A fluorescence-based colorimetric droplet platform for biosensor application to the detection of α-fetoprotein

Droplet methods have been successfully applied in DNA hybridization analysis and protein-protein interaction. Existing assay methods implemented in droplet platforms are severely limited by expensive and high-maintenance equipment. As a convenient detection method, colorimetry provides a new path for microscale assay since it can enhance assay efficiency and simplify the detection procedure. Here, a microscale immunoassay for α-fetoprotein (AFP) was developed for the first time by the incorporation of colorimetry and droplet platform. Ru(bpy)₂(mcbpy-O-Su-ester)(PF₆) ₂ complex (Ru) was coupled with the monoclonal antibody (Ab) of AFP to form a stable red Ru-Ab complex both as a quencher for green CdTe quantum dots (QDs) and as a capture probe for AFP. In the absence of AFP, the mixed droplet showed a red color. With the increase of AFP concentration, the color change of the droplet was from red to green as a result of the competition of AFP with QDs for Ru-Ab. The biosensor exhibited not only good sensitivity and specificity for AFP with a detection limit of 0.06 ng ml^-1, but also satisfactory performance in diluted human sera with a detection limit of 0.4 ng ml^-1. Notably, a visual droplet platform for screening cancer biomarkers by the naked eye based on this principle is anticipated.