An efficient multi-channel UV–vis spectral, fluorescence and RGB-based method for trace S2 − detection in agricultural foods

Herein, a novel Bi 3+ -doped Au-Ag quantum dot (Bi 3+ -Au-Ag QD) system was synthesized, by homogeneous reduction method using AgNO 3 , Bi(NO 3 ) 3 ·5 H 2 O and HAuCl 4 ·4 H 2 O as raw materials, for multi-channel detection of S 2− in agricultural foods. Owing to the strong interaction between S 2− and the Ag/Au components, trace amounts of S 2− led to the formation of Ag 2 S or Au 2 S sulphides on the surface of the Bi 3+ -Au-Ag QDs. This surface modification altered the intrinsic fluorescence property of QDs, and their catalytic ability to the oxidization of colourless 3,3′,5,5′-tetramethylbenzidine (TMB) into its blue oxide (oxTMB), resulting in noticeable changes in spectral responses, colour and RGB values. Under optimised experimental conditions, Bi 3+ -Au-Ag QDs were employed successfully for ‘turn-on’ fluorescence, UV–vis and smartphone-assisted red-green-blue (RGB) detection of S 2− in real agricultural food samples. The limits of detection –4.75 × 10 −10 , 6.48 × 10 −9 and 7.22 × 10 −9 mol·L −1 respectively, well met the safety threshold set by the World Health Organization.