How similar is the antibacterial activity of silver nanoparticles coated with different capping agents?

Silver nanoparticles (AgNPs) represent one of the most commercialised metal nanomaterials, with an extensive number of applications that span from antimicrobial products to electronics. Bare AgNPs are very susceptible to aggregation, and capping agents are required for their protection and stabilisation. The capping agents can endow new characteristics which can either improve or deteriorate AgNPs (bio)activity. In the present work, five different capping agents were studied as stabilizing agents for AgNPs: trisodium citrate (citrate), polyvinylpyrrolidone (PVP), dextran (Dex), diethylaminoethyl-dextran (Dex^DEAE) and carboxymethyl-dextran (Dex^CM). The properties of the AgNPs were studied using a set of methods, including transmission electron microscopy, X-ray diffraction, thermogravimetric analysis and ultraviolet-visible and infrared spectroscopy. Coated and bare AgNPs were also tested against Escherichia coli, methicillin-resistance Staphylococcus aureus and Pseudomonas aeruginosa to analyse their capacity to suppress bacterial growth and eradicate biofilms of clinically relevant bacteria. The results showed that all the capping agents endow long-term stability for the AgNPs in water; however, when the AgNPs are in bacterial culture media, their stability is highly dependent on the capping agent properties due to the presence of electrolytes and charged macromolecules such as proteins. The results also showed that the capping agents have a substantial impact on the antibacterial activity of the AgNPs. The AgNPs coated with the Dex and Dex^CM were the most effective against the three strains, due to their better stability which resulted in the release of more silver ions, better interactions with the bacteria and diffusion into the biofilms. It is hypothesized that the antibacterial activity of capped AgNPs is governed by a balance between the AgNPs stability and their ability to release silver ions. Strong adsorption of capping agents like PVP on the AgNPs endows higher colloidal stability in culture media; however, it can decrease the rate of Ag⁺ release from the AgNPs and reduce the antibacterial performance. Overall, this work presents a comparative study between different capping agents on the properties and antibacterial activity of AgNPs, highlighting the importance of the capping agent in their stability and bioactivity.