Colorimetric Response of Dithizone Product and Hexadecyl Trimethyl Ammonium Bromide Modified Gold Nanoparticle Dispersion to 10 Types of Heavy Metal Ions: Understanding the Involved Molecules from Experiment to Simulation

A new kind of analytical reagent, hexadecyl trimethyl ammonium bromide (CTAB), and dithizone product-modified gold nanoparticle dispersion, is developed for colorimetric response to 10 types of heavy metal ions (Mn+), including Cr(VI), Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+. The color change of the modified gold nanoparticle dispersion is instantaneous and distinct for Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+. The color change results from the multiple reasons, such as electronic transitions, cation−π interactions, formation of coordination bonds, and Mn+-induced aggregation of gold nanoparticles (AuNPs). The different combining capacity of heavy metal ions to modifiers results in the different broadening and red-shifting of the plasmon peak of modified AuNPs. In addition, Cr(VI), Cu2+, Co2+, Ni2+, and Mn2+ cause the new UV–vis absorption peaks in the region of 360–460 nm. The interactions between the modifiers and AuNPs, and between the modifiers and Mn+, are investigated by using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results confirm that AuNPs are modified by CTAB and dithizone products through electrostatic interactions and Au–S bonds, respectively, and the Mn+–N bonds form between Mn+ and dithizone products. Furthermore, the experimental and density functional theory calculated IR spectra prove that dithizone reacts with NaOH to produce C6H5O– and [SCH2N4]2-. The validation of this method is carried out by analysis of heavy metal ions in tap water.