Nanowire Biosensors

A variety of nanoscale structures are under development for biosensing applications [11,12,18-20]. Nanoparticles and nanocrystals have received a large amount of attention because they offer convenient spectroscopic properties that allow sensitive detection of both nucleic acid and protein analytes [21,22]. Nanowires and nanotubes, however, are extremely attractive because electrical or electrochemical readout strategies are enabled by the use of structures that can be electrically contacted.

FIGURE 6.1 (See color insert following page 18-18.) Schematic illustration of a DNA-modified nanowire and a strategy for nucleic acids detection using electrocatalysis. Using templated gold deposition within a polycarbonate membrane followed by oxygen plasma etching, gold nanowires can be generated (left). The deposition of a thiolated oligonucleotide (red) produces a single-stranded DNA film (middle) that serves as bait for an analyte sequence (blue). After hybridization of the target analyte sequence, an electrocatalytic reporter system is used to readout the presence of the bound strand. The reporter groups are Ru(NH3)f+ and Fe(CN)|_, which serve as a primary and secondary electron acceptor, respectively. The accumulation of the positively charged electron acceptor on double-stranded DNA provides a handle for monitoring hybridization.

Field-effect devices based on single semiconductor nanowires have enabled the sensitive electrical detection of a variety of chemical and biological analytes [13,17,18]. Boron-doped silicon nanowires exhibit conductivity changes in response to variations in their electrostatic environment. This effect can be used to monitor the binding of charged biomolecules to surface-immobilized receptors, the binding of ions to biomolecular receptors, or to detect nucleic acids binding or extension in situ. The generation of silicon nanowire arrays, and the demonstration that these structures are robust enough to detect analytes within serum, indicates that this configuration will provide the basis for powerful multiplexed sensors if the electronics required for such measurements can be made practical. Indium oxide (In2O3) nanowires have also been explored for biosensing and chemical-sensing applications. Methods for the derivatization of In2O3 nanowires have been developed [12,15], and excellent sensitivity with chemical analytes has been achieved, with detection levels of biomolecular analytes not yet determined.

Collections of nanowires implemented as a sensing ensemble [11,20,23] are also useful platforms for biomolecular detection. Templated synthesis of gold nanowires provides an inexpensive and robust means to access and address these structures (Figure 6.1 and Figure 6.2). Moreover, the metallic structures act as excellent electrodes, and permit the use of straightforward solution electrochemistry for sensitive biomolecular detection. It is this approach that will be the focus of the remainder of this review.

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