Hydroquinone (HQ) (1,4-DHB), an isomer of diphenols (Fig. 1.6), is important in a number of biological and industrial processes, such as papermaking, coal tar production, and photography [169]. HQ has been widely used in skin lightening, cosmetics, antioxidants, polymers, dye production in the pharmaceutical industry and as an anticancer agent. HQ is a potential carcinogen with severe effects on the central nervous system [170]. In fact, HQ is a serious environmental pollutant, e.g. exposure to HQ can irritate eyes, nose, throat, skin and nail discoloration [171]. Therefore, it is necessary to identify and quantify HQ in order to estimate its possible impact. Recently, constructing enzyme-based glucose biosensors using HQ instead of labile oxygen as an electron transfer medium has received increasing attention.
HQ and catechol (CC) are central phenolic resins widely used in film developers, dyes, pesticides, and cosmetics (Huo et al., 2011). Therefore, they can be released into the atmosphere without any trouble through the manufacture of these products. Due to its high toxicity, even trace concentrations can cause great harm to the atmosphere and humans. Therefore, a practical and rapid quantifiable determination of CC and HQ is highly desirable in environmental analysis. They coexist in the hydroquinone structure  environment, making it difficult to detect or isolate them immediately (Yuan et al., 2013). A rapid, precise, and susceptible system is needed for the immediate detection of CC and HQ.
A key issue in the EC detection of HQ and CC is that their oxidation potentials overlap on unmodified GCE, implying the need for novel NMs to modify bare electrodes. Recently, some NMs have been prepared to encounter this situation and have effectively realized the instant detection of HQ and CC, demonstrating the feasibility of the EC method for real-time detection of these two isomers (Ma et al., 2013; Guo et al. al. , 2014). It was reported that ECS based on Ce-MOF/CNTs NCs treated with NaOH/H2O2 mixture could simultaneously detect HQ and CC. Structural evaluation was performed using SEM, TEM, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Post-processing changes the morphology of Ce-MOF from rod-like structure to granular structure. The bivalent state of Ce3+/4+, the high surface area provided by MOFs, and the enhanced conductivity provided by CNTs endow EC with high capability for HQ and CC detection. Differential pulse voltammetry (DPV) recordings showed high selectivity of Ce-MOF/GCE in the presence of other environmental pollutants and high recovery of HQ/CC from local river water samples (Huang et al. , 2021a).
A growing family of two-dimensional transition metals, known as MXenes, have stimulated extensive research interest due to their outstanding electrical conductivity and natural stability. Nevertheless, due to van der Waals exchange and hydrogen bonding on both sides of the MXene sheet. MXenes are persuaded to restack, which results in a drastic drop in surface area and electrical conductivity, limiting their EC performance (Shang et al., 2019). A simple self-assembled system based on a heterostructure (Ti3C2/MOF) was developed to circumvent this limitation. The electrode is composed of alk-Ti3C2/N-PC and MOF-5-NH2 derived nitrogen-doped porous carbon (N-PC) through acid etching and preliminary intercalation treatment of Ti3C2. This approach effectively prevents the restacking of Ti3C2 NSs. It enables the detection of benzenediol via a hydrogen-bonding interface transported by abundant -OH functional moieties on alk-Ti3C2 and frequent Ce-N bonds on NePC. Due to the enhanced conductivity of Mxene and the large surface area of NePC, HQ, and CC were detected based on their benzoquinone/benzenediol redox reactions. Under adjusted conditions, a broad linear range of 0.5–150 μM and low DLs of 4.8 nM and 3.1 nM were obtained for HQ and CC, respectively. Alk-Ti3C2/NePC ECS was used to detect HQ and CC in industrial wastewater with satisfactory recoveries. In the future, fabrication of alk-Ti3C2 with NePC is an advanced technique to develop a variety of alk-Ti3C2-based NCs for detection of phenolic compounds and ecological analysis (Huang et al., 2020).