Biosensors are bioanalytical devices which employ biological recognition properties to detect the target analyte. Biosensors utilise a wide variety of technologies from electrochemical to biomechanical. Most biosensors rely on specific molecular recognition events such as ligand-receptor interactions and detection via direct or indirect techniques.
DNA biosensors are generally constructed by immobilising a DNA molecule on a suitable transducer such as a quartz crystal microbalance. These sensors form an extremely promising area of research due to the potential cost and sensitivity benefits associated with these types of devices.
Piezoelectric mass-sensing devices - quartz crystal microbalance
Piezoelectric mass-sensing devices are sensors that enable the label free detection of molecules such as nucleic acids. At the core of a quartz crystal microbalance is a specifically manufactured quartz plate with a fundamental resonance frequency in the range of 5-30 MHz. The crystal is excited to or near to resonance and the effect of molecular absorption monitored.
The quartz crystal microbalance (QCM) comprises thin film electrodes, usually gold, deposited on each face of a crystal. A voltage is applied across these electrodes to deform the crystal plate, so there is relative motion between the two parallel crystal surfaces. The crystal is induced to oscillate at a specific resonant frequency. Therefore, any change in the mass of the material on the surface will alter the resonant frequency of the crystal. A linear relationship exists between deposited mass and frequency response for quartz crystals. This characteristic of QCMs can be exploited to develop bioanalytical tools.
Piezoelectric mass sensing devices are capable of the label-free detection of oligonucleotides (click 
for a diagram). Central to the success of this approach is the immobilisation of single-stranded oligonucleotides on the sensor surface through chemical modification.
More information can be found at the web site of Q-Sense AB (http://www.q-sense.com/) who market QCM based systems.
Nanopores
The detection and characterisation of individual nucleic acid molecules is a thriving area of research and development with potential diagnostic applications. Single nucleic acid molecules can be characterised using electric fields to drive RNA or DNA strands through a single protein pore (Click for a figure). The associated change in ionic current flowing through the pore enables base discrimination. This allows direct nucleic acid characterisation without the need for amplification or probes/dyes.
The nanopores are capable of discriminating between individual DNA strands up to 30 bases in length differing by a single base substitution. Possible applications for this technology include SNP determination, array formats and mini-sequencing. However at present the technology is not commercially available.
For more information on these and other novel technologies visit the UK Bio-measurement network and down load the following report from the Document Library:
'Survey on emerging technologies potential for highly sensitive detection.'