Biology Reference
In-Depth Information
thiolated oligonucleotides, using only a single biochemical step.
It also eliminates various problems that may occur using semi-
conductor or insulator surfaces, which are prone to uncontrolled
modifications, contaminations, or hydration. These may lead to
a change in the intrinsic properties of the insulator, such as its
dielectric constant, which are critical for the stable operation of
FETs.
Polycrystalline silicon thin film transistors have also been
employed for the detection of DNA hybridization [16]. A mixed
self-assembled monolayer of thiolated DNA probes and mercapto-
hexanol was immobilized onto the gold gate of an extended gate
poly-SiTFT.Ashiftofthe
I
-
V
characteristicsontheorderof300mV
was obtained upon hybridization of the immobilized probe with a
fully complementary strand. The shift is independent of electrode
area, so microarrays can be constructed where a known DNA probe
is immobilized on each FET. The inherent miniaturization and com-
patibility with microfabrication technologies make the technique
highly promising forthe development oflow-cost portable devices.
6.3.1
Physical Mechanisms of Detection
A better understanding of the physical mechanisms involved in the
field-effect detection of DNA is fundamental in the development
of reliable DNA microarrays based on FETs. Several aspects play a
role in the detection mechanism. Counterion condensation theory
can be used to evaluate the effective charge density of the DNA
layer in contact with an electrolyte, which partly screens its charge,
its dependence on the ionic strength of the electrolyte, and the
reduction of the charge fraction observed upon hybridization.
Mathematicalmodelshavebeenusedtodescribetheobservedshifts
in the
I
-
V
curves ofthe field-effect transistors.
The immobilization of the nucleic acid probe is crucial in deter-
miningtheperformanceofthebiosensor.Toachievehighsensitivity
and selectivity, the hybridization e
ciency must be maximized
and the non-specific adsorption minimized. Immobilization should
produce a stable layer of well-defined probe orientation, readily
accessible to the target. There are a wide variety of immobilization
methods, depending on the transducer surface and application. For
