Biomedical Engineering Reference
In-Depth Information
Fig. 5.21 ( a ) A schematic diagram showing the operating principle of the negatively charged
biomolecules and the expected dI cp = dV h versus V h [ 53 ]. ( b ) The dI cp = dV h versus V h characteris-
tics. The shift direction in the dI cp = dV h
V h plot indicates the charge polarity of the biomolecules
[ 53 ]. ( c ) The measured dI cp = dV h data with two different pH solutions. When streptavidin is at pH
3.5, the expected charge polarity of streptavidin is positive. Therefore, the peak of dI cp = dV h .V C /
is shifted to the left side from initial (V 0 / value due to the locally increased V T near the nanogap
region (Copyright 2010 American Institute of Physics)
5.5.1.2
1/f Noise Measurement
The 1=f (flicker) noise in FET systems has been studied for more than four decades.
It is known that the 1=f noise comes from the random trapping and detrapping pro-
cesses of charges in the interface traps located at the Si=SiO 2 interface (channel/gate
dielectric). The charge fluctuation results in fluctuation of the surface potential,
which in turn modulates the channel carrier density and conductance. This type
of noise is one of the limiting factors for biosensing. To distinguish the signal from
noise clearly, the noise spectra in the frequency domain may allow contributions
from different noise sources to be analyzed directly.
As shown in Fig. 5.22 a, the concentration-dependent conductance change indi-
cates that it is difficult to distinguish the signal from noise when the PSA
concentration is at or below 0.15 pM. In contrast, when the device is in pure buffer,
a clear 1=f spectrum (noise) can be observed in the frequency domain (Fig. 5.22 b).
When solutions of 0.15 pM PSA were delivered, the power spectra showed a curved-
shape signal that was clearly different from that measured in buffer.
Therefore, the main benefit of 1=f noise technique is that sub-picomolar detec-
tion has been routinely achieved based on the fact that the characteristic frequencies
associated with protein binding are well separated from other noise sources. An
increase in the detection sensitivity of more than tenfold has been achieved with the
frequency domain compared to time domain measurements from the same device.
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