# Enhancing Disease Diagnosis: Biomedical Applications of Surface-Enhanced Raman Scattering
## 1 Introduction
Of particular interest now is the emergence of portable handheld Raman systems and reproducible enhancing substrates which are directing SERS for point-of-care (PoC) biosensing. This may overcome biosafety concerns associated with transportation of infectious biological samples to the laboratory especially during an outbreak of highly contagious diseases. Additionally, the inherent selectivity and ultrasensitivity of SERS would enable bespoke clinical diagnostics of subclinical infections like pancreatic adenocarcinoma, which are asymptomatic until the late stage of infection [14]. Since its discovery, SERS has been successfully applied to a wide range of fields including environmental [15; 16], antibiotic resistance [17; 18], food [19; 20] and pharmaceuticals [21; 22]. However, it is in the last decade that SERS has become an exciting hypothesis-driven tool for in vitro and in vivo characterisation of diseases in biofluids, cells or biopsy [23; 24]. This is driven by the fact that SERS profiles reveal detailed molecular structure and abundance dynamics which define specific diseases with minimal sample handling. Recently, SERS involving direct mixing of human biofluids and bare NPs yielded promising results for detecting global molecular profiles for diseased and healthy status [25]. [PERSON] et al. segregated blood plasma of patients with nasopharyngeal carcinoma from healthy controls with classification sensitivity and specificity of 90.7% and 100%, respectively [25]. Similarly, [PERSON] et al. applied a noninvasive Raman system equipped with a 785 nm laser, and, based on SERS fingerprints measured from direct mixtures of Ag colloid and serum, cohorts of liver disease and normal subjects were discriminated with the aid of multivariate analysis [26]. Whilst these studies demonstrate the sensitivity of SERS, it should be acknowledged that untargeted analysis of biomarkers in chemically diverse samples is very challenging; given that SERS registers signals of all biomolecules such as proteins, carbohydrates and lipids, present in a clinical specimen [27]. Thus, SERS spectral bands are composites of complex convoluted vibrational modes which are extremely difficult to interpret and employ for targeted therapeutics. Needless to say, untargeted SERS phenotypic signals do not reveal the structural identity of the actual diagnostic markers in complex backgrounds. We reiterate here that the major analytical strength of SERS in clinical diagnostics typically lies in chemical/molecular specificity in combination with the ultrasensitivity of this technique. On this premise, to realise the maximum benefits of SERS, there is a need to target known biomarkers identified _a priori_ by other techniques, such as LC-MS or GC-MS, polymerase chain reaction (PCR), etc. As a case in point, [PERSON] and coworkers used reversed-phase LC to separate therapeutic drugs in human urine followed by quantitative detection by SERS [28]. Furthermore, specific disease markers can be detected by SERS method, which has been extensively reported by [PERSON] and [PERSON] and coworkers [23; 29]. Here, SERS signals of target analytes are measured indirectly through SERS-active reporter molecules and recognition elements adsorbed onto NP surface. Notably, [PERSON] et al. employed AgNPs labelled with a fluorescent reporter and biotin modification probes to detect multiple DNA sequences extracted from etiological agents of meningitis in clinical samples [30], extending on the picomolar quantitative detection of actual meningitis pathogens reported previously [31]. When utilised in this manner, SERS produces unique, sharp and well resolved analyte-specific signature peaks of exceptionally narrow bandwidth and fluorescence-free background. In this context, SERS can provide answers to the 'yes' or 'no' diagnostic questions pertaining to the presence or absence of disease, and importantly, quantitative information based on specific single or multiple biomarkers.