Visual quantitative point-of-care immunoassay based on signal transduction amplification and hue-recognition analysis

Visual based immunoassay employs both human eye interpretation and machine-vision (smartphone imaging) quantification for equipment-independent diagnosis of biomarkers, fitting a variety of scenarios such as home-testing. Here we use hue-recognition strategy to create vivid and sensitive color changes against C-reactive protein (CRP) for accurate visual analysis. The red-to-green tonality gradient presentation was realized by ratiometric fluorescent probe using dual-emissive quantum dots (QDs) structure. The sandwich immuno-capturing process of antigen was integrated on ELISA plate, with the signal amplified and transduced by silver particle labels etching and interaction with the ratiometric QDs probe. The narrow-emissive QDs with selective response to released Ag⁺ ions produced high color fidelity and prominent hue variation towards analyte concentration. This portable immunoassay enabled eye-perception of 5 specific CRP concentration points (0, 5, 50, 200, 500 ng/mL) and 4 concentration intervals (0-5, 5-50, 50-200, 200-500 ng/mL) with reading accuracy of 97.5 % and 96.4 %, respectively, which is superior against traditional lightness-gradient based reading mode. With the aid of smartphone imaging and software color-analyzing, an elaborated quantification of CRP was achieved via color information. The broad CRP responsive range (0-500 ng/mL), low limit of detection (0.062 ng/mL) and high specificity against protein substrates allowed a robust point-of-care CRP clinical diagnosis application.