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Nitrogen-doped hollow carbon nanospheres wrapped with MoS2 nanosheets for simultaneous electrochemical determination of acetaminophen and 4-aminophenol
Journal of Electroanalytical Chemistry 847 (2019) 113229
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Nitrogen-doped hollow carbon nanospheres wrapped with MoS2 nanosheets for simultaneous electrochemical determination of acetaminophen and 4aminophenol Depeng Zhang1, Junjuan Qian1, Yinhui Yi, Odoom Jibrael Kingsford, Gangbing Zhu
T
⁎
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China
ARTICLE INFO
ABSTRACT
Keywords: Acetaminophen Aminophenol MoS2 Hollow carbon spheres Electrochemical sensor
Since acetaminophen (ACAP) and 4-aminophenol (4-AP, the degradation product from ACAP) have different properties and toxicities, the simultaneous electrochemical determination of ACAP and 4-AP is very important. In this work, nitrogen-doped hollow carbon spheres wrapped with MoS2 nanosheets (MoS2@NHCSs) novel hybrids were synthesized via a simple step-by-step method and consecutively the obtained MoS2@NHCSs was successfully used to construct a reliable electrochemical sensor for simultaneously detecting acetaminophen (ACAP) and 4-aminophenol (4-AP). Owing to the synergetic properties of each component in MoS2@NHCSs nanohybrids (MoS2 has highly catalytic activity and can offer many active sites; NHCSs can impede the aggregation tendency and enhance the effective surface area of MoS2 as well as increasing the conductivity of the nanohybrids), the experimental results demonstrated that the electrochemical responses of both 4-AP and ACAP at MoS2@NHCSs modified electrode are much better than those at NHCSs or MoS2 modified electrodes. Importantly, the MoS2@NHCSs nanohybrids can show excellent electrochemical sensing performance for the sensitive and selective as well as simultaneous determination of ACAP and 4-AP. After optimizing various conditions, the proposed MoS2@NHCSs-based sensor has wide linear range and low detection limit for ACAP and 4-AP analysis.
1. Introduction A widely used analgesic and antipyretic drug, acetaminophen (ACAP) does not pose any harmful effects but an overdose could give rise to serious negative effects [1–4]. 4-Aminophenol (4-AP) is the primary hydrolytic degradation product from ACAP and it is highly toxic and has nephrotoxicity and teratogenic properties [5–7]. As a consequence, it is very important to develop an effective technique for the simultaneous determination of ACAP and 4-AP. Up to now, many electrochemical methods have been devoted to detect ACAP and 4-AP separately, owing to the advantages of rapid response, inexpensive instrument and high sensitivity as well as simple operation [8–11]. Although some reports have focused on the simultaneous electrochemical sensing for ACAP and 4-AP [12–15], it is still a great challenge to develop an electrochemical sensor with high selectivity and high sensitivity. For improving the analytical performance of sensors, specific nanomaterials are usually needed to modify electrodes. Among a great
deal of novel electrode materials, a typical member of the family of 2 dimensional transition metal dichalcogenide with similar layered structure of graphene, molybdenum disulfide (MoS2) nanosheet is becoming a research focus [16–18]. MoS2 nanosheets are composed of a metal Mo layer sandwiched between two sulfur layers and generally stacked together by weak Van der Waals interactions [19–24]. Up to now, MoS2 received many attentions in the fields of lithium battery [25], catalysts [26], supercapacitor [27], lubricants [28] and sensors [29,30] owing to its unique properties, such as outstanding catalytic ability, large surface area, remarkable electron mobility and high density of electronic states. The interesting advantages of MoS2 also make it regarded as one of the most promising layered materials in the development of next-generation sensors [22]. Recent reports indicated that MoS2 shows excellent electrocatalysis properties toward phenolic compounds [16], which predicate that MoS2 nanosheet maybe a desirable nanomaterial for electrochemical sensing ACAP and 4-AP. Nevertheless, the layered structure of MoS2 can give some defects including inevitable stack and lower electronic conductivity, which
Corresponding author. E-mail address: [email protected] (G. Zhu). 1 Were co-first authors. ⁎
https://doi.org/10.1016/j.jelechem.2019.113229 Received 15 February 2019; Received in revised form 11 June 2019; Accepted 12 June 2019 Available online 13 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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restrict its applications in electrochemical sensing [31–34]. It's well known that most of the carbonaceous materials have high specific surface area and good electric conductivity that are always selected as functional active additives to enhance the dispersity and conductivity of electrode materials. To overcome the issues of MoS2, two conducting carbonaceous materials, carbon nanotubes (CNTs) and reduced graphene (RG), have been introduced to combine MoS2 for obtaining the nanohybrids which can exhibit the synergistic properties of MoS2 and carbonaceous materials [35–38]. Meanwhile, considerable attention has been paid to hollow nanostructures due to their excellent properties (e.g., large specific surface area, low density, shell permeability) and potential applications in many fields [39,40]. Among the family of hollow nanostructures, hollow carbon spheres (HCSs) is very intriguing owing to their high thermal stability, large interior void, high porosity and excellent conductivity [41–43]. Compared with RG and CNT, HCSs is more suitable as support to integrate MoS2 for improving its dispersibility and conductivity owing to the special spherical structure of HCSs [44,45]. Presently, some MoS2-HCSs nanohybrid has been used in H2 evolution reaction and capacitors [46], but there is no report presented for electrochemical sensing based on this nanohybrid. Therefore, developing a simple method to prepare a nanohybrid composed of MoS2 and HCSs and using the nanohybrids for electrochemical sensing is of great significance. By using MnO2 nanotubes as template and dopamine (DA) as carbon source, MoS2 nanosheets grown on nitrogen-doped carbon coated TiO2 tubes were achieved most recently for enhanced lithium storage [47,48]; in the absence of template, our group prepared solid carbon spheres coated with MoS2 by using glucose as carbon source recently [49]. Inspired by this insight, via introducing SiO2 nanospheres as template and DA as carbon source, in this work, ultrathin MoS2 nanosheets uniformly wrapped nitrogen-doped HCSs nanohybrids (MoS2@NHCSs; the N-doping is beneficial to further improve the electrochemical properties of NHCSs [50]) were prepared successfully via a simple three-step processes (Scheme 1): (a) producing silica@ polydopamine (SiO2@PDA) nanocomposites via the auto-polymerization of dopamine on SiO2 surface; (b) carbonizing and etching of SiO2@ PDA to form NHCSs; (c) wrapping NHCSs with MoS2 to obtain MoS2@ NHCSs nanohybrids. This smart design could effectively prevent aggregation, increase electric conductivity, and provide more active sites for electrochemical sensing. Then, the produced MoS2@NHCSs nanostructures were used to construct an electrochemical sensor for the simultaneous determination of 4-AP and ACAP. The experimental results showed that MoS2@NHCSs can exhibit the synergistic advantages from
NHCSs and MoS2 in electrochemical sensing. Under the optimized conditions, the MoS2@NHCSs-based sensor possesses high sensitivity and selectivity for 4-AP and ACAP coupled with low detection limits and wide linear range. Furthermore, the proposed sensor was applied successfully to simultaneously determine of 4-AP and ACAP in real samples including paracetamol tablet products and human serum. It's believed that the produced MoS2@NHCSs have important applications in electrochemical sensing of 4-AP/ACAP and other analytes. 2. Experimental section 2.1. Reagents and apparatus Na2MoO4·2H2O, 4-AP, ACAP, tris(hydroxymethyl)aminomethane (Tris), L-cysteine were purchased from Aladdin Industrial Inc. (Shanghai, China). Dopamine hydrochloride and SiO2 particles were purchased from Alfa Aesar (USA). As the supporting electrolyte, 0.1 M phosphate buffer solution (PBS) was prepared with Na2HPO4, NaH2PO4 and KCl. All other chemicals not mentioned were of analytical grade and used as received. Aqueous solutions used throughout were prepared with ultrapure water (Resistivity: ≥18 MΩ∗cm at 25 °C), obtained from a Millipore purification system. All the electrochemical measurements were performed on a CHI 660E Electrochemical Workstation (Chenhua Instrument Company of Shanghai, China). A conventional three-electrode system was used with a glass carbon electrode (GCE; 3 mm diameter) as working electrode, a platinum wire as auxiliary electrode, and an Ag/AgCl electrode (saturated KCl) as reference electrode. Unless otherwise specified, all the measurements were carried out in PBS (0.1 M, pH 6.0) at room temperature (25 ± 2 °C). All the potentials in this work were referred to Ag/AgCl electrode (saturated KCl). 2.2. Synthesis of MoS2@NHCSs (a) SiO2@PDA: SiO2@PDA was synthesized via an auto-polymerization process according to the previous report [51]. In brief, silica particles (80 mg) were mixed with dopamine (80 mg) in Tris-buffer (25 mL, 10 mM; pH 8.5) by mechanical stirring for 24 h. The SiO2@ PDA was collected by centrifugation and drying in vacuum at 60 °C for 8 h. (b) NHCSs: By carbonizing SiO2@PDA under N2 atmosphere at 800 °C for 3 h with a heating rate of 5 °C min−1, and sequentially removing the SiO2 template in 1 M NaOH aqueous solution for 24 h, the uniform NHCSs structures were obtained. (c) MoS2@NHCSs: MoS2@NHCSs nanohybrids were prepared through a facile hydrothermal process. HCS (0.05 g) was dispersed in DI water (30 mL) by ultrasonication for 30 min. Sodium molybdate (0.1 g) and L-cysteine (0.4 g) were then added into the solution under vigorous stirring. The mixture was transferred into a 50.0 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 12 h. The autoclave was then left to cool to room temperature in the oven. The resulting black product was collected by centrifugation, filtrated and washed with water and ethanol for several times, and then dried at 60 °C overnight. After collection, the as-prepared products were annealed under N2 atmosphere at 800 °C for 2 h with a heating rate of 2 °C min−1. For comparison, MoS2 nanosheets were prepared under the similar conditions without the addition of NHCSs. 2.3. Preparation of the modified electrodes 0.5 mg/mL MoS2@NHCSs suspension was prepared by dissolving 5.0 mg MoS2@NHCSs nanocomposite in 10.0 mL ultrapure water. MoS2@NHCSs modified glass carbon electrode (GCE) (MoS2@NHCSs/ GCE) was prepared by casting 10.0 μL MoS2@NHCSs suspension on the
Scheme 1. The procedures for preparing MoS2@NHCSs nanocomposite and electrochemically sensing of 4-AP and ACAP. 2
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Fig. 1. TEM images of NHCSs (A), MoS2 (B) and MoS2@NHCSs (C, D).
preprocessed GCE and drying under an infrared lamp. For comparison, NHCSs/GCE and MoS2/GCE were prepared via the same procedure. The electrochemical determination of 4-AP and ACAP was performed in 0.1 M phosphate buffer solution (PBS) consisting of two consecutive steps: (1) pre-concentration of 4-AP and ACAP from the solution to the modified electrodes with a definite accumulation time; and (2) electrochemical detecting by cyclic voltammetry (CV) or differential pulse voltammetry (DPV).
MoS2 nanosheets and the surface of the NHCSs became rough. These were consistent with TEM images. The crystallographic structure of the as-prepared samples was examined by X-ray powder diffraction (XRD), and the result was shown in Fig. S2. The XRD spectrum of hollow carbon spheres reveals three principles and highly disperse peaks arising at 2θ = 12.32°, 30.45°, 41.96°, which may ascribe to the amorphous carbon derived from dopamine. It can be found that MoS2@NHCSs displays five diffraction peaks at 13.8°, 33.1°, 39.4°, 49.5°, and 59.3°, which is consistent with the (002), (100), (103), (105), and (110) planes of hexagonal MoS2 (JCPDS card No. 37-1492). Besides, similar diffraction peaks can be observed from MoS2, demonstrating the successful formation of pure MoS2 crystal. No diffraction peaks from impurities or residues have been discovered in the XRD pattern. Raman spectroscopy was further utilized to investigate NHCSs, MoS2 and MoS2@NHCSs (Fig. S3). In high-frequency region, the spectrum of NHCSs reveals two characteristic peaks at 1363 and 1581 cm−1, assigning to the D and G bands of carbon materials, respectively. For pure MoS2, two intensity peaks at the band of 378 and 402 cm−1 can be observed clearly, corresponding to the characteristic in-plane E2g1 and out-of-plane A1g vibration modes of S-Mo-S bonds of the MoS2 crystal. It is observed that the frequency difference between E2g1 and A1g modes is large (∆f = 24 cm−1), indicating the layer thickness of MoS2 nanosheets is few-layers [52]. The relative intensity ratio of D band to G band (ID/IG) is widely utilized to clarify the graphitization degree of carbon materials. Two characteristic peaks of NHCSs were located at 1339 and 1586 cm−1, corresponding to the D and G bands, respectively. The value of ID/IG was evaluated to be 0.88 for NHCSs. Compared with the NHCSs, the value of ID/IG was calculated to 0.81 for MoS2@NHCSs, which demonstrates the successful formation of MoS2@NHCSs nanocomposite.
3. Results and discussion 3.1. Characterization of MoS2@NHCSs and related nanomaterials The morphology and microstructure of MoS2@NHCSs and related nanomaterials (NHCSs and MoS2) were characterized by transmission electron microscopy (TEM) and scanning electron microscope (SEM). As shown in Fig. 1A, the uniform NHCSs can be clearly observed, and the thickness of the carbon layer was ~10 nm. TEM image of MoS2 in Fig. 1B displays the nanosheets are close to transparency, indicating that they are ultrathin. The visible lattice fringes on the edge view of some sheet-like structures can be observed and the interplanar distance was about ~0.65 nm, which were consistent with (002) crystalline planes of hexagonal MoS2. Fig. 1C–D were TEM images of MoS2@ NHCSs at different magnifications, it can be observed that the NHCSs was uniformly wrapped by dense MoS2 nanosheets with abundant exposed edges, thus impeding the aggregation of MoS2. The SEM images of MoS2@NHCSs were shown in Fig. S1, it is observed that the NHCSs have smooth surfaces and were uniformly distributed (Fig. S1(A)). As shown in Fig. S1(B), MoS2 has a flower-like structure composed of nanosheets and the nanosheets extensively aggregate and overlap with each other. The morphology of MoS2@NHCSs (Fig. S1(C)) reveals that the NHCSs were densely wrapped with many ultrathin and interlaced 3
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3.2. Electrochemical characterization of fabricated electrode Electrochemical behaviors of different modified GCE were investigated by CV in 5.0 mM K3[Fe(CN)6]3−/4− containing 0.1 M KCl (Fig. S4). As shown in Fig. S4(A), the GCE shows the lower redox peak current (curve a) than that of other modified electrode, and the peak currents increase when MoS2 is modified on GCE (curve b). While NHCSs are coated on GCE, the electronic transmission is highly accelerated owing to the excellent conductivity of NHCSs, which result in a greatly intensive current response (curve c). As for the MoS2@NHCSs nanohybrid, it displays higher redox peak current (curve d) than MoS2 due to the presence of NHCSs in nanohybrid. Electrochemical impedance spectroscopy (EIS) was further used to study the electrochemical behaviors of the electrodes. Fig. S4(B) depicted the EIS plots of different modified electrodes recorded in 5.0 mM Fe(CN)63−/4− containing 0.1 M KCl with a frequency range of 0.1 Hz–10 kHz. By simulating obtained impedance data with Randles equivalent circuit (inset of Fig. S4(B)), the charge transfer resistance (Rct) of the bare GCE, MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE were 309.5, 134.4, 31.2, and 69.8 Ω, respectively. These results clearly demonstrated that the resistance decreased significantly with NHCSs/ GCE, owing to the rapid electron transfer property of NHCSs. Compared to the MoS2/GCE, the Rct value decreased on MoS2@NHCSs modified GCE, which further implies that the presence of carbon nanomaterials in MoS2@NHCSs can highly enhance the conductivity of composite electrode. 3.3. Electrochemical behavior of 4-AP and ACAP at modified electrodes The electrochemical behaviors of 5.0 μM 4-AP and ACAP at different modified electrodes (bare GCE, MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE) investigated by CV were shown in Fig. 2A. On the bare GCE, there was only a pair of weak and broad redox peaks for 4-AP and ACAP in the potential range of −0.2 to 0.8 V (curve a), suggesting that bare GCE cannot achieve directly the simultaneous determination of 4-AP and ACAP. Compared with the bare GCE, two well-separated oxidation peaks were clearly observed at MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE due to their excellent advantages. However, the oxidation peak currents of both 4-AP and ACAP at MoS2@NHCSs/GCE were increased remarkably and outperformed all the other modified electrodes owing the synergetic effects of each component in MoS2@ NHCSs: the MoS2 has highly catalytic activity and offer many active sites; NHCSs impede the aggregation tendency and enhance the effective surface area of MoS2 as well as increasing the conductivity of the nanohybrids. Furthermore, the oxidation peak potentials for 4-AP and ACAP were 0.119 and 0.411 V, respectively, presenting a peak-to-peak separation of 0.292 V. Fig. 2B shows the CV responses of 5.0 μM 4-AP (a), ACAP (b), and their mixture solution (c) at MoS2@NHCSs modified electrode. It's noted that the oxidation/reduction peaks of single 4-AP and ACAP appear at 0.121 and 0.411 V, respectively. When MoS2@ NHCSs/GCE was used to measure the CV response of the mixture solution containing both 4-AP and ACAP, the redox peaks of 4-AP and ACAP have little changes compared to single analyte. These data implied that the MoS2@NHCSs nanocomposite was a very promising material with excellent sensing performance for the simultaneous selective determination of 4-AP and ACAP.
Fig. 2. (A) CV responses of 5.0 μM 4-AP and ACAP on bare GCE (a), MoS2/GCE (b), NHCSs/GCE (c) and MoS2@NHCSs/GCE (d) 0.1 M PBS. (B) CV of 5.0 μM 4AP (a), ACAP (b) and the mixture containing 4-AP and ACAP at MoS2@NHCSs/ GCE in 0.1 M PBS.
from 5.0 to 6.0. The result may be ascribed to the high concentration of protons in the solution, which can replace phenol molecules on the adsorption sites of MoS2@NHCSs/GCE surface. Further increasing pH to 9.0 results in a decrease in the oxidation peak current, as hydroxyl anions may prevent phenol compounds from entering adsorption sites on the surface of MoS2@NHCSs/GCE. Fig. S5(B) further demonstrates the above conclusions. Fig. S5(C) displays that both the peak potentials of 4-AP and ACAP shifted negatively with the increase of pH. The linear regression equations can be expressed as: Epa (V) = 0.54–0.064 pH (R2 = 0.992) for 4-AP and Epa (V) = 0.78–0.061 pH (R2 = 0.995) for ACAP. The slope value of ~64 mV/pH and ~61 mV/pH was close to the theoretical value of ~57.6 mV/pH, indicating that the number of protons and electrons occurred in the electrode reaction is equal. The effect of scan rate on the current response of a mixed solution of 5.0 μM 4-AP and ACAP in 0.1 M PBS (pH 6.0) at MoS2@NHCSs/GCE was further investigated by CV (Fig. S6). Redox peak currents increased continuously with the increase of the scan rate. It can be observed that the redox peak currents (Ip) exhibited good linear relationship with the square root of the scan rate (υ1/2), indicating that the electrode reaction of 4-AP and ACAP on the MoS2@NHCSs/GCE was a typical diffusioncontrolled process. Linear regression equations between Ip and υ1/2 for 4-AP and ACAP can be expressed as the following Eqs. (1) and (2),
3.4. Optimization of the conditions for the electrochemical determination of 4-AP and ACAP The electrochemical responses of 4-AP and ACAP were influenced by the acidity of the electrolyte because protons are involved in the electrode reaction. Effect of buffer pH on the electrochemical behavior of 5.0 μM 4-AP and ACAP at MoS2@NHCSs/GCE was investigated by DPV at a pH range of 5.0–9.0. Fig. S5(A) reveals that the oxidation peak currents of 4-AP and ACAP gradually increase with the pH increasing 4
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Fig. 3. DPV curves at MoS2@NHCSs/GCE in 0.1 M PBS (pH 6.0) containing (A) constant concentrations of ACAP with different concentrations of 4-AP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM); (B) constant concentrations of 4-AP with different concentrations of ACAP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM); (C) different concentrations of 4-AP and ACAP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM). (D) The corresponding calibration curves. Table 1 Comparison with other proposed sensor for detection of 4-AP and ACAP. Electrode
Linear range (μM) 4-AP
MoS2 nanoclusters Au/Pd/reduced graphene oxide Graphene-polyaniline CdS quantum dots-graphene Graphene TiO2-graphene/poly(methyl red) Reduced graphene oxide@poly(3,4-ethylenedioxythiophene) nanotubes Graphene Paper-based microfluidic device Poly(chromium Schiff base complex) Poly(3,4-ethylenedioxythiophene) This work
ACAP
0.04–17.0 1–300.0 0.2–100.0 0.05–3.5
0.05–2000.0 0.008–0.133 4–320 0.05–20
respectively:
Ip (µA) = 6.885
1/2
(mV s 1)
25.99 (R2 = 0.9925) (For 4
Ip (µA) = 8.557
1/2
(mV s 1)
34.96 (R2 = 0.9933) (For ACAP)
AP)
5.0–800.0 0.25–50.0 1.0–35.0 0.1–20.0 0.008–0.125 1–100
Detection limit (μM)
R2
4-AP
4-AP
0.03 0.12 0.065 0.023
10.0 0.0056 1.2 0.013
ACAP
Refs. ACAP
0.989
1.2 0.025 0.4 0.032 25.0 0.0068 0.4 0.02
0.996 0.992 0.999 0.9959 0.9955 0.9985
0.986 0.999 0.9945 0.9945 0.9956
[53] [14] [54] [11] [55] [56] [8] [57] [58] [12] [59]
the electrochemical reaction of 4-AP and ACAP on the MoS2@NHCSs/ GCE. Fig. S7 reveals that the effect of MoS2@NHCSs amounts on the oxidation peak currents of 4-AP and ACAP. As shown in Fig. S7(A), both peak currents of 4-AP and ACAP increase gradually with increasing the MoS2@NHCSs amount from 4.0 μL to 10.0 μL, and reach a maximum at 10.0 μL, and then decrease obviously. Thus, optimal coating amount (10.0 μL) was utilized to prepare the modified electrode in this study.
(1) (2)
Apart from the pH of PBS and scan rate, the coating amount of MoS2@NHCSs suspension and the accumulation time are essential for 5
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Fig. S7(B) displays that the oxidation peak currents of 4-AP and ACAP increase with the increase of the accumulation time and the maximum are observed at 120 s and 100 s, respectively. As the peak current of ACAP at 120 s is similar to the current at 100 s, 120 s was chosen as the optimal accumulation time in this work.
3.7. Real sample analysis The MoS2@NHCSs modified electrode was applied to determine 4AP and ACAP in paracetamol tablet products. Prior to determination, the commercial paracetamol tablets were finely comminuted in an agate mortar and accurately weighed in triplicate (100 mg/part). Then the sample was extracted with 30 mL anhydrous ethyl alcohol for 30 min. After centrifugation for 5 min and filtration, the filtrate was collected in a volumetric flask and diluted with anhydrous ethyl alcohol. Subsequently, electrochemical measure was used to determine 4AP and ACAP in an appropriate amount of these diluted samples. The standard addition method was utilized to evaluate the recovery rate and the results were shown in Table S1. It was observed that the results were in accordance with the original contents of paracetamol and the RSD of three parallel detections was ≪4.22%. The recovery of 4-AP was in the range of 95.0%–104.8%. These results suggested that the MoS2@ NHCSs/GCE could be efficiently applied to the monitoring of 4-AP and ACAP in pharmaceutics with satisfactory results.
3.5. Simultaneous determination of 4-AP and ACAP Under the optimized conditions, the quantitative and simultaneous determination of 4-AP and ACAP was performed in 0.1 M PBS (pH 6.0) at MoS2@NHCSs/GCE by DPV because of its high current sensitivity and selectivity. The individual determination of 4-AP and ACAP in their mixtures was first performed, where the concentration of one species remained constant while the concentration of the other species increased. As shown in Fig. 3A, the peak currents linearly increased with increasing concentration of 4-AP from 0.05 μM to 20.0 μM, while the concentration of ACAP remained constant at 2.0 μM. The regression equation was Ip (μA) = 0.1978 + 2.087C (μM) (R2 = 0.9987) and the detection limit for 4-AP was evaluated to be 0.011 μM (S/N = 3). Fig. 3B shows the DPVs of ACAP with different concentrations in the presence of 5.0 μM 4-AP. The peak current of ACAP linearly increased with increasing concentration from 0.05 μM to 20.0 μM. The regression equation was Ip (μA) = 0.2712 + 2.755C (μM) (R2 = 0.9965) and the detection limit for ACAP was evaluated to be 0.015 μM (S/N = 3). Furthermore, the simultaneous determination of 4-AP and ACAP was studied by varying their concentrations simultaneously. As shown in Fig. 3C, two separated and well-defined oxidation peaks for 4-AP and ACAP at about 130 and 410 mV, respectively, while the oxidation peak currents were increased consistently. The linear ranges for the determination of 4-AP and ACAP were 0.05–20.0 μM with the detection limits of 0.013 μM and 0.020 μM (S/N = 3), respectively. The regression equations could be expressed as Ip (μA) = 0.1425 + 2.016C4-AP (μM) (R2 = 0.9985), and Ip (μA) = 0.3529 + 2.695CACAP (μM) (R2 = 0.9956), respectively. All these results demonstrated that the MoS2@NHCSs modified electrode could be successfully utilized for individual or simultaneous determination of 4-AP and ACAP with high selectivity and sensitivity. The performance of the proposed MoS2@ NHCSs modified electrode was compared with other modified electrodes reported in previous studies (Table 1), which showed that the developed MoS2@NHCSs/GCE exhibits wider linear range and lower detection limit; meanwhile, the sensor shows many advantages in the aspect of sensitivity, selectivity, stability, facility and economy.
4. Conclusion In this paper, a facile three-step processes was applied to fabricate MoS2@NHCSs novel nanostructures, then the obtain MoS2@NHCSs was used to construct a reliable electrochemical sensor for the simultaneous determination of 4-AP and ACAP. The experimental results demonstrated that the MoS2@NHCSs nanostructures can exhibit the synergetic advantages of NHCSs and MoS2, and the redox peak currents of both 4AP and ACAP at the MoS2@NHCSs/GCE are much higher than those at the other corresponding modified electrodes. Under the optimal conditions, the proposed MoS2@NHCSs/GCE exhibited low detection limit (0.013 μM for 4-AP and 0.020 μM for ACAP), wide linear range (0.05–20.0 μM) and good anti-interference ability for the determination of 4-AP and ACAP, which indicates that the MoS2@NHCSs-based sensor has great promising applications in the electrochemical simultaneous sensing of 4-AP and ACAP. It is also believed that the produced MoS2@ NHCSs will have great significance in lithium-ion battery, sensors, catalysis, supercapacitors oxygen-reduction, etc. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment
3.6. Reproducibility, stability and interference of MoS2@NHCSs/GCE
We acknowledge the support from the National Natural Science Foundation of China (21607061), the Natural Science Foundation of Jiangsu Province (BK20190543), the Opening Project of State Key Laboratory of Chemo/Biosensing and Chemometrics of Hunan University (2018019), the China Postdoctoral Science Foundation (2019M561596) the Priority Academic Program Development of Jiangsu Higher Education Institutions, Collaborative Innovation Center for Water Treatment Technology and Materials, and the Program of Young Backbone Teachers in Jiangsu University (2015).
The reproducibility and stability of the MoS2@NHCSs/GCE were further investigated by measuring the current response of the MoS2@ NHCSs/GCE in the mixed solution containing 5.0 μM 4-AP and 5.0 μM ACAP. The reproducibility was estimated by using seven individual modified electrodes prepared by the same procedure and the relative standard deviation (RSD) was 4.6% for 4-AP and 3.2% for ACAP, which indicated the modified electrode had good reproducibility. MoS2@ NHCSs/GCE was stored at 4 °C for 4 weeks, the oxidation peak currents of 4-AP and ACAP decreased only 3.67% for 4-AP and 2.76% for ACAP, respectively. The result suggests that the as-prepared sensor has excellent stability. The response of the MoS2@NHCSs modified electrode toward 4-AP and ACAP was also estimated by DPV in the presence of some potential interfering substances in pharmaceuticals or biological fluids, such as acetanilide, 4-chloroaniline, 4-Nitroaniline, ascorbic acid, uric acid, lactic acid, dopamine, caffeine, methionine, glucose, phenylalanine, naproxen, cholesterin, cysteine, ribose and deoxyribose. It was observed that 10-folds concentration of the interfering substances did not interfere with the oxidation currents of 5 μM 4-AP and ACAP (peak current change below ± 8%).The results indicated that the MoS2@ NHCSs modified electrode had a good anti-interference ability.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Nitrogen-doped hollow carbon nanospheres wrapped with MoS2 nanosheets for simultaneous electrochemical determination of acetaminophen and 4aminophenol Depeng Zhang1, Junjuan Qian1, Yinhui Yi, Odoom Jibrael Kingsford, Gangbing Zhu
T
⁎
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China
ARTICLE INFO
ABSTRACT
Keywords: Acetaminophen Aminophenol MoS2 Hollow carbon spheres Electrochemical sensor
Since acetaminophen (ACAP) and 4-aminophenol (4-AP, the degradation product from ACAP) have different properties and toxicities, the simultaneous electrochemical determination of ACAP and 4-AP is very important. In this work, nitrogen-doped hollow carbon spheres wrapped with MoS2 nanosheets (MoS2@NHCSs) novel hybrids were synthesized via a simple step-by-step method and consecutively the obtained MoS2@NHCSs was successfully used to construct a reliable electrochemical sensor for simultaneously detecting acetaminophen (ACAP) and 4-aminophenol (4-AP). Owing to the synergetic properties of each component in MoS2@NHCSs nanohybrids (MoS2 has highly catalytic activity and can offer many active sites; NHCSs can impede the aggregation tendency and enhance the effective surface area of MoS2 as well as increasing the conductivity of the nanohybrids), the experimental results demonstrated that the electrochemical responses of both 4-AP and ACAP at MoS2@NHCSs modified electrode are much better than those at NHCSs or MoS2 modified electrodes. Importantly, the MoS2@NHCSs nanohybrids can show excellent electrochemical sensing performance for the sensitive and selective as well as simultaneous determination of ACAP and 4-AP. After optimizing various conditions, the proposed MoS2@NHCSs-based sensor has wide linear range and low detection limit for ACAP and 4-AP analysis.
1. Introduction A widely used analgesic and antipyretic drug, acetaminophen (ACAP) does not pose any harmful effects but an overdose could give rise to serious negative effects [1–4]. 4-Aminophenol (4-AP) is the primary hydrolytic degradation product from ACAP and it is highly toxic and has nephrotoxicity and teratogenic properties [5–7]. As a consequence, it is very important to develop an effective technique for the simultaneous determination of ACAP and 4-AP. Up to now, many electrochemical methods have been devoted to detect ACAP and 4-AP separately, owing to the advantages of rapid response, inexpensive instrument and high sensitivity as well as simple operation [8–11]. Although some reports have focused on the simultaneous electrochemical sensing for ACAP and 4-AP [12–15], it is still a great challenge to develop an electrochemical sensor with high selectivity and high sensitivity. For improving the analytical performance of sensors, specific nanomaterials are usually needed to modify electrodes. Among a great
deal of novel electrode materials, a typical member of the family of 2 dimensional transition metal dichalcogenide with similar layered structure of graphene, molybdenum disulfide (MoS2) nanosheet is becoming a research focus [16–18]. MoS2 nanosheets are composed of a metal Mo layer sandwiched between two sulfur layers and generally stacked together by weak Van der Waals interactions [19–24]. Up to now, MoS2 received many attentions in the fields of lithium battery [25], catalysts [26], supercapacitor [27], lubricants [28] and sensors [29,30] owing to its unique properties, such as outstanding catalytic ability, large surface area, remarkable electron mobility and high density of electronic states. The interesting advantages of MoS2 also make it regarded as one of the most promising layered materials in the development of next-generation sensors [22]. Recent reports indicated that MoS2 shows excellent electrocatalysis properties toward phenolic compounds [16], which predicate that MoS2 nanosheet maybe a desirable nanomaterial for electrochemical sensing ACAP and 4-AP. Nevertheless, the layered structure of MoS2 can give some defects including inevitable stack and lower electronic conductivity, which
Corresponding author. E-mail address: [email protected] (G. Zhu). 1 Were co-first authors. ⁎
https://doi.org/10.1016/j.jelechem.2019.113229 Received 15 February 2019; Received in revised form 11 June 2019; Accepted 12 June 2019 Available online 13 June 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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restrict its applications in electrochemical sensing [31–34]. It's well known that most of the carbonaceous materials have high specific surface area and good electric conductivity that are always selected as functional active additives to enhance the dispersity and conductivity of electrode materials. To overcome the issues of MoS2, two conducting carbonaceous materials, carbon nanotubes (CNTs) and reduced graphene (RG), have been introduced to combine MoS2 for obtaining the nanohybrids which can exhibit the synergistic properties of MoS2 and carbonaceous materials [35–38]. Meanwhile, considerable attention has been paid to hollow nanostructures due to their excellent properties (e.g., large specific surface area, low density, shell permeability) and potential applications in many fields [39,40]. Among the family of hollow nanostructures, hollow carbon spheres (HCSs) is very intriguing owing to their high thermal stability, large interior void, high porosity and excellent conductivity [41–43]. Compared with RG and CNT, HCSs is more suitable as support to integrate MoS2 for improving its dispersibility and conductivity owing to the special spherical structure of HCSs [44,45]. Presently, some MoS2-HCSs nanohybrid has been used in H2 evolution reaction and capacitors [46], but there is no report presented for electrochemical sensing based on this nanohybrid. Therefore, developing a simple method to prepare a nanohybrid composed of MoS2 and HCSs and using the nanohybrids for electrochemical sensing is of great significance. By using MnO2 nanotubes as template and dopamine (DA) as carbon source, MoS2 nanosheets grown on nitrogen-doped carbon coated TiO2 tubes were achieved most recently for enhanced lithium storage [47,48]; in the absence of template, our group prepared solid carbon spheres coated with MoS2 by using glucose as carbon source recently [49]. Inspired by this insight, via introducing SiO2 nanospheres as template and DA as carbon source, in this work, ultrathin MoS2 nanosheets uniformly wrapped nitrogen-doped HCSs nanohybrids (MoS2@NHCSs; the N-doping is beneficial to further improve the electrochemical properties of NHCSs [50]) were prepared successfully via a simple three-step processes (Scheme 1): (a) producing silica@ polydopamine (SiO2@PDA) nanocomposites via the auto-polymerization of dopamine on SiO2 surface; (b) carbonizing and etching of SiO2@ PDA to form NHCSs; (c) wrapping NHCSs with MoS2 to obtain MoS2@ NHCSs nanohybrids. This smart design could effectively prevent aggregation, increase electric conductivity, and provide more active sites for electrochemical sensing. Then, the produced MoS2@NHCSs nanostructures were used to construct an electrochemical sensor for the simultaneous determination of 4-AP and ACAP. The experimental results showed that MoS2@NHCSs can exhibit the synergistic advantages from
NHCSs and MoS2 in electrochemical sensing. Under the optimized conditions, the MoS2@NHCSs-based sensor possesses high sensitivity and selectivity for 4-AP and ACAP coupled with low detection limits and wide linear range. Furthermore, the proposed sensor was applied successfully to simultaneously determine of 4-AP and ACAP in real samples including paracetamol tablet products and human serum. It's believed that the produced MoS2@NHCSs have important applications in electrochemical sensing of 4-AP/ACAP and other analytes. 2. Experimental section 2.1. Reagents and apparatus Na2MoO4·2H2O, 4-AP, ACAP, tris(hydroxymethyl)aminomethane (Tris), L-cysteine were purchased from Aladdin Industrial Inc. (Shanghai, China). Dopamine hydrochloride and SiO2 particles were purchased from Alfa Aesar (USA). As the supporting electrolyte, 0.1 M phosphate buffer solution (PBS) was prepared with Na2HPO4, NaH2PO4 and KCl. All other chemicals not mentioned were of analytical grade and used as received. Aqueous solutions used throughout were prepared with ultrapure water (Resistivity: ≥18 MΩ∗cm at 25 °C), obtained from a Millipore purification system. All the electrochemical measurements were performed on a CHI 660E Electrochemical Workstation (Chenhua Instrument Company of Shanghai, China). A conventional three-electrode system was used with a glass carbon electrode (GCE; 3 mm diameter) as working electrode, a platinum wire as auxiliary electrode, and an Ag/AgCl electrode (saturated KCl) as reference electrode. Unless otherwise specified, all the measurements were carried out in PBS (0.1 M, pH 6.0) at room temperature (25 ± 2 °C). All the potentials in this work were referred to Ag/AgCl electrode (saturated KCl). 2.2. Synthesis of MoS2@NHCSs (a) SiO2@PDA: SiO2@PDA was synthesized via an auto-polymerization process according to the previous report [51]. In brief, silica particles (80 mg) were mixed with dopamine (80 mg) in Tris-buffer (25 mL, 10 mM; pH 8.5) by mechanical stirring for 24 h. The SiO2@ PDA was collected by centrifugation and drying in vacuum at 60 °C for 8 h. (b) NHCSs: By carbonizing SiO2@PDA under N2 atmosphere at 800 °C for 3 h with a heating rate of 5 °C min−1, and sequentially removing the SiO2 template in 1 M NaOH aqueous solution for 24 h, the uniform NHCSs structures were obtained. (c) MoS2@NHCSs: MoS2@NHCSs nanohybrids were prepared through a facile hydrothermal process. HCS (0.05 g) was dispersed in DI water (30 mL) by ultrasonication for 30 min. Sodium molybdate (0.1 g) and L-cysteine (0.4 g) were then added into the solution under vigorous stirring. The mixture was transferred into a 50.0 mL Teflon-lined stainless-steel autoclave and heated at 200 °C for 12 h. The autoclave was then left to cool to room temperature in the oven. The resulting black product was collected by centrifugation, filtrated and washed with water and ethanol for several times, and then dried at 60 °C overnight. After collection, the as-prepared products were annealed under N2 atmosphere at 800 °C for 2 h with a heating rate of 2 °C min−1. For comparison, MoS2 nanosheets were prepared under the similar conditions without the addition of NHCSs. 2.3. Preparation of the modified electrodes 0.5 mg/mL MoS2@NHCSs suspension was prepared by dissolving 5.0 mg MoS2@NHCSs nanocomposite in 10.0 mL ultrapure water. MoS2@NHCSs modified glass carbon electrode (GCE) (MoS2@NHCSs/ GCE) was prepared by casting 10.0 μL MoS2@NHCSs suspension on the
Scheme 1. The procedures for preparing MoS2@NHCSs nanocomposite and electrochemically sensing of 4-AP and ACAP. 2
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Fig. 1. TEM images of NHCSs (A), MoS2 (B) and MoS2@NHCSs (C, D).
preprocessed GCE and drying under an infrared lamp. For comparison, NHCSs/GCE and MoS2/GCE were prepared via the same procedure. The electrochemical determination of 4-AP and ACAP was performed in 0.1 M phosphate buffer solution (PBS) consisting of two consecutive steps: (1) pre-concentration of 4-AP and ACAP from the solution to the modified electrodes with a definite accumulation time; and (2) electrochemical detecting by cyclic voltammetry (CV) or differential pulse voltammetry (DPV).
MoS2 nanosheets and the surface of the NHCSs became rough. These were consistent with TEM images. The crystallographic structure of the as-prepared samples was examined by X-ray powder diffraction (XRD), and the result was shown in Fig. S2. The XRD spectrum of hollow carbon spheres reveals three principles and highly disperse peaks arising at 2θ = 12.32°, 30.45°, 41.96°, which may ascribe to the amorphous carbon derived from dopamine. It can be found that MoS2@NHCSs displays five diffraction peaks at 13.8°, 33.1°, 39.4°, 49.5°, and 59.3°, which is consistent with the (002), (100), (103), (105), and (110) planes of hexagonal MoS2 (JCPDS card No. 37-1492). Besides, similar diffraction peaks can be observed from MoS2, demonstrating the successful formation of pure MoS2 crystal. No diffraction peaks from impurities or residues have been discovered in the XRD pattern. Raman spectroscopy was further utilized to investigate NHCSs, MoS2 and MoS2@NHCSs (Fig. S3). In high-frequency region, the spectrum of NHCSs reveals two characteristic peaks at 1363 and 1581 cm−1, assigning to the D and G bands of carbon materials, respectively. For pure MoS2, two intensity peaks at the band of 378 and 402 cm−1 can be observed clearly, corresponding to the characteristic in-plane E2g1 and out-of-plane A1g vibration modes of S-Mo-S bonds of the MoS2 crystal. It is observed that the frequency difference between E2g1 and A1g modes is large (∆f = 24 cm−1), indicating the layer thickness of MoS2 nanosheets is few-layers [52]. The relative intensity ratio of D band to G band (ID/IG) is widely utilized to clarify the graphitization degree of carbon materials. Two characteristic peaks of NHCSs were located at 1339 and 1586 cm−1, corresponding to the D and G bands, respectively. The value of ID/IG was evaluated to be 0.88 for NHCSs. Compared with the NHCSs, the value of ID/IG was calculated to 0.81 for MoS2@NHCSs, which demonstrates the successful formation of MoS2@NHCSs nanocomposite.
3. Results and discussion 3.1. Characterization of MoS2@NHCSs and related nanomaterials The morphology and microstructure of MoS2@NHCSs and related nanomaterials (NHCSs and MoS2) were characterized by transmission electron microscopy (TEM) and scanning electron microscope (SEM). As shown in Fig. 1A, the uniform NHCSs can be clearly observed, and the thickness of the carbon layer was ~10 nm. TEM image of MoS2 in Fig. 1B displays the nanosheets are close to transparency, indicating that they are ultrathin. The visible lattice fringes on the edge view of some sheet-like structures can be observed and the interplanar distance was about ~0.65 nm, which were consistent with (002) crystalline planes of hexagonal MoS2. Fig. 1C–D were TEM images of MoS2@ NHCSs at different magnifications, it can be observed that the NHCSs was uniformly wrapped by dense MoS2 nanosheets with abundant exposed edges, thus impeding the aggregation of MoS2. The SEM images of MoS2@NHCSs were shown in Fig. S1, it is observed that the NHCSs have smooth surfaces and were uniformly distributed (Fig. S1(A)). As shown in Fig. S1(B), MoS2 has a flower-like structure composed of nanosheets and the nanosheets extensively aggregate and overlap with each other. The morphology of MoS2@NHCSs (Fig. S1(C)) reveals that the NHCSs were densely wrapped with many ultrathin and interlaced 3
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3.2. Electrochemical characterization of fabricated electrode Electrochemical behaviors of different modified GCE were investigated by CV in 5.0 mM K3[Fe(CN)6]3−/4− containing 0.1 M KCl (Fig. S4). As shown in Fig. S4(A), the GCE shows the lower redox peak current (curve a) than that of other modified electrode, and the peak currents increase when MoS2 is modified on GCE (curve b). While NHCSs are coated on GCE, the electronic transmission is highly accelerated owing to the excellent conductivity of NHCSs, which result in a greatly intensive current response (curve c). As for the MoS2@NHCSs nanohybrid, it displays higher redox peak current (curve d) than MoS2 due to the presence of NHCSs in nanohybrid. Electrochemical impedance spectroscopy (EIS) was further used to study the electrochemical behaviors of the electrodes. Fig. S4(B) depicted the EIS plots of different modified electrodes recorded in 5.0 mM Fe(CN)63−/4− containing 0.1 M KCl with a frequency range of 0.1 Hz–10 kHz. By simulating obtained impedance data with Randles equivalent circuit (inset of Fig. S4(B)), the charge transfer resistance (Rct) of the bare GCE, MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE were 309.5, 134.4, 31.2, and 69.8 Ω, respectively. These results clearly demonstrated that the resistance decreased significantly with NHCSs/ GCE, owing to the rapid electron transfer property of NHCSs. Compared to the MoS2/GCE, the Rct value decreased on MoS2@NHCSs modified GCE, which further implies that the presence of carbon nanomaterials in MoS2@NHCSs can highly enhance the conductivity of composite electrode. 3.3. Electrochemical behavior of 4-AP and ACAP at modified electrodes The electrochemical behaviors of 5.0 μM 4-AP and ACAP at different modified electrodes (bare GCE, MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE) investigated by CV were shown in Fig. 2A. On the bare GCE, there was only a pair of weak and broad redox peaks for 4-AP and ACAP in the potential range of −0.2 to 0.8 V (curve a), suggesting that bare GCE cannot achieve directly the simultaneous determination of 4-AP and ACAP. Compared with the bare GCE, two well-separated oxidation peaks were clearly observed at MoS2/GCE, NHCSs/GCE and MoS2@NHCSs/GCE due to their excellent advantages. However, the oxidation peak currents of both 4-AP and ACAP at MoS2@NHCSs/GCE were increased remarkably and outperformed all the other modified electrodes owing the synergetic effects of each component in MoS2@ NHCSs: the MoS2 has highly catalytic activity and offer many active sites; NHCSs impede the aggregation tendency and enhance the effective surface area of MoS2 as well as increasing the conductivity of the nanohybrids. Furthermore, the oxidation peak potentials for 4-AP and ACAP were 0.119 and 0.411 V, respectively, presenting a peak-to-peak separation of 0.292 V. Fig. 2B shows the CV responses of 5.0 μM 4-AP (a), ACAP (b), and their mixture solution (c) at MoS2@NHCSs modified electrode. It's noted that the oxidation/reduction peaks of single 4-AP and ACAP appear at 0.121 and 0.411 V, respectively. When MoS2@ NHCSs/GCE was used to measure the CV response of the mixture solution containing both 4-AP and ACAP, the redox peaks of 4-AP and ACAP have little changes compared to single analyte. These data implied that the MoS2@NHCSs nanocomposite was a very promising material with excellent sensing performance for the simultaneous selective determination of 4-AP and ACAP.
Fig. 2. (A) CV responses of 5.0 μM 4-AP and ACAP on bare GCE (a), MoS2/GCE (b), NHCSs/GCE (c) and MoS2@NHCSs/GCE (d) 0.1 M PBS. (B) CV of 5.0 μM 4AP (a), ACAP (b) and the mixture containing 4-AP and ACAP at MoS2@NHCSs/ GCE in 0.1 M PBS.
from 5.0 to 6.0. The result may be ascribed to the high concentration of protons in the solution, which can replace phenol molecules on the adsorption sites of MoS2@NHCSs/GCE surface. Further increasing pH to 9.0 results in a decrease in the oxidation peak current, as hydroxyl anions may prevent phenol compounds from entering adsorption sites on the surface of MoS2@NHCSs/GCE. Fig. S5(B) further demonstrates the above conclusions. Fig. S5(C) displays that both the peak potentials of 4-AP and ACAP shifted negatively with the increase of pH. The linear regression equations can be expressed as: Epa (V) = 0.54–0.064 pH (R2 = 0.992) for 4-AP and Epa (V) = 0.78–0.061 pH (R2 = 0.995) for ACAP. The slope value of ~64 mV/pH and ~61 mV/pH was close to the theoretical value of ~57.6 mV/pH, indicating that the number of protons and electrons occurred in the electrode reaction is equal. The effect of scan rate on the current response of a mixed solution of 5.0 μM 4-AP and ACAP in 0.1 M PBS (pH 6.0) at MoS2@NHCSs/GCE was further investigated by CV (Fig. S6). Redox peak currents increased continuously with the increase of the scan rate. It can be observed that the redox peak currents (Ip) exhibited good linear relationship with the square root of the scan rate (υ1/2), indicating that the electrode reaction of 4-AP and ACAP on the MoS2@NHCSs/GCE was a typical diffusioncontrolled process. Linear regression equations between Ip and υ1/2 for 4-AP and ACAP can be expressed as the following Eqs. (1) and (2),
3.4. Optimization of the conditions for the electrochemical determination of 4-AP and ACAP The electrochemical responses of 4-AP and ACAP were influenced by the acidity of the electrolyte because protons are involved in the electrode reaction. Effect of buffer pH on the electrochemical behavior of 5.0 μM 4-AP and ACAP at MoS2@NHCSs/GCE was investigated by DPV at a pH range of 5.0–9.0. Fig. S5(A) reveals that the oxidation peak currents of 4-AP and ACAP gradually increase with the pH increasing 4
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Fig. 3. DPV curves at MoS2@NHCSs/GCE in 0.1 M PBS (pH 6.0) containing (A) constant concentrations of ACAP with different concentrations of 4-AP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM); (B) constant concentrations of 4-AP with different concentrations of ACAP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM); (C) different concentrations of 4-AP and ACAP (0.05, 0.1, 0.5, 1.0, 2.0, 5.0, 7.0, 10.0, 12.0, 15.0 and 20.0 μM). (D) The corresponding calibration curves. Table 1 Comparison with other proposed sensor for detection of 4-AP and ACAP. Electrode
Linear range (μM) 4-AP
MoS2 nanoclusters Au/Pd/reduced graphene oxide Graphene-polyaniline CdS quantum dots-graphene Graphene TiO2-graphene/poly(methyl red) Reduced graphene oxide@poly(3,4-ethylenedioxythiophene) nanotubes Graphene Paper-based microfluidic device Poly(chromium Schiff base complex) Poly(3,4-ethylenedioxythiophene) This work
ACAP
0.04–17.0 1–300.0 0.2–100.0 0.05–3.5
0.05–2000.0 0.008–0.133 4–320 0.05–20
respectively:
Ip (µA) = 6.885
1/2
(mV s 1)
25.99 (R2 = 0.9925) (For 4
Ip (µA) = 8.557
1/2
(mV s 1)
34.96 (R2 = 0.9933) (For ACAP)
AP)
5.0–800.0 0.25–50.0 1.0–35.0 0.1–20.0 0.008–0.125 1–100
Detection limit (μM)
R2
4-AP
4-AP
0.03 0.12 0.065 0.023
10.0 0.0056 1.2 0.013
ACAP
Refs. ACAP
0.989
1.2 0.025 0.4 0.032 25.0 0.0068 0.4 0.02
0.996 0.992 0.999 0.9959 0.9955 0.9985
0.986 0.999 0.9945 0.9945 0.9956
[53] [14] [54] [11] [55] [56] [8] [57] [58] [12] [59]
the electrochemical reaction of 4-AP and ACAP on the MoS2@NHCSs/ GCE. Fig. S7 reveals that the effect of MoS2@NHCSs amounts on the oxidation peak currents of 4-AP and ACAP. As shown in Fig. S7(A), both peak currents of 4-AP and ACAP increase gradually with increasing the MoS2@NHCSs amount from 4.0 μL to 10.0 μL, and reach a maximum at 10.0 μL, and then decrease obviously. Thus, optimal coating amount (10.0 μL) was utilized to prepare the modified electrode in this study.
(1) (2)
Apart from the pH of PBS and scan rate, the coating amount of MoS2@NHCSs suspension and the accumulation time are essential for 5
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Fig. S7(B) displays that the oxidation peak currents of 4-AP and ACAP increase with the increase of the accumulation time and the maximum are observed at 120 s and 100 s, respectively. As the peak current of ACAP at 120 s is similar to the current at 100 s, 120 s was chosen as the optimal accumulation time in this work.
3.7. Real sample analysis The MoS2@NHCSs modified electrode was applied to determine 4AP and ACAP in paracetamol tablet products. Prior to determination, the commercial paracetamol tablets were finely comminuted in an agate mortar and accurately weighed in triplicate (100 mg/part). Then the sample was extracted with 30 mL anhydrous ethyl alcohol for 30 min. After centrifugation for 5 min and filtration, the filtrate was collected in a volumetric flask and diluted with anhydrous ethyl alcohol. Subsequently, electrochemical measure was used to determine 4AP and ACAP in an appropriate amount of these diluted samples. The standard addition method was utilized to evaluate the recovery rate and the results were shown in Table S1. It was observed that the results were in accordance with the original contents of paracetamol and the RSD of three parallel detections was ≪4.22%. The recovery of 4-AP was in the range of 95.0%–104.8%. These results suggested that the MoS2@ NHCSs/GCE could be efficiently applied to the monitoring of 4-AP and ACAP in pharmaceutics with satisfactory results.
3.5. Simultaneous determination of 4-AP and ACAP Under the optimized conditions, the quantitative and simultaneous determination of 4-AP and ACAP was performed in 0.1 M PBS (pH 6.0) at MoS2@NHCSs/GCE by DPV because of its high current sensitivity and selectivity. The individual determination of 4-AP and ACAP in their mixtures was first performed, where the concentration of one species remained constant while the concentration of the other species increased. As shown in Fig. 3A, the peak currents linearly increased with increasing concentration of 4-AP from 0.05 μM to 20.0 μM, while the concentration of ACAP remained constant at 2.0 μM. The regression equation was Ip (μA) = 0.1978 + 2.087C (μM) (R2 = 0.9987) and the detection limit for 4-AP was evaluated to be 0.011 μM (S/N = 3). Fig. 3B shows the DPVs of ACAP with different concentrations in the presence of 5.0 μM 4-AP. The peak current of ACAP linearly increased with increasing concentration from 0.05 μM to 20.0 μM. The regression equation was Ip (μA) = 0.2712 + 2.755C (μM) (R2 = 0.9965) and the detection limit for ACAP was evaluated to be 0.015 μM (S/N = 3). Furthermore, the simultaneous determination of 4-AP and ACAP was studied by varying their concentrations simultaneously. As shown in Fig. 3C, two separated and well-defined oxidation peaks for 4-AP and ACAP at about 130 and 410 mV, respectively, while the oxidation peak currents were increased consistently. The linear ranges for the determination of 4-AP and ACAP were 0.05–20.0 μM with the detection limits of 0.013 μM and 0.020 μM (S/N = 3), respectively. The regression equations could be expressed as Ip (μA) = 0.1425 + 2.016C4-AP (μM) (R2 = 0.9985), and Ip (μA) = 0.3529 + 2.695CACAP (μM) (R2 = 0.9956), respectively. All these results demonstrated that the MoS2@NHCSs modified electrode could be successfully utilized for individual or simultaneous determination of 4-AP and ACAP with high selectivity and sensitivity. The performance of the proposed MoS2@ NHCSs modified electrode was compared with other modified electrodes reported in previous studies (Table 1), which showed that the developed MoS2@NHCSs/GCE exhibits wider linear range and lower detection limit; meanwhile, the sensor shows many advantages in the aspect of sensitivity, selectivity, stability, facility and economy.
4. Conclusion In this paper, a facile three-step processes was applied to fabricate MoS2@NHCSs novel nanostructures, then the obtain MoS2@NHCSs was used to construct a reliable electrochemical sensor for the simultaneous determination of 4-AP and ACAP. The experimental results demonstrated that the MoS2@NHCSs nanostructures can exhibit the synergetic advantages of NHCSs and MoS2, and the redox peak currents of both 4AP and ACAP at the MoS2@NHCSs/GCE are much higher than those at the other corresponding modified electrodes. Under the optimal conditions, the proposed MoS2@NHCSs/GCE exhibited low detection limit (0.013 μM for 4-AP and 0.020 μM for ACAP), wide linear range (0.05–20.0 μM) and good anti-interference ability for the determination of 4-AP and ACAP, which indicates that the MoS2@NHCSs-based sensor has great promising applications in the electrochemical simultaneous sensing of 4-AP and ACAP. It is also believed that the produced MoS2@ NHCSs will have great significance in lithium-ion battery, sensors, catalysis, supercapacitors oxygen-reduction, etc. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment
3.6. Reproducibility, stability and interference of MoS2@NHCSs/GCE
We acknowledge the support from the National Natural Science Foundation of China (21607061), the Natural Science Foundation of Jiangsu Province (BK20190543), the Opening Project of State Key Laboratory of Chemo/Biosensing and Chemometrics of Hunan University (2018019), the China Postdoctoral Science Foundation (2019M561596) the Priority Academic Program Development of Jiangsu Higher Education Institutions, Collaborative Innovation Center for Water Treatment Technology and Materials, and the Program of Young Backbone Teachers in Jiangsu University (2015).
The reproducibility and stability of the MoS2@NHCSs/GCE were further investigated by measuring the current response of the MoS2@ NHCSs/GCE in the mixed solution containing 5.0 μM 4-AP and 5.0 μM ACAP. The reproducibility was estimated by using seven individual modified electrodes prepared by the same procedure and the relative standard deviation (RSD) was 4.6% for 4-AP and 3.2% for ACAP, which indicated the modified electrode had good reproducibility. MoS2@ NHCSs/GCE was stored at 4 °C for 4 weeks, the oxidation peak currents of 4-AP and ACAP decreased only 3.67% for 4-AP and 2.76% for ACAP, respectively. The result suggests that the as-prepared sensor has excellent stability. The response of the MoS2@NHCSs modified electrode toward 4-AP and ACAP was also estimated by DPV in the presence of some potential interfering substances in pharmaceuticals or biological fluids, such as acetanilide, 4-chloroaniline, 4-Nitroaniline, ascorbic acid, uric acid, lactic acid, dopamine, caffeine, methionine, glucose, phenylalanine, naproxen, cholesterin, cysteine, ribose and deoxyribose. It was observed that 10-folds concentration of the interfering substances did not interfere with the oxidation currents of 5 μM 4-AP and ACAP (peak current change below ± 8%).The results indicated that the MoS2@ NHCSs modified electrode had a good anti-interference ability.
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