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Novel nanomaterials for bioassay applications represent a rapidly progressing field of nanotechnology and nanobiotechnology. Here, we present an exploration of single-walled carbon nanotubes as a platform for investigating surface-protein and proteinprotein binding and developing highly specific electronic biomolecule detectors. Nonspecific binding on nanotubes, a phenomenon found with a wide range of proteins, is overcome by immobilization of polyethylene oxide chains. A general approach is then advanced to enable the selective recognition and binding of target proteins by conjugation of their specific receptors to polyethylene oxide-functionalized nanotubes. This scheme, combined with the sensitivity of nanotube electronic devices, enables highly specific electronic sensors for detecting clinically important biomolecules such as antibodies associated with human autoimmune diseases. R ecent years have witnessed significant interest in biological applications of novel inorganic nanomaterials such as nanocrystals (1, 2), nanowires (3), and nanotubes (4, 5) with the motivation to create new types of analytical tools for life science and biotechnology. Single-walled carbon nanotubes (SWNTs) are interesting molecular wires (diameter Ϸ1-2 nm) with unique electronic properties that have been spotlighted for future solid-state nanoelectronics (6, 7). Bridging nanotubes with biological systems, however, is a relatively unexplored area, with the exception of a few reports on nanotube probe tips for biological imaging (4), nonspecific binding (NSB) of proteins (8-10), functionalization chemistry for bioimmobilization on nanotube sidewalls (5), and one study on biocompatibility (11).Previously, we and others have shown that the electrical conductance of a nanotube is highly sensitive to its environment and varies significantly with changes in electrostatic charges and surface adsorption of various molecules (12)(13)(14). This research has hinted at possible SWNT-based miniature sensors for detecting biological molecules in fluids. Here, we systematically explore how nanotubes interact with and respond to various proteins in solution, how chemical functionalization can be used to tailor these interactions, and how the resulting understanding enables highly selective nanotube sensors for the electronic detection of proteins. Using atomic force microscopy (AFM) and quartz crystal microbalance (QCM) and electronic transport measurements, we first reveal that proteins in general exhibit a high degree of NSB on nanotubes, a phenomenon undesirable for potential biosensors. We then demonstrate a functionalization scheme involving irreversible adsorption of Tween 20 or triblock copolymer chains on nanotubes to prevent this general NSB, while at the same time enabling the binding of specific proteins of interest that can be detected electronically without the need for labeling. Further, we demonstrate specific detection of mAbs to the human autoantigen U1A, a prototype target of the autoimmune response in patients with systemic lupu...

The interface between biological molecules and novel nanomaterials is important to developing new types of miniature devices for biological applications. Here, the streptavidin/biotin system is used to investigate the adsorption behavior of proteins on the sides of single-walled carbon nanotubes (SWNTs). Functionalization of SWNTs by coadsorption of a surfactant and poly(ethylene glycol) is found to be effective in resisting nonspecific adsorption of streptavidin. Specific binding of streptavidin onto SWNTs is achieved by co-functionalization of nanotubes with biotin and protein-resistant polymers.

It has been reported that protein adsorption on single-walled carbon nanotube field effect transistors (FETs) leads to appreciable changes in the electrical conductance of the devices, a phenomenon that can be exploited for label-free detection of biomolecules with a high potential for miniaturization. This work presents an elucidation of the electronic biosensing mechanisms with a newly developed microarray of nanotube "micromat" sensors. Chemical functionalization schemes are devised to block selected components of the devices from protein adsorption, self-assembled monolayers (SAMs) of methoxy(poly(ethylene glycol))thiol (mPEG-SH) on the metal electrodes (Au, Pd) and PEG-containing surfactants on the nanotubes. Extensive characterization reveals that electronic effects occurring at the metal-nanotube contacts due to protein adsorption constitute a more significant contribution to the electronic biosensing signal than adsorption solely along the exposed lengths of the nanotubes.