Colorimetric detection of d-amino acids based on anti-aggregation of gold nanoparticles

Abstract

Abstract A new method has been proposed to realize the visual detection of d -amino acids (DAAs) via the anti-aggregation of 4-mercaptobenzoic acid modified gold nanoparticles (AuNPs) in the presence of d -amino acid oxidase (DAAO). The negatively charged AuNPs were prepared using sodium citrate as a reducer and stabilizer. The presence of 4-mercaptobenzoic acid (4-MBA) and Cu 2+ induces the aggregation of AuNPs, resulting in a color change from ruby red to royal purple. However, DAAO could oxidize DAAs to generate H 2 O 2 . In the presence of H 2 O 2 , the mercapto ( SH) group in 4-mercaptobenzoic acid can be oxidized to form a disulfide ( S S ) bond. Based on these facts, the pre-incubation of DAAs and 4-mercaptobenzoic acid with DAAO would significantly reduce the concentration of free 4-mercaptobenzoic acid molecules, thus the aggregation of AuNPs was interrupted since due to the lack of inducer. As the concentration of DAAs increases, the color of the AuNPs solution would progress from royal purple to ruby red. Consequently, DAAs could be monitored by the colorimetric response of AuNPs using a UV–vis spectrophotometer or even naked eyes. This DAAO mediated visual detection method could determine d -alanine ( d -Ala) as a representative DAA with concentrations ranging from 1.5 × 10 −7 mol L −1 to 3.0 × 10 −5 mol L −1 , and the detection limit was as low as 7.5 × 10 −8 mol L −1 . The proposed method is convenient, low-cost and free of complex equipment, making it feasible to analyze the concentration of d -Ala in real samples of β -amyloid peptide (A β 1–42 ).