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Article Cite This: ACS Omega 2019, 4, 1191−1200
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Bayberry-Kernel-Derived Wormlike Micro/Mesoporous Carbon Decorated with Human Blood Vessel-Like Structures and Active Nitrogen Sites as Highly Sensitive Electrochemical Sensors for Efficient Lead-Ion Detection Kunquan Li,* Zeqing Wan, Jiamin Liu, and Gulnaz Guliyeva
ACS Omega 2019.4:1191-1200. Downloaded from pubs.acs.org by 79.110.19.207 on 01/16/19. For personal use only.
College of Engineering, Nanjing Agricultural University, Nanjing 210031, China ABSTRACT: The biomass-derived nitrogen-doped wormlike micro-/mesoporous carbon (BNWMC) was synthesized for highly sensitive lead-ion detection. The influence of carbonization temperature (CT) from 500 to 900 °C on the transformation of nitrogen/carbon-bonding configurations, pore structures, and electrochemical properties was investigated. The results showed that the BNWMCs have been equipped with active nitrogen/carbon bonding functionalities and unique human blood vessel-like structures, which are more pronounced with increasing CTs for a more effective generation of new criss-crossing micro/mesopores and high active quaternary-N, graphitic-C, and pyridinic-N sites on the C−C/C−N skeletons. The cyclic voltammetry response of the BNWMC-modified glassy carbon electrode increased with CT, further indicating that the unique physicochemical characteristics are crucial for improving electrochemical sensitivity for their possessing shorter path interpenetrating micro/ mesopores and more active C/N-active sites. An excellent linear relationship (R2 = 0.995) between peak currents and lead concentrations in a broad range of 10−800 mg L−1 was achieved, indicating that the BNWMC is a promising electrochemical modifier for developing lead detection sensor.
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INTRODUCTION Lead is a widely used material at present in the industrial and agricultural production. Inevitably, some lead ions would be released into the environment, ecosystem, and the food chain during producing, transporting, and using and thus harm food safety and human health. Even at lower concentrations, lead may damage the brain and nervous system and thus decrease intelligence, creating behavioral problems and slowing a child’s growth because it is a type of very toxic heavy metal.1 Thus, it is becoming an important hot spot in recent research to develop a sensitive and selective sensor to detect lead ion in the environment.2 Many technologies have been explored to analysis lead ions such as X-ray fluorescence spectrometry, atomic absorption spectroscopy, inductively coupled plasma, electrochemical detection, and so on.3,4 Among these technologies, electrochemical technique is the most widely adopted technology because of its low energy requirement, high sensitivity, short time consumption, and operation simplicity.5,6 The functionalization of electrodes is an effective way to improve sensitivity of analytic detection for metal ions by the electrochemical technique.7,8 As sustainable metal-free materials, multiple categories of porous nanocarbons including activated carbon, carbon nanotube, and mesoporous carbon (MC) have been employed to modify electrodes including the © 2019 American Chemical Society
glassy carbon electrode (GCE) for the determination of heavy metals.2,3,5,9 Recently, MC has been becoming a promising modifier for an electrochemical sensor because of its big surface area, low resistance, good conductivity, high mechanical stability, large potential window, and long-term oxidation resistance. To achieve the trace targets including heavy metals and to enhance the electrochemical-sensing accuracy of MCmodified electrode, surface chemical functionality modification by nitrogen, sulfur, boron, phosphorus, and so forth is strongly proposed because these heteroatoms can facilitate proton and electron transfer and improve the efficiency of charge transfer at the interface between MC-modified electrode and electrolyte through adjusting the electronic structure.6,8,10 Among the heteroatoms, nitrogen is the most common and effective element to improve the electrocatalytic activity of carbon electrode for oxygen reduction reaction (ORR) or sensors because the inserted nitrogen into carbon lattice can not only modify the surface chemical composition but also improve the electronic property of MC.11−13 As is known, the introduction of nitrogen on MC can be obtained through post-treatment or in situ pyrolysis preparaReceived: October 26, 2018 Accepted: January 3, 2019 Published: January 14, 2019 1191
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prepared at different temperatures were compared, and the role of N-bonding configurations including pyridinic-N, pyrrolic-N, and quaternary-N was also investigated. Moreover, the optimum conditions for lead detection on the NWMC-900 modified electrode were explored.
tion using nitrogen-containing modifiers. The in situ preparation is a more preferred approach for its simple craft, economical energy, and free from pollution, whereas the posttreatment is often involved in high-energy consumption, highpollution process, low nitrogen, weak stability, and nonhomogeneous-functionalized surface.14 Nitrogen-containing precursors including melamine, urea, ammonium, and polypyrrole have been employed as modifiers for nitrogen introduction on MC by in situ preparation. The possible introduced N-bonding configurations on in situ prepared MC often include amides, lactames, pyrrolic, pyridinic, and quaternary nitrogen groups, which might play a different role in electrochemical processes for the difference of electron density and structure of nitrogen in a carbon framework. Except for chemistry, the characteristic of porous structure is the other important factor that might influence electrochemical performance of carbon-modified electrode. For example, both pore size distribution and pore pathway may exert important effects on electrical transmission.15−18 It has been reported that abundant mesopores with suited size are good for mass transfer and ion diffusion.19−22 The structures of wormlike MC with rich inserted micropores on mesoporous wall are as similar to “human blood vessel networks”. The unique structures have many interpenetrating micro/mesopores, which can provide many shorter pathways as rapid electronic or adsorbate transmission channels for ion adsorption or chemical modification. Moreover, the abundant interpenetrating micro/mesoporous structures can effectively prevent blocking of adsorbates or modifiers and provide carbon-based composites with large specific surface area, more active sites, and fast electron transport kinetics. On the basis of the above analysis, we speculated that N-doped wormlike MC (NWMC) with rich micropores could be an excellent electrode candidate in electrochemical sensing applications. On the other hand, in situ pyrolysis method and preparation parameters, especially the pyrolysis temperature, have been proved to greatly influence the types of nitrogen functionality structures, amounts of different nitrogen-bonding configurations, and pore properties of MC,23 thus influencing its electrochemical properties of ORR,24,25 and its effect of electrochemical sensing has not been reported so far. Moreover, it is still a complicated problem to identify which active sites play the main role in nitrogen-doped MC. In addition, the bayberry kernel (BK), a waste biomass precursor, has been successfully synthesized to ordered MCs for heavy metal removal in our recent research because of their nontoxicity, low cost, and excellent chemical stability.26 To the best of our knowledge, no research has been reported on the electrochemical sensing detection application of NWMC with the before-mentioned interesting human blood vessel-like network structures. Hence, in this work, the unique NWMCs were developed by controlling the in situ carbonization temperature using BK as a precursor and an electrochemical sensor for the effective determination of lead ions. The physicochemical structures of N-doped MCs were characterized by scanning electron microscopy (SEM), specific surface area analysis, pore size distribution, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The transformation of nitrogen, carbon, and oxygen atoms and the change of nitrogen and oxygen N-bonding configurations were analyzed at different temperatures ranging from 500 to 900 °C. The electrochemical properties of BK-derived NWMCs
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RESULTS AND DISCUSSION Effect of Carbonization Temperature on Physicochemical Properties. As shown in Figure 1, the FTIR curves
Figure 1. FTIR spectra of BNWMC-500, BNWMC-700, and BNWMC-900.
show that three obvious bands around 1380, 1560, and 3400 cm−1 dominate the spectra of all biomass-derived nitrogendoped wormlike micro/MC (BNWMC) samples, which can be ascribed to C−N stretching vibration, N−H in-plane deformation vibration, and N−H antisymmetric stretching vibrations, respectively.27−29 The latter band around 3400 cm−1 is slightly shifted to a higher wavenumber as the carbonization temperature of material rises, and it is stronger and broader for BNWMC-900 than others.30 The above FTIR results confirm the effective doping of nitrogen into the carbon frames. Meanwhile, the weak bands at 2910−2930 and 2840− 2870 cm−1 refer to symmetric and antisymmetric stretching vibration of C−H bond, respectively, which proves the presence of aliphatic groups.31 Obviously, there are several bands in the range of 400−1250 cm−1 on the spectra of BNWMC-500 and BNWMC-700 commonly, whereas it is no longer observed on BNWMC-900, suggesting the presence of C−C, CC, C−H groups, and CN heterocycles on the former two BNWMCs other than BNWMC-900.32,33 Compared with the former two BNWMCs, the appearance of the new peak at 1450 cm−1 on BNWMC-900 can be attributed to s-νCO symmetric vibrations.34 It is worth mentioning that the new band at 1110 cm−1 for BNWMC-700 and BNWMC900 may be identified as originating from CN stretching vibrations.35 According to the above description, it is evident that a variety of nitroxyl functional groups still exist on the materials of high-temperature carbonization. The chemical constituents of the three materials are further confirmed by XPS spectra. As shown in Figure 2, the C, N, and O peaks are clearly presented in materials, confirming that the nitrogen atom has been successfully incorporated into the carbon framework which is well-consistent with the FTIR analysis results. The nitrogen content reaches 4.42, 3.63, and 1.24 at. % in the three prepared BNWMCs at 500, 700, and 900 °C, respectively. It can be observed that the nitrogen content decreases with the increasing carbonization temper1192
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representing the CN bond and the CO bond are located at 285.1 and 288 eV, respectively.6 Moreover, after deconvolution of N 1s, peaks at 398.2 and 399.7 eV are originated from the pyridinic-N species and the pyrrolic-N species, respectively. The peak with a higher binding energy of 400.8 eV can be attributed to the quaternary-N bound within a graphitic nitrogen framework.39−41 In addition, the peak centered at 404.2 eV corresponds to the nitrogen oxide species.42 The basic constant of pyridinic nitrogen, a great decrease of pyrrolic nitrogen, and a significant increase of quaternary nitrogen with the increasing temperature show that the thermal stability of pyrrolic-N is weaker than quaternary/pyridinic-N in carbon.23 Therefore, what are the advantages of these nitrogen species of prepared carbons for the electrode material? Because the pyridine-N-type substance at the edge of the graphene layer guides the attraction of ions, more pseudocapacitance is obtained, and the quaternary-N in graphitic-C skeleton and pyridinic-N are recognized as active species which effectively improve the electrochemical performance of the carbons.25 Simultaneously, the positively charged quaternary nitrogen interacts with negatively charged pyridine nitrogen to promote the electron transfer of the BNWMC, thereby increasing the electrical conductivity.43 It is noticeable that the possible active N-binding species including quaternary-N and pyridinic-N on the comparative material NACF are 19.45 and 49.82%, which are more than those on BNWMC-900. The pore structure of the three BNWMC materials was characterized by nitrogen adsorption/desorption isotherms (Figure 4a). According to the definition of IUPAC, the N2 adsorption/desorption curves of the three BNWMCs correspond to a typical IV isotherm. The significant H3-type hysteresis loops of the three materials as P/P0 > 0.4 indicate that they have plenty of mesopores.10 The nitrogen adsorption increases with the increase of carbonization temperature so that BNWMC-900 has the highest surface area. From the density functional theory (DFT) curves (Figure 4b), the materials have numerous mesopores centering at around 3 and 5 nm and micropores in the range of 0.8−2 nm. The narrow pore-size distribution may correspond to the good thermal stability of the concretelike TEOS-decorated carbon wall. As shown in Table 2, the Brunauer−Emmett−Teller (BET) surface area (SBET), average pore size (Pdia), and micro/ mesopore volume of BNWMC rise with the increasing carbonization temperature, which may be because of the loss of some surface heteroatoms that can enter the pores,
Figure 2. XPS spectra of BNWMC-500, BNWMC-700, BNWMC900, and NACF, respectively.
ature which also, in previous studies, can be reported.36 Because of the poor thermostability of CN bonds, the reduction of the nitrogen amount caused by the thermal decomposition of the CN bonds becomes serious with the increasing carbonization temperature.37 This phenomenon can also be seen from the compared nitrogen-enriched activated carbon fiber (NACF), the nitrogen content of which is only 1.99 at. % (Table 1), which is less than that of BNWMC-700 and BNWMC-500. The loss of nitrogen functional groups can provide defect sites for the material, which contributes to the improvement of the electron transfer rate.6 It is worth noting that the N content is not directly associated with the electrocatalytic activity of the N-doped nanocarbon material, which has been proven to be valid.38 On the other hand, the oxygen content reaches a peak on BNWMC prepared at 900 °C, which is again consistent with the FTIR analysis results. To explore the states of carbon/nitrogen-bonding configurations, high-resolution XPS was also performed. As shown in Figure 3, the C 1s peak consists of four components of carbon, which are located at 284.6, 285.1, 286.1, and about 288.0 eV, respectively. As seen in Table 1, it can be clearly observed that the graphitic carbon at 284.6 eV accounts for a large proportion of three materials, and the XPS analysis also confirms that the prepared materials have graphite pore walls. In previous studies, it is known that the degree of graphitization increases as the carbonization temperature increases, resulting in better conductivity.6 This has also proved by our next electrochemical experimental part. The peak at 286.1 eV is mainly involved in alcohols, ethers, and CN bonds (C−OH, C−O−C, and C−N).39 The binding energies
Table 1. Atomic % of Carbon (C 1s), Nitrogen (N 1s), and Oxygen (O 1s) in the Carbon Samples from Full XPS Spectra and the Binding Energy and Relative Atomic % of Different Carbon and Nitrogen States from High-Resolution Scans carbon C 1s graphitic-C C−OH, C−O−C, C−N CO CN N 1s pyridinic-N pyrrolic-N quaternary-N NOx O 1s
binding energy (eV) 284.6 286.1 288.9 285.1 398.2 399.7 400.8 404.2
BNWMC-500 (%)
BNWMC-700 (%)
BNWMC-900 (%)
NACF (%)
84.87 57.12 13.16 9.89 19.82 4.42 31.99 34.98 25.82 7.21 10.71
88.27 57.09 9.16 23.50 10.25 3.63 29.46 17.15 43.99 9.4 8.10
84.08 57.21 23.43 11.72 7.65 1.24 21.30 9.69 46.31 22.7 14.68
87.82 58.67 13.52 20.23 7.58 1.99 19.45 30.73 49.82
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Figure 3. High-resolution survey spectra of C 1s (a−d) and N 1s (e−h) of BNWMC-500, BNWMC-700, BNWMC-900, and NACF, respectively.
including H2, NH3, CO2, CO, and CH4.23 Among the three BNWMCs, BNWMC-900 shows the maximum SBET, Vmeso, and Vmicro of 1201 m2 g−1, 1.4828 mL g−1, and 0.4478 mL g−1, respectively. According to SEM and transmission electron microscopy (TEM) morphologies (Figure 4c,d), there are many number of wormlike structures as well as interpenetrating micro/mesopores, similar to human blood vessel networks, on the surface of BNWMC-900. The unique structures should be very favorable for exposure to active sites and provide short pathways as rapid electronic transmission channel of ORR-related species.44 Specifically, the sufficient mesopores doped with interpenetrating micropores are critical for the rapid electrolyte transfer of the electrodes because of the fact that the mesopores can provide shorter channels for ion penetration and transport, whereas the high specific surface area contributes to the large electrode electrolyte interface. Moreover, the unique pore characteristics
of the BNWMC-900 are also favorable for the proximity of the chemical species, which can adjust the electronic structure, promote the process of proton and electron transfer, and enhance the efficiency of charge transfer between the electrode and the electrolyte interface.45 Compared with BNWMC-900, the comparative carbon NACF has more micropore volumes of 0.6647 mL g−1 and similar N- and C-binding functional groups except mesopores, which can be an effective-control carbon to certify the role of human blood vessel-like structure, resulting from the rich mesopores and the interpenetrating micropores on the mesopore wall. Electrochemical Studies of the BNWMC-Modified GCE. The electrochemical reaction of three modified electrodes on 5 mM K3[Fe(CN)6]/0.1 M KCl solution was studied using potassium ferricyanide as a probe. As shown in Figure 5a, the peak current of BNWMC-900/GCE, BNWMC-700/GCE, and BNWMC-500/GCE is arranged from large to small, 1194
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Figure 4. Nitrogen adsorption−desorption isotherms (a) and DFT pore-size distributions (b) of BNWMCs and NACF, SEM (c), and TEM (d) morphologies of BNWMC-900.
proportional to the electrochemical active surface area, whereas other conditions are controlled and unchanged; hence, BNWMC-900/GCE was found with the best electroactive surface area. In addition, it is observed that the peak potential difference ΔE of the three electrodes is 220 mV for BNWMC500/GCE, 209 mV for NACF, 119 mV for BNWMC-700/ GCE, and 90 mV for BNWMC-900/GCE, which is a function of the pair of electron transfer rate constants. The smaller the difference of the potential, the larger the electron transfer constant, indicating that the electrode with a smaller ΔE has a larger rate constant.46 Hence, the minimum potential difference of BNWMC-900/GCE indicates the strongest electron transfer rate among the three modified GCEs. Figure 5b is a Nyquist plot of a modified electrode in which the electron transfer resistance (Ret) and the electrolyte ion diffusion rate correspond to the diameter of the semicircular portion of the higher frequency range and the linear part of the sloped line of the lower frequency range, respectively.8 The poor conductivity of the BNWMC-500/GCE can be confirmed by the larger semicircle in the Nyquist plot (Ret = 2172 Ω). Almost no semicircularity in the Nyquist diagram of BNWMC-700/GCE and BNWMC-900/GCE can be found, implying their small electron-transfer resistances. In a lower
Table 2. BET Surface Area (SBET), Pore Diameter (Pdia), Micropore Volume (Vmicro), and Mesopore Volume (Vmeso) of the Prepared BNWMCs and NACF sample
SBET (m 2 −1 g )
V (mL g−1)
Vmeso (mL g−1)
Vmicro (mL g−1)
Pdia (nm)
BNWMC-500 BNWMC-700 BNWMC-900 NACF
798 1102 1201 1408
0.6687 1.1055 1.3004 0.7644
0.6568 1.2632 1.4828 0.1738
0.3102 0.4055 0.4478 0.6647
3.3533 4.0098 4.3341 2.1710
indicating the best ability of the charge transfer of BNWMC900/GCE because higher electrochemical signal corresponds to higher charge transfer rate. In addition, the electroactive surface area can be calculated according to the Randles−Sevcik equation: i p = 2.99 × 105nACD1/2ν1/2
where ip is the peak current (A), n is the number of electrons involved in the redox reaction, A is the electroactive surface area (cm2), D is the diffusion coefficient of the molecule in the solution, C is the concentration of the redox probe, and ν is the scanning rate (V/s).38 On the basis of the equation, ip is
Figure 5. CVs (a) and EIS (b) of different BNWMC-modified GCE in 5.0 mM [Fe(CN)6]3−/4− with 0.1 M KCl. 1195
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electrode shows a greater response current and smaller peak potential difference, which again demonstrates the most excellent electrochemical behavior of BNWMC-900/GCE ability.8 Simultaneously, the peak currents of BNWMC-900/ GCE and BNWMC-700/GCE are significantly larger and more stable than BNWMC-500/GCE. A larger reaction current proves that the electrode has a larger active area, suggesting a better electrochemical reaction capability. This improved electrochemical behavior is likely to be related to the defective potential of its surface physicochemical characteristics, resulting from the rich defect sites and the functional groups for the material because of nitrogen doping and hightemperature carbonation processes.47 From the experimental results of BNWMC-700/GCE and BNWMC-900/GCE, it can be speculated that the electrochemical properties of the modified electrode are together controlled by physical and chemical structures of the modified materials and the concrete analysis as follows. First, BNWMC900 material has a higher SBET, unique interpenetrating micro/ mesoporous structure, and high conductivity caused by graphitization, which are conducive to the electrolyte penetration, thereby improving the electrochemical performance.48 On the other hand, the BNWMC-700 has more pyridine nitrogen and quaternary nitrogen, although it is at a moderate level in the specific surface area, pore size, and micro/mesopore volume. It is known that N doping contributes to the generation of abundant defect sites and facilitates rapid electron transport rates, and those materials with more pyridine nitrogen content and quaternary nitrogen may also contribute to electron-transfer kinetics. Although the BNWMC-500 material has a high nitrogen content, its small specific surface area, low graphitization structure, and quaternary nitrogen might lead to the lowest conductivity and electron transfer rate. Moreover, the comparative NACF/ GCE has a lower electrochemical ability than BNWMC-900/
frequency range, the plots of BNWMC-700/GCE and BNWMC-900/GCE are more vertical, indicating that the unique pore structure and N-binding configurations provide excellent transport pathway and electrical conductivity for the electrolyte, which is in good agreement with cyclic voltammetry (CV) results. Detection Performance of Lead(II) by BNWMC-900/ GCE. As shown in Figure 6, the CV curves of BNWMC-900/
Figure 6. Electrochemical behavior in lead solution of different BNWMC-GCEs in 0.2 M NaAc−HAc solution containing 100 μg L−1 of lead(II) solution.
GCE, BNWMC-700/GCE, BNWMC-500/GCE, and comparative material NACF were also investigated in 0.2 M NaAc− HAc solution containing 100 μg L−1 of lead(II) solution. All of the three modified BNWMC/GCEs have obvious stripping peaks from Figure 6, whereas the bare GCE has no oxidation stripping peak from the pretest, which indicates that the modified BNWMC has a strong affinity for the lead ion. The characteristics of CV curves of the three BNWMC/GCEs are similar in the lead solution to those in the potassium ferricyanide solution. The higher temperature carbon-modified
Figure 7. Effect of experimental parameters on the peak currents of BNWMC-900/GCE in 0.2 M NaAc−HAc solution containing 100 μg L−1 of lead(II) solution: (a) NBMC-900 dose; (b) deposition potential; (c) deposition time; and (d) pH value. 1196
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GCE although it has more micropore volumes and similar Nbinding functional groups, which further confirm that except for the active N-binding configurations, the human blood vessel-like interconnected micro/mesopore structures play an important role in the electrochemical process. The above description shows that the BNWMC-900/GCE has the best electrochemical performance for its high micro/mesoporous volume, appropriate nitrogen-containing functional groups, rich graphitization structure, and carbon defects. Therefore, BNWMC-900/GCE was employed for lead(II) ion detection in the subsequent experiments. To explore the high-performance stripping analysis of lead(II), the effects of various parameters including BNWMC-900 dose, solution pH, deposition potential, and deposition time on the stripping current performance of lead(II) were investigated. Figure 7a presents the effect of BNWMC-900 dose from 2 to 4 μL on the stripping peak current of BNWMC-900/GCE, which shows that a 2.5 μL dose has the most sensitive dissolution signal. Less modifiers cause the electrode’s affinity for lead ions to drop, resulting in lower peak currents; and excessive modifiers may affect the conductivity of the electrode itself and cause the peak current to drop. Figure 7b,c includes optimizations of the deposition potential in the range of −1.4 to −0.6 V and deposition time from 200 to 280 s, respectively. As the potential increases, the peak current gradually increases, reaching a peak at −1.0 V. The reason why the peak current is relatively small between −0.6 and −1.0 V is as follows: when the enrichment potential is not sufficiently negative, the amount of lead ions deposited on the surface of the modified electrode is relatively small; hence, the dissolution signal becomes correspondingly weaker. However, when the potential is too negative, the hydrogen evolution and reduction can also become interference factors. Thus, −1.0 V is preferred for the enrichment potential. Figure 7c shows that the peak current of lead(II) also increases with the prolongation of time, reaching its maximum value at 240 s, which is the increase of lead-ion enrichment with time, and the response current increases and then tends to saturate; therefore, 240 s was preferred for the deposition time. The pH of the solution was an important factor affecting the accumulation of lead ions on the electrode materials. Figure 7d shows the influence of pH on voltammetric responses of BNWMC-900/GCE toward lead(II). The peak current first increases and then decreases with the rising pH, and the maximum peak occurs at 3.8. Therefore, the acetate solution with a pH of 3.8 was preferred to be the electrolyte for the subsequent experiments. As shown in Figure 8, the connection between the CV elution peak current of BNWMC-900/GCE and lead(II) concentration was analyzed under optimized conditions. The CV peak current of lead ion is obviously increased with the increase of the solution lead concentration, and the inset graph in Figure 8 gives a linear relationship between the CV peak current and the solution lead concentration in the range of 10−800 μg L−1. The linear equation is y = 0.00791x + 5.04438 with a high linear correlation coefficient R2 (0.99524) and a high detection limit (5 μg L−1, S/N = 3). In the equation, y and x are the peak current (μA) and the solution lead-ion concentration (μg L−1), respectively. The peak currents measured many times are almost the same (n = 5) with a low relative standard deviation of 1.45%, indicating that the BNWMC-900/GCE-modified electrode has an excellent stability. In addition, the comparison of other modified
Figure 8. CV current peaks of different lead-ion concentrations on BNWMC-900/GCE electrode. The inset graph: the linear relationship between the peak currents and the change of lead-ion concentration.
electrode measurements of lead(II) is shown in Table 3.5,7,9,49−51 Obviously, the BNWMC-900/GCE has a broader linear range and higher sensitivity for lead determination among the listed electrochemical sensors, which is most likely attributed to the enhanced conductivity by the unique wormlike micro/mesoporous structures, appropriate nitrogencontaining functional groups, and rich carbon−nitrogen graphitization structures.
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CONCLUSIONS The BK-derived NWMC was successfully synthesized as highly sensitive electrochemical sensors for lead-ion detection under different carbonization temperatures from 500 to 900 °C. The results showed that the BNWMCs have been equipped with different nitrogen functionalities and interesting wormlike structures with rich micropores on the mesoporous wall, which is similar as “human blood vessel networks”. The CV peak currents of unique BNWMC-modified electrode were increased obviously with carbonization temperature, indicating that raising the temperature favored the electrochemical sensitive activity of BNWMC/GCE. The proportion of quaternary-N and graphitic-C greatly increased, and pyridinic-N remained relative stable with the increasing carbonization temperature from 500 to 900 °C, whereas pyrrolic-N and total nitrogen were greatly reduced. The results imply that the graphitic-C, quaternary-N, and pyridinic-N binding configurations played an important role. Moreover, the comparative micropore/nitrogen-rich NACF has a lower electrochemical activity than 900 °Cprepared BNWMC900further confirming the dominant role of human blood vessel-like structures in BNWMC/GCEs. The CV elution peak currents of lead on BNWMC/GCE greatly varied with solution parameters such as solution pH, deposition potential, BNWMC dose, and deposition time. Under optimal conditions, a good linear relationship of R2 = 0.995 between elution peak current and lead-ion concentrations with a broad measure range of 10−800 mg L−1 and the lower sensitive detection limit of 5 μg L−1 confirmed that the BNWMC is a very promising electrode-modifier candidate to develop a highly sensitive electrochemical sensor for lead-ion detection.
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METHODS/EXPERIMENTAL SECTION Preparation of Biomass Prepolymer. The biomass material as a carbon source is derived from the pretreated
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Table 3. Comparison of Different Electrodes for Lead-Ion Determination modified electrode GA-CTS-CN OMC-IL-chitosan/CLLE Bi/MGF-Nafion/GCE PA/PPy/GO P(DPA-co-2ABN)/GCE Bi/graphene BNWMC-900/GCE
linear range (mol L−1) 9.90 −2.00 4.83 −1.4 0.24 −5.3 2.41 −7.25 1.26 −2.83 3 −1.7 4.83 −3.86
× × × × × × × × × × × × × ×
−8
10 10−6 10−8 10−6 10−8 10−7 10−8 10−7 10−6 10−4 10−7 10−6 10−8 10−6
linear range (μg L−1)
detection limit (mol L−1)
detection limit (μg L−1)
−8
refs
20.5−414
5.7 × 10
11.8
49
10−290
2.5 × 10−8
5.2
50
0.5−110
0.48 × 10−9
0.1
5
0.198 × 10−8
0.41
7
7.96 × 10−7
165
9
62−352
3 × 10−8
6.2
51
10−800
2.42 × 10−8
5.0
this work
5−150 261−58581
Characterization of BNWMCs. The specific surface area was obtained according to the monolayer adsorption amount and the BET formula from the nitrogen adsorption− desorption curve, and the pore-size distribution was calculated using the DFT method. FTIR and XPS were utilized to determine the structures and chemical bonds of the BNWMCs. Preparation of Modified Electrode and Electrochemical Testing. A GCE (3 mm in diameter) was sequentially polished on the suede with a 1.00, 0.30, and 0.05 μm Al2O3 powder to a mirror surface and ultrasonically purged with nitrogen gas to dry it. The BNWMC was dissolved in ethanol and 1.0% Nafion, and then the mixture was sonicated into a homogeneous dispersion. Then, a microsyringe was used to remove the quantitatively dispersed droplets and apply them evenly to the surface of the treated GCE. After being allowed to dry naturally, it was placed under an infrared lamp for 15 min. The BNWMC-modified electrode (BNWMC/GCE) was finally formed and dried. Potassium ferricyanide was used as a probe to study the electrode surface state via the CV method. In addition, impedance analysis was performed, and the initial potential was obtained by opencircuit potential test. Electrochemical Test of BNWMC/GCE and Optimizing Conditions for Lead Detection. The dissolution signal generated by CV represents the content of lead in solution. To get the best dissolution response signal, a series of conditions including dispersant dose, electrolyte pH, deposition potential, and deposition time were tested to optimize the process. Under optimized conditions, lead-ion solutions with different concentrations were configured to study the relationship between peak current and heavy metal ion concentration.
preservative BK. According to our previous article, biomass liquefaction was first prepared from BK.26 Then, 40.5 g of BK liquefaction was mixed with 6.5 g of 20% NaOH solution and stirred for 10 min. The difference from the previous article is that at this stage, 20 g of melamine was added as a nitrogen source. Then, 52.5 g of 37% formaldehyde solution was slowly added, and the solution temperature was raised to 75 °C and kept for 100 min. The mixture was cooled to room temperature, and the pH value was adjusted to 5.0 with hydrochloric acid. Finally, the yellow colloidal substance, which was obtained by rotary evaporation at 55 °C for 60 min, was dispersed in anhydrous ethanol and formulated into a 20% ethanol solution. The ethanol solution of BK prepolymer was ready for use. Synthesis of Bayberry-Derived MC. First, 8.0 g of Pluronic F127 (EO106−PO70−EO106, Mw = 12 600, SigmaAldrich) was dissolved in 40 g of anhydrous ethanol to form a transparent solution, and 5.0 g of hydrochloric acid (0.2 M) was added and the mixture was stirred at 40 °C for 1 h. Then, 10.4 g of TEOS and 25 g of ethanol solution of BK prepolymer with a mass ratio of 20 wt % were added successively. After stirring for 2 h, the mixture was transferred to a flat-bottomed evaporating dish and the solvent was evaporated at 25 °C for 6 h. Petri dishes were placed in a drying oven and thermally polymerized at 100 °C for 24 h to form a soft film. Then, the film was carbonized in a tubular furnace at 350 °C for 3 h and at 500 °C/700 °C/900 °C for 2 h under 20 mL min−1 nitrogen flow. In this process, the Pluronic F127 in the composite film was removed as the carbonization temperature > 350 °C, and large narrow mesopores were generated. In addition, some micropores could be produced because the composite film could be activated by carbon monoxide, hydrogen generated from the carbonization process, and the impurity oxygen in the nitrogen protection gas. After carbonization, the wormlike carbon−silicon oxide composite was obtained. Then, the resulting wormlike carbon−silicon oxide composite was etched with excess 3 M NaOH for 24 h, and many micro/mesopores interpenetrated the formerly produced mesopores by the removal of SiO2. Last, the BK-derived NWMC: BNWMC-500, BNWMC-700, and BNWMC-900 with interpenetrating micro/mesopores as human blood vessel-like structures were obtained. Also, the visualized image of human blood vessel-like pore structures on BNWMC was presented in a graphical abstract. For comparison, the commercial NACF was selected from China Nantong carbon fiber Co., Ltd.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86-25-58606570. Fax: +86-25-58606630 (K.L.). ORCID
Kunquan Li: 0000-0001-5763-0044 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the research grants provided by China under the Natural Science Foundation (no. 21876086), 1198
DOI: 10.1021/acsomega.8b02954 ACS Omega 2019, 4, 1191−1200
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the Key research and development plan of Jiangsu Province (no. BE2018708), and the China Postdoctoral Science Fund (no. 2014M560429).
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