# Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives
## 4 Localized Surface Plasmon Resonance Phenomenon-Based Optical Sensor for Phenolic Compounds  
LSPR has been responsible for an enhanced electromagnetic field, inducing surface-enhanced spectroscopic technologies [87]. Several scientists are increasingly interested in plasmon resonance-based optical sensors, especially noble metal nanostructured materials such as Pt, Au, and Ag nanoparticles with various shapes and sizes, exhibiting plasmonic features that have been successfully used as a powerful analytic technique [6,88]. Here, these plasmonic nanostructures were practically employed as efficient transducers that convert changes in the spectral location of refractive index, thereby shifting the LSPR peak upon binding of analytes to either the surface of bare plasmonic nanostructures or specific receptor-conjugated nanomaterials. By coating unique probe molecules on the surface of plasmonic nanomaterials, LSPR-shift assay-based sensor techniques were introduced for specific analytics, considering a change in the signal localization of refractive index around their surfaces [89]. In addition, graphene oxide-based nanostructured hybrids have been successfully used as a novel material for enhanced SPR sensors thanks to their significant features, including strong adsorbed molecules, amplificated signals, high electronic bridge, and simple fabrication. Moreover, regarding amplification of signals obtained by plasmonic materials, graphene-based nanostructure composites can significantly increase the sensitive detection of analytes up to fM [90]. Owing to their sensitivity and the selectivity of the spectral location of the refractive index, SPR-based optical sensors have been widely  
Figure 5: (**A**) Schematic illustration of AuNC-assisted and inner filter effect (IFE)-based fluorescence detection of 4-nitrophenol (4-NP) by means of selectively induced quenching of 410 nm fluorescence intensity of residual di-tyrosine (diTyr) using fluorescence intensity of AuNCs [85] at 630 nm in a complex of bovine serum albumin (BSA)-stabilized AuNCs. (**B**) UV/Vis spectroscopy of AuNCs@BSA before and after 4-NP addition, illustrated by a black line and a red line, respectively. (**C**) Under excitation at 330 nm, photoluminescence emission spectroscopy of AuNCs@BSA before and after 4-NP addition is demonstrated by a black line and a red line, respectively. Adapted from [85]. investigated in sensing applications such as mercury ion detection [8; 91], CO\({}_{2}\) detection [1], and gas sensors [2]. Some special emphasis on plasmonic nanostructures has been developed for the fabrication of novel surfaces, exposing high SPR-based optical sensors for phenolic compounds, such as GNP impregnation in TiO\({}_{2}\) structure-assisted hydroquinone detection [24], Au- and tyrosinase-modified graphene oxide film-introduced detection of phenol [9], and polymeric film-based phenol determination [92]. As shown in Figure 6A, an SPR-based optical sensor method was successfully implemented for dopamine detection based on a sensing layer of chitosan and graphene quantum dots, which were fabricated on Au thin film. The system of the SPR angle shift-based optical sensor showed a detection limit of 1.0 fM of dopamine [93]. In contrast, Figure 6B shows an innovative polyaniline/Pt-coated fiber optic-based SPR sensor used for ultra-sensitive detection of 4-nitrophenol, with a limit of detection of 0.34 pM [23]. Figure 6: (**A**) Schematic diagram of dopamine detection using composites of chitosan/graphene quantum dots/Au thin film by means of SPR spectroscopy. Reproduced with permission from [93]. (**B**) Schematic diagram of 4-nitrophenol detection using innovative polyaniline/Pt-coated fiber optic-based SPR sensing substrate. Adapted from [23]. Recently, SPR-based sensor systems have also been introduced for application in photoelectrochemistry.