Surface nitrogen-enriched carbon nanotubes for uniform dispersion of platinum nanoparticles and their electrochemical biosensing property

Electrochimica Acta 143 (2014) 10–17

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Surface nitrogen-enriched carbon nanotubes for uniform dispersion of platinum nanoparticles and their electrochemical biosensing property Chunhui Xiao a,∗ , Qiong Zou b , Yuhai Tang a a

Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xianning West Road, Xi’an, 710049, P.R. China State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Lushan South Road, Changsha, 410082, P.R. China b

a r t i c l e

i n f o

Article history: Received 29 April 2014 Received in revised form 15 July 2014 Accepted 17 July 2014 Available online 4 August 2014 Keywords: carbon nanotubes nitrogen doped polydopamine platinum nanoparticles amperometric biosensor

a b s t r a c t Nitrogen containing carbon layer-coated carbon nanotubes (CNx-CNTs) was prepared as the support for platinum nanoparticles (Pt NPs) and their enhanced properties for electrochemical biosensor has been demonstrated in this paper. The CNx-CNTs were obtained from pyrolysis of polydopamine-wrapped CNTs, which were synthesized by a single deposition process based on the oxidative self-polymerization of dopamine on CNTs. It is found that Pt NPs are deposited on the surface of the CNx-CNTs (Pt/CNx-CNTs) with highly dispersion and small particle size (with an average diameter of 1.7 ± 0.3 nm). Compared to the nitrogen-free CNTs supported Pt NP composite (Pt/CNTs), the Pt/CNx-CNTs modified glassy carbon (GC) electrode exhibits superior electrocatalytic performance towards the oxidation of hydrogen peroxide (increase by about 55% of response current). Taking glucose oxidase (GOD) as the model, the proposed amperometric enzyme biosensor based on the Pt/CNx-CNTs shows excellent analytical characteristics to glucose detection, such as excellent sustainability in large range of pH values, high sensitivity (66.51 ␮A (mmol dm−3 )−1 cm−2 ), wide linear range (0.01-6.1 mmol dm−3 ) and low detection limit (0.4 ␮mol dm−3 ). © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) have been employed as support for transition metal nanoparticles (NPs) in a broad range of applications (eg. biosensor, fuel cell and heterogeneous catalysis) [1–9], owing to their fascinating properties like large accessible surface areas, low electric resistance, and high chemical and electrochemical stability. The performance of CNTs/NPs is strongly affected by many factors, such as the sizes, amount and uniformity of the metal NPs and the total surface area of carbon supports. In order to establish a uniform coverage of metal particles on the surface of CNTs, it is necessary to activate the graphitic surface of the tubes, which is extremely inert, to introduce more reactive sites [10]. Several methods have been applied and typically involve harsh chemical or electrochemical oxidation at defect sites of CNTs [11,12], grafting of polymer [4,13,14], etc. Unfortunately, these treatments considerably harm the mechanical and/or electronic properties of the tubes due to the introduction of either large numbers of oxidative damage on graphite structures or non-conductive functional groups. In

∗ Corresponding author. E-mail address: [email protected] (C. Xiao). http://dx.doi.org/10.1016/j.electacta.2014.07.093 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

addition, the weak bonding interaction between metal and support could potentially lead to detachment of NPs from the CNTs surface, and thereby affect the durability and utilization of composite material [15–17]. Alternatively, doping carbon materials with heteroatoms (e.g., nitrogen) represents a feasible pathway to provide chemically active sites on carbon surface whereas without significant degradation of their electrical properties, hence improving the dispersion of metal NPs on carbon materials and retarding the degradation of metal nanoparticles [17–24]. Nitrogen-doped CNTs (NCNTs) have been reported as excellent support materials for uniform Pt NPs dispersion with significantly better electro-catalytic performance and durability than that of nitrogen-free Pt/CNT in the application of fuel cell and sensing [10,23–26]. Furthermore, nitrogen-doped carbon itself is also already known to act as metal-free catalyst for oxygen reduction [27–32]. Currently, one of the most common methods to synthesis NCNTs is direct CVD deposition by using nitrogen-containing carbon precursor. However, the NCNTs prepared by CVD usually have low nitrogen content, resulting in a dispersion and particle size of noble metal NPs on CNTs which are still unsatisfactory. If the nitrogen content of NCNTs is extensively increased by a direct nitrogen-doping strategy, the nature of the CNTs may change due to the introduction of lots of disordered sites

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into the CNT lattice during the nitrogen-doping process, leading to the formation of a less stable structure of NCNTs. To minimize the above disadvantages, it is highly desirable to develop an effective surface-nitrogen-enrichment method in order to increase the surface nitrogen content of CNTs but cause limited structural damage to CNTs. A promising method to form nitrogen containing CNTs is based on the pyrolysis of nitrogen-containing polymer functionalized CNTs to obtain a nitrogen-containing carbon layer (CNx) covered on the surface of CNTs [10]. Polydopamine has great adhesive properties on a range of substrates (CNTs [33–35] and SiO2 [36] etc), which can be easily obtained via exposing a solution of dopamine to air at room temperature. Pyrolyzing dopamine and its polymer to synthesize nitrogen-containing carbon materials has advantages of generalized and robust synthesis, film thickness control, without heating and avoiding metal and organic catalysts. Recently we have employed this approach to generate hollow nitrogendoped carbon spheres (HNCS) for applications of electrochemical sensors and fuel cells [32,37,38]. Due to the presence of nitrogen species, the HNCS has shown admirable properties towards electrocatalytic oxidation of small molecular compounds (DA, UA, AA and oxygen) as well as Au NPs uniformly loading. Herein, we successfully synthesized nitrogen-doped carbon layer-coated CNTs (CNx-CNTs) composite by heating PDA wrapped CNTs to temperature of 1000 ◦ C, which were fabricated by the spontaneous self-polymerization of dopamine onto CNTs surface. The surfacenitrogen-enriched CNTs not only introduce a uniform distribution of nitrogen atoms on the CNT surface to anchor and grow platinum NPs with ultra-uniform dispersion and narrow size distribution, but also maintain the intrinsic nature of CNTs (as shown in Scheme 1). Furthermore, taking glucose oxidase (GOD) as a model, this Pt/CNxCNTs was utilized as an enzyme-immobilization electrode material in an amperometric biosensor with significant superior characteristics than nitrogen free CNTs supported Pt NPs (Pt/CNTs).

2. Experimental 2.1. Reagents and Apparatus Multi-walled carbon nanotubes (CNTs, >95%, diameter 2060 nm, length 5-15 ␮m) prepared by CVD were purchased from Shenzhen Nanotech Port Ltd. Glucose oxidase (E.C. 1.1.3.4, Type X-S, 136100 U/G from Aspergillus niger), Dopamine hydrochloride was purchased from Alfa Aesar (USA) and used as received. 5 wt% Nafion 117 solution was purchased from Sigma-Aldrich (USA). Glucose stock solution was stored overnight at room temperature before use. All other chemicals were of analytical grade. Aqueous solutions used throughout were prepared with ultra-pure water obtained from a Millipore system (>18 M cm). All of the electrochemical measurements were carried out at room temperature (25 ± 2 ◦ C). Micrographs of transmission electron microscopy (TEM) were obtained on a JEOL 3010 transmission electron microscope operating at 200 kV. Nitrogen sorption isotherms and Brunauer–Emmett–Teller (BET) surface areas of the materials were determined by an ASAP 2010 Micrometrics sorptometer (America). Nitrogen content in CNx-CNTs was determined by elemental analyzer (TCH-600, America). Thermogravimetric analysis (TGA) of CNx-CNTs was carried out on Luxx STA 409 PC with 5 K min−1 heating rate with oxygen flow. All of the electrochemical measurements were performed on a CHI 660D electrochemical workstation (Chenhua Instrument Company of Shanghai, China) with a conventional three-electrode system. Platinum wire was used as counter electrode, Ag/AgCl electrode as reference electrode, and modified GC electrode as working electrode.

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2.2. Preparation of CNx-CNTs The preparation of CNx-CNTs consists of two steps. Firstly, PDA/CNTs composite was synthesized according to the literature [33,36]. In brief, 100 mg CNTs was washed thoroughly with 50 mM tris(hydroxymethyl)aminomethane-chloride acid (Tris-HCl) buffer (pH = 8.5) and centrifuged three times alternatively. The deposit was re-suspended in 100 ml 50 mM Tris buffer (pH = 8.5) containing 2 mg mL−1 DA, followed by vigorous stirring for 24 h to form PDA/CNTs composite. Then the dark-coloured precipitate was centrifuged (5000 rpm, 2 min) and washed with fresh Tris buffer (three times) to remove the tan solution, followed by drying in vacuum at 60 ◦ C overnight. Finally, CNx-CNTs were synthesized by the pyrolysis of the prepared PDA/CNTs in a nitrogen atmosphere at 1000 ◦ C for 2 h. 2.3. Preparation of Pt/CNx-CNTs Deposition of the Pt NPs on the CNx-CNTs was carried out via a microwave-assisted reduction process in ethylene glycol solution. The details were as follows: 20 mg of CNx-CNTs was mixed with 665 ␮L H2 PtCl6 (38.6 mmol dm−3 ) in 25 mL ethylene glycol solution under ultra-sonication for 30 min. The pH value of the solution was adjusted to 10 with 1.0 M KOH aqueous solution. Then, the mixture was placed in a microwave oven and heated by microwave irradiation (800 W) for 10 min at 120 ◦ C. The products were centrifuged and washed three times with distilled water. The CNx-CNTs-supported Pt NPs, denoted as Pt/CNx-CNTs, was dried in a vacuum oven at 60 ◦ C overnight. For comparison, Pt NPs supported on the acid-treated CNTs, labeled as Pt/CNTs, was prepared under the same procedure as described above. 2.4. Preparation of GOD/Pt/CNx-CNTs/GC electrode Prior to use, GC electrodes (diameter = 3 mm) were carefully polished to a mirror-like plane with 0.5 and 0.05 ␮m alumina slurries, successively. Afterward, the electrodes were washed thoroughly with excess amount of water and dried under nitrogen gas. The CNx-CNTs modified GC (CNx-CNTs/GC) electrode was prepared by casting 5 ␮L CNx-CNTs suspension (2 mg mL−1 CNx-CNTs in mixture of water and ethanol (v:v = 1:1)) on GC surface and dried at room temperature for 24 hours. For comparison, the CNTs/GC electrode was also prepared by the same procedure. For fabricating GOD biosensor, a 5 ␮L of GOD solution (10 mg mL−1 GOD in PBS, pH 7.0) was dropped on the CNx-CNTs/GC electrode and dried at less than 4 ◦ C overnight, the obtained GOD/CNx-CNTs modified electrode was then rinsed carefully with double-distilled water and dried, followed by coating 4 ␮L of Nafion ethanol solution (1 wt%) onto the electrode surface to avoid the leak of the GOD. The Nafion/GOD/CNx-CNTs electrode was washed thoroughly with water. For comparison, the CNTs modified electrode was also prepared to immobilize GOD via the same procedure. 3. Results and discussion 3.1. Characterization of CNx-CNTs and Pt/CNx-CNTs Fig. 1A and B shows the TEM images of obtained CNx-CNTs composite. It displays clearly the tubular morphology and typical graphite layered structure in the inner wall of the tubes, indicating that the framework of CNTs is well-preserved after the pyrolysis in nitrogen atmosphere. Furthermore, it can be observed that the outside surface of CNTs is covered by an amorphous structured shell (CNx) with thickness of ∼12 nm, which should be the carbonization product of PDA on the surface of CNTs.

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Scheme 1. Schematic routine for the preparation of Pt/CNx-CNTs.

Raman spectroscopy was carried out in order to gauge the degree of structural deformations present in the CNx-CNTs and CNTs (Fig. 1C). The D band observed at approximately 1319 cm−1 for our samples, is disorder induced, being ascribed to structural defects on the graphitic plane of CNTs. The G band, observed at approximately 1578 cm−1 , is commonly observed for all graphitic structures and attributed to the E2 g vibrational mode present in the sp2 bonded graphitic carbons. The extent of the defects in graphite materials upon surface modification can be quantified by the intensity ratio of the D to G bands (ID /IG ). The CNx-CNTs is found to have an ID /IG ratio of 1.42, significantly larger than that for CNTs (1.3). The larger ID /IG ratios for CNx-CNTs is a result of the structural defects caused by nitrogen containing carbon layer on the walls of CNTs. The CNx-CNTs and CNTs were characterized by TGA-DSC analysis, which was performed at a heating rate of 5 K min−1 in O2 atmosphere. The TGA curve of CNTs shows a one-step weight loss and DSC curve has an exothermic peak at around 877 K. However, the TGA curve of CNTs modified with CNx has two-step weight

losses and the curve of DSC has two exothermic peaks. The former peak at lower temperature (801 K) should result from thermal oxidation of CNx on the surface of CNTs, while the latter (853 K) is assigned to combustion of CNTs. The amount of CNx in the composite of 18.7 wt% could be estimated by subtracting the weight loss in CNTs from that in CNx-CNTs at 801 K. On the other hand, the result of the elemental analysis (Table 1) shows that the content of the nitrogen element in CNx-CNTs was 0.52 wt%, which is much higher than that in primary CNTs (less than 0.06%). The surface area of this carbon material was also measured by using the Brunauer-Emmet-Teller (BET) method at a bath temperature of 77 K using nitrogen gas as adsorbate. Representative nitrogen adsorption/desorption isotherms of the obtained CNTs and CNx-CNTs are shown in Fig. A1 (See in appendices). Note that the N2 adsorption–desorption isotherms exhibits obvious hysteresis at high relative pressure, suggesting the presence of mesopores in CNx-CNTs. The measured BET surface area of the CNx-CNTs (349.1 m2 g−1 ) was as 2.9 times as that of CNTs (119.4 m2 g−1 ).

Fig. 1. TEM (A) and HRTEM (B) images of CNx-CNTs; Raman spectra (C) and TGA-DSC (D) of CNTs and CNx-CNTs.

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Table 1 Physicochemical properties of CNx-CNTs and CNTs.

CNTs CNx-CNTs

ID /IG

BET (m2 g−1 )

C (wt%)

N (wt%)

Pt loading (wt%)

1.42 1.30

119.4 349.1

95.87 94.33

NA 0.52

17.5% 18.4%

The CNx-CNTs with features of specific core/shell structure, high surface area and nitrogen atom-doping, is expected to favor more metal nanoparticles being anchored on the surface, and thus enhance the electro-catalytic performance. Fig. 2 displays the TEM images of the Pt/CNx-CNTs and Pt/CNTs composites along with their respective particle size histograms. The CNx-CNTs (Fig. 2A) are decorated densely with well-dispersed Pt NPs and no obvious aggregation of NPs was observed. Their size distribution was evaluated statistically by measuring the diameter of 200 Pt NPs in the selected TEM images. The size of Pt NPs is mainly distributed between 1.0-2.6 nm with an average diameter of 1.7 ± 0.3 nm (Fig. 2C). However, for the nitrogen-free CNTs, a scatter of Pt NPs are attached on the CNT surface and aggregation of the NPs could be observed (Fig. 2B). The size distribution of Pt NPs is broad (2-9 nm) and the average diameter is ca. 5.0 ± 2.0 nm (Fig. 2D). Apparently, coating a layer of CNx on CNTs surface significantly improves the dispersion of Pt NPs loading. As is well-known, Pt NPs tends to deposit on the localized defect sites of CNTs where are usually not uniform during the growth and post-synthesis process, leading to poor dispersion and aggregation. In the case of CNx-CNTs, the existence of CNx layer provides a high surface content and uniform distribution of nitrogen atoms that enables to immobilize Pt precursors through the coordinative action [10]. Therefore, higher degree dispersion of uniformly sized platinum NPs can be observed on the surface of CNx-CNTs.

On the other hand, the Pt loading mass in the Pt/CNx-CNTs and Pt/CNTs were determined by ICP-AES and the corresponding results are shown in Table 1. The loading mass of the Pt NPs on the CNxCNTs (18.4 wt%) is a little higher than that on the CNTs (17.5 wt%), which may provide evidence for better NPs loading property on the CNx-CNTs. It is expected that the as-prepared Pt/CNx-CNTs will exhibit superior electrocatalytic performance in H2 O2 oxidation and further biosensor applications. 3.2. Electrochemical properties of Pt/CNx-CNTs/GC electrode toward H2 O2 oxidation The cyclic voltammograms of bare GC, CNTs/GC, CNx-CNTs/GC, Pt/CNTs/GC and Pt/CNx-CNTs/GC electrodes in 0.1 M PBS (pH 7.0) with (solid line) or without (dotted line) 1.0 mM hydrogen peroxide are presented in Fig. 3. The oxidation currents of H2 O2 being started at around +0.42 V, which are consistent with that reported in the literatures [8], could be observed obviously at all of the bare GC, CNTs/GC and CNx-CNTs/GC electrodes. Also, the oxidation current of H2 O2 at CNx-CNTs/GC electrode is higher than that at the bare GC and GC/CNTs electrode, indicating the higher electrocatalytic activity of CNx-CNTs. This result matches well with many other positive observations of improving electronic and catalytical properties of bulk carbon materials by N-doping [37,39–41]. For Pt/CNx-CNTs/GC electrode, the oxidation of H2 O2 starts from a more negative

Fig. 2. TEM images and size distribution of Pt/CNx-CNTs(A, C) and Pt/CNTs (B, D).

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Fig. 4. Amperometric responses to successive additions of H2 O2 and calibration curves (inset) on Pt/CNx-CNTs/GC (a) and Pt/CNTs/GC (b) electrodes in stirred 0.1 M PBS. Applied potential: +0.3 V.

Fig. 3. Cyclic voltammograms obtained at bare GC(a, b), CNx-CNTs/GC(c, e), CNTs/GC(d, f), Pt/CNx-CNTs/GC(g, i) and Pt/CNTs/GC(h, j) electrodes in 0.1 M PBS with (solid line) and without (dotted line) 1.0 mmol dm−3 H2 O2 . Scan rate: 50 mV s−1 .

potential (∼ + 0.15 V), which is apparently induced by the catalytic effect of Pt NPs. The oxidation response current of H2 O2 at Pt/CNxCNTs/GC electrode is higher than that at Pt/CNTs/GC electrode, though the background current of Pt/CNx-CNTs/GC electrode is larger than that of CNTs/GC electrode because of its larger specific surface area. In order to confirm the electrocatalytic behavior of the Pt/CNxCNTs towards H2 O2 , the amperometric responses to successive additions of H2 O2 at an applied potential of +0.3 V in 0.1 M PBS (pH 7.0) were measured at Pt/CNx-CNTs/GC and Pt/CNTs/GC electrodes. It can be clearly observed from Fig. 6 that Pt/CNx-CNTs/GC electrode shows enhanced amperometric response comparing with Pt/CNx-CNTs/GC electrode does. The calibrations of H2 O2 at these two electrodes are also presented in the inset of Fig. 4. At both electrodes, the current responses increase linearly with the increasing of H2 O2 concentration from 10 to 900 ␮mol dm−3 . Both of electrodes respond quickly to H2 O2 (t95% ≈ 5 s). However, the response sensitivity of Pt/CNx-CNTs/GC electrode (678.6 nA (␮mol dm−3 )−1 cm−2 ) is 55% more than that of Pt/CNTs/GC electrode (437.2 nA (␮mol dm−3 )−1 cm−2 ), indicating its excellent electrocatalytic activity for H2 O2 oxidation. The smaller size and better dispersion of Pt NPs on CNx-CNTs lead to a better electrocatalytic activity towards the oxidation of H2 O2 , making the Pt/CNx-CNTs as a promising supporting material for amperometric enzyme biosensors. 3.3. Response to glucose at GOD/Pt/CNx-CNTs/GC electrode and effect of pH value and applied potential The excellent performance of the composite modified electrode toward the detection of H2 O2 makes it attractive for the fabrication of oxidase-based biosensors. A glucose biosensor was fabricated by immobilization of glucose oxidase onto a Pt/CNx-CNTs/GC electrode using a thin layer of Nafion. Fig. 5 shows cyclic voltammetries as a function of glucose concentration at GOD/Pt/CNx-CNTs/GC

Fig. 5. Cyclic voltammograms at GOD/Pt/CNx-CNTs/GC electrode in 0.1 M PBS containing different concentrations of glucose (0, 1, 2, 3, 4, 5 and 6 mmol dm−3 , respectively), Scan rate: 50 mV s−1 .

electrode. Note that the electrode is sensitive toward glucose and the response increases with the glucose concentration increasing from 0 to 6 mmol dm−3 . According to previously reported literature [42], the mechanism of the biosensor should be as follow: GOD

glucose + O2 −→Gluconolactone + H2 O2 −

H2 O2 → H2 O + O2 + 2e

(1) (2)

The reaction firstly involves equation 1. The product, H2 O2 , then undergoes oxidation on the composite of Pt/CNx-CNTs with large active surface area and high electrocatalytic activity, releasing two electrons (equation 2). The effect of pH value of the buffer solution on amperometric response of GOD/Pt/CNx-CNTs/GC electrode has been investigated and the results are shown in Fig. 6A. In the pH value range of 6.0-8.0, the current responses to a certain concentration of glucose (0.1 mmol dm−3 ) remain substantially unchanged. Even under more acidic conditions (pH = 5.0), the response value still remains 66% of the maximum value at pH 7.0. For comparison, the maximum response current of GOD/Pt/CNx-CNTs/GC electrode (565 nA) is as 2.2 times as that of GOD/Pt/CNTs/GC electrode (258.5 nA). Moreover, only 40% of the response left at pH 5.0 on

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Fig. 6. Effect of pH value (A) and applied potential (B) on the steady-state response current of GOD/Pt/CNx-CNTs/GC and GOD/Pt/CNTs/GC electrodes in 0.1 M PBS containing 0.1 mmol dm−3 glucose.

GOD/Pt/CNTs/GC electrode. The GOD/Pt/CNx-CNTs/GC electrode not only increases the sensitivity of glucose sensing, but also improves the sustainability in large range of pH values, which is very meaningful for the practical application of glucose biosensor. Fig. 6B shows the influence of applied potential on the amperometric behavior of GOD/PtNPs/CNx-CNTs/GC electrode. The current responses significantly increase with increasing of applied potential and gradually reach to a platform after a potential of +0.5 V, which is almost identical with other literatures observed [8,43]. The response of the enzyme electrode is controlled by the electrochemical oxidation of hydrogen peroxide in the lower potential range, limited by the rate-limiting process of enzyme kinetics and substrate diffusion. Considering that the higher the applied potential is, the easier the electroactive interferents to be oxidized and cause the response current, we set the applied potential of +0.5 V for the amperometric determination of glucose. 3.4. Amperometric determination of glucose at GOD/Pt/CNx-CNTs/GC electrode Fig. 7 shows the amperometric determination of glucose at GOD/Pt/CNx-CNTs/GC electrode and the calibration curve of the response current to glucose concentration. Setting at the optimized conditions, the current response (I)

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Fig. 7. Amperometric responses to the successive additions of 1 mmol dm−3 glucose and calibration curves obtained at the GOD/Pt/CNx-CNTs/GC and GOD/Pt/CNTs/GC electrode in 0.1 M PBS. The applied potential: +0.5 V.

increases linearly with increasing glucose concentration in the range of 0.01-6.1 mmol dm−3 and linear equation is I(␮A) = 0.2174 + 4.6554cglu (mmol dm−3 ) (R2 = 0.9986). Comparing with that to H2 O2 at/Pt/CNx-CNTs/GC electrode, the response time to glucose at the GOD/Pt/CNx-CNTs/GC electrode was slightly prolonged (t95% ≈ 7 s) due to the nafion blocking. The value of apparent Michaelis-Menten constant (KMapp ), which gives an indication of the enzyme-substrate kinetics, is determined as 15.2 mmol dm−3 , according to the Lineweaver-Burk equation [5]. This value is similar to the reported value for the free enzyme, illustrating the nondenaturating character of the enzyme-anchoring procedure. The as-prepared enzyme sensor shows a sensitivity of 66.51 ␮A (mmol dm−3 )−1 cm−2 and the detection limit for glucose is determined to be 0.4 ␮mol dm−3 (S/N = 3). It exhibits superior or comparable performance comparing with previously reported Pt NPs modified CNTs glucose sensor. Table 2 lists the data reported in the literature in recent years regarding the Pt-modified electrodes for glucose detection [8,42,44–51]. Only CS-GA-GOx/Nafion/PtPd-MWCNTs/GCE has higher sensitivity than the GOD/Pt/CNx-CNTs/GC electrode, but its detection limit is not satisfactory (31 ␮mol dm−3 ). The detection limit of GOD/Pt/CNxCNTs/GC electrode is one of the lowest among these electrodes. (Con A/GOD)3 /Pt-CNTs-CS/GCE has the same value of detection

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Table 2 Comparison of the analytical performance of different modified electrodes for glucose detection. Electrodes

Sensitivity (␮A (mmol dm−3 )−1 cm−2 )

GOD/Aunano /Ptnano /CNTs/Au Nafion/GOD/PtNPs-MWNTs/GC Ni-PMA/Au-PtNPs/NFs/CNTGCE Pt-CNTs-GOD/GCE CS-GA-GOx/Nafion/PtPdMWCNTs/GCE (Pt-DEN/GOx)4 /CNTs/Pt (Con A/GOD)3 /Pt-CNTs-CS/GCE GOD/PtNPs/PIL-CNTs/GCE GOx/Pt-DENs/PANI/CNT PyOx–IL–Pt–MnOx/GCE GOD/Pt/CNx-CNTs/GCE

9.55 18.87 5.10 112 30.64 41.9 28.28 42.0 66.51

Linear range (mmol dm−3 )

Detection Limit (␮mol dm−3 )

Applied potential (V)

Response time (s)

Ref.

0.5–17.5 1–7 0.001–0.1

400 3 0.55

+0.6 (SCE) +0.7 (Ag/AgCl) +0.35 Ag/AgCl

7 -

[44] [42] [45]

0.16–11.5 0.062–14.07

55 31

-0.1 Ag/AgCl +0.6 (SCE)

5 5

[46] [47]

0.005–0.65 0.0012–2 0.1–12 0.001–12 0.01–0.75 0.01–6

2.5 0.4 10 0.5

0 +0.3 (SCE) +0.6 (SCE) 0 (Ag/AgCl) +0.2 (Ag/AgCl) +0.5 (Ag/AgCl)

5 7

[48] [49] [8] [50] [51] present

limit with the as-prepared electrode, but its detection linear range is very narrow. 3.5. The anti-interference ability, reproducibility, stablity and real sample analysis Some compounds, such as uric acid (UA) and ascorbic acid (AA), coexisted with glucose in the blood can also be oxidized and contribute to the response currents in the potential range for glucose determination. Herein, the interferences of electroactive compounds (AA and UA) to the glucose response were examined in the presence of glucose (500 ␮mol dm−3 ) and the corresponding results are shown in Fig. A2 (See in appendices). Current responses observed from the additions of 50 ␮mol dm−3 UA and 10 ␮mol dm−3 AA are 4.5% and 3.8% of the response of 500 ␮mol dm−3 glucose, respectively. Considering lower concentrations of these interferences in the normal biological blood, the resulting GOD/PtNPs/CNx-CNTs/GC electrode exhibited a satisfactory anti-interference performance. The good anti-interference ability attributes to the Nafion polymer, which has been reported as a negatively charged polyelectrolyte matrix reducing the permeability of anionic biological interferences [52,53]. To evaluate the reproducibility of the electrode, six parallel enzyme electrodes were prepared for detection of 0.1 mmol dm−3 glucose, and the relative standard deviation (RSD) is 3.3%, demonstrating a good reproducibility of the biosensor. Additionally, when the GOD/Pt/CNx-CNTs/GC electrode was stored at 4 ◦ C in 0.1 M PBS (pH 7.0), the current response to 0.1 mmol dm−3 glucose decreased by only 2% after placed seven days, 85% of its initial current response was retained a month later. The excellent stability of as-prepared electrode may be attributed to the uniform nitrogen atom distribution of the nitrogen doped-carbon layer, its role in platinum nanoparticle stabilization, and also electrostatic interaction between the positive charged surface and the negative charged GOD in maintaining the activity of GOD. The practical application of the GOD/Pt/CNx-CNTs/GC electrode was also evaluated by determining the glucose in human plasma Table 3 Glucose concentration in human blood samples. Sample number

1 2 3 4

Provided by the local hospital (mmol dm−3 )

Determined by current method* (mmol dm−3 )

4.21 5.39 5.88 9.29

4.37 5.26 6.08 9.01

*three separate measurements

± ± ± ±

0.23 0.28 0.36 0.39

Relative error (%)

0.4

samples. Fresh plasma samples were provided by the local hospital. Then the samples were assayed with the GOD/Pt/CNx-CNTs/GC electrode by adding 0.1 mL plasma into 10 mL 0.1 M PBS (pH 7.0), and the response current was obtained at +0.5 V. The concentration of glucose in blood can be calculated from the calibration curve. The results are shown in Table 3, which are satisfactory and agree closely with those measured by the biochemical analyzer in the hospital. 4. Conclusions In summary, pyrolysis of polydopamine-modified CNTs has been proved to be a generalized and robust way to prepare surface-nitrogen-enriched carbon nanotubes. Due to the high surface content and uniform distribution of nitrogen atoms provided by CNx, The Pt NPs loaded on CNx-CNTs had a smaller particle size, better dispersion than that on nitrogen-free CNTs. The Pt/CNxCNTs show significantly enhanced electro-catalytic activity toward oxidation of hydrogen peroxide. More importantly, as an enzyme electrode material, it avoided typical spillover sensitivities of ordinary glucose sensor in a large range of pH value, making the enzyme electrode possesses superior performance for glucose sensing. This suggests that the CNx-CNTs composite could, indeed overcome the limitations of traditional nitrogen-doped CNTs, present a suitable, sustainable, and cheap alternative carbon material for the loading of metal NPs in the biosensors application and other fields. Acknowledgment Financial support from the Fundamental Research Funds for the Central Universities (No. 08143099) are gratefully acknowledged. We thank Shan Gao at First Affiliated Hospital of Xi’an Jiaotong University Health Science Center, for providing and analyzing fresh plasma samples. We gratefully thank Prof. Jinhua Chen (Hunan University) for constructive discussions and long-term support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.07.093. References

+3.8 -2.5 +3.4 -3.0

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