Mercaptopropionic acid-capped Mn-doped ZnS quantum dots as a probe for selective room-temperature phosphorescence detection of Pb2+ in water

Abstract

In this study, highly water-soluble Mn-doped ZnS quantum dots (QDs) capped by 3-mercaptopropionic acid (MPA) were successfully synthesized by three different synthetic methods. By comparison, QDs prepared by hydrothermal synthesis method at 100 °C heating for 2 h had the best phosphorescence emission properties. Room-temperature phosphorescence (RTP) could be remarkably and selectively quenched by Pb²⁺ in a pH 3.8 NaAc–HAc buffer solution. Based on the abovementioned finding, a simple, sensitive, and selective phosphorescence method for rapid detection of Pb²⁺ was successfully developed using MPA-capped Mn-doped ZnS QDs as a probe. Under the optimized conditions, the RTP intensity ratio of P₀/P had a good linear relationship with Pb²⁺ concentration in the concentration ranges of 4 × 10⁻⁸–90 × 10⁻⁸ mol L⁻¹ and 90 × 10⁻⁸–600 × 10⁻⁸ mol L⁻¹ with the correlation coefficients of 0.9967 and 0.9970, which well followed the Stern–Volmer quenching equation, and the Stern–Volmer quenching constants were 8.41 × 10⁵ L mol⁻¹ and 3.452 × 10⁶ L mol⁻¹, respectively. The detection limit was 3.69 × 10⁻⁸ mol L⁻¹, and the relative standard deviation for 11 repeated detections of 60 × 10⁻⁸ mol L⁻¹ Pb²⁺ was 2.8%. According to the analysis of the Stern–Volmer quenching equation, UV-Vis absorption spectra, decay curves of phosphorescence emission, and phosphorescence lifetime, the quenching mechanism may imply that energy transfer and charge transfer in the interaction of MPA-capped Mn-doped ZnS QDs at the excited state with Pb²⁺ made QDs lose excitation energy. This resulted in phosphorescence of QDs causing dynamic quenching. The proposed method was successfully applied to detect Pb²⁺ in real water samples with satisfactory results, and the recoveries ranged from 93% to 109.3%. The developed method was simple, rapid, and specific and could effectively avoid fluorescence interference of the system, which opened up a promising prospect for the sensitive, convenient, and fast sensing and monitoring of small molecule pollutants in water based on phosphorescence sensors.