###### Abstract  
We demonstrate the development of an active multicomponent Ag/PDA/ZnO@GMF surface-enhanced Raman scattering (SERS) substrate via introducing bio-inspired polydopamine (PDA) in between a noble metal (AgNPs) and ZnO nanorods. The insertion of PDA enabled efficient charge redistribution between metal and semiconductor through their aromatic cores. The substrate exhibited a high enhancement factor (EF) of \(10^{10}\) for the organic pollutant dye Rhodamine B (RhB). Subsequent exposure of a RhB-loaded substrate to an external UV light source developed an efficient pathway for RhB degradation and replenished the substrate for multiple usage cycles with remarkable photostability. Thus, enhanced performance of the substrate in terms of light-harvesting capability and high charge-separation efficiency was observed. In addition, the much larger surface area of the branched ZnO nanostructures served as a template for PDA assisted synthesis and controlled deposition of AgNPs, which further improved the SERS effect. Our work seeks to understand the contributions of the noble metal and semiconductor components and the synergistic effects of combining them with a facile charge transport medium to enable the fabrication of highly efficient SERS substrates for use in industrial and environmental applications. zinc oxide; polydopamine; surface-enhanced Raman scattering; Rhodamine B; photocatalytic degradation +
Footnote †: journal: Applied Sciences  
SERS-based analyte detection and surface plasmons to assist or induce catalytic action to derive a variety of chemical conversions with the additional advantage of confirming reaction products [3,4]. Several efforts have been made to construct a SERS substrate with MNPs and metal-oxide semiconductors such as ZnO, CuO, Cu\({}_{2}\)O, and TiO\({}_{2}\)[5, 6, 7, 8, 9, 10]. Among them, ZnO nanostructures are the most commonly used, due to the use of their wide bandgap (3.37 eV) in several applications [11, 12] and SERS techniques [13, 14, 15, 16]. Additionally, their different crystal facets generated through different optimized conditions are quite fascinating and have different applications [17, 18]. Several attempts in the past few decades were directed toward the development of SERS substrates that were specifically composed of one or two noble metals directly deposited onto hierarchical ZnO structures (Table 1) [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. However, the fabrication of a SERS active substrate having a significant enhancement factor (EF) by utilizing noble metals and semiconductors is still challenging. The incorporation of an adhesive layer consisting of a long-range \(\pi\) conjugated segment such as polydopamine (PDA) may be beneficial in this sense because the strong adhesion of a PDA layer is able to bind together the noble metal and metal oxide and provide a uniform and well-packed substrate for effective charge redistribution [31]. The synergistically improved charge transfer through an active charge transport layer of PDA and the increased number of adsorption sites due to branched nanostructures of ZnO are highly effective in that they produce a large enhancement of SERS activity. In addition, ZnO nanostructures may contribute to the chemical effect (CE) and their high refractive index promotes strong light confinement. Thus, a PDA layer in between a noble metal and the branched nanostructures of ZnO, owing to the unique organic semiconductor nature, is highly beneficial for overall SERS enhancement. Developing the as mentioned SERS substrates to realize improved SERS effects is thus essential. The SERS-based label-free trace detection of organic molecules from a highly dilute solution and the visualization of their degradation on a designed substrate is quite attractive and promising to monitor pollutants for a clean and safe environment [32]. Consequently, the SERS technique has been widely used by research groups for the trace detection of various organic dyes, pesticides, and biomolecules down to the single-molecule level.