# Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives
## 5 Plasmonic Resonance-Based Raman Sensors for Detection of Phenolic Compounds
### Pure Noble Metal Nanomaterial-Based Raman Sensors  
Plasmonic nanostructured materials have been well known as one of the most significant keys leading to the success of sensing technologies thanks to their effective Raman-active substrates. Among various nanomaterials, Au and Ag as noble metals have been adapted as a potential platform, which has contributed to the sensitive Raman detection [5,100]. Furthermore, the unique shape and size of noble metals induce highly dense plasmonic hotspots to increase the sensitivity of the Raman method. Many scientists have discovered more novel structures of noble metals that aim to maximize the generation of a high density of plasmonic hotspots, inducing enhanced electromagnetic fields and amplified Raman signals. Besides the preparation of new structures, the modification of the surface of nanostructures were performed by specific probes for the sensitivity and selectivity of Raman sensing. For example, great efforts have been made toward this goal. AgNP-assisted Raman detection for bisphenol A determination [101] and GNP-induced Raman platforms for methyl parathion detection [102] have been considered. Interestingly, Figure 7A shows SEM images of unique nanostructures of Ag nanorod bundles. Herein, Ag nanorods were well aligned as vertical bundles distributed on an Au/Cu template. This uniformity of Ag nanorod bundles demonstrates a potential application as a Raman-active platform used for trace detection of phenolic pollutants, including 4-chlorobiphenyl, methyl parathion, 2,4-dichlorophenoxyayacetic acid (2,4-D), and two pesticides mixtures in water environments. Vertical bundles of Ag nanorod were well designed by combining porous anodic aluminum oxide membranes and spherical polystyrene templates. Based on the well-controlled \(\sim\)65 nm diameter of AAO pores, the diameter of the Ag nanorods was also well achieved, such as the size of AAO pores. Figure 7A(a,b) show that vertical bundles of Ag-nanorod were well arranged with a P6 mm hexagonal symmetry. Each vertical bundle was surrounded by three other bundles and comprised an \(\sim\)800 nm length size of 30-45 nanorods with a \(\sim\)65 nm diameter (Figure 7A(c,d)). This unique structure exhibits a small gap (2 nm), leading to highly dense hot spots, inducing an enhanced Raman signal up to 10\({}^{8}\). As shown in Figure 7B, to further illustrate the practical application of this Raman platform, vertical bundle arrays of Ag nanorods were used to detect phenolic pollutants of methyl parathion and 2,4-D in the environment using Raman spectroscopy, whose limits of detection were estimated as 21.5 \(\times\) 10\({}^{-9}\) M and 61.9 \(\times\) 10\({}^{-9}\) M, respectively [103]. Figure 7: **(A)** SEM images of vertical bundles of Ag nanorods including top views with different scale bars of (a) 1 \(\upmu\)m and (b) 2 \(\upmu\)m, (c) side view of vertical bundle arrays of Ag nanorods, and (d) top view of one unique bundle of Ag-nanorods. (**B**) Vertical bundle arrays of Ag nanorod-based Raman detection of phenolic pollutants of (a) methyl parathion (0.3 \(\times\) 10\({}^{-6}\) M). (b) Linear curve of logarithmic concentration of methyl parathion-correlated Raman intensities (1260 cm\({}^{-1}\)). (c) Raman spectra of 2,4-D (2 \(\times\) 10\({}^{-6}\) M). (d) Linear curve of logarithmic concentration of 2,4-D-correlated Raman intensities (1177 cm\({}^{-1}\)). Adapted from [103]. An easy and rapid detection of bisphenol A detection has been successfully developed using Raman spectroscopy and a matrix of molecularly imprinted polymers (MIPs), where silver nanoparticles (AgNPs) were fabricated in situ inside this matrix. AgNPs have been well distributed in the matrix, leading to highly dense hotspots.