Article pubs.acs.org/ac
Electrochemical Functionalization of N‑Methyl-2-pyrrolidoneExfoliated Graphene Nanosheets as Highly Sensitive Analytical Platform for Phenols Can Wu, Qin Cheng, and Kangbing Wu* Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *
ABSTRACT: Graphene nanosheets (GS) were easily prepared from graphite via a one-step ultrasonic exfoliation approach using N-methyl-2-pyrrolidone (NMP) as the solvent. Compared with the widely used graphene oxide (GO) obtained by multistep chemical oxidation, the NMP-exfoliated GS exhibited apparently better electrochemical activity toward the oxidation of a series of phenols like hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol. Interestingly, the electrochemical activity of GS toward these phenols can be further enhanced by simply anodizing at 1.8 V for 2 min (denoted as EGS), reflected by the apparently enlarged oxidation peak currents in voltammograms and the obviously reduced charge transfer resistance in electrochemical impedance spectra (EIS). Characterizations by techniques like X-ray photoelectron spectra (XPS), Raman spectra, and atomic force microscopy (AFM) demonstrated that the introduction of new oxygen-containing groups or edge-plane defects and the enhanced surface roughness were responsible for the enhanced activity of EGS. Thereafter, a simple electrochemical method for the highly sensitive detection of phenols was established and the detection limits were 0.012 μM, 0.015 μM, 0.01 μM, and 0.04 μM for hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol, respectively. The facile synthesis of EGS, together with its high electrochemical activity, thus created a novel platform for developing highly sensitive electrochemical sensing systems.
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electrochemical treatments have been employed for the surface functionalization of carbon materials, including glassy carbon,15−18 activated carbon,19 carbon fibers,20 screen-printed carbon paste,21 and single-walled carbon nanotubes.22 Graphene, the thinnest known material comprising a single atom thick sheet of hexagonally arranged carbon atoms, offers great opportunities for application developments in nanotechnology. Until now, numerous technologies have been developed for the preparation of high-quality graphene, including chemical vapor deposition (CVD),23 epitaxial growth (EG),24 chemical oxidation exfoliation,25 and solvent exfoliation.26,27 Among these methods, solvent exfoliation has obtained considerable attention due to its facile and gentle synthesis process. However, no works have reported the electrochemical activation of solvent-exfoliated graphene. Here, graphene nanosheets (GS) were easily obtained through a one-step ultrasonic exfoliation approach from graphite powder using N-methyl-2-pyrrolidone (NMP) as the solvent. The prepared GS was employed for the electrochemical sensing of phenols. Compared with the bare glassy
key strategy in electrochemical sensing is to control the electrode interface structure because the performance of electrochemical sensors is highly relevant to the surface characteristics. A large number of investigations have proved that surface modification and functionalization are highly efficient strategies to improve the performance of electrochemical sensors.1−5 Carbon materials such as fullerene, carbon nanotubes, and carbon nanoparticles have attracted great attention in electrochemical sensing because of their unique physicohemical properties and high electrochemical activity. Meanwhile, many studies have focused on the further activation of these carbon-based sensing materials by methods like chemical oxidation or reduction,6−8 UV radiation,9 γ-ray radiation,10 covalent bonding,11,12 and self-assembling.13,14 Electrochemical activation has been demonstrated to be an effective approach to the activity improvements of various functional materials, owing to several advantages compared with other technologies: first, it is very eco-friendly because the driving forces during the treatment process only involve the migration of electron which can be easily achieved by a power source; second, it is convenient to fulfill in situ and rapid surface treatments; third, the reaction conditions can be precisely and easily controlled via regulating the applied potential, current, and duration time. In recent years, © XXXX American Chemical Society
Received: November 18, 2014 Accepted: February 28, 2015
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Figure 1. SEM (A, C, E) and AFM (B, D, F) images of GCE (A, B), GO/GCE (C, D), and GS/GCE (E, F).
using a 532 nm laser (Horiba JobinYvon, France). X-ray photoelectron spectra (XPS) were carried out on an AXIS ULTRA DLD-600W spectrometer (Kratos Company, England). Preparation of GS and GO. GS was prepared via ultrasonic exfoliation of graphite powder in NMP. In a typical procedure, 0.25 g of graphite powder was added to 50.0 mL of NMP and then sonicated in a KQ-100B ultrasonicator (40 kHz, 100 W) for 48 h. After 2 h quiescence to naturally subside down large-sized graphite particles, the supernatant was transferred to a glass vial for electrode modification. The concentration (c) of GS was determined via spectrophotometry by exploiting the Beer−Lambert equation A = αcl, where A is the absorbance, l [m] is the length of the optical path, and α [L g−1 m−1] is the absorption coefficient. On the basis of the previous reports,26 α was evaluated to be 3620 L−1 g−1 m−1 for graphene exfoliated by NMP. Thus, the concentration of GS was calculated to be 1.50 mg mL−1. GO was prepared by a widely used modified Hummer’s method25 and employed for comparison. In a typical process, the graphite powder was first oxidized by H2SO4, K2S2O8, and P2O5 at 80 °C for 5 h and then oxidized in the mixture of concentrated H2SO4 and KMnO4 in an ice bath for 2 h. After that, the mixture was diluted, filtered, and then washed successively with 10% HCl solution, ultrapure water, and ethanol. Finally, the obtained solid samples were dried at 30 °C for 12 h and dispersed into NMP in a similar manner to a concentration of 1.50 mg mL−1. Electrochemical Functionalization of GS. A GCE (CHI, 3 mm in diameter) was polished with 0.05 μm alumina slurry and then cleaned ultrasonically in ultrapure water and ethanol, respectively. After that, 2.0 μL of the GS or GO suspension was cast onto the surface of the GCE and the solvent was evaporated under an infrared lamp. The resulting GS and GO modified GCE were denoted as GS/GCE and GO/GCE, respectively. For the fabrication of the EGS modified GCE (EGS/GCE), GS/GCE was anodized at 1.8 V for 2 min in pH 5.6 acetate buffer solution.
carbon electrode (GCE) and the graphene oxide modified glassy carbon electrode (GO/GCE), the NMP-exfoliated GS modified GCE (GS/GCE) was found to remarkably increase the oxidation peak currents of several phenols including hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol. Moreover, the NMP-exfoliated GS with 2 min anodization at 1.8 V (denoted as EGS) exhibited much higher electrochemical activity for these phenols. The possible mechanism for this signal enhancement was explored by electrochemical impedance spectra (EIS), X-ray photoelectron spectra (XPS), Raman spectra, and atomic force microscopy (AFM). It was found that the charge transfer rate, oxygen-containing groups, edge-plane defects, and surface roughness were increased after electrochemical functionalization. On the basis of the excellent electrochemical activity of EGS, a highly sensitive electrochemical platform was developed for the rapid detection of the four phenols at nanomolar levels.
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EXPERIMENTAL SECTION Reagents. All chemicals were of analytical grade and used as received. Hydroquinone, catechol, 4-chlorophenol, 4-nitrophenol, graphite, NMP, sulfuric acid (H2SO4), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), and potassium permanganate (KMnO4) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Ultrapure water (18.2 MΩ) was obtained from a Milli-Q water purification system and used throughout. Instruments. Electrochemical measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China). A conventional three-electrode system, consisting of a GC working electrode, a KCl saturated Ag/AgCl reference electrode, and a platinum wire counter electrode, was employed. Scanning electron microscopic (SEM) measurements were conducted with a Quanta 200 microscope (FEI Company, The Netherlands). Atomic force microscopy (AFM) was performed with a SPM 9700 microscope (Shimadzu, Japan). Raman spectra were recorded using a LabRAM HR800 confocal Raman microscopy system B
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RESULTS AND DISCUSSION Surface Morphology of NMP-Exfoliated GS. The surface morphology of GCE, GO/GCE, and GS/GCE was characterized by SEM and AFM. As shown in Figure 1A,B, the surface of unmodified GCE is smooth and virtually featureless. After modification with GO (Figure 1C,D), a rough surface made up of wrinkled GO nanosheets is clearly observed. Much different from GO/GCE, the surface of GS/GCE is coated by a stacked layer of rigid graphene nanosheets with sizes of several micrometers (Figure 1E), suggesting that graphite powder was mainly exfoliated into graphene nanoflakes instead of singlelayer graphene. AFM images reveal that GS/GCE has a much rougher surface compared with either GCE or GO/GCE (Figure 1F). Therefore, the GS/GCE displays higher surface roughness, which may lead to larger effective sensing area and provide more active sites. Electrochemical Responses of Phenols at GS/GCE. The electrochemical activities of GCE, GO/GCE, and GS/GCE were compared by using the oxidation signals of four kinds of phenols, i.e., hydroquinone, catechol, 4-chlorophenol, and 4nitrophenol (Figure 2). No oxidation peaks are observed on the
electrochemical responses of phenols on GCE, which together with the scalable synthesis of GS suggests its promising applications in electrochemical sensing. Electrochemical Responses of Phenols at EGS/GCE. Electrochemical treatments have been proven to be a simple and effective strategy for improving the electrochemical performance of carbon-based electrodes. Here, GS/GCE was anodized at 1.8 V for 2 min in a pH 5.6 acetate buffer solution. The resulting electrochemically activated GS/GCE (denoted as EGS/GCE) is found to further apparently improve the responses of hydroquinone, catechol, 4-chlorophenol, and 4nitrophenol (Figure 3). Also, the background currents at
Figure 3. DPV curves of 5.0 μM hydroquinone (A), catechol (B), 4chlorophenol (C), and 4-nitrophenol (D) on GS/GCE (curve a) and EGS/GCE (curve b), accumulation time: 3 min. Insets: Nyquist plots in 0.1 M, pH 5.6 HAc-NaAc solutions containing 50.0 mM hydroquinone (A), catechol (B), 4-chlorophenol (C), and 4nitrophenol (D) on GS/GCE (curve d) and EGS/GCE (curve c). Frequency range, 1.5 MHz to 1 Hz; amplitude, 5 mV.
extremely positive potentials are obviously repressed, leading to significantly improved signal-to-noise ratios. Electrochemical impedance spectroscopy (EIS) was employed to explore the origin of the enhanced signals of the phenols on EGS/GCE. For all the phenols, a small partially resolved semicircle attributed to the electrolyte resistance appears in high frequency ranges, suggesting the negligible electrolyte resistance.30,31 Meanwhile, in low frequency ranges, a large semicircle is observed, of which the diameter corresponds to the charge transfer resistance (Rct) of active species on the electrode surfaces. The values of Rct on GS/GCE fitted from the semicircles are 7.7, 17.6, 46.2, and 53.8 kΩ for hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol, respectively. The corresponding values of Rct on EGS/GCE decrease to 1.7, 4.0, 23.1, and 38.7 kΩ, respectively. The remarkably reduced Rct values again demonstrate that the electrochemically activated GS can facilitate the electron transfer of phenols. As a result, the oxidation peak potentials of the phenols negatively shift and the oxidation peak currents are apparently enlarged on EGS/GCE. Therefore, electrochemical polarization provides a simple but effective approach
Figure 2. DPV curves of 5.0 μM hydroquinone (A), catechol (B), 4chlorophenol (C), and 4-nitrophenol (D) on GCE (a), GO/GCE (b), and GS/GCE (c). Accumulation time, 3 min; pulse amplitude, 50 mV; pulse width, 40 ms; scan rate, 40 mV s−1.
differential pulse voltammetric (DPV) curves at GCE (curve a) and only very weak oxidation peaks appear at GO/GCE (curve b), suggesting the poor electrochemical activities of these phenols at both electrodes. Here the improved responses at GO/GCE is probably attributed to the large surface roughness and high density of edge plane-like sites and defects.28,29 The oxidation peaks currents of the phenols are apparently enhanced at GS/GCE, especially for hydroquinone, catechol, and 4-chlorophenol (curve c). Also, the oxidation peak potentials apparently shift to negative directions, demonstrating that GS prepared by NMP exfoliation exhibits much higher electrochemical activity as compared with GO. Therefore, the high effective surface area can significantly promote the C
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Figure 4. (A) XPS survey spectra of GS and EGS, (B) XPS C1s spectra of pristine GS, and (C) XPS C1s spectra of EGS.
to the further performance improvement of graphene-based functional materials for electrochemical sensing applications. In order to understand the enhanced electrochemical activities of EGS, XPS was used to characterize the change of composition and structure after anodization (Figure 4). The results indicate that the content of oxygen-related groups obviously increase after electrochemical oxidation, i.e., compared with the O/C ratio of 0.029 for pristine GS, the value of the O/C ratio rises up to 0.079 for EGS (Figure 4A). Moreover, the XPS C1s spectra indicate that the carbonyl group (−CO) and carboxyl group (−COOH) are introduced onto the surface of EGS (Figure 4B,C), which is the main source for the intensity enlargement of the O1s peak. Therefore, we speculate that the newly created carbonyl group and carboxyl group not only facilitate electron transfer kinetics by introducing electrochemically active sites but also enhance the adsorption or accumulation of phenols on EGS by possible interactions like hydrogen bonding. To elucidate the change of structure during electrochemical activation, Raman spectra of graphite, GS, and EGS were collected (Figure 5). Three disorder-related bands, including
further increases to 1.05 for EGS, suggesting that anodization treatments effectively deconstruct the partial sp2 bond lattice to form sp3 bond by introducing oxygen-related defects.34 Therefore, the increased ID/IG value for EGS reveals that the surface electronic structure is altered and more oxygen-related defects are formed on edge planes, which is well coincident with the XPS analysis. Therefore, electrochemical anodization can efficiently enhance the defect level on the edge planes of GS and consequently provide more active sites and better electrochemical activity. The structure change of EGS compared with GS was also supported by AFM characterizations. As seen from Figure 6,
Figure 5. Raman spectra of graphite powder, pristine GS, and EGS.
Figure 6. AFM images of pristine GS (A) and EGS (B).
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the D band at 1350 cm , the G band at 1580 cm , and the 2D band at 2700 cm−1, are clearly observed for these graphene materials.32,33 On the Raman spectra of the pristine graphite powder, the intensity of the D band is very weak, and the D-toG band intensity ratio (ID/IG) is just 0.29, indicative of a rather low edge plane defect level. In the case of GS, the intensity of the D band increases greatly and ID/IG rises up to 0.81, suggesting that many defect sites are introduced on graphene sheets during the liquid exfoliation process. The value of ID/IG
more corrugation structures are formed, and the surface becomes much rougher after the electrochemical anodization of GS. Apparently, the increased surface roughness enhances the effective sensing areas and leads to stronger signal enhancement effects. Highly Sensitive Electrochemical Sensing of Phenols at EGS/GCE. On the basis of the above discussions, it is clear the NMP-exfoliated GS exhibits superior electrochemical D
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chlorophenol, and 4-nitrophenol. Moreover, the performance of GS can be further improved with 2 min oxidation at 1.8 V in pH 5.6 acetate buffer. Results from EIS, AFM, XPS, and Raman spectroscopy indicated that the enhanced electrochemical performance of EGS relied on the creation of oxygen-related active defects and the increase of effective surface areas by anodization. On the basis of this, a novel electrochemical sensing platform with high sensitivity for the detection of different phenols was developed, which exhibited apparently higher sensitivity for the four phenols compared with previous works.
activity after electrochemical anodization due to the increase of active defects sites and surface roughness. Thus, EGS is used here as an excellent sensing material for constructing a highly sensitive analytical platform of phenols. Figure 7 shows the
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ASSOCIATED CONTENT
S Supporting Information *
Magnifying DPV curves, analytical properties of studied phenols, and performance comparisons of electrochemical sensors. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86-27-87543632. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 7. DPV curves of hydroquinone (A), catechol (B), 4chlorophenol (C), and 4-nitrophenol (D) with different concentrations on EGS/GCE in 0.1 M pH 5.6 acetate buffer solution. The insets show the corresponding calibration plots. Accumulation time, 3 min. Error bars represent the standard deviations of three measurements.
ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (973 Program, Grant No. 2015CB352100), the National Natural Science Foundation of China (Grant No. 21375041), and the Program for New Century Excellent Talents in University (Grant NCET-11-0187). The Center of Analysis and Testing of Huazhong University of Science and Technology was also acknowledged for its help in the SEM, AFM, XPS, and Raman observation.
DPV responses of hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol with different concentrations on EGS/GCE. Clearly, the oxidation peak currents (Ip) increased linearly with the concentration of the phenols. Moreover, the magnifying DPV curves of trace level of hydroquinone (A), catechol (B), 4chlorophenol (C), and 4-nitrophenol (D) on EGS/GCE are shown in Figure S-1 in the Supporting Information, and oxidation waves are clearly observed. In addition, the detection limits (DL) are calculated as recommended by the International Union of Pure and Applied Chemistry (IUPAC)35−37 using equation DL = kSb/m, where k is a numerical coefficient, Sb is the standard error of parallel blank measurements and m is the slope of the linear regression equation. In accord with IUPAC recommendations, a k value of 3 is applied, which corresponds to a 99.87% confidence level. Here, detection limits of 0.012 μM, 0.015 μM, 0.01 μM, and 0.04 μM are calculated for hydroquinone, catechol, 4-chlorophenol, and 4-nitrophenol, respectively. The corresponding peak potentials, linear regression equations, linear ranges, and detection limits are summarized in Table S-1 in the Supporting Information. Obviously, the detection limits of the four phenols are all at nanomolar levels, which are apparently lower than most of the previous works38−47 (Table S-2, Supporting Information) and demonstrate the overwhelming advantages of EGS for electrochemical sensing of phenols.
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CONCLUSIONS Compared with the widely used GO that obtained by multistep chemical oxidation, the NMP-exfoliated GS prepared via onestep ultrasonication exhibited higher activity toward the electrochemical oxidation of hydroquinone, catechol, 4E
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