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

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.