Instrumentation & Measurement Magazine 25-3 - 14

non-destructiveness. Attractive solutions have been developed
to differentiate between metals [35], to classify different
bi-metallic coins [36], to detect the presence of cracks in aluminum
plates for aerospace applications [37], and to characterize
thin layers of aqueous solutions [29]. Furthermore, solutions
have been developed to extract materials' physical properties
such as conductivity, permeability, yield, ultimate tensile
strength, residual stresses and thickness [38]. The measured
impedance spectra for different materials can be used for distance
correction and infer the conductivity and permeability
by comparing them to the simulated inductance using an analytical
model [39]. The extracted values of conductivity and
permeability can establish the relation to mechanical properties
such as hardness, stress and thickness in the sample. For
instance, the change in permeability due to the hardening process
can be measured [38].
Impedance spectroscopy-based multi-sensor systems
have demonstrated the capability for analyzing magnetic anisotropy
in cold-rolled ferromagnetic metals. The magnetic
anisotropy is quantified using a multi-magnetic sensor system
and a primary excitation coil [40]. The system also provides the
provision to understand tilting between the test sample and
sensor system [40].
Future Trends
With the development of new materials, trends are visible in
material characterization and analysis, thus increasing the demand
for impedance spectroscopy-based devices in industrial
applications. There is a great need for portable embedded solutions
with robust signal processing, including more sensor
elements to extract more information from the test samples
and eliminate influencing effects in the measurement [41].
There is also a trend towards including post-processing data
models and application of artificial intelligence-based signal
processing on embedded systems for real-time analysis.
Electrochemical Sensors
Electrochemical Impedance Spectroscopy (EIS) is gaining
importance in systems that use electrochemical sensors and
biosensors, especially for the quantitative measurement of
the analytical concentrations of ions [43], [43]. It is a very effective
technique for electrochemical characterization. It is
sensitive to surface modifications, and the binding events occurring
on the transducer surface [44]. It can be implemented
for various substances using functionalized and label-free
sensors, which realize high sensitivities even in the femtoand
atto-molar ranges [45. EIS is used to measure the Faradic
current or the non-Faradic current and measure impedance
using two interdigitated electrodes (IDE) [46]. In general, it
has many advantages, such as surface characterization of
the enlarged electroactive surface area of functionalized sensors
[47] (Fig. 4a), a broad linear detection range, a low limit
of detection (LoD), and it provides possibilities for a label-free
operation [45]. Similar to electrochemical biosensors, impedimetric
sensors use the interactions of biomolecules with a
transducer surface [48]. The sensor principle is based on a
guest host principle. A recognition compound between the target
sensing biomolecule and the analyte acts as a bioreceptor
at the transducer's surface. It changes the electrical properties
of the electrode surface directly or indirectly. These interactions
result in a modification of the interfacial electron transfer
kinetics from the solution to the electrode. Thereby, the chargetransfer
resistance increases depending on the concentration
of the bound target species. Thus, the sensor's sensitivity,
selectivity, response time, and detection limit are strongly associated
with the use of suitable materials and catalysts [50].
Several researchers have investigated the use of EIS in biosensors,
e.g., for Endocrine disruptors detection [50], SARS-CoV-2
antibody detection [51], heavy metals pollutants detection in
water [52] and various other contaminants [53].
Advances
Impedimetric biosensors use different functionalization strategies,
self-assembled mono- as well as multi-layers (SAM)
recognition, nanocomposites and biofunctionalization with
enzyme, DNA, antibody or biomimetic that gives the specificity
to the sensor [54]. Nanocomposites based on different
nanomaterials (Fig. 4b), such as metallic nanoparticles, carbon
Fig. 4. (a) Example of impedance spectra for a bare and functionalized electrode; (b) Label-free impedimetric biosensors based on nanomaterials and
nanocomposites.
14
IEEE Instrumentation & Measurement Magazine
May 2022

Instrumentation & Measurement Magazine 25-3

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-3