Strategy for Highly Sensitive Electrochemical Sensing: In Situ

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Strategy for Highly Sensitive Electrochemical Sensing: In Situ Coupling of MOF with Ball-mill-exfoliated Graphene Xiaoyu Li, Caoling Li, Can Wu, and Kangbing Wu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00556 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Analytical Chemistry

Strategy for Highly Sensitive Electrochemical Sensing: In Situ Coupling of MOF with Ball-mill-exfoliated Graphene

Xiaoyu Li 1, Caoling Li 1, Can Wu 2*, Kangbing Wu 1*, 1

Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

2 Hubei

Collaborative Innovation Center for Advanced Organic Chemical Materials,

Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China ∗Corresponding author. E-mail: [email protected] (C. Wu); [email protected] (K.B. Wu).

ABSTRACT: A highly sensitive electrochemical sensing system is developed via in situ

integration

of

Cu-based

metal-organic

frameworks

(Cu-BTC)

and

high-conductivity ball-mill-exfoliated graphene (Cu-BTC@GS) by a simple method. The

as-synthesized

Cu-BTC@GS

hybrids

display

remarkably

enhanced

electrochemical activity due to the synergistic effect resulting from the integration. Compared to those of the pristine GS, the introduction of Cu-BTC nanoparticles leads to significant improvement in the surface area and porosity, as revealed by the nitrogen adsorption-desorption analysis. In addition, the oxidation behavior of 1

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nicotinamide adenine dinucleotide studied using the rotating ring disk electrode further reveals a superior electron-transfer rate constant (k) for the composite, indicating higher catalytic ability. Moreover, double potential step chronocoulometry of biomolecules (xanthine and hypoxanthine) and phenolic pollutants (bisphenol A and p-chlorophenol) reveals that the prepared composite possesses greatly-enhanced adsorption properties, resulting in much higher response signals and detection sensitivity. Benefiting from the superior reactivity, a highly sensitive electrochemical sensing platform for wide targets is successfully fabricated. It was used in the analysis of plasma, urine, receipt and wastewater samples, and the results were highly consistent with those obtained by high-performance liquid chromatography. We believe that this study provides an effective strategy for the construction of high-performance electrochemical sensing systems.

In recent years, metal-organic frameworks (MOFs) have been widely applied in the field of electrochemical sensors due to their unique structural advantages, such as large surface areas, high porosities and tunable pore structures.1-3 To exploit their structural advantages and further improve their electrochemical sensing properties, coupling MOFs with other materials has been confirmed as an effective strategy. 4 To date, various materials, including multiwalled carbon nanotubes,5 macroporous carbon,6 metal nanoparticles7-9 and conducting polymers,10,11 have been employed to combine with MOFs to enhance their electrochemical performances. With the development of graphene research, graphene-based composites have 2


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drawn great attention.12,13 Recently, graphene-based MOFs composites have been demonstrated that their integration certainly produces remarkable synergistic effect.14,15 Until now, the used graphene to combine with MOFs are almost graphene oxides16 (GO) and reduced graphene oxides17,18 (rGO) that prepared by chemical oxidation exfoliation of graphite. As is well known, the chemically exfoliated GO and rGO have many intrinsic drawbacks, such as the use of a large amount of toxic reagents, complicated and dangerous processes, et al.19 Unlike to chemical oxidation exfoliation, physical exfoliation such as micromechanical cleavage,20 liquid phase ultrasonication21 and ball milling22,23 has been proved to be an effective, mild and simple approach to prepare graphene. Moreover, the physically exfoliated graphene has been reported to have superior sensing properties compared with GO and rGO.24,25 Among these physical exfoliation methods, ball milling is a superior approach to obtain graphene considering high yields and good quality.26 However, there are few investigations about the electrochemical sensing of ball-mill-exfoliated graphene, especially, the studies about its composites with MOFs are more rare. Therefore, it is urgently necessary and fascinating to investigate the preparation method and potential application of their composites. Herein, a gentle and facile method is reported for the in situ synthesis of Cu-BTC (BTC=1,3,5-benzenetricarboxylic acid) frameworks-decorated ball-mill-exfoliated graphene (Cu-BTC@GS). In the hybrids, Cu-BTC frameworks with size of about 30 nm were uniformly anchored on the surface of ball-mill-exfoliated GS. The results of nitrogen adsorption-desorption analysis and rotating ring disk electrode measurements 3



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showed that the introduction of Cu-BTC frameworks effectively enhanced the surface area, porosity and electron-transfer rate constant. Furthermore, the as-synthesized Cu-BTC@GS hybrids exhibited greatly-enhanced reaction activity and adsorption capacity toward biomolecules (xanthine and hypoxanthine) and phenolic pollutants (bisphenol A and p-chlorophenol), offering much higher response signal and detection sensitivity. Due to the synergistic effect between the ball-mill-exfoliated graphene and the in situ grown Cu-BTC nanoparticles, an electrochemical sensing platform with high sensitivity was firstly fabricated. It was successfully used in different real samples,

and

the

results

were

validated

using

high-performance

liquid

chromatography (HPLC). This work is in favor of preparing other MOFs composites with physically exfoliated 2D materials, and also helpful for developing high-performance electrochemical sensing systems.

EXPERIMENTAL SECTION Reagents Copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), 1,3,5-benzenetricarboxylic acid (H3BTC), trimethylamine, N,N-dimethylformamide (DMF) and graphite powder (99.95%, 1200 mesh) was purchased from Aladdin (Shanghai, China). Cetyltrimethyl ammonium bromide (CTAB) was purchased from Sinopharm Chemical Reagent Company (Shanghai, China). Nicotinamide adenine dinucleotide (NADH), Xanthine (XA), hypoxanthine (HXA), bisphenol A (BPA) and p-chlorophenol (CP) were purchased from Sigma Aldrich (United States). The stock solutions of NADH, XA 4


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and HXA, and BPA and CP (1 mg mL-1) were prepared using ultrapure water, 0.1 M NaOH, and ethanol, respectively. All chemicals were of analytical grade, and ultrapure water (18.2 MΩ cm) was obtained with a Milli-Q water purification system.

Apparatus Ball milling was performed on a planetary ball mill (MiQi Instrument Equipment Co., Ltd., China). Scanning electron microscopy (SEM) images were obtained on Hitachi SU8010 (Hitachi Limited, Japan). High-resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray spectroscopy (EDX) elemental maps were obtained on a Tecnai G2 F30 microscope (FEI Company, Netherlands). Fourier-transform infrared (FTIR) spectra were collected on an Equinox-55 FTIR spectrometer (Bruker Company, Germany) using KBr pellets. Raman spectra were obtained on a LabRAM HR800 confocal Raman microscope (Horiba Jobin Yvon, France) using a 532 nm laser. X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS-ULTRA DLD-600W spectrometer (Shimadzu, Japan). Thermogravimetric analysis (TGA) was performed on STA449F3 (NETZSCH, Germany) with a heating rate (10 C min−1) in air. Nitrogen adsorption-desorption analysis was performed on an ASAP2420-4MP analyzer (Micromeritics Instrument Corporation, US). Rotating disk electrode experiments were performed using the RRDE-3A apparatus (BAS Company, Japan). Electrochemical measurements were performed on a CHI 830D electrochemical workstation with a conventional three-electrode system: a glass carbon electrode as 5



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the working electrode, platinum wire electrode as the counter electrode, and saturated calomel

electrode

as

the

reference

electrode.

High-performance

liquid

chromatography (HPLC) was performed on UltiMate 3000 (Thermo Fisher Scientific, Germany) with a C-18 analytical column (4.6 mm  150 mm  5 mm). All actual samples for HPLC detection were pretreated. Plasma and urine samples were collected from the Wuhan Tongji hospital (Hubei province, China) and stored at -20 C. On the day of HPLC analysis, 1 mL plasma was deproteinized with 9 mL of 0.4 M perchloric acid at 4 C, and then centrifuged at 15000 rpm for 10 min to collect the supernatant liquid. The urine samples were acidified to a pH below 3 with 1 M H2SO4, and centrifuged at 15000 rpm for 10 min to remove impurities; subsequently, the supernatant was diluted with ammonium acetate solution by a factor of 10. Shopping receipts (thermal paper) were collected from a local market and cut into small pieces, subsequently, 1.0 g of the receipt sample was exactly weighed and added to 20 mL methanol under 1 h sonication, and the extracting solution was centrifuged at 10000 rpm for 20 min to collect the supernatant liquid. Wastewater was collected from a local chemical plant and pretreated by centrifuging at 10000 rpm for 20 min. All the supernatant samples were filtered through a 0.45 µm Millipore filter and the filtrate was collected. The HPLC detection conditions for XA and HXA,27 BPA28 and CP29 were maintained the same as those reported in the literature. Preparation of GS Until now, ball milling has been successfully used to exfoliate graphite powder to graphene30 or with the assistance of nonionic surfactant (naphthol polyoxyethylene 6


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ether).31 Here, the graphene nanosheets (GS) were prepared by ball-mill exfoliation with the aid of CTAB. In brief, graphite powder (300 mg) was mixed with an equal amount of CTAB, and then dispersed homogeneously in 30 mL of water/ethanol mixture (volume ratio = 17:3). The resulting solution was then transferred to a 100 mL milling jar with two kinds of grinding balls with diameters 8 mm (5 balls) and 2 mm (30 balls). Ball milling was performed at 300 rpm (to avoid damage to the graphene basal planes) for 12 h. Subsequently, the solution was subjected to first low-speed centrifugation (2000 rpm, 20 min) to remove oversized graphene sheets, and then high-speed centrifugation (9000 rpm, 20 min) to obtain the solid. Finally, the obtained product was washed repeatedly with water and ethanol and dried by blast drying.

Synthesis of Cu-BTC@GS Cu(NO3)2·3H2O (0.7 g) was homogeneously dissolved in DMF (50 mL) under ultrasonication, and then GS powder (50 mg) was added under stirring for 0.5 h. Next, H3BTC (0.42 g) was dispersed in 50 mL DMF and directly added in the mixed solution. Then, 1% (v/v) trimethylamine/DMF (1 mL) was slowly dropped into the mixed solution under stirring. After stirring for another 0.5 h, the composite was collected by centrifugation, rinsed carefully with ethanol three times, and finally, dried at 30 C for 24 h. For comparison, pure Cu-BTC was also prepared through the same process without adding the GS powder. For electrochemical measurements, 2 mg mL−1 slurries of GS, Cu-BTC, and Cu-BTC@GS samples were prepared by 7



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dispersing the samples in DMF and sonicating for 20 min.

Electrochemical Measurements First, a glassy carbon electrode (GCE, φ = 3 mm) was polished with 50 nm alumina powder to a mirror-like surface and then ultrasonically washed with ultrapure water and ethanol. Then, 5 μL of the prepared dispersion was directly dropped onto the clean GCE surface, and then dried under an infrared lamp. Cyclic voltammetry (CV),

differential

pulse

voltammetry

(DPV),

and

double

potential

step

chronocoulometry measurements were performed in 0.1 M phosphate buffer solution (pH = 7.0). The enrichment time was 120 s, pulse amplitude was 50 mV, pulse width was 40 ms, and the scan rate was 40 mV s-1.

RESULTS AND DISCUSSION Morphology and Structure Characteristics of Cu-BTC@GS The composition and crystal structures of the as-prepared materials were investigated by XRD. The XRD pattern of Cu-BTC@GS (Figure 1a) exhibited all the characteristic peaks of Cu-BTC, and the pattern matched well with that reported in the literature,32 demonstrating the effectiveness of the synthesis procedure. In addition, the XRD patterns of both GS and Cu-BTC@GS exhibited a strong peak at around 26.58, which was assigned to the (002) plane of graphite. Thus, the XRD results confirmed that the obtained composite consisted of the Cu-BTC framework and GS. To examine the morphology and microstructure of Cu-BTC@GS, we performed SEM 8


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and TEM measurements. The original GS sample was composed of thin flake-like structures with micron sizes, which could provide large surface areas for loading Cu-BTC (Figure 1b). In the Cu-BTC@GS composite, Cu-BTC particles with size about 30 nm and irregular morphology were found to be homogeneously deposited on the surfaces of the GSs (Figure 1c-d). Moreover, no obvious morphology change was observed as to the pure Cu-BTC particles without the existence of GS (Figure S1, Supporting Information). These results indicated the successful synthesis of Cu-BTC@GS composites. The elemental composition and structural features of the Cu-BTC@GS composite were further investigated by TEM, as illustrated in Figure 1e-f. The images showed that ~30 nm Cu-BTC particles were uniformly distributed on well-defined graphene substrates, in agreement with the SEM results. During the synthesis procedure, the copper ions gradually adsorbed on the surface of the ball-mill-exfoliated GS, and subsequently, combined with H3BTC to form Cu-BTC. The existence of graphene prevented the aggregation of crystalloids and increased dispersion, which led to the formation of well-dispersed and nanosized Cu-BTC particles on the surface of the GS. Besides, the EDX maps of Cu-BTC@GS (Figure 1g-k) confirmed the existence and homogeneous distribution of the constituent elements C, O and Cu, further verifying the presence of GS and Cu-BTC in the composite.

9



Figure 1. (a) XRD pattern of the as-synthesized GS, Cu-BTC and Cu-BTC@GS. (b) SEM image of GS. (c-d) Different magnified SEM images of Cu-BTC@GS. (e-f) Different magnified HRTEM images of Cu-BTC@GS. (g-j) EDX element mapping images of Cu-BTC@GS.

To understand the interaction between Cu-BTC and GS, we performed FTIR spectroscopy (Figure 2a). The C-OH absorption peak of H3BTC at 1278 cm-1 were not detected for the Cu-BTC sample; however, additional strong absorption bands at 1375 and 1563 cm-1 attributed to the asymmetric and symmetric stretching vibrations of the carboxylate group in Cu-MOFs were detected, indicating the coordination of organic ligands with copper centers and the formation of Cu-BTC.33 In addition, for Cu-BTC@GS, the characteristic absorption peaks of Cu-BTC and GS were observed, confirming the composition of Cu-BTC@GS. Meanwhile, to investigate the 10


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symmetric structural vibration modes, Raman spectroscopy was also performed on Cu-BTC@GS (Figure 2b). The D, G and 2D bands derived from GS were detected for the Cu-BTC@GS sample. Moreover, the dominant peaks in the spectra of Cu-BTC@GS and Cu-BTC were those assigned to the C=C and C-H bonds in the MOF. Raman bands corresponding to the υ(C=C) modes of the benzene ring were observed at 1611 and 1004 cm−1; the bands at 824 and 742 cm−1 were assigned to the out-of-plane ring (C-H) bending and out-of-plane ring bending vibrations. The bands at 1540 and 1460 cm-1 corresponded to the υasym(C-O2) and υsym(C-O2) stretching modes. The band at 500 cm−1 was associated with the (Cu-Cu) modes.34,35 Thus, the Raman results further confirmed that the obtained composite was primarily composed of Cu-BTC and GS. To obtain more information about the chemical composition of the prepared composite, we performed XPS (Figure 2c). The full XPS profile confirmed that the Cu-BTC@GS hybrid was composed of the elements C, O and Cu, which is consistent with the EDX element mapping analysis. In order to adequately determinate the content of the individual component in the hybrids, thermal stabilities of GS, Cu-BTC and Cu-BTC@GS were determined by TGA under an O2 atmosphere. As seen in Figure 2d, the Cu-BTC and Cu-BTC@GS samples began to lose weight below 100 °C due to the thermal desorption of the water molecules physically adsorbed onto the MOF. Notably, significant weight loss was observed at around 300 °C, which indicated the removal of organic ligands from the framework.36 When the sample was heated up to 500 °C, the second weight loss step attributed to the oxidation degradation of the GSs was observed. At temperatures above 700 °C, the 11



TGA curves became stable, indicating that no significant weight loss occurred. From the TGA analysis, the weight fractions of Cu-BTC and GSs in the composite were calculated to be about 63.1% and 36.9%, respectively. At last, the specific surface areas and pore structures were characterized by N2 adsorption-desorption measurements at 77 K. As shown in Table 1, the introduction of Cu-BTC particles led to a significant improvement in the BET specific surface area, pore volume and pore size compared with the pristine GS, which is expected to provide the composite material with good adsorption ability.

Figure 2. (a) FTIR spectra of H3BTC, Cu-BTC, GS and Cu-BTC@GS. (b) Raman spectra of GS, Cu-BTC and Cu-BTC@GS. (c) XPS spectrum of Cu-BTC@GS. (d) TGA curves of GS, Cu-BTC and Cu-BTC@GS.

12


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Table 1. N2 sorption data of GS and Cu-BTC@GS Sample

SBET/(m2 g-1)

VTot/(cm3 g-1)

Dav/(nm)

GS

15.5

0.0239

7.30

Cu-BTC@GS

35.8

0.119

13.5

Electrochemical Behavior of Cu-BTC@GS The electrochemical properties of Cu-BTC@GS were first investigated using NADH, which is used as a common electrochemical probe in previous report.37 In this study, the electron-transfer rate constant (k), which is generally regarded as a symbolic parameter of intrinsic electrocatalytic ability of a coating material on electrode surface, was measured in 1.5 g L-1 NADH solution using a rotating disk electrode.38,39 The values of k calculated for bare GCE, GS, Cu-BTC and Cu-BTC@GS according to the Koutecky-Levich equation were 0.0016, 0.0038, 0.0045 and 0.0053 cm s−1 (Figure 3). Notably, the larger value of k manifests higher catalytic ability. Thus, the electrocatalytic ability was determined to be the highest for the composite, which could be attributed to the synergistic effect of the high electronic conductivity of GS and the strong adsorption capacity of Cu-BTC.

13



Figure 3. Steady-state voltammograms of GCE (a), GS (b), Cu-BTC (c) and Cu-BTC@GS (d) in 0.1 M pH 7.0 phosphate buffer solution containing 1.5 g L-1 NADH at different rotation speed with a scan rate of 50 mV s-1. (e) The Kouteckye-Levich plots for different materials modified electrodes.

In addition, to confirm the electrochemical activity observed for Cu-BTC@GS, small biological molecules (XA and HXA) and phenolic pollutants (BPA and CP) were chosen as probe molecules. The electrochemical activities of Cu-BTC, GS and Cu-BTC@GS were studied using DPV by measuring the oxidation signals corresponding to different organic molecules (Figure 4). Very weak oxidation peaks were observed for GCE, suggesting the poor electrochemical performance of bare GCE. However, the introduction of GS led to a significant improvement in the oxidation peak current, which could be attributed to the accelerated electron transfer and enhanced specific surface area. In addition, the Cu-BTC modified electrodes also exhibited increased oxidation peak current relative to bare GCE, demonstrating the 14


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strong adsorption capability of Cu-BTC caused by the high porosity. Nonetheless, the Cu-BTC@GS composite exhibited the highest oxidation current, testifying that the Cu-BTC@GS hybrid possessed the advantages of both Cu-BTC and GS, which led to superior electrochemical activity due to the synergistic effect.

Figure 4. DPV curves of 0.3 mg L-1 XA and HXA (a), 2 mg L-1 BPA (b) and 2 mg L-1 CP (c) in 0.1 M pH 7.0 phosphate buffer on different modified electrodes and GCE.

To clearly understand the mechanism of synergetic effect of Cu-BTC and GS, we further explored the electrochemical adsorption tests by double potential step chronocoulometry in phosphate buffer solution (pH = 7.0) containing 0.2 mg L-1 XA, 0.2 mg L-1 HXA, 1 mg L-1 BPA and 1 mg L−1 CP, respectively. The charge (Q)-time (t) curves were individually recorded during the forward and reverse steps. The values of Qads, corresponding to the oxidation of the reactant adsorbed on different materials, were calculated according to the Cottrell theory, as shown in Figure S2a-d (Supporting Information).40 The Qads values for different analytes on GCE, GS, Cu-BTC and Cu-BTC@GS are listed in Table 2. It was found that the Qads values of all analytes on Cu-BTC were higher than those on GCEs, this result could be attributed to the larger surface area, microporous structure and strong adsorption 15



Page 16 of 31

ability of Cu-BTC towards the probe molecules. Also, the Qads values of GS were higher compared with bare GCEs owing to the good conductivity and large specific surface area. As expected, the highest Qads values were determined for Cu-BTC@GS, indicating high conductivity (inherited from GS) of the hybrid and abundant adsorption sites (inherited from Cu-BTC) on its surface, which greatly enhanced the electrochemical activity of the hybrid electrode. Therefore, the oxidation peak currents of XA, HXA, BPA and CP on the Cu-BTC@GS surface were higher than those on the surfaces of Cu-BTC or GS.

Table 2. Qads values (µC) for different molecules on different material surface Electrode

Qads-XA

Qads-HXA

Qads-BPA

Qads-CP

GCE

0.977

0.365

0.0281

0.0569

GS

4.74

1.20

1.81

0.368

Cu-BTC

3.72

0.561

0.492

0.218

Cu-BTC@GS

9.40

4.47

2.74

0.724

Highly-sensitive Electrochemical Sensing Platform The sensing properties and application of Cu-BTC@GS toward these organic molecules were further studied. As shown in Figure 5, the oxidation peak currents (Ip) of the analytes increased linearly with the increase of their concentrations (C), and the correlation coefficient (R) was higher than 0.99, indicating good linearity. In addition, we also found that the oxidation signals of XA and HXA had no mutual interferences, as confirmed from Figure 5a and Figure 5b. So the developed Cu-BTC@GS sensing 16


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platform is qualified for the simultaneous determination of XA and HXA. To demonstrate the sensing properties, the important quantitative data such as linear range, detection limit and sensitivity are given in Table 3. Our results were compared with those previously reported, as summarized in Table S1-3 (Supporting Information).37,41-55 As seen, clearly, the performance of the Cu-BTC@GS sensing platform developed in this work was comparable or better than those of others.

Figure 5. DPV curves of XA (a), HXA (b), BPA (c) and CP (d) with different concentrations on Cu-BTC@GS modified GCE.

Table 3. Sensing properties of Cu-BTC@GS for different molecules 17



Analyte

Linear range (mg L-1)

Regression equation

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Detect limit

Sensitivity

(mg L-1)

(µA mg-1 L cm-2)

XA

0.003-1.5

Ip (µA) = 13.63 C

0.0011

195

HXA

0.01-1.5

Ip (µA) = 13.82 C

0.0073

197

BPA

0.002-6

Ip (µA) = 1.230 C

0.0012

17.6

CP

0.005-15

Ip (µA) = 1.020 C

0.0019

14.6

Furthermore, the repeatability of the measurements with the fabricated electrochemical platform was examined using 0.2 mg L-1 of XA and HXA mixed solution, 1 mg L-1 of BPA, and 1 mg L-1 of CP. The relative standard deviation (RSD) values of five measurements were 3.2%, 5.2%, 5.0% and 4.6% for XA, HXA, BPA and CP, indicating excellent repeatability. Moreover, the stability of the electrochemical platform was also examined in 0.2 mg L-1 of XA and HXA mixed solution, 1 mg L-1 of BPA, and 1 mg L-1 of CP using the Cu-BTC@GS-modified GCEs stored in air at room temperature. After seven days, the current responses were measured; the results showed that the electrode could retain 95.6%, 94.1%, 97.0% and 96.4% of the original values for XA, HXA, BPA and CP. Thus, the results revealed the high stability of the Cu-BTC@GS. The potential interferences of other species that maybe existed in practical samples were studied. No influence was observed for 0.2 mg L-1 XA and HXA with the addition of glucose at 50 mg L-1; dopamine, ascorbic acid, uric acid or L-cysteine at 5 mg L-1. For 1 mg L-1 BPA and CP, no obvious interferences were found after the addition of Cd2+ or Pb2+ at 50 mg L-1, hydroquinone, catechol or p-nitrophenol at 25 18


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mg L-1; phenol at 10 mg L-1. To evaluate the applicability of the developed electrochemical sensing platform, we measured the amount of XA, HXA, BPA and CP in the real samples using the Cu-BTC@GS-modified GCEs. For XA and HXA, the used real samples were clinical plasma and urine samples. Before detection, the samples were subjected to high-speed centrifugation at 15000 rpm to remove impurities. Subsequently, 200 μL (plasma) or 500 µL (urine) supernatant was directly added to 5 mL pH 7.0 phosphate buffer solution for electrochemical detection. For the detection of BPA in the shopping receipt, similar to sample pretreatment for HPLC, 100 μL extracting sample solution was added to 5 mL phosphate buffer solution (pH 7.0). For CP detection, 5 mL wastewater filtrate was mixed with 5 mL pH 7.0 phosphate buffer solution. Each sample was subjected to three parallel measurements, and the RSD was below 5.0%. The contents were determined by the standard addition method. To check the accuracy of the method, we performed HPLC on the samples, and the results are summarized in Table 4. It was found that the results obtained by this method and HPLC were in good agreement with less than ±10% of relative errors, indicating that the proposed electrochemical method is highly accurate and suitable for real sample detection.

Table 4. Practical application of sensing platform Samples

Analyte

Plasma (mg L-1)

XA

By HPLC By Cu-BTC@GS/GCE 0.160

0.150 19


Relative error -6.25%


HXA

0.650

0.630

-3.07%

XA

12.5

12.8

2.40%

HXA

18.7

17.8

-4.81%

CP

0.0220

0.0240

9.09%

BPA

10.6

10.4

-1.89%

Urine

Wastewater Receipt (mg g-1)

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CONCLUSIONS In summary, an effective and convenient method was developed for the in situ synthesis of graphene-MOFs sensing materials. The ball-mill-exfoliated graphene served as deposition substrate and prevented the aggregation of Cu-BTC nanoparticles, so the Cu-BTC@GS possessed many unique properties such as large surface area, good conductivity, high catalytic ability, strong absorbability, et al. As revealed by the electrochemical tests, Cu-BTC@GS, which possessed advantages of both Cu-BTC and GS, exhibited superior electrochemical reactivity toward phenolic pollutants and small biological molecules. Based on the composite, an electrochemical sensing platform with high sensitivity and promising application was constructed. Thus, the study shows that the proposed synthesis strategy could provide a new possibility for the design and fabrication of other highly sensitive sensing system for various applications in the future.

ASSOCIATED CONTENT Supporting Information Available 20


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The Supporting Information is available free of charge on the http://pubs.acs.org. SEM images of Cu-BTC (Figure S1), the double potential step tests for XA, HXA, BPA and CP (Figure S2), Comparison of sensing performance of different sensors for XA, HXA (Table S1), BPA (Table S2) and CP (Table S3).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C. Wu). *E-mail: [email protected] (K.B. Wu). ORCID Can Wu: 0000-0002-1665-2957 Kangbing Wu: 0000-0003-3927-4300 Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work was supported by the National Basic Research Program of China (973 Program, No. 2015CB352100), the National Natural Science Foundation of China (Nos. 21775050 & 21804031), and the Hubei Provincial Natural Science Fund for 21



Distinguished Young Scholars (2016CFA039). The Center of Analysis and Testing of Huazhong University of Science and Technology was also acknowledged for the help in TEM and XPS measurements.

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Table of contents

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Strategy for Highly Sensitive Electrochemical Sensing: In Situ

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