Semiconductor-Based Biosensing Chip for Point-of-Care Diagnostics - Point-of-Care Diagnostics on a Chip - page 130

Fig. 6.14 V G I D electrical characteristic of DNA-based FET

by measuring a shift of the V T . When a n-channel FET is used, the V T shifts
in the positive direction in response to DNA molecules. When intercalators are
introduced into the double-stranded DNA after hybridization, the V T of the genetic
FET shifts in the negative direction, because the intercalators are positively charged.
Since the intercalators react specifically with double-stranded DNA, an undesirable
background noise caused by nonspecific adsorption of single-stranded target DNA
can be eliminated, and a more precise and reliable detection of a hybridization event
can be realized.
A specific binding of charged biomolecules at the gate surface can be detected
as a shift of the V T , which can be determined in the gate voltage V G and
the drain current I D .V G I D / characteristics of the genetic FET (Fig. 6.14 ).

The V G I D characteristics of the genetic FET shifted along the gate voltage

V G axis in the positive direction after immobilization of oligonucleotide probes.
Immobilized oligonucleotide probes were a normal type for R353Q locus of
factor VII gene (probe: 5 0 -amino group-CCACTACCGGGGCACGT-3 0 (17mer),

(17mer), melting temperature: 60 ı C)
[ 18 ]. In order to evaluate the V T shift in more detail, the local area shown in
Fig. 6.14 b (surrounded area) was magnified. The V T shifts after hybridization and
specific binding of DNA binder are also shown in Fig. 6.14 . When oligonucleotide
probes were immobilized on the gate surface, the V T shifted along the V G axis
by the amount of 32 mV. The positive shift is due to negative charges induced
at the gate surface after immobilization process including cleaning, silanization,
glutaraldehyde treatment, immobilization of oligonucleotide probes, and blocking
with glycine. Immobilization of oligonucleotide probes and glycine blocking is
considered to contribute to the V T shift to a large extent. When the complementary
target DNA was introduced to the gate surface and hybridized with oligonucleotide
probes, the V T shifted in the positive direction by the amount of 12 mV. This is
due to increase of negative charges of the target DNA by hybridization. After
hybridization, a DNA binder, Hoechst 33258, was introduced to the gate surface.
The V T shifted in the negative direction by the amount of 14 mV. The negative shift

target: 5 0 ACGTGCCCCGGTAGTGG 3 0