Photothermal antibacterial biomaterials utilising gold nanorods and near-infrared light

Licentiate thesis, 2024

Biomaterials serve an integral role in modern medicine, providing essential functions for therapeutic, prosthetic, and diagnostic purposes. However, once introduced into a biological setting, biomaterials are prone to bacterial colonisation, resulting in hard-to-treat infections with severe clinical consequences. Biomaterial-associated infections (BAIs) are commonly caused by biofilm-forming bacteria. The reduced sensitivity of biofilm-forming bacteria to antibiotics, along with the increased occurrence of antibiotic resistant pathogens, significantly limits the effectiveness of conventional antibacterial strategies. As our growing life expectancy is accompanied by an increased use of biomaterials, there is a pressing need for novel strategies that can combat the infections associated with these materials. A promising approach is the development of antibacterial biomaterial modifications. In this thesis, a photothermal antibacterial modification strategy using surface-immobilised gold nanorods and near-infrared (NIR) light has been investigated.

Procedures were developed for immobilisation of gold nanorods on glass and titanium substrates. Material characterisation was conducted using UV-Vis spectroscopy, electron microscopy, and X-ray photoelectron spectroscopy. In situ X-ray diffraction studies were performed to evaluate the photothermal properties of the gold nanorods on glass, and the in situ datasets were corroborated with scanning electron microscopy and UV-Vis spectroscopy characterisation. The findings revealed that the supported gold nanorods displayed a linear temperature increase with NIR laser power until an onset of morphological changes around 120 °C, and the slope of the temperature increase demonstrated a dependence on the surface coverage of gold nanorods. The in vitro antibacterial activity of the gold nanorods on glass and titanium upon illumination with NIR light was evaluated against Staphylococcus aureus. On titanium, the antibacterial activity was attributed to NIR light absorption of the titanium leading to heating of the substrate, with no evident effect from plasmonic heating of the gold nanorods. In contrast, on glass a significant NIR light-intensity dependent antimicrobial activity from plasmonic heating of the gold nanorods was observed. The findings provide insights into the photothermal behaviour of surface-immobilised gold nanorods and highlight the important role of the support material for the antibacterial activity of the systems. Altogether, the results are of relevance for advancing the design of antibacterial biomaterial modifications using supported gold nanorods and near-infrared light.