Streptavidin-Displaying E. coli Scaffolds Enable Target-Mediated Gold Nanoparticle Assembly for Colorimetric Biosensing.

Enhancing the visual detection sensitivity of point-of-care (POC) optical sensors without resorting to complex, multistep chemical or enzymatic signal amplification remains a major challenge in sensor nanotechnology. Here, we present a multifunctional bio-nanostructured hybrid platform that integrates engineered Escherichia coli cellular scaffolds with antibody-functionalized gold nanoparticles (AuNPs) for high-sensitivity, solution-phase colorimetric sensing. By exploiting the biotin-streptavidin interaction through the Lpp-OmpA outer-membrane display system, the engineered bacterial cells serve as high-capacity, multivalent biological interfaces with a large effective surface area and high recognition-site density. Using SARS-CoV-2 nucleocapsid protein as a model biomarker, target-mediated bridging between AuNP-antibody probes and bacterial scaffolds promotes localized AuNP assembly and enhanced plasmonic coupling, resulting in a distinct visual color change within 30 min. The optimized assay achieved a naked-eye limit of detection (LOD) of 10-12 mol, representing a 10-fold sensitivity enhancement over conventional scaffold-free AuNP assays. This study provides a proof of concept for leveraging engineered living cells as multifunctional nanostructured templates. The modular, plug-and-play architecture of the biotin-streptavidin coupling interface highlights its potential for adaptation to clinical diagnostics and environmental monitoring.