Label-free colorimetric assay for biological thiols based on ssDNA/silver nanoparticle system by salt amplification

We report a novel method for biological thiols detection using ssDNA/silver nanoparticles system. The adsorbing ssDNA supplies silver nanoparticles high density charge to rescue nanoparticles from aggregation induced by salt. However, homocysteine (cysteine or glutathione) is conjugated more powerfully than ssDNA to AgNPs via Ag-S bond, which holds back ssDNA binding to AgNPs surface. When salt is added, AgNPs aggregation occurs and the corresponding color changes from yellow to brown after these biological thiols is introduced. A high sensitivity can be achieved using salt as an amplifier to assay thiols. In our study, a favorable linear correlation between the A(0)/A(x) ratio and homocysteine concentration was obtained in the range of 10 to 500 nM with a low detection limit of 10 nM, indicating that homocysteine could be analyzed at low concentration. A concentration as low as 300 nM homocysteine caused a visible color change. As well as, cysteine and glutathione can be detected at a detection limit of 50 nM and 100 nM, respectively. In addition, study on the selectivity of this method shows that only homocysteine, cysteine and glutathione can generate signal response.