12 research outputs found
Multicompartment Nanoparticles of Poly(4-vinylpyridine) Graft Block Terpolymer: Synthesis and Application as Scaffold for Efficient Au Nanocatalyst
Multicompartment nanoparticles (MCBNs)
constructed with the brush
block terpolymer of [poly(<i>p</i>-chloromethylstyrene)-<i>graft</i>-poly(4-vinylpyridine)]-<i>block</i>-polystyrene
(PCMS-<i>g</i>-P4VP)-<i>b</i>-PS are prepared
through dispersion polymerization of styrene in the methanol/water
mixture mediated by the brush macro-RAFT agent of poly(<i>p</i>-chloromethylstyrene)-<i>graft</i>-poly(4-vinylpyridine)
trithiocarbonate. During the dispersion RAFT polymerization, the molecular
weight of the brush (PCMS-<i>g</i>-P4VP)-<i>b</i>-PS block terpolymer linearly increases with the monomer conversion.
Ascribed to the brush (PCMS-<i>g</i>-P4VP) block, MCBNs
including a PS core and discrete subdomains of (PCMS-<i>g</i>-P4VP) on the PS core dispersed in water are formed. The reasons
leading to formation of MCBNs are discussed, and the immiscibility
of the brush (PCMS-<i>g</i>-P4VP) block with the PS core,
the low number density of the brush (PCMS-<i>g</i>-P4VP<sub>25</sub>)<sub>21</sub> chains tethered on per surface area of the
PS core, and the high molecular weight but the low polymerization
degree of the brush (PCMS-<i>g</i>-P4VP) block are ascribed.
Au nanoparticles are immobilized on the bulgy PCMS-<i>g</i>-P4VP subdomains on MCBNs and show high catalytic efficiency in the
aerobic alcohol oxidation
Efficient Synthesis of Molecularly Imprinted Polymers with Enzyme Inhibition Potency by the Controlled Surface Imprinting Approach
A facile, general, and highly efficient
approach to prepare uniform
core–shell molecularly imprinted polymer (MIP) particles with
enzyme inhibition potency is described for the first time, which involves
the combined use of molecular imprinting and controlled/“living”
radical polymerization (CRP) techniques as well as surface-anchoring
strategy. The thickness of the enzyme-imprinted surface layers of
the core–shell MIP microspheres had a significant influence
on their binding properties, and only those with their thickness comparable
with the diameters of the targeted enzymes could afford enzyme-MIPs
with optimal specific bindings. The as-prepared enzyme-MIPs were found
to have homogeneous binding sites and high template binding capacities,
affinity, and selectivity, and they proved to show much higher enzyme
inhibition potency than the small inhibitor by 3 orders of magnitude
(i.e., the enzyme inhibition constant of every binding site of the
MIP microspheres was about one-thousandth of that of the small inhibitor),
mainly due to the formation of strong long-range secondary interactions
between enzymes and imprinted pockets. In addition, the general applicability
of our strategy was confirmed
Synthesis and Self-Assembly of Amphiphilic Janus Laponite Disks
Materials with asymmetric structures
are attractive for wide applications
in chemistry and materials science. Two-dimensional Janus disks or
nanosheets are particularly appealing because of the unique shape
and the distinctive self-assembled structures. A facile and versatile
method for the synthesis of amphiphilic Janus Laponite disks is proposed
in this paper. Positively charged PS spheres were prepared by ATRP
emulsion polymerization. Upon addition of aqueous dispersion of negatively
charged Laponite disks into PS emulsions, the nanosized disks were
adsorbed onto the surface of PS particles via electrostatic interaction.
One side of a Laponite disk touches the surface of a colloidal particle,
and the other side faces the medium. After addition of positively
charged polymeric micelles or quaternized poly(2-(dimethylamino)ethyl
methacrylate) (q-PDMAEMA) chains into the aqueous dispersions of the
colloidal particles, the micelles or polymer chains were immobilized
onto the Laponite disks, and Janus disks were produced on particle
templates. After centrifugation and redispersion of the colloidal
particles into a good solvent, amphiphilic Janus Laponite disks with
PS chains on one side and hydrophilic q-PDMAEMA or polymeric micelles
on the other side were obtained. Transmission electron microscopy
(TEM) and atomic force microscopy (AFM) were used to characterize
the Janus disks. Self-assembly of the Janus disks at liquid–liquid
interface and in selective solvents was investigated. Similar to small
molecular surfactants, the amphiphilic Janus disks can self-assemble
at liquid–liquid interface, resulting in a decrease of the
interfacial tension and emulsification of oil droplets in water. In
a THF–methanol mixture at a volume ratio of 1:6, PS brushes
on the Janus disks collapse forming two-layer face-to-face stacks.
The distinctive self-assembled structures were analyzed by TEM and
AFM
Synergy between Polyamine and Anionic Surfactant: A Bioinspired Approach for Ordered Mesoporous Silica
A novel bioinspired approach for
ordered mesoporous silica was developed on the basis of the synergic
coassembly between polyamine and an anionic surfactant as a template.
With the help of cationic polyamine, anionic surfactant micelles could
be utilized as a mesostructure template, whereas with the aid of the
anionic surfactant micelles the cationic polyamine chains underwent
aggregation to exert their ability to induce silica condensation.
Mesoporous silicas with well-ordered mesostructure of <i>Fd</i>-3<i>m</i> symmetry and 3D hexagonal close-packed mesostructure
(hcp) were fabricated. Because of the abundant types of anionic surfactants
and polyamines, the synthesis approach can be regarded as a general
method for anionic-surfactant-templated mesoporous silica, and new
mesostructures and morphologies are expected
Thickness Dependence of Glass Transitions Measured by AC-Chip Calorimetry in Films with Controlled Interface
When most prior studies on thin polymer
films have shown that glass
transition temperature (<i>T</i><sub>g</sub>) decreases
under nanoconfinement, the differential alternating current chip (ac-chip)
calorimetric method shows little dependence of <i>T</i><sub>g</sub> on thickness for supported film. To reveal this contradiction,
we have manipulated a controlled interface by spin-coating polystyrene
(PS) with immiscible surfactants such as tetraoctylammonium bromide
or citric acid. Since the immiscible surfactants did not show plasticizing
effect for PS, there was no observable reduction of <i>T</i><sub>g</sub> from the bulk value ether in powdered blends or in thick
films. However, the ultrathin film with thickness <i>h</i> ∼ 25 nm, consisting of 95 wt % PS and 5 wt % surfactants,
showed a reduction of <i>T</i><sub>g</sub> by 6–7
°C, as compared to thick film with the same composition. We propose
that the surfactant molecules assembled on the interface between thin
film and substrate due to phase separation. The molecular mobility
of molecules at the interface was dramatically increased, which was
detected by <sup>1</sup>NMR with dipolar filter sequence. It appeared
that the deviation range was not so large as that measured by other
methods. But considering that we were measuring <i>T</i><sub>g</sub> at a high frequence (10 Hz), this amount of deviation
was quite significant for ac-chip calorimetry. As a result, ac-chip
calorimetry measured <i>T</i><sub>g</sub> data unambiguously
demonstrate that thickness dependence of <i>T</i><sub>g</sub> is a real property of confined thin film
Reversible Interactions of Proteins with Mixed Shell Polymeric Micelles: Tuning the Surface Hydrophobic/Hydrophilic Balance toward Efficient Artificial Chaperones
Molecular chaperones can elegantly
fine-tune its hydrophobic/hydrophilic
balance to assist a broad spectrum of nascent polypeptide chains to
fold properly. Such precious property is difficult to be achieved
by chaperone mimicking materials due to limited control of their surface
characteristics that dictate interactions with unfolded protein intermediates.
Mixed shell polymeric micelles (MSPMs), which consist of two kinds
of dissimilar polymeric chains in the micellar shell, offer a convenient
way to fine-tune surface properties of polymeric nanoparticles. In
the current work, we have fabricated ca. 30 kinds of MSPMs with finely
tunable hydrophilic/hydrophobic surface properties. We investigated
the respective roles of thermosensitive and hydrophilic polymeric
chains in the thermodenaturation protection of proteins down to the
molecular structure. Although the three kinds of thermosensitive polymers
investigated herein can form collapsed hydrophobic domains on the
micellar surface, we found distinct capability to capture and release
unfolded protein intermediates, due to their respective affinity for
proteins. Meanwhile, in terms of the hydrophilic polymeric chains
in the micellar shell, poly(ethylene glycol) (PEG) excels in assisting
unfolded protein intermediates to refold properly via interacting
with the refolding intermediates, resulting in enhanced chaperone
efficiency. However, another hydrophilic polymer-poly(2-methacryloyloxyethyl
phosphorylcholine) (PMPC) severely deteriorates the chaperone efficiency
of MSPMs, due to its protein-resistant properties. Judicious combination
of thermosensitive and hydrophilic chains in the micellar shell lead
to MSPM-based artificial chaperones with optimal efficacy
Accessing Structure and Dynamics of Mobile Phase in Organic Solids by Real-Time T<sub>1C</sub> Filter PISEMA NMR Spectroscopy
The structure and dynamic behavior of mobile components play a significant role in determining properties of solid materials. Herein, we propose a novel real-time spectrum-editing method to extract signals of mobile components in organic solids on the basis of the polarization inversion spin exchange at magic angle (PISEMA) pulse sequence and the difference in <sup>13</sup>C T<sub>1</sub> values of rigid and mobile components. From the dipolar splitting spectrum sliced along the heteronuclear dipolar coupling dimension of the 2D spectrum, the structural and dynamic information can be obtained, such as the distances between atoms, the dipolar coupling strength, the order parameter of the polymer backbone chain, and so on. Furthermore, our proposed method can be used to achieve the separation of overlapped NMR signals of mobile and rigid phases in the PISEMA experiment. The high efficacy of this 2D NMR method is demonstrated on organic solids, including crystalline l-alanine, semicrystalline polyamide-6, and the natural abundant silk fibroin
Confinement-Induced Deviation of Chain Mobility and Glass Transition Temperature for Polystyrene/Au Nanoparticles
The mobility and glass transition
temperature (<i>T</i><sub>g</sub>) for polymers under nanoscale
confinement differ substantially
from the bulk. Whereas many studies have focused on the one-dimensional
confinement, it has great significance to extend studies to higher
geometries. Here, we systematically investigate the mobility by dipolar-filter
sequence in solid-state NMR and <i>T</i><sub>g</sub> by
DSC for thiolated polystyrene (PS-SH) on gold nanoparticles. The increase
in <i>T</i><sub>g</sub> and signal suppression in NMR spectra
clearly indicate that the surface confinement dominates molecular
mobility as well as <i>T</i><sub>g</sub>. The molecular
weight of PS-SH and nanoparticles size show significant influence
on the immobilization and <i>T</i><sub>g</sub>. Our results
can be fitted with a core–two shell model; the inner shell
is under strong constraints while the outer shell with less confinement.
This work is essential to better understand the confinement effect
and also provides a step toward the ultimate desire to tailor the
properties of nanomaterials
Investigation on the Mechanism of the Synthesis of Gold(I) Thiolate Complexes by NMR
In this article, we characterized
the polymeric gold(I) thiolates
that precipitated from the intermediate solutions during the synthesis
process of gold nanoparticles (GNPs) by the Brust–Schiffrin
two-phase method and investigated the formation mechanism of the polymeric
gold(I) thiolates. The solution <sup>1</sup>H NMR confirmed the complete
reduction from Au(III) to Au(I) with the addition of the first two
equivalents of thiols, while only the third and fourth equivalents
of thiols were found to participate in forming gold(I) thiolates.
Gold(I) thiolates, [Au(I)SR]<sub><i>n</i></sub>, precipitated
from these solutions were further characterized by <sup>1</sup>H solid-state
NMR spectroscopy under fast magic angle spinning (MAS), Raman spectroscopy,
and thermogravimetric analysis. Further quantitative studies revealed
that the composition of [Au(I)SR]<sub><i>n</i></sub> could
be controlled by changing the order of addition of the third and fourth
equivalents of thiols. This work has great significance to better
understand the mechanism of gold nanoparticle formation and thus to
tailor the properties of the final products
Viscoelasticity and Structures in Chemically and Physically Dual-Cross-Linked Hydrogels: Insights from Rheology and Proton Multiple-Quantum NMR Spectroscopy
Hydrogels have received considerable
attention as an innovative
material due to their widespread applications in various fields. As
a soft and wet material, its mechanical behavior is best understood
in terms of the viscoelastic response to the periodic deformation,
which is closely related to the microscopic chemically/physically
cross-linked structures. Herein, a dual-cross-linked (DC) hydrogel,
where a physically cross-linked network by ionic coordination (Fe<sup>3+</sup>) is imposed on a chemically cross-linked poly(acrylamide-<i>co</i>-acrylic acid) network, was studied in detail by rheology
and proton multiple-quantum (MQ) NMR spectroscopy. Rheology experiments
revealed the diverse temperature- and strain-frequency-dependent viscoelastic
behaviors for DC hydrogels induced by the dynamic Fe<sup>3+</sup> coordination
interactions, in contrast to the single chemically cross-linked (SC)
hydrogels. During the shear experiment, the trivalent Fe<sup>3+</sup> complex with moderate/weak binding strength might transform to those
with strong binding strength and serve as permanent-like cross-linkages
to resist the periodic deformation when a large strain frequency was
applied. The viscoelastic behaviors of the DC hydrogels were strongly
affected by the monomer ratio (<i>C</i><sub>AAc</sub>/<i>C</i><sub>AAm</sub>) and Fe<sup>3+</sup> concentrations; however,
the chemically cross-linked density did not change with <i>C</i><sub>AAc</sub>/<i>C</i><sub>AAm</sub>, while the physically
cross-linked density was greatly enhanced with increasing Fe<sup>3+</sup> concentrations. Besides, the DC hydrogels have less contents of
network defects in comparison to the SC hydrogels. The heterogeneous
structural evolution with increasing the Fe<sup>3+</sup> concentration
and monomer ratio was also quantitatively determined and elucidated
by proton MQ NMR spectroscopy. In addition, the moduli (<i>G</i>′, <i>G</i>″) of DC hydrogels were almost
an order magnitude higher than that of the corresponding SC hydrogels,
demonstrating the significant contribution of Fe<sup>3+</sup> coordination
to the mechanical properties, in consistent with the high activation
energy of viscoelasticity for the physically cross-linked network
as obtained from the variable-temperature shear rheology experiments.
The experimental findings obtained from the rheology and proton MQ
NMR experiments can be correlated with and complementary to each other.
Herein, a combination of rheology and proton solid-state NMR is well
demonstrated as an effective and unique way for establishing the relationship
between microscopic structures and macroscopic viscoelastic properties