Biofilm Eradication by Photothermal Near-Infrared Activated Spherical Silver Nanoaggregates

Near-infrared (NIR) light finds many important applications in biomedicine due to the ability of NIR light to penetrate tissue which provides allows for minimally invasive therapeutic and diagnostic devices in-vivo. Typical NIR extinction optimisation strategies rely on plasmonic nanostructures with complex, non-spherical geometries with typically difficult synthetic routes. This provides an immediate limitation for the translation of derived technologies to commercial therapeutic products. The photothermal effect, whereby the absorbed light is converted into heat due to non-radiative decay processes, is often applied to derive biomedical treatments. The temperature increases which can be produced can be harnessed to kill cancerous or infected tissue. Biofilms are a highly resistant form of bacterial infections often found in-vivo on infected implanted medical devices such as catheters. Biofilms form a dense and inhomogeneous structure and their treatment by typical strategies such as antibiotics is challenging. This study presents the single-step aerosol self-assembly of plasmonic fractal-like nanoaggregates by flame spray pyrolysis that are formed of spherical silver nanoparticles with tuneable extinction from the visible to NIR wavelengths. The extinction spectrum of the nanoaggregates was tuned by introducing a SiO₂ spacer during flame aerosol synthesis allowing optimisation of NIR extinction at the readily available laser line at 808 nm. Simulations of the optical properties of such fractal nanoaggregates were performed using the coupled dipole approximation to further elucidate the origin of the optical performance. Aerosol self-assembly of particle films was performed on polydimethylsiloxane coated substrates directly from the flame synthesis and further encased with a top polymer layer, forming plasmonic photothermal nanocomposite films. These photothermal nanocomposites were applied to eradicate established biofilms of clinically-relevant <i>E. coli</i> and <i>S. aureus</i> strains using 808 nm NIR irradiation. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC Grant agreement # 758705). Funding from the Karolinska Institutet Faculty Board, the Swedish Research Council (2016-03471), the Torsten Söderberg Foundation (M87/18), and the Swedish Foundation for Strategic Research (FFL18-0043) is kindly acknowledged.