## 5 Summary and Perspectives  
As described in this review, there are various kinds of structures bimetallic nanoparticles can adopt depending on the atomic ordering of the two metals. The structures can be broadly classified as mixed structures and segregated structures. Synthetic methods can be divided into simultaneous (e.g., co-reduction, sonochemical synthesis, thermal decomposition, microwave irradiation, and radiolytic synthesis) and successive methods (e.g., seed-mediated growth). Bimetallic nanoparticles prepared via these methods offer advances in a variety of biological applications that range from diagnostics (e.g., sensing and imaging) to therapeutics (e.g., tumor ablation and drug delivery). Bimetallic nanoparticles with plasmonic properties, such as Au/Ag/Cu composites, are typically employed as plasmonic biosensors due to the sensitivity of their LSPR to the surrounding medium. Most imaging techniques, such as magnetic resonance imaging (MRI), computed tomography (CT), and dual modal imaging, require the use of a contrast agent. Due to the magnetic nature of Fe, Co, or Ni bimetallic nanoparticles, these types of materials serve as good contrast agents in imaging techniques. Additionally, magneto-plasmonic materials, such as Au-Fe nanoparticles, have useful in multimodal biological detection and imaging diagnostic techniques due to their combined optical and magnetic properties.  
Both plasmonic and magnetic bimetallic nanoparticles have been used in therapeutic applications, including thermal treatments (hyperthermia) and drug delivery. By taking advantage of the tunable LSPR of plasmonic nanoparticles (e.g., Au-based bimetallics) or the relaxation of magnetic nanoparticles (e.g., Fe-based bimetallics), specific cells can be targeted for treatment without damaging the surrounding tissues. Additionally, Au-based nanoparticles have been proven to be excellent candidates for drug delivery due to their biocompatibility. Moreover, Fe-based materials coupled with pH- or temperature-sensitive coatings can be magnetically controlled for targeted drug release. The optical and magnetic properties inherent in bimetallic nanoparticles render these types of materials promising candidates for improving the efficiency of diagnostic and therapeutic applications. As described in this review, various kinds of bimetallic nanoparticles have been used in diagnostic applications such as imaging and sensing. However, there are comparatively fewer examples in the literature for therapeutic applications, especially for use in the treatment of cancer. This paucity coupled with emerging synthetic strategies in the synthesis of bimetallic nanoparticles and technological advances will hopefully fill the void. Further, the use of multi-component nanoparticles comprised of three or more metals have also emerged as materials that can enhance the optical and magnetic properties discussed herein. Undoubtedly, we will continue to see creative advances in the synthesis and applications of these unique dual-component and multi-component nanomaterials.  
[PERSON], [PERSON]-T.C., [PERSON], M.D.M., and T.R.L. discussed, commented, and wrote the manuscript.  
We thank the Air Force Office of Scientific Research (AFOSR FA9550-18-1-0094), the Robert A. Welch Foundation (E-1320), and the Texas Center for Superconductivity for generously supporting this research.  
The authors declare no competing financial interest.