Magnetic Tunnel Junctions for Biosensing Applications
Polo Regionale di Como - Conference Room, via Anzani 42
November 24th, 2010
Biochips and biosensors are largely used in the biomedical field for many applications like, for example, molecular recognition, genotyping of pathogens, biomarkers and proteins recognition, environmental analysis, etc.
The most widely used biodetection systems are today based on the optical detection of fluorescent markers which are opportunely bonded to the molecules to be analyzed. The use of these systems is typically subordinated to the employment of particular amplification techniques, such as PCR (polymerase Chain Reaction), which have the aim to increment the concentration of targeted molecules by different orders of magnitude, to increase the sensitivity of the bioassay.
An alternative consists in using magnetic markers, instead of fluorescent ones, which can be detected by magnetoresistive (MR) sensors, whose electrical resistance changes under the effect of an external magnetic field, which is in this case generated by the magnetic markers themselves. Such devices allow to realize biodetection systems characterized by a higher sensitivity, a better portability and, potentially, a lower cost.
Magnetic tunnel junctions (MTJs) are the magnetoresistive devices which are characterized by the highest values of magnetoresistance (more than 600%) and sensitivity (few %/Oe) now available and are thus currently studied by many research groups in the world. In order to obtain good performances in terms of signal-to-noise ratio, however, a good engineering and biasing of the device are needed, as well as a good understanding of the noise characteristics of it so to adopt the appropriate measurement techniques. An example is the use of lock-in amplifiers for reducing the impact of 1/f noise components on the measurements results. Thanks to such techniques, limit of detection values of about 33 pg/l for solutions containing DNA molecules have been obtained with MTJ-based biochips and are reported in literature. Furthermore methods for magnetic particle detection based on the measurement of their frequency response to AC magnetic fields (magnetorelaxometry) are particularly appealing for developing novel bioassay protocols, including the possibility of using “magnetically coloured” tags.
In the final part of this work, measurements performed on some devices manufactured at L-NESS laboratories of Politecnico di Milano will be described. In particular, the measurements consisted in static (I-V) characterization, magnetoresistance (TMR) measurement and electric/magnetic frequency characterizations of some MgO-based junctions.
Sensors and instrumentation