AuAgNPs). These metallic "bumps" serve as stable anchoring sites for thiol-modified probe DNA, enabling sequence-specific target recognition. In a magnetic sandwich assay, target DNA mediates sandwich hybridization between DNA-AgCl@AuAgNPs and DNA-modified magnetic microparticles (DNA-MMPs). After selective isolation of target-bound complexes using an external magnetic field, the captured nanoprobes undergo rapid in situ chemical transformation triggered by NaBH4. During this process, the captured AgCl@AuAgNP nanoprobes are efficiently transformed into a large ensemble of small, Ag-rich nanoparticles, increasing the accessible catalytic surface area and, thus, the number of catalytically active sites. This structural evolution markedly accelerates the catalytic reduction of 4-nitrophenol to 4-aminophenol, producing a clear color change from yellow to colorless. The reaction kinetics, monitored by UV-vis absorbance at 410 nm, show a clear concentration dependence on target DNA concentration. The developed platform demonstrates high sensitivity and reliable performance in both buffer and serum-containing samples, establishing these hybrid nanostructures as promising catalytic nanoprobes for biomolecular sensing in resource-limited settings.