Au-ag quantum dot nanozyme identified for multi-mode fluorescent, UV–vis and intelligent RGB monitoring and removal of toxic Hg2+ and S2− in food samples

To conveniently monitor and remove toxic Hg²⁺ and S²⁻ in food samples, AuAg quantum-dot nanozyme (Au-Ag@FA QDs) was identified. The strong synergistic effect among folic acid (FA), Ag and Au endowed Au-Ag@FA QDs with superior peroxidase-mimic activity to accelerate 3,3′,5,5′-tetramethylbenzidine (TMB) redox with K_m/V_max of 0.28 mmol·L⁻¹/6.40 × 10⁻⁸ mol·L⁻¹·s⁻¹. Trace Hg²⁺ or S²⁻ could selectively alter its peroxidase-like activity and inherent fluorescence performance, with obvious hyperchromic/“turn-on” fluorescent effect for Hg²⁺ or hypochromic/“turn-off” one for S²⁻. And red-green-blue (RGB) values analyzed from smartphones changed regularly with c_Hg²⁺ or c_S²⁻. Under the optimized conditions, Au-Ag@FA QDs were successfully applied for multi-mode UV–vis, fluorescent, and RGB monitoring of Hg²⁺ or S²⁻ in food samples. The detection limits were 7.38 × 10⁻⁷/5.74 × 10⁻⁸/4.93 × 10⁻⁶ g·L⁻¹ and 1.99 × 10⁻⁶/4.48 × 10⁻⁸/5.79 × 10⁻⁶ g·L⁻¹ respectively. Importantly, over 93.8 % Hg²⁺ or 90.5 % S²⁻ could be removed by simple centrifugal-separation after detection. The selective recognition mechanisms to Hg²⁺ and S²⁻ were also proposed.