Comparison of physicochemical and functional properties of elemental sulfur, sulfur nanoparticles, and sulfur quantum dots for sustainable biological applications

Abstract

Elemental sulfur (ES) and sulfur nanoparticles (SNPs) have been widely used as antibacterial ingredients, especially in dermatological formulations. However, their use in biological systems is greatly limited because of their inability to form hydrophilic and stable aqueous suspensions. On the other hand, sulfur quantum dots (SQDs) are surface-passivated nano-sulfur particles with less than 10 nm, are hydrophilic, and exhibit superior biological activity to ES or SNPs. It is also a sustainable material produced from sublimated sulfur, obtained as a by-product in huge quantities from petroleum refineries. However, systematic comparisons of these sulfur particles' physicochemical and functional properties have not yet been performed, resulting in knowledge gaps. The present work has characterized and compared the physicochemical and functional properties of ES, SNPs, and SQDs. SQDs had the highest zeta potential (−11.9 mV) of these three sulfur forms and were most stable in aqueous suspension. Surface-passivated SQDs with polyethylene glycol were more biologically compatible without cytotoxicity to L929 mouse fibroblasts. Unlike ES or SNPs, SQDs showed high antioxidant activity, showing 100% oxidative free radical scavenging activity in an aqueous medium. In addition, SQDs showed potent antibacterial activity against pathogenic bacteria such as Listeria monocytogenes and Escherichia coli and the fungal strains such as Penicillium chrysogenum and Aspergillus niger. With its aqueous stability and diverse biological activities, SQDs are expected to have tremendous potential as multifunctional components in various biological and related applications such as biomedical engineering and food packaging.