Small molecule L-cysteine assisted synthesis of green fluorescent sulfur quantum dots for nimesulide sensing and bioimaging

Abstract

As a new kind of heavy-metal-free fluorescent nanomaterials, synthesis of sulfur quantum dots (SQDs) using small molecules as capping agents remains a big challenge. In this work, a novel small molecule assisted synthesis strategy of SQDs was reported. L-cysteine was employed as capping reagent for the first time to prepare green fluorescent SQDs. The as-synthesized L-cysteine protected SQDs (Cys-SQDs) samples were further effectively purified by a simple treatment with sodium ion exchange resin. Cys-SQDs samples were characterized in detail using various characterization techniques. In the light of good fluorescence property, excellent optical stability and low biological toxicity, the potential applications of the obtained Cys-SQDs in sensing and bioimaging aspects were investigated. Results showed that Cys-SQDs were highly sensitive and selective towards drug molecule nimesulide (Nim), which can be applied for the detection of Nim. The detection limit was calculated as 160 nM. The developed method was successfully used for the determination of Nim in real samples, demonstrating good recoveries of 93.23 ∼ 110.01% with RSDs of 0.78 ∼ 5.68%. The fluorescence quenching process of Cys-SQDs by Nim mainly involved dynamic quenching mechanism and fluorescence resonance energy transfer. Moreover, Cys-SQDs nanoprobe exhibited low biological toxicity to three different cell lines. By comparing the staining regions of three cell lines using Hoechst dye and followed colocalization analysis, it was definitely confirmed that the synthesized Cys-SQDs can stain both cytoplasm and nucleus, and it can also identify Nim in living cells. This research demonstrated the potential applications of Cys-SQDs in sensing and bioimaging fields.