## 2 Optical and Magnetic Properties of Bimetallic Nanoparticles for Biological Applications. The unique optical and magnetic properties of bimetallic nanoparticles render them attractive for use in biological applications. One of the most interesting optical properties of noble metal nanoparticles, such as gold, silver, and copper, is the localized surface plasmon resonance  
Figure 1: The synthesis and properties of bimetallic nanoparticles for biological applications. (LSPR) [27; 28]. Surface plasmon resonance occurs when the frequency of incident light matches the collective oscillation of surface electrons in the conduction band of the metal, as illustrated in Figure 2. Moreover, the LSPR extinction band can be tuned by varying the size, shape, and composition of the nanoparticles [29]. Furthermore, noble metal bimetallic nanoparticles exhibit strong plasmon resonances that can be controlled by varying the composition of the metals in the nanostructures [30]. Over the past several years, there has been increasing interest in utilizing the LSPR of nanoparticles for biomedical applications such as biosensors, drug-delivery devices, cell imaging, diagnostics, and therapeutics [31; 32; 33; 34]. Specifically, surface enhanced Raman scattering (SERS) has been used in biological applications due to SERS being highly effective in the near-infrared (NIR) region, where the signal from Raman scattering is greater than autofluorescence from surrounding tissue [35]. By varying the composition and structure of bimetallic nanoparticles, the optical resonances can be tuned over a broad range of wavelengths (e.g., UV-vis to NIR). Perhaps most important are optical resonances in the NIR region, which includes wavelengths between 800 and 1200 nm, and is referred to as the \"Tissue Transparency Window\", where such light can penetrate both water and human tissue [36]. Similarly, the magnetic component of bimetallic nanoparticles provides them with unique nanomagnetism that allows for control of the motion of the particles in addition to offering magnetic spin behavior, which is critical in magnetic detection techniques, imaging, and thermal treatment methods [37]. Magnetic nanoparticles are typically characterized by the following physical characteristics: magnetization (M), coercivity (H\({}_{\text{C}}\)), and magnetocrystalline anisotropy constant (K). Figure 3 illustrates the fundamental magnetic properties of magnetic nanoparticles: saturation magnetization (M\({}_{\text{s}}\)), remanent magnetization (M\({}_{\text{r}}\)), and the coercivity (H\({}_{\text{C}}\)). When used in biological applications, the ability to manipulate the magnetic properties, such as saturation magnetization (M\({}_{\text{s}}\)), relaxation, and the size of the magnetic nanoparticles are critical parameters [37]. Saturation magnetization is the maximum magnetization value exhibited by the material, where the magnetic moments of the particles align with the direction of an applied external field. Relaxation can be categorized as Neel relaxation and Brownian relaxation, where the former arises from spin-spin relaxation within the particles and the latter from the mechanical rotation of the particles [37]. Neel relaxation dominates when the size of the particle is less than a single domain size and when the size of the nanoparticle is small; it is the critical factor for magnetic hyperthermia applications. On the other hand, Brownian relaxation dominates for larger particles and sizes greater than a single domain size; moreover, it is the critical factor in thermal applications [37; 38]. Figure 2: Localized surface plasmon oscillations in noble metal nanoparticles. In general, higher saturation magnetization gives higher relaxation rates for magnetic resonance imaging (MRI), higher heat emission rates for magnetic hyperthermia treatments, and stronger translational attractive forces for magnetic separation [37; 39].