ABSTRACT
CNTs have high aspect ratios, high mechanical strength, high surface area, excellent chemical and thermal stability, and rich electronic and optical properties [8]. These features make CNTs important transducer nanomaterials in biosensors, though CNTs, in general, come in a mixture with different sizes, lengths, and various electronic properties, such as metallic and semiconductor CNTs. The semiconducting behavior exhibited by some types of CNTs makes them ideal for FET-based biosensors detecting single-molecule events [9]. Generally, the surface of CNTs needed to be functionalized for biosensor applications [5]. One kind of functionalization is noncovalent adsorption, or wrapping of various bioaffinitive agents around the nanotubes. In this case, a nonspecific binding effect should be mitigated [10]. The other is covalent attachment of the functional groups to CNTs. This involves reactions with carboxylic groups on CNTs, which are produced by oxidation in strong acids, and thus the structures of the CNTs can be altered. In some instances the functionalized CNTs can directly react with bioaffinitive agents, targeting either the amino site or the thiol site of the agents. For instance, secondary antibodies (Ab2) were linked to the carboxylfunctionalized CNTs using the standard N-hydroxy succimide/ carbodiimide chemistry [11]. In other instances, an additional bifunctional spacer is first coupled to the modified CNTs and the
resulting structure is then linked to bioaffinitive agents, for example, thiolated single-stranded DNA (ssDNA) [12]. The potential for identification of specific biomolecular recognition events with carboxyl-functionalized single-walled CNTs (SWCNTs) was first demonstrated by detecting hybridization of peptide nucleic acid (PNA), an uncharged DNA analogue, with DNA molecules as resolved with atomic force microscopy in 2002 [13]. Detection of DNA hybridization events was further exploited using a SWCNT network-based FET by measuring conductance change [14]. In the work, incubation of the SWCNT network-based FET with 12basepair (bp) ssDNA probes led to a shift in the transconductance curve toward a more negative voltage, assuming that adsorption of the ssDNA to the sidewall of the SWCNTs resulted in electron doping in the SWCNTs. Hybridization of the ssDNA with the complementary DNA led to a further shift in the transconductance curve, showing the possibility of detecting the DNA hybridization event. This report represents one of the early demonstrations of electrical identification of DNA hybridization. Similarly, Gui et al. [15] used CNT networks for the detection of DNA hybridization, where the initial ssDNA molecules were bonded noncovalently to the CNTs. To minimize nonspecific DNA adsorption on the CNTs and increase the specificity of the biosensing process, Martinez et al. [16] functionalized CNTs through nonvalent attachment of a methacrylate copolymer that contains ethylene glycol and N-succinimidyl groups to the nanotubes, thereby attempting to limit nonspecific DNA adsorption on the CNTs and simultaneously providing stable binding for DNA probes through robust amide linkages.
