Magneto-responsive photochromic acrylic copolymer nanoparticles: An investigation into the mutual interactions and photoisomerization kinetics

Article abstract of DOI:10.1016/j.polymer.2021.123524

Photoresponsive and magneto-responsive smart materials have received specific attention. Here, novel bifunctional polyacrylic nanoparticles were prepared via miniemulsion polymerization in which, magnetite nanoparticles (10–15 nm) were resided in the core and poly (methyl methacrylate-co-spiropyran ethyl acrylate) (poly (MMA-co-SPEA)) was located in the shell. Chemical bonding of spiropyran onto the surface of magnetite provided their close vicinity, leading to effective interactions which have not been studied before. The interactions between the excited state of merocyanine (MC) zwitterion from spiropyran (SP) group and magnetic field of the superparamagnetic Fe₃O₄ NPs were investigated. The results revealed the existence of mutual interactions and successful increment in the absorption intensity (up to 176%) and optical stability, 9% changes in the rate of photoisomerization and increment in the half life time of MC isomers. These were achieved in the presence of magnetite as well as enhancement in the magnetic properties by 10% under UV irradiation at 365 nm. Such synergistic and two-way interactions between magnetic field of the superparamagnetic Fe₃O₄ NPs and the produced instant dipole moments in MC may offer a new route toward preparation of dual responsive nanocomposite particles for light-triggered magnetic switches, magnetic-triggered photo switches and simultaneous sensing and separation probes.

Full text of DOI:10.1016/j.polymer.2021.123524

Polymer 218 (2021) 123524
Contents lists available at ScienceDirect
Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Magneto-responsive photochromic acrylic copolymer nanoparticles: An
investigation into the mutual interactions and photoisomerization kinetics
Alireza Mouraki, Zeinab Alinejad, Samira Sanjabi, Hamid Salehi-Mobarakeh,
Ali Reza Mahdavian*
Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran, 1497713115, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Photoresponsive and magneto-responsive smart materials have received specific attention. Here, novel bifunc-
tional polyacrylic nanoparticles were prepared via miniemulsion polymerization in which, magnetite nano-
particles (10–15 nm) were resided in the core and poly (methyl methacrylate-co-spiropyran ethyl acrylate) (poly
(MMA-co-SPEA)) was located in the shell. Chemical bonding of spiropyran onto the surface of magnetite pro-
vided their close vicinity, leading to effective interactions which have not been studied before. The interactions
between the excited state of merocyanine (MC) zwitterion from spiropyran (SP) group and magnetic field of the
superparamagnetic Fe3O4 NPs were investigated. The results revealed the existence of mutual interactions and
successful increment in the absorption intensity (up to 176%) and optical stability, 9% changes in the rate of
photoisomerization and increment in the half life time of MC isomers. These were achieved in the presence of
magnetite as well as enhancement in the magnetic properties by 10% under UV irradiation at 365 nm. Such
synergistic and two-way interactions between magnetic field of the superparamagnetic Fe3O4 NPs and the
produced instant dipole moments in MC may offer a new route toward preparation of dual responsive nano-
composite particles for light-triggered magnetic switches, magnetic-triggered photo switches and simultaneous
sensing and separation probes.
Photochromic
Spiropyran
Magnetite
Miniemulsion polymerization
Stimuli-responsive
1. Introduction
In the past 50 years, spiropyran as a photochromic compound has
widely been studied according to its spectacular features. Upon UV
Smart or stimuli-responsive polymers are high-performance poly-
mers which have attained many attentions in recent years. Their prop-
erties can be changed based on external environmental stimuli such as
temperature, humidity, pH, solvent, mechanical force, light, electrical or
magnetic fields, ions, sound, enzymes and biomolecules [1–5]. The re-
sponses can be either physically or chemically like color or trans-
parency, conductivity, permeability, solubility, shape and etc., but the
changes must importantly be reversible [4,6]. Because of the
non-destructive, harmless, quick and remote controllability of light,
photoresponsive polymers are promising candidates in optics,
photo-switches, photo-mechanical systems, micropatterning, and
non-linear optical devices [7]. During photoisomerization of such
compounds, some physicochemical properties including oxidation/re-
duction potential, refractive index, dielectric constant, geometrical
structure and luminescence are changed in addition to their absorption
spectra [8–10].
irradiation, spiropyran (SP) to merocyanine (MC) isomerization occurs
with an intense color change through ring opening and cleavage of C–O
(spiro) bond to give zwitterion form. Merocyanine returns to the
colorless closed SP by either visible light or heat [11,12]. Copolymeri-
zation has already been accepted as an efficient route for incorporation
of spiropyran groups into a polymeric substrates [13–16]. As a result of
chemical bonding between spiropyran and polymer chains, these pho-
toresponsive copolymers represent prominent aspects including pre-
vention of the dye leakage or aggregation, reducing the probability of
undesired negative photochromism, enhancement in photostability and
photofatigue resistance of the photoactive moieties [17,18].
Magnetic nanoparticles (MNPs) and magnetic composite nano-
particles are being applied in various fields such as magnetic resonance
imaging (MRI), coatings, cell separation, catalysis, drug delivery sys-
tems, cancer therapy and magnetic separation due to their high response
rate widely [19–22]. Among several MNPs, Fe3O4 NPs have been
* Corresponding author.
E-mail address: a.mahdavian@ippi.ac.ir (A.R. Mahdavian).
https://doi.org/10.1016/j.polymer.2021.123524
Received 8 September 2020; Received in revised form 7 December 2020; Accepted 7 February 2021
Available online 11 February 2021
0032-3861/© 2021 Elsevier Ltd. All rights reserved.

A. Mouraki et al.
Polymer 218 (2021) 123524
recognized as the best nominee because of their excellent features like
reasonable biocompatibility, ease of synthesis and modification, high
ability to control size and shape, proper chemical stability and
remarkable magnetic properties [23,24]. These NPs, can be synthesized
in both hydrophobic and hydrophilic media [25]. The synthetic methods
include solvothermal [26], co-precipitation [27], polyol [28], sono-
chemical reactions [29], sol-gel [30] and thermal decomposition of the
organometallics [24,31]. However, adjustment of the size (even below
20 nm) and morphology with narrow particle size distribution are still
serious challenges to obtain superparamagnetic iron oxides nano-
particles (SPIONs) and can be achieved by thermal decomposition
method preferably. Reaction time, heating rate, surrounding atmo-
sphere, solvent, ligand and additives are the key points for this purpose
[32]. On the other hand, several strategies are being employed for
preparation of magnetic composite nanoparticles and encapsulation of
MNPs in the polymer particles [33,34], particularly in emulsion-based
systems [27]. Coverage of MNPs by polymeric shells will avoid their
agglomeration, protect them against environmental effects, improve
their chemical stability and reduce their toxicity [35].
2. Experimental section
2.1. Materials
2,3,3-Trimethylindolenin (Sigma-Aldrich Chemical Co.), iron (III)
acetylacetonate (Fe(acac)3) (Sigma-Aldrich Chemical Co.), 1,2-hexade-
canediol (Sigma-Aldrich Chemical Co.), oleic acid (Sigma-Aldrich
Chemical Co.), oleylamine (Sigma-Aldrich Chemical Co.), diphenyl
ether (Merck Chemical Co.), 3-(trimethoxysilyl)propyl methacrylate
(MPS) (Sigma-Aldrich Chemical Co.), sodium dodecyl sulfate (SDS)
(Sigma-Aldrich Chemical Co.) as an anionic surfactant, and Triton X-100
(Merck Chemical Co.) as a nonionic surfactant were used without further
purification. All solvents, 2-bromoethanol, 2-hydroxy-5-nitro-
benzaldehyde, methyl methacrylate (MMA), triethylamine, acryloyl
chloride (AC), sodium bicarbonate (NaHCO3) as a buffer, hexadecane
(HD) as a co-stabilizer and potassium persulfate (KPS) as the initiator
were supplied by Merck Chemical Co, and used as received. Tetrahy-
drofuran (THF) was dried over sodium and distilled off. Other solvents
and all reagents were used without purification. Deionized (DI) water
was used in all experiments.
Dual responsive polymeric nanoparticles to light and magnetic field
are among the latest trends in smart materials. The exploitation of these
polymeric nanoparticles in early diagnosis, MRI, targeted therapy, cell
separation, drug delivery, sensors and nanodevices are developing now
[36–39]. Osborne et al. [40] reported the synthesis of a reversible T2
agent using magnetite NPs and spiropyran in MRI and evaluated the
relaxation time in response to light irradiation. However, they did not
study the relationship between photochromic and magnetic properties.
In 2003, the intercalation of iron oxide NPs into photoresponsive
amphiphilic spiropyran vesicles were investigated together with their
interactions [41,42]. The observed aggregation after UV-irradiation led
2.2. Synthesis of 1′-(2-acryloxyethyl)-3′,3′-dimethyl-6-nitrospiro-(2H-1-
benzopyran-2,2′-indoline) (SPEA)
SPEA was synthesized in a four-step process and based on the pre-
viously reported procedure by our group [11]. Briefly, synthesis of SPEA
involved following steps: (1) substitution nucleation reaction between 2,
3,3-trimethylindolenine and 2-bromoethanol in methyl ethyl ketone
(MEK) under N2 to give 1-(2-hydroxyethyl)-2,3,3-trimethyl-3H-indo-
lium bromide pink solid; (2) isomerization reaction of the product from
step (1) in the presence of KOH/H2O to yield (R/S)-9,9,9a-trimethyl-2,3,
to an irreversible phenomenon as
a drawback for responsive
photochromic-magnetic systems. No close interaction between these
two stimuli-responsive moieties was taken into consideration, i.e.
through chemical bondings. Apart from this, prolonged UV irradiation
(about 180 min) with minor enhancement in magnetic properties might
be a questionable issue due to their poor mutual interactions.
9,9a-tetrahydrooxazolo[3,2-a] indole (R/S) as a yellow oil; (3)
condensation reaction between the product from step (2) and 2-hydrox-
y-5-nitrobenzaldehyde in ethanol (EtOH) under N2 which gives
(R/S)-2-(3′,3′-dimethyl-6-nitro-3′H-spiro-[chromene-2,2′-indole]-1′-yl)
ethanol (R/S) (SPOH) as a purple solid; (4) modification of SPOH with
acryloyl chloride in dry THF in the presence of triethylamine to produce
yellow precipitate of SPEA.
To the best of our knowledge, there is not any specific study on the
possible intervention between photoactive groups with magnetite
nanoparticles after putting them together and in a close contact. In
addition, such investigations may pave the way for preparation of light-
triggered magnetic switches or memories, magnetic-triggered photo-
switches and also simultaneous sensing and separation probes. In other
words, the aim of this study is obtaining photoresponsive super-
paramagnetic nanoparticles with dual responsivity as well as studies on
their mutual interactions. For real-time exploitations, desirable satura-
tion magnetism with near zero loss of magnetization during
magnetization-demagnetization cycles and optimized UV irradiation
time for photo-magnetization should be considered essentially. This
work reports the preparation of multifunctional composite nanoparticles
with photochromic and magnetic properties. Spiropyran ethyl acrylate
(SPEA) monomer and acrylic-modified Fe3O4 NPs were synthesized first.
The polymer-magnetite nanoparticles (PMNPs) were then prepared by
encapsulating Fe3O4 NPs with 10–15 nm size in poly (methyl
methacrylate-co-SPEA) (poly (MMA-co-SPEA)) through miniemulsion
polymerization. These new PMNPs with simultaneous response to
UV–Vis light and magnetic field were in the range of 65–95 nm and the
stimuli-responsive compartments were all covalently bonded in each
nanoparticle. Next, photochromic behavior of PMNPs under UV–Vis
irradiation, magnetic parameters, photofatigue resistance and long-term
photostability were investigated comprehensively. Covalent bonding
and embedment in the polymeric matrix provided the situation to study
the probable and exclusive interactions. Herein, the correlation between
the magnetic field of Fe3O4 NPs and the photochromic properties of
spiropyran groups has been considered as a novel aspect.
2.3. Preparation of Fe3O4 NPs and the modified magnetite (m-Fe3O4)
nanoparticles
Fe3O4 NPs were synthesized via a thermal decomposition method
[24]. 2.12 g (6 mmol) Fe(acac)3, 4.81 g (18 mmol) oleylamine, 5.08 g
(18 mmol) oleic acid and 7.75 g (10 mmol) 1,2-hexadecanediol were
dissolved in 60 mL diphenyl ether and the reaction mixture was heated
to 200 ◦C and maintained at this temperature for 1 h under a flow of
nitrogen gas, while stirring at 500 rpm. Then the mixture was further
heated to the reflux conditions (~260 ◦C) and kept for 2 h, until a stable
black suspension was obtained. The resulting black mixture was cooled
down to room temperature. At ambient conditions, 25 mL ethanol was
added and the synthesized Fe3O4 NPs were separated by a magnet,
followed by three times washing with ethanol and drying for 48 h under
vacuum at 40 ◦C.
Next, the prepared Fe3O4 NPs were modified by 3-(trimethoxysilyl)
propyl methacrylate (MPS). 1 g of Fe3O4 NPs was dispersed in 70 mL
toluene under ultrasonic irradiation for 10 min. The mixture was
transferred to a three-necked round bottom flask equipped with me-
chanical stirrer, condenser and nitrogen gas inlet. 1 g MPS was added to
the above dispersion and stirred at 700 rpm and 75 ◦C for 6 h. The
product (m-Fe3O4 NPs) was washed with ethanol (3 × 25 mL), separated
with a magnet and dried for 24 h under vacuum at 25 ◦C.
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A. Mouraki et al.
Polymer 218 (2021) 123524
2.4. Preparation of m-Fe3O4@poly(MMA-co-SPEA) nanocomposite
particles through miniemulsion polymerization
Table 2
Composition of the samples with various amounts of m-Fe3O4 NPs and SPEA.
Sample
m-Fe3O4 NPs (g)
SPEA (g)
m-Fe3O4 NPs (wt%)a
SPEA (wt%)a
The oil phase containing MMA, hexadecane and m-Fe3O4 NPs was
prepared and probe-sonicated for 2 min to achieve a good dispersion of
m-Fe3O4 NPs in MMA. To proceed miniemulsion polymerization, the
above dispersion was mixed with a SDS solution (as ionic surfactant),
Triton X-100 (as non-ionic surfactant) and NaHCO3 (as buffer) in DI
water and then, SPEA was added to this mixture. The mixture was probe-
sonicated for 20 min in an ice bath to form a pre-emulsion, followed by
transferring into a three-necked round bottom flask equipped with a
mechanical stirrer, condenser and nitrogen gas inlet. After addition of
KPS as the initiator at 70 ◦C and stirring at 300 rpm, polymerization
reaction was continued for 4 h to obtain stable latex with high conver-
sion (>95%) with almost no coagulum. The starting materials and their
amounts in preparation of typical latex (MNP0-S0) have been listed in
Table 1.
MNP0-
S0
0
0
0
0
MNP1-
S0
0.03
0.06
0
0
1
2
0
0
1
1
1
2
0
0
1
2
1
2
3
2
MNP2-
S0
0
MNP0-
S1
0.03
0.06
0.03
0.06
0.09
0.06
MNP0-
S2
0
MNP1-
S1
0.03
0.03
0.03
0.06
MNP1-
S2
MNP1-
S3
MNP2-
Samples with various contents of SPEA and m-Fe3O4 NPs, and similar
to the above recipe were prepared and amounts of stimuli-responsive
components have given in Table 2.
S2
a
Relative to MMA amount.
Finally, the latexes were coagulated by ethanol (as the non-solvent)
to determine the amount of covalently bonded SPEA to the acrylic
backbone. This was estimated to above 97% based on absorbance of the
remained serum relative to the standard solution. In addition, this
copolymerization efficiency was also confirmed, while the nanoparticles
were separated magnetically instead of coagulation and were analyzed
by absorption intensity measurement.
sputter coater (England). EDX (electron diffraction X-ray) spectroscopy
was coupled with SEM to identify the elemental composition. Mor-
phologies of the prepared products were detected using a Philips
EM208S transmission electron microscope (TEM; The Netherlands) at an
accelerating voltage of 150 kV. To prepare TEM samples, a drop of the
diluted latex was placed onto a copper grid and then left to dry at 30 ◦C
for 15 min. Molecular weight and its distribution was determined in THF
at a flow rate of 1 mL minꢀ 1 using a GPC Agilent 1100 (CA, USA) with a
refractive index detector. Narrow PS standards were employed for cal-
ibrations. Nanoparticles were centrifuged, washed with fresh DI water
and vacuum dried prior to dissolution in THF.
2.5. Characterization
The products were identified by means of a FT-IR BRUKER-IFS48
spectrophotometer (Germany) with a resolution of 4 cmꢀ 1 in the range
of 400–4000 cmꢀ 1. A small amount of sample powder was grinded with
KBr and the mixture was pressed into a disk for analysis. Photochromic
properties of PMNPs were evaluated through UV–Vis analysis by a
Shimadzu-UV2550 UV–Vis Spectrophotometer (Japan). For these ana-
lyses, the initial latexes were diluted to about 0.5 wt% with DI water. A
UV lamp (365 nm), CAMAG 12VDC/VAC (50/60 Hz, 14VA,
Switzerland) was used to stimulate the changes in structure and ab-
sorption bands of the photochromic SP moieties. Moreover, the source
for visible light was a 15 W LED lamp with white light. In all in-
vestigations, UV and Vis irradiations time were set 5 min for all the
samples. X-ray diffraction (XRD, D-5000 Diffractometer model, Siemens,
Germany) measurements were carried out using a radiation source of Cu
Magnetic properties of the samples were determined by vibrating
sample magnetometer (VSM, model LBKFB, Kavir Magnet Co., Kashan,
Iran). A small amount of powder was weighed and placed on the probe
and the magnetic field was applied in perpendicular direction to the
sample surface.
Sonication was carried out by means of an ultrasound generator,
SONOPULS Ultrasonic homogenizer, Model HF-GM 2200 (BANDELIN
Electronics GmbH and Co. KG, Berlin, Germany) and a titanium microtip
KE-76 probe with 6 mm diameter was employed as the probe.
3. Results and discussion
Photochromic properties and isomerization rate of SPEA depend on
its interaction with the surrounding environment, e.g. chemical struc-
ture, solvent characteristics and pH [4,43]. For photochromic-magnetic
polymer nanoparticles, there is no information available in the literature
about these interactions with magnetic field of the nanoparticles and
any attempt in this area will be worthwhile.
K
α
(2θ: 10–90) with a step size of 0.02◦ and a scan step time of 1 s.
Particle size and polydispersity index (PDI) of the diluted latexes (up
to 100 folds) were measured by dynamic light scattering (DLS), while
zeta potential of the diluted photochromic-magnetic latexes was deter-
mined by micro-electrophoretic method, both using Malvern Zetasizer
ZEN3600. Scanning electron microscopy (SEM, Vega II, TESCAN In-
strument, Czech Republic) was employed to study size and morphology
of the obtained PMNPs. One drop of the diluted latex was placed on a
sample holder and dried under vacuum at 25 ◦C. The sample was then
evacuated under vacuum and gold-sputtered by using EMITECH K450x
Here, superparamagnetic Fe3O4 NPs were synthesized through
thermal decomposition method and chemical modification of their
surface was carried out with 3-(trimethoxysilyl) propyl methacrylate
(MPS). Emulsion-based systems are among the most effective proced-
ures for preparation of multifunctional polymer nanoparticles. PMNPs
containing MMA, SP groups and m-Fe3O4 NPs were prepared via mini-
emulsion polymerization, through which m-Fe3O4 NPs were designed to
locate in the core and poly (MMA-co-SPEA) in the shell (Scheme 1). The
simultaneous presence of SP groups and m-Fe3O4 NPs in the polymeric
composite nanoparticles will provide photochromic and magnetic
properties, respectively. This strategy will guarantee preventing of some
disadvantages regarding to each segment, such as dye leakage or ag-
gregation, irreversible oxidation of Fe3O4 NPs, easy processability,
tailoring the switchability plus bio- and solvent-compatibility.
Table 1
Miniemulsion polymerization in preparation of
MNP0-S0 latex.
Component
Amount
DI water (mL)
MMA (g)
27
3
KPS (g)
0.075
0.124
0.177
0.060
0.075
Triton X-100 (g)
SDS (g)
NaHCO3 (g)
HD (g)
3

A. Mouraki et al.
Polymer 218 (2021) 123524
Scheme 1. Preparation of SPEA (up), superparamagnetic Fe3O4 NPs plus their surface modification (middle), and miniemulsion polymerization with MMA and SPEA
(below). SP⇄MC isomerization under UV (365 nm) and visible irradiations has been shown in the bottom.
3.1. Preparation and surface modification of the magnetite nanoparticles
pattern of Fe3O4 NPs show a cubic spinel structure. The strong six
discrete peaks at 2θ of 30.6◦, 36.4◦, 43.3◦, 53.4◦, 56.9◦ and 63.1◦ refer to
the lattice planes of [220], [311], [400], [422], [511] and [440],
respectively, with no impurity in Fe3O4 NPs. All diffraction peaks in the
XRD pattern comply with the characteristic diffractions of standard
Fe3O4 NPs inverse spinel structure (Joint Committee for Powder
Diffraction Studies, JCPDS, Card No. 19–0629). Average size of the
Preparation of SPIONs was performed through thermal decomposi-
tion method in order to achieve a good control on their both size and size
distribution with fine dispersion in the organic medium. Crystalline
structure of the prepared Fe3O4 NPs was identified by XRD analysis
(Fig. 1S, Supporting Information). The characteristic peaks in the XRD
Fig. 1. TEM micrographs of the prepared Fe3O4 cubic magnetic nanoparticles with different magnifications. The inset represents SAED pattern of the magnetite with
assigned lattice indexes.
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A. Mouraki et al.
Polymer 218 (2021) 123524
prepared Fe3O4 NPs crystallite was estimated from XRD results, using
Scherrer’s equation [24] and it was found 11 nm.
(coercive force) and Mr/Ms for the prepared samples were extracted
from VSM analysis and summarized in Table 3. High Ms, low Mr and low
Hc values and also approaching of Mr/Ms to zero are good indications for
obtaining SPIONs with magnetic single domains here. These illustrate
that both samples have no hysteresis loop. Also, Ms value of m-Fe3O4
NPs (56.42 emu/g) did not significantly reduce in comparison with that
of Fe3O4 NPs (58.05 emu/g), implying that the crystalline and magnetic
domains have not been influenced during this modification reaction.
Morphology and size of the obtained Fe3O4 NPs were investigated by
TEM and the images are shown in Fig. 1. It can be seen that the particles
possess almost cubic shape with regular distribution and size of about
15 nm. SAED (selected area electron diffraction) pattern from TEM
image is in accordance with the recorded XRD pattern too. Hence and
according to the small size and shape of the particles, it is expected to
have excellent superparamagnetic properties.
FT-IR spectra of Fe3O4 NPs and m-Fe3O4 NPs were taken (Fig. 2S,
3.2. Particle size and morphology of PMNPs
Supporting Information). The characteristic peaks at 443 and 588 cmꢀ 1
belong to the stretching vibration of Fe3+–O and Fe2+–O bonds,
To achieve suitable photochromic properties, particles size in the
latexes must necessarily be smaller than half of the incident beam
wavelength and usually below 100 nm [11]. Here, all affecting param-
eters except SPEA concentration and m-Fe3O4 NPs content were kept
constant in the recipes.
respectively. The bands at 3440 and 1625 cmꢀ 1 (merged with C
C
–
–
bond) correspond to O–H stretching and bending vibrations respec-
tively, and confirming the synthesis of Fe3O4 NPs. The peaks at 1406
(sym) and 1565 (asym) cmꢀ 1 are related to the stretching vibration of
–COOˉ bond from the added oleic acid (Fig. 2Sa). The bands appeared at
1108, 1460, 1540, 2850 and 2920 cmꢀ 1 are attributed to C–N
(stretching), CH2 (bending), NH2 (bending) and aliphatic C–H (sym and
asym), respectively, due to the presence of oleylamine on the surface of
Fe3O4 NPs [44]. The presence of MPS moieties on the surface of Fe3O4
NPs was confirmed by FT-IR spectrum of m-Fe3O4 NPs (Fig. 2Sb). The
appearance of new peaks at 802 cmꢀ 1 (Si–O–Si bending), 1013 cmꢀ 1
(Si–O–Si symmetric stretching), 1095 cmꢀ 1 (Si–O–Si asymmetric
Average particles size (Dz) and polydispersity index (PDI) of the
prepared latexes were determined by dynamic light scattering (DLS)
analysis and their zeta potentials were also measured (Fig. 3). The ob-
tained particles sizes were all laid in the range of 65–95 nm. It could be
found that Dz of the samples did not change significantly by increasing
the number of m-Fe3O4 NPs in the polymerization recipe. Similarly, no
obvious variation in Dz was found with the increase in SPEA concen-
tration at constant m-Fe3O4 NPs content. These results illustrate that the
considered amounts of SPEA and modified magnetite nanoparticles did
not affect the polymerization process, particle formation and growth. On
the other hand, the particles were all below 100 nm and the latexes had
almost narrow particles size distributions (Table 4). It is expected that
the presence of SPEA in the particles shell would cause some changes in
stretching), 1170 cmꢀ 1 (C–O stretching), 1260 cmꢀ 1 (Si–C stretching),
ꢀ 1
1455 cmꢀ 1 (C–H bending), 1637 cmꢀ 1 (C C stretching) and 1717 cm
–
–
–
–
(C O stretching) implies successful chemically surface modification of
magnetite NPs with MPS.
Energy dispersive X-ray (EDX) analysis as well as the corresponding
elemental map for Si and Fe were carried out to confirm the co-existence
of MPS and Fe3O4 NPs too (Fig. 3S, Supporting Information). EDX
pattern shows the existence of Si (3.51 wt%), C (11.43 wt%), O (34.36
wt%) and Fe (50.70 wt%), depicting the inclusion of MPS on the surface
of Fe3O4 NPs. However, Si- and Fe-mapping reveal uniform distribution
of the components.
Table 3
Magnetic parameters from VSM analysis for the prepared Fe3O4 and m-Fe3O4
nanoparticles.
Sample
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Mr/Ms
Magnetic properties of Fe3O4 NPs and m-Fe3O4 NPs were evaluated
by vibrating sample magnetometer (VSM) analysis (Fig. 2). The values of
Ms (saturation magnetization), Mr (remanence magnetization), Hc
Fe3O4 NPs
58.05
56.42
0.008
0.006
0.181
0.142
0.00014
0.0001
m-Fe3O4 NPs
Fig. 2. VSM analysis for Fe3O4 (a) and m-Fe3O4 (b) nanoparticles. The insets reveal the corresponding expanded regions.
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A. Mouraki et al.
Polymer 218 (2021) 123524
Fig. 3. DLS analysis of the prepared PMNPs with different SPEA concentrations and m-Fe3O4 NPs contents.
Table 4
Extracted data from DLS analysis for the prepared latexes.
Sample
MNP0-S0
MNP0-S2
MNP2-S0
MNP2-S2
MNP1-S0
MNP1-S1
MNP1-S2
MNP1-S3
MNP1-S2 (after UV)
Dz (nm)
88
93
80
69
86
83
78
76
–
PDI
0.187
ꢀ 37.14
0204
ꢀ 25.82
0.225
–
0.156
–
0.191
–
0.172
ꢀ 27.69
0.179
ꢀ 26.03
0.164
ꢀ 24.23
–
Zeta Potential (mV)
ꢀ 20.73
their surface properties, especially after UV irradiation. Zeta potential
measurements were used to verify this, and the results for some typical
samples have been given in Table 4.
Due to the presence of SDS as the anionic surfactant, negative zeta
potentials are predictable. It should be noted that the amount of SDS in
all PMNPs samples is identical. However, zeta potential of PMNPs
Fig. 4. SEM micrographs of (a) MNP0-S0, (b) MNP2-S0, (c) MNP0-S2 and (d) MNP1-S2 latex particles.
6

A. Mouraki et al.
Polymer 218 (2021) 123524
increased by the increase in SPEA content slightly. This could be
attributed to the localization of SPEA on the particles surface, which
suppresses the negative charge density with approximately similar sizes
in PMNPs. This was strictly approved by measurement of zeta potential
of MNP1-S2 after UV irradiation at 365 nm typically. Zeta potential is
expected to change after UV irradiation due to the conversion of SP to
MC with zwitterionic character, and this was found by the increase from
ꢀ 26.03 to ꢀ 20.73 mV, revealing the variation in charge density around
PMNPs after MC formation. On the other hand, this analysis for MNP0-
S2 and MNP1-S2 showed no significant changes with respect to the
engulfment of m-Fe3O4 NPs inside PMNPs and their entrapment by the
copolymer shell.
colored merocyanine form upon UV irradiation (365 nm), which could
be identified by strong absorption bands in the ranges of 350–450 and
450–650 nm. All of the latex samples had almost similar λmaxs to SPEA at
388 and 570 nm, referring to SP⇄MC isomerization upon UV–Vis irra-
diation. Maximum absorbance (Amax) for each peak was measured and
has been summarized in Table 5. By entrance of SP into the polymeric
nanoparticles, λmax for MC shifted from 514 to 570 nm (
π
→π
* electronic
transition of the merocyanine form), while λ for n→
π
* electronic
max
transition of the chromene moiety (388 nm) did not change. This would
be explained by the environmental effect of the polymeric matrix on
stabilization of MC isomer and increase in conjugation length. All of the
prepared latexes had favorable photochromic properties and were
identifiable evidently (Fig. 6c). The presence of SP inside nanocomposite
nanoparticles was also traced by XRD analysis typically for MNP1-S2
(Fig. 6S, Supporting Information), representing an extra peak at 2θ of
12.3◦ and the intensified peak at 15◦ relative to magnetite (Fig. 1S,
Supporting Information). These two new peaks would be related to the
presence of crystalline SP in the obtained nanocomposite nanoparticles.
The effects of increasing amounts of m-Fe3O4 NPs and SPEA on
photochromic properties were studied, and it was found that the ab-
sorption intensity improved by the increase in magnetite content. As
expected, Amax increased from MNP0-S1 to MNP0-S2 with the increase
in SPEA content (Table 5), this was significantly observed for MNP1-S2
(104%) and MNP2-S2 (176%), while the amount of SPEA was kept
constant in these samples (Fig. 6b). These notable variations in Amax
values were accompanied by a slight red shift (5 nm in the corre-
sponding λmaxs. This increment in the absorption intensity might origi-
nate from one or more triggers like enhanced light absorption by Fe3O4
NPs, stabilization of MC form in vicinity of Fe3O4 NPs and stacking of
MC isomers. If stacking occurs, another absorption band at longer
wavelengths of 570 nm (~620 nm) representing the formation of J-ag-
gregates of MC isomers will be expected [42]. Due to the absence of new
absorption band at longer wavelength, this phenomenon is less likely for
observing such an enhancement in intensity by introducing magnetite.
On the other hand, magnetite nanoparticles are able to strongly absorb
light at a wide range of wavelength with a more preference in UV region
(Fig. 8S, Supporting Information). This is the reason for detecting high
absorption of UV irradiation beside the incorporated photochromic
groups. There are also some reports on the enhanced light absorption by
aggregates of magnetite nanoparticles which are consistent with the
above findings [19,45]. Either enhancement in absorption intensity or a
slight bathochromic shift in λmax would be the result of possible stabi-
lization of MC zwitterionic form with the vicinal Fe3O4 NPs through
dipolar interactions, and also through dipole-magnetic interactions
(with magnetic field of Fe3O4 NPs).
As a complementary characterization, MNP0-S0 (as a test sample
that had similar polymerization recipe without any insoluble Fe3O4 NPs)
was chosen for GPC analysis to measure molecular weights of the
copolymer and their distribution (Fig. 4S, Supporting Information). The
obtained weight and number average molecular weights were found
1.85 × 105 and 7.2 × 104 g molꢀ 1, respectively, with PDI of 2.6.
SEM and TEM analyses were used to investigate the morphology and
particle size of the obtained latex particles. Fig. 4 demonstrates that the
obtained particles are spherical and their sizes are in good agreement
with DLS results.
TEM image of MNP1-S2 has been shown in Fig. 5 typically. It is
evident that magnetite NPs have been located inside the polymer par-
ticles and the predicted core-shell morphology is obvious. No extra ring
was observed in SAED pattern of the prepared nanocomposite nano-
particles, implying negligible amount of silica. The approximate amount
of Fe3O4 NPs was estimated in the range of 1.5–2 wt% by TGA analysis of
the prepared nanocomposite samples (Fig. 5S, Supporting Information)
with respect to the applied procedure. Totally, DLS, SEM and TEM an-
alyses are all consistent and confirm the spherical core-shell morphology
of the latex nanoparticles with average sizes below 100 nm with
encapsulated multiple m-Fe3O4 NPs.
3.3. Study on the interaction between photochromic and magnetic
components
After approving the structural characteristics of the obtained PMNPs,
the possible interactions between magnetite nanoparticles and photo-
chromic moieties were studied for the first time. For this reason,
photochromic properties of the latexes (0.5 wt%) were assessed by
UV–Vis analysis (Fig. 6). UV irradiation causes isomerization of the
closed ring and nonpolar form of SP (colorless) into the open form and
zwitterionic merocyanine (colored) together with color changes in the
latex. This will result in appearance of strong absorption bands in the
ranges of 350–450 and 450–650 nm with maximum absorption wave-
length (λmax) at 388 nm and 570 nm. SPEA aqueous solution was
exposed to UV irradiation and its UV spectra has been shown in Fig. 7S
(Supporting Information). The colorless SPEA was converted to the
As a consequence, the coexistence and close contact of magnetite
nanoparticles with photochromic moieties may provide high efficiency
for the applications at which multi-responsivity accompanied by
improved absorption intensity are specially demanded.
Fig. 5. TEM micrographs of MNP1-S2 with different magnifications. The dark region corresponds to the magnetite NPs and the grey region reveals the polymeric
layer. The inset represents SAED pattern of the prepared nanocomposite particles.
7

A. Mouraki et al.
Polymer 218 (2021) 123524
Fig. 6. UV–Vis spectra of the stimuli-responsive PMNPs containing different SPEA and m-Fe3O4 NPs ratios after UV illumination (365 nm, 5 min) (a and b). Color
changes of the latexes (0.5 wt %) during SP ⇄ MC isomerization with UV and visible irradiations (c). (For interpretation of the references to color in this figure
legend, the reader is referred to the Web version of this article.)
In other words, SP to MC isomerization and in-situ formation of the
Table 5
zwitterionic MC form enhance magnetization of Fe3O4 NPs in prolonged
The extracted numerical data from UV–Vis spectra.
UV irradiation without affecting superparamagnetic properties. There
Sample
Amax at
570 nm
Amax at
388 nm
Increment in Amax at
570 nm (%)
Increment in Amax at
388 nm (%)
have recently been few reports on light-induced magnetization of mo-
lecular nanoparticles, including Prussian blue analogs, Hofmann-type
systems, single-molecule magnets and those based on transition metal
oxides like Fe3O4 [47,48]. These are in accordance with our observa-
tions and probable induced photo-magnetization of Fe3O4 NPs. Note-
worthy that photo-magnetism is detectable at temperatures below the
block temperature of nano magnetite. The presence of SP groups adja-
cent to Fe3O4 NPs may have shifted this process toward ambient tem-
perature. On the other hand, anisotropic energy barrier of Fe3O4 NPs
should be lowered in order to observe light-induced magnetization [48].
It seems that zwitterionic MC isomers have been able to reduce magnetic
energy barrier for spin-flip transitions far more than what temperature
reduction do and promote this photo-magnetism. Dipole moments of SP
and MC are ~4–6 D and ~14–18 D, respectively [49,50]. MC isomer has
quite different affinity to chemical compounds due to its higher polarity,
and particularly to metal ions [51,52]. It could be concluded that the
established efficient dipole-magnetic interaction and subsequent
reduced magnetic energy barrier as a result of interaction between Fe
cations and MC zwitterions are responsible for such observations. More
detailed elucidation needs further complementary experiments which is
not in the scope of the present work and we hope these may open up new
ways for practical application of spin-based electronics and photonics.
MNP0-
S1
0.056
0.111
0.080
0.227
0.265
0.306
0.028
0.056
0.039
0.110
0.135
0.148
–
–
MNP0-
S2
98a
100a
MNP1-
S1
42a
39a
MNP1-
S2
305a/104b
373a
293a/96b
382a
MNP1-
S3
MNP2-
446a/176b
429a/164b
S2
a
b
Increment in absorption intensity compared to MNP0-S1.
Increment in absorption intensity compared to MNP0-S2.
With respect to the advantages of MNP1-S2 and MNP2-S2 samples,
their magnetic behaviors were studied as well. As already shown, closely
related magnetite NPs had synergistic effect on photochromic properties
and enhanced absorption intensities together with a slight red shift in
λmax. To check whether this interaction is mutual, i.e. photoactivated
moieties have any effect on the magnetic properties of the adjacent
Fe3O4 NPs, VSM analysis was recorded for these two samples in the
presence and absence of UV irradiation at 365 nm (Fig. 7). The results
have been extracted and shown in Table 6. A slight change (4% for
MNP1-S2) in magnetic parameters was found after exposure to UV (365
nm) for 5 min and this was more sensed for MNP2-S2 (10%) with higher
Fe3O4 NPs content. This improvement is remarkable in comparison with
previous reports, such as 0.1 emu.gꢀ 1 elevation upon 180 min UV
irradiation (in the magnetic field of ꢀ 3500 to +3500 Oe) [41] and also
up to 5% increment in magnetization after 250 min UV irradiation [42].
It has been demonstrated that the photoisomerization alters the
distance between Fe3O4 NPs or their coordination sphere, and these
changes are reflected in Ms, Mr and Hc of the magnetite NPs [39,40,46].
3.4. Kinetic of isomerization
The kinetic of photoisomerization and variations in the absorption
intensity of all latexes (0.5 wt%) in different time intervals upon UV
irradiation have been demonstrated in Fig. 8. In order to inspect the
effect of m-Fe3O4 NPs on switching rate of SP to MC (Fig. 8a–f) and
reversible isomerization (Fig. 8e and f) after irradiations, the absorption
intensities of MC form was measured at the corresponding λmaxs. All the
samples attained their maximum absorption intensities under UV irra-
diation (365 nm) after about 60 s with a steep slope before reaching to
8

A. Mouraki et al.
Polymer 218 (2021) 123524
Fig. 7. VSM analyses of (a) MNP2-S2 (after UV), (b) MNP2-S2 (before UV), (c) MNP1-S2 (after UV) and (d) MNP1-S2 (before UV). The insets reveal the corre-
sponding expanded regions and UV irradiation was carried out at 365 nm for 5 min.
Table 6
Magnetic parameters obtained from VSM analysis for MNP1-S2 and MNP2-S2 samples before and after UV irradiation at 365 nm (for 5 min).
Sample
Before exposure to UV irradiation
After exposure to UV irradiation
Variation upon UV exposure (%)
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Mr/Ms
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Mr/Ms
ΔMs (emu/g)
ΔMr (emu/g)
ΔHc (Oe)
ΔMr/Ms
MNP1-S2
MNP2-S2
0.5036
0.8645
0.00026
0.00088
1.154
2.103
0.0005
0.0010
0.5238
0.9552
0.00032
0.00101
1.272
2.401
0.0006
0.0011
4.0
23.1
14.8
10.2
14.2
20
10
10.5
the plateau region. The complete sweep cycle has been demonstrated in
Fig. 8e and f in which, a direct correlation between the absorption in-
tensity and UV irradiation time is clearly observed according to the
formation of MC isomer. The results of absorption intensity show an
exponential dependency of the isomerization rate on irradiation time in
both directions (SP⇄MC) for MNP1-S2 and MNP2-S2 typically.
Totally, SP to MC isomerization kinetic for all latexes could be well-
described by Equation (1), while that for MC to SP isomerization was
fitted by Equation (2) and the later was studied for those in Fig. 8e and f.
These equations are applicable to both λmaxs at 388 and 570 nm.
of m-Fe3O4 NPs on the isomerization kinetic and photochromic prop-
erties was investigated for the samples with similar SPEA concentration
in the polymer particles (MNP1-S2 and MNP2-S2). kC or kD and the
absorption intensity are good indications to study the responsivity in
such dual responsive latex particles during photoisomerization. Kinetic
equations and the corresponding parameters, i.e. kC, kD, R2 (coefficient
of determination), and the required time to reach half of the final
absorbance for color changes (T1/2) have been extracted from Fig. 8 and
summarized in Table 7. R2 values depict good fitting of Equations (1)
and (2) with the experimental results. It is worth mentioning that kC or
kD and T1/2 are approximately the same for each sample at both λmax
s
Norm alized absorbance (t)
= A ꢀ A′ exp( ꢀ kCt)
Eq 1
(388 and 570 nm), depicting absorption consistency at these two
wavelengths and formation of similar molecular structures in the pres-
ence and absence of m-Fe3O4 NPs. According to the obtained parameters
for different samples in the presence of m-Fe3O4 NPs, maximum
responsivity and minimum T1/2 for SP to MC isomerization were
observed for MNP2-S2. These illustrate that m-Fe3O4 NPs have got
involved in the photoisomerization process and facilitated this process
due to the enhanced light absorption. Coexistence of photochromic
compound with magnetite nanoparticles not only led to facile formation
of MC isomers, but also reduced the rate of reverse isomerization reac-
tion (almost 50%) with respect to kD and T1/2 (about 40 s). The latter
could be attributed to the stabilization of MC zwitterions through effi-
cient interaction with Fe3O4 NPs lattices which has been discussed in
previous section and its consequent less tendency toward interchange to
SP form. Similarly, rapid SP to MC isomerization as well as slow MC to
SP isomerization have previously been reported by changing in polarity
of the media or pH, where MC isomer would become more stable [4].
Norm alized absorbance (0)
Norm alized absorbance (t)
Norm alized absorbance (0)
= A∘ + A′′exp( ꢀ kDt)
Eq 2
where A, A∘, A′ and A′′ are constant values; kC and kD represent the rate
constants for SP to MC (Equation (1)) and MC to SP (Equation (2))
photoisomerization upon UV and visible light irradiations, respectively.
The normalized absorbance values are the maximum absorbance in-
tensities of MC form in the photoresponsive latexes at 388 or 570 nm
after UV or visible light irradiations at times t and 0.
It has previously been shown that polarity, pH of the environment
and chain flexibility are the influential factors on isomerization kinetics
of photoactive groups [4,17]. Here, the proposed interaction between
magnetic field of Fe3O4 NPs and instantly produced dipole moments in
MC isomers (by UV light) was followed by kinetic studies of the isom-
erization process as a new parameter beside the above factors. The effect
9

A. Mouraki et al.
Polymer 218 (2021) 123524
Fig. 8. Normalized absorbance values ( and ) and exponential fitting (
and
) curves for the kinetic of SP ⇄ MC isomerization of the prepared stimuli-
responsive PMNPs latexes at the corresponding λmaxs under UV (365 nm) and visible irradiations: (a) MNP0-S1, (b) MNP0-S2, (c) MNP1-S1, (d) MNP1-S3, (e) MNP1-
S2 and (f) MNP2-S2.
3.5. Photofatigue resistance and photo-switchability
during 9 cycles and the obtained absorption intensities were measured
at 388 and 570 nm (Fig. 9). The absorbances were immediately
measured after each irradiation within a 5 min interval between each
cycle. This was carried out to analyze the role of incorporated Fe3O4 NPs
on photofatigue resistance of SPEA. It had previously been approved
that photoreversibility and photofatigue resistance of photochromic
compounds are substantially boosted by inclusion into a hydrophobic
Photoreversibility and photofatigue resistance of SPEA-containing
latexes are important parameters in evaluation of the life-time and
photochromic behavior in multi-responsive polymers. Here, the pre-
pared diluted latex samples (0.5 wt%) were exposed to alternating
illumination cycles of UV (365 nm, 5 min) and visible (5 min) lights
10

A. Mouraki et al.
Polymer 218 (2021) 123524
Table 7
Coloration and discoloration rate equations and the corresponding kinetic parameters for the prepared stimuli-responsive PMNPs latexes under UV (365 nm) and
visible light irradiations for the samples containing various SPEA and m-Fe3O4 NPs ratios.
Isomerization
Sample
Absorbance at 388 nm
Absorbance at 570 nm
Equation
kC (sꢀ 1
)
kD (sꢀ 1
)
T1/2
(s)
R2
Equation
kC (sꢀ 1
)
kD (sꢀ 1
)
T1/2
(s)
R2
SP to MC
MNP0-
S1
0.03346–0.03346 exp
(-0.10625t)
0.10625
0.10796
0.04187
0.03658
0.03451
0.04324
–
–
–
–
–
–
7
0.99
0.99
0.99
0.99
0.99
0.99
0.06553–0.06553 exp
(-0.10631t)
0.10631
0.10807
0.04193
0.03667
0.03461
0.04336
–
–
–
–
–
–
7
0.99
0.99
0.99
0.99
0.99
0.99
MNP0-
S2
0.06703–0.06703 exp
(-0.10796t)
6
0.13661–0.13661 exp
(-0.10807t)
6
MNP1-
S1
0.04302–0.04302 exp
(-0.04187t)
17
19
20
16
0.09541–0.09541 exp
(-0.04193t)
17
19
20
16
MNP1-
S2
0.11985–0.11985 exp
(-0.03658t)
0.25545–0.25545 exp
(-0.03667t)
MNP1-
S3
0.13502–0.13502 exp
(-0.03451t)
0.29312–0.29312 exp
(-0.03461t)
MNP2-
S2
0.16079–0.16079 exp
(-0.04324t)
0.34579–0.34579 exp
(-0.04336t)
MC to SP
MNP1-
S2
0.00815 + 24.84378 exp
(-0.01297t)
–
–
0.01297
0.01195
53
58
0.99
0.99
0.01009 + 57.75152 exp
(-0.01299t)
–
–
0.01299
0.01198
53
58
0.99
0.99
MNP2-
S2
0.00841 + 23.16548 exp
(-0.01195t)
0.00724 + 52.04732 exp
(-0.01198t)
Fig. 9. Photofatigue resistance of the prepared stimuli-responsive PMNPs latexes upon alternating UV (365 nm, 5 min) and visible (5 min) irradiations: (a) MNP0-S2,
(b) MNP1-S2 and (c) MNP2-S2.
polymeric matrix via covalent bonding [15]. Here, the obtained results
exhibited excellent photostability and photoreversibility for the exam-
ined samples. It is evident from Fig. 9 that the internalized m-Fe3O4 NPs
have caused significant enhancement in the photostability. Higher
m-Fe3O4 NPs content in the sample made lower fluctuation of absor-
bance for the dual functional composite nanoparticles. It has already
been reported that photofatigue resistance is controlled by the dose of
absorbed light beside chemical stability of the photochromic compound
[53]. Therefore, the observed steady absorbances in the presence of
magnetite nanoparticles can be attributed to the enhanced contribution
of Fe3O4 NPs in light absorption, resulting in limited fluctuations.
These confirm that such polymeric magnetic photoactive materials
11

A. Mouraki et al.
Polymer 218 (2021) 123524
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4. Conclusions
In this study, some novel, photochromic-magnetic polymer nano-
particles (PMNPs) were designed as efficient dual responsive probes via
miniemulsion polymerization due to the presence of magnetite and
spiropyran groups simultaneously. According to SEM, TEM and DLS
analyses, all stimuli-responsive latex particles were spherical with sizes
in the range of 65–95 nm. Zeta potential analysis revealed that SP groups
in the prepared PMNPs have been localized in the outer layer. UV–Vis
and VSM analyses were comprehensively followed to investigate prob-
able mutual correlation between Fe3O4 NPs and MC form through
magnetic field and dipolar interactions, because of their close in-
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photochromic properties had positive synergy and improved signifi-
cantly. These were observed by measurement of magnetic parameters
and UV absorptions intensities for several samples with various
magnetite and SPEA contents. Studies on the kinetic of photo-
isomerization were another strong evidence to prove the above-
mentioned effective interactions. However, photofatigue resistance
and photo-switchability were improved for the prepared PMNPs with
respect to those without Fe3O4 NPs.
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Declaration of competing interest
[18] J.K. Rad, A.R. Mahdavian, H. Salehi-mobarakeh, A. Abdollahi, FRET phenomenon
in photoreversible dual-color fluorescent polymeric nanoparticles based on
azocarbazole/spiropyran derivatives. https://doi.org/10.1021/acs.macromo
l.5b02401, 2016.
This is also notable that there is no conflict of interest with this
submission to Polymer.
[19] T.-T.D. Pham, Y.H. Seo, D. Lee, J. Noh, J. Chae, E. Kang, J. Park, T.J. Shin, S. Kim,
Acknowledgement
J. Park, Ordered assemblies of Fe3O4 and a donor-acceptor-type π-conjugated
polymer in nanoparticles for enhanced photoacoustic and magnetic effects,
Polymer 161 (2019) 205–213, https://doi.org/10.1016/j.polymer.2018.12.020.
We wish to express our gratitude to Iran Polymer and Petrochemical
Institute (IPPI) for financial support of this work (Grant# 24761185).
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Appendix A. Supplementary data
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