Colorimetric Au Nanoparticle Probe for Speciation Test of Arsenite and Arsenate Inspired by Selective Interaction between Phosphonium Ionic Liquid and Arsenite

The exposure of millions of people to unsafe levels of arsenite (As^III) and arsenate (As^V) in drinking waters calls for the development of low-cost methods for on-site monitoring these two arsenic species in waters. Herein, for the first time, tetradecyl (trihexyl) phosphonium chloride ionic liquid was found to selectively bind with As^III via extended X-ray absorption fine structure (EXAFS) analysis. Based on the finding, an As^III-specific probe was developed by modifying gold nanoparticles with the ionic liquid. Futhermore, Hofmeister effect was primarily observed to significantly affect the sensitivity of gold nanoparticle probe. With the colorimetric probe, we developed a protocol for naked eye speciation test of As^III and As^V at levels below the World Health Organization (WHO) guideline of 10 μg L^–1. This method featured with high tolerance to common coexisting ions such as 10 mM PO₄^3–, and was validated by assaying certified reference and environmental water samples