## 3 Results  
A number of groups have attempted to develop ZnO based SERS substrates over the past few years. The details are shown below in Table 1. Most efforts were mainly focused on the direct deposition of noble metals onto the hierarchical nanostructures of ZnO. In this study, we demonstrate a PDA mediated deposition of AgNPs onto the nanostructures of ZnO for the development of a multicomponent Ag/PDA/ZnO@GMF SERS substrate. We used GMF because of its commercially availability, low-cost, and its superior physical adsorption capability over silicon wafers. Furthermore, the 3D steric-construction structure of GMF offers a high surface area for immobilizing MNPs. The SERS substrate Ag/PDA/ZnO@GMF was prepared by the deposition of ZnO on GMF, followed by the incorporation of PDA layer on ZnO@GMF, and finally, in-situ formation of AgNPs. The substrate morphology was characterized using scanning electron microscopy (SEM). We found that GMF existed as long tubular fibers with a relatively smooth surface. Rod-like structures were formed after being coated by ZnO and heated to a temperature of 90 \({}^{\circ}\)C (Figure 1). Following application of the PDA film, the in-situ chemical reduction of silver ions over PDA/ZnO@GMF formed AgNPs that decorated the PDA/ZnO structures as shown in Figure A2. Additionally, we confirmed the ZnO deposition at room temperature. However, under such conditions, ZnO revealed a decreased crystallinity and formed cluster-like aggregates of irregular shapes (Figure A1). We further investigated the optimum time for AgNP deposition between 2 and 12 h on the PDA/ZnO@GMF substrates. As shown in Figure A2, there were no AgNPs observed to form on ZnO surfaces for deposition times of 2 h and 4 h. This indicates that the particle size of the AgNPs was quite small during that reaction time. When the deposition time was increased to 6 h, relatively uneven distributions of AgNPs on ZnO were observed. At a deposition time of 8 h, the particle distribution was relatively uniform, and the average particle size was 28 \(\pm\) 7 nm. The average size of the AgNPs was estimated from the SEM image shown in Figure A2d using the image J software program (version Java 1.8.0). Finally, at a deposition time of 12 h, the AgNPs aggregated, nearly  
Figure 1: SEM images of ZnO@GMF substrates at different resolutions (**a**) 2 \(\upmu\)m, (**b**) 1 \(\upmu\)m, (**c**) 500 nm, respectively. covering the original ZnO crystal structures. We therefore conclude that the optimal time for AgNP deposition on PDA coated ZnO was 8 h.  
We next used Xray Photoelectron Spectroscopy (XPS) to confirm the chemical composition of the Ag/PDA/ZnO@GMF substrates. First, we performed XPS measurements to detect silver on the substrate (Ag-8h/PDA/ZnO@GMF) where Ag ions were directly reduced on PDA/ZnO@GMF. We observed signal peaks at 368.6 and 374.6 eVs, which were assigned to the Ag 3d\({}_{5/2}\) and Ag 3d\({}_{3/2}\), respectively, indicating the presence of AgNPs on the substrate. Next, XPS spectra of the Zn 2p\({}_{3/2}\) transition was collected and compared from substrates with and without adsorbed AgNPs. As shown in Figure 2, the XPS spectrum of ZnO@GMF revealed a distinct peak at 1023.9 eV, which corresponds to the binding energy of zinc oxide. The introduction of PDA reduced the binding energy to 1023.7 eV. The consequent addition and in-situ generation of AgNPs on PDA/ZnO@GMF further reduced the binding energy to 1022.7 eV. Such peak shifts for Ag and ZnO can be attributed to the CT effect between them, where PDA played the role of conducting layer for charge redistribution. The CT effect from the metal-molecule-semiconductor interface is illustrated by the energy band diagram of zinc oxide and silver shown in Figure 3. In general, the electrons within solid materials require more energy relative to the vacuum level to overcome lattice adsorption forces to become free electrons [33; 34].