an amperometric glucose biosensor fabricated with pt nanoparticle-decorated

ELSEVIE Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb An amperometric glucose biosensor fabricated wit

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ELSEVIE

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journal homepage: www.elsevier.com/locate/snb

An amperometric glucose biosensor fabricated with Pt nanoparticle-decorated carbon nanotubes/TiOz nanotube arrays composite

Xinyu Pang, Dongmei He, Shenglian Luo, Qingyun Cai*

State Key Laboratory of Chemo/Biosensing and Chemometrics, Department of Chemistry, Hunan University, Changsha 410082, People’s Republic of China

Article history:

Received 14 August 2008

Received in revised form

27 September 2008

Accepted 30 September 2008

Available online 15 October 2008

Carbon nanotubes (CNTs)-modified titania nanotube (NT) arrays are prepared by vapor-growing CNTs

in the inner of titania NTs Pt nanoparticles of ~3nm in diameter are uniformly decorated on the as synthesized titania-supported CNTs (TiO2/CNTs) electrode, showing remarkably improved catalytic activities for the oxidation of hydrogen peroxide The consequent glucose biosensor fabricated by modifying TiO2/CNT/Pt electrode with glucose oxidase (GOx) presents a high sensitivity of 0.24.AmM—'! cm~? to glucose in the range of 0.006 mM to 1.5 mM with a response time of less than 3 s and a detection limit of

Keywords:

Carbon nanotube

Titania nanotube

Platinum nanoparticles

Glucose biosensor

5.7 HM at 3 signal/noise ratio

1 Introduction

Since the development of the first glucose biosensor, much

attention has been focused on the improvement of the response

performances of enzyme electrodes for biosensor research [1-8]

Wireless glucose biosensors were developed by using a pH-

responding polymer [4] or a glucose-responding mass-changing

polymer [9] as the sensing coating Significant research and efforts

concerning preparation of glucose sensor for blood glucose mon-

itoring have been reported [2-4] and most of them are based on

the determination of hydrogen peroxide [5-8] Carbon nanotubes

(CNT) have attracted increasing research interests due to their

unique electrical, geometrical, and mechanical properties [10] that

make them excellent materials for the construction of ultrasensi-

tive electrochemical biosensors which has led more recently to an

upsurge of research on incorporating CNTs into biosensing plat-

forms [7,8] It has been demonstrated that electrodes based on

the combination of metal nanoparticles and CNT exhibits highly

sensitive and selective responses to hydrogen peroxide generated

by enzymatic reactions [7,8,11] Wang et al [11] reported that

multi-wall CNTs dissolved in Nafion could be applied to construct

amperometric sensor for hydrogen peroxide Electrochemists have

also successfully taken advantage of CNT for accelerating the elec-

* Corresponding author Tel.: +86 731 8822170; fax: +86 731 8821848

E-mail address: qycai0001@hnu.cn (Q, Cai)

doi:10.1016/j.snb.2008.09.051

tron transfer reaction involving electrocatalytic activities toward

H»2O, NADH, cysteine, ascorbic acid, nucleic acids, and homocys-

teine [12,13]

Enzyme immobilization is a key step in fabrication of a sensitive and stable biosensor Generally in biosensors enzymes are immobilized to the sensor surface by either cross-linking with, e.g glutaraldehyde [14] or being protected with a thin gel or polymer layer of, e.g Nafion [15,16] to avoid the loss of enzymes In particular, new materials and methods were researched for get- ting more active and stable biosensors in immobilizing enzyme [17,18] Nano-architectured TiO, has also attracted considerable interest due to the superior properties such as large specific surface area, high uniformity, and excellent biocompatibility [19-25], and has been applied in a variety of fields including highly efficient photocatalysis [20,21], fuel cells [22], biosensors [23], and hydrogen sensor [24,25] Liu and Chen [23] fabricated a biosensor for measuring HO, utilizing a TiO nanotube array on which horseradish peroxidase and thionine were adsorbed The high uniformity and superior semi-conductivity of the TiO2 NT array makes

it a sensitive hydrogen gas-responsive material [24,25] TIO› film has also been utilized in the immobilization of proteins for bioanalytical applications due to its stability and biocompatibility [26-28] Topoglidis et al reported the adsorption of protein on nanocrystalline TiOz films based on the electrostatic interactions between the negatively and positively charged groups of them [28] These successful applications indicate that nanocrystalline TiO, materials are promising ideal functional materials for biosensor substrate

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Here, an electrocatalytic enzyme biosensor based on Pt

nanoparticles-decorated TiO02/CNT composite substrate was devel-

oped The Electrochemical catalytic activity of the as-prepared

TiO2/CNT/Pt electrode in response to H202 was investigated An

amperometric glucose oxidase (GOx) biosensor was fabricated

by modifying GOx on the as-prepared electrode, and a sensitive

response to glucose was achieved

2 Experimental

2.1 Reagents and apparatus

Titanium foil (99.8%, 0.127mm thick) was purchased from

Aldrich (Milwaukee, WI) Sodium fluoride, citric acid, hexachloro-

platinic (IV) acid and glucose of analytical reagent grade were

purchased from commercial sources and used as supplied GOx

(type: X-S, Aspergillus niger, 127 units/mg) were obtained from

Aldrich-Sigma (St Louis, MO) The supporting electrolyte was

0.067 M pH 7.2 phosphate buffer solutions (PBS) Double distilled

water was used throughout the experiments

The catalyst topology was characterized using a field-emission

scanning electron microscope (FE-SEM) operating at 5kV (JSM

6700F; JEOL, Tokyo, Japan) An energy dispersive X-ray (EDX) spec-

trometer fitted to the scanning electron microscope was used

for elemental analysis Transmission electron microscopy (TEM)

images were obtained using a JEM 3010 (JEOL; Tokyo, Japan)

operating at 300kV Cyclic voltammetry (CV) and amperometric

measurements were performed in a standard three-electrode con-

figuration with the modified electrodes as working electrode, a

platinum flake auxiliary electrode, and Ag/AgCl reference electrode

(saturated by KCl) by an electrochemical working station (CHI 660B;

CH Instruments, Inc., Austin, TX)

2.2 Preparation of Ti02/CNT/Pt/GOx electrode

The construction process of the electrode is schemati-

cally shown in Fig 1 Prior to anodization, the titanium foil

(3mm x 15mm) was degreased by sonication in acetone and then

ethanol The cleaned titanium ribbon was anodized at anodiza-

tion voltages of 15 V in an electrolyte containing 0.2 M citric acid

and 0.1 M NaF at room temperature for 5h in a conventional two-

electrode system with a platinum cathode The titanium sample

was only partially immersed in the electrolyte, with the upper

un-anodized portion used as an electrical contact The efficient

geometrical area of the anodized part (both sides) is 0.6cm? The resulting nanotube arrays were ~90 nm in diameter

CNTs were prepared by chemical vapor deposition (CVD) accord- ing to paper [29] Nickel cations were introduced into the TiO, nanotubes by capillary action and electric field, and electrochemi- cally reduced to form Ni nanoparticles as the catalyst for the growth

of CNTs CNTs were grown from the inner of TiOz nanotubes through catalytic decomposition of acetylene over Ni nanoparticles in a vac- uum tube furnace for 1 h at 700°C

On the CNT-modified TiO2 NTs, Pt nanoparticles were electode-

posited using chronopotentiometry at a current density of 5 mA/s

in a standard three-electrode system with a TiO2/CNT working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode The catalytic activity of TiO2/CNT/Pt electrode was deter- mined by cyclic voltammetry with 1 mM H>0O, as the testing probe

at a scan rate of 100 mVs~! in a solution containing 10 mL pH 7.2 (measured with Mettler-Toledo Delta 320 pH meter) phosphate buffer solution (PBS) and 0.1 M NaCl

The enzyme solution was prepared by dissolving 36 mg of GOx

in 1 mLof0.067 M PBS (pH 7.2) containing 0.1 M NaCl A TiO02/CNT/Pt

electrode was loaded with 20 wL enzyme solution, and dried at 4°C

overnight The resultant electrode was rinsed with PBS to remove free enzymes When not in use, the enzyme biosensor was stored in phosphate buffer (pH 7.2) at 4°C in a refrigerator Amperometry of glucose were performed in 10 mL phosphate buffer solution under stirring at ambient temperature

3 Results and discussion 3.1 Characterization of TiO2/CNT/Pt electrode

To investigate the microstructure of the as synthesized TiO2/CNT/Pt electrode, it was characterized by SEM and TEM Fig 2 shows the topography of the TiOz nanotubes and configuration of TiO2/CNT/Pt electrode Fig 2a shows that uniform TiO nanotubes were formed with a pore size of about 90nm in diameter and a length of about 320nm (shown in the inset of Fig 2a) Fig 2(b) shows that long CNTs with an average diameter of about 50 nm are grown on TiO; NTs The CNTs were grown from the inside of the Ti02 NTs [30] as the Ni catalyst was electrodeposited inside the TiO, NTs

To confirm this speculation the surface CNTs was removed by son-

ication, and a CNTs-embedded porous annealed TiO, substrate is

seen as shown in Fig 2(c), where the TiO2 NTs were collapsed during the growth process of CNTs Since the CNTs were formed at around 650°C [31] before the collapse of TiOz NTs at around 680°C [32],

(

A — ~ -7 ee 5 L2 ee

aN

Fig 1 Schematic showing the construction of the Pt nanoparticle-decorated CNT/TiO2 NT electrode, step (a): electrodeposition of Ni nanoparticles into TiO NTs; step (b): CNTs development from the inner of TiO2 by chemical vapor deposition; and step (c): electrodeposition of Pt nanoparticles on CNTs.

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Fig 2 SEM micrographs showing the morphology of TiO2/CNT/Pt electrode: (a) TiOz NTs prepared at 15 V with 90 nm in diameter and 300 nm in length (the inset), (b) Pt nanopaticles-deposited CNTs grown from the inner of TiO2 NTs; the corresponding TEM image is shown in the inset and the corresponding EDX pattern is shown in (d) (c) The SEM image of TiO2/CNT/Pt electrode after removing the surface CNTs

CNTs were embedded in TiO, substrate which existed in the mix-

ture of rutile and anatase phase based on the former results [19,32]

The CNTs-embedded TiO, substrate facilitates the electron trans-

fer due to its enhanced conductivity, and the disorderly distributed

CNTs provide a large surface area for Pt nanoparticles dispersion

and a conductive network for electrons transfer The TEM imaging

(the inset in Fig 2b) shows that Pt nanoparticles at an average size

~3 nm are uniformly dispersed within the CNT network The corre-

sponding EDX spectrum given in Fig 2(d) shows the presence of Ti,

Pt, C, and O, with aTi:O atomic ratio of 1:2 The uniformly dispersed

small Pt nanoparticles are essential to a high catalytic activity

3.2 Detection of hydrogen peroxide

While the quantification of glucose is based on the electro-

chemical detection of the enzymatically liberated H2O>, the sensor

sensitivity is dependent on the electrochemical response of the sen-

sor to HạOs; electrodes with high catalytic efficiency to H202 would

achieve high sensitivity to glucose The electrochemical response

of the TiO2/CNT/Pt electrode to H2O2 was firstly investigated Fig 3

shows the cyclic voltammograms of the TiO2/CNT/Pt electrode in

the absence (curve d) and in the presence of increasing H202 con-

centration (curves a-c) The response of the TiO02/CNT/Pt electrode

in 0.067 M pH 7.2 PBS displays a distinct background current, which

indicates that the incorporation of CNT with TiO, NTs facilitates

the electron transfer between electrode and H203 A pair of well-

defined redox peaks (Epa, 0.45 V; Epc, 0.0 V) of H2O02 were observed,

and the H202 oxidation peak potential at the TiO2/CNT/Pt electrode

is at 0.45 V, which is 230 mV lower than that at the Pt bulk electrode

(0.67 V) This suggests the catalytic activity of Pt nanopartilces to the oxidation of HzO2 [14] and a faster electron transfer rate at the TiO2/CNT/Pt electrode Hall et al [33] investigated the mechanism of electrochemical oxidation of H2O2 at platinum electrodes, concluding that the oxidation reaction is an adsorption-controlled

-6 4

Potential/V(vs.Ag/AgCl)

Fig 3 Cyclic voltammograms of TiO2/CNT/Pt electrode in 0.067 M pH 7.2 phosphate buffer containing: (a) 15, (b) 5, (c) 2.5, and (d) O mM H203 Scan rate: 100 mV/s.

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0.301

< 0.20 R2=0.997

=

^

0.10

<i 4 -0.4 0.0 05

E -0,

©

-0.8 =

-1.0

Time/ s

Fig.4 Ampermetric responses of TiO2/CNT/Pt electrode to continuously injection of

1 pmol/L Hz 02 (pH 7.2, 0.067 mol/L PBS) Working potential: 400 mV (vs Ag/AgCl);

the inset shows the calibration curve

mechanism, which depends on the potential, temperature, phos-

phate buffer, pH value, and chloride concentration The mechanism

should be applicable to the oxidation of HzO, at the TiO,/CNT/Pt

electrode; the high adsorbability of TiO2 and CNT to H20O, facil-

itate the oxidation reaction of H203 Such electrocatalytic action

facilitates low-potential amperometric measurements of hydrogen

peroxide

The redox currents of H2O2 increase with the increase of H202

concentration The peak potential difference is 0.45 V, indicating

an irreversible redox reaction of H2Q> at the TiO2/CNT/Pt electrode

The catalytic activity toward H202 of the TiO2/CNT/Pt electrode was

assessed by quantitative analysis of the amperometric response to

successive injection of 1M H2O2 As shown in Fig 4, a current

response (Aji) of 0.145 LA/jMM was achieved at applied poten-

tial of 400 mV, with a liner response in the range of 0.001 mM

to 2mM with a slope of 0.134 AM (R2 =0.997) and a limit of

detection (LOD) of 1 uM at a signal-to-noIse ratio of 3 as shown

in the inset The achieved LOD is much lower than what was

obtained at a horseradish peroxidase-modified multi-wall carbon

nanotubes/chitosan sensor (10 4M) [34], and at the mesoporous

Pt microelectrode (4.5 uM) [35], indicating that the TiO2/CNT/Pt

electrode is with a superior electrocatalytic activity toward the

oxidation of HzO

3.3 Detection of glucose

A glucose biosensor was fabricated by modifying GOx onto

the as prepared TiO2/CNT/Pt electrode which detects H202 gen-

erated by enzymatic reactions Fig 5 shows the amperometric

responses of the glucose sensor to the successive additions of 1 mM

glucose at applied potential of 400 mV vs Ag/AgCl reference elec-

trode An evident current response (Aji) of 0.138 LA was achieved

Successive addition of 1 mM glucose (n=7) showed a relative stan-

dard deviation (R.S.D.) of 3.2% verifying a good repeatability of

the sensor As shown in the inset, the nanocomposite biosensor

exhibits a linear response from 0.01 mM to 1.5 mM with a response

slope of 0.146 4AmM~! (R?2=0.998) A sensitivity of as high as

0.24 4A mM"! cm~? was achieved The LOD was 5.7 uM at a signal-

to-noise ratio of 3, and the response time was less than 3s The

proposed sensor represents a simple and fast approach to the detec-

tion of glucose

The inter-sensor reproducibility was investigated at a glucose

concentration of 0.01 mM Three independently made GOx sensors

showed an acceptable reproducibility with a variation coefficient of

0

0.20}

<0.15}

\ 0.05Ƒ

3 -0.4 0.0 0.3 0.6 0.9 1.2 15

2

5

-0.8 F

~—

Time/ s

Fig 5 Ampermetric responses of Ti02/CNT/Pt/GOx sensor to continuously injection of 1 mmol/L glucose (pH 7.2, 0.067 mol/L PBS) Working potential: 400 mV (vs Ag/AgCl); the inset shows the calibration curve

4.8% (n=5) for the current determination at 0.01 mM glucose The sensor response to 0.01 mM glucose was decreased by 18% after one month mostly due to the decrease in the enzyme activity

3.4 Performances of biosensors The functions of the CNT and Pt nanoparticles were investigated by fabricating GOx sensors with (a) bare TiOz NTs, (b) Pt-deposited TiO, NTs (TiO2/Pt), (c) Pt-deposited TiO NTs/CNT composite (TiO/CNT/Pt), and (d) the electrode c but the surface CNT was removed (TiO2/(CNT)/Pt) as substrate, respectively The Pt loading was 0.126 mg/cm? Fig 6 shows the amperometric responses of the four sensors upon subsequent additions of 1mM glucose The TiO02/CNT/Pt/GOx sensor exhibits the highest electrocatalytic activity toward glucose (Fig 6c), while the bare TiO2 NT/GOx sensor exhibits negligible response (Fig 6a), indicating the catalytic activity of Pt nanoparticles The current response (Ai) at TiO2/CNT/Pt/GOx sensor (Fig 6c) is ten times that at the TiO, /Pt/GOx sensor (Fig 6b), indicating that the CNT enhances the catalytic activity of Pt due to the large surface area of the CNT net-

Fig 6 Amperometric responses of sensors: (a) TiOz/GOx, (b) TiO2/Pt/GOx, (c) TiO2/CNT/Pt/GOx, and (d) TiO, (CNT)/Pt/GOx in 10mM PBS (pH 7.2) to continu- ous injection of 1 mM glucose at 400 mV working potential (vs Ag/AgCl) Scan rate:

100 mvs".

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work which increases the Pt loading and facilitates the electron

transfer In addition, CNT network provides superior adsorption

ability toward glucose The interesting result is that even the sur-

face CNT has been removed (TiO2 (CNT)/Pt/GOx sensor), the current

response (Fig Gd) is still three times that at the TiO2/Pt/GOx sen-

sor (Fig 6b), confirming again that the CNTs were grown from the

inner of TiO2 NTs, and the CNTs-embedded TiOz showed sensitive

responses to glucose As for practical applications, we can remove

the surface CNTs to obtain a stable electrode structure There would

be no potential bio-hazards resulted from the lose of CNTs

4 Conclusions

A Pt nanoparticle-decorated electrode was fabricated by using

the TiO2/CNT composite as substrate The electrode exhibits a high

catalytic efficiency to the oxidation of hydrogen peroxide with a

slop of 0.134 jLA/mM The resultant GOx sensor by modifying GOx

on the as-prepared electrode displays a good response to glucose

with a sensitivity of 0.24 wAmM~—! cm~? and a LOD of 5.7 uM The

sensor-to-sensor reproducibility was measured on three indepen-

dently made GOx sensors with a variation coefficient of 4.8% The

high catalytic activity is ascribed to the large surface area and good

conductivity of CNT network, the highly dispersed Pt nanoparticles,

and the excellent biocompatibility of TiO2 to HzO

Acknowledgment

Funding for this work by the National Science Foundation of

China under Grants No 20775024, and the Specialized Research

Fund for the Doctoral Program of Higher Education under grant

20050532024 is gratefully acknowledged Professor Shenglian

Luo gratefully acknowledges partial support of this work by the

National Science Foundation for Distinguished Young Scholars

under Grant No 50725825

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Biographies

Xinyu Pang received BS degree in chemistry from Hunan University, PR China in

2006 Now he is pursuing his MS degree in analytical chemistry at Hunan University

in the research group of professor Qingyun Cai

Dongmei He received BS degree in chemistry from Henyang Normal University, PR China in 2005 and MS degree in analytical chemistry from Hunan University, PR China in 2008 She is currently an assistant researcher at Kangde Solar Energy Int Co., Dongguan, Guandong, PR, China

Qingyun Cai received BA degree in 1983 and MS degree in 1986, both in chemistry from Hunan University, PR China Since then he has been on the faculty at Hunan University He earned the PhD in chemistry in 1996 from Hunan University He is currently a full-time professor in the Department of Chemistry at Hunan University, PR China His primary research interests concern the chemo/biosensors and functional (nano) materials.

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