Universidade de São Paulo
Biblioteca Digital da Produção Intelectual - BDPI
Departamento de Física e Ciência Interdisciplinar - IFSC/FCI
Artigos e Materiais de Revistas Científicas - IFSC/FCI
2014-02
Ionically conducting Er3+-doped DNA-based
biomembranes for electrochromic devices
Electrochimica Acta, Amsterdam : Elsevier, v. 120, p. 327-333, Feb. 2014
http://www.producao.usp.br/handle/BDPI/50690
Downloaded from: Biblioteca Digital da Produção Intelectual - BDPI, Universidade de São Paulo
Electrochimica Acta 120 (2014) 327–333
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Ionically conducting Er3+ -doped DNA-based biomembranes for
electrochromic devices
R. Leones a , M. Fernandes b , F. Sentanin c , I. Cesarino c , J.F. Lima d , V. de Zea Bermudez b ,
A. Pawlicka c , C.J. Magon d , J.P. Donoso d , M.M. Silva a,∗
a
Centro de Química, Universidade do Minho, Gualtar, 4710-057 Braga, Portugal
Departamento de Química e CQ-VR, Universidade de Trás-os-Montes e Alto Douro, 5000-801 Vila Real, Portugal
c
IQSC, Universidade de São Paulo, 13566-590 São Carlos, SP–Brazil
d
IFSC, Universidade de São Paulo, POBox 369, 13566-590, São Carlos, SP, Brazil
b
a r t i c l e
i n f o
Article history:
Received 7 June 2013
Received in revised form 4 December 2013
Accepted 4 December 2013
Available online 22 December 2013
Keywords:
DNA
Ionic conductivity
Natural polymer
Rare earth
a b s t r a c t
Biopolymer-based membranes have particular interest due to their biocompatibility, Biodegradability,
easy extraction from natural resources and low cost. The incorporation of Er3+ ions into natural macromolecule hosts with the purpose of producing highly efficient emitting phosphors is of widespread
interest in materials science, due to their important roles in display devices. Thus, biomembranes may be
viewed as innovative materials for the area of optics. This paper describes studies of luminescent material
DNA-based membranes doped with erbium triflate and demonstrates that their potential technological
applications may be expanded to electrochromic devices. The sample that exhibits the highest ionic conductivity is DNA10 Er, (1.17 × 10−5 and 7.76 × 10−4 S.cm−1 at 30 and 100 ◦ C, respectively). DSC, XRD and
POM showed that the inclusion of the guest salt into DNA does not change significantly its amorphous
nature. The overall redox stability was ca. 2.0 V indicating that these materials have an acceptable stability
window for applications in solid state electrochemical devices. The EPR analysis suggested that the Er3+
ions are distributed in various environments. A small ECD comprising a Er3+ -doped DNA-based membrane was assembled and tested by cyclic voltammetry and chronoamperometry. These electrochemical
analyses revealed a pale blue color to transparent color change and a decrease of the charge density from
-4.0 to -1.2 mC.cm−2 during 4000 color/bleaching cycles.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Solid polymer electrolytes (SPEs) are interesting materials as
they can be used in solid state batteries [1] and other electrochemical devices [2,3] due to their high energy density, high ionic
conductivity and electrochemical stability. SPEs are defined as
host polymers with conducting ions under the effect of an electric field [4–6].Different host polymers have been used for SPEs,
among which poly(ethylene) (PE) [7], poly(propylene) (PP) [8],
poly(ethylene oxide) (PEO) [9,10] and poly(acrylonitrile) (PAN)
have been the most widely explored [9,11]. Among these PEO
seems to have the best properties although the corresponding SPEs
present ionic conductivity values [12]. To solve this problem and
thus produce PEO-based systems with high ionic conductivities,
some strategies, such as the incorporation of plasticizers and the
production of polymer blends have been proposed [13,14]. These
modifications lead to the reduction of the crystalline fraction and
∗ Corresponding author.
E-mail address: [email protected] (M.M. Silva).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.12.006
to the increase of the polymer segmental mobility, which result in
a larger charge carrier content and mobility for ionic transport.
More recently new types of electrolytes based on natural polymers (like cellulose derivatives, chitosan, gelatin, starch or natural
rubber) have been proposed due to their biodegradability, low
production cost, good physical and chemical properties and good
performance as SPEs [15–17]. DNA has been investigated for several electrochemical applications [18,19] and can be relatively easy
Extracted from the fish industry waste, generating a high molecular weight material, which can be transformed into free-standing
transparent membranes [18]. From the fundamental point of view,
DNA is an interesting molecule due to the presence of heteroatoms
in its structure, able to complex Li+ ions, protons or anions, or
even release the Na+ ions and thereby promote ionic conduction in
these systems [20,21]. It can also show ion conduction properties
in association with ionic liquids [22] and/or exhibit electrochromic
properties upon alkyl viologen addition [23].
As mentioned above the motivation for the development of
SPEs is associated with their possible application in practical electrochemical devices including electrochromic windows. The main
advantages of SPEs when compared with ceramic systems are: (i)
328
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
Table 1
Samples composition.
Sample name
DNA (g)
Er(CF3 SO3 )3 (g)
Glycerol (g)
DNA matrix
DNA50 Er
DNA25 Er
DNA16.7 Er
DNA12.5 Er
DNA10 Er
0.5
0.5
0.5
0.5
0.5
0.5
0.00
0.01
0.02
0.03
0.04
0.05
0.5
0.5
0.5
0.5
0.5
0.5
easy processing such as flexible thin films and easy processing on
large areas; (ii) no need for separators; (iii) good thermal and potential stability windows; (iv) wide operating temperature range; (v)
low chemical reactivity; (vi) good safety as well as good optical
contrast, memory effect, image stability, tolerance to shock, vibration and mechanical deformation properties; (vii) possibility to act
as binder because they facilitate the good electrical contact with
the electrodes/good adherence to the neighbouring electrochromic
layers leading to compact large area device fabrication and (viii) frequently enhanced endurance to varying electrode volume changes
cycling [24–27].
In this paper we report the properties of electrolytes synthesized through the addition of controlled quantities of europium
triflate (Er(CF3 SO3 )3 ) to DNA host networks prepared by the solvent casting method. The samples were examined by means of
differential scanning calorimetry (DSC), X-ray diffraction (XRD),
complex impedance spectroscopy, cyclic voltammetry (CV), polarized optical microscopy (POM), atomic force microscopy (AFM) and
electron paramagnetic resonance (EPR). In addition, a preliminary
evaluation of the performance of an all solid-state ECD including as
electrolyte a sample belonging to the DNA-Er3+ family was carried
out.
2. Experimental
and at a heating rate of 5 ◦ C.min−1 using a Mettler DSC 821e. Samples were transferred to 40 ␮L aluminium cans with perforated lids
within a dry argon-filled glove box.
2.2.3. Electrochemical stability
The evaluation of the electrochemical stability window of electrolyte compositions was carried out under an argon atmosphere
using a two-electrode cell configuration. The preparation of a
25 ␮m diameter gold microelectrode surface, by polishing it with
a moist cloth and 0.05 ␮m alumina powder (Buehler) was completed outside the dry box. The cell was assembled by locating
a clean lithium disk counter electrode (cut from Aldrich, 99.9%,
19 mm diameter, 0.75 mm thick) on a stainless steel current collector and centring a sample of electrolyte on the electrode
surface. 2 ␮L of THF was placed on the microelectrode surface
and the microelectrode was then located on the electrolyte surface and supported firmly by means of a clamp. The use of
THF to soften the electrolyte was necessary to achieve a reproducible microelectrode/electrolyte interfacial contact. An Autolab
PGSTAT-12 (Eco Chemie) was used to record cyclic voltammograms
at scan rates of 10, 20, 40, 60, 80 and 100 mV.s−1 . Measurements were performed at room temperature and within a Faraday
cage.
2.2.4. X-ray diffraction (XRD)
XRD measurements were performed at room temperature with
a PANalytical X‘Pert Pro equipped with a X‘Celerator detector using
monochromated CuK␣ radiation with ␭ = 1.541 Å over the 2␪ range
between 10 and 60◦ .
2.2.5. Polarized optical microscopy (POM)
POM images were recorded in an OPTIKA B-600 Pol microscope.
The images were obtained through a digital camera 8Mpixel Digital
Photo and were analysed with an Optika Vision Pro software.
2.1. Polymer electrolyte preparation
Salmon sperm-DNA (0.5 g, Ogata Research Laboratory, Japan)
was dispersed in 20 mL of Milli-Q® water and heated under magnetic stirring for a few minutes and up to 60 ◦ C for a complete
dissolution. To this solution different quantities of Er(CF3 SO3 )3 [28]
ranging from 0.01 to 0.05 g, and 0.5 g of glycerol (Himedia, 99.5%),
acting as plasticizer, were added. The resulting solution was poured
onto Petri plates and let to dry up at room temperature to form
transparent membranes. The membranes were then transferred to
an oven at 60 ◦ C and aged for 8-12 days. The composition of the
samples is depicted in Table 1.
2.2.6. Atomic force microscopy (AFM)
The atomic force microscopy (AFM) images were obtained with
Nanosurf easyScan 2 AFM System (Nanosurf AG, Switzerland). In all
AFM analyses the noncontact mode was employed by using silicon
AFM probes with a force constant of 48 N.m−1 and the resonance
frequency was 190 kHz.
2.2.7. Electron paramagnetic resonance (EPR)
The X-band cw-EPR spectra were recorded at 6 K on a Bruker
Elexsys E580 spectrometer operating at 9.478 GHz, equipped with
a continuous flow liquid helium Oxford cryogenic system.
2.2. Measurements
2.2.1. Ionic conductivity
The total ionic conductivity of the samples was determined
by locating an electrolyte disk between two 10 mm diameter ion-blocking gold electrodes (Goodfellow, > 99.95%) to form
a symmetrical cell. The electrode/polymer electrolyte/electrode
assembly was secured in a suitable constant volume support and
installed in a Büchi TO51 tube oven with a K-type thermocouple
placed close to the electrolyte disk to measure the sample temperature. Bulk conductivities of the electrolyte samples were obtained
during heating cycles using the complex plane impedance technique with an Autolab PGSTAT-12 (Eco Chemie) between 25 and
100 ◦ C and at approximately 7 ◦ C intervals.
2.2.2. Differential Scanning Calorimetry (DSC)
Samples from dry membranes were subjected to thermal analysis under a flowing argon atmosphere between -60 and 200 ◦ C
2.2.8. Electrochromic cell assembly
Electrochromic devices with the glass/ITO/WO3 /electrolyte/
CeO2 -TiO2 /ITO/glass configurations were obtained by assembling
the 2 pieces of coated glasses. Electrolytes were located on
glass/ITO/WO3 substrate and 1 cm free space was left for the electrical contact. Then the other CeO2 -TiO2 /ITO/glass substrate was
pressed onto the membrane in such a way that the WO3 and CeO2 TiO2 faced each other inside the assembled window. A 1 cm wide
Cu-conducting tape (3 M® ) was glued to the free edge of each substrate for electrical connection. The windows were finally sealed
with a protective tape (3 M® ) to avoid the air humidity.
The electrochemical measurements of ECDs were performed
with an Autolab 302 N apparatus. The cyclic voltammetry measurements were performed in the −2.0 - +1.8 V range and scan velocity
of 50 mV s−1 . The chronoamperometric measurements were
performed by applying a square wave potentials of -2.0 and +1.8 V
for 15 s each.
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
Fig. 1. Variation of the ionic conductivity of DNAn Er polymer electrolytes with
inverse temperature. (Inset: Physical appearance of the DNA10 Er sample).
3. Results and discussion
Fig. 1 shows the variation of the ionic conductivity of the DNAbased materials with 1/T. The ionic conductivity (␴i ) was calculated
according to:
i =
d
Rb A
(1)
where Rb is the bulk resistance, d is the thickness and A is the
area of the sample. Rb is obtained by interception of the imaginary part of the impedance (minimum value of Z”) with the slanted
line in the real part of the impedance (Z’). It is possible to observe
that electrolytes exhibit ionic conductivity results similar to those
of comparable materials based on PEO [26,28] and one order of
magnitude higher than those based on d-U(600) [27,29]. The most
conducting sample, DNA10 Er, exhibits an ionic conductivity equal
to 1.17 × 10−5 S.cm−1 at 30 ◦ C and 7.76 × 10−4 S.cm−1 at 100 ◦ C.
If practical applications in electrochemical devices are envisaged,
high ionic conductivity (higher than 10−5 S.cm−1 [28,30]), is definitely one of the most important requirements that need to be
fulfilled. In the present case this reference value is exceeded by
DNA10 Er. Besides that and, as it can be seen in the inset of Fig. 1,
the samples studied here were obtained in the form of transparent
and flexible membranes, a clear advantage for their application in
see-through and/or flexible devices.
The ionic conductivity usually depends strongly on many factors, such as the types of cation and anion, salt concentration,
temperature, etc. [5]. This dependence gives information on the
329
specific interaction between the salt and the polymer matrix. The
increase in the ionic conductivity with the increase of salt concentration is related to the increase in the number of mobile charge
carriers [4,5]. In the case of the samples based on DNA it was
observed that the addition of the erbium salt promoted an increase
of the ionic conductivity of the samples of one order with respect
to the DNA-matrix (10−7 S.cm−1 at 25 ◦ C) and DNA-Er membranes.
However, almost no difference in the conductivity values at room
temperature between the samples with 0.05 and 0.02 g of erbium
salt was registered.
An increase in ionic conductivity with temperature may be
interpreted as a hopping mechanism between coordinating sites
together with local structural relaxations and segmental motions
of the polymer salt complexes [12]. As temperature increases the
polymer chain acquires faster internal modes in which bond rotations produce segmental motion. This, in turn, favours inter-chain
hopping and intra-chain ion movements, which will increase the
electrolyte conductivity [4,5]. Similarly to the DNA-ionic liquids
[31] or DNA-LiClO4 [32] systems, the DNA-matrix and DNA-Er
membranes evidenced a linear increase of the ionic conductivity values as a function of the inverse temperature, indicating an
Arrhenius behaviour (Fig. 1). Moreover, above 60 ◦ C all the samples of DNA-Er exhibited almost the same ionic conductivity value
(around 10−4 S.cm−1 ). However, the Ea values of 36.14 kJ.mol−1 and
24.56 kJ.mol−1 for DNA-matrix and DNA10 Er, respectively, revealed
a considerable decrease with addition of erbium triflate due probably to the enhancement of segmental motion of the polymer chains
and increase of ionic mobility.
Fig. 2 shows the DSC curves obtained between -60 and 200 ◦ C
for different samples. These thermograms confirm that the DNAbased electrolytes are amorphous. At 180 ◦ C, for some samples, one
can observe an exothermic peak, which is the onset temperature of
thermal degradation and a weak endothermic peak around 60 ◦ C is
tentatively attributed to the formation of polymer-salt complexes.
The microelectrode cyclic voltammogram of the DNA10 Er sample over the -2.0 to 6.0 V potential range obtained at room
temperature is depicted in Fig. 3. This voltammogram demonstrates
that in the anodic region the sample is stable up to about 3.5 V
versus Li/Li+ , whereas in the cathodic region it is stable up to about
1.5 V versus Li/Li+ . This means that the overall redox stability of
DNA10 Er spans about 2.0 V as already observed in other studies on
DNA membranes [31].
Fig. 3 also shows that increasing the scan rate of the samples
analysed here caused an increase in the peaks current and a shift
into more cathodic and anodic peak potentials. The effect of scan
rate on the voltammogram suggests the irreversible behaviour of
the electrochemical reaction. While in a Nernstian, i.e., reversible
system, the peak potential is scan rate independent, in irreversible
systems it does depend on the scan rate. Furthermore, the linear
nature of the plot of the anodic peak current versus the square root
Fig. 2. DSC thermograms of the DNA and erbium triflate based polymer electrolytes.
330
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
Fig. 3. Cyclic voltammogram of the DNA10 Er at room temperature with a 25 ␮m
gold microelectrode as working electrode and lithium as counter and reference electrodes. The initial sweep direction is anodic and the sweep rates are 10, 20, 40, 60,
80 and 100 mV.s−1 . (Inset: Plot of the anodic peak current against the square root of
scan rate).
of scan rate shown in the inset of Fig. 3 indicates that the mass
transfer to the electrode surface was diffusion-controlled [31,33].
The XRD patterns of the DNA-based materials, reproduced in
Fig. 4, reveal that the DNA matrix and the doped samples are
predominantly amorphous. The broad Gaussian peak centred at
about 21◦ in the XRD patterns of these electrolytes is also produced by other systems based on natural macromolecules [20,34].
The diffractogram of DNA10 Er (Fig. 4) shows, however, that further addition of erbium triflate salt to DNA gives rise to the small
diffraction peak at 28◦ , which is indicative of the existence of
crystalline domains.The POM images of Fig. 5 demonstrate that
the samples are birefringent. The anisotropy of these electrolytes
Fig. 4. XRD patterns of selected samples.
suggests that at the submicrometer scale, crystalline phases are
formed. This effect might be associated with the formation of the
polymer-salt complexes detected by DSC.
Aiming to analyse the morphological characteristics of the
DNA-based membranes doped with Er3+ the samples were also
characterized by atomic force microscopy (AFM) measurements.
Fig. 6 displays typical AFM images of the DNA10 Er and DNA25 Er
membranes where one can observe that the samples with higher
quantity of the erbium salt (Fig. 6a) promoted a formation of a
membrane with rougher surface than that of the less concentrated
sample (Fig. 6b). This behaviour can be attributed to the poor
dissolution of salt when a high amount of Er3+ is used in the preparation of the membranes. The roughness mean square (RMS) of the
DNA25 Er was 12.0 nm and of the DNA10 Er was 22.8 nm leading to
a 1.9-fold increase.
Fig. 5. POM images (under crossed polarizers) (a) DNA50 Er; (b) DNA12,5 Er and (c) DNA10 Er.
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
331
Fig. 6. AFM images of (a) DNA10 Er and (b) DNA25 Er.
The EPR spectra of the DNA-based membranes doped with Er3+
with three different concentrations are shown in Fig. 7. Free Er3+
ions have a 4f11 electronic configuration with a 2S+1 LJ = 4 I15/2 ground
state, where S, L and J are the spin, orbital and total momenta,
respectively. The Er3+ ion is paramagnetic and therefore EPR active.
Erbium has five isotopes with nuclear spin I = 0 and one stable isotope, 167 Er, with I = 7/2 (22.9% natural abundance). Therefore, in
crystals the EPR spectrum is expected to show a hyperfine structure
composed by a set of eight lines, resulting from the dipole-dipole
interaction between the magnetic moment of the 167 Er nuclei and
the electronic moment of the paramagnetic Er3+ ion [35,36]. When
Er3+ is incorporated into a host, the16-fold degenerate free ion
ground-state energy level splits into a number of Stark levels. For
Er3+ ions in cubic or tetragonal symmetry sites the 4 I15/2 ground
state splits into two Kramers doublets, 6 and 7 , with an effective
spin S = ½ and into three quadruplets 8 , each with an effective
spin S = 3/2 [36,37]. The ground state is expected to be one of the
Kramers doublets, 6 or 7 , according to the ratio of the parameters
332
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
Fig. 7. X-band EPR spectra of the DNAn Er3+ membranes measured at 6 K. The plot
corresponding to sample DNA25 Er is vertically amplified by a factor 3.
B4 and B6 of the crystal-field Hamiltonian [37]. The g-values associated with transition within the 6 and 7 states are calculated to
be g ≈ 6.0 and g ≈ 6.8, respectively [36,38]. The EPR spectra in Fig. 7
are similar to those reported previously for Er-doped glasses and
disordered materials [36,38,39]. The EPR spectra are composed of
a very broad and intense signal (line width around 1500 G) located
at low field (g ≈ 6.7). The position of the resonance is consistent
with a 6 or a 7 Kramers doublet ground state in a cubic-like
environment. The large inhomogeneous broadening suggests that
the Er3+ ions can be considered as isolated in the sense of absence
of clustering, and are distributed in various environments in the
membrane.
Finally the DNA12.5 Er membrane was used to assemble a small
ECD with WO3 /DNA12.5 Er/CeO2 –TiO2 configuration. Fig. 8 shows
the cyclic voltammograms of this ECD, recorded at different cycles
(Fig. 8a) and scan rates of 20, 50, 100, 200, 300 and 500 mV.s−1
(Fig. 8b) obtained by sweeping the potential in the range of -2.0 V
to +1.8 V. From Fig. 8a one can observe an increase of the cathodic
current to -0.3 mA.cm−2 at -1.8 V, due to the reduction and intercalation process occurring in the WO3 film and consequently its
switch to the pale blue color; at 0.0 V for the scan rate of 50 mV.s−1
an anodic peak is observed and it is due to the oxidation reaction
and consequently to the bleaching process [20]. Fig. 8b reveals that
an increase of the voltammetric scan rate promotes an increase of
the cathodic current to -6.2 mA.cm−2 and also shifts the anodic peak
to more positive potentials (0.2 V at 500 mV.s−1 ).
Aiming to evaluate the reversibility of the ECD, the samples were
subjected to chronoamperometric cycling by applying potentials of
-2.0 and +1.8 V for 15s/15s. Fig. 9 shows the results of the charge
density for the 10th and 4000th cycles. The color variation between
blue and transparent showed a change during the test, where a
decrease of the charge density from -4.0 mC.cm−2 to -1.2 mC.cm−2
was observed after 4000 color/bleaching cycles. The charge density
value for the initial sweeps was higher than -1.8 mC.cm−2 at -2.5 V,
reported for the ECD with DNA-based electrolytes [20], but lower
than -6.6 mC.cm−2 at -3.5 V [40].
The transmission spectra obtained in the wavelength range
between 340 and 800 nm for the colored and bleached states were
recorded (results not shown). All the spectra displayed a series
of maxima and minima in this spectral region assigned to interference resulting from the presence of a multilayer structure. The
transmittance of ECD in the VIS region (550-633 nm) was 38% for
the bleached state, being 31% for the colored state. The change in
optical density ((OD)) (where (OD) = log (Tcolored/Tbleached))
in the VIS region is thus 0.085. We recall that the human eye is
Fig. 8. Cyclic voltammetry studies for the ECD with WO3 /DNA12.5 Er/CeO2 –TiO2 configuration for different cycles (a) and different scan rates (b).
Fig. 9. Charge density results at 10th and 4000th cycles for electrochromic window with WO3 /DNA12.5 Er/CeO2 –TiO2 composition. (Inset: The current density as a
function of number of cycles).
sensitive to light waves for which the wavelength is between
about 400 and 700 nm (VIS region). Under abundant illumination
(daylight) the maximum eye sensitivity lies in the green region at
555 nm (photopic vision). The low transmittance value of the window in the bleached state is due to the electrolyte that is slightly
opaque. The only 7% of the transmittance change after potential
switching to -2.0 V can be due to the several factors as Er3+ ionic
radius [41], diffusion coefficient [42], or softness parameter [5].
However, no data concerning this subject was until now reported.
R. Leones et al. / Electrochimica Acta 120 (2014) 327–333
4. Conclusions
Er3+ -doped DNA-based biomembranes have been prepared and
analyzed by DSC, X-ray, AFM, POM, cyclic voltammetry and EPR
spectroscopy. All the samples analyzed showed good mechanical properties, good transparency and useful adhesive functions.
From the obtained results it was concluded that the most conducting sample, DNA10 Er, exhibited an ionic conductivity equal
to 1.17 × 10−5 S.cm−1 at 30 ◦ C and 7.76 × 10−4 S.cm−1 at 100 ◦ C.
In all the samples the ionic conductivity as a function of the
inverse temperature showed Arrhenius type behavior. Techniques
like DSC and XRD, demonstrated that the inclusion of the guest
salt does not change significantly the amorphous nature of the
samples. Moreover, the overall redox stability of about 2.0 V is an
indication that this material displays have an acceptable stability
window for an application in solid state electrochemical devices.
The results of EPR analysis suggest that the Er3+ ions are distributed
in various environments. To evaluate the use of these membranes in electrochemical devices, a small ECD incorporating the
DNA12.5 Er membrane was assembled and tested by cyclic voltammetry and chronoamperometry. These electrochemical analyses
revealed color change between the pale blue and transparent and a
decrease of the charge density, from -4.0 mC.cm−2 to -1.2 mC.cm−2
during 4000 color/bleaching cycles.
Acknowledgements
The authors are pleased to acknowledge the financial support provided by the University of Minho and the Fundação
para a Ciência e Tecnologia supported by FEDER contracts
(PTDC/CTM-BPC/112774/2009, PEst-C/QUI/UI0686/2011 and Pest14 C/CTM/LA0011/2011), grant SFRH/BD/90366/2012 (R. Leones)
for laboratory equipment and research staff. The support of
COST Action MP1202 “Rational design of hybrid organic-inorganic
interfaces” is also acknowledged. The authors are also indebted
to CNPq, FAPESP and CAPES for the financial support given to
this research. M. M. Silva acknowledges to FAPESP and FCT
(SFRH/BSAB/1312/2013).
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