Semiconductor-Based Biosensing Chip for Point-of-Care Diagnostics - Point-of-Care Diagnostics on a Chip - page 131

Fig. 6.15 Direct detection of extension reaction

of the V T indicates increase of positive charges at the gate surface and is due to
specific binding of Hoechst 33258 to the double-stranded DNA. This is in contrast
to the positive change of the V T due to negatively charged DNA molecules. Thus,
the charge density change at the gate surface after each molecular recognition event
can be successfully detected using genetic FET.
DNA recognition events such as primer extension reaction can be also directly
detected as electrical signal by use of the genetic FET. The 11-base ligonucleotide
probes on the genetic FET were hybridized with the 21-base target DNA at first.
In order to extract small changes of the output voltages of the genetic FET after
extension reaction, the output voltages before the introduction of DNA polymerase
for both genetic and reference FET were initialized and adjusted to zero as shown
in Fig. 6.15 by adding or subtracting offset voltages, although the absolute output
voltages are not zero and different between active and reference FET. After washing,
the genetic FETs were immersed in a reaction mixture and thermostable DNA
polymerase was introduced into the gate surface. The V T of the FET changed
during primer extension reaction as shown in Fig. 6.15 . Differential measurement
was performed using a pair of FETs; one is the genetic FET with immobilized
oligonucleotides probes, and the other is the reference FET without oligonucleotide