## 4 Biological Applications
### Diagnostic Applications
#### 4.1.1 Biosensors  
A biosensor is a device that provides qualitative and quantitative information about the chemical composition of the molecules in which the sensor is situated [156]. Plasmonic bimetallic nanoparticles have been used for biomolecular analyte detection based on their unique optical properties, including LSPR (_vide supra_). An additional factor affecting the wavelength at which the LSPR of plasmonic nanoparticles undergoes excitation is the refractive index of the medium surrounding the nanoparticles [157]. A medium with a relatively high refractive index, such as a buffer solution or biomolecules, will shift the LSPR to longer wavelengths [158]. Since most biomolecules have a higher refractive index than the buffer solution, the LSPR of the plasmonic nanoparticles conjugated with biomolecules will experience a red shift [159]. Another factor affecting the LSPR of nanoparticles is the degree of particle aggregation [160]. Specifically, certain biological molecules such as peptide nucleic acids (PNA), DNA aptamers, and protein antigens, can induce nanoparticle aggregation; a shift in the extinction peaks of the nanoparticles will then indicate the presence of the targeted molecules [161; 162]. For example, [PERSON] et al. used Au@Ag nanorods for the colorimetric sensing of the biological molecule alpha-fetoprotein, a biomarker for hepatocellular carcinoma in human serum [163]. Surface-enhanced Raman scattering (SERS) coupled with plasmonic materials has also been used for the detection of biological species due to the high selectivity and sensitivity of the technique; thorough reviews of the use of SERS in sensing applications can be found in the literature [164; 35; 165]. Recently, [PERSON] et al. synthesized a label-free biosensing cellulose strip sensor with SERS-encoded bimetallic silver-gold core-shell nanoparticles (Ag@Au NPs) [76]. The biosensing material was characterized with Raman spectroscopy to evaluate the feasibility of detecting representative prohibited drugs with hazardous substances taken from patients including aniline, sodium azide, and malachite green. Recently, [PERSON] et al. proposed an ultrasensitive biosensor based on gold-silver core-shell SERS nanotags and photonic crystal beads for the detection of proteins over a wide range of concentrations [166]. This biosensor showed a limit of detection of 672 fg mL\({}^{-1}\) with a linear dynamic detection from 10 pg mL\({}^{-1}\) to 10 ug mL\({}^{-1}\), showing improved analytical performance for the Au@Ag NPs compared to that reported for simple Au NP-based nanotags. Magnetic nanoparticles have also been used as biosensing materials [113; 167]. Compared to their single-component oxides, bimetallic nanoparticles offer higher magnetic moments, which give rise to sensing materials with lower detection limits [109; 120]. Magnetic bimetallic nanoparticles have been successfully applied as sensing agents in a variety of magnetic-based detection techniques. For example, sensing based on magnetic resistant biosensors utilizes the local magnetic dipole field produced by the change in the magnetoresistance signal of the magnetic nanoparticles. The higher magnetic moment of bimetallic nanoparticles offers stronger dipole fields and thus greater sensing potential [109]. Similarly, bimetallic nanoparticles have been used to improve the sensitivity of giant magnetoresistance (GMR) sensors used for biosensing applications through the modification of the particle size and shape [112; 113; 168]. By reducing the size of the particle or changing the shape from spherical to cubic, the binding density of the particle and the magnetic response both increase [169; 170]. For example, 12 nm FeCo nanocubes have been used to quantify interleukin-6 (IL-6) and endoglin from unprocessed body fluids [112; 113; 168]. In addition, magnetic nanoparticles offer unique magnetic separation methods for multimodal detection, allowing for the capture and detection of target biomolecules at low concentrations [170; 144; 171]. For example, [PERSON] et al.