A preliminary exploration on Au nanoparticles-mediated colorimetric analysis of Cr(III)-carboxyl complexes in synthetic and authentic water samples

Toxic metal-organic complexes are ubiquitous in contaminated waters but more challenging than free ion counterparts for water decontamination. Currently, almost all the available analytical methods, including Au nanoparticles-mediated colorimetric assay, cannot discriminate metal-organic complexes from the total metal content, which greatly impedes the development of specific decontamination strategies for metal-organic complexes. In this study, we found that nitrilotriacetic acid (NTA) functionalized AuNPs could be used to determine various Cr(III) complexes (Cr(III)-lactate, Cr(III)-malonate, Cr(III)-oxalate, Cr(III)-tartrate, Cr(III)-citrate and Cr(III)-EDTA) in water. Mechanism study indicated that at acidic pHs (pH=1.6), the protonation of organic ligands resulted in partial dissociation of Cr(III) from the corresponding complexes to expose its accessible orbitals for re-coordination with NTA. Such re-coordination induces the aggregation of the dispersed NTA-AuNPs and thereafter causes considerable changes both in color and UV-Vis spectrum of the solution. The nanosensor is highly selective to Cr(III)-organic complexes with detection limits of 0.029 mu M for Cr(III)-lactate, 0.043 mu M for Cr (III)-malonate, 0.35 mu M for Cr(III)-oxalate, 0.079 mu M for Cr(III)-tartrate, 2.58 mu M for Cr(III)-citrate and 0.90 mu M for Cr(III)-EDTA, respectively. The feasibility of the proposed colorimetric assay has been verified by using the authentic natural and industrial water samples. We believe this study will be of fundamental significance to push forward the application of Au NPs colorimetric assay in authentic waters contaminated by toxic metals.