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RESEARCH ARTICLE
|
JULY 12 2023
Ion transport in thermally responsive pectin film
Linghui W
ang
;
Tae Hyun Kim
;
Vincenzo Costanza
;
Nicholas J. Higdon
;
Chiara Daraio
Appl. Phys. Lett.
123, 021903 (2023)
https://doi.org/10.1063/5.0142020
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Ion transport in thermally responsive pectin film
Cite as: Appl. Phys. Lett.
123
, 021903 (2023);
doi: 10.1063/5.0142020
Submitted: 10 January 2023
.
Accepted: 26 June 2023
.
Published Online: 12 July 2023
Linghui
Wang,
1
Tae Hyun
Kim,
1
Vincenzo
Costanza,
1
Nicholas J.
Higdon,
2
and Chiara
Daraio
1,a)
AFFILIATIONS
1
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125, USA
2
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
a)
Author to whom correspondence should be addressed:
daraio@caltech.edu
ABSTRACT
The ionic conductivity of CaCl
2
-crosslinked pectin was found to exhibit a record-high temperature response, suggesting its potential applica-
tions in wearable devices and infrared sensors [R. Di Giacomo
et al.
, Sci. Rob.
2
, eaai9251 (2017)]. However, little was known about its ion
conduction mechanisms and the origin of its high-temperature sensitivity. In this study, we perform controlled experiments and identify cal-
cium ions as the dominant current carriers. By analyzing infrared spectra at different temperatures, we find that the temperature response is
due to changes in ion mobility, rather than variations in ion number density. We compare measurements and modeling results of nine differ-
ent multivalent ions and find a positive correlation between their temperature responses and their binding energy to pectin. While these find-
ings are fundamental in nature, they provide relevant guidance for the future design of temperature-sensitive polymers and other materials
for organic electronics.
Published under an exclusive license by AIP Publishing.
https://doi.org/10.1063/5.0142020
Thermal sensing technologies have been developed for a wide
range of applications, including wearable devices, robotics, infrared
sensing, and internet of things.
2–4
Recent reports discovered a new
type of thermal sensing material:
1
CaCl
2
-crosslinked pectin. Its electri-
cal conductivity is highly sensitive to temperature changes, over a wide
range of temperatures. Compared to the state-of-art temperature sens-
ing materials, such as vanadium oxide, pectin has been found to be at
least an order of magnitude higher in its response.
1
Furthermore, pec-
tin is transparent, solution processable, biodegradable, inexpensive,
and lightweight, making it a great candidate for large-scale tempera-
ture mapping applications, such as electronic skins.
Pectin is a structural polysaccharide, which can be extracted from
plant cells. It has been frequently used in the food and pharmaceutical
industry as gelling agents. It is polydisperse and has a complex struc-
ture.
5
The pectin backbone is mainly composed of homogalacturonan,
rhamnogalacturonan I (RG I), and II (RG II). RG I and RG II with
complicated side chains, making it difficult to understand the exact
molecular structure.
6
However, homogalacturonan, which consists of
at least 65% of the pectin molecule, is composed of a relatively simple
linear chain of 1,4-linked
a
-d-galacturonic acids with C-6 carboxyl
groups partially esterified.
7
When the degree of esterification is low,
calcium ions can bind with carboxyl groups of the pectin chain result-
ing in the gelation of pectin solutions.
8
This binding interaction has
been experimentally observed using Fourier transform infrared spec-
troscopy (FTIR).
9,10
Previous simulation studies also found that
calcium ions can bind with carboxyl groups on the pectin chain to
form an egg-box-like structure, as shown in
Fig. 1
. Some studies found
that in a similar system, hydroxyl groups also participate in ion coordi-
nation. When water molecules are present, calcium ions are partially
solvated by water.
11,12
Other divalent ions, such as copper and zinc
ions, have also been shown to bind with pectin to different degrees.
13
In previous studies, it has been shown that the interaction
between calcium chloride and pectin is essential to temperature sensi-
tivity.
1
However, little is known about pectin’s ion transport mecha-
nisms and the origin of its temperature response. The lack of
knowledge about the underlying molecular mechanisms governing
petcin’s temperature response hinders the optimization of this poly-
electrolyte for sensing applications. Nevertheless, probing the ion
transport mechanisms in this biopolymer is challenging. First of all,
the composition and the structure of the RG section of pectin are
extremely complicated and not well understood. This renders nuclear
magnetic resonance spectroscopy and many other spectroscopic
results noisy and hard to decipher. Second, multiple factors could
affect the interaction between cations and pectin. Type of ions, pH,
degree of acetylation, and distribution pattern of carboxyl groups all
can have a great impact on the ion–pectin coordination geometry and
pectin’s binding capacity.
14
Third, there are multiple types of mobile
ions present in the system: calcium ions, chloride ions, and protons
from carboxyl groups. They all could be contributing to the overall
ionic current and temperature sensitivity.
Appl. Phys. Lett.
123
, 021903 (2023); doi: 10.1063/5.0142020
123
, 021903-1
Published under an exclusive license by AIP Publishing
Applied Physics Letters
ARTICLE
pubs.aip.org/aip/apl
27 October 2023 22:01:04
This paper focuses on the identification of the dominant current
carrier in pectin and elucidates the causes of its high-temperature sen-
sitivity. By comparing electrical responses of systems with different
amounts of chloride ions and protons, we found that calcium ions are
the main current carrier in CaCl
2
-crosslinked pectin. To understand
the origin of the temperature sensitivity, we studied the electrical
responses of pectin films crosslinked with nine types of multivalent
ions. Binding interactions between these ions and pectin were also
investigated via infrared spectroscopy and density functional theory
(DFT) modeling. Combining these results, we conclude that the tem-
perature sensitivity is correlated with the binding energy between the
metal ion and pectin.
First, we identified the main charge carrier responsible for the
large temperature response. Three types of mobile ions present in the
system were considered: (i) calcium ions, (ii) chloride ions, and (iii)
protons from the carboxyl group. Assuming ideal electrolytes, the ion
conductivity could be written as the sum of contributions from all
mobile ions
r
ð
T
Þ¼
X
i
r
i
ð
T
Þ¼
X
i
n
i
ð
T
Þ
l
i
ð
T
Þ
q
i
;
(1)
where
n
i
,
l
i
,and
q
i
are the number density, the mobility, and the
charge of the
i
th type of mobile ions. T refers to the temperature. In
general, both number densities and mobilities can depend on tempera-
ture, and the temperature response in this paper is defined as
r
ð
T
1
Þ
=
r
ð
T
2
Þ
where
T
1
>
T
2
. If protons are the dominant current car-
riers, then the temperature response of CaCl
2
-crosslinked pectin
should be similar to the uncrosslinked pectin, where protons are the
only possible current carrier. However, previous studies found that the
temperature response of uncrosslinked pectin films is less than one-
fourth of the temperature response of CaCl
2
-crosslinked pectin.
1
To
further separate the contribution of (i) calcium ions from (ii) chloride
ions and (iii) protons, we performed controlled experiments compar-
ing the electrical properties of samples containing two different con-
centrations of chloride ions and protons. Pectin solution was drop cast
on the gold electrodes [
Fig. 2(a)
] and then crosslinked with CaCl
2
solution (see the supplementary material for more details). One sam-
ple was immersed in DI water for a day to remove the chlorides and
unbound protons. Calcium ions that deprotonated the carboxylic acid
groups formed strong coordination with the negatively charged
carboxylate groups that remained during the DI rinse. Both samples
were dehydrated and subjected to a gold-dissolution electrochemical
test [
Fig. 2(b)
]. By applying 1 V of voltage to the pectin-deposited elec-
trodes for 7 h at room temperature, the surface of the gold anode was
monitored. As expected, the unwashed samples turned red due to the
formation of auric chloride from the reaction between gold and chlo-
ride ions in the acidic environment,
15
while no color change was
observed from the DI rinsed samples. This confirmed that the rinsed
pectin film contained significantly fewer chloride ions. Moreover, since
this system must be electrically neutral after rinsing, the same number
of protons must have been removed. Finally, the electrical responses of
both rinsed and unrinsed samples were characterized by procedures
described in the supplementary material. Their electrical conductance
at varying temperatures is plotted in
Fig. 2(c)
. No significant difference
in the electrical response was observed, suggesting that chloride ions
and protons are not the dominant current carriers that cause the high-
temperature response in the system.
Although we confirmed that calcium ions play an important role
in pectin’s high-temperature response, questions remain on what
mechanisms gives rise to pectin’s high-temperature response. Both the
FIG. 2.
(a) Schematic diagram of samples fabricated for electrical measurements.
Pectin was deposited on the gold-interdigitated electrodes and then crosslinked
with salt solutions. After dehydration, it was then sealed with polyimide tape. (b)
Optical microscopy images of the gold interdigitated electrodes. The first (second)
row corresponds to the electrodes before (after) the gold-dissolution electrochemi-
cal test. Compared to the first column, the sample measured using the electrodes
in the second column was washed in de-ionized water before measurements. (c)
The electrical properties of the washed sample (red circle) compared to the
unwashed one (blue square). No significant difference was observed. The error
bars are on the order of graph point size.(d) FTIR spectra of pectin cross-linked
with five concentrations of CaCl
2
: 0, 30, 80, 100, and 300 mM. Dotted lines corre-
spond to three fitted Gaussian peaks to the spectrum of pectin cross-linked with
300 mM CaCl
2
between 1550 and 1800 cm
–1
.
FIG. 1.
(a) Schematic diagram showing divalent-cation crosslinked pectin. The
divalent cations (orange) can deprotonate polygalacturonic acid and cross-link mul-
tiple polymer chains to form a connected network. The proton (light blue) disassoci-
ated from the galacturonic acid can form ion pairs with the anion (green) from the
cross-linking salt, when the water content in the network is low. (b) Coordination
between calcium ion and galacturonates.
Applied Physics Letters
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Appl. Phys. Lett.
123
, 021903 (2023); doi: 10.1063/5.0142020
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, 021903-2
Published under an exclusive license by AIP Publishing
27 October 2023 22:01:04
number density and the mobility of the current carriers could be tem-
perature dependent and contribute to the thermal response. We first
inspect the possibility of the charge carrier density being responsible
for the pectin’s temperature response. In general, the number density
of unbound calcium ions is expected to increase with increasing tem-
perature.
16
Since these unbound calcium ions are much more mobile
than the bound calcium ions, the number density would be expected
to increase, leading to higher conductivity. To test such hypothesis, we
monitored how the number density of the bound calcium ions changes
with respect to the applied voltage and temperature using infrared (IR)
spectra. Three major absorption peaks were present from the cross-
linked pectin in the range between 1500 and 1800 cm

1
. Each corre-
sponded to the asymmetric stretch of the carboxyl group (1733 cm

1
),
the asymmetric stretch of the metal–ion-bound carboxylate group
(1593 cm

1
), and the scissoring bend of the water molecules
(1620 cm

1
).
9,10
As shown in
Fig. 2(d)
, with increasing concentration
of cross-linking CaCl
2
, the peak corresponding to the metal–ion-
bound carboxylate group rises while the peak corresponding to the
carboxyl group diminishes. Therefore, the concentration of bound cal-
cium ions could be measured using IR spectra. Interestingly, all four
IR spectra of the CaCl
2
crosslinked pectin that were measured under
different temperatures and conditions overlapped [
Fig. 3(a)
], sugges-
ting that no significant change in the number density of bound/free
calcium ions had occurred. Therefore, we can conclude that pectin’s
thermal response originates from the change in ion mobility.
We postulate that the steep dependence of ion mobility on tem-
perature arises from the strong binding strength between the calcium
ions and pectin. With an applied electric field [
Fig. 3(b)
], bound cal-
cium ions can hop to another site by crossing energy barriers [
Fig.
3(c)
]. At elevated temperatures, this hopping frequency increases due
to an increase in thermal energy resulting in a higher ion mobility and
ion conductivity. Such phenomenon can be explained using the
Arrhenius theory.
17,18
To verify our claim, the electrical properties and
binding strengths of pectin samples crosslinked with different multiva-
lent salts were evaluated.
For electrical characterizations, we prepared samples crosslinked
with nine multivalent salts solution with the same concentration
30 mM: CaCl
2
,CuCl
2
,FeCl
2
,FeCl
3
,BaCl
2
,CoCl
2
, ZnCl
2
,AlCl
3
,and
MgCl
2
. Each sample was deposited on the electrode surfaces. We
applied a 100 mV step voltage and measured the current as a function
of time. The ionic current value was extracted from the time-
independent plateau region (before the charge polarization was fully
developed).
19
The temperature was cycled from 18 to 47

C. The tem-
perature response is defined as
r
45
8
C
=
r
20
8
C
¼
I
45
8
C
=
I
20
8
C
,where
r
45
8
C
;
r
20
8
C
;
I
45
8
C
,and
I
20
8
C
are the electrical conductivity and electri-
cal current magnitude at 45 and 20

C, respectively. From the tempera-
ture responses measured over different cross-linking salts [
Fig. 4(a)
], we
observed that the temperature response is highly dependent on the type
FIG. 3.
(a) Infrared spectra of CaCl
2
crosslinked pectin film taken at 24

C, with no
voltage (light blue), 24

C with 3 V applied across the sample (dotted dark blue),
40

C with no voltage (red), and 40

C with 3 V applied across the sample (orange)
the red line. (b) Schematic illustration showing how calcium ions hop from one sol-
vation site to another in pectin chains. (c) Schematic showing the free energy sur-
face for a calcium ion at a solvation site and its hopping activation energy E
A
.
FIG. 4.
(a) Temperature responses of pectin samples crosslinked by nine different
multivalent ions. Error bars are the standard deviations of at least three indepen-
dent samples. The salts are ordered such that the ion size increases from left to
right. (b) Arrhenius plots of pectin films crosslinked by the nine different multivalent
cations. Slopes of fitted lines are summarized in Table S1. The symbols used in the
curves serve only to distinguish between different curves and are not indicative of
data points. (c) Binding curves for Ca
2
þ
,Fe
2
þ
, and Cu
2
þ
. The inset shows a typi-
cal fitting of a FTIR with four Gaussian peaks (shown as cyan, pink, blue, and pur-
ple curves). The red dashed line corresponds to the overall fitting, and the yellow
curve is the experimentally measured absorption data. (d) Activation energy
E
A
is
compared with the binding energy
E
B
obtained from DFT computation. The error
bar for the copper ion is smaller than the graph point size.
Applied Physics Letters
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, 021903 (2023); doi: 10.1063/5.0142020
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, 021903-3
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of cross-linking metal ion, providing additional evidence to our previous
conclusion that metal ions are the dominating current carriers. Also,
copper shows the highest response value compared to other ions.
However, no apparent trend was recognized between the temperature
response and the size of ions or their valency. Nevertheless, all samples
followed the Arrhenius relation:
I
¼
I
o
exp
ð
E
A
=
k
B
T
Þ
,where
E
A
is the
activation energy for hopping,
k
B
is the Boltzmann constant, T is the
temperature, and
I
0
is the pre-exponential factor, respectively [
Fig. 4(b)
].
Such ion transport behavior in polymeric systems indicates independent
ion hopping, without coupling with the polymer chains’ segmental
motion.
17,20
Finally, the activation energy from each ion was extracted
[
Fig. 4(d)
]. Since the response is defined as a current ratio, temperature
responses are exponentially dependent on activation energies.
To characterize and compare the affinity of ions to pectin, bind-
ing curves of CuCl
2
,CaCl
2
,FeCl
2
,andMgCl
2
crosslinked pectin were
measured using infrared spectroscopy [
Fig. 4(c)
]. Binding curves
explain how the percentage of bound metal ions (i.e., the binding ratio)
changes with the concentration of the cross-linking solution. To obtain
the binding ratios, Gaussian peaks were fitted to each FTIR spectrum
between 1500 and 1850 cm

1
[
Fig. 4(c)
]. For simple reactions, i.e.,
M
þ
L
ML, and independent binding events, the dissociation con-
stant
K
d
equals to the corresponding [L] when
f
ratio
¼
0.5.
21
In
these cases, the binding constant
K
a
¼
1
=
K
d
and binding free energy
D
G
¼
k
B
TlnK
a
can be calculated easily. In the metal crosslinked
pectin system, the binding process could be more complicated, includ-
ing multiple binding steps and correlations between adjacent binding
sites. However, to qualitatively compare the binding affinity between
different ions, we use
½
L

f
¼
0
:
5
as an estimate of
K
d.
22
We found that the
binding affinity decreases as follows: Cu
2
þ
>
Fe
2
þ

Ca
2
þ

Mg
2
þ
[
Fig. 4(c)
]. This order matches their corresponding order in tempera-
ture responses.
For further evaluation, binding energies between divalent ions
and galacturonate were calculated using density functional theory
(DFT) with ORCA package
23
(see the supplementary material for
more details). For each calculation, a negatively charged galacturonate
molecule, a metal ion, and six water molecules were included in the
simulation. Finally, the binding energy was estimated using the follow-
ing equation:
24
D
E
¼
E
Gal

M
2
þ

6H
2
O

E
Gal


6H
2
O

E
M
2
þ
;
(2)
where
E
Gal

M
2
þ

6H
2
O
corresponds to the energy of the optimized galac-
turonate–metal–ion–water complex.
E
Gal


6H
2
O
is the single point
energy of Gal


6H
2
O calculated at the optimized geometry of
Gal

M
2
þ

6H
2
O. To confirm whether our model can properly repre-
sent the binding interaction between the metal ions and pectin in the
metal–ion-crosslinked pectin system, frequency calculations were per-
formed to generate the corresponding infrared spectra. The calculated
spectra showed qualitative agreement with experimental results
regarding the locations of symmetric and antisymmetric vibration
peaks of the metal-bound carboxylate group, at around 1420 and
1590 cm

1
, respectively
25
[Fig. S1(a)]. In the region below 1500 cm

1
,
called the “a fingerprint region” for polysaccharides, many peaks that
correspond to glycosidic linkage vibration are not present in the simu-
lated spectra, because only one monomer is included in the DFT cal-
culation.
9
In the region above 1500 cm

1
, peaks corresponding to
metal-bound carboxylate groups and water were observed in both IR
spectra. The concentration ratio of these molecules used in the DFT
calculation does not necessarily match the one observed experimen-
tally, and thus, the relative peak heights are not expected to agree
quantitatively. Due to the relativistic quantum effect that was not
implemented in our calculation, the binding energy calculated from
barium ions was inaccurate and excluded from the comparison.
26
Figure 4(d)
shows the binding energies between metal ions and galac-
turonate E
A
obtained from the DFT simulations, plotted against the
activation energies E
B
extracted from the Arrhenius plots in
Fig. 4(b)
.
Overall, a positive correlation was observed. However, compared to
Ca
2
þ
,Mg
2
þ
showed surprisingly high binding energy with galacturo-
nate, a result not observed in our experiments. This incongruity
between experiments and numerical results is due to the high affinity
of Mg
2
þ
to water molecules. Earlier studies also found that Mg
2
þ
has
a much higher binding affinity to polygalacturonate than Ca
2
þ
.
13
However, Mg
2
þ
only polycondensates on polygalacturonic acid
instead of cross-linking, because it is energetically unfavorable for
Mg
2
þ
to release water molecules from its solvation shell to bind with
galactronate.
13
In our simulations, no ions bind with the hydroxyl
group on the galacturonate molecule. This is because only one galac-
turonate molecule was included in the model, and it was sterically hin-
dered for an ion to bind to both the carboxylate and hydroxyl groups.
However, metal ions do coordinate with the hydroxyl groups in water
molecules. With more galaturonate molecules present in the simula-
tions, metal ions would also likely bind to the hydroxyl groups from
other galacturonate molecules, as observed in other similar systems.
12
Considering the positive correlation observed between the activation
and binding energies, from both experimental and simulation results,
we conclude that the highly temperature dependent ion conductivity
originates from the tight binding between calcium ion and pectin.
Similar results were also demonstrated in a recent report, describing a
polyelectrolyte similar to pectin, composed of 2-hydroxyethyl acrylate
(HEA) and acrylic acid (AA).
27
These findings further confirmed that
the binding of calcium ions to carboxyl groups and hydroxyl groups is
key to achieving high responsivity.
In this study, we elucidate the ion transport mechanism in dehy-
drated calcium pectinate. We identify calcium ions as the main current
carrier of the system and determine that conductivity is dominated by
ion hopping, which is independent from the polymer segmental relax-
ation. We also demonstrate that the temperature dependency origi-
nates from changes in charge carrier mobility, rather than its number
density. In addition, we show that the high activation energy is related
to the strong binding between cation and galacturonate, specifically
the carboxyl and hydroxyl groups. Finally, we highlight that Cu
2
þ
cross-linked pectin is three times more temperature responsive than
Ca
2
þ
cross-linked pectin, and therefore, the use of copper pectinate
can significantly improve the thermal sensing performance for future
developments and applications.
See the supplementary material for additional details on the sam-
ple fabrication, electrical measurements, infrared spectroscopy, and
DFT calculation.
This research was funded in part by the Samsung Electronic
GRO Program and in part by the HMRI program at Caltech. We
are grateful to the Molecular Materials Research Center (MMRC) in
the Beckman Institute of the California Institute of Technology
(Caltech) for access to their experimental facilities. We thank
Applied Physics Letters
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Appl. Phys. Lett.
123
, 021903 (2023); doi: 10.1063/5.0142020
123
, 021903-4
Published under an exclusive license by AIP Publishing
27 October 2023 22:01:04
Dr. Bruce S. Brunschwig from MMRC and Dr. Yeong Suk Choi
from Samsung for useful discussions. The computations presented
here were conducted in the Resnick High Performance Computing
Center, a facility supported by the Resnick Sustainability Institute at
the California Institute of Technology.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Linghui Wang:
Conceptualization (lead); Data curation (lead);
Formal analysis (lead); Investigation (lead); Methodology (lead);
Software (lead); Validation (lead); Visualization (lead); Writing –
original draft (lead); Writing – review & editing (lead).
Tae Hyun
Kim:
Data curation (supporting); Methodology (supporting);
Resources (supporting); Writing – review & editing (supporting).
Vincenzo Costanza:
Conceptualization (supporting); Resources (sup-
porting); Validation (supporting); Writing – review & editing (sup-
porting).
Nicholas J. Higdon:
Formal analysis (supporting);
Validation (supporting); Writing – review & editing (supporting).
Chiara Daraio:
Conceptualization (equal); Formal analysis (equal);
Funding acquisition (equal); Investigation (equal); Methodology
(equal); Project administration (equal); Resources (equal); Supervision
(equal); Validation (equal); Writing – review & editing (equal).
DATA AVAILABILITY
Most of the data support the findings of this study are available
within the paper. The data that support the findings of this study are
available from the corresponding author upon reasonable request.
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Applied Physics Letters
ARTICLE
pubs.aip.org/aip/apl
Appl. Phys. Lett.
123
, 021903 (2023); doi: 10.1063/5.0142020
123
, 021903-5
Published under an exclusive license by AIP Publishing
27 October 2023 22:01:04