**1. Introduction**

Traditional gold-standard diagnostic techniques combined with advances in nucleic acid-based assays, enzyme-linked immunoassays, and rapid diagnostic assays are widely used for the detection of diseases. Despite the advances presented by these techniques several hurdles such as false positives/negatives, expensive infrastructure or equipment, non-specificity, complicated sample preparation, and assay result

analysis limit their use. This is evidenced by the continued world health organization cases reported from low-resource regions. The need for affordable, specific, simple, user-friendly, rapid, and sensitive, diagnostics remains.

Plasmonic-based diagnostics or biosensors offer an attractive solution in the detection and management of diseases. They can achieve enhanced sensitivity, rapidity, real-time and label-free detection of pathogenic biomarkers [1, 2]. The plasmonic phenomenon yields various techniques: Surface Plasmon Resonance (SPR), Localized Surface Plasmon Resonance (LSPR), colorimetric Plasmonic assays, Surface-Enhanced Raman spectroscopy (SERS) and its variants. Benefiting the diagnostic fraternity, Plasmonic-based assays are merited with a multiplexing potential and smartphone integration [1]. The Plasmonic based sensing platform extends to environmental sensing. Water contamination due to pathogens and biological molecules such as Covid-19 benefits from Plasmonic sensors. In addition to detection, the plasmonic sensors can quantify the pathogens in the drinking waters to inform water treatment measures to be taken.

SPR biosensing is based on the excitation of the free electrons by a polarized light on a metal film which onsets the electrons' collective oscillation [1–3]. A plasmonic surface is immobilized with biomarker receptors, which bind the analyte and induce a change in the local refractive index. The change is perceived through changes in the incident light used for the excitation of the free electrons to the SPR state [1]. This principle has resulted in paradigms for different diseases: malaria [4–6], tuberculosis [7–9], HIV [10, 11], and Covid-19 [12].

The LSPR is based on the confined oscillation of electrons at the metallic surface and the localized SPR distinguishes LSPR from the propagating SPR biosensor. Interaction of the receptor with an analyte in LSPR biosensing induces changes that prompt a wavelength shift in the excitation spectrum. The LSPR-based sensors are excellent for both the detection and quantification of biomolecules/diseases [1, 3]. The potential for LSPR biosensing has been tested on Covid-19 [13], glucose [14], and cancer cells [15].

Colorimetric Plasmonic assays are driven by LSPR of metallic nanostructures such as gold (Au) and silver (Ag) that yield enhanced magnetic fields in the visible/NIR range. This phenomenon yields color changes observed by the naked eye. The color changes merit colorimetric plasmonic assays for point-of-care testing [1, 16].

SERS principle uses the plasmonic effect of the metallic substrates to enhance weak traditional Raman signals. Raman spectroscopy is a fingerprinting tool that is used to study characteristic peaks of molecules. However, its sensitivity is compromised for some molecules especially biomolecules limiting its use in diagnostics. SERS alleviates the low signal challenge by amplifying the weaker signals and availing the technique for biosensing. It is used as either label-free for Raman active analytes or labeled for non-Raman active molecules. Zhou et al. developed an AgNPs based sensor for the detection of bacteria from drinking water. The SERS substrate, AgNPs coats on the cell wall of the bacteria and enhances the Raman signal of the analyte by 30-folds compared to a non-coating AgNPs colloids. The chapter examines the potential for plasmonic-based assays in the detection of diseases and common water pathogens. It compares literature and prototypes in the field and future expectations.
