from Theron et al. in Nanotechnology in Water Treatment Applications

Since their discovery, carbon nanotubes (CNTs) have attracted great attention as nanoscale building blocks for micro- and nanodevices. CNTs can be divided essentially into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) based on the principle of hybridized carbon atom layers in the walls of CNTs. Whereas SWCNTs have diameters ranging from 0.3 to 3 nm, the MWCNTs are composed of a concentric arrangement of many cylinders and can reach diameters of up to 100 nm. CNTs are considered to be ideal for use in biosensors for detecting individual biomolecules and other biological agents. Not only do they have high surface-to-volume and surface-to-weight ratios, but they are also conducting, act as electrodes, and can be derivatized with functional groups that allow immobilization of biomolecules.

The use of nano-size electrodes based on CNTs has the potential to greatly improve the sensitivity in recognizing DNA hybridization events. Li et al. developed DNA microarrays containing sensing pads constructed from MWCNTs and built into a matrix within a silicon nitride template. The upper (open) ends of the tubes act as nanoelectrodes and are functionalized with ssDNA probes. Target DNA that hybridizes to the ssDNA probe on the ends of the electrically conductive MWCNTs is detected using an electrochemical method that relies on guanine oxidation. The hybridization of less than a few attomoles of oligonucleotide targets was demonstrated. In an alternative approach, CNTs coated with alkaline phosphatase enzymes was used for the detection of amplified DNA. This assay employs a magnetic microparticle modified with oligonucleotides that are complementary to one-half of the target DNA sequence, and alkaline phosphatase-coated carbon nanotubes that are modified with oligonucleotides that are complementary to the other half of the target DNA sequence. Binding of the target DNA promotes the formation of a magnetic microparticle/target/carbon nanotube sandwich, which is magnetically separated from the assay medium. After separation, the enzyme substrate alpha-naphthyl phosphate is added to the mixture, resulting in formation of alpha-naphthol product that is ultimately detected at a CNT-modified electrode via chronopotentiometric stripping. This method detected target DNA at concentrations as low as 54 aM.

In addition to their potential in recognizing DNA hybridization events, the use of CNTs displaying ligands for the capturing or recognition of bacterial pathogens has recently been described. The solubilization of SWCNTs via fictionalization with derivatized galactose has been reported and it was subsequently shown that the nanotube-bound galactose could serve as polyvalent ligands that strongly interacted with receptors on E. coli O157:H7, resulting in significant cell agglutination. This work has subsequently been extended to the preparation of immuno-carbon tubes. For this purpose, SWCNTs and MWCNTs are functionalized with bovine serum albumin (BSA) to attain aqueous solubility and then further conjugated with an E. coli O157:H7-specific antibody to form immuno-carbon tubes. Limited quantitative data was provided, but the results suggest that the immuno-carbon tubes are capable of sensitively capturing the target bacteria.

While CNTs currently are not as easily functionalized as QDs or nanoparticles, they offer the distinct advantage of rapid, real-time detection and may thus become viable options as nanostructured biodiagnostic devices. In addition to challenges at the fabrication level (e.g., production of pure and uncontaminated nanotubes is costly, continuous growth of defect-free CNTs to macroscopic lengths is difficult to obtain and dispersion of CNTs onto a polymer matrix is very difficult), another important issue related to the use of CNTs is their toxicity. Although results suggest that chemically modifying CNTs can reduce their cytotoxicity to a certain extent, more research is required to address the effect of CNTs on biological systems, as well as information related to safety issues.

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