Poly (N-isopropylacrylamide) microgel-based assemblies for organic dye removal from water

Silica@poly(chitosan-N-isopropylacrylamide-methacrylic acid) microgels: Extraction of palladium (II) ions and in situ formation of palladium nanoparticles for pollutant reduction

Most of the transition metal ions and organic dyes are toxic in nature. Therefore, their removal from water is imperative for human health. For this purpose, various types of systems have been developed to tackle either transition metal ions or organic dyes individually. A core-shell microgel system is introduced which is capable of effectively removing both types (toxic organic dyes and transition metal ions) of pollutants. A long-rod-shaped silica@poly(chitosan-N-isopropylacrylamide-methacrylic acid) S@P(CS-NIPAM-MAA) S@P(CNM) core-shell microgel system was developed by free radical precipitation polymerization method (FRPPM). S@P(CNM) was utilized as an adsorbent for extracting palladium (II) (Pd (II)) ions from water under different concentrations of S@P(CNM), several agitation times, palladium (II) ion content, and pH levels. The adsorption data of Pd (II) ions on S@P(CNM) was evaluated by various adsorption isotherms. The kinetic study was investigated by employing pseudo-2nd order (Ps2O), Elovich model (ElM), intra-particle diffusion (IPDM), and pseudo-1st order (Ps1O). Additionally, palladium nanoparticles (Pd NPs) were generated via in-situ reduction of adsorbed Pd (II) ions within the P(CNM) shell region of S@P(CNM). The resulting Pd NPs loaded S@P(CNM) exhibited the capability to reduce organic pollutants like methyl orange (MeO), 4-nitrophenol (4NiP), methylene blue (MeB), and Rhodamine B (RhB) from aqueous medium. 0.766 min-1, 0.433 min-1, 0.682 min-1, and 1.140 min-1 were the values of pseudo 1st order rate constant (kobs) for catalytic reduction of MeB, 4NiP, MeO, and RhB respectively. The S@Pd-P(CNM) system exhibits significant catalytic potential for various organic transformations.

Water treatment using stimuli-responsive polymers

Stimuli-responsive polymers are a new category of smart materials used in water treatment via a stimuli-induced purification process and subsequent regeneration processes.

Influence of Purification Process on the Function of Synthetic Polymer Nanoparticles

Multifunctional synthetic polymers can bind to target molecules and are therefore widely investigated in diagnostics, drug delivery carriers, and separation carriers. Because these polymers are synthesized from nonbiological components, purification processes (e.g., chromatography, dialysis, extraction, and centrifugation) must be conducted after the synthesis. Although several purification methods are used for polymer purification, few reports have revealed the influence of purification process on the functions of polymer. In this study, we demonstrated that the characteristics, function, and stability of synthetic polymer depend on the purification process. N-Isopropylacrylamide-based polymer nanoparticles (NPs) and melittin (i.e., honey bee venom) were used as a model of synthetic polymer and target toxic peptide, respectively. Synthesized NPs were purified by dialysis in methanol, acetone precipitation, or centrifugation. NPs purified by dialysis in ultrapure water were used as control NPs. Then, NP size, surface charge, toxin neutralization effect, and stability were determined. NP size did not considerably change by purification with centrifugation; however, it decreased by purification using dialysis in methanol and acetone precipitation compared with that of control NPs. The ζ-potential of NPs changed after each purification process compared with that of control NPs. The melittin neutralization efficiency of NPs depended on the purification process; i.e., it decreased by acetone precipitation and increased by dialysis in methanol and centrifugation compared with that of control NPs. Of note, the addition of methanol and acetone decreased NP stability. These studies implied the importance of considering the effect of the purification method on synthetic polymer function.

Recyclable polymer microgel stabilized rhodium nanoparticles for reductive degradation of para-nitrophenol

The purpose of present work is to fabricate rhodium nanoparticles in Poly(N-isopropylmethacrylamide-acrylic acid) [p(NMAA)] microgel system. Synthesized polymer [p(NMAA)] microgels and rhodium nanoparticles loaded [Rh-p(NMAA)] microgels were analyzed by FTIR (Fourier Transform Infra-red) spectroscopy, XRD (X-ray Diffraction) analysis and UV/Vis (Ultraviolet–Visible) spectroscopy. Catalytic reductive conversion of P-nitrophenol (P-Nph) into P-aminophenol (P-Aph) via Rh-p(NMAA) was used to evaluate the catalytic activity of the hybrid microgel [Rh-p(NMAA)]. Kinetic study of catalytic reductive conversion of P-Nph was explored by considering various reaction parameters. It was found that the value of first order observed rate constant (k obs) was varied from 0.019 to 0.206 min−1 with change in concentration of sodium borohydride (SBH) from 3 to 14 mM at given temperature. However, further increment in concentration of SBH from 14 to 17 mM, reduced the value of k obs from 0.206 to 0.156 min−1. The similar dependence of k obs on concentration of P-Nph was observed at specific concentration of SBH and Rh-p(NMAA) at constant temperature. Kinetic study reveals that conversion of P-Nph to P-Aph takes place on the surface of rhodium nanoparticles (RhNPs) by adopting different reactions intermediates and obeys the Langmuir-Hinshelwood mechanism. Reduction efficiency of recycled Rh-p(NMAA) catalytic system was also measured and no significant reduction in the percentage catalytic activity was obtained up to four cycles for P-Nph conversion into P-Aph.

Stimuli-responsive polymer-based systems for diagnostic applications

Stimuli-responsive polymers exhibit properties that make them ideal candidates for biosensing and molecular diagnostics. Through rational design of polymer composition combined with new polymer functionalization and synthetic strategies, polymers with myriad responsivities, e.g., responses to temperature, pH, biomolecules, CO2, light, and electricity can be achieved. When these polymers are specifically designed to respond to biomarkers, stimuli-responsive devices/probes, capable of recognizing and transducing analyte signals, can be used to diagnose and treat disease. In this review, we highlight recent state-of-the-art examples of stimuli-responsive polymer-based systems for biosensing and bioimaging.

Adsorption of Anionic Dyes Using a Poly(styrene-block-4-vinylpyridine) Block Copolymer Organogel

An organogel was prepared by chemically cross-linking a poly(styrene-block-4-vinylpyridine) diblock copolymer using dibromododecane in dimethylformamide. Analysis of the prominent structure peak in small-angle X-ray scattering along with the results of light scattering and rheological profile suggests the bridging of the spherical micelles to one another to form an interconnected network after gelation. The use of this organogel as a selective adsorbent for removing anionic dyes from individual aqueous dye solutions and in a mixture of cationic and anionic dye solutions has shown more than 90% removal of the anionic dyes within 2 h. The regeneration and reusability studies showed that even after 20 cycles, the adsorption property of the organogel holds extremely well still beyond 90%. These results are indicative of the potential use of poly(styrene-block-4-vinylpyridine) organogel for the anionic ions removal in wastewater treatment.

Construction of Self-Assembled Polyelectrolyte/Cationic Microgel Multilayers and Their Interaction with Anionic Dyes Using Quartz Crystal Microbalance and Atomic Force Microscopy

This study aimed to reveal the interaction between self-assembled multilayers and dye molecules in the environment, which is closely related to the multilayers' stable performance and service life. In this work, the pH-responsive poly (N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate) microgels were prepared by free-radical copolymerization and self-assembled with sodium alginate (SA) into multilayers by the layer-by-layer deposition method. Quartz crystal microbalance (QCM) and atomic force microscopy (AFM) results confirmed the construction of multilayers and the absorbed mass, resulting in a decrease in the frequency shift of the QCM sensor and the deposition of microgel particles on its surface. The interaction between the self-assembled SA/microgel multilayers and anionic dyes in the aqueous solution was further investigated by QCM, and it was found that the electrostatic attraction between dyes and microgels deposited on the QCM sensor surface was much larger than that of the microgels with SA in multilayers, leading to the release of the microgels from the self-assembled structure and a mass loss ratio of 27.6%. AFM observation of the multilayer morphology exposed to dyes showed that 29% of the microgels was peeled off, and the corresponding microgel imprints were generated on the surface. In contrast, the shape and size of the remaining self-assembled microgel particles did not change.

Physicochemical aspects of inorganic nanoparticles stabilized in N-vinyl caprolactam based microgels for various applications

The vinyl caprolactam (VCL) based microgel system has become the center of great attention due to its versatile properties. Copolymerization of VCL with an ionic monomer imparts pH responsive properties into the microgel system in addition to thermo-sensitivity. Stimuli responsive behavior of VCL-based microgels makes them prospective and appealing candidates for practical applications covering the fields of drug delivery, catalysis and optical devices. In the last few years, VCL-based microgels have been used as microreactors and stabilizers for the synthesis and stabilization of inorganic nanoparticles to obtain hybrid microgels. The present review article provides a summary of the present-day progress of fabrication, stabilization, categorization and analysis of VCL-based microgels and their hybrids with different morphologies. The stimuli responsive properties and applications of VCL-based hybrid microgels have been reviewed critically. The remaining problems which need to be addressed have been pointed out for further advancement in this field.

Well-Defined Thermo-Responsive Copolymers Based on Oligo(Ethylene Glycol) Methacrylate and Pentafluorostyrene for the Removal of Organic Dyes from Water

Thermo-responsive copolymers based on oligo(ethylene glycol) methacrylate (OEGMA, Mn = 300 g/mol) and pentafluorostyrene (PFS), coded PFG, were synthesized by RAFT polymerization, using a trithiocarbonate (CTTPC) as controlling agent. Different molar masses were targeted and dispersities lower than 1.51 were obtained. The thermally triggered self-assembly of the resulting PFG copolymers in water was investigated by dynamic light scattering (DLS). The lower critical solution temperature (LCST) slightly increased with the molecular weight in the 26-30 °C temperature range, whereas the sizes of the intermicellar aggregates formed upon self-assembly tended to decrease with increasing molecular weights (ranging from 1415 to 572 nm). The resulting thermally-induced polymer aggregates were then used to encapsulate and remove organic contaminants from water. Nile Red (NR) and Thiazole yellow G (TYG) were employed as hydrophobic and hydrophilic model contaminants, respectively. Experimental results evidenced that higher molecular weight copolymers removed up to 90% of NR from aqueous solution, corresponding to about 10 mg of dye per g of copolymer, regardless of NR concentration. The removal of TYG was lower with respect to NR, decreasing from about 40% to around 20% with TYG concentration. Finally, the copolymers were shown to be potentially recycled and reused in the treatment of contaminated water.

Hybrid Ionic Silver and Magnetite Microgels Nanocomposites for Efficient Removal of Methylene Blue

The ionic crosslinked 2-acrylamido-2-methylpropane sulfonic acid-co-acrylic acid hydrogel, AMPS/AA and its Ag and Fe₃O₄ composites were synthesized using an in situ technique. The surface charge, particle sizes, morphology, and thermal stability of the prepared AMPS/AA-Ag and AMPS/AA-Fe₃O₄ composites were evaluated using different analytical techniques and their adsorption characteristics were evaluated to remove the methylene blue cationic dye, MB, from their aqueous solutions at optimum conditions. Also, the same monomers were used to synthesize AMPS/AA microgel and its Ag and Fe₃O₄ nanocomposites, which were synthesized using the same technique. The AMPS/AA-Fe₃O₄ nanocomposite was selected as conventional iron-supported catalyst due to the presence of both Fe(II) and Fe(III) species besides its magnetic properties that allow their easy, fast, and inexpensive separation from the aqueous solution. It was then evaluated as a heterogeneous catalyst for complete MB degradation from aqueous solution by heterogeneous Fenton oxidation. It achieved a high rate of degradation, degrading 100 mg L^-1 of MB during a short time of 35 min as compared with the reported literature.

Nanoadsorbents Based on NIPAM and Citric Acid: Removal Efficacy of Heavy Metal Ions in Different Media

Heavy metal ions in aqueous solutions are harmful to human health, but exploring and exploiting nanoadsorbents with a high adsorption capacity and low cost should be an effective method for overcoming this problem. In this study, a novel nanoadsorbent termed poly(N-isopropylacrylamide-co-citric acid) (PNCA) was designed and synthesized via free-radical polymerization. PNCA exhibits good solubility in aqueous solutions and can self-assemble into spherical nanoaggregates with a mean hydrodynamic diameter of approximately 723.1 nm. After freeze-drying, the solid powder of PNCA exhibited a loose porous structure. When PNCA is dissolved in water, the resulting copolymer solution exhibits high removal rates for Cu2+ and Pb2+ of over 80%; meanwhile, over 97% of the PNCA is precipitated with metal ions. The adsorption process of PNCA chelated with Cu2+ ions fit the Freundlich model. The adsorption capacity is independent of the media pH, but could be affected by the temperature. Except for herbal medicines with alkaloids as active ingredients, PNCA also presents good adsorption capacity for Cu2+ in herbal medicine decoctions, with a removal rate of over 80%. The cell cytotoxicity in vitro and system toxicity in vivo demonstrate the desirable biosafety of PNCA. These results suggest that PNCA with good biosafety can be utilized as a nanoadsorbent to remove the metal ions, especially Cu2+, in different media.

Transfer Free Energies and Partitioning of Small Molecules in Collapsed PNIPAM Polymers

A central quantity in the design of functional hydrogels used as nanocarrier systems, for instance, for drug delivery or adaptive nanocatalysis, is the partition ratio, which quantifies the uptake of a molecular substance by the polymer matrix. By employing all-atom molecular dynamics simulations, we study the solvation and partitioning (with respect to bulk water) of small subnanometer-sized solutes in a dense matrix of collapsed poly( N-isopropylacrylamide) (PNIPAM) polymers above the lower critical solution temperature in aqueous solution. We examine the roles of the solute's polarity and its size on the solubility properties in the thermoresponsive polymer. We show that the transfer free energies of nonpolar solutes from bulk water into the polymer are favorable and scale in a good approximation with the solute's surface area. Even for small solute size variation, partitioning can vary over orders of magnitude. A polar nature of the solute, on the other hand, generally opposes the transfer, at least for alkyl solutes. Finally, we find a strong correlation between the transfer free energies in the gel and the adsorption free energies on a single extended polymer chain, which enables us to relate the partition ratios in the swollen and collapsed state of a PNIPAM gel.

Enrichment of methanol inside pNIPAM gels in the cononsolvency-induced collapse

From Raman, we determined an enrichment of methanol inside the polymer in the cononsolvency-induced collapse and donor-type hydrogen-bonding of methanol with pNIPAM.

Flexible 3D Fe@VO2 core-shell mesh: A highly efficient and easy-recycling catalyst for the removal of organic dyes

Nowadays, it is extremely urgent to search for efficient and effective catalysts for water purification due to the severe worldwide water-contamination crises. Here, 3D Fe@VO₂ core-shell mesh, a highly efficient catalyst toward removal of organic dyes with excellent recycling ability in the dark is designed and developed for the first time. This novel core-shell structure is actually 304 stainless steel mesh coated by VO₂, fabricated by an electrophoretic deposition method. In such a core-shell structure, Fe as the core allows much easier separation from the water, endowing the catalyst with a flexible property for easy recycling, while VO₂ as the shell is highly efficient in degradation of organic dyes with the addition of H₂O₂. More intriguingly, the 3D Fe@VO₂ core-shell mesh exhibits favorable performance across a wide pH range. The 3D Fe@VO₂ core-shell mesh can decompose organic dyes both in a light-free condition and under visible irradiation. The possible catalytic oxidation mechanism of Fe@VO₂/H₂O₂ system is also proposed in this work. Considering its facile fabrication, remarkable catalytic efficiency across a wide pH range, and easy recycling characteristic, the 3D Fe@VO₂ core-shell mesh is a newly developed high-performance catalyst for addressing the universal water crises.

Poly(N-isopropylacrylamide) microgel-based etalons for the label-free quantitation of estradiol-17β in aqueous solutions and milk samples

A novel estradiol-17β (E2) biosensor was constructed from poly(N-isopropylacrylamide) (pNIPAm) microgel-based etalons by modification of their outermost Au layer with an E2 binding 75-mer DNA aptamer. When E2 is not present in the solution, the aptamer forms a loose/linear structure that allows ions to pass through and into the microgel layer. The ions can change the solvation state of the microgels, which changes the optical properties of the etalon. When E2 is present in the solution, the aptamer binds the E2 and undergoes a conformational change to a form that can block the diffusion of salt ions into the microgel layer. This blocking decreases the response of the device to salt exposure, which can be related to the concentration of E2 in solution. Using this approach, E2 sensor showed a dynamic range of 0.9-200 pg/mL with a calculated detection limit of 0.9 pg/mL (3.2 pM) E2, and the lowest measured concentration of E2 is 5.0 pg/mL. This sensor also showed low cross reactivity with progesterone, a similar steroid hormone. Moreover, this sensor could be regenerated five times without losing its sensitivity. Finally, we demonstrated that the sensor could also be used to quantify E2 in commercial skim and 2% milk, as well as farm milk directly without any pre-treatment. The successful quantitation of E2 in unprocessed milk demonstrates its potential use as a "cow-side" testing device for the dairy industry. Graphical abstract ᅟ.

Selective solute adsorption and partitioning around single PNIPAM chains

Thermoresponsive polymer architectures have become integral building blocks of 'smart' functional materials in modern applications. For a large range of developments, e.g. for drug delivery or nanocatalytic carrier systems, the selective adsorption and partitioning of molecules (ligands or reactants) inside the polymeric matrix are key processes that have to be controlled and tuned for the desired material function. In order to gain insights into the nanoscale structure and binding details in such systems, we here employ molecular dynamics simulations of the popular poly(N-isopropylacrylamide) (PNIPAM) polymer in explicit water in the presence of various representative solute types with a focus on aromatic model reactants. We study a single polymer chain and explore the influence of its elongation, stereochemistry, and temperature on the solute binding affinities. While we find that the excess adsorption generally increases with the size of the solute, the temperature-dependent affinity to the chain is highly solute specific and has a considerable dependence on the polymer elongation (i.e. polymer swelling state). We elucidate the molecular mechanisms of the selective binding in detail and eventually present how the results can be extrapolated to macroscopic partitioning of the solutes in swollen polymer architectures, such as hydrogels.

Single-Step Binary Electrostatic Directed Assembly of Active Nanogels for Smart Concentration-Dependent Encryption

Anionic and cationic (N-isopropylacrylamide derivatives) active colloidal hydrogel nanoparticles, i.e., nanogels, are electrostatically assembled on surfaces to form microscale patterns with complex geometries. While using mixed dispersions of these two kinds of nanogels, we demonstrate the capability of sorting the nanogels in one step to form binary nanogel patterns on a surface. These patterns appear independently or simultaneously depending on the relative proportion of each nanogel type in the mixture. Hence, the resulting nanogel patterns provide quantitative information regarding the dispersion composition and can be used to achieve smart concentration-dependent nanogel encryption. Moreover, atomic force microscopy characterization measurements performed in liquid prove that the assembled nanogels maintain their swelling/deswelling properties once attached to the surface. Consequently, this method paves the way for applying such active nanogel patterns to produce smart coatings and sensors.

Microgels as efficient adsorbents for the removal of pollutants from aqueous medium

Due to their responsive behavior, high stability, and reusability, microgels have gained importance as adsorbents for the removal of aqueous pollutants such as heavy metals, nitroarenes, organic matter, and toxic dyes. However, there are few challenges that need to be addressed to make microgels as potential adsorbents for the removal of aqueous pollutants. This review article encircles the recent developments in the field of microgel usage as adsorbents for the extraction of aqueous pollutants. Many factors that influence the adsorption of pollutants such as pH, temperature of the medium, agitation time, pollutant concentration, microgel dose, and feed contents of microgels have been discussed in detail. Different adsorption isotherms as well as the kinetic and thermodynamic aspects of the adsorption process have also been enlightened to interpret the insight of the adsorption process. Microgel recovery from the reaction mixture as well as reusability is discussed from the financial point of view. The biodegradability of microgels induced due to the incorporation of specific biomacromolecules is also discussed.

Synthesis of poly (N-isopropylacrylamide)-co-(acrylic acid) microgel-entrapped CdS quantum dots and their photocatalytic degradation of an organic dye

CdS quantum dots (CdSQDs) were generated inside the network structure of poly (N-isopropylacrylamide)-co-(acrylic acid) (pNIPAm-co-AAc) microgels and their ability to photocatalytically degrade organic dyes was evaluated using rhodamine B (RhB).

Terpyridine-functionalized stimuli-responsive microgels and their assembly through metal–ligand interactions

We developed a terpyridine-functionalized microgel (tpy-mG) for its supramolecular assembly.

Supramolecular Microgels Fabricated from Supramonomers

This letter describes a new method for fabricating supramolecular microgels from supramonomers. To this end, we designed and assembled supramonomers with one acrylate moiety on each end on the basis of noncovalent host-guest interactions, which could be utilized as a cross-linker. Then supramolecular microgels were fabricated through the copolymerization of supramonomers and N-isopropylacrylamide (NIPAm). The supramolecular microgels not only showed temperature-responsive properties as expected from conventional PNIPAm-based microgels but also exhibited stimuli-responsive and degradable properties benefiting from the dynamic nature of supramonomers. In addition, it was found that the degradation kinetics of the supramolecular microgels was related greatly to the structure of the microgels, providing a way to tune the degradation kinetics of the supramolecular microgels. Various supramolecular microgels with desired structure and function are supposed to be facilely fabricated from supramonomers. It is anticipated that the supramolecular microgels can enrich the application of microgels by easily endowing the microgels with stimuli-responsive and degradable properties.

Microfluidically fabricated pH-responsive anionic amphiphilic microgels for drug release

Novel amphiphilic microgels with hydrophobic and hydrophilic monomer units on the polymer chains were fabricated with an on-chip polymerisation methodology using a novel chip design.

Synthesis and multi-responsiveness of poly(N-vinylcaprolactam-co-acrylic acid) core–shell microgels via miniemulsion polymerization

We report herein a facile and large fabrication of multi-responsive poly(N-vinylcaprolactam-co-acrylic acid) microgels with a core–shell structure via seed miniemulsion polymerization. The multi-responsive microgels can reversibly swell and shrink in response to pH and temperature variation.

Influence of hydrophilic dyes on the phase transition of a thermoresponsive hyperbranched polymer

The influences of hydrophilic dyes on the phase transition behaviors of HPEI-IBAm in the neutral and charged state were different.

Stimuli-responsive microgel-based etalons for optical sensing

Responsive polymers have found numerous applications over the years. This review highlights their use as components of photonic materials, with emphasis on responsive polymer-based etalons. The use of these materials for sensing and biosensing is detailed.

Tailoring pH-responsive acrylic acid microgels with hydrophobic crosslinks for drug release

Amphiphilic microgels that were able to encapsulate and release both hydrophobic and hydrophilic moieties were fabricated via a new methodology using a lab-on-a-chip device.

Thickness and substrate dependences of phase transition, drug release and antibacterial properties of PNIPAm-co-AAc films

Normalized absorption coefficient at 590 nm of solutions of extracted CV from 50 nm and 500 nm thick PNIPAm-co-AAc films deposited on Au substrates. Releases were measured for temperatures below (25 °C) and above (40 °C) the LCST.

Overview of the effect of monomers and green solvents on thermoresponsive copolymers: phase transition temperature and surface properties

The thermoresponsive and surface properties of PNIPAM based copolymers were investigated in response to green solvents, ionic liquids.

Relationship between temperature-induced changes in internal microscopic structures of poly(N-isopropylacrylamide) microgels and organic dye uptake behavior

Temperature-induced changes in the internal structures of poly(N-isopropylacrylamide) (pNIPAm) microgels were evaluated by small-angle X-ray scattering (SAXS), and the results were used to explain organic dye uptake by the microgels. The dye uptake experiments were conducted using two organic dyes: cationic rhodamine 6G (R6G) and anionic erythrosine. In the SAXS investigation, the internal structures of the microgels were characterized in terms of the correlation length, ξ, and the distance, d*, which originated from the local packing of the isopropyl groups of two neighboring chains. With increasing temperature up to the volume phase transition temperature (VPTT) of the microgels, the correlation length, ξ, was increased and the distance, d*, was decreased. At the same time, the amounts of the dyes taken up by the pNIPAm microgels were increased, despite a decrease in the volume of the microgels. The results indicated that the pNIPAm chains were closer to each other due to the hydrophobic association of isopropyl groups, which resulted in the growth of the hydrophobic domains. Thus, the hydrophobic interactions between the dyes and pNIPAm were probably accompanied by the domain formation. With a further increase of temperature above the VPTT, the correlation length, ξ, was decreased and then not defined because the Ornstein-Zernike type contribution disappeared, and the distance, d*, was not largely changed. At the same time, the uptake amounts of the dyes per unit volume of the microgels were also not largely changed, which behaved similar to the distance, d*. It was probably due to the fact that the internal structures of the microgels were not largely changed because the isopropyl groups were in contact with each other. The view was supported by the result of the uptake study of the nonthermoresponsive microgels which did not have the hydrophobic isopropyl groups.

Poly (N-isopropylacrylamide) microgel-based optical devices for sensing and biosensing

Responsive polymer-based materials have found numerous applications due to their ease of synthesis and the variety of stimuli that they can be made responsive to. In this review, we highlight the group's efforts utilizing thermoresponsive poly (N-isopropylacrylamide) (pNIPAm) microgel-based optical devices for various sensing and biosensing applications.

Thermoresponsive magnetic hydrogels as theranostic nanoconstructs

We report the development of thermoresponsive magnetic hydrogels based on poly(N-isopropylacrylamide) encapsulation of Fe3O4 magnetic nanostructures (MNS). In particular, we examined the effects of hydrogels encapsulated with poly-ethylene glycol (PEG) and polyhedral oligomeric silsesquioxane (POSS) surface modified Fe3O4 MNS on magnetic resonance (MR) T2 (transverse spin relaxation) contrast enhancement and associated delivery efficacy of absorbed therapeutic cargo. The microstructural characterization reveal the regular spherical shape and size (∼200 nm) of the hydrogels with elevated hydrophilic to hydrophobic transition temperature (∼40 °C) characterized by LCST (lower critical solution temperature) due to the presence of encapsulated MNS. The hydrogel-MNS (HGMNS) system encapsulated with PEG functionalized Fe3O4 of 12 nm size (HGMNS-PEG-12) exhibited relaxivity rate (r2) of 173 mM(-1) s(-1) compared to 129 mM(-1) s(-1) obtained for hydrogel-MNS system encapsulated with POSS functionalized Fe3O4 (HGMNS-POSS-12) of the same size. Further studies with HGMNS-PEG-12 with absorbed drug doxorubicin (DOX) reveals approximately two-fold enhance in release during 1 h RF (radio-frequency) field exposure followed by 24 h incubation at 37 °C. Quantitatively, it is 2.1 μg mg(-1) (DOX/HGMNS) DOX release with RF exposure while only 0.9 μg mg(-1) release without RF exposure for the same period of incubation. Such enhanced release of therapeutic cargo is attributed to micro-environmental heating in the surroundings of MNS as well as magneto-mechanical vibrations under high frequency RF inside hydrogels. Similarly, RF-induced in vitro localized drug delivery studies with HeLa cell lines for HGMNS-PEG-12 resulted in more than 80% cell death with RF field exposures for 1 h. We therefore believe that magnetic hydrogel system has in vivo theranostic potential given high MR contrast enhancement from encapsulated MNS and RF-induced localized therapeutic delivery in one nanoconstruct.

Functional elastic hydrogel as recyclable membrane for the adsorption and degradation of methylene blue

Developing the application of high-strength hydrogels has gained much attention in the fields of medical, pharmacy, and pollutant removal due to their versatility and stimulus-responsive properties. In this presentation, a high-strength freestanding elastic hydrogel membrane was constructed by clay nanosheets, N, N-dimethylacrylamide and 2-acrylamide-2-methylpropanesulfonic acid for adsorption of methylene blue and heavy metal ions. The maximum values of elongation and Young’s modulus for 0.5% AMPSNa hydrogel were 1901% and 949.4 kPa, respectively, much higher than those of traditional hydrogels. The adsorptions were confirmed to follow pseudo-second kinetic equation and Langmuir isotherm model fits the data well. The maximum adsorption capacity of hydrogel towards methylene blue was 434.8 mg g⁻¹. The hydrogel also exhibited higher separation selectivity to Pb²⁺ than Cu²⁺. The methylene blue adsorbed onto the hydrogel membrane can be photocatalytically degraded by Fenton agent and the hydrogel membrane could be recycled at least five times without obvious loss in mechanical properties. In conclusion, this presentation demonstrates a convenient strategy to prepare tough and elastic clay nanocomposite hydrogel, which can not only be applied as recyclable membrane for the photocatalytic degradation of organic dye, but also for the recovery of valuables.

Responsive polymers for biosensing and protein delivery

Responsive polymers have found their way into numerous sensing and drug delivery platforms; some examples of biosensing and protein delivery are highlighted here.

The influence of deposition solution pH and ionic strength on the quality of poly(N-isopropylacrylamide) microgel-based thin films and etalons

Poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based thin films and etalons were fabricated via "painting" a pNIPAm-co-AAc microgel monolayer on a Au-coated substrate, followed by the deposition of another Au overlayer. Herein, in situ observation of how the pH and ionic strength (I.S.) of the painting solution influenced microgel deposition and, ultimately, the optical homogeneity and pH sensitivity of the etalon was carried out. It was shown that microgels closely pack on the Au substrate when they are deposited at pH 3.0, leading to a good optical homogeneity. Additionally, increasing the painting solution I.S. leads to a slight decrease in microgel packing density on the substrate, but enhances the ability of the microgel layer to swell, exhibiting thicker polymer layers when immersed in pH 3.0 solutions. When painting at pH 7.5, the optical homogeneity of the etalon is improved at the expense of swellability, exaggerated high I.S. We also determined the device's sensitivity to pH changes and found a maximum sensitivity when the microgels were deposited at pH 7.5 with an I.S. of 10 mM.