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Gold nanoparticle/chitosan@N,S co-doped multi-walled 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02840 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Sustainable Chemistry & Engineering

Gold nanoparticle/chitosan@N,S co-doped multi-walled 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, Yanying Wang* College of Science, Sichuan Agricultural University, Xin Kang Road, Yucheng District, Ya’an 625014, China, P.R.China

Corresponding author at: College of Science, Sichuan Agricultural University, Ya’an 625014, China, P.R.China. E-mail addresses: [email protected] or [email protected]

The authors wish it to be known that, in their opinions, Hanbing Rao and Yiting Liu should be regarded as joint First Authors.

ACS Paragon Plus Environment


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ABSTRACT Given the catechol and nitrite can be easily released into the environment and cause serious damage to our physical and ecological environment, it’s 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 multi-walled carbon nanotubes (CS@N,S co-doped MWCNTS) composite modified glassy carbon electrode (GCE). The preliminary prepared composite materials were characterized by transmission electron microscope (TEM)， scanning electron microscope (SEM), X-ray

diffraction

(XRD),

X-ray

photoelectron

spectroscopy

(XPS)

and

thermogravimetric analysis (TGA). The AuNPs/CS@N,S co-doped MWCNTS modified electrode shows a good linear relationship between 1 µmol/L (µM) to 5000 µM for catechol and the detection limit was calculated as 0.2 µM. For nitrite the linear relationship was 1 µM to 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, multi-walled carbon nanotubes, chitosan, catechol, nitrite, electrochemical sensor


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Introduction Nowadays, the development of methods for sensitive and selective phenolic 1

. Even at very low

compounds detection are of great emergence interest

concentrations, these compounds are extremely toxic to human health and difficult to degrade in the ecological system 2, that’ s why the US Environmental Protection Agency (EPA) and the European Union (EU) regard them as environmental pollutants 3-5

. Catechol (1,2-dihydroxybenzene) is one of phenolic compounds which is

widespreadly applied in several fields such as cosmetics, textile, antioxidant, dye, photography, petroleum refinery, plastic, agricultural chemicals and medicines 6. In the process of producing and application in these fields

7, 8

, the catechol might be

casually released into the environment as pollutants. Skin contact with catechol leads to eczema in humans 9, while numerous contact to catechol through skin adsorbing, inhaling steam or direct ingesting can lead to serious burns and deleterious effects on liver, heart, lung and central nervous system

10-13

. So far, there are several methods

with good detection limit have been established, such as fluorescence chromatography

15

, high performance liquid chromatography

14

, gas

16

, chemiluminescence

17

. Nevertheless, major defects of current methods contain higher cost, more

laboratory setting, relatively high limit of detection and inability to 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 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 be very easy to react with secondary amines to constitute the teratogenetic, mutagenic and carcinogenic species, bring on the esophagus and gastric cancer

22, 23

. On the

grounds of the World Health Organization (WHO), the maximum NO2- allowable content in drinking water is 43.48 µM, while the excess quantity in water can result in breathe shortness and “Blue Baby Syndrome” diseases

23


. Up to now, numerous


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analytical approaches have been already used for the determination of nitrite, comprising spectrophotometry 25

microextraction spectroscopy

24

, micro-spectrophotometry with liquid-phase

, chromatography

30

26, 27

, capillary electrophoresis

and electrochemical method

31,

28, 29

, Raman

32

. Under compares, the

electrochemical techniques, on the score of the facile manufacture of electrode material, straightforward operation, relatively inexpensive instrument, rapid response time and high sensitivity are often prior 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. Recent years, interests have focused on nano-scaled 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 account of strengths, such as their mass transport, improved active area and electrocatalytic effect

38, 39

.

Compared to majority electrodes, these merits can provide a better analytical performance

40-43

. Besides, in terms of new biosensors, the nano-sized AuNPs show

good electrical properties, good mechanical resistance and good biocompatibility, and have large active surface area which can increase the electron transfer, electrode conductivity and enhance the analytical sensitivity

44

. Therefore, several researches

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 recent two decades 37. Multi-walled carbon nanotubes (MWCNTS), a nanoscale tubular material made of graphitic laminas rolled into closed concentric cylinders 45, 46, is one kind of excellent electrode


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material, because of their unique physical-chemical properties such as chemical stability, electric conductivity as well as high surface area 47, 48. They have significant applications such as batteries, electron field emission sources, chemical sensors and nano-electronic 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 carbon framework are incorporated of 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 electronegativities elements

dopants can bring about a synergistic influence because of a unique electron distribution 54-57. Finally, chitosan (CS) is a non-toxic and inexpensive support for nanomaterials. Its high porosity character and hydrophilic causes low steric hindrance and less resistance mass transfer to the nanomaterials. Chitosan can 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 as-prepared electrode shows high abundant porosity and large specific surface area, securing the high electrochemical performance of our sensor. The property of the catechol and nitrite biosensor was evaluated

by cyclic voltammetry and

amperometric,

including sensitivity,

reproducibility and stability. Importantly, the sensor is performing a well-respond current in both catechol and nitrite detection. Hence, the AuNPs/CS@N,S co-doped



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MWNTS modified electrode is a viable candidate for electrochemical biosensor.

Scheme. 1 Schematic illustration for fabricating AuNPs/CS@N,S co-doped MWCNTS.

Experimental Materials and reagents Multi-walled carbon nanotubes (diam.40-60nm, 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, viscosity90.0%) was purchased from


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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 GCE (Φ=3mm), 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. XRD data were gathered in a D/Max-RA Diffractometer (DX-2700, Dan Dong, China) with Cu Kα-radiation (λ=0.1548nm) operated at 40 kV and 100 Ma. TGA data were analyzed on a TGA209F3A Tarsus instrument (NETZSCH-Gerätebau GmbH, Germany).SEM pictures were collected on a JSM4800F instrument (JEOL, Japan) and TEM pictures were obtained from JEOL2100F (JEOL, Japan). 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. 500mg of MWCNTS were mixed with 15 ml 65% HNO3 and 45 ml 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 nearly neutral environment, 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



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mg of oxMWCNTS were first finely grinded 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 under nitrogen flow and then cooled down to room temperature. The formed product was cleaned using ultrapure water and dried overnight in a vacuum oven at 50 °C afterwards. Preparation of CS@N,S co-doped MWCNTS CS@N,S-doped MWCNTS were prepared according to a common method. 100mg chitosan powder was dissolved in 2% (v/v) acetic acid solution. 100mg N,S-doped MWCNTS were added to the solution and gently stirred until homogenous suspension was 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 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. 5 mg CS@N,S-doped MWCNTS were dispersed in 5 ml deionized water using ultrasonication method to acquire a homogeneous black suspension liquid. Whereafter, 10 µl of CS@N,S-doped 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 was consisted of 1 mM HAuCl4 (0.1M KNO3). CS@N,S-doped MWCNTS/GCE

electrode

was

immersed

in

the

plating

bath

using

chronoamperometry at the potential from -0.2 V to 0 V for 200 s. In fact, different electrodes can be obtained by changing the step numbers.

Results and discussions Characterization of AuNPs/CS@N,S co-doped MWCNTS Morphological and structural characterization


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Fig. 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, (G) AuNPs/CS@N,S co-doped MWCNTS. TEM images of (H) MWCNTS, (I) oxMWCNTS, (J) N,S co-doped MWCNTS, (K)CS@N,S co-doped MWCNTS.

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 Fig. 1A and 1H, we can clearly see 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 (Fig. 1B and 1I). Fig. 1C and 1J sustained that nitrogen and sulfur co-doping did not break the tubular structure of MWCNTS. By comparison, Fig. 1D and 1K display a larger size, the boundary is not as smooth as that from Fig. 1J, and we cannot see the middle-hollowed structure clearly. It indicates that the chitosan was successfully wrapped around the carbon



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nanotubes. The structures of electrodeposition AuNPs were shown in Fig. 1E-G. The AuNPs are nearly spherical and the AuNPs are strongly assembled onto the electrode surface (Fig. 1E) on account of the electrodeposition method. When N,S co-doped MWCNTS prior dispensed onto the electrode surface, the electrodeposition of AuNPs proceed. 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 (Fig. 1F), but the sample exhibits significant aggregation. As seen in Fig. 1G, the number of cavernous caves and the aggregation effect are both declined. It is because that chitosan increases the biocompatibility between AuNPs and MWCNTS, making a better dispersion of AuNPs on MWCNTS. X-ray diffraction analysis

Fig. 2 The XRD patterns of (A) MWCNTS, (B) oxMWCNTS (C) N,S co-doped MWCNTS, (D) CS@N,S co-doped MWCNTS, (E) AuNPs/CS@N,S co-doped MWCNTS.

The XRD data was used to ascertain the chemical compositions and phases of the products. XRD patterns of MWCNTS, oxMWCNTS, N,S co-doped MWCNTS, CS@N,S co-doped MWCNTS and AuNPs/CS@N,S co-doped MWCNTS were shown in Fig. 2. for a better understanding of the structure and phase of the


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nanocomposite. The XRD pattern peaks in Fig. 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 Fig. 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 co-doped MWCNTS (Fig. 2A-D) were calculated from C (002) diffraction peak to be 2.36 nm, 2.37 nm, 2.37 nm 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 co-doped MWCNTS is slightly bigger than others, and the result manifested that the chitosan may successfully wrap onto the surface of MWCNTS. X-ray photoelectron spectroscopy analysis X-ray photoelectron spectroscopy (XPS) is sensitive to the surface nature and composition 63. The survey spectrum (Fig. 3(a)) indicates the presence of Au, C, O, N and S in AuNPs/CS@N,S co-doped MWCNTS. As seen from Fig. 3(b), the black line (Fig. 3(b-1)) is the pure MWCNTS with no treatment, and only the peaks of carbon element exist. Under the oxidation of mixed acid, carboxyl groups were added onto MWCNTS framework, and the peaks of oxygen element appeared (Fig. 3(b-2)). After co-doping treatment of nitrogen and sulfur, corresponding peaks also appeared in the survey curve (Fig. 3(b-3)). Since the main elements of chitosan are carbon, nitrogen and oxygen, no other peaks appeared in the survey curve in Fig. 3(b-4). With the use of 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 (Fig. 3(c)) 64. As shown in Fig. 3(e), the N1s 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 graphitic-N

65

. Fig. 3(f) shows the N1s peak components in the

spectra of CS@N,S co-doped MWCNTS obtained at a BE = 398.8 eV for pyridinic-N,



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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 Fig. 3(f). Sulfur dopants were mainly bound in structure in the form of thiophenic S (C-S-C), locating at 162.5 and 164.8 eV, and the other formation of oxidized S (C-Sox-C) were observed at 168.6 eV (Fig. 3(d)) 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.


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Fig. 3 XPS survey spectra of the (a) AuNPs/CS@N,S co-doped MWCNTS nanocomposite, (b) 1-MWCNTS, 2-oxMWCNTS, 3-N,S co-doped MWCNTS and 4-CS@N,S co-doped MWCNTS, (c) Au4f region, (d) S2p region, (f) N1s region in AuNPs/CS@N,S co-doped MWCNTS and (e) N1s region in N,S co-doped MWCNTS.

Thermogravimetric analysis Thermogravimetric analysis (TGA) data further identify the weight loss of MWCNTS in different treatments. Fig. S1 shows TGA thermograms of MWCNTS, oxMWCNTS, N,S co-doped MWCNTS and CS@N,S co-doped MWCNTS in the temperature range from room temperature to 900 °C under condensed Ar at a rate of 10 °C/min. For MWCNTS (Fig. 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 the previous literature

67

. In Fig. 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 °C to 400°C is attributed to the disintegration of -OH, -COOH and epoxy groups from MWCNTS. The main weight loss from 450 °C to 650 °C is attributed to the carbon skeleton destructing of the MWCNTS framework. By comparison with CS@N,S co-doped MWCNTS (Fig. S1D), a weight loss from 200 °C to 300 °C is related to the deacetylation, cleavage of 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 (0.1M KNO3). CS@N,S-doped MWCNTS/GCE electrode was immerged in the electroplating bath using chronoamperometry at the potential from -0.2 V to 0 V for 200 s

9, 70

. The

electrochemical behavior of electrodeposited AuNPs using chronoamperometry method was shown in Fig. S2(a) and Fig. S2(b). For the observation of the influence of electrodeposition step number, AuNPs were deposited on CS@N,S co-doped MWCNTS/GCE by applying a potential from 0



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V~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 Fig. S3, 3 times of the electrodeposition step number was considered as 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 reduces the electrochemical active surface area of the electrode, which is attributed to the negative affect of large-sized nanoparticles on electrocatalytic activity. Electrochemical measurements of catechol Electrochemical responses of catechol on AuNPs/CS@N,S co-doped MWCNTS Fig. 4 shows the cyclic voltammograms of 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. Accordingly, the dotted lines of Fig. 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 Fig. 4 shows 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 display an optimal capability, higher than the electrodes modified with AuNPs/N,S co-doped MWCNTS and AuNPs. 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 V and 0.1 V were obtained. The peak at 0.4 V was assumably 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.


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

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 Fig. S4(a), 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 Fig. S4(b). It can be observed that with pH value increasing from 5.0 to 8.0 the anodic peak potentials of catechol shift towards to the low potential, evincing that the protons are 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 two protons and two electrons process, proposing that the electrochemical catechol redox reaction at AuNPs/CS@N,S



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co-doped MWCNTS suppose 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:

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 10-100 mV/s was investigated by CV (Fig. S5(a)). The anodic and cathodic peak currents of catechol at AuNPs/CS@N,S co-doped MWCNTS was boosted with the scan rate increased. Fig. S5(b) indicates that the determination of redox peak current densities were linearly dependent on the scanning rate (mV/s) 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 is a surface controlled 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) n3/2AD1/2 v1/2 C

(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 redox species (2 mM of catechol), and v is the scan rate (20 mVs-1). The calculated D values for AuNPs/CS@N,S co-doped MWCNTS is 3.48×10-5 cm2/s. The electro-active 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 electron stoichiometry, D is the calculated


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diffusion coefficient and C is the concentration of the redox species (2 mM catechol). The effective surface area of the AuNPs/CS@N,S co-doped 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 was measured in the bulk solution. Fig. 5(a) 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 the concentrations of catechol in PBS, revealing that AuNPs/CS@N,S co-doped MWCNTS can accelerate electrochemical oxidation rate of catechol to o-benzoquione. The inset of Fig. 5(a) shows the current value variation of low concentrations. In Fig. 5(b), the corresponding current-concentration calibration plots obviously shows that the linear regression equations are Ipa1 (µA·cm-2) =-7.202+0.9081C (µM), with R2=0.9992 at a concentration of 1 µM-1000 µM; and Ipa2 (µA·cm-2) =450.1+0.4104C (µM), with R2=0.9962 at a concentration of 1000 µM-5000 µM. In general, a large linear range of 1 µM-5000 µM with a moderate sensitivity of 0.9081 µA·µM-1·cm-2 and a low detection limit of 0.2 µM were observed. Nevertheless, the linearity with a concentration higher than 1000 µM was not as fine as that in the concentration range of 1 µM-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 (Fig. 6(a)), proposing a faster electrode response time, which demonstrates a good electrooxidation 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



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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.

Fig. 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.

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 Fig. 6(b). As shown in Fig. 6(b), 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 slightly increase in the current response, while their current increase extents are 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.


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Fig. 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.

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 (Fig. S6(a)). From these results, it is identified that the AuNPs/CS@N,S co-doped MWCNTS has significant reproducibility, which make 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 datum of the sensor were obtained. As shown in Fig. S6(b), during the first 6 days AuNPs/CS@N,S co-doped MWCNTS exhibited good stability, the current response is still 91% of its original response. 14 days later, the current response is 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.



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Table 1 Performance comparison of different modified electrodes for the determination of catechol with our proposed sensor. Electrode

Method

Medium

Sensitivity

Detection

Linear

(µA·µM-1·cm-2)

limit (µM)

range

Reference

2 µM-0.5 mM NPC thin film

Amperometric

PBS pH 7.2

0.0930

2.0

0.5 mM-10

76

mM AuNPs/Fe3O4Amperometric

PBS pH 7.4

0.1271

0.8

2-145 µM

9

DPV

PBS pH 7.0

-

0.18

5-120 µM

77

Amperometric

PBS pH 5.0

0.9320

0.085

0.2-209.7 µM

78

MOF-ERGO-5

DPV

PBS pH 6.0

0.5392

0.1

0.1-566 µM

79

Au-PdNF/rGO

DPV

PBS pH 7.0

0.3171

0.8

2.5-100 µM

13

DPV

PBS pH 6.0

0.3968

0.13

0.4-33.8 µM

80

Amperometric

PBS pH 7.4

0.9081

0.2

APTES-GO P-rGO GR-CMF/lacca se/SPCE

Au/Ni(OH)2/rG O AuNPs/CS@N, 1 µM-1 mM S co-doped

This work 1 mM-5 mM

MWCNTS *DPV: Different pulse voltammetry

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 Fig. 5(b). 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


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differences between the two methods are acceptable. As seen in Table 2, the results displayed that catechol wasn’t found in the tap water sample, and the recoveries were calculated to be approximately 99.03%-99.66% for catechol, demonstrating the satisfactory and the reliability of the submitted sensor.

Table 2 Determination of catechol in Ya’An tap water at various concentrations Detected by this method

Detected by HPLC

Added Sample

Found

Recovery

RSD

Found

Recovery

RSD

(µM)

(%)

(n=3)

(µM)

(%)

(n=3)

40

39.86

99.65

2.19%

38.93

97.33

2.37%

500

503.51

100.70

2.99%

487.75

97.55

2.25%

1500

1490.75

99.38

2.67%

1488.06

99.20

1.52%

(µM)

Tap water

Electrochemical analysis of NO2Cyclic voltammetric characterization of NO2- on AuNPs/CS@N,S co-doped MWCNTS Fig. 7 shows the cyclic voltammograms (CVs) of 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 Fig. 7, the dotted lines display CVs 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 activities of NO2- (3 mM) oxidation on different electrodes in PBS were shown in Fig. 7 (solid lines). The results reveal that the AuNPs/CS@N,S co-doped MWCNTS modified electrode display a peak current at



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about 0.9 V, that is higher than 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 the previous reports. Accordingly, the oxidation of NO2occurrence through two-electron oxidation from NO2- to NO3- and the reaction mechanism can be written by the followed equation 81. NO2- + H2O → NO3- + 2H+ + 2e-

(3)

The electrochemical response current of the AuNPs/N,S co-doped 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 electrooxidation performance. Owing to the hydrogen bonding between chitosan molecules and MWCNTS’ functional groups, AuNPs/CS@N,S co-doped MWCNTS has a better electrical conductivity.

Fig. 7 The CV behaviors obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), AuNPs/CS@N,S co-doped


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MWCNTS (f) in pH 7.4 PBS with 3 mM nitrite. Scan rate=50 mV/s.

Influence of scan rate The effect of scan rate on NO2- oxidation behavior was investigated using AuNPs/CS@N,S co-doped MWCNTS modified electrode. Fig. S7(a) displayed the CV response of the present electrode in PBS involving 3 mM nitrite at different scan rates from 10 to 100 mV/s. 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 Fig. S7(b), a plot of the oxidation peak current response of NO2- vs. the square root of scan rate was established, and the plot was found to be linear from 10 to 100 mV/s 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 calculated by equation (1). The value of A and v are same to those of catechol, while the C of NO2- is 3 mM. According to the equation, the calculated D values of AuNPs/CS@N,S co-doped MWCNTS is 5.98×10-5 cm2/s for NO2-. The electro-active surface area of the AuNPs/CS@N,S co-doped MWCNTS was determined from the calculated D and the equation (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 Fig. S8(a), 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 V, 0.90 V and 0.95 V, respectively. Fig. S8(a) 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



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oxidation peak potential (Fig.7) acquired from the CV analysis. Therefore, in the next experiments 0.90 V was selected as the sub-optimal working potential. The steady-state current signal obtained at AuNPs/N,S co-doped MWCNTS is lower than 5 s (Fig. S8(b)), providing a faster electrode response time, which reveals a fast electron exchange response and a good electrooxidation 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 Fig. 8(a). 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 µM to 7000 µM. The linear regression equations (Fig. 8(b)) were expressed as Ipa (µA·cm-2) =0.7595C (µM)+46.99, with R2=0.9960. And the limit of detection (LOD) is obtained to be 0.2 µM on the basis of 3 signal-noise ratio. The sensitivity was estimated to be 0.7595 µA·uM-1·cm-2. These results clearly confirm that AuNPs/CS@N,S co-doped MWCNTS has excellent sensitivity for nitrite detection by comparing 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.

Fig. 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: +0.9V.


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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 anion 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 concentration of interfering substance are 100 fold of NO2- and experimental conditions are same to Fig. 8(a). Firstly, 10 µM NaNO2 was added twice, followed by 100-fold the concentration of NaCl, KCl, CaCl2, MgCl2, Na2CO3, MgSO4, NaNO3, KBr, NaF, NaAc. Subsequently 10 µM NaNO2 was added again, and then 10-fold concentration of glucose and urea was added. The selectivity results are shown in Fig. 9(a), 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 has 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 co-doped MWCNTS modified electrodes and the results are shown in Fig. 9(b). 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 co-doped MWCNTS electrode.

Fig. 9 (a) Amperometric response of AuNPs/CS@N,S co-doped MWCNTS to the successive addition of 100 µM



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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 electrode prepared in the same conditions.

The long-term stability of the modified electrode was evaluated regularly up to 20 days and the results are shown in Fig. 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 83.06% of initial current response to NO2- after 20-day 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.

Table 3 Comparison of different modified electrodes for nitrite sensing.

Electrode

Method

Medium

Sensitivity

Detection

Linear

(µA·µM-1·cm-2)

limit (µM)

range

Reference

Fe1.833(OH)0.5O Amperometric

PBS pH 7.4

0.2614

0.027

0.1-1275 µM

83

Amperometric

PBS pH 7.0

0.3943

0.19

0.5-1621 µM

84

Amperometric

PBS pH 5.0

-

0.22

5-1000 µM

85

Amperometric

PBS pH 5.5

0.0239

0.21

1-5400 µM

86

NGE/PdNC

Amperometric

PBS pH 6.0

0.3424

0.11

0.5-1510 µM

87

AuCu NCN

DPV

PBS pH 7.0

0.2514

0.2

10-4000 µM

88

CV

PBS pH 7.4

0.1720

0.06

0.1-1600 µM

89

Amperometric

PBS pH 7.4

0.7595

0.2

1-7000 µM

This work

2.5/NG@PDDA

Au2Pt1NPs/PyT s-NG PtNPs-ErGO Co3O4-HRP/rG O

CcR-SAM-GN P-PPy-SPCE AuNPs/CS@N, S co-doped MWCNTS


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*DPV: Different pulse voltammetry CV: Cyclic voltammetry

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) 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 the 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.



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Table 4 The analytical results for detection nitrite in real samples Detected by UV-vis Detected by this method Type of

Initial Added

samples

(µM)

(µM)

spectrophotometer Found

Recovery

RSD

Found

Recovery

RSD

(µM)

(%)

(n=3)

(µM)

(%)

(n=3)

10

10.35

97.00

2.87%

9.71

91.00

1.93%

70

67.70

95.80

2.08%

70.28

99.45

2.65%

100

101.87

101.19

2.34%

101.72

101.04

3.11%

15

14.57

90.55

2.75%

15.24

94.72

2.45%

60

62.70

102.64

1.82%

62.80

102.80

2.07%

150

153.37

101.51

2.16%

150.88

99.86

2.54%

5

5.25

89.60

3.01%

5.17

88.23

2.89%

50

53.96

106.10

2.66%

48.23

94.82

2.67%

150

157.38

104.32

2.39%

154.67

102.25

2.02%

Fermented 0.67 bean curd

Ham 1.09 sausage

Mustard 0.86 tuber

*Above ingredients were obtained from the local supermarket.

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 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 towards the detection of catechol and nitrite, with high sensitivity, low detection limit, eminent selectivity and long-term stability. Hence, it is thought that the AuNPs/CS@N,S co-doped MWCNTS nanocomposite can be considered as a promising material to biosensors.


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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, PR China (Grant No. 16ZA0039). We thank the anonymous reviewers for their valuable suggestions.

Supporting Information Supporting information associated with this article can be found in the online version.

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Fe1.833(OH)0.5O2.5-decorated N-doped graphene ternary hierarchical nanocomposite. Sensors and Actuators B: Chemical. 2017, 243, 184-194. http://doi.org/10.1016/j.snb.2016.11.124. 84. Li, Z.; An, Z.; Guo, Y.; Zhang, K.; Chen, X.; Zhang, D.; Xue, Z.; Zhou, X.; Lu, X. Au-Pt bimetallic nanoparticles supported on functionalized nitrogen-doped graphene for sensitive detection of nitrite. Talanta. 2016, 161, 713-720. http://doi.org/10.1016/j.talanta.2016.09.033. 85. Vijayaraj, K.; Jin, S.-H.; Park, D.-S. A Sensitive and Selective Nitrite Detection in Water Using Graphene/Platinum Nanocomposite. Electroanalysis. 2017, 29 (2), 345-351. 10.1002/elan.201600133. 86. Liu, H.; Guo, K.; Lv, J.; Gao, Y.; Duan, C.; Deng, L.; Zhu, Z. A novel nitrite biosensor based on the direct electrochemistry of horseradish peroxidase immobilized on porous Co3O4 nanosheets and reduced graphene oxide composite modified electrode. Sensors and Actuators B: Chemical. 2017, 238, 249-256. http://doi.org/10.1016/j.snb.2016.07.073. 87. Shen, Y.; Rao, D.; Bai, W.; Sheng, Q.; Zheng, J. Preparation of high-quality palladium nanocubes heavily deposited on nitrogen-doped graphene nanocomposites and their application for enhanced electrochemical sensing. Talanta. 2017, 165, 304-312. http://doi.org/10.1016/j.talanta.2016.12.067. 88. Huang, S.-S.; Liu, L.; Mei, L.-P.; Zhou, J.-Y.; Guo, F.-Y.; Wang, A.-J.; Feng, J.-J. Electrochemical sensor for nitrite using a glassy carbon electrode modified with gold-copper nanochain networks.



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Microchimica Acta. 2016, 183 (2), 791-797. 10.1007/s00604-015-1717-z. 89. Santharaman, P.; Venkatesh, K. A.; Vairamani, K.; Benjamin, A. R.; Sethy, N. K.; Bhargava, K.; Karunakaran, C. ARM-microcontroller based portable nitrite electrochemical analyzer using cytochrome c reductase biofunctionalized onto screen printed carbon electrode. Biosensors and Bioelectronics. 2017, 90, 410-417. http://doi.org/10.1016/j.bios.2016.10.039.

Figure captions Scheme. 1. Schematic illustration for fabricating AuNPs/CS@N,S co-doped MWCNTS. Fig. 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, (G) AuNPs/CS@N,S co-doped MWCNTS. TEM images of (H) MWCNTS, (I) oxMWCNTS, (J) N,S co-doped MWCNTS, (K)CS@N,S co-doped MWCNTS. Fig. 2 The XRD patterns of (A) MWCNTS, (B) oxMWCNTS (C) N,S co-doped MWCNTS, (D) CS@N,S co-doped MWCNTS, (E) AuNPs/CS@N,S co-doped MWCNTS. Fig. 3 XPS survey spectra of the (a) AuNPs/CS@N,S co-doped MWCNTS nanocomposite, (b) 1-MWCNTS, 2-oxMWCNTS, 3-N,S co-doped MWCNTS and 4-CS@N,S co-doped MWCNTS, (c) Au4f region, (d) S2p region, (f) N1s region in AuNPs/CS@N,S co-doped MWCNTS and (e) N1s region in N,S co-doped MWCNTS. Fig. 4 CVs obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 2 mM catechol. Scan rate=50 mV/s. Fig. 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. Fig. 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. Fig. 7 The CV behaviors obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 3 mM nitrite. Scan rate=50 mV/s. Fig. 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: +0.9V. Fig. 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 electrode prepared in the same conditions.


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Scheme. 1



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Fig. 1

Fig. 2


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Fig. 3



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Fig. 4

Fig. 5


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Fig. 6

Fig. 7



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Fig. 8

Fig. 9


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Table 1 Performance comparison of different modified electrodes for the determination of catechol with our proposed sensor. Electrode

Method

Medium

Sensitivity

Detection

Linear

(µA·µM-1·cm-2)

limit (µM)

range

Reference

2 µM-0.5 mM NPC thin film

Amperometric

PBS pH 7.2

0.0930

2.0

0.5 mM-10

76

mM AuNPs/Fe3O4Amperometric

PBS pH 7.4

0.1271

0.8

2-145 µM

9

DPV

PBS pH 7.0

-

0.18

5-120 µM

77

Amperometric

PBS pH 5.0

0.9320

0.085

0.2-209.7 µM

78

MOF-ERGO-5

DPV

PBS pH 6.0

0.5392

0.1

0.1-566 µM

79

Au-PdNF/rGO

DPV

PBS pH 7.0

0.3171

0.8

2.5-100 µM

13

DPV

PBS pH 6.0

0.3968

0.13

0.4-33.8 µM

80

Amperometric

PBS pH 7.4

0.9081

0.2

APTES-GO P-rGO GR-CMF/lacca se/SPCE

Au/Ni(OH)2/rG O AuNPs/CS@N, 1 µM-1 mM S co-doped

This work 1 mM-5 mM

MWCNTS *DPV: Different pulse voltammetry



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Table 2 Determination of catechol in Ya’An tap water at various concentrations Detected by this method

Detected by HPLC

Added Sample

Found

Recovery

RSD

Found

Recovery

RSD

(µM)

(%)

(n=3)

(µM)

(%)

(n=3)

40

39.86

99.65

2.19%

38.93

97.33

2.37%

500

503.51

100.70

2.99%

487.75

97.55

2.25%

1500

1490.75

99.38

2.67%

1488.06

99.20

1.52%

(µM)

Tap water

Table 3 Comparison of different modified electrodes for nitrite sensing.

Electrode

Method

Medium

Sensitivity

Detection

Linear

(µA·µM-1·cm-2)

limit (µM)

range

Reference

Fe1.833(OH)0.5O Amperometric

PBS pH 7.4

0.2614

0.027

0.1-1275 µM

83

Amperometric

PBS pH 7.0

0.3943

0.19

0.5-1621 µM

84

Amperometric

PBS pH 5.0

-

0.22

5-1000 µM

85

Amperometric

PBS pH 5.5

0.0239

0.21

1-5400 µM

86

NGE/PdNC

Amperometric

PBS pH 6.0

0.3424

0.11

0.5-1510 µM

87

AuCu NCN

DPV

PBS pH 7.0

0.2514

0.2

10-4000 µM

88

CV

PBS pH 7.4

0.1720

0.06

0.1-1600 µM

89

Amperometric

PBS pH 7.4

0.7595

0.2

1-7000 µM

This work

2.5/NG@PDDA

Au2Pt1NPs/PyT s-NG PtNPs-ErGO Co3O4-HRP/rG O

CcR-SAM-GN P-PPy-SPCE AuNPs/CS@N, S co-doped MWCNTS *DPV: Different pulse voltammetry CV: Cyclic voltammetry


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Table 4 The analytical results for detection nitrite in real samples Detected by UV-vis Detected by this method Type of

Initial Added

samples

(µM)

(µM)

spectrophotometer Found

Recovery

RSD

Found

Recovery

RSD

(µM)

(%)

(n=3)

(µM)

(%)

(n=3)

10

10.35

97.00

2.87%

9.71

91.00

1.93%

70

67.70

95.80

2.08%

70.28

99.45

2.65%

100

101.87

101.19

2.34%

101.72

101.04

3.11%

15

14.57

90.55

2.75%

15.24

94.72

2.45%

60

62.70

102.64

1.82%

62.80

102.80

2.07%

150

153.37

101.51

2.16%

150.88

99.86

2.54%

5

5.25

89.60

3.01%

5.17

88.23

2.89%

50

53.96

106.10

2.66%

48.23

94.82

2.67%

150

157.38

104.32

2.39%

154.67

102.25

2.02%

Fermented 0.67 bean curd

Ham 1.09 sausage

Mustard 0.86 tuber *Above ingredients were obtained from the local supermarket.



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For Table of Contents Use Only

The AuNPs/CS@N,S co-doped MWCNTS shows significant activity in catechol and nitrite detection due to the modified materials, demonstrating a superior development prospect.


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Scheme. 1. Schematic illustration for fabricating AuNPs/CS@N,S co-doped MWCNTS. 297x209mm (300 x 300 DPI)



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Fig. 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, (G) AuNPs/CS@N,S co-doped MWCNTS. TEM images of (H) MWCNTS, (I) oxMWCNTS, (J) N,S co-doped MWCNTS, (K)CS@N,S co-doped MWCNTS. 475x403mm (300 x 300 DPI)


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Fig. 2 The XRD patterns of (A) MWCNTS, (B) oxMWCNTS (C) N,S co-doped MWCNTS, (D) CS@N,S co-doped MWCNTS, (E) AuNPs/CS@N,S co-doped MWCNTS. 287x201mm (300 x 300 DPI)



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Fig. 4 CVs obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 2 mM catechol. Scan rate=50 mV/s. 299x201mm (300 x 300 DPI)


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Fig. 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. 539x200mm (300 x 300 DPI)



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Fig. 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. 527x183mm (300 x 300 DPI)


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Fig. 7 The CV behaviors obtained at AuNPs (a), AuNPs/N,S co-doped MWCNTS (c), AuNPs/CS@N,S co-doped MWCNTS (e) in pH 7.4 PBS and AuNPs (b), AuNPs/N,S co-doped MWCNTS (d), AuNPs/CS@N,S co-doped MWCNTS (f) in pH 7.4 PBS with 3 mM nitrite. Scan rate=50 mV/s. 304x201mm (300 x 300 DPI)



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Fig. 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: +0.9V. 539x195mm (300 x 300 DPI)


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Fig. 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 electrode prepared in the same conditions. 544x195mm (300 x 300 DPI)


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