Silver (Ag) and Ag-based compounds have been used as broad-spectrum antimicrobial agents for centuries for their great antimicrobial activities against many different strains of bacteria, fungi, and viruses. Though the development of Ag-based antimicrobial agents have made a great advancement in the past decades, many challenges remain to be addressed: 1) constructing sustained-release delivery system of antimicrobials to ensure prolonged antimicrobial efficacy; 2) reducing antimicrobial agents’ toxicity to human; 3) minimizing the risk of causing potential environmental pollution by incompletely degraded antimicrobial agents residues. To meet these challenges, two novel Ag-based antimicrobial agents have been developed: Ag nanoclusters (AgNCs) and alkynyl Ag modified CS (Ag-CS). Further studies were conducted to investigate their synthesis routes, chemical and physical properties, antimicrobial efficiencies, cytotoxicity, and potential applications.

AgNCs with particle size of 2-4 nm were successfully synthesized and embedded into zein films to form a novel antimicrobial coating material. AgNCs endowed bright fluorescence with emissions between 620-650 nm and the synthesis conditions were optimized by adjusting the irradiation time, light sources, and concentrations of synthesis materials. The antimicrobial efficacy and cytotoxicity of AgNCs were then systematically evaluated and compared with those of AgNO3 as well as larger Ag nanoparticles (AgNPs) with diameters of 10 nm (AgNP10) and 60 nm (AgNP60). AgNCs presented a comparable dose-dependent antimicrobial efficacy to AgNO3, but had a significantly lower toxicity towards human cells than that of AgNO3. Further, AgNCs presented a much greater antimicrobial capacity than AgNP10 and AgNP60, which indicated that the administration dose of AgNCs for antimicrobial applications could be dramatically reduced compared to that of AgNPs.

Ag-CS were prepared by modifying the alkynyl Ag group on the backbone of chitosan (CS) through chemical reactions. Experiment results indicated that Ag-CS exhibited a sustained release of Ag for over 5 days whereas Ag acetate (AgOAc) infused CS released 91.182.07 % of its Ag in 4 h. Ag-CS was proved to have potent bacteria inhibition and inactivation efficiencies that were stronger than AgOAc at the same equivalent Ag concentration. By coating on shrimps and strawberries, Ag-CS greatly extent their shelf lives under cold storage conditions.