Novel cellulose polyampholyte-gold nanoparticle-based colorimetric competition assay for the detection of cysteine and mercury(II)

We provide a highly sensitive and selective assay to detect cysteine (Cys) and Hg²⁺ in aqueous solutions using Au nanoparticles (NPs) stabilized by carboxylethyl quaternized cellulose (CEQC). This method is based on the thiophilicity of Hg²⁺ and Au NPs as well as the unique optical properties of CEQC-stabilized Au NPs. CEQC chains are good stabilizing agents for Au NPs even in a high-salt solution. The addition of Cys results in the aggregation of CEQC-stabilized Au NPs, which induces the visible color change and obvious redshift in UV-visible absorption spectra. On the other hand, Hg²⁺ is more apt to interact with thiols than Au NPs; thus, it can remove the Cys and trigger Au NP aggregate redispersion again. By taking advantage of this mechanism, a novel off-on colorimetric sensor has been established for Cys and Hg²⁺ detection. This new assay could selectively detect Cys and Hg²⁺ with the detection limits as low as 20 and 40 nM in aqueous solutions, respectively.