New materials for electrochemical sensing VI: Carbon nanotubes

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Abstract

Carbon nanotubes (CNTs) combine in a unique way high electrical conductivity, high chemical stability and extremely high mechanical strength. These special properties of both single-wall (SW) and multi-wall (MW) CNTs have attracted the interest of many researchers in the field of electrochemical sensors. This article demonstrates the latest advances and future trends in producing, modifying, characterizing and integrating CNTs into electrochemical sensing systems.

CNTs can be either used as single probes after formation in situ or even individually attached onto a proper transducing surface after synthesis. Both SWCNTs and MWCNTs can be used to modify several electrode surfaces in either vertically oriented “nanotube forests” or even a non-oriented way. They can be also used in sensors after mixing them with a polymer matrix to form CNT composites.

We discuss novel applications of CNTs in electrochemical sensors, including enzyme-based biosensors, DNA sensors and immunosensors, and propose future challenges and applications.

Introduction

The trend of using novel materials in electrochemical sensing systems is constant, with their success largely due to the continuous design and development that meets the needs of modern electrochemical (bio)sensor technology. Materials ranging from carbon composites (Parts I and II of this series) [1], [2], beads or microspheres (Part III) [3], molecular imprinted polymers (MIP) (Part IV) [4] or quantum dots (Part V) [5] are playing an important role in these sensing systems.

Since their discovery in 1991 [6], carbon nanotubes (CNTs) have generated great interest in future applications based on their field emission and electronic transport properties [7], their high mechanical strength [8] and their chemical properties. There is increasing potential for CNTs to be used as field emission devices [9], nanoscale transistors [10], tips for scanning microscopy [11] or components for composite materials [12].

CNTs are one of the most commonly used building blocks of nanotechnology. With 100 times the tensile strength of steel, thermal conductivity better than all but the purest diamond, and electrical conductivity similar to copper, but with the ability to carry much higher currents, they are very interesting.

CNTs include both single-walled and multi-walled structures (Fig. 1). Single-wall CNTs (SWCNTs) (Fig. 1A) comprise of a cylindrical graphite sheet of nanoscale diameter capped by hemispherical ends. The closure of the cylinder is result of pentagon inclusion in the hexagonal carbon network of the nanotube walls during the growth process. SWCNTs have diameters typically ∼1 nm with the smallest diameter reported to date of 0.4 nm. This corresponds to the theoretically predicted lower limit for stable SWCNT formation based on consideration of the stress energy built into the cylindrical structure of the SWCNT.

The multi-wall CNTs (MWCNTs) (Fig. 1B) comprise several to tens of incommensurate concentric cylinders of these graphitic shells with a layer spacing of 0.3–0.4 nm. MWCNTs tend to have diameters in the range 2–100 nm. The MWCNT can be considered as a mesoscale graphite system, whereas the SWCNT is truly a single large molecule.

CNT metal–insulator–semiconductor capacitors were examined theoretically by Guo et al. [13]. CNTs can have metallic or semiconducting properties. Because of the small diameter of CNTs, quantum-mechanical effects determine their electronic structure. This means that the quantization conditions along the nanotube perimeter determine whether a nanotube acts as a metal or a semiconductor. These properties are being considered as the basis of future nanoelectronics, which represent one of the most important applications of CNTs.

The aim of this part of the “New materials for electrochemical sensing” series is to cover only particular aspects related to integrating CNTs into electrochemical sensing systems. We will describe the different methods of preparing CNTs, the possible ways of modifying and solubilizing CNTs, with the major part devoted to describing how CNTs are integrated into sensors and biosensors. We will also set out novel ideas on using and integrating CNTs in electrochemical sensors.

Section snippets

Arc-discharge method

There are several ways of preparing CNTs, the arc-discharge method [14], [15] being the first. This method creates MWCNTs through arc-vaporization of two graphite rods placed end to end, separated by approximately 1 mm, in an enclosure usually filled with inert gas (helium, argon) at low pressure (between 50 and 700 mbar). After applying a dc arc voltage between two separated graphite rods by modifying apparatus for producing SiC powder, the evaporated anode generates fullerenes in the form of

Boron and nitrogen doping

The electronic, chemical and mechanical properties of CNTs can be tailored by replacing some of the carbon (C) atoms with either boron (B) or nitrogen (N). If B (with one electron less than C) or N (one electron more than C) replaces some C atoms, p or n type conductors can be formed, respectively. From the chemical point of view, these doped structures would be more likely to react with donor or acceptor molecules, depending on the doping.

B- or N-doped CNTs can be obtained by the arc method by

Solubilization

Before application to (bio)analytical assays, CNTs (prepared as described in Section 2 above) must be first modified to be transformed to a soluble product. The preparation of homogeneous dispersions of CNTs, suitable for processing into thin films or for other applications, is of a great importance. Various methods can be used for this purpose (detailed information in Table 1 and typical examples in Fig. 2).

End [35] and/or sidewall [36] functionalization, use of surfactants with sonication [37]

Integration of carbon nanotubes

CNTs can be integrated into a variety of configurations to perform electrochemical detections. The current formats (Fig. 3) can be classified in groups:

  • individual CNT configurations;

  • conventional electrodes that are modified with CNTs, in both oriented or non-oriented configurations; and,

  • CNTs integrated into a polymer matrix, creating a CNT composite.

Enzymes

One of the key issues in biosensor design is the establishment of a fast electron-transfer between the active site of the enzyme and the electrochemical transducer. This is a significant challenge in designing enzyme-based sensors, taking into consideration the additional restrictions applied when miniaturization of the device is attempted.

The majority of reported articles (see Section 5) have demonstrated that CNTs promote electron-transfer reactions at low overpotentials (see Table 2). This

Conclusions and future prospects

We have described several possible applications of CNTs, with emphasis on material science-based applications of interest to sensor design. We have remarked on electrochemical applications of CNTs.

The main message that we would like to convey is that the unique structure, topology and dimensions, along with electrochemical properties of CNTs, have created a new material, which can be considered suitable for a variety of interesting possibilities in the design of sensors. The remarkable physical

Acknowledgments

This work was financially supported by:

  • (1)

    the Ministry of Education and Culture (MEC), Spain (Projects BIO2004-02776 and MAT2004-05164, and Grant MEC 2003-022, given to M. Pumera);

  • (2)

    the Spanish foundation Ramón Areces (Project ‘Bionanosensores’); and,

  • (3)

    the “Ramón y Cajal” program of MEC, Spain, that supports A. Merkoçi.

Arben Merkoçi was awarded his PhD in chemistry from the University of Tirana, Albania, in 1991 and then did post-doctoral researches in Greece, Hungary, Italy, Spain and USA. His main interest has been electroanalytical methods for several applications in sensors and biosensors. Currently, he is “Ramon y Cajal” researcher and professor at the Sensor & Biosensor Group, Chemistry Department, Autonomous University of Barcelona (AUB), Spain. His main research interests concern the design of

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    Arben Merkoçi was awarded his PhD in chemistry from the University of Tirana, Albania, in 1991 and then did post-doctoral researches in Greece, Hungary, Italy, Spain and USA. His main interest has been electroanalytical methods for several applications in sensors and biosensors. Currently, he is “Ramon y Cajal” researcher and professor at the Sensor & Biosensor Group, Chemistry Department, Autonomous University of Barcelona (AUB), Spain. His main research interests concern the design of composites, biocomposites and nanobioconjugate materials for enzyme-, immuno- and DNA-based electrochemical sensors.

    Martin Pumera received his PhD in Analytical Chemistry from Charles University, Prague, The Czech Republic, in 2001. Shortly after that he became post-doctoral researcher at Prof. Joseph Wang’s SensoChip Laboratory at NMSU, USA, where he developed new concepts of electrochemical detection on Lab-on-a-Chip devices for space (JPL/NASA) and security/forensic applications (US Navy). Currently, he conducts research focused on integration of nanobiotechnology (CNTs, quantum dots, DNA-gold nanoparticle conjugates) on the Lab-on-a-Chip platform at Sensor and Biosensor Group at the AUB.

    Xavier Llopis obtained chemistry and food technology diplomas at the AUB, in 1998 and 2000, respectively. Since 2000, he has been a PhD student at Sensor & Biosensor Group in the Chemistry Department of the AUB. His research activity is focused on developing novel flow-through injection and Lab-on-a-Chip biosystems with electrochemical detection based on magnetic particles, biocomposites and lastly on CNTs.

    Briza Pérez got her diploma in chemistry in Mexico in 2000 and her master’s degree in chemistry at Polytechnical University of Catalonia, Spain, in 2003. In 2004, she joined the Sensor & Biosensor Group at the AUB to carry out her PhD thesis. Her research is focused on the study of electrochemistry of CNTs and their integration into biosensing systems.

    Manuel del Valle got his PhD thesis at AUB in 1992. He was made Associate Professor at the AUB in 1997. His research lines include chemical sensors (including those for non-ionic or anionic surfactants), biosensors, ISEs, ISFETs, computer-based instrumentation, sensor arrays, electronic tongues, automated analytical systems (flow-injection analysis, sequential injection analysis), conducting polymers, nanostructurated materials, and impedance spectroscopy.

    Salvador Alegret was made professor of Analytical Chemistry at the AUB in 1991. He is head of the Sensor & Biosensor Group in the Chemistry Department. Currently, he is devoted to the development of electrochemical chemo- and biosensors based on amperometric, potentiometric and ISFET transducers in chemical, enzymatic, immunological and DNA-recognition systems. The resulting sensor devices are being applied in automated analytical systems based on bio- or biomimetic instrumentation concepts for monitoring and process control in different fields, such as biomedicine, the environment and the chemical industry.

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