from Theron et al. in Nanotechnology in Water Treatment Applications

Nanocantilevers, which are typically made of silicon, silicon nitride or silicon oxide, require only minute changes in compressive or tensile surface stress on either their upper surface or lower surface to cause measurable deflection of the cantilever, and are capable of converting biomolecular recognition reactions into micromechanical motion. Consequently, nanocantilevers offer an opportunity for the development of highly sensitive, miniature and label-free detection systems.

Direct, label-free detection of DNA typically involves the use of silicon cantilevers with a thin gold coating on the top surface that is functionalized with thiolated capture DNA strands. Binding of the capture DNA strands with the introduced target DNA causes deflection of the cantilever, which can be measured accurately using optical detection methods. Using this approach, Fritz et al. demonstrated the hybridization of complementary oligonucleotides with a detection limit of 10 nM and showed that a single mismatch between two 12-mer oligonucleotides is clearly detectable. Similarly promising results were obtained by Hansen et al. with a 10-mer oligonucleotide.

In recent years, cantilever immunosensors have been developed to detect bacteria and viruses. Typically, the devices detect the additional mass loading that results from the interaction between specific antibodies, immobilized on the surface of the cantilever, and antigens on the cell membrane surface. In an early experiment, a cantilever biosensor was constructed and used to detect E. coli O157:H7, following immersion of the cantilever in a suspension containing 10⁶-10⁹ cells/ml. The detection of 16 E. coli O157:H7 cells were demonstrated and no frequency shifts were observed when buffer alone or buffer containing S. enterica serovar Typhimurium was incubated with the cantilevers. A magnetoelastic cantilever immunosensor has been developed that uses a magnetic field to induce oscillation of the sensor. The sensor surface is coated with antibodies to permit specific capture of the desired target agent after which alkaline phosphatase-labelled antibodies to the target are added to amplify the signal, thereby increasing the total mass on the sensor. The sensor was tested with E. coli O157:H7 and a sensitivity of 10² cells/ml were reported. More recently, resonant cantilever biosensors have been developed for detection of Listeria innocua and vaccinia virus, and the detection of a single L. innocua cell and vaccinia virus particle were demonstrated.

Despite being in its early days, cantilevers provide an opportunity for label-free, real-time measurements in fluids in a single-step reaction, and can potentially serve as a powerful platform for sensitive, multiplexed detection of biomolecules. Although cantilevers can be microfabricated by standard low-cost silicon technology, leading to a decrease in production costs and allowing the possibility of integrating multiple functional devices onto the same platform, some challenges must be overcome before cantilever array sensors can come into widespread use. These challenges relate especially to methods for detecting nanoscale motion and the development of immobilization techniques that can efficiently transduce the stress involved in biochemical interaction to the cantilever substrate.

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