Gold nanoparticles (Au NPs) are used as delivery vehicles for cancer treatments due to their non-toxicity and biocompatibility; and controlling the size of Au NPs is critical for these applications. On the other hand, silver nanoparticles (Ag NPs) are extremely toxic to microorganisms such as bacteria; therefore, they are widely used as antibacterial agents in healthcare applications and consumer goods. Previous studies demonstrated, mainly by chemical approaches, that the bactericidal efficacy of Ag NPs was size dependent where smaller size presented higher antibacterial activity; in addition, the stability of Ag NPs might also affect the durability of the bactericidal activity significantly. Therefore, this project aims to produce ultra-fine Au NP and Ag NP colloids by laser ablation of their metal plates in various liquid solutions, and study the stability of these nanoparticles in different storage conditions, and also investigate other factors, including surface charge of Ag NPs on their antibacterial activity. Au NPs were produced by laser ablation of a pure gold plate in four liquids: deionised water, Cetrimonium bromide (CTAB), Sodium dodecyl sulphate (SDS) and Polyvinylpyrrolidone (PVP), with different synthesized conditions by varying concentrations and laser-operating parameters. The stability of the produced nanoparticles was studied over a period of 30 days. The results showed that Au NPs in the range of 2 to 4.5 nm were produced by the one-step laser ablation technique in the PVP solution whilst Au NPs with a range from 2 to 10 nm were produced in the SDS solution; both were highly dispersed and more stable compared with those in deionised water and CTAB. Ag NPs were synthesized by laser ablation of a pure Ag plate in deionised water, sodium borohydride (NaBH4) and PVP solutions with different concentrations. For NaBH4 and PVP solutions, 2-10 nm sized Ag NPs were obtained and showed a higher antibacterial activity against Escherichia coli (E. coli) bacteria than those of a larger size of 10-100 nm in deionised water, and decreased the minimum inhibition concentration (MIC) from 20 mg/L for bigger-sized Ag NPs to 10 mg/L. Comparative study of the Ag NPs produced in deionised water and PVP solution was also carried out, in terms of the particle size, the stability of the NPs and the durability of anti-bacterial activity over a period of six months under different storage conditions. The results showed that the Ag NPs generated in deionised water were in the range of 10 to 100 nm while Ag NPs in the range of 2 to 10 nm were produced in PVP solutions, which exhibited a higher antibacterial efficacy with a MIC of 10 mg/L. It was also shown that all the samples tested in this work including those from laser ablation in deionised water and PVP solution maintained their antibacterial activities after six months regardless of their storage conditions, although these samples presented different in stability, depending on the storage conditions. Studies on the effects of surface charges of Ag NPs (negative charge in SDS, positive charge in CTAB plus PVP and neutral in PVP solution) on antibacterial activities against Gram-negative E. coli, Pseudomonas aeruginosa (P. aeruginosa) and Gram-positive Staphylococcus aureus (S. aureus) were carried out. For Gram-negative bacteria, both Ag NPs with a positive charge and those with a neutral charge showed better antibacterial activity than those with a negative charge. For Gram-positive bacteria, the solutions of CTAB and SDS exhibited a major contribution towards antibacterial activity. Interestingly, it was found that SDS surfactant enhanced the antibacterial activity of Ag NPs against Gram-positive bacteria by eight folds. Finally, Au@Ag core@shell NPs in the range of 2 to 10 nm were fabricated by a sequential two-step laser ablation method in a PVP solution. The stability of these Au@Ag core@shell NPs was investigated over three months and the results showed the nanoparticles had a high stability over this period of time. The efficacy of the antibacterial activity of the Au@Ag core@shell NPs was higher than that of Ag NPs of 2-10 nm against E. coli bacteria.
|Date of Award||1 Aug 2019|
- The University of Manchester
|Supervisor||Lin Li (Supervisor) & Zhu Liu (Supervisor)|
- high purity