## 3 Results and Discussion  
Figure 1 shows the morphology of the different metallic nanoparticles used to fabricate the assemblies on the paper. GNSs of an average size of 70 nm and Au@AgNRs of an aspect ratio of 1.6 were synthesized. The electron microscopy images show a narrow size distribution for the nanoparticles that will then be self-assembled using the PDMS-paper hybrid device. Additionally, UV-vis spectra for both types of NPs are shown in the Supplementary Information (Figure S1). Figure S1A shows the extinction spectrum of the AuNRs and Au@AgNRs with the longitudinal plasmon band maxim absorbance at 545 and 688 nm, and the transverse plasmon band maximum absorbance at 395 and 526 nm, respectively. The extinction spectrum of the GNSs used along this work is shown in Figure S1B, and the maximum absorbance for the plasmon band corresponding to the core and tips are 561 and 832 nm, respectively. For the fabrication of the PDMS-paper device, an AutoCAD design for microfluidic reservoirs was adapted from previous works [36]. For the preparation of the hybrid self-assembling platform, the process depicted in Figure 2 was followed. The PDMS replicas of the reservoirs (Figure 2A) were bonded to standard wax weighing paper by plasma activation (Figure 2B). The bonding, despite not being permanent, allowed the injection of the anisotropic NPs dispersed in isopropanol by using  
Figure 1: Transmission electron microscopy images of (**A–C**) Au nanostars (GNSs) and (**D–F**) silver coated gold nanorods (Au@AgNRs) at increasing magnifications. connection tubing (Figure 2D). The fact of the bonding not being permanent benefits the system, since the researcher can choose to introduce the sample via tubing, or, for on-field application, the paper piece can be submerged in the sample to test. After introducing the NPs, they were allowed to dry, and a homogeneous assembly of NPs was deposited on the surface of the paper (Figure 2E). Lastly, the PDMS part was unsealed from the paper substrate for analysis (Figure 2F). The use of PDMS was crucial for the development of this substrate because it permits the control of the evaporation of the solvent to achieve homogeneous deposition and also the use of reduced NP and sample volumes. Figure S2 shows additional SEM images of the paper substrates after the self-assembly process and after de-bonding of the PDMS for different repetitions of the same experiment. The high magnification images of Figure S2F demonstrates that the self-assembly lead to a closed packed arrangement of the NPs, and, thus, to the formation of hot-spots along the substrate. The internal volume of the PDMS reservoir (depth 60 \(\upmu\)m) used for the assembly was 1.85 \(\upmu\)L. In this way, the amount of NPs needed for the fabrication of this SERS substrate is considerably lower than the volume needed with previous dip-coating approaches [37; 38] or with the silver mirror reaction method, which is usually no less than 10 mL [23; 39]. The active area of the assembly was that of the reservoir design, 32.88 mm\({}^{2}\) (Figure 3A), and, thus, the active area for sensing can be extended when compared to previous approaches [40]. The surface coverage for the GNSs and NRs sensors were 0.6 mM mm\({}^{-2}\) and 0.3 mM mm\({}^{-2}\), respectively. Despite the very low volume of NPs needed for the assembly, the naked-eye assessment of the homogeneity (Figure 3B) of the deposition and concentration of NPs was comparable to alternative strategies. For example, [PERSON] et al. [40] reported on the fabrication of a gold nanoparticle array on paper by drop-casting using different NP concentrations. In order to achieve a close packed NP arrangement, they needed to use a 50-fold pre-concentrated NPs solution of the well-known [PERSON] [41] gold NPs. The latter is equivalent to a gold concentration of 25 mM. The pre-concentration step is needed and the process took about 1 hour. Notably, the drying process using the microfluidics-based strategy presented in this work took a maximum of 30 min.