Introduction

Efficient biological sensing requires directly interrogating individual biomolecules with a physical dimension of about 1 to 100 nm. Development in this field strongly demands techniques and probing tools extending to similar length scales. This has been one of the major driving forces for nanotechnology in recent years. As the size of the materials is reduced to the nanometer regime, they show many new electronic, optical, and mechanical properties, which are more directly associated with the environment and target samples. Although the majority of raw nanomaterials are nanoparticles, high-aspect ratio one-dimensional (1D) materials such as carbon nanotubes (CNTs) and various nanowires (NWs) are more attractive as building blocks in the fabrication of

From: Methods in Molecular Biology, vol. 300: Protein Nanotechnology, Protocols, instrumentation, and Applications Edited by: T. Vo-Dinh © Humana Press inc., Totowa, NJ

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Fig. 1. Transmission electron microscopy (TEM) images of (A) a SWCNT and (B,C) two types of MWCNTs, and (D) scanning electron microscopy (SEM) image of an array of ZnO NWs. (Panel [A] is reprinted from ref. 10 with permission and panels [B] and [C] are reprinted from ref. 9 with permission. Copyright 1991 and 1993, respectively, Nature Publishing Group, http://www.nature.com.)

devices. The potential of CNTs and NWs as sensing elements and tools for biological applications as well as for detecting gases and small chemicals has been recently recognized. Promising results in improving sensitivity, lowering detection limit, reducing sample amount, and increasing the degree of multiplex and miniaturization have been reported based on CNTs (1,2) and NWs (3,4). Even though these studies are still in early research stages, they have shown great potential for future biotechnologies. This chapter summarizes, from a technology development point of view, the recent progress in the development of biological sensors using CNTs and NWs, it is our hope to attract interest in exploring the potential applications of such technologies in biomedical research.

CNTs belong to a family of materials consisting of seamless graphitic cylinders with extremely high aspect ratios (5-8). The typical diameter varies from about 1 nm to hundreds of nanometers, and the length spans from tens of nanometers to hundreds of microns or even centimeters. Scientists in NEC Corporation originally discovered the cylindrical structure of CNTs in 1991 (9). A CNT may consist of one graphitic layer, referred to as single-walled CNTs (SWCNTs) (10), as shown in Fig. 1A, or multi graphitic layers, referred to as multiwalled CNTs (MWCNTs) (9), as shown in Fig. 1B,C. SWCNTs normally give a smaller diameter (down to 7 A) and show electronics properties strongly

Fig. 1. Transmission electron microscopy (TEM) images of (A) a SWCNT and (B,C) two types of MWCNTs, and (D) scanning electron microscopy (SEM) image of an array of ZnO NWs. (Panel [A] is reprinted from ref. 10 with permission and panels [B] and [C] are reprinted from ref. 9 with permission. Copyright 1991 and 1993, respectively, Nature Publishing Group, http://www.nature.com.)

dependent on the helicity, namely the (m,n) lattice vector in the graphitic sheet along which it is rolled into a tube (11). It is known that SWCNTs are semiconducting if their chirality (m,n) satisfies m - n ^ 3 x integer (5-8). MWCNTs consist of a random mixture of all possible helicities in each shell (12).

Owing to the intriguing nanometer-scale structures and unique properties, CNTs have quickly attracted intensive attention in the past few years in many fields such as nanoelectronic devices (12-20), composite materials (21), fieldemission devices (22,23), atomic force microscope probes (24-27), and hydrogen/lithium ion storage (28,29). Many studies reported potential for ultrahigh sensitivity sensors as well (1,20,30-32). The extremely high surface-to-volume ratio of CNTs is ideal for efficient adsorption. The (1D) quantum wire nature makes their electronic properties extremely sensitive to gas or chemical adsorption. Both of these factors are essential for high-sensitivity sensors. In the past few years, CNT sensors have been demonstrated in many applications involving gas molecules, liquid-phase chemicals, and biomolecules, showing improved performance compared with conventional sensors utilizing micro- or macromaterials and thin films (2). Depending on applications, sensing devices can be fabricated using single free-standing CNTs (25,26), semiconducting SWCNT field-effect-transistors (FETs) (1,20,33,34), well-defined nanoelectrode arrays (35), or porous films (32,36,37).

NWs, on the other hand, typically refer to highly crystalline, wirelike 1D materials consisting of metals (38), semiconductors (3,39), or inorganic compounds (4,40). They can be synthesized with a high aspect ratio similar to that of CNTs, i.e., about a few to hundreds of nanometers in diameter and more than microns in length. Figure 1D shows a well-aligned array of ZnO NWs with an average diameter of approx 100 nm and length more than microns. Besides showing similar properties, such as high aspect ratio and large surface-to-volume ratio, CNTs, NWs have well-defined crystalline structure. The broad choice of various crystalline materials and doping methods makes the electronic and optical properties of NWs tunable with a high degree of freedom and precision. With the development of new synthesis methods, NWs have attracted more and more attention for sensor applications.

Another category of NWs is based on templating methods. These materials are the assembly of intrinsically heterogeneous biological and solid-state nanomaterial components instead of homogeneous structures. The nano-/bio-assembly NWs consist of two approaches: (1) CNTs or crystalline NWs serving as templates on which biomolecules aggregate into nanoscale wirelike structures (41), and (2) DNA or protein macromolecular backbones or assembly serving as templates on which nanoparticles are deposited to form nanoscale wirelike structures (42). Both of these approaches have been demonstrated in heterogeneously integrated nano-biosystems for biological sens ing, which combine the biorecognition-driven self-assembly functionalities with desired solid-state electronics properties.

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