**4. Plasmonic nanostructures in diagnostics**

### **4.1 Molecular diagnostics assays**

Historically, molecular diagnostics assays polymerase chain reaction (PCR) have played a key role in society in curbing the spread and effects of various infectious diseases through early detection [36, 62]. Even though, this technology provides fast and reliable results and do not require any post processing, its limitations are still the expensive machinery, lengthy assay times and sophisticated operational processes that often require a trained personnel [63, 64]. The fast development of nanotechnology and their use in biotechnology has shone a new light towards the use of molecular diagnostics. Specifically, the use of plasmonic nanoparticles has had a significant impact on the clinical and life science [65]. Plasmonic optical properties make them ideal for use in molecular diagnostics because they improve sensitivity, selectivity, efficiency in drug delivery and specificity [66, 67]. K. Jiang et al. reported the use plasmonic magnetic nanoparticles covered with silica core shell for the detection of DNA ranging from 0.5 ng μL−1 to 3 fg μL−1 within 20 minutes using cPCR [62, 67]. This study shows the importance of using plasmonic nanoparticles for the detection of diseases in real time that is acquit of traditional molecular diagnosis limitations such as lengthy assay times. In this study, the plasmonic magnetic nanoparticles were used for their dual function of thermal cycling and magnetic separation and detectable color change [62].

### **4.2 Surface-enhanced Raman scattering immunoassays**

SERS-based immunoassays are a promising tool and integral for the identification of biological threats through early detection of biomarkers, which is crucial for disease control [68]. The SERS technology the plasmonic nanoparticles are functionalized with Raman reporters, which are attached to protein binding membranes. The binding membrane facilitates the detection of diseases and shows high sensitivity [69]. Gold, silver, copper, and platinum nanoparticles have been used to improve sensitivity and enhancement factor of the SERS substrate [70]. The schematic illustration of a typical SERS immunoassay is shown in **Figure 6**.

However, in recent years the SERS technology has been moving away from the use of pristine spherical nanoparticles and moving towards the different morphologies and composites. The reason for the observed shifts is due to the realization that plasmonic using pristine spherical nanoparticles does not realize the full potential of SERS in point of care tests. S. Nyembe et al. performed a comparison study of gold nanowires (diameter of 10 nm) with various gold nanoparticles (14, 30 and 40 nm) and the results showed that gold nanowires had a better enhancement factor than

*Application of Plasmonic Nanostructures in Molecular Diagnostics and Biosensor Technology… DOI: http://dx.doi.org/10.5772/intechopen.108319*

**Figure 6.**

*Schematic illustration of a typical SERS sandwich immunoassay for biomarker detection [71].*

the spherical gold nanoparticles [72]. The higher enhancement factor was due to better adsorption capacity due to higher surface area and from entrapment caused by interstices formed by their network as shown in **Figure 7** [72].
