External field-driven property localization in liquids of responsive macromolecules

We explore theoretically the effects of external potentials on the spatial distribution of particle properties in a liquid of explicitly responsive macromolecules. In particular, we focus on the bistable particle size as a coarse-grained internal degree of freedom (DoF, or "property"), σ, that moves in a bimodal energy landscape, in order to model the response of a state-switching (big-to-small) macromolecular liquid to external stimuli. We employ a mean-field density functional theory (DFT) that provides the full inhomogeneous equilibrium distributions of a one-component model system of responsive colloids (RCs) interacting with a Gaussian pair potential. For systems confined between two parallel hard walls, we observe and rationalize a significant localization of the big particle state close to the walls, with pressures described by an exact RC wall theorem. Application of more complex external potentials, such as linear (gravitational), osmotic, and Hamaker potentials, promotes even stronger particle size segregation, in which macromolecules of different size are localized in different spatial regions. Importantly, we demonstrate how the degree of responsiveness of the particle size and its coupling to the external potential tune the position-dependent size distribution. The DFT predictions are corroborated by Brownian dynamics simulations. Our study highlights the fact that particle responsiveness can be used to localize liquid properties and therefore helps to control the property- and position-dependent function of macromolecules, e.g., in biomedical applications.

Synergistic chemomechanical dynamics of feedback-controlled microreactors

The experimental control of synergistic chemomechanical dynamics of catalytically active microgels (microreactors) is a key prerequisite for the design of adaptive and biomimetic materials. Here, we report a minimalistic model of feedback-controlled microreactors based on the coupling between the hysteretic polymer volume phase transition and a volume-controlled permeability for the internal chemical conversion. We categorize regimes of mono- and bistability, excitability, damped oscillations, as well as sustained oscillatory states with tunable amplitude, as indicated by experiments and representable by the FitzHugh-Nagumo dynamics for neurons. We summarize the features of such a "colloidal neuron" in bifurcation diagrams with respect to microgel design parameters, such as permeability and relaxation times, as a guide for experimental synthesis.

Active responsive colloids driven by intrinsic dichotomous noise

We study the influence of intrinsic noise on the structure and dynamics of responsive colloids (RCs), which actively change their size and mutual interactions. The colloidal size is explicitly resolved in our RC model as an internal degree of freedom (DOF) in addition to the particle translation. A Hertzian pair potential between the RCs leads to repulsion and shrinking of the particles, resulting in an explicit responsiveness of the system to self-crowding. To render the colloids active, their size is internally driven by a dichotomous noise, randomly switching ("breathing") between growing and shrinking states with a predefined rate, as motivated by recent experiments on synthetic active colloids. The polydispersity of this dichotomous active responsive colloid (D-ARC) model can be tuned by the parameters of the noise. Utilizing stochastic computer simulations, we study crowding effects on the spatial distributions, relaxation times, and self-diffusion of dense suspensions of the D-ARCs. We find a substantial influence of the "built-in" intrinsic noise on the system's behavior, in particular, transitions from unimodal to bimodal size distributions for an increasing colloid density as well as intrinsic noise-modified diffusive translational dynamics. We conclude that controlling the noise of internal DOFs of a macromolecule or cell is a powerful tool for active colloidal materials to enable autonomous changes in the system's collective structure and dynamics towards the adaptation of macroscopic properties to external perturbations.

Active responsive colloids coupled to different thermostats

We introduce a model of active responsive colloids (ARCs) in which an internal degree of freedom (DoF) of a single colloidal particle is "activated" by coupling it to a different thermostat than for the translational DoFs. As for the responsive internal DoF, we consider specifically the size (diameter) of the spherical particles, which is confined by a harmonic parent potential being either entropic or energetic in nature. The ARCs interact via a repulsive Hertzian pair potential, appropriate to model hydrogels or elastic colloids, and are studied for various densities using Brownian dynamics simulations. We tune the internal activity in the nonequilibrium steady state by scanning through a wide range of internal temperatures, both smaller ("colder") and larger ("hotter") than the translational temperature. The results show a rich and intriguing behavior for the emergent property distributions, colloidal pair structure, and the diffusive translational dynamics controlled by the internal activity, substantially depending on whether the internal DoF moves in an entropic or energetic potential. We discuss theoretical thermal limits and phenomenological models which can explain some of the nonequilibrium trends qualitatively. Our study indicates that particle dynamical polydispersity as well as the structure and dynamics of dense macromolecular suspensions can be vastly tuned by internal activity in terms of internal "hot" or "cold" fluctuating states.

Multifunctional Core-Shell Microgels as Pd-Nanoparticle Containing Nanoreactors With Enhanced Catalytic Turnover

In this work, we present core-shell microgels with tailor-made architecture and properties for the incorporation of palladium nanoparticles. The microgel core consists of poly-N-isopropylacrylamide (PNIPAM) copolymerized with methacrylic acid (MAc) as anchor point for the incorporation of palladium nanoparticles. The microgel shell is prepared by copolymerization of NIPAM and the UV-sensitive comonomer 2-hydroxy-4-(methacryloyloxy)-benzophenone (HMABP). The obtained core-shell architecture was analyzed by means of photon correlation spectroscopy, while the incorporated amount of HMABP was further confirmed via Fourier transform infrared spectroscopy. Subsequently, the microgel system was used for loading with palladium nanoparticles and their size and localization were investigated by transmission electron microscopy. The catalytic activity of the monodisperse palladium nanoparticles was tested by reduction of 4-nitrophenol to 4-aminophenol. The obtained reaction rate constants for the core-shell system showed enhanced activity compared to the Pd-loaded bare core system. Furthermore, it was possible to recycle the catalyst several times. Analysis via transmission electron microscopy revealed, that the incorporated palladium nanoparticles emerged undamaged after the reaction and subsequent purification process since no aggregation or loss in size was observed.

Electrostatic Reaction Inhibition in Nanoparticle Catalysis

Electrostatic reaction inhibition in heterogeneous catalysis emerges if charged reactants and products with similar charges are adsorbed on the catalyst and thus repel the approaching reactants. In this work, we study the effects of electrostatic inhibition on the reaction rate of unimolecular reactions catalyzed on the surface of a spherical model nanoparticle using particle-based reaction-diffusion simulations. Moreover, we derive closed rate equations based on an approximate Debye-Smoluchowski rate theory, valid for diffusion-controlled reactions, and a modified Langmuir adsorption isotherm, relevant for reaction-controlled reactions, to account for electrostatic inhibition in the Debye-Hückel limit. We study the kinetics of reactions ranging from low to high adsorptions on the nanoparticle surface and from the surface- to diffusion-controlled limits for charge valencies 1 and 2. In the diffusion-controlled limit, electrostatic inhibition drastically slows down the reactions for strong adsorption and low ionic concentration, which is well described by our theory. In particular, the rate decreases with adsorption affinity because, in this case, the inhibiting products are generated at a high rate. In the (slow) reaction-controlled limit, the effect of electrostatic inhibition is much weaker, as semiquantitatively reproduced by our electrostatic-modified Langmuir theory. We finally propose and verify a simple interpolation formula that describes electrostatic inhibition for all reaction speeds ("diffusion-influenced" reactions) in general.

Rhodium nanoparticles inside well-defined unimolecular amphiphilic polymeric nanoreactors: synthesis and biphasic hydrogenation catalysis

Rhodium nanoparticles (Rh NPs) embedded in different amphiphilic core-crosslinked micelle (CCM) latexes (RhNP@CCM) have been synthesized by [RhCl(COD)(TPP@CCM)] reduction with H₂ (TPP@CCM = core-anchored triphenylphosphine). The reduction rate depends on temperature, on the presence of base (NEt₃) and on the P/Rh ratio. For CCMs with outer shells made of neutral P(MAA-co-PEOMA) copolymer chains (RhNP@CCM-N), the core-generated Rh NPs tend to migrate toward the hydrophilic shell and to agglomerate depending on the P/Rh ratio and core TPP density, whereas the MAA protonation state has a negligible effect. Conversely, CCMs with outer shells made of polycationic P(4VPMe⁺I^-) chains (RhNP@CCM-C) maintain core-confined and well dispersed Rh NPs. All RhNP@CCMs were used as catalytic nanoreactors under aqueous biphasic conditions for acetophenone, styrene and 1-octene hydrogenation. Styrene was efficiently hydrogenated by all systems with high selectivity for vinyl reduction. For acetophenone, competition between benzene ring and carbonyl reduction was observed as well as a limited access to the catalytic sites when using CCM-C. Neat 1-octene was also converted, but the activity increased when the substrate was diluted in 1-nonanol, which is a better core-swelling solvent. Whereas the molecular Rh^I center was more active than the Rh⁰ NPs in 1-octene hydrogenation, the opposite trend was observed for styrene hydrogenation. Although Rh NP migration and agglomeration occurred for RhNP@CCM-N, even at high P/Rh, the NPs remained core-confined for RhNP@CCM-C, but only when toluene rather than diethyl ether was used for product extraction before recycling.

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.

Structure and dynamics of responsive colloids with dynamical polydispersity

Dynamical polydispersity in single-particle properties, for example a fluctuating particle size, shape, charge density, etc, is intrinsic to responsive colloids (RCs), such as biomacromolecules or microgels, but is typically not resolved in coarse-grained mesoscale simulations. Here, we present Brownian dynamics simulations of suspensions of RCs modeling soft hydrogel colloids, for which the size of the individual particles is an explicitly resolved (Gaussian) degree of freedom and dynamically responds to the local interacting environment. We calculate the liquid structure, emergent size distributions, long-time diffusion, and property (size) relaxation kinetics for a wide range of densities and intrinsic property relaxation times in the canonical ensemble. Comparison to interesting reference cases, such as conventional polydisperse suspensions with a frozen parent distribution, or conventional monodisperse systems interacting with an effective pair potential for one fixed size, shows a significant spread in the structure and dynamics. The differences, most apparent in the high density regimes, are due to many-body correlations and the dynamical coupling between property and translation in RC systems, not explicitly accounted for in the conventional treatments. In particular, the translational diffusion in the RC systems is surprisingly close to the free (single RC) diffusion, mainly due to a cancellation of crowding and size compression effects. We show that an effective monodisperse pair potential can be constructed that describes the many-body correlations reasonably well by convoluting the RC pair potential with the density-dependent emergent size distributions and using a mean effective diffusion constant.

Facilitating target search in polymer networks: Effects of target size and mixed one-dimensional and three-dimensional diffusion

We theoretically investigate the problem of diffusive target search and mean first passage times (MFPTs) of a tracer in a three-dimensional (3D) polymer network with a particular focus on the effects of combined one-dimensional (1D) diffusion along the polymer chains and 3D diffusion within the network. For this, we employ computer simulations as well as limiting theories of a single diffusive tracer searching for a spherical target fixed at a cross-link of a homogeneous 3D cubic lattice network. The free parameters are the target size, the ratio of the 1D and 3D friction constants, and the transition probabilities between bound and unbound states. For a very strongly bound tracer on the chains, the expected predominant set of 1D lattice diffusion (LD) is found. The MFPT in the LD process significantly depends on the target size, yielding two distinct scaling behaviors for target sizes smaller and larger than the network mesh size, respectively. In the limit of a pointlike target, the LD search becomes a random walk process on the lattice, which recovers the analytical solution for the MFPT previously reported by S. Condamin, O. Bénichou, and M. Moreau [Phys. Rev. Lett. 95, 260601 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.260601]. For the very weakly bound tracer, the expected 3D free diffusion (FD) dominates, extrapolating to the well-known Smoluchowski limit. A critical target size is found above which the MFPT in the FD process is faster than in the LD process. For intermediate binding, i.e., a combination of LD and FD processes, the target search time can be minimized for an optimal range of target sizes and partitions between FD and LD, for which the MFPTs are substantially faster when compared to the limiting FD or LD processes. Our study may provide a theoretical basis to better understand and predict search and reaction processes in complex structured materials, thereby contributing to practical applications such as designing nanoreactors where catalytic targets are immobilized in polymer networks.

How the Shape and Chemistry of Molecular Penetrants Control Responsive Hydrogel Permeability

The permeability of hydrogels for the selective transport of molecular penetrants (drugs, toxins, reactants, etc.) is a central property in the design of soft functional materials, for instance in biomedical, pharmaceutical, and nanocatalysis applications. However, the permeation of dense and hydrated polymer membranes is a complex multifaceted molecular-level phenomenon, and our understanding of the underlying physicochemical principles is still very limited. Here, we uncover the molecular principles of permeability and selectivity in hydrogel permeation. We combine the solution-diffusion model for permeability with comprehensive atomistic simulations of molecules of various shapes and polarities in a responsive hydrogel in different hydration states. We find in particular that dense collapsed states are extremely selective, owing to a delicate balance between the partitioning and diffusivity of the penetrants. These properties are sensitively tuned by the penetrant size, shape, and chemistry, leading to vast cancellation effects, which nontrivially contribute to the permeability. The gained insights enable us to formulate semiempirical rules to quantify and extrapolate the permeability categorized by classes of molecules. They can be used as approximate guiding ("rule-of-thumb") principles to optimize penetrant or membrane physicochemical properties for a desired permeability and membrane functionality.

Structure and position-dependent properties of inhomogeneous suspensions of responsive colloids

Responsive particles, such as biomacromolecules or hydrogels, display a broad and polymodal distribution of conformations and have thus the ability to change their properties (e.g., size, shape, charge density, etc.) substantially in response to external fields or to their local environment (e.g., mediated by cosolutes or pH). Here we discuss the basic statistical mechanics for a model of responsive colloids (RCs) by introducing an additional "property" degree of freedom as a collective variable in a formal coarse-graining procedure. The latter leads to an additional one-body term in the coarse-grained (CG) free energy, defining a single-particle property distribution for an individual polydisperse RC. We argue that in the equilibrium thermodynamic limit such a CG system of RCs behaves like a conventional polydisperse system of nonresponsive particles. We then illustrate the action of external fields, which impose local (position-dependent) property distributions leading to nontrivial effects on the spatial one-body property and density profiles, even for an ideal (noninteracting) gas of RCs. We finally apply density-functional theory in the local density approximation to discuss the effects of particle interactions for specific examples of (i) a suspension of RCs in an external field linear in both position and property, (ii) a suspension of RCs with highly localized properties (sizes) confined between two walls, and (iii) a two-component suspension where an inhomogeneously distributed (nonresponsive) cosolute component, as found, e.g., in the studies of osmolyte- or salt-induced collapse or swelling transitions of thermosensitive polymers, modifies the local properties and density of the RC liquid.

Kinetic and equilibrium study of (poly amido amine) PAMAM dendrimers for the removal of chromium from tannery wastewater

Water gets polluted by industrial effluents, mainly composed of heavy metals and organic materials. Water soluble heavy metals can be taken up by living organisms. Chromium mainly occurs in the form of chromate and cationic hydroxo complexes in water. Apart from conventional methods of heavy metal removal, there are some novel approaches such as using dendrimers for removal of heavy metal. Dendrimers are extremely branched nano sized polymers with a three-dimensional symmetry around a core that imparts poly functionality. PAMAM (poly amido amine) dendrimers having ethylene diamine as core and methyl acrylate as repeating unit was divergently synthesized. Characterization of PAMAM dendrimers was evaluated by UV–Vis spectroscopy, zeta sizer, scanning electron microscopy (SEM) and Fourier-transform infrared (FT-IR) spectroscopy. Zero generation dendrimers have amine terminal groups, showed intense amide group peak at 1596.76 cm−1. The λ max value was 278 nm. SEM exhibited spherical shape for full generation while needle like structure for −0.5 generation. Evaluation of chromium removal from wastewater has been done by atomic absorption spectroscopy (AAS). The data revealed that optimal removal of Cr occurs at dendrimer concentration of 5 mL, Cr concentration of 300 ppm, contact time of 2 min and pH 7. The synthesized dendrimers have effectively removed Cr from tannery wastewater.

Morphological Transitions in Patchy Nanoparticles

Nanoparticles (NPs) decorated with topographically or chemically distinct surface patches are an emerging class of colloidal building blocks of functional hierarchical materials. Surface segregation of polymer ligands into pinned micelles offers a strategy for the generation of patchy NPs with controlled spatial distribution and number of patches. The thermodynamic nature of this approach poses a question about the stability of multiple patches on the NP surface, as the lowest energy state is expected for NPs carrying a single patch. In the present work, for gold NPs end-grafted with thiol-terminated polymer molecules, we show that the patchy surface morphology is preserved under conditions of strong grafting of the thiol groups to the NP surface (i.e., up to a temperature of 40 °C), although the patch shape changes over time. At higher temperatures (e.g., at 80 °C), the number of patches per NP decreases, due to the increased lateral mobility and coalescence of the patches as well as the ultimate loss of the polymer ligands due to desorption at enhanced solvent quality. The experimental results were rationalized theoretically, using a scaling approach. The results of this work offer insight into the surface science of patchy nanocolloids and specify the time and temperature ranges of the applications of patchy NPs.

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.

Observation of Unusual Thermoresponsive Volume Phase Transition Behavior of Cubic Poly(N-isopropylacrylamide) Microgels

Here, we report the observation of an unusual thermoresponsive volume phase transition behavior of cubic poly(N-isopropylacrylamide) (PNIPAM) microgels. Cubic PNIPAM microgels with a mean edge size of 125 ± 41 nm were synthesized via electrochemical-initiated radical polymerization with a photovoltaic cell as power supply. In turbidity and laser light scattering studies on dilute aqueous dispersions of these cubic microgels, both the light attenuation and hydrodynamic radius variations with temperature reveal an additional transition at about 25.0 °C, besides the widely reported volume phase transition at the PNIPAM LCST that is typically found for (quasi-)spherical microgels. This unusual thermoresponsive volume phase transition behavior of the cubic microgels can be elucidated by using a core-corona model, with the contribution from each part varying at different temperatures. The finding is also checked by thermal analysis.

Tuning catalysis of boronic acids in microgels by in situ reversible structural variations

The catalysis of boronic acids immobilized in polymer microgels can be modulated by bubbling with N₂/CO₂ gas, and in some cases by adding glucose, making their catalytic activity comparable or even superior to that of the corresponding free boronic acid monomers homogeneously dispersed in solutions and, more importantly, making these boronic-acid-containing polymer microgels able to catalyze alternate reactions that may extend the usefulness. This enhanced catalytic function of these boronic-acid-containing microgels as organoboron acid catalysts is plausibly achieved via in situ reversibly structural variations. Kinetic studies have been carried out on the model boronic-acid-catalyzed aza-Michael addition, aldol, amidation, and [4 + 2] cycloaddition reactions in order to better understand the catalytic process.

Influence of patch size and chemistry on the catalytic activity of patchy hybrid nonwovens

In this work, we provide a detailed study on the influence of patch size and chemistry on the catalytic activity of patchy hybrid nonwovens in the gold nanoparticle (Au NP) catalysed alcoholysis of dimethylphenylsilane in n-butanol. The nonwovens were produced by coaxial electrospinning, employing a polystyrene solution as the core and a dispersion of spherical or worm-like patchy micelles with functional, amino group-bearing patches (dimethyl and diisopropyl amino groups as anchor groups for Au NP) as the shell. Subsequent loading by dipping into a dispersion of preformed Au NPs yields the patchy hybrid nonwovens. In terms of NP stabilization, i.e., preventing agglomeration, worm-like micelles with poly(N,N-dimethylaminoethyl methacrylamide) (PDMA) patches are most efficient. Kinetic studies employing an extended 1^st order kinetics model, which includes the observed induction periods, revealed a strong dependence on the accessibility of the Au NPs' surface to the reactants. The accessibility is controlled by the swellability of the functional patches in n-butanol, which depends on both patch chemistry and size. As a result, significantly longer induction (t _ind) and reaction (t _R) times were observed for the 1^st catalysis cycles in comparison to the 10^th cycles and nonwovens with more polar PDMA patches show a significantly lower t _R in the 1^st catalysis cycle. Thus, the unique patchy surface structure allows tailoring the properties of this "tea-bag"-like catalyst system in terms of NP stabilization and catalytic performance, which resulted in a significant reduction of t _R to about 4 h for an optimized system.

Diffusion-influenced reactions on non-spherical partially absorbing axisymmetric surfaces

The calculation of the diffusion-controlled reaction rate for partially absorbing, non-spherical boundaries presents a formidable problem of broad relevance. In this paper we take the reference case of a spherical boundary and work out a perturbative approach to get a simple analytical formula for the first-order correction to the diffusive flux onto a non-spherical partially absorbing surface of revolution. To assess the range of validity of this formula, we derive exact and approximate expressions for the reaction rate in the case of partially absorbing prolate and oblate spheroids. We also present numerical solutions by a finite-element method that extend the validity analysis beyond spheroidal shapes. Our perturbative solution provides a handy way to quantify the effect of non-sphericity on the rate of capture in the general case of partial surface reactivity.

Temperature-Controlled Catalysis by Core-Shell-Satellite AuAg@pNIPAM@Ag Hybrid Microgels: A Highly Efficient Catalytic Thermoresponsive Nanoreactor

A novel wet-chemical protocol is reported for the synthesis of "temperature-programmable" catalytic colloids consisting of bimetallic core@shell AuAg nanoparticles encapsulated into poly(N-isopropylacrylamide) (pNIPAM) microgels with silver satellites (AgSTs) incorporated within the microgel structure. Spherical AuNPs of 50 nm in diameter are initially synthesized and used for growing a pNIPAM microgel shell with temperature stimulus response. A silver shell is subsequently grown on the Au core by diffusing Ag salt through the hydrophilic pNIPAM microgel (AuAg@pNIPAM microgel). The use of allylamine as a co-monomer during pNIPAM polymerization facilitates the coordination of Ag⁺ with the NH₂ nitrogen lone pair of electrons, which are reduced to Ag seeds (∼14 nm) using a strong reducing agent, obtaining thus AuAg@pNIPAM@Ag hybrid microgels. The two systems are tested as catalysts toward the reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Amp) by NaBH₄. Both exhibit extremely sensitive temperature-dependent reaction rate constants, with the highest K₁ value of the order of 0.6 L/m² s, which is one of the highest values ever reported. The presence of plasmonic entities is confirmed by UV-vis spectroscopy. Dynamic light scattering proves the temperature responsiveness in all cases. Transmission electron microscopy and energy-dispersive X-ray (EDX) elemental mapping highlight the monodispersity of the synthesized hybrid nanostructured microgels, as well as their size and metallic composition. The amount of gold and silver in both systems is obtained by thermogravimetric analysis and the EDX spectrum. The reduction reaction kinetics is monitored by UV-vis spectroscopy at different temperatures for both catalytic systems, with the AuAg@pNIPAM@Ag microgels showing superior catalytic performance at all temperatures because of the synergistic effect of the AuAg core and the AgSTs. The principal novelty of this study lies in the "hierarchical" design of the metal-polymer-metal core@shell@satellite nanostructured colloids exhibiting synergistic capabilities of the plasmonic NPs for, among others, temperature-controlled catalytic applications.

Improved Smart Microgel Carriers for Catalytic Silver Nanoparticles

Acrylamide-based, thermoresponsive core-shell microgels with a linear phase transition region are used as improved carriers for catalytically active silver nanoparticles in the present study. In this context, we investigated the swelling behavior of the carriers and the stability of the silver nanoparticles inside the polymer network with photon correlation spectroscopy, transmission electron microscopy, and by following the surface plasmon resonance of the nanoparticles. Depending on the cross-linker content of the microgel core, we observed very good stability of the nanoparticles inside the microgel network, with nearly no bleeding or aggregation of the nanoparticles over several weeks for core cross-linker contents of 5 and 10 mol %. The architecture of the hybrid particles in the swollen state was investigated with cryogenic transmission electron microscopy. The particles exhibit a core-shell structure, with the silver nanoparticles located mainly at the interface between the core and shell. This architecture was not used before and seems to grant advanced stability to the nanoparticles inside the network in combination with good switchability of the catalytic activity. This was measured by following the reduction of 4-nitrophenole, which is a well-studied model reaction. The obtained Arrhenius plots show that similar to previous works, the swelling of the core and shell can influence the catalytic activity of the silver nanoparticles. As mentioned before, the cross-linker content of the core seems to be a very important parameter for the switchability of the catalytic activity. A higher cross-linker content of the core seems to be connected to a stronger influence of the carrier swelling degree on the catalytic activity of the silver nanoparticles.