**1. Introduction**

Surface plasmon resonance (SPR) biosensors have become one of the most promising, standard, and affordable technology due to prompt research and expansion of SPR phenomenon in the last two decades. Nowadays, SPR sensors are broadly implemented for numerous biological and biochemical analytes identification and characterization due to its high sensitivity, real-time monitoring, level free detection assay, small sample size, and reusable sensor chip [1–5]. To be detailed, the SPR biosensors are adopted to agriculture and food quality monitoring [6], security and safely analysis [7], in need of medical diagnostics, environmental monitoring, bio-imaging [8–10], cancer detection [11, 12], DNA hybridization [13, 14], enzyme detection [15], protein-protein, protein-DNA, and protein-virus hybridization [16, 17], microorganisms identifying [18], industrial appliance's condition monitoring, temperature monitoring [19], gas sensing [20, 21], chemical and biochemical analysis [22, 23], pharmaceutical and biological molecule analysis [24, 25], oil condition monitoring [26], and so on. In the year 1902, Wood [27] first

observed unexpected optical power attenuation characteristic at the time of measuring the reflection of light from metallic gratings. This phenomenon occurs due to absorbance and conversion of photon energy to surface plasma wave (SPW) which is the result of combined oscillation of excited electrons called surface plasmon polaritons (SPPs). This oscillating electron consumes maximum energy at a certain wavelength for a specific angle of incidence of light which is called resonance condition. That is why this phenomenon is named surface plasmon resonance (SPR). In 1968, Otto [28] and Kretschmann [29] introduced attenuated total internal reflection (ATR), which encouraged scientists and researchers to concentrate on the implementation of SPR sensing technology practically. In 1982, the SPR sensing technique was first demonstrated by Nylander and Liedberg [4, 30] for the practical application of gas sensing. After that, SPR sensing technology has been getting ceaselessly developing consideration from the scientific and academic network. In 1990, the SPR sensing instrument was first commercially produced and introduced to the market by Biacore AB. Since then a considerable number of manufacturers e.g. IBIS Technologies B.V., Graffinity pharmaceuticals, GWC Technologies, Bio-Red, AutoLab, Farfield Sensors, Genoptics Bio Interactions, Microvaccum, Biosensing Instrument, and SPR Navi have launched their SPR instruments to the market [17, 31].

Different optical techniques are currently proposed for sensing purposes, including Ramman scattering based sensors [32, 33], grating coupled sensors [34, 35], prism coupled sensors [36, 37], optical fiber-based sensors [38, 39], planner waveguide-based sensors [40, 41] etc. The optical biosensors basically work with the measurement of change in input incident light and detected light at the output terminal. To be specific, the change in phase, amplitude, wavelength, frequency, or polarization of light is measured at the output terminal of the sensors and the changes in these parameters are observed. Among them, the commonly used technique is observing the reflected light angle where maximum light is attenuated. This method is called angular interrogation approach with attenuated total internal reflection (ATR) that is applied usually in prism coupled devices. The performance of an optical sensor is basically measured in terms of its sensitivity, detection accuracy or detection limit, the figure of merits (FOM) and quality factor (QF), etc. The researchers and scientists are continuously working for the improvement of the performances of the SPR sensors [31, 42–44].

In SPR biosensors, the most crucial parameters determining the characteristics of the sensors are plasmonic materials. Materials with adequate free electrons at their valance bands can be used as plasmonic materials. To be specific, metals e.g. gold (Au), aluminum (Al), silver (Ag), copper (Cu), etc. are a good candidate to be used as a plasmonic material [45, 46]. Al and Cu have not gained much interest to be used because of their high damping nature, prone to oxidation, corrosion, and interband transition characteristics. But Silver (Ag) can be nominated as a potential candidate for SPR sensors as it attributes outstanding optical properties, such as no interband transfer at the visible light frequency, small optical damping, and sharper resonance peak [46–48], etc. Using Ag in SPR sensors, better sensitivity can be captured, but it shows poor chemical stability as it creates brittle oxide layers with liquid analyte [49]. Some researchers have reported that applying bimetallic layer on the Ag surface can resolve this problem [50, 51]. On the other hand, Au is more chemically stable compared to Ag and free of corrosion and oxidation problems. But, gold offers a slightly higher damping loss and widen SPR curve that restricts the detection accuracy and figure of merits (FOM) of the sensors [52]. The sensitivity of Au-based sensors is also slightly lower because of the low biomolecular adsorption characteristics of the gold surface. In order to improve the sensitivity of the sensors, researchers recommended various approaches in which the application

