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.

Molecular docking in organic, inorganic, and hybrid systems: a tutorial review.

Molecular docking simulation is a very popular and well-established computational approach and has been extensively used to understand molecular interactions between a natural organic molecule (ideally taken as a receptor) such as an enzyme, protein, DNA, RNA and a natural or synthetic organic/inorganic molecule (considered as a ligand). But the implementation of docking ideas to synthetic organic, inorganic, or hybrid systems is very limited with respect to their use as a receptor despite their huge popularity in different experimental systems. In this context, molecular docking can be an efficient computational tool for understanding the role of intermolecular interactions in hybrid systems that can help in designing materials on mesoscale for different applications. The current review focuses on the implementation of the docking method in organic, inorganic, and hybrid systems along with examples from different case studies. We describe different resources, including databases and tools required in the docking study and applications. The concept of docking techniques, types of docking models, and the role of different intermolecular interactions involved in the docking process to understand the binding mechanisms are explained. Finally, the challenges and limitations of dockings are also discussed in this review.

2H NMR Study of Polymer Segmental Dynamics at Varying Cross-Linking in Poly(N-isopropylacrylamide) Microgels.

A microscopic understanding of the internal structure and dynamics of poly(N-isopropylacrylamide) (PNIPAM) chains, in microgel colloids, is developed using deuterium NMR (2H NMR) to study deuterated PNIPAM suspensions as functions of temperature and pressure for four cross-linker molar fractions (f). The PNIPAM polymers were labeled with deuterons at the backbone (d3-PNIPAM) or on side chains (d7-PNIPAM). 2H NMR spectra of the d3-PNIPAM suspensions for all cross-linker molar fractions indicated freely moving chains at low temperature and a nearly immobilized fraction above ∼35 °C. Polymer segments in the collapsed phase of the d3-PNIPAM suspension were more mobile than those in the dry powder. This is direct microscopic evidence that the polymer remains significantly hydrated in the collapsed phase, consistent with strong, indirect evidence from recent light scattering and rheology measurements from our laboratory. However, the observation of a small fraction of immobilized segments in the swollen phase for higher cross-linker molar fraction suggests that, particularly for high levels of cross-linking, some polymer is nonhydrated even in the swollen phase. Finally, variable-pressure NMR (up to 90 MPa) showed a slight increase in transition temperature with pressure for lower cross-linker molar fractions and a larger increase in transition temperature with pressure for higher cross-linker molar fractions. This is consistent with a previously reported dependence of collapse transition enthalpy on cross-linker molar fraction.

On the role of softness in ionic microgel interactions.

Thermoresponsive microgels are a popular model system to study phase transitions in soft matter, because temperature directly controls their volume fraction. Ionic microgels are additionally pH-responsive and possess a rich phase diagram. Although effective interaction potentials between microgel particles have been proposed, these have never been fully tested, leading to a gap in our understanding of the link between single-particle and collective properties. To help resolve this gap, four sets of ionic microgels with varying crosslinker density were synthesised and characterised using light scattering techniques and confocal microscopy. The resultant structural and dynamical information was used to investigate how particle softness affects the phase behaviour of ionic microgels and to validate the proposed interaction potential. We find that the architecture of the microgel plays a marked role in its phase behaviour. Rather than the ionic charges, it is the dangling ends which drive phase transitions and interactions at low concentration. Comparison to theory underlines the need for a refined theoretical model which takes into consideration these close-contact interactions.

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 pKa and another at pH above pKa 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.

AC Electric Field Mediated Assembly of Bacterial Tetrads.

Understanding spatiotemporal organization in bacteria under an external AC electric field is not only very interesting from a perspective of studying assembly and disassembly in a model biofilm but also provides insight into the intricate role of anisotropic interaction with bacterial dynamics that can generate interesting complex structures. In the current study, using confocal microscopy, we demonstrate such complex assemblies of monodisperse tetrad clusters of Micrococcus luteus, an environmental bacterium synthesized under a controlled growth condition. These clusters under the AC field produce a range of interesting structures such as chains, double helix, and bundles, which are instantaneously reversible when the field is switched off. Our studies can provide important insights into the natural organization of the clustered bacterium (with relevance in biofilm-like states) and generate strategies for biomaterial fabrication with a switchable functionality.

Stimuli-responsive photoluminescence soft hybrid microgel particles: synthesis and characterizations.

Stimuli-responsive (pH and temperature sensitive) photoluminescence hybrid particles are prepared using oppositely charged cationic microgels of Poly(N-isopropyl polyacrylamide) (PNIPAm) and anionic cadmium sulfide (CdS) quantum dots (QDs). A facile synthetic strategy such as in situ/post incorporation of QDs along with pH tuneable electrostatic interactions is optimized to obtain hybrid microgels with maximum photoluminescence. Transmission electron microscope (TEM), fluorescence spectroscopy and dynamic light scattering (DLS) methods are used for characterizing the synthesized hybrid particles. TEM studies directly confirm the successful loading of QDs onto microgels whereas fluorescence spectroscopy reveals higher photo luminosity of the hybrid microgels prepared via in situ compared to post incorporation method. The pH-dependent photoluminescence supported by temperature-dependent swelling studies using DLS suggest that the hybrid microgels prepared at low pH have greater fluorescence with less thermoresponsivity and these behaviors follow an opposite trend with increasing pH. Further, these results are compared with the hybrid microgels prepared using similar charged anionic microgels and anionic quantum dots under same experimental condition (via in situ) and suggest that maximum photoluminescence can be achieved only through oppositely charged species.

Soft particles in an electric field - a zero average contrast study.

We report on the structural properties of ionic microgel particles subjected to alternating electric fields, using small-angle neutron scattering. The experiments were performed under so-called zero average contrast conditions, which cancel the structure factor contribution to the scattered intensity, allowing us to obtain direct information on the single particle size and structure as particles align in field-induced strings. Our results reveal only a marginal compression of the particles as they align in strings, and indicate considerable particle overlap at higher field strengths. These findings provide further insight into the origins of the previously reported unusual path dependent field-induced crystal-crystal transition found for these systems (P. S. Mohanty et al., Phys. Rev. X, 2015, 5, 011030).

Structural parameters of soft PNIPAM microgel particles as a function of crosslink density.

HYPOTHESIS: The temperature dependences of hydrodynamic radii in thermo-sensitive microgel suspensions, known as collapse curves, are commonly fitted to the benchmark Flory-Rehner theory but parameters obtained often yield little physical insights. Our study of poly(N-isopropylacrylamide) (PNIPAM) microgel suspensions in water is driven by the hypothesis that fitting to Flory-Rehner theory can yield meaningful parameters that separate into ones that are insensitive to crosslink density or deuteration and ones that are not. EXPERIMENTS: Dynamic light scattering (DLS) and rheology experiments were done on 8 microgel variants, protonated and deuterated PNIPAM for four crosslink densities, synthesized under otherwise identical conditions. FINDINGS: Remarkably, polymer volume fractions in the microgel particle at collapse, ϕcollapse, obtained via rheology, are independent of crosslink density. Along with collapse curves from DLS, this determines the temperature dependence of microgel water and polymer volume fractions. Fitting collapse curves to Flory-Rehner theory yields reference polymer volume fractions, ϕ0, associated with microgel particle elasticity. ϕ0 is much lower than ϕcollapse, and increases with crosslink density. For all microgel sample variants, a crossover temperature, where the elastic contribution to osmotic pressure changes sign, is found to approximate the final temperature after microgel synthesis and also to the free polymer θ temperature.

Alternating electric-field-induced assembly of binary mixtures of soft repulsive ionic microgel colloids.

An external alternating electric field is used to study the assembly of a binary mixture of Poly(N-isopropylacrylamide-co-acrylic acid) microgels in their swollen form at hydrodynamic size ratio 2:1 under deprotonated state. The AC field experiments were carried out at a fixed frequency of 100 kHz in the fluid regime for three number density ratios 1:3, 1:1 and 3:1 of big-to-small microgels using a confocal microscope. Strings with different types of co-assembly structures such as buckled, ring, flame and sandwich have been observed at low and intermediate field strengths at ratio 1:3, 1:1. In buckled and ring type, one or two small particles sit at the contact of two big particles and in the flame type, small particles arrange like a cone at end of the string. In the sandwich structure, several double small particle layers lie in between big particles. At high field strength, aggregation of strings and a phase separation into individual aggregates of strings from both big and small microgels have been observed. At higher ratio 3:1, the string formation is mostly dominated by big particles. Our experimental results are discussed with the recent simulation and experimental works on AC field induced structures in binary hard sphere mixtures.

Deswelling behaviour of ionic microgel particles from low to ultra-high densities.

The swelling of ionic microgel particles is investigated at a wide range of concentrations using a combination of light, X-ray and neutron scattering techniques. We employ a zero-average contrast approach for small-angle neutron scattering experiments, which enables a direct determination of the form factor at high concentrations. The observed particle size initially decreases strongly with the particle concentration in the dilute regime but approaches a constant value at intermediate concentrations. This is followed by a further deswelling at high concentrations above particle overlap. Theory and experiments point at a pivotal contribution of dangling polymer ends to the strong variation in size of ionic microgels, which presents itself mainly through the hydrodynamics properties of the system.

Self-Assembly of Ionic Microgels Driven by an Alternating Electric Field: Theory, Simulations, and Experiments.

The structural properties of a system of ionic microgels under the influence of an alternating electric field are investigated both theoretically and experimentally. This combined investigation aims to shed light on the structural transitions that can be induced by changing either the driving frequency or the strength of the applied field, which range from string-like formation along the field to crystal-like structures across the orthogonal plane. In order to highlight the physical mechanisms responsible for the observed particle self-assembly, we develop a coarse-grained description, in which effective interactions among the charged microgels are induced by both equilibrium ionic distributions and their time-averaged hydrodynamic responses to the applied field. These contributions are modeled by the buildup of an effective dipole moment at the microgels backbones, which is partially screened by their ionic double layer. We show that this description is able to capture the structural properties of this system, allowing for very good agreement with the experimental results. The model coarse-graining parameters are indirectly obtained via the measured pair distribution functions and then further assigned with a clear physical interpretation, allowing us to highlight the main physical mechanisms accounting for the observed self-assembly behavior.

Contrasting the dynamics of elastic and non-elastic deformations across an experimental colloidal Martensitic transition.

We present a framework to segregate the roles of elastic and non-elastic deformations in the examination of real-space experiments of solid-solid Martensitic transitions. The Martensitic transformation of a body-centred-tetragonal (BCT) to a body-centred-orthorhombic (BCO) crystal structure has been studied in a model system of micron-scale ionic microgel colloids (P. S. Mohanty, P. Bagheri, S. Nöjd, A. Yethiraj and P. Schurtenberger, Phys. Rev. X, 2015, 5, 011030). Non-affine fluctuations, i.e., displacement fluctuations that do not arise from purely elastic (affine) deformations, are detected in particle configurations acquired from the experiment. Tracking these fluctuations serves as a highly sensitive tool in signaling the onset of the Martensitic transition and precisely locating particle rearrangements occurring at length scales of a few particle diameters. Particle rearrangements associated with non-affine displacement modes become increasingly favorable during the transformation process. The nature of the displacement fluctuation modes that govern the transformation are shown to be different from those predominant in an equilibrium crystal. We show that BCO crystallites formed through shear may, remarkably, co-exist with those resulting from local rearrangements within the same sample.

Interpenetration of polymeric microgels at ultrahigh densities.

Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance a s can be smaller than the initial unperturbed particle diameter σ 0, due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕ eff above close packing (ϕ cp ), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕ cp using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕ eff . We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕ eff ≫ ϕ cp , a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.

Effective interactions between soft-repulsive colloids: experiments, theory, and simulations.

We describe a combined experimental, theoretical, and simulation study of the structural correlations between cross-linked highly monodisperse and swollen Poly(N-isopropylacrylamide) microgel dispersions in the fluid phase in order to obtain the effective pair-interaction potential between the microgels. The density-dependent experimental pair distribution functions g(r)'s are deduced from real space studies using fluorescent confocal microscopy and compared with integral equation theory and molecular dynamics computer simulations. We use a model of Hertzian spheres that is capable to well reproduce the experimental pair distribution functions throughout the fluid phase, having fixed the particle size and the repulsive strength. Theoretically, a monodisperse system is considered whose properties are calculated within the Rogers-Young closure relation, while in the simulations the role of polydispersity is taken into account. We also discuss the various effects arising from the finite resolution of the microscope and from the noise coming from the fast Brownian motion of the particles at low densities, and compare the information content from data taken in 2D and 3D through a comparison with the corresponding simulations. Finally different potential shapes, recently adopted in studies of microgels, are also taken into account to assess which ones could also be used to describe the structure of the microgel fluid.