Platinum-doped and calcium phosphate-encapsulated silver nanoparticles: synthesis, development, and characterization of antimicrobial properties

Chronic oral infection is a critical public health issue that can lead to several life-threatening systemic diseases, e.g., endocarditis. Oral infections include dental caries, periodontal disease, and infections around the dental implant, a.k.a. peri-implantitis. Several non-surgical approaches have been proposed to deal with peri-implantitis; unfortunately, it cannot be effectively treated with local or systematic antibiotic drugs. As a result, silver nanoparticles have been proposed as an alternative agent to combat oral infections, including peri-implantitis.

Silver nanoparticles are nano-sized particles of silver with well-established, wide spectrum antimicrobial properties. However, although silver nanoparticles are being used in protective clothing, food storage, and hospital settings, long-term high-dose usage may negatively affect human health. To overcome these issues, silver nanoparticles can be altered with small amounts of platinum, aiming that a combination of silver-platinum nanoparticles will enhance their antimicrobial activity. Another option pursued in this work is combining calcium phosphate and silver nanoparticles, with the expectation that silver nanoparticles will be preserved at the desired sites under physiological conditions but activated when triggered by acidic conditions associated with infection.

In this dissertation, the background, history, and applications of metallic nanoparticles, e.g., silver and platinum are reviewed. The production of metallic nanoparticles is discussed, including the top-down and bottom-up approaches. It was shown that silver nanoparticles exhibited antimicrobial properties. This was expected through 3 different processes, including direct interaction with bacterial cell membranes, generation of reactive oxygen species (highly reactive chemical derived from oxygen), and the interruption of bacterial functions by silver ions, eventually leading to cell death. In addition, the development of silver platinum nanoparticles is also investigated. Combining a small amount of platinum with silver nanoparticles led to an enhanced antimicrobial effect for silver nanoparticles. Furthermore, fundamental properties of calcium phosphate are reviewed, including their compositions, crystallographic structures, chemical properties, and their applications. Owing to its distinctive properties, calcium phosphate could be considered as a promising candidate for drug delivery system.

The antimicrobial activity of silver-platinum nanoparticles was investigated. With small additions of platinum to silver nanoparticles, mixed structures of spherical, oval particles and ribbon-like structures were observed. The amount of ionic silver released increased with the level of platinum doping. As a result, silver-platinum nanoparticles exhibited greater antimicrobial activity against pathogenic bacteria, compared to silver or platinum nanoparticles.

In the last part of this work, silver nanoparticles were enclosed in calcium phosphate, forming a core-shell structure. These features were refined using bovine serum albumin (extracted proteins) and polyethylene glycol (polymer derivative). It was shown that the amount of silver ions released from core-shell structures increased when pH and calcium phosphate shell thickness decreased. In addition, these core-shell nanostructures were successfully coated on titanium surfaces and exhibited an antimicrobial activity against both S. aureus and P. aeruginosa, specifically under acidic environments.

In conclusion, we developed an alternative antimicrobial agent based on doped or encapsulated silver nanoparticles. Silver nanoparticles doped with platinum led to a larger release of silver ions, resulting in enhanced antimicrobial efficacy. Furthermore, silver nanoparticles encapsulated in calcium phosphate shells showed a pH-triggered release of silver at acidic pH, exhibiting antimicrobial activities upon exposure to infectious-acidosis microorganisms. With these unique properties, our silver-based composite particles could be considered as a potential of coating agents on dental titanium implant, preventing periimplantitis. Furthermore, we anticipated that our research could be widely expanded upon to further development and characterized the silver-based composite particles, improving their antimicrobial properties as well as investigating their biological properties for further use clinically as a dental implant.