Functionalization of Carbon Nanotubes for Biocompatibility and Biomolecular Recognition

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. Recent years have witnessed a significant interest in biological applications of novel solid-state nanomaterials. 1-6 The unique physical properties of molecular- or nanoscale solids (dots or wires) when utilized in conjunction with the remarkable biomolecular recognition capabilities could lead to miniature biological electronics and optical devices including probes and sensors. Not only could these devices exhibit advantages over existing technology in size but also in performance. Several issues are important regarding nanomaterial/biosystems. One of them is biocompatibility, especially for in-vivo applications of implantable bioelectronic devices. Another is specificity that requires biofunctionalization of nanomaterials for recognition of only one type of target biomolecule in solution and rejection of others. Central to tackling these issues is surface functionalization of nanomaterials and elucidating the interfaces and interactions between nanomaterials and biosystems. Single-walled carbon nanotubes (SWNTs) are novel molecular scale wires exhibiting useful properties for various potential applications including miniature biological devices. For instance, nanotubes can be used as electrodes for detecting biomolecules in solutions, similar to commonly used conventional carbon based electrode materials. Also, the electrical properties of SWNTs are sensitive to surface charge transfer and changes in the surrounding electrostatic environment, undergoing drastic changes by simple adsorptions of certain molecules or polymers. 7-10 SWNTs are therefore promising for chemical sensors for detecting molecules in the gas phase and biosensors for probing biological processes in solutions. Nevertheless, significant effort is required in order to understand interactions between nanotubes and biomolecules and how to impart specificity and selectivity to nanotube-based bioelectronic devices. Motivated by the biological application prospects of solidstate nanomaterials, this work investigates (1) nonspecific binding (NSB) of proteins to SWNTs, (2) functionalization of nanotubes for resisting nonspecific interactions, and (3) enabling specific binding of proteins to functionalized nanotubes. We find that streptavidin nonspecifically binds to as-grown SWNTs and show that prevention of NSB of streptavidin on SWNTs is achieved by coating nanotubes with a surfactant and poly(ethylene glycol), PEG. Selective binding of streptavidin is introduced by co-functionalization of SWNTs with PEG and biotin. The results have implications to the nanotube biocompatibility 11 issue and specificity of potential bioelectronic devices based on nanotubes. In an earlier communication, we have shown that protein binding to SWNTs is reliably enabled via a noncovalent sidewall functionalization scheme. 5 In this manner, a variety