## 4 Biological Applications
### Therapeutic Applications
#### Drug Delivery  
Delivering therapeutic compounds to target sites has been a major obstacle in many applications. A solution to the problem is cell-specific targeting treatments, which can be achieved by attaching drugs onto individually designed carriers [195]. Nanoparticles have been used as carriers in targeted drug delivery to deliver therapeutic agents at the disease site, improve the uptake of poorly soluble drugs, and improve drug bioavailability [196]. Furthermore, the small sizes and unique physicochemical and biological properties render nanoparticles to be promising candidates as drug carriers [195]. Gold-based bimetallic nanoparticles are particularly promising candidates as drug carriers due to their small size, biocompatibility, and facile surface modification [197]. For example, [PERSON] et al. altered the plasmonic properties of porous-silica-coated AuAg alloy nanoparticles with built-in hotspots where the electromagnetic field was sufficiently strong to amplify the Raman scattering of target molecules [198]. These types of materials are suitable for drug delivery and photothermal therapeutics owing to their size in the range of nanometers, porous structure of both the core and shell, and the unique optical activity in the NIR region [198]. Other examples of modified gold-based nanoparticles for drug delivery include the cationic BSA embedded AuAg bimetallic nanocluster system developed by [PERSON]. In their study, the authors used BSA to synthesize the unique architecture to serve as a carrier for the pDNA encoding of the enzyme _E. coli cytosine deaminase uracil phosphoribosyltransferaseense_ (CD-UPRT) [155]. The nanoclusters were then converted into composite nanoparticles and bound to the negatively charged pDNA. Successful uptake of the composite nanoparticles was achieved by the cancer cells. To track the delivery of pDNA into the cells, the luminescence of the bimetallic nanoclusters was used. Separately, AuNi nanorods have been designed as selectively functionalized carriers for gene delivery [83]. The nanorods were double functionalized with transferrin on the Au segment and DNA plasmids on the Ni segment. The dual-functionalized nanorods demonstrated their potential use as carriers for gene-delivery into human cells for targeted protein expression [83]. In addition to exploring the drug-delivery efficacy, recent nanoparticle-based research has also focused on developing additional capabilities that are beneficial to the patients receiving the treatments, such as reducing the systemic toxicity. [PERSON] et al., for example, synthesized multifunctional Au@Pt nanoparticles that allow for the loading of the anticancer drug, doxorubicin (DOX) [95]. The DOX-loaded nanoparticles serve as a chemo-photothermal combination therapy platform with oxidative stress reduction (Figure 12). In addition to gold-based bimetallic nanoparticles, [PERSON] et al. demonstrated that mesoporous silica-coated Pd@Ag nanoparticles (Pd@Ag@mSiO\({}_{2}\)) can be used as a nanoplform for photodynamic therapy (PDT) in drug delivery [190]. These nanocomposites were utilized as nanocarriers for the photosensitizer Ce6 in photothermally enhanced photodynamic therapy. Magnetically guided capsules using an external magnetic field have also gained attention as a potential method for controlled drug delivery. The use of pH- and temperature-sensitive polymeric materials as coatings on magnetic nanoparticles have proven to be good candidates for controlled drug release [19; 80; 107; 176]. [PERSON] et al. optimized polyelectrolyte embedded Co@Au nanoparticles for magnetically controlled permeability [80]. The permeability of the nanoparticles was achieved by applying an appropriate frequency to cause the oscillating magnetic field to \"twist and shake\" the nanoparticles. Another example of a potential magnetically guided drug-delivery system is the polycation hybrid FePt nanoparticle-coated capsules developed by [PERSON] et al. [107]. [PERSON] et al.