Specific detection of cysteine and homocysteine in biological fluids by tuning the pH values of fluorosurfactant-stabilized gold colloidal solution

► Cysteine is discriminated from homocysteine by tuning the pH values of the FSN-capped AuNPs. ► A novel colorimetric assay for cysteine and homocysteine was developed without separation. ► This method has been successfully applied to analyze human urine and plasma samples. This study describes the use of 14 nm nonionic fluorosurfactant-capped gold nanoparticles (FSN-capped AuNPs) for the simultaneous detection of cysteine (Cys) and homocysteine (Hcy) using colorimetric method, requiring no use of separation techniques. It was found that the kinetics of Cys/Hcy-induced aggregation of the 14 nm FSN-capped AuNPs strongly depends on the pH value of gold colloidal solution. At a pH of 6.5, the Cys-induced aggregation kinetics of the FSN-capped AuNPs was almost identical to that induced by Hcy, facilitating simultaneous detection of total Cys and Hcy up to a concentration as low as 0.15 μM; while at pH 12.0, the kinetics of Cys-induced aggregation was much faster than that inducted by Hcy, leading to selective detection of Cys at concentration as low as 1.0 μM in the presence of Hcy. The applicability of the method was validated by spiking known amount of Cys and Hcy in human urine and plasma samples, obtaining a recovery of 95.4–105.5%. The present approach is simple, high selective and provides high reproducibility, and has a great potentiality in disease diagnosis.