Synthesis of silver doped titanium dioxide nanocomposites using tea extract from Aspalathus linearis and evaluation of their antibacterial effects.
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Despite the wide success of antimicrobial agents against waterborne pathogens, waterborne disease continues to pose a threat to both mankind and animals. A major concern is that certain bacteria have developed resistance to antimicrobial agents, as a result of their overuse. Silver (Ag) nanoparticles are widely used for antibacterial purposes such as medical dressings. However, they are highly toxic to human cells. Hence, there is a great interest in developing next generation antibacterial nanoparticles that are as effective as Ag nanoparticles for antibacterial functions, while having less toxicity to human cells. Several methods can be used to generate these antimicrobial nanoparticles, one of which is green nanotechnology. Green nanotechnology uses natural plants such as tea to synthesise nanoparticles rather than chemicals, thus reduce human and animal harm and improve sustainability of antibacterial agents. Silver-titanium nano-composites (Ag-TiO2 NCs) were synthesised with the hydrothermal method using a tea extract from Aspalathus linearis (Rooibos, RB), and distilled water in the presence of nitrogen. The resulting structures were characterised with high resolution transmission electron microscopy (HRTEM), energy-dispersive spectroscopy (EDS) analysis X-Ray Diffraction (XRD) and Thermogravimetric Analysis (TGA). The antibacterial characteristics of these new NCs were evaluated against 3 bacteria: Bacillus cereus, Cupriavidus metallidurans, and Escherichia coli. The optimum processing conditions to produce 6-nm spherical NPs included maintaining the temperature at 90 °C, the pH at 4.35, and using RB extract at a concentration of 2 mg/mL. The size of silver NPs was reduced in acidic conditions, agglomerated in neutral conditions, and highly reduced in alkaline conditions. Increasing the pH decreased the particle size and narrowed the particle size distribution. Gram-positive B. cereus showed slight resistance or tolerance to the Ag-TiO2 nanocomposite compared to the gram-negative bacteria E. coli and C. metallidurans. The treatment concentration required for total inhibition of E. coli and C. metallidurans growth was 100 mg/mL. Supported silver nanoparticles has shown to be a suitable way to obtain highly dispersed silver over higher surface area. This approach allowed Ag-TiO2 nanocomposite to be an efficient bactericide, with less silver amount employed.