Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 62

reference
electrode

V G

DNA

S

D

G

SiO 2

V D

p -Si

Fig. 2.4 The label-free detection of DNA using a MOSFET-like transistor

region of the FET, the drain current has decreased as the DNA concentration has
increased, producing a positive shift in V th as

I D D n C ox W. V th V D V D =2/=L G : (2.13)

The threshold voltage is shifted due to changes in the work function ˆ ms or
dielectric changes in capacitances:

V th D ˆ ms Q eff =C ox : (2.14)

The experiments indicated that ˆ ms is about 180 meV and Q eff is 9 nC cm 2 .
Considering that the Debye length of 0:25m contains about 9 10 6 molecules
above the gate, this change in the total charge corresponds to 2:4e=molecule. Taking
into account that the area of the FET is 1 8m 2 , the sensitivity of the label-free
detector is 0.1 mV, indicating that even a single molecule can be detected in the
device presented in Fig. 2.5 . The electrodes are covered with a polymer to integrate
the sensor with a microfluidic channel.
Very recently, a FET having as channel a thin doped silicon wire fabricated
by electron beam techniques, with the width of 80 nm and the length of 15m,
was used to detect a breast cancer biomarker ( Chen et al. 2010 ). In this label-free
sensor, the gate is silanized, as the FET in Fig. 2.4 , and the drain-source changes
in the differential conductance G are monitored. First, antibiotin in solution was
detected with this transistor at very small concentrations (0.2-1.7 nM), the detection
limit being of 20 pM. The conductance varies almost linearly in the range 40-
100 nS as a function of antibiotin concentration. Next, the cancer antigen CA15.3
is dialyzed in solution with 10M phosphate buffer and 150M NaCl. Then, the
FET is functionalized with antibody CA15.3. The G is varying linearly with the
concentration m of CA15.3 due to the specific binding of the biomarker. This linear