The use of field effect sensors for biological and chemical sensing is widely employed due to its ability to make
detections based on charge and surface potential. Because proteins and DNA almost always carry a charge , silicon
can be used to micro fabricate such a sensor. The EIS structure (Electrolyte on Insulator on Silicon) provides a novel,
label-free and simple to fabricate way to make a field effect DNA detection sensor.
The sensor responds to fluctuating capacitance caused by a depletion layer thickness change at the surface of the silicon
substrate through DNA adsorption onto the dielectric oxide/PLL (Poly-L-Lysine) surface. As DNA molecules diffuse
to the sensor surface, they are bound to their complimentary capture probes deposited on the surface. The negative
charge exhibited by the DNA forces negative charge carriers in the substrate to move away from the surface. This
causes an n-type depletion layer substrate to thicken and a p-type to thin.
The depletion layer thickness can be measured by its capacitance using an LCR meter. This experiment is conducted
using the ConVolt (constant voltage) approach. Nucleic acids are amplified by an on chip PCR (Polymerase Chain
Reaction) system and then fed into the sensor. The low ionic solution strength will ensure that counter-ions do not
affect the sensor measurements. The sensor surface contains capture probes that bind to the pathogen. The types of
pathogens we’ll be detecting include salmonella, campylobacter and E.Coli DNA. They are held onto the sensor surface
by the positively charged Poly-L-Lysine layer. The electrolyte is biased through a pseudo-reference electrode.
Pseudo reference electrodes are usually made from metals such as Platinum or Silver. The problem associated with
“floating” biasing electrodes is they cannot provide stable biasing potentials . They drift due to surface charging
effects and trapped charges on the surface. To eliminate this, a differential system consisting of 2 sensors that share a
common pseudo-reference electrode is used to cancel out this effect.
This paper will look at a differential system for multi-arrayed biosensors fabricated on silicon.