Antimicrobial properties of Ag zeolites and Ag zeolite containing polymers

Belkhair, Sama Salem (2016) Antimicrobial properties of Ag zeolites and Ag zeolite containing polymers. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Ag zeolites have been studied in this thesis for antibacterial applications in powder form or in composite materials. The influence of the textural characteristics (crystal size and mesoporosity) of silver-exchanged FAU- and BEA-type zeolites on their antimicrobial efficacy was studied. Two pairs of zeolite X and zeolite beta with different sizes as well as a mesoporous zeolite beta were tested against Escherichia coli (E. coli) and Candida albicans (C. albicans). Reduction of crystal size resulted in a slight decrease in the killing efficacy against both microorganisms. Zeolite beta showed higher activity than zeolite X despite the lower Ag content in that sample, which was attributed to the higher concentration of silver released from zeolite beta samples to the medium. Introduction of mesoporosity had a beneficial effect on the antimicrobial efficacy. Cytotoxicity measurements using peripheral blood mononuclear cells indicated that Ag zeolite X was more toxic compared to Ag beta, particularly the nanosized sample. The influence of the form of silver (metallic or ionic) on the antimicrobial efficacy was determined using Ag-loaded EMT zeolites. The killing efficacy of pure EMT, Ag+-EMT and Ag0-EMT against E. coli was studied semi-quantitatively. The antibacterial activity increased with increasing Ag content for both types of samples (Ag+-EMT and Ag0-EMT). The Ag0-EMT samples show slightly enhanced antimicrobial efficacy compared to that of Ag+-EMT, however, the differences are not substantial and the preparation of Ag nanoparticles is not viable considering the complexity of preparation steps. A multidisciplinary approach has been applied to the preparation of antibacterial Ag zeolite/silicone elastomer composites aimed at products that satisfy a range of requirements, namely good mechanical properties after zeolite incorporation and strongly antibacterial. The composites showed strong efficacy against E. coli and Staphylococcus epidermidis (S. epidermidis). Organic functionalisation of the zeolite with organo-silanes prior blending with the matrix usefully improved composite mechanical properties and reduced colour development in Ag zeolite containing silicone elastomers. The same approach was applied for the preparation of dental acrylic resins. Antibacterial dental resin containing Ag zeolites at loadings of 0.2, 0.4, 0.7 and 2 wt.% were prepared. The composites showed strong efficacy against E. coli and no viable E. coli were detected after 5 hours of incubation. The presence of Ag zeolites within the acrylic resin resulted in resin discolouration, which was more pronounced at high Ag zeolite loadings. A sustained Ag release over long periods of time (54 days) was observed with a greater Ag release in artificial saliva compared to that in distilled water. Dental resins loaded with Ag-free zeolites followed by post-fabrication insertion of Ag by ion-exchange were prepared to overcome discolouration. The materials prepared using this approach had a similar colour to that of the original polymer as determined by the colour measurements. These composites showed strong efficacy against E. coli and no viable E. coli cells were detected after 5 hours of incubation.