Although antibiotics have saved many lives over the past century, their overuse and misuse have contributed to the emergence of antibiotic-resistant bacteria (ARB), limiting treatment options and increasing mortality rates. One alternative strategy is the use of antibacterial metals, which have been utilized for thousands of years. For example, ancient Egyptians and Greeks used these metals to treat wounds and fabricate plates and cups to reduce bacterial contamination of drinks and foods. Rather than using bulk metals, these metals can be engineered into nanoparticles (NPs), which are approximately 50 times smaller than a bacterial cell and can enhance antibacterial treatment. In particular, inorganic NPs such as iron oxide (IO), silver (Ag), and gold (Au) have demonstrated significant potential in biomedical applications. The intrinsic magnetism of iron oxide nanoparticles (IONPs) provides several benefits for antibacterial treatment, as IONPs can be readily guided, concentrated, and removed from targeted sites using an external magnetic field. Additionally, Au and Ag exhibit antibacterial properties that can effectively inhibit bacterial growth. By combining these metals with IONPs, multifunctional metal-decorated NPs can be developed for both antibacterial treatment and bacterial capture. In this study, silica-coated IONPs (SIONPs) were synthesized to facilitate surface decoration with either Au or Ag. The primary objective of this research was to apply the hard-soft acid-base (HSAB) theory to investigate the affinity of Au or Ag for SIONPs functionalized with one of three silane capping agents containing amine (AP-SIONP), hydroxyl (T-SIONP), or thiol (MP-SIONP) groups. Inductively coupled plasma optical emission spectrophotometry (ICP-OES) was used to quantify and compare the amount of metal loading on the antibacterial decorated SIONPs. Moreover, the optimized Au- and Ag-decorated SIONPs were evaluated against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) to investigate their antibacterial activity and capturing potential. Our findings demonstrated that Ag@MP-SIONPs exhibited superior antibacterial performance, yielding minimum inhibitory concentrations (MICs) of 62.5 and 500 µg/mL for E. coli and S. aureus, respectively. Additionally, Ag@MP-SIONPs achieved a minimum bactericidal concentration (MBC) of 62.5 µg/mL against E. coli but did not reach an MBC against S. aureus under the tested conditions. ICP-OES was further used to study the kinetic release of Au and Ag ions and evaluate its contributions to antibacterial activity. Lastly, the Ag@MP-SIONPs were modified with a photosensitizer (PS) to assess the synergistic effects of photodynamic activation following irradiation with either green or red light-emitting diodes (LEDs). Preliminary data suggest that PS incorporation increases antibacterial activity through controlled Ag ion release at concentration up to four times lower than the MIC for Ag@MP-SIONPs. However, antibacterial efficacy was strongly influence by the irradiation wavelength. Overall, this systematic approach establishes a framework for the development of optimized antibacterial metal-decorated SIONP through the application of HSAB theory and light-triggered activation. The results demonstrate that the Ag and PS incorporation produces a promising multifunctional nanoplatform with both antibacterial activity and magnetic bacterial capturing capabilities, offering a valuable foundation for future research in antimicrobial nanomedicine.
