Aiping Chang

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.

A colloidal supra-structure of responsive microgels as a potential cell scaffold

A colloidal supra-structure that regulates essential cellular functions such as cell proliferation rate can better mimic the extracellular matrix for engineering complex tissues. Such a colloidal supra-structure can be built from the thermo-driven gelation of the colloidal dispersion of poly(N-isopropylacrylamide-co-acrylamide) (poly(NIPAM-co-AAm)) microgels, which exhibit a reversible and continuous volume phase transition in water at a temperature of ≈35 °C and remain partially swollen and soft at 37 °C. In our experimental size range (hydrodynamic radius ∼100 to 250 nm at 37 °C), the microgels of a larger particle size displayed a larger critical effective concentration ϕeffective, above which the microgel dispersions undergo a sol-to-gel transition to form a solid-like colloidal supra-structure under the physiological conditions. While the syneresis kinetics of all colloidal supra-structures fitted well to the Tanaka–Fillmore model, indicating a polymer-like nature, the colloidal supra-structures built from the larger microgel particles exhibited a smaller degree of syneresis. The colloidal supra-structures can serve as scaffolds for cell culture. The larger the microgel particles used to build the colloidal supra-structures for scaffolds, the greater the enhancement in the relative cell viability. The relative small difference in size (2.5-fold) of the building microgel particles could lead to a large difference in the cell proliferation rate (as high as 4.5-fold). These results indicate that the size of microgel building blocks takes an important role in controlling the sol-to-gel transitions, degree of syneresis, and permeability of the constructed colloidal supra-structures, thus providing a simple way to construct desirable cell scaffolds to regulate the cellular functions.

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).

Synthesis and characterization of ureido-derivatized UCST-type poly(ionic liquid) microgels

We report a class of stimuli-responsive poly(ionic liquid) microgel that possesses an upper critical solution temperature (UCST). Such a microgel is synthesized from ureido polymers via quaternary ammonization and post synthetic modification by anion-exchange treatment. These microgels can undergo reversible UCST-type volume phase transitions in response to the changes in temperature of water, methanol, and water/methanol mixtures, such that the temperature-responsive volume phase transition behavior can be readily tuned over a significantly wide range in a miscible mixture of water/methanol by varying the mol ratio of the two solvents, resulting in a phase diagram displaying the UCST-type co-nonsolvency phase separation phenomenon. At a set temperature in our experimental temperature window of 20.0–64.0 °C, the microgels can undergo an isothermal re-entrant swelling–shrinking–swelling transition as the methanol content increases in the water/methanol mixture. Moreover, these microgels as catalysts in a model esterification reaction of palmitic acid with methanol can display merits of both homogeneous (considerably high efficient catalytic activity) and heterogeneous (excellent recyclability) catalysis. With UCST-type responsive phase behavior and catalytic properties harnessed on the same object, the microgels react to external stimuli which allows their catalytic properties to be altered accordingly, to a certain extent, in a novel non-monotonous way different from that reported previously on the catalyst systems using LCST-type microgels.

Assembly of polythiophenes on responsive polymer microgels for the highly selective detection of ammonia gas

A class of smart composite materials based on the assembly of conjugated polymers on responsive polymer microgels has been prepared. We have chosen poly(3-((2-(2-methoxyethoxy)ethoxy)methyl)-thiophene) as the model conjugated polymer and an ammonia-responsive microgel of phenoxazinium-functionalized poly(N-isopropylacrylamide-co-propargyl acrylate) as the model template. Under this design, the composite materials can combine the electrical conductivity of the conjugated polymers and the ammonia recognisability of the ammonia-responsive polymer microgels; the cooperation of these properties allows the reversible control of electrical conductivity by ammonia gas. Those composite materials can not only adapt to ammonia gas, but also convert changes in the concentration of ammonia into conductance, allowing the electrical detection of ammonia gas with high selectivity. This makes the composite materials different from the conductive polymer platforms reported previously, which may also respond to non-ammonia gases and the response induced by non-ammonia gases is close to that induced by ammonia gas. Using these composite materials as sensing materials for the electrical detection of ammonia gas, the detection limit can reach as low as 1.1 ppb. These features enable their use for the electrical detection of ammonia in breath.