Magnetic polymer bowl for enhanced catalytic activity and recyclability

Article abstract of DOI:10.1039/d1ra00453k

This work introduces the fabrication of a magnetic polymer bowl for enhanced catalytic activity and recyclability, which involves the synthesis of silica-coated Fe₃O₄magnetic clusters, seeded dispersion polymerization using the magne

Full text of DOI:10.1039/d1ra00453k

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Magnetic polymer bowl for enhanced catalytic
activity and recyclability†
Cite this: RSC Adv., 2021, 11, 13545
cd
Sang Gi Hong,a Eunmi Im,‡b Da In Kim,a Eun Jin Jeong,b Jongbok Kim,
b
a
*
*
Geon Dae Moon and Dong Choon Hyun
This work introduces the fabrication of a magnetic polymer bowl for enhanced catalytic activity and
recyclability, which involves the synthesis of silica-coated Fe3O4 magnetic clusters, seeded dispersion
polymerization using the magnetic clusters, and transformation into a bowl-like structure via a phase
separation route. The additional treatment with tannic acid (TA) on the bowls allows the in situ formation
of silver nanoparticles (AgNPs) on their surfaces. The openness and larger surface area of the bowls, as
compared with those of other structured particles, such as spheres and flowers, enable a considerably
higher immobilization of AgNPs, thus leading to an excellent catalytic reduction for 4-nitrophenol (4-
NP), methylene blue (MB), and rhodamine B. Furthermore, the strong magnetic response originating
from the magnetic clusters inside the bowls endows a good magnetic recovery and an excellent
reusability for the repeated reduction of the organic dyes without loss of catalytic activity.
Received 19th January 2021
Accepted 7th April 2021
DOI: 10.1039/d1ra00453k
rsc.li/rsc-advances
catalysts, which can convert organic dyes into non-/less toxic or
useful products through change in their molecular structures,
Introduction
have recently gained considerable attention owing to its high
efficiency, low cost, and simple setups.8,9 For example, 4-NP can
be reduced to 4-aminophenol (4-AP) in the presence of catalysts
and reductants, which is a useful compound in pharmaceutical
industry.10 Similarly, rhodamine B and MB can be reduced to
colorless compounds (leuco rhodamine B and leuco MB) with
the use of proper catalysts in the presence of NaBH4.11,12 When
catalytic activity and chemical/physical stability are concerned,
noble metal nanoparticles (NPs), including platinum (Pt), silver
(Ag), and gold (Au) have been considered as the optimal cata-
lysts.2,13,14 However, the poor colloidal stability, originating from
their high surface area, and difficulty in separation cause the
decrease of active sites and catalytic performance for the
reduction of organic dyes.15 In addressing this issue, the use of
various supports to deposit noble metal NPs on their surfaces
has been reported.16 Among them, polystyrene (PS) particles
have shown numerous advantages as a catalytic support mate-
rial, such as their simple and large scalable synthesis, mono-
dispersity and controllability in size and shape, good dispersion
in various media, and easy surface modication.17–20 Because
most of previously reported studies have used, however, solid PS
particles with a relatively low surface area,20–26 it is difficult to
increase the loading amount of noble metal NPs. Alternatively,
numerous studies have reported the fabrication of PS particles
with hollow structures to provide large surface areas.27 However,
the closed shells in most of the hollow particles allow the
deposition of noble metal NPs only on the outer surfaces, not
exhibiting a higher deposition of noble metal NPs compared
with solid PS particles. Furthermore, their limited recovery from
With the rapid growth of industrialization and population,
modern society has suffered from environmental pollution.1
Especially, inow of effluent containing organic dyes into water
resources causes serious environmental problems because the
strong color of dyes can block oxygen and sunlight from pene-
trating the water.2 Furthermore, toxicity of organic dyes can
threaten human health.3 As a representative organic dye
distributed from agricultural and industrial areas, 4-nitro-
phenol (4-NP), which is responsible for methemoglobinemia,
can cause cyanosis, confusion, and unconsciousness as well as
damage to the liver and kidneys.4,5 Rhodamine B and methylene
blue (MB) exhibit toxicity and carcinogenicity, which can cause
numerous severe diseases.6,7 Thus, the development of an effi-
cient approach to remove harmful organic dyes from contami-
nated water is essentially needed.
Among several techniques developed for the removal of
organic dyes, chemical approaches using nanomaterial
aDepartment of Polymer Science and Engineering, School of Applied Chemical
Engineering, Polymeric Nano-materials Laboratory, Kyungpook National University,
Daegu 41566, South Korea. E-mail: dong.hyun@knu.ac.kr
bDongnam Division, Korea Institute of Industrial Technology, Busan 46938, Korea.
E-mail: gmoon@kitech.re.kr
cDepartment of Materials Science and Engineering, Kumoh National Institute of
Technology, Gumi, Gyeongbuk 39177, Korea
dDepartment of Energy Engineering Convergence, Kumoh National Institute of
Technology, Gumi 39177, Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra00453k
‡ These authors contributed equally to the preparation of this paper.
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reaction system by centrifugation or ltration causes a poor
reusability during cycles of catalytic process, consequently
hindering their practical applications to the treatment of toxic
organic dyes.
Synthesis of Fe3O4 magnetic clusters and their coating with
silica
Using the modied solvothermal method, Fe3O4 magnetic
clusters were synthesized, as reported previously.28 Briey,
a PSSMA solution was rst prepared by dissolving 0.5 g of the
polymer into 20 mL of EG under vigorous stirring, followed by
additionally introducing 0.54 g of FeCl3$6H2O and 1.5 g of
NaOAc. Aer sealing the mixture solution in a Teon-lined
stainless-steel autoclave, the system was heated to 200 ꢀC for
10 h. The sample was then cooled to room temperature, fol-
lowed by centrifugation (4500 rpm, 15 min), washing with DI
water, and vacuum-drying for 24 h. For coating the Fe3O4
magnetic clusters with silica, the dried clusters were mixed with
a co-solvent consisting of 100 mL of ethanol and 15 mL of DI
water, followed by injecting 5 mL of NH4OH solution and 1 mL
of TEOS. Aer 3 h, the resultant clusters, separated by a magnet,
were consecutively washed with ethanol and DI water.
In this study, we demonstrate the fabrication of a magnetic
polymer bowl for the enhanced catalytic activity and recycla-
bility. The magnetic polymer bowl is prepared through
synthesis of silica-coated Fe3O4 magnetic clusters, their surface
treatment with vinyl group, and seeded dispersion polymeri-
zation under the use of the magnetic clusters, followed by phase
separation for the transformation into bowl-like structure
(Scheme 1). The additional treatment with tannic acid (TA) on
the bowls allows the in situ formation of AgNPs on their
surfaces. The openness and larger surface area of the bowls, as
compared with those of other structured-particles such as
spheres and owers, enable a considerably higher immobili-
zation of AgNPs, leading to an excellent catalytic reduction for 4-
NP, rhodamine B, and MB. Furthermore, the strong magnetic
response arising from the magnetic clusters inside the bowls
endows a good magnetic recovery and an excellent reusability,
achieving twenty consecutive reduction of the dye molecules
with no loss in catalytic activity.
Fabrication of magnetic polymer spheres and owers
Aer the silica-coated magnetic clusters (0.5 g) were mixed with
100 mL of ethanol, MPS was injected. The mixture solution was
mechanically agitated at room temperature to induce the
formation of vinyl groups on the surfaces of the silica-coated
magnetic clusters. Subsequently, the mixture solution was
washed with ethanol and DI water. The MPS-treated clusters (0.12
g) were introduced into a co-solvent with 70 mL of ethanol and
30 mL of DI water, together with 0.06 g of AIBN, 0.03 g of NaSS, and
3 mL of St. Aer mechanical agitation for 30 min, the mixture
solution was heated to 70 ꢀC for 6 h under a nitrogen (N2) atmo-
sphere. Then, the resultant particles were separated with a magnet,
washed with ethanol and DI water, and dried in vacuum.
Experimental
Materials
Poly(4-styrenesulfonic acid-co-maleic acid, SS : MA ¼ 3 : 1)
sodium salt (PSSMA, MW ¼ 20 000), styrene (St), tetraethyl
orthosilicate (TEOS), silver nitrate (AgNO3), decane, TA, 4-NP,
rhodamine B, MB, anhydrous sodium acetate (NaOAc), sodium 4-
vinylbenzenesulfonate (NaSS), ethylene glycol (EG), ammonium
hydroxide (NH4OH) solution, 2-ethylhexyl methacrylate (EHMA), 3-
(trimethoxysilyl)propyl methacrylate (MPS), ferric chloride (FeCl3),
isoamyl alcohol, ferric chloride hexahydrate (FeCl3$6H2O), and
azobisisobutyronitrile (AIBN) were obtained from Sigma-Aldrich.
All chemicals were used as received without further purication.
Transformation of magnetic polymer sphere into magnetic
polymer bowls
Deionized (DI) water with a resistivity of 18.2 MU cm, puried by First, 0.025 g of the magnetic polymer spheres was put into
a water ultra-purication system (ROMAX, Human Science), was a glass vial with 10 mL of methanol and 2 mL of DI water, fol-
utilized for silica coating, preparation of organic dye solutions, and lowed by successively adding 0.006 g of AIBN, 0.642 mL of
dispersion of particles.
decane, and 0.124 mL of EHMA. Under N2 atmosphere and
Scheme 1 Schematic illustration of the procedure for fabricating magnetic polymer bowl with highly immobilized AgNPs.
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mechanical agitation, the mixture solution was heated to 70 ꢀC 7700, Hitachi) equipped with an energy-dispersive X-ray spec-
for 2 h. Aer the resultant particles were separated with trometer (EDS). The high resolution TEM (HRTEM) image of the
a magnet, the decane, unreacted reagents, and polymerized magnetic cluster was taken using a Titan G2 ChemiSTEM
EHMA were removed from the particles using ethanol and iso- operated at 200 kV (FEI Company). The surface charge proper-
amyl alcohol.
ties of the magnetic polymer particles treated with FeCl3 and TA
were measured by a zeta-potential analyzer (Nano ZS zetasizer,
Malvern Instruments). X-ray diffraction (XRD) pattern of Fe3O4
magnetic clusters was obtained with the use of an X-ray
diffractometer (D/MAX-2500, Rigaku) with a Cu Ka radiation
source. The elements present in the magnetic polymer particles
with attached AgNPs were investigated using an X-ray photo-
electron spectrometer (XPS, Quantera SXM, ULVAC-PHI). The
number of AgNPs attached on a magnetic polymer particle was
determined by employing inductively coupled plasma mass
spectrometry (ICP-MS, NexION 2000, PerkinElmer).
In situ formation of AgNPs
An aqueous FeCl3 solution (200 mL, 10 mg mLꢁ1) was injected
into 20 mL of an aqueous dispersion of magnetic polymer
particles (1 mg mLꢁ1) under mechanical agitation. Aer 1 min,
the magnetic polymer particles were separated by a magnet,
followed by washing with DI water and dispersing again in
20 mL of DI water. Subsequently, 200 mL of an aqueous TA
solution (40 mg mLꢁ1) was injected into the dispersion under
mechanical agitation, followed by separating with a magnet and
washing with DI water. The resultant particles were treated
again with FeCl3 and TA in the same way as described previously Results and discussion
and then dispersed again in 10 mL of DI water. To form AgNPs
To enable repeated use of magnetic polymer particles, they
on the magnetic polymer particles, 30 mL of Ag(NH3)2OH solu-
tion, which was prepared using an aqueous AgNO3 solution (10
mL, 0.1 M) and an NH4OH solution (200 mL), was injected into
the dispersion of the magnetic polymer particles. Aer mild
agitation for 1 h, separation with a magnet and washing with DI
water were consecutively carried out.
should be easily collected in response to moderate magnetic
eld. Magnetic nanocrystals with dimensions on the order of
10 nm are known to exhibit low magnetization per particle,29
thus essentially requiring the use of an extremely strong eld.
Growth of the nanocrystal in size exhibits an increase in satu-
ration magnetization, but at the expense of the super-
paramagnetism.29 Magnetic clusters with assembly of multiple
magnetic nanocrystals have many merits, including high
magnetic susceptibility, superparamagnetism, and low coercive
force.30,31 With these regards, we chose to use Fe3O4 magnetic
clusters as seeds to synthesize magnetic polymer particles. A
TEM image of the magnetic clusters synthesized by employing
the modied solvothermal method is shown in Fig. 1A. The
formation of particles with nearly spherical shape was observed,
and their mean diameter, which was obtained using the TEM
image, was 254 ꢃ 24 nm. Fig. 1B shows an HRTEM image for the
white-boxed region in Fig. 1A. Many nanocrystals with a diam-
eter of 7–9 nm were assembled together to form the magnetic
clusters. The corresponding electron diffraction (ED) pattern
recorded on the single cluster contained 220, 311, 400, 511, and
440 diffractions, as shown in the inset. A similar result was also
observed through XRD measurement. As shown in Fig. 1C, six
characteristic peaks appeared at 2q ¼ 30.2ꢀ, 35.5ꢀ, 43.1ꢀ, 53.4ꢀ,
57.1ꢀ, and 62.8ꢀ, which corresponded to (220), (311), (400),
(422), (511), and (440) of Fe3O4, respectively.14 These results
indicate that the small nanocrystals were Fe3O4. Using the Scher-
rer's equation and the strongest diffraction peaks indexed to (311),
we could estimate the average crystallite size and the calculated
value was 7.5 nm, which was in line with to the result reported
previously.28 The saturation magnetization of the magnetic clusters
was measured with a vibrating sample magnetometer (VSM),
which was determined to be 75.8 emu gꢁ1 at room temperature
(Fig. S1, ESI†). This high magnetization moved all the clusters in
Catalytic test
To examine the catalytic performance of magnetic polymer
particles with immobilized AgNPs on their surfaces, an aqueous
NaBH4 solution (1 mL, 40 mM) was mixed with an aqueous 4-NP
solution (1 mL, 0.4 mM), to which 3 mL of a dispersion of AgNP-
immobilized, magnetic polymer particle (2.1 ꢂ 107 particles
mLꢁ1) was additionally injected. For rhodamine B, a dispersion
of AgNP-immobilized, magnetic polymer particles (3 mL, 2.1 ꢂ
107 particles mLꢁ1) was introduced into 2 mL of an aqueous
solution with rhodamine B solution (0.05 mM) and NaBH4 (5
mM). The reduction of MB was tested by introducing a disper-
sion of AgNP-immobilized, magnetic polymer particles (3 mL,
2.1 ꢂ 107 particles mLꢁ1) into 2 mL of a mixture solution pre-
containing 10 mM of NaBH4 and 0.1 mM of MB. The reduc-
tion reactions of the organic dye molecules were monitored via
UV-vis spectrometry (T60, PG Instruments). Moreover, the
conversion extents were determined from the absorbance peak
intensities (I400, I554, and I664) at 400, 554, and 664 nm for 4-NP,
rhodamine B, and MB, respectively. To test the reusability of the
AgNP-immobilized, magnetic polymer particles for catalytic
reduction, they were separated from the reaction solution with
a magnet aer each cycle of the reduction and washed with DI
water. Then, the resultant particles were utilized for the next
cycle of reduction.
Characterization
Scanning electron microscopy (SEM, SU-8220, Hitachi) was their aqueous dispersion toward an external magnet in 2 min
used to study the surface structure of the magnetic polymer (Fig. 1D), indicating that they were magnetically responsive.
particles. The inside structures of the magnetic polymer parti-
The surfaces of the magnetic clusters were silica-coated for
cles and the immobilization of AgNPs on their surfaces were enhancing their chemical stability and acid resistance, as well
examined using a transmission electron microscope (TEM, HT- as for conducting further chemical reactions to make magnetic
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Fig. 1 (A) TEM image of the Fe3O4 magnetic clusters synthesized using the modified solvothermal method. (B) HRTEM image of the white-boxed
region in (A). The inset shows an ED pattern of the single magnetic cluster. (C) XRD pattern of the as-synthesized Fe3O4 magnetic clusters. (D and
E) Photographs of the Fe3O4 magnetic clusters dispersed in DI water: (D) without and (E) with application of a magnet.
polymer particles. In this study, the Stober method, which has increased the value of QMPS to 4.5 mmol mꢁ2. Fig. 2B shows
¨
been mainly employed for silica coating,32 was used. Fig. 2A a TEM image of the particles made by the polymerization at
shows a TEM image of the magnetic clusters obtained aer the 70 ꢀC for 6 h under the use of the MPS-treated seeds with QMPS
treatment using the Stober method, demonstrating the forma- of 1 mmol mꢁ2, which demonstrates the formation of ower-like
¨
tion of spherical particles that consist of single magnetic cores particles, consisting of PS nodules attached to one seed. This
and silica shells with uniform thickness. The silica-coated result was because the MPS treatment promoted favorable
magnetic clusters had a mean diameter of 401 ꢃ 21 nm. interactions between the seed and polymer species, leading to
Using this result and the diameter of the magnetic clusters in a covalent graing of PS chains on the surfaces of the seeds.33 As
Fig. 1A, the coating layer thickness was calculated to be about shown in Fig. S4A (ESI†), all the particles exhibited the ower-
73 nm, which was also conrmed in the inset image. The XRD like structure, suggesting that the large-scale preparation of
pattern of the silica-coated sample was the same as that of the ower-like particles is possible. The mean diameter of each
magnetic clusters in Fig. 1C (Fig. S2A, ESI†), suggesting that the nodule was measured to be 355 ꢃ 12 nm, and the number of PS
coating did not affect the crystalline structure of the magnetic nodules per seed was 8 on average. However, the use of the
cluster cores. Consequently, the silica-coated particles still seeds with QMPS of 4.5 mmol mꢁ2 formed the spherical particles
retained a strong magnetic response driven by the magnetic with MPS-treated seed cores and PS shells (Fig. 2C). The mean
cores (Fig. S2B and C†).
diameter of the spheres was 752 ꢃ 21 nm. For all the spheres,
These silica-coated particles were employed as seeds for encapsulation of the seeds was observed, as shown in Fig. S4B.†
preparing magnetic polymer particles. When we carried out the This structural change can be explained using the difference in
polymerization using the particles in Fig. 2A as seeds, the PS QMPS value. When QMPS is high, the increased affinity of PS to
particles separated from the magnetic seeds were made (Fig. S3, the functionalized seed surfaces allows the collapsing of
ESI†), indicating no interaction between both of the two growing polymer chains or the coalescence of freshly nucleated
components. To improve the interaction, the silica-coated polymer domains, eventually forming a polymer shell fully
particles were modied with MPS to induce the formation of coating the seeds.34 For low QMPS, however, the high interfacial
vinyl groups on their surfaces. We controlled the graing energy between the seed and PS blocks the spreading of the PS
density of MPS by varying its treatment conditions, such as the chains on the surfaces of the seeds or the merging of the
volume of MPS and treatment time, followed by determining polymer domains, consequently generating the polymer
the density value (QMPS, mmol mꢁ2) using UV-vis spectrometry nodules around the seeds.33
(see ESI†). When the silica-coated magnetic clusters were
treated with 1 mL of MPS for 24 h, the calculated QMPS was 1 70 C for 2 h together with decane, EHMA, and AIBN for their
mmol mꢁ2. The incubation with 2 mL of MPS for 5 days transformation into bowl-like structure. At the elevated
The spherical particles in Fig. 2C were additionally treated at
ꢀ
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Fig. 2 (A) TEM image of the silica-coated Fe3O4 magnetic clusters obtained using the Stober method. (B) TEM image of the flower-like particles
¨
obtained after the polymerization at 70 ꢀC for 6 h in the presence of the MPS-treated seeds with QMPS of 1 mmol mꢁ2. (C) TEM image of the
spherical particles made by the polymerization at 70 ꢀC for 6 h in the presence of the MPS-treated seeds with QMPS of 4.5 mmol mꢁ2. (D) TEM
image of the particles made by treating the spheres in (C) at 70 ꢀC for 2 h, together with decane, EHMA, and AIBN.
temperature, the decane molecules tend to absorb into the Before the immobilization, Fe3+ and TA were consecutively
polymer phase. Due to the phase separation between the deposited on the magnetic polymer particles. When Fe3+ was
absorbed molecules and polymer phase, the small-sized decane injected into the dispersion of the magnetic polymer particles,
domains starts to form within the polymer, followed by merging the cation was adsorbed on the negatively charged particles,
into single-phased interiors over time. The interior voids and which led to the change in their zeta potential (Fig. S7, ESI†).
surface holes are created during the removal of decane.19 Fig. S5 Subsequently, the addition of TA, which is a green reducing agent,
(ESI†) shows an SEM image of the particles obtained through induced the chelation of the molecules with the Fe3+ layer. The
the thermal treatment and washing in ethanol and isoamyl repetition of this process formed a highly stable coating of TA–Fe
alcohol, demonstrating that the core–shell spheres were with good uniformity on the surfaces of the magnetic polymer
successfully transformed into bowl-like particles with hollow particles, which could provide a homogeneous reduction envi-
+
voids and large surface openings. The formation of the voids ronment on the highly curved surfaces.36 When Ag(NH3)2 was
and open holes could be more clearly observed in the TEM injected into the dispersion of the magnetic polymer particles
image of Fig. 2D. Furthermore, it turned out that each seed with treated with Fe3+ and TA, the in situ reduction of the ion occurred
a magnetic cluster and a silica shell was still encapsulated in the on the magnetic polymer particles because of the catechol groups
bowl-like particle aer the transformation, as shown in the in TA molecules deposited on the particles.
inset TEM image, indicating that a sequence of treatments,
Fig. 3A shows an SEM image of the resultant sample aer the
such as MPS modication, polymerization, and transformation, repeated deposition of Fe3+ and TA on the particles of Fig. S5,†
did not affect the structure of the seed. Maintaining the seed followed by the reduction process. The particles exhibited the
structure rendered the polymer bowls magnetically responsive uniform distribution without aggregation and the bowl-like
(Fig. S6, ESI†), with a saturation magnetization of 51.2 emu gꢁ1 structure with hollow interior and large surface opening,
(Fig. S1†) that was much higher than the result reported in which suggest no structural change aer a series of processes
a previous study.12 Similar results were observed in the ower- involving the deposition and reduction. The TEM image in the
like and spherical particles, as shown in the insets in Fig. 2B inset shows the formation of the small spherical NPs on the
and C, along with Fig. S6.†
surface of the magnetic polymer bowl. They exhibited a good
To provide a catalytic property to the magnetic polymer uniformity in size, with a mean diameter of 6 nm. To investigate
particles, we immobilized AgNPs on the polymer particles the elements present in the bowls, EDS analysis was carried out.
owing to their low cost, unique reactivity and selectivity, and C, O, Fe, Si, and Ag were observed (Fig. S8, ESI†). The existence
good stability, as well as recyclability in catalytic reactions.35 of these elements was also conrmed using XPS measurement,
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Fig. 3 SEM and TEM (inset) images of the polymer particles obtained after the repeated deposition of Fe3+ and TA, followed by the reduction of
Ag precusor: (A) magnetic polymer bowls, (B) magnetic polymer flowers, (C) magnetic polymer spheres, and (D) polymer spheres without the
magnetic cluster seeds.
as shown in Fig. S9 (ESI†), which suggests the in situ formation bowls with immobilized AgNPs (Fig. 4C and D). The I400 values
of AgNPs. These AgNP-immobilized, magnetic polymer bowl for the owers and bowls decreased by 67 and 85% in the initial
still retained a high magnetization (Fig. S1†), which suggests 10 min, whereas the magnetic polymer spheres with immobi-
that their rapid recovery from reaction system is achievable lized AgNPs exhibited a decrease in I400 by 46% during the same
using a magnet. Similar results were observed for the magnetic period. Assuming that each magnetic polymer particle with
polymer owers and spheres. They could immobilize AgNPs on immobilized AgNPs worked as each catalytic center, we could
their surfaces without any structural change, as shown in determine the turnover frequency (TOF), which quanties the
Fig. 3B and C. Furthermore, the AgNP-immobilized, non- specic activity of a catalytic center for a reaction under dened
magnetic polymer spheres were prepared using PS particles conditions,37 of the AgNP-immobilized magnetic polymer
with a mean diameter of 750 ꢃ 18 nm (Fig. 3D).
particles (see ESI†). The TOF value for the magnetic polymer
We examined the catalytic performance of these particles for bowl with attached AgNPs was calculated to be 3.01 ꢂ
the reduction of 4-NP. Aer the AgNP-immobilized, magnetic 1011 minꢁ1, which was larger than those for the other magnetic
polymer spheres in Fig. 3C were introduced into an aqueous particles (9.11 ꢂ 1010 minꢁ1 for the magnetic polymer sphere
system containing 4-NP and NaBH4, the temporal change of UV- and 1.61 ꢂ 1011 minꢁ1 for the magnetic polymer ower). This
ꢀ
vis absorbance spectra at 25 C for the system was monitored result indicates that the magnetic polymer bowl-based system
(Fig. 4A). While the intensity located at 400 nm (I400) decreased had more enhanced catalytic activity for the reduction of 4NP
over time, a new peak started to appear at 300 nm and its than the other particles.
intensity increased gradually, indicating that 4-AP formed as
To further study the catalytic performance of the AgNP-attached
a resultant product from the reduction of 4-NP.36 A similar particles in Fig. 3, we also examined the reduction behaviors at
reduction behavior was observed in the AgNP-attached polymer 25 ꢀC of rhodmaine B and MB under the use of these particles. As
spheres (Fig. 4B). In contrast, there was no change in I400 in the shown in Fig. 5, all the samples exhibited a gradual decrease in I554
absence of the AgNP-attached, magnetic polymer spheres over time, indicating the reduction of rhodamine B.6,12 However,
(Fig. S10A, ESI†). The addition of the magnetic polymer spheres the reduction rate differed in each sample. As in the case of the 4-
without immobilized AgNPs exhibited no change in I400, either NP, the fastest reduction was observed in the bowl sample, with
(Fig. S10B†). These results indicate that the reduction of 4-NP a decrease in I554 by 74%, followed by the magnetic polymer ower
was induced by the attached AgNPs, which can assist the elec- (52% decrease) aer the initial 10 min. Conversely, the magnetic
ꢁ
tron transfer from BH4 to the dye molecules adsorbed on the and non-magnetic polymer spheres exhibited very slow reduction
NPs.12 Interestingly, the reduction behavior of 4-NP was more reactions, with I554 decrease by 34%. The catalytic reduction order
enhanced under the use of the magnetic polymer owers and was also same in the case of MB (Fig. 6).
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Fig. 4 Changes in UV-vis absorbance spectra at 25 ꢀC over time of 4-NP using various particles with immobilized AgNPs: (A) magnetic polymer
spheres, (B) polymer spheres without the magnetic cluster seeds, (C) magnetic polymer flowers, and (D) magnetic polymer bowls.
Additionally, the reduction of these organic dyes was exam- concentration of NaBH4 was one hundred times higher than
ined at different temperatures (4 and 37 ꢀC), as shown in that of each dye, it could be assumed that the reduction of the
Fig. S11–S16 (ESI†). In all the reduction tests, because the dye molecules was not affected by the change in the
Fig. 5 Changes in UV-vis absorbance spectra at 25 ꢀC over time of rhodamine B using various particles with immobilized AgNPs: (A) magnetic
polymer spheres, (B) polymer spheres without the magnetic cluster seeds, (C) magnetic polymer flowers, and (D) magnetic polymer bowls.
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Fig. 6 UV-vis absorbance spectra at 25 ꢀC for MB over time under the use of various particles with immobilized AgNPs: (A) magnetic polymer
spheres, (B) polymer spheres without the magnetic cluster seeds, (C) magnetic polymer flowers, and (D) magnetic polymer bowls.
concentration of NaBH4 during the whole reactions. Under this and 0.319 minꢁ1, which were larger than those at the corre-
assumption, the rst-order reaction kinetics could be used to sponding temperatures for the other particles (Table 1).
ꢀ
understand the reduction of the dyes. Based on the linear Furthermore, the k value at 25 C was higher than the results
relation between ꢁln(It/I0), where It and I0 are the absorbance reported in previous studies.38–41 The high k value for the
peak intensities at time t and zero, and the time for each reac- magnetic polymer bowls was also observed in rhodamine B and
tion (Fig. S17–S19, ESI†), the reaction rate constant (k) for each MB. Conversely, the Ea values for the reduction of each dye were
reduction system was evaluated. From the obtained value of k, similar, irrespective of the type of AgNP-immobilized particles,
the activation energy (Ea) for the reduction reaction of each dye which suggests that the reduction mechanism of each dye was
could be estimated using the Arrhenius equation, i.e., ln k ¼ the same for all the particle types. It is well known that the
ln A ꢁ Ea/RT, where A and R denote the pre-exponential factor catalytic reduction of dye molecules, including 4-NP, rhoda-
and ideal gas constant, respectively. For 4-NP, the k values at 4, mine B, and MB, in the presence of NaBH4 takes place on the
25, and 37 ꢀC for the magnetic polymer bowls were 0.082, 0.202, surface of noble metal nanocatalysts.42–44 BH4ꢁ ions, which are
Table 1 Reaction rate constant (k) at different temperatures and activation energy (Ea) for the reduction of various dyes driven by AgNP-
immobilized particles with different structures
k (minꢁ1
4 ꢀ
)
Dye
Particle
C
25 ꢀ
C
37 ꢀ
C
Ea (kJ molꢁ1
)
4-NP
Non-magnetic sphere
Magnetic sphere
Magnetic ower
Magnetic bowl
Non-magnetic sphere
Magnetic sphere
Magnetic ower
Magnetic bowl
Non-magnetic sphere
Magnetic sphere
Magnetic ower
Magnetic bowl
0.025
0.024
0.043
0.082
0.014
0.014
0.023
0.046
0.030
0.030
0.051
0.096
0.062
0.061
0.108
0.202
0.042
0.042
0.073
0.142
0.073
0.070
0.127
0.231
0.098
0.101
0.179
0.319
0.073
0.073
0.130
0.244
0.115
0.112
0.199
0.366
29.6
30.2
30.5
29.2
35.8
35.8
37.4
36.3
29.1
28.5
29.0
28.9
Rhodamine B
MB
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Scheme 2 Schematic illustration of magnetic polymer particles for calculating their volumes and surface areas: (A) sphere, (B) flower, and (C) bowl.
produced from ionization of NaBH4, are adsorbed on the estimated from the Rseed, Rnodule, and Nnodule values obtained
surface of the nanocatalysts, forming surface-bound hydrogen using the TEM images of the magnetic polymer owers. In the
as metal hydride. At the same time, the dye molecules are also case of the magnetic polymer bowl, V can be expressed as V ¼ (1/
attached on the surface of the nanocatalysts. The hydrogen 3)pRpolymer3(2 + 3cosqpolymer ꢁ cos3qpolymer) ꢁ (1/3)pRvoid3(2 ꢁ
interacts with the adsorbed dye molecules, causing their cosqvoid + cos3qvoid),47 where Rpolymer and Rvoid denote the radii of
reduction to 4-AP, leuco methylene blue, and leuco rhodamine the magnetic core-encapsulated polymer and void phases,
B.9,42,45 As shown in Table 1, the Ea values for all the organic dyes respectively, and qpolymer and qvoid are the angles opposite to the
were in the range of 8–40 kJ molꢁ1, corresponding to the result line segment connecting from the intersection of the two pha-
of surface catalyzed reactions,46 which suggests that surface ses to center of each phase (Scheme 2C). Assuming that there is
catalysis induced the reduction of the dye molecules.
no loss of polymer chains during the transformation into bowl-
To understand the higher k value in the magnetic polymer like particles, its volume is the same as that of a magnetic
bowls for all the dyes, we calculated the volume (V) and surface polymer sphere. The S of a magnetic polymer bowl can be
2
area (S) of a magnetic polymer bowl, followed by comparing the expressed as S ¼ 4pRpolymer ꢁ 2pRpolymer2(1 ꢁ cosqpolymer) +
values with those of the other-structured particles. For the 2pRvoid2(1
ꢁ
cosqvoid).19 The additional relations among
calculation of V and S of each type of magnetic polymer particle, Rpolymer, Rvoid, qpolymer, and qvoid are given as DH ¼ 2Rpolymer
-
simple models were used, as illustrated in Scheme 2. For the sinqpolymer ¼ 2Rvoidsinqvoid by taking the diameter of the single
magnetic polymer sphere, a simple model, consisting of the opening (DH) on the surface of the bowl into consideration. The
silica-coated Fe3O4 seed and surrounding polymer, was value of qpolymer could be theoretically calculated using Rpolymer
considered (Scheme 2A). The V and S of a magnetic polymer and DH, which were determined from the TEM images for the
2
sphere can be expressed as V ¼ (4/3)pRouter3 and S ¼ 4pRouter
,
magnetic polymer bowls. Taking the V of the magnetic polymer
where Router denotes the outer radius of the sphere, which could sphere together with DH, the values of Rvoid and qvoid could be
be determined using the TEM images of the magnetic polymer estimated. Using the obtained qpolymer, qvoid, Rpolymer, and Rvoid
spheres. For the magnetic polymer ower, V can be expressed as values, we calculated S of the magnetic polymer bowl. Fig. S20
V ¼ (4/3)pRseed3 + (4/3)pRnodule Nnodule, where Rseed, Rnodule, and (ESI†) shows the calculated V for each type of magnetic polymer
3
Nnodule denote the radius of the magnetic seed, radius of the particle. The values of V for the non-magnetic polymer sphere
polymer nodule, and number of polymer nodule attached to the and magnetic polymer particles (sphere, ower, and bowl) were
magnetic seed, respectively (Scheme 2B). Assuming that the very similar to each other, whereas the S values were different.
surface of the silica-coated magnetic core is fully covered with As shown in Fig. 7A, the magnetic polymer bowl exhibited the
polymer nodules, the S of a magnetic ower can be expressed as largest S value (5.86 ꢂ 106 nm2), followed by the magnetic
2
S ¼ 4pRnodule Nnodule. The V and S of the ower could be ower and both the spheres.
Fig. 7 (A) Surface areas (S) for the different types of magnetic polymer particles. (B) Numbers of immobilized AgNPs per magnetic polymer
particle (XAg).
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Fig. 8 (A) Conversion extent of rhodamine B (a) after its reduction for 30 min. (B) Reaction rate constants (k) during twenty reduction cycles of
rhodamine B in the presence of the magnetic polymer particles.
Furthermore, we evaluated the number of AgNPs immobi- magnetic clusters, which were prepared via modied solvothermal
¨
lized on one magnetic polymer particle (XAg) via ICP-MS analysis approach and the Stober method, as seeds during seeded disper-
(see ESI†). The XAg for all the polymer particles with attached sion polymerization allowed the formation of magnetic polymer
AgNPs is shown in Fig. 7B. The value of XAg of the magnetic spheres and owers. The additional phase-separation between the
polymer bowl was 8.2 ꢂ 103, which was larger than those of the magnetic polymer spheres and decane led to their transformation
other particles. This result indicates that the bowls were able to into bowl-like particles still containing the magnetic clusters
more effectively immobilize the AgNPs on their surfaces. To inside. Their larger surface area, compared with those of the
correlate the obtained S and XAg values, the normalization for S magnetic spheres and owers, and openness enabled a much
and XAg was conducted. As shown in Fig. S21 (ESI†), the normal- higher immobilization of AgNPs on the surfaces. Such high
ized values in S for the magnetic polymer particles were consistent immobilization endowed the bowls with an excellent catalytic
of those in XAg for the corresponding particles, which indicates activity for the reduction of 4-NP, rhodamine B, and MB. Moreover,
that the openness of the bowls allowed the deposition of AgNPs on the strong magnetic response, which was given by the magnetic
both their outer and inner surfaces. Moreover, the values were in clusters present in their inside, allowed a rapid, magnetic sepa-
good agreement with the normalized results in k, which were ob- ration and an excellent reusability, achieving twenty cycled
tained by dividing k at each temperature for each type of magnetic reduction of the dye molecules without loss of catalytic activity.
polymer particles by that for the non-magnetic polymer spheres Although this study mainly focused on the reduction of toxic
(Table 1). This similarity suggests that the larger surface area and organic dyes, these particles are applicable to the removal of
openness of the magnetic polymer bowls led to the greater pathogenic microorganisms owing to the bactericidal ability of
immobilization of AgNPs, which functioned as the reactive sites, highly immobilized AgNPs.48 Moreover, the attachment of other
thus inducing the enhanced catalytic activity and higher k value.
functional nanoparticles can expand the applicability of the
We also performed the reusability test of the magnetic polymer magnetic polymer bowls in diverse and advanced elds.
particles with immobilized AgNPs by examining their catalytic
performances during repeated cycles of rhodamine B reduction.
Upon the application of a magnet, the particles could be separated
in 3 min from the reaction solution aer each cycle of the reduction
The authors declare that they have no conict of interest.
reaction and were then reused as catalysts. Fig. 8A shows the extent
Conflicts of interest
of conversion (a) of the organic dye, which is given as a ¼ (I0 ꢁ It)/I0,
aer the reaction for 30 min at 25 ꢀC. The AgNP-attached, magnetic
polymer bowls exhibited the highest a (over 0.98), and the values
Acknowledgements
were similar for twenty cycles. Similarly, the high k value of the DCH thanks for the support from the National Research
bowls was maintained even aer the repeated reduction reactions
(Fig. 8B). These results demonstrate the excellent stability and
reusability of the magnetic polymer bowls with immobilized
AgNPs, making them useful as a recoverable and efficient system
for enhanced and repeated reduction of toxic organic dyes.
Foundation of Korea (NRF) grant funded by the Korea govern-
ment (MSIT and MOE) (NRF-2018R1D1A1B07043878 and NRF-
2021R1A2C1007129). GDM acknowledges the support from
the Korea Institute of Industrial Technology through Develop-
ment of ber-based technology for reduction of hazardous
substances in the air (kitech EO-21-0001).
Conclusion
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