# Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives
## 3 Plasmonic Resonance-Based Fluorescence Sensor for Phenolic Compounds  
As a normal type of optical sensor, fluorescence techniques have been extensively applied in many fields, such as fast detection of phenolic substances [22], bioimaging [74], detection of aniline [75], determination of peroxynitrite products [76], analysis of total phenolic substances in teas [21], quantification of bisphenol A and its derivatives [77], determination of phenolics [78], and aptamer-labeled fluorescent detection of bisphenol A [79]. Plasmonic resonance-based fluorescence method has been used predominantly, because of its excellent plasmonic resonance mechanism. Various scientific studies have reported the successful fabrication of novel plasmonic resonance nanomaterials that exhibit uniform geometries and enhanced plasmon, aiming to assist fluorescence technology. Great efforts have been made on new nanomaterials with high plasmon resonance, such as AgNP-based fluorescence sensors for trace detection of dopamine [80], bimetallic Au-Ag nanocluster-assisted fluorescent biosensing of dicofol [81], polyethyleneimine-modified ovalbumin-stabilized gold nanoclusters (AuNCs) used for fluorescence sensor of tetracyclines [82], and MOF-based fluorescence detection of bisphenol substances [40]. A fluorescent sensor of 2,6-dimethyl phenol in seawater has been successfully performed relying on Eu\({}^{3+}\)-2-aminoterephthalate immobilized on mesoporous silica nanoparticles [83]. As shown in Figure 4, although dopamine has been known as one of the most significant neurotransmitters in the human body, its specific monitoring remains many chal  
Figure 3: **(A)** Schematic diagram of Au@Ni/rGO-based colorimetric detection and degradation of phenol. **(B)** Schematic diagram of mechanism of core-shell nanocomposite-assisted colorimetric detection of phenol. Adapted from [72]. lenging. Using a one-pot process, the authors successfully fabricated bimetallic Au-Ag nanoclusters using a protein template as bovine serum albumin. These initial Au-Ag hybrid nanoclusters have been found to exhibit a weak level of fluorescent intensity. However, dopamine has been introduced to enhance the extremely strong fluorescent intensity of nanoclusters, simultaneously causing a red shift in the spectrum. Using electrochemical spectroscopy, dopamine has contributed to reducing Au-Ag hybrid nanoclusters, enhancing their fluorescence, which has been adopted for trace and sensitive detection of dopamine. This facile and efficient method provided a limit of detection of dopamine at 6.9 nM [84]. Figure 5 shows a dual-emissive AuNC-based sensor technique used for detecting 4-NP by means of fluorescence quenching. Herein, the first AuNCs were synthesized and stabilized by BSA to form the AuNCs@BSA complex. Subsequently, the inner filter effect (IFE), indicated that 4-NP selectively induced fluorescence quenching at 410 nm (F\({}_{410}\)) of residual di-tyrosine (diTyr) more effectively than that at 630 nm (F\({}_{630}\)) of AuNCs. Within 1 min, this sensor was successfully employed for trace detection of 4-NP by measuring the ratio of fluorescence intensity of F\({}_{410}\)/F\({}_{630}\). Based on the 4-NP concentration-correlated ratio of F\({}_{410}\)/F\({}_{630}\), its limit of detection was estimated at 13.8 nM (1.9 ng/mL). Furthermore, this technique shows highly sensitive and selective detection of 4-NP, despite the presence of eight other common phenol derivatives. Thus, this method was practically applied for an efficient analysis of trace 4-NP in actual river water samples [85]. Figure 4: **(A)** Schematic diagram of bimetal Au-Ag nanocluster-based fluorescence detection of dopamine. **(B)** (a) Fluorescence intensity-correlated different concentrations of dopamine samples at 0, 0.01, 0.08, 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 6, 8, and 10 uM labeled from \"a\" to \"m\". (b) A linear curve of fluorescence intensities correlated with the concentrations of dopamine at pH 8.5 Tris-HCl buffer. Fluorescence measurements under 370 nm excitation after 15 min incubations.