This technology is an ultra-sensitive colorimetric assay, based on an enzyme-catalyzed gold nanoparticle growth process, for detection of disease-associated proteins (biomarkers) and disease diagnosis. Current detection methods, such as ELISA immunoassays, measure concentrations above 0.1 ng/mL in a sample. PCR, although more sensitive than ELISA, requires expensive and specialized equipment and reagents, skilled labor, and complex analysis techniques. This assay detects fg/mL to pg/mL concentrations, allowing detection and diagnosis in the earliest stage of disease or infection. A simple to read colorless-to-red change of gold nanoparticle is read with the naked eye, without the need for advanced instruments. This assay can be performed in a standard ELISA plate. Prototype, proof of concept tests using this platform have been designed for enterovirus 71 (EV71) and prostate specific antigen (PSA). The limit of detection (LOD) for a PSA prototype exceeded the commercial ELISA by more than four orders of magnitude. This assay may be particularly well suited for field use/point-of-care detection of infections and early stage disease.