Highly sensitive electrochemical sensor for chloramphenicol based on MOF derived exfoliated porous carbon

Talanta 167 (2017) 39–43

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Highly sensitive electrochemical sensor for chloramphenicol based on MOF derived exfoliated porous carbon ⁎

MARK

⁎

Lili Xiaoa,b, Ruiyu Xua,b, Qunhui Yuana,c, , Fu Wanga,

a Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics and Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China

A R T I C L E I N F O

A BS T RAC T

Keywords: MOF derived carbon Exfoliated porous carbon Square wave voltammetry Chloramphenicol

Benefit from the advantages in costless, simplicity and efficiency, solvent exfoliation has been widely used in preparation of two-dimensional nanosheets with enhanced performances in electronics, photonics, and catalysis. In this work, solvent exfoliation was first applied to prepare exfoliated porous carbon (EPC) from an isoreticular metal-organic framework-8 (IRMOF-8) derived porous carbon (DPC). The obtained EPC with high surface area (1854 m2 g−1) and improved dispersibility was used as electrode modifier for glassy carbon electrode (GCE) in square wave voltammetry (SWV) detection of chloramphenicol (CAP). The sensitivity of EPC modified GCE (EPC/GCE) was greatly improved in compare with that of the DPC modification. The corresponding linear ranges are 1×10−8–1×10−6 mol L−1 and 1×10−6–4×10−6 mol L−1. The detection limit was calculated to be 2.9×10−9 mol L−1 (at a signal-to-noise ratio of 3, S/N=3). In addition, the proposed sensor was successfully applied in the analysis of CAP in honey and achieved satisfying recovery.

1. Introduction Chloramphenicol (CAP) is a broad-spectrum antibiotic which has notable efficiency in against a wide variety of bacteria [1]. The extensive usages of CAP in treating infectious diseases of humans and animals have resulted in increasing worries about food-safety relating to the CAP residues [2]. Moreover, the CAP has serious side-effects in human health which can cause haemopoietic system abnormality and aplastic anaemia [3]. It is thereby critically important to develop a highsensitive and fast detection method for CAP. Presently, techniques used for determination of CAP include high-performance liquid chromatography, photon induced chemiluminescence, gas chromatography-mass spectrometry and electrochemical technique [2,4]. In comparison, the electrochemical method has attracted extensive attentions and been considered as a promising approach in practical applications owing to its simplicity, low-cost and sensitivity [5,6]. Porous carbon with high surface area is particularly suitable for electrochemical analysis due to its attractive features such as superior adsorption capacity, good stability, and mass productivity [7–11]. Recently, a rapidly growing attention has been focused on metalorganic framework (MOF) derived porous carbon, in which MOF was utilized as a precursor because of its diverse structure, tunable

property, high surface area, and large pore volume [10,12]. For example, a nitrogen doped porous carbon derived from a zeolitic imidazolate framework was demonstrated to be an excellent electrode modifier for simultaneous detection of ascorbic acid, dopamine and uric acid [13]. Our previous studies found that the nitrogen doped microporous carbon or hierarchical porous carbon derived from MOF possesses excellent electrochemical performance in detecting Cd(II) and Pb(II) [14,15]. As an effective strategy in further improving the performance of electrode modifier, solvent exfoliation has recently been comprehensively adopted in electrochemical sensing. For example, exfoliated graphene nanosheets prepared by Wu et al. [16,17] exhibited excellent electrochemical activity in sensing phenols and biological molecules. Keeley et al. [18] also found that N,N-dimethylformamide (DMF)exfoliated graphene showed better electrochemical properties than reduced graphitic oxide. Inspired by these works, we hereby successfully extend the solvent exfoliation into MOF derived porous carbon. The EPC was prepared from DPC by ultrasonication approach using Nmethyl-2-pyrrolidone (NMP) as solvent, which is adopted in solvent exfoliation of two-dimensional materials due to its aromatic donors, surface tension and high viscosity [19–21]. In comparison, the assynthesized EPC exhibited improved dispersibility and high surface

⁎ Corresponding authors at: Laboratory of Environmental Sciencce and Technology, The Xinjiang Technical Institute of Physics and Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China. E-mail addresses: [email protected] (Q. Yuan), [email protected] (F. Wang).

http://dx.doi.org/10.1016/j.talanta.2017.01.078 Received 18 November 2016; Received in revised form 20 January 2017; Accepted 26 January 2017 Available online 03 February 2017 0039-9140/ © 2017 Published by Elsevier B.V.

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Fig. 1. SEM images of DPC at (a) low-magnification and (b) high-magnification. SEM images of EPC at (c) low-magnification and (d) high-magnification.

washing with DMF, ethanol, and then drying at 80 °C for 12 h in a vacuum oven. DPC was prepared through a simple carbonization process that about 1 g of IRMOF-8 was heated at 1000 °C for 5 h with a heating rate of 5 °C min−1 under a nitrogen environment. Synthesis of EPC: The as-obtained DPC (80 mg) was dispersed in 80 mL of solvent (water, ethanol, isopropanol or NMP), and then transferred to a 100 mL reagent bottle. The sealed bottle was sonicated in a KQ-5200 ultrasonicator for 24 h (40 kHz, 100 W). After 2 h quiescence, the EPC was obtained by filtering the supernatant through a 0.22 µm membrane filter, washing with ethanol, and drying at 60 oC for 12 h.

area. The EPC modified GCE (EPC/GCE) was further used to detect CAP by SWV method, demonstrating excellent performances in electrochemical determination of CAP with wide linear ranges and a low detection limit. 2. Materials and methods 2.1. Chemicals and characterization apparatus All chemicals were of analytical grade and used without further treatment. 2,6-naphthalenedicarboxylic acid, zinc nitrate hexahydrate and CAP were purchased from Aldrich, Aladdin and TCI Chemicals, respectively. Anhydrous ethanol, NMP and DMF were purchased from Tianjin Oubokai Chemical Industry. Honey was purchased from a local supermarket. 0.01 mol L−1 standard solution of CAP was prepared with anhydrous ethanol. Deionized water (≥18.2 ΩM cm) was used to prepare all aqueous solutions. 0.1 mol L−1 phosphate buffer solution (PBS) with different pH was prepared by mixing the stock solutions of 0.1 mol L−1 NaH2PO4 and 0.1 mol L−1 Na2HPO4. The transformation of IRMOF-8 to porous carbon was proceeded in a vacuum-tube furnace (GSL-1500X, Kejing Technology, Hefei, China). The ultrasonic treatment was carried out by using a KQ-5200 sonifier (100 W, Kunshan ultrasonic instruments Co., Ltd.). Scanning electron microscopy (SEM) measurements were carried out on a Zeiss (Germany) microscope. Transmission electron microscopy (TEM) measurements were operated at FEI microscope (TecnaiG2 F20, at 200 kV). N2 adsorption–desorption isotherms were recorded on an Autosorb-IQ-MP gas sorption analyzer (USA).

2.3. Fabrication of EPC modified glassy carbon electrode (EPC/GCE) The GCE was polished successively before using with 1.0, 0.3 and 0.05 µm alumina powder, then ultrasonic cleaned with deionized water and ethanol, blow dried with nitrogen flow. 5.0 mg of EPC was dispersed in DMF to get 1 mg mL−1 EPC-DMF suspension. A certain volume of EPC-DMF suspension was dropped onto the cleaned GCE and then dried under infrared lamp for fabrication of EPC/GCE. 2.4. Electrochemical measurements An electrochemical workstation (CHI 660E, CH Instruments, China) was used to measure all electrochemical experiments in which the modified GCE (diameter 3 mm; Gaoss Union, China), a platinum wire (CH Instruments, China) and a saturated calomel electrode (SCE, Gaoss Union, China) were used as the working, counter and reference electrodes, respectively. SWV was recorded from −0.6 to 0.4 V with an increment step of 4 mV, amplitude of 25 mV, and frequency of 15 Hz.

2.2. Synthesis of DPC and EPC

3. Results and discussion

Synthesis of DPC: IRMOF-8 was synthesized according to previous literature methods [15,22]. Typically, 1.19 g of zinc nitrate hexahydrate and 0.43 g of 2,6-naphthalenedicarboxylic acid were dissolved in 40 mL DMF under ultrasonication for 5 min. Then the clear solution was transferred to a 50 mL Teflon vessel in an autoclave and kept at 120 °C for 20 h. Yellow IRMOF-8 was obtained by filtration and

3.1. Characterization of DPC and EPC As shown in Fig. 1a and b, the DPC exhibits a feature of threedimensional bulk carbon enriched with “cheese-like” pores. After 40

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Fig. 2. TEM images of DPC (a) and EPC (b).

3.3. Optimization of experimental conditions The effects of solvents for ultrasonication were investigated by using water, ethanol, isopropanol and NMP as solvent. It was found that water and ethanol were not suitable solvents for exfoliation because of many carbon bulks retaining (Fig. S1) and their bad electrochemical performances (Fig. S2). In contrast, exfoliation proceeded in isopropanol and NMP exhibited promising optimation from the observation of similar fine crushing and uniform morphology (Fig. S1). However, in comparison with EPC/GCE, the GCE modified with isopropanol exfoliated porous carbon showed a weaker SWV response for CAP (Fig. S2), which might because of the isopropanol exfoliated porous carbon possessing less spreadability (Fig. S3) and lower specific surface area (326 m2 g−1) (Fig. S4). Therefore, NMP was chosen as solvent for exfoliation of DPC. As another critical parameter in exfoliation, the ultrasonication time exhibited significant influences on the morphology and SWV performance of EPC. It can be seen from Fig. S5 that the DPC is gradually exfoliated into carbon fragments with increasing ultrasonication time. The effect of the ultrasonication time on the peak current of CAP was evaluated by SWV from 4 h to 30 h (Fig. S6 a). The SWV response of EPC/GCE for CAP increased when the ultrasonication time ranged from 4 h to 24 h and then drastically decreased, which might because of the pores suffered a collapse in longer ultrasonication time. As shown in Fig. S6b, the BET surface area of EPC with NMPexfoliation in 30 h is 138 m2 g−1. Therefore, 24 h was chosen as the optimal ultrasonication time for preparing EPC. The volume of EPCDMF suspension for dropping on GCEs was also optimized by using SWV (Fig. S7a). The peak current of CAP increased sharply with the volume from 2 μL to 5 μL, and then decreased gradually because of excessively increasing EPC amount may hinder the mass transference of CAP [25]. Therefore the volume of EPC-DMF suspension coated on GCEs was chosen as 5 μL. The effect of adsorption time was investigated by SWV in the range of 60–360 s (Fig. S7b). It was found that peak current of CAP increased greatly from 60 s to 180 s, and then kept almost unchanged, which indicated that the adsorption of CAP was saturation. Thus 180 s was chosen as optimal adsorption time. The influence of pH on the electrochemical response of CAP at EPC/GCE was investigated in a pH range of 5.0–9.0 (Fig. S8). The peak current of CAP gradually increased with increasing pH from 5.0 to 7.5, and then decreased when further increasing pH which could be attributed to hydrolysis of CAP [25]. Therefore, pH of 7.5 was chosen as the optimized pH value. Moreover, the peak potential was proportional to pH with a slope of −56.7 mV pH−1, indicating that the number of protons and electrons involved in the electrode reaction were equal [1].

Fig. 3. Nitrogen adsorption–desorption isotherms of DPC and EPC.

ultrasonic treatment, the DPC bulk was successfully exfoliated to wellspread nanoscale carbon fragments (Fig. 1c, d). The exfoliation could be further confirmed in TEM images (Fig. 2), which clearly demonstrate large carbon bulk turned into thin carbon fragments after sonication in NMP. The nitrogen adsorption–desorption isotherms exhibit a sharp increase at low relative pressure and a H3-type hysteresis loop at relative pressure from 0.45 to 1.0 (Fig. 3), respectively, indicating both DPC and EPC possess micropores and mesopores [23,24]. The Brunauer-Emmett-Teller (BET) surface area were calculated to be 1336 m2 g−1 and 1854 m2 g−1 for DPC and EPC, respectively. The increased area is contributive to increase the capacity of EPC/GCE for adsorption of CAP.

3.2. Electrochemical behaviors of various electrodes As shown in Fig. 4a, no obvious electrochemical response for CAP is observed at GCE. On the contrary, well-defined peaks of CAP are observed at about −0.16 V for DPC/GCE and EPC/GCE indicating their high electrocatalytic activity. Moreover, the peak current of CAP at EPC/GCE improved about 10 times than that at DPC/GCE which could be attributed to the increased surface area and good dispersibility of EPC caused by NMP-exfoliation. The electrochemical redox behavior of CAP at EPC/GCE was investigated using cyclic voltammetry (CV) and the results are shown in Fig. 4b. It is obvious that no peak for CAP is observed at EPC/GCE in blank PBS. When adding 5×10−6 mol L−1 CAP, a pair of strong redox peaks and a weak reduction peak are observed at −0.12 V (Pa1), −0.15 V (Pc1) and −0.63 V (Pc2), respectively. The peak at −0.63 V could be assigned to the irreversible reduction of the nitro group of CAP, where the peaks of Pa1 and Pc1 are related to the reversible redox of hydroxylamine group [1,25]. 41

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Fig. 4. (a) SWVs of different electrodes in 0.1 mol L-1 PBS (pH=7.5) containing 1×10-6 mol L-1 CAP by accumulation of 180 s on open circuit. (b) CVs of EPC/GCE in 0.1 mol L-1 PBS (pH=7.5) by accumulation of 180 s on open circuit, scan from −0.8 to 0.4 V at 100 mV s−1.

free from large amount of tetracycline and p-nitroaniline, otherwise a purification step is needed.

3.4. Determination of CAP with different concentrations With the optimized experimental conditions, the SWVs of different concentrations of CAP from 0 to 6×10−6 mol L−1 were further recorded. As shown in Fig. 5, the peak current of CAP increase upon the addition of CAP. The increasement of peak current vs concentration exihibits two stages that 1×10−8−1×10−6 mol L−1 and 1×10−6−6×10−6 mol L−1. The regression equations were I (μA)=129.1C (μmol L−1)+10.8 (R2=0.991) and I (μA)=26.2C (μmol L−1)+123.5 (R2=0.992) for 1×10−8 −1×10−6 mol L−1 and 1×10−6 −4×10−6 mol L−1, respectively. The detection limit was calculated as 2.9×10−9 mol L−1 (S/N=3). The EPC/GCE exhibits higher sensitivity, lower detection limit and a reasonable linear range for CAP in comparison with these directly electrochemical sensors [25–29], indicating the present EPC is a promising sensor for CAP detection.

3.6. Repeatability, reproducibility and stability of EPC/GCE Five SWVs of 1×10−6 mol L−1 CAP at a single EPC/GCE were successively recorded to evaluate the repeatability. The relative standard deviation (RSD) of the current response on the successive SWVs was 3.6%. The SWVs of 1×10−6 mol L−1 CAP at five electrodes fabricated with the same procedure were measured to investigate the producibility. The RSD of peak current at the five EPC/GCEs was 2.8%. In addition, the SWV response at the EPC/GCE was retained more than 96% of their initial values after one week storage at 4 °C. These results indicate that the EPC/GCE holds a good repeatability, reproducibility and stability.

3.5. Interferences studies 3.7. Real sample analysis

The anti-interferences ability of EPC/GCE was evaluated by using SWV in the presence of some metal ions and organic compounds (Fig. S9). It was found that 100-fold metal ions, such as Al(III), Ca(II), Mg(II), Co(II), Zn(II), Mn(II), Cd(II), Pb(II), Hg(II) and Fe(III) had little interference on CAP detection with a signal change below 7%. At the same time, 100-fold of glucose, fructose, cysteine, glutamine acid, ascorbic acid, 50-fold of uric acid, 20-fold p-nitrophenol, nitrobenzoic acid, and 10-fold metronidazole made a little of change (≤7%) on the peak current of CAP. However, over 2-fold tetracycline and p-nitroaniline had a serious interference on CAP detection. Therefore, the proposed sensor can be applied to analysis of CAP in complex samples

As a kind of food easily polluted by CAP [30,31], honey was used to evaluate the practicality of the proposed EPC/GCE. The honey sample was pretreated according to previous reports [1,25]. Briefly, about 1.0g of honey was added into 9 mL of deionized water and fiercely shaked for 5 min. The mixture was centrifuged at 4000 rpm for 10 min and the supernatant was extracted by 30 mL of ethyl acetate for three times. The organic extraction was evaporated and mixed with 10 mL PBS (pH 7.5) by fiercely shaking, and then filtered by a 0.22 µm membrane filter. The pretreated honey sample was diluted by 10-fold pure PBS (pH 7.5) and used as the detection solution. As shown in Table 1, there is no detectable CAP in diluted honey sample without addition of CAP. After the addition of CAP standard solutions at various concentrations, satisfying recoveries of 96.0–110.0% are achieved, indicating the developed EPC/GCE is a promising candidate for rapid and sensitive detection of CAP in honey. Table 1 Determination of CAP in honey samples by using the standard addition method (Each sample was tested for three times).

Fig. 5. SWV of the EPC/GCE in 0.1 mol L−1 PBS (pH 7.5) containing different concentration of CAP by accumulation of 180 s on open circuit and plot of the peak currents as a function of CAP concentrations.

Added /μmol L−1

Founda /μmol L−1

RSD /%

Recovery /%

0 0.10 0.30 0.50 1.50

0 0.11 ± 0.01 0.30 ± 0.02 0.48 ± 0.01 1.53 ± 0.03

– 9.1 6.7 2.1 2.0

– 110.0 100.0 96.0 102.0

a

42

Mean value ± Standard deviation.

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4. Conclusion In summary, exfoliated porous carbon was facilely obtained by a one-step solvent exfoliation with the use of an IRMOF-8 derived carbon as precursor. The exfoliated porous carbon exhibits a remarkably improved electrochemical activity of chloramphenicol compared with its parental carbon precursor, owing to its increasing surface area and improved dispersibility. The EPC modified GCE demonstrated a high sensitivity (129.1 μA μmol−1 L) and a low detection limit (2.9×10−9 mol L−1) and proved capable in analysis of chloramphenicol residue in honey. The present solvent exfoliation strategy is an efficient method in the preparation of electrode materials with abundant porosity, which may further extend its applications areas in batteries, electrocatalysis and supercapacitors.

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Financial support by the ‘1000 Talent Program’ (The Recruitment Program of Global Experts), the National Natural Science Foundation of China (21473247) and the ‘Scientific Research Foundation for the New Teacher’ of the Harbin Institute of Technology (Shenzhen) are gratefully acknowledged.

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Appendix A. Supporting information

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Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2017.01.078.

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