Target-induced charge reduction of aptamers for visual detection of lysozyme based on positively charged gold nanoparticles.

Positively charged gold nanoparticles ((+)AuNPs) are electrostatically adsorbed on the negatively charged aptamers and the solution displays a blue color due to the aggregation of the (+)AuNPs, while the binding of the target lysozyme with the aptamers reduces the charge of the aptamers, which stabilizes the (+)AuNPs and the (+)AuNPs solution remains red in color.