Affinity sensors are the most frequently used devices for biological and chemical sensing research and applications. These can be segregated in two major categories depending on the target binding techniques applied; labeled and label-free. Labeled sensors use immunoassay approach for binding the target molecule that is purely laboratory-based technique involving expensive, time-consuming and hi-tech laboratory protocols, hence, rendering real-time measurements impossible. Sensors for direct, label-free quantification of analytes are attractive to researchers for a number of reasons like direct detection of analytes with minimal or no sample preparation, cost effective, simple and real-time. These involve optical, mass-sensitive and electrochemical detection routes for measurement of analyte in bulk sample. Optical sensors are the most studied but the electrochemical sensors are relatively more sensitive and display comparatively lower detection limit capabilities with a requirement of least complicated support instrumentation. On the other hand, optical and mass-sensitive techniques claim state-of-the-art electronics and bulky instrumentation for interpretation of optical and mass variation data into comprehendible information.
.The term ‘interdigital’ (ID) refers to digit-like or finger-like parallel in-plane electrodes built in a periodic pattern in order to exploit the capacitive effect produced as a result of applied alternating electric field that fringes through the material sample and carries useful information about it. This term is often replaced by ‘interdigitated’, ‘combed’ and ‘microstrip’ in the research literature but strictly speaking it is the name of a specialized geometric structure shown in Figure 3.1, which owns important advantages in accessing the material properties. The most important advantage is a single side access to the material under test (MUT). This advantage provides freedom of penetrating the sample with electric, magnetic or acoustic field only from one side which encourages in situ measurements and detections. Other benefits of interdigital
structures include its capability to be used for non-destructive testing which makes it more useful for process control and inline testing applications. The penetration depth of qausi-static electric field lines can be varied thus making it possible to obtain dielectric profiling and conduction properties of semi insulating materials. These dielectric properties profiling can be further helpful in evaluating other physical parameters of the material under test like density, structural integrity and chemical content in MUT [47].
Figure 3.1 Planar interdigital sensor geometry
The technology of planar interdigital sensors is under development since last three decades. These sensors are employed in a number of applications like photosensitive detection [48-51], humidity detection [52-55], chemicals and gas sensing [56-59], moisture sensing applications [60,61] and measurement of electrolytic conductivity [62]. The dielectric characterization to evaluate the material properties is one of the most interesting areas where interdigital sensors along with meander coils are successfully employed to study the dielectric properties of meat, leather, dairy products and saxophone reeds [63-66].
Conventional interdigital sensors operate on the principle of parallel plate capacitor. Parallel plate capacitors offer uniformity of the applied electric field and the equations related to the dielectric material properties are simple, but these 3D structures do not offer single side and non-invasive testing freedom. Interdigital sensors, on the other hand, are 2D structures (coplanar geometry) and provide non-destructive, in situ and single side access to the material under test. The transformation of parallel plate capacitor to interdigital one is shown in Figure 3.2 [47].
Figure 3.2 Concept of transformation (a) parallel plate capacitor (b) transformation to planar geometry (c) coplanar structure.
The transformation occurred at the cost of electric field which changed from uniform electric field to ‘fringing electric field’ penetrating through the material under test. That is the reason conventional interdigital sensors are also referred as fringing field dielectrometry sensors in literature [47]. The basic idea behind this transformation is to apply spatially periodic electric field to the material under test using single side access to the material. The penetrating field lines combined with the variation of applied excitation frequency carry useful dielectric spectroscopic information about the MUT. A change in the material dielectric properties is a function of various chemical, physical and structural properties of the material, therefore a comparative study of electrochemical impedance spectroscopy of MUT reflect the changes in material properties. The well-known expression for capacitance is given by:
ܥ
=
ߝ
ߝ
ܣ
݀
(3.1)Where C is the capacitance,
ߝ
ois the permittivity of free space (8.8554 x 10-12F/m),ߝ
r is the relative permittivity of the dielectric medium used, A is the area, and d is thespacing between the positive and negative electrode. The value of capacitance varies directly with the area A of electrodes, according to equation (3.1), and for interdigital sensors the area of electrode is sacrificed considerably due to its coplanar structure. The value of the capacitance in ID structure is reduced to an extent that it approaches the stray capacitance of the conductors connecting excitation source and electrodes. This limitation necessitates the use of repeated coplanar structures connected in parallel to keep the signal to noise ratio at an acceptable range. Important concepts related to this technology are discussed in detail in [67-70].