# Enhancing Disease Diagnosis: Biomedical Applications of Surface-Enhanced Raman Scattering
## 2 Surface Enhanced Raman Scattering: A Brief Tutorial  
Previously theorised by [PERSON] in 1923 [32], the Raman effect was discovered and experimentally demonstrated by Sir [PERSON] in 1928 using simple optical materials and instrumental setup [33]. Raman spectroscopy is a specific optical readout platform which involves scattering of irradiated light following interaction with polarisable molecules under interrogation with a monochromatic laser source. By far the largest proportion of scattered photons has the same energy as incident light (known as Rayleigh scattering), and so does not carry significant chemical information [34]. However, a very small fraction of emergent radiation is inelastically scattered (Raman scattering), with photons of lower frequency than incident radiation (Stokes scattering) being measured in conventional Raman spectroscopy. Raman spectroscopy instrumentation has greatly evolved mechanically and technically over time. Nowadays, powerful and stable lasers, sensitive detectors, optical fibres and software have accelerated the application of Raman spectroscopy in bioanalytical chemistry. Despite this, the scattering cross-section of the Raman Effect is still extremely inefficient with a photon conversion rate of less than 1 per 10\({}^{6}\)-10\({}^{8}\) incident photons. So to obtain Raman spectra of good signal-to-noise ratio, high power density, long collection time and clinically unrealistic concentrations are usually required. Such measurement parameters often initiate fluorescence and photodegradation which frequently mask Raman spectral lines. This limits the utility of Raman spectroscopy for diagnosis of diseases in biofluids or cells, where biomarkers may be in trace quantities and preserving sample integrity is desirable [29]. It is indeed exciting that significant attention has now shifted to SERS to overcome the quantum inefficiency of conventional Raman spectroscopy. The SERS effect was discovered accidentally by [PERSON] and coworkers in 1974 at the University of Southampton (UK), when they observed a dramatic increase in Raman signals of a monolayer of pyridine adsorbed onto electrochemically roughened Ag electrodes [35]. At that time, the anomalous observation was thought to be a combined effect of the large surface area of roughened Agelectrodes and increased local concentration of adsorbed pyridine molecules. This discovery marked the beginning of an excitingly new era of physical and surface chemistry, attracting much attention from analytical scientists and engineers. In 1977, [PERSON] and colleagues [36] and [PERSON] and coworkers [37] independently concluded that the observed anomaly in Raman signals of pyridine was rather due to increased cross-section as a result of enhanced electric fields induced by roughened Ag electrodes. It was at this time when [PERSON] enlightened the scientific community and coined the term'surface-enhanced Raman scattering (SERS)' as we know it today. In principle, SERS involves interactions between electromagnetic radiation and molecules adsorbed onto, or in close proximity to, nanoscale rough metallic particles of smaller diameter than the wavelength of excitation radiation [38]. Although there is no comprehensive explanation for the SERS phenomenon to date, and the topic is under active debate [39], the independent research by [PERSON] and [PERSON], combined with selection rules of molecules adsorbed on metal surfaces [40], proposed two simultaneously operative theories. The electromagnetic (EM) theory, thought to be the dominant mechanism, is a physical effect which involves optical excitation of electric fields and charge motions created by collective oscillations of electrons in the conduction band (surface plasmon) of NPs. This interaction creates localised surface plasmon resonance (LSPR), the so-called 'hotspots' around NP surfaces [41]. Therefore, analyte molecules that interact with LSPR and produce intensified spectral signals with enhancement factor (EF) of \(10^{6}\)-\(10^{8}\) compared to conventional Raman lines.