Fabrication of the protonated graphitic carbon nitride nanosheets as enhanced electrochemical sensing platforms for hydrogen peroxide and paracetamol detection

Electrochimica Acta 206 (2016) 259–269

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Fabrication of the protonated graphitic carbon nitride nanosheets as enhanced electrochemical sensing platforms for hydrogen peroxide and paracetamol detection Lin Liua , Hongying Lva , Chengyin Wanga,* , Zhimin Aob,** , Guoxiu Wangb a College of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Environmental Engineering and Monitoring, Yangzhou University, 180 Si-WangTing Road, Yangzhou, 225002, China b Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, University of Technology, Sydney, P.O. Box 123, Broadway, Sydney, NSW 2007, Australia

A R T I C L E I N F O

Article history: Received 19 January 2016 Received in revised form 19 April 2016 Accepted 22 April 2016 Available online 27 April 2016 Keywords: Graphitic carbon nitride Protonation Hydrogen peroxide Paracetamol

A B S T R A C T

In this research, graphitic carbon nitride (g-C3N4) is synthesized through the direct pyrolysis of the melamine, and the pristine g-C3N4 is further treated by sufficient protonation and ultrasonication. The resultant g-C3N4 nanosheets, with two-dimensional thin nature, exhibit enhanced ionic conductivity and large specific surface area. Density function theory (DFT) calculations of the electrical properties of the protonated g-C3N4 nanosheets demonstrate that the higher level of protonation enables g-C3N4 to have better conductivity. In addition, the protonated g-C3N4 nanosheets also show excellent electro-catalytic activity and have been employed as electrochemical sensing platforms for the non-enzymatic electrochemical sensing hydrogen peroxide (H2O2) and the selective determination of paracetamol (PCM). The results demonstrate that the protonated g-C3N4 nanosheets, as sensor materials, achieve superior electrochemical sensing performance. The exfoliated g-C3N4 nanosheets have great potential for application in further sensor development and biomedical analysis. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Graphitic carbon nitride (g-C3N4) as a new two-dimensional material has aroused a great deal of interest in recent decades. Owing to its unique physical and chemical properties, g-C3N4 has been applied in catalysis, electronics, biomedical imaging and in sensor fields [1–6]. For sensor applications, g-C3N4 and its composites have been employed to fabricate fluorescent, photoelectrochemical, electrogenerated chemiluminescent (ECL), optical and electrochemical sensors [6–15]. The electrochemical sensing performance of pristine g-C3N4 is limited to its chemical inertness, specific surface area and the conductivity. There have been scarce reports about the fabrication of electrochemical sensors based on g-C3N4. In order to broaden the application of gC3N4 in electrochemical sensing, various methods have been developed to modify g-C3N4 and enable g-C3N4 to be incorporated

* Corresponding author. Fax: +86 514 87975244; Tel: +86 514 87888454. ** Corresponding author. Fax: +61 2 95141628; Tel: +61 2 9514 1722. E-mail addresses: [email protected], [email protected] (C. Wang), [email protected] (Z. Ao). http://dx.doi.org/10.1016/j.electacta.2016.04.123 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

with other nanomaterials when applied in the fabrication of electrochemical sensors [13]. Functionalized g-C3N4 and its composites as electrochemical sensing platforms had been used for H2O2, nitrobenzene, and NADH, glucose and mercuric ions detection [12–15]. Hence, developing a way of tailoring and functionalizing g-C3N4 is a major research task. There has been some progress in the modifying g-C3N4 that liquid-exfoliation method, chemical oxidation and protonation can be employed to alter the properties of g-C3N4 [16–20]. Liquidphase exfoliation methods were carried out through intercalation and surface passivation with solvent molecules to exfoliate g-C3N4 [16–18]. This method was then followed by sonication treatment or thermal shock. Though this method can achieve exfoliation of gC3N4, suitable solvents are required for the preparation of uniform dispersion and the sonication time is too long. The chemical oxidization can be regarded as an efficient way for exfoliating gC3N4 and introducing some active groups to g-C3N4. The pristine gC3N4 when treated with oxidizing agent will produce hydroxyl and carboxyl groups on its basal plane, thus improving the chemical activity of g-C3N4 [19]. The key to the chemical oxidation of g-C3N4 is the selection of proper reactants and oxidants, which exerts a

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great effect on the degree of oxidation and the planar atomic structure of g-C3N4 [19,21]. Owing to the rich nitrogen in carbon nitride, direct protonation is a very convenient modification route. The protonation of g-C3N4 was firstly proposed by Zhang et al. The protonation has been commonly recognized as an efficient way to improve the specific surface area and ionic conductivity of g-C3N4 [20,22]. Hydrogen peroxide (H2O2) is often used as a versatile oxidant in various fields such as food, pharmaceutical industry and environmental analysis. H2O2 is also considered as a signaling molecule in various biological processes such as vascular remodeling, immune cell activation and root growth [23–25]. In addition, H2O2 is a side product generated from some biochemical reactions that some enzymes, such as glucose oxidase (GOx) and alcohol oxidase (AlOx), are taken as catalysts. Due to the significance of H2O2 in the academic research of biological systems and practical applications, it is imperative to develop efficient methods for H2O2 detection. The non-enzymatic H2O2 sensor can achieve accurate H2O2 detection with high sensitivity. Pharmaceutical analysis plays an important role in drug dosage control and quality monitoring. Paracetamol (Nacetyl-p-aminophenol, acetaminophen, PCM) is the most widely used antipyretic and analgesic drug in the world. PCM is commonly applied to reduce fever and relieve colds and pain. However, the clinical experiments had verified that overdoses of PCM would cause acute liver necrosis, inducing morbidity and mortality in humans because of the toxic metabolite accumulation [26–28]. Hence, developing simple and accurate approaches to detecting PCM is very significant in the pharmaceutical management. The electrochemical method is preferred to be used in the PCM detection, because it is very fast, simple and lowcost and can also achieve good accuracy. In this paper, a simple method was proposed for the protonation and exfoliation of the pristine g-C3N4, and the asprepared g-C3N4 nanosheets exhibited thin layers and twodimensional structure. Various techniques were used to characterize the protonated g-C3N4 nanosheets. Simulation and density function theory (DFT) calculations were used to study the structures and electrical property of the protonated g-C3N4 nanosheets, and the results demonstrated that the high level protonation enabled g-C3N4 to have good conductivity. Furthermore, the protonated g-C3N4 nanosheets modified electrode was successfully utilized to achieve the electro-catalytical oxidation of H2O2 and highly selective determination of PCM with a wide linear range and low detection limit. The protonated g-C3N4 nanosheets modified electrode presented good anti-interference performance and excellent stability. 2. Experimental

microscopy (HRTEM: Tecnai-G2 F30 S-Twin, the accelerating voltage: 300 kV) were utilized to characterize the morphologies of the g-C3N4 samples. The energy-dispersive X-ray (EDX) analysis was also conducted on the Tecnai-G2 F30 at the accelerating voltage of 300 kV. X-ray diffraction (XRD) patterns of the powders were collected with a 2u range from 5 to 50 on a Bruker AXS D8 ADVANCE X-ray diffractometer with Cu/Ka radiation (1.5406A ). X-ray photoelectron spectroscopy (XPS) technology was used to study the bonding states of the elements on an EASY ESCA spectrometer (VG ESCA LAB MKII). The Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Varian Cary 610/670 FTIR microspectrometer, with the prepared powders diluted in KBr pellets. The Brunauer-Emmett-Teller (BET) surface area (SBET) was obtained by nitrogen adsorption/desorption isotherm measurements at 77 K on a Micromeritics Accelerated Surface Area and Porosimetry (ASAP) 2020 system, and the corresponding pore diameter distributions were calculated using the BJH (Barrett–Joyner–Halenda) method. Electrochemical experiments were performed with a CHI 660A electrochemical workstation (Chenhua Instruments Company, Shanghai, China) with a conventional three-electrode system. The acid density of the protonated g-C3N4 was determined using the non aqueous acid– base titration method. 2.2. Synthesis and protonation of g-C3N4 The g-C3N4 powders were synthesized via direct pyrolysis of the melamine method based on the report [29], but some modification was made for this reported method. Melamine was set in a porcelain combustion boat with a cover and put into a tube furnace. In the first step, it was heated to 500  C and held at this temperature for 2 hours. It was then heated again to 550  C and held at this temperature for 6 hours. The above heating procedures must be in the nitrogen atmosphere. When cooled to room temperature, the porcelain boat was taken out and the pale yellow powdered g-C3N4 was prepared. The typical protonation was carried out by stirring g-C3N4 with hydrochloric acid (HCl, 37%) for 3 hours at room temperature, centrifugal washing with water until the neutral condition and drying at 105  C in air-circulating oven overnight, which was based on the literature [20]. The as-prepared sample was denoted as the g-C3N4 H+. The g-C3N4 H+ was further exfoliated g-C3N4 using concentrated nitric acid (HNO3, 60%) and ultrasonication. The as-obtained g-C3N4 H+ powder were put into the concentrated HNO3 and stir the mixture for 1 hour. The dispersion was then heated to 80  C and ultrasonically treated for 4 hours. Finally, the mixture was centrifugally washed with water until it reached a neutral condition and dried at 60  C in a vacuum drying oven. Finally, the as-prepared product was the protonated g-C3N4 nanosheets.

2.1. Chemicals and apparatus 2.3. Fabrication modified electrode Analytical grade melamine (Shanghai Chemical Reagents Company, China) was recrystallized before use. The phosphate buffer solutions (PBS) were prepared from NaH2PO4 and Na2HPO4. Paracetamol (PCM), dopamine (DA), ascorbic acid (AA), 4-aminophenol, caffeine and chitosan (CTS, M.W. 100,000–300,000, deacetylation degree95%) were purchased from Sigma-Aldrich Chemical Reagent Company (Shanghai, China). All reagents were analytical grade. Dipyrone, aspirin, naproxen, penicillins and other drugs were purchased from a local pharmacy. All the other reagents were analytical reagent grade, and used without further purification. All water used for experimentation in this work was re-distilled. Transmission electron microscope (TEM: Tecnai-12, the accelerating voltage: 120 kV) and high resolution transmission electron

Firstly, the glassy carbon electrodes (GCEs) were firstly polished with a-alumina powders, then rinsed with deionized water after each polishing step. Secondly, these electrodes were successively sonicated in 1:1 nitric acid, acetone and deionized water. Finally, the GCE was dried at room temperature. The protonated g-C3N4 nanosheets, the g-C3N4 H+ and the pristine g-C3N4, as modifier materials, were attached onto the GCE surfaces using the biopolymer chitosan (CTS) as an immobilization matrix. The modifier materials including the pristine g-C3N4, gC3N4 H+ and the protonated g-C3N4 nanosheets were added into CTS solutions to prepare suspensions, respectively. 10 mL of this suspension was dropped onto the GCE surface and allowed to dry at room temperature.

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2.4. Simulation details of the protonated g-C3N4

3.2. The effect of protonation on the electrical properties of g-C3N4

To better understand the experiment results of the effect of protonation on the electrical property of g-C3N4, density function theory (DFT) calculations were performed using DMOL3 code [30]. The generalized gradient approximation (GGA), with revised Perdew-Burke-Ernzerhof (RPBE) functional, was employed as the exchange-correlation functional [31]. A double numerical plus polarization (DNP) was used as the basis set, and the treatment for electron core was employed. Spin polarization was considered in the calculations. The convergence tolerance of the energy was set to 10 5 Ha (1 Ha = 27.21 eV), and the maximum allowed force and displacement were 0.02Ha and 0.005 Å, respectively. In the simulations, three-dimensional periodic boundary conditions were imposed, and all the atoms were allowed to relax. As reported, there are two possible building blocks for g-C3N4 [32] as shown in Fig. 1. Therefore, both of the two structures were considered in the calculations.

It was reported that the protonation of g-C3N4 can adjust the electronic band gap of C3N4, and that the protonated g-C3N4 has higher conductivity, which is proportional to the degree of protonation [20,32]. To understand the effect of protonation of g-C3N4 on its electronic properties, the band structure before and after protonation for both structures was calculated, and the results are shown in Fig. 2. The band gaps before protonation for gC3N4 with tri-s-triazine and triazine structures were around 1.53 and 1.64 eV, respectively. The results were consistent with the reported result that the band gap of g-C3N4 is around 2 eV [38]. After protonation, the conductivity of g-C3N4 was remarkably improved [20]. The band structure after protonation showed that the protonated g-C3N4 as shown in Fig. 2(c) and (f) (high protonation concentration with one proton per simulation cell as shown in Fig. 1) became a conductor for both structures, where no band gap was found. If reducing the protonation concentration to one proton per supercell as shown in Fig. 1, the resultant g-C3N4 is still a semiconductor with a reduced band gap, i.e. low protonation rate g-C3N4 with triazine and tri-s-triazine structures were around 1.04 and 1.34 eV, respectively as shown in Fig. 2(b) and (d).

3. Results and discussion 3.1. The determination of g-C3N4 and the protonated g-C3N4 structures Owing to the lack of experimental data, there have been intensive discussions about the actual existence of g-C3N4 [32]. Inspired by the structure, graphite, triazine (C3N3) had been proposed as elementary building blocks of g-C3N4 as shown in Fig. 1(b) [33–35]. Another possible building block of g-C3N4, tri-striazine (heptazine) rings, had been proposed recently [36,37], which was structurally related to the hypothetical polymer melon as shown in Fig. 1(a). Therefore, both the blocks were considered in the calculations. After DFT calculation, it was found that the energies of the two structures are close to each other, and the energy of g-C3N4 with triazine block is 0.012 eV/atom lower than that of the tri-s-triazine block. Because of the very slight difference between the two structures, the two structures may co-exist when synthesized. For the protonation of g-C3N4, positive charged H atoms prefer to bind with N atoms, which have higher electronegativity than C atoms. In order to determine the position of the H on g-C3N4, H bonded with N atom at different positions were considered, as shown as 1, 2, 3 in Fig. 1(a), and 1, 2 in Fig. 1(b). After geometry optimization, the H atom at position 2 for both cases has the lowest energy. Thus, the H atom prefers to bind with the N atom at position 2 in the two structures.

3.3. Characterizations of the protonated g-C3N4 nanosheets The morphologies of the bulk g-C3N4 and the protonated g-C3N4 were characterized using TEM and HRTEM. Fig. 3(a, b) presented the TEM images of the pristine g-C3N4 and the protonated g-C3N4 nanosheets. Comparing these two TEM images, it can be found that the protonated g-C3N4 nanosheets were well exfoliated with a large surface area. Furthermore, HRTEM and scanning transmission electron microscopy (STEM) were employed to observe the morphology of the protonated g-C3N4 nanosheets, and the corresponding HRTEM and STEM images were shown in Fig. 3(c, d). From Fig. 3(c, d), it can be seen that the protonated g-C3N4 is mainly composed of interconnected thin layers stacked with a two-dimensional structure. From comprehensive results of the TEM, HRTEM and STEM, it is easy to find that the protonation method proposed in this research enables the g-C3N4 to reduce the thickness and size, and the resulting g-C3N4 exhibited the twodimensional and thin nature. In addition, the EDX spectrum of the protonated g-C3N4 shown in Fig. 3(e) revealed the presence of C, N and O elements in the sample and the corresponding element mapping of C and N was inserted in Fig. 3(e), while the signal of Cu was attributed to the copper grids. The O element presented in g-

Fig. 1. Two possible building blocks for g-C3N4, (a) tri-s-triazine (heptazine) rings, and (b) triazine (C3N3). The red rhombus denotes the primary simulation cell. The numbers denote the possible position of H atom on g-C3N4 during protonation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Band structure of g-C3N4 with triazine structures [(a) before and after protonation with low protonation concentration (b), one H proton per simulation cell in Fig. 1(b) and high protonation concentration (c), one H proton per supercell in Fig. 1(b)] and tri-s-triazine [(d) before and after protonation with low protonation concentration (e), one H proton per simulation cell in Fig. 1(a) and high protonation concentration (f), one H proton per supercell in Fig. 1(a)].

C3N4 nanosheets can be attributed to the absorbed H2O or CO2 molecules on the surface. The nitrogen adsorption desorption isotherms were carried out to measure the BET specific surface areas and pore structures of the pristine g-C3N4 and protonated g-C3N4 nanosheets. The results are shown in Fig. 4. The N2 adsorption isotherms shows that the BET surface area of the protonated g-C3N4 nanosheets can reach to 130.9 m2 g 1, which is almost 13 times larger than that of the bulk g-C3N4 (10.7 m2 g 1). The corresponding pore diameter distributions (the insert in Fig. 4) were calculated using the BJH method. The N2 adsorption–desorption isotherm of the protonated g-C3N4 nanosheets is type IV (Brunauer, Deming, Deming, and Teller, BDDT classification) with a H3 hysteresis loop at high relative pressure between 0.5 and 1.0, suggesting the presence of mesopores (2– 50 nm) and macropores (>50 nm) [39]. It is reported that the type H3 hysteresis loop at 0.45 < P/P0 < 1.00 in the isotherm is often observed on the aggregates of plate-like particles giving rise to slitshaped pores, which agrees well with the nanosheet-like morphology [39,40]. In addition, the TEM images and HRTEM images of the as-obtained sample further prove that the protonated g-C3N4 (in Fig. 3) is mainly composed of nanosheets. The increased surface area of the protonated g-C3N4 nanosheets can be ascribed to slit-shaped pores caused by the stack of interconnected thin layers. In addition, the XRD patterns, XPS spectra and the FTIR spectra of the pristine g-C3N4 and the protonated g-C3N4 nanosheets were investigated to determine the effect of protonation on the composition and structure of g-C3N4. The results are shown in Fig. S1. The XRD patterns of the pristine g-C3N4 and the protonated g-C3N4 nanosheets demonstrate that protonation can decrease the crystal size and the orderliness of the planar structure of g-C3N4. However, the protonation enables g-C3N4 to keep the original XRD pattern and has not destroyed the typical layered structure of gC3N4. XPS spectra (in Fig. S1 (B) and (C)) of the pristine g-C3N4 and the protonated g-C3N4 nanosheets show that the protonation provides g-C3N4 with the increase in the percentage of C-NH2 and

amino groups, indicating that the protonation introduces more active groups and the protonated g-C3N4 nanosheets has more defective sites compared with the pristine g-C3N4. The FTIR spectra of the pristine g-C3N4 and the protonated g-C3N4 nanosheets further prove that the protonation contributes to the existence of the C N stretching bonds in the aromatic ring, which were related to the extended network [41]. The detailed description of the XRD patterns, XPS spectra and the FTIR spectra for the pristine g-C3N4 and the protonated g-C3N4 nanosheets are listed in the Supporting Information. Electrochemical impedance spectroscopy (EIS) analysis was employed to study the interfacial properties of surface-modified electrode and evaluate the ionic conductivity of the g-C3N4 with varying degrees of protonation. There was a semicircle portion and a linear portion in the EIS spectrum. The semicircle diameter at higher frequencies corresponds to the electron-transfer resistance (Ret). Fig. 5 shows the Nyquist plots for g-C3N4 (curve a), g-C3N4 H+ (curve b), the protonated g-C3N4 nanosheets (curve c) modified electrodes and the bare GCE (curve d), which is recorded at the open circuit potential in 0.1 mol
L 1 KCl solution containing 5 mmol L 1 [Fe(CN)6]3 /4 . As shown in Fig. 5 (curve a), a biggest semicircle was observed in the EIS spectrum of the pristine g-C3N4 modified electrode. When the g-C3N4 was protonated by concentrated hydrochloric acid and the resultant g-C3N4 H+ were attached on the surface of GCE (curve b), there was a decrease in the Ret value of the g-C3N4 H+ modified electrode. The result demonstrated that the protonation could improve electric conductivity of g-C3N4 and accelerate the electron transfer. But when the g-C3N4 H+ was further treated with concentrated nitric acid and ultrasonication, the Ret value of the prepared protonated g-C3N4 nanosheets modified electrode was smaller compared with that of the g-C3N4 H+ modified electrode. This indicated that the second-step treatment with HNO3 and can further increase ionic conductivity of g-C3N4. According to the DFT calculation of the protonated gC3N4, the higher level of the protonation enables g-C3N4 to have better conductivity. The protonated g-C3N4 as a solid acid and its

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Fig. 3. Characterization of g-C3N4 and the protonated: (a) g-C3N4 and (b) the protonated g-C3N4 of TEM images, (c) HRTEM image of the protonated g-C3N4, (d) HADDF-STEM image of the protonated g-C3N4, and (e) the EDX spectrum of the protonated g-C3N4 along with element mapping of C and N.

acid density can be detected through acid–base titration method. The determination of acid density of the protonated g-C3N4 was described in detail in the Supporting Information section. The results demonstrated that the acid density of the protonated gC3N4 nanosheets (1.95 mmol/g) is higher than that the g-C3N4 H+ (1.42 mmol/g), suggesting that the protonated g-C3N4 nanosheets have higher level of protonation and better conductivity than the g-C3N4 H+. This result is consistent with the EIS result. However, theoretic stimulation is carried out under ideal conditions, and the full protonation will enable the g-C3N4 to have a zero band gap and

become a conductor. Though two strong acids were used to protonated g-C3N4, the complete protonation was still hard to achieve. The possible reason was that the pristine g-C3N4 was not exfoliated into single layers, and there was not enough room between each layer for the intercalation of acid molecules. As seen in Fig. 5, the Ret value of the protonated g-C3N4 nanosheets modified electrode is slightly bigger compared with the bare GCE, which is partly attributed to the use of CTS as an adhesion matrix. However, compared with the first step of protonation, the further protonation really improves the ionic conductivity of g-C3N4.

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3.4. Electrochemical behavior of hydrogen peroxide and paracetamol Owing to large surface area and good conductivity as discussed above, the protonated g-C3N4 nanosheets was used to fabricate modified electrodes for electro-oxidation of H2O2. Fig. 6(A) exhibited the cyclic voltammograms (CVs) of the bare GCE (curve a), the g-C3N4 (curve b), g-C3N4 H+ (curve c) and the protonated gC3N4 nanosheets (curve d) modified electrodes in the presence of 1.20 mM H2O2 in PBS (pH 7.0) at a scan rate of 100 mV/s. The bare GCE and g-C3N4/CTS GCE only generated a weak cationic current due to the oxidation of H2O2, while the g-C3N4 H+/CTS GCE showed an increase in cationic current at around 1.2 V. However, as shown in Fig. 6(A), the H2O2 oxidation at the protonated g-C3N4 nanosheets modified electrode can generate stronger current response at lower potential compared with that at other three modified electrodes. The results indicated that the protonated gC3N4 nanosheets exhibited a good catalytic activity towards H2O2 oxidation. The ampermoteric technique was further used to

investigate the electrochemical performance of these three modified electrodes. The amperometric curves of the g-C3N4 (curve a), g-C3N4 H+ (curve b), the protonated g-C3N4 nanosheets (curve c) towards detecting H2O2 at the applied potential of 1.0 V were inserted in Fig. 6(A). The results demonstrated that the electrochemical oxidation of H2O2 on the protonated g-C3N4 nanosheets modified electrode can generate stronger current response compared with other two modified electrodes. From the results in Fig. 6(A), it can be concluded that the second-step treatment with concentrated nitric acid and ultrasonication enable g-C3N4 to have the enhanced electrochemical property. The protonated g-C3N4 nanosheets as a modifier material exhibited good electrochemical performance towards H2O2 detection. The protonated g-C3N4 nanosheets modified electrode was then employed for PCM detection. Fig. 6(B) showed the CVs of 0.15 mM paracetamol (PCM) on the bare GCE (curve a), g-C3N4 H+/CTS-GCE (curve b) and the protonated g-C3N4 nanosheets/CTS-GCE (curve c) in 0.1 M PBS solution (pH 7.0). PCM showed an irreversible electrochemical behavior at all five electrodes. As shown in Fig. 6(B), on the bare GCE, the PCM could oxidize at the potential of 0.60 V. After GCE was coated with the g-C3N4 H+, the oxidation peak potential of PCM shifted negatively to 0.50 V. For the protonated g-C3N4 nanosheets modified electrode, it can be seen that the PCM oxidation occurred at the potential of 0.50 V and generated strong oxidation peak current, which can be attributed to the good ionic conductivity and large surface area of the protonated g-C3N4 nanosheets. Compared the electrochemical performance of the protonated g-C3N4 nanosheets modified electrode with the g-C3N4 H+ modified electrode, a conclusion can be drawn that the protonation can improve the electrochemical property of g-C3N4, and the resultant protonated g-C3N4 nanosheets as modifier material can exhibit good electrochemical performance towards PCM detection. Though protonation can improve the conductivity g-C3N4 nanosheets, it is still difficult for g-C3N4 nanosheets to obtain good conductivity as a conductor. The above results demonstrated that the protonated g-C3N4 nanosheets as a modifier material can show good electrochemical performance for the detection of PCM and H2O2. It is also suggested that sufficient protonation can improve the surface area and conductivity of g-C3N4. After sufficient protonation and exfoliation, g-C3N4 can be as a good sensor material towards the detection of H2O2 and PCM. 3.5. Electrochemical detection of hydrogen peroxide and paracetamol

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Z'(KΩ) Fig. 5. EIS spectra of the g-C3N4 (curve a), g-C3N4 H+ (curve b) and the protonated gC3N4 nanosheets (curve c) modified electrodes and bare GCE (curve d) in 5.0 mM [Fe (CN)6]3 /4 containing 0.1 M KCl.

The protonated g-C3N4 nanosheets modified electrode was then employed to detect H2O2 and PCM. The amperometry has been proved to be a good electrochemical technique to detect H2O2. Before H2O2 detection, some experimental conditions were optimized, like the amount of protonated g-C3N4 nanosheets and CTS, the buffer solution, applied potential. Under optimal experimental conditions, the protonated g-C3N4 nanosheets modified electrode was used to detect H2O2. Fig. 7(a) shows a typical current–time plot of the protonated g-C3N4 nanosheets/ CTS-GCE on successive step changes of H2O2 concentration in the stirring PBS (0.005 M, pH 7.0) at an applied potential of 1.0 V. With the addition of H2O2 into the stirring buffer solution (0.005 M, pH 7.0), the current response rapidly increased to the substrates and could achieve 95% of the steady-state current within 2 s, which was faster than those in most other reports. The linear relationship between the oxidation currents and the concentrations of H2O2 is shown in Fig. 7(b). The oxidation current was linear with the concentration of H2O2 from 1.60 mM to 3.72 mM with a low detection limit (S/N = 3) of 0.40 mM. It can be concluded that the protonated g-C3N4 nanosheets can be as an excellent candidate of electrochemical sensing platform for H2O2 determination. Carbon

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0.8

Potential (V/ vs. SCE)

Fig. 6. (A) Cyclic voltammograms of 1.20  10 3 M H2O2 on the GCE (curve a), g-C3N4/CTS GCE (curve b), g-C3N4 H+/CTS–GCE (curve c) and protonated g-C3N4 nanosheets/ CTS–GCE (curve d) in 0.005 M PBS (pH 7.0). Scan rate: 100 mV s 1. The inserted figure is the amperometric response of 2.510 4 H2O2 on the GCE (curve a), g-C3N4/CTS–GCE (curve b), g-C3N4 H+/CTS–GCE (curve c) and the protonated g-C3N4 nanosheets/CTS–GCE (curve d) in 0.005 mol/L PBS (pH 7.0), the applied potential: 1.0 V. (B) Cyclic voltammograms of 1.5010 4 M PCM on the bare GCE (curve a), g-C3N4 H+/CTS–GCE (curve b) and protonated g-C3N4 nanosheets/CTS–GCE (curve c) in 0.1 M PBS (pH 7.0). Scan rate: 100 mV s 1.

materials have also been used as modifier materials for the estimation of hydrogen peroxide through eletrochemical oxidation. Carbon nanotubes paste electrode has been used to electrochemically oxidize H2O2 at the applied potential of 0.95 V with a sensitivity of 0.8 mA mM 1, and the detection limit was 20 mM [42]. Pumera et al. reported the multi-walled carbon nanotubes-epoxy composite (CNTEC) electrodes were utilized for electrochemical oxidation of H2O2 at the applied potential of 0.95 V over linear concentration range of 0.0–2.0 mM, and the sensitivity was 18.0 mA mM 1 [43]. Carbon nanofiber modified GCE was used to detect H2O2 at the applied potential of 0.65 V with a sensitivity of 3.20 mA mM 1, and the detection limit was 4.0 mM [44]. Compared with the above-mentioned carbon materials, the protonated

0.1

20 μΜ 80 μΜ

(a)

0.0 -0.5

2 μΜ

0.3 0.2

-1.0 -1.5

0.1

-2.0

-0.4

0

-0.5

(b)

Current / -mA

0.4 mM

-0.1

-0.3

0.4

0.16 mM

Current (μΑ)

Current / mA)

0.0

-0.2

g-C3N4 nanosheets as a modifier material exhibited better electrochemical performance with a wider linear range (1.60 mM–3.72 mM), higher sensitivity (106.9 mA mM 1) and lower detection limit (0.4 mM) towards the oxidation of H2O2. However, the applied potential of the electrode modified by the protonated g-C3N4 nanosheets for the detection of H2O2 was slightly more positive than those of the carbon materials modified electrodes, reported previously. It has been verified that the more positive potential used for electrochemical oxidation of H2O2 produces larger oxidation current. In order to make a fair comparison, the asobtained g-C3N4 nanosheets modified electrodes were further investigated to electrochemically oxidize H2O2 at the potential of 0.6, 0.7, 0.8, 0.9, 1.0 and 1.1 V. The sensitivity of the protonated

0

100 200 300 400 500 600 Time (s)

400

800

1200

0.0

1600

Concentration (mM)

Time / s

(c)

(d)

a

-6

-9

l

-12

60

Current / -μA

Current / μA

-3

40

20

0

0.30

0.45

0.60

0.75

Potential / V (vs. SCE)

0.0

0.5

1.0

1.5

2.0

Concentration / mM

Fig. 7. (a) The amperometric response of different concentrations of H2O2 on the the protonated g-C3N4 nanosheets/CTS-GCE for in 0.005 mol/L PBS (pH 7.0), the applied potential: 1.0 V; (b) The corresponding calibration curve for H2O2 detection. (c) Differential pulse voltammograms of the protonated g-C3N4 nanosheets/CTS-GCE in PBS containing different PCM concentrations: (a)-(l) are 0, 1.7010 6, 9.5010 6, 1.5010 5, 2.8010 5, 4.8010 5, 1.0010 4, 1.3010 4, 1.6010 4, 2.2510 4, 2.5010 4, 2.8010 4 M, respectively. Accumulation potential under stirring: 0.10 V; Accumulation time: 50 s; Quiet time: 30 s; Scan rate: 0.004 V s 1; Pulse height: 0.050 V; Sampling width: 0.05 s; Pulse period: 0.2 s; Sensitivity: 5.010 6A V 1; (d) The calibration curve for PCM detection.

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g-C3N4 nanosheets modified electrodes towards oxidation of H2O2 at the potential of 0.6, 0.7, 0.8, 0.9, 1.0 and 1.1 V were 5.66, 11.31, 20.20, 55.85, 106.9 and 100.4 mA mM 1, respectively. Compared with the above-mentioned carbon materials, the protonated gC3N4 nanosheets as a modifier material for the detection of H2O2 presented higher sensitivity at similar applied potentials. The highest sensitivity was obtained at 1.0 V. In order to detect H2O2 with a high sensitivity and a low detection limit, the applied potential of the proposed g-C3N4 nanosheets modified electrode was set at 1.0 V. Another important aspect is that high applied potential for electrochemical sensing will weaken the antiinterference performance of the modified electrode. In order to electrochemically oxidize H2O2 at a low potential and detect H2O2 with a high sensitivity, carbon materials have been incorporated with other materials to exert a synergic effect on enhancing electrochemical oxidation of H2O2. There have been some modifiers based on carbon material composites for the electrochemical oxidation of H2O2, such as graphene/AuNPs/chitosan, MnO2/graphene/carbon nanotubes, PtPd/multi-walled carbon nanotubes, cobalt oxide nanoparticles/electrochemically reduced graphene oxide (CoOxNPs/ERGO) hybrids, and cobalt-tetraphenylporphyrin/reduced graphene oxide (CoTPP/RGO) nanocomposite [45–49]. Compared with these carbon material composites, the protonated g-C3N4 nanosheets modified electrode presented comparable electrochemical performance in terms of the detection limit, sensitivity and linear range, but it still needs further improvement in lowering the applied potential for electrochemical oxidation of H2O2 while ensuring the high sensitivity. For PCM detection, the first was to select the optimum operation condition. Differential pulse voltammetry (DPV) was employed for the quantitative determination of PCM. Owing to its accumulation process and imposing pulse potential, differential pulse voltammetry shows a good sensitivity and a low detection limit. Some factors including the amount of the protonated g-C3N4 nanosheets and CTS, the buffer solution, the accumulation potential and accumulation time was investigated, which exerted strong effect on PCM detection. Fig. 7(c) shows the differential pulse voltammetric curves of the protonated g-C3N4 modified electrode in 0.1 M PBS solution (pH 7.0) in the presence and absence of PCM. As shown in Fig. 7(c), an obvious oxidation peak around 0.45 V emerged in the presence of PCM, indicating a strong response even in the lower PCM concentration. With a further increase of PCM concentration, the peak current dramatically increased along with the oxidation peak, but we can see that the oxidation peak was sharper and increased more. Under the optimum operational conditions, the protonated g-C3N4 nanosheets modified electrode was utilized to determinate PCM. The oxidation peak current was proportional to PCM concentrations in

the range of 1.70 mM 2.02 mM with a correlation coefficient R2 = 0.996 as shown in Fig. 7(d). The detection limit for determination of PCM was 0.15 mM in terms of the role of signal-to-noise ratio of 3:1 (S/N = 3). Carbon materials modified electrodes, such as the carbon nanotube-modified basal-plane pyrolytic graphite electrodes (MWCNT-BPPGE), graphene/GCE, carboxylated MWCNTs/GCE and C60-modified GCE, had been used to detect PCM [50–53]. Compared with the above mentioned modified electrodes, the protonated g-C3N4 nanosheets modified electrode for the detection of PCM was not best in terms of the detection limit. However, the electrode modified by the protonated g-C3N4 nanosheets obtained a wider linear range (1.70 mM– 2.02 mM) than C60/GCE (0.05–1.5 mM), graphene/GCE (0.1– 20 mM), MWCNT-BPPGE (0.1–25 mM), and carboxylated MWCNTs/GCE (1.0–200 mM). In order to construct good electrochemical assays, carbon materials are mostly incorporated with other nanomaterials to prepare hybrid materials as modifier materials. Table 1 displays reported works on modified electrodes based on carbon material composites for the detection of PCM. From Table 1, it can be observed that most of the electrodes modified by carbon material composites for PCM detection, retained a lower detection limit than the protonated g-C3N4 nanosheets modified electrode. However, from the perspective of the linear range, the protonated g-C3N4 nanosheets obtained a wider linear range than most of electrodes modified by carbon materials composites. The protonated g-C3N4 nanosheets as a modifier material for the detection of PCM still needs improvement in the detection limit. After a comparison of carbon materials and their composites, the protonated g-C3N4 nanosheets as modifier materials for the detection of H2O2 and PCM still needs improvement in terms of the detection limit and the applied potential. However, it can detect H2O2 with a high sensitivity in the wide linear range while ensuring good anti-interference performance. Furthermore, the protonated g-C3N4 nanosheets modified electrode was used for the determination of PCM in the wide linear range. In addition, the low cost and the simple and the “green” preparation method of the gC3N4 nanosheets modified electrodes make it promising for electrochemical sensing. Inspired by carbon materials, the incorporation with other nanomaterials would be an important research direction for g-C3N4 to broaden its applications in the field of electrochemical sensing. 3.6. The anti-interference for electrodetection of hydrogen peroxide and paracetamol Avoiding interference from these organic substances is undoubtedly challenging for non-enzymatic H2O2 detection. Thus,

Table 1 Comparison of reported works with different electrochemical platforms for electrochemical detection paracetamol. Electrochemical platform

pH used

Linear range

Limit of detection (mM)

Reference

P4VPa /MWCNT/GCE (rGO–PEDOT NT)b /GCE (AuNPs-DNS)e /MWCNT/GCE Fe3O4@SiO2/MWCNTs-CPEc b-CDd/RGO/GCE SWCNT–GNSe/GCE graphene–chitosan (GR–CS)/GCE MWCNTs/CTS–Cu/GCE Protonated g-C3N4/CTS-GCE

7.0 7.0 7.0 6.0 7.0 7.0 7.0 7.0 7.0

0.02–450 mM 1–35 mM 0.8–400 mM 0.60 mM–0.10 mM 0.01–0.80 mM 0.05–64.5 mM 1.0 mM–0.1 mM 0.1–200 mM 1.70 mM–2.02 mM

0.00169 0.4 0.050 0.13 2.3 0.038 0.30 0.024 0.15

[54] [55] [56] [57] [58] [59] [60] [61] This work

a b c d e

poly (4-vinylpyridine). reduced graphene oxide (rGO) and poly(3,4-ethylenedioxythiophene) nanotubes (PEDOT NTs). dopamine nanospheres functionalized with gold nanoparticles. b-cyclodextrin. the single-walled carbon nanotube (SWCNT)–graphene nanosheet (GNS) hybrid film.

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the proposed sensor has good selectivity towards PCM. Therefore, we can conclude from the above results that the protonated g-C3N4 nanosheets is a good sensing material for H2O2 and PCM detection owing to its good anti-interference.

H2O2 Glucose

0

UAAA DA

Current (μΑ)

-4 -8

3.7. Long-term stability and reproducibility of the protonated g-C3N4 modified electrode

H2O2

-12 -16 -20 -24 500

1000

1500

2000

267

2500

Time (s) Fig. 8. Current–time curve recorded on the protonated g-C3N4 nanosheets/CTS-GCE for addition of 50 mmol/L H2O2 and 10 mmol/L glucose, UA, AA and DA in 0.005 mol/ L PBS (pH 7.0) at 1.0 V under gently stirring.

anti-interference of the protonated g-C3N4 nanosheets for H2O2 detection should be investigated, which is also the important criterion in evaluating the performance of the protonated g-C3N4 for electrochemical detection of H2O2. Considering the lower applied potential and the relatively large current response, For the amperometric sensing, modified electrodes are generally evaluated by measuring the current response at a fixed potential and adding the analyte and possible interfering species, and the detection potential was set at 1.0 V. Fig. 8 displays the amperometric responses of four electroactive species (glucose, UA, AA, DA, 10 mM, respectively) and H2O2 (50 mM) on the protonated g-C3N4 nanosheets/CTS-GCE at a applied potential of 1.0 V. From Fig. 8, the UA, AA and DA cause little response current, which could be ignored. In addition, the anti-interference of the protonated g-C3N4 nanosheets modified electrode for PCM detection was also investigated. We had studied the protonated g-C3N4 nanosheets modified electrode towards sensing PCM in the presence of some possible coexisting substrates including UA, AA, 4-Aminophenol (4-AP), caffeine, penicillins, dipyrone, naproxen and aspirin. Then, we recorded the peak potential of each substrate and counted the peak current change caused by the addition of each interferent, which was shown in Table 2. 500-fold of caffeine and penicillins, 100-fold of citrate, malic acid, cysteine, vitamin B6, CuSO4, aspirin, 100-fold of 4-AP, AA and DA, 50-fold of UA, Fe(NO3)3 and dipyrone had no interference. In addition, no interferences were observed in the presence of 1000-fold concentrations of NaCl, KNO3, KCl, NH4Cl, ZnSO4, Al(NO3)3, Ca(NO3)2. The above results suggest that

The long-term storage and operational stability of the modified electrode is essential for the monitoring of H2O2 and PCM. The stability of the present electrode was examined using the same the protonated g-C3N4 nanosheets/CTS-GCE for 10 repetitive measurements in successive additions of 0.05 mM H2O2 at 1.0 V, and the relative standard deviation was 3.5%, confirming that the assynthesized electrode for H2O2 sensing is stable. In order to determine the reproducibility, the solution containing 50 mM PCM was repeatedly detected more than 10 times using the same modified electrode. The average current was 2.075 mA with the relative standard deviation (RSD) of 3.2%, which indicated that the modified electrode had a good reproducibility. The long-term storage was evaluated by measuring its sensitivity to H2O2 and PCM over a period of two months. The sensor was stored in air and the sensitivity was tested each day. The results demonstrated that the electrode can respectively retain 95% and 98.7% of its initial response after two-month-storage. This stability can be acceptable for most practical applications. These results indicated that the protonated g-C3N4 nanosheets exhibited good long-term storage ability and excellent stability, which can be acceptable for most practical applications. 4. Conclusions In conclusion, a simple method was employed to sufficiently exfoliate and protonate graphitic-like g-C3N4 and obtained the protonated g-C3N4 nanosheets. Furthermore, the protonated gC3N4 nanosheets were used to fabricate modified electrode for high-effective non-enzymatic electrochemical detecting H2O2 and selective detection of PCM. The superior electro-catalytic activity of this material was attributed to unique properties of the protonated g-C3N4 nanosheets, which can greatly improve the electrochemical performance towards some probes. The protonated g-C3N4 nanosheets as a modifier material exhibited excellent performance toward H2O2 and PCM oxidation with high catalytic current and stable response. The results demonstrated that the protonated g-C3N4 nanosheets modified electrode for electrochemical sensing exhibited a lot of advantages, such as high sensitivity, fast analysis speed and good stability, which further enriches applications of g-C3N4 in sensor fabrication. The application of graphitic carbon nitride in the detection of PCM and H2O2 in our work can be regarded as a way to broaden innovative materials in the application of the electrochemical sensing. Acknowledgements

Table 2 Interferences of different possible co-existing substrates on peak current of paracetamol. Interferents

Peak potential (V)

Concentration (mol/L)

4-AP AA UA Caffeine Dipyrone Aspirin Paracetamol

0.094 0.167 0.218 1.324 0.506 0.884 0.450

5.0010 5.0010 2.5010 2.5010 2.5010 5.0010 5.0010

4 4 4 3 4 4 6

Change of peak current (%) 3.3 3.5 3.4 3.2 3.7 3.4 –

C. Wang acknowledges the financial supports from the National Natural Science Foundation of China (Grant No. 21375116), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, a research program on the Analytical Methods and Techniques on the Shared Platform of Large-scale Instruments and Equipment in Jiangsu province (BZ 201409), and the Graduate Research and Innovation Projects in Jiangsu Province (KYLX15-1357). Z. Ao acknowledges the financial supports from the Chancellor's Research Fellowship Program of the University of Technology, Sydney. This research was supported by the National

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Computational Infrastructure (NCI) through the merit allocation scheme and used NCI resources and facilities in Canberra, Australia. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.04.123. References [1] M. Shalom, S. Inal, C. Fettkenhauer, D. Neher, M. Antonietti, Improving carbonnitride photocatalysis by supramolecular preorganization of monomers, J. Am. Chem. Soc. 135 (2013) 7118. [2] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76. [3] Y.J. Zhang, T. Mori, J.H. Ye, Polymeric carbon nitrides: Semiconducting properties and emerging applications in photocatalysis and photoelectrochemical energy conversion, Sci. Adv. Mater. 4 (2012) 282. [4] Y.S. Fu, J.W. Zhu, C. Hu, X.D. Wu, X. Wang, Covalently coupled hybrid of graphitic carbon nitride with reduced graphene oxide as a superior performance lithium-ion battery anode, Nanoscale 6 (2014) 12555. [5] L.S. Lin, Z.X. Cong, J. Li, K.M. Ke, S.S. Guo, H.H. Yang, G.N. Chen, Graphitic-phase C3N4 nanosheets as efficient photosensitizers and pH-responsive drug nanocarriers for cancer imaging and therapy, J. Mater. Chem. B 2 (2014) 1031. [6] S.W. Zhang, J.X. Li, M.Y. Zeng, J.Z. Xu, X.K. Wang, W.P. Hu, Polymer nanodots of graphitic carbon nitride as effective fluorescent probes for the detection of Fe3+ and Cu2+ ions, Nanoscale 6 (2014) 4157. [7] Q.B. Wang, W. Wang, J.P. Lei, N. Xu, F.L. Gao, H.X. Ju, Fluorescence quenching of carbon nitride nanosheet through its interaction with DNA for versatile fluorescence sensing, Anal. Chem. 85 (2013) 12182. [8] H. Xu, J. Yan, X.J. She, L. Xu, J.X. Xia, Y.G. Xu, Y.H. Song, L.Y. Huang, H.M. Li, Graphene-analogue carbon nitride: novel exfoliation synthesis and its application in photocatalysis and photoelectrochemical selective detection of trace amount of Cu2+, Nanoscale 6 (2014) 1406. [9] H. Dai, S.P. Zhang, G.F. Xu, L.S. Gong, M. Fu, X.H. Li, S.Y. Lu, C.Y. Zeng, Y.W. Jiang, Y.Y. Lin, G.N. Chen, A sensitive arecoline photoelectrochemical sensor based on graphitic carbon nitride nanosheets activated by carbon nanohorns, RSC Adv. 4 (2014) 11099. [10] C.M. Cheng, Y. Huang, X.Q. Tian, B.Z. Zheng, Y. Li, H.Y. Yuan, D. Xiao, S.P. Xie, M. M.F. Choi, Electrogenerated chemiluminescence behavior of graphite-like carbon nitride and its application in selective sensing Cu2+, Anal. Chem. 84 (2012) 4754. [11] J. Li, X.Y. Zhang, H.M. Ma, D. Wu, Y. Zhang, B. Du, Q. Wei, Cathodic electrochemiluminescence immunosensor based on nanocomposites of semiconductor carboxylated g-C3N4 and graphene for the ultrasensitive detection of squamous cell carcinoma antigen, Biosens. Bioelectron. 55 (2014) 330. [12] Y.F. Zhang, X.J. Bo, A. Nsabimana, C. Luhana, G. Wang, H. Wang, M. Li, L.P. Guo, Fabrication of 2D ordered mesoporous carbon nitride and its use as electrochemical sensing platform for H2O2, nitrobenzene, and NADH detection, Biosens. Bioelectron. 53 (2014) 250. [13] H.Q. Zhang, Q.T. Huang, Y.H. Huang, F.M. Li, W.X. Zhang, C. Wei, J.H. Chen, P.W. Dai, L.Z. Huang, Z.Y. Huang, L.P. Kang, S.R. Hua, A.Y. Hao, Graphitic carbon nitride nanosheets doped graphene oxide for electrochemical simultaneous determination of ascorbic acid, dopamine and uric acid, Electrochim. Acta 142 (2014) 125. [14] J.Q. Tian, Q. Liu, C.J. Ge, Z.C. Xing, A.M. Asiri, A.O. Al-Youbi, X.P. Sun, Ultrathin graphitic carbon nitride nanosheets: a low-cost, green, and highly efficient electrocatalyst toward the reduction of hydrogen peroxide and its glucose biosensing application, Nanoscale 5 (2013) 8921. [15] M. Sadhukhan, S. Barman, Bottom-up fabrication of two-dimensional carbon nitride and highly sensitive electrochemical sensors for mercuric ions, J. Mater. Chem. A 1 (2013) 2752. [16] X.D. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, Enhanced photoresponsive ultrathin graphitic-phase C3N4 nanosheets for bioimaging, J. Am. Chem. Soc. 135 (2013) 18. [17] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P.M. Ajayan, Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Adv. Mater. 25 (2013) 2452. [18] X.J. She, H. Xu, Y. Xu, J. Yan, J. Xia, L. Xu, Y. Song, Y. Jiang, Q. Zhang, H. Li, Exfoliated graphene-like carbon nitride in organic solvents: enhanced photocatalytic activity and highly selective and sensitive sensor for the detection of trace amounts of Cu2+, J. Mater. Chem. A 2 (2014) 2563. [19] H.J. Li, B.W. Sun, L. Sui, D.J. Qian, M. Chen, Preparation of water-dispersible porous g-C3N4 with improved photocatalytic activity by chemical oxidation, Phys. Chem. Chem. Phys. 17 (2015) 3309. [20] Y.J. Zhang, A. Thomas, M. Antonietti, X.C. Wang, Activation of carbon nitride solids by protonation: morphology changes, enhanced ionic conductivity, and photoconduction experiments, J. Am. Chem. Soc. 131 (2008) 50.

[21] P. Niu, L.L. Zhang, G. Liu, H.M. Cheng, Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv. Funct. Mater. 22 (2012) 4763. [22] T.Y. Ma, Y. Tang, S. Dai, S.Z. Qiao, Proton-functionalized two-dimensional graphitic carbon nitride nanosheet: An excellent metal-/label-free biosensing platform, Small 10 (2014) 2382. [23] A.S. Baas, B.C. Berk, Differential activation of mitogen-activated protein kinases by H2O2 and O2 in vascular smooth muscle Cells, Circulation Res. 77 (1995) 29. [24] C.G. Lux, S. Joshi-Barr, T. Nguyen, E. Mahmoud, E. Schopf, N. Fomina, A. Almutairi, Biocompatible polymeric nanoparticles degrade and release cargo in response to biologically relevant levels of hydrogen peroxide, J. Am. Chem. Soc. 134 (2012) 15758. [25] S.C. Cho, Y.Y. Chao, C.Y. Hong, C.H. Kao, The role of hydrogen peroxide in cadmium-inhibited root growth of rice seedlings, Plant Growth Regul. 66 (2012) 27. [26] F. Martin, A. McLean, Comparison of paracetamol-induced hepatotoxicity in the rat in vivo with progression of cell injury in vitro in rat liver slices, Drug Chem. Toxicol. 21 (1998) 477. nchez, F.S. Rojas, C.B. Ojeda, Determination of paracetamol: [27] M.E. Bosch, A.J.R. Sa historical evolution, J. Pharm. Biomed. Anal. 42 (2006) 291. [28] H.S. Yin, K. Shang, X.M. Meng, S.Y. Ai, Voltammetric sensing of paracetamol, dopamine and 4-aminophenol at a glassy carbon electrode coated with gold nanoparticles and an organophillic layered double hydroxide, Microchim. Acta 175 (2011) 39. [29] C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397. [30] B. Delley, From molecules to solids with the approach, J Chem. Phys. 113 (2000) 7756. [31] B. Hammer, L.B. Hanse, J.K. NLrskov, Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals, Phys. Rev. B 59 (1999) 7413. [32] Y. Wang, X. Wang, M. Antonietti, Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: From photochemistry to multipurpose catalysis to sustainable chemistry, Angew. Chem. Int. Ed. 51 (2012) 68. [33] E. Kroke, M. Schwarz, Novel group 14 nitrides, Coord. Chem. Rev. 248 (2004) 493. [34] D.M. Teter, R.J. Hemley, Low-compressibility carbon nitrides, Science 271 (1996) 53. [35] Z.H. Zhang, K. Leinenweber, M. Bauer, L.A.J. Garvie, P.F. McMillan, G.H. Wolf, High-pressure bulk synthesis of crystalline C6N9H3 center dot HCl: A novel C3N4 graphitic derivative, J. Am. Chem. Soc. 123 (2001) 7788. [36] E. Kroke, M. Schwarz, E. Horath-Bordon, P. Kroll, B. Noll, A.D. Norman, Tri-striazine derivatives. Part I. From trichloro-tri-s-triazine to graphitic C3N4 structures, New J. Chem. 26 (2002) 508. [37] J. Sehnert, K. Baerwinkel, J. Senker, Ab Initio calculation of solid-state NMR spectra for different triazine and heptazine based structure proposals of gC3N4, J. Phys. Chem. B 111 (2007) 10671. [38] A. Du, S. Sanvito, S.C. Smith, First-principles prediction of metal-free magnetism and intrinsic half-metallicity in graphitic carbon nitride, Phys. Rev. Lett. 108 (2012) 197207. [39] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Surface area and pore texture of catalysts, Catal. Today 41 (1998) 207. [40] G.H. Dong, L.Z. Zhang, Porous structure dependent photoreactivity of graphitic carbon nitride under visible light, J. Mater. Chem. 22 (2012) 1160. [41] J.L. Zimmerman, R. Williams, V.N. Khabashesku, J.L. Margrave, Synthesis of spherical carbon nitride Nanostructures, Nano Lett. 1 (2001) 731. [42] F. Valentini, A. Amine, S. Orlanducci, M.L. Terranova, G. Palleschi, Carbon Nanotube Purification: Preparation and characterization of carbon nanotube paste electrodes, Anal. Chem. 75 (2003) 5413. [43] M. Pumera, A. Merkoc, S. Alegret, Carbon nanotube-epoxy composites for electrochemical sensing, Sens. Actuators B 113 (2006) 617. [44] Z.Z. Li, X.L. Cui, J.S. Zheng, Q.F. Wang, Y.H. Lin, Effects of microstructure of carbon nanofibers for amperometric detection of hydrogen peroxide, Anal. Chim. Acta 597 (2007) 238. [45] L. Zheng, D. Ye, L. Xiong, J. Xu, K. Tao, Z. Zou, D.L. Huang, X. Kang, S. Yang, Preparation of cobalt-tetraphenylporphyrin/reduced graphene oxide nanocomposite and its application on hydrogen peroxide biosensor, Anal. Chim. Acta 768 (2013) 69. [46] D. Ye, H. Li, G. Liang, J. Luo, X. Zhang, S. Zhang, H. Chen, J. Kong, A threedimensional hybrid of MnO2/graphene/carbon nanotubes based sensor for determination of hydrogen-peroxide in milk, Electrochim. Acta 109 (2013) 195. [47] Q. Xi, X. Chen, D.G. Evans, W. Yang, Gold nanoparticle-embedded porous graphene thin films fabricated via layer-by-layer self-assembly and subsequent thermal annealing for electrochemical sensing, Langmuir 28 (2012) 9885. [48] S.J. Li, J.M. Du, J.P. Zhang, M.J. Zhang, J. Chen, A glassy carbon electrode modified with a film composed of cobalt oxide nanoparticles and graphene for electrochemical sensing of H2O2, Microchim. Acta 181 (2014) 631. [49] K.J. Chen, K.C. Pillai, J. Rick, C.J. Pan, S.H. Wang, C.C. Liu, B.J. Hwang, Bimetallic PtM (M = Pd Ir) nanoparticle decorated multi-walled carbon nanotube enzyme-free, mediator-less amperometric sensor for H2O2, Biosens. Bioelectron. 33 (2012) 120. [50] R.T. Kachoosangi, G.G. Wildgoose, R.G. Compton, Sensitive adsorptive stripping voltammetric determination of paracetamol at multiwalled carbon nanotube

L. Liu et al. / Electrochimica Acta 206 (2016) 259–269

[51]

[52]

[53] [54]

[55]

[56]

modified basal plane pyrolytic graphite electrode, Anal. Chim. Acta 618 (2008) 54. X.H. Kang, J. Wang, H. Wu, J. Liu, I.A. Aksay, Y. Lin, A graphene-based electrochemical sensor for sensitive detection of paracetamol, Talanta 81 (2010) 754. Y. Li, S. Feng, S. Li, Y. Zhang, Y. Zhong, A high effect polymer-free covalent layer by layer self-assemble carboxylated MWCNTs films modified GCE for the detection of paracetamol, Sens. Actuators B 190 (2014) 999. R.N. Goyal, S.P. Singh, Voltammetric determination of paracetamol at C60modified glassy carbon electrode, Electrochim. Acta 51 (2006) 3008. H. Ghadimi, R.M.A. Tehrani, A.S.M. Ali, N. Mohamed, S.A. Ghani, Sensitive voltammetric determination of paracetamol by poly (4-vinylpyridine)/ multiwalled carbon nanotubes modified glassy carbon electrode, Anal. Chim. Acta 765 (2013) 70. T.Y. Huang, C.W. Kung, H.Y. Wei, K.M. Boopathi, C.W. Chu, K.C. Ho, A high performance electrochemical sensor for acetaminophen based on a rGO– PEDOT nanotube composite modified electrode, J. Mater. Chem. A 2 (2014) 7229. X. Liu, X.Y. Zhang, L.L. Wang, Y.Y. Wang, A sensitive electrochemical sensor for paracetamol based on a glassy carbon electrode modified with multiwalled

[57]

[58]

[59]

[60]

[61]

269

carbon nanotubes and dopamine nanospheres functionalized with gold nanoparticles, Microchim. Acta 181 (2014) 1439–1446. M. Arvand, M. Hassannezhad, Magnetic core–shell Fe3O4@SiO2/MWCNT nanocomposite modified carbon paste electrode for amplified electrochemical sensing of uric acid, Mater. Sci. Eng. C 36 (2014) 160. L. Fu, G. Lai, A. Yu, Preparation of b-cyclodextrin functionalized reduced graphene oxide: application for electrochemical determination of paracetamol, RSC Adv. 5 (2015) 76973. Chen, J. Zhu, Q. Xi, W.S. Yang, A high performance electrochemical sensor for acetaminophen based on single-walled carbon nanotube-graphene nanosheet hybrid films, Sens. Actuators B 161 (2012) 648. M.X. Zheng, F. Gao, Q.X. Wang, X.L. Cai, S.L. Jiang, L.Z. Huang, F. Gao, Electrocatalytical oxidation and sensitive determination of acetaminophen on glassy carbon electrode modified with graphene–chitosan composite, Mater. Sci. Eng. C 33 (2013) 1514. A.R. Mao, H.B. Li, D.Q. Jin, L.Y. Yu, X.Y. Hu, Fabrication of electrochemical sensor for paracetamol based on multi-walled carbon nanotubes and chitosan– copper complex by self-assembly technique, Talanta 144 (2015) 252.