Shrunken State Research Articles

Linear and nonlinear interfacial rheology of responsive microgels at the oil-water interface

Microgelation can significantly enhance the interfacial activity of polysaccharide and the ability to stabilize emulsion, but the interfacial behaviour of microgels and the intrinsic mechanism of stabilizing emulsion is not fully understood. In this study, chitosan microgels (h-CSMs) with uniform size were successfully prepared by electrostatic crosslinking hydrophobically modified chitosan (h-CS) with sodium phytate (SP) using microfluidic technique. The microfluidic conditions for fabricating of h-CSM were optimized: the mass ratio of h-CS: SP = 5: 1 with h-CS concentration of 0.5 mg/ml, and the pressure ratio of Ph-CS (Pa):PSP (Pa) = 1600:1000. The prepared h-CSMs were pH-responsive, with a swollen state at pH 5 and a shrunken state at pH 3 and 7. Further, the interfacial adsorption kinetics, and the interfacial rheology particularly the nonlinear interfacial rheology were systematically studied. The instantaneous Si-factor was proposed to describe the nonlinear responses of the microgel interface during an individual dilatational oscillation cycle. The results showed that microgelation significantly improved the interfacial ability of chitosan molecules at the oil-water interface. The h-CSM interface exhibited strain-softening response during extension and strain-hardening response during compression. The interface formed by h-CSMs under the swollen state had higher viscoelastic modulus, lower degree of nonlinearity and higher emulsifying capacity compared to that formed under the shrinking state. Low ionic strength significantly increased the viscoelastic modulus and reduced the nonlinearity and enhanced emulsifying capacity of the h-CSM interface, whereas high ionic strength disrupted the interaction between the h-CSMs and increased the nonlinear of the h-CSM interface due to too strong electrostatic screening effect.

Phase Transition Behaviors of Poly(N-isopropylacrylamide) Nanogels with Different Compositions Induced by (−)-Epigallocatechin-3-gallate and Ethyl Gallate

Phase transition behaviors of poly(N-isopropylacrylamide) nanogels with different compositions induced by (−)-epigallocatechin-3-gallate (EGCG) and ethyl gallate (EG) has been investigated systematically. Monodisperse poly(N-isopropylacrylamide-co-N-hydroxymethyl acrylamide) (P(NIPAM-co-NMAM)) and poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (P(NIPAM-co-HEMA)) nanogels with different feeding monomer ratios were prepared by emulsion polymerization. P(NIPAM-co-NMAM) nanogels exhibit rapid isothermal phase transition behavior in EGCG solutions with low concentration (10−3 mol/L) in less than 10 minutes. The thermosensitive phase transition behaviors of nanogels are affected not only by the copolymerized monomers but also by the concentrations of EGCG and EG in aqueous solutions. Nanogels remain in a shrunken state and do not exhibit thermosensitive phase transition behaviors in EGCG solutions (≥5 mmol/L), whereas they display thermo-responsive phase transition behaviors in EG solutions. The volume phase transition temperature (VPTT) shifts to lower temperatures with increasing EG concentration. The diameters of P(NIPAM-co-NMAM) nanogels decrease with increasing EG concentration at temperatures between 29 and 33 °C. In contrast, the diameters of P(NIPAM-co-HEMA) nanogels increase with increasing EGCG concentration at temperatures between 37 and 45 °C. The results demonstrate the potential of nanogels for simple detection of EG and EGCG concentrations in aqueous solutions over a wide temperature range, and EGCG can serve as a signal for the burst-release of drugs from the P(NIPAM-co-NMAM)-based carriers at physiological temperature.

Dynamic Behavior of Heavy Water Inside and Outside of Poly (N-Isopropylacrylamide) (PNIPAM) Microgel Using Broadband Dielectric Spectroscopy

One pot free radical precipitation polymerization was used to synthesize an aqueous suspension of non-ionic poly ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -isopropylacrylamide) [PNIPAM] microgel particles. For dielectric measurements in the frequency range of 100 MHz to 50 GHz and temperature range of 288 K to 323 K, 10 wt. % of freeze-dried PNIPAM microgels dispersed in heavy water were used. The relaxation process was seen at around 15 GHz, which corresponds to the entire rotational motion of the heavy water, both within and outside the microgel. Furthermore, in the high frequency relaxation spectrum, heavy water outside the microgel (h1-process with a relaxation time of h1, τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h1</sub> , fixed as that of pure heavy water) and confined heavy water within the microgel (h2-process with a relaxation time of h2, τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h2</sub> ) were evaluated in view of the two-water model’s assumption and contribution. Under volume phase transition temperature (VPTT), τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h2</sub> is about 4 to 5 times larger than τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h1</sub> , and it does not change much even though the temperature is raised. However, τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h2</sub> rapidly increases above VPTT from 7 to 14, which reveals that the dynamics of heavy water are severely constrained inside the shrunken state of the PNIPAM microgel. The obtained results are compared to the dynamics of pure water in PNIPAM microgels aqueous suspension to better understand the effect of deuterium substitution for hydrogen on the dynamics of heavy water inside and outside of PNIPAM microgels.

Transport of ionic species affected by interactions with a pH-sensitive monolayer of microgel particles attached to electrode surface

In this work, we present the use of pH-sensitive microgel based on acrylic acid crosslinked with N,N′-bis(acryloyl)cystamine in the modification of Au electrode surface through the chemisorption process. The strong interactions between the crosslinker disulfide bridges and the gold surface were employed. The shape and morphology of the microgel particles were determined using scanning electron microscopy (SEM). The gel sensitivity to pH was examined using the dynamic light scattering technique (DLS). The impact of pH on electrode processes at the modified Au surface was studied using cyclic voltammetry (CV). It was found that at appropriate pH the microgel particles formed a loose monolayer on the electrode surface, which, in the swollen state, was able to block the transport of negatively charged redox probe to the electrode surface. On the other hand, when the microgels were in the shrunken state, the electrode surface was partially uncovered, and that allowed the redox probe to reach the electrode surface. The anchoring process of the microgels on the Au electrode surface was monitored with a quartz crystal microbalance with dissipation technique (QCM-D). The electrochemical properties of the modified with the pH-sensitive microgel layer electrodes were examined in the presence of either positively or negatively charged redox probes using CV and electrochemical impedance spectroscopy (EIS). It turned out that the electrochemical signals were strongly affected by both: degree of swelling of the microgel and protonation of the carboxylic groups of acrylic acid in the polymer network. The switching of electrode behavior was fully reversible and relatively fast.

Glucose-Responsive Boronic Acid Hydrogel Thin Films Obtained via Initiated Chemical Vapor Deposition.

Glucose-responsive materials are of great importance in the field of monitoring the physiological glucose level or smart insulin management. This study presents the first vacuum-based deposition of a glucose-responsive hydrogel thin film. The successful vacuum-based synthesis of a glucose-responsive hydrogel may open the door to a vast variety of new applications, where, for example, the hydrogel thin film is applied on new possible substrates. In addition, vacuum-deposited films are free of leachables (e.g., plasticizers and residual solvents). Therefore, they are, in principle, safe for in-body applications. A hydrogel made of but-3-enylboronic acid units, a boronic acid compound, was synthesized via initiated chemical vapor deposition. The thin film was characterized in terms of chemical composition, surface morphology, and swelling response toward pH and sucrose, a glucose–fructose compound. The film was stable in aqueous solutions, consisting of polymerized boronic acid and the initiator unit, and had an undulating texture appearance (rms 2.1 nm). The hydrogel was in its shrunken state at pH 4–7 and swelled by increasing the pH to 9. The pKa was 8.2 ± 0.2. The response to glucose was observed at pH 10 and resulted in thickness shrinking.

Tuning the Volume Phase Transition Temperature of Microgels by Light (Adv. Funct. Mater. 2/2022)

Temperature-Responsive Microgels In article number 2107946, Svetlana Santer and co-workers demonstrate that at any fixed temperature in the range between 32 °C and 80 °C, one can reversibly change the radius of the microgel up to 3 times (7 μm to 21 μm) by irradiating either with UV (shrunken state) or green light (swollen state).

Thermo- and Photoresponsive Behaviors of Dual-Stimuli-Responsive Organogels Consisting of Homopolymers of Coumarin-Containing Methacrylate.

Coumarin-containing vinyl homopolymers, such as poly(7-methacryloyloxycoumarin) (P1a) and poly(7-(2′-methacryloyloxyethoxy)coumarin) (P1b), show a lower critical solution temperature (LCST) in chloroform, which can be controlled by the [2 + 2] photochemical cycloaddition of the coumarin moiety, and they are recognized as monofunctional dual-stimuli-responsive polymers. A single functional group of monofunctional dual-stimuli-responsive polymers responds to dual stimuli and can be introduced more uniformly and densely than those of dual-functional dual-stimuli-responsive polymers. In this study, considering a wide range of applications, organogels consisting of P1a and P1b, i.e., P1a-gel and P1b-gel, respectively, were synthesized, and their thermo- and photoresponsive behaviors in chloroform were investigated in detail. P1a-gel and P1b-gel in a swollen state (transparent) exhibited phase separation (turbid) through a temperature jump and reached a shrunken state (transparent), i.e., an equilibrium state, over time. Moreover, the equilibrium degree of swelling decreased non-linearly with increasing temperature. Furthermore, different thermoresponsive sites were photopatterned on the organogel through the photodimerization of the coumarin unit. The organogels consisting of homopolymers of coumarin-containing methacrylate exhibited unique thermo- and photoresponsivities and behaved as monofunctional dual-stimuli-responsive organogels.

Nanocomposite hydrogel coatings: Formation of metal nanostructures by electrodeposition through thermoresponsive hydrogel layer

Electrode surface was modified with a layer of a nanocomposite. The nanocomposite consisted of electrodeposited nanosized silver structures and thermosensitive hydrogel. The modification procedure consisted of two steps. In the first one, the thermosensitive hydrogel, based on poly(N-isopropylacrylamide) (pNIPA), was electrodeposited. At appropriate temperatures (lower or higher than circa 32 °C) this hydrogel existed either in the swollen- or the shrunken state. Even in the shrunken state, the hydrogel deposited on the electrode surface still contained pinholes and microchannels. Due to this fact, small electroactive redox probes could diffuse to the electrode surface. In the second preparation step the silver nanoobjects were electrodeposited. It was done through the shrunken pNIPA layer. The hydrogel layer acted as a template and finally became one of the components of the composite. We found that the composite adhered satisfactorily to the GC and Pt electrode surfaces. The properties of the composite layers were examined with cyclic voltammetry and scanning electron microscopy. Finally, it was demonstrated that the GC electrodes modified in the proposed manner exhibited electrocatalytic activity. As an example the electroreduction of H2O2 was selected. It appeared that hydrogen peroxide could be successfully determined. The detection limit was found to be 48 μM. The presented approach should enable electrodeposition of other potentially useful metals nanostructures.

Criteria for colloidal gelation of thermo-sensitive poly(N-isopropylacrylamide) based microgels

HypothesisSuspensions of the poly(N-isopropylacrylamide) (PNIPAM) based temperature(T)-sensitive microgels can undergo colloidal gelation forming a three-dimensional sparse network-like structure in the hydrophobic and shrunken state of T > T* (T*: volume transition temperature), despite their considerably low particle volume fractions (<0.2). The effective surface charge density is expected to be a key factor governing the colloidal gelation and gel modulus. ExperimentsThe combined analysis of the viscoelasticity and electrophoretic mobility (EPM) was performed varying systematically pH and ionic strength (I). The microgels containing the extremely small content of electrolyte (0.1 mol%) with the T* and swelling degree being insensitive to pH and I were employed to facilitate the exclusive analysis of their effects on colloidal gelation. FindingsThe results unambiguously reveal (1) that the gelation requires the adequate suppressions of the interparticle charge repulsion, and (2) that a reduction in the interparticle charge repulsion results in an increase in gel modulus by several orders of magnitude. The long-term linear creep behavior show that the colloidal gels are identified as a viscoelastic fluid with a long relaxation time and a high viscosity whereas they behave elastically at relatively short timescale in conventional oscillatory tests.

Switching of Fluorescence Wavelength Caused by Phase Separation of Pyrene in Poly(N-isopropylacrylamide) Gel

Abstract Pyrene (Py) contained in poly(N-isopropylacrylamide) gel (NIPA gel) that is swollen by water emits excimer-like derived fluorescence. However, as dehydration proceeded with increasing temperature, the luminescence changed to the monomer-derived one. Changes of it occurred continuously and reversibly around the lower critical solution temperature of NIPA gel. Because the gel interior becomes a hydrophobic environment in the shrunken state, the Py molecules can diffuse. However, it gathers like an oil droplet in the swollen gel by water. Reversible change of fluorescence wavelength with phase separation of Py according to hydration of NIPA gel represents an unprecedented type of chromism.

P-NIPAM in water-acetone mixtures: experiments and simulations.

The lower critical solution temperature (LCST) of poly-N-isopropylacrylamide (p-NIPAM) diminishes when a small volume of acetone is added to the aqueous polymer solution, and then increases for further additions, producing a minimum at a certain acetone concentration. Here this behavior is observed through the variation of the hydrodynamic radius RH of p-NIPAM microgels with temperature, measured by dynamic light scattering (DLS), when adding increasing amounts of acetone in the molar fraction range of 0.00 to 0.25. This size trend of microgels with temperature is well captured by all-atom molecular dynamic simulations, which are implemented for a single 30-mer, at similar solvent and temperature conditions. Both DLS measurements and simulations indicate that the shrunken state continuously augments its size with increasing acetone content. This, in turn, leads to a minimum of the globule-to-coil transition temperature, which should correspond to the minimum of the LCST. Furthermore, density profiles, as obtained by considering a membrane arrangement of oligomers, reveal a preferential interaction of the polymer with acetone to the detriment of water. We observe how the membrane loses water content as the temperature is increased while keeping a similar amount of acetone in its interior. This competition between water and acetone for the polymer surface plays a major role in the enthalpy driven dependence of the critical temperature with acetone concentration.

Thickness-Dependent Swelling Behavior of Vapor-Deposited Smart Polymer Thin Films.

In this contribution, the temperature-dependent swelling behavior of vapor-deposited smart polymer thin films is shown to depend on cross-linking and deposited film thickness. Smart polymers find application in sensor and actuator setups and are mostly fabricated on delicate substrates with complex nanostructures that need to be conformally coated. As initiated chemical vapor deposition (iCVD) meets these specific requirements, the present work concentrates on temperature-dependent swelling behavior of iCVD poly(N-isopropylacrylamide) thin films. The transition between swollen and shrunken state and the corresponding lower critical solution temperature (LCST) was investigated by spectroscopic ellipsometry in water. The films’ density in the dry state evaluated from X-ray reflectivity could be successfully correlated to the position of the LCST in water and was found to vary between 1.1 and 1.3 g/cm3 in the thickness range 30–330 nm. This work emphasizes the importance of insights in both the deposition process and mechanisms during swelling of smart polymeric structures.

Micro- and nanoelectrode array behavior at regularly sized electrode modified with a thin film of thermoresponsive polymeric gel

Regularly sized platinum electrodes were modified with a layer of thermosensitive hydrogel based on poly(N-isopropylacrylamide) and containing different amounts of crosslinker. The layers were formed by using the electrochemically induced free radical polymerization. The transport of electroactive species through the layers was examined in function of crosslinker content and for two possible forms of the hydrogel. It was found that in the shrunken state, within a specific scan-range, the shape of the voltammograms was typical for a set of independent micro/nanoelectrodes, while in the swollen state of the layers the voltammograms were peak-shaped in the entire scan-rate range. This was a result of either through-film transport (swollen layer) or pinhole/pore (shrunken layer) diffusion of electroactive species. 2% content of crosslinker caused the biggest limitation to the probe transport to the electrode surface.

Austerity: Neoliberal dreams come true?

The 2008 global economic crisis paved the way for the construction of a new, elite-driven, capital-centric, shrunken welfare state project founded on ideology disguised as pragmatism and objective ‘truths’. Today, welfare states exist in a context in which a new politics of austerity sets the parameters of the debate. Austerity incorporates the neoliberal desire to shrink the (social welfare) state, deregulate labour markets and emphasise private markets as the drivers of growth, enabling a reconfiguration of the interests of capital, the needs of people and the role of the state. The new politics of austerity looks like a ‘dream come true’ for neoliberals. Or is it? There is also a powerful counter-narrative that suggests that the global crisis exposed the fundamental weaknesses and limitations of neoliberalism and forced policy makers to question core principles and change direction. Focusing on the International Monetary Fund (IMF), perhaps the pre-eminent global neoliberal interlocutor, and using quantitative textual analysis, the article locates some evidence of movement, but little to suggest that the fundamental assumptions of neoliberalism have been displaced.

Fabrication of Thermoresponsive Plasmonic Core–Satellite Nanoassemblies with a Tunable Stoichiometry via Surface‐Initiated Reversible Addition–Fragmentation Chain Transfer Polymerization from Silica Nanoparticles

This work presents a fabrication of thermoresponsive plasmonic core–satellite nanoassemblies. The structure has a silica nanoparticle core surrounded by gold nanoparticle satellites using thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) chains as scaffolds. The thiol‐terminated PNIPAM shell is densely grafted on the silica core via surface‐initiated reversible addition–fragmentation chain transfer polymerization and used to anchor numerous gold nanoparticle satellites with a tunable stoichiometry. Below and above lower critical solution temperature, the chain conformation of PNIPAM reversibly changes between swollen and shrunken state. The reversible change of the polymer size varies the refractive index of the local medium surrounding the satellites and the distance between them. The two effects together lead to the thermoresponsive plasmonic properties of the nanoassemblies. Under different satellite densities, two distinctive plasmonic features appear.

Elastic modulus and equilibrium swelling of stranded and particulate protein hydrogels at acid pH

The effect of the pH and ionic strength on the gelation of protein systems, resulting in very different microstructures, has been extensively studied in the past, yet the swelling of such gels is poorly understood. A systematic study on the swelling of two protein systems, whey protein isolate and egg white, is presented here at acidic pH relevant to foods. The swelling behavior is critically determined by the general gel micro-structure, stranded or particulate, and not for example by the nature of the proteins. Transparent stranded gels swollen at pH > 6 > pI can be modeled considering the net charge of the proteins and the ionic strength, but at lower pH up to ∼4 the gels collapse to a highly shrunken state regardless of the pH or the salt concentration. At pH < 4 < pI stranded gels swell again, but much less than expected using models developed at higher pH, suggesting higher crosslinking degrees or lower net charges. Opaque particulate gels are only mildly affected by the pH or salts concentration, due to their rigid structure that cannot accommodate large volume changes meanwhile the forming aggregates are stable. The shear modulus of gels at different swelling degrees can be explained using power laws developed for polymeric networks, but no effect is seen by the swelling pH in the apparent crosslink density, implying that the chemical interactions in the gels are not substantially modified during swelling.

A target-responsive and size-dependent hydrogel aptasensor embedded with QD fluorescent reporters for rapid detection of avian influenza virus H5N1

A target-responsive hydrogel based fluorescent aptasensor was developed for rapid detection of avian influenza virus (AIV) H5N1. A specific aptamer against H5N1 was selected and two single-stranded DNAs (ssDNA1 and ssDNA2) were designed to be partially complementary to two ends of the aptamer. Both aptamer and ssDNA1 were functionalized with acrydite at 5’-terminal for hydrogel synthesis by polymerization. Quantum dots (QDs), used as fluorescence reporters, were labeled at 5’-terminal of ssDNA2 and quenchers for QDs were conjugated at 3’-terminal of the aptamer. Hybridization between aptamer and ssDNAs formed the crosslinker to complete the QD-aptamer hydrogel. Initially, without targets, the prepared hydrogel remained in a shrunken state because of the crosslinking in the polymer network and the QD was quenched. With target AIVs, the crosslinking was dissociated due to the binding reaction between the aptamer and target, resulting in the abrupt swelling of the hydrogel and the release of aptamer-quencher and ssDNA2-QD. The response of aptamer upon target binding was demonstrated by quartz crystal microbalance (QCM) method and the microstructure of hydrogel was characterized by SEM. Spectrophotometric results indicated fluorescent changes at the emission peak were correlated to the titers of H5N1. The total detection time from sampling to results was 30min. The lowest detection limit was 0.4HAU with a detecting range of 2−1.2 to 26 HAU 20μL−1. The size-dependent property of this aptasensor was examined quantitatively and qualitatively. This simple, low-cost, specific, and label-free aptasensor has great potential for the in-field rapid detection of AIV H5N1.

Offline: The (higher) responsibilities of science and medicine

Offline: The (higher) responsibilities of science and medicine

Diffusional permeability characteristics of positively K+-responsive membranes caused by spontaneously changing membrane pore size and surface wettability

Diffusional permeability characteristics of K+-responsive membranes with grafted poly(N-isopropylacrylamide-co-acryloylamidobenzo-15-crown-5) (poly(NIPAM-co-AAB15C5)) chains in membrane pores are investigated systematically. Upon specifically recognizing K+, the grafted membrane gates dramatically switch from swollen and hydrophilic state to shrunken and hydrophobic state. The K+-responsive solute diffusion is controlled not only by the K+-triggered isothermal change of membrane pores from “closed” to “open” state but also heavily by the hydrophilic/hydrophobic switching of grafted gates. For K+-responsive diffusional permeability characteristics of the positively K+-responsive membranes, there presents a threshold value of the molecule size, which divides the solutes being promoted or restricted to diffuse across the grafted membrane upon presence of K+ in environment. With size smaller than the threshold value, the solutes are promoted to diffuse across the grafted gating membrane in the K+ solution compared with that in water; when the solute size is larger than the threshold value, the diffusion of solutes across the membrane is resisted in K+ solution. The longer and the denser the grafted chains, the easier the membrane pores being choked with the grafted copolymer chains in the shrunken state. As a result, the larger the grafting yield of poly(NIPAM-co-AAB15C5), the smaller the threshold value of the molecule size. The results in this study are valuable for rationally designing K+-responsive controlled release systems for biomedical purpose.

Local structure of temperature and pH-sensitive colloidal microgels.

The temperature dependence of the local intra-particle structure of colloidal microgel particles, composed of interpenetrated polymer networks, has been investigated by small-angle neutron scattering at different pH and concentrations, in the range (299÷315) K, where a volume phase transition from a swollen to a shrunken state takes place. Data are well described by a theoretical model that takes into account the presence of both interpenetrated polymer networks and cross-linkers. Two different behaviors are found across the volume phase transition. At neutral pH and T ≈ 307 K, a sharp change of the local structure from a water rich open inhomogeneous interpenetrated polymer network to a homogeneous porous solid-like structure after expelling water is observed. Differently, at acidic pH, the local structure changes almost continuously. These findings demonstrate that a fine control of the pH of the system allows to tune the sharpness of the volume-phase transition.