Colorimetric sensing of iodide ions based on unmodified gold nanoparticles and the distinctive antiaggregation-to-aggregation process.

A highly sensitive and selective colorimetric analysis method based on unmodified gold nanoparticles (AuNPs) to detect iodide ions (I- ) in solution in the presence of hexadecyl trimethyl ammonium bromide (CTAB) and mercury ions (Hg2+ ) has been successfully developed. Hg2+ could form a gold amalgam with AuNPs to protect AuNPs from CTAB-induced aggregation caused by the electrostatic attraction between CTAB and citrate ion-covered AuNPs. When a mixture of Hg2+ and I- was added to the solution of AuNPs, the formation of the HgI2 complex destroyed the protection of Hg2+ for AuNPs, which led to the aggregation of AuNPs accompanied with the change in colour of the solution from red to grey black and decrease in the absorbance of AuNPs at 520 nm. There was a good linear relationship between A520nm and I- concentration from 0 to 800 nM with a low limit of detection (LOD) of 4.2 nM. Furthermore, this simple and reliable colorimetric sensor has been applied successfully to the detection of I- in practical samples.