# Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives
## 2 Plasmonic Resonance-Based Colorimetric Sensor for Phenolic Compounds  
The colorimetric sensor is a simple method that has been widely used in the selective detection of analytes using color changes in probes under unique conditions. This method can be widely applied in environmental gas sensing [62] and trace detection of tetracyclines in foods [63] thanks to its convenient performance. For example, a paper-based bioassay has been developed by assembling alginate and chitosan layer-by-layer. A tyrosinase enzyme has been used to bind these layers, leading to successful bioassay-based colorimetric sensing of not only phenol, but also bisphenol A, catechol, and cresols. The detection limit of these phenolic substances has been estimated at 0.86 (\(\pm\)0.1) \(\mu\)g/L for each of the analytes [64]. A colorimetric sensor, based on plasmonic resonance nanomaterials, is one of the best analytical techniques for the efficient detection of various phenolic compounds by observing the color change of nanomaterials with either UV/Vis or the naked eye [39]. The basic principle of colorimetric technology is based on the specific interaction between the analytes with either pristine or probe-modified nanomaterials. For illustration, single-stranded DNA-regulated gold nanoparticle (GNP)-based colorimetric sensors have been demonstrated as an effective indicator for the sensitive detection of phenols [65]. As shown in Figure 2A, colorimetric detection of phenol has been successfully performed using Fenton reaction with a mixture of single-stranded DNA and initial GNPs. The authors have found that single-stranded DNA assisted in making initial GNPs more stabilized owing to electrostatic repulsion, leading to the prevention of GNP aggregation under the NaCl-introduced condition. However, these single-stranded DNAs have been destroyed by the Fenton reagents (OH free-radical) generating small fragments as mono- or oligonucleotide, resulting in the disruption of AuNP stability. In the presence of the phenolic substances, a reduction-oxidation between Fenton reagents and phenolic compounds will firstly occur to avoid the destruction of single-stranded DNA to assist the stability of GNPs from NaCl-induced aggregation. Meanwhile, Figure 2B illustrates that OH free-radical generated from Fenton reagents could attack catechol, leading to a reduction in phenolic compounds. Either with the naked eye or using UV/Vis spectroscopy, a GNP-assisted sensor exhibits rapid micromolar detection of catechol and hydroquinone. Figure 2: (**A**) AuNP-based colorimetric sensor for phenols’ detection by means of Fenton reaction. (**B**) Schematic diagram of the mechanism of catechol oxidation owing to OH free-radicals generated from Fenton reagents. Adapted from [65]. Colorimetric detection of bisphenol A has been successfully performed using specific aptamer-based and cationic polymer-assisted GNP aggregation with a limited detection as low as 1.50 nM [66]. In contrast, an aptasensor-based and GNP-assisted colorimetric technique has been developed for ultra-sensitive detection of bisphenol A. In this study, the authors have reported that GNPs have been modified with the two types of specific aptamers as truncated 38-mer and 12-mer, leading to the limits of bisphenol A detection of 7.60 pM and 14.41 pM, respectively [67]. On the other hand, elliptical or sphere shapes of Fe\({}_{3}\)O\({}_{4}\) nanoparticles with an average size of 7 nm indicate successful colorimetric detection of phenol in a range of 1-200 mM concentration [19]. Among various plasmon resonance nanomaterials, spherical Au nanostructures are well-known as the best colorimetric sensing units. Recently, special MOF structures have emerged as a novel material construction successfully applied in colorimetric detection. Numerous MOF structures with tunable colorimetric characteristics have been well designed and synthesized, aiming to be applied for sensing various analytes, including organic compounds and gaseous pollutants [68].