Biomedical Engineering Reference
In-Depth Information
transducers are typically not well miniaturized. Thus, the portability, which is meant
to be the primary advantage of POCT systems, is sacrificed. The use of electronic
devices for POCT systems circumvents this problem, enabling label-free detection,
miniaturization, and low costs [
3
]. Label-free detection is made possible by direct
electrical measurement of the sample molecules, which works by monitoring
changes in their intrinsic electrical properties. Miniaturization and the integration of
sensors and readout circuitry have been enabled by industrialized microfabrication
technology. If the sensors and circuitry are monolithically integrated on the same
substrate, then the fabrication cost can be remarkably reduced.
Label-free electrical detection is usually based on the electrical properties of
biomolecules. The binding of charged molecules leads to changes in the surface
potential, which can be measured by changes in conductivity or capacitance.
Although this type of charge-based detection achieves high sensitivity, the sensor
signal can be adversely affected by environmental conditions such as pH and ionic
strength. Moreover, weakly charged or neutral biomolecules can be difficult to
detect with charge-based methods. However, dielectric detection, which is a type
of detection based on the dielectric properties of biomolecules, is less sensitive to
environmental variations, which allows it to be used to detect weakly charged or
neutral biomolecules.
The aim of this chapter is to describe recent advances in the dielectric detection
of biomolecules for POCT systems. Several electrical detection techniques will be
reviewed. A nanogap-embedded device that is well suited to detecting dielectric
changes will be described, and experimental results obtained with this device will be
discussed. The discussions will also address the structural modifications of dielectric
sensors, different options for sensing metrics, and the effects of environmental
conditions on this technology.
5.2
Electrical Detection Based on Dielectric and Charge
Properties
5.2.1
Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) is suitable for the electrical detec-
tion of biomolecular interactions on the transducer surface [
4
,
5
]. In EIS, a voltage
perturbation with a small amplitude applied to an electrochemical cell generates a
current response. The current response depends on the impedance of biomolecules,
which is related to the resistive and capacitive properties of the biomolecules. The
impedance is defined as the ratio of the applied voltage and the current response.
The impedance between the electrode and the electrolyte solution can be simply
modeled using the Randles equivalent circuit, as shown in Fig.
5.1
a[
4
], where R
s
denotes the resistance of the electrolyte solution. The charge can be stored in the
electrical double layer at the interface, resulting in the double layer capacitance C
dl
.
