Reduced graphene oxide-cyclodextrin-chitosan electrochemical sensor: Effective and simultaneous determination of o- and p-nitrophenols

Accepted Manuscript Title: Reduced Graphene Oxide-cyclodextrin-chitosan Electrochemical Sensor: Effective and Simultaneous Determination of o- and p-Nitrophenols Author: Chengkun Li Zhiliang Wu Hua Yang Liu Deng Xiaoqing Chen PII: DOI: Reference:

S0925-4005(17)30879-1 http://dx.doi.org/doi:10.1016/j.snb.2017.05.059 SNB 22343

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

13-12-2016 11-5-2017 12-5-2017

Please cite this article as: C. Li, Z. Wu, H. Yang, L. Deng, X. Chen, Reduced Graphene Oxide-cyclodextrin-chitosan Electrochemical Sensor: Effective and Simultaneous Determination of o- and p-Nitrophenols, Sensors and Actuators B: Chemical (2017), http://dx.doi.org/10.1016/j.snb.2017.05.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reduced Graphene Oxide-cyclodextrin-chitosan Electrochemical Sensor:

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Effective and Simultaneous Determination of o- and p-Nitrophenols

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Chengkun Li1, Zhiliang Wu1, Hua Yang1, Liu Deng1, Xiaoqing Chen1,2,*

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College of Chemistry and Chemical Engineering, Central South University,

Changsha, 410083, China 2

Collaborative

Innovation

Center

of

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E-mail address: [email protected]

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*Corresponding author: Tel: +86-731-88830833

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Resource-conserving

Environment-friendly Society and Ecological Civilization, Changsha, China

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Page 1 of 42

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Abstract

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novel

electrochemical

sensor

based

on

cyclodextrin

and

ip t

A

chitosan-functionalized reduced graphene oxide composite film was designed and

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prepared to detect o- and p-nitrophenols simultaneously. Apart from the synergistic

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effect of large specific surface area of reduced graphene oxide (RGO), the adsorption

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of nitrophenols (NPhs) through electrostatic interaction benefited from large

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quantities of host–guest recognition sites on cyclodextrin (CD) and abundant

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functional groups on chitosan. By taking advantage of the salient features of RGO,

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CDs, and chitosan, p-nitrophenol (p-NPh) and o-nitrophenol (o-NPh) can be

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determined simultaneously based on their nitroaromatic/hydroxylamine redox

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reactions. Under the optimized experimental conditions, a wide linear range was

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obtained over o-NPh concentration from 0.12 µM to 0.28 µM and 5 µM to 40 µM with a detection limit of 0.018 µM (S/N = 3) as well as p-NPh concentration from 0.06 µM to 0.16 µM and 5 µM to 40 µM along with a detection limit of 0.016 µM

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(S/N

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oxide-cyclodextrin-chitosan

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satisfactory reproducibility, good stability, and adequate efficiency in simultaneously

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detecting o-NPh and p-NPh in aqueous samples.

=

3).

In

addition,

the

newly

(RGO-CD-CS)

developed

sensor

reduced

displayed

high

graphene accuracy,

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Keywords: electrochemical detection; reduced graphene oxide-cyclodextrin-chitosan

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composite; nitrophenols; second-order derivative differential pulse voltammetry

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1. Introduction

Nitrophenols (NPhs), including o-nitrophenol, p-nitrophenol, and m-nitrophenol,

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are widely used in the manufacture of dyes, pesticides, and pharmaceuticals [1]. Due

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to their high toxicity even at low concentrations, nitrophenols adversely affect

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animals, plants, and human beings seriously and are being listed as priority toxic

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pollutants. Therefore, it is of great importance and urgency to develop reliable and

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simple method to detect trace amount of nitrophenols in environment. So far, various

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settings as extensive pre-purification and time-consuming or complex equipments

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were usually requisite. Obviously, highly operational, effective, and selective

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analytical techniques for determining nitrophenols contamination are still in demand.

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methods have been developed to detect and quantify nitrophenols, such as UV-Vis spectrophotometry [2], gas chromatography [3], liquid chromatography [4], fluorescence detection [5], capillary electrophoresis [6], and other integrated methods. Many of the reported technologies were found inconvenient in routine environmental

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Page 3 of 42

Electrochemical techniques have attracted considerable interest because they

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usually are stable, simple, less time-consuming, and environment-friendly [7, 8]. A

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variety of modified electrodes, such as metal nanoparticles [9], carbon materials [10],

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and conducting polymers [11], have been applied to detect NPhs. However, these

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methods are suffering from a common drawback as they are hardly able to determine

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p-NPh and o-NPh simultaneously. As a consequence, it is highly desirable to develop

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novel electrodes to realize the simultaneous detections of o-NPh and p-NPh.

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In recent years, graphene oxide (GO)-based materials have been extensively

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studied, particularly with respect to electrochemical applications [12]. GO has been

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used to detect a variety of species, including small organic molecules (hydroquinone

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[13] and aminophenol [14]), heavy metal ions (Pb2+ ion [15]), fungicides

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different polymers usually afforded some attractive properties, such as high

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conductivity, good dispersibility, and prominent thermal stability. As well known,

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cyclodextrins are cyclic oligosaccharides consisting of α-1,4-D-glucopyranose units,

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(carbendazim [16]), and pesticides (methyl parathion [17]). Notably, RGO can be readily manufactured in large quantity through chemical or thermal reduction of GO. Compared with GO, RGO possesses a higher conductivity, holding a great potential for electrochemical sensing applications. Besides, the combination of RGO with

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which are widely used in molecular recognition by forming “host–guest” inclusion

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complexes with small molecules, especially for those bearing aromatic moieties.

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Unfortunately, CDs were rarely employed as modifiers for voltammetric electrodes

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due to two inherent drawbacks - poor electrical conductivity and unpleasant aqueous

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solubility [18]. It can be rationalized that the employment of RGO as the supporting

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materials might effectively circumvent the drawbacks of CDs, owing to the high

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conductivity and the large surface area of RGO. However, RGO are prone to

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agglomerate due to the strong van der Waals force among the tubes, resulting in poor

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dispersion in most solvents [19]. It has been found that introducing polar molecule -

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chitosan onto the surface or sidewalls of RGO could facilitate the dispersion of RGO

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in many solvents. Additionally, chitosan can enhance the sensing performance of the

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simultaneous and sensitive voltammetric determination of o-nitrophenol and

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p-nitrophenol.

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main body of the sensors, and also exhibits excellent film-forming capability [20]. As a consequence, it can be expected that the rational integration of the merits of RGO, CDs, and Chitsan could significantly upgrade the electrochemical performance of the corresponding hybride composite, which might serve as a platform for the

In this work, we successfully developed the electrochemical sensor based on 5

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cyclodextrin and chitosan-functionalized reduced graphene oxide composite for the

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first time. Impressively, RGO-CD-CS electrochemical sensor exhibited extraordinary

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high

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electrochemical performance, and high selectivity towards nitrophenols, which

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enabled the simultaneous and accurate determination of p-NPh and o-NPh in aqueous

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solutions.

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2. Experiment

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2.1 Materials

conductivity,

excellent

electrochemical

stability,

superior

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electrical

Graphite powder (≤100 mesh) was obtained from XF NANO (Nanjing, China).

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L-(+)-Ascorbic acid, o-nitrophenol and p-nitrophenol were obtained from Alfa Aesar

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(Shanghai, China). L-tryptophan, chitosan, β-cyclodextrin, NaOH, KH2PO4, KMnO4, concentrated H2SO4, concentrated HCl, concentrated H3PO4 and 30% H2O2 aqueous solutions were all analytical-grade and obtained from Beijing Chemical Reagents Company (Beijing, China). The phosphate buffer solution (PBS, 0.2 M) was prepared

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using KH2PO4. Analytical samples were taken from Xiangjiang River and Jade

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Ribbon River.

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2.2 Apparatus

Fourier transform infrared (FT-IR) spectra were carried out on a Bruker Model

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IFS 66v/s spectrophotometer. The structures of GO, RGO, and RGO-CD-CS were

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determined on a TESCAN MIRA3 LMH/LMU field-emission scanning electron

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microscope (FE-SEM). All electrochemical experiments were performed on a

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CHI650D electrochemical workstation (CH Instrumental Co., China) with a

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conventional three-electrode cell. A saturated calomel electrode (SCE) was used as

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reference electrode and a platinum wire was used as auxiliary electrode, respectively.

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A modified glassy carbon electrode (GCE, d = 3 mm) was used as the working

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2.3 Preparation

te

electrode.

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resulting mixture under vigorous stirring, and the temperature was well maintained

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under 10 °C. Then, the mixture was ultrasonicated at ∼50 °C for 30 min and stirred

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Graphene oxide was prepared according to the modified Hummers method [21].

Phosphoric acid (4 mL) was added into concentrated sulfuric acid (36 mL) at 0 °C gradually. Graphite powder (0.3 g) and KMnO4 (1.5 g) were gradually added to the

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Page 7 of 42

for another 12 h before it was cooled to room temperature and slowly added into ice

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water (130 mL). Subsequently, H2O2 (30%) was added dropwise to it until the colour

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of solution turned bright yellow. The precipitate was filtrated and washed with HCl

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aqueous solution (1 M) and H2O, respectively. Finally, the resulting solid was dried

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under vacuum at 60 °C for 3 days.

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RGO was synthesized through reducing graphene oxide by ascorbic acid [16].

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An aqueous mixture solution of graphene oxide (20 mg), L-(+)-ascorbic acid (1 g),

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L-tryptophan (0.4 g), and NaOH (80 mg) in H2O (200 mL) was ultrasonicated for 0.5

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h. Then the mixture was maintained at 80 °C for 24 h. After that, the mixture was

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cooled to room temperature and ultrasonicated for 1 h. Ultimately, reduced graphene

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oxide was prepared via vacuum filtration by using a membrane filter (47 mm in

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mg) in water under ultrasonication for 30 min. Prior to modification, a bare glassy

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carbon electrode was polished to form a mirror-like surface with 0.3 and 0.05 µm

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alumina slurry on micro-cloth pads, and then washed with deionized water. Then 5.0

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diameter, 0.22 µm pore size). The resulting solid was washed with ultrapure water to completely remove the excessive ascorbic acid and L-tryptophan. As for the fabrication of modified electrodes, a suspension was prepared by

dispersing reduced graphene oxide (10 mg), chitosan (5 mg), and β-cyclodextrin (200

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Page 8 of 42

µL of the as-prepared suspension was coated onto the fresh glassy carbon electrode

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surface using a micropipette followed by being dried in air.

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3. Results and discussion

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3.1 Characterizations of RGO-CD-CS

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3.1.1 Morphology analysis

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GO, RGO, and RGO-CD-CS were first characterized by FT-IR spectra (Fig. 1).

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The IR spectrum of GO shows absorption bands at 3400 cm-1, 1630 cm-1, and 1161

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cm-1, which are attributed to hydroxyl, carboxyl, and epoxy groups, respectively.

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However, the vibration bands at 3400 cm-1 and 1430 cm-1 were weakened after GO

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was reduced, confirming that most of oxygen-containing functionalities in RGO were

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removed. As shown in Fig. 1C, the FT-IR peaks of RGO-CD-CS exhibit typical CD absorptions of vibration at 3400, 1645, and 1028 cm-1. Meanwhile, the intensities of C–O (epoxy) stretching vibration peak at 1157 cm-1, and C–O (alkoxy) stretching

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peak at 1028 cm-1 decreased dramatically. These phenomena indicated that CDs were

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successfully attached to GO through nucleophilic reactions, in which hydroxyl groups

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on CD at the secondary face were converted into alkoxides in alkaline solution.

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Moreover, the stretching vibration peak of C=O at 1644 cm-1 and the vibration and 9

Page 9 of 42

deformation peaks of O–H groups at 3400 cm-1 and 1417 cm-1 are much smaller than

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those in GO and RGO, indicating that the hydrogen bonds between hydroxyl groups

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in CD have been destroyed. In addition, the absorption at 1157 cm-1 could be ascribed

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to amino groups in chitosan. All these results clearly confirm that CD and chitosan

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have been attached onto the surface of RGO [22].

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Fig. 2 displays SEM images of GO (a), RGO (b), and RGO-CD-CS (c). As

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shown in Fig. 2a, GOs were efficiently exfoliated to form separated thin sheets, while

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they were scrolled and corrugated in a way of crumpled silk veils. As shown in Fig.

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2b, RGOs are curlier and thicker than GOs. Many graphene sheets were present in

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RGOs, reflecting the folding nature throughout the morphology. Upon addition of CD

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and chitosan (Fig. 2c), RGO-CD-CS sheets tended to form small aggregates through

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the cross-linked RGO with CD and chitosan, which was also observed in previous work [23].

3.1.2 Cyclic voltammetry

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Figure 3a displays cyclic voltammograms (CVs) of differently modified

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electrodes in K3Fe(CN)6 solution (1.0 mmol·L-1) at a scan rate of 50 mV/s. Upon

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using a bare glass carbon electrode (GCE) (Fig. 3a-A), a pair of redox peaks appeared

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with a peak-to-peak separation (∆Ep) of 72 mV. After casting CD and chitosan on the 10

Page 10 of 42

glass carbon electrode, an obvious increase in ∆Ep as 85 mV was observed (Fig.

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3a-C), possibly due to the hindered diffusion of Fe(CN)63-/4- towards the electrode

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surface by inert electron and mass transfer blocking layer of CD and chitosan. When

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the electrode was modified by RGO-CD, the redox peaks increased obviously with

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∆Ep value as 94 mV due to the excellent electrical conductivity of RGO present on

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the electrode surface (Fig. 3a-D). Moreover, a remarkable current increase was

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observed at the RGO-CD-CS modified electrode (RGO-CD-CS/GCE), which could

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be ascribed that chitosans are positively charged species and can adsorb negatively

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charged ferricyanides. However, the background of RGO-CD-CS/GCE was larger

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than that of GCE and CD-CS/GCE, which could be attributed to the good

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conductivity and large specific area of RGO. At this stage, we can conclude that

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RGO-CD-CS exhibits the supreme electron-transfer efficiency with the largest active electrode surface, which is desirable in practical electrochemical application.

3.1.3 Electrochemical impedance spectroscopy

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Electrochemical impedance spectroscopy (EIS) was performed to investigate the

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electronic transfer properties of the electrodes after different surface modifications. As

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shown in Fig. 3b, Nyquist plots of bare GCE, RGO/GCE, RGO-CD/GCE,

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RGO-CS/GCE and RGO-CD-CS/GCE were obtained under open-circuit potential 11

Page 11 of 42

conditions using Fe(CN)63−/4− (0.2 mM, containing 0.2 M KCl) as the electrochemical

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probes at an frequency varying from 0.01 to 100 kHz. The charge transfer resistance

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(Rct) for GCE, RGO-CD/GCE, RGO-CS/GCE, RGO/GCE and RGO-CD-CS/GCE

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were 4223 Ω, 6260 Ω, 28.94 Ω, 37.19 Ω, and 225 Ω, respectively. It is clear that bare

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GCE and RGO-CD/GCE exhibit larger quasi-semicircular diameters than other

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modified electrodes, reflecting the greatest charge-transfer resistance. Rct’s of

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RGO-CS/GCE and RGO/GCE are lower than that of bare GCE, reflecting the

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superior electrical conductivity of RGO facilitates the electron-transfer and builds up

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the expanded ionic highways for electrons to access the surface of electrode [24]. The

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electrical conductivity of RGO-CD-CS is slightly less than that of pristine RGO and

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RGO-CS. Undoubtedly, the presence of CD would stumble the electron transfer.

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3.2 Electrochemical response of RGO-CD-CS/GCE and nitrophenols

Figure 4 depicts CVs for 20 µM o-NPh and 20 µM p-NPh at bare GCE (A),

RGO/GCE (B) and RGO-CD/GCE (C) and RGO-CD-CS/GCE (D) in PBS solution

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(pH 5.0, 0.2 M). As displayed in Figure 4 B-D, one large cathodic peak (R1) appears

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at around -0.6 V for these two NPhs, which can be attributed to the irreversible

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reduction of NPh to hydroxylamine derivative as the following four-electron transfer

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reaction: 12

Page 12 of 42

1 As for four CV curves in Fig. 4a, a small reduction peak current of p-NPh can be

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observed at bare GCE, and the reduction peak current of p-NPh at RGO/GCE

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increases for 26 times compared to bare GCE. Besides, a positive shift of 55 mV for

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the peak potentials was observed. The positive shift and signal enhancement of

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reduction peak potential both indicate that RGO is efficient in reducing p-NPh owing

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to its large surface area, high conductivity, and large adsorption capability. However,

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compared to RGO/GCE, the reduction peak current for RGO-CD/GCE increased 1.8

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times, caused by the formation of 1:1 inclusion complex between CD and

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nitrobenzene [25]. Moreover, a prominent positive shift of 9 mV at peak potentials

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was also observed at RGO-CD/GCE compared to that of RGO/GCE. Since the

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used in this work is 1200 kDa with pKa between 6.39 and 6.51. Thus, amino group of

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chitosan is prone to be protonated to form –NH3+ when pH is lower than pKa of

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chitosan (in this work, pH = 5.0). This electrostatic interaction between chitosan and

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cyclodextrin attached on the surface of RGO can form host–guest complexes with p-NPh, higher current response and positive shift could be attributed to the autoprotonation mechanism [26]. Interestingly, peak current of the cathodic reduction at RGO-CD-CS/GCE was 2.3 times higher than that at RGO-CD/GCE. The chitosan

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Page 13 of 42

NPh would facilitate the migration of NPh molecule to the electrode surface.

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Therefore, an increase in the reduction peak current was observed. Similar

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phenomena were observed for the reduction behavior of o-NPh on GCE, RGO/GCE,

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RGO-CD/GCE, and RGO-CD-CS/GCE under identical conditions (Fig. 4b). All the

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results reveal that RGO-CD-CS/GCE demonstrates excellent electrocatalytic activity

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towards the reduction of NPhs. In addition, p-NPh and o-NPh on RGO-CD/GCE

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show different reduction potentials, -0.522 V and -0.429 V respectively, and peak

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current response is relatively high. Meanwhile, reduction potentials are so

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distinguishable that they can be easily measured at the same time.

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In order to assess the separation degree of peak potentials for p-NPh and o-NPh,

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simultaneous differential pulse voltammetric (DPV) responses of p-NPh and o-NPhs

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RGO-CD-CS/GCE, which would further reduce the background interference to

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improve the corresponding sensitivity. Two sharp and separated reduction peaks of

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o-NPh and p-NPh were observed accurately at -0.384 V and -0.468 V, respectively.

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were performed at RGO-CD-CS/GCE (real surface area: 0.159 cm2, Fig. 5). Two sharp and separated reduction peaks of p-NPh and o-NPh were observed distinctly at -0.482 V and -0.366 V, respectively. Fig. S1 shows the second-order derivative differential pulse voltammetry (2D-DPV) curves of p-NPh and o-NPh on

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The distinct separation of peak potentials for these two NPhs ensured the effective

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discrimination for individual nitrophenol on RGO-CD-CS/GCE in aqueous solution.

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3.3 Optimization of detection

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Since the electrochemical oxidation of phenols are always involved with proton

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transfers to deliver quinones [27], pH value of solution would have great impact on

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the electrochemical responses of nitrophenols. As a consequence, the effect of pH

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value on the electrochemical response of p-NPh and o-NPh at RGO-CD-CS/GCE was

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investigated in a range of pH 4.0–9.0 in PBS buffer solution (0.2 M). As shown in Fig.

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6a, the largest redox peak current was obtained at pH 5.0. When pH of solution was

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lower than 5.0 (pKa of chitosan), chitosan was positively charged through the

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protonation of -NH2. The electrostatic interaction of chitosan with NPh would enrich

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NPhs on the electrode surface, leading to a redox peak current. While pH was higher than 5.0 and amino groups on chitosan were unlikely protonated, a decrease in the corresponding redox peak current of NPh was observed. Same phenomena were

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observed for o-NPh under identical conditions (Fig. 6b). Accordingly, pH value of 5.0

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was used in the following experiments. Furthermore, as can be seen in Fig. 6c,

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reduction peak potentials of p-NPh and o-NPh shifted negatively along with an

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increase in pH. The linear regression equations were expressed as follows: 15

Page 15 of 42

p-NPh

Epc(V) = -0.047pH-0.438 (R^2=0.9870)

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o-NPh

Epc(V) = -0.038pH-0.384 (R^2=0.9876)

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, indicating that the proton was directly involved in the electrochemical redox

ip t

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0.0592 m pH + b [28] (n is n

process of NPh. According to Nernst equation of E p =

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number of electron, m is number of proton), the value of m/n was calculated to be

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0.79 times and 0.64 times than electron respectively, showing that equal numbers of

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protons and electrons were involved in the electrochemical redox reaction of p-NPh

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and o-NPh.

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The content and proportion of RGO, CD, and CS are very important factors to

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the structure and composition of this newly developed material. As shown in Fig. 7,

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the optimum concentrations for RGO, CD, and CS for the preparation of

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RGO-CD-CS composite were found to be 1 mg/mL, 20 mg/mL, and 0.5 mg/mL, respectively. As RGO, CD, and CS played different roles in the composite, a balance between RGO, CD, and CS was well achieved under the optimal conditions.

3.4 Simultaneous Determination of p-NPh and o-NPh

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Under the optimal experimental conditions, 2D-DPV was performed on

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RGO-CD-CS/GCE for the quantitative analysis of p-NPh and o-NPh. The peak

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current at -0.732 V increased linearly with an increase in the concentration of o-NPh. 16

Page 16 of 42

And the linear relation between o-NPh concentration and the peak current

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(Ipa-Cp-NPh) can be seen in the inset of Fig. 8 and Fig. S2. The peak current for

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o-NPh increased with the concentration changing from 0.12 µM to 0.28 µM and 5 µM

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to 40 µM with regression coefficients of 0.9961 and 0.9951, indicating that the

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presence of p-NPh has little influence on the o-NPh determination. Two different

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slopes were observed on the linear curves since the mechanisms in these two different

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ranges of concentrations were different. The electron-withdrawing effect of

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RGO-CD-CS skeleton dominated with the low levels of NPhs in solution. However,

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when the concentrations of NPhs in the solution were high, there existed

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hydrogen-bonding between NPh molecules, besides the electron-withdrawing effect

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in RGO-CD-CS skeleton [29]. 2D-DPV peaks of p-NPh in a series of mixture

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0.06 µM to 0.16 µM and 5 µM to 40 µM with regression coefficients of 0.9995 and

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0.9901. These results clearly indicate that the concentrations of NPhs in the mixture

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solution can be accurately determined. The detection limits (S/N = 3) of p-NPh and

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solutions were investigated under the same experimental conditions. Typical 2D-DPV responses and standard curves for the determination of p-NPh are shown in Fig. S3 and Fig. 9. The current of sharp peak at -0.621V increased with an increase of concentration of o-NPh. Fig. 9 presents a favourable linear relationship in a range of

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o-NPh for individual analysis were estimated to be 0.016 µM and 0.018 µM

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respectively. These observations clearly demonstrated that RGO-CD-CS could serve

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as a suitable sensing composite for the determination of p-NPh and o-NPh. Pleasingly,

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the sensitivity of RGO-CD-CS/GCE is much higher than that of the previously used

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electrodes (Table 1). This could be ascribed to three reasons: Firstly, due to its high

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conductivity and large surface area, RGO is an ideal functional material for

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electrochemical analysis; Secondly, the reduction of NPhs on RGO-CD-CS/GCE can

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be enhanced by host-guest recognition and the facilitated electron transfer; Thirdly,

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chitosan can act as an efficient mediator to facilitate the migration of NPh towards the

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electrode surface.

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3.5 Anti-interference ability and repeatability study

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Selectivity is one of the most desirable features for a proposed sensor in the

practical analysis. Therefore, anti-interference of RGO-CD-CS/GCE was investigated by measuring the amperometric response signals of o-NPh (20 µM), p-NPh (20 µM)

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and several coexisting species (KCl, Fe2+, Zn2+, Co2+, phenol, and aminophenol, 2

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mM, respectively) in practical samples. The amperometric response current ratios to

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p-NPh and the potential interfering agents on RGO-CD-CS/GCE are shown in Fig. 10.

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No obvious interference was observed in the presence of those potential interfering 18

Page 18 of 42

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agents. The above results suggest that RGO-CD-CS/GCE possesses good selectivity

2

towards p-NPh and o-NPh owing to its good anti-interference capability. In order to evaluate the practicability and precision of the proposed method, the

4

intra-day precision of RGO-CD-CS/GCE was studied by analyzing a mixed solution

5

of p-NPh and o-NPh (20 µM respectively) for 6 successive times with the same

6

electrode in a day. Moreover, the inter-day precision of RGO-CD-CS/GCE was

7

studied by determining a mixed solution of p-NPh and o-NPh for 5 times with the

8

same electrode in 5 days. The between-run precision of RGO-CD-CS/GCE was

9

investigated by surveying a mixed solution for 6 times with different electrodes. Not

10

surprisingly, this developed electrode possesses a satisfactory repeatability with an

11

acceptable relative standard deviation (RSD) ≤ 3.23% (intra-day precision), ≤ 5.24%

13

14

cr

us

an

M

d

te

Ac ce p

12

ip t

3

(inter-day precision), and ≤ 5.98% (between-run precision). It can be concluded that the precision and reliability of this analytical method surely meet the expectation.

3.6 Detection of p-NPh and o-NPh in real samples

15

Ultimately, RGO-CD-CS/GCE was used for the simultaneous detection of p-NPh

16

and o-NPh in real water samples from Xiangjiang River and Jade Ribbon River.

17

Electrochemical signals of NPhs were unable to be detected without the addition of

18

p-NP and o-NP into samples, which may be due to the low concentrations of NPhs in 19

Page 19 of 42

water samples. Then, the standard addition method was used for calculating the

2

concentration of p-NPh and o-NPh under the optimal conditions. The results of p-NPh

3

and o-NPh in water samples and the recoveries are shown in Table 2. The recoveries

4

in this work were ranged from 98.30% to 100.59%. The detection of each sample was

5

run for 6 times (RSD below 2.17%) and the average value was adopted, confirming

6

the feasibility and effectiveness of RGO-CD-CS composite modified electrode.

7

4. Conclusions

In this work, a RGO-CD-CS-composite-modified electrochemical sensor was

9

prepared and successfully applied to the simultaneous determination of p-NPh and

10

o-NPh. RGO-CD-CS hybrid nanosheets show a synergistic effect of individual

11

14 15

the detection of p-NPh and o-NPh. More broadly, this work would offer an alternative

16

solution for fabricating chemical sensors to recognize and detect NPh isomers in

17

environmental control and chemical industry.

12 13

te

d

8

Ac ce p

M

an

us

cr

ip t

1

components towards the reduction of NPh isomers. Under the optimized conditions, the reduction peak currents of p-NPh and o-NPh were linear to their concentrations in a wide range with a low detection limit. The RGO-CD-CS modified electrode displayed good reproducibility, high stability, and favorable sensing performance in

20

Page 20 of 42

2 3

Acknowledgements

We gratefully acknowledge the financial support from National Natural Science Foundation of China (21475152 and 21576296).

ip t

1

cr

4

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carbon electrode for determination of gatifloxacin, Sensors & Actuators B Chemical, 228 (2016) 59-65.

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Nitrophenol Isomers Based on β-Cyclodextrin Functionalized Reduced Graphene

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2

mesoporous carbon modified electrode, Electrochimica Acta, 106 (2013) 127-134.

3

[31] Z. Liu, X. Ma, H. Zhang, W. Lu, H. Ma, S. Hou, Simultaneous Determination of

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Nitrophenol Isomers Based on β-Cyclodextrin Functionalized Reduced Graphene

5

Oxide, Electroanalysis, 24 (2012) 1178-1185.

6

[32] P. Deng, Z. Xu, J. Li, Simultaneous voltammetric determination of 2-nitrophenol

7

and 4-nitrophenol based on an acetylene black paste electrode modified with a

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graphene-chitosan composite, Microchimica Acta, 181 (2014) 1077-1084.

9

[33] H.B. Böhm, J. Feltes, D. Volmer, K. Levsen, Identification of nitrophenols in

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rain-water by high-performance liquid chromatography with photodiode array

11

detection, Journal of Chromatography A, 478 (1989) 399-407.

16 17 18 19 20

cr

us

an

M

te

15

Ac ce p

14

d

12 13

ip t

1

21 22 23 24 25

Page 25 of 42

1 2

Table 1 Performances comparison of RGO-CD-CS/GCE for the simultaneous determination of o-NPh and p-NPh with other electrochemical sensors Detection limit Linear range (µM) Sensor

Reference

o-NPh

p-NPh

OMCs/GCEd

0.5–90

2–90

DPV

CD-RGO/GCEe

7.2–64.7

7.2–72.0

o-NPh 0.08

an

Derivative

p-NPh 0.1

[30]

0.14

0.36

[31]

0.2

0.08

[32]

0.324

0.216

[33]

0.018

0.016

This work

us

DPVa

ip t

(µM)

cr

Technique

0.1–20 and

Gr-Chit/ABPE

f

0.4–80

b

LSV

Photodiode array

1.44–719.4

1.44–719.4

d

HPLCc

M

20–80

0.06–0.16 and

5–40

5–40

0.16–0.28 and

te

RGO-CD-CS/GCE

6

Ac ce p

2D-DVP

7

e β-Cyclodextrin functionalized reduced graphene oxide modified glassy carbon

8

electrode

9

f Acetylene black paste electrode modified with a graphene-chitosan composite film

3 4 5

a Differential pulse voltammetry b Derivative linear sweep voltammetry c High-performance liquid chromatography d Ordered mesoporous carbons modified glassy carbon electrode

26

Page 26 of 42

1

Table 2 Determination results of p-NPh and o-NPh in water samples and recoveries Added.

Detected

Recovery

(µM)

(%)

RSD

Samples

6.0

5.898

98.30

1.16

o-NPh

6.0

6.012

100.20

1.98

p-NPh

6.0

5.972

99.53

2.17

o-NPh

7.5

7.544

100.59

1.49

us

M d te Ac ce p

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

an

Ribbon river

cr

p-NPh Xiangjing river

Jade

(%, n = 6)

ip t

(µM)

23 27

Page 27 of 42

Fig. 1 FT-IR of GO, RGO and RGO-CD-CS

2

Fig. 2 SEM images of GO (a), RGO (b) and RGO-CD-CS (c)

3

Fig. 3 a: Cyclic voltammograms of 1 mM K3Fe(CN)6 solution on GCE (A), CD/GCE

4

(B), CD-CS/GCE (C), RGO-CD/GCE (D) and RGO-CD-CS/GCE (E) at a scan rate of

5

50 mV/s.

6

b: Nyquist plots of different electrodes in 5.0 mM Fe(CN)63−/4− solution (containing

7

0.10 M KCl) at open-circuit potential conditions (AC frequency range: 0.01 Hz to 10

8

kHz; AC amplitude: 5.0 mV): (A) bare GCE; (B) RGO-CD/GCE; (C) RGO-CS/GCE;

9

(D) RGO/GCE and (E) RGO-CD-CS/GCE

M

an

us

cr

ip t

1

Fig. 4 Cyclic voltammograms of 20 µM p-nitrophenol (a) and 20 µM o-nitrophenol (b)

11

on GCE, RGO/GCE, RGO-CD/GCE and RGO-CD-CS/GCE at a scan rate of 50 mV/s

15

Ac ce p

te

d

10

16

buffer solution (0.2 M): p-nitrophenol (a), o-nitrophenol (b) and the influence on

17

reduction peak potential (c)

18

Fig. 7 Second-order derivative differential pulse voltammetry of 20 µM p-nitrophenol

12 13 14

Fig. 5 Differential pulse voltammetry curves of PBS (pH 5, 0.2 M) (A), 20 µM p-nitrophenol (B), 20 µM o-nitrophenol (C), 20 µM p-nitrophenol and 20 µM o-nitrophenol (D) on RGO-CD-CS/GCE (real surface area: 0.159 cm2) Fig. 6 The influence of different pH (4.0–9.0) on the reduction peak current in PBS

28

Page 28 of 42

and o-nitrophenol on the RGO-CD-CS/GCE with different amounts of RGO (a), CD

2

(b) and CS (c)

3

Fig. 8 Standard curve of different concentrations of o-nitrophenol: (a) 5~40 µM; (b)

4

0.12~0.28µM

5

Fig. 9 Standard curve of different concentrations of p-nitrophenol: (a) 5~40 µM; (b)

6

0.06~0.16µM

7

Fig. 10 The amperometric response current ratios to p-NPh and the potential

8

interfering agents (KCl, Fe2+, Zn2+, Co2+, phenol and aminophenol, 2 mM,

9

respectively) on RGO-CD-CS/GCE

12 13 14 15

cr

us

an

M

d te

11

Ac ce p

10

ip t

1

16 17 18 29

Page 29 of 42

1

Fig. 1 A B C

B

GO RGO RGO-CD-CS

ip t

1630.29

A 1630 3400

1644.96 1417.16

2928.32

cr

C

us

Transmittance

3436.16

1157.31 3400.14

1028.08

3200

2400

1600

an

4000

800

-1

Wavenumbers( cm )

2

M

3

7 8 9

te

6

Ac ce p

5

d

4

10 11 12 13 30

Page 30 of 42

1

Fig. 2

b

c

ip t

a

cr

2

us

3

an

4 5

M

6

10 11 12

te

9

Ac ce p

8

d

7

13 14 15 16 31

Page 31 of 42

Fig. 3

cr

ip t

a

us

I (mA)

0.14

GCE CD/GCE CD-CS/GCE RGO-CD/GCE RGO-CD-CS/GCE

A B C D E

0.07

an

1

M

0.00

0.0

2

0.3

0.6

d

E (V)

te

3

Ac ce p

4

500

8000

400

C D E

A B C D E

RGO-CS/GCE RGO/GCE RGO-CD-CS/GCE

-Z'' (ohm)

300

6000

GCE RGO-CD/GCE RGO-CS/GCE RGO/GCE RGO-CD-CS/GCE

b

200

100

-Z'' (ohm)

0

100

200

300

400

500

Z' (ohm)

4000

2000

Cdl Rs

0 0

2000

4000

Ret 6000

w 8000

10000

Z' (ohm)

5 6 32

Page 32 of 42

1

Fig. 4

A B C D

0.08 0.06

GCE RGO/GCE RGO-CD/GCE RGO-CD-CS/GCE

a

ip t

0.04

0.00

cr

I (mA)

0.02

-0.02

-0.06 -0.08 -1.0

-0.5

0.0

0.5

1.0

2 0.06

GCE RGO/GCE RGO-CD/GCE RGO-CD-CS/GCE

0.04

2.0

b

M

A B C D

1.5

an

E (V)

us

-0.04

d

0.00

-0.02

Ac ce p

-0.04

te

I (mA)

0.02

-0.06

-1.0

3 4 5

-0.5

0.0

0.5

1.0

1.5

2.0

E (V)

6 7 8 33

Page 33 of 42

1

Fig. 5 A B C D

-17

pbs p-nitrophenol o-nitrophenol p-nitrophenol and o-nitrophenol

ip t

-19 -20

cr

-21 -22 -23 -24 -25 -26 -0.6

-0.5

2

E (V)

7 8 9 10

te Ac ce p

6

d

4 5

-0.3

M

3

-0.4

an

-0.7

us

-2

Current density (µA cm )

-18

11 12 13 34

Page 34 of 42

1

Fig. 6

2.6

a

p-nitrophenol

2.4

ip t

2.2 2.0

1.6 1.4

cr

I (mA)

1.8

1.2 1.0

0.6 0.4 4

5

6

7

pH

M

2.4 2.2

1.8

d

I(mA)

2.0

1.6

Ac ce p

0.8

te

1.4

1.0

4

3

9

b

o-nitrophenol

2.6

1.2

8

an

2

us

0.8

5

6

7

8

9

pH

-0.55

p-nitrophenol o-nitrophenol

c

7

9

-0.60

E(V)

-0.65

-0.70

-0.75

-0.80

-0.85

-0.90 4

5

6

8

pH

4 5 35

Page 35 of 42

1

Fig. 7

0.40

p-nitrophenol o-nitrophenol

a

0.35

ip t

I(mA)

0.30

0.25

cr

0.20

0.10

5 mg

10 mg

15 mg

p-nitrophenol o-nitrophenol

0.50

M

b 0.45

d

0.35

Ac ce p

0.25

te

I(mA)

0.40

0.30

100 mg

3

0.50

20 mg

an

2

us

0.15

150 mg

200 mg

250 mg

300 mg

p-nitrophenol o-nitrophenol

c

0.45

I (mA)

0.40

0.35

0.30

0.25

3 mg

4 mg

5 mg

6 mg

7 mg

4 36

Page 36 of 42

1

Fig. 8

a

1.6

Ipc(A)=3.640e^(-5) C(µM)+1.081e^(-4) (R^2=0.9951)

ip t

1.4

1.0 0.8

cr

current/mA

1.2

0.4 0.2 0

5

10

15

20

25

35

40

45

an

concentration/µM

30

us

0.6

b

M

2

0.040

Ipc(A)=1.190e^(-4) C(µM)+4.470e^(-6) (R^2=0.9961)

0.025

d te

0.030

Ac ce p

current/mA

0.035

0.020

0.015 0.10

3 4 5

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

concentration/µM

6 7 8 37

Page 37 of 42

1

Fig. 9

a

1.2

Ipc(A)=2.673e^(-5) C(µM)+2.721e^(-5) (R^2=0.9901)

ip t

1.0

cr

0.6

0.4

0.2

0.0 0

5

10

15

20

25

30

35

40

45

an

concentration/µM

us

current/mA

0.8

0.09

M

2

d

0.07

Ac ce p

te

current/mA

0.08

0.06

b

Ipc(A)=4.023e^(-4) C(µM)+2.538e^(-5) (R^2=0.9995)

0.05

0.06

3 4 5

0.08

0.10

0.12

0.14

0.16

concentration/µM

6 7 8 38

Page 38 of 42

1

Fig. 10

100

103.761

ip t

100

cr

60

40

20

0.386

0.277

KCl

Fe

0.406 0.567

0

o-NPh

2+

2

phenol aminophenol

5 Cheng-Kun Li

te

d

4

Cheng-Kun Li is a postgraduate student at the College of Chemistry and Chemical Engineering at Central South University, Changsha, China. His scientific interest is focused on the synthesis of graphene-based composites materials for electrochemical sensing applications.

12

Ac ce p

7 8 9 10

3+

Co

1.275

M

3

6

2+

Zn

2.051

an

p-NPh

us

Current Ratio (%)

80

13 14 15 16 17 18

Zhi-Liang Wu is currently pursuing the Ph.D. degree at the College of Chemistry and Chemical Engineering at Central South University, Changsha, China. His work is focused on the modification and functionalization of carbon materials (carbon nanotubes, graphene, and graphene nanoribbons) for the application of adsoption, preconcentration and separation techniques, and electrochemical sensors.

11

Zhi-Liang Wu

39

Page 39 of 42

1 Hua Yang

3

Hua Yang received the Ph.D. degree in Organic Chemistry in 2006 from

4

West Virginia University, West Virginia, America. He is currently a

5

professor and working at College of Chemistry and Chemical

6

Engineering at Central South University, Changsha, China. He has long

7

been committed to organic synthesis, asymmetric catalysis, and total

8

synthesis of chiral drug molecules, hosted by the National Natural

9

Science Foundation, science and Technology Department of Hunan

10

province key project, participated in a number of American NSF and NIH

11

of the fund research work.

15

Ac ce p

te

d

M

an

us

cr

ip t

2

16

Changchun Institute of Applied Chemistry Chinese Academy of Sciences,

17

Changchun, China. She is currently an associate professor and working at

18

College of Chemistry and Chemical Engineering at Central South

12 13 14

Liu Deng

Liu Deng received the Ph.D. degree in Analytical Chemistry in 2011 from

40

Page 40 of 42

1

University, Changsha, China.

2

18 19 20 21 22

ip t

d

M

an

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Xiao-Qing Chen received the Ph.D. degree in Applied Chemistry in 2006 from Central South University, Changsha, China. She is currently a professor and working at College of Chemistry and Chemical Engineering at Central South University, Changsha, China, and she is a senior engineer and working at Collaborative Innovation Center of Resource-conserving & Environment-friendly Society and Ecological Civilization, Changsha, China. She has been active in the investigation of several aspects of the application of carbon-based materials. Her research interests are in the modification and functionalization of carbon materials (carbon nanotubes, graphene, and graphene nanoribbons) for the application of adsoption, preconcentration and separation techniques, and electrochemical sensors.

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• A novel composite — reduced graphene oxide-cyclodextrin-chitosan was developed. • The self-assembly of composite was achieved under the chemical effects of ultrasound. • The composite based electrochemical sensor displayed good performance for simultaneous detection of nitrophenols. • The composite possessed high selectivity towards nitrophenols.

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