Enhancement of the colorimetric sensitivity of gold nanoparticles with triethanolamine to minimize interparticle repulsion

Cysteine and thioglycolic acid were immobilized on gold nanoparticles via established thiolgold surface chemistry. It is found that calcium ions rapidly induce the aggregation of the functional gold nanoparticles due to the complexation of Ca(II) by immobilized cysteine. It was also found that triethanolamine enhances the effect of calcium ions by decreasing the electrostatic repulsion between the gold nanoparticles. Transmission electron microscopy, electrophoresis, zeta potential measurements and absorptiometry were used to investigate the mechanism. Under the optimum experimental condition, the cysteine/thioglycolate/triethanolamine-modified nanoparticles were highly sensitive (the detection limit being 0.3 mu M) and selective towards calcium and magnesium ions, with a linear detection range between 1.0 mu M and 14 mu M. Based on these findings, a rapid and selective colorimetric method was developed for assaying Ca(II) ions in serum.