Qingshi Wu

Responsive Au@polymer hybrid microgels for the simultaneous modulation and monitoring of Au-catalyzed chemical reaction

The simultaneous modulation and monitoring of catalysis is possible when using metal@polymer hybrid microgels by rational design. Such hybrid microgels are made of Au nanoparticles covered with a temperature and pH dual-responsive copolymer gel shell of poly(N-isopropylacrylamide-co-allylamine). The Au nanoparticle cores can act as catalysts in a model electron-transfer reaction between hexacyanoferrate(III) and borohydride ions. The introduction of a smart polymer gel shell onto the Au nanoparticles can not only allow modulation of the catalysis of the Au nanoparticle cores through varying the solution temperature, but also allow label-free in situ localized surface plasmon resonance (LSPR) monitoring of the kinetics and thermodynamics of the catalyzed chemical reaction. Unlike conventional spectroscopic methods that only reflect the overall information occurring in the reaction system, the label-free in situ LSPR monitoring gives local information occurring on the catalytic surface and therefore has the potential to advance our understanding of the catalyzed chemical reaction.

Synthesis and characterization of responsive poly(anionic liquid) microgels

We report a class of poly(anionic liquid) microgels that undergo stimuli-responsive volume phase transitions. Such a microgel is synthesized by free radical precipitation polymerization of a tetrabutylphosphonium 4-styrenesulfonate monomer and a cross-linker N,N’-methylenebisacrylamide. These microgels can not only undergo reversible volume phase transitions in response to changes in temperature in water, methanol, or water/methanol mixtures, but also undergo re-entrant swelling–shrinking–swelling transitions as the methanol content increases in water/methanol mixtures at a set temperature in our experimental temperature window of 25.0–64.0 °C. Such microgels can be post functionalized, e.g., via ion-exchange treatment with HSO3CF3 to partially transform sulfonate to the catalytic Bronsted acidic –SO3H, whilst the yielded microgels can inherit the responsive properties. With both responsive and catalytic properties simultaneously harnessed on the same object, the functionalized microgels potentially can be used as a highly efficient catalyst for the esterification reaction of palmitic acid and the transesterification reaction of tripalmitin with methanol (as model reactions) at 65.0 °C (temperature of an oil bath), and allow the catalytic activity to be modulated to a certain extent in a non-monotonous way, making it possible to boost the reactions at a relatively lower temperature (e.g., 42.0 °C), while maintaining considerable catalytic activity. These features underlie the feasible use of the microgels in biodiesel production (with waste cooking oil as a model feedstock).

Switchable glucose-responsive volume phase transition behavior of poly(phenylboronic acid) microgels

We develop a class of poly(phenylboronic acid) microgels, which are made of 3-aminophenylboronic acid covalently bonded to oligo(ethylene glycol)-based polymers, to demonstrate the feasibility of on-site tailoring of the glucose-responsive volume phase transition behavior of poly(phenylboronic acid) gels. Different from the poly(phenylboronic acid) gels reported previously that typically undergo a fixed type (swelling and/or shrinking) of glucose-responsive volume phase transition behavior, the presented microgels can display switchable behavior upon adding glucose: shrinking (at temperature ≤29.0 °C), unresponsive (29.0–33.0 °C), and swelling (≥33.0 °C). The underlying mechanism for such an on-site tailoring is possibly associated with a competition of glucose-induced increase in the Donnan potential (favoring swelling; due to the formation of glucose-boronates or glucose-bis-boronates) and additional cross-links (favoring shrinking; due to the formation of glucose-bis-boronates) at a particular temperature. Accompanied by this on-site tailoring, the photoluminescence of the microgels can be tuned from “turn-off” (e.g., at 25.0 °C) to “turn-on” (e.g., at 37.0 °C) upon adding glucose, which may provide a functional basis for biosensors for prospective biomedical applications.

Glucose-responsive microgels based on apo-enzyme recognition

Glucose-responsive polymer microgels that can undergo highly selective, reversible, and rapid volume phase transitions in response to fluctuations in blood glucose concentration have the potential to regulate insulin delivery to improve patient compliance and health. Herein, we report such a glucose-responsive polymer microgel, which is made of a representative apo-enzyme, apo-glucose oxidase (apo-GOx), interpenetrated in a chemically-crosslinked network of poly(N-isopropylacrylamide) (pNIPAM). Introduction of apo-GOx into the pNIPAM network made the newly developed semi-interpenetrating-structured microgels responsive with high selectivity to glucose over a glucose concentration range of 0–20 mM at a physiological pH value of 7.4. While the microgels could swell and reach stability shortly (<1 s) after adding glucose over a concentration range of 50.0 μM–20.0 mM, the changes in the average hydrodynamic diameter and the size distribution of the microgels could be fully reversible within experimental error even after twenty cycles of adding/removing glucose. The association rate constant was determined to be ca. 1.0 mM−1 s−1 with a ca. 10.1 s−1 dissociation rate constant, indicating a fast reversible time response to the glucose concentration change of the microgels. With the microgels as carriers, in vitro insulin release could be modulated in a pulsatile profile in response to glucose concentrations, and in vivo studies revealed that these formulations may improve glucose control in streptozotocin-induced diabetic mice subcutaneously administered with the insulin loaded microgels.

Facile synthesis of silver nanoparticles in a crosslinked polymeric system by in situ reduction method for catalytic reduction of 4-nitroaniline

ABSTRACT In this study, poly(N-isopropylmethacrylamide-co-methacrylic acid) microgels prepared by free radical precipitation polymerization were used as micro-reactors for the synthesis and stabilization of silver nanoparticles. UV-Visible spectroscopy, Transmission Electron Microscopy and Fourier-transform infrared spectroscopy were used to characterize both pure and hybrid microgels. The catalytic reduction of 4-nitroaniline was carried out in the presence of hybrid microgels to test their catalytic activity, and the catalysis mechanism was explored by varying the concentrations of reacting species like 4-nitroaniline and NaBH4, as well as the dose of the catalyst. The kinetic data indicates that this reaction follows pseudo-first order. The variation in apparent rate constant (kapp) with respect to NaBH4 concentration also discloses it to be the following Langmuir–Hinshelwood mechanism. The relationship between catalyst concentration and apparent rate constant was found to be increasing in a linear manner. The data obtained also confirmed that silver nanoparticles loaded microgels have the potential to be used as an excellent micro-reactor for selective reduction of 4-nitroaniline to p-phenylenediamine.

Synthesis and characterization of ammonia-responsive polymer microgels

We report a type of polymer microgel that undergoes rapid, reversible, and highly sensitive volume phase transitions upon varying ammonia concentrations in milieu. Such an ammonia-responsive microgel is made by tethering of a phenoxazinium, N-(5-(3-azidopropylamino)-9H-benzo[a]-phenoxazin-9-ylidene)-N-methylmethanaminium chloride, to the network chains of poly(N-isopropylacrylamide-co-propargyl acrylate) via a copper(I)-catalyzed azide–alkene cycloaddition. Tethering of the ammonia-recognizable phenoxazinium onto the polymer network chains makes the microgels responsive to ammonia. While a fast (<0.1 s) and stable shrinkage of the microgels can be achieved upon addition of ammonia over a clinically relevant range (0.25–2.9 ppm), the microgels can convert the elevated concentrations of the solution/gas-phase ammonia into enhanced photoluminescence signals. This makes the microgels different from the phenoxazinium, or its analogs reported in previous studies, that exhibit ammonia-induced quenching of photoluminescence. With the microgels as probes, the detection limit was ca. 7.3 × 10−2 and 3.9 ppb for the solution and the gas-phase ammonia, respectively. These features enable “turn-on” photoluminescence detection of ammonia in breath.

Highly efficient solid polymer electrolytes using ion containing polymer microgels

A solid polymer electrolyte exhibiting high ionic conductivity (reaching ca. 10−4.8 S cm−1 at 25 °C) is fabricated using ion containing polymer microgels of lithium tris(perfluorophenyl) (2,3,5,6-tetrafluoro-4-(2-(2-(vinyloxy)ethoxy)ethoxy)phenyl) borate. This solid polymer electrolyte shows great possibilities for use in large-capacity lithium ion batteries.

Engineering of responsive polymer based nano-reactors for facile mass transport and enhanced catalytic degradation of 4-nitrophenol

Silver nanoparticles with average diameter of 10±3nm were synthesized within the sieves of poly(N-isopropylacrylamide-2-hydroxyethylmethacrylate-acrylic acid) (p(NIPAAm-HEMA-AAc)) polymer microgels. Free radial emulsion polymerization was employed for synthesis of p(NIPAAm-HEMA-AAc) polymer microgels. Silver nanoparticles were introduced within the microgels sphere by in situ reduction method. Microgels and hybrid microgels were characterized by Fourier transform infrared spectroscopy, ultra violet-visible spectroscopy, transmission electron microscopy and dynamic light scattering measurements. Catalytic activity of Ag-p(NIPAAm-HEMA-AAc) hybrid microgels was studied using catalytic reduction of 4-nitrophenol (4-NP) as a model reaction in aqueous media. The influence of sodium borohydride (NaBH4) concentration, catalyst dose and 4-NP concentration on catalytic reduction of 4-NP was investigated. A linear relationship was found between catalyst dose and apparent rate constant (kapp). The mechanism of catalysis by hybrid microgels was explored for further development in this area. The deep analysis of catalytic process reveals that the unique combination of NIPAAm, HEMA and AAc does not only stabilize silver nanoparticles in polymer network but it also enhances the mass transport of hydrophilic substrate like 4-NP from outside to inside the polymer network.

Bioinspired synthesis of poly(phenylboronic acid) microgels with high glucose selectivity at physiological pH

A highly selective response to glucose is possible by using a poly(phenylboronic acid) microgel prepared by a rational design. Such a microgel is made of a commercial 4-vinylphenylboronic acid crosslinked with N,N′-bis(propene)perylene-3,4,9,10-tetracarboxyldiimide. The introduction of a suitable amount of perylene bisimides into the poly(phenylboronic acid) network can make the newly developed microgels responsive with high selectivity to glucose over a clinically relevant concentration range of 0.0–30.0 mM at a physiological pH of 7.4. Unlike poly(phenylboronic acid) gels reported in previous reports that swell but show poor glucose selectivity, or shrink upon adding glucose, the proposed microgels swell and the swelling ratio is much larger than that upon adding other common monosaccharides (natural stereoisomers of glucose, e.g., fructose, galactose and mannose). The responsive volume phase transitions of the microgels give excellent fits to a 1 : 1 binding model, to estimate an association constant of 212 M−1 for glucose binding, which is remarkably larger than those of 78, 42 and 52 M−1 respectively, for fructose, galactose and mannose binding. The proposed microgels also feature a class of built-in signalling systems in the form of highly selective glucose-dependent fluorescence emission. The highly selective glucose-response of the microgels suggests the potential for development into sensors for detection of glucose in serums of fasting bloods and even non-fasting bloods (as models of complex bio-systems).

Electrochemical synthesis of polymer microgels

We describe an electrochemical approach for the synthesis of polymer microgels through polymerization of the monomer in the presence of the crosslinker. This electrochemical approach means initiation by the electron transfer processes which occur at the electrodes, in that by controlling the applied potential it is possible to control the generation of free radicals and/or other reactive species. Upon applying a suitable potential above the electrochemical oxidation waves of N-isopropylacrylamide (as a model of the monomer) and N,N′-methylenebisacrylamide (as a model of the crosslinker), the polymerization and crosslinking are able to proceed to obtain nearly monodisperse polymer microgels with high yield. The apparent rate constant was determined to be 1.69 × 10−2 min−1 based on the evolution of light scattering intensity, or 1.43 × 10−2 min−1 based on the average hydrodynamic diameter. The underlying formation mechanism to reach polymer microgels instead of macrogels, even at high monomer concentrations, is possibly due to the limitation of the primary chain length such that bridging between growing microgel regions can be eliminated. The microgel size can be tuned by varying the applied potential. The reaction medium can be recycled, and reused directly without a notable impact on the next cycle of synthesis. This electrochemical approach can be extended to synthesize microgels of poly(acrylamide) or poly(acrylic acid) (as the additional models).