gold nanoparticle encapsulated-tubular tio 2 nanocluster as a scaffold...
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Article
A gold nanoparticle encapsulated-tubular TiO2 nanoclusteras a scaffold for development of thiolated enzyme biosensors
Xiaoqiang Liu, Jiamei Zhang, Shanhu Liu, Qing-You Zhang, Xiuhua Liu, and Danny K.Y. WongAnal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac303420a • Publication Date (Web): 12 Apr 2013
Downloaded from http://pubs.acs.org on April 16, 2013
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A GOLD NANOPARTICLE ENCAPSULATED-TUBULAR TIO2 NANOCLUSTER
AS A SCAFFOLD FOR DEVELOPMENT OF
THIOLATED ENZYME BIOSENSORS
Xiaoqiang Liu∗, Jiamei Zhang, Shanhu Liu, Qingyou Zhang, Xiuhua Liu, and
Danny K.Y.Wong#••
Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical
Engineering, Henan University, Kaifeng, Henan 475004, P. R. China
# Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, NSW
2109, Australia
* Corresponding author: Tel: +86-378-2825854; Fax: +86-378-2825854; E-mail: [email protected].
••
Corresponding author: Tel: +61-2-9850-8300; Fax: +61-2-9850-8313; E-mail: [email protected].
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Abstract
In this work, a highly sensitive and stable sensing scaffold consisting of gold nanoparticle-
encapsulated TiO2 nanotubes, the hydrophilic ionic liquid, 1-decyl-3-methylimidazolium
bromide, and Nafion was developed for the fabrication of electrochemical enzyme biosensors.
A significant aspect of our work is the application of 12-phosphotungstic acid as both a highly
localised photoactive reducing agent to deposit well-dispersed gold nanoparticles on TiO2
nanotubes and an electron mediator to accelerate the electron transfer between an enzyme and
the electrode. After characterising the nanocomposite component of the scaffold by Fourier
transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy,
thiolated horseradish peroxidase (as a model enzyme) was immobilised on the scaffold and the
biosensor was applied to the detection of H2O2. The direct electron transfer between the
enzyme and the electrode was promoted by the excellent biocompatibility and conductivity of
the scaffold. In addition, a thiolated enzyme has significantly improved the stability and direct
electron transfer of horseradish peroxidase on the biosensor, which could be ascribed to the
strong affinity between the sulfhydryl group on the enzyme and gold nanoparticles on the
biosensor surface. Cyclic voltammetry, chronoamperometry and square wave voltammetry
were used to study the electrochemistry and analytical performance of the biosensor. A
dynamic range from 65 to 1600 µM, a limit of detection of 5 µM and a sensitivity of (18.1±
0.43)×10-3 µA µM-1 H2O2 were obtained. The sensing scaffold based on the nanocomposite was
demonstrated to be effective and promising in developing enzyme biosensors.
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INTRODUCTION
Redox enzyme modified electrodes have attracted increasing attention due to their
potential applications to the development of biosensors, bioelectronics and biofuel cells.1,2
In
these applications, satisfactory electrical communication between the active sites of enzymes
and the electrodes is critical to achieve an optimal performance.3 In this respect, the material
and method used to immobilise the redox enzyme were considered among the most important
factors affecting the direct electron transfer between the enzyme and electrode.4, 5
Owing to
good biocompatibility, large surface area and strong ability to facilitate the electron transfer
kinetics for redox reactions, TiO2 nanomaterials were often applied as a matrix for
immobilising enzymes to the development of electrochemical and photoelectrochemical
biosensors.3, 6
More recently, noble metals, particularly Au nanoparticles, were used to modify
TiO2 nanomaterials for use in constructing enzyme biosensors.7, 8
For example, Zhu et al.
fabricated a photoelectrochemical biosensor based on gold nanoparticle-modified TiO2
nanoneedles, where gold nanoparticles were deposited by incubating the TiO2 nanoneedles in a
gold nanoparticle suspension for 12-15 h.7 The as-prepared biosensor exhibited a 4-fold
increase in sensitivity for detection of H2O2 under visible light illumination, compared to that
without the corresponding illumination. Similarly, a photoelectrochemical sensor based on a
gold nanoparticle-modified nanostructured anatase electrode was constructed by incubating
sintered TiO2 anatase nanoparticle films in a gold nanoparticle dispersion for 20 min.9 This has
resulted in more than 10% increase in photocurrent in 0.05 M NaOH. In contrast, Wang et al.
used a one-step method to reduce gold nano-seeds on TiO2 nanoparticles to construct a
horseradish peroxidase (HRP) biosensor.10
The biosensor not only exhibited a response within
3 s and a low limit of detection of 5.9 µM H2O2, but also retained 90% of its initial response
after a 2-week storage. In their work, the Au nanoparticles provided a favourable
microenvironment for biomolecules,11
and they also acted as an electron transfer bridge for
direct electrical contact between redox proteins and electrode support.12
Unfortunately, there
are several limitations associated with gold nanoparticle deposition methods hitherto reported,
which may affect the performance of gold nanoparticle-TiO2 nanomaterial based enzyme
biosensors. For example, the incubation method often resulted in weak binding between gold
nanoparticles and TiO2, causing the gold nanoparticles to easily detach from TiO2 during the
detection process.7 In addition, the chemical reduction method also yielded gold nanoparticles
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that tended to agglomerate and they were shown to be non-uniformly dispersed in the TiO2
nanomaterial, all of which will negatively affect the repeatability and reproducibility of
biosensors constructed.13
Furthermore, both the incubation method and chemical reduction
method produce a mixture of free gold nanoparticles and gold nanoparticle modified-TiO2,
which is undesirable for practical applications.
Alternatively, noble metal (Au, Pt) nanoparticles can be attached to semiconductor
nanoparticles using polyoxometalates (e.g. PW12
O40
3- , SiW12
O40
4- and W10
O32
4- ) as a linker.14
The
significance of this technique is that the noble metal nanoparticles were found to be well
dispersed and they strongly adhered to the semiconductor nanoparticle surface without any free
noble metal nanoparticle contamination.14,15
Moreover, polyoxometalates were shown to
exhibit photocatalytic capability, thermal stability and electrical communication ability.14
In
particular, polyoxometalates have demonstrated ability to undergo reversible multi-electron
redox processes without decomposition, making them excellent electrochemical catalysts in the
detection of nitrite, H2O2 and iodate.16,17
All of above features will benefit the development of
electrochemical and photoelectrochemical biosensors.
In comparison to TiO2 nanoparticle-based biosensors, TiO2 nanotube-based biosensors
have exhibited dramatically improved performance due to their relatively higher specific
surface area, better biocompatibility and higher conductivity of the special tube-like geometry.8
For example, the special hollow structure of TiO2 nanotubes, compared to the bulkier
nanoparticle spheres, will potentially facilitate a stable immobilisation of small biological
molecules on the inner wall of the nanotubes, which may enhance the performance of
nanocomposite-based biosensors.
In this work, we will initially use the polyoxometalate anion of 12-phosphotungstic acid
(PTA; PW12
O40
3- ) as a linker to attach gold nanoparticles to TiO2 nanotubes to construct a
scaffold for the development of an electrochemical enzyme biosensor using HRP as a model
enzyme. In this scaffold, photochemically reduced PTA acts not only as a localised reducing
agent to deposit gold nanoparticles on TiO2 nanotube surface, but also as an electron mediator
to improve the enzyme catalytic reaction. To further increase the stability of the biosensor and
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to enhance direct electron transfer at the biosensor interface, it is necessary to strengthen the
contact between HRP and gold nanoparticle-PTA modified TiO2 nanotubes. One strategy to
achieve this is by introducing a sulfhydryl group to HRP using a dithio crosslinker so that it can
readily bind to the gold nanoparticles. In this way, the strong interaction between thiolated
HRP and gold nanoparticles is expected to enhance the stability of HRP in this scaffold.
Previously, the amine groups on Protein G and antibodies were successfully modified in a
similar manner to facilitate an orientation-controlled immobilisation on gold electrodes for the
construction of electrochemical immunosensors with improved performance.18,19
Finally, the
ionomer, Nafion, and the hydrophilic ionic liquid, 1-decyl-3-methylimidazolium bromide
([Demim]Br), were applied to bind the thiolated HRP | gold nanoparticle-PTA-TiO2 nanotube
scaffold to the electrode surface. Specifically, [Demim]Br was used because it offers excellent
conductivity, biocompatibility, wide electrochemical windows and high chemical stability.20,21
The electrocatalytic response to the reduction of H2O2 by HRP at the thiolated HRP | gold
nanoparticle-PTA-TiO2 nanotube | [Demim]Br-Nafion biosensor (hereafter referred to as a
thiolated HRP biosensor) was then studied by cyclic voltammetry, chonoamperometry and
square wave voltammetry.
EXPERIMENTAL SECTION
Chemicals and Materials. TiO2 nanotubes were prepared using a procedure reported
prviously.22
Briefly, hydroxyl functionalised TiO2 nanotubes were synthesised by reacting
polycrystalline TiO2 with NaOH solution at 110°C for 20 h in a in a high pressure Teflon
reactor. HRP, N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP), dithiothreitol (DTT),
propan-2-ol, bicinchoninic acid (BCA) QuantiPro Protein Assay Kit, HAuCl4·3H2O and
dimethylsulfoxide (DMSO) were purchased from Sigma–Aldrich, USA. PTA was obtained
from J&K Scientific Ltd., Beijing, China. [Demim]Br was purchased from Shanghai Chengjie
Chemical Co., Ltd., China. Zeba™ spin desalting columns were purchased from Thermo
Fisher Scientific Inc. All solutions were prepared in Milli-Q ultrapure water.
Instrumentation. Electrochemical measurements were performed using a CHI630
electrochemical workstation (CH Instruments, Shanghai, China) with a conventional three-
electrode system consisting of a 3-mm diameter glassy carbon working electrode, a platinum
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wire counter electrode and a Ag|AgCl (3.0 KCl) reference electrode. All electrodes were
purchased from Gaoshiruilian Co. Ltd., Wuhan, China. During electrochemical measurements,
the working solutions were deoxygenated with N2 gas for 15 min and then a nitrogen
atmosphere was kept over the solutions until the measurements were completed. Philips
tubular ultraviolet low-pressure 16-W mercury lamps were purchased from Philips Lighting
B.V., The Netherlands. Morphological characterisation of the nanocomposites was
investigated by Fourier transform infrared spectroscopy (FT-IR; Nicolet 170, USA), X-ray
diffraction (XRD; X-PertPro, Netherland) with Cu Kα radiation (λ = 1.5406 nm) and field
emission transmission electron microscopy (FETEM; Tecnai G2 20, FEI Co. Ltd., USA). The
bioactivity of thiolated HRP was determined using uv–vis spectrophotometry (UV-1750
spectrophotometer, SHIMADZU, Japan) and FT-IR.
Preparation of Nanocomposites. Firstly, TiO2 nanotubes (20 mg) was dispersed in 20 mL
PTA aqueous solution (10 mM) and left overnight under mechanical stirring. The products
were then centrifuged and washed with deionised water to facilitate the removal of
uncoordinated PTA molecules. Next, 4 mL of PTA–modified TiO2 nanotubes (4 mg mL–1
)
solution was mixed with 1 mL of propan-2-ol in a quartz cell, purging with N2 gas for 15 min.
The mixed solution was photoexcited under four UV lamps (16 W, λex = at 253 nm) for 4 h to
reduce PTA on the TiO2 nanotubes before 5 mL of 1 mM HAuCl4 solution was added. After
the colour of the mixture solution had changed from dark blue to light purple, the mixture was
allowed to mature for 2 h. Finally, the suspension was centrifuged to obtain the gold
nanoparticle-PTA-TiO2 nanocomposite.
Thiolation of HRP. ZebaTM
desalt spin columns used in the enzyme thiolation process contain
a high-performance resin which offers exceptional purification, desalting and buffer-exchange
for protein samples, and they therefore do not require any chromatographic system and
cumbersome preparation or equilibration procedures. The spin-column method also eliminates
delay caused by samples emerging at the end of a column. Compared with our previous
enzyme thiolation method,19,23
the whole thiolation time has been reduced from approximately
1 day to half day and the enzyme recovery has been increased from 44% to approximately 65%
using the spin column method. The specific procedure was described as follows. Firstly, 1 mg
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SPDP was dissolved in 160 µL of DMSO, and then 12.5 µL of the 20 mM SPDP solution and 1
mg HRP was dissolved in 500 µL of phosphate buffer saline containing 1 mM EDTA (PBS-
EDTA), and this was allowed to react for 30–60 min at room temperature. Two desalt spin
columns were centrifuged at 2500 rpm for 5 min to remove storage solution, and acetate buffer
was exchanged 2-3 times to equilibrate two desalting columns, then the SPDP-modified HRP
was centrifuged to remove reaction by-products and excess non-reacted SPDP. Next, 200 µL
DTT solution was added to 500 µL SPDP-modified HRP solution, followed by a 30-min
incubation to produce thiolated HRP. Finally, PBS-EDTA solution was employed to
equilibrate desalting columns and thiolated HRP was desalted using the equilibrated columns to
remove the excess DTT. The recovery of thiolated HRP was determined by estimating the
quantity of residual protein content in the final solution using a BCA-based QuantiPro Protein
Assay Kit. This detection procedure was similar to that reported previously,18,19
except that
thiolated HRP was used in place of Protein G.
Preparation of a thiolated HRP Biosensor. Initially, a glassy carbon electrode was
successively polished on a polishing cloth with 0.3 µm and 0.05 µm alumina powder and rinsed
with deionised water and ethanol. The polished electrode was then allowed to dry at room
temperature. The mixture of thiolated HRP, gold nanoparticle-PTA-TiO2, [Demim]Br and
Nafion was kept agitated at 2°C for 4 h in a full temperature oscillation incubator before being
applied to the electrode surface. Specifically, 300 µL thiolated HRP aqueous solution (1.5 mg
µL-1
) and 50 µL Nafion were added in [Demim]Br and gold nanoparticle-PTA-TiO2 composite
(1:2, w/w). Finally, 4 µL of the suspension was applied to the glassy carbon electrode and
dried at 4°C for 2 h to obtain an HRP biosensor.
RESULTS AND DISCUSSION
Briefly, an electrochemical HRP biosensor was developed in this work by initially
immobilising on a glassy carbon electrode a scaffold consisting of gold nanoparticle-modified
TiO2 nanotubes using PTA as a linker. Thiolated HRP was then allowed to interact with the
gold nanoparticles before the scaffold was anchored on an electrode using Nafion and
[Demim]Br. One novel aspect of this work lies in the application of PTA to synthesise the gold
nanoparticle-TiO2 composite such that the gold nanoparticles are uniformly dispersed,
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minimising the aggregation of thiolated HRP that was subsequently immobilised on the
composite. PTA was also exploited as an electron mediator in the direct electron transfer of
HRP at the electrode. In the following sections, we applied a range of techniques to
characterise the individual components of the biosensor scaffold, before the thiolated HRP
biosensor was used to analytically detect H2O2.
FT-IR, XRD and FETEM Characterisation of Nanocomposites.
We have initially used FT-IR to examine whether PTA was successfully attached to TiO2.
Figure 1A shows the FT-IR spectra of TiO2 alone, PTA alone and PTA-TiO2. As expected, the
FT-IR spectrum of TiO2 nanotubes alone (trace a) did not exhibit any significant features above
800 cm-1
.14
In the FT-IR spectrum of PTA alone (trace b), there are four characteristics bands
at 1079, 983, 893, and 800 cm-1
attributable to the four bonds linking the tungsten atoms by
oxygen atoms with a phosphorus atom at the centre of a tetrahedron (P-Oa, W-Od, W-Ob-
W and W-Oc-W) in PTA.24,25
Then, the FT-IR spectrum of PTA-TiO2 (trace c) shows a
minor shift of the four PTA characteristic bands relative to those of PTA alone in trace b. We
attributed this to the effect on the vibrational modes of the oxygen atoms in PTA after being
attached to TiO2 nanotubes. Accordingly, we shall use this to infer successful adsorption of
PTA on TiO2 nanotubes by mechanical stirring in our work.
Next, XRD was used to characterise the crystal structure of TiO2 nanotubes, PTA-TiO2
nanotubes, and gold nanoparticle-PTA-TiO2 nanocomposite. The results are shown in Figure
1B. As reported previously, the diffraction peak at 2θ angle of 9° in trace a is a notable feature
of the TiO2 nanotubes successfully synthesised using a hydrothermal method.26
Moreover, the
peaks at 25.0°, 38.1°and 48.5° correspond to the spacing of (101), (004) and (200) of the
anatase (tetragonal) phase of the TiO2 nanotubes.6 However, the binding of PTA to TiO2
nanotubes did not result in any significant change in the XRD pattern (trace b) relative to that
of TiO2 nanotubes alone. This might be due to the presence of only a very thin PTA coating on
the TiO2 nanotube surface.14
Similar to a previous report,27
the peaks at 38.3°, 44.6°, 64.8°
and 77.8° in trace c can be assigned to (110), (200), (220), (311) reflection of gold
nanoparticles, which clearly indicate that gold nanoparticles were successfully deposited on
TiO2 nanotubes.
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Finally, the morphology of TiO2 nanotubes and gold nanoparticle-PTA-TiO2
nanocomposite synthesised in our laboratory was characterised using FETEM and the results
are shown in Figure 2A and 2B, respectively. In Figure 2A, the TiO2 nanotubes show a
relatively uniform geometrical dimension, with a mean diameter of approximately 5 nm and
length of a few hundred nm. The FETEM image of a gold nanoparticle-PTA-TiO2
nanocomposite shown in Figure 2B depicts relatively well dispersed gold nanoparticles
(denoted by dark spheres) in the TiO2 nanotube matrix with no observable free gold
nanoparticles. This also demonstrated the successful immobilisation of gold nanoparticles on
TiO2 nanotubes using PTA as a linker. The average particle size of gold nanoparticles on TiO2
nanotubes is approximately 15 nm and this is in good agreement with 15.4 nm evaluated using
the Scherrer equation28,29
based on the XRD data obtained above,
d =0.9×λ
kα
B(2θ)
cos θmax
where λkα is the wavelength of X-rays used (0.154056 nm), B(2θ) is the width of the peak at half
height (with the peak height evaluated from the baseline), and θmax is the Bragg angle at the
peak. In Figure 2B, the bird-nest like TiO2 nanotube matrix will offer a biocompatible
microenvironment for encapsulating the enzymes and sustaining their bioactivity. The high
specific area of TiO2 nanotube will also accommodate a substantial quantity of gold
nanoparticles and thereby promoting the binding of high load of thiolated HRP.
UV–Vis and FT-IR spectroscopic analysis of sulfhydryl modified HRP. Uv–vis
spectrometry was previously employed to investigate the native structure of HRP.30,31
The
Soret band shift in a uv–vis spectrum will aid in determining whether denaturation in heme
protein has occurred after modifying the microenvironment around the heme group.32
Figure
3A shows the respective uv-vis spectra of HRP, sulfhydryl-modified HRP and sulfhydryl-
modified HRP in the gold nanoparticle-PTA-TiO2 nanotube | [Demim]Br scaffold. In these
spectra, the Soret band absorption for HRP (trace a) and sulfhydryl-modified HRP (trace b) is
located at nearly the same wavelength (~403 nm), suggesting that the heme structure of HRP
was not significantly altered after the thiolation process. There was also no obvious shift in the
Soret band after HRP was mixed with the gold nanoparticle-PTA-TiO2 nanotube composite and
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[Demim]Br (trace c), indicating that the composite materials provided biocompatibility that
allowed HRP to remain in the native heme structure and to maintain its bioactivity.
FT-IR spectroscopy was also conducted to provide additional information for the secondary
structure of HRP. Previously, the amide I band at 1600–1700 cm−1
was attributed to the C=O
stretching vibration of the peptide bond in the backbone of HRP, while the amide II band
between 1500 and 1600 cm−1
was assigned to the combination of N–H bending and C–N
stretching vibration.33,34
Denaturation or any changes in the structure of an enzyme molecule is
expected to lead to diminished intensity of these bands or even their disappearance.35
As
supported by the results in Figure 3B, there was no significant difference in the amide I and
amide II bands between the FT-IR spectrum of HRP before thiolation (trace a) and that of
thiolated HRP (trace b). Furthermore, even after being incorporated in the nanocomposite film,
the amide I and amide II bands of HRP molecules exhibited minor changes in their position and
intensity (trace c). All of above results demonstrated that the native secondary structure of
HRP was retained after being thiolated and entrapped in the composite material. As the
bioactivity of HRP is strongly related to its heme group and secondary structure, its bioactivity
has not been significantly affected based on non-detectable changes in the FT-IR features.
Electrochemical characterisation of thiolated HRP biosensors. Figure 4A shows the cyclic
voltammograms at different modified electrodes in N2-saturated PBS at 50 mV s−1
. Initially,
the cyclic voltammogram (trace a) obtained at the PTA-TiO2 nanotube | [Demim]Br | Nafion-
coated electrode did not show any obvious redox peaks. Notably, PTA-modified TiO2
nanotubes were obtained in this work by incubating the two components together, followed by
copious rinsing, that only a relatively small quantity of PTA would be present on the TiO2
nanotubes. Therefore, PTA was not expected to yield any observable signals in the cyclic
voltammogram. After immobilising HRP on the gold nanoparticle-PTA-TiO2 nanocomposite,
a pair of redox peaks at 0.126 V and 0.010 V (peak separation of ~116 mV) arising from direct
electron transfer of HRP in the matrix was observed in the cyclic voltammogram (trace b).
Here, both the gold nanoparticles and PTA acted as electron transfer bridges to accelerate the
electron transfer between the enzyme and the electrode, while gold nanoparticles and TiO2
nanotubes provided a biocompatible microenvironment for HRP. Next, a thiolated HRP,
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instead of HRP, was immobilised on the nanocomposite and the cyclic voltammogram obtained
is shown in trace c. The reduction-to-oxidation peak current ratio was decreased from 2.98
(trace b) to 1.40 (trace c) and a corresponding decrease in the peak separation from 116 mV to
72 mV was also observed, indicating an improved reversibility of the direct electron transfer of
HRP at the thiolated HRP biosensor. In this work, the thiolation of HRP was achieved by
modifying the enzyme with a dithio crosslinker, SPDP. The sulfhydryl groups would facilitate
strong binding of HRP to the gold nanoparticle surface. After being stably anchored on the
electrode, direct electron transfer of HRP takes place more readily, giving rise to a reduction-
to-oxidation peak current ratio that is closer to unity and a smaller peak separation that are
indicative of a more electrochemically reversible reaction.
Next, the thiolated HRP biosensor was placed in 0.05 M PBS (pH 7.0) containing 0.1,
0.2 and 0.3 mM H2O2, respectively, before square wave voltammetry was performed and the
results obtained are shown in Figure 4B. The thiolated HRP exhibited a strong catalytic effect
towards H2O2, which can be used as additional evidence that the bioactivity and catalytic ability
of HRP was sustained after being thiolated and immobilised on the nanocomposite.
Accordingly, the proportional increase of catalytic current as a function of H2O2 concentration
demonstrated that the thiolated HRP biosensor could be applied to the determination of H2O2.
Current-Time Relationship for the H2O2 Measurement. The chronoamperometric responses
of (a) a HRP | TiO2 nanotube | [Demim]Br | Nafion biosensor, (b) a HRP | gold nanoparticle-
PTA-TiO2 nanotube | [Demim]Br | Nafion biosensor and (c) a thiolated HRP | gold
nanoparticle-PTA-TiO2 nanotube | [Demim]Br | Nafion biosensor to the successive additions of
H2O2 into continuously stirred 0.1 M PBS solution (pH 7.0) at -0.4 V are shown in Figure 5A.
As observed in trace (a), the current increased sluggishly after injection of H2O2, indicating the
slow response of the biosensor in the absence of PTA and gold nanoparticles. On the contrary,
the chronoamperometric responses of both biosensor (b) and (c) were enhanced dramatically
once H2O2 was injected, demonstrating that both PTA and gold nanoparticles aided in
significantly reducing the response time of the biosensors. Based on the slopes of the
calibration plots, the corresponding sensitivity for the three designs of sensors was estimated to
be (a) (1.72 ± 0.11)×10-3
µA µM-1
, (b) (10.7 ± 0.13)×10-3
µA µM-1
and (c) (18.1 ± 0.43)×10-3
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µA µM-1
respectively. Notably, biosensor (b) has displayed a larger than 500% enhancement in
sensitivity over biosensor (a), which can be explained by the presence of gold nanoparticles and
PTA in the scaffold of biosensor (b). However, based on our previous results,13
gold
nanoparticles alone were unlikely to yield such a dramatic increase in sensitivity of HRP
biosensors. This led us to consider PTA as another contributing factor affecting the
chronoamperometric response of the HRP biosensors. Previously, polyoxometalates including
PTA were reported to have directly catalysed the electrochemical reduction of H2O2 in strongly
acidic solutions.16,17
However, direct electrochemical catalysis by PTA was unlikely in our
work as H2O2 reduction occurred in a neutral phosphate buffer throughout our experiments.
Instead, we speculate that PTA played the role of an electron mediator, for example,
hydroquinone, in the redox reaction of HRP.8 In such a mechanism, Fe(III) within the heme
group of HRP was initially reduced by the reduced form of PTA yielding Fe(II) and the
oxidised form of PTA at -0.4 V.
HRP(Fe(III)) + PTA(Red) →HRP(Fe(II)) + PTA(Ox)
HRP(Fe(II)) was then oxidised back to HRP(Fe(III)) by H2O2. Finally, PTA(Ox) was reduced
back to PT(Red), which further enhanced the reduction current.
HRP(Fe(II)) + H2O2 → HRP(Fe(III)) + H2O
PTA(Ox) → PTA(Red)
The analytical performance of the thiolated HRP biosensor and several other HRP
biosensor designs are listed Table 1. Compared to the other HRP biosensors, the thiolated
HRP biosensor exhibited a superior sensitivity and a comparable limit of detection. Both
characteristics of the thiolated HRP biosensor may be ascribed to the following features. In the
gold nanoparticle-PTA-TiO2 nanotube | [Demim]Br | Nafion scaffold, the porous structure of
TiO2 nanotube-cluster provides a biocompatible matrix, exposes the gold nanoparticles to the
adsorbed enzyme and aids in electrolyte accessibility. Gold nanoparticles and [Demim]Br are
also biocompatible materials that would have facilitated electron transfer between enzyme and
the underlying electrodes.19,31
In addition, the uniform distribution of gold nanoparticles on
TiO2 nanotubes and high specific surface area of both gold nanoparticles and TiO2 nanotubes
have increased the load of enzyme molecules and decreased their aggregation in the scaffold.
Furthermore, the strong binding between sulfhydryl of HRP and gold nanoparticles has
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minimised the leakage of HRP and improved the electron transfer between enzyme and
electrode, which is also beneficial to the sensitivity of HRP biosensors. Finally, PTA acted as
an electron mediator to enhance the sensitivity and to significantly reduce the response time of
the thiolated HRP biosensor.
Stability study of the biosensors. Cyclic voltammetry of the thiolated HRP biosensor in 0.05
M PBS was conducted to evaluate its stability. For comparison, cyclic voltammetry of a non-
thiolated HRP biosensor was also conducted. More specifically, 20 repeated cyclic
voltammetric scans were conducted at the respective electrode at 50 mV s-1
and the results are
shown in Figure 5B. The HRP redox currents at the thiolated HRP biosensor exhibited a 12%
decrease after 20 continuous scans (trace a), while that at the non-thiolated HRP biosensor
decreased by ~26% (trace b), indicating a higher stability of the former biosensor than the
latter. Here, the strong interaction between sulfhydryl of HRP and gold nanoparticles has
minimised the leakage of the enzyme molecules from the biosensor. In addition, the uniformly
deposited, rather than agglomerated, gold nanoparticles would have aided in maintaining the
dispersion and therefore the bioactivity of HRP molecules. In summary, all these advantages of
the gold nanoparticle-PTA-TiO2 nanotube | [Demim]Br | Nafion scaffold contributed to the
improved stability and prolonged life span of the thiolated HRP biosensor.
CONCLUSIONS
We presented in this work a scaffold consisting of gold nanoparticle-PTA-TiO2 nanotube,
[Demim]Br and Nafion for the development of an electrochemical enzyme biosensor. In this
scaffold, PTA was employed as both a UV-switchable reducing agent to immobilise gold
nanoparticles on the TiO2 nanotube surface and as an electron mediator to accelerate enzyme
catalytic reaction. The FT-IR, XRD and FETEM results demonstrated that gold nanoparticles
were well dispersed in the TiO2 nanotube matrix and no observable free metal nanoparticles
were produced. In our work, the model enzyme, HRP, was modified with sulfhydryl using a
spin-column method to evaluate the performance of the scaffold. Uv-vis spectrometry, FT-IR
and voltammetry provided evidence for sustained bioactivity of thiolated HRP immobilised on
the scaffold. Finally, the chroamperometric and cyclic voltammetric results demonstrated that
high sensitivity and stability were achievable using the described scaffold on an
electrochemical enzyme biosensor.
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ACKNOWLEDGMENT
This work was financially supported by the China Postdoctoral Science Foundation funded
project (No. 2012M511569) and National Natural Science Foundation of China (No.
20875022, No.21105021).
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FIGURE CAPTIONS
Figure 1 (A) FT-IR spectra of (a) TiO2 nanotubes alone, (b) PTA alone and (c) PTA-TiO2
nanocomposite; (B) XRD patterns of (a) TiO2 nanotubes, (b) PTA-TiO2 nanotubes,
and (c) gold nanoparticle-PTA-TiO2 nanocomposite.
Figure 2 FETEM image of (A) TiO2 nanotubes; (B) gold nanoparticle-encapsulated TiO2
nanotubes.
Figure 3 (A) Uv–vis absorption spectra and (B) FT-IR spectra of (a) HRP, (b) sulfhydryl
modified HRP and (c) sulfhydryl modified HRP in gold nanoparticle-PTA-TiO2
nanotube | [Demim]Br composite.
Figure 4 (A) Cyclic voltammograms at (a) a PTA-TiO2 nanotube | [Demim]Br | Nafion-
coated electrode, (b) a HRP-gold nanoparticle-PTA-TiO2 nanotube | [Demim]Br |
Nafion-coated electrode, (c) a thiolated HRP-gold nanoparticle-PTA-TiO2 nanotube
| [Demim]Br | Nafion-coated electrode in N2-saturated PBS at 50 mV s−1
; (B)
square wave voltammograms of a thiolated HRP-gold nanoparticle-PTA-TiO2
nanotube | [Demim]Br | Nafion-coated electrode in 0.05 M PBS (pH 7.0) in the
presence of 0.1, 0.2 and 0.3 mM H2O2 (from trace a to c) at 50 mV s
-1.
Figure 5 (A) Chronoamperometric response of (a) a HRP | TiO2 nanotube | [Demim]Br |
Nafion electrode, (b) HRP-gold nanoparticle-PTA-TiO2 nanotube | [Demim]Br |
Nafion-coated electrode, and (c) a thiolated HRP-gold nanoparticle-PTA-TiO2
nanotube | [Demim]Br | Nafion-coated electrode to the successive additions of
H2O2 into continuously stirred 0.1 M N2-saturated PBS solution (pH 7.0) at -0.4 V.
The inset shows an enlarged portion of trace (c). (B) Twenty successive cyclic
voltammetric scans of (a) a thiolated HRP biosensor and (b) a non-thiolated HRP
biosensor in 0.05 M PBS (pH 7.0) at a scan rate of 50 mV s-1
.
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Table 1 Analytical characteristics of several HRP biosensors in H2O2 detection. 1
2
HRP biosensors Sensitivity × 10
3 /
µA µM-1
Dynamic range /
µM
Limit of detection /
µM References
Thiolated HRP | gold nanoparticle -PTA-TiO2
nantube | [Demim]Br biosensor 18.1 65 – 1600 5 This work
HRP | gold nanoparticle -PTA-TiO2 nantube |
[Demim]Br biosensor
10.7 90 – 1400 6.5 This work
Nafion | HRP–gold nanoseed–TiO2 | glassy carbon
electrode biosensor
0.23 41 – 630 5.9 10
HRP | halloysite nanotube | chitosan | glassy carbon
electrode biosensor 12.3 2 – 75 0.7
30
HRP | flowerlike ZnO nanoparticle | gold
nanoparticle | Nafion | glassy carbon electrode
biosensor
6.36 15 – 1100 9 36
3
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Figure 1 4
5
6 7
8
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Figure 2 9
10
11
12
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Figure 3 13
14
15 16
17
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Figure 4 18
19
20 21
22
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Figure 5 23
24
25
26
27
28
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For TOC only 29
30
SPDP/DTT SH HRP HRP ZebaTM columns
PTA, N2/15 min,
UV 4 h,
HAuCl4, Mature
4 h
Tubular TiO2
nanocluster
Gold nanoparticle-
Tubular TiO2
nanocluster
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