Efficient Conversion of Elemental Sulfur to Robust Ultrabright Fluorescent Sulfur Quantum Dots Using Sulfur-Ethylenediamine Precursor

Abstract

The direct utilization of abundant and cheap elemental sulfur (S) to synthesize sulfur quantum dots (SQDs) with satisfactory yield and photoluminescence quantum yield (PLQY) remains a big challenge. Here, we report a simple and effective method for mass-producible synthesis of ultrabright fluorescent SQDs based on one-pot solvothermal treatment of S-ethylenediamine (S-EDA) solution. The PLQY of synthesized SQDs and the corresponding S-to-SQDs conversion efficiency (CE) can reach as high as 87.8% and 15.9%, respectively, both of which are the highest values to date. In addition, the SQDs simultaneously show good water solubility, superior storage stability, stable fluorescence against ionic strength variation and pH change, and low toxicity, which enable them promising for numerous applications such as fabrication of fluorescent polymer composite and ink. Furthermore, SQDs also present strong two-photon emission, and their use for two-photon fluorescence imaging is reported here for the first time. This work thus highlights enormous opportunities not only for bringing SQDs from laboratory research to real applications but also for the value-added utilization of S resource.

Synthesis of Fluorescent Sulfur Quantum Dots for Bioimaging and Biosensing

“…In addition to the elemental S-NaOH system, Zhou and co-workers recently reported the solvothermal treatment of elemental S-ethylenediamine solution to prepare highly fluorescent SQDs ( Figure 1E ). The fluorescence QY of prepared SQDs and the corresponding S-to-SQD conversion efficiency reach as high as 87.8 and 15.9%, respectively, both of which are the highest values to date ( Gao et al, 2022 ). This study provides a new reaction system for large-scale preparation of ultrabright fluorescent SQDs from elemental S.…”

“…When the concentration of SQDs was up to 375 μg/ml, the cell viability was still higher than 85%, suggesting the low cytotoxicity of SQDs. In addition to cytotoxicity evaluation, Zhou et al also assessed the biocompatibility of SQDs by cultivating soybean seeds in SQDs solution ( Gao et al, 2022 ). The results showed that even in the high concentration of SQDs solution (200 μg/ml), the presence of SQDs had a negligible effect on plant growth, revealing the fine biocompatibility of SQDs.…”

“…Copyright 2021, Royal Society of Chemistry. (D) Reproduced, with permission, from Gao et al (2022) . Copyright 2022, American Chemical Society.…”

“…On the other hand, it is also possible to overcome this disadvantage by using the two-photon or multi-photon fluorescence imaging technology. Very recently, Zhou and co-workers used SQDs as a fluorescent probe for two-photon fluorescence imaging of HeLa cells ( Gao et al, 2022 ) ( Figure 2D ). The HeLa cells can emit bright two-photon fluorescence in the cytoplasm with excitation at 800 nm.…”

One-pot synthesis of robust dendritic sulfur quantum dots for two-photon fluorescence imaging and “off–on” detection of hydroxyl radicals and ascorbic acid

“…1 Additionally, the SQDs can now be straightforwardly synthesized from cheap elemental sulfur. [2][3][4][5] However, most of the surface ligands reported so far cannot completely stabilize SQDs, resulting in the poor photostability of SQDs and the difficulty of tolerating long time UV-light illumination, which seriously limits the practical applications of SQDs. 4,6,7 For instance, the fluorescence intensity of polyethylene glycol (PEG)capped SQDs declined by 48% after exposure to UV lamp for 10 min.…”