Review article
Catalytic Effect of Nanostructurated Carbon Forms
for Biosensor Applications
Briza Pérez López1,2, Maria Guix1,2, Salvador Alegret2, Arben Merkoçi1,2*
1 Nanobioelectronics & Biosensors Group, Institut Català de Nanotecnologia & ICREA
2 Grup de Sensors & Biosensors, Departament de Química
Universitat Autònoma de Barcelona
08193 Bellaterra, Catalonia, Spain.
Corresponding author: E-mail: [email protected]
Abstract
The purpose of this review is to summarize recent studies on the electrocatalytic behaviour of carbon
nanomaterials (CNM) such as carbon nanotubes (CNT) and carbon nanofibers (CNF) and at the
same time to show the development of new designs of electrochemical sensors making use of several
electrode surfaces modifications procedures with interest for future biosensing applications. Several
alternatives for CNMs integration into electrochemical biosensing systems have been developed.
These are based on modifications of electrode surfaces with CNTs, CNF or in the use of CNT based
epoxy composites. For comparison purposes the CNM modified electrodes have been compared with
electrodes modified/prepared with carbon microparticles (CMP).
Keywords: Carbon nanotubes, carbon nanofibers, carbon microparticles, composite, modified
electrodes, biosensors.
delocalized ð electrons on the basal planes, and
this property imparts a weak basic character to
the material in which the polarity can be
manipulated by selected chemical treatments
[12].
CNFs represent carbon fibers with a
nanometer-size diameter and no hollow core,
but with many edge sites on the outer wall [13].
It has been recognized as one of the very
promising materials based on its nanostructure
and particular properties [14] and is expected to
be used in various applications such as
catalysts or catalyst supports [15], probe tips
[16], and fuel cells [17].
1. Introduction
The advancement of nanoscience and
nanotechnology is making possible the creation
of new materials based on carbon structures
with
unique
properties
and
potential
various
technological
applications
of
nanostructures [1,2].
The rapid development of new
nanomaterials and nanotechnologies has
provided
many
new
opportunities
for
electroanalysis. In particular the unique
properties of carbon materials (CMs), such as
graphite/carbon microparticles (CMPs), carbon
nanotubes (CNTs), and carbon nanofibers
(CNFs) have shown significant improvements in
the development of novel sensors and
biosensors [3- 7]. High-surface area of CMs
may also lead to new signal transduction
processes [8] and to increased sensitivity in
sensing applications [9- 11].
CMPs represent a highly ordered form
of carbon. It is a solid that possesses
CNTs can be divided into single-wall
carbon nanotubes (SWCNT) and multi-wall
carbon nanotubes (MWCNT). SWCNT possess
a cylindrical nanostructure formed by rolling up a
single graphite sheet into a tube. SWCNT can
thus be viewed as molecular wires with every
atom on the surface. MWCNT comprise of an
array of such nanotubes that are concentrically
nested like rings of a tree trunk [18].
10
oxidase (GOx) within the CNT epoxy-composite
matrix prepared by dispersion of multi-wall CNT
inside the epoxy resin. The use of CNT, as the
conductive part of the composite, ensures better
incorporation of enzyme into the epoxy matrix
and faster electron transfer rates between the
enzyme and the transducer [5]. In the same
manner, but to measure catechol, tyrosinase
(Tyr) integrating CNT and immobilizing the
enzyme in the same way as for the glucose
biosensor before mentioned has been used in
order to perform another composite biosensor
[28].
Although there is a large amount of size
overlap between CNT and CNF, there are
significant structural differences. Distinctive
properties of CNTs such as high surface area,
ability to accumulate analyte, minimization of
surface fouling and electrocatalytic activity are
very attractive for electrochemical sensing
[7,19]. The multi-wall CNTs (MWCNTs)
comprise several to tens of incommensurate
concentric cylinders of these graphitic shells
which have a layer spacing of 0.3–0.4 nm [7].
CNF consist of graphitic layers that stack either
at an angle, perpendicular or parallel to the tube
axis [20- 23]. They present nanometer-size
diameter and no hollow core, but many edge
sites on the outer wall [24 ].
Considering each of the applications of
carbon materials (CNT, CNF CMP) above
mentioned we can say that they have
advantages as electrode modifiers in terms of
the new electrocatalytic properties giving to the
sensors at the same time a good
electrochemical
response
and
having
reasonable cost.
The combination of biological molecules
and novel nanomaterial components is of great
importance in the process of developing new
nanoscale devices for future biological, medical,
and electronic applications [25]. The electrical
contacting of redox enzymes with electrodes is a
subject of extensive research over the last
decade, with important implications for
developing biosensing enzyme electrodes,
biofuel cells, and bioelectronic systems [26,18].
In this context carbon nanomaterials (CNM)
such as CNT and CNF are offering a great
promise for developing electrochemical sensors.
In the present review we will discuss the
progress achieved in the field of CNMs
integration into electrochemical biosensing
systems based on modifications of electrode
surfaces with CNTs [19] and CNF [27] or in the
use of CNT based epoxy composites [5,28]. We
have studied CNMs catalytic effects, for the first
case, integrating CNTs onto a glassy carbon
electrode with interest for NADH detection
based on the use of poly(vinylchloride) (PVC) as
a novel matrix for CNTs dispersion as well as to
ensure better mechanical/robustness properties
of the sensor membrane compared to the use of
a CNT layer only, additionally glutaraldehyde as
a matrix linker [19]; also chemically treated and
untreated carbon materials (CNF, CNT and
CMP) have been used as modifiers of glassy
carbon and gold electrode surfaces, to compare
their efficiencies for electrochemical oxidation of
NADH with great future promises for biosensors
applications [27]. In addition the results obtained
by the use of a carbon nanotube matrix linked
with glutaraldehyde for the electrochemical
detection of phenols using screen printed
electrode modified with CNTs will be shown.
2. Carbon materials: general
considerations
Carbon based materials have attracted
extensive attention from both the scientific and
industrial communities since the discovery of
C60, carbon nanofibers (CNFs) and carbon
nanotubes (CNTs) [29].
In recent periods the modification of the
electrodes [19,27] and fabrication of enzymebased electrodes [5,28] using nanomaterials
have been active subjects. To have good sensor
performances, the fabrication of an efficient
electrode with proper selection of a substrate
matrix for dispersing the sensing material is
crucial. It is very important to use the substrates
that would favour effective loading of enzymes
to have high sensitivity towards the
biomolecules. The host material (substrate)
having high-surface area, optimum porosity,
high-thermal stability, chemical inertness and
minimum or negligible swelling in aqueous and
non-aqueous solutions would be an ideal choice
to obtain high performance for the biosensors.
Among these nanomaterials, carbon
nanostructures have been shown to be ideally
suited for biosensor applications since they are
conductive, biocompatible, easily functionalized
while they have very large surface areas. The
present review will discuss the progress
achieved in the field of nanostructurated carbon
forms such as carbon nanotubes [5,19,28] and
carbon
nanofibers
[27]
for
biosensor
applications.
For the CNT epoxy composite case, a
glucose biosensor based on a rigid and
renewable
carbon
nanotube
(CNT)
biocomposite [5] will be shown. The biosensor
was based on the immobilization of glucose
11
carbon (GC) and gold (Au) electrode promote
better the electron transfer between the βnicotinamide adenine dinucleotide (NADH) and
the electrode surface in comparison to the
CMPs film [27].
2.1. Carbon nanotubes
The history of CNTs becomes with the
discovery in 1985 of buckminsterfullerene that
opened a new era for the chemistry of carbon
and for novel materials. The Japanese Sumi
Ijima discovered the CNTs in 1991 [30]. From
their discovering the CNTs have generated
great interest for various applications based on
their field emission and electronic transport
properties, their high mechanical strength and
their chemical properties [7]. From this arises an
increasing potential for use as field emission
devices [31], nanoscale transistors [32], tips for
scanning microscopy [33] or components for
composite materials [34].
2.3. Graphite/carbon microparticles
Graphite/carbon microparticles (CMPs)
structure consists of carbon layers (in which the
carbon atoms are arranged in an open
honeycomb network) situated ones over others
and the electrical conductivity between them is
very poor [43]. The impurity species may
occupy some interstitial position between the
graphene layer planes, which are bonded by a
weak van der Waals force. These weak
interplanar bonds at graphite allow dopant
atoms or molecules to be intercalated between
the carbon layers to form intercalation
compounds [43].
CNTs can be thought as all sp2 carbons
arranged in graphene sheets, which have been
rolled up to form a seamless hollow tube. The
tubes can be capped at the ends by a fullerene
type hemisphere and can have lengths ranging
from tens of nanometers to several microns
[35]. They can be subdivided into SWCNT and
MWCNT as it is written previously. The
conductivity of CNTs, as consequence of their
electronic properties, is highly important for the
role that plays these nanomaterials in
electrochemistry. MWCNTs are regarded as
metallic conductors, a highly attractive property
for an electrode. It is not so clear for SWCNTs,
which can be metallic, semi-conductors or small
band gap semi-conductors depending on their
diameter and chirality [35- 37].
3. Applications of carbon
nanomaterials in electrochemical
biosensing systems
The rapid development of new nanomaterials
and nanotechnologies has provided many new
opportunities for electroanalysis. In particular,
the unique properties of CNTs make them
extremely attractive for the fabrication of
electrochemical (bio)sensors. Recent studies
have demonstrated that CNTs can enhance the
electrochemical reactivity of biomolecules and
promote the electron–transfer reactions of
proteins [5,28]. The high conductivity of CNTs
permits their use as highly sensitive nanoscale
sensors.
2.2. Carbon nanofibres
CNFs are cylindrical nanostructures
composed of graphite layers stacked around the
fiber’s axis in different variations giving distinct
well-organized structures. The diameter of
nanofibers varies from 5 to 200 nm while their
length is in the scale of micrometers [38].
Due to the unique characteristics of
CNF there is a wide range of applications. The
thermal and chemical stability, high surface
area, low-ohmic resistance, and the surface
properties are the unique qualities that are
exploited when CNF are used as catalytic
support materials [25-28,39,40]. The surface at
the tip of a CNF possesses a large amount of
exposed edges due to the arrangement of the
graphitic layers [41].
The
solubility
of
CNF
in
dimethylformamide
(DMF)
facilitates
its
manipulation, including the coating on electrode
surfaces
for
electrochemical
biosensing
applications [27], because the insolubility of
carbon materials in most solvents is a major
obstacle in implementing their widespread use
[42]. The presence of a CNFs film on a glassy
Different configurations of CNTs based
(bio)sensors have been reported. CNTs have
been (a) aligned onto a conductor platform, (b)
randomly distributed onto a conducting platform
(c) mixed with a polymer forming CNTs
composites or (d) single CNTs have been used
as sensors.
3.1. Carbon nanotubes based composites
As electrode materials, CNTs can
facilitate
electron-transfer
between
the
electroactive species and electrode, and provide
a new avenue for fabricating chemical sensors
or biosensors [44,45].
The effective immobilization of enzymes
(i.e. glucose oxidase and tyrosinase) is one of
the key features for successful application of
amperometric biosensors. The developed
carbon nanotube epoxy composites have been
used to study electrocatalytic oxidation
12
processes with interest for compounds such as
glucose or catechol.
3.1.1. Carbon nanotubes based enzymatic
It has high sensitivity, good stability [5] along
with advantages such as simple in making; it is
cheap in application, and easy in operating.
biosensors
3.1.1.2. Catechol biosensor
Integration of biomaterials such as
proteins, enzymes, antigens, antibodies or DNA
with the CNTs essentially provides new hybrid
systems that combine the conductive or semiconductive properties of the CNTs with the
recognition or catalytic properties of the
biomaterials. Superior to other kinds of carbonbased materials, the internal cavities or external
sides of CNTs walls provide the platform for the
accommodation of the biomolecules.
Tyrosinase (Tyr) has been frequently
used for the detection of phenolic compounds.
The development of a biosensor based on this
enzyme integrated into a CNT epoxy composite
electrode (CNTEC) in order to perform
measurements of catechol have been
developed in a similar way as for the case of
glucose biosensor [28] (see figure 2).
3.1.1.1. Glucose biosensor
The effective immobilization of glucose
oxidase (GOx) is one of the key features for
successful
application
of
amperometric
biosensors. Hence, it is pertinent to explore and
develop a simple and reliable method to
integrate CNT with enzymes. A glucose
biosensor based on a rigid and renewable CNT
based on biocomposite have been developed
[5]. The biosensor was based on the
immobilization of GOx within the CNT epoxycomposite matrix prepared by dispersion of
multi-wall CNT inside the epoxy resin (see
schematic presentation in Figure 1). The use of
CNT, as the conductive part of the composite,
ensures better incorporation of enzyme into the
epoxy matrix and faster electron transfer rates
between the enzyme and the transducer, being
the same case with the tyrosinase enzyme.
Figure 2. Schematic representation of the
reaction of catechol with tyrosinase.
3.1.2. Electrochemical comparison with
graphite biocomposites
The developed CNT biocomposite
based biosensors for glucose and catechol are
compared with graphite based epoxybiocomposite (GECE) ones showing a better
response in terms of working potential decrease
and higher current besides the better stability
(see figure 3). The same comparison for
catechol is also performed (see reference 28).
Figure 1. Schematic of the glucose biosensors based on
GEC (A) and CNTEC (B).Shown, the left side, are also the
SEM images of the dried mixtures of glucose oxidase with
graphite (A) and CNT (B) before mixing with the epoxy
polymer to form GEC and CNTEC pastes. A better
entrapment of GOx inside the sponge like structure of the
CNT bundles compared to graphite particle is clearly
visualized in the SEM images. Electrodes composition:
Carbon, 18% (w/w); GOx, 2% (w/w); epoxy, 80% (w/w).
Figure 3. Current-time recordings obtained from
amperometric experiments at GOx-GEC (a) and at GOxCNTEC (b) for successive additions of 0.5 mM glucose.
Working potential: (A) +0.90 V and (B) +0.55 V in 0.1 M
phosphate buffer pH 7.0. The insets show the corresponding
13
calibration plots.Electrodes composition: as described in the
Figure 1.
(see figure 4) are further supported by the
corresponding ATR-IR spectra of GC modified
electrodes [19] and the differences in the
electrochemical activity between the CNTsmodified electrode and the unmodified electrode
(GC treated in the same way but without the
polymeric matrix) are shown in the figure 5.
3.2. Carbon materials as modifiers of the
electrodes surfaces.
The derivatisation of carbon surfaces
allows electrochemists to tailor-make electrodes,
which offer distinct advantages for catalysis,
analysis and biological applications, and this
area of electrochemistry has attracted
considerable attention over the last few years
[46- 49].
3.2.1. Electrochemical response of NADH
The study of the oxidation of βnicotinamide adenine dinucleotide (NADH), a
cofactor
in
NAD+/NADH-dependent
dehydrogenases, has been the subject of
numerous studies related to the development of
amperometric biosensors.
The most important problems to anodic
detection are the large overvoltage encountred
for NADH oxidation at ordinary electrodes and
surface fouling associated with the accumulation
of reaction products [50]. For the above reasons
significant efforts have been dedicated for
identifying new electrode materials that will
reduce the overpotential for NADH oxidation and
minimize surface passivation effects [19,27]. Is
the case of the integration of a CNTs PVC
based matrix modified with glutarldehyde
suitable for biosensor applications [19].
Figure 4. Images of the MWCNTs in THF (A), THFţ PVC
(B), and in THFţPVCţGA solution (C). Lower part: SEM
images of the same solutions as in the upper images.
Figure 5. Current – time recordings including (as inset) the
corresponding calibration plots obtained from amperometric
experiments at unmodified (a) and at modified (b) GC
electrode for successive additions of 1 mM NADH in 0.1 M
phosphate buffer pH 7. Working potential: +0.7 V. The
stability of the response (as zoom of the left recordings) is
shown for each electrode used.
3.2.1.1. Carbon nanotube PVC based matrix
A new alternative for the integration of
CNTs onto a glassy carbon (GC) electrode with
interest for NADH detection have been
developed. It is based on the use of PVC as a
matrix for CNTs dispersion aimed to ensure
better mechanical / robustness properties of the
sensor membrane compared to an unmodified
GC electrode, additionally with glutaraldehyde
as a matrix linker with great future promises for
biosensors applications due to its ability for
covalent binding of biological molecules.
3.2.1.2. Carbon nanofibers DMF based layer
The electrochemical response for NADH
oxidation showing the response efficiencies of
the GC and Au electrodes modified with CNFs
or CMPs and using dimethylformamide (DMF)
as dispersing agent is also reported [27]. The
responses were significantly different due to the
peculiar physical (see figure 6) and chemical
characteristics of each doping material.
CNTs were purified prior to use by a 2
M nitric acid solution. The GC electrode was
modified
with
a
mixture
of
MWCNTs+tetrahydrofuran (THF)+PVC prepared
via dispersing 1.4 mg MWCNTs and 4 mg PVC
into a 750 μL of THF by ultrasonication agitation
for about 10 min. The GC electrode was coated
with 4 μL of this mixture and finally incubated
during 30 seconds in a GA solution, obtaining a
MWCNTs-TPG film onto the GC electrode
surface. The results obtained by SEM images
14
Figure 6. Schematic representation (not in scale) of the
possible interactions of the NADH molecules with: (A)
graphene layers for the CMPs and (B) graphene layers for
CNFs.
CNFs has been recognized as one of
the very promising materials based on its
nanostructure and particular properties and is
expected to be used in various applications such
as catalysts or catalyst supports showing
significant improvements in the development of
novel sensors and biosensors.
Figure 7. SEM images for CMP and CNF films on GC and
Au electrode surfaces; without treatment (a) and with
treatment (b) in HNO3. The same acceleration voltage (5 kV)
and resolution are used.
3.2.2. Comparison between CNFs and CMPs
Table 1 summarizes all the values (potential and
currents) for each electrode and the
corresponding modifications. The shown
differences can be attributed to the different
structural forms of the carbon materials used.
Carbon nanofibers (see figure 6) provide a very
high-surface area and, consequently, a very
high number of binding sites for NADH. This
high density of binding sites may increase
sensitivity in the same manner as in porous
materials [51]. Generally, electron transfer may
be more facile at samples containing a higher
proportion of binding sites, producing a faster
electron transfer.
The dispersion of the CNF–DMF or
CMP–DMF onto the electrode surfaces (GC and
Au) is an important issue in producing carbon
films onto the electrode surfaces. Scanning
electron microscopy was used to analyze the
carbon films on GC and Au electrode surfaces.
Figure 7 compares SEM images of treated CNF
and CMP and untreated CNF and CMP films
deposited onto the surfaces of GC and Au
electrodes. It can be appreciated that the CMPs
are more scattered over the gold electrode
surface than in the GC electrode. However,
untreated
CNF
create
a
better
uniform/homogeneous film and even more
coverage over the surface of GC and Au
electrodes, while treated CNF creates more
scattered and even thinner layer onto the GC
and Au surfaces [27], which seem to be
available to act as an electrochemical sensing
surface for NADH detection.
3.2.3. Electrochemical detection of phenol
Carbon nanotubes matrix linked with
glutaraldehyde for the electrochemical detection
of phenols using screen printed electrode
modified with CNTs actually is being developed
at our laboratories.
CNTs works as electron mediators and
at the same time provides a very good matrix for
enzyme immobilization.
15
Table 1. Summary of responses toward a solution of NADH 2.5mM at 0.1M phosphate buffer pH 7 obtained by two different
electrode transducers (GC and Au) modified with carbon nanofibers (CNFs) + dimethylformamide (DMF) and graphite +DMF
Ip: the peak of the NADH oxidation current; Eo: NADH oxidation potential; ΔE: shift of the oxidation potentials between bare
and modified electrodes.
The adsorption technique used to
modify the screen printed electrode with CNTs
results the most interesting approach in relation
to the preparation of tyrosinase-based
biosensors for the analysis of phenolic
compounds. This work is still in process at our
laboratories and the results will be published in
the future.
nanomaterials on different substrates toward
different analytes are still unclear and are under
investigation. A possible explanation of such an
effect beside the carboxylic groups and the
possible metal impurities is the different contact
resistances of carbon materials with the
electrode surfaces.
Acknowledgements
The WARMER Project Reference: FP6034472-2005-IST-5 and MAT2008-03079/NAN
(From MEC, Madrid) are acknowledged.
4. Conclusions
Carbon nanomaterials represent a novel
alternative for biosensor development. Due to
unique characteristics, such as high surface
area, ability to accumulate analyte, minimization
of surface fouling and electrocatalytic activity,
carbon nanomaterials are used as catalytic
support materials and they had shown very
attractive
properties
for
electrochemical
biosensing systems.
The principal reasons for different
“electrocatalytic”
properties
of
carbon
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