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Coatings 2023, 13, 213. https://doi.org/10.3390/coatings13010213 www.mdpi.com/journal/coatings
Article
Statistical Study of the Effectiveness of Surface Application
of Graphene Oxide as a Coating for Concrete Protection
Andrea Antolín-Rodríguez 1, Daniel Merino-Maldonado 1, Álvaro Rodríguez-González 1, María Fernández-Raga 2,
José Miguel González-Domínguez 3, Andrés Juan-Valdés 1 and Julia García-González 1,*
1 Department of Engineering and Agricultural Sciences, School of Agricultural and Forest Engineering,
University of Leon, Av. De Portugal 41, 24071 Leon, Spain
2 Department of Chemistry and Applied Physics, Industrial Engineering School, University of Leon,
Vegazana Campus S/N, 24071 Leon, Spain
3 Instituto de Carboquímica (ICB-CSIC), Group of Carbon Nanostructures and Nanotechnology (G-CNN),
C/Miguel Luesma Castán 4, 50018 Zaragoza, Spain
* Correspondence: julia.garcia@unileon.es
Abstract: Improving the protection of concrete by applying graphene oxide (GO) as a surface treat-
ment has become the objective of the present study. This study focuses on performing a statistical
analysis to study different levels of GO application as an exterior coating, thus observing the effec-
tiveness of the coating and the optimization of the treatment material for concrete protection. Four
tests were performed to define concrete durability, such as pressurized water penetration, capillary
absorption, freeze-thaw resistance and carbonation resistance. The results showed an increase in
concrete durability with any level of GO application on the surface, considering that the optimum
amount of application for water impermeability and freeze-thaw resistance is 26.2 µg/cm2, since it
was possible to reduce pressurized water penetration by 45%, capillary water absorption by 57%
and freeze-thaw detachment by 25%. However, the optimum application rate for carbonation re-
sistance is 52.4 µg/cm2, reducing carbonation by almost 60%. In conclusion, if the concrete is going
to be exposed to less aggressive environments, the application of a mild surface coating of GO is
sufficient for its protection, and if the concrete is going to be exposed to more aggressive environ-
ments, it is necessary to increase the amount of GO. The performance of GO as a coating signifi-
cantly increased the degree of protection of the concrete, increasing its service life and proving to
be a promising treatment for concrete surface protection.
Keywords: graphene oxide; statistical analysis; coating; durability; concrete
1. Introduction
Throughout their useful life, concrete structures are exposed to different environ-
mental conditions that cause their degradation and deterioration. Generally, this deterio-
ration is caused by various agents (carbonation, chlorides, etc.) that lead to a shortening
of the service life of the concrete elements [1]. For this reason, protection against degrada-
tion has gained significant attention, since this problem has caused great damage in the
construction industry, being also the origin of costly repairs or even replacements with
the consequent economic and environmental cost [2].
The most commonly employed strategy in recent years has been the use of high-per-
formance concretes, which are distinguished by their low permeability and high strength.
However, new more economical approaches are being sought that focus on providing
additional protection to the materials [3,4], such as the use of corrosion inhibitors or sur-
face treatment [5,6]. Nowadays, surface treatment is one of the most widely accepted ap-
proaches due to its effectiveness in preventing the entry of aggressive substances from the
environment and protecting not only against corrosion inside the structures, improving
Citation: Antolín-Rodríguez, A.;
Merino-Maldonado, D.;
Rodríguez-González, Á.;
Fernández-Raga, M.;
González-Domínguez, J.M.;
Juan
-Valdés, A.; García-González, J.
Statistical Study of the Effectiveness
of Surface Application of Graphene
Oxide as a Coating for Concrete
Protection. Coatings 2023, 13, 213.
https://doi.org/10.3390/
coatings13010213
Academic Editor: Andrea Nobili
Received: 17 December 2022
Revised: 11 January 2023
Accepted: 1 January 2023
Published: 16 January 2023
Copyright: © 2023 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
ativecommons.org/licenses/by/4.0/).
Coatings 2023, 13, 213 2 of 11
their mechanical behavior, but also protecting the whole set that integrates the cementi-
tious matrix, thus extending its usefulness as a consequence of a greater durability of the
treated elements [7].
Recently, one of the most promising methods is the use of nanomaterials [8], thanks
to their extraordinary potential to penetrate through cracks and pores in concrete [9]. So
far, the literature has focused little on the study of surface treatments with nanomaterials
[10–14].
In this study, a graphene-based material has been used as a possible protective treat-
ment for concrete with the objective of studying whether it can be a potential candidate
for its preservation, taking into account its cost with other coatings and the range of pro-
tection it offers.
Graphene oxide (GO) is an excellent nanomaterial, as it has successively demon-
strated important functions and broad prospects in the field of coating fabrication [15]. So
far, GO has been employed as a coating modifier material to improve the barrier and
strength of coatings, such as GO and ETEO-modified epoxy resin coatings, GO-modified
silane emulsions, GO/ITBS coatings or GO-modified epoxy coatings [16–20], due to its
unique properties and highly specific surface area [20,21]. However, GO as a unique coat-
ing material has been scarcely reported in the literature [22,23], being this a good starting
point for its study. In this regard, one of the most relevant properties of 2D GO assemblies
should be highlighted; it has been shown that metal ions (Al3+, Fe3+, Ca3+, Mg2+, etc.) are
excellent crosslinking agents for GO membranes, making them stronger and more re-
sistant to washing [24]. These alkaline earth cations are some of the main components of
cement used in the manufacture of concrete, so GO could be an excellent protective coat-
ing for concrete due to a possible chemical bonding to its surface by virtue of the afore-
mentioned cations.
However, in addition to studying GO spraying as a surface coating of concrete, it is
also necessary, for practical reasons, to establish the optimal amount while keeping intact
its degree of protection, which is the objective of this article. There is not any report, to the
best of our knowledge, that establishes the optimum protective coating for concrete; there-
fore, a statistical study is conducted to determine the surface effectiveness of GO as a pro-
tective coating. Consequently, water transmission, resistance to freeze/thaw cycles and
resistance to carbonation were studied; all of them with defining parameters of concrete
durability. Concrete specimens without any coating and concrete specimens with various
amounts of GO surface coatings were evaluated. The results of this research provide fur-
ther information on the suitability of a GO coating as a protective surface treatment for
concrete, as well as to optimize the amount of GO required for an efficient concrete pro-
tection, always with a view to achieving a “functional coating,” that is, a coating that is
economically viable and increases the durability of the concrete, taking into account the
optimization of resources.
2. Materials and Methods
2.1. Materials
2.1.1. Hardened Concrete
The cement used was blast furnace slag CEM IIIA 42.5 N/SR, which complies with
the requirements of EN 197-1 [25]. The aggregates used were of a siliceous nature, using
4/12.5 mm gravel as coarse aggregate and 0/4 mm sand as fine aggregate. These aggregates
are considered suitable for concrete production in accordance with EN 12620:2003+A1 [26]
and the Eurocode [27].
The dosage applied for the manufacture of the concrete complies with the mechanical
and durability requirements of the European standards (EN) and is presented in Table 1.
Coatings 2023, 13, 213 3 of 11
Table 1. Concrete mixture components.
Components (Quantity/m3) Conventional Concrete
Gravel (kg) 1030.7
Sand (kg) 650.5
Cement (kg) 390.0
Water (L) 198.0
Concrete batching was established according to the De La Peña method, based on the desired char-
acteristic strength of the final concrete [28].
2.1.2. Graphene Oxide
Synthesis of GO
The GO used for the surface coating of the hardened concrete samples in the present
study was obtained by exfoliation of the graphitic oxide in water and subsequent appli-
cation of mild ultrasound. Graphitic oxide was produced by an oxidation method known
as the Hummers method, with certain particular modifications. The complete experi-
mental process for obtaining graphene oxide is reported in [23], as well as the complete
characterization of graphitic oxide and GO thereof.
Figure 1 shows a transmission electron microscopy (TEM) image of freshly exfoliated
graphene oxide, showing single-layer thick GO sheets of approximately (2–4) µm lateral
size. The two-dimensional and thin nature of this material allows its physical and flexible
adaptability, thus confirming that GO is a convenient material at micro-nanoscale to act
as a protective coating.
Figure 1. TEM image of a GO flake. Recorded with a JEOL-200FXII (JEOL, Tokyo, Japan) microscope
working at 200 kV and with 0.28 nm and with 0.28 nm point-to-point resolution (Electron Micros-
copy Sciences, ref CF-400CU).
2.2. Treatment
The treatment consists of the application of a dispersion of GO in aqueous suspension
(0.5 mg/mL), without any other additives or adjuvants, over the surface of the hardened
concrete. The treatment is applied by means of an airbrush with horizontal movement and
slow speed, controlling the amount of treatment deposited on the surface (26.2 µg/cm2 per
each coating passage). The application time between passages is set at 1 h and after the
complete treatment, the concrete specimens are left to dry at a temperature of 20 ± 2 °C
and a relative humidity of 45 ± 15 % for two days.
The concrete samples were cured for 90 days at a temperature of 20 ± 2 °C and relative
humidity >95% prior to surface application of the GO coating.
Coatings 2023, 13, 213 4 of 11
2.3. Experimental Design
The experimental design included four trials with six treatment levels and three rep-
licates of each treatment (72 samples in total). The treatments analyzed were the following:
one treatment with one GO coating (1-GO coating), one treatment with two GO coatings
(2-GO coatings), one treatment with three GO coatings (3-GO coatings), one treatment
with four GO coatings (4-GO coatings), one treatment with five GO coatings (5-GO coat-
ings) and one control treatment, that is, uncoated concrete specimens, without any treat-
ment (CC). Table 2 lists each treatment level and the amount of GO contained in each.
Table 2. Treatment levels and surface content of each level.
Treatment Levels (Coating) GO Content (µg·cm2)
CC 0.0
1-GO coating 26.2
2-GO coatings 52.4
3-GO coatings 78.6
4-GO coatings 104.8
5-GO coatings 131.1
2.4. Methods
The research method involved the development of four tests focused on the durabil-
ity assessment of concrete. Subsequently, a statistical analysis was performed to each of
the tests, with an aim at evaluating the minimum significant differences between the treat-
ment levels, thus determining which is the minimum GO surface concentration necessary
for the full concrete protection to occur.
2.4.1. Depth of Water Penetration under Pressure
Standard cylindrical concrete specimens (150 mm Ø x 300 mm height) were tested to
determine the penetration depth of pressurized water according to UNE-EN 12390-8 [29].
The test was conducted over a period of 72 h in a water penetration equipment, which
allows a hydrostatic pressure of 5 bar (0.5 MPa) on the upper base of the specimen. After
this period of pressurized water application, the specimens are broken by splitting tensile
strength according to the protocol established in the EN 12390-6 [30] standard, which al-
lows observing the water penetration front, which goes from the base towards the interior
of the cylindrical specimen. For the calculation of the area enclosed by the penetration
front, the ImageJ software program is used, which allows the calculation of areas. To de-
termine the average depth of penetration (Pm), Equation (1) contained in the UNE-EN
12390-8 [29] standard is used, written as follows:
Pm =
Apf
(1)
where the area of the penetration front Apf (mm2) was divided by the specimen diameter
d (mm), thus obtaining the average penetration depth Pm (mm).
2.4.2. Water Absorption by Capillarity
Cubic specimens (100 mm x 100 mm x 100 mm) were tested for capillary water ab-
sorption according to UNE 83982 [31]. The test consisted of placing the concrete speci-
mens, after conditioning according to the UNE 83966 [32] standard, in a plastic container
with a leveling grid, on which the specimens were placed in contact with a layer of deion-
ized water about 5 mm high. The capillary absorption coefficient (K) was determined by
means of the following Equation (2), contained in the UNE 83982 [31] standard:
K =
· ɛ
· √
(2)
Coatings 2023, 13, 213 5 of 11
where K is the capillary absorption coefficient (kg/m2 · min0.5), ɛe is the effective porosity
of concrete (cm3/cm3), δa is the density of water (the value of 1 g/cm3 is considered) and m
is the resistance to water penetration by capillary absorption (min/cm2).
2.4.3. Resistance to Freezing/Thawing with De-Icing Salts
Truncated conical specimens (110 mm Ø x 75 mm Ø x 85 mm height) were tested
with a layer of (5 ± 2) mm of a 3% NaCl solution in potable water as de-icing salt. The
assembly is subjected to 28 freeze-thaw cycles according to EN 1339-Anex D [33], which
each cycle lasting 24 h alternating temperature decreases from (20 ± 5) °C to (-20 ± 5) °C
for 17 h, and temperature increases from (-20 ± 5) °C to (20 ± 5) °C for 7 h. The freeze-thaw
resistance of the hardened concrete was determined by recording the weight of material
released per unit area (kg/m2).
2.4.4. Resistance to Carbonation at Atmospheric Levels of CO2
Hardened concrete cubic specimens (100 mm x 100 mm x 100 mm) are exposed to a
natural environment for a period of 6 months and protected from direct rain as described
in EN 12390-10 [34]. After the exposure period, the specimens were divided by tensile
strength (EN 12390-6 [30]) and a phenolphthalein solution was sprayed. Once the solution
was applied and following the provisions of standard EN 12390-10 [34], the carbonation
depth was determined perpendicular to the surface of the concrete specimen, using a cal-
iper. Three central points were measured, located at 0.25, 0.5 and 0.75 of the edge length.
From the calculated carbonation depth, the carbonation resistance of the concrete was de-
termined by calculating the carbonation speed according to the following Equation (3):
=
√
(3
)
where V is the carbonation speed (mm/year0.5), x is the carbonation depth (mm) and t
is the exposure time (years).
2.4.5. Statistical Analysis
The data were subjected to an ANOVA one-way to comparison of means, consider-
ing the durability parameters (pressurized water penetration depth, capillary water ab-
sorption, freeze/thaw resistance and carbonation resistance) as variable factors and GO
treatment level as a fixed factor. Differences (p ≤ 0.05) between durability parameters and
between GO treatment levels were examined by comparison of means using the LSD
(Least Significant Difference) post-hoc test.
All analyses were performed with the SPSS Software (Statistical Package for the So-
cial Sciences) version 21 (IBM, Statistics, SPSS Inc., Chicago, IL, US).
3. Results and Discussion
The four parameters analyzed indicated that the increase in the protection of concrete
was conditioned by the level of treatment applied to its surface, i.e., the surface concen-
tration of deposited GO, showing a direct correlation between the protective efficacy and
the amount of coating. However, when studying the increase in concrete protection, it is
necessary to consider the optimization of the treatment material, so the optimal treatment
level for concrete protection was determined from the statistical analysis performed for
each durability parameter. The final decision about the quantity to add should take into
consideration also the cost that the treatment has in comparison with the improvement on
the durability parameters. So, it was selected per each use the minimum product that will
represent a significative difference in properties.
Coatings 2023, 13, 213 6 of 11
3.1. Depth of Penetration of Water under Pressure
The greatest depth of water penetration under pressure (20.9 mm) occurs in the un-
coated control samples (CC). The application of any GO coating on the surface of the con-
crete specimens implies a decrease in the water penetration depth, there being a direct
relationship between the decrease in the water penetration depth and the amount of GO
applied on the surface of the concrete. The results of the statistical analysis showed that
the 1-GO treatment, which is the first level of treatment, presented significant differences
with the CC samples, which are the samples without any coating, since the penetration
depth of the 1-GO samples (11.4 mm) was significantly lower (F = 42.951; df = 5,12; p ≤
0.001) than that of the control samples (20.8 mm), implying a decrease of 45% (Figure 2).
Figure 2. Depth of water penetration under pressure at different treatment levels. Capital letters
indicate minimum significant differences between treatment levels. Vertical bars represent the mean
and the standard error (SE). Statistical differences are indicated by different letters (LSD test, p <
0.05).
The 1-GO treatment showed no significant differences with the subsequent level of
treatment (2-GO), with the application of the 3-GO treatment being necessary for minimal
significant differences to exist. The samples with three GO coating passages (3-GO)
showed a water penetration depth of 7.2 mm, which implies a decrease of 37% with re-
spect to the 1-GO samples, but an increase in the amount of GO applied on the surface of
the concrete samples of 52.4 µg/cm2. Therefore, it is established that the improvement in
protection achieved with one GO coating passage is sufficient, considering it as the opti-
mal level for the protection of the concrete surface against the penetration of pressurized
water.
3.2. Capillary Absorption
The highest capillary water absorption coefficient (0.035 kg/m2min0.5) is observed in
the control samples (CC), without any type of coating. The application of any level of GO
coating on the surface of the concrete samples, implies a decrease in the capillary absorp-
tion coefficient, there being a direct relationship between the decrease in the “K” coeffi-
cient and the amount of GO deposited on the surface of the concrete. Regarding the results
obtained in the statistical analysis of the capillary absorption coefficient, it is established
that the first level of treatment, compared to those samples without any type of coating,
presented significant differences, since the capillary absorption coefficient was
Coatings 2023, 13, 213 7 of 11
significantly lower for the 1-GO treatment (F = 29.090; df = 5,12; p ≤ 0.001) than that ob-
tained in the control samples, being 0.015 kg/m2min0.5, which represents a decrease of 57%
(Figure 3).
Figure 3. Coefficient of capillary water absorption at different treatment levels. Capital letters indi-
cate minimum significant differences between treatment levels. Vertical bars represent the mean
and the standard error (SE). Statistical differences are indicated by different letters (LSD test, p <
0.05).
The 1-GO treatment showed no significant differences with the 2-GO treatment,
which is the subsequent level of treatment, with the application of the 3-GO treatment
being necessary for minimal significant differences to exist. The 3-GO samples showed a
k coefficient of 0.007 kg/m2min0.5, which implies a decrease of 52% with respect to the 1-
GO samples, but a neat increase in the amount of GO applied of 52.4 µg/cm2. Therefore, it
is established that the improvement in concrete protection achieved with 1-GO coating is
sufficient, considering it the optimal level for the protection of concrete against capillary
water absorption.
3.3. Freeze/Thaw Resistance
The greatest mass loss due to freeze/thaw cycles occurs in uncoated specimens (CC),
with a mass loss of 6.8 kg/m2 in this type of sample. The application of any GO coating
treatment implies an increase in the resistance of the concrete to freeze/thaw cycles by
decreasing the amount of material detached from the surface; there being a direct rela-
tionship between the decrease in such amount of material detached and the amount of
GO deposited on the surface of the concrete. Looking at Figure 4, the statistical analysis
performed to evaluate freeze/thaw resistance showed that the first treatment level with
which the CC samples showed significant differences was the 1-GO treatment, since the
mass loss is significantly lower (F = 9.752; df = 5,12; p = 0.001) than that obtained in the CC
samples, 5.1 kg/m2, implying a 25% decrease.
Coatings 2023, 13, 213 8 of 11
Figure 4. Mass loss after freeze-thawing cycles at different treatment levels. Capital letters indicate
minimum significant differences between treatment levels. Vertical bars represent mean and stand-
ard error (SE). Statistical differences are indicated by different letters (LSD test, p < 0.05).
The 1-GO treatment showed no significant differences with the subsequent two treat-
ment levels (2-GO and 3-GO), with the application of the 4-GO treatment being necessary
for minimal significant differences to exist. The specimens with four GO coating passages
(4-GO) showed a mass loss of 4 kg/m2, which implies a decrease of 22% with respect to
the 1-GO specimens, but an increase in the amount of GO applied on the concrete surface
of 78.6 µg/cm2. Therefore, it is established that the improvement in protection achieved
with one GO coating passage is sufficient, being considered as the optimal level for the
protection of the concrete surface against freezing/thawing.
3.4. Resistance to Carbonation
The highest carbonation speed is found in the samples without any type of coating
(CC), with 7.4 mm/year0.5. The application of any GO coating treatment implies an increase
in the resistance of the concrete to carbonation, as the carbonation speed decreases; there
is a direct relationship between the decrease in the carbonation speed and the amount of
GO deposited on the surface of the concrete. As displayed in Figure 5, the results of the
statistical analysis indicated that the 1-GO treatment is the first treatment with which the
CC samples presented significant differences, since the carbonation speed of this treat-
ment was significantly lower (F = 15.096; df = 5,12; p ≤ 0.001) than that produced in the CC
samples, 5.0 mm/ year0.5, which represents a decrease of 32%.
Coatings 2023, 13, 213 9 of 11
Figure 5. Carbonation speed at different treatment levels after 6 months exposure. Capital letters
indicate minimum significant differences between treatment levels. Vertical bars represent mean
and standard error (SE). Statistical differences are indicated by different letters (LSD test, p < 0.05).
The 1-GO treatment showed significant differences with the subsequent treatment
level (2-GO), presenting a carbonation speed of 3.0 mm/ year0.5, which implies a 40% de-
crease compared to the 1-GO specimens. The 2-GO treatment did not show significant
differences with the rest of the higher treatment levels (3-GO, 4-GO and 5-GO). Therefore,
it is established that the improvement in protection achieved with a GO coating is not
enough, being necessary to reach two GO coating passages, considering this as the opti-
mal level for the protection of the concrete surface against the carbonation process.
From all the results obtained in the four-durability test, it is observed that for water
impermeability of the concrete surface and for freeze/thaw resistance the optimal amount
of coating is 26.2 µg·cm2, while for carbonation resistance the optimal amount of coating
is 52.4 µg·cm2. This difference may be justified by the type of exposure to which the con-
crete is subjected to, in each test, since different kinds of concrete exposure may produce
slight changes with respect to durability requirements. According to Eurocode [27], in the
water penetration and capillary absorption test, the concrete is in exposure X0, which cor-
responds to a kind of exposure without risk of corrosion. In the freeze/thaw test, the con-
crete in in exposure XF2, which corresponds to an exposure class for freeze/thaw attack in
which the concrete is moderately saturated with melting salts. Finally, in the carbonation
resistance test, the concrete is in an XC3 exposure, which corresponds to a carbonation-
induced corrosion exposure kind in which the concrete is subjected to natural exposure
with medium-high humidity but protected from rain.
Exposures X0 and XF2 involve less aggressive environments and therefore the opti-
mal amount of protection is 26.2 µg·cm2; however, exposure XC3 involves a much more
aggressive environment and the application of 26.2 µg·cm2 is not sufficient and it is nec-
essary to extend the optimum amount up to 52.4 µg·cm2.
4. Conclusions
The application of a GO coating improved the protection of concrete surfaces by in-
creasing its durability. The results of the durability tests showed that any application of
GO on the surface of the concrete resulted in an increase in its protection and with a direct
correlation between the protection efficacy and the deposited amount of GO. However, it
Coatings 2023, 13, 213 10 of 11
is necessary to determine an optimal coating extent, balancing between the effective pro-
tection and the optimization of the material used treatment. Therefore, the results of the
statistical analysis showed that the optimal GO surface concentration for the water imper-
meability and freeze-thaw resistance tests was 26.2 µg·cm2, while for the carbonation re-
sistance it was 52.4 µg·cm2.
In conclusion, if the concrete is going to be exposed to less aggressive environments,
the application of a mild surface coating of GO (26.2 µg·cm2) is sufficient for its protection,
if the concrete is going to be exposed to more aggressive environments, such as carbona-
tion, it is necessary to increase the amount of GO (52.4 µg·cm2), to maintain an adequate
protection of the concrete.
In addition, GO dispersion is commercially widely available and relatively afforda-
ble, as it is the cheapest graphene derivative available, being Spain and Europe where it
is produced the most. Currently, a GO dispersion with a concentration of 0.05% wt is com-
mercially available at around 70 €/liter. Therefore, taking into account the economic cost
of this product, the optimization of the product that has been studied and the protection
it provides to the concrete, GO can be accepted as an excellent surface treatment, which
can be practical for large surfaces or specific applications due to the small amount used
during its application.
Author Contributions: Conceptualization, A.J.-V., J.G.-G. and A.A.-R.; methodology, A.A.-R., Á.R.-
G. and D.M.-M.; software, A.A.-R.; formal analysis, A.A.-R., J.M.G.-D., M.F.-R. and J.G.-G.; investi-
gation, A.A.-R., D.M.-M. and Á.R.-G.; resources, J.M.G.-D. and M.F.-R.; data curation, A.A.-R., Á.R.-
G. and D.M.-M.; writing—original draft preparation, A.A.-R., A.J.-V., Á.R.-G. and J.G.-G.; writing—
review and editing, A.A.-R., A.J.-V., J.G.-G., J.M.G.-D., M.F.-R. and Á.R.-G.; supervision, J.G.-G. and
A.J.-V. All authors have read and agreed to the published version of the manuscript.
Funding: A.A.R. greatfully acknowledge to Aid from the Junta de Castilla y Leon to finance the pre-
doctoral hiring of research personnel, co-financed by the European Social Fund, resolved in the OR-
DER EDU/875/2021, of July 13. D.M.M. greatfully acknowledge to Aid from the Junta de Castilla y
Leon to finance the pre-doctoral hiring of research personnel, co-financed by the European Social
Fund, resolved in the ORDEN EDU/601/2020, of July 3. M.F.R and J.M.G.-D. greatfully acknowledge
Spanish Ministry of Science and Innovation (MICINN) and the Spanish Research Agency (AEI) for
the financial support provided by the NANOSHIELD project (PID2020-120439RA-I00).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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