Biology Reference
In-Depth Information
down to tens of micrometers, which sets the minimum size of the
sensing area of each individual sensor element, and consequently
the density ofthe sensor array [40].
The transducing methods also limit the size of sensor elements.
For potentiometric detection, although the signal-to-noise ratio is
independent of the FET dimension, having a larger extended gate
structure can significantly improve the sensitivity of the transducer
[4]. According to the requirements on the sensitivity, it is preferred
to have a ratio between the areas of the extended gate and the
FET gate at 10-100, which results in individual sensor dimensions
of 10-100 μ m based on modern microfabrication technologies.
For amperometric detection methods such as EIS, reducing the
size of sensor electrodes leads to a decrease of the current to be
measured. The resistance of the biomolecular layer varies within a
large range—typically between 30 k cm 2 and 5 M cm 2 . A reliable
measurement of sub-pA current requires very high performance
electronic devices and complicated circuit design. Therefore, the
typical dimension of amperometric sensors based on ac methods
cannotbe smaller than tens of micrometers.
Electronic biosensor transducers, either potentiometric or
amperometric/EIS-based, also require atomically flat surfaces or at
least surfaces with controlled roughness. The underlying consider-
ation is the density of immobilized biomolecular probes and hence
the target captured by the probes in the biomolecular interaction.
It has been shown that the immobilization density depends on the
microscopicareaofthesensorsurface,whichisdeterminedbyboth
the geometry area and the roughness factor [39]. For measurement
techniques where the amount of charge is of concern, such as
potentiometricdetectionorchronocoulometricdetection,auniform
surfacewithregularroughnessfactorisneededfortheentiresensor
array.
Another important consideration is the working frequency. To
characterize the sheer number of sensors in the same array in
real time, both the switching circuit and measurement units need
to work at high enough frequency. The speed needed eventually
depends on the nature and kinetics of the biomolecular interaction
to be measured. For example, considering a typical array with 1,000
elementstobemeasuredin1second,thelogiccircuittoaddressthe
 
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