Biocompatible Ag2S quantum dots for highly sensitive detection of copper ions

Abstract

A simple aqueous approach for synthesizing biocompatible red and near-infrared (NIR) emitting Ag₂S quantum dots (QDs) at low temperature without a required oxygen-free process has been developed. The fluorescence of the obtained red emitting Ag₂S QDs could be selectively quenched by Cu²⁺. Based on this, a novel Ag₂S QD based fluorescent sensor for highly selective and sensitive detection of Cu²⁺ was developed. This method showed a low limit of detection (LOD) of 27.6 nM for Cu²⁺ sensing in a wide linear range concentration of 25 nM–10 μM. The quenching mechanism was discussed by the time-resolved photoluminescence, absorption spectra, TEM and energy-dispersive X-ray (EDX) results. Cation exchange of Ag₂S QDs between Cu(II) and Ag(I), and dynamic quenching due to excited state electron transfer from Ag₂S QDs to Cu²⁺ were considered to be the quenching mechanism. Furthermore, to understand the interaction between Ag₂S QDs and Cu²⁺, the thermodynamic process was investigated by means of isothermal titration calorimetry (ITC). The values of ΔH, ΔS and ΔG were calculated from the ITC results to be −79.43 kJ mol⁻¹, −142.18 J K⁻¹ mol⁻¹ and −37.06 kJ mol⁻¹, respectively. The Ag₂S QD based Cu²⁺ detection had advantages of nontoxicity, fast response, high sensitivity and selectivity, wide linear range and easy preparation.