Anal Bioanal Chem (2008) 390:293
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298
DOI 10.1007/s00216-007-1686-0
TRENDS
Carbon nanotubes (CNTs)-based electroanalysis
M. Teresa Fernández-Abedul & Agustín Costa-García
Published online: 11 November 2007
#
Springer-Verlag 2007
Introduction
In the context of electrochemical detection methods, suitable for integration in miniaturised devices, the search for favourable working electrodes is a field of constant research.
When a new material is discovered, carbon nanotubes (CNTs) in this case, research is stimulated aimed at looking for promising methodologies and applications.
Carbon has traditionally been used as an alternative solid electrode material to metal. Structures based on different forms of sp-hybridized carbon and planar graphite sheets have been used.
Although in 1978 Wiles and Abrahamson [
] observed a thick mat of fine fibres of graphite anodes with diameters ranging from 4 to 100 nm composed of graphitic layers with a hollow core, the discovery of carbon nanotubes was attributed to Iijima [
] who prepared carbon structures consisting of needle-like tubes, each needle comprising coaxial tubes of graphitic sheets. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis, resulting in high aspect ratio (length to diameter) materials. The ensuing revolution in the electrochemical field is mainly a result of the CNTs
’ electrical conductivity but mechanical, chemical and thermal properties are also outstanding. Another important material is the C
60 buckminsterfullerene [
3 ], but its low conductivity meant that it
did not attract as much attention.
Dedicated in memory of Prof. Lorenzo Pueyo Casaus
M. T. Fernández-Abedul
:
Departamento de Química Física y Analítica,
Universidad de Oviedo,
Oviedo 33006, Spain e-mail: costa@fq.uniovi.es
A. Costa-García (
*
)
Carbon nanotubes are considered to be composed of only one material; however, CNTs can be present in different forms and therefore their properties and applications can be diverse. Apart from variations in diameter or length, two main classes of CNTs can be distinguished: multiple-wall carbon nanotubes (MWCNTs) comprising several concentric tubes and single-wall carbon nanotubes (SWCNTs) in which only one graphite sheet is rolled up (Fig.
a). On the other hand, electronic properties depend on the structure of
SWCNTs, mainly diameter and chirality. SWCNTs can be classified as metallic (armchair) or semiconducting (zigzag or chiral) [
] (Fig.
b). In the case of MWCNTs,
“ hollowtube ” , “ herringbone ” or “ bamboo ” morphological variations can be found (Fig.
1 c). Moreover, closed- or open-ended
CNTs can be found. Chemically functionalised carbon nanotube with groups such as – COOH, – OH, – SH or – NH
2 are available and other functionalisations are also possible.
Thus, even when CNTs have a very simple chemical composition and atomic bond configuration, they can exhibit extreme diversity in structure and in turn in properties and behaviour. A homogeneous reagent is not available yet and fabrication methodologies have to be directed towards this goal.
The intriguing properties of carbon nanotubes have led to an explosion of research efforts worldwide. Therefore, the number of relevant publications that can be found in the literature is enormous, even when only analytical applications are considered. Among them, carbon nanotubes have been most widely used as components in electrochemical sensors, although other important applications can be found
[ 5 , 6 ]. As this is not a review article, only general con-
siderations will be made here. Other articles can be examined
for a more detailed coverage [ 4 , 5
–
], including specific
ones on CNT-based electroanalysis [ 8 , 9 ] as well as special journal issues [ 10
].
294
Fig. 1 a Single- (SWCNT) and multiple- (MWCNT) wall configurations, b SWCNT categories, and c schematic cross section through different
MWCNTs showing the orientation of the graphene sheets
]-Reproduce by permission of The Royal
Society of Chemistry
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Solubilisation
Most applications of CNTs rely on the modification of working electrodes. Abrasive methodology in which CNTs are attached by gently rubbing a polished electrode on a paper containing nanotubes or a CNT-ionic liquid gel can be employed but is not common. Inclusion in the paste of a carbon paste electrode or in the ink of a screen-printed electrode is also a possibility for the fabrication of electrochemical sensors. However, most of the applications involve the use of a CNT suspension that is dropped onto the electrode or this is dipped into the CNTs solution for further washing or drying in both cases. Disaggregation into individual tubes is then required. Different solvents can be employed but it has to be taken into account that not all solvents are valid for all type of nanotubes. Depending on the provider or the functionalisation their solubilisation is totally different and therefore more studies are necessary.
The mass/volume ratio has to be considered as well as the procedure, commonly sonication and centrifugation. Oxidative acid treatment or covalent functionalisation produces groups (at the end or on the walls) that commonly enhance solubility.
Aqueous solutions with surfactants are adequate media for dispersing CNTs because the hydrophobic tails interact with the hydrophobic nanotubes while the hydrophilic part improves water solubility. However one must bear in mind that surfactants can have an extra effect due to their cationic/anionic or hydrophobic nature.
Non-covalent weak interactions can be used to attach small or big molecules including polymer chains that can help solubilisation. In fact, wrapping with polymers has provided a supramolecular approach to solubilise CNTs and also to prepare composite materials. A special case is the use of the polyanionic polymer Nafion that possesses a polar side chain and produces CNT solubilisation. However, one must take into account that this process forms a membrane that can act as a diffusion barrier on the surface of the electrode as well as acting as an ion exchanger.
Similarly, dispersion can be obtained with chitosan, which in this case is a polycation polymer. In this context and as a biopolymer, DNA can be also employed for solubilisation purposes. It has been reported that single stranded DNA
(ssDNA) interacts strongly with CNTs to form a stable
DNA
–
CNT hybrid that effectively disperses CNTs in
π -stacking interactions with the sidewall of carbon nanotubes and the positioning of the hydrophilic sugar
– phosphate backbone to the exterior are responsible for this.
In all cases, apart from variables such as the stability of the solutions, the influence of the solubilisation and surface
] on the electrochemical be-
Anal Bioanal Chem (2008) 390:293
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298 haviour has to be studied. In this way, comparison of various dispersing strategies is very useful because the electrochemical response can vary to a high degree from two different CNT-dispersed solutions. On the other hand, it is extremely important that the effect of the dispersing reagent by itself, without containing nanotubes, is thoroughly studied, because the influence of e.g. surfactants or polymers on the electrochemical behaviour of molecules is well know. These studies are not commonly found in the literature and sometimes definite behaviour can be erroneously attributed to nanotubes.
Improvements of the electrochemical behaviour
As commented above, carbon is a traditional and common material for the construction of solid electrodes and is employed in different forms. In the case of nanotubes, their special geometry and unique electronic, mechanical, chemical and thermal properties make them tremendously attractive for the development of electrochemical devices.
Since the first modification of a paste electrode in 1996
], it has been proven that they have an outstanding ability to mediate fast electron-transfer kinetics for a wide range of electroactive species. Reducing overpotentials is important because the use of high potentials is not necessary and possible interferences are thereby avoided. Moreover, such systems allow the resolution of the overlapping response of several analytes. The enhancement in the reversibility of the reactions is also documented as well as the resistance to surface fouling. These advantages imply that better analytical characteristics are obtained.
Elegant research aimed at explaining the electrocatalytic behaviour of CNTs has been performed and this behaviour is attributed to the presence of edge
– plane-like sites [
fact the importance of oxygenated species at the ends of carbon nanotubes has been reported for their favourable
electrochemical properties [ 15
]. Improvement in the electrochemical behaviour is also the goal of the co-immobilisation of nanoparticles and nanotubes. Platinum or gold nanoparticles can be employed and they can be generated previously or electrochemically deposited on a CNT-modified electrode. Electrical contact of the electrode through the CNT enables the whole structure to be used as an electrode. As commented above, the separate influence of several modifiers has to be known in order to avoid errors in attributing observed benefits.
Most of the studies have been performed on conventional electrodes, commonly glassy carbon. However, miniaturisation and simplicity are two necessary requirements of the analytical tool designed for future analysis. Screen-printed electrodes (SPEs) (Fig.
) posses both characteristics and have been demonstrated to be perfectly compatible with CNTs.
Nanotubular electrodes
Miniaturisation has for some time been an important goal in electroanalytical research. In the case of nanotubular electrodes, their small dimensions and conductivity means that they can be considered as the smallest possible electrodes. Among all the properties their small size, which can be an inconvenience for other techniques, could be an advantage when dealing with electrochemical techniques.
The nanoelectrode has a large effective surface, increased mass transport rate and decreased susceptibility to the solution resistance. The smallest nanoneedle-type biosensor was fabricated by attaching an MWCNT to a tungsten tip
].
Multiple CNTs can also be constructed on a conducting layer in certain architectures to fabricate nanoelectrode arrays (NEAs) or ensembles (NEEs). They can produce a much higher current than a single nanoelectrode and improve the signal to noise ratio. The interspacing of the individual electrodes should be much larger than the radius of each electrode: otherwise it will behave similar to a macroelectrode as happens with some CNT-forest electrodes.
Usually the CNT array is grown on nickel catalyst film in such a way that site density of vertically aligned nanotubes is
]. In some cases, bottom-up and top-down miniaturisation approaches are joined together because e.g.
lithographically patterned silicon pillars (top-down) with transferred catalyst materials onto their tops form the basis for CNT growth (bottom-up). On the other hand, if a dielectric is encapsulated on a forest-like vertically aligned array, leaving only the very end of the CNTs exposed, an inlaid NEA with diminished background noise is formed.
Electrochemical biosensors
295
It is usually the case that the transduction efficiency determines many of the analytical characteristics of the biosensors. Many different conductive materials, compatible with the recognition element can be used as support.
The synthesis of nanomaterials, CNTs among them, has provided a new field for development of novel transduction matrices. A recent review classifies the analytical use of
CNTs based on their sorption, electronic or other properties
[ 6 ] in such a way that electrochemical applications were
included in the second approach. However, electroanalytical methodologies take advantage of both electronic and sorption properties (actually adsorptive stripping voltammetry on CNT-modified electrodes takes advantage of
sorption phenomena [ 5 ]). These are very important in the
development of biosensors where apart from covalent, noncovalent interactions can facilitate the adsorption of analytes.
It has to be considered that nanotubes posses a large active
296
Fig. 2 a Schematic diagram of a screen-printed carbon electrode.
b SEM images of a bare
SPCE and a MWCNT
–
COOHmodified SPCE [
]
Anal Bioanal Chem (2008) 390:293
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298 a
Counter
Electrode (C)
Working
Electrode (C)
Pseudoreference electrode (Ag)
3.3 cm
1.0 cm b
Screen-Printed Carbon
Electrode
5
µ m
1
µ m
MWCNT-COOH SPCE
5
µ m
1
µ m surface that when properly functionalised, favour adsorptive processes. On the other hand, the performance of a biosensing electrode is strongly dependent on the type of raw nanotube material used, the transduction electrode (miniaturised and simple if possible) and the modification/ immobilisation procedure.
Excellent reviews on electrochemical biosensors are available [
5 , 7 – 9 ]. Enzymes (oxidase, dehydrogenase, peroxidase
Fig. 3 Schematic representation of the analytical protocol:
A capture of the ALP-loaded CNT tags by the streptavidin-modified magnetic beads by a sandwich DNA hybridization ( a
) or Ab
–
Ag
–
Ab interaction
( b
);
B enzymatic reaction;
C electrochemical detection of the product of the enzymatic reaction at the CNT-modified glassy carbon electrode.
MB magnetic beads,
P
1
DNA probe 1,
T
DNA target,
P
2
DNA probe 2,
Ab
1 first antibody,
Ag antigen,
Ab
2 secondary antibody,
S substrate and
P product of the enzymatic reaction,
GC glassy carbon electrode,
CNT carbon nanotube layer. Reprinted with permission from IACS 2004 126:3010
–
3011. Copyright 2004 American Chemical Society
Anal Bioanal Chem (2008) 390:293
–
298 and catalase classes) are the most common biorecognition molecules employed, and direct electron transfer between
CNT and enzymes has been demonstrated. Usually, the redox centre of an enzyme is electrically insulated by a protein shell and therefore the whole enzyme cannot suffer redox processes at any potential. CNTs allow molecular wiring and electron transport, thereby acting as electrical wires or conductive nanoneedles [
detection through CNT-modified electrodes is also possible since non-covalent or covalent attachment between antibodies/DNA and CNTs is feasible.
An elegant amplification strategy for immuno and DNA sensors involves application of CNTs with two different uses (Fig.
): a CNT-modified electrode for immobilising the primary antibody/DNA probe and a CNT for immobilising both the secondary antibody/DNA probe and multiple enzyme labels [
297
Biosensing is a crucial field of analytical chemistry which has undoubtedly been propelled forward by using CNTs. The adsorptive properties of CNTs convert the modified surface of a working electrode not only into an adequate surface for stripping measurements but also for immobilising biorecognition elements. It is the time for joining bio- and nanotechnology and developing first-class methodologies based on molecular wiring, self-assembly, DNA wrapping... Application to nanomedicine is very promising owing to the formation of supramolecular complexes with proteins, polysaccharides and nucleic acids. Even studies on the metabolism, toxicity and mechanism of elimination of watersoluble carbon nanotubes in order to evaluate their impact on health and validate the concept of CNT as new delivery systems have been initiated [
The material of the electrode employed as the support for the biosensing event is also of importance as well as the design that must be simple and user friendly. In this context, screen-printed electrodes seem a very adequate support for successful development of analytical tools. Moreover, sensitive detection is necessary not only in chemical or biochemical sensing but also in other techniques such as separation methods.
In summary, CNTs represent an impressive and promising material whose numerous applications are only just starting to be envisioned by researchers. Much more effort and research are necessary to fully take advantage of the extraordinary properties of CNTs.
Outlook
In terms of the different types of CNTs that can be produced and the way they can be used, the possible applications are enormous and have not been exploited enough. For example and considering only functionalisation alternatives, it is surprising that the nothing has been reported on NH
2
–
CNTs.
The effect of the curvature on nanotubes by introducing pentagons, the storage of guest molecules on the hollow core or the chemical doping is still unexplored.
Referring to the solubilisation, much work will have to be done when it has been proven that different CNTs need different disaggregation procedures. Work has to be performed in parallel with advances in fabrication that will produce more homogeneous reagents. Moreover, procedures need to be
“ validated
” for electrochemical purposes. Similarly, several designs commonly employed in electrochemical sensing, such as self-assembly that employs Au
–
S bonding, has not been entirely exploited and reports on thiol-functionalised nanotubes on gold electrode surfaces is scarce.
Concerning the electrochemical behaviour, more basic studies with comparison of the electrochemical reactivity of modified electrodes with different CNTs are needed. Combination with other nanostructures, deposition of mixed layers as well as the studies on metal
– nanotube interactions
[ 4 ] has to be explored more.
As soon as technology advances a
“ smaller
” electrode is always possible. Alternative methods of generating arrays or NE ensembles and ways of applying them are also at their beginning. The fabrication of patterned carbon nanotubes on predefined catalyst layers, which benefits from both bottom-up and top-down approaches, has to be exploited as well as their use in a forest-like configuration or embedding them in a dielectric media.
Acknowledgement This work has been supported by Project No.
BIO2006-15336-C04-01 from the Spanish Ministry of Science and
Technology.
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