# Multimodal Magnetic-Plasmonic Nanoparticles for Biomedical Applications
## 3 Biomedical Applications of Magnetic-Plasmonic Nanocomposites
### Photothermal Therapy  
Photothermal therapy is an attractive approach to cancer therapy as it is can be a highly selective theranostic approach if nanoparticles (most commonly gold) are delivered to tumour sites. An external NIR laser can be applied, causing the oscillation of the plasmons and resulting in the nanoparticles generating heat, which has been shown to be highly effective in the thermal ablation of cancer  
Figure 7: HR-TEM image of silica coated Ag-Fe\({}_{2}\)O\({}_{3}\) dumbbell synthesised by flame spray pyrolysis Reprinted with permission from [56]. Copyright ACS, 2013.  
cells [61; 62]. Nanoparticles based on gold are the most popular choice for photothermal agents as it is biocompatible enabling a thiol functionalisation which allows the easy attachment of antibodies and other targeting agents. Gold nanoparticles also have an effective light to heat conversion, and small sizes which can be tuned to efficiently absorb NIR light, which penetrates tissue better than other wavelengths of light. Utilising multimodal magnetic plasmonic nanoparticles means that additional functionalities are now possible. Not only can the gold component be used to facilitate photothermal therapy, but the magnetic component can be used to direct the nanoparticles to the tumour site using an external magnet, the gold nanoparticles can be used for dark field imaging and the magnetite could potentially act as an MRI agent during the process. Targeting moieties could be conjugated to the surface of the gold to endow extra cell specificity. The litany of possible applications for these multimodal nanosystems ensures that they are the subject of intensive research efforts. Some magnetic plasmonic nanoparticle systems proposed for use in photothermal therapy are discussed below.  
Gelatin was used as a mediator in the synthesis of Fe\({}_{3}\)O\({}_{4}\)@Au nanoparticles proposed for use as photothermal agents. This is of particular importance as gelatin can be readily degraded by a subclass of metalloproteinases in the body, which further ensures that the system is suitable for use in vivo [49]. The biocompatibility and bioelimination of this system was closely examined, and it was found that in the acidic tumour microenvironment the Fe\({}_{3}\)O\({}_{4}\)@Au nanoparticle assemblies were successfully broken in conjunction with the metalloproteinases. An 808 nm NIR laser was found to cause significant temperature increases, and high doses of the nanoparticle composites were found to be nontoxic, thus illustrating its potential for use in vivo as a photothermal agent. This material also showed potential for uses in MRI, CT and PAT (photoacoustic therapy). A similar chitosan Fe\({}_{3}\)O\({}_{4}\)@Au nanomaterial showed a temperature increase at various concentrations under illumination of a NIR laser [27]. Chitosan is a polysaccharide often used due to its interesting biopharmaceutical properties; for example, it's high mucoadhesion and suitability for facilitating drug delivery, and is the subject of many reviews [63; 64; 65; 66]. The authors also highlighted the multiple uses of the synthesised nanoparticles--not only as photothermal agents but for use in simultaneous MRI and dark field imaging. While these proposed photothermal agents have under-gone heating tests using NIR lasers ex vivo, more comprehensive biocompatibility and in vivo testing on these multimodal magnetic photothermal agents is urgently required.