from Theron et al. in Nanotechnology in Water Treatment Applications

Nanowires have been explored as signal transduction motifs in the electrical detection of DNA, proteins and microbial pathogens. Nanowire sensors operate on the basis that the change in chemical potential accompanying a target binding event can act as a field-effect gate upon the nanowire, thereby changing its conductance. The ideal nanowire sensor is a lightly doped, high-aspect ratio, single-crystal nanowire with a diameter between 10 and 20 nm. Recently, detailed protocols for fabrication of silicon nanowire devices, including their covalent modification with biorecognition molecules, have been described.

Real-time, label-free detection of DNA has been demonstrated by Hahm and Lieber using silicon nanowires functionalized with peptide nucleic acid (PNA). The conductance of the PNA-functionalized silicon nanowire bridging two electrodes was measured in the presence of target DNA and mutant DNA with three consecutive base deletions. Introduction of target DNA into the assay caused a rapid and immediate change in conductance, while the effect of mutant DNA was negligible. Furthermore, the conductance changes scale with target concentration, and target DNA at concentrations as low as 10 fM were detected.

Semiconducting silicon nanowires have also been used for the electrical detection of viruses in solutions. Patolsky et al. interfaced nanowires functionalized with antibodies specific for influenza A virus particles with a microfluidic sampling system. The nanowire sensing system was able to detect influenza A at the single-virus level, demonstrating that single virus/nanowire recognition events can be detected by measuring real-time changes in nanowire conductivity. In addition to silicon nanowires, metallic multi-striped nanowires have also shown great promise as potential platforms for multiplex immunoassays. These nanowires are built through submicrometer layering of different metals, e.g., gold, silver and nickel, by electrodeposition within a porous alumina template. Due to the permutations in which the metals can be deposited, a large number of unique yet easily identifiable encoded nanowires can be fabricated. The multi-striped nanowires, when coated with target-specific antibodies, were reported to efficiently and accurately allow multiplex detection of Bacillus globiggi spores, MS2 bacteriophage and ovalbumin protein. The sensitivity of detection for B. globiggi spores, MS2 bacteriophage and ova protein was estimated to be 1 x 10⁵ cfu/ml, 1 x 10⁵ pfu/ml and 5 ng/ml, respectively, and is comparable with results obtained using microarrays.

Nanowires are set apart from other available nanobiotechnologies due to several key features such as direct, label-free, real-time electrical signal transduction, as well as ultrahigh sensitivity, exquisite selectivity and the potential for integration of addressable arrays. However, an intrinsic limitation of nanowires is that the detection sensitivity depends on solution ionic strength. Consequently, for samples with a high ionic strength, diagnostics will require a desalting step before analysis to achieve the highest sensitivity. Furthermore, a practical constraint might be that the synthesis and fabrication of nanowire biosensor devices require some technologies that are not common to most laboratories.

Tags: Microbial Detection | Pathogen Detection | Biodiagnostics | Biodetection Assays | Biomolecular Detection