## 1 Introduction  
Accurate and timely diagnosis of infectious diseases is crucial for effective disease management and epidemic preparedness. Infectious diseases caused by parasites, viruses, bacteria, and fungi are responsible for 15 million deaths each year, while more than 95% of these deaths take place in low-income countries [1]. Among them, most of the mosquito-borne and bacterial infectious diseases are included in the list of neglected tropical diseases declared by World Health organization (WHO) that need to be controlled by 2030 [2; 3], which emphasizes the current urge to develop rapid diagnostic tests for the early detection of infectious diseases. Current diagnostic modalities rely on highly sensitive polymerase chain reaction (PCR), which is technically challenging and expensive for widespread implementation in resource-limited settings, or light microscopy-based cytological examination, which has high specificity but poor sensitivity. Therefore, it is a timely need to develop new detection modalities that bring affordable, accurate, and highly sensitive characteristics to diagnostic tests. Among new detection modalities, surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical technique for molecular analysis, which can be particularly advantageous for diagnostic purposes when the Raman spectroscopy is combined with inherent optical and chemical properties of plasmonic nanoparticles [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. SERS is the main method of enhancing the inherently weak Raman intensity, such that Raman scattering can be widely utilized in analytical applications. In general, the Raman signal enhancement (commonly known as the SERS enhancement) is proposed to be a result of a combination of two main contributing phenomena, (i) electromagnetic enhancement (then main contributor) and (ii) chemical enhancement [14]. The electromagnetic enhancement is known to originate from the unique optical properties of plasmonic nanoparticles such as gold (Au) and silver (Ag). Plasmonic nanoparticles are known to generate localized electromagnetic field when their size is smaller than the wavelength of the interacting light, which is known as localized surface plasmon resonance (LSPR) [15; 16]. In SERS, this intense localized electromagnetic field of the plasmonic substrate is exploited to amplify the intrinsically weak Raman scattering cross-section of analytes when they are either on or in close proximity to a plasmonic substrate [14]. The LSPR and the magnitude of SERS enhancement can be modulated by changing the size, composition, shape, and the local environment of the NP [17; 18; 19; 20; 21; 22; 23; 24; 25]. In the context of SERS, plasmonic nanomaterial are referred to as SERS substrates and their effectiveness as a SERS substrate is evaluated by the SERS enhancement factor [26; 27; 28; 29]. In SERS-based sensing applications, the optical transducer (SERS substrate) and the molecular functional interface are the two main elements that dictate the performance of sensors. The optical properties of a SERS substrate affect the analytical sensitivity while the functional interface of a SERS substrate determines the detection specificity. A great deal of work has been done in designing different types of plasmonic nanoscale substrates to improve the SERS enhancement factor, hence the analytical sensitivities. The first generation of SERS substrates were mainly composed of spherical gold or silver nanoparticles or substrates [30; 31; 32; 33]. Anisotropic nanoparticles such as rod-, cubic-, triangle-, and star-shaped plasmonic nanoparticles and two-dimensional plasmonic substrates were then introduced [25; 34; 35; 36; 37; 38; 39]. Due to the higher electromagnetic field concentration imparted by the antenna effect of the sharp features of the second-generation SERS substrates, higher SERS enhancement factors could be achieved [38].