Fluorescent silver hydrosol for the dual fluorometric sensing of gallic acid and Cd2+

Herein, silver-enhanced fluorescence was obtained using a small molecule of salicylaldehyde (SL) in an alkaline solution in the presence of silver nitrate. Ag⁺ was reduced to Ag⁰, while SL was oxidized to the quinone form OSL. OSL was a very weak fluorophore and exhibited metal-enhanced fluorescence (MEF) in the presence of proximal silver nanoparticle surfaces. MEF was selectively quenched by gallic acid and restored by Cd²⁺. Thus, a dual-sensing platform was obtained for gallic acid [limit of detection (LOD) = 1.77 × 10⁻⁵ M; linear detection range = 10⁻⁸ to 5 × 10⁻⁵ M)] and Cd²⁺ (LOD = 2.4 × 10⁻⁶ M; linear detection range = 5 × 10⁻⁸ to 10⁻⁶ M) in a one-pot. The observed lightning rod effect and higher radiative decay rate were ascribed to MEF, while replacing the capping agents was responsible for the tuning of MEF. The sensing protocol was applied to natural samples from real sample sources, and satisfactory results were obtained.