Advancing silver nanostructures towards antibacterial applications
Creators: name: Li, Vivian Ling Wei
Publisher: name: RMIT University
Year: 2024

description: There is a growing concern on the emergence and re-emergence of drug-resistant pathogens such as multi-resistant bacterial strains. Hence, the development of new antimicrobial compounds or the modification of those that already exist in order to improve antimicrobial activity is a high priority area of research. Silver has a strong antimicrobial potential, which has been exploited since ancient times. Therefore, the exploration of various Ag-based nanomaterials for antimicrobial applications was pursued. A range of Ag nanomaterials were synthesised, characterised and tested for their antibacterial activity. Different shapes of Ag nanoparticles (spheres, cubes and prisms) were synthesised and tested for their antibacterial activity against model strains of Gram negative bacteria <em>Escherichia coli</em> and Gram positive bacteria <em>Staphylococcus albus</em>. All shapes of Ag nanoparticles exhibited antibacterial activity against both bacterial strains. However, Ag nanocubes exhibited the highest antibacterial activity. Tyrosine reduced spherical Ag nanoparticles were selected as the model Ag nanoparticles for further studies. These nanoparticles were employed as antibiotic carriers for antibacterial activity. Traditional antibiotics (ampicillin, penicillin G and polymyxin B) were utilised as functional conjugates to influence antibacterial capabilities on the surface of Ag nanoparticles. The combination of Ag nanoparticles and antibiotics demonstrated synergistic effects at lower concentrations of silver and revealed physical mode of action against bacteria causing cell wall cleavage and lysis. The control of surface functionalisation and composition of nanoparticles via a green approach was also achieved. In particular, three particular phenolic compounds including tyrosine, curcumin and epigallocatechin gallate (EGCG) were utilised as reducing as well as capping agent to synthesise functional Ag nanoparticles. These phenolic compounds incorporate one or multiple phenolic groups which were instigated as organic surface coronas. Ag nanoparticles containing equimolar and various mole ratios of these phenolic compounds were synthesised. These functionalised Ag nanoparticles were tested for their antibacterial activity and a correlation between surface coronas and composition of nanoparticles was studied. The functionalised Ag nanoparticles of various mole ratios all exhibited significant antibacterial activity with physical damage to bacterial cells. Efforts were made to understand the role of surface functionalisation of Ag nanoparticles in dictating the ability of these nanoparticles to differentially interact with bacterial membranes. This led to the mechanistic insight into the antibacterial performance of Ag nanomaterials. Electrochemical and biological techniques were elucidated to understand the proposed mechanism of Ag nanomaterials’ interaction with Gram negative and Gram positive bacterial cells. The interaction of nanoparticles with bacterial membrane proteins was also studied. Membrane protein studies demonstrated that EGCG-Ag nanoparticles exhibited more affinity for <em>E. coli</em> membrane proteins compared to curcumin-Ag nanoparticles. However, for <em>S. albus</em> membrane proteins, curcumin-Ag nanoparticles demonstrated a 30 fold increase in affinity compared to EGCG-Ag nanoparticles. These studies have demonstrated that the antibacterial activity of silver can be controllably enhanced as outlined within this thesis. Several factors can influence the microbial effectiveness of silver nanomaterials, including their shape, surface functionality, as well as their conjugation with well-proven traditional antibacterial compounds.<br>