## 3 Results
The combination of a semiconductor and a metal induces electron flow from higher to lower Fermi level--bringing the two Fermi levels to a new equilibrium. The work functions of zinc oxide and silver are 5.3 eV and 4.1 eV, respectively. Therefore, when these two materials combine, the electrons flow from the AgNPs to ZnO. Hence, the electron density of the silver is reduced. Moreover, we can observe that the XPS signal of Ag 3d shifted towards higher binding energy, and the Zn 2p signal shifted towards lower binding energy--indicating that there is a tendency for electrons to move from the AgNPs to ZnO. Since ZnO and AgNPs were separated by  
Figure 2: X-ray photoelectron spectroscopy of (**a**) Zn 2p\({}_{3/2}\), (**b**) Ag 3d\({}_{5/2}\) and Ag 3d\({}_{3/2}\)-(i) glass fiber filter paper, (ii) zinc oxide substrate, (iii) polydopamine deposited on zinc oxide substrate, (iv) polydopamine reduced silver nanoparticles on zinc oxide substrate. a PDA film, the resonance structure of the PDA served as a bridge for electron transfer between these two. In addition, PDA itself promoted the process of electron transport and thereby contributed to the signal enhancement. Next, we performed SERS measurements of RhB on the designed substrates-GMF, Ag/ZnO@GMF, ZnO@GMF, Ag/PDA@GMF, and Ag/PDA/ZnO@GMF, to determine the contribution of the CT effect on SERS signal enhancement. Figure 4 shows that no RhB signal could be observed on GMF nor on ZnO@GMF substrates, suggesting that neither GMF nor ZnO@GMF exhibit SERS activity. On the ZnO@GMF substrate, however, ZnO directly reduces the AgNPs, and a relatively weak SERS signals was detected. Next, we observed that the AgNPs on PDA in the absence of ZnO exhibit SERS activity, but the enhancement is limited. Finally, we tested the SERS signal of the Ag/PDA/ZnO@GMF substrate and observed a very strong signal intensity. Therefore, as we hypothesized, the combination of zinc oxide and AgNPs, when combined with PDA, can significantly enhance the SERS effect, even greater than the enhancement effect of AgNPs themselves. Moreover, we believe that the catechol functional groups of PDA can further undergo oxidation to form orthoquinone functional group. This property further allowed PDA to release electrons during the oxidation process and act as an efficient electron contributor [35]. Therefore, zinc oxide was more exposed to electrons, and the energy required to excite electrons was reduced. It is worth noting that the conjugated aromatic core of PDA helped in efficient electron transport. Next, we measured the SERS enhancement on Ag/PDA/ZnO@GMF at different AgNP deposition times (2, 4, 6, 8, 10, and 12 h). As the AgNP deposition time increased from 2 to 8 hours, the detectable concentration of RhB gradually decreased from \(10^{-4}\), \(10^{-6}\), \(10^{-7}\), to \(10^{-9}\) M, respectively. However, for longer deposition times, the detectable concentration of RhB increased for \(10^{-6}\) M at 10 h and \(10^{-5}\) M at 12 h. These results indicated that the optimum deposition time of 8 h exhibited the largest SERS enhancement effect with the lowest detection limit of RhB (Figure 5). In order to clearly define the extent of SERS enhancement, we further calculated the EF for the substrate Ag/PDA/ZnO@GMF using the following equation [36]:  
\[\text{EF}=\text{I}_{\text{SERS}}/\text{I}_{\text{bulk}}\times\text{(concentration factor)}\]  
where I\({}_{\text{SERS}}\) represents the SERS signal intensity of the RhB peak on the Ag/PDA/ZnO@GMF substrate at 1364 cm\({}^{-1}\), and I\({}_{\text{bulk}}\) represents the original Raman signal intensity of RhB solution on the top of a silicon wafer under otherwise identical conditions. According to the calculated process [37], the EF has already excluded the possibility of electronic transitions of the RhB molecules themselves. These results are presented in Table 1. We can see that there is a minimum detection limit of \(10^{-9}\) M for an 8 h AgNP deposition time and the enhancement factor is \(10^{10}\).