Chitosan@N,S Co-doped Multiwalled Carbon

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10926-10939

Gold Nanoparticle/Chitosan@N,S Co-doped Multiwalled Carbon Nanotubes Sensor: Fabrication, Characterization, and Electrochemical Detection of Catechol and Nitrite Hanbing Rao,† Yiting Liu,† Ji Zhong, Zhaoyi Zhang, Xun Zhao, Xin Liu, Yuanyuan Jiang, Ping Zou, Xianxiang Wang, and Yanying Wang* College of Science, Sichuan Agricultural University, Xin Kang Road, Yucheng District, Ya’an 625014, People’s Republic of China Downloaded via FORDHAM UNIV on June 29, 2018 at 15:44:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Given that catechol and nitrite can be easily released into the environment and cause serious damage to our physical and ecological environment, it is necessary to develop a fast and reliable detection method for catechol and nitrite analysis. In our work, an electrochemical catechol and nitrite sensor was created based on gold nanoparticles (AuNPs) deposited on chitosan@N,S co-doped multiwalled carbon nanotubes (CS@N,S co-doped MWCNTS) composite modified glassy carbon electrode. The preliminary prepared composite materials were characterized by transmission electron microscope, scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The AuNPs/CS@N,S co-doped MWCNTS modified electrode shows a good linear relationship between 1 μmol·L−1 (μM) and 5000 μM for catechol, and the detection limit was calculated as 0.2 μM. For nitrite the linear relationship was 1−7000 μM with the detection limit of 0.2 μM. The sensitivity of the created sensor was evaluated as 0.9081 μA·μM−1·cm−2 for catechol, and for nitrite it was 0.7595 μA·μM−1·cm−2. Also the sensor shows significant selectivity, reproducibility, and stability. Moreover, the modified electrode is also applied in tap water samples for the catechol measurement and nitrite in fermented bean curd, ham sausage, and mustard tuber samples. KEYWORDS: Gold electrodeposition, Multiwalled carbon nanotubes, Chitosan, Catechol, Nitrite, Electrochemical sensor

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inability to use for field analysis. Consequently, there is a strong demand to develop a low-cost, simple, and reliable detection method with high sensitivity and selectivity for the analysis of catechol. Nitrite (NO2−), a food additive extensively used in the food industry, can prevent bacterial growth and food oxidative degradation.18−20 NO2− can easily interact with hemoglobin and cause the occurrence of methemoglobinemia.21 Moreover, NO2− can very easily react with secondary amines to constitute teratogenetic, mutagenic, and carcinogenic species, bringing on esophagus and gastric cancer.22,23 On the grounds of standards of the World Health Organization (WHO), the maximum NO2− allowable content in drinking water is 43.48 μM, while an excess quantity in water can result in shortness of breath and “Blue Baby Syndrome” diseases.23 Up to now, numerous analytical approaches have been already used for the determination of nitrite, comprising spectrophotometry,24 microspectrophotometry with liquid-phase microextraction,25 chromatography,26,27 capillary electrophoresis,28,29 Raman spectroscopy,30 and electrochemical method.31,32 Under

INTRODUCTION Nowadays, the development of methods for sensitive and selective phenolic compounds detection is of great emerging interest.1 Even at very low concentrations, these compounds are extremely toxic to human health and difficult to degrade in the ecological system;2 that is why the U.S. Environmental Protection Agency (EPA) and the European Union (EU) regard them as environmental pollutants.3−5 Catechol (1,2dihydroxybenzene) is one of the phenolic compounds which has widespread application in several fields such as cosmetics, textiles, antioxidants, dyes, photography, petroleum refinery, plastics, agricultural chemicals, and medicines.6 In the process of production and application in these fields,7,8 catechol might be casually released into the environment as pollutants. Skin contact with catechol leads to eczema in humans,9 while numerous contacts with catechol through skin adsorbing, inhaling steam, or direct ingesting can lead to serious burns and deleterious effects on liver, heart, lungs, and the central nervous system.10−13 So far, there are several methods with good detection limit that have been established, such as fluorescence,14 gas chromatography,15 high-performance liquid chromatography,16 and chemiluminescence.17 Nevertheless, major defects of the current methods include higher cost, more laboratory setting, relatively high limit of detection, and © 2017 American Chemical Society

Received: August 17, 2017 Revised: September 15, 2017 Published: September 27, 2017 10926

DOI: 10.1021/acssuschemeng.7b02840 ACS Sustainable Chem. Eng. 2017, 5, 10926−10939

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration for Fabricating AuNPs/CS@N,S Co-doped MWCNTS

physical−chemical properties such as chemical stability and electric conductivity as well as high surface area.47,48 They have significant applications such as batteries, electron field emission sources, chemical sensors, and nanoelectronic devices.49 When used as the electrode material, the electronic properties of MWCNTS manifesting that they have the capacity to accelerate electron transfer reaction.50 The open three-dimensional network structure of MWCNTS can minimize mass transport limitations and imitate the inverse structure of traditional porous metal catalysts, which are usually used in heterogeneous catalysis.51 Furthermore, the structural modification of the carbon framework is incorporated with adventitious heteroatoms (N, S, B, or P) that have a positive effect on their hydrophilicity and conductivity,52 and in certain cases their electrocatalytic properties.53 Especially, co-doping with two or three different electronegativity elements’ dopants can bring about a synergistic influence because of a unique electron distribution.54−57 Finally, chitosan (CS) is a nontoxic and inexpensive support for nanomaterials. Its high-porosity and hydrophilic character causes low steric hindrance and less resistance mass transfer to the nanomaterials. Chitosan can be bound with nanomaterials easily due to hydroxyl (-OH) and amino (-NH2) groups.58,59 The intention of this work was to construct a novel biosensor based on AuNPs/CS@N,S co-doped MWCNTS composite nanomaterials modified electrode for the first time. In our approach, MWCNTS were first processed using a reflux condensation and calcination to obtain N,S co-doped MWCNTS. Then the chitosan was wrapped around the N,S co-doped MWCNTS surface through a simple reflux reaction. Afterward, AuNPs were deposited on the electrode by an electrodeposition method, as shown in Scheme 1. The asprepared electrode shows high abundant porosity and large specific surface area, securing the high electrochemical performance of our sensor. Properties of the catechol and nitrite biosensor were evaluated by cyclic voltammetry and amperometric, including sensitivity, reproducibility, and

comparison of electrochemical techniques, on the score of the facile manufacture of electrode material, straightforward operation, a relatively inexpensive instrument, rapid response time, and high sensitivity are often priorities for sensing experiments. Electrochemical techniques have been developed for years by virtue of their simplicity, reasonable exactitude, low cost, and rapidity. There is no requirement for derivitizing reactions or extraction of time-consuming steps in comparison with other techniques.33,34 The modification of electrode surfaces is one of the important developments in recent years.35 When modified with proper materials on the purified electrode, it has the ability to promote the electrode process via significantly decreasing the overpotential of the purified electrode.36 Hence, we selected the following materials to modify our electrode. In recent years, interests have focused on nanoscaled materials, particularly metallic nanoparticles, and their extensive use in analytical chemistry including catalysis, biological sensors, optics, and electronics.37 In electrochemical analysis, nanoparticles have been applied for electrode modification, on the account of strengths such as their mass transport, improved active area, and electrocatalytic effect.38,39 Compared to the majority of electrodes, these merits can provide a better analytical performance. 40−43 Besides, in terms of new biosensors, the nanosized gold nanoparticles (AuNPs) show good electrical properties, good mechanical resistance, and good biocompatibility and have a large active surface area which can increase the electron transfer and electrode conductivity and enhance the analytical sensitivity.44 Therefore, several research contributions have probed into the application of AuNPs in biosensors. Furthermore, the production of carbon-supported materials has also made remarkable progress with proper cost during the most recent 2 decades.37 Multiwalled carbon nanotubes (MWCNTS), a nanoscale tubular material made of graphitic laminas rolled into closed concentric cylinders,45,46 are one kind of excellent electrode material, because of their unique 10927


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ACS Sustainable Chemistry & Engineering

obtained. The suspension was transferred into a round-bottom flask and refluxed at 90 °C for 8 h. Then the reaction mixture was washed thoroughly with ultrapure water and collected after drying under vacuum at 50 °C. Electrodeposition of AuNPs on the CS@N,S Co-doped MWCNTS. CS@N,S-doped MWCNTS film was prepared on the surface of the GCE electrode by the drop-coating method. The GCE was polished by alumina powder and cleaned respectively with 1:1 (v/ v) HNO3, absolute ethyl alcohol, and ultrapure water in an ultrasonic bath. A 5 mg amount of CS@N,S-doped MWCNTS was dispersed in 5 mL of deionized water using ultrasonication method to acquire a homogeneous black suspension liquid, whereafter 10 μL of CS@N,Sdoped MWCNTS suspension was dropped onto the electrode surface and dried at room temperature. The electrodeposition of AuNPs on CS@N,S-doped MWCNTS/GCE electrode was performed in an electroplating bath which consisted of 1 mM HAuCl4 (0.1 M KNO3). CS@N,S-doped MWCNTS/GCE electrode was immersed in the plating bath using chronoamperometry at the potential from −0.2 to 0 V for 200 s. In fact, different electrodes can be obtained by changing the step numbers.

stability. Importantly, the sensor performs with a current that responds well in both catechol and nitrite detection. Hence, the AuNPs/CS@N,S co-doped MWNTS modified electrode is a viable candidate for electrochemical biosensor.

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EXPERIMENTAL SECTION

Materials and Reagents. Multiwalled carbon nanotubes (diameter, 40−60 nm; length, 97%) was purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China). Thiourea (CH4N2S, ≥99.8%), sodium carbonate (Na2CO3, ≥99.0%), potassium nitrate (KNO3, ≥99.0%), magnesium sulfate (MgSO4, ≥99.0%), sodium nitrate (NaNO3, ≥99.0%), calcium chloride (CaCl2, ≥96.0%), sodium chloride (NaCl, ≥99.5%), sodium fluoride (NaF, ≥98.0%), potassium chloride (KCl, ≥99.5%), magnesium chloride (MgCl2, ≥98.0%), sodium nitrite (NaNO2, ≥99.0%), sodium acetate (NaAc, ≥99.0%), disodium hydrogen phosphate (Na2HPO4, ≥99.0%), sodium dihydrogen phosphate (NaH2PO4, ≥99.0%), glucose (C6H12O6), urea (CO(NH2)2, ≥99.0%), hydroquinone (HQ, C6H6O2, ≥98.0%), and ascorbic acid (AA, C6H8O6, ≥99.7%) were purchased from KeLong (Chengdu, China). Chitosan (CS; viscosity, 90.0%) was purchased from LanJi. (Shanghai, China). Potassium bromide (KBr) was purchased from Xiya Chemical Industry Co. Ltd. (Shandong, China). Phenol (C6H6O) and hydrogen peroxide (H2O2, ≥30.0%) were purchased from XiLong. (Guangdong, China). Catechol (C6H6O2, 99.5%), dopamine (DA, C8H11NO2, 98.0%), guaiacol (C7H8O2, >99.0%), o-phenylenediamine (OPD, C6H8N2, 99.5%), resorcinol (C6H6O2), and chloroauric acid (HAuCl4, 99.0%) were purchased from Macklin Co. Ltd. (Shanghai, China). All chemicals were of analytical grade, and all the solutions were prepared using deionized water (DW, 18.2 MΩ·cm−1). Apparatus. Glassy carbon electrode (GCE; o.d., 3 mm), platinum (Pt) wire electrode, and saturated calomel electrode (SCE) were purchased from Tianjin Incole Union Technology Co., Ltd. (Tianjin, China). All electrochemical measurements were performed on a CHI660E electrochemical workstation (Chen Hua, Shanghai, China) at room temperature with a traditional three-electrode system. In this system, the Pt wire was used as the auxiliary electrode, the SCE was used as the reference electrode, and the modified GCE was used as the working electrode. X-ray diffraction (XRD) data were gathered in a D/ Max-RA diffractometer (DX-2700, Dan Dong, China) with Cu Kα radiation (λ = 0.1548 nm) operated at 40 kV and 100 Ma. Thermogravimetric analysis (TGA) data were analyzed on a TGA209F3A Tarsus instrument (NETZSCH-Gerätebau GmbH, Germany). Scanning electron microscopy (SEM) pictures were collected on a JSM4800F instrument (JEOL, Japan), and transmission electron microscopy (TEM) pictures were obtained from JEOL2100F (JEOL). X-ray photoelectron spectroscopy (XPS) data were collected from ESCALAB 250Xi (Boyue, Shanghai, China). Preparation of N,S Co-doped MWCNTS. N,S-doped MWCNTS were synthesized by the thermal decomposition method. A 500 mg amount of MWCNTS was mixed with 15 mL of 65% HNO3 and 45 mL of 98% H2SO4 in a round-bottom flask and refluxed at 75 °C for 3 h to obtain oxidized MWCNTS (oxMWCNTS). The reaction mixture was diluted with plenty of deionized water until a nearly neutral environment and then filtrated and washed thoroughly with deionized water and ethanol until the solution was neutral. The resultant oxMWCNTS were collected after drying under vacuum at 45 °C for 12 h. Then 200 mg of oxMWCNTS was first finely ground with 200 mg of thiourea for several minutes and then transferred into a quartz boat-like container. The container was placed into a tube furnace, calcined at 350 °C for 1 h with a heating rate at 10 °C·min−1 under nitrogen flow, and then cooled to room temperature. The formed product was cleaned using ultrapure water and dried overnight in a vacuum oven at 50 °C afterward. Preparation of CS@N,S Co-doped MWCNTS. CS@N,S-doped MWCNTS were prepared according to a common method. A 100 mg amount of chitosan powder was dissolved in 2% (v/v) acetic acid solution. A 100 mg amount of N,S-doped MWCNTS was added to the solution and gently stirred until a homogeneous suspension was

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RESULTS AND DISCUSSION Characterization of AuNPs/CS@N,S Co-doped MWCNTS. Morphological and Structural Characterization. The morphologies and structures of the MWCNTS, oxMWCNTS, N,S co-doped MWCNTS, CS@N,S co-doped MWCNTS, AuNPs, AuNPs/N,S co-doped MWCNTS, and AuNPs/CS@N,S co-doped MWCNTS nanocomposites were confirmed by SEM and TEM. In Figure 1a,h, we can clearly see

Figure 1. SEM images of (a) MWCNTS, (b) oxMWCNTS, (c) N,S co-doped MWCNTS, (d) CS@N,S co-doped MWCNTS, (e) AuNPs, (f) AuNPs/N,S co-doped MWCNTS, and (g) AuNPs/CS@N,S codoped MWCNTS. TEM images of (h) MWCNTS, (i) oxMWCNTS, (j) N,S co-doped MWCNTS, and (k) CS@N,S co-doped MWCNTS.

a tubular frame with diameters ranging from approximately 5 to 10 nm, which is the most typical structure of MWCNTS. When MWCNTS were oxidized by mixed acid, they were cut into smaller pieces but the structure did not change much (Figure 1b,i). Panels c and j of Figure 1 verified that nitrogen and sulfur co-doping did not break the tubular structure of MWCNTS. By comparison, panels d and k of Figure 1 display a larger size, the boundary is not as smooth as that from Figure 1j, and we cannot see the middle-hollowed structure clearly. It indicates 10928


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manifested that the chitosan may successfully wrap onto the surface of MWCNTS. X-ray Photoelectron Spectroscopy Analysis. XPS is sensitive to the surface nature and composition.63 The survey spectrum (Figure 3a) indicates the presence of Au, C, O, N, and S in AuNPs/CS@N,S co-doped MWCNTS. As seen from Figure 3b) the black line (Figure 3b-1) is the pure MWCNTS with no treatment, and only the peaks of the carbon element exist. Under the oxidation of mixed acid, carboxyl groups were added onto the MWCNTS framework, and the peaks of the oxygen element appeared (Figure 3b-2). After co-doping treatment of nitrogen and sulfur, corresponding peaks also appeared in the survey curve (Figure 3b-3). Since the main elements of chitosan are carbon, nitrogen, and oxygen, no other peaks appeared in the survey curve in Figure 3b-4. With the use of the Gaussian fitting method, the XPS peaks of Au0 4f in AuNPs/CS@N,S co-doped MWCNTS are located at 84.35 and 88.1 eV (Figure 3c).64 As shown in Figure 3e, the N 1s peak components in the spectra of N,S co-doped MWCNTS can be observed at a binding energy (BE) = 398.2 eV for pyridinic-N, BE = 399.1 eV for pyrrolic-N, and BE = 400.2 eV for graphiticN.65 Figure 3f shows the N 1s peak components in the spectra of CS@N,S co-doped MWCNTS obtained at a BE = 398.8 eV for pyridinic-N, BE = 400.8 eV for graphitic-N, and BE = 399.6 eV can be regarded as chitosan-N.66 The peak of pyrrolic-N is close to chitosan-N, and the content of chitosan-N is higher than that of pyrrolic-N. Thus, we cannot see the peak of pyrrolic-N in Figure 3f. Sulfur dopants were mainly bound in structure in the form of thiophenic S (C−S−C), located at 162.5 and 164.8 eV, and the other formation of oxidized S (C− Sox−C) was observed at 168.6 eV (Figure 3d).57 The results manifested that N and S atoms were doped into the carbon nanotube framework successfully. In addition, the peak of chitosan-N shows that chitosan macromolecules were wrapped onto the MWCNTS. Thermogravimetric Analysis. TGA data further identify the weight loss of MWCNTS in different treatments. Supporting Information Figure S1 shows TGA thermograms of MWCNTS, oxMWCNTS, N,S co-doped MWCNTS, and CS@N,S codoped MWCNTS in the temperature range from room temperature to 900 °C under condensed Ar at a rate of 10 °C/min. For MWCNTS (Figure S1a), there is almost no weight loss below 650 °C, and the thermal degradation starts at about 650 °C which demonstrate the existence of disordered and amorphous carbons on the surface of MWCNTS, and it is close to the result of previous literature.67 In Figure S1b,c, the prime weight loss which occurred below 160 °C is induced by the removal of moisture and bound water. A slower weight loss from 220 to 400 °C is attributed to the disintegration of -OH, -COOH, and epoxy groups from MWCNTS. The main weight loss from 450 to 650 °C is attributed to the carbon skeleton destruction of the MWCNTS framework. By comparison with CS@N,S co-doped MWCNTS (Figure S1d), weight loss from 200 to 300 °C is related to the deacetylation, cleavage of the glycosidic linkage, dehydration, and deamination of chitosan, etc., which is similar to the reported literature.68,69 All information above further demonstrates the presence of chitosan on the surface of N,S co-doped MWCNTS. Electrochemical Behavior of Electrodeposited AuNPs. The electrodeposition of AuNPs on CS@N,S-doped MWCNTS/ GCE electrode was performed in a plating bath constitutive of 1 mM HAuCl4 (0.1 M KNO3). CS@N,S-doped MWCNTS/ GCE electrode was immersed in the electroplating bath using

that the chitosan was successfully wrapped around the carbon nanotubes. The structures of electrodeposition AuNPs were shown in Figure 1e−g. The AuNPs are nearly spherical, and the AuNPs are strongly assembled onto the electrode surface (Figure 1e) on account of the electrodeposition method. The electrodeposition of AuNPs proceeds when N,S co-doped MWCNTS are prior dispensed onto the electrode surface. Numerous cavernous caves appeared because of the deposition of AuNPs on the MWCNTS, which acts as supporting material, forming an approximate, poor porosity holes structure (Figure 1f), but the sample exhibits significant aggregation. As seen in Figure 1g, the number of cavernous caves and the aggregation effect have both declined, because chitosan increased the biocompatibility between AuNPs and MWCNTS, making a better dispersion of AuNPs on MWCNTS. X-ray Diffraction Analysis. The XRD data were used to ascertain the chemical compositions and phases of the products. XRD patterns of MWCNTS, oxMWCNTS, N,S codoped MWCNTS, CS@N,S co-doped MWCNTS, and AuNPs/CS@N,S co-doped MWCNTS were shown in Figure 2 for a better understanding of the structure and phase of the

Figure 2. XRD patterns of (a) MWCNTS, (b) oxMWCNTS, (c) N,S co-doped MWCNTS, (d) CS@N,S co-doped MWCNTS, and (e) AuNPs/CS@N,S co-doped MWCNTS.

nanocomposite. The XRD pattern peaks in Figure 2e at 2θ = 38.15°, 44.36°, 64.64°, and 77.61° can be respectively indexed to the (111), (200), (220), and (311) reflections of the standard patterns of the spinel Au nanoparticles phase, which is in agreement with JCPDS Card No. 04-0784.60 On the basis of Scherrer’ equation,61 the average particle size of AuNPs was calculated from the Au(111) diffraction peak to be 3.52 nm for AuNPs/CS@N,S co-doped MWCNTS. In Figure 2a−d, the typical characteristic peaks near 26.16°, 43.36°, and 53.86° can be assigned to the (002), (100), and (004) reflections of MWCNTS.57,62 The mean particle sizes of MWCNTS, oxMWCNTS, N,S co-doped MWCNTS, and CS@N,S codoped MWCNTS (Figure 2a−d) were calculated from C(002) diffraction peak to be 2.36, 2.37, 2.37, and 2.42 nm, respectively, indicating that the carboxylation reaction and the nitrogen and sulfur co-doping reaction did not change the mean size of MWCNTS. Yet, the particle size of CS@N,S codoped MWCNTS is slightly bigger than others, and the result 10929


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Figure 3. XPS survey spectra of the (a) AuNPs/CS@N,S co-doped MWCNTS nanocomposite, (b-1) MWCNTS, (b-2) oxMWCNTS, (b-3) N,S codoped MWCNTS, and (b-4) CS@N,S co-doped MWCNTS, (c) Au 4f region, (d) S 2p region, (f) N 1s region in AuNPs/CS@N,S co-doped MWCNTS, and (e) N 1s region in N,S co-doped MWCNTS.

chronoamperometry at the potential from −0.2 to 0 V for 200 s.9,70 The electrochemical behavior of electrodeposited AuNPs using chronoamperometry method was shown in Figure S2a,b. For the observation of the influence of the electrodeposition step number, AuNPs were deposited on CS@N,S co-doped MWCNTS/GCE by applying a potential from 0 to 0.2 V for different step numbers (1, 2, 3, and 4 times). In order to settle the optimum value, the responses results of the prepared

electrodes were then evaluated. As shown in Figure S3, 3 times of the electrodeposition step number was considered the most optimum value. After 3 times of electrodeposition step number, the current response declined. This outcome might be due to the addition of the deposition step number reducing the electrochemical active surface area of the electrode, which is attributed to the negative affect of large-sized nanoparticles on electrocatalytic activity. 10930


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ACS Sustainable Chemistry & Engineering Electrochemical Measurements of Catechol. Electrochemical Responses of Catechol on AuNPs/CS@N,S Codoped MWCNTS. Figure 4 shows the cyclic voltammograms of

According to the report, the catechol anodic oxidation was possibly involved in the formation of quinones, trihydroxybenzenes, and phenoxy-type radicals.71 In the reverse sweep, two reductions peaks at approximately 0.4 and 0.1 V were obtained. The peak at 0.4 V presumably arose from the gold oxides reduced, yet the peak at 0.1 V can be allocated to a reversible transition between catechol and quinones.72 Effect of pH. The acidity of electrolyte can influence the adsorption capacity of catechol; therefore it is a momentous factor in the electrochemical oxidation of catechol.8 In this context, the effects of pH on the oxide peak currents and potentials of catechol at AuNPs/CS@N,S co-doped MWCNTS were evaluated by CV technique from pH 5.0 to 8.0. As shown in Figure S4a, when the pH was increased from 5.0 to 7.4, the anodic peak currents of catechol increased and then declined after pH 7.4. Additionally, the relationship between pH condition and the anodic peak potential was evaluated and the results were given in Figure S4b. It can be observed that with pH value increasing from 5.0 to 8.0 the anodic peak potentials of catechol shift toward to the low potential, evincing that the protons participated in the catechol oxidation reaction directly.73 The linear regression equations could be expressed as follows: Epa (mV) = −58.4pH + 624.91; R2=0.9933. The slopes of the regression equations are close to the theoretical value of −58 mV·pH−1 (20 °C) for the two protons and two electrons process, proposing that the electrochemical catechol redox reaction at AuNPs/CS@N,S co-doped MWCNTS is supposed be a two protons and two electrons process. Given the above results, the probable reaction mechanism of catechol at AuNPs/CS@N,S co-doped MWCNTS can be described as follows:2

Figure 4. CVs obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), and AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), and AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 2 mM catechol. Scan rate = 50 mV/s.

the AuNPs, AuNPs/N,S co-doped MWCNTS, and AuNPs/ CS@N,S co-doped MWCNTS in PBS (pH 7.4) solution with and without the existence of 2 mM catechol at a scan rate of 50 mV·s−1. Accordingly, the dotted lines of Figure 4 demonstrate the CVs of different electrodes in PBS solution. As the CVs show, there is a pair of redox (anodic and cathodic) peaks, and the existence of these redox peaks in the PBS solution is because of the onset of gold redox. The solid lines in Figure 4 show the recorded current was derived from the oxidation of catechol generated at about 0.1 V and peaked at 0.2 V before the onset of gold oxidation at 0.85 V in the positive scanning. The result reveals that the AuNPs/CS@N,S co-doped MWCNTS electrode displays an optimal capability, higher than the electrodes modified with AuNPs/N,S co-doped MWCNTS and AuNPs.

Effect of Scan Rate. The potential relationship between peak current and scan rate can obtain information about electrochemical mechanism. The effect of scan rate on the electro-oxidation of 2 mM catechol in PBS (pH 7.4) between 10 and 100 mV/s was investigated by CV (Figure S5a). The anodic and cathodic peak currents of catechol at AuNPs/CS@

Figure 5. (a) Amperometric responses to successive additions of catechol and (b) calibration curves obtained at the AuNPs/CS@N,S co-doped MWCNTS electrode in pH 7.4 PBS. The applied potential: +0.2 V. 10931


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Figure 6. (a) Plot of response time and (b) amperometric response of AuNPs/CS@N,S co-doped MWCNTS to the successive addition of 100 μM catechol and 200 μM NaCl, glucose, urea, H2O2, phenol, resorcinol, HQ, OPD, DA, AA, and methoxyphenol at the applied potential of 0.2 V.

Table 1. Performance Comparison of Different Modified Electrodes for the Determination of Catechol with Our Proposed Sensor

a

detection limit (μM)

methoda

medium

sensitivity (μA·μM−1·cm−2)

NPC thin film

amperometric

PBS pH 7.2

0.0930

2.0

AuNPs/Fe3O4-APTES-GO P-rGO GR-CMF/laccase/SPCE MOF-ERGO-5 Au-PdNF/rGO Au/Ni(OH)2/rGO AuNPs/CS@N,S co-doped MWCNTS

amperometric DPV amperometric DPV DPV DPV amperometric

PBS PBS PBS PBS PBS PBS PBS

0.1271

0.8 0.18 0.085 0.1 0.8 0.13 0.2

electrode

pH pH pH pH pH pH pH

7.4 7.0 5.0 6.0 7.0 6.0 7.4

0.9320 0.5392 0.3171 0.3968 0.9081

linear range 2 μM−0.5 mM 0.5 mM−10 mM 2−145 μM 5−120 μM 0.2−209.7 μM 0.1−566 μM 2.5−100 μM 0.4−33.8 μM 1 mM−5 mM 1 μM−1 mM

ref 76 9 77 78 79 13 80 this work

DPV, different pulse voltammetry.

electron stoichiometry, D is the calculated diffusion coefficient, and C is the concentration of the redox species (2 mM catechol). The effective surface area of the AuNPs/CS@N,S codoped MWCNTS was 0.13 cm2 for catechol. The value is higher than that of the GCE surface area. Hence, AuNPs/CS@ N,S co-doped MWCNTS can successfully increase the electron transfer for the catechol redox process. Amperometric Response of Catechol at AuNPs/CS@N,S Co-doped MWCNTS Sensor. Generally, the amperomeric response is usually investigated by detecting the current responses at immovable working potentials in the existence of the particular analyte. Thus, the amperometric signals of catechol at AuNPs/CS@N,S co-doped MWCNTS were measured in the bulk solution. Figure 5a shows the classic amperometric response of AuNPs/CS@N,S co-doped MWCNTS for successive injections of different concentrations of catechol into PBS (pH 7.4) at an applied potential of 0.2 V. Apparently, a gradual boost in the oxidation currents values of catechol was obtained with stepwise increasing of the concentrations of catechol in PBS, revealing that AuNPs/ CS@N,S co-doped MWCNTS can accelerate the electrochemical oxidation rate of catechol to o-benzoquione. The inset of Figure 5a shows the current value variation of low concentrations. In Figure 5b, the corresponding current−concentration calibration plots obviously show that the linear regression equations are Ipa1/(μA·cm−2) = −7.202 + 0.9081(C/μM), with

N,S co-doped MWCNTS were boosted with the scan rate increased. Figure S5b indicates that the determination of redox peak current densities were linearly dependent on the scanning rate (mV·s−1) with linear regression values (R2) of 0.9923 for the anodic and 0.9919 for the cathodic peak potentials, indicating that the reaction processes of catechol are a surfacecontrolled process rather than a diffusion-controlled process.8 The diffusion coefficient of the redox species from the catechol to the AuNPs/CS@N,S co-doped MWCNTS was calculated using the following Randles−Sevcik equation:74 Ip = (2.65 × 105)n3/2AD1/2v1/2C

(1)

where Ip is the peak current of the AuNPs/CS@N,S co-doped MWCNTS (Ipa anodic and Ipc cathodic), n is the number of electrons involved or electron stoichiometry, A is the surface area of the electrode (0.07 cm2), D is the diffusion coefficient, C is the concentration of the redox species (2 mM catechol), and v is the scan rate (20 mV·s−1). The calculated D value for AuNPs/CS@N,S co-doped MWCNTS is 3.48 × 10−5 cm2·s−1. The electroactive surface area (Ae) of the AuNPs/CS@N,S co-doped MWCNTS was determined from the calculated D and the Randles−Sevck equation.75 Ae = S /(2.65 × 105)n3/2D1/2C

(2)

where S is the slope of the straight line obtained from the graph Ip versus scan rate 1/2, n is the number of electrons involved or 10932


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ACS Sustainable Chemistry & Engineering Table 2. Determination of Catechol in Ya’An Tap Water at Various Concentrations detected by this method

detected by HPLC

sample

added (μM)

found (μM)

recovery (%)

RSD (%) (n = 3)

found (μM)

recovery (%)

RSD (%) (n = 3)

tap water

40 500 1500

39.86 503.51 1490.75

99.65 100.70 99.38

2.19 2.99 2.67

38.93 487.75 1488.06

97.33 97.55 99.20

2.37 2.25 1.52

R2=0.9992 at a concentration of 1−1000 μM; and Ipa2/(μA· cm−2) = 450.1 + 0.4104(C/μM), with R2=0.9962 at a concentration of 1000−5000 μM. In general, a large linear range of 1−5000 μM, with a moderate sensitivity of 0.9081 μA·μM−1·cm−2 and a low detection limit of 0.2 μM, was observed. Nevertheless, the linearity with a concentration higher than 1000 μM was not as fine as that in the concentration range of 1−1000 μM (R2 = 0.9992 versus R2 = 0.9962). The steady-state current signal is lower than 2 s on AuNPs/CS@N,S co-doped MWCNTS (Figure 6a), proposing a faster electrode response time, which demonstrates a good electro-oxidation performance and a quick electron exchange reaction of the AuNPs/CS@N,S co-doped MWCNTS modified electrode. A comparison of the AuNPs/CS@N,S co-doped MWCNTS electrode with some recently reported catechol biosensors is summarized in Table 1. Under comparison, the AuNPs/CS@N,S co-doped MWCNTS showed a higher sensitivity and a relatively low detection limit. Selectivity Reproducibility and Stability of the AuNPs/CS@ N,S Co-doped MWCNTS Sensor. In order for the exploration of the selectivity of the sensor, the response currents of the modified electrode were studied under optimum conditions, and the various potential interfering substances related to catechol were selected as testing sample. Probable interference was studied by adding various ions and biological compounds to PBS (pH 7.4) in the existence of catechol, and the results are given in Figure 6b. As shown in Figure 6b, no perceptible signals were observed with the addition of NaCl, glucose, urea, H2O2, phenol, resorcinol, and methoxyphenol. However, the addition of HQ, OPD, DA, and AA caused slight increases in the current response, while their current increase extents were lower than the catechol response. The good selectivity of AuNPs/CS@N,S co-doped MWCNTS to catechol can be due to its low oxidation potential in neutral electrolyte while some other organic compounds may require a higher voltage to be oxidized on AuNPs/CS@N,S co-doped MWCNTS. Importantly, it can be observed that AuNPs/CS@N,S co-doped MWCNTS could selectively detect catechol in PBS (pH 7.4) solution at the potential of 0.2 V. The reproducibility was examined from the CV response to catechol at six modified electrodes prepared under the same condition. By calculation, a relative standard deviation (RSD) of 2.25% suggests excellent reproducibility (Figure S6a). From these results, it is identified that the AuNPs/CS@N,S co-doped MWCNTS has significant reproducibility, which makes it applicable for practical use. The long-term stability experiments were implemented to determine the property of the AuNPs/CS@N,S co-doped MWCNTS sensor. By testing the current responses of 2 mM catechol among 20 days, the storage stability data of the sensor were obtained. As shown in Figure S6b, during the first 6 days AuNPs/CS@N,S co-doped MWCNTS exhibited good stability and the current response was still 91% of its original response. Fourteen days later, the current response was about 81% of its original response. Afterward, a 23% activity reduction was

observed after 18 days. The sensor was stored under ambient conditions when it was not in use. These results propose that the AuNPs/CS@N,S co-doped MWCNTS possesses good long-term stability. Determination in Real Samples. For evaluating the property of the prepared sensor in practical measurement, AuNPs/CS@N,S co-doped MWCNTS was applied for the analysis of catechol in tap water sample by addition of the standard solution of catechol. The concentrations of catechol were then calculated from the calibration curves given in Figure 5b. Every concentration was detected three times in parallel. For validating the accuracy of the proposed method, the detection results were compared with those acquired through high-performance liquid chromatography (HPLC) method. Under comparison, the differences between the two methods are acceptable. As seen in Table 2, the results displayed that catechol was not found in the tap water sample, and the recoveries were calculated to be approximately 99.03−99.66% for catechol, demonstrating the satisfactory performance and the reliability of the submitted sensor. Electrochemical Analysis of NO2−. Cyclic Voltammetric Characterization of NO2− on AuNPs/CS@N,S Co-doped MWCNTS. Figure 7 shows the cyclic voltammograms of

Figure 7. CV behaviors obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), and AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), and AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 3 mM nitrite. Scan rate = 50 mV/s.

AuNPs, AuNPs/N,S co-doped MWCNTS, and AuNPs/CS@ N,S co-doped MWCNTS in PBS (pH 7.4) solution with and without the presence of 3 mM NO2− at a scan rate of 50 mV· s−1. As shown from Figure 7, the dotted lines display CV curves of different electrodes in PBS solution. Apparently a pair of redox (anodic and cathodic) peaks was obtained. The appearances of these redox peaks in the PBS solution are because of the onset of gold redox. The electrochemical 10933


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Figure 8. (a) Amperometric responses to successive additions of nitrite and (b) calibration curves obtained at the AuNPs/CS@N,S co-doped MWNCTS electrode in pH 7.4 PBS. The applied potential: +0.9 V.

activities of NO2− (3 mM) oxidation on different electrodes in PBS were shown in Figure 7 (solid lines). The results reveal that the AuNPs/CS@N,S co-doped MWCNTS modified electrode displays a peak current at about 0.9 V, which is higher than that of the electrodes modified with AuNPs/N,S co-doped MWCNTS and AuNPs. The heightened peak current clearly proves that the AuNPs/CS@N,S co-doped MWCNTS electrode has a more effective catalysis ability to oxidize NO2− than AuNPs/N,S co-doped MWCNTS and AuNPs. The oxidation mechanism of NO2− on metal nanoparticles and carbon materials are well-discussed and -documented in previous reports. Accordingly, the oxidation of NO 2 − occurrence through two-electron oxidation from NO2− to NO3− and the reaction mechanism can be written by the following equation.81 NO2− + H 2O → NO3− + 2H+ + 2e−

calculated by eq 1. The values of A and v are the same as those of catechol, while the C of NO2− is 3 mM. According to the equation, the calculated D value of AuNPs/CS@N,S codoped MWCNTS is 5.98 × 10−5 cm2·s−1 for NO2−. The electroactive surface area of the AuNPs/CS@N,S co-doped MWCNTS was determined from calculated D and eq 2. On the basis of the equation, the effective surface area was calculated as 0.16 cm2. Amperometric Performance of NO2− on AuNPs/CS@N,S Co-doped MWCNTS. Normally, the amperometric response is studied by analyzing the current response at an immovable working potential in the existence of a particular analyte. On account of the unconspicuous nitrite oxidation peak potential, it is necessary to select the optimized working potential for the electrochemical determination of nitrite. As shown in Figure S8a, the I−T plots were obtained from the amperometric response of AuNPs/CS@N,S co-doped MWCNTS with consecutive additions of nitrite (100 μM) under 0.85, 0.90, and 0.95 V, respectively. Figure S8a displays that the sensor shows optimal performance when the potential is under 0.90 V. In addition, the highest electrode response current observed at 0.90 V matches well with anodic oxidation peak potential (Figure 7) acquired from the CV analysis. Therefore, in the next experiments 0.90 V was selected as the suboptimal working potential. The steady-state current signal obtained at AuNPs/N,S co-doped MWCNTS is lower than 5 s (Figure S8b), providing a faster electrode response time, which reveals a fast electron exchange response and a good electro-oxidation performance of the AuNPs/CS@N,S co-doped MWCNTS modified electrode. The amperometric method was employed for investigating the performance of the optimized sensor. The fast and sensitive response is observed on AuNPs/CS@N,S co-doped MWCNTS as nitrite is successively injected in Figure 8a. In the case of successive adding of NO2− into the PBS solution with continuous stirring at the working potential of 0.90 V, the Au/NPs CS@N,S co-doped MWCNTS exhibits a ladder rising of current. It can be observed that a clearly defined oxidation current had a good linearity with the concentration of nitrite from 1 to 7000 μM. The linear regression equation (Figure 8b) was expressed as Ipa/(μA·cm−2) = 0.7595(C/μM) + 46.99, with R2 = 0.9960. And the limit of detection (LOD) is obtained to be 0.2 μM on the basis of a 3 signal-noise ratio. The sensitivity

(3)

The electrochemical response current of the AuNPs/N,S codoped MWCNTS electrode is highly superior to that of the AuNPs and GCE electrode, which is attributed to the remarkable electronic conductivity and large surface area with the presence of carbon nanotubes. N,S co-doping of MWCNTS can improve the electro-oxidation performance. Owing to the hydrogen bonding between chitosan molecules and MWCNTS’ functional groups, AuNPs/CS@N,S co-doped MWCNTS have a better electrical conductivity. Influence of Scan Rate. The effect of scan rate on NO2− oxidation behavior was investigated using AuNPs/CS@N,S codoped MWCNTS modified electrode. Figure S7a displayed the CV response of the present electrode in PBS involving 3 mM nitrite at different scan rates from 10 to 100 mV·s−1. It can be observed that as the scan rate increases, the oxidation peak current response boosts, while the oxidation peak potential slightly moved to the high potential direction. As shown in Figure S7b, a plot of the oxidation peak current response of NO2− versus the square root of the scan rate was established, and the plot was found to be linear from 10 to 100 mV·s−1 with a correlation coefficient of 0.9969. The result reveals that the NO2− oxidation at AuNPs/CS@N,S co-doped MWCNTS modified electrode is a traditional diffusion-controlled electrochemical process, and it is consistent with the published reports.82 The diffusion coefficient of the redox species from the NO2− to AuNPs/CS@N,S co-doped MWCNTS was 10934


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ACS Sustainable Chemistry & Engineering Table 3. Comparison of Different Modified Electrodes for Nitrite Sensing

a

electrode

methoda

Fe1.833(OH)0.5O2.5/NG@PDDA Au2Pt1NPs/PyTs-NG PtNPs-ErGO Co3O4−HRP/rGO NGE/PdNC AuCu NCN CcR-SAM-GNP-PPy-SPCE AuNPs/CS@N,S co-doped MWCNTS

amperometric amperometric amperometric amperometric amperometric DPV CV amperometric

medium PBS PBS PBS PBS PBS PBS PBS PBS

pH pH pH pH pH pH pH pH

sensitivity (μA·μM−1·cm−2)

7.4 7.0 5.0 5.5 6.0 7.0 7.4 7.4

0.2614 0.3943 0.0239 0.3424 0.2514 0.1720 0.7595

detection limit (μM)

linear range

ref

0.027 0.19 0.22 0.21 0.11 0.2 0.06 0.2

0.1−1275 μM 0.5−1621 μM 5−1000 μM 1−5400 μM 0.5−1510 μM 10−4000 μM 0.1−1600 μM 1−7000 μM

83 84 85 86 87 88 89 this work

DPV, different pulse voltammetry; CV, cyclic voltammetry.

Figure 9. (a) Amperometric response of AuNPs/CS@N,S co-doped MWCNTS to the successive addition of 100 μM nitrite and each 100-fold addition of interferences NaCl, KCl, CaCl2, MgCl2, Na2CO3, MgSO4, NaNO3, KBr, NaF, NaAc, glucose, and urea at the applied potential of 0.9 V. (b) CV response to nitrite at six AuNPs/CS@N,S co-doped MWCNTS modified electrodes prepared in the same conditions.

was estimated to be 0.7595 μA·uM−1·cm−2. These results clearly confirm that AuNPs/CS@N,S co-doped MWCNTS have excellent sensitivity for nitrite detection by comparison with the recently reported chemically modified electrodes, as shown in Table 3. Therefore, the AuNPs/CS@N,S co-doped MWCNTS can be regarded as an electrochemical sensor for nitrite detection with high sensitivity, wide linear range, and low detection limit. Influence of Interferences and Availability of the Sensor. The selectivity of the sensor is crucial for obtaining further access to real sample analysis, because other metal cations and anions may interfere on the prepared electrode surface. Consequently, we have investigated the selectivity of the sensor for NO2− detection in the existence of metal cations and anions using amperometry; the concentrations of interfering substances are 100-fold that of NO2−, and the experimental conditions are the same as those in Figure 8a. First, 10 μM NaNO2 was added twice, followed by 100-fold the concentrationa of NaCl, KCl, CaCl2, MgCl2, Na2CO3, MgSO4, NaNO3, KBr, NaF, and NaAc. Subsequently 10 μM NaNO2 was added again, and then 10-fold concentrations of glucose and urea were added. The selectivity results are shown in Figure 9a, and it can be broadly seen that there are obvious changes in current response while nitrite was added into the solution. Most importantly, these changes can quickly reach a stable value. On the contrary, while the interference species were added, there is no significant current change. These results reveal that AuNPs/CS@N,S co-doped MWCNTS have

excellent selectivity and anti-interference capability for nitrite detection. The repeatability of the created sensor was investigated in 3 mM NO2− by CV. The repeatability data for the oxidation peak current was evaluated at six different AuNPs/CS@N,S codoped MWCNTS modified electrodes, and the results are shown in Figure 9b. The relative standard deviation of NO2− detection at six different electrodes was calculated as 3.90%, which denotes good repeatability of the AuNPs/CS@N,S codoped MWCNTS electrode. The long-term stability of the modified electrode was evaluated regularly up to 20 days, and the results are shown in Figure S9. Before testing, the AuNPs/CS@N,S co-doped MWCNTS modified electrode was stored under ambient conditions, and the NO2− (3 mM) oxidation peak current response was investigated in PBS solution by CV. The fabricated electrode persists at 83.06% of initial current response to NO2− after 20 days of storage. The result demonstrates the adequate stability of the modified electrode, and that is on account of the high chemical stability of the AuNPs/CS@N,S co-doped MWCNTS. Real Sample Analysis. A good sensor is not limited to the standard samples but can be used in actual samples. For the purpose of assessing the applicability for practical use of the suggested method, we applied the AuNPs/CS@N,S co-doped MWCNTS sensor in various real samples to determine nitrite. Fermented bean curd, ham sausage, and mustard tuber were obtained from local markets (Ya’An, China) 10935


Research Article

ACS Sustainable Chemistry & Engineering Table 4. Analytical Results for Detection Nitrite in Real Samples detected by this method

a

detected by UV−vis spectrophotometer

type of samplea

initial (μM)

added (μM)

found (μM)

recovery (%)

RSD (%) (n = 3)

found (μM)

recovery (%)

RSD (%) (n = 3)

fermented bean curd

0.67

ham sausage

1.09

mustard tuber

0.86

10 70 100 15 60 150 5 50 150

10.35 67.70 101.87 14.57 62.70 153.37 5.25 53.96 157.38

97.00 95.80 101.19 90.55 102.64 101.51 89.60 106.10 104.32

2.87 2.08 2.34 2.75 1.82 2.16 3.01 2.66 2.39

9.71 70.28 101.72 15.24 62.80 150.88 5.17 48.23 154.67

91.00 99.45 101.04 94.72 102.80 99.86 88.23 94.82 102.25

1.93 2.65 3.11 2.45 2.07 2.54 2.89 2.67 2.02

Ingredients were obtained from the local supermarket.

Author Contributions

The extraction of nitrite ions from fermented bean curd, ham sausage, and mustard tuber was accomplished by leaving a certain amount of minced samples in deionized water at about 90 °C for 15 min and further gathering the remaining liquid through filtering. The nitrite detection in real samples was performed by standard addition method. Meanwhile, we have used UV−vis spectrophotometry to validate the accuracy and reliability of the modified electrode. The results are summarized in Table 4. The results obtained with the modified electrode are sound and comparable to those of the UV−vis spectrophotometry. Moreover, the satisfactory recoveries of the three measurements prove that the prepared electrode can be practically applied in determining nitrite in real samples.

†

H.R. and Y.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a grant from the Two-way Support Programs of Sichuan Agricultural University (Project No.03572228) and the Education Department of Sichuan Provincial, People’s Republic of China (Grant No. 16ZA0039). We thank the anonymous reviewers for their valuable suggestions.

■

■

CONCLUSIONS We have exploited a highly sensitive catechol and nitrite sensor employing a glassy carbon electrode modified with AuNPs/ CS@N,S co-doped MWCNTS. The SEM and TEM results demonstrated that AuNPs were well-distributed when deposited on the electrode. The XRD and XPS results confirmed the successful doping of N and S into the nanotube framework. The sensor indicated an excellent analytical feature toward the detection of catechol and nitrite, with high sensitivity, low detection limit, eminent selectivity, and longterm stability. Hence, it is thought that the AuNPs/CS@N,S co-doped MWCNTS nanocomposite can be considered as a promising material for biosensors.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02840. TGA thermograms, time−current and −voltage curves, electrodeposition step number effect, pH effects on peak currents and potentials, CV and current densities plots, CV response to catechol and sensor stability plots, and amperometric current responses and response time (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hanbing Rao: 0000-0003-2453-2426 10936


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Chitosan@N,S Co-doped Multiwalled Carbon

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