A facile gold nanoparticles embeded hydrogel for non-enzymatic sensing of glucose

Accepted Manuscript Title: A facile gold nanoparticles embedded hydrogel for non-enzymatic sensing of glucose Authors: Jialin Zhao, Xiaojun Hu, Xing Huang, Xin Jin, Kwangnak Koh, Hongxia Chen PII: DOI: Article Number:

S0927-7765(19)30548-X https://doi.org/10.1016/j.colsurfb.2019.110404 110404

Reference:

COLSUB 110404

To appear in:

Colloids and Surfaces B: Biointerfaces

Received date: Revised date: Accepted date:

4 May 2019 20 July 2019 27 July 2019

Please cite this article as: Zhao J, Hu X, Huang X, Jin X, Koh K, Chen H, A facile gold nanoparticles embedded hydrogel for non-enzymatic sensing of glucose, Colloids and Surfaces B: Biointerfaces (2019), https://doi.org/10.1016/j.colsurfb.2019.110404 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.

A facile gold nanoparticles embedded hydrogel for non-enzymatic sensing of glucose Jialin Zhaoa,1, Xiaojun Hua,1, Xing Huanga, b, Xin Jina, Kwangnak Kohc, *, Hongxia Chena, * a

Center for Molecular Recognition and Biosensing, School of Life Sciences, Shanghai University, Shanghai 200444, P. R. China

b

Shanghai Key Laboratory of Bio-Energy Crop, School of Life Sciences, Shanghai University, Shanghai 200444, P. R. China of General Education, Pusan National University, Pusan 46241, Republic of Korea

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*Corresponding author. E-mail addresses: [email protected] (H. Chen); [email protected] (K. Koh) 1 These authors contribute equally. 1

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Graphical abstract

Highlights

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1. A simple strategy for in-situ preparation of D-gel@AuNPs was reported.

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2. Reduction of AuCl4- occurred in the hydrogel to yield AuNPs located on the gel nanofibers.

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3. The catalytic process of D-gel@AuNPs on glucose is highly promoted.

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Abstract

The assembly of nanoparticle into electrodes with precise structure and uniform core sizes is important for electrocatalysis. In this study, we reported on a simple strategy for in-situ preparation of gold nanoparticles embedded D-sorbitol hydrogel (D-gel@AuNPs). D-sorbitol hydrogel with acyl hydrazide (D-gel) was synthesized and characterized. AuNP’s stable electronic structure, high surface coverage and good conductivity was

achieved

enabled

D-gel@AuNPs 2

exhibits

the

enhanced

electrocatalytic performance. The electrochemical results reveal that the catalytic progress is highly promoted by the D-gel@AuNPs with a detection limit of 0.067 mM and detection range of 0.1-30 mM. The high enzymatic activity and stability provide the high possibility for the development of high value glucose sensors. This mechanistically novel strategy expands the scope of assembly of NPs method for the development of enhanced other electrochemical properties such as amperometric sensing and photcatalysis applications, as well as electrocatalysis.

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Keywords: Hydrogel; In-situ reduction; AuNPs; Three-dimensional structure;

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Glucose sensing

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1. Introduction Nanoparticle-functionalized electrodes have been studied for electrochemical applications such as amperometric sensing [1,2], photcatalysis [3,4], as well as electrocatalysis [5,6]. In electrocatalysis applications, the addition of nanoparticles (NPs) to an electrode surface promotes electron transfer and results in the increment of electrode’s catalytic activity. The enhanced catalytic activity of NP-functionalized

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electrodes attributed to the NP’s crystal facets, surface chemistry, the interface

between the NP and the electrode support as well as the NP’s electronic structure,

important to enhance catalytic activity [7,8].

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density on the electrode surface. Therefore, the NP assembly methods are very

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Many assembly methods exist, including vacuum evaporation [9,10], electrode position drop-cast deposition [11,12], chemical assembly [13,14], and electrophoretic

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deposition [15,16]. In case of vacuum evaporation or electrode position, their

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interfaces with the electrode are difficult to control, making it challenging to attribute observed electrochemical properties to specific structures. Such deposition methods

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also make it hard to control the resulting NP core size distribution or coverage on the electrode. The solution deposition of preformed nanoparticles includes drop-casting

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[17], spin-coating [18], chemical assembly [13,14] and dip-coating [19] as alternative strategies to fabricate NP-functionalized electrodes. Though these methods are convenient, there are several drawbacks making it difficult to directly relate NP morphology to observed electrochemical behavior. Solution deposition methods offer limited control of the NP surface coverage, the NP-electrode interface and/or 4

interactions between the NPs, all of which influence electrocatalytic activity. Though chemical assembly can control the NP coverage and attachment on electrodes, it is sensitive to surface pretreatment steps and/or NP desorption may occur over time. In order to simplify the modification procedure, it is challenging to find the facile immobilization methods. Three-dimensional (3D) scale assembly provides a suitable bottom-up approach

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for the design of NP functionalized electrodes for electrocatalysis application [20]. Eguchi et al. reported a DNA-gold nanoparticle hybrid hydrogel consisting of AuNP

cross-linked with enzymatically synthesized DNA. [21]. The AuNPs and the

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bio-frame structure combined composite material, which not only avoids the

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preaggregation of the AuNPs, but also composes the signal of the 3D structure to enlarge the output, and enhances the performance of electrocatalysis. However, these

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bio-frame structures composed of DNA or proteins are economic consuming.

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In order to find a facile, efficient and economic approach for nanoparticle functionalization on the electrode surface, here we investigate a simple strategy for

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in-situ preparation of gold nanoparticles embedded hydrogel. D-sorbitol hydrogel with acyl hydrazide (D-gel) was synthesized with good pH stability and

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bio-satisfaction [22]. We define that the hydrazide groups in D-gel has the strong reducibility to react with Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O) to form AuNPs embedded D-gel in one step (D-gel@AuNPs) (Fig. 1). The uniform AuNPs core size distribution and 3D structure on the electrode was proved. Owing to AuNP’s stable electronic structure, high surface coverage and good conductivity, the 5

enhancing electrocatalytic performance was achieved [23]. Glucose, one of the essential substances in human’s living, is also an important component of food, drugs and industrial products. Efficient approaches for glucose detection with high sensitivity, fast response, and facile manipulation is of high importance [24]. Take the glucose as a model, with this newly defined method, we rationally design D-gel@AuNPs system for glucose sensor with good stability and repeatability.

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Therefore, in-situ assembly of AuNPs hydrogels provided a simple NP functionalized electrodes method for the development of enhanced electrocatalysis.

2. Experimental

Hydrogen

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2.1. Reagents and Materials

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tetrachloroaurate

trihydrate

(HAuCl4·3H2O),

β-D-(+)-glucose,

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L-ascorbic, D-Sorbitol, p-Toluenesulfonic acid monohydrate and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich, Inc. (St. Louis, USA). Tetrahydrofuran

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and cyclohexane were received from Aladdin, Inc. (Shanghai, China). Methy

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4-formyibenzoate and methanol were bought from Sangon Biotech Co., Ltd. (Shanghai, China). Hydrazine monohydrate was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All reagents and solvents were used without further purification. All solutions were prepared with deionized water (18.2 MΩ∙cm) purified with a Millipore Milli-Q purification system (Barnstead, USA). 2.2. Apparatus and Measurements 6

All electrochemical measurements were performed using Metrohm Autolab B.V. (The Netherlands). The modified glassy carbon electrode (GCE) was used as the working electrode. Saturated calomel (1 M KCl) and Platinum foil were used as the reference electrode and the counter electrode, respectively. The scanning electron microscope (SEM) studies were performed using the FEI Nova NanoSEM 450 instrument. The transmission electron microscopy (TEM) was carried out with a

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JEM-2010F instrument with an operating accelerating voltage of 200 kV. The sample was imaged in tapping mode using atomic force microscopy (AFM, Agilent 5500, Agilent Technologies Co Ltd, USA). X-ray photoelectron spectroscopy (XPS) was

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recorded on an ESCALAB 250Xi instrument (Thermofisher, England) (condition:

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spot size 650 μm, energy step size 1.0 eV and pass energy 150.0 eV). UV−vis spectra collected by a UV−vis spectrophotometer from the Shimadzu UV-2450 PC model,

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with a path length of 10 mm and 50 μL quartz cuvette was loaded. 1 H NMR

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spectrum was carried out on a Jeol 400 spectrometer; the samples were recorded as solutions in deuterated NMR solvents. Mass spectral information was obtained

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through the Agilent LC-MS (TOF) system (USA). 2.3. Synthesis of D-gel

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D-gel was synthesized and according to the slightly modified method reported by

Okesola et al [22]. Briefly, 0.98 g D-sorbitol was mixed with 7 mL of cyclohexane and 2 mL methanol in a three-necked round-bottomed flask. The mixture was stirred under nitrogen gas protection for 25 min. 1.5 g of methy 4-formyibenzoate and p-toluene sulfonic acid hydrate 0.2 g were dissolved in 4 mL of methanol, stirred for 7

25 min at room temperature, and then added drop by drop into the round bottom flask. The reaction temperature was stirred for 2 h at 70 °C. The resulting white paste was washed well with methanol (80 mL), boiling water (100 mL) and toluene (100 mL) to remove derivatives, respectively. Finally, the resulting intermediate product was dried for 3 h under vacuum and then air-dried. Yield: 76.7%. 1H NMR (400 MHz, DMSO-d6): δ 7.84-7.98 (m, 4H, ArH), 7.54-7.60 (m, 4H, ArH), 5.76 (s, 2H, Ar-CH),

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4.47 (br, 1H, CH2OH), 4.17-4.26 (q, 3H, sugar (overlap)), 4.01 (s, 1H, sugar), 3.86 (d,

1H, sugar, J = 9.2), 3.84 (s, 6H, OCH3 ), 3.77 (br, 1H, sugar), 3.61-3.63 (m, 1H, sugar), 3.45-3.48 (m, 1H, sugar).

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0.1375 g of the intermediate product was dissolved in 5 mL of tetrahydrofuran,

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after which 0.8 mL of hydrazine monohydrate was added and the reaction was refluxed at 70 °C for 12 h. The resulting white precipitate was thoroughly washed

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with deionized water (100 mL) and then dried under vacuum for 3 h and transferred

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into an oven at 85 °C. Yield: 90.6%. 1H NMR (400 MHz, DMSO-d6): δ 9.81 (s, 2H, CONHNH2 ), 7.54-7.84 (m, 8H, Ar-H), 5.72 (s, 2H, ArCH), 4.94-4.96 (d, 1H, CHOH),

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4.46 (s, 4H, CONHNH2 ), 4.42-4.46 (t, 1H, CH2OH), 4.17 (q, 2H, CH2OH), 3.98 (s, 1H, CHOH), 3.86 (d, 2H, J = 9.2 Hz, sugar), 3.74-3.77 (m, 1H, sugar), 3.51-3.58 (m,

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1H, sugar), 3.45-3.48 (m, 1H, sugar). ESI-MS C22H26N4O8, m/z calculated 474.1751, found 475.1819 [M+H] +, 497.1639 [M+Na] +. 2.4. Synthesis of D-gel@AuNPs 2.0 mg of D-gel was completely dissolved in 0.3 mL of H2O by ultrasound and heat treatment, followed by cooling at room temperature for 4 h. 100uL of 8

HAuCl4·3H2O (8mM) was added to the prepared D-gel and left overnight at room temperature to obtain D-gel@AuNPs. 2.5. Electrocatalytic experiments A glassy carbon electrode (GCE, diameter 3 mm) was sequentially polished using alumina polishing powders of 1.0 and 0.3 μm, and then ultrasonically washed in water, ethanol and water 3, 5, and 3 min, respectively. The washed GCE was dried with

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nitrogen to make the following modifications. 30 μL of the prepared D-gel@AuNPs

was deposited on the purified GCE and dried in a blast oven at 50 °C for 20 min. 0.1 M NaOH solution was used as supporting electrolyte in the experiment. Using an

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electrochemical instrument for cyclic voltammetry (CV) measurement and collecting

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data near +0.2 V, we performed CV at a scan rate of 100 mV/s over a scan range of

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-0.7 to 0.7 V and pulse amplitude of 50 mV.

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3. Results and discussion

3.1. Characterization of the synthesized nanomaterials

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The supramolecular D-gel with a crisscrossing 3D network structure was synthesized and the in-situ reduction method was used to obtain AuNPs embedded

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hydrogels. The intermediate product was characterized by NMR spectral (Fig. S1), and the final product D-gel was characterized by NMR spectral (Fig. S2) and mass spectrometry (Fig. S3) and proved to be successfully synthesized. UV−vis spectrum and color change of D-gel@AuNPs before and after addition of HAuCl4·3H2O are shown in Fig. 2(A). Au3+ in AuCl4- was directly reduced to Au0 at room temperature 9

and gradually diffused into the network structure of the gel fibers, at the same time, the original yellow reaction solution became clear due to the reduction of Au3+. The absorption peak of AuNPs was shown around 538 nm, and UV−vis absorption spectroscopy indicated the successful formation of AuNPs. Fig. 2(B) shows the XPS spectra of D-gel@AuNPs. In the presence of Au, the O1s and C1s peaks were shifted using XPS. However, we found that the N1s peak was broadened and reduced,

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potentially revealed intense interactions between N and Au due to various complex

environments. No peaks were observed corresponding to Au3+, demonstrating formation of complete Au0 nanoparticle. As shown in Fig. 2(C), S4 and Fig. 2(D),

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after thoroughly drying the sample with an infrared lamp, basically the same shape

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was seen in TEM and SEM. AFM images confirmed that D-gel@AuNPs was a 3D crisscrossing network structure with a fiber diameter of about 10 nm (Fig. S5).

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Compared with D-gel, D-gel@AuNPs embedded with gold nanoparticles showed

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uneven surface. The prepared 3D gel network structure could convert planar carbon material carriers into porous, high surface area, and solvated electrochemically active



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surfaces.

A large number of dendritic fiber branches were loaded with relatively uniform

gold nanoparticles with a diameter of about 8 nm. The reduced AuNPs by the acyl hydrazides functional groups of D-gel were substantially all present in the gel fibers,

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while almost none was observed in the in solvent pockets within the hydrogel fiber network, the reason was due to the ligand-metal interaction between gold ions and gel fibers [25]. SEM image shows the D-gel@AuNPs was immobilized on the electrode surface with relative porous. These results further confirmed the success synthesis and modification of D-gel@AuNPs. 3.2. Modification of D-gel@AuNPs on GCE and its electrocatalysis activity

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To prove the principle of the detection system, the interface properties of the

modified electrode were studied step by step (Fig. 3A). Electrochemical impedance

spectroscopy (EIS) is a common interface characterization method that can effectively

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characterize electrode modification processes. The EIS data were supplied to an

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equivalent electrical circuit to obtain quantitative information [26]. The simulation and calculation were carried out by the Randles equivalent circuit, as shown in Fig.

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3A (inset). The model used Warburg impedance (W) and Rct linear diffusion electrode

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surfaces, which could be calculated using a custom Nova (Metrohm Autolab B.V., The Netherlands). The change in the state of the electrode surface could be reflected

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by the significant change in the impedance value. A semicircle with a diameter of ~310 ohm was measured at the bare GCE (Fig.3A, black), which was a typical feature

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of the diffusion limiting step of the electrochemical process. After modified with AuNPs, the conductive AuNPs synergetic electron transfer, and leading to decreased impedance (Fig. 3A, red). The D-gel only modified electrode showed an electron transfer resistance (Rct) of 521 ohm dues to the weak conductivity of D-gel (Fig. 3A, green). At the three-dimensional D-gel@AuNPs modified GCE (Fig.3A, blue), the 11

diameter of the high frequency semicircle was dramatically reduced, and the Rct value was 53 ohm. These results confirmed that D-gel@AuNPs improved GCE’s conductivity due to the large increase of the electron transport rate. The D-gel@AuNPs-modified GCE described the electrochemical performance of AuNPs in alkaline solutions (Fig. 3B). No electrochemical signal was generated in D-gel-modified

GCE

when

AuNPs

was

absent

(the

blue

line).

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D-gel@AuNPs-modified GCE anodic oxidation current started to appear at 0.2 V,

corresponding to the reduction peak produced by the negative potential scan (the black line). After the addition of a certain concentration of glucose, a pair of

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well-defined anodic peak was observed due to the oxidation of glucose by

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D-gel@AuNPs modified GCEs in the positive potential scan. An anodic peak of glucose was appeared again in the negative potential scan once the oxide layer was

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reduced (red line). In the meantime, we studied the catalytic effect of AuNPs on

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glucose (green line). The D-gel@AuNPs-modified GCEs showed good activity in glucose oxidation, which was better than AuNPs only and other reported in the

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literature [27−30]. These controlled experiments proved that D-gel@AuNPs-modified GCEs showed higher catalytic activity. The catalytic activity was about 2.27 times as

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much as the AuNPs.

For purpose of better study the electrochemical properties of D-gel@AuNPs, EIS

and

electrochemical

activity

D-gel@AuNPs-modified

GCE

of

bare

were

GCE,

AuNPs-modified

compared,

respectively

GCE,

and

(Table

1).

D-gel@AuNPs-modified electrode showed a smaller Rct and higher catalytic activity, 12

which meant that the electron transfer kinetics of the electrode surface was much faster. The ECSA value of the D-gel@AuNPs-modified GCE calculated according to the Randalls-Sevcik equation was 28.7 m2 g−1, which indicated that it showed a denser active site with higher catalytic performance. These results confirmed that D-gel@AuNPs enhanced the electron transfer and conductivity. In addition, the porous structures of D-gel@AuNPs allowed fast transport of glucose by the

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electrolyte or electrode interface. Meanwhile, they could ensure full contact with a larger reaction surface as well because of the high surface area.

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Table 1. Electrochemical properties of AuNPs and D-gel@AuNPs modified

GCE

CV (µA)

ECSA (m2 g−1)

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NDa

ND

174.0

90.4

13.88

53.5

213.3

28.72

EIS (ohm)

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Comparisons

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glassy carbon electrodes.

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AuNPs/GCE

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D-gel@AuNPs/GCE a

ND means non detected.

3.3. Optimization of modification conditions In order to investigate the experimental conditions, we analyzed several important factors in the experiment. Since the AuNPs obtained by in-situ reduction need to be properly embedded in the gel fiber network, and the size or density of the AuNPs 13

directly affects its unique electrochemical characteristics. Therefore, it is necessary to control the concentration of oxidant in the reaction system and modification temperature in the experiment. Respective data and figures are given in Fig.4. In short, the following experimental conditions were found to give best results: (a) Optimal concentration of HAuCl4·3H2O: 8 mM; (b) Best modification temperature: 50 °C.

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3.4. Electrocatalytic performance for glucose

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Due to the high reactivity and non-enzymatic catalysis, Nano scale gold is increasingly attracting interest in catalysis and detection. Besides, AuNPs have the

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function of assisting electrons and energy transfer, exhibiting a larger surface area and

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more reactive sites. As a widely used nano-enzyme, AuNPs have peroxidase catalytic activity and exhibit good non-enzymatic catalytic ability. In the process of catalyzing

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glucose, molecular oxygen is used as an oxidant to achieve environmentally friendly green oxidation of glucose. Therefore, the Au hydrogels prepared in this work could

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be tested as a high-level electrocatalyst that catalyzes the oxidation of glucose because

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of its sensitivity to sugar in human blood and its potential role in the development of fuel energy cells.

The analysis performance of the prepared electrochemical sensor was demonstrated by measuring CV response to glucose with different concentrations (0.1

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to 30 mM). Under the optimal experimental conditions, as illustrated in Fig. 5(A), the CV signal enhanced gradually with the increment of glucose concentrations. The linear regression equations were clarified according to the amperometric response opposite the glucose concentration in Fig. 5(B) and two good linear relationships were obtained in the concentration range of 0.1 to 4 mM and 4 to 30 mM. Meanwhile, the linear equation was I (μA) = 31.27 Cglucose + 8.17 (R2 = 0.991) in the concentration

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range of 0.1 to 4 mM with a low detection limit of 0.067 mM (S/N = 3) according to

the means of 3's blank criterion. The D-gel@AuNPs modified GCEs exhibited highlighted electrocatalytic performance for glucose detection in the matter of the

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sensitivity due to the large amount of AuNPs embedded in the 3D fiber structure. The

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detection limit was better than electrodes modified with AuNPs and was comparable to other originally intended methods (Table 2).

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Table 2. Parameter comparison of different glucose sensors. Detection range

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Electrode

Sensitivity

Referencs

limit (mM) (μA·μM−1 ·cm−2)

(mM)

0.1–27

-

0.85

[31]

ZnO-EnFET

0.2–40

0.14

3.3

[32]

Nafion/GOx/ZnO/Ag/PET

0.0–80

0.07

22.3

[33]

NPG/NiCo2O4

0.01–21 NRs/Ag/PET

0.01

0.3871

[34]

D-gel@AuNPs/GCE

0.0–30

0.067

0.32

This work

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RGo-Gox biocomposite

Detection

3.5. Selectivity and stability of D-gel@AuNPs/GCE

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Furthermore, we collected a similar electrochemical response after mixing the same concentration (10 mM) of ascorbic acid with glucose (Fig. 6A), the D-gel@AuNPs hydrogel modified GCE showed slight electrochemical response on ascorbic acid, an existing interference, reflecting its good selectivity for glucose [27,35]. Long-term stability experiments were researched by storing the prepared electrodes at the same time at room temperature and measured their electrochemical

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signals after a specified period of time. After 10 consecutive scans, the D-gel@AuNPs hydrogel modified electrode showed an initial catalytic current of over

92.8% (data not shown), and there was little change in the electrochemical response

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of oxidized glucose after storage at room temperature for 1 month (Fig. 6B). These

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results indicated that the D-gel@AuNPs hydrogel exhibited high sensitivity and great stability, showing the promise of the D-gel@AuNPs as a nonenzymatic stable sensing

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material and providing the foundation for high-level analytical applications.



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4. Conclusions

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In conclusion, we have successfully synthesized stable hydrogels with hydrazide functional groups and obtained AuNPs (8 nm) embedded in the gel fibers by in-situ reduction. Owing to this hybrid material’s stable electronic structure, high surface coverage

and

good

conductivity,

D-gel@AuNPs

exhibited

the

enhanced

electrocatalytic performance on glucose. In-situ assembly of AuNPs hydrogels provided a simple AuNPs functionalized electrodes method for the development of 16

enhanced electrocatalysis sensor. Therefore, D-gel@AuNPs had potentially high value and widespread applications in nanoelectronics. However, the alkaline environment required for this method to catalyze glucose limited practical applications, and subsequent studies would explore better catalytic materials to overcome the shortcomings of sensitive responses in neutral environments.

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Conflicts of interest There are no conflicts to declare.

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Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant

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No. 61275085).

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Figure legends Fig. 1. Scheme of in situ preparation of AuNPs embedded in the hydrogel for catalyzing glucose. Fig. 2. (A) UV−vis spectra before (black) and after (red) addition of HAuCl4·3H2O. Inset shows the photo of corresponding colors of hydrogel before (A) and after (B) adding HAuCl4·3H2O; (B) XPS wide scan; (C) TEM and (D) SEM images of the D-gel@AuNPs. Fig. 3. (A) Nyquist plots before (black) and after (blue) electrode modification of

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D-gel@AuNPs and AuNPs (red), D-gel (green) modified GCE in 5 × 10−3 mol/L [Fe(CN)6]3−/4− (1:1) solution containing 0.1 mol/L KCl. Insert: Randle's equivalent

circuit using a constant phase element for the D-gel@AuNPs modified GCE. (B) CV responses for D-gel (blue) and D-gel@AuNPs (black) modified GCE in the absence

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of 10 mM glucose, and CV responses of the AuNPs (green), D-gel@AuNPs (red) in the presence of 10 mM glucose.

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Fig. 4. (A) The catalytic effect of HAuCl4·3H2O concentrations on glucose (10 mM) and (B) temperature optimization of glucose catalyzed by modified electrode.

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Fig. 5. (A) CV responses of the developed method to different concentrations (0.1 to 30 mM) of glucose at +0.2 V (scan rate: 100 mV/s, scan range: -0.7V-0.7 V). (B)

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linear relationships of the electrocatalytic current of glucose vs concentration (0.1–4 mM and 4–30 mM) of the modified GCEs. Fig. 6. (A) CV responses at the D-gel@AuNPs hydrogel modified GCE in 10 mM

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glucose in 0.1 M NaOH solution before (black line) and after (red line) the addition of 10 mM ascorbic acid. Scan rate: 50 mV s−1. (B) Histograms of the electrocatalytic

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current vs time of the modified GCEs.

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