As an important chemical raw material, phenol is widely used in the industrial production of pesticides, dyes, plastics, etc. However, phenol is also a major pollutant in environmental groundwater and surface water. Therefore, the development of online sensors for selective and accurate detection of phenol concentration in water is crucial to protect the water environment and human health. In order to realize online detection of the concentration of phenolic compounds in water, electrochemical, photoelectrochemical, and fiber-optic sensors have been widely studied. Among them, the fiber-optic sensor is one of the most promising sensors for online detection of the concentration of phenolic compounds due to its advantages of long-distance transmission, anti-electromagnetic interference, quasi-distributed measurement, etc. However, fiber-optic sensors with photocatalyst or oxidase have low selectivity for the catalysis or oxidation of phenol, which makes the sensor not selective for the detection of phenol in water. In addition, there is no report on the development of horseradish peroxidase (HRP)-coated fiber-optic phenol concentration sensor. Therefore, it is necessary to develop an HRP-coated fiber-optic sensor with high sensitivity and selectivity for detecting phenol concentration by using silica optical fibers.

To increase the sensitivity and selectivity of the fiber-optic sensor for phenol concentration, a novel surface plasmon resonance (SPR) fiber-optic biosensor composed of horseradish peroxidase (HRP)-coated SPR optical fiber and phenol permeable membrane was created. In order to obtain HRP-coated SPR fibers, first, a layer of polydopamine was polymerized on the surface of the optical fibers, which was used to adsorb gold nanoparticles to form a gold film to stimulate the SPR effect, and then the polydopamine was polymerized on the surface of the gold film to immobilize HRP. Second, the phenol permeable membrane prepared with PEBA2533 and β-cyclodextrin was sealed on the HRP-coated fiber. Third, an online analytical platform for detecting the phenol in water was constructed by using polymethyl methacrylate (PMMA) plates. Fourth, the principle of phenol detection by the sensor was analyzed. Fifth, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the surface morphology and elements of the samples. Furthermore, the influence of the preparation conditions of the sensor on its performance was studied experimentally. Lastly, the output spectrum, sensitivity, response time, selectivity, and detection limit of the sensor were tested.

The experimental results highlight that when the HRP-coated fiber-optic biosensor was prepared under the optimum conditions and the sampling time of the biosensor was set to 300 s, the sensitivity and lower detection limit (LOD) of the prepared sensor reached 224.84 pm·mmol^-1·L and 159 nmol/L, respectively (Fig. 9). The optimum conditions were set as follows: 1) the polymerization time and temperature of the polydopamine were 20 min and 25 ℃, respectively (Fig. 6); 2) the adsorption time of gold nanoparticles was 3 h (Fig. 7); 3) the concentration and fixation time of HRP were 0.10 mg/mL and 3 h, respectively (Fig. 8); 4) the thickness of the phenol permeable membrane was 30 μm. The good sensitivity and LOD of the prepared biosensor were generated by the strongest SPR resonance intensity caused by the formed gold film and oxidized products (insoluble polymer) with a higher refractive index than the phenol (the products were produced during the reaction between the phenol and HRP with the assistance of H₂O₂). The prepared phenol biosensor also showed high selectivity in the 4-chlorophenol, 4-fluorophenol, 2, 4-difluorophenol, 2, 4-dichlorophenol, 2, 4, 6-trichlorophenol, 2, 3, 5-trifluorophenol, NaCl, urea, and glucose solutions. The good selectivity of the biosensor can be attributed to the fact that the phenol permeable membrane prepared with PEBA2533 and β-cyclodextrin shows high phenol selectivity and permeability.

In this paper, a novel SPR fiber-optic biosensor with high sensitivity and selectivity for detecting the concentration of phenol in aqueous solutions was developed. The sensor used HRP to catalyze H₂O₂ to oxidize phenol, so as to produce an insoluble polymer that adheres to the surface of the optical fiber, make the refractive index on the surface of the optical fiber change, and increase the shift of the SPR wavelength and sensitivity of the sensor. At the same time, the phenol permeable membrane prepared with PEBA2533 and β-cyclodextrin was sealed on the HRP-coated fiber, which promoted the sensor to achieve selective detection of phenol in water. The research in this paper will promote the development and engineering application of optical fiber sensing technology and phenol concentration online detection technology.