Colorimetric determination of cysteine by a paper-based assay system using aspartic acid modified gold nanoparticles

A method is described for cysteine (Cys) determination on paper-based analytical devices using aspartic acid modified gold nanoparticles (Asp-AuNPs). The Asp-AuNPs were characterized by their size, zeta potential, and UV-visible absorption spectrum. After the addition of Cys, it will interact with Asp-AuNPs selectively and leads to the aggregation of Asp-AuNPs. A color change from red to blue can be observed on the paper-based analytical devices. The results were recorded using a cell phone and subsequently analyzed using the Photoshop software. The ratiometric color intensity at red channel and blue channel (Red/Blue) increased linearly in the range 99.9-998.7 mu M for Cys (R = 0.9984), and the limit of detection was 1.0 mu M. The effects of assay conditions have been investigated and are discussed. The Cys concentration was determined as (0.27 +/- 0.02 mM) in human plasma, and the recovery was from 99.2 to 101.1%.