Carbohydrates-Derived Nitrogen-Doped Hierarchical Porous Carbon

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 17391−17401

pubs.acs.org/journal/ascecg

Carbohydrates-Derived Nitrogen-Doped Hierarchical Porous Carbon for Ultrasensitive Detection of 4‑Nitrophenol Lingling Hu,† Fei Peng,† Dehua Xia,*,†,‡ Huanjunwa He,† Chun He,*,†,‡ Zekun Fang,† Jingling Yang,† Shuanghong Tian,†,‡ Virender K. Sharma,*,§ and Dong Shu∥ †

School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, China § Department of Environmental and Occupational Health, School of Public Health, Texas A&M University, College Station, Texas 77843, United States ∥ Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China

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S Supporting Information *

ABSTRACT: A facile, cost-effective approach to obtain sensor electrode materials with excellent electrochemical performance for sensitive and fast detection of 4-nitrophenol (4-NP) is of great importance to the environment and human health. Herein, a smart strategy was proposed for fabrication of nitrogen-doped hierarchical porous carbon (NPC) material with large surface area and unique hierarchical porous structure derived from conveniently available carbohydrates via a facile process. NPC combined with chitosan (CTS) was used to modify an indium-tin oxide (ITO) electrode, referred to as a CTS/NPC/ITO electrode, in which CTS acted as dispersant and immobilization reagent. On the basis of the optimum conditions, 4-NP was successfully deposited on a CTS/NPC/ITO electrode and the cathodic deposit of 4-NP showed reversible characteristics in a potential range between −0.22 and −0.00 V as well as high ionic-electronic conductivity. Moreover, the electrochemical reaction kinetics and mechanism of 4-NP were explored in detail by CVs, FTIR spectra, and LC−MS. The response sensitivities of the electrode for 4-NP were obtained as 4.85 μA μM−1, 2.212 μC μM−1, and 0.118 μA μM−1 (RSD ∼ 5%) while detection limits (S/N = 3) were 27.55, 30.10, and 5.44 μM by applying cyclic voltammetry, chronocoulometry, and differential pulse voltammetry, respectively. The results were presented to demonstrate that the CTS/NPC/ITO electrode had excellent reproducibility, repeatability, good stability, and high selectivity for detecting 4-NP in real water samples. KEYWORDS: Cathodic deposit, Electrochemical detection, Nitrogen-doped hierarchical porous carbon, Reaction kinetics, 4-Nitrophenol

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INTRODUCTION

(4-NP) is highly toxic, which can damage the liver, kidney, and human blood as well as the central nervous system.4 Therefore, 4-NP is considered as one of the priority pollutants by United States Environmental Protection Agency, which has set the allowable limit of 60 ppb in drinking water.3,5 It is imperative to develop a simple and a highly sensitive method to detect 4-

Nitrogen-containing aromatic compounds like nitrobenzene, nitrotoluenes, and nitrophenols are widely used in industrial manufacturing, e.g., in the production of pesticides, drugs, and dyes and in the leather production as a fungicide.1,2 These compounds ultimately release to the environment in which they remain for a long period of time and have therefore been found in freshwater and in marine environments.3 Among the numerous nitro-compounds in the environment, 4-nitrophenol © 2018 American Chemical Society

Received: October 8, 2018 Published: October 29, 2018 17391

DOI: 10.1021/acssuschemeng.8b05169 ACS Sustainable Chem. Eng. 2018, 6, 17391−17401

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic for preparation procedure of CTS/NPC/ITO; (b) SEM image, (c) TEM image, (d) XRD pattern, (e) N2 adsorption− desorption isotherms, (f) high-resolution C 1s XPS spectrum, and (g) high-resolution N 1s XPS spectrum of as-prepared NPC material.

NP in water.6 Conventional identification and quantification techniques for determining 4-NP include high-performance liquid chromatography (HPLC),7 liquid chromatography− mass spectrometry (LC−MS),8 gas chromatography−mass spectrometry (GC−MS),9 and fluorescence measurement.10 However, these methods are time-consuming, involve enormous instrumentation, and require complex sample pretreatment steps, thus unsuitable for routine monitoring of 4-NP in water.11 In contrast, the electrochemical methods have a great potential for environmental analysis because they are usually simple, inexpensive, and rapid as well as sufficiently sensitive for a large-scale monitoring.12,13 The electrochemical response

intensity toward 4-NP is extremely dependent on the surface properties and composition of electrode materials.14 Therefore, an approach has been done for designs of new electrochemical sensors capable to provide better surface properties to detect 4NP in terms of selectivity, sensitivity, reliability, low cost, and ease of fabrication and use.15 Carbon materials including porous carbon and nanocarbons such as carbon nanotubes, nanofibers, graphene, etc., have been widely used as electrode materials for diverse application in fields of energy conversion devices (e.g., solar cells, fuel cells, supercapacitors, and batteries) and sensor fabrication.16−18 By far, the properties of nanocarbons are well-documented; 17392


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superhydrophilic adsorbent, which had ultrastrong adsorption capacities for water-soluble contaminants.38 To our knowledge, in our study, NPC derived from pomelo peel was first used as electrode materials to fabricate an electrochemical sensor for detecting contaminants. In this study, our strategy is to develop a facile and sustainable approach to synthesize N-doped porous carbons (NPC) derived from pomelo peel with abundant active sites, excellent electron and reactant transfer rate, thus fabricating an ultrasensitive electrochemical sensor for 4-NP detection. Chitosan (CTS) is a biocompatible polymer, which contains a large number of amino and hydroxyl groups,39 and under specific conditions, it is able to form highly water swellable hydrogels and have the excellent membrane forming ability.40 For these reasons, CTS could increase the bonding effect between NPC and ITO electrode, subsequently leading to decrease electron transfer resistance and increase response current. This paper thus proposes combining NPC with CTS to modify an ITO electrode, called a CTS/NPC/ITO electrode, for electrochemical detection of 4-NP in water samples. The novelty of the CTS/NPC/ITO electrode as a sensor was demonstrated herein by (i) preparing and characterizing NPC and CTS/NPC/ITO electrode; (ii) investigating the voltammetric behavior, electrochemical reaction kinetics, and mechanism of 4-NP on the CTS/NPC/ITO electrode; (iii) examining the reproducibility, stability, and sensitivity of the CTS/NPC/ITO electrode for detecting 4-NP using three electrochemical techniques, including cyclic voltammetry (CV), chronocoulometry, and differential pulse voltammetry (DPV); (iv) establishing the feasibility of the developed sensor to detect 4-NP in real water samples.

however, porous carbon, especially activated porous carbon, remains the first choice because of the merits of low cost and easy preparation.19,20 Large surface area and unique pore structure of porous carbon are two key parameters for an ideal electrode to develop electrochemical sensors and biosensors, since they can provide plenty of active sites and help reactant/ product transfer.21 For example, Fu et al. have successfully synthesized a graphitic mesoporous carbon membrane with dimensionally networked nanotunnels and applied as monolithic matrix for electrochemical sensing.22 Silva et al. reported that a porous diamond-like carbon (DLC) electrode was highly sensitive and presented low limits for sensing hormones, neurotransmitters, and endocrine disruptors.23 Consequently, a wide range of porous carbon synthesis routes have been developed, including using a metallic compound as a template, employing expensive zeolites or silica oxides as a sacrificial scaffold, carbonizing polypyrrole microsheets, and cross-linking of terephthalonitrile monomers to produce a carbon precursor.24−28 However, the uneconomic carbon sources, templates, and complicated preparation processes used for mass production of porous carbon materials are highly challenging. Carbohydrates, common natural products, which consist of carbon, oxygen, and hydrogen, are largely synthesized by plants through photosynthesis (billion tons per year). They are abundant, environmentally benign, and easily accessible. Converting these carbohydrates to high valueadded functional carbon materials seems to be a sensible approach for environmental sustainability.29,30 Pomelo peel is a sustainable, inexpensive, and abundant resource supplied by nature. Usually, it is considered as agricultural wastes and often directly discarded in garbage dumps in south China, which is harmful to the environment. In fact, pomelo peel contains a large quantity of carbohydrates, which is usually considered as an ideal carbon precursor to produce porous carbon. Futhermore, it could also provide an excellent platform for further optimizing the structure and properties of derived carbon since the middle white layer of pomelo peel possesses superior absorption capacity toward organic and inorganic solutions due to its sponge-like structure.30,31 Therefore, pomelo peel could be optimized to offer a good way for facile, sustainable, and low-cost development of metal-free porous carbon electrode materials. Recently, it is reported that doping of heteroatoms (e.g., nitrogen, sulfur, phosphorus) into carbon materials is an efficient method to improving their chemical activity and electronic property.24,32−34 Among these heteroatoms, nitrogen is considered as a peerless dopant since it could significantly improve the surface wettability and conductivity of carbon materials, thus enhancing electrochemical performances and possessing long-term stability.30,35−37 To date, some researchers have successfully synthesized nitrogen-doped carbon materials using pomelo peel as carbon precursor. The nitrogen-doped carbon materials possessed many advantages including the excellent chemical stability, high surface area, versatile structures, high conductivity, and relatively low cost. Yuan et al. displayed a nitrogen-doped nanoporous carbon derived from waste pomelo peel, which showed excellent performance for the oxygen reduction reaction.21 Qu et al. and Xiao et al. reported the nitrogen-doped carbon modified electrodes derived from pomelo peel possessed a remarkably large capacitance with good rate capability and excellent longterm cycling stability.30,35 Li et al. found that N-doped carbon microspheres (NCMSs) derived from pomelo peel were a

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RESULTS AND DISCUSSION Physicochemical Properties of NPC. The synthetic NPC was derived from pomelo peel precursor through hydrothermal and activation processes. Figure 1a shows a diagrammatic drawing to expound the synthetic process (see Text S1 for preparation details). Pomelo peel mainly consists of abundant small cellulose fibers to form a sponge-like structure (Figure S1). Due to its sponge-like structure, pomelo peel has a strong adsorption capacity, which could favor to adsorb melamine. Thus, it is believed that nitrogen-doped porous carbon material could be obtained from carbonization of pomelo peel after adsorbing melamine. Figure 1b showed that NPC consisted of a large number of ultrafine spherical carbon particles, which were connected to each other to form a porous structure. The TEM image in Figure 1c also confirmed porous carbon frameworks of NPC. The unique porous structures were beneficial to increase the surface area of NPC material, thus providing more active sites to effectively improve the electrochemical activity toward 4-NP. The NPC was further studied by XRD and Raman spectroscopy. As shown in Figure 1d, the XRD pattern of NPC exhibited two broad peaks at 2θ of 24.5° and 43.5°, which could be ascribed to the (002) and (100) index planes of amorphous carbon.31 The Raman spectra (Figure S2) showed two separate characteristic bands of the D-band peak at 1350 cm−1 and G-band peak at 1600 cm−1. The D/G intensity ratio intensity was approximating to 1, which further indicated the amorphous carbon in NPC in agreement with the Raman results of honeycomb-like porous carbon.19 The BET surface area and pore structure of NPC samples were 17393


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Figure 2. (a) CVs for ITO, PC/ITO, CTS/PC/ITO, and CTS/NPC/ITO in 0.4 mM 4-NP/50 mM NaCl solution; (b) CVs for ITO and CTS/ NPC/ITO in 0.05 mM K3[Fe(CN)6]/K4[Fe(CN)6]; (c) CVs for ITO and (d) CVs for CTS/NPC/ITO electrode in 0.05 mM K3[Fe(CN)6]/ K4[Fe(CN)6] at different scan rates (v = 0.02−0.4 V s−1). (The inset indicated the relation of anodic peak current with the square root of scan rate.)

Electrochemical Properties of CTS/NPC/ITO Electrodes. The CVs of bare electrode, CTS/PC/ITO electrode, and CTS/NPC/ITO electrode at the scan rate of 0.1 V s−1 in 50 mM NaCl were investigated. Compared with CV spectra of the bare electrode and CTS/PC/ITO electrode (Figure S3), the CTS/NPC/ITO electrode showed the excellent electrochemical activity toward 4-NP. The electrochemical properties of the CTS/NPC/ITO electrode were further investigated in 0.4 mM 4-NP/50 mM NaCl solution at pH 7.0 by CVs shown in Figure 2a. The response current of the PC/ITO electrode increased significantly compared to the bare ITO. When CTS was introduced, the voltammetric response of the CTS/PC/ ITO electrode was further slightly increased. CTS was a dispersant and immobilization reagent, which can decrease electron transfer resistance and then increase the response current. Compared with the former three electrodes, the voltammetric response of CTS/NPC/ITO increased significantly, indicating that NPC possessed an excellent electrochemical activity toward 4-NP. The extra performance was attributed to the cooperative synergistic effect, including good accessibility to reactant benefiting from the hierarchical porous structure as well as highly active sites deriving from nitrogen doping.21 Next, the electrochemical reversible behavior of electrodes was evaluated by performing CVs of ITO and CTS/NPC/ITO electrodes in 0.05 mM K3[Fe(CN)6] and K4[Fe(CN)6] solutions containing 50 mM NaCl as the supporting electrolyte. The 1st to 20th CVs of ITO and CTS/NPC/ ITO electrodes for Fe3+/Fe2+ redox pair were observed (Figure

determined using N2 adsorption−desorption isotherms. Figure 1e shows the curve exhibited a type-I sorption isotherm with steep uptakes below P/P0 = 0.05, confirming the presence of abundant micropores. In addition, the obscure hysteresis loop indicated the existence of mesopores and macropores, confirming the formation of a hierarchical porous structure in NPC with pore size distribution entered at 1.8 and 9 nm. NPC also possessed high BET surface area (1071 m2 g−1) and high pore volume (0.60 cm3 g−1). NPC with hierarchical porous structure, high surface area, and high pore volume would provide a favorable path for electrolyte penetration and transportation.35,36,41 Thus, it is expected that NPC as an electrode material could show a high electrochemical property for electrochemical sensing of 4-NP. The chemical state in the carbonaceous matrixes was studied by XPS analysis. The high-resolution C 1s XPS spectrum displayed four peaks at 284.7, 285.4, 286.5, and 289.1 eV for NPC, which were referred to as C−C, C−N, C−O, and O− CO, respectively (Figure 1f).19,21 Similarly, the highresolution N 1s spectra (Figure 1g) can be fitted into four peaks located at 398.6, 400.3, 401.3, and 404.3 eV, attributed to pyridinic-N, pyrrolic-N, graphitic-N, and oxidic-N, respectively.21,24 The N binding configuration was mainly in the form of pyridinic-N, pyrrolic-N, graphitic-N, which have been reported as active sites for electrochemical reaction, thus enhancing electrochemical properties of porous carbon.42 Furthermore, these oxygen and nitrogen functional groups could also improve the wettability of the NPC-based electrodes in the aqueous electrolytes.19 17394


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ACS Sustainable Chemistry & Engineering 2b). A pair of well-defined redox peaks can be seen at the ITO electrode with a peak-to-peak separation (ΔEp) of 140 mV, and the ratio between anodic and cathodic peak currents (Ipa/ Ipc) was ∼1. When the electrode was modified with CTS/NPC materials, the peak currents of redox peaks increased by 8% compared to that of ITO electrode. Moreover, the decrease of ΔEp (72 mV) was observed at the CTS/NPC/ITO electrode, indicating that the CTS/NPC/ITO electrode could improve the reversibility of the redox reaction. The effect of scan rates (v) was examined using the Fe3+/ Fe2+ redox pair (Figure 2c,d and Figure S4). The electrochemical active surface area A (cm2) for each electrode was estimated using the Randles−Sevcik equation43 Ip = 2.69 × 105n3/2AD1/2ν1/2c

(1)

where Ip is the anodic or cathodic current peak, n is the number of exchanged electrons, D is the diffusion coefficient (6.39 × 10−6 cm2 s−1), and C is the concentration of electroactive species (mol cm−3). The slope values of the plots of Ipa vs v1/2 gave the effective electro-active areas for ITO, CTS/ PC/ITO and CTS/NPC/ITO electrodes as 0.0437, 0.0619, and 0.0701 cm2, respectively. The result indicated that high surface area and nitrogen doping of NPC were beneficial to increase the electrochemical active surface area. The electrochemical impedance spectroscopy (EIS) was further conducted to study the interface properties of electrodes. In general, the diameter of the semicircle part at high frequency was equivalent to charge transfer resistance (RCT), depicting the charge transfer rate through the electrode/electrolyte interface.44 As displayed in Figure S5, the CTS/NPC/ITO electrode exhibited a smaller semicircle diameter than that of the CTS/PC/ITO electrode, indicating that nitrogen doping has a positive effect on carrier transfer efficiency and electronic conductivity of the electrode. On the basis of these results, CTS/NPC material coating on an ITO electrode was able to optimize the properties of electrodes successfully, confirmed by the increase in electroactive area, carrier transfer efficiency, and electronic conductivity as well as improvement in reversibility of the redox reactions. Therefore, CTS/NPC could provide a good opportunity for the modified electrode to detect 4-NP with high selectivity. Voltammetric Behavior of 4-NP on CTS/NPC/ITO Electrode. Initially, the CVs on the CTS/NPC/ITO electrode were obtained in 50 mM NaCl electrolyte solution with and without 0.4 mM 4-NP (Figure S6). No redox signals were observed without 4-NP, suggesting that the CTS/NPC/ITO was an electrochemically inactive electrode in the selected potential range. Figure 3a shows multiple CVs on the CTS/ NPC/ITO electrode in 0.4 mM 4-NP when applying voltage ranging between 0.6 V and −1.4 V at a scan rate of 0.1 V s−1. A distinct reduction peak appeared at −1.182 V (peak I) in the first CV sweep. The reduction peak was attributed to the reduction of 4-NP.45 Following the 10th of multiple CVs, a pair of sharp redox peaks emerged in the potential region along with the new cathodic peak (−0.273 V, peak II) and the anodic peak (−0.025 V, peak III). With successive potential scanning, the reduction peak (peak I) showed a gradual decrease and remained unchanged thereafter. The currents of the redox peaks continuously increased until a maximum was attained, indicating the formation of a cathodic deposition layer. Interestingly, when potential scan range was set between 0.6

Figure 3. CVs for CTS/NPC/ITO electrode in 0.4 mM 4-NP/50 mM NaCl solution at different scan potential ranges of (a) −1.4 to 0.6 V, (b) −1.2 to 0.6 V, and (c) −0.8 to 0.6 V (v = 0.1 V s−1).

and −1.2 V (Figure 3b), the peak II and peak III became smaller than the voltammograms under the potential scan up to −1.4 V. Next, the potential scan range was set between 0.6 and −0.8 V in order to elucidate the origin of the redox pair. Under this voltage range, reduction of the nitryl group did not occur. As shown in Figure 3c, the redox peaks even disappeared in multiple CVs, revealing that the formation of peak I, namely, the reduction of 4-NP, was responsible for the appearance of the reversible redox couple. Notably, the response current of peak I was much higher than that of peaks II and III, further suggesting that the cathodic electroreduction of 4-NP likely corresponded to a reaction possessing high electron transfer number. Kinetic Studies of 4-NP on CTS/NPC/ITO Electrode. The CVs at different potential scan rates were carried out to 17395


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The regression equation could be written as Ip.a (μA) = 52.639 v + 3.252. The number of electrons involved in the redox reaction at the CTS/NPC/ITO electrode was ascertained by eq 248

learn details of peak current and peak potential. The CVs on the CTS/NPC/ITO electrode with different potential scan rates in 0.4 mM 4-NP are shown in Figure 4a. The reduction

n = 0.0565(Ep − Ep/2)−1

(2)

where Ep was the peak potential and Ep/2 was the half peak potential. The calculated value of n was ∼ 1 (peak III of Figure 4b was taken as an example to determine n). This suggested that one electron was involved in the redox process of peaks II and III. The surface coverage for cathodic deposits of 4-NP on the electrode surfaces could be estimated from the slope of Ip.a vs v and using eq 346 n 2F 2 ·vA Γ∞ 4RT

Ip.a =

(3)

where A is the effective area of the electrode (calculated from eq 1) and F is Faraday’s constant (95 485 C mol−1). The calculation result showed that the surface coverage for cathodic deposits of 4-NP was 8.01 × 10−11 mol cm−2. With the increase of the scan rate, the potential of the reduction peak (peak II) shifted negatively and the potential of the oxidation peak (peak III) shifted positively. The Ep.a varied linearly with ln v and the regression equation was Ep.a = 0.03548 ln v + 0.03575. According to the Laviron’s theory, the relationship between Ep.a and scan rate can be expressed by eq 447 Ep.a = E o +

RT RT ijj αnF yzz ln v + lnj z αnF αnF jjk RTks zz{

(4)

where E is the formal standard potential, α is the charge transfer coefficient, and ks is the standard heterogeneous reaction rate constant (s−l). The linear fitting of the data gave a slope of 0.035, from which an estimated value of α was obtained as 0.778 by considering one electron was involved in the redox reaction. From the intercept, the ks value of 0.36 s−1 was obtained using E0 = −0.149 V (vs SCE at pH 7.0). This result indicated a reasonably fast electron transfer between the cathodic deposits of 4-NP and the CTS/NPC/ITO electrode. The ionic-electronic conductivity of cathodic deposits of 4NP was studied by EIS. Figure 5 shows the Nyquist plots of the CTS/NPC/ITO electrode with cathodic 4-NP deposits in comparison with the CTS/NPC/ITO electrode at open circuit o

Figure 4. (a) CVs of 4-NP and (b) CVs of cathodic deposits of 4-NP on CTS/NPC/ITO electrode at different scan rates (v = 0.02−0.4 V s−1). (The insets are the relation of peak currents and peak potentials with scan rate.)

peak current increased with the increase in scan rate. Significantly, the current was proportional to the square root of scan rate in the range of 0.02−0.4 V s−1, indicating that the cathodic reduction of 4-NP on the CTS/NPC/ITO electrode was a diffusion-controlled process. The peak potential (Ep.c) of peak I shifted negatively with increase in the scan rate. There was a linear relationship between Ep.c and ln v in the studied range, and the regression equation was Ep.c = −0.0312 ln v − 1.0657 (r2 = 0.99). According to Laviron’s theory,46 RT/αnF was calculated as 0.0312. On the basis of the assumption that the charge transfer coefficient α was 0.5 for a totally irreversible electrode reaction process,47 the calculated number of electrons was ∼2.0, which suggests that two electrons were involved in the reduction of 4-NP. The influence of the scan rate on the redox peak current and potential was also investigated, and the kinetic parameters including charge transfer coefficient (α), number of electrons involved (n), surface coverage (Γ), and reaction rate constant Ks could be experimentally calculated. The cathodic deposit of 4-NP, obtained by CVs of 20 cycles, showed pairs of well-defined redox peaks in 0.4 mM 4-NP/50 mM NaCl solution (Figure 4b). The cathodic peak heights were close to anodic peaks for all the scan rates and simultaneously showed a linear increase as a function of the scan rate from 0.02 to 0.3 V s−1 (inset in Figure 4b). This suggests that the redox process was a surface-confined process.

Figure 5. EIS Nyquist plots of CTS/NPC/ITO electrode and CTS/ NPC/ITO electrode with cathodic 4-NP deposits in 50 mM NaCl under open circuit conditions. 17396


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Figure 6. LC-MS spectra of (a) 4-NP and (b) the reaction products extracted from the modified electrodes for 10 times. (c) Reaction process of 4NP on CTS/NPC/ITO.

Electrochemical Reaction Mechanism of 4-NP on CTS/NPC/ITO. The number of electrons involved in the electrochemical reaction of 4-NP was carried out in Figure 4a,b. It was concluded that two electrons were involved in the reduction of 4-NP and one electron was involved in the redox process. In order to identify electrochemical reaction products of 4-NP, the reaction products extracted from the modified electrodes for 10 times were measured by FTIR spectra to examine the characteristic bands. As shown in Figure S7, the FTIR spectra revealed CO stretching vibration at 1638 cm−1, NN stretching vibration at 1480 cm−1, and C-O-C vibration of aromatic series at 1312 cm−1, indicating that the reaction products of 4-NP contained the functional groups of CO, NN, C-O-C. Furthermore, the electrochemical reaction products of 4-NP, obtained by chloroform extraction from the CTS/NPC/ITO after CVs of 500 cycles in 0.4 mM 4-NP solution with potential scan range setting between 0.6 and −1.4 V, were analyzed by LC−MS. As shown in Figure 6a, 4-NP standard solution showed a peak at m/z 138. Compared with 4-NP standard solution, three new products with m/z values 276.9, 485.5, and 623.9 were found (Figure 6b). According to the intensity of the three peaks, the substance

potential (Eocp). Randles equivalent circuit, electrolyte solution resistance (RS), charge transfer resistance (RCT), and double layer capacitance (Cd), were used to fit experimental data. The calculated parameters of the simulated equivalent circuit are listed in Table S1. The calculated value of RCT on the CTS/ NPC/ITO electrode from the EIS measurements was 6780 Ω, whereas RCT on the CTS/NPC/ITO electrode with cathodic 4-NP deposits was 397 Ω. Such lower charge transfer resistance on the CTS/NPC/ITO electrode revealed that the cathodic deposits of 4-NP possessed mixed ionic and electronic conductivity and hence acted as ion-to-electron transducers.49 According to the discussion above, the cathodic deposited layer of 4-NP could be successfully coated on the CTS/NPC/ ITO electrode by multiple cyclic voltammetry and the deposited layer showed reversible characteristics in the low potential range and also had high ionic-electronic conductivity. Moreover, the electron transfer rate for the redox reaction of cathodic 4-NP deposits was fast. Thus, it was reasonable to conclude that the redox current response on the CTS/NPC/ ITO electrode could easily be used to detect 4-NP with high sensitivity and selectivity. 17397


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ACS Sustainable Chemistry & Engineering with m/z value 485.5 (tetramer) was the main electrochemical reaction product. On the basis of LC−MS analysis, two electrons involved in 4-NP reduction and one electron involved in the redox process, the scheme for electrochemical reaction of 4-NP could be speculated, and the proposed pathway was plotted as shown in Figure 6c. First, along with the processes of two-electron transfer, the large reduction peak (I) was attributed to the reduction of 4-NP to the corresponding nitrosophenol or benzoquinone monoxime. The intermediate products immediately reacted with each other to form a dimer with one benzoquinonyl through polymerization reaction, and the process had no electron transfer. As the functional group O-NN-O in the dimer was a strong electron acceptor, the O−H bond of the benzene ring was easily oxidized at the active site of the modified electrode to benzoquinonyl, which was a reversible reaction with one electron transferring. Finally, the dimer with double benzoquinonyl (m/z = 276.9) can transform to a tetramer (m/z = 485.5) because of the unstable functional group ONN-O. Electrochemical Determination of 4-NP on CTS/NPC/ ITO Electrode. Figure S8 shows a group of CVs, recorded in different concentrations of 4-NP solutions to develop a relationship between the current responses and the concentrations of 4-NP. Under the optimum conditions, the anodic peak current of peak III on the 5th CVs was linearly proportional to the concentration of 4-NP in the range of 0.05−0.4 mM (r2 = 0.99). The average response sensitivity of three replicates was 4.85 μA μM−1 with the relative standard deviation (RSD) of 4.6%. The limit of detection (LOD, S/N = 3) was estimated as 27.55 μM. The chronocoulometry method was also employed to directly detect 4-NP. The transient current as a function of time was recorded (Figure S9). The chronocoulometric response to 4-NP concentration was linear in the concentration range from 0.02 to 0.1 mM (r2 = 0.99). The average response sensitivity of three replicates was 2.212 μC μM−1 with an RSD of 6.3%. The LOD (S/N = 3) was calculated as 30.10 μM. Finally, the DPV method was carried out using different concentrations of 4-NP solutions (Figure 7). The Ip.c was linear with the concentration of 4-NP in the range of 0.002−0.4 mM. The response sensitivity of 4-NP was 0.118 μA μM−1 and the

RSD was 3.8% (n = 3). The LOD (S/N = 3) was estimated as 5.44 μM. The comparison of three electrochemical detection methods is presented in Table S2. The DPV technique gave the best results in view of LOD (5.44 μM), linear response range (0.002−0.4 mM), and RSD (3.8%). However, in terms of the sensitivity, the cyclic voltammetry technique (4.85 μA μM−1) showed a better result than the DPV technique (0.118 μA μM−1). Thus, selection parameters would determine which of the three methods may be applied to determine the concentration of 4-NP. Overall, results established successful fabrication of the CTS/NPC/ITO electrode to detect 4-NP. Furthermore, the analytical performance of the present CTS/NPC/ITO-based sensor for 4-NP sensing was compared with those of other reported sensors in terms of linear range and LOD. As shown in Table S3, a well-linear range and LOD were attained without employing any nanomaterials or tedious electrode modification process to fabricate a CTS/NPC/ITObased sensor, demonstrating CTS/NPC/ITO could be a promising sensor electrode for electrochemical determination of 4-NP. Durability, Stability, and Selectivity of CTS/NPC/ITO Electrode. The reproducibility of repeated electrodes was estimated by comparing the oxidation peak current of the cathodic deposit of 0.4 mM 4-NP. The RSD was 5.5% for 10 repeated electrode preparations. Repetitive measurements for the same electrode were carried out in 0.4 mM 4-NP solution to evaluate the reusability of the CTS/NPC/ITO electrode. Between the repeated measurements, the used electrode was washed by dipping in acetone for 10 min to remove the reaction products of 4-NP, followed by thoroughly washing using ethanol and deionized water. Then CVs on the treated electrodes after the first cycle to the sixth cycle were performed in 50 mM NaCl solution as shown in Figure S10. It was found that no peak signals were observed in CVs, suggesting that the reaction products of 4-NP were completely removed from the CTS/NPC/ITO electrode. As shown in Figure S11a, there was no significant decrease (6%) in the current response until seven repeated measurements, revealing a good reusability of the CTS/NPC/ITO electrode. The long-term stability of the CTS/NPC/ITO electrode was investigated by measuring the current response of 0.4 mM 4-NP among 40 days using the DPV technique. As shown in Figure S11b, during the first 20 days, the CTS/NPC/ITO electrode exhibited good stability and the current response maintained 95.1% of its original response. Afterward, 85.4% of the original current response was retained after 40 days. Moreover, it was found that the morphology of the CTS/NPC/ITO electrode was unchanged after 200 CV sweeps in the potential range between 0.6 and −1.4 V, according to the corresponding SEM images (Figure S12). These results suggested that the CTS/NPC/ITO electrode displayed a relatively long-term stability. The selectivity of CTS/NPC/ITO on the determination of 4-NP was also studied (the detail is given in Text S2 and Table S4). It was concluded that the inorganic ions and organic compounds, which possibly present in real water samples, had no significant influence on the electrochemical response signals of 4-NP. Real Water Samples Detection. The detection reliability of the fabricated electrode was examined by measuring 4-NP in Guangzhou Zhujiang River water and tap water with the standard addition method. The contrast between the HPLC and our method (DPV) in water sample detection was

Figure 7. DPV for CTS/NPC/ITO electrode under different 4-NP concentrations. The inset is the relation of anodic peak current with 4-NP concentration. 17398


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ACS Sustainable Chemistry & Engineering Table 1. Determination Results of 4-NP in Water Samples and Recovery Value this method

HPLC method

samples

add. (μM)

detection (μM)

recovery (%)

RSD (%)

detection (μM)

recovery (%)

RSD (%)

river water 1 river water 2 river water 3 tap water 1 tap water 2 tap water 3

40 60 120 40 60 120

37.8 58.4 122.9 38.6 59.3 120.8

94.6 97.3 102.4 96.5 98.9 100.7

3.25 2.30 2.75 3.75 2.83 2.08

38.7 61.1 121.8 39.2 59.6 120.5

96.75 101.8 101.5 98.0 99.3 100.4

3.51 3.02 2.36 3.59 3.25 3.15

investigated, and the obtained results are shown in Table 1. The recoveries of this work were ranged from 94.6% to 102.4% which was in good accordance with the range obtained by the HPLC method. Each of the samples was measured for six times (RDS below 4%), and the average values were adopted, suggesting that the CTS/NPC modified electrode was effective and feasible. Thus, the modified electrode can be successfully used for the determination of 4-NP in practical samples.

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CONCLUSIONS We herein reported a facile, cost-effective approach to prepare nitrogen-doped hierarchical porous carbon (NPC) as electrochemical sensor materials. The as-obtained NPC material exhibited a unique hierarchical porous structure with abundant micro/meso/macropores and high surface area (up to 1071 m2 g−1), favor to the performance of electrochemical sensor. The modified electrode, CTS/NPC/ITO, exhibited a high sensitivity and selectivity toward 4-NP. The cathodic deposits layer of 4-NP on the modified electrode formed by multiple sweep voltammetry showed well-defined redox signals in the low potential range and had high ionic-electronic conductivity. Moreover, the electrochemical reaction kinetics and mechanism of 4-NP were explored by CVs, FTIR spectra, and LC− MS. Under the optimized conditions of three electroanalytical methods, the DPV technique had the widest linear range and lowest detection limit to detect 4-NP. The oxidation peak current in the DPV method varied linearly in the range from 0.02 to 0.4 mM while the response sensitivity and detection limit were 0.118 μA μM−1 (RSD = 3.8%) and 5.44 μM (S/N = 3), respectively. Comparatively, the CV technique had higher sensitivity to detect 4-NP than that of the DPV method. The anodic peak current in CV varied linearly in the range from 0.05 to 0.4 mM while the response sensitivity and detection limit were 4.85 μA μM−1 (RSD = 4.6%) and 27.55 μM (S/N = 3), respectively. The modified sensor was able to detect 4-NP in real water samples with good precision. The proposed sensor demonstrated several advantages including easy fabrication, high selectivity, good reproducibility, high stability, and low cost.

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Raman spectra of NPC, CVs and EIS on different electrodes, CVs at different scan rate, CVs with and without 4-NP, FTIR of 4-NP reaction products, 4-NP detection using CV and chronocoulometry methods, CVs on the treated CTS/NPC/ITO electrode after different cycles, repetitive measurements and long-term current response, SEM images of CTS/NPC/ITO electrode after 200 CV sweeps, DPV of 4-NP in the presence of interferents (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.X.). *E-mail: [email protected] (C.H.). *E-mail: [email protected] (V.K.S.). ORCID

Chun He: 0000-0002-3875-5631 Virender K. Sharma: 0000-0002-5980-8675 Dong Shu: 0000-0001-6915-6714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the National Natural Science Foundation of China (Nos. 51578556, 21876212, 21673086, 41603097), the Natural Science Foundation of Guangdong Province (Nos. 2015A030308005, S2013010012927, S2011010003416), Science and Technology Research Programs of Guangdong Province (No. 2014A020216009) for financially supporting this work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05169. Additional details on texts showing experimental section and selectivity of CTS/NPC/ITO electrode, table showing fitting values based on equivalent circuit, tables highlighting the superior performance and high selectivity of CTS/NPC/ITO electrode toward 4-NP sensing, optical microscopy image of pomelo peel, 17399


Research Article

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Carbohydrates-Derived Nitrogen-Doped Hierarchical Porous Carbon

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