Investigation of the Physical and Physiochemical Relationships in Nanoparticles and their Attributions to Antimicrobial Activities

Abstract

Antibiotic resistance is one of the greatest threats to global health, as bacteria are becoming increasingly resistant to antibiotics leading to failure of treatments. This PhD project investigated the physical, chemical and physiochemical properties of a total of eleven potential antimicrobial nanomaterials. Some of which were engineered with antiviral function using TesimaTM thermal plasma technology, others were commercially available. The aims of this project are to select 2-3 nanomaterial candidates with the best physiochemical and antimicrobial performances and to exploit them into a single antimicrobial formulation, which would generically deactivate a wide spectrum of pathogens and suitable to be applied as a coating/impregnating agent for the applications in biomedical instrument.

In order to understand how nanoparticles such as Tungsten (W), Copper (Cu), Silver (Ag), Zinc (Zn) and their derivatives lead to the inactivation of Gram-negative bacteria (P. aeruginosa) and Gram-positive bacteria (S. aureus), different instrumentation analyses were used to examine these nanoparticles in powder and aqueous suspension forms. These analyses were then used to reflect on the associated antimicrobial testing results. SEM was used to reveal the shapes and approximate sizes of the nanoparticle powder. Powder X-ray diffraction identified the exact crystal phases in the raw nanopowders. FTIR and Raman spectroscopy were used to trace the presence of organic impurities. ICP-OES was used to quantify the elemental compositions in nanoparticles and the amount of metallic ions saturated in the aqueous media. Nanoparticle tracking analysis (NTA) was used to not measure the concentration and size distribution of nanoparticles in the aqueous media, but also reveal the difference between nanoparticle suspensions and their powder forms. Zetasizer was used to measure their surface charge, which affects their stability. The physiochemical study provided details of the hydrodynamic particle sizes, distributions and stabilities of the selected materials, which had direct reflection on their exposure and static interactions to different microbial cells. Different dispersing methods were investigated, in order to optimize the processing parameters to obtain uniform and stable nanoparticle suspensions, of which are the basic requirements for enhancing fabrication compatibility and antimicrobial efficacy with hybridizable substrates for biomedical devices.

In general, different shapes of nanoparticles were found in both commercial and engineered samples, nevertheless, they all met the physical criteria as nanoparticles. Engineered nanoparticles, especially the Ag nanoparticles, appeared to be less pure when comparing to the commercial ones, however it was found to be the most effective materials against P. aeruginosa. The ion release as the main antimicrobial mechanism performed differently depending on types of nanoparticle forms, pH levels and salt effect in the aqueous media. In this study, the pH values of all samples in water were basically neutral, and the zeta potential values were mostly negative. Overall, the best way to obtain stable nanoparticle suspension is to disperse raw nanoparticle in aqueous medium using high frequency liquid processor (sonicator) for 2 minutes in the present of surface treatment. NTA detected that most of the tested metallic nanoparticles and formulations prepared with using this method showed to exhibit good dispersion, while excessive sonication can cause overheating, particle collisions and contamination. Besides, a processing method based on microwave reactor was developed for poorly dispersed Cu nanoparticle, which enhanced dispersion with increasing the heating temperature. The level of antimicrobial efficacy appeared to be highly dependent on their hydrodynamic sizes and stability. A 30-minute dispersant measurement using NTA showed good antimicrobial nanoparticle agents (Ag, CuAg and AMNP2 suspensions) remained relatively high concentrations and small sizes in the end. The combination of multi-elements played a stronger role in killing bacteria. Therefore, alloy nanoparticles, such as CuAg and CuZn showed the most promising physiochemical and antimicrobial characteristics. Their formulas were calculated (CuAg42 and Cu2.3Zn1) based on the atomic ratio of elements from elemental analysis.