Review Carbon-Nanotube Based Electrochemical Biosensors: A Review Joseph Wang* Department of Chemistry and Biochemistry, New Mexico State University Las Cruces, NM 88003, USA *e-mail: joewang@nmsu.edu Received: April 6, 2004 Final version: May 11, 2004 Abstract This review addresses recent advances in carbon-nanotubes (CNT) based electrochemical biosensors. The unique chemical and physical properties of CNT have paved the way to new and improved sensing devices, in general, and electrochemical biosensors, in particular. CNT-based electrochemical transducers offer substantial improvements in the performance of amperometric enzyme electrodes, immunosensors and nucleic-acid sensing devices. The greatly enhanced electrochemical reactivity of hydrogen peroxide and NADH at CNT-modified electrodes makes these nanomaterials extremely attractive for numerous oxidase- and dehydrogenase-based amperometric biosensors. Aligned CNT “forests” can act as molecular wires to allow efficient electron transfer between the underlying electrode and the redox centers of enzymes. Bioaffinity devices utilizing enzyme tags can greatly benefit from the enhanced response of the biocatalytic-reaction product at the CNT transducer and from CNT amplification platforms carrying multiple tags. Common designs of CNT-based biosensors are discussed, along with practical examples of such devices. The successful realization of CNT-based biosensors requires proper control of their chemical and physical properties, as well as their functionalization and surface immobilization. Keywords: Carbon nanotubes, Biosensors, Glucose oxidase, Molecular wires, Nanomaterials, Enzyme electrodes, DNA, Immunosensors 1. Introduction Carbon nanotubes (CNT) have become the subject of intense investigation since their discovery [1]. Such consid- erable interest reflects the unique behavior of CNT, includ- ing their remarkable electrical, chemical, mechanical and structural properties. CNT can display metallic, semi- conducting and superconducting electron transport, possess a hollow core suitable for storing guest molecules and have the largest elastic modulus of any known material [2]. CNT can be made by chemical vapor deposition, carbon arc methods, or laser evaporation and can be divided into single- wall carbon-nanotubes (SWCNT) and multi-wall carbon- nanotubes (MWCNT). SWCNT possess a cylindrical nano- structure (with a high aspect ratio), formed by rolling up a single graphite sheet into a tube (Figure 1). MWCNT comprise of several layers of grapheme cylinders that are concentrically nested like rings of a tree trunk (with an interlayer spacing of 3.4 A [2, 3]. The unique properties of carbon nanotubes make them extremely attractive for the task of chemical sensors, in general, and electrochemical detection, in particular [4]. Recent studies demonstrated that CNT can enhance the electrochemical reactivity of important biomolecules [4, 5], and can promote the electron-transfer reactions of proteins (including those where the redox center is embedded deep within the glycoprotein shell) [6, 7]. In addition to enhanced electrochemical reactivity, CNT-modified electrodes have been shown useful to accumulate important biomolecules (e.g., nucleic acids) [8] and to alleviate surface fouling effects (such as those involved in the NADH oxidation process) [5]. The remarkable sensitivity of CNT conductivity to the surface adsorbates permits the use of CNT as highly sensitive nanoscale sensors. These properties make CNT extremely attractive for a wide range of electrochemical biosensors ranging from amperometric enzyme electrodes to DNA hybridization biosensors. To take advantages of the remarkable properties of these unique nanomaterials in such electrochemical sensing applications, the CNT need to be properly functionalized and immobilized. It is the aim of this article to summarize recent advances in the use of CNT for electrochemical biosensors. Specifically, we will discuss: (i) amperometric (oxidase or dehydrogen- ase) enzyme electrodes based on the accelerated oxidation of NADH or hydrogen peroxide and based on the use of CNT molecular wires for achieving efficient electron trans- fer to enzyme redox centers; (ii) bioaffinity devices (par- ticularly DNA biosensors) based on the enhanced detection of the product of the enzyme label or of the target guanine, and the use of CNT support platforms. Common designs, Fig. 1. Structure of SWCNT. 7 Electroanalysis 2005, 17, No. 1  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200403113