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MoS2-ionic liquid functionalized ordered mesoporous carbon
Accepted Manuscript Title: A novel electrochemical quercetin sensor based on Pd/MoS2 -ionic liquid functionalized ordered mesoporous carbon Authors: Bingjie Xu, Lite Yang, Faqiong Zhao, Baizhao Zeng PII: DOI: Reference:
S0013-4686(17)31368-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.130 EA 29768
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
20-4-2017 22-6-2017 22-6-2017
Please cite this article as: Bingjie Xu, Lite Yang, Faqiong Zhao, Baizhao Zeng, A novel electrochemical quercetin sensor based on Pd/MoS2ionic liquid functionalized ordered mesoporous carbon, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel electrochemical quercetin sensor based on Pd/MoS2-ionic liquid functionalized ordered mesoporous carbon Bingjie Xu, Lite Yang, Faqiong Zhao, Baizhao Zeng
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, P. R. China
Graphical Abstract A 3D Pd/MoS2-IL-OMC nanocomposite is prepared and used for the electrochemical detection of QR; it presents good performance.
Highlights
IL-OMC was used as support to improve the conductivity of MoS2. 3D Pd/MoS2-IL-OMC composite was employed for constructing quercetin sensor. The sensor showed high sensitivity and stability in sensing quercetin.
Corresponding author. Tel: 86-27-68752701, Fax: 86-27-68754067. E-mail address: [email protected] (BZ Zeng)
1
Abstract A quercetin (QR) sensor based on Pd/MoS2-ionic liquid functionalized ordered mesoporous carbon (Pd/MoS2-IL-OMC) composite is developed. The Pd/MoS2-IL-OMC composite is prepared by using IL as anchor and OMC as substrate. MoS2 nanosheets are in situ grown on IL-OMC surface and then Pd nanoparticles are prepared on MoS2 nanosheets via chemical reduction method. As the nanocomposite combines the high conductivity of IL-OMC and the electrocatalytic activity of Pd nanoparticles and MoS2, the resulting sensor exhibits good performance towards QR, better than MoS2-IL-OMC and Pd/IL-OMC nanocomposites modified electrodes. The electrochemical sensor allows for the selective determination of QR, with a detection limit of 8.0 nM (S/N = 3), a linear range of 0.020 μM - 10 μM, and a sensitivity of 150.1 μA μM-1 cm-2. It also shows good reproducibility and stability, and can be applied to the detection of QR in real samples. Keywords Ionic liquid; Ordered mesoporous carbon; MoS2; Pd nanoparticles; Quercetin 1. Introduction In last few years, two-dimensional (2D) transition metal dichalcogenide nanomaterials caused much concern because of their intriguing properties. Among them, the graphene-like layered molybdenum disulfide (MoS2) played an important role in various applications due to its tunable band gap and versatile chemistry [1-4], such as in sensors [5, 6], electrode materials [7-9] and catalysts [10]. Although it presented many advantages, the poor conductivity impeded its application in electrochemical analysis. Therefore, many strategies were attempted to overcome the shortcoming, including preparing ultrathin MoS2 film [11], changing interfacial chemistry [12] and promoting phase transition (from semiconducting 2H phase to metallic 1T phase) [13]. Compared with these methods, incorporating with carbon materials [14] or metal nanoparticles [15] was more easily carried out so that is a better approach to improve its electrochemical property. Among various materials, ordered mesoporous carbon (OMC) and Pd nanoparticles (Pd NPs) were often applied to improve electrochemical performance [16, 17]. Generally, OMC was used as 2
support as it has extremely well-ordered pore structure, large pore volume, good conductivity and high surface-to-mass ratio; Pd NPs was selected for the modification of MoS2 due to its high electrocatalytical activity and antioxidant capacity [15, 18]. For examples, Liu’s group prepared a MoS2/OMC composite by hydrothermal method and used it for electrochemical hydrogen evolution; it exhibited excellent catalytic activity and remarkable electrical conductivity [18]. Zhu et al. constructed a Pd-MoS2 catalyst for the oxygen reduction, which showed high electrocatalytic activity and stability [15]. Recently, ionic liquid (IL) functionalized OMC (IL-OMC) as substrate material was widely applied in electrochemical systems, because IL could improve the electrochemical capability of OMC and introduce charges for binding precursors of nanomaterials [19, 20]. Taking into account the advantages of MoS2, Pd NPs and IL-OMC, their combination was expected to present good electrochemical performance. Quercetin (QR), one of the most abundant flavonoids, not only widely exists in natural plants, but also has various biological functions, such as antioxidant, anti-inflammatory, antiallergy and anticancer [21]. These efficacies are related to its content and are very beneficial for human’s health. Hence, its detection is very important, especially in natural pharmaceutical chemistry, biochemistry and clinical medicine. Compared with HPLC-UV and spectrophotometry for QR determination, electrochemical detection methods get the favour of many researchers as they are low cost, simple, rapid and sensitive [22, 23]. For instance, Manokaran and co-workers developed an electrochemical sensor for QR, based on platinum-polydopamine @SiO2 nanocomposite [24]. Yola et al. used p-aminothiophenol functionalized graphene oxide and gold nanoparticles to modify glassy carbon electrode (GCE) for QR determination [25]. These electrochemical approaches displayed some merits, but their sensitivity was not high enough. As far as we know, there is no report about MoS2 hybrids based sensor for QR. In this work, an IL-OMC was used as support for MoS2. The modification of OMC with IL was expected to introduce positive charges that benefited the immobilization of the precursor of MoS2 on OMC surface. Meanwhile, Pd NPs were 3
efficiently prepared on the surface of MoS2 via chemical reduction method. Thus, the composite material had large surface, also possessed high electrocatalysis because of the synergistic effect of Pd and MoS2. The obtained nanocomposite was applied for the construction of QR sensor, and it showed good performance. 2. Experimental 2.1. Reagents and apparatus The OMC was obtained from Nanjing XF NANO Materials Tech Co., Ltd (Nanjing, China). Na2PdCl4 (purity: ≥ 99.95%) was purchased from Energy Chemical (Shanghai, China). Sodium molybdate (Na2MoO4•2H2O) and thiourea (NH2CSNH2) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The ionic liquid 1-vinyl-3-ethylimidazolium bromide ([VEIM]Br, purity > 99%) was provided by Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as received. QR (purity: ≥ 97%) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). A 0.10 M acetate buffer solution (ABS, pH 5.0) was employed as supporting electrolyte, which was prepared with HAc and NaAc. Other chemicals used were of analytical reagent grade. All solutions were prepared with ultrapure water. Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry（CV） experiments were performed with a CHI 604D electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was applied. The working electrode was a modified GCE (diameter: 3 mm), and the auxiliary and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The scanning electron microscope (SEM) image was obtained by using field emission SEM (ZEISS, Germany). Transmission electron microscopy (TEM) image was obtained by using a JEM-2100 transmission electron microscope with an accelerating voltage of 200 kV. X-ray diffraction data (XRD) were recorded with a PANalytical X'Pert Pro diffractometer (Holland) using Cu Kα radiation (40 kV, 40 mA) with a Ni filter. X-ray photoelectron spectroscopy (XPS) was recorded with a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer with Al Kα X-ray radiation for excitation. N2 4
sorption analysis was carried out on a Quantachrome Autosorb-iQ gas sorption analyzer. Before gas analysis, the samples were evacuated for 12 h at 120 °C under vacuum. 2.2. Preparation of IL-OMC The IL-OMC was prepared as follows [20]: 60 mg OMC was dispersed in 10 mL methanol by ultrasonication for 30 min. Then 16 mg IL and 20 mg 2,2’-azobisisobutyronitrile (AIBN) were added to the above dispersion. After ultrasonication for another 30 min, the mixture was transferred to a round-bottomed flask and refluxed at 80 °C for 12 h under vigorous stirring and N2 protection. The precipitate was collected by centrifugation and washed with acetone and ultrapure water for several times. The product was dried in an oven at 60 °C. 2.3. Synthesis of Pd/MoS2-IL-OMC The MoS2-IL-OMC hybrids were synthesized according to the literature with minor modification [26]. Briefly, 0.5 mmol Na2MoO4 and 4.5 mmol NH2CSNH2 were dissolved in 30 mL IL-OMC suspension (1 mg mL-1) under stirring for 1 h. Then, the resultant solution was transferred into a Teflon-lined stainless steel autoclave and heated at 200 °C for 12 h. The black precipitation was washed with water and ethanol for three times respectively, followed by drying at 60 °C in a vacuum oven overnight to obtain MoS2-IL-OMC. In order to load Pd NPs, 15 mg of as-prepared MoS2-IL-OMC was dispersed into 15 mL ultrapure water by ultrasonication of 1 h, then 1.0 mL 10 mM Na2PdCl4 aqueous solution was added. Subsequently, 2.0 mL fresh aqueous solution of NaBH4 (0.10 M) was introduced to reduce Na2PdCl4. After stirring for 1 h, the mixture was centrifuged and washed with ultrapure water until pH 7. 2.4. Preparation of modified electrode Before modification, the bare GCE was polished with alumina slurry (0.05 μm) and then washed thoroughly with ultrapure water, 7 M HNO3 solution, ethanol, and ultrapure water with the aid of ultrasonication, respectively. Then, 0.5 mg of the as-obtained Pd/MoS2-IL-OMC was dispersed into 2.0 mL DMF to give homogeneous suspension, and 6.0 μL suspension (0.25 mg ml-1) was dropped onto the surface of 5
cleaned GCE and dried under an infrared lamp. Thus, a Pd/MoS2-IL-OMC film coated electrode (i.e. Pd/MoS2-IL-OMC/GCE) was obtained. 2.5. Sample preparation Dried blueberries and grape juice were obtained from a local supermarket and treated by following ways. Firstly, dried blueberries were grinded thoroughly with a agate mortar, and 1.0 g of the resulting paste was dissolved in 30 mL ethanol under ultrasonication for 30 min. Then, the solution was centrifugated at 3000 rpm for 10 min. Subsequently, 1.0 mL of the supernatant was diluted to 10 mL with ABS (pH 5.0) for electrochemical detection. Grape juice was also diluted for 10-fold with ABS (pH 5.0) before determination. 3. Results and discussion 3.1. Characterization of Pd/MoS2-IL-OMC composites Scheme 1. illustrated the preparation procedure of Pd/MoS2-IL-OMC/GCE. Firstly, OMC was functionalized with IL, so its surface was full of positive charges due to highly charged nature of IL [19]. Then, molybdate anions, the precursor of MoS2, were adsorbed on the surface of IL-OMC through electrostatic interaction, and after hydrothermal treatment MoS2 was prepared on IL-OMC. After that, Pd NPs were prepared on the MoS2 by reducing the precursor with NaBH4 at room temperature. As Pd NPs and MoS2 had strong interaction concerning bonding mechanism of Pd atom and MoS2, Pd NPs were more likely to be loaded on MoS2 to form stable configuration [27]. The prepared composite was coated on GCE surface for detecting the electrochemical oxidation current of QR. The IL-OMC was characterized by FTIR spectra as shown in Fig. 1A. The peaks at 1549 cm-1 and 1170 cm-1 were attributed to C=N and C-N stretching vibrations of IL, respectively [20], indicating that the IL had been attached to OMC. The elemental compositions of the hybrid were determined by XPS. N 1s XPS spectrum (Fig. 1B) was marked by four peaks at 401.3 eV (graphitic N), 399.5 eV (pyrrolic N), 398.2 eV (pyridinic N) and 394.6 eV (Mo 3p3/2), respectively [29, 30]. High-resolution XPS spectrum of C 1s was displayed in Fig. 1C. The strong peak located at 284 eV belonged to graphitic N of OMC [30]. The two peaks at 285.7 eV and 287.7 eV were 6
assigned to C-O and C=O, respectively [30]. The additional peak at 286.4 eV could be indexed as C-N, originating from imidazole ring of IL, which further confirmed the presence of IL [30]. In Fig. 1D, the Pd 3d5/2 and Pd 3d3/2 peaks for the sample appeared at 336.6 eV and 341.9 eV, in accordance with that of metallic Pd0, indicating that Pd material was prepared [31]. Fig. 1E depicted four signals corresponding to Mo4+ 3d3/2, Mo4+ 3d5/2, S 2s and Mo6+ 3d [31]. Meanwhile, Fig. 1F exhibited the typical peaks of S2- 2p3/2 and S2- 2p1/2 [31]. XRD and Raman spectrum experiments were performed to investigate the structure of MoS2, MoS2-IL-OMC and Pd/MoS2-IL-OMC. As shown in Fig. 2A, the diffraction peaks of MoS2-IL-OMC at 32.6°, 35.3°, 43.6°, 57.6° corresponded to (100), (102), (104) and (110) of MoS2 [26, 32], respectively. In addition, the broad weak peak at 25.0° could be indexed to (002) of OMC, demonstrating that the IL-OMC was decorated with MoS2 [33]. The XRD pattern of Pd/MoS2-IL-OMC presented representative diffraction peaks at 40.0°, 46.7° and 68.4°, assigned to (111), (200) and (220) of Pd, indicating the existence of Pd [15]. It was noticed that the (100) and (102) peaks were blended, which might be related to poor crystallinity [31]. Moreover, a new weak peak emerged at 59.7° for Pd/MoS2-IL-OMC hybrid. This phenomenon could be explained by the increased interlayer distance of MoS2 [33]. Since the edges of MoS2 layers were catalytic active, such change of MoS2 led to the exposure of more active sites [26, 34]. In Fig. 2B, D and G peaks for OMC appeared in both Raman spectra of MoS2-IL-OMC and Pd/MoS2-IL-OMC. The peaks at 376, 380 and 382 cm-1 were associated with E12g modes of the MoS2-based composites (insert, a and b) [35]. Compared
with
pure
MoS2,
E2g1
both
modes
of
MoS2-IL-OMC
and
Pd/MoS2-IL-OMC had a red-shift and the distance between E2g1 and A1g peaks decreased (the A1g peaks of MoS2-IL-OMC and Pd/MoS2-IL-OMC occurred at 408 and 409 cm-1, respectively) [35]. From above results, it could be thought that the MoS2 in the composites consisted of fewer layers than pure MoS2, because the staking of MoS2 nanosheets was suppressed by OMC and Pd NPs [5, 35, 36]. Additionally, when incorporated with Pd, the position of A1g and E2g1 peaks further shifted toward 7
higher frequency, suggesting that the semiconductor characteristics and electronic structure of the MoS2 had changed [37]. This considerable change might be associated with the improvement of material properties [37]. The morphology of pristine MoS2 displayed spherical structure via flakes highly randomly staking (Fig. 3A). The diameter of MoS2 spheres was about 2.2 μm and the thickness per layer was about 1.8 nm. Once IL-OMC presented, MoS2 flakes piled up into nanoflowers on IL-OMC surface and the number of staked layers markedly reduced (Fig. 3B) [27]. Fig. 3C showed the SEM survey of the final material, in which lots of Pd NPs located on the edges of MoS2 nanosheets and some existed between interlayer, leading to larger layer-spacing. In Fig. 3D, the MoS2 consisted of few layers tightly incorporated with IL-OMC, and Pd NPs were mostly anchored by MoS2. The HR-TEM image of Pd NPs was shown in Fig. 3D, the lattice distance corresponding to (111) plane of metallic Pd phase was 0.22 nm, which was in agreement with that reported [38]. On the basis of the observation, it could be sure that Pd/MoS2-IL-OMC hybrid was successfully fabricated. The nitrogen adsorption/desorption isotherms were shown in Fig. S1. It could be seen that the isotherm curves belonged to typical IV isotherm, indicating the existence of mesopore structure [32]. The specific surface area of Pd/MoS2-IL-OMC was ca.589.99 m2 g-1, lager than that of MoS2-IL-OMC (558.17 m2 g-1) and IL-OMC (327.67 m2 g-1), which benefited from the Pd NPs. This was beneficial to the adsorption of target molecules and catalytic process. 3.2. Electrochemical properties of Pd/MoS2-IL-OMC/GCE To examine the electrochemical property of the Pd/MoS2-IL-OMC/GCE etc, the EIS and CV curves of various modified electrodes were recorded in Fe[(CN)6]3-/4solution. As shown in Fig. 4A, the EIS of MoS2/GCE presented a smaller semicircle as compared with that of bare GCE, indicating a low impedance. When GCE was modified by MoS2-IL-OMC and Pd/MoS2-IL-OMC, the impedance curves almost became straight line, manifesting the formation of high charge conduction pathways at the interface. This change was related to the excellent electrical conductivity of Pd NPs and IL-OMC. It was observed that Fe[(CN)6]3-/4-showed a pair of poor redox 8
peaks at the MoS2/GCE (Fig. 4B (d)), indicating the low electron-transfer rate of MoS2. But on MoS2-IL-OMC/GCE the peaks became more reversible, indicating that IL-OMC could improve the conductive property of MoS2 layer. Compared with MoS2-IL-OMC/GCE
and
Pd/IL-OMC/GCE,
the
redox
peak
current
at
Pd/MoS2-IL-OMC/GCE apparently increased (Fig. 4B (e)). This improvement probably resulted from the synergism of MoS2 and Pd NPs in facilitating electron transfer. Different noble metals modified MoS2-IL-OMC/GCE were compared in Fig. S2. The result indicated that the electrocatalytic activity of Pd NPs for QR was better than Au and Pt NPs in this case. This could be explained as follows: 1) Au with relatively stronger s-orbital contribution was different from Pd with significant d-orbital characteristics [39]. The electronic structures of the metals influenced their incorporation with the substrate metal and their catalytic performance [40]. 2) The properties of Pt and Pd are similar; in general, Pt shows higher catalysis than Pd. However, Pt displayed poorer poisoning-tolerance, which affected the catalytic efficiency [41]. Thus, Pd NPs were selected in this work. Then, the electrochemical response of QR at different electrodes was tested under the same conditions. As could be seen in Fig. 4C, the MoS2 modified GCE (Fig. 4C (b)) had no response toward QR. QR caused a very small oxidation peak at bare GCE (Fig. 4C (a)) because of its weak adsorption at GCE. As for IL-OMC/GCE, MoS2-IL-OMC/GCE and Pd/IL-OMC/GCE (Fig. 4C (c, d and e)), the peak current increased due to their high specific surface area and good conductivity as well as good catalysis for QR. Furthermore, the Pd/MoS2-IL-OMC/GCE (Fig. 4C (f)) exhibited higher peak current, thanks for the synergistic catalytic effect of Pd NPs and MoS2 [42]. Besides, the specific surface area of Pd/MoS2-IL-OMC/GCE was higher, which also contributed to the increase of peak current. Therefore, the sensitive current response at Pd/MoS2-IL-OMC/GCE could be explained as follows: 1) after the introduction of Pd NPs, the number of staked MoS2 layers decreased and the interlayer spacing of MoS2 increased, thus more active sites became available [1, 26]; 2) when Pd NPs were prepared on the MoS2, the specific surface area of the composite increased further, and it could adsorb more QR 9
[17]; 3) the combination of MoS2 and Pd NPs altered the electronic structure, which was in favor of electron transfer [17, 37]. The influence of scan rates (ν) on the current response of QC was also investigated. As shown in Fig. S3, the peak current (Ip) was linear to ν (i.e. Ip = 0.48 ν - 1.12, ν: mV s-1, R=0.998), indicating that the oxidation process of QC is adsorptioncontrolled [43]. At the same time, the peak potential (Epa) shifted positively with increasing scan rate, and Epa and ln v had such relationship as Epa = 0.022 ln ν + 0.19 (R=0.996), meaning the electron-transfer rate was not very fast. 3.3. Optimization of conditions The electrochemical performance of the sensor was affected by several factors, such as the amount of MoS2 and Pd NPs used, the solution pH and accumulation time. In order to get good performance, some experimental conditions were optimized. The synergistic effect of Pd NPs and MoS2 played a crucial role in the electrochemical response of QR. Thus, the MoS2 content and Pd content in the composite were taken into account. Fig. S4A depicted that the oxidation peak current of QR increased with the concentration of MoO2-4 increasing from 3 to 17 mM for preparing the composite. However, when MoO2-4 concentration further increased, the peak current decreased, because too much MoS2 would lead to poor conductivity. Therefore, the optimal MoO2-4 concentration was 17 mM. The peak current of QR increased dramatically with increasing PdCl-4 concentration up to 0.7 mM (Fig. S4B). This could be explained as follows: when Pd content raised, the catalysis for QR enhanced, but too much Pd NPs would agglomerate each other so that the specific surface area decreased; excessive Pd resulted in weak synergistic catalytic effect due to the decrease of MoS2 proportion. Consequently, 0.7 mM of PdCl-4 was adopted. The influence of pH value of ABS was presented in Fig. S4C. The results revealed that the oxidation peak current increased as the pH value increased and the highest peak current was obtained at 5.0. When pH exceeded 5.0 the oxidation peak current decreased. This was because proton was significant in electrochemical oxidation-reduction of QR. In addition, at higher proton concentration QR (Scheme 2) was neutral, while at lower proton concentration, the two hydroxyl groups, assigned 10
to C-1 and C-2, became dissociated forms [23, 44], affecting the accumulation of QR at the sensor surface. Therefore, ABS with pH 5.0 was selected as the supporting electrolyte. It could be seen that the oxidation peak current of 4.0 μM QR increased with increasing preconcentration time and trended toward unchanged above 4 min (Fig. S4D), indicating the saturated adsorption of QR. Thus, 4 min was adopted for accumulation in the experiment. 3.4. Analytical performance LSV method was applied to detect QR under the optimized conditions (Fig. 5). The result showed that the peak current of QR at about 0.3 V varied linearly with the addition of QR from 0.020 to 10.0 μM, with a linear equation of Ip (μA) = 10.6 C (μM) - 0.05 (R=0.999). The LOD and sensitivity were evaluated to be 8.0 nM and 150.1 μA μM-1 cm-2, respectively. The sensor was compared with other QR sensors reported and the result was listed in Table 1. It could be concluded that the sensor had a wide linear range and low detection limit. 3.5. Interference, reproducibility and stability The anti-interference capability of Pd/MoS2-IL-OMC/GCE for QR determination was investigated (Fig. 6). The LSV curves were recorded for 4.0 μM QR solution containing different concentrations of potential interferents. It was found that 100-fold sucrose, lactose, fructose and glucose, 50-fold L-cysteine, glycine and citric acid and 5-fold ascorbic acid didn’t generate interferences (the change of peak current was less than 5 %). Thus, the sensor had desirable anti-interference ability and could be used for the determination of QR in some samples.. To estimate the reproducibility and stability of the sensor, five modified electrodes were tested by measuring the peak current of 4.0 μM QR under the same experimental conditions and the relative standard deviation (RSD) was 6.8%. Furthermore, one electrode was used for five detections; the RSD was 4.1 %. After storing for one week at room temperature, the peak current maintained 95.2% of the initial value (Fig. S5). These reflected the good reproducibility and stability of the sensor. 11
3.6. Applications The Pd/MoS2-IL-OMC/GCE was used for detecting QR in real samples to evaluate its practicability. Dried blueberries and grape juice samples were pretreated according to the method mentioned previously, and the sample solutions were determined. The QR contents in dried blueberry and grape juice were calculated to be 0.13 mg g-1 and 4.9 mg L-1, respectively. The recoveries for QR standards added varied from 82 % to 108.0 % (Table 2). The contents of QR were also detected by HPLC (Fig. S6). Correspondingly, the results were 0.11 mg g-1 (RSD: 3.8%, n = 3) and 4.6 mg L-1 (RSD: 2.1%, n = 3), respectively. According to F-test, the F values were calculated as 5.92 and 2.27. When confidence level was 95% and n = 3, the statistic F value was 19, bigger than the calculated F values, indicating that there was no significant difference between the methods. Then the average values were evaluated by t-test, the calculated t values were 0.77 and 1.73, less than t0.05,4 = 2.78. Therefore, the proposed method in this work is reliable. 4. Conclusion In this work, a novel electrochemical sensor for QR was constructed, which was based on a 3D Pd/MoS2-IL-OMC nanocomposite. As the IL functionalized OMC had high specific surface, conductivity and binding action, and the Pd NPs and MoS2 had strong electrocatalysis, the composite not only exhibited large specific surface area but also showed high electrocatalytic activity toward QR. Hence, the resulting sensor gave good performance. It had a wide liner range (0.020 - 10.0 μΜ) and low detection limit (8.0 nM), also displayed good reproducibility and stability. The sensor could be used for the detection of QR in real samples. 5. Acknowledgements This work got the financial support from the National Natural Science Foundation of China (Grant No.: 21277105).
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17
Captions Table 1 . Comparison of different QR sensors. Table 2. Determination results of QR in samples with the prepared sensor. Fig. 1. FT-IR spectra of IL, IL-OMC(A); high-resolution XPS spectra of N 1s (B), C 1s (C), Pd 3d (D), Mo 3d (E) and S 2p (F) of Pd/MoS2-IL-OMC. Fig. 2. (A) XRD of MoS2, MoS2-IL-OMC and Pd/MoS2-IL-OMC; (B) Raman spectra of MoS2-IL-OMC (a) and Pd/MoS2-IL-OMC (b). Insert: Raman spectrum of MoS2. Fig. 3. SEM images of MoS2 (A), MoS2-IL-OMC (B) and Pd/MoS2-IL-OMC (C). TEM images of Pd/MoS2-IL-OMC (D). Insert: HR-TEM image of Pd NPs in Pd/MoS2-IL-OMC. Fig. 4. (A) EIS of GCE (a), pure MoS2/GCE (b), MoS2-IL-OMC/GCE (c) and Pd/MoS2-IL-OMC/GCE MoS2-IL-OMC/GCE
(b),
(d).
(B)
CV
Pd/IL-OMC/GCE
curves (c),
of pure
IL-OMC/GCE MoS2/GCE
(d)
(a), and
Pd/MoS2-IL-OMC/GCE (e). Solution composition for EIS and CV: 0.10 M KCl containing 5.0 mM Fe[(CN)6]3-/4- (1:1) . (C) LSV curves of QR at different electrodes: bare GCE (a), pure MoS2/GCE (b), IL-OMC/GCE (c), MoS2-IL-OMC/GCE (d), Pd/ IL-OMC/GCE (e) and Pd/MoS2-IL-OMC/GCE (f). Solution condition: 0.10 M ABS containing 4.0 μM QR. Fig. 5. LSV curves of QR at Pd/MoS2-IL-OMC/GCE in 0.10 M ABS (pH 5.0). QR concentration (from a to l): 0.020, 0.090, 0.20, 0.40, 0.80, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 and 15.0 μM. Accumulation time: 4 min; scan rate: 100 mV/s. Insert: the calibration curve. Fig. 6. Influence of coexistents on the peak current of QR at the sensor. Solution composition: 4.0 μM QR, 4.0 μM QR plus 100-fold sucrose, 4.0 μM QR plus 100-fold lactose, 4.0 μM QR plus 100-fold fructose, 4.0 μM QR plus 100-fold glucose, 4.0 μM QR plus 50-fold L-cysteine, 4.0 μM QR plus 50-fold glycine, 4.0 μM QR plus 50-fold citric acid and 4.0 μM QR plus 5-fold ascorbic acid. Other conditions as in Fig. 5.
18
Scheme 1. Illustration of the fabrication procedure of Pd/MoS2-IL-OMC/GCE. Scheme 2. Chemical structure of QR.
19
Fig. 1.
20
Fig. 2.
21
Fig.3.
22
Fig.4.
23
Fig. 5.
24
Fig. 6.
25
Scheme 1.
26
Scheme 2.
27
Table 1. Comparison of different QR sensors. Linear range (μM) LOD for QR (nM) References
Sensors Pt-PDAa@SiO2/GCE
0.05–0.383
16
[24]
0.50–330
100
[45]
0.0165-0.3309
12
[46]
CD/AuNPs/MWCNTs/GCE
0.005-7
6.4
[47]
MIPc/GO/GCE
0.6-15
48
[48]
Pd/MoS2-IL-OMC/GCE
0.02-10
8.0
This work
Co3O4/GCE Activated silica gel/CPEb
a
Polydopamine.
b
Carbon paste electrode.
c
Molecularly Imprinted Polymer.
Table 2. Determination results of QR in samples with the prepared sensor. Sample Dried blueberries
Grape juice
Added (μM) 0.00 1.00 2.00 3.00 0.00 1.00 2.00 3.00
Detected (μM) 1.43 2.49 3.60 4.38 1.61 2.43 3.59 4.77
28
Recovery (%) 106 108 98.3 82 99 105.3
RSD (n = 3) (%) 2.5 3.2 0.2 2.7 2.7 4.5 3.2 2.1
S0013-4686(17)31368-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.130 EA 29768
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
20-4-2017 22-6-2017 22-6-2017
Please cite this article as: Bingjie Xu, Lite Yang, Faqiong Zhao, Baizhao Zeng, A novel electrochemical quercetin sensor based on Pd/MoS2ionic liquid functionalized ordered mesoporous carbon, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel electrochemical quercetin sensor based on Pd/MoS2-ionic liquid functionalized ordered mesoporous carbon Bingjie Xu, Lite Yang, Faqiong Zhao, Baizhao Zeng
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, P. R. China
Graphical Abstract A 3D Pd/MoS2-IL-OMC nanocomposite is prepared and used for the electrochemical detection of QR; it presents good performance.
Highlights
IL-OMC was used as support to improve the conductivity of MoS2. 3D Pd/MoS2-IL-OMC composite was employed for constructing quercetin sensor. The sensor showed high sensitivity and stability in sensing quercetin.
Corresponding author. Tel: 86-27-68752701, Fax: 86-27-68754067. E-mail address: [email protected] (BZ Zeng)
1
Abstract A quercetin (QR) sensor based on Pd/MoS2-ionic liquid functionalized ordered mesoporous carbon (Pd/MoS2-IL-OMC) composite is developed. The Pd/MoS2-IL-OMC composite is prepared by using IL as anchor and OMC as substrate. MoS2 nanosheets are in situ grown on IL-OMC surface and then Pd nanoparticles are prepared on MoS2 nanosheets via chemical reduction method. As the nanocomposite combines the high conductivity of IL-OMC and the electrocatalytic activity of Pd nanoparticles and MoS2, the resulting sensor exhibits good performance towards QR, better than MoS2-IL-OMC and Pd/IL-OMC nanocomposites modified electrodes. The electrochemical sensor allows for the selective determination of QR, with a detection limit of 8.0 nM (S/N = 3), a linear range of 0.020 μM - 10 μM, and a sensitivity of 150.1 μA μM-1 cm-2. It also shows good reproducibility and stability, and can be applied to the detection of QR in real samples. Keywords Ionic liquid; Ordered mesoporous carbon; MoS2; Pd nanoparticles; Quercetin 1. Introduction In last few years, two-dimensional (2D) transition metal dichalcogenide nanomaterials caused much concern because of their intriguing properties. Among them, the graphene-like layered molybdenum disulfide (MoS2) played an important role in various applications due to its tunable band gap and versatile chemistry [1-4], such as in sensors [5, 6], electrode materials [7-9] and catalysts [10]. Although it presented many advantages, the poor conductivity impeded its application in electrochemical analysis. Therefore, many strategies were attempted to overcome the shortcoming, including preparing ultrathin MoS2 film [11], changing interfacial chemistry [12] and promoting phase transition (from semiconducting 2H phase to metallic 1T phase) [13]. Compared with these methods, incorporating with carbon materials [14] or metal nanoparticles [15] was more easily carried out so that is a better approach to improve its electrochemical property. Among various materials, ordered mesoporous carbon (OMC) and Pd nanoparticles (Pd NPs) were often applied to improve electrochemical performance [16, 17]. Generally, OMC was used as 2
support as it has extremely well-ordered pore structure, large pore volume, good conductivity and high surface-to-mass ratio; Pd NPs was selected for the modification of MoS2 due to its high electrocatalytical activity and antioxidant capacity [15, 18]. For examples, Liu’s group prepared a MoS2/OMC composite by hydrothermal method and used it for electrochemical hydrogen evolution; it exhibited excellent catalytic activity and remarkable electrical conductivity [18]. Zhu et al. constructed a Pd-MoS2 catalyst for the oxygen reduction, which showed high electrocatalytic activity and stability [15]. Recently, ionic liquid (IL) functionalized OMC (IL-OMC) as substrate material was widely applied in electrochemical systems, because IL could improve the electrochemical capability of OMC and introduce charges for binding precursors of nanomaterials [19, 20]. Taking into account the advantages of MoS2, Pd NPs and IL-OMC, their combination was expected to present good electrochemical performance. Quercetin (QR), one of the most abundant flavonoids, not only widely exists in natural plants, but also has various biological functions, such as antioxidant, anti-inflammatory, antiallergy and anticancer [21]. These efficacies are related to its content and are very beneficial for human’s health. Hence, its detection is very important, especially in natural pharmaceutical chemistry, biochemistry and clinical medicine. Compared with HPLC-UV and spectrophotometry for QR determination, electrochemical detection methods get the favour of many researchers as they are low cost, simple, rapid and sensitive [22, 23]. For instance, Manokaran and co-workers developed an electrochemical sensor for QR, based on platinum-polydopamine @SiO2 nanocomposite [24]. Yola et al. used p-aminothiophenol functionalized graphene oxide and gold nanoparticles to modify glassy carbon electrode (GCE) for QR determination [25]. These electrochemical approaches displayed some merits, but their sensitivity was not high enough. As far as we know, there is no report about MoS2 hybrids based sensor for QR. In this work, an IL-OMC was used as support for MoS2. The modification of OMC with IL was expected to introduce positive charges that benefited the immobilization of the precursor of MoS2 on OMC surface. Meanwhile, Pd NPs were 3
efficiently prepared on the surface of MoS2 via chemical reduction method. Thus, the composite material had large surface, also possessed high electrocatalysis because of the synergistic effect of Pd and MoS2. The obtained nanocomposite was applied for the construction of QR sensor, and it showed good performance. 2. Experimental 2.1. Reagents and apparatus The OMC was obtained from Nanjing XF NANO Materials Tech Co., Ltd (Nanjing, China). Na2PdCl4 (purity: ≥ 99.95%) was purchased from Energy Chemical (Shanghai, China). Sodium molybdate (Na2MoO4•2H2O) and thiourea (NH2CSNH2) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The ionic liquid 1-vinyl-3-ethylimidazolium bromide ([VEIM]Br, purity > 99%) was provided by Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as received. QR (purity: ≥ 97%) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). A 0.10 M acetate buffer solution (ABS, pH 5.0) was employed as supporting electrolyte, which was prepared with HAc and NaAc. Other chemicals used were of analytical reagent grade. All solutions were prepared with ultrapure water. Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry（CV） experiments were performed with a CHI 604D electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was applied. The working electrode was a modified GCE (diameter: 3 mm), and the auxiliary and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The scanning electron microscope (SEM) image was obtained by using field emission SEM (ZEISS, Germany). Transmission electron microscopy (TEM) image was obtained by using a JEM-2100 transmission electron microscope with an accelerating voltage of 200 kV. X-ray diffraction data (XRD) were recorded with a PANalytical X'Pert Pro diffractometer (Holland) using Cu Kα radiation (40 kV, 40 mA) with a Ni filter. X-ray photoelectron spectroscopy (XPS) was recorded with a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer with Al Kα X-ray radiation for excitation. N2 4
sorption analysis was carried out on a Quantachrome Autosorb-iQ gas sorption analyzer. Before gas analysis, the samples were evacuated for 12 h at 120 °C under vacuum. 2.2. Preparation of IL-OMC The IL-OMC was prepared as follows [20]: 60 mg OMC was dispersed in 10 mL methanol by ultrasonication for 30 min. Then 16 mg IL and 20 mg 2,2’-azobisisobutyronitrile (AIBN) were added to the above dispersion. After ultrasonication for another 30 min, the mixture was transferred to a round-bottomed flask and refluxed at 80 °C for 12 h under vigorous stirring and N2 protection. The precipitate was collected by centrifugation and washed with acetone and ultrapure water for several times. The product was dried in an oven at 60 °C. 2.3. Synthesis of Pd/MoS2-IL-OMC The MoS2-IL-OMC hybrids were synthesized according to the literature with minor modification [26]. Briefly, 0.5 mmol Na2MoO4 and 4.5 mmol NH2CSNH2 were dissolved in 30 mL IL-OMC suspension (1 mg mL-1) under stirring for 1 h. Then, the resultant solution was transferred into a Teflon-lined stainless steel autoclave and heated at 200 °C for 12 h. The black precipitation was washed with water and ethanol for three times respectively, followed by drying at 60 °C in a vacuum oven overnight to obtain MoS2-IL-OMC. In order to load Pd NPs, 15 mg of as-prepared MoS2-IL-OMC was dispersed into 15 mL ultrapure water by ultrasonication of 1 h, then 1.0 mL 10 mM Na2PdCl4 aqueous solution was added. Subsequently, 2.0 mL fresh aqueous solution of NaBH4 (0.10 M) was introduced to reduce Na2PdCl4. After stirring for 1 h, the mixture was centrifuged and washed with ultrapure water until pH 7. 2.4. Preparation of modified electrode Before modification, the bare GCE was polished with alumina slurry (0.05 μm) and then washed thoroughly with ultrapure water, 7 M HNO3 solution, ethanol, and ultrapure water with the aid of ultrasonication, respectively. Then, 0.5 mg of the as-obtained Pd/MoS2-IL-OMC was dispersed into 2.0 mL DMF to give homogeneous suspension, and 6.0 μL suspension (0.25 mg ml-1) was dropped onto the surface of 5
cleaned GCE and dried under an infrared lamp. Thus, a Pd/MoS2-IL-OMC film coated electrode (i.e. Pd/MoS2-IL-OMC/GCE) was obtained. 2.5. Sample preparation Dried blueberries and grape juice were obtained from a local supermarket and treated by following ways. Firstly, dried blueberries were grinded thoroughly with a agate mortar, and 1.0 g of the resulting paste was dissolved in 30 mL ethanol under ultrasonication for 30 min. Then, the solution was centrifugated at 3000 rpm for 10 min. Subsequently, 1.0 mL of the supernatant was diluted to 10 mL with ABS (pH 5.0) for electrochemical detection. Grape juice was also diluted for 10-fold with ABS (pH 5.0) before determination. 3. Results and discussion 3.1. Characterization of Pd/MoS2-IL-OMC composites Scheme 1. illustrated the preparation procedure of Pd/MoS2-IL-OMC/GCE. Firstly, OMC was functionalized with IL, so its surface was full of positive charges due to highly charged nature of IL [19]. Then, molybdate anions, the precursor of MoS2, were adsorbed on the surface of IL-OMC through electrostatic interaction, and after hydrothermal treatment MoS2 was prepared on IL-OMC. After that, Pd NPs were prepared on the MoS2 by reducing the precursor with NaBH4 at room temperature. As Pd NPs and MoS2 had strong interaction concerning bonding mechanism of Pd atom and MoS2, Pd NPs were more likely to be loaded on MoS2 to form stable configuration [27]. The prepared composite was coated on GCE surface for detecting the electrochemical oxidation current of QR. The IL-OMC was characterized by FTIR spectra as shown in Fig. 1A. The peaks at 1549 cm-1 and 1170 cm-1 were attributed to C=N and C-N stretching vibrations of IL, respectively [20], indicating that the IL had been attached to OMC. The elemental compositions of the hybrid were determined by XPS. N 1s XPS spectrum (Fig. 1B) was marked by four peaks at 401.3 eV (graphitic N), 399.5 eV (pyrrolic N), 398.2 eV (pyridinic N) and 394.6 eV (Mo 3p3/2), respectively [29, 30]. High-resolution XPS spectrum of C 1s was displayed in Fig. 1C. The strong peak located at 284 eV belonged to graphitic N of OMC [30]. The two peaks at 285.7 eV and 287.7 eV were 6
assigned to C-O and C=O, respectively [30]. The additional peak at 286.4 eV could be indexed as C-N, originating from imidazole ring of IL, which further confirmed the presence of IL [30]. In Fig. 1D, the Pd 3d5/2 and Pd 3d3/2 peaks for the sample appeared at 336.6 eV and 341.9 eV, in accordance with that of metallic Pd0, indicating that Pd material was prepared [31]. Fig. 1E depicted four signals corresponding to Mo4+ 3d3/2, Mo4+ 3d5/2, S 2s and Mo6+ 3d [31]. Meanwhile, Fig. 1F exhibited the typical peaks of S2- 2p3/2 and S2- 2p1/2 [31]. XRD and Raman spectrum experiments were performed to investigate the structure of MoS2, MoS2-IL-OMC and Pd/MoS2-IL-OMC. As shown in Fig. 2A, the diffraction peaks of MoS2-IL-OMC at 32.6°, 35.3°, 43.6°, 57.6° corresponded to (100), (102), (104) and (110) of MoS2 [26, 32], respectively. In addition, the broad weak peak at 25.0° could be indexed to (002) of OMC, demonstrating that the IL-OMC was decorated with MoS2 [33]. The XRD pattern of Pd/MoS2-IL-OMC presented representative diffraction peaks at 40.0°, 46.7° and 68.4°, assigned to (111), (200) and (220) of Pd, indicating the existence of Pd [15]. It was noticed that the (100) and (102) peaks were blended, which might be related to poor crystallinity [31]. Moreover, a new weak peak emerged at 59.7° for Pd/MoS2-IL-OMC hybrid. This phenomenon could be explained by the increased interlayer distance of MoS2 [33]. Since the edges of MoS2 layers were catalytic active, such change of MoS2 led to the exposure of more active sites [26, 34]. In Fig. 2B, D and G peaks for OMC appeared in both Raman spectra of MoS2-IL-OMC and Pd/MoS2-IL-OMC. The peaks at 376, 380 and 382 cm-1 were associated with E12g modes of the MoS2-based composites (insert, a and b) [35]. Compared
with
pure
MoS2,
E2g1
both
modes
of
MoS2-IL-OMC
and
Pd/MoS2-IL-OMC had a red-shift and the distance between E2g1 and A1g peaks decreased (the A1g peaks of MoS2-IL-OMC and Pd/MoS2-IL-OMC occurred at 408 and 409 cm-1, respectively) [35]. From above results, it could be thought that the MoS2 in the composites consisted of fewer layers than pure MoS2, because the staking of MoS2 nanosheets was suppressed by OMC and Pd NPs [5, 35, 36]. Additionally, when incorporated with Pd, the position of A1g and E2g1 peaks further shifted toward 7
higher frequency, suggesting that the semiconductor characteristics and electronic structure of the MoS2 had changed [37]. This considerable change might be associated with the improvement of material properties [37]. The morphology of pristine MoS2 displayed spherical structure via flakes highly randomly staking (Fig. 3A). The diameter of MoS2 spheres was about 2.2 μm and the thickness per layer was about 1.8 nm. Once IL-OMC presented, MoS2 flakes piled up into nanoflowers on IL-OMC surface and the number of staked layers markedly reduced (Fig. 3B) [27]. Fig. 3C showed the SEM survey of the final material, in which lots of Pd NPs located on the edges of MoS2 nanosheets and some existed between interlayer, leading to larger layer-spacing. In Fig. 3D, the MoS2 consisted of few layers tightly incorporated with IL-OMC, and Pd NPs were mostly anchored by MoS2. The HR-TEM image of Pd NPs was shown in Fig. 3D, the lattice distance corresponding to (111) plane of metallic Pd phase was 0.22 nm, which was in agreement with that reported [38]. On the basis of the observation, it could be sure that Pd/MoS2-IL-OMC hybrid was successfully fabricated. The nitrogen adsorption/desorption isotherms were shown in Fig. S1. It could be seen that the isotherm curves belonged to typical IV isotherm, indicating the existence of mesopore structure [32]. The specific surface area of Pd/MoS2-IL-OMC was ca.589.99 m2 g-1, lager than that of MoS2-IL-OMC (558.17 m2 g-1) and IL-OMC (327.67 m2 g-1), which benefited from the Pd NPs. This was beneficial to the adsorption of target molecules and catalytic process. 3.2. Electrochemical properties of Pd/MoS2-IL-OMC/GCE To examine the electrochemical property of the Pd/MoS2-IL-OMC/GCE etc, the EIS and CV curves of various modified electrodes were recorded in Fe[(CN)6]3-/4solution. As shown in Fig. 4A, the EIS of MoS2/GCE presented a smaller semicircle as compared with that of bare GCE, indicating a low impedance. When GCE was modified by MoS2-IL-OMC and Pd/MoS2-IL-OMC, the impedance curves almost became straight line, manifesting the formation of high charge conduction pathways at the interface. This change was related to the excellent electrical conductivity of Pd NPs and IL-OMC. It was observed that Fe[(CN)6]3-/4-showed a pair of poor redox 8
peaks at the MoS2/GCE (Fig. 4B (d)), indicating the low electron-transfer rate of MoS2. But on MoS2-IL-OMC/GCE the peaks became more reversible, indicating that IL-OMC could improve the conductive property of MoS2 layer. Compared with MoS2-IL-OMC/GCE
and
Pd/IL-OMC/GCE,
the
redox
peak
current
at
Pd/MoS2-IL-OMC/GCE apparently increased (Fig. 4B (e)). This improvement probably resulted from the synergism of MoS2 and Pd NPs in facilitating electron transfer. Different noble metals modified MoS2-IL-OMC/GCE were compared in Fig. S2. The result indicated that the electrocatalytic activity of Pd NPs for QR was better than Au and Pt NPs in this case. This could be explained as follows: 1) Au with relatively stronger s-orbital contribution was different from Pd with significant d-orbital characteristics [39]. The electronic structures of the metals influenced their incorporation with the substrate metal and their catalytic performance [40]. 2) The properties of Pt and Pd are similar; in general, Pt shows higher catalysis than Pd. However, Pt displayed poorer poisoning-tolerance, which affected the catalytic efficiency [41]. Thus, Pd NPs were selected in this work. Then, the electrochemical response of QR at different electrodes was tested under the same conditions. As could be seen in Fig. 4C, the MoS2 modified GCE (Fig. 4C (b)) had no response toward QR. QR caused a very small oxidation peak at bare GCE (Fig. 4C (a)) because of its weak adsorption at GCE. As for IL-OMC/GCE, MoS2-IL-OMC/GCE and Pd/IL-OMC/GCE (Fig. 4C (c, d and e)), the peak current increased due to their high specific surface area and good conductivity as well as good catalysis for QR. Furthermore, the Pd/MoS2-IL-OMC/GCE (Fig. 4C (f)) exhibited higher peak current, thanks for the synergistic catalytic effect of Pd NPs and MoS2 [42]. Besides, the specific surface area of Pd/MoS2-IL-OMC/GCE was higher, which also contributed to the increase of peak current. Therefore, the sensitive current response at Pd/MoS2-IL-OMC/GCE could be explained as follows: 1) after the introduction of Pd NPs, the number of staked MoS2 layers decreased and the interlayer spacing of MoS2 increased, thus more active sites became available [1, 26]; 2) when Pd NPs were prepared on the MoS2, the specific surface area of the composite increased further, and it could adsorb more QR 9
[17]; 3) the combination of MoS2 and Pd NPs altered the electronic structure, which was in favor of electron transfer [17, 37]. The influence of scan rates (ν) on the current response of QC was also investigated. As shown in Fig. S3, the peak current (Ip) was linear to ν (i.e. Ip = 0.48 ν - 1.12, ν: mV s-1, R=0.998), indicating that the oxidation process of QC is adsorptioncontrolled [43]. At the same time, the peak potential (Epa) shifted positively with increasing scan rate, and Epa and ln v had such relationship as Epa = 0.022 ln ν + 0.19 (R=0.996), meaning the electron-transfer rate was not very fast. 3.3. Optimization of conditions The electrochemical performance of the sensor was affected by several factors, such as the amount of MoS2 and Pd NPs used, the solution pH and accumulation time. In order to get good performance, some experimental conditions were optimized. The synergistic effect of Pd NPs and MoS2 played a crucial role in the electrochemical response of QR. Thus, the MoS2 content and Pd content in the composite were taken into account. Fig. S4A depicted that the oxidation peak current of QR increased with the concentration of MoO2-4 increasing from 3 to 17 mM for preparing the composite. However, when MoO2-4 concentration further increased, the peak current decreased, because too much MoS2 would lead to poor conductivity. Therefore, the optimal MoO2-4 concentration was 17 mM. The peak current of QR increased dramatically with increasing PdCl-4 concentration up to 0.7 mM (Fig. S4B). This could be explained as follows: when Pd content raised, the catalysis for QR enhanced, but too much Pd NPs would agglomerate each other so that the specific surface area decreased; excessive Pd resulted in weak synergistic catalytic effect due to the decrease of MoS2 proportion. Consequently, 0.7 mM of PdCl-4 was adopted. The influence of pH value of ABS was presented in Fig. S4C. The results revealed that the oxidation peak current increased as the pH value increased and the highest peak current was obtained at 5.0. When pH exceeded 5.0 the oxidation peak current decreased. This was because proton was significant in electrochemical oxidation-reduction of QR. In addition, at higher proton concentration QR (Scheme 2) was neutral, while at lower proton concentration, the two hydroxyl groups, assigned 10
to C-1 and C-2, became dissociated forms [23, 44], affecting the accumulation of QR at the sensor surface. Therefore, ABS with pH 5.0 was selected as the supporting electrolyte. It could be seen that the oxidation peak current of 4.0 μM QR increased with increasing preconcentration time and trended toward unchanged above 4 min (Fig. S4D), indicating the saturated adsorption of QR. Thus, 4 min was adopted for accumulation in the experiment. 3.4. Analytical performance LSV method was applied to detect QR under the optimized conditions (Fig. 5). The result showed that the peak current of QR at about 0.3 V varied linearly with the addition of QR from 0.020 to 10.0 μM, with a linear equation of Ip (μA) = 10.6 C (μM) - 0.05 (R=0.999). The LOD and sensitivity were evaluated to be 8.0 nM and 150.1 μA μM-1 cm-2, respectively. The sensor was compared with other QR sensors reported and the result was listed in Table 1. It could be concluded that the sensor had a wide linear range and low detection limit. 3.5. Interference, reproducibility and stability The anti-interference capability of Pd/MoS2-IL-OMC/GCE for QR determination was investigated (Fig. 6). The LSV curves were recorded for 4.0 μM QR solution containing different concentrations of potential interferents. It was found that 100-fold sucrose, lactose, fructose and glucose, 50-fold L-cysteine, glycine and citric acid and 5-fold ascorbic acid didn’t generate interferences (the change of peak current was less than 5 %). Thus, the sensor had desirable anti-interference ability and could be used for the determination of QR in some samples.. To estimate the reproducibility and stability of the sensor, five modified electrodes were tested by measuring the peak current of 4.0 μM QR under the same experimental conditions and the relative standard deviation (RSD) was 6.8%. Furthermore, one electrode was used for five detections; the RSD was 4.1 %. After storing for one week at room temperature, the peak current maintained 95.2% of the initial value (Fig. S5). These reflected the good reproducibility and stability of the sensor. 11
3.6. Applications The Pd/MoS2-IL-OMC/GCE was used for detecting QR in real samples to evaluate its practicability. Dried blueberries and grape juice samples were pretreated according to the method mentioned previously, and the sample solutions were determined. The QR contents in dried blueberry and grape juice were calculated to be 0.13 mg g-1 and 4.9 mg L-1, respectively. The recoveries for QR standards added varied from 82 % to 108.0 % (Table 2). The contents of QR were also detected by HPLC (Fig. S6). Correspondingly, the results were 0.11 mg g-1 (RSD: 3.8%, n = 3) and 4.6 mg L-1 (RSD: 2.1%, n = 3), respectively. According to F-test, the F values were calculated as 5.92 and 2.27. When confidence level was 95% and n = 3, the statistic F value was 19, bigger than the calculated F values, indicating that there was no significant difference between the methods. Then the average values were evaluated by t-test, the calculated t values were 0.77 and 1.73, less than t0.05,4 = 2.78. Therefore, the proposed method in this work is reliable. 4. Conclusion In this work, a novel electrochemical sensor for QR was constructed, which was based on a 3D Pd/MoS2-IL-OMC nanocomposite. As the IL functionalized OMC had high specific surface, conductivity and binding action, and the Pd NPs and MoS2 had strong electrocatalysis, the composite not only exhibited large specific surface area but also showed high electrocatalytic activity toward QR. Hence, the resulting sensor gave good performance. It had a wide liner range (0.020 - 10.0 μΜ) and low detection limit (8.0 nM), also displayed good reproducibility and stability. The sensor could be used for the detection of QR in real samples. 5. Acknowledgements This work got the financial support from the National Natural Science Foundation of China (Grant No.: 21277105).
12
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Captions Table 1 . Comparison of different QR sensors. Table 2. Determination results of QR in samples with the prepared sensor. Fig. 1. FT-IR spectra of IL, IL-OMC(A); high-resolution XPS spectra of N 1s (B), C 1s (C), Pd 3d (D), Mo 3d (E) and S 2p (F) of Pd/MoS2-IL-OMC. Fig. 2. (A) XRD of MoS2, MoS2-IL-OMC and Pd/MoS2-IL-OMC; (B) Raman spectra of MoS2-IL-OMC (a) and Pd/MoS2-IL-OMC (b). Insert: Raman spectrum of MoS2. Fig. 3. SEM images of MoS2 (A), MoS2-IL-OMC (B) and Pd/MoS2-IL-OMC (C). TEM images of Pd/MoS2-IL-OMC (D). Insert: HR-TEM image of Pd NPs in Pd/MoS2-IL-OMC. Fig. 4. (A) EIS of GCE (a), pure MoS2/GCE (b), MoS2-IL-OMC/GCE (c) and Pd/MoS2-IL-OMC/GCE MoS2-IL-OMC/GCE
(b),
(d).
(B)
CV
Pd/IL-OMC/GCE
curves (c),
of pure
IL-OMC/GCE MoS2/GCE
(d)
(a), and
Pd/MoS2-IL-OMC/GCE (e). Solution composition for EIS and CV: 0.10 M KCl containing 5.0 mM Fe[(CN)6]3-/4- (1:1) . (C) LSV curves of QR at different electrodes: bare GCE (a), pure MoS2/GCE (b), IL-OMC/GCE (c), MoS2-IL-OMC/GCE (d), Pd/ IL-OMC/GCE (e) and Pd/MoS2-IL-OMC/GCE (f). Solution condition: 0.10 M ABS containing 4.0 μM QR. Fig. 5. LSV curves of QR at Pd/MoS2-IL-OMC/GCE in 0.10 M ABS (pH 5.0). QR concentration (from a to l): 0.020, 0.090, 0.20, 0.40, 0.80, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 and 15.0 μM. Accumulation time: 4 min; scan rate: 100 mV/s. Insert: the calibration curve. Fig. 6. Influence of coexistents on the peak current of QR at the sensor. Solution composition: 4.0 μM QR, 4.0 μM QR plus 100-fold sucrose, 4.0 μM QR plus 100-fold lactose, 4.0 μM QR plus 100-fold fructose, 4.0 μM QR plus 100-fold glucose, 4.0 μM QR plus 50-fold L-cysteine, 4.0 μM QR plus 50-fold glycine, 4.0 μM QR plus 50-fold citric acid and 4.0 μM QR plus 5-fold ascorbic acid. Other conditions as in Fig. 5.
18
Scheme 1. Illustration of the fabrication procedure of Pd/MoS2-IL-OMC/GCE. Scheme 2. Chemical structure of QR.
19
Fig. 1.
20
Fig. 2.
21
Fig.3.
22
Fig.4.
23
Fig. 5.
24
Fig. 6.
25
Scheme 1.
26
Scheme 2.
27
Table 1. Comparison of different QR sensors. Linear range (μM) LOD for QR (nM) References
Sensors Pt-PDAa@SiO2/GCE
0.05–0.383
16
[24]
0.50–330
100
[45]
0.0165-0.3309
12
[46]
CD/AuNPs/MWCNTs/GCE
0.005-7
6.4
[47]
MIPc/GO/GCE
0.6-15
48
[48]
Pd/MoS2-IL-OMC/GCE
0.02-10
8.0
This work
Co3O4/GCE Activated silica gel/CPEb
a
Polydopamine.
b
Carbon paste electrode.
c
Molecularly Imprinted Polymer.
Table 2. Determination results of QR in samples with the prepared sensor. Sample Dried blueberries
Grape juice
Added (μM) 0.00 1.00 2.00 3.00 0.00 1.00 2.00 3.00
Detected (μM) 1.43 2.49 3.60 4.38 1.61 2.43 3.59 4.77
28
Recovery (%) 106 108 98.3 82 99 105.3
RSD (n = 3) (%) 2.5 3.2 0.2 2.7 2.7 4.5 3.2 2.1