Stimuli-responsive one-dimensional copolymer nanostructures fabricated by metallogel template polymerization and their adsorption of aspirin

Article abstract of DOI:10.1039/c4sm00121d

pH responsive poly(N,N′-methylenebisacrylamide-co-4-vinylpyridine) (P(MBA-4VP)) one dimensional (1D) nanostructures have been prepared by metallogel template copolymerization, which was carried out in an Ag(i)-coordinated organogel with benzoyl peroxide (

Full text of DOI:10.1039/c4sm00121d

Soft Matter
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Stimuli-responsive one-dimensional copolymer
nanostructures fabricated by metallogel template
polymerization and their adsorption of aspirin
Cite this: Soft Matter, 2014, 10, 3960
*
Xing Wen, Liming Tang and Lu Qiang
pH responsive poly(N,N0-methylenebisacrylamide-co-4-vinylpyridine) (P(MBA-4VP)) one dimensional (1D)
nanostructures have been prepared by metallogel template copolymerization, which was carried out in
an Ag(I)-coordinated organogel with benzoyl peroxide (BPO) as the initiator. The product has been
characterized using infrared spectroscopy, scanning electron microscopy and transmission electron
microscopy. The experimental results reveal that the gel fiber is a crucial template for polymerization.
Due to the degradation of the template in copolymerization, nanofibers of metallogel were transcribed
to copolymer nanowires. The introduction of co-monomer 4-vinylpyridine (4VP) imparts to the 1D
copolymer nanostructures pH sensitivity and the possible use as an adsorption material of aspirin.
Adsorbed 1D copolymer nanostructures could be regenerated using proton solvent, acid medium and
salt solution. In addition, silver nanoparticle loaded copolymer nanowires have been produced from the
reduction of silver ions instead of template removal, where silver ions act both as the template and as
the nanoparticle growth substrate.
Received 17th January 2014
Accepted 6th March 2014
DOI: 10.1039/c4sm00121d
www.rsc.org/softmatter
nanostructures such as stimuli responsive nano-objects have
rarely been addressed.
Introduction
In recent years, the study of 1D polymer nanostructures,
such as nanowires, nanoribbons and nanotubes, has attracted
considerable attention, and much effort has been focused on
the exploration of their unique properties and their synthesis
and fabrication. Their 1D morphology and anisotropic nano-
structures offer an unlimited amount of potential applications
relevant to electromechanical nanoscale devices,21 separa-
tions,22,23 drug delivery24,25 and catalyst supports.26 So far, three
different strategies have been developed for the preparation of
1D polymer nanostructures, including electrospinning,27 self-
assembly of block copolymers28 and template methods. Most of
the template methods focus on the polymerization in porous
hard templates,29,30 which are tedious to remove fully aer the
synthesis. Every strategy has its own advantages and disadvan-
tages. Therefore, it is still required to develop novel and facile
approaches for 1D polymer nanostructures from a practical view
point.
Recently, our group has reported the fabrication of 1D
polymer nanostructures using a metallogel as a so template,
which can be very easily removed.31 Metallogels, the supramo-
lecular gels in some way incorporating metallic elements,32 have
heat-stable properties and a facile preparation procedure,33
which have been exploited to fabricate and assemble inorganic
nanomaterials,34 as well as polymer nanostructures.33 In our
previous articles, we have developed a silver metallogel and
based on which we managed to fabricate 1D polymer nano-
structures by the free radical polymerization of multifunctional
The pursuit of stimuli responsive polymers with a desired
structure and novel utilization has persisted over many
decades,1 and covers chemistry, physics, biology, and materials
science.2 Stimuli-responsive polymers (also called intelligent,3
smart,4,5 or environmentally sensitive6 polymers) can undergo
relatively large and abrupt physical or chemical changes in
response to small external environmental stimuli, such as
temperature, pH, electrolyte concentration, light, electric or
magnetic elds, and a combination of any of them.7 Among
these stimuli, pH is the most intensively investigated in various
laboratories and industries8 due to its rapid response,9,10 rela-
tive effective control11,12 and a versatile application range.13
A
pH-response can be obtained either by using weakly acidic
monomers (e.g. acrylic acid) whose ionization status can be
changed by pH variation, or by using weakly basic monomers
(e.g. 4-vinylpyridine) that can be rendered water-soluble by
protonation at low pH.11,14 pH responsive polymers have been
widely investigated for drug delivery, separation, and diagnos-
tics applications.13 Presently, a great deal of work has been
dedicated to different physical forms of stimuli responsive
polymers, such as nanogels/microgels,15–17 micelles,18 brushes,19
membranes20 and so on, that can be craed into new smart
materials.1 However, one dimensional (1D) polymer
Key Laboratory of Advanced materials of Ministry of Education of China, Department
of Chemical Engineering, Tsinghua University, Beijing 100084, P. R. China. E-mail:
tanglm@tsinghua.edu.cn
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monomers, such as N,N0-methylenebisacrylamide (MBA)31 and purchased from Beijing Chemicals and puried by recrystalli-
diacryloyl-2,6-diaminopyridine (DADAP).35,36 Heat-initiated zation using methanol. N,N0-Methylenebisacrylamide (MBA)
polymerization took place on the gel bers, and the original gel purchased from Sigma Aldrich was used as received. Benzene-
bers were successively easily removed by using excess 1,3,5-tricarbonyl trichloride, 3-hydroxyl pyridine, pyridine,
ammonia. However, these works were based on the homo- 4-vinylpyridine (4VP), AgNO3 and aspirin purchased from Alfa
polymerization of a cross-linkable monomer bearing coordi- Aesar were used as received. Other solvents and reagents,
nation groups, which limit the selection of monomers seriously. including deionized water, ethanol, dichloromethane, chloro-
Therefore, the purpose of this paper is to introduce other mono- form and so on, were used without further purication.
functional monomers to copolymerize with MBA to synthesis
1D copolymer nanostructures. That means more selections for
co-monomer types, copolymer compositions and functions of
1D copolymer nanostructures.
Synthesis of tripyridin-3-yl benzene-1,3,5-tricarboxylate
(ligand L)
In this paper, we attempted to fabricate pH responsive 1D First, 3.81 g (40 mmol) of 3-hydroxyl pyridine and 3.2 g of
copolymer nanostructures via copolymerizing MBA with pH pyridine (40 mmol) were dissolved in 70 mL dehydrated THF.
sensitive co-monomer 4-vinylpyridine (4VP) using the silver 2.65 g (10 mmol) benzene-1,3,5-tricarbonyl trichloride dissolved
metallogel as a template. Compared to DADAP, 4VP is easy to in another 30 mL of dehydrated THF was dropped into the
obtain and renders the copolymer to be sensitive to pH solution in 1 h at room temperature. Aer addition, the reaction
changes.37 Furthermore, the pyridine group can form the mixture was allowed to stir at 85 ꢁC for 12 h. Aer that, the
double hydrogen bonding system with carboxylic acid group cooled resulting solution was evaporated at reduced pressure
with the association constant up to 107 Mꢀ1 in chloroform.38,39 and 50 mL deionized water was added to dissolve impurities.
Therefore, the product is capable of ‘uptaking’ some carboxylic The resulting white solid was ltered and puried by column
compounds (e.g. aspirin) from dilute solution effectively, which chromatography using dichloromethane/THF (5/1) as the
1
makes the 1D copolymer nanostructures to be used as an eluent to give a 2.91 g product (66% yield). H NMR (300 MHz,
adsorption material in drug production, extraction and recovery [D6] DMSO, d/ppm): 9.08 (s, 3H, aryl H), 8.70 (d, 3H, 2-pyridyl H),
industries. Additionally, we also tried to fabricate silver nano- 8.58 (d, 3H, 6-pyridyl H), 7.91(d, 3H, 4-pyridyl H), 7.59 (d ꢂ d,
particle loaded copolymer nanowires by using the reducing 3H, 5-pyridyl H). FT-IR (KBr, cmꢀ1): 3085, 3066 (aryl and pyridyl
agent to treat the copolymerized product, instead of ammonia C–H), 1740 (C]O, strong), 1589, 1578, 1477, 1427 (aryl, pyridyl
treatment to remove the metallogel templates. Different from C]C and pyridyl C]N).
the traditional method which involves the entrapment and
subsequent reduction of silver ions,40 silver ions originally exist
inside the copolymer nanowires without additional incorpora-
Fabrication of 1D copolymer nanostructures
tion steps in this system. Silver ions act both as the template In a typical process, 10 mg ligand L, 8 mg MBA and a certain
and as the nanoparticle growth substrate. That means a more molar ratio of 4VP were dissolved in 0.4 g dehydrated THF in a
deep application of 1D copolymer nanostructures and a simple vial, then a 0.4 g THF solution of 6 mg AgNO3 and 2 mg BPO was
approach for one dimensional metal–organic hybrid injected quickly by a needle. A gel (2 wt%) was formed within 10
nanomaterials.
seconds. The tightly sealed vial containing the gel was put into
The obtained 1D copolymer nanostructures were character- an oil bath at 63 ꢁC to carry out the radical copolymerization of
ized by Fourier-transform infrared (FT-IR), whereas their MBA and 4VP. Aer a desired period of reaction, excess
morphology was conrmed by transmission electron micros- ammonia was added to remove any remaining templates. The
copy (TEM) and scanning electron microscopy (SEM) analyses white solid was collected by ltration, washed repeatedly with
and the rheological properties of the gel samples were tested by ammonia and THF, and nally dried in a vacuum.
a rheometer. The pH-responsive behaviors of 1D copolymer
nanostructures formed at various monomer compositions were
investigated as the change of pH values. Their adsorption and
Characterization of 1D copolymer nanostructures
desorption of aspirin were detected by a UV-Vis spectrometer. The resulting copolymer was characterized by a Nicolet 560
The morphology of silver nanoparticle loaded copolymer Fourier-transform infrared (FT-IR) spectroscope. The
nanowires was conrmed by TEM and the silver nanoparticles morphology of 1D copolymer nanostructures was characterized
were characterized using electron dispersive X-ray (EDX) spec- using transmission electron microscopy (TEM) and scanning
troscopy. Based on these results, we think that this simple electron microscopy (SEM). TEM images were obtained on a
approach reported herein may be extended to prepare more JEOL J2010 transmission electron microscope operating at an
kinds of functional 1D polymer nanostructures.
accelerating voltage of 200 kV by drop casting the sample
dispersion on copper grids coated with a carbon lm. SEM
images were obtained on a JEOL FESEM 6704F electron
microscope operating at an accelerating voltage of 20 kV aer
the sample was coated with a thin layer of gold. The rheological
Experimental
Materials
Tetrahydrofuran (THF, Tianjin Chemical Reagents) was freshly properties of the metallogels and copolymer gels were tested by
distilled from sodium prior to use; benzoyl peroxide (BPO) was a Physica MCR300 rheometer with a plate xture.
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pH-responsive behaviors of 1D copolymer nanostructures
Results and discussion
Fabrication of 1D copolymer nanostructures
The equilibrium swelling ratio was determined to characterize
the pH-responsive behavior of 1D copolymer nanostructures.
The equilibrium swelling ratio (Q) was determined as follows:
As reported in our previous article, the silver ions constituting
the metallogel bers on the surface could adsorb molecules
bearing coordination groups.31 Therefore the monomer used for
polymerization should have certain groups that can interact
Q ¼ (Ws ꢀ Wd)/Wd
where Ws and Wd are the weights of the swollen and dry gels, with silver ions, like amide groups of MBA and the pyridine
respectively.
group of DADAP, which make polymerized products adhere
To determine the equilibrium swelling behavior, known onto the surfaces of the template bers during polymerization.
amounts of dried copolymer gels aer the ammonia treatment
In this study, six random copolymers poly(N,N0-methyl-
were put into 5 mL buffer solutions at different pH values, enebisacrylamide-co-4-vinylpyridine) (dened as P(MBA-4VP))
until a constant weight was attained. The buffer solutions of were prepared by the free radical copolymerization of the two
different pH values were prepared with 0.2 mol Lꢀ1 disodium monomers within the THF metallogel. The co-monomer 4VP
hydrogen phosphate solution, 0.1 mol Lꢀ1 citric acid solution, whose pyridine groups can coordinate with silver ions may be
0.05 mol Lꢀ1 disodium hydrogen phosphate solution and responsible for copolymers adhering onto the surfaces of the
0.1 mol Lꢀ1 sodium hydroxide solution.
template, as well as MBA. However, the coordination ability of
pyridine groups is stronger than that of amide groups,35 and
thus excess 4VP will affect the gelation time or appearance of
the gel. The molar ratios of MBA/4VP in the feed of six copoly-
mers were 1/1, 1/2, 1/3, 1/4, 1/5 and 1/6, and for convenience in
the discussion, the corresponding six random copolymers were
dened as P(MBA-4VP-1), P(MBA-4VP-2), P(MBA-4VP-3), P(MBA-
4VP-4), P(MBA-4VP-5) and P(MBA-4VP-6), respectively.
The preparation procedure is illustrated in Scheme 1. The
two co-monomers and BPO were separately added into the THF
solutions of ligand L and AgNO3, and the gel was rapidly
produced in 10 s by injecting AgNO3 solution into ligand L
solution. Aerwards, the gel was treated at 63 ꢁC to carry out the
Adsorption and desorption of aspirin
The rate of guest molecule adsorption was measured by placing
10 mg of dried copolymer gels aer the ammonia treatment in 2
mL aspirin–chloroform solution (50 mg Lꢀ1). The absorbance
spectrum of the solution was recorded with a UV757CRT spec-
trophotometer, and the characteristic absorbance of aspirin at
242 nm was used to monitor its concentration. For the
desorption studies, the aspirin-loaded copolymer gels were
immersed in 5 mL solvents including hot (65 ꢁC) ethanol, buffer
solution of pH 2.2 and sodium chloride solution respectively,
then ltered aer a certain time. The absorbance spectra of
these ltrates were recorded with a UV757CRT spectropho-
tometer. Aer that, new solvents were added again to immerse
the aspirin-loaded copolymer gels and then ltered, and the
absorbance spectra of these ltrates were recorded. This
process was repeated until the absorbance of ltrates in UV-Vis
spectra was less than 0.3.
Fabrication and characterization of silver nanoparticle loaded
copolymer nanowires
The copolymerization process was the same as that of 1D
copolymer nanostructures. Aer 24 hours of copolymeriza-
tion, the resulting copolymer gel with the template was dried
to remove THF, and then 2 mL ethylene glycol solution was
added in the vial to swell the copolymer gel. The tightly
sealed vial containing the gel and ethylene glycol solution
ꢁ
was put into an oil bath at 120 C to carry out the reduction
reaction of silver ions for 2 hours. The high temperature
ethylene glycol solution acted as a reducing agent for
the silver ions. Aer reduction, the yellow solid was
collected by ltration, washed repeatedly with deionized
water, and nally dried in a vacuum. The morphology of
silver nanoparticle loaded copolymer nanowires was char-
acterized using a JEOL J2010 TEM and the silver nano-
particles were characterized using electron dispersive X-ray
(EDX) spectroscopy performed on a JEOL J2010 with an
Oxford spectrometer.
Scheme 1 Illustration of the procedure for the fabrication of 1D
copolymer nanostructures.
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radical copolymerization of MBA and 4VP. Aer polymerization,
the resulting gel-like sample was treated with excess ammonia
to remove the remaining metallogel templates and then dried in
a vacuum oven.
Infrared spectroscopy
The 1D copolymer nanostructures are characterized by FT-IR
spectroscopy. Fig. 1A shows the FT-IR spectrum of pure poly-
(N,N0-methylenebisacrylamide) (PMBA) nanotubes, where the
broad peak at 3269 cmꢀ1 represents N–H stretching and 1655,
1529, and 1213 cmꢀ1 correspond to amide I, II, and III bands,
respectively; the small peak at 3066 cmꢀ1 indicates a residual
double bond that had not been fully polymerized. Fig. 1B
shows the FT-IR spectrum of P(MBA-4VP-2) 1D nanostructures.
Except for the peaks corresponding to secondary amide in
PMBA, characteristic peaks of pyridine rings of P(MBA-4VP-2)
are manifested by 1601 cmꢀ1 representing C]N stretching
and 1417 cmꢀ1 representing C]C stretching.
Fig. 2 (A) Photograph of the metallogel as the template. (B) TEM
image of metallogel nanofibers.
Morphology characterization
As reported in our previous article, ligand L could form 3D
networks of bers with silver ions in THF, therefore forming the
metallogel.41 In this work, the brillar networks of the metal-
logel (2 wt%) were used as a template to fabricate 1D copolymer
nanostructures, and the formation of the metallogel was
conrmed by the inverted-test-tube method (Fig. 2A). The TEM
image in Fig. 2B reveals the morphology of the metallogel,
therein numerous bers entangle to form a network structure.
Most of the bers have their diameters in the range of 30–50 nm
and can extend several micrometers in length, demonstrating
their high aspect ratio.
Aer polymerization, P(MBA-4VP) was synthesized on the
template gel bers, giving 1D copolymer nanostructures as a
white gel (Fig. 3A). The SEM image of P(MBA-4VP-2) 1D nano-
structures aer the ammonia treatment is shown in Fig. 3B.
Similar to the original gel bers, the networks of 1D copolymer
nanostructures are branched and interconnected. Lengths of
Fig. 3 (A) Photograph of the copolymer gel. (B) SEM image of P(MBA-
4VP-2) 1D nanostructures.
the nanowires can reach tens of micrometres and their uniform
diameter is 80 nm.
The TEM images of P(MBA-4VP) 1D nanostructures aer the
ammonia treatment with different molar ratios of MBA/4VP are
displayed in Fig. 4. In the sample with lower 4VP molar
contents, both nanotubes and nanowires appeared (Fig. 4A).
The arrows in Fig. 4A point to the hollow nanotubular structures
in P(MBA-4VP-1), and the average inner diameter of the nano-
tubes is 20 nm, slightly smaller than the diameters of the gel
template shown in Fig. 2B. With the increase of the MBA/4VP
molar ratio from 1/2 to 1/5, only copolymer nanowires were
obtained (Fig. 4B–F), and the yield of copolymer nanowires
could all be higher than 95%. When the molar ratio of MBA/4VP
is further increased to 1/6, coordination between the pyridyl
group of 4VP and the silver ions constituting the metallogel will
affect the formation of the gel template, resulting in copolymer
nanowires with more defects and fracture (as indicated by
arrows in Fig. 4F) and the decrease of the yield of copolymer
nanowires (below 60%).
In order to investigate the template effect for copolymeriza-
tion and the fabrication mechanism of copolymer nanowires,
we monitored the polymerization process in different stages via
TEM observation. The TEM images of P(MBA-4VP-2) 1D nano-
structures obtained at different polymerization times (without
ammonia treatment) are displayed in Fig. 5. In the early stage
(Fig. 5A), a core–shell 1D nanostructure was obtained consisting
of the copolymer shell and the nanober core. As the
Fig. 1 FT-IR spectra of (A) PMBA and (B) P(MBA-4VP-2).
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Fig. 4 TEM images of P(MBA-4VP) 1D nanostructures. (A) P(MBA-4VP-1), (B) P(MBA-4VP-2), (C) P(MBA-4VP-3), (D) P(MBA-4VP-4), (E) P(MBA-
4VP-5), and (F) P(MBA-4VP-6). The inset of the magnified image shows the hollow nanotubular structures in P(MBA-4VP-1).
collapse quickly resulting in the disappearance of the narrow
inner diameter and only nanowires were observed.
The fabrication mechanism of copolymer nanowires could
be described by the ‘self-degraded template polymerization’
mechanism.35 The metallogel templates were eroded gradually
during the polymerization, and the eroding process could be
ascribed to the pyridyl groups of 4VP which were inclined to
coordinate with the silver ions of the template gel.
Rheological properties
The metallogel is rather fragile and could be destroyed into a
free-owing liquid by shaking or sonication, especially the
addition of monomers into the gel could lower the gel strength
to some extent.31 That might be attributed to the coordination
interaction between amide groups and pyridyl groups with
silver ions, which lowers the density of silver ions as the cross-
linker, thus lowering the gel strength. Aer copolymerization of
MBA and 4VP, the product was obtained as a gel which could
not be destroyed by shaking or sonication, indicating the
Fig. 5 TEM images of P(MBA-4VP-2) 1D nanostructures after (A) 2 h,
(B) 6 h, (C) 14 h and (D) 24 h of copolymerization.
formation of a copolymer gel.
The mechanical properties of the metallogel and the copol-
ymer gels were measured by using a rheometer. In Fig. 6, curves
of the storage modulus (G0) and the loss modulus (G00) against
polymerization time gets longer, the metallogel templates were frequency are presented. Obviously, the copolymer gel displays
eroded gradually (Fig. 5B–D). The hollow nanotubular struc- a higher strength than the metallogel. Both G0 and G00 of the
tures were obtained even before the ammonia treatment, as P(MBA-4VP-2) gel are higher than those of the P(MBA-4VP-4) gel,
indicated by arrows in Fig. 5B–D. Their average inner diameter which indicates that the incorporation of mono-functional
is 5 nm, smaller than that of P(MBA-4VP-1) nanotubes in monomer 4VP could cause the decrease of crosslinking degree
Fig. 4A. Aer removing the template, these nanotubes might of the copolymer making the gel strength lowered to some
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Fig. 6 Storage modulus and loss modulus of the copolymer gels and
the metallogel at different frequencies.
extent. In contrast to the metallogel, the copolymer gel shows a
typical elastic behavior, and is strong enough to be used as a
specic adsorbing material.
pH-responsive behaviors of 1D copolymer nanostructures
Fig. 7 Photographs of pH-responsive behavior of P(MBA-4VP-4) 1D
nanostructures immersed into buffer solutions of pH 2.2 (A) and pH 11
(B) respectively. (C) Dried sample and the same sample saturated by
buffer solutions of pH 2.2 (D) and pH 11 (E) respectively.
4VP is a weak basic monomer, and its polymer or copolymer
could show pH-responsive behavior that undergoes a phase
transition under pH 5 owing to the protonation of pyridine
groups.11,37 The pyridine groups contribute protons under acidic
conditions and accept them under basic conditions, which
results in the transition from an expanded state to a collapsed
state that can be explained by the osmotic pressure exerted by
mobile counterions neutralizing the network charges.14
Therefore, P(MBA-4VP) 1D nanostructures could respond to
a pH change in buffer solutions. The photographs in Fig. 7
reveal pH-responsive behavior of P(MBA-4VP-4) 1D nano-
structures immersed into buffer solutions of pH 2.2 (Fig. 7A)
and 11 (Fig. 7B) respectively. The size of the P(MBA-4VP-4) gel
decreased as the pH increased from 2.2 (Fig. 7D) to 11 (Fig. 7E),
due to the deprotonation of the 4VP pyridine groups. That
means P(MBA-4VP-4) 1D nanostructures swell at pH 2.2 while
deswell at pH 11.
To investigate the pH-responsive behaviors of P(MBA-4VP)
1D nanostructures with different molar contents of 4VP, a
known amount of dried copolymer gels aer the ammonia
treatment was immersed into 5 mL of different pH buffer
solutions at room temperature, and their equilibrium swelling
ratio (Q) was measured aer 1 day. The curves of Q against pH
are shown in Fig. 8, which is characteristic behavior of the
polymer with weakly basic groups.42 In Fig. 8, the most notice-
able feature is the sharp change in the Q values between pH 3
and 5, which corresponds to a transition from the charged to
uncharged state on the network. Additionally, Fig. 8 shows that
the pH sensitivity weakens with the decrease of molar contents
of 4VP. The result indicates that the co-monomer 4VP is
responsible for the pH sensitivity of P(MBA-4VP) nano-
structures, and the increase in crosslinking degree with the
Fig. 8 pH sensitivity of P(MBA-4VP) 1D nanostructures. (A) P(MBA-
4VP-1), (B) P(MBA-4VP-2), and (C) P(MBA-4VP-4).
The feature is also noticeable in Fig. 9, which reveals the
reversible pH response of P(MBA-4VP) 1D nanostructures
alternatively kept at the two extreme pH values. Known amounts
of dried copolymer gels with different molar contents of 4VP
were alternatively kept in a 5 mL buffer solution of pH values 2.2
and 11 for 12 hours.
Adsorption and desorption of aspirin
decrease in molar contents of 4VP is another cause that limits Generally, pyridine groups in P(MBA-4VP) nanostructures can
the copolymer gels from swelling.
form the double hydrogen bonding system with carboxylic acid
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Fig. 9 The reversible pH response of P(MBA-4VP) 1D nanostructures
Fig. 11 Mass increase of adsorbed aspirin in P(MBA-4VP) 1D nano-
alternatively kept in a buffer solution of pH values 2.2 and 11 for 12 structures with respect to time. (A) P(MBA-4VP-1), (B) P(MBA-4VP-2),
hours. (A) P(MBA-4VP-1), (B) P(MBA-4VP-2), and (C) P(MBA-4VP-4).
and (C) P(MBA-4VP-4).
groups, making the products capable to uptake some carboxylic of P(MBA-4VP) 1D nanostructures contribute to the different
compounds from dilute solution effectively. That provides the
adsorption amounts of guest aspirin. However, not all pyridine
possibility to functionalize the P(MBA-4VP) 1D nanostructures. groups of 1D copolymer nanostructures were involved in aspirin
Aspirin (chemical name is 2-acetylsalicylic acid) is an adsorption. Take P(MBA-4VP-4) for example, a 10 mg sample
important medicine used to reduce fever and to relieve mild to used for adsorption contained 0.07 mmol pyridine groups,
moderate pain from headaches, colds, toothaches, and muscle while the equilibrium molar content of aspirin adsorbed in
aches. The carboxylic acid group in the chemical structure P(MBA-4VP-4) 1D nanostructures was 0.000344 mmol, much
makes aspirin possible to be immobilized on P(MBA-4VP) 1D less than the molar content of pyridine groups, indicating that
nanostructures. To investigate the adsorption of aspirin, a only the pyridine groups distributed on the surface of 1D
known amount of dried copolymer gels aer the ammonia
treatment was immersed into dilute aspirin–chloroform solu-
copolymer nanostructures could participate in adsorption.
The 1D copolymer nanostructures could be regenerated
tion (50 mg Lꢀ1) for a certain time. Their time-dependent UV-Vis through desorption of aspirin from the copolymer gels, and
spectra in Fig. 10 show that the intensity of the peaks decreases there are three methods to do this. The rst is according to the
with respect to time, from 2 h to 12 h, which indicates that all reversible nature of hydrogen-bonding, desorption of aspirin
samples are capable of adsorbing aspirin from the organic will take place when a heated proton solvent is used. So we
phase.
The mass of aspirin adsorbed in P(MBA-4VP) 1D nano- edly, then aspirin was removed from the copolymer gels thor-
structures increased with the adsorption time (shown in
treated the aspirin-loaded sample with heated ethanol repeat-
oughly. The other is to place the aspirin-loaded sample in a
Fig. 11), and basically tended to be stable aer 24 h to reach the buffer solution of pH 2.2, and the protonation of pyridine
equilibrium. The maximum adsorption increased with molar groups could destroy the double hydrogen bonding system
contents of 4VP which suggests the formation of the double between the gel sample and aspirin, which is based on the pH
hydrogen bonding system, and the contents of pyridine groups sensitivity of P(MBA-4VP) 1D nanostructures. The third is based
Fig. 10 UV-Vis spectra of time-dependent adsorption of aspirin from chloroform solution by using P(MBA-4VP) 1D nanostructures with different
molar contents of 4VP.
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on the effect of ionic interaction on hydrogen-bonding. Aspirin
was released from the copolymer gels when placed in a 0.1
mol Lꢀ1 sodium chloride solution, because salt solution could
easily break hydrogen bonds. Therefore, we immersed aspirin-
loaded P(MBA-4VP-4) and P(MBA-4VP-2) 1D nanostructures in
hot ethanol, a buffer solution of pH 2.2 and a 0.1 mol Lꢀ1
sodium chloride solution respectively. The UV-Vis spectra of
ethanol solutions, buffer solutions of pH 2.2 and sodium
chloride solutions at different times are presented in Fig. 12–14
respectively. With the increase of times of immersion, the
concentration of aspirin released in ltrates decreased,
which indicates the removal of guest molecules from the
copolymer gels.
Aer desorption, the aspirin-released sample was placed in
initial aspirin–chloroform solution to absorb aspirin again for
12 hours, and then desorbed using hot ethanol. The test was
repeated three times. The mass of aspirin adsorbed in P(MBA-
Fig. 14 UV-Vis spectra of 0.1 mol Lꢀ1 sodium chloride solutions at
different times. The curves from up to down correspond to 2 h, 4 h and
6 h respectively.
Fig. 12 UV-Vis spectra of ethanol solutions at different times. The
curves from up to down correspond to 2 h, 4 h, 6 h, 8 h and 10 h,
respectively.
Fig. 15 Mass of aspirin adsorbed in P(MBA-4VP-4) 1D nanostructures
at different experimental times.
4VP-4) 1D nanostructures each time is summarized in Fig. 15,
which is basically identical. This suggests that P(MBA-4VP) 1D
nanostructures could be regenerated and used as an adsorption
material repeatedly.
Fabrication of silver nanoparticle loaded copolymer
nanowires
In the fabrication process of P(MBA-4VP) 1D nanostructures
using the silver metallogel as a template, silver ions play an
important role in the induced deposition of copolymerized
products onto the surfaces of the template bers, which should
be removed aer copolymerization. However, if we use the
reducing agent to treat the copolymerized products, instead of
ammonia treatment to remove the metallogel templates, silver
nanoparticle loaded copolymer nanowires would be produced
from the reduction of silver ions.
Fig. 13 UV-Vis spectra of buffer solutions of pH 2.2 at different times.
The curves from up to down correspond to 2 h, 4 h and 6 h
respectively.
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Paper
There are a variety of reduction methods of silver ions. In nanoparticles exist in copolymer nanowires. In this system,
this study, we use high temperature ethylene glycol solution as a silver ions act both as the template and as the nanoparticle
reducing agent for reducing silver ions to silver nanoparticles, growth substrate.
which is a classical method of silver nanowire synthesis.43
Additionally, we have also found that different reduction
As the reduction time gets longer, the color transition methods and reduction times have a great inuence on the size
process of the copolymer gel from light yellow to brown yellow and distribution of silver nanoparticles. The overall and
was observed, which indicates the generation of silver nano- detailed experimental results will be published in a future
particles.44 A typical TEM image of silver nanoparticle loaded article.
P(MBA-4VP-2) nanowires is shown in Fig. 16A. The silver
nanoparticles rather than silver nanowires are dispersed
throughout the P(MBA-4VP-2) nanowires, and they are almost
Conclusions
spherical with diameters of about 5–15 nm. However, under the
same reduction conditions, the small black particles have not
In conclusion, using a metallogel as a template, we successfully
fabricated pH responsive 1D copolymer nanostructures via
been observed on pure PMBA nanotubes (Fig. 16B). The reason
copolymerizing MBA with pH sensitive co-monomer 4VP. The
is the leaching of silver ions due to swelling of PMBA during
combination of 1D polymer nanostructures with stimuli
reduction. The introduction of 4VP could improve the coordi-
responsive polymers could impart to materials the unique
nation ability of copolymerized products with silver ions, so as
structures and smart properties, and broaden their application
to enhance the binding capacities of P(MBA-4VP) nanowires for
eld. This approach is very simple and efficient, involving
copolymerization in a gel system at a moderate temperature.
silver nanoparticles to prevent their leaching. These observa-
tions indicate that pyridine groups of 4VP play an important
TEM and SEM analyses of the reaction product demonstrate the
role in the immobilization of silver nanoparticles inside
morphology of 1D copolymer nanostructures including nano-
copolymer nanowires. EDX analysis of the selected region in
wires and nanotubes, which duplicate the networks of nano-
Fig. 17 shows the peaks of silver, further proving that silver
bers of the template metallogel. The fabrication mechanism of
copolymer nanowires could be described by the ‘self-degraded
template polymerization’ mechanism. The introduction of
co-monomer 4VP imparts to the 1D copolymer nanostructures
pH sensitivity and the possible use as an adsorption material of
special compounds, such as aspirin. Adsorbed 1D copolymer
nanostructures could be regenerated using proton solvent, acid
medium or salt solution. The pH-responsive behaviors and
reversible pH response of various 1D copolymer nanostructures
were investigated. Their adsorption and desorption of aspirin
were detected by a UV-Vis spectrometer. Using this strategy, a
variety of functionalized 1D copolymer nanostructures might be
prepared by different kinds of co-monomers. In addition, silver
Fig. 16 TEM images of (A) P(MBA-4VP-2) and (B) PMBA 1D nano-
nanoparticle loaded copolymer nanowires have been produced
from the reduction of silver ions instead of template removal,
where silver ions act both as the template and as the nano-
particle growth substrate.
structures with the template after the 2 h reduction.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 20874055 and 21174079) and
the National Basic Research Program of China (2014CB932202).
Notes and references
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