Ion-selective electrodes (ISEs) have been proven particularly useful in water
analysis. They are usually used as single-rod measuring chains in different
designs, which are manufactured using precision mechanical manufacturing and
assembling technologies. The paper describes a microsystem technology approach
for the fabrication of miniaturized electrochemical sensors. The ceramic HTCC (high-temperature co-fired ceramic)
and LTCC (low-temperature co-fired ceramic) multilayer technology enables suitable processes for the
manufacturing of robust and miniaturized sensor arrays with a high functional
density. Design, manufacture, and electrochemical performance of the novel
ceramic multilayer-based sensor array are presented in the paper using various
examples. An adapted material and process development was carried out for the
sensitive functional films. Special thick-film pastes for the detection of the
pH value as well as NH4+, K+, Ca2+,
and Cu2+ ion concentrations in aqueous solutions were
developed. Ion-sensitive thick-film membranes were deposited on a ceramic
multilayer sensor platform by means of screen-printing. All ISEs, integrated
in the sensor array, showed suitable electrochemical performances including a
very quick response (several seconds) combined with a high sensitivity
(exhibiting Nernstian behaviour) in the tested measuring range. The obtained
sensitivities were around 57 mVperdecade: for the pH sensor,
30 mVperdecade for calcium, 53 mVperdecade for potassium,
and 57 mVperdecade for ammonium. Depending on the application,
different sensitive electrodes on the ceramic sensor array can be combined as
required.
Introduction
Monitoring water quality is essential to protect the environment and health. In
applications like drinking water and wastewater monitoring, intelligent
agriculture, and the food industry, the measurement of different ion
concentrations is required. Users have increasing demands on the sensors
regarding their measuring properties, robustness, miniaturization, geometry
variability, and costs. The sensor requirements are particularly aimed at
functional integration and smart, cheap, miniaturized multi-sensors for mass
applications. According to the state of the technology, usually potentiometric
sensors for ion concentration measurements, so-called ion-selective electrodes
(ISEs), are used. These ISEs are based on various, mostly precision mechanical
manufacturing and assembling technologies. Their manufacture is quite
expensive, and the miniaturization potential is limited. These restrictions
can be overcome by using ceramic multilayer technology: LTCC – low-temperature co-fired ceramic (sintering temperatures at
700–950 ∘C) and HTCC – high-temperature co-fired ceramic
(sintering temperatures >950∘C). The ceramic multilayer
technology was originally developed for the manufacturing of multilayer
printed circuit boards and enables the design of complex three-dimensional
structures in a simple manner, layer by layer. Components made by ceramic
multilayer technology have already conquered broad areas of application in
electronics and microsystem technology.
The manufacturing process of LTCC systems is simple, fast, and inexpensive
(Golonka, 2006). The advantages of LTCC ceramics like high mechanical
strength, high reliability, the ability of making 3D microstructures with
cavities and channels, and a high level of integration and miniaturization (several
sensors in one LTCC module) are responsible for its successful use as a sensor
platform. In a very similar manner, HTCC modules are manufactured (Sebastian
and Jantunen, 2017). HTCC ceramics are used if higher chemical and thermal
stabilities are necessary. HTCC tapes require sintering temperatures above
950 ∘C and need compatible melting point conductors such as
tungsten, molybdenum, or platinum for co-firing. In contrast, lower melting
point conductors like silver, gold, and copper can be used in the LTCC co-firing
process.
The literature shows examples for LTCC-based electrochemical sensors, e.g. a
miniaturized pH sensor based on an Ag / AgCl reference electrode and an iridium
oxide (IrOx) sensing electrode for environmental and food quality
monitoring (Amor et al., 2016). A miniaturized, low-cost pH sensor made of a
commercial RuO2-based paste and
RuO2–Ta2O5 glass paste fabricated on LTCC substrates
for online solution monitoring has been described (Manjakkal et al.,
2016). The performance of the prepared LTCC-based pH electrodes enables their
applicability in the next generation of solid-state pH sensors.
In many cases, electrochemical pH sensors based on metal oxides (MOx) are
used for the measuring of the pH value in food processing, health monitoring,
agriculture, and water quality monitoring, etc. (Manjakkal et al., 2020). Ion-sensitive MOx are alternative materials to glass membranes that are
mainly used for electrochemical pH sensors. Actual MOx-based pH sensors
have attracted significant attention due to their high sensitivity, fast
response time, long lifetime, ease of miniaturization, and stability in
different atmospheres. However, despite their performance, MOx-based pH
sensors are not suitable for mass use due to the high material price of the
MOx (RuO2, IrOx). Due to their lower manufacturing and
processing costs, pH-sensitive glasses are more cost-effective for broad
application.
There are already developments of miniaturized multi-sensors for water quality
monitoring, for example miniaturized water quality monitoring pH and
conductivity sensors (Banna et al., 2014). Interdigitated electrodes were
formed for the pH and conductivity sensors, implemented in a serpentine
channel interface, and covered with hydrogel for the pH sensor. The electrical
properties of the hydrogel change during the swelling and de-swelling process
and are used as indicators of the pH change. A multi-sensor using thick-film
technology for water quality control has been developed on Rubalit alumina
substrate, designed to measure parameters like temperature, conductivity,
redox potential, pH value, and dissolved oxygen (Martinez-Manez et al.,
2005). Here, a thick-film RuO2 electrode was screen-printed and used
for potentiometric pH measurements. Again, a RuO2 sensing electrode
on an alumina substrate was used for water quality assessment by an integrated
multi-sensor (Zhuiykov et al., 2009). The multi-sensor was designed to detect
the pH value, oxygen content, temperature, conductivity, and
turbidity. Furthermore, a miniature multi-parameter sensor chip for water
quality assessment to detect the pH value, conductivity, and temperature was
fabricated using microelectromechanical system (MEMS) techniques and once
more iridium oxide film as the pH-sensing material (Zhou et al., 2017).
To measure heavy metal ion concentrations in water, electrochemical electrodes
based on selective membranes of chalcogenide glass (CG) can be
used. Different authors have described the measuring of Cu2+ ions
with ISEs based on CG (Baker and Trachtenberg, 1971), as well as the synthesis and
physical and electrochemical properties of Cu2+-sensitive
CG materials (Vlasov et al., 1986) and the realization of Cu2+-ion-sensitive CG membranes by means of radio frequency (RF) co-sputtering on ion-sensitive field effect transistors (Vlasov, 1993). This physical procedure
caused problems regarding the stoichiometric deposition of CG membranes that
is of importance for the functionality of the CG-based ISEs. By means of the
process of pulsed laser deposition (PLD), CG membranes with remaining
stoichiometry of glass composition may be manufactured (Schubert et al.,
1999). PLD indicates high equipment costs, while the deposition of thin
CG membranes is very time-consuming.
Thick-film technology is an alternative process for the fabrication of
low-cost miniaturized chalcogenide-glass-based ISEs. The preparation and
results of measurements with screen-printed Cu2+-sensitive
CG electrodes on ceramic have been reported (Enseleit et al., 2018). This work
is also based on thick-film-based ISEs, whereby the manufacturing process of
Cu2+-sensitive CG was optimized. It was possible to melt the glass
at significantly lower temperatures (far below 1100 ∘C).
Ion-selective electrodes based on a screen-printing technique for the determination
of calcium (Goldberg et al., 1994 and Wang et al., 2003), potassium, and
ammonium ions (Goldberg et al., 1994) were studied. The ion-selective
membranes are prepared from a mixture composed of a high-molecular-weight
polymer, a plasticizer, an ionophore, and lipophilic cationic or anionic salts,
dissolved in tetrahydrofuran (THF) as organic solvent. Because of the high
evaporation rate of THF, the viscosity of the paste changes significantly during
screen-printing, which results in drying out of the paste and in blocking of the
screen meshes. This disadvantage was overcome by a simple and cheap method for
the manufacture of solvent polymeric ion-selective membranes for potassium,
ammonium, and nitrate ions on flexible polyester foils (Koncki et al.,
1999). The chemical cocktail has been calculated according to the classical
composition of an ion-selective membrane. In contrast, however, the modified
pastes are manually produced mixing ionophore, plasticizer, and additives in an
insulating ink. In this way, the solvent that evaporates easily is avoided.
This paper presents the development of a ceramic multilayer-based,
cost-effective, and miniaturized sensor array for water analysis in numerous
analytical and industrial processes. Manufacturing processes of thick-film and
ceramic multilayer technology-based electronics are used for sensor
manufacturing. Fine mechanics or precision mechanical manufacturing and
assembling are being replaced by screen-printing and the fabrication of
multiple printed panels, which allows for the automation of sensor production as
well as sensor miniaturization and therefore cost reduction. The outstanding
feature of ceramic multilayer technology is the ability for the manufacturing
of hermetic 3D multi-material ceramic components. In the case of
multilayer-based pH sensors, besides different sensor functions,
electromagnetic shielding is embedded inside the multilayer. The above-mentioned hermetic tightness is important to prevent cross-influences such
as leakage currents. Thus, the ceramic multilayer technology offers a high
potential as a ceramic-based integration platform for various miniaturized
ion-sensitive sensors. This work demonstrates an example of a sensor array for
the determination of the pH value and the concentrations of ammonium,
potassium, calcium, and copper ions in water.
Development and design of the multilayer-based miniaturized sensor arrayCeramic multilayer technology
The manufacture of ceramic multilayers is based on a characteristic technology
sequence. The still unfired, green ceramic tapes are structured using
micromachining processes such as punching, laser cutting, or embossing. After
the vias have been inserted and filled, screen-printing of, for example,
conductors and sensor structures takes place. Then the single, individual
layers are stacked and laminated under pressure at elevated
temperatures. During the subsequent sintering process, the organic components
are burned out, and the green body is densified to a pore-free monolithic
ceramic. Finally, further functional elements of the sensor array are printed
and fired in thick-film technology in the post-firing process. Since the
production takes place in multiple panels, the individual elements must be
separated by laser cutting or wafer sawing.
HTCC-based sensor array
A partially stabilized ZrO2 tape from ESL (Electro-Science Laboratories) was
used to manufacture the ceramic HTCC multilayer-based sensor platform. The
inner metallization structures and vias were screen-printed with platinum
pastes from ESL too. After lamination, the unfired sensor body was sintered at
1400 ∘C. Due to its high mechanical strength and chemical
resistance, the fired ceramic HTCC ZrO2 substrate is very well
suited as the sensor substrate for use in water analysis. In the post-firing
process, the outermost platinum vias were gold-covered (conductor paste, ESL)
to obtain gold pads in the case of the ISEs and silver-covered (conductor
paste, Heraeus) to obtain silver pads for the application in the reference
electrode. The Ag conductor paste (Heraeus) was also used for the contact pads
for sensor connection. All post-fire metallization films were fired at
850 ∘C. The functional layers of the all-solid-state sensors
were subsequently deposited on the top and bottom layers of the multilayer
using screen-printing and were finally sintered.
Figure 1 visualizes a ceramic multilayer with metallization of the inner and
outer layers (interconnects, vias, contact pads) and a possible arrangement of
the ISEs in the sensor array on the top layer to determine the pH value and
potassium, ammonium, and calcium ions. The Cu ISE and reference electrode are
placed on the bottom layer of the sensor array (Fig. 2a). Figure 2b and c
visualize the shielding against electromagnetic fields and inner metallization
for electrical contact of the ISEs, which are all located in inner ceramic
layers. The geometric arrangement of the ISEs is variable. It can be adapted
and changed to the respective measurement needs. All ion-selective
electrodes, which are integrated in the described sensor array, are based on the
potentiometric measuring principle.
The HTCC-based sensor array (CAD drawing) – top view with
ion-selective electrodes, e.g. pH, potassium, ammonium, and calcium ions.
The HTCC-based sensor array (CAD drawing): (a) bottom view with
reference electrode and ion-selective electrode, e.g. copper ion, (b)
shielding against electromagnetic fields, and (c) inner metallization for
potential derivation.
LTCC-based sensor array
The LTCC sensor body was designed comparable to the above-described HTCC
sensor body, whereby the LTCC technology allows for the use of low-temperature
sintering silver and gold pastes. The LTCC-based sensor array was made of a
lead-free LTCC tape (CT708, Fraunhofer IKTS). Compatible co-fired silver pastes
(conductor pastes, DuPont) were used for inner metallization structures,
solder contacts, and vias. Only the external vias for the ISEs were printed
with a compatible co-fired gold paste (conductor paste, Du Pont). In a
post-firing process, the external vias were gold-covered with a gold paste
(conductor paste for post-firing, DuPont) to achieve gold pads in the case of
the ISEs and silver-covered (conductor paste for post-firing, DuPont) to
obtain a silver pad in the case of the reference electrode. The LTCC
multilayer was fired at 860 ∘C. In the subsequent post-firing
process, the sensor films were deposited in the same manner as in HTCC sensor
array.
LTCC is a glass ceramic material whose glass component can react chemically
with aqueous solutions. The reaction of glass in acidic solutions can cause
leaching of the network modifier. The reaction in alkaline solutions causes
dissolution of the glass network. As a sensor substrate, the LTCC must be
chemically resistant over a wide pH range and must be stable against any
solubility or corrosion. The suitability of LTCC CT708 as sensor substrate was
tested by stability investigations in the range of pH 1.68 to pH 12 and in
0.1 M concentrated potassium chloride solutions at
25 ∘C. The chemical stability studies were performed with LTCC
pieces of the same size and continued over a period of 6 months. The samples
were weighed periodically after they have been cleaned with distilled water
and dried for 1 h at 150 ∘C. The resulting mass changes of
the exposure tests are shown in Fig. 3. The maximum mass loss of 0.07 % in the
pH 1.68 solution is very low over the relatively long period. Since the samples
weighed 245 mg on average, a mass loss of 0.07 % means a
mass decrease of 0.17 mg. This value is in the range of the weighing
error of the precision balance used (Satorius Entris 224i-1S, weighing error
≤±0.1mg). Thus, the LTCC CT708 can be considered as being stable in the long term in the investigated pH range.
Mass changes of LTCC CT708 in various media.
Sensor materialspH-sensitive glass
Important selection criteria for a suitable pH-sensitive glass are a coefficient
of thermal expansion (CTE) adapted to the ceramic substrate and a process
ability using thick-film technology. These criteria are met by a
SiO2–Na2O–MgO–MnO2 glass. Figure 4
shows the thermal expansion behaviour of the pH glass bulk material in
comparison to that of the ceramics HTCC ZrO2, LTCC CT708, and
Al2O3 (96 %). The thermal expansion behaviour was
measured with a DIL 402ES/1 thermodilatometer from NETZSCH-Gerätebau
GmbH. The thermal expansion of the pH glass is very similar to that of the
HTCC ceramic, so that densely sintered, crack-free glass films are obtained on
this ceramic. In the case of LTCC CT708, it is also possible to achieve crack-free
glass films, although the LTCC CT708 expands less thermally, and the glass gets
under tension on this ceramic during the firing process. In contrast,
Al2O3 (96 %) as an inexpensive ceramic substrate cannot
be used. Due to the large mismatch in the thermal expansion of glass and
Al2O3, a strong tensile stress acts on the glass during the
firing process, and as a result the glass cracks significantly.
Thermal expansion behaviour of pH glass, HTCC ZrO2, LTCC CT708,
and Al2O3 (96 %).
For the preparation of the thick-film pH electrode, the glass bulk material was
milled to fine glass powder. The glass powder was dispersed in an organic
binder consisting of a solvent and a polymer. The glass paste was subsequently
processed on a three-roll mill to obtain a homogeneous, screen-printable glass
suspension. Afterwards the glass paste was screen-printed over one of the gold
pads on the ceramic multilayer substrate. The printed and dried pH glass film
was fired at 650 ∘C according to the sintering temperature of
the glass. Typical thickness of the deposited glass films was 40 µm.
A chalcogenide glass (CG) containing copper serves as a sensitive electrode
material for Cu2+ ISE. Based on investigations of glass formation in
the quaternary system, CuAgAsSe, a copper-containing chalcogenide glass, was synthesized from the elementary raw materials. The powder mixture
was melted in closed, evacuated quartz glass ampoules at 600 ∘C
for 8 h in a chamber furnace. The melt was then quenched in
air. Studies on the sintering behaviour of the chalcogenide glass showed that
the glass sinters densely at temperatures above 200 ∘C without
the occurrence of sinter blockades by crystallization. The bulk material was
milled into a fine glass powder, dispersed in a binder, and processed into a
printable suspension on the three-roll mill. The copper glass paste was screen-printed over one of the gold pads of the ceramic multilayer substrate. The
copper glass film was finally fired at 230 ∘C according to the
sintering temperature of the copper glass. The thickness of the membranes was
approximately 25 µm.
The ion-sensitive membranes of the Ca2+, K+, and
NH4+ ISEs consist of a polymeric support matrix (silicone rubber)
into which plasticizers (bis(2-ethylhexyl)sebacat – DOS), ionophore
(Ca2+: ETH129, K+: valinomycin, NH4+: nonactin), and
additive (potassium tetrakis(p-chlorophenyl)borate – KTpClPB) are mixed. The
mixture has been calculated according to the classical composition of
polymer-based ion-sensitive electrodes (Koncki et al., 1999). The
suspensions were deposited by manually screen-printing over one of the gold
pads and were cured at room temperature. The final processed sensitive membranes
have a thickness of approximately 20 µm.
Reference electrode
The reference electrode is a second type of electrode for potentiometric
measurements. It is based on the Ag / AgCl / NaCl system, whereby the saturated
chloride reservoir consists of a mixture of NaCl and glass. A
SiO2–B2O3 glass was selected for this. The
screen-printed NaCl glass film was sintered at 400 ∘C. The
thickness of the fired NaCl glass film was about 90 µm. An AgCl
film was screen-printed under the chloride reservoir and sintered at
400 ∘C. To protect the film against leaching, the chloride
reservoir was covered with a polymer film (epoxy resin) using screen-printing. After curing, it was laser-opened to enable the contact with the
measurement solution. The NaCl glass paste and AgCl paste were also
manufactured according to the procedure described for the pH glass paste. The
solid particles were dispersed in an organic binder. The pastes were then
processed on a three-roll mill to form a homogeneous, screen-printable
suspension.
Results and discussion
HTCC- and LTCC-based sensor arrays with varying arrangements of the ISEs were
produced. Figure 5 shows an example of an HTCC multilayer with two variants of
the sensor array on the top of the multilayer. Variant 1 is designed for a
triangular-shaped pH electrode and three other ISEs and variant 2 for a
circular-shaped pH electrode and another ISE. The construction was carried out
on a tile with 55 sensors. After completion of the sensor assembly, the
individual sensor arrays are separated from the multiple printed board by
laser cutting. In Fig. 6, an example of a separated sensor array, with a
reference electrode on the bottom and the pH electrode, Cu2+ ISE, and two
other ISEs like Ca2+, K+, or NH4+ on the
top, is shown.
HTCC multilayer in multiple use with sensor array variant 1
(triangular-shaped pH electrode and three other ISEs) and variant 2
(circular-shaped pH electrode and another ISE) on the top.
Separated HTCC-based sensor array variant 1/ Bottom left:
reference electrode; top right: pH electrode, Cu2+ ISE, and two other
ISEs like Ca2+, K+, or
NH4+ ISEs.
Using LTCC technology, for example, a “simple” miniaturized pH sensor was
designed (Fig. 7). The assembly was manufactured on a tile with 45 sensors
with the pH electrode on the top and the reference electrode on the bottom.
Figure 8 shows the separated sensor.
The ISEs are potentiometric indicator electrodes that respond to the activity
of the type of ion to be determined. Here, potentiometry, as an
electro-analytical method of quantitative analysis, uses the concentration
dependence of the electrochemical potential. The changes of potential of all
ISEs were measured with a Keithley electrometer 6512 with an input resistance
in the tera-ohm (TOhm) range in connection with the saturated silver / silver chloride reference electrode at 25 ∘C. Triaxial cables were used for
the measurement, and a shielding cage was installed around the device under
test. The chip contacts were covered with silicone after soldering the
measuring cable to protect the contacts from moisture by measuring liquid and
to avoid leakage currents. In the following, the measurement results of ISEs
using the circular electrode shape are represented.
pH sensor
Potentiometric measurements with HTCC-based and LTCC-based miniaturized pH
sensors were carried out in four standard buffer solutions of pH 1.68, pH 4.01,
pH 6.86, and pH 9.18. The pH sensors were preconditioned in 0.1 M KCl
solution at least 2 h before the measurements at room temperature (23±2∘C). Regardless of the ceramic material
(HTCC ZrO2, LTCC), the pH sensors show a sensitivity of 57.0±1.0mV/pH, which is in accordance with the Nernst equation for
monovalent ions. One calibration curve of a LTCC-based pH sensor is exemplarily
shown in Fig. 9.
Calibration curve of a LTCC-based pH sensor.
Furthermore, the potential stability continuously over time of the pH sensor
as an important parameter for its practical application was examined. It was
measured in the standard buffer solutions of pH 1.68, pH 4.01, pH 6.86, and pH
9.18 at 25 ∘C. Between the single measurements of the
long-term test, the pH sensor was stored in a 0.1 M KCl solution at room
temperature (23±2∘C). During the study period of
175 d, the pH sensor showed extremely stable potentials in all
investigated buffer solutions at 25 ∘C (Fig. 10).
Potential stability of a LTCC-based pH sensor at 25 ∘C
in standard buffer solutions.
Cu<inline-formula><mml:math id="M109" display="inline"><mml:msup><mml:mi/><mml:mrow><mml:mn mathvariant="normal">2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math></inline-formula> ISE
The thin glass membranes manufactured by screen-printing of chalcogenide glass
paste are copper-ion-sensitive and can be used on various ceramic substrates
(HTCC ZrO2, LTCC). The sensitivity of the CuAgAsSe thick-film
electrode is in the linear measuring range from 10-1 to
10-4M Cu(NO3)2, 0.1 KNO3 solution at 30.5±1.0mV/pCu2+. This value corresponds to Nernst's
behaviour for divalent ions. Figure 11 represents an example of the calibration
curve of a LTCC-based Cu2+ ISE.
Calibration curve of Cu2+ ISE in the measurement range
of 10-1 to 10-4M Cu(NO3)2, 0.1 MKNO3.
The polymer-based ion-sensitive membranes of the K+,
NH4+, and Ca2+ ISEs show suitable electrochemically
properties independent of the ceramic substrate material (HTCC ZrO2,
LTCC). All ISEs show a rapid response (several seconds) and high sensitivity
in the studied measuring range at 25 ∘C (Fig. 12).
The Ca2+ ISE was checked for functionality in
Ca2+-containing solutions of different concentrations. The
sensitivity of the Ca2+ ISE thick-film electrode is in the measuring
range from 10-1 to 10-4M Ca(NO3)2, 0.1 MKNO3 solutions at 29.0±1.0mV/pCa2+. This
value is in accordance with Nernst's behaviour for divalent ions.
The sensitivity of the K+ ISE thick-film electrode is in the measuring
range from 10-1 to 10-4MKNO3, 0.05 MMgSO4 solutions
at 54.5±1.0mV/pK+ and thus exhibits Nernstian behaviour for
monovalent ions.
The NH4+ ISE was checked for its functionality in
NH4+-containing solutions in the concentration range 10-1 to
10-4MNH4Cl, 0.05 MMgSO4. The
sensitivity is in this measuring range at 56.0±1.0mV/pNH4+. This value also corresponds to the
Nernst behaviour for monovalent ions.
Calibration curves of ISEs (K+, NH4+, and
Ca2+) in the measurement range of 10-1 to 10-4M
solution.
Conclusions
Based on different examples, the suitability of HTCC and LTCC multilayer
technology for the manufacturing of ceramic-based miniaturized sensor arrays was
demonstrated. Using this microsystem technology, a platform for the
integration of electrochemical sensors for water analysis has been
developed. Depending on the design, sensor arrays of different customized
layouts can be manufactured. The individual sensitive films are screen-printed
using special thick-film pastes. This included the paste and process
development for the deposition of the ion-sensitive thick-film membranes on
the ceramic sensor platform. The sensitive films on the sensor array are freely
combinable regarding its application. In the presented examples, the
functional proof for the measuring of parameters like the pH value and the Cu2+
heavy metal ion concentration, as well as the concentration of cations like
Ca2+, NH4+, and K+ in aqueous solutions, was
demonstrated. In terms of manufacturing costs, LTCC technology is the more
cost-effective option compared to HTCC technology. Both variants allow for the
manufacturing of smart, robust sensor arrays with a high functional density.
Code availability
All relevant codes presented in the article are stored according to institutional requirements and, as such, are not available online.
Data availability
All data used in this paper can be made available upon request to the authors.
Author contributions
CF and UP created the concept of the project and were responsible for raising the funds. The sensor layouts, all sensor designs, validation, and measurement data analysis were carried out by CF in close consultation with UP. All authors contributed to the review and editing of the final paper. UP supervised the work.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Sensors and Measurement Science International SMSI 2020”. It is a result of the Sensor and Measurement Science International, Nuremberg, Germany, 22–25 June 2020.
Financial support
This research has been supported by the ZIM programme of the German Federal Ministry for Economic Affairs and Energy (grant nos. KF2087359KM4 and ZF4076443RE8).
Review statement
This paper was edited by Andreas Schütze and reviewed by two anonymous referees.
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