Colorimetric determination of melamine based on the reversal of the mercury(II) induced inhibition of the light-triggered oxidase-like activity of gold nanoclusters

We describe a colorimetric assay for the quantitation of melamine (MA). It relies on the observation that MA reverses the inhibition of the light-triggered activity of horseradish peroxidase (HRP) functionalized gold nanoclusters (HRP-Au NCs) by Hg(II) ions. The HRP-Au NCs catalyze the oxidation of the substrate 3,3',5,5'-tetramethylbenzidine (TMB) by dissolved oxygen if exposed to visible light provided by a 300 W Xe lamp (lambda a parts per thousand yen 400 nm). The addition of Hg(II) suppresses the enzymatic activity due to the metallophilic interaction between Hg(II) and Au(I) present on the surface of the HRP-Au NCs. MA with its triazole ring can restore the catalytic activity of the HRP-Au NCs by inhibiting the metallophilic interaction. As a result of these findings, a colorimetric "turn-on" assay for MA was worked out that can be used to determine MA in the 0.2 to 15 mu mol center dot L-1 concentration range and with a 72 nmol center dot L-1 detection limit, which is much lower than the established safety limit. The method was successfully applied to the quantitation of MA in (spiked) raw milk and milk powder, with recoveries ranging from 98.5 % to 101.5 %.