Simultaneous and sensitive electrochemical detection of dihydroxybenzene isomers with UiO-66 metal-organic framework/mesoporous carbon

Highlights

• A simple preparation of zirconium-based MOF (UiO-66)/mesoporous carbon (MC) composite for the first time.

• The first electrochemical sensing platform base on UiO-66 as a catalytic center.

• UiO-66/MC-3 shows high electrocatalytic activity to dihydroxybenzene isomers oxidation.

• A large peak-to-peak potential separation between CT and HQ.

• Simultaneous and sensitive detection of dihydroxybenzene isomers.

Abstract

The zirconium-based MOF (UiO-66)/mesoporous carbon (MC) composite was synthesized using conventional hydrothermal method for the first time. The surface morphology and structure of UiO-66/MC composite were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). A novel electrochemical sensor based on UiO-66/MC was constructed for simultaneous and sensitive determination of dihydroxybenzene isomers (DBIs) of hydroquinone (HQ), catechol (CT) and resorcinol (RS). The proposed sensor displays excellent electrocatalytic activity toward the oxidation of HQ, CT and RS. The peak-to-peak potential separations between CT and HQ, and RS and CT are 0.130 V and 0.345 V, respectively. Under the optimized conditions, the electrochemical sensor shows a wide linear response in the concentration range of 0.5–100 μM, 0.4–100 μM and 30–400 μM with a detection limit of 0.056 μM, 0.072 μM and 3.51 μM (S/N = 3) for HQ, CT and RS, respectively. In addition, the sensor has superior sensitivity and electrochemical stability along with good reproducibility and anti-interference properties. The fabricated sensor was also applied for the determination of DBIs in the real water samples with satisfying results.

Introduction

Hydroquinone (HQ), catechol (CT) and resorcinol (RS) are three typical dihydroxybenzene isomers (DBIs) of phenolic compounds, which usually coexist in environmental samples [1]. They are widely used in the synthesis of antioxidants, pesticides, photostabilizers, dyes, paints, cosmetics and in other synthetic chemicals manufacturing industries [2], [3], [4]. Moreover, they have also been certified as a toxic environmental pollutant, caused by their high toxicity and low degradability in the ecological environment [5]. These phenolic compounds can quickly enter the body through contact with the skin or mucous membranes caused by poisoning symptoms. Moreover, the DBIs may induce tiredness, headache, dizziness, pale, sometimes even kidney and liver function damage, these isomers can also cause nervous system lesion [19], [20], [23]. In the process of its production and use, dihydroxybenzene water pollution accidents are occasionally induced by improper operation or accidental leakage, which will be bad for the environment and human health, even at very low concentration. It is high time that the researchers should develop sensitive and rapid analytical techniques for the determination of them. Up to date, a number of analytical methods have been established to determine phenolic compounds, including high-performance liquid chromatography [6], fluorescence [7], [8], [9], [10], [11], chemiluminescence [12], photolithography [13], spectrophotometry [14], mass spectrometry [15], capillary electrochromatography [16], and electrochemical methods [17], [18], [19], [20]. The electrochemical detections have attracted significant attention in the simultaneous determination of DBIs on account of many intrinsic advantages such as straightforward operation, low cost, fast response, high sensitivity, good selectivity and so on. However, due to three isomers have similar stereochemical structure and contiguous redox potentials on conventional electrode, the simultaneous determination of them is nearly impossible. To overcome these shortcoming, various nanomaterials modified electrodes including metal sulfides [21], quantum dots [22], metal/metallic oxide nanoparticles [23] and carbon nanotubes [24] have been used to individual or simultaneous determination of them. Nevertheless, it is noticed that the synthesis process of above mentioned materials should be simplified and the sensitivity for the simultaneous detection of three isomers needs to be promoted. Therefore, it is an urgent need for researchers to design ingenious and convenient sensing platform for the simultaneous and sensitive determination of DBIs.

Metal-organic frameworks (MOFs), which consist of metal ions/clusters connected by organic linker groups, have recently emerged as a type of novel three-dimensional (3D) coordination compounds with unique properties, such as ordered crystalline structure, permanent porosity, high specific surface areas, excellent thermal stability, and tailorable chemistry. As a result of these fascinating structures and properties, MOFs have been widely used in gas capture and separation [25], catalysis [26], energy storage [27], drug delivery [28] and sensing [29], [30] in the past few years. Recently, MOFs have received special attentions in electrochemical applications. Whereas, since their intrinsic weaker conductivity and electrochemical stability, the electrochemical applications of single-phase MOFs still exist certain limitation. Thus, recent research interest has gradually shifted from single-phase MOFs to MOFs-base composites. Thus, heterogeneous nanocomposites with multiple functional components, especially conductive carbon materials, have been proposed. The nanocomposites have significantly improved electronic conductivity and electrochemical stability of MOFs materials. For instance, Zhang and co-workers reported MOF-macroporous carbon hybrid material for the oxidation of hydrazine and the reduction of nitrobenzene [31]. The combination of Cu-based metal organic frameworks (Cu-MOFs) and macroporous carbon (MPC) was investigated, which show enhanced stability and good electrocatalytic ability for ascorbic acid and hemoglobin [32]. Multi-walled carbon nanotubes (MWCNTs) was implanted into manganese-based MOFs (Mn-BDC) via one-step solvothermal method, this material can be realized simultaneous detection of biomolecules in body fluids [33]. An AuNP@MOF composite modified carbon paste electrode for the determination of bisphenol A (BPA) was developed by Silva et al. [34]. The selective detection of Bisphenols at the MOF-chitosan sensing platform was reported [35]. Herein, the combination of carbon materials with MOFs for the electrochemical sensing platforms are highly desirable. Mesoporous carbon (MC), one of carbon materials, is promising as a support platform for MOFs owing to their large specific surface area and high electrical conductivity [36].

All kinds of MOF materials, we turned our attention to UiO-66 MOF due to its excellent thermal, aqueous and acid stability [37]. As a zirconium-based MOF, the UiO-66 has a face-centered cubic crystal structure. The structure of UiO-66 composed of octahedral cages and tetrahedral cages, and these cages is provided by narrow triangular windows with a diameter close to 0.6 nm [38]. These unique features of UiO-66 have motivated researches for wide applications. But there is extremely rare work on UiO-66 and their composites for the electrochemical areas. To date, the UiO-66 MOF has been utilized as the catalytic carrier of noble metals for the detection of H₂O₂ [39] and telomerase [40]. Nevertheless, to the best of our knowledge, it is not found that the UiO-66 MOF and their composites as an electrochemical sensing material are used for electrochemical studies. Therefore, we hope to fabricated a novel electrochemical sensing platform base on UiO-66 for the catalytic center, which not only avoids the use of noble metals but also further extends the application of UiO-66 and other MOFs. In addition, there is not found the application of UiO-66 and their composites for the highly selective and sensitive detection of isomers.

In this study, we combined the advantages of zirconium-based MOF (UiO-66) and mesoporous carbon (MC) to fabricate a novel electrochemical sensing platform for the simultaneous detection of DBIs through simple and convenient solvothermal treatment of the UiO-66 precursor mixture and MC for the first time (Scheme 1). The obtained composites are defined as UiO-66/MC-x, and its morphology were characterized. The electrochemical measurements show that UiO-66/MC-3 exhibits drastically enhanced sensitivity with regard to simultaneously determination of DBIs. The sensitivity of HQ, CT and RS is 0.360 μA μM⁻¹, 0.142 μA μM⁻¹ and 0.034 μA μM⁻¹, respectively. Compared with the existing reports, the peak-to-peak potential separation for the oxidation peaks of the CT and HQ achieved a maximum. Furthermore, the developed sensor was used for the analysis of real water samples successfully.