Biosensors and Bioelectronics 53 (2014) 250–256
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Fabrication of 2D ordered mesoporous carbon nitride and its use as electrochemical sensing platform for H2O2, nitrobenzene, and NADH detection Yufan Zhang, Xiangjie Bo, Anaclet Nsabimana, Charles Luhana, Guang Wang, Huan Wang, Mian Li, Liping Guo n Faculty of Chemistry, Northeast Normal University, 130024 Changchun, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2013 Received in revised form 19 September 2013 Accepted 2 October 2013 Available online 9 October 2013
Two-dimensional ordered mesoporous carbon nitride (OMCN) has been successfully prepared for the first time using SBA-15 mesoporous silica and melamine as template and precursor respectively, by a nano hard-templating approach. A series of OMCN-x samples with different pyrolysis temperatures have been reported. The formation of these composite materials was verified by detailed characterization (e.g., Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, N2 adsorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy). The results showed that the materials were structurally well ordered with two-dimensional porous structure, high surface area and large pore volume. The influence of BET surface area and different amounts of N-bonding configurations formed at different pyrolysis temperatures of OMCN-x for the electrocatalysis towards hydrogen peroxide, nitrobenzene, and nicotinamide adenine dinucleotide were investigated in detail. Results indicated that OMCN treated at 800 1C with largest BET surface area and highest amounts of pyrindinic N showed improved electrocatalytic activity for H2O2, nitrobenzene, and NADH in neutral solution. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon nitride Pyridinic N Electrocatalysis H2O2 Nitrobenzene NADH
1. Introduction Seeking highly active electrochemical catalysts has been an intensifying endeavor worldwide, mainly due to their promising applications for electrocatalytic and electrochemical analysis. Carbon material, as one of the most famous electrochemical catalysts, has been still used widely, due to the excellent electronic conductivity and good stability (Huang et al., 2013a; Liu et al., 2013; Parlak et al., 2013; Walcarius, 2012; Xiong et al., 2010; Zhou et al., 2007). However, single-phased carbon material is limited in performance gradually because of their intrinsic weaker material properties in electrocatalytic ability. Despite the success of precious metals and metal oxides incorporated into carbon catalysts for electrochemical applications, the prohibitively high cost and limited supply of noble metals have been raised as major issues to be addressed (Antolini et al., 2005; Boxall and Lukehart 2001; Dong et al., 2012; Giovanni et al., 2012; Li et al., 2012; Rajesh et al., 2000; Suresh et al., 2012; Vig et al., 2009; Yu et al., 2013; Zhang et al., 2011; Zhang et al., 2013). In recent years, incorporation of heteroatoms (e.g., N, B, P, and S) into the carbon supports so as to modify their surface and physicochemical properties have been
n
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investigated (Choi et al., 2011; Jin et al., 2013; Poh et al., 2013; Wang et al., 2011; Xiao et al., 2011; Yang et al., 2012; Yang et al., 2011a). Among them, N-containing carbons have received considerable attentions due to the strong electron donor nature of N which should promote enhancement in π bonding, leading to improved stability, electron transfer rate, and hence durability of the carbon supports during electrocatalytic processes (Gong et al., 2009; Sheng et al., 2012; Silva et al., 2012; Wong et al., 2013; Yang et al., 2013; Yu et al., 2010). Therefore, a variety of N-containing carbon nanostructures including carbon nanotubes, graphene, and porous carbon has been widely reported. The nitrogen groups have been demonstrated to play a critical role in catalytic activity. According to previous reports, nitrogen atoms can be substitutionally incorporated into the basal plane of carbon in the form of “pyridinic”, “pyrolic”, and “graphitic” nitrogen bonding configurations. It is believed that nitrogen environment affects the electronic structure of doped carbon materials and results in different mechanisms of activity enhancement. In general, N-doping can be achieved by either “post-doping”, namely post-synthesis treatment of carbons by N-containing chemicals or in situ doping, i.e., direct synthesis of nanostructured carbons involving N-containing precursors. In this regard, carbon nitride, a carbonaceous material that is enriched with very high nitrogen content and that can be readily obtained through the pyrolysis of cyanamide (Kwon et al., 2012; Wang et al., 2009), melamine (Thomas et al., 2008; Vinu et al., 2007),
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or ethylenediamine/carbon tetrachloride (Vinu et al., 2005), may serve as a potential precious metal-free catalyst for electrochemical applications. Insights from the published report confirm that edge plane sites or defects of carbon-based catalysts are the predominant origin of electrochemical activity and the basal plane is electro-catalytically inert. It has been reported that carbon nitride with high edge plane sites or defects possesses a high density of electronic states, which is a key factor in determining the heterogeneous electron transfer rate of electrode. This happens because the doping of N heteroatoms can disrupt the basal surface and the generation of edge plane like-sites or defects. Carbon nitride was mainly produced by heating the carboncontaining and nitrogen-containing precursors. Various efforts have been concentrated on the synthesis of carbon nitride during the past few years. For instance, Chang et al. (2013) developed a modification of carbon nitride through co-pyrolysis of melamine and sodium nitrateor or potassium nitrate. A carbon nitride powder was obtained through directly heating dicyandiamine at a given temperature (Bu et al., 2013). The synthesis of the Uddin and Yang (2009) reported well-crystallized carbon nitride nanostructures with clear hexagonal morphology and size range of 50–500 nm on stainless steel substrates from triethylenetetramine using a sol–gel method. In another work, Lu et al., (2007) demonstrated successful fabrication of carbon nitride nanowires and pseudocubic carbon nitride polycrystalline nanoparticles by the reaction between C3N3Cl3 and NaN3 with Zn powder as catalyst. Unfortunately, the bulk carbon nitride compound gives lower surface area exposure. Therefore, lower current density is observed with such metal-free carbon nitride compounds used as electrocatalyst. Among various carbon nitride materials, ordered mesoporous carbon nitride (OMCN) is a well known and fascinating material that has attracted worldwide attention because the incorporation of nitrogen atoms in the carbon nanostructure enhances the mechanical, conducting, field-emission, and energy-storage properties (Huang et al., 2013b; Vinu, 2008). Herein we report the preparation of the OMCN, which was synthesized by using melamine as carbon and nitrogen sources and mesoporous silica SBA-15 as template. The uniform pore size and highly ordered arrangement of OMCN homogeneously covered on glassy carbon electrode (GCE) could be very attractive for various electrochemical applications. Hydrogen peroxide (H2O2), a well-known oxidizing agent, is not only a by-product of a large number of oxidase enzymes, but also an essential intermediate in biomedical, food production, pharmaceutical, industrial and environmental fields (Liu et al., 2013; Palanisamy et al., 2012). Nicotinamide adenine dinucleotide (NADH) is an important coenzyme involved in metabolic processes, widely existing in every cell of living organisms. It alternates between the reduced state NADH and the oxidized state NAD þ for the production of ATP, and serves as a hydrogen and electron carrier in cellular respiration and photosynthesis. The investigations of the redox reactivity of NADH and NAD þ are important, because a large number of dehydrogenase enzymes (4300) use these compounds as cofactors (Baskar et al., 2012; Teymourian et al., 2012). Nitrobenzene (NB) is an important raw material and solvent in the manufacturing of aniline, dye, pesticide, explosives, and pharmaceuticals and also as a solvent in products like paints, shoes and floor metal polishes. A large amount of NB has been released into the environment annually due to excessive use and improper handling of the wastewater (Chen et al., 2011; Zhang et al., 2012). Therefore, the rapid and accurate detection of H2O2, NADH, and NB is of great significance. The electrochemical results showed that the OMCN exhibited significant electrocatalytic activity towards H2O2, NB, and NADH in neutral solution, which proved that the as-synthesized OMCN could be used as environmental and electrochemical sensors and biosensors.
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2. Experimental 2.1. Chemical reagents H2O2, NB, NADH, and N,N′-dimethylformamide (DMF) (HPLC grade) were used as purchased from Beijing Dingguo Biotechnology Co. Ltd. Pluronic P123, melamine, and tetraethoxysilane were obtained from Sigma-Aldrich. The 0.1 M phosphate buffer solution (PBS pH 7.0), which was made up from NaH2PO4, Na2HPO4, and H3PO4, was employed as a supporting electrolyte. All other reagents were of analytical grade, and all solutions were prepared with double distilled water.
2.2. Instrumentation All the electrochemical experiments were performed with a CHI 830B electrochemical workstation (CH Instruments, Shanghai Chenhua Instrument Corporation, China). A conventional three electrode cell was used, the used working electrode was GCE or the modified electrode, a platinum electrode was applied as the counter electrode and an Ag/AgCl (in saturated KCl solution) electrode served as a reference electrode. All potentials in this paper were measured and reported versus Ag/AgCl. In this study, all the sample solutions were purged with purified nitrogen for 20 min to remove oxygen prior to the beginning of a series of experiments and all experiments were carried out at laboratory temperature. X-Ray diffraction (XRD) patterns were obtained on an X-ray D/max-2200vpc (Rigaku Corporation, Japan) instrument operated at 40 kV and 20 mA using CuKα radiation (k¼0.15406 nm). Fourier transform infrared (FT-IR) spectroscopy of the sample was recorded with Nicolet Magna 560 FT-IR spectrometer with a KBr plate. N2 adsorption-desorption isotherms were measured on ASAP 2020 Micromeritics (USA) at 77 K. The Brunauer–Emmett– Teller (BET) method was utilized to calculate the specific surface area. Scanning electron microscopy (SEM) images were determined with a Philips XL-30 ESEM operating at 3.0 kV. X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCA LAB spectrometer (USA). Transmission electron microscopy (TEM) images were obtained using a JEM-2100F transmission electron microscope JEOL (Japan) operating at 200 kV.
2.3. Preparation of the modified electrodes Prior to the modification, GCE (model CHI104, 3 mm diameter) was polished before each experiment with 1, 0.3 and 0.05 mm alumina power, rinsed thoroughly with doubly distilled water between each polishing step, and then sonicated successively in 1:1 nitric acid, absolute alcohol, and double distilled water. The cleaned electrode was dried with a high-purify nitrogen stream for the next modification. To prepare the modified electrodes, 5 mg of the as-prepared samples were dispersed into 1 mL DMF to give homogeneous suspension upon bath sonication. The voltammetric response is closely related to the thickness of OMCN catalyst film. The relationship between the amount of OMCN-800 suspension and the K3[Fe(CN)6] peak current was examined, as shown in Fig. S1. The peak current gradually increases as improving the volume of OMCN-800 suspension from 0.0 to 5.0 mL. Further increasing the volume of OMCN-800 suspension to 7.0 mL, the peak current almost kept stable. However, the peak current is diminished when the volume of OMCN suspension is above 8.0 mL, which is probably attributed to the slowdown of mass transport and charge transfer rate due to the over thickness of the OMCN film. Therefore, the optimized amount of OMCN is chosen as 5.0 mL.
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2.4. Preparation of OMCN-x samples The mesoporous SBA-15 was prepared using Pluronic P123 as a surfactant and tetraethoxysilane as a silica source (Jun et al., 2000; Ryoo et al., 1999). OMCN-x materials were prepared using SBA-15 mesoporous silica and melamine as a template and a precursor, respectively. The syntheses were performed by varying the pyrolysis temperature. The OMCN-x materials are referred to as OMCN-500, OMCN-650, OMCN-800, and OMCN-950, where 500, 650, 800, and 950 1C represent the different temperatures of pyrolysis in the synthesis procedure, respectively. Briefly, in an optimized procedure for OMCN-x, 1 g of SBA-15 was added to a solution obtained by dissolving 2.5 g of melamine in 10 g of H2O by vigorous stirring at room temperature for 1 h. The mixture was dried in a 60 1C drying oven. The pyrolysis was completed by heating the product to typically 500, 650, 800, and 950 1C under a flow of N2. The SBA-15 silica template was then etched away by overnight dissolution in 10% aqueous HF to leave behind the OMCN-x. Illustration of the preparation of OMCN-x is presented in Scheme 1.
3. Results and discussion 3.1. Characterization of OMCN-x Fig. 1 shows SEM and TEM images of the OMCN-800 replica. In Fig. 1A, the SEM image of the OMCN-800 replica indicates that the wormlike morphology of SBA-15 was maintained after the replication. TEM was used to examine the structural order and morphology of the OMCN-800. The cross-sectional TEM image of the OMCN-800 viewed along the channels shows the hexagonal arrays of uniform mesopores (Fig. 1B), which were typically observed in images of SBA-15 silica. Bright contrast strips on the planar TEM image represent the pore-wall images, whereas dark contrast cores display empty channels (Fig. 1C). The pore-structure ordering of the OMCN-x along with the parent SBA-15 silica was further investigated by powder XRD measurements. Fig. S2 shows the small-angle XRD patterns for SBA-15 template (a) and OMCN-x (b–e). The small-angle XRD patterns of OMCN-x samples exhibit (100), (110), and (200) diffraction peaks that can be indexed on a two-dimensional hexagonal lattice (pбmm), which indicates a well-ordered hexagonal mesostructure of the SBA-15 template preserved after the replication. The FT-IR spectra of the OMCN-x samples present several bands (Fig. S3). The band around at 3430 cm 1 is assignable to Fig. 1. (A): SEM image of OMCN-800. (B) and (C): TEM images of OMCN-800.
Scheme 1. Illustration of the preparation of OMCN-x samples.
carboxylic acid O–H stretch vibration, while the O–H vermicular vibration is responsible for the band around 1185 cm 1. The band at 1640 cm 1 can be assigned to the vibrations of benzene rings from the carbon matrix as well as the CQN bonds. The peaks located at 1565 and 1495 cm 1 are assigned to the stretching vibration of CQN in –NQquinoidQN– and CQC in benzenoid ring, further suggesting the formation of carbon nitride (Guo et al., 2011). The band centered around 1245 cm 1 is attributed to aromatic C–N stretching bond. The band at 650 cm 1 corresponds to the out of plane N–H deformation vibrations (Liu et al., 2011; Wu et al., 2012). Fig. S4 shows the N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of OMCN-x samples. Typical type-IV isotherms with a clear hysteresis loop at P/P0 ¼0.5– 0.8, indicating mesoporous character (Datta et al., 2011; Liu et al., 2011). The textural properties of the final carbon nitride materials calcined at different temperatures are shown in Table S1. The textural properties of OMCN-x samples reveal that the obtained
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materials contain significant mesoporous structures. Notably, all the samples represent a pore size distribution in the center of 5.24–5.26 nm, suggesting that the ordered mesostructure is wellpreserved after pyrolysis at the four treatment temperatures. This is important for the fast electrolytes transfer because the mesopores and interconnections provide a more favorable path for penetration and transportation of ions. From Table S1, the BET surface area and total pore volume of the OMCN-x were increased with increasing pyrolysis temperature between 500 and 800 1C, and decreased along with the pyrolysis temperature up to 950 1C. All the above characterizations confirm that OMCN-x materials with ordered hexagonal mesostructure has been synthesized by simply template method. X-ray photoelectron spectroscopy (XPS) measurements were performed to probe the amounts of N-bonding configurations in the OMCN-x samples. The high resolution N1s XPS spectra can be deconvoluted into three different signals with binding energies of 398.0, 400.0, and 401.3 eV that correspond to pyridinic N, pyrrolic N, and graphitic N, respectively (Wang et al., 2012; Yang et al., 2011b) (Fig. 2). Remarkably, the shape of these three peaks significantly changes when the pyrolysis temperature is increased, hence implying that different amounts of N-bonding configurations are formed at different pyrolysis temperatures. In the case of OMCN-500, pyrrolic N is dominant, thus accounting for 50% of the overall N content. When the pyrolysis temperature is increased to 650 and 800 1C, the amount of pyridinic N and graphitic N remains constant, whereas that of pyrrolic N largely decrease; and with the increase of pyrolysis temperature from 800 to 950 1C, both the content of pyridinic N and pyrrolic N are largely decreased, hence implying that these species are less stable at high temperatures. Thus, such different amounts of N-bonding configurations in OMCN-x samples must exert a large influence on their electrocatalytic performances. According to recent reports (Liu et al., 2010; Qu et al., 2010; Yang et al., 2011b; Yu et al., 2010), it has been suggested that the content of nitrogen, especially the pyrindinic N portion, is crucial for the promotion of the electrocatalytic reaction. Given that pyrindinic N atoms with strong electron-accepting ability can create a net positive charge on the adjacent carbon atoms in the OMCN, they are favorable for the adsorption of small molecules (e.g., H2O2, NADH, and NB) and can readily attract electrons from the anode, thus facilitating the electrochemical reaction.
3.2. Electrocatalysis of H2O2, NB, and NADH and their detection In Fig. 3A, the CVs for H2O2 reduction at different electrodes were compared. It shows a weak electrocatalytic reduction current toward H2O2 at bare GCE (curve a). For OMCN-x/GCE (curve b–e), the reduction results exhibit obvious marked decrease in overpotential with increase in peak current for H2O2 reduction compared with GCE. In curve d, the catalytic activity of OMCN-800/ GCE is evident from a decrease in overpotential as well as response current increase for H2O2 reduction compared with other OMCNx/GCE. Obviously, the presence of OMCN-800 made the electron transfer much easier compared with that of other OMCN-x samples. Thus, we focused on the investigation of the electrochemical properties of OMCN-800. Fig. 3B displays the current-time responses of OMCN-800/GCE for H2O2 detection at pH¼ 7.0 with the applied potential of 0.19 V. Insets a and b of Fig. 3B show the amperometric response of low concentration of H2O2 at OMCN-800/GCE. The current response of OMCN-800/GCE generally reached a steady-state level within 1.5 s after the H2O2 addition (inset c of Fig. 3B). The corresponding calibration plot for the reduction of H2O2 at OMCN-800/GCE was shown in Fig. 3C. Error bars are the standard deviation of five repetitive experiments (RSD¼4.6%). The H2O2 sensor displays a linear range of 4 and 40 μM (R2 ¼ 0.999, n¼10) with a sensitivity of
Fig. 2. High-resolution N1s XPS spectra of (A): OMCN-500, (B): OMCN-650, (C): OMCN800, and (D): OMCN-950. (E): The content of three nitrogen species in OMCN-x samples.
45.41 mA mM 1 and 40 to 12400 μM (R2 ¼0.999, n¼59) with a sensitivity of 20.34 mA mM 1. The detection limit is calculated as
1.52 μM with the signal to noise ratio of three (S/N ¼3). The reproducibility of the sensor was also investigated by current– time method for five repetitive measurements with additions of H2O2 concentration of 2.0 mM at 0.19 V (pH¼ 7.0). The RSD of the sensitivity was less than 3.0%. When the OMCN-800/GCE was stored at 4 1C for two weeks, the current response to 2.0 mM H2O2 remained 92.6% of its original value, suggesting the long-term stability of the modified electrode. The performance of the OMCN800/GCE was also compared with other H2O2 sensors (Table S2).
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Fig. 3. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 2.0 mM H2O2 (pH ¼7.0). Scan rate: 50 mV s 1. (B): Typical amperometric current–time curve of OMCN-800/GCE with successive additions of H2O2 (pH ¼7.0). Insets a and b: The amperometric response with successive addition of H2O2 at lower concentration. Inset c: The current response time after the H2O2 addition at OMCN-800/GCE. (C): The linear dependence of between H2O2 concentration and current signal for OMCN-800/GCE. Inset: the amperometric response with successive addition of H2O2 at lower concentration.
Fig. 4A displays the CVs of different electrodes in the presence of NB. There is almost no electrochemical response at bare GCE (curve a). However, the electrochemical reduction results exhibit obvious marked decrease in overpotential with increase in peak current for NB reduction at OMCN-x/GCE compared with bare GCE (curve b–e). Additionally, the reduction current of NB at the OMCN-800/GCE exhibits an increased current signal, which is 2.9-, 1.8-, and 1.5-fold higher than that of the OMCN-500/GCE, OMCN-650/GCE, and OMCN-950/GCE, respectively. Thus, OMCN800/GCE was selected as an amperometric sensor for NB. The differential pulse voltammetry (DPV) was employed to detect NB at OMCN-800/GCE in this study (Fig. 4B). Clearly, a series of the DPV curves (a-l) were obtained from different concentrations of NB. The calibration curve of reduction current is depicted in Fig. 4C, which exhibits steady amperometric response towards NB in the linear concentration range of 0.5–1000 mM with a correlation coefficient R2 ¼0.997; the data are the average for parallel determinations (n ¼5) at the each content value of NB. The detection limit is 0.18 mM and the sensitivity is calculated as 674.87 mA mM 1. The reproducibility of the sensor was also investigated by DPV method. The RSD of current signal for 100 mM NB was
Fig. 4. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 100 μM NB (pH¼ 7.0). Scan rate: 50 mV s 1. (B): DPV curves of NB (a-l): 0.5, 5, 20, 30, 50, 75, 100, 200, 300, 400, 700, and 1000 μM at the potential of 0.7 V in PBS (0.1 M, pH 7.0) at the OMCN-800/GCE. (C): The linear dependence of the current response with the different concentration of NB.
less than 3.2% for five measurements for the same electrode. After being stored at 4 1C for two weeks, 9.3% current loss at OMCN800/GCE was obtained by the amperometric response of 100 mM NB. The detailed comparison of NB detection performance using different NB sensors is summarized in Table S3. The electrocatalytic properties of the different electrodes toward NADH detection were also investigated (Fig. 5). Fig. 5A displays the CVs behaviors of the bare GCE (curve a) and OMCN-x/ GCE (curve b–e) in 0.1 M PBS (pH¼ 7.0) solution in the presence of 1.0 mM NADH over a potential range of 0.2–0.6 V at a scan rate of 50 mV s 1. Clearly, there is almost no electrochemical response at bare GCE. However, there is a significantly enhanced electrochemical response towards NADH oxidation after modification by the OMCN-x. Additionally, the OMCN-800 sample performs the best electrocatalytic activity towards NADH oxidation among the synthesized electrode materials. Thus, we focused on the investigation of the NADH at the OMCN-800/GCE. Fig. 5B shows a typical amperometric current–time curve of OMCN-800/GCE with successive additions of NADH (pH 7.0). The best potential to be applied was chosen at þ 0.2 V based on the CVs measurements. Inset a of
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Therefore, the OMCN-800 sample with high edge plane-like defective sites and electroactive sites could offer a favorite microenvironment for transferring species in solution through the pores of OMCN material, and also would be beneficial for accelerating electron transfer between the electrode and species in solution.
4. Conclusions 2D OMCN materials with ordered and uniform mesoporous as well as large BET surface areas have been successfully prepared for the first time using mesoporous silica SBA-15 as template and melamine as carbon and nitrogen sources by a nano hardtemplating approach. The OMCN-x samples have been synthesized by different pyrolysis temperatures. These materials were verified by detailed characterization analyses and electrochemical investigation. From the results of XPS measurements and nitrogen adsorption, different pyrolysis temperatures have an influence on the BET surface areas and N-bonding configurations, which may be the two key factors in determining the electrochemical activity of OMCN-x. The OMCN-800 with largest BET surface areas and suitable N-bonding configurations made an increase electrocatalytic activity towards H2O2, NB, and NADH in neutral solution. A sensitive electrochemical sensor for H2O2, NB, and NADH was developed based on the OMCN-800/GCE, which showed wide linear range, low detection limit, high sensitivity, and good stability. As a result, successful fabrication of OMCN-800 not only holds great promise for the design of electrochemical sensors, but also promotes the development of new electrode materials.
Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 21075014), the State Key Laboratory of Electroanalytical Chemistry, CIAC, CAS (No. SKLEAC201201) and the Fundamental Research Funds for the Central Universities (No. 12SSXT145).
Fig. 5. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 1 mM NADH (pH ¼7.0). Scan rate: 50 mV s 1. (B): Typical amperometric current–time curve of OMCN-800/GCE with successive additions of NADH (pH ¼ 7.0). Inset a: The amperometric response with successive addition of NADH at lower concentration. Inset b: The current response time after the NADH addition at OMCN-800/GCE. (C): The linear dependence of between NADH concentration and current signal for OMCN-800/GCE.
Fig. 5B shows the amperometric response of low concentration of NADH at OMCN-800/GCE. The OMCN-800/GCE responds very rapidly to the changes in the level of NADH, producing steadystate signals less than 2 s (inset b of Fig. 5B). The relationship between NADH concentration and current signal for OMCN-800/ GCE is illustrated in Fig. 5C. The current increased linearly with the good linear ranges from 2 to 2200 μM (R2 ¼0.999, n ¼27) for NADH detection with a sensitivity of 13.17 mA mM 1 and a detec-
tion limit of 0.82 μM (S/N ¼3). The performance of the OMCN-800/ GCE was also compared with other NADH sensors (Table S4). So, from these results of electrochemical experiments, the OMCN-800 material performs the best electrocatalytic activity among the synthesized samples so far in this study. This may be caused by the largest BET surface areas and suitable N-bonding configurations in the OMCN-800 sample, which makes the OMCN800 with highest edge plane sites or defects and possesses a highest density of electronic states among these samples. It is a key factor in determining the heterogeneous electron transfer rate of electrode.
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Fabrication of 2D ordered mesoporous carbon nitride and its use as electrochemical sensing platform for H2O2, nitrobenzene, and NADH detection Yufan Zhang, Xiangjie Bo, Anaclet Nsabimana, Charles Luhana, Guang Wang, Huan Wang, Mian Li, Liping Guo n Faculty of Chemistry, Northeast Normal University, 130024 Changchun, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2013 Received in revised form 19 September 2013 Accepted 2 October 2013 Available online 9 October 2013
Two-dimensional ordered mesoporous carbon nitride (OMCN) has been successfully prepared for the first time using SBA-15 mesoporous silica and melamine as template and precursor respectively, by a nano hard-templating approach. A series of OMCN-x samples with different pyrolysis temperatures have been reported. The formation of these composite materials was verified by detailed characterization (e.g., Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, N2 adsorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy). The results showed that the materials were structurally well ordered with two-dimensional porous structure, high surface area and large pore volume. The influence of BET surface area and different amounts of N-bonding configurations formed at different pyrolysis temperatures of OMCN-x for the electrocatalysis towards hydrogen peroxide, nitrobenzene, and nicotinamide adenine dinucleotide were investigated in detail. Results indicated that OMCN treated at 800 1C with largest BET surface area and highest amounts of pyrindinic N showed improved electrocatalytic activity for H2O2, nitrobenzene, and NADH in neutral solution. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon nitride Pyridinic N Electrocatalysis H2O2 Nitrobenzene NADH
1. Introduction Seeking highly active electrochemical catalysts has been an intensifying endeavor worldwide, mainly due to their promising applications for electrocatalytic and electrochemical analysis. Carbon material, as one of the most famous electrochemical catalysts, has been still used widely, due to the excellent electronic conductivity and good stability (Huang et al., 2013a; Liu et al., 2013; Parlak et al., 2013; Walcarius, 2012; Xiong et al., 2010; Zhou et al., 2007). However, single-phased carbon material is limited in performance gradually because of their intrinsic weaker material properties in electrocatalytic ability. Despite the success of precious metals and metal oxides incorporated into carbon catalysts for electrochemical applications, the prohibitively high cost and limited supply of noble metals have been raised as major issues to be addressed (Antolini et al., 2005; Boxall and Lukehart 2001; Dong et al., 2012; Giovanni et al., 2012; Li et al., 2012; Rajesh et al., 2000; Suresh et al., 2012; Vig et al., 2009; Yu et al., 2013; Zhang et al., 2011; Zhang et al., 2013). In recent years, incorporation of heteroatoms (e.g., N, B, P, and S) into the carbon supports so as to modify their surface and physicochemical properties have been
n
Corresponding author. Tel./fax: þ 86 431 85099762. E-mail address: [email protected] (L. Guo).
0956-5663/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bios.2013.10.001
investigated (Choi et al., 2011; Jin et al., 2013; Poh et al., 2013; Wang et al., 2011; Xiao et al., 2011; Yang et al., 2012; Yang et al., 2011a). Among them, N-containing carbons have received considerable attentions due to the strong electron donor nature of N which should promote enhancement in π bonding, leading to improved stability, electron transfer rate, and hence durability of the carbon supports during electrocatalytic processes (Gong et al., 2009; Sheng et al., 2012; Silva et al., 2012; Wong et al., 2013; Yang et al., 2013; Yu et al., 2010). Therefore, a variety of N-containing carbon nanostructures including carbon nanotubes, graphene, and porous carbon has been widely reported. The nitrogen groups have been demonstrated to play a critical role in catalytic activity. According to previous reports, nitrogen atoms can be substitutionally incorporated into the basal plane of carbon in the form of “pyridinic”, “pyrolic”, and “graphitic” nitrogen bonding configurations. It is believed that nitrogen environment affects the electronic structure of doped carbon materials and results in different mechanisms of activity enhancement. In general, N-doping can be achieved by either “post-doping”, namely post-synthesis treatment of carbons by N-containing chemicals or in situ doping, i.e., direct synthesis of nanostructured carbons involving N-containing precursors. In this regard, carbon nitride, a carbonaceous material that is enriched with very high nitrogen content and that can be readily obtained through the pyrolysis of cyanamide (Kwon et al., 2012; Wang et al., 2009), melamine (Thomas et al., 2008; Vinu et al., 2007),
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or ethylenediamine/carbon tetrachloride (Vinu et al., 2005), may serve as a potential precious metal-free catalyst for electrochemical applications. Insights from the published report confirm that edge plane sites or defects of carbon-based catalysts are the predominant origin of electrochemical activity and the basal plane is electro-catalytically inert. It has been reported that carbon nitride with high edge plane sites or defects possesses a high density of electronic states, which is a key factor in determining the heterogeneous electron transfer rate of electrode. This happens because the doping of N heteroatoms can disrupt the basal surface and the generation of edge plane like-sites or defects. Carbon nitride was mainly produced by heating the carboncontaining and nitrogen-containing precursors. Various efforts have been concentrated on the synthesis of carbon nitride during the past few years. For instance, Chang et al. (2013) developed a modification of carbon nitride through co-pyrolysis of melamine and sodium nitrateor or potassium nitrate. A carbon nitride powder was obtained through directly heating dicyandiamine at a given temperature (Bu et al., 2013). The synthesis of the Uddin and Yang (2009) reported well-crystallized carbon nitride nanostructures with clear hexagonal morphology and size range of 50–500 nm on stainless steel substrates from triethylenetetramine using a sol–gel method. In another work, Lu et al., (2007) demonstrated successful fabrication of carbon nitride nanowires and pseudocubic carbon nitride polycrystalline nanoparticles by the reaction between C3N3Cl3 and NaN3 with Zn powder as catalyst. Unfortunately, the bulk carbon nitride compound gives lower surface area exposure. Therefore, lower current density is observed with such metal-free carbon nitride compounds used as electrocatalyst. Among various carbon nitride materials, ordered mesoporous carbon nitride (OMCN) is a well known and fascinating material that has attracted worldwide attention because the incorporation of nitrogen atoms in the carbon nanostructure enhances the mechanical, conducting, field-emission, and energy-storage properties (Huang et al., 2013b; Vinu, 2008). Herein we report the preparation of the OMCN, which was synthesized by using melamine as carbon and nitrogen sources and mesoporous silica SBA-15 as template. The uniform pore size and highly ordered arrangement of OMCN homogeneously covered on glassy carbon electrode (GCE) could be very attractive for various electrochemical applications. Hydrogen peroxide (H2O2), a well-known oxidizing agent, is not only a by-product of a large number of oxidase enzymes, but also an essential intermediate in biomedical, food production, pharmaceutical, industrial and environmental fields (Liu et al., 2013; Palanisamy et al., 2012). Nicotinamide adenine dinucleotide (NADH) is an important coenzyme involved in metabolic processes, widely existing in every cell of living organisms. It alternates between the reduced state NADH and the oxidized state NAD þ for the production of ATP, and serves as a hydrogen and electron carrier in cellular respiration and photosynthesis. The investigations of the redox reactivity of NADH and NAD þ are important, because a large number of dehydrogenase enzymes (4300) use these compounds as cofactors (Baskar et al., 2012; Teymourian et al., 2012). Nitrobenzene (NB) is an important raw material and solvent in the manufacturing of aniline, dye, pesticide, explosives, and pharmaceuticals and also as a solvent in products like paints, shoes and floor metal polishes. A large amount of NB has been released into the environment annually due to excessive use and improper handling of the wastewater (Chen et al., 2011; Zhang et al., 2012). Therefore, the rapid and accurate detection of H2O2, NADH, and NB is of great significance. The electrochemical results showed that the OMCN exhibited significant electrocatalytic activity towards H2O2, NB, and NADH in neutral solution, which proved that the as-synthesized OMCN could be used as environmental and electrochemical sensors and biosensors.
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2. Experimental 2.1. Chemical reagents H2O2, NB, NADH, and N,N′-dimethylformamide (DMF) (HPLC grade) were used as purchased from Beijing Dingguo Biotechnology Co. Ltd. Pluronic P123, melamine, and tetraethoxysilane were obtained from Sigma-Aldrich. The 0.1 M phosphate buffer solution (PBS pH 7.0), which was made up from NaH2PO4, Na2HPO4, and H3PO4, was employed as a supporting electrolyte. All other reagents were of analytical grade, and all solutions were prepared with double distilled water.
2.2. Instrumentation All the electrochemical experiments were performed with a CHI 830B electrochemical workstation (CH Instruments, Shanghai Chenhua Instrument Corporation, China). A conventional three electrode cell was used, the used working electrode was GCE or the modified electrode, a platinum electrode was applied as the counter electrode and an Ag/AgCl (in saturated KCl solution) electrode served as a reference electrode. All potentials in this paper were measured and reported versus Ag/AgCl. In this study, all the sample solutions were purged with purified nitrogen for 20 min to remove oxygen prior to the beginning of a series of experiments and all experiments were carried out at laboratory temperature. X-Ray diffraction (XRD) patterns were obtained on an X-ray D/max-2200vpc (Rigaku Corporation, Japan) instrument operated at 40 kV and 20 mA using CuKα radiation (k¼0.15406 nm). Fourier transform infrared (FT-IR) spectroscopy of the sample was recorded with Nicolet Magna 560 FT-IR spectrometer with a KBr plate. N2 adsorption-desorption isotherms were measured on ASAP 2020 Micromeritics (USA) at 77 K. The Brunauer–Emmett– Teller (BET) method was utilized to calculate the specific surface area. Scanning electron microscopy (SEM) images were determined with a Philips XL-30 ESEM operating at 3.0 kV. X-ray photoelectron spectroscopy (XPS) was measured using Thermo ESCA LAB spectrometer (USA). Transmission electron microscopy (TEM) images were obtained using a JEM-2100F transmission electron microscope JEOL (Japan) operating at 200 kV.
2.3. Preparation of the modified electrodes Prior to the modification, GCE (model CHI104, 3 mm diameter) was polished before each experiment with 1, 0.3 and 0.05 mm alumina power, rinsed thoroughly with doubly distilled water between each polishing step, and then sonicated successively in 1:1 nitric acid, absolute alcohol, and double distilled water. The cleaned electrode was dried with a high-purify nitrogen stream for the next modification. To prepare the modified electrodes, 5 mg of the as-prepared samples were dispersed into 1 mL DMF to give homogeneous suspension upon bath sonication. The voltammetric response is closely related to the thickness of OMCN catalyst film. The relationship between the amount of OMCN-800 suspension and the K3[Fe(CN)6] peak current was examined, as shown in Fig. S1. The peak current gradually increases as improving the volume of OMCN-800 suspension from 0.0 to 5.0 mL. Further increasing the volume of OMCN-800 suspension to 7.0 mL, the peak current almost kept stable. However, the peak current is diminished when the volume of OMCN suspension is above 8.0 mL, which is probably attributed to the slowdown of mass transport and charge transfer rate due to the over thickness of the OMCN film. Therefore, the optimized amount of OMCN is chosen as 5.0 mL.
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2.4. Preparation of OMCN-x samples The mesoporous SBA-15 was prepared using Pluronic P123 as a surfactant and tetraethoxysilane as a silica source (Jun et al., 2000; Ryoo et al., 1999). OMCN-x materials were prepared using SBA-15 mesoporous silica and melamine as a template and a precursor, respectively. The syntheses were performed by varying the pyrolysis temperature. The OMCN-x materials are referred to as OMCN-500, OMCN-650, OMCN-800, and OMCN-950, where 500, 650, 800, and 950 1C represent the different temperatures of pyrolysis in the synthesis procedure, respectively. Briefly, in an optimized procedure for OMCN-x, 1 g of SBA-15 was added to a solution obtained by dissolving 2.5 g of melamine in 10 g of H2O by vigorous stirring at room temperature for 1 h. The mixture was dried in a 60 1C drying oven. The pyrolysis was completed by heating the product to typically 500, 650, 800, and 950 1C under a flow of N2. The SBA-15 silica template was then etched away by overnight dissolution in 10% aqueous HF to leave behind the OMCN-x. Illustration of the preparation of OMCN-x is presented in Scheme 1.
3. Results and discussion 3.1. Characterization of OMCN-x Fig. 1 shows SEM and TEM images of the OMCN-800 replica. In Fig. 1A, the SEM image of the OMCN-800 replica indicates that the wormlike morphology of SBA-15 was maintained after the replication. TEM was used to examine the structural order and morphology of the OMCN-800. The cross-sectional TEM image of the OMCN-800 viewed along the channels shows the hexagonal arrays of uniform mesopores (Fig. 1B), which were typically observed in images of SBA-15 silica. Bright contrast strips on the planar TEM image represent the pore-wall images, whereas dark contrast cores display empty channels (Fig. 1C). The pore-structure ordering of the OMCN-x along with the parent SBA-15 silica was further investigated by powder XRD measurements. Fig. S2 shows the small-angle XRD patterns for SBA-15 template (a) and OMCN-x (b–e). The small-angle XRD patterns of OMCN-x samples exhibit (100), (110), and (200) diffraction peaks that can be indexed on a two-dimensional hexagonal lattice (pбmm), which indicates a well-ordered hexagonal mesostructure of the SBA-15 template preserved after the replication. The FT-IR spectra of the OMCN-x samples present several bands (Fig. S3). The band around at 3430 cm 1 is assignable to Fig. 1. (A): SEM image of OMCN-800. (B) and (C): TEM images of OMCN-800.
Scheme 1. Illustration of the preparation of OMCN-x samples.
carboxylic acid O–H stretch vibration, while the O–H vermicular vibration is responsible for the band around 1185 cm 1. The band at 1640 cm 1 can be assigned to the vibrations of benzene rings from the carbon matrix as well as the CQN bonds. The peaks located at 1565 and 1495 cm 1 are assigned to the stretching vibration of CQN in –NQquinoidQN– and CQC in benzenoid ring, further suggesting the formation of carbon nitride (Guo et al., 2011). The band centered around 1245 cm 1 is attributed to aromatic C–N stretching bond. The band at 650 cm 1 corresponds to the out of plane N–H deformation vibrations (Liu et al., 2011; Wu et al., 2012). Fig. S4 shows the N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of OMCN-x samples. Typical type-IV isotherms with a clear hysteresis loop at P/P0 ¼0.5– 0.8, indicating mesoporous character (Datta et al., 2011; Liu et al., 2011). The textural properties of the final carbon nitride materials calcined at different temperatures are shown in Table S1. The textural properties of OMCN-x samples reveal that the obtained
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materials contain significant mesoporous structures. Notably, all the samples represent a pore size distribution in the center of 5.24–5.26 nm, suggesting that the ordered mesostructure is wellpreserved after pyrolysis at the four treatment temperatures. This is important for the fast electrolytes transfer because the mesopores and interconnections provide a more favorable path for penetration and transportation of ions. From Table S1, the BET surface area and total pore volume of the OMCN-x were increased with increasing pyrolysis temperature between 500 and 800 1C, and decreased along with the pyrolysis temperature up to 950 1C. All the above characterizations confirm that OMCN-x materials with ordered hexagonal mesostructure has been synthesized by simply template method. X-ray photoelectron spectroscopy (XPS) measurements were performed to probe the amounts of N-bonding configurations in the OMCN-x samples. The high resolution N1s XPS spectra can be deconvoluted into three different signals with binding energies of 398.0, 400.0, and 401.3 eV that correspond to pyridinic N, pyrrolic N, and graphitic N, respectively (Wang et al., 2012; Yang et al., 2011b) (Fig. 2). Remarkably, the shape of these three peaks significantly changes when the pyrolysis temperature is increased, hence implying that different amounts of N-bonding configurations are formed at different pyrolysis temperatures. In the case of OMCN-500, pyrrolic N is dominant, thus accounting for 50% of the overall N content. When the pyrolysis temperature is increased to 650 and 800 1C, the amount of pyridinic N and graphitic N remains constant, whereas that of pyrrolic N largely decrease; and with the increase of pyrolysis temperature from 800 to 950 1C, both the content of pyridinic N and pyrrolic N are largely decreased, hence implying that these species are less stable at high temperatures. Thus, such different amounts of N-bonding configurations in OMCN-x samples must exert a large influence on their electrocatalytic performances. According to recent reports (Liu et al., 2010; Qu et al., 2010; Yang et al., 2011b; Yu et al., 2010), it has been suggested that the content of nitrogen, especially the pyrindinic N portion, is crucial for the promotion of the electrocatalytic reaction. Given that pyrindinic N atoms with strong electron-accepting ability can create a net positive charge on the adjacent carbon atoms in the OMCN, they are favorable for the adsorption of small molecules (e.g., H2O2, NADH, and NB) and can readily attract electrons from the anode, thus facilitating the electrochemical reaction.
3.2. Electrocatalysis of H2O2, NB, and NADH and their detection In Fig. 3A, the CVs for H2O2 reduction at different electrodes were compared. It shows a weak electrocatalytic reduction current toward H2O2 at bare GCE (curve a). For OMCN-x/GCE (curve b–e), the reduction results exhibit obvious marked decrease in overpotential with increase in peak current for H2O2 reduction compared with GCE. In curve d, the catalytic activity of OMCN-800/ GCE is evident from a decrease in overpotential as well as response current increase for H2O2 reduction compared with other OMCNx/GCE. Obviously, the presence of OMCN-800 made the electron transfer much easier compared with that of other OMCN-x samples. Thus, we focused on the investigation of the electrochemical properties of OMCN-800. Fig. 3B displays the current-time responses of OMCN-800/GCE for H2O2 detection at pH¼ 7.0 with the applied potential of 0.19 V. Insets a and b of Fig. 3B show the amperometric response of low concentration of H2O2 at OMCN-800/GCE. The current response of OMCN-800/GCE generally reached a steady-state level within 1.5 s after the H2O2 addition (inset c of Fig. 3B). The corresponding calibration plot for the reduction of H2O2 at OMCN-800/GCE was shown in Fig. 3C. Error bars are the standard deviation of five repetitive experiments (RSD¼4.6%). The H2O2 sensor displays a linear range of 4 and 40 μM (R2 ¼ 0.999, n¼10) with a sensitivity of
Fig. 2. High-resolution N1s XPS spectra of (A): OMCN-500, (B): OMCN-650, (C): OMCN800, and (D): OMCN-950. (E): The content of three nitrogen species in OMCN-x samples.
45.41 mA mM 1 and 40 to 12400 μM (R2 ¼0.999, n¼59) with a sensitivity of 20.34 mA mM 1. The detection limit is calculated as
1.52 μM with the signal to noise ratio of three (S/N ¼3). The reproducibility of the sensor was also investigated by current– time method for five repetitive measurements with additions of H2O2 concentration of 2.0 mM at 0.19 V (pH¼ 7.0). The RSD of the sensitivity was less than 3.0%. When the OMCN-800/GCE was stored at 4 1C for two weeks, the current response to 2.0 mM H2O2 remained 92.6% of its original value, suggesting the long-term stability of the modified electrode. The performance of the OMCN800/GCE was also compared with other H2O2 sensors (Table S2).
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Fig. 3. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 2.0 mM H2O2 (pH ¼7.0). Scan rate: 50 mV s 1. (B): Typical amperometric current–time curve of OMCN-800/GCE with successive additions of H2O2 (pH ¼7.0). Insets a and b: The amperometric response with successive addition of H2O2 at lower concentration. Inset c: The current response time after the H2O2 addition at OMCN-800/GCE. (C): The linear dependence of between H2O2 concentration and current signal for OMCN-800/GCE. Inset: the amperometric response with successive addition of H2O2 at lower concentration.
Fig. 4A displays the CVs of different electrodes in the presence of NB. There is almost no electrochemical response at bare GCE (curve a). However, the electrochemical reduction results exhibit obvious marked decrease in overpotential with increase in peak current for NB reduction at OMCN-x/GCE compared with bare GCE (curve b–e). Additionally, the reduction current of NB at the OMCN-800/GCE exhibits an increased current signal, which is 2.9-, 1.8-, and 1.5-fold higher than that of the OMCN-500/GCE, OMCN-650/GCE, and OMCN-950/GCE, respectively. Thus, OMCN800/GCE was selected as an amperometric sensor for NB. The differential pulse voltammetry (DPV) was employed to detect NB at OMCN-800/GCE in this study (Fig. 4B). Clearly, a series of the DPV curves (a-l) were obtained from different concentrations of NB. The calibration curve of reduction current is depicted in Fig. 4C, which exhibits steady amperometric response towards NB in the linear concentration range of 0.5–1000 mM with a correlation coefficient R2 ¼0.997; the data are the average for parallel determinations (n ¼5) at the each content value of NB. The detection limit is 0.18 mM and the sensitivity is calculated as 674.87 mA mM 1. The reproducibility of the sensor was also investigated by DPV method. The RSD of current signal for 100 mM NB was
Fig. 4. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 100 μM NB (pH¼ 7.0). Scan rate: 50 mV s 1. (B): DPV curves of NB (a-l): 0.5, 5, 20, 30, 50, 75, 100, 200, 300, 400, 700, and 1000 μM at the potential of 0.7 V in PBS (0.1 M, pH 7.0) at the OMCN-800/GCE. (C): The linear dependence of the current response with the different concentration of NB.
less than 3.2% for five measurements for the same electrode. After being stored at 4 1C for two weeks, 9.3% current loss at OMCN800/GCE was obtained by the amperometric response of 100 mM NB. The detailed comparison of NB detection performance using different NB sensors is summarized in Table S3. The electrocatalytic properties of the different electrodes toward NADH detection were also investigated (Fig. 5). Fig. 5A displays the CVs behaviors of the bare GCE (curve a) and OMCN-x/ GCE (curve b–e) in 0.1 M PBS (pH¼ 7.0) solution in the presence of 1.0 mM NADH over a potential range of 0.2–0.6 V at a scan rate of 50 mV s 1. Clearly, there is almost no electrochemical response at bare GCE. However, there is a significantly enhanced electrochemical response towards NADH oxidation after modification by the OMCN-x. Additionally, the OMCN-800 sample performs the best electrocatalytic activity towards NADH oxidation among the synthesized electrode materials. Thus, we focused on the investigation of the NADH at the OMCN-800/GCE. Fig. 5B shows a typical amperometric current–time curve of OMCN-800/GCE with successive additions of NADH (pH 7.0). The best potential to be applied was chosen at þ 0.2 V based on the CVs measurements. Inset a of
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Therefore, the OMCN-800 sample with high edge plane-like defective sites and electroactive sites could offer a favorite microenvironment for transferring species in solution through the pores of OMCN material, and also would be beneficial for accelerating electron transfer between the electrode and species in solution.
4. Conclusions 2D OMCN materials with ordered and uniform mesoporous as well as large BET surface areas have been successfully prepared for the first time using mesoporous silica SBA-15 as template and melamine as carbon and nitrogen sources by a nano hardtemplating approach. The OMCN-x samples have been synthesized by different pyrolysis temperatures. These materials were verified by detailed characterization analyses and electrochemical investigation. From the results of XPS measurements and nitrogen adsorption, different pyrolysis temperatures have an influence on the BET surface areas and N-bonding configurations, which may be the two key factors in determining the electrochemical activity of OMCN-x. The OMCN-800 with largest BET surface areas and suitable N-bonding configurations made an increase electrocatalytic activity towards H2O2, NB, and NADH in neutral solution. A sensitive electrochemical sensor for H2O2, NB, and NADH was developed based on the OMCN-800/GCE, which showed wide linear range, low detection limit, high sensitivity, and good stability. As a result, successful fabrication of OMCN-800 not only holds great promise for the design of electrochemical sensors, but also promotes the development of new electrode materials.
Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 21075014), the State Key Laboratory of Electroanalytical Chemistry, CIAC, CAS (No. SKLEAC201201) and the Fundamental Research Funds for the Central Universities (No. 12SSXT145).
Fig. 5. (A): CVs of bare GCE and OMCN-x/GCE in the presence of 1 mM NADH (pH ¼7.0). Scan rate: 50 mV s 1. (B): Typical amperometric current–time curve of OMCN-800/GCE with successive additions of NADH (pH ¼ 7.0). Inset a: The amperometric response with successive addition of NADH at lower concentration. Inset b: The current response time after the NADH addition at OMCN-800/GCE. (C): The linear dependence of between NADH concentration and current signal for OMCN-800/GCE.
Fig. 5B shows the amperometric response of low concentration of NADH at OMCN-800/GCE. The OMCN-800/GCE responds very rapidly to the changes in the level of NADH, producing steadystate signals less than 2 s (inset b of Fig. 5B). The relationship between NADH concentration and current signal for OMCN-800/ GCE is illustrated in Fig. 5C. The current increased linearly with the good linear ranges from 2 to 2200 μM (R2 ¼0.999, n ¼27) for NADH detection with a sensitivity of 13.17 mA mM 1 and a detec-
tion limit of 0.82 μM (S/N ¼3). The performance of the OMCN-800/ GCE was also compared with other NADH sensors (Table S4). So, from these results of electrochemical experiments, the OMCN-800 material performs the best electrocatalytic activity among the synthesized samples so far in this study. This may be caused by the largest BET surface areas and suitable N-bonding configurations in the OMCN-800 sample, which makes the OMCN800 with highest edge plane sites or defects and possesses a highest density of electronic states among these samples. It is a key factor in determining the heterogeneous electron transfer rate of electrode.
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