## 1 Introduction  
Key enabling technologies (KETs) such as nanotechnology, surface-enhanced Raman scattering (SERS) spectroscopy, and microfluidics have been thriving in recent years. Actually, one of the techniques that most benefited from advances in nanotechnology is SERS. This ultrasensitive analytical technique overcomes the intrinsic low efficiency of Raman by using nanoparticles (NPs), which reach up to enhancing factors of \(10^{12}\)-\(10^{14}\) orders of magnitude [1; 2]. The high sensitivity of this analytical tool allows it to be used to detect chemical and biological analytes that interact with active SERS substrates, even at very low detection limits [3; 4; 5; 6]. However, one of the major challenges hindering the establishment of SERS as a standard analytical technique is the difficulty of controlling the reproducibility of those SERS substrates. SERS substrates are based in plasmonic NPs of different compositions (Ag, Au, Pt, Cu) and shapes [7]. Despite the availability of several top-down or bottom-up synthetic strategies, such as chemical and physical vapor deposition, mechanical or biological synthesis, which is the chemical approach leading to colloidal suspensions, has been the most used by far [8]. Metallic NPs in colloidal dispersions can be used as SERS substrates directly as they are or supported over solid substrates [9]. The use of metallic nanostructures as synthesized allows average measurements and avoids sample damage due to the Brownian motion of particles in liquid [10]. However, when performing average SERS, reaching very low detection limits can be challenging and the reproducibility highly depends on the NP batch. Furthermore, SERS is a distance effect and the analyte needs to be close to the surface of the nanostructure to undergo the SERS effect. In the case of colloidal dispersions, this is only possible for molecules that have affinity for the surface of the NP. Additionally, SERS performance in terms of sensitivity often relies on the formation of hot-spots, which are areas with an extra SERS enhancement occurring when NPs are in close vicinity [11; 12; 13; 14; 15]. Supporting NPs over different solid substrates for the development of SERS sensors increases the reproducibility of their optical properties, the control of hot-spots, and the physical adsorption of molecules with no-affinity for those SERS active nanostructures [16]. In addition, SERS substrates using anisotropic particles, such as gold nanostars (GNSs) and nanorods (NRs), offer an extra signal enhancement and facilitate label-free detection strategies [17]. Paper-based SERS substrates are popular due to their low cost, ease of use, flexibility, and suitability for on-field measurements [18; 19; 20]. However, reproducible deposition of NPs in paper is not straightforward, especially when using anisotropic NPs [18; 19]. In recent years, several approaches for the fabrication of paper-based SERS substrates have been reported, including physical vapor deposition [21], laser driven deposition [22], dip coating [23; 24], pen [25] and printer-based plasmonic links [26], or painting brushing [27]. For instance, [PERSON] et al. [25] developed a pen-on-paper approach using plasmonic inks to write SERS arrays. Those SERS substrates showed a higher SERS performance when compared to commercial SERS substrates. [PERSON] and co-workers focused their research on overcoming a common limitation of NP assemblies, which the coffee-ring effect caused when drop casting both the NPs and the sample [28]. In this work, they leveraged the coffee ring effect to achieve higher sensitivity by measuring at the ring zone in which the enhancement is superior. However, in all above-mentioned approaches, several limitations restrict the use of these paper sensors. Dip coating or soaking of paper usually takes a long time to achieve sufficient loading of NPs and requires high volumes, ink-based strategies may clog and there is a high dead volume of NPs, and laser or vapor deposition require high-tech instrumentation and expert personnel.