Polymerized stimuli-responsive microgel hybrids of silver nanoparticles as efficient reusable catalyst for reduction reaction

We have showcased the potential of polymerized hydrogels (PGMs) with uniform-sized stimuli-responsive microgel particles as promising alternatives to prevent aggregation in solution based nanoparticle systems. In the current work, we implemented the PGM concept by embedding anionic stimuli-responsive microgels (PNIPAM-co-AAc)-silver (Ag) hybrids within a hydrogel matrix. These PGM@AgNP hybrid materials are used as catalysts for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride. UV-VIS spectroscopy is used for studying catalytic activity. In the solution based system, the complete reduction of 4-NP to 4-AP took 30 minutes with pure Ag nanoparticles, 24 minutes with PNIPAM-Ag hybrid (Neutral) microgels and 15 minutes with PNIPAM-co-AAc-Ag (Anionic) hybrid microgels. In contrast PGM containing PNIPAM-co-AAc-Ag hybrids achieved full reduction in just 15 minutes, along with a 3-minute induction period. For pure Ag nanoparticles, the first-order rate constant is found to be 0.25 min-1, for PNIPAM-Ag hybrid (Neutral), it is 0.21 min-1 and for PNIPAM-co-AAc-Ag (Anionic), it is 0.5 min-1 where as for PGM containing anionic microgel hybrids it is found to be 0.8 min-1. Furthermore, the reusability of the PGM-Ag (anionic) materials for catalytic activity remains unaltered even after several washings. In summary, our study highlights the effectiveness of PGM@AgNP materials as efficient catalysts for the reduction of 4-nitrophenol to 4-aminophenol, indicating their versatile potential in various catalytic applications.

Catalytic innovations: Improving wastewater treatment and hydrogen generation technologies

The effective reduction of hazardous organic pollutants in wastewater is a pressing global concern, necessitating the development of advanced treatment technologies. Pollutants such as nitrophenols and dyes, which pose significant risks to both human and aquatic health, making their reduction particularly crucial. Despite the existence of various methods to eliminate these pollutants, they are not without limitations. The utilization of nanomaterials as catalysts for chemical reduction exhibits a promising alternative owing to their distinguished catalytic activity and substantial surface area. For catalytically reducing the pollutants NaBH4 has been utilized as a useful source for it because it reduces the pollutants quiet efficiently and it also releases hydrogen gas as well which can be used as a source of energy. This paper provides a comprehensive review of recent research on different types of nanomaterials that function as catalysts to reduce organic pollutants and also generating hydrogen from NaBH4 methanolysis while also evaluating the positive and negative aspects of nanocatalyst. Additionally, this paper examines the features effecting the process and the mechanism of catalysis. The comparison of different catalysts is based on size of catalyst, reaction time, rate of reaction, hydrogen generation rate, activation energy, and durability. The information obtained from this paper can be used to steer the development of new catalysts for reducing organic pollutants and generation hydrogen by NaBH4 methanolysis.

Modulation of Transaminase Activity by Encapsulation in Temperature-Sensitive Poly(N-acryloyl glycinamide) Hydrogels

Smart hydrogels hold much potential for biocatalysis, not only for the immobilization of enzymes, but also for the control of enzyme activity. We investigated upper critical solution temperature‐type poly N‐acryloyl glycinamide (pNAGA) hydrogels as a smart matrix for the amine transaminase from Bacillus megaterium (BmTA). Physical entrapment of BmTA in pNAGA hydrogels results in high immobilization efficiency (>89 %) and high activity (97 %). The temperature‐sensitiveness of pNAGA is preserved upon immobilization of BmTA and shows a gradual deswelling upon temperature reduction. While enzyme activity is mainly controlled by temperature, deactivation tended to be higher for immobilized BmTA (≈62–68 %) than for free BmTA (≈44 %), suggesting a deactivating effect due to deswelling of the pNAGA gel. Although the deactivation in response to hydrogel deswelling is not yet suitable for controlling enzyme activity sufficiently, it is nevertheless a good starting point for further optimization.

Phase Transition Behavior and Catalytic Activity of Poly(N-acryloylglycinamide-co-methacrylic acid) Microgels

Poly(N-acryloyl glycinamide) is a well-known thermoresponsive polymer possessing an upper critical solution temperature (UCST) in water. By copolymerizing N-acryloyl glycinamide (NAGA) with methacrylic acid (MAA) in the presence of a crosslinker, poly(N-acryloyl glycinamide-co-methacrylic acid) [P(NAGA-MAA)] copolymer microgels with an MAA molar fraction of 10-70 mol % were obtained. The polymerization kinetics suggests that the copolymer microgels have a random structure. The size of the microgels was between 60 and 120 nm in the non-aggregated swollen state in aqueous medium and depending on the solvent conditions, they show reversible swelling and shrinking upon temperature change. Their phase transition behavior was studied by a combination of methods to understand the process of the UCST-type behavior and interactions between NAGA and MAA. P(NAGA-MAA) microgels were loaded with silver nanoparticles (AgNPs) by the reduction of AgNO₃ under UV light. Compared with the chemical reduction of AgNO₃, the photoreduction results in smaller AgNPs and the amount and size of the AgNPs are dependent on the comonomer ratio. The catalytic activity of the AgNP-loaded microgels in 4-nitrophenol reduction was tested.

Impact of Thermal History on the Kinetic Response of Thermoresponsive Poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol)methyl ether methacrylate) Thin Films Investigated by In Situ Neutron Reflectivity

The impact of thermal history on the kinetic response of thin thermoresponsive diblock copolymer poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate), abbreviated as PMEO₂MA-b-POEGMA₃₀₀, films is investigated by in situ neutron reflectivity. The PMEO₂MA and POEGMA₃₀₀ blocks are both thermoresponsive polymers with a lower critical solution temperature. Their transition temperatures (TTs) are around 25 °C (TT₁, PMEO₂MA) and 60 °C (TT₂, POEGMA₃₀₀). Thus, by applying different temperature protocols (20 to 60 or 20 to 40 to 60 °C), the PMEO₂MA-b-POEGMA₃₀₀ thin films experience different thermal histories: the first protocol directly switches from a swollen to a collapsed state, whereas the second one switches first from a swollen to a semicollapsed and finally to a collapsed state. Although the applied thermal histories differ, the response and final state of the collapsed films are very close to each other. After the thermal stimulus, both films present a complicated response composed of an initial shrinkage, followed by a rearrangement. Interestingly, a subsequent reswelling of the collapsed film is only observed in the case of having applied a thermal stimulus of 20 to 40 °C. The normalized film thickness and the D₂O amount of each layer in the PMEO₂MA-b-POEGMA₃₀₀ films are consistent at the end of the two different thermal stimuli. Hence, it can be concluded that the thermal history does not influence the final state of the PMEO₂MA-b-POEGMA₃₀₀ films upon heating. Based on this property, these thin films are especially suitable for the temperature switches on the nanoscale, which may experience different thermal histories.

Smart microgel-metal hybrid particles of PNIPAM-co-PAA@AgAu: synthesis, characterizations and modulated catalytic activity

Smart pH and thermoresponsive, poly(N-isopropyl acrylamide co acrylic acid) (PNIPAM-co-PAA) microgel particles are used as microreactors to prepare hybrids of gold (Au) and silver (Ag) nanoparticles (PNIPAM-co-PAA@AgAu) using a facile two steps in situ approach. These hybrid particles are characterized using the transmission electron microscope (TEM), UV-VIS spectrometer, and dynamic light scattering (DLS). TEM directly confirms the successful loading of metal nanoparticles onto microgels and the hybrid particles have a narrow size distribution. UV-VIS spectroscopy at different concentration ratios of silver/gold chloride strongly reveals the presence of plasmon peaks of both silver and gold between 10% to 25% of gold chloride concentration. DLS studies demonstrate that these hybrid microgels exhibit both pH and thermoresponsive properties comparatively with a lesser swelling than the pure microgels without loaded nanoparticles. Further, the catalytic activities of PNIPAM-co-PAA@AgAu hybrids are studied through a reduction of 4-nitrophenol (4-NP)-to-4-aminophenol (4-AP) in the presence of sodium borohydride at different pH. Interestingly, these hybrid particles exhibit modulating catalytic activity with variation in pH. The reduction kinetics decreases with increasing pH and the corresponding apparent rate constant exhibits two linear regimes with one at pH below pK_a and another at pH above pK_a of acrylic acid. This pH-modulated catalytic behavior of PNIPAM-co-PAA@AgAu hybrids is discussed based on pH-induced swelling/deswelling transition, the core-shell nature of microgel particles, and its intrinsic interplay with the diffusion of nitrophenols within the microgel network. Finally, our results are compared and discussed in the context of previously studied catalytic activities in different polymer-metal hybrids.

Glucose and Maltose Surface-Functionalized Thermoresponsive Poly(N-Vinylcaprolactam) Nanogels

Soft nanoparticles are interesting materials due to their size, deformability, and ability to host guest molecules. Surface properties play an essential role in determining the fate of the particles in biological medium, and coating of the nanoparticles (and polymers) with carbohydrates has been found to be an efficient strategy for increasing their biocompatibility and fine-tuning other important properties such as aqueous solubility. In this work, soft nanogels of poly(N-vinylcaprolactam), PNVCL, were surface-functionalized with different glucose and maltose ligands, and the colloidal properties of the gels were analyzed. The PNVCL nanogels were first prepared via semibatch precipitation polymerization, where a comonomer, propargyl acrylate (PA), was added after preparticle formation. The aim was to synthesize "clickable" nanogels with alkyne groups on their surfaces. The nanogels were then functionalized with two separate azido-glucosides and azido-maltosides (containing different linkers) through a copper-catalyzed azide-alkyne cycloaddition (CuAAc) click reaction. The glucose and maltose bearing nanogels were thermoresponsive and shrank upon heating. Compared to the PNVCL-PA nanogel, the carbohydrate bearing ones were larger, more hydrophilic, had volume phase transitions at higher temperatures, and were more stable against salt-induced precipitation. In addition to investigating the colloidal properties of the nanogels, the carbohydrate recognition was addressed by studying the interactions with a model lectin, concanavalin A (Con A). The binding efficiency was not affected by the temperature, which indicates that the carbohydrate moieties are located on the gel surfaces, and are capable of interacting with other biomolecules independent of temperature. Thus, the synthesis produces nanogels, which have surface functions capable of biorelevant interactions and a thermoresponsive structure. These types of particles can be used for drug delivery.

Resolving the internal morphology of core-shell microgels with super-resolution fluorescence microscopy

We investigate the internal morphology of smart core-shell microgels by super-resolution fluorescence microscopy exploiting a combination of 3D single molecule localization and structured illumination microscopy utilizing freely diffusing fluorescent dyes. This approach does not require any direct chemical labeling and does not perturb the network structure of these colloidal gels. Hence, it allows us to study the morphology of the particles with very high precision. We found that the structure of the core-forming seed particles is drastically changed by the second synthesis step necessary for making the shell, resulting in a core region with highly increased dye localization density. The present work shows that super-resolution microscopy has great potential with respect to the study of soft colloidal systems.

Hybrid Microgels for Catalytic and Photocatalytic Removal of Nitroarenes and Organic Dyes From Aqueous Medium: A Review

Polymer microgels loaded with inorganic nanoparticles have gained much attention as catalytic systems for reduction of toxic chemicals. Enhanced catalytic properties of hybrid microgels are related to the stimuli responsive nature of microgels and extraordinary stability of nanoparticles within network of polymer microgels. Catalytic properties of hybrid microgels can be tuned very easily by slight variation in environmental conditions. Herein we have reviewed catalytic reduction of toxic chemicals such as nitroarenes and organic dyes in the presence of appropriate hybrid microgel catalytic systems under different operating conditions of reaction. Recent advancements in catalytic behavior of hybrid microgels with special emphasis on their ability to catalytically degrade various toxic chemicals has been presented in this review.

Reduction of nitroarenes catalyzed by microgel-stabilized silver nanoparticles

Poly(N-isopropylacrylamide-co-acrylamide) (PNA-BIS-2) microgels were synthesized by free radical precipitation polymerization in aqueous medium. Spherical Ag nanoparticles with diameter of 10-20 nm were fabricated inside the PNA-BIS-2 microgels by in-situ reduction of silver nitrate using sodium borohydride as reducing agent. The Ag nanoparticles- loaded hybrid microgels were characterized by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Energy dispersive X-ray (EDX), Scanning transmission electron microscopy (STEM), Ultraviolet visible spectroscopy (UV Visible), Thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Ag contents in the hybrid system were determined by inductively coupled plasma - optical emission spectrometry (ICP-OES). Various nitroarenes were successfully converted into their respective aromatic amines with good to excellent yields (ranging from 75% to 97%) under mild reaction conditions. The catalyst has ability to successfully convert substituted nitroarenes into desired products keeping many functionalities intact. The catalyst can be stored for long time without any sign of aggregation and can be used multiple times without any significant loss in its catalytic activity.

Hydration and Dehydration Kinetics: Comparison between Poly( N-isopropyl methacrylamide) and Poly(methoxy diethylene glycol acrylate) Films

Thermoresponsive films of poly( N-isopropyl methacrylamide) (PNIPMAM) and poly(methoxy diethylene glycol acrylate) (PMDEGA) are compared with respect to their hydration and dehydration kinetics using in situ neutron reflectivity. Both as-prepared films present a homogeneous single-layer structure and have similar transition temperatures of the lower critical solution temperature type (TT, PNIPMAM 38 °C and PMDEGA 41 °C). After hydration in unsaturated D₂O vapor at 23 °C, a D₂O enrichment layer is observed in PNIPMAM films adjacent to the Si substrate. In contrast, two enrichment layers are present in PMDEGA films (close to the vapor interface and the Si substrate). PNIPMAM films exhibit a higher hydration capability, ascribed to having both donor (N-H) and acceptor (C═O) units for hydrogen bonds. While the swelling of the PMDEGA films is mainly caused by the increase of the enrichment layers, the thickness of the entire PNIPMAM films increases with time. The observed longer relaxation time for swelling of PNIPMAM films is attributed to the much higher glass transition temperature of PNIPMAM. When dehydrating both films by increasing the temperature above the TT, they react with a complex response consisting of three stages (shrinkage, rearrangement, and reswelling). PNIPMAM films respond faster than PMDEGA films. After dehydration, both films still contain a large amount of D₂O, and no completely dry film state is reached for a temperature above their TTs.

Triply responsive coumarin-based microgels with remarkably large photo-switchable swelling

Using two different wavelengths of UV light enables remarkably strong photo-switchable swelling of pH- and temperature-responsive microgels and photo-release of doxorubicin.

Design of smart polyacrylates showing thermo-, pH-, and CO2-responsive features

A novel acrylate monomer was synthesized by facile sequential two-step reactions and polymerized using RAFT polymerization to yield a well-defined thermo-, pH- and CO₂-responsive homopolymer.

Poly(N-acryloyl glycinamide): a fascinating polymer that exhibits a range of properties from UCST to high-strength hydrogels

This feature article introduces the diverse intriguing properties of poly(N-acryloyl glycinamide) aqueous systems spanning from low to high concentrations.