MWCNTs nanocomposites modified glassy carbon electrode for electrochemical simultaneous detection of hydroquinone and catechol

Ecotoxicology and Environmental Safety 184 (2019) 109619

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Self-assembled Ti3C2 /MWCNTs nanocomposites modified glassy carbon electrode for electrochemical simultaneous detection of hydroquinone and catechol

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Runmin Huanga, Sisi Chena, Jingang Yua,b, Xinyu Jianga,b,∗ a b

School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China Key Laboratory of Hunan Province for Water Environment and Agriculture Product Safety, Changsha, 410083, China

A R T I C LE I N FO

A B S T R A C T :

Keywords: MXene Ti3C2-MWCNTs nanocomposites Hydroquinone Catechol Electrochemical sensor

A versatile electrochemical sensor based on titanium carbide (Ti3C2) and multi-walled carbon nanotubes (MWCNTs) nanocomposite was constructed to detection catechol (CT) and hydroquinone (HQ). To prepare this novel nanocomposite, a self-assembled process was conducted by blending two-dimensional (2D) hierarchical Ti3C2 and MWCNTs under ultrasonic-assisted. X-ray diffraction (XRD), High resolution transmission electron microscopy (HR-TEM) and Scanning electron microscopy (SEM) methods as well as electrochemical technique, such as Electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Differential pulse voltammetry (DPV) were performed to characterize the Ti3C2-MWCNTs nanocomposite and illuminate the electrochemical oxidation process. Under the optimum conditions, wide linear range from 2 μM to 150 μM for both HQ and CT and low detection limit of 6.6 nM for HQ and 3.9 nM (S/N = 3) for CT have been achieved. Impressively, the sensor possesses superior selectivity, ultra-stability, and good repeatability, which was successfully applied for detecting CT and HQ in real industrial waste water sample with recovery of 96.9%–104.7% and 93.1%–109.9% for HQ and CT, respectively. Hence, Ti3C2 nanosheeets were proved to be a promising platform to construct electrochemical oxidation sensor in environmental analyses and phenolic isomers detection.

1. Introduction Transition metal carbides, nitrides and carbonitrides (MXenes), a new addition to the two-dimensional (2D) world, have aroused considerable attention and great research enthusiasm in recent years, which benefit from their large surface area, excellent metallic conductivity, hydrophilic property and environment-friendly (Li et al., 2018), endowing them with outstanding characteristics for an extensive range of application, including electrochemical energy storage field (Xie et al., 2016; Zhao et al., 2018; Zhou et al., 2018b), electrocatalysis (Rasheed et al., 2018), sensor (Lorencova et al., 2018) and biosensor (Liu et al., 2015; Wang et al., 2015). MXenes are obtained from the removal of “A” layer of the precursor MAX phase with acidic etchant, such as hydrofluoric acid (HF) (Jiang et al., 2018b), fluoride salts (Feng et al., 2017) or hydrochloric acid (HCl) and lithium fluoride (LiF) (Jiang et al., 2018a; Kumar et al., 2018; Zhang et al., 2018b). The exfoliated transition metal carbides and carbonitrides resemble exfoliated graphite, then labeled MXenes (Li et al., 2017). Generally, their formula is

∗

Mn+1XnTx (n = 1–3), in which M is an early transition metal, X represent nitrogen or carbon, T denotes terminal groups and x are their number (Yang et al., 2018). Titanium carbide (Ti3C2), as a member of MXenes family, has been vastly explored compared with other members. There are two reasons for this: one is Ti3C2 possesses not only all the advantages of the MXenes family, but also facile preparation and structural stability; the other is plenty of active sites exist in Ti3C2 (Xiao et al., 2019). Significantly, in view of these merits, Ti3C2 is popular in electrochemical detection, biosensor and sensor field lately (Sinha et al., 2018; Zhu et al., 2017a). For example, electrochemical determination of divalent metal ion with alkaline intercalation of Ti3C2MXene (Zhu et al., 2017b); Ti3C2-MXene immobilize enzyme for detecting phenol (Wu et al., 2018); the sensing of H2O2 and small molecules using Ti3C2Tx (MXene) with modified by Pt nanoparticles (Lorencova et al., 2018) and a mediator-free biosensor consisting of TiO2/Ti3C2 MXene nanocomposite encapsulating hemoglobin (Wang et al., 2015). These works paved the way for the using of Ti3C2-MXene nanomaterial in sensor field. Noteworthy, delaminating multilayered

Corresponding author. School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected] (X. Jiang).

https://doi.org/10.1016/j.ecoenv.2019.109619 Received 3 July 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

Ecotoxicology and Environmental Safety 184 (2019) 109619

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with diameter of 20–40 nm and length of 5 μm, was purchased from Shenzhen Nanotech Port Co., Ltd. Sodium dihydrogen phosphate (NaH2PO4, Purity: 99%) and disodium hydrogen phosphate (Na2HPO4, Purity: 99%) were provided by Shanghai Chemical Reagent Co., Ltd. Ultrapure water provided from a Millipore Milli-Q water purification system (Millipore, Milford, MA) was used throughout all experiments. All Reagents used in this paper were analytical purity, and without further purified before use.

Ti3C2 into few-layered nanosheets can extremely promote the electrochemical performance of Ti3C2 (Zhou et al., 2018a). However, there exist hydrogen bonds and van der Waals interactions between the Ti3C2 layers might cause restacking and aggregation, giving a great difficulty on research works and limiting electrochemical performance, which result in decreasing surface area and active redox surface (Anasori et al., 2017; Zhao et al., 2015). So it is enormously important to purposefully construct Ti3C2-based nanocomposite to elevate the electrochemical performance. After much research, introducing interlayer spacers is proved to be an effective strategy and some carbon nanomaterial, including carbon nanotubes (CNTs), fullerenes and graphene nanosheets (GNPs) have been acted as intercalator to avoid Ti3C2MXene from aggregation (Yu et al., 2018; Zhao et al., 2015). With hollow structure, large surface area and good electronic transport capacity, multi-walled carbon nanotubes (MWCNTs) was chosen as spacer to synthesize Ti3C2-MWCNTs nanocomposite to enhance the electrochemical performance of individual Ti3C2 (Zheng et al., 2018). The incorporation of MWCNTs into hierarchical Ti3C2 can ensure that expanding more open spaces and exposing more active sites. Recently, various kinds of pollutants have posed great threat to the environment (Chen et al., 2019; Mo et al., 2018; Zhang and Jiang, 2019). Especially two isomers of the benzenediol, catechol (CT) and hydroquinone (HQ) are supposed to be serious environmental pollutants (Huang et al., 2019), which show high toxicity to biological and ecological environment and thus should be monitored carefully. However, it is still a big challenge to detect them simultaneously owing to mutual interference caused by their similar structure and property. Thus, some different techniques have been carried out to detect HQ and CT, such as high-performance liquid chromatography, liquid chromatography-mass spectrometry and electrochemical method (Qi et al., 2019; Shen et al., 2017). Among these techniques, electrochemical technique has aroused vast attentions in recent years, which are attributed to inexpensive, simplicity, timesaving, high sensitivity and fast response. For instance, Fe, N and Co co-doped carbon spheres to detect HQ and CT; S, N co-doped graphene detect CT and HQ; in situ synthesis MOF-Pt-MOF-rGO for electrochemical sensing of dihydroxybenzene isomers (Qi et al., 2019; Yang et al., 2019; Ye et al., 2019). However, no works are available on the electrochemical sensing of the Ti3C2MWCNTs nanocomposite towards the detection of CT and HQ. Inspired by above considerations, we constructed an electrochemical sensor platform based on Ti3C2-MWCNTs nanocomposite to detect HQ and CT for the first time. The 2D hierarchical Ti3C2 has been prepared by removing aluminum layer from the precursor titanium aluminum carbide (Ti3AlC2) with HCl and LiF. And a self-assembled process was conducted to form the hierarchical accordion-like Ti3C2MWCNTs nanocomposite. Following, this nanocomposite was characterization by X-ray diffraction (XRD), Scanning electron microscopy (SEM), High resolution transmission electron microscopy (HR-TEM) and some electrochemical characterization methods such as Electrochemical impedance spectroscopy (EIS) and Cyclic voltammetry (CV). Meanwhile, its electrochemical behaviors were investigated by Differential pulse voltammetry (DPV). Satisfactorily, it turns out to be a successful electrochemical sensor, showing excellent performance for simultaneous determination of CT and HQ with good linear range, ultra-stability and low detection limit. A schematic diagram of the preparation and detection is shown in Scheme 1. Hence, this paper highlighted the use of Ti3C2-based nanohybrid in environmental analysis and electrochemical sensor.

2.2. Apparatus and methods XRD (Rigaku Corp., Tokyo, Japan) was performed to analyze the crystal structure of Ti3AlC2, Ti3C2 and Ti3C2-MWCNTs nanocomposite. SEM images were provided by a field emission scanning electron microscopy (FE-SEM, TESCAN MIRA3 LMH/LMU, Czech). The microstructure of Ti3C2-MWCNTs nanocomposite was obtained on HR-TEM (JEM-2100F, JEOL Ltd., Tokyo, Japan). All the electrochemical experiments were carried out using a CHI660E-Electrochemical Workstation (Chenhua Instrument Co., Ltd. Shanghai, China) with a classical three-electrode system, in which platinum wire worked as counter electrode, Ag/AgCl electrode acted as reference electrode and glassy carbon electrode utilized as working electrode. EIS measurements were conducted in a solution containing 1.0 mM K3 [Fe (CN) 6]/K4 [Fe (CN) 6] and 0.1 M KCl. CV was used to explore the voltammetric performance of HQ and CT, which were performed from 0 V to +0.6 V with a scan rate of 50 mV/s. DPV was recorded between −0.1 V and 0.4 V at a pulse period of 2s. 2.3. Synthesis of Ti3C2-MXene Ti3C2-MXene was prepared by the specific removal of aluminum layer from the precursor Ti3AlC2 powder using HCl and LiF as etchant (Chen et al., 2018). Typically, 20 mL 6 mol/L HCl was added into 1.5 g of LiF under vigorous stirring to obtain etchant. After LiF was totally dissolved, 2 g of Ti3AlC2 powder was slowly poured into the above solution and reacted for 24 h at 35 °C under magnetically stirring. The resulting product was entirely washed with ultra-purified water for several times until the pH value of the supernatant reached 6, and then centrifuged at 3500×g for 5 min. The resulting solid was collected and freezing dried. 2.4. Preparation of Ti3C2 based MWCNTs sensor The Ti3C2 based MWCNTs sensor was synthesized by a facile casting method. First of all, 1 mg of Ti3C2 and 5 mg of MWCNTs were added into 1 mL ultra-purified water and sonicated for 100 min (details of material optimization in Fig. S-1). Then, before modification, the glassy carbon electrodes (GCE) was burnished to a glazed surface using 0.05 μm Al slurries and washed with ultra-purified water, followed by sonicating for 2 min. The GCE was dried in N2 atmosphere. Finally, 8 μL Ti3C2-MWCNTs mixture solution was dispensed onto the surface of precleaned GCE, marked as Ti3C2-MWCNTs/GCE. The modified GCE was dried at room temperature. 3. Results and discussion 3.1. Structural characterization The crystalline structures of Ti3AlC2, Ti3C2 and Ti3C2-MWCNTs nanocomposites were characterized by XRD (Fig. 1A). We can see clearly that the peak (104) in the pattern of Ti3C2 disappear after selective removal of Al layer from the precursor Ti3AlC2 powder. Meanwhile, the (002) peak of Ti3C2 shifts to lower angle (from 9.54° to 8.64°), suggesting an increase in spacing for Ti3C2 layers compared with Ti3AlC2. The results show above confirms that Ti3AlC2 has been exfoliated successfully to Ti3C2 (Xu et al., 2016; Zhao et al., 2017). And

2. Experimental 2.1. Reagents HQ (Purity: 99%) and CT (Purity: 98%) were supplied by Alfa Aesar. Ti3AlC2, (Particle size: 200 mesh, Purity: 98%) was obtained from Forsman Scientific Co., Ltd., Beijing, China. MWCNTs (Purity: 97%) 2

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Scheme 1. Schematic of Ti3C2-MWCNTs nanocomposite synthesis as well as HQ and CT assay.

electrochemical performance compared with other modified electrodes. It is also necessary for the analysis of the electron transfer behaviors of all modified electrodes to implement EIS. The Nyquist plot consists of semicircular and linear. The electron transfer-limited process depends on semicircle diameter at higher frequencies that can reflect the resistance value (Rct), while the linear of lower frequencies has something to do with the diffusion process. Fig. 2B shows the Nyquist plot of Bare/GCE, Ti3C2/GCE, MWCNTs/GCE and Ti3C2-MWCNTs/GCE. The semicircle diameter of bare GCE is the largest because of the higher resistance and the Rct value is 130.6 Ω. Compared with bare GCE, the Rct value of Ti3C2/GCE and MWCNTs/GCE are relatively smaller and measured as 119.1 Ω and 104.8 Ω, respectively. It shows that the presence of Ti3C2 or MWCNTs with good electrical conductivity. When it comes to Ti3C2-MWCNTs/GCE, the Rct value decreases remarkably to 96.94 Ω. Obviously, the result implies that the incorporation of the Ti3C2 and MWCNTs speeds up the electron transfer on the surface of electrode, which increases the electrical conductivity. The above results conform to the CV measurement results. Fig. 2C demonstrates the DPV responses to 0.1 M PBS with 100 μM HQ and CT on the bare GCE, Ti3C2/GCE, MWCNTs/GCE and Ti3C2MWCNTs/GCE with a scan rate of 50 mV/s. Clearly, the bare GCE displays the poorest determination sensitivity towards HQ and CT. On the contrary, the MWCNTs/GCE and Ti3C2-MWCNTs/GCE can completely separate HQ and CT with a potential difference of 100 mV, proving they are qualified to detect HQ and CT synchronously. Moreover, compared with MWCNTs/GCE, Ti3C2-MWCNTs/GCE exhibits much higher peak current and is 5.7 times higher than the peak current of MWCNTs/GCE. With current response enhancing, we speculate that Ti3C2-MWCNTs nanocomposite with high specific surface area, outstanding catalytic activity, excellent electronical conductibility and strong adsorptive property due to more active sites can produce more efficient channels to promote electron transfer and supply sufficient conduction pathways for the oxidation process of HQ and CT. Furthermore, the DPV response to Ti3C2-MWCNTs/GCE in the absence of HQ and CT was also inspected, there is no oxidation peak appear.

observe from the XRD patterns of Ti3C2-MWCNTs nanocomposite, there appear a distinctly wide peak that belongs to the characteristic peak of MWCNTs (2θ = 25.96°) and the characteristic peaks of Ti3C2 ((002), (102), (105), (110)) still exist in the meantime, which implying that Ti3C2-MWCNTs nanocomposite was synthesized successfully. However, the addition of MWCNTs attenuate the diffraction peak of Ti3C2. The SEM technique was carried out to characterize the micro morphology of Ti3AlC2, Ti3C2 and Ti3C2-MWCNTs nanocomposite. As shown in Fig. 1B, the pristine Ti3AlC2 powder exhibits a closely aligned structure like bulky rocks. Compared to Ti3AlC2, as seen in Fig. 1C and D, almost all pristine Ti3AlC2 have been etched successfully, and the exfoliated Ti3C2 show an obvious layered structure similar to accordion. In other words, there exist many different scale spaces between the layers, which offer a high surface area. Fig. 1E is the SEM pattern of Ti3C2-MWCNTs nanocomposite, when MWCNTs are intercalated into Ti3C2 layers, the structure of Ti3C2 does not collapse, on the contrary, the spaces are enlarged, which leads to increasing surface area and providing more channels beneficial for ion transfer (Yu et al., 2018). To further analyze the microstructure of the Ti3C2-MWCNTs nanocomposite, TEM was conducted. The TEM image (Fig. 1F) indicates thin and transparent Ti3C2 nanosheets and 1D carbon nanotubes, which can be easy to distinction. Besides, TEM image obviously shows that the Ti3C2 and MWCNTs were already well combined. It means Ti3C2 nanosheets are parallel arranged and MWCNTs serve as interlayer spacer, resulting in building a robust and effective structure that can take full advantages of not only Ti3C2 but also MWCNTs. The result is in line with the SEM images.

3.2. Electrochemical characterization We performed CV experiment to study the catalytic capability of bare GCE, Ti3C2/GCE, MWCNTs/GCE and Ti3C2-MWCNTs/GCE in 1.0 mM [Fe (CN) 6]3-/4- solution containing 0.1 M KCl at a scan rate of 50 mV/s. As observed from Fig. 2A, the CV current responses of Ti3C2/ GCE, MWCNTs/GCE and Ti3C2-MWCNTs/GCE are better than the bare GCE, but Ti3C2-MWCNTs/GCE is the best among various modificated electrodes. It turns out that the conductivity of Ti3C2/MWCNTs nanocomposite is the best among these materials due to a combination of metallic conductivity of Ti3C2 and excellent electroconductibility of MWCNTs. Besides, the peak separation (ΔE) of bare GCE, Ti3C2/GCE, MWCNTs/GCE and Ti3C2-MWCNTs/GCE is 248 mV, 124 mV, 54 mV and 43 mV, respectively. It indicates that Ti3C2/MWCNTs nanocomposite have excellent catalytic activity. A well-designed structure is of significant importance in sensor, just like Ti3C2/MWCNTs nanocomposite form an accordion-like structure that tremendously improve

3.3. Optimization of the experiment conditions To improve the HQ and CT voltammetric sensing system, some experimental parameters were investigated, such as the amount of modification material, buffer solution, pH of the electrolyte solution, and scan rate. 3.3.1. Effect of the amount of modification material The influence of the amount of modification material on the sensor 3

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Fig. 1. (A) XRD patterns of Ti3AlC2, Ti3C2, and Ti3C2-MWCNTs (mass ratio 1:5) nanocomposite; SEM images of (B) Ti3AlC2; (C) and (D) Ti3C2 under different magnifications; (E) Ti3C2-MWCNTs nanocomposite; (F) TEM image of Ti3C2-MWCNTs nanocomposite.

3.3.2. Effects of the pH of the electrolyte solution and the type of buffer solution The effect of the type of buffer solution on simultaneous detecting HQ and CT at Ti3C2-MWCNTs/GCE was studied. Four kinds of buffer solution (B-R, NaAc-HAc, K2HPO4-CA and PBS) were investigated. As observed from Fig. 3C, the B-R, NaAc-HAc and K2HPO4-CA buffer solution show poor current response to HQ and CT. While the PBS exhibits unexceptionable effect, and the peak current is 4 times, 3.6 times and 2.5 times higher than that of the B-R, NaAc-HAc and K2HPO4-CA buffer solution, respectively. So the PBS buffer solution is used as the support electrolyte. The effect of the pH of the electrolyte solution is significative, since the protons involved in the electrochemical oxidation process can be

response to HQ and CT was studied using the Ti3C2-MWCNTs/GCE (Fig. 3A). With the Ti3C2-MWCNTs nanocomposite loading increases from 6 μL to 8 μL, the current response increases gradually and appears the maximum current response when 8 μL Ti3C2-MWCNTs nanocomposite solution was used. Due to the increase of dripping amount leads to the increase of active sites, so the current response becomes higher. However, the current response decreases when the amount of Ti3C2-MWCNTs nanocomposite loading increases from 8 μL to 10 μL. This result may be attributed to too much loading would lengthen the electron transfer distance, which leads to the decrease of peak current (Liu et al., 2017). Hence, 8 μL is looked as the optimal coat loading for the following experiments (Fig. 3B).

4

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Fig. 2. (A) CV behaviors; (B) Nyquist plots of different modified electrodes (a. bare GCE; b. Ti3C2/GCE; c. MWCNTs/GCE; d. Ti3C2-MWCNTs/GCE) in 1.0 mM [Fe (CN)6]3-/4- solution containing 0.1 M KCl at a scan rate of 50 mV/s (C) DPV response of bare GCE, Ti3C2/GCE, MWCNTs/GCE and Ti3C2-MWCNTs/GCE in 0.1 M PBS in the presence and absence of 100 μM HQ and CT.

MWCNTs/GCE is an adsorption-controlled electrochemical process.

influenced by the acidic or alkaline nature of the detection medium (Zhang et al., 2018a). DPV analysis was performed from pH 5.5 to pH 7.5. From Fig. 3D, an increase in peak current is seen from pH 5.5 to 6.5. On the contrary, the peak current reduces when pH value rises from 6.5 to 7.5. The reason for this is that HQ and CT with pKa (HQ) = 9.85 and pKa (CT) = 9.4 tend to deprotonate in higher pH atmosphere, which might weaken the adsorption capacity of HQ and CT on the Ti3C2-MWCNTs/GCE, so the peak current response goes down (Mohammed Modawe Alshik Edris et al., 2019). Therefore, pH 6.5 is selected for the later experiments (Fig. 3E). Meanwhile, we can see that the peak potential shifts negatively while the pH increases, which shows the participation of protons in the electrochemical process. Following linear regression equations: CT is Epa (V) = −0.064 pH + 0.6176 (R2 = 0.9975) and HQ is Epa (V) = −0.064 pH + 0.5176 (R2 = 0.9975). The slopes of two equations are −0.064 V, which verge to the theoretical value of −0.059 V obtained from Nernst equation: dEp/dpH = 2.303 mRT/nF, demonstrating two electrons and two protons participated in the electrochemical oxidation process (Fig. 3F). The mechanism of HQ and CT on Ti3C2-MWCNTs/GCE is depicted in Fig. S-2.

3.4. Determination of CT and HQ Under optimal conditions, HQ and CT were detected by DPV method. Fig. 4C, shows the DPV response to HQ and CT at Ti3C2MWCNTs/GCE in the presence of 20 μM CT with varied concentrations of HQ from 2 μM to 150 μM in 0.1 M PBS (pH = 6.5) at 50 mV/s. The linear regression equation of HQ is Ip (μA) = 0.4793 C (μM) - 0.03221 with regression coefficient of 0.9967 (Fig. 4D). Similarly, the DPV response of CT with concentrations from 2 μM to 150 μM in 0.1 M PBS (pH = 6.5) at 50 mV/s under the existence of 20 μM HQ is provided by Fig. 4E and the regression equation is Ip (μA) = 0.4789 C (μM) + 1.3158 with regression coefficient of 0.9965 (Fig. 4F). The limit of detections of HQ and CT are 6.6 nM and 3.9 nM (S/N = 3), respectively. This sensor exhibits ultra-low detection limit and wide linear range towards the simultaneous detection of CT and HQ compared with other reported works (Table 1). 3.5. Interference studies, reproducibility, stability and repeatability To estimate the anti-interference property of Ti3C2-MWCNTs/GCE, 100-fold concentration of some inorganic ions (K+, Na+, Zn2+, Al3+, Ca2+, Mg2+, Cu2+, NO3−, Cl−) and some same concentration of organic molecules, such as glucose (Glc), ascorbic acid (AA), bisphenol A (BPA), resorcinol (Re), paranitrophenol (p-NP) were added into 100 μM HQ and CT. In Fig. S-3, the peak currents of HQ and CT have nothing tremendous change (less than ± 7.55%), indicating outstanding selectivity and anti-interference property. The reproducibility, stability and repeatability of Ti3C2-MWCNTs/ GCE were evaluated by DPV. Six parallel detections were carried out on different Ti3C2-MWCNTs/GCE in 0.1 M PBS containing 100 μM HQ and CT. The relative standard deviations (RSD) are 1.6% and 1.0% for HQ and CT, respectively (Fig. S-4A), which indicate that this sensor owns excellent reproducibility. In addition, to investigate repeatability of

3.3.3. The effect of scan rate For the purposes to explore the reaction kinetics, CV was used to study the influence of scan rate on the oxidation and reduction process of CT and HQ at Ti3C2-MWCNTs/GCE from 10 mV/s to 400 mV/s. Fig. 4A presents that increasing scan rate, the anodic and cathode currents increase (Rajkumar et al., 2018). At the same time, the reduction peak potentials become more negative while the oxidation peak potentials become more positive. The linear regression equations of HQ are Ipa (μA) = 0.2385 v (mV/s) + 0.00931 (R2 = 0.9958) and Ipc (μA) = −0.2461 v (mV/s) － 0.4839 (R2 = 0.9952). The linear regression equations of CT are Ipa (μA) = 0.2476 v (mV/s) + 0.8969 (R2 = 0.9962) and Ipc (μA) = −0.2353 v (mV/s) + 0.0117 (R2 = 0.9942) (Fig. 4B). From the above results we can draw a conclusion that the oxidation and reduction of HQ and CT on Ti3C25

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Fig. 3. (A) Effect of the amount of modification material using Ti3C2-MWCNTs/GCE with different volume (6, 7, 8, 9 and 10 μL), scan rate: 50 mV/s; (B) The volumecurrent curves; (C) Current response of Ti3C2-MWCNTs/GCE towards 100 μM HQ and CT in different supporting electrolytes (K2HPO4-CA, PBS, B-R, NaAc-HAc) with scan rate of 50 mV/s; (D) DPV signal of 100 μM HQ and CT using Ti3C2-MWCNTs/GCE in different pH (5.5, 6.0, 6.5, 7.0, 7.5) with scan rate of 50 mV/s; (E) The pHcurrent curves; (F) The linearity of pH value vs. peak potential.

Ti3C2-MWCNTs/GCE, the same electrode was tested five times in one day with RSD of 2.4% and 1.7% for HQ and CT, respectively (Fig. S–4B). After five days’ tests in sequence, the peak current still kept unchanged and the RSD is 3.1% and 2.7% for HQ and CT, respectively (Fig. S–4C). Surprisingly, 50 times successive measurements using the same electrode were performed to study the stability of Ti3C2MWCNTs/GCE. The peak currents retained 90.8% and 92.6% as well as the RSD is 4.4% and 3.0% for HQ and CT, respectively (Fig. S-4D), which is a strong evidence to prove the excellent stability and repeatability of our constructed sensor.

93.1%–109.9%, respectively. Meanwhile, the RSD of real sample detection are less than ± 2.1%, which confirm that this electrochemical sensor based on Ti3C2-MWCNTs/GCE hold excellent stability. Consequently, the convincing results reveal the Ti3C2-MWCNTs sensor is feasible and reliable and HQ and CT in real samples can be detected.

4. Conclusions To sum up, an ultra-sensitive, hyperstable and simple sensor based on self-assembled 2D hierarchical Ti3C2 nanosheets and MWCNTs was constructed to detect HQ and CT. The fabricated sensor possesses not only biocompatibility, superior metallic conductivity and large specific area, but also excellent electronic conductivity. Benefiting from these merits, the sensor owns selective electrocatalytic oxidation ability towards HQ and CT. Interestingly, the prepared sensor shows ultra-stability, high selectivity and satisfactorily electrochemical responses: an ultra-low detection limit of 6.6 nM and 3.9 nM for HQ and CT with a good linear ranging from 2 to 150 μM for HQ and CT. Besides, HQ and

3.6. Real sample application Practical application of Ti3C2-MWCNTs/GCE towards HQ and CT was investigated by testing HQ and CT in industrial wastewater samples through standard addition method. The standard HQ and CT of different known concentrations were added into the wastewater samples. Tables S–1 shows the recoveries of HQ and CT are 96.9%–104.7%, 6

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Fig. 4. (A) CV curves of Ti3C2-MWCNTs/GCE in pH 6.5 with different scan rates (10, 20, 50, 100, 150, 200, 250, 300, 350, 400 mV/s); (B) Calibration plot between cathode/anodic peak currents and scan rates; (C) DPV response of HQ with different concentrations (2, 5, 10, 20, 50, 100, 150 μM) in the presence of 20 μM CT; (D) Calibration plot of HQ; (E) DPV response of CT with different concentrations (2, 5, 10, 20, 50, 100, 150 μM) in the presence of 20 μM HQ; (F) Calibration plot of CT.

Table 1 Comparison of different materials for simultaneous detection of HQ and CT. Materials NRCu/rGO N-Co-Fe-HCS M@Pt@M-rGO HMCCSs [email protected] SNGO c-MWCNTs/CTS/Au AgNP/MWCNT Ni/N-MWCNT rGO-Fe3O4–Au Ti3C2-MWCNT

Linear Range (μM) HQ 0.13–131.5 0.5–500 0.05-20,20-200 0.3–1000 0.5-10,10-70 10–320 0.5–1500 2.5–260 0.3–300 0.1–500 2–150

Detection Limit (μM) HQ 0.049 0.075 0.015 0.12 1.08,0.37 0.15 0.17 0.16 0.011 0.17 0.0066

CT 0.13–131.5 0.5–1500 0.1–160 2.0–2000 0.5-10,10-70 10–320 5–900 20–260 0.1–300 0.05–550 2–150

7

Ref. CT 0.052 0.080 0.032 0.19 1.17,0.47 0.28 0.89 0.2 0.009 0.02 0.0039

Sabbaghi (2019) Yang et al. (2019) Ye et al. (2019) Ren (2019) Chen et al. (2019) Qi et al. (2019) Shen et al. (2017) Goulart (2018) Rajkumar et al. (2018) Kong (2019) This work

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CT in industrial wastewater were further tested by the Ti3C2-MWCNTs/ GCE sensor with a desirable recovery of 96.9%–104.7% for HQ and 93.1%–109.9% for CT. Therefore, Ti3C2 are regarded as a candidate for the combination with carbon materials and have potential application in environmental analyses and phenolic isomers detection.

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