Plasmonic nanoparticles play an important role in biomedical applications today as they can serve as superior, optically stable bioimaging agents, be employed in biosensor devices for the early diagnosis of diseases, and exhibit promising results in vivo as therapeutic agents. For several bioapplications, however, nanoparticles that express more than one functionality are often advantageous. This has led to the synthesis of multifunctional plasmonic nanostructures that combine the attractive plasmonic properties with other functionalities such as magnetism, photoluminescence, dispersibility in aqueous solutions, and resistance to degradation. Such multifunctional nanoparticles can be detected by multiple imaging techniques including magnetic resonance imaging and fluorescence microscopy. Furthermore, their performance in diagnostics and therapy (theranostics) can be significantly improved facilitating the early detection of diseases. The possibility to tune the desired properties enables such hybrid nanoparticles to be employed in vivo as therapeutic agents that can actively target tumor sites and destroy them by external means. This makes such bionanoprobes ideal candidates for non- or minimally invasive cancer treatments. Through rational design and engineering, 'smart' multifunctional nanomaterials with unprecedented properties can be made that will lead the nano-based theranostics in the future.
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