Rapid Colorimetric Detection of Free and Complexed Cr3+ Based on a Tetrabutylphosphonium Bromide/Gold Nanoparticle Anti-Aggregation System

Abstract

Existing colorimetric sensing strategies based on gold nanoparticles (AuNPs) are largely limited to the detection of free metal ions, leaving metal–organic complexes widely present in real water bodies undetectable. Herein, we develop a simple colorimetric method based on the anti-aggregation effect of bare AuNPs for the simultaneous detection of free Cr3+ and its organic complexes. Tetrabutylphosphonium bromide (TPB) dissociates into phosphonium cations ([P(C4H9)4]+), which neutralize the negatively charged surface of AuNPs via electrostatic attraction, thereby inducing particle aggregation and a color change from wine-red to blue. Under strongly alkaline conditions, Cr3+ converts to negatively charged [Cr(OH)6]3− species. These anionic species then compete with the AuNPs for the phosphonium cations, blocking the aggregation process and preserving the wine-red color. This competitive mechanism is further extended to the detection of Cr3+-EDTA, Cr3+-tartrate, and Cr3+-oxalate complexes, as they also transform into [Cr(OH)6]3− under alkaline conditions. Without the need for tedious surface modification, the proposed method is simple to operate and exhibits excellent sensitivity and selectivity for both free and complexed Cr3+ in real water samples. It thus provides a reliable analytical tool for monitoring chromium pollution in environmental waters.