Shell Microgels Research Articles

Non-close-packed plasmonic Bravais lattices through a fluid interface-assisted colloidal assembly and transfer process

AbstractThe assembly of colloids at fluid interfaces followed by their transfer to solid substrates represents a robust bottom-up strategy for creating colloidal monolayers over large, macroscopic areas. In this study, we showcase how subtle adjustments in the transfer process, such as varying the contact angle of the substrate and controlling deposition speed and direction, enable the realization of all five two-dimensional Bravais lattices. Leveraging plasmonic core–shell microgels as the building blocks, we successfully engineered non-close-packed plasmonic lattices exhibiting hexagonal, square, rectangular, centered rectangular, and oblique symmetries. Beyond characterizing the monolayer structures and their long-range order, we employed extinction spectroscopy alongside finite difference time domain simulations to comprehensively investigate and interpret the plasmonic response of these monolayers. Additionally, we probed the influence of the refractive index environment on the plasmonic properties by two methods: first, by plasma treatment to remove the microgel shells, and second, by overcoating the resulting gold nanoparticle lattices with a homogeneous refractive index polymer film. Graphical Abstract

Temporal resistance fluctuations during the initial filtration period of colloidal matter filtration

Membrane filtration processes with flux reversal for backwashing are employed to manage fouling during colloidal fluid treatment. This flux reversal accompanies filtration cycles where the total resistance increases over time. However, little is known about the initial phase of filtration. We focus on early filtration phase and present a microfluidic filtration system that allows precise observation of filtration resistances under constant flux or constant pressure operation for soft matter dead-end filtration with subsequent percolation of pure solvent. We identified temporal hydraulic resistance fluctuations during the start-up phase, which significantly differed from the steady-state condition. We focus on the experimental filtration of soft core–shell poly(N-isopropylacrylamide)-co-acrylic-acid microgels and observed a distinct peak in filter cake resistance with a subsequent reduction down to 50% of the initial peak resistance. To comprehend the phenomenon, we complement the experiments with temporal and spatial confocal microscopy studies linking cake compression and cake density gradients to its resistance. By comparing filtration, dialysis, and centrifugation as different microgel purification methods, we discover microgel shell degradation leading to detached polymer chains permeating in the downstream which causes the observed resistance reduction. This result demonstrates the permeation-induced degradation of macromolecular colloids during membrane filtration.

Correlating structural changes in thermoresponsive hydrogels to the optical response of embedded plasmonic nanoparticles.

Stimuli-responsive microgels, composed of small beads with soft, deformable polymer networks swollen through a combination of synthetic control over the polymer and its interaction with water, form a versatile platform for development of multifunctional and biocompatible sensors. The interfacial structural variation of such materials at a nanometer length scale is essential to their function, but not yet fully comprehended. Here, we take advantage of the plasmonic response of a gold nanorod embedded in a thermoresponsive microgel (AuNR@PNIPMAm) to monitor structural changes in the hydrogel directly near the nanorod surface. By direct comparison of the plasmon response against measurements of the hydrogel structure from dynamic light scattering and nuclear magnetic resonance, we find that the microgel shell of batch-polymerized AuNR@PNIPMAm exhibits a heterogeneous volume phase transition reflected by different onset temperatures for changes in the hydrodyanmic radius (RH) and plasmon resonance, respectively. The new approach of contrasting plasmonic response (a measure of local surface hydrogel structure) with RH and relaxation times paves a new path to gain valuable insight for the design of plasmonic sensors based on stimuli-responsive hydrogels.

Liquid Metal Powders Wrapped with Robust Shell by Bimetallic Ions Chelation Strategy for Energy Harvesting and Flexible Electronics

AbstractExcellent electrical and fluidic properties render liquid metal (LM) a promising material for flexible electronics, soft robotics, and stretchable circuits. Although producing LM micro/nano droplets by nanometerization addresses the tricky issue of the processing result from high surface tension, it still remains challenging to obtain stable LM droplets because of their dynamic surfaces. In this study, LM powders wrapped with robust shells are fabricated by bimetallic ions chelation strategy. During sonicating LM (e.g., EGaIn) in an alginate solution, a microgel shell is first formed by the coordination of Ga3+ and carboxyl groups. Subsequently, LM powders are achieved by introducing Ca2+ as secondary metal ions to chelate with redundant carboxyl groups. Besides high chemical stability in water and air, these nonstick LM powders also exhibit superior photothermal effects for energy harvesting, being capable of recovering electrical conductivity by mechanical sintering for flexible electronics. These conductive circuits based on LM powders not only work as electro‐thermal heaters, and electromagnetic interference shielding material but also can be designed as smart actuators. Thus, this bimetallic ions chelation strategy may start a pathway to produce stable, transportable LM powders with multifunctional properties, applicable in stretchable electronics, smart actuators, and electric skins.

A Simple and Fast Compression‐Based Method to Fabricate Responsive Gold‐pNIPAM Hybrid Materials: From Thin Films to Anisotropic Microgels

AbstractIn recent years, hydrogel‐based soft materials with hybrid properties have found widespread use in various technological fields, including tissue engineering, soft actuators, and flexible electronics. The proper implementation of these smart multifunctional materials into real‐world applications requires the development of simple, cost‐effective, and large‐scale fabrication methods. Herein, a simple compression‐ and colloid‐based method is presented to fabricate responsive Au‐poly(N‐isopropylacrylamide) (pNIPAM) hybrid films using photopolymerizable resin containing Au‐pNIPAM core–shell microgels as building blocks. Uniform Au‐pNIPAM hybrid films of 25 × 25 mm with adjustable thickness in the micron‐size range (2.3–1.2 µm) w ere successfully fabricated on glass substrates and flexible commercial acetate sheets. The resulting flexible Au‐pNIPAM films exhibit robust optical and mechanical properties, even after repeated edge‐to‐edge bending cycle tests. Additionally, using patterned light to polymerize the Au‐pNIPAM films allows synthesizing of anisotropic Au‐pNIPAM microgels with high width‐to‐height aspect ratios, such as square, circular, and rectangular microgels, adding a new dimension to the proposed fabrication method.

Bactericidal and biocatalytic temperature responsive microgel based self-cleaning membranes for water purification

The present study focuses on creating an antimicrobial and biocatalytic smart gating membrane by synthesizing unique core–shell microgels. The core–shell microgels are synthesized by grafting short chains of poly(ethylenimine) (PEI) onto a poly((N-isopropyl acrylamide)–co-glycidyl methacrylate)) (P(NIPAm-co-GMA)) core. Subsequently, the produced microgels are utilized as a substrate for synthesizing and stabilizing silver nanoparticles (Ag NPs) through an in-situ approach. These Ag NPs immobilized microgels are then suction filtered over a polyethylene terephthalate (PET) track-etched support to create cross-linked composite microgel membranes (CMMs). After structural and permeation characterization of the prepared CMMs, the laccase enzyme is then covalently grafted to the surface of the membrane and tested for its effectiveness in degrading Reactive red-120 dye. The laccase immobilized biocatalytic CMMs show effective degradation of the Reactive red-120 by 71%, 48%, and 34% at pH 3, 4, and 5, respectively. Furthermore, the immobilized laccase enzyme showed better activity and stability in terms of thermal, pH, and storage compared to the free laccase, leading to increased reusability. The unique combination of Ag NPs and laccase on a thermoresponsive microgel support resulted in a responsive self-cleaning membrane with excellent antimicrobial and dye degradation capabilities for environmentally friendly separation technology.

Liquid Metal-Doped Conductive Hydrogel for Construction of Multifunctional Sensors.

Interest in wearable and stretchable multifunctional sensors has grown rapidly in recent years. The sensing elements must accurately detect external stimuli to expand their applicability as sensors. However, the sensor's self-healing and adhesion to a target object have been major challenges in developing such practical and versatile devices. In this study, we prepared a hydrogel (LM-SA-PAA) composed of liquid metal (LM), sodium alginate (SA), and poly(acrylic acid) (PAA) with ultrastretchable, excellent self-healing, self-adhesive, and high-sensitivity sensing capabilities that enable the conformal contact between the sensor and skin even during dynamic movements. The excellent self-healing performance of the hydrogel stems from its double cross-linked networks, including physical and chemical cross-linked networks. The physical cross-link formed by the ionic interaction between the carboxyl groups of PAA and gallium ions provide the hydrogel with reversible autonomous repair properties, whereas the covalent bond provides the hydrogel with a stable and strong chemical network. Alginate forms a microgel shell around LM nanoparticles via the coordination of its carboxyl groups with Ga ions. In addition to offering exceptional colloidal stability, the alginate shell has sufficient polar groups, ensuring that the hydrogel adheres to diverse substrates. Based on the efficient electrical pathway provided by the LM, the hydrogel exhibited strain sensitivity and enabled the detection of various human motions and electrocardiographic monitoring. The preparation method is simple and versatile and can be used for the low-cost fabrication of multifunctional sensors, which have broad application prospects in human-machine interface compatibility and medical monitoring.

Extraction of iron (III) ions by core-shell microgel for in situ formation of iron nanoparticles to reduce harmful pollutants from water

Since the most transition metal ions and organic dyes are hazardous, it is crucial to remove them from water. For this purpose, various systems are reported for removal of only transition metal ions or organic dyes. I have introduced a new system which is suitable for removal of both metal ions and organic dyes. The free radical precipitation polymerization method was used to create the spherical system of silica surrounded by poly(N-vinylcaprolactam-acrylic acid) S@P(NVCP-AA) core shell microgels and then characterized by using FTIR, DLS, XRD, and STEM. The S@P(NVCP-AA) microgel system was applied as adsorbent to extract iron (III) ions from water under various concentrations of S@P(NVCP-AA), iron (III) ions content, pH, and agitation time. The adsorption process of iron (III) ions on S@P(NVCP-AA) microgels was investigated by different adsorption isotherms. The kinetics of adsorption of iron (III) ions on S@P(NVCP-AA) microgel system was also examined by intra-particle diffusion model (InPDM), pseudo-1st order (P1O), pseudo-2nd order (P2O) and Elovich model (EM). Iron nanoparticles were formed by in-situ reduction of iron (III) ions that had been adsorbed in the P(NVCP-AA) shell region of the S@P(NVCP-AA) microgel. The organic pollutants and toxic transition metal ions like 4-nitrophenol (4NiP), methyl red (MeR), methylene blue (MeB) and chromium (VI) ions (CMI) were reduced from an aqueous medium by using the iron nanoparticles loaded in S@P(NVCP-AA) microgels. The pseudo 1st order rate constant values for the catalytic reduction of 4NiP, MeR, MeB and CMI were found to be 0.778 min−1, 0.928 min−1, 0.943 min−1 and 0.142 min−1 respectively. The resulting S@Fe-P(NVCP-AA) system can act as an efficient catalyst for a wide range of additional organic transformations.

Core–Shell Spheroid‐Laden Microgels Crosslinked under Biocompatible Conditions for Probing Cancer‐Stromal Communication

Multicellular cancer spheroids (MCSs) have emerged as a promising in vitro model that recaptures many features of solid tumours in vivo. To generate cancer spheroids, cells are encapsulated in microgels with high throughput. While the biophysical properties of the cancer spheroid and biomaterial influence the function and behavior of the cells, characterization of these properties remains largely unexplored. In addition, many existing techniques lack control over cell positioning, resulting in the formation of spheroids with large variability. Herein, a versatile, microfluidic platform for generating biocompatible core–shell microgels with uniform cancer spheroids is reported. In addition, an in situ micromechanics measuring device to determine the stiffness of individual artificial cancer niches is used. The power of the microfluidics‐based method by making MCS‐laden microgels to model the interactions of stromal cancer cells is demonstrated. Together with in vivo data, it is shown that tumor cell proliferation is promoted by cocultured fibroblasts.

Catalytic degradation of organic dyes using Au-poly(styrene@N-isopropylmethacrylamide) hybrid microgels

Poly([email protected]N-isopropylmethacrylamide) [P([email protected]NPM)] microgel particles were produced by seed facilitated free radical precipitation polymerization technique and laterly gold (Au) nanoparticles (NPs) were integrated into the crosslinked polymeric network of shell of P([email protected]) core shell microgel particles to form gold-P([email protected]) hybrid microgel system by reduction of HAuCl4.3H2O within microgel dispersion. The synthesized microgel and hybrid microgel particles were characterized with FTIR, UV–Vis., DLS, TEM, and XRD methods. Then gold-P([email protected]) system was used as catalyst for reduction/degradation of rhodamine-B (RhB), congo red (CoR), methyl orange (MeO), eriochrome black T (ErBT), methylene blue (MeB), methyl red (MeR), and eosin (ES) dyes in the presence of NaBH4 at ambient temperature. Their reduction/degradation kinetics was also studied. Au-P([email protected]) was also applied as catalyst to reduce/degrade the wastewater sample containing various dyes to study the real application of the catalytic system. The Au-P([email protected]) is recyclable without any substantial loss of activity.

Magnetic ion‐imprinted microspheres for the removal of heavy metal ions from aqueous solution

AbstractMagnetic ion‐imprinted microspheres (IIM) with core‐shell structure were successfully prepared by reverse emulsion polymerization and applied to adsorb heavy metal ions from sewage. The semi‐interpenetrating polymer network composed of cross‐linked polyacrylamide (PAM) and linear chitosan (CTS) was used as the microgel shell of microspheres, which can not only fully retain the active sites of CTS but also tightly encapsulate magnetic particle nuclei. In addition, ion imprinting technology was used to further improve the adsorption capacity. In this study, the effects of PAM to CTS ratio on the thermal stability, magnetic properties, and microstructure of microspheres were investigated. Adsorption studies showed that IIM exhibited excellent selective adsorption of Cu(II), and the effects of initial concentration of metal ions, adsorption time, adsorbent dosage, pH, ionic strength, and cycle times on adsorption of Cu(II) by IIM‐2 were also studied. In addition, it was revealed that pseudo‐second‐order kinetic model and Langmuir isotherm model could better simulate the adsorption kinetic and isotherm of IIM‐2 for Cu(II), respectively. At 30°C and pH 5.0, the theoretical maximum adsorption capacities for Cu(II) by IIM‐2 were 151.13 mg/g. X‐ray photoelectron spectroscopy and Zeta potential study showed that the adsorption mechanism of IIM‐2 was a combination of electrostatic interaction and ion exchange.

Scattering of Visible Light by Au–PNIPAM Core–Shell Microgels

Scattering of Visible Light by Au–PNIPAM Core–Shell Microgels

Machine-learning effective many-body potentials for anisotropic particles using orientation-dependent symmetry functions.

Spherically symmetric atom-centered descriptors of atomic environments have been widely used for constructing potential or free energy surfaces of atomistic and colloidal systems and to characterize local structures using machine learning techniques. However, when particle shapes are non-spherical, as in the case of rods and ellipsoids, standard spherically symmetric structure functions alone produce imprecise descriptions of local environments. In order to account for the effects of orientation, we introduce two- and three-body orientation-dependent particle-centered descriptors for systems composed of rod-like particles. To demonstrate the suitability of the proposed functions, we use an efficient feature selection scheme and simple linear regression to construct coarse-grained many-body interaction potentials for computationally efficient simulations of model systems consisting of colloidal particles with an anisotropic shape: mixtures of colloidal rods and non-adsorbing polymer coils, hard rods enclosed by an elastic microgel shell, and ligand-stabilized nanorods. We validate the machine-learning (ML) effective many-body potentials based on orientation-dependent symmetry functions by using them in direct coexistence simulations to map out the phase behavior of colloidal rods and non-adsorbing polymer coils. We find good agreement with the results obtained from simulations of the true binary mixture, demonstrating that the effective interactions are well described by the orientation-dependent ML potentials.

Polymer Stabilized Metal Nanoparticles for Catalytic Degradation of Methylene Blue in Water

Methylene blue is highly toxic and releases from various industries. It must be transformed into less toxic compounds. The Core-Shell microgels p (Pst core), Pstcore-NIPMamm-MAa and Ag in Pst-p NIPMamm-MAa have been synthesized using the Core-Shell hybrid micro gelling NIPMamm-MAa emulsion polymerization process. The 0.086mM, MB 6.2mM NaBH4 and 0.2916 mg / mL catalysts in the cuvette were measured using UV-visible spectrophotometers. Spectrums were measured at a one-minute interval. The peak at 600 nm steadily decreased over time and was completely eliminated after 11 minutes. Without the catalyst, MB decreases with NaBH4 which showed that the reaction decreases were slow and MB very high within 120 minutes. The Psty core of FT-IR core microgels, pNIpmam-MMAA, and Ag-pNIpmam-MAA core microgels are hybrids. At 2955 and 2845 cm−1 FT-IR spectra, Psty NiPMaM – MaA and Ag-p NiPMaM – MaA were used for core shell microgels, with C-H vibrations expanding the aromatic ring. In this study degradation of Methylene were carried out with Ag- Nanocomposites at different interval of time to check the degradation at minimum time. The degradation of MB dye

Extraction of copper ions from aqueous medium by microgel particles for in-situ fabrication of copper nanoparticles to degrade toxic dyes

Abstract Most of the transition metal ions are toxic and their removal from water is important. For this purpose, nearly monodisperse spherical core shell microgel particles with diameter of 88 ± 3 nm have been synthesized by free radical precipitation polymerization method and characterized by fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), dynamic light scattering (DLS), and transmission electron microscopy (TEM). Extraction of copper ions from water under several conditions of pH, copper ions content and core shell microgel concentrations was undertaken. Several adsorption isotherms were tested to explore the process of adsorption of copper ions on the microgel particles. Kinetics of adsorption process was examined by pseudo first order, pseudo second order, intra-particle diffusion and Elovich models. Copper ions adsorbed in shell region of core shell microgel were reduced to copper nanoparticles. The hybrid microgel was used to reduce organic pollutants such as 4-nitrophenol (4NP), methylene blue (MB), and methyl orange (MO) in aqueous medium. The value of pseudo first order rate constant for catalytic reduction of 4NP, MB, and MO was found 0.602, 0.831, and 0.874 min−1 respectively. The resultant core shell hybrid microgel system can serve as efficient catalyst for numerous other organic transformations.

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.

SAXS Investigation of Core–Shell Microgels with High Scattering Contrast Cores: Access to Structure Factor and Volume Fraction

To explore dense packings of soft colloids, scattering experiments are ideal to access the structure factor. However, for soft microgels determination of the structure factor is difficult because of the low contrast of the polymer network and potential microgel interpenetration and deformation that change the form factor contribution. Here, we employ small-angle X-ray scattering (SAXS) to study soft, thermoresponsive microgels with poly-N-isopropylacrylamide (PNIPAM) shells and gold nanoparticle cores. The scattering of the gold cores dominates the scattering patterns and allows precise determination of the microgel volume fraction over a broad range of concentrations. At high volume fractions we find distinct patterns with sharp Bragg peaks allowing extraction of the structure factor and characterization of the phases combined with UV-Vis spectroscopy. The unique scattering contrast of our core-shell microgels combined with SAXS opens up new ways to investigate dense packings of soft microgels including in situ studies of phase transitions.

Interfacial Assembly of Anisotropic Core-Shell and Hollow Microgels.

Microgels, cross-linked polymers with submicrometer size, are ideal soft model systems. While spherical microgels have been studied extensively, anisotropic microgels have hardly been investigated. In this study, we compare the interfacial deformation and assembly of anisotropic core-shell and hollow microgels. The core-shell microgel consists of an elliptical core of hematite covered with a thin silica layer and a thin shell made of poly(N-isopropylacrylamide). The hollow microgels were obtained after a two-step etching procedure of the inorganic core. The behavior of these microgels at the oil-water interface was investigated in a Langmuir-Blodgett trough combined with ex situ atomic force microscopy. First, the influence of the architecture of anisotropic microgels on their spreading at the interface was investigated experimentally and by dissipative particle dynamic simulations. Hereby, the importance of the local shell thickness on the lateral and longitudinal interfacial deformation was highlighted as well as the differences between the core-shell and hollow architectures. The shape of the compression isotherms as well as the dimensions, ordering, and orientation of the microgels at the different compressions were analyzed. Due to their anisotropic shape and stiffness, both anisotropic microgels were found to exhibit significant capillary interactions with a preferential side-to-side assembly leading to stable microgel clusters at low interfacial coverage. Such capillary interactions were found to decrease in the case of the more deformable hollow anisotropic microgels. Consequently, anisotropic hollow microgels were found to distribute more evenly at high surface pressure compared to stiffer core-shell microgels. Our findings emphasize the complex interplay between the colloid design, anisotropy, and softness on the interfacial assembly and the opportunities it therefore offers to create more complex ordered interfaces.

Core-shell microgels synthesized in continuous flow: deep insight into shell growth using temperature-dependent FTIR.

While core-shell microgels have been intensively studied in their fully synthesized state, the formation mechanism of the shell has not been completely understood. Such insight is decisive for a customization of microgel properties for applications. In this work, microgels based on a N-isopropylmethacrylamide (NiPMAM) core and a N-n-propylacrylamide (NnPAM) shell are synthesized in a continuous flow reactor. The shell growth is studied depending on the solution's time of residence inside the reactor. PCS experiments reveal a significant decrease of the volume phase transition temperatures of the core and the shell, with increasing residence time. At early stages, a decreased swelling capacity is found before a discrete NnPAM shell is formed. Temperature-dependent FTIR spectroscopy shows that the decreased swelling capacity originates from a pronounced interpenetrated network (IPN) between NnPAM and NiPMAM. AFM images resolve heterogeneously distributed shell material after 3 min, pointing to an aggregation of NnPAM domains before the distinct shell forms. The combination of diffusional properties, AFM images and vibrational information confirms a deeply interpenetrated network already at early stages of the precipitation polymerization, in which the shell material heavily influences the swelling properties.

Polymer microgels for the stabilization of gold nanoparticles and their application in the catalytic reduction of nitroarenes in aqueous media.

Polymer microgels containing a polystyrene core and poly(N-isopropylmethacrylamide) shell were synthesized in aqueous media following a free radical precipitation polymerization. Au nanoparticles were fabricated into the shell region of the core–shell microgels denoted as P(STY@NIPM) by the in situ reduction of chloroauric acid with sodium borohydride. Various characterization techniques such as transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV-visible) and Fourier transform infrared spectroscopy (FTIR) were used for the characterization of Au–P(STY@NIPM). The catalytic potential of Au–P(STY@NIPM) toward the reductive reaction of 4-nitrophenol (4NP) under various reaction conditions was evaluated. The Arrhenius and Eyring parameters for the catalytic reduction of 4NP were determined to explore the process of catalysis. A variety of nitroarenes were converted successfully into their corresponding aminoarenes with good to excellent yields in the presence of the Au–P(STY@NIPM) system using NaBH4 as a reductant. The Au–P(STY@NIPM) system was found to be an efficient and recyclable catalyst with no significant loss in its catalytic efficiency.