Instrumentation & Measurement Magazine 25-3 - 15

nanotubes, and graphene, play an important role because of
their high affinity to biomolecules and high aspect ratio [55].
They are frequently used as electrode coating to enlarge their
effective surface area and immobilize various biomolecules
(e.g., proteins, enzymes, and antibodies) [55].
Impedance spectroscopy is widely used as a characterization
tool for nanocomposites or biofunctionalized-based
sensors to reveal their surface properties and conduction
mechanism. It is also an analytical tool for measuring their
sensing performance. The possibilities with no-Faradaic IS
to detect more than one analyte were demonstrated in [56] by
optimizing the frequency of ZnO-based electrodes and antibody-antigen
interaction. It exhibited an LoD of 0.10 ng mL−1
for PCT and 0.10 μg mL−1
for CRP in human serum. CNT/
PDMS polymer-based DNA sensors were developed, and EIS
measurements achieved 25 pM LoD and 30 minutes response
time with an improvement of 50% compared to voltammetric
methods [57]. An impedimetric sensor based on molecular
imprinting for kanamycin detection was fabricated based on
graphene oxide and polymerization of pyrrole [58]. The analysis
by EIS showed 5 nM LoD and no significant interference.
Electrode structures are also fundamental. Novel electrode
designs combining twisted and interdigitated electrodes to
serpentine electrodes significantly increase the sensitivity. Various
techniques such as photolithography, printing, and laser
patterning in flexible and rigid substrates are promising [59].
Different stretchable electrode structures were developed for
wearable impedimetric biosensors [60]. However, several issues
must be overcome, such as selectivity and specificity, to
enhance multifunctionality and repeatability.
Future Trends
Several challenges exist for impedimetric electrochemical sensors
regarding system stability, understanding phenomena in
the corresponding frequency range, shielding, and artifacts,
especially in high Ohmic systems. But at the same time, impedimetric
sensing enables simultaneous determination of
various bio species based on electrodes array as it is label-free
and non-invasive because of the small amplitude perturbation
from steady-state [45].
A future trend can be observed towards combining different
potentiometry methods and impedance biosensing. In this
case, it is possible to measure the same binding molecules/
ions on the same active sensing surface [61]. In addition, highresolution
measurements with a low limit of detection down
to single ion/molecules are realizable using nano-porous
sensors. However, in the future, it is required to make more
progress in this respect.
Also, the direction of impedance measurement is moving
toward the miniaturization of sensing platforms to develop
IS in portable and wearable devices with high accuracy and
cost-effectiveness. Such systems are designed for different applications,
such as points of care and diagnosis of patients,
online contaminants monitoring and corrosion applications
using artificial intelligence. Big data and digitalization will
subsequently play an important role [62].
May 2022
From a nanomaterial point of view and their application
within the impedimetric biosensors, it still in the research
phase, which demands ongoing work to develop new nanomaterials
and nanocomposites for various targets to improve
their biocompatibility, required for some specific applications
such as health care diagnosis and environmental conditions
monitoring [63].
Battery Diagnosis
Impedance spectroscopy is suitable for diagnosis due to its
non-invasiveness and the deep insight it provides into systems,
materials, and intrinsic material structures. Good
examples are batteries [64], wires and connectors [65], and
material corrosion [4]. With the increasing market share of
Li-ion batteries, battery management systems have evolved
from simple voltage measurement circuits to smart circuits
that evaluate both State-of-Charge (SoC) [64] and State-ofHealth
(SoH) [66], [67]. The SoC estimation based on simple
voltage measurements is insufficient due to its non-linear relationship
to SoC and its dependence on other factors such as
temperature, pressure, and aging SoH. Coulomb counting is
the standard in the SoC estimation. It is based on integrating
the current to estimate the change in charge from the battery's
initial charge. In practice, due to sampling uncertainties (e.g.,
timing and jitter, ADC resolution) and environmental noise
and non-linearity of the electronic components in the current
measurement circuit, the measured remaining charge deviates
from the actual value over time. Moreover, the initial battery
charge required in the calculation is sensitive to temperature
and aging, leading to further uncertainties. Kalman-based filters
can address these problems but cause more measurement
confusion due to the oversimplification of the battery's impedance
spectrum.
Impedance spectroscopy provides a potential alternative
for estimating the SoC and SoH. It characterizes the different
battery phenomena as a function of frequencies (diffusion,
charge transfer or intercalation) and provides information
about the battery structure (electrodes, electrolyte, solid electrolyte
interphase layer (SEI)). Therefore, modeling is required
to obtain and interpret these values [3]. This leads to the inference
of SoC and SoH without relying on battery usage history.
Advances
The impedance spectrum reflects the status of the battery. For
instance, the impedance increases, and the porosity decreases
due to diffusion processes, forming a basis for SoH assessment.
These phenomena take place in the low mHz frequency
range (Fig. 5). Similarly, the SoC can be inferred by tracking
the charge transfer and the total impedance [64]. These phenomena
take place in the middle Hz-range frequencies, which
also include the electrode porosity information. The highfrequency
range (> 100 Hz) contains inductive effects due to
connectors and current flow in the battery structure.
While the measurements are usually performed with sophisticated
lab equipment, a more recent trend approach
is towards embedded systems [57]. A significant challenge
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