## 4 Biological Applications
### Diagnostic Applications
#### 4.1.2 Bioimaging
The authors of the study used the polyol method to synthesize 3, 6, and 12 nm FePt nanoparticles and concluded that the 12 nm FePt nanoparticles offered the highest amount of serum concentration as well as provided long-term circulation in both CT and MRI applications. Surface coatings on bimetallic nanoparticles open new avenues for the creation of nanoparticles that offer higher stability, lower toxicity, and access toward a variety of functional groups [111; 174]. Using chemical vapor deposition, [PERSON] et al. synthesized FeCo nanoparticles having a graphitic shell [111]. The particles exhibited high r\({}_{1}\) and r\({}_{2}\) relaxivities and high optical absorbances in the NIR region. Preliminary in vivo experiments showed that these nanoparticles offer high-contrast in the MRI of the targeted tissue and NIR laser-triggered tumor destruction. [PERSON] et al. showed that silica-coated FePt nanoparticles functionalized with cysteamine provided a six-fold increase in the relaxivity, r\({}_{2}\), and lower cytotoxicity than the commercially offered colloidal suspension of superparamagnetic iron oxide particles known as Feridex [177]. [PERSON] et al. designed and synthesized 3 nm FePt@Fe\({}_{2}\)O\({}_{3}\) yolk-shell nanoparticles through the Kirkendall effect [34]. The iron oxide surface of the nanoparticle allows for slow oxidation; thus, the yolk-shell nanoparticles were found to exhibit a strong MR contrast enhancement (effective relaxivity, r\({}_{2}\)\({}^{*}\) = 3.462 (\(\mathrm{\SIUnitSymbolMicro g}\)/mL)\({}^{-1}\) s\({}^{-1}\)). In addition, the low cytotoxicity of the FePt@Fe\({}_{2}\)O\({}_{3}\) nanoparticles makes them a potential candidate for use in drug delivery. Separately, [PERSON] et al., demonstrated the use of infrared-active FePt nanoparticles as a quadruple-contrast in multiscale molecular imaging in CT, MRI, photoacoustic imaging, and high-order multiphoton luminescence microscopy [102]. The FePt nanoparticles gave a higher photoacoustic response than gold nanorods and allowed for the visualization of in vivo targeting with four techniques at the same time, making them flexible and versatile imaging contrasts. Magneto-plasmonic materials have also been employed for use as MRI, CT, and SERS imaging [78; 79; 84]. For example, [PERSON] et al. synthesized AuFe nanoalloys, a magneto-plasmonic material conjugated with thiolated PEG or dithiolated sulforhodamine [78; 79]. The synthesized AuFe nanoparticles showed simultaneous MRI/CT imaging and in situ SERS imaging with amplification of Raman signals. Additionally, surface functionalization of the AuFe particles was found to render them biocompatible. Separate studies by [PERSON] et al. have led to the preparation of polyethyleneimine-coated AuCu nanoparticles with hollow structures as contrast enhancers in both T\({}_{1}\) and T\({}_{2}\) imaging, with T\({}_{2}\) imaging done at low doses [84]. Further, the FePt-Au heterodimer metallic nanoparticles have been utilized in multimodal biological detection [178]. The Au segment offers high water solubility  
Figure 11: Schematic illustrations of (**a**) the surface functionalization of Anti-Her2 antibodies conjugated onto cysteamine capped FePt particles, and (**b**) in vitro and in vivo MRI and CT imaging using FePt nanoparticles as dual contrast agents. Reproduced with permission from reference [106]. Copyright 2010 American Chemical Society. and biocompatibility after functionalization with dihydrolipoic acid through gold-sulfur bonds. The heterodimer nanoparticles were used in an avidin-biotin biochip assay and in MRI applications.