# Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives
## 5 Plasmonic Resonance-Based Raman Sensors for Detection of Phenolic Compounds
### Metal-Organic Frameworks Structure-Introduced Raman Sensors  
As shown in Figure 11A, a core-shell nanostructure of Cu2O and SiO2 was coated with \(\sim\)4 nm porous Zn-based MOFs (ZIF-8) using an organic bridge as 5-mercapto-1-methyltetrazolole, successfully yielding a novel Cu2O@SiO2@ZIF-8. Subsequently, in situ synthesis of AgNPs with various sizes from 2 nm to 29 nm on the surface of these MOFs was performed, inducing new MOFs and Cu2O@SiO2@ZIF-8@Ag. Owing to the strong interaction with ZIF-8, AgNPs were uniformly distributed in the structures of Cu2O@SiO2@ZIF-8. Interestingly, 4 nm AgNPs, assembled on Cu2O@SiO2@ZIF-8 templates, exhibited an excellent limit of detection, as low level as 5.76 \(\times\) 10\({}^{-12}\) M concentration of phenol red in real samples. These Cu2O@SiO2@ZIF-8@Ag as 3D substrates could be employed for Raman monitoring of environmental contaminants [36]. Figure 11B shows the electromagnetic and chemical enhancement mechanisms of these Raman substrates. Firstly, owing to the many porous structures in Cu2O@SiO2@ZIF-8, AgNPs can be powerfully adsorbed and well loaded by the strong interaction with N and S atoms inside the ZIF-8 structure, thereby effectively preventing AgNP aggregation. Second, Cu2O@SiO2@ZIF-8@Ag exhibited a large surface area, assisting in the absorption of many phenol red molecules and strong binding between Ag and S atom via the Ag-S bond. However, owing to the high density of AgNPs on the surface of the MOF template, this SERS substrate generated many hotspots, enhancing Raman signals of phenol red that correspond to the electromagnetic mechanism. In contrast, charge transfer occurs as a result of the chemical enhancement mechanism between MOFs and phenol red molecules. Here, AgNPs provided an SPR phenomenon and encouraged charge transfer in the SERS system, increasing SERS signals. Importantly, a small size \(\sim\)4 nm of AgNPs provides a maximum SERS signal of an MOF template because of more AgNPs located in porous ZIF-8. Nevertheless, different sizes of AgNPs will not properly match ZIF-8 pores, yielding weak Raman intensities [36]. As shown in Figure 12, the Raman sensor was combined with MIPs, inducing the distribution of a new Raman detection for selectivity of chloro-phenols. The Raman platforms as Ag@MIL-101(Fe)@MIPs were successfully fabricated by in situ synthesis of AgNPs on the surface of MOFs octahedral structures as MIL-101(Fe), subsequently, by polymerized precipitation between a template and a functional monomer as 2,6-DCP and acrylamide, respectively. MOF-based nanocomposite-assisted Raman monitoring of 2,6  
Figure 11: (**A**) Schematic diagram of the fabrication process of unique MOFs’ structure as Cu2O@SiO2@ZIF-8@Ag. (**B**) Mechanism of Raman detection of phenol red using MOFs templates based on Cu2O@SiO2@ZIF-8@Ag. Adapted from [36]. DCP with a limit of detection of 4.5 nmol/L. This method provides a potential Raman template not only for sensing other chlorophenols, but also for real sample analysis using Raman spectroscopy [42]. As summarized in Table 1, based on plasmonic nanomaterials, numerous novel optical sensor technologies have been successfully developed for trace detection of phenol and phenolic compounds, with improvements in aspects such as selectivity and sensitivity as well as economical and reproducible benefits. The following aspects were studied and evaluated on the current spectroscopic and optical sensor methods.