Interfacing Langmuir-Blodgett and Pickering Emulsions for the Synthesis of 2D Nanostructured Films: Applications in Copper Ion Adsorption.

This research focuses on developing a 2D thin film comprising a monolayer of silica nanoparticles functionalized with polyethyleneimine (PEI), achieved through a novel integration of Langmuir-Blodgett (L-B) and Pickering emulsion techniques. The primary aim was to create a nanostructured film that exhibits dual functionality: iridescence and efficient metal ion adsorption, specifically Cu(II) ions. The methodology combined L-B and Pickering emulsion polymerization to assemble and stabilize a nanoparticle monolayer at an oil/water interface, which was then polymerized under UV radiation to form an asymmetrically structured film. The results demonstrate that the film possesses a high adsorption efficiency for Cu(II) ions, with the enhanced mechanical durability provided by a reinforcing layer of polyvinyl alcohol/glycerol. The advantage of combining L-B and Pickering emulsion technology is the ability to generate 2D films from functional nanoparticle monolayers that are sufficiently sturdy to be deployed in applications. The 2D film's practical applications in environmental remediation were confirmed through its ability to adsorb and recover Cu(II) ions from aqueous solutions effectively. We thus demonstrate the film's potential as a versatile tool in water treatment applications owing to its combined photonic and adsorptive properties. This work paves the way for future research on the use of nanoengineered films in environmental and possibly photonic applications focusing on enhancing the film's structural robustness and exploring its broader applicability to other pollutants and metal ions.

Reversible Cu-Nanoparticle Formation in Soft Hydrogel Composites: Towards Write-Erase Displays and Fluorescence Detection.

In this study, we introduce a hydrogel-polymer microsphere (HPM) composite material constituted of PVA, glycerin, and polymer microspheres obtained from Pickering emulsions that are capable of adsorbing Cu2+ ions. The obtained HPM composite is soft, flexible, can be fully saturated with Cu2+ ions, and exhibits a reversible color transition from blue to black upon electrode contact or interaction with a reducing agent, due to in situ generation of copper nanoparticles (Cu-NPs). Because of the color contrast between the locally generated Cu-NPs and the background, the HPM can be used as substrate for stamping different shapes or writing text. Further, the surface can be erased by an acidic solution, which makes it interesting as flexible write-erase displays. A second feature of the HPM is that it can function as a fluorescence detector of cyanide ions. An HPM whose surface has been stamped with an electrode, upon contacting an aqueous solution containing cyanide ions, begins fluorescing a yellow-green light around the patterned area. The displayed luminescence is irreversible and is preserved even after HPM's drying or lyophilization. This work lays a foundational framework for future exploration of the HPM composites in various technological applications, for sensing, circuit printing, and flexible displays.

Morphological Design and Synthesis of Nanoparticles.

Nanoparticles are particles with dimensions measured in nanometers, and exist at a scale where the physical, chemical, and biological properties of materials can differ significantly from those at a larger scale [...].

Water-Floating Hydrogel Polymer Microsphere Composites for Application in Hydrological Mining of Cu(II) Ions.

Innovative materials and technologies capable of extraction and recovery of technologically relevant metal ions from various water sources, such as lakes, oceans, ponds, or wastewater reservoirs, are in great demand. Polymer beads are among the most well-known solid-phase adsorbents and ion exchangers employed in metal ion recovery. On the other hand, hydrogels are an emerging platform for producing innovative adsorbents, which are environmentally friendly and biocompatible materials. In this work, we take advantage of both technologies and produce a new type of material by loading nanostructured polymer microsphere adsorbent into a PVA matrix to obtain a hydrogel polymer microsphere (HPM) composite in the form of a block. The main role of the poly(4-vinylpyrridine-co-methacrylic acid) microspheres is to adsorb metal ions, such as Cu(II), from model water samples. The secondary role of these microspheres in the hydrogel is to change the hydrogel morphology by softening it and stabilizing it under a foam-like morphology. The foam-like morphology endows these composites with the capability of floating on water surfaces. In this work, we report, for the first time, an HPM composite capable of floating on water surfaces and extracting Cu(II) ions from model water samples. This could enable more environmentally friendly hydrological mining technologies by simply deploying adsorbents on water surfaces for metal ion extraction and recovery, thus eliminating the need for water pumping and mechanical processing steps.

Asymmetrically Nanostructured 2D Janus Films Obtained from Pickering Emulsions Polymerized in a Langmuir-Blodgett Trough.

Low-dimensional structures, such as two-dimensional (2D) Janus films, can be useful in studying fundamental interactions or in applications at the nanoscale. In this work, we report the fabrication of 2D polymer Janus films consisting of one smooth and another nanostructured facet on which silica nanoparticles (NPs) are self-assembled in a compact monolayer shield. The 2D films are made from Pickering emulsions of monomers in water, stabilized by NPs, which are spread over the surface of the water in a Langmuir-Blodgett trough. Following the spreading of the colloidosomes, oil droplets stabilized by NPs collapse, and the interfaces reorganize such that the NP monolayer is found exclusively at the oil/water interface. Upon compression followed by UV polymerization, a 2D solid film is formed, with one smooth and another nanostructured face. The film can be removed from the surface of the water and handled with tweezers. The 2D films exhibit different surface properties on the two sides, such as differences in water wettability. On the nanostructured side, water wettability can be tuned by tuning the surface energy of the nanoparticles, namely by changing their surface functional groups. Upon removal of NPs, the surface can be patterned with an array of circular traces.

Thermal and Dielectric Investigations of Polystyrene Nanoparticles as a Viable Platform-Toward the Next Generation of Fillers for Nanocomposites.

Nanoparticles are often used as fillers for enhancing various properties of polymer composites such as mechanical, electrical, or dielectric. Among them, polymer nanoparticles are considered ideal contenders because of their compatibility with a polymer matrix. For this reason, it is important that they are synthesized in a surfactant-free form, to obtain predictable surface and structural properties. Here, we synthesized a series of polystyrene nanoparticles (PS NPs), by emulsion polymerization of styrene, using varying amounts of divinylbenzene as a crosslinking agent and sodium 4-vinylbenzenesulfonate as a copolymerizing monomer surfactant-"surfmer". Using "surfmers" we obtained surfactant-free nanoparticles that are monodisperse, with a high degree of thermal stability, as observed by scanning electron microscopy and thermogravimetric investigations. The prepared series of NPs were investigated by means of broadband dielectric spectroscopy and we demonstrate that by fine-tuning their chemical composition, fine changes in their dielectric and thermal properties are obtained. Further, we demonstrate that the physical transformations in the nanoparticles, such as the glass transition, can be predicted by performing the first derivative of dielectric permittivity for all investigated samples. The glass transition temperature of PS NPs appears to be inversely correlated with the dielectric permittivity and the average diameter of NPs.

Monitoring the Surface Energy Change of Nanoparticles in Functionalization Reactions with the NanoTraPPED Method.

Performing chemical functionalization on the surface of nanoparticles underlies their use in applications. Probing that a physicochemical transformation has indeed occurred on a nanoparticles' surface is rather difficult. For this reason, we propose that a macroscopic parameter, namely the surface energy γ, can monitor the physicochemical transformations taking place at the surface of nanoparticles. Determining the surface energy of macroscopic surfaces is trivial, but it is very challenging for nanoparticles. In this work we demonstrate that the Nanoparticles Trapped on Polymerized Pickering Emulsion Droplet (NanoTraPPED) method can be successfully deployed to monitor the evolution of surface energies γ, with its γp polar and γd dispersive components of the silica nanoparticles at each stage of two surface reactions: (i) amination by siloxane chemistry, coupling reaction of a 2,4-dihydroxy benzaldehyde and formation of a Schiff base ligand, followed by coordination of metal ions and (ii) epoxide ring opening and formation of azide. The change in surface energy and its components are discussed and analyzed for each step of the two reactions. It is observed that large variations in surface energy are observed with the complexity of the molecular structure attaching to nanoparticle surface, while functional group replacement leads to only small changes in the surface energies.

Extraction of Metal Ions by Interfacially Active Janus Nanoparticles Supported by Wax Colloidosomes Obtained from Pickering Emulsions.

Most common wastewater treatment technologies for ion extraction and recovery rely on pumping wastewater through ion-exchange columns, filled with surface-functionalized polymer microspheres. To avoid the energetically intensive process of pumping large quantities of water through ion-exchange columns, alternative technologies are being developed, such as water-floating membranes containing ligands. In this context, innovative materials could be deployed. Here, we report nanostructured paraffine wax microspheres capable of floating on water, a design based on Pickering emulsion technology, where Janus nanoparticles act both as emulsion stabilizers and as ligand carriers. In the process of emulsification of molten wax in water, followed by cooling, the branched polyethylenimine (bPEI) carrying Janus nanoparticles are trapped at the molten wax/water interface, forming spherical microspheres or colloidosomes decorated with nanoparticles. The paraffine wax colloidosomes stabilized by ligand-carrying Janus nanoparticles are capable of floating on water and show high metal ion extraction capacities towards Cr(VI), Co(II), Ni(II), Cu(II) and Zn(II). In addition, we demonstrate that the ions can be recovered from the colloidosomes and that the colloidosomes can withstand several extraction/recovery cycles with little or no loss in the absorption capacity.

Flexible Composites with Variable Conductivity and Memory of Deformation Obtained by Polymerization of Polyaniline in PVA Hydrogel.

Flexible materials that provide an electric, magnetic, or optic response upon deformation or tactile pressure could be important for the development of smart monitors, intelligent textiles, or in the development of robotic skins. In this work we demonstrate the capabilities of a flexible and electrically conductive polymer material that produces an electrical response with any deformation, namely the electrical resistance of the material changes proportionally with the deformation pressure. Furthermore, the material exhibits a memory effect. When compressed beyond the elastic regime, it retains the memory of the plastic deformation by increasing its resistance. The material was obtained by in situ polymerization of semiconducting polyaniline (PANi) in a polyvinyl alcohol/glycerol (PVA/Gly) hydrogel matrix at -17 °C. Upon drying of the hydrogel, an elastomer composite is obtained, with rubber-like characteristics. When compressed/decompressed, the electrical resistance of the material exhibits an unusually long equilibration/relaxation time, proportional with the load applied. These phenomena indicate a complex relaxation and reconfiguration process of the PANi/PVA elastomer matrix, with the shape change of the material due to mechanical stress.

Homologous Series of Polyaniline Derivatives Block Copolymers with Amphiphilic and Semiconducting Properties.

Semiconducting polymers with amphiphilic properties can play an increasing role in future organic and unimolecular electronic devices, especially due to their excellent processability and ease of self-assembly into thin films, but they could also be used as intermediate layers to improve electron transport in metal-organic junctions. In this work, we synthesized a homologous series of amphiphiles by copolymerization of aniline with aniline-N-propanesulfonic acid. The polymerization was first initiated with aniline, and the latter monomer was added at different time intervals: 2, 10, 20, 30, 40, and 60 min, spaced from the time of initiation. Thus, the poly(aniline-co-aniline-N-propanesulfonic acid) (PANi-co-PANs) homologous series of copolymers obtained had the same length of the water soluble PANs chain, and a variable length of the water insoluble PANi chain. We demonstrated that there is a strong structure-activity relationship in the homologous series of PANi-co-PANs copolymers, evidenced in the tensiometry and wettability studies, as well as in-depth conductivity with frequency and temperature investigations. We observed a gradual change in solubility, interfacial activity, and conductivity in the homologous series of amphiphiles within the boundaries set by the electrically insulating, hydrophilic PANs chain and the semiconducting, hydrophobic PANi chains; representing a viable platform toward designing polymers with tunable conductivity.

Role of Surface Energy of Nanoparticle Stabilizers in the Synthesis of Microspheres via Pickering Emulsion Polymerization.

Polymer microspheres are important for a variety of applications, such as ion exchange chromatography, catalyst supports, absorbents, etc. Synthesis of large microspheres can be challenging, because they cannot be obtained easily via classic emulsion polymerization, but rather by more complex methods. Here, we present a facile method for obtaining polymer microspheres, beyond 50 µm, via Pickering emulsion polymerization. The method consists in creating oil-in-water (o/w) Pickering emulsion/suspension from vinyl bearing monomers, immiscible with water, whereas silica nanoparticles (NPs), bearing glycidyl functionalities, have a stabilizing role by adsorbing at the monomer/water interface of emulsion droplets. The emulsion is polymerized under UV light, and polymer microspheres decorated with NPs are obtained. We discovered that the contact angle of the NPs with the polymer microsphere is the key parameter for tuning the size and the quality of the obtained microspheres. The contact angle depends on the NPs' interfacial energy and its polar and dispersive contributions, which we determine with a newly developed NanoTraPPED method. By varying the NPs' surface functionality, we demonstrate that when their interfacial energy with water decreases, their energy of adhesion to water increases, causing the curvature of the polymer/water interface to decrease, resulting in increasingly larger polymer microspheres.

NanoTraPPED-A New Method for Determining the Surface Energy of Nanoparticles via Pickering Emulsion Polymerization.

Surface energy with its polar and disperse components describes the physicochemical state of nanoparticles' (NPs) surfaces, and can be a valuable parameter for predicting their bulk behavior in powders. Here, we introduce a new method, namely, Nanoparticles Trapped on Polymerized Pickering Emulsion Droplets (NanoTraPPED), for measuring the surface energy of a series of silica NPs bearing various surface functional groups. The method consists in creating Pickering emulsions from vinyl bearing monomers, immiscible with water, whereas NPs of interest have a stabilizing role, and in the process, become trapped at the monomer/water interface of emulsion droplets. The Pickering emulsion is polymerized, and polymer microspheres (colloidosomes) decorated with NPs are obtained. NanoTraPPED relies on measuring contact angles from the immersion depth of nanoparticles at the interface of various polymer colloidosomes with the electron microscope. The contact angle values are used as input for the Owens-Wendt-Rabel-Kaelble (OWRK) model, to quantitatively determine the total surface energy with water γNP/water, air γNP, and the corresponding polar and dispersive interaction components of NPs carrying -NH2, -SH, -OH, -CN and -C8 surface functional groups, ranking these according to their polarity. Our findings were confirmed independently by calculating the interfacial desorption energies of NPs from contact angles.

Semiconductor-Insulator (Nano-)Couples with Tunable Properties Obtained from Asymmetric Modification of Janus Nanoparticles.

Coupling a semiconductor with an electrical insulator in a single amphiphilic nanoparticle could open new pathways for manufacturing and assembling organic electronic devices. Here, a poly(3,4-ethylenedioxythiophene)/polyaniline (PEDOT/PANI) bilayer is confined on the surface of one lobe of snowman-type Janus nanoparticles (JNPs), such that one lobe is semiconducting and the other is electrically insulating. The PEDOT/PANI bilayer is constructed in two synthesis steps, by asymmetric modification of the JNPs with PANI followed by PEDOT. The addition of the PEDOT layer onto the PANI-modified JNPs leads to an enhancement in the conductivity of up to 2 orders of magnitude. Further, we demonstrate that JNPs are very versatile supports for semiconducting polymers because by tuning their size and geometry the overall conductivity of the JNP powders can be modulated within several orders of magnitude.

A Diversity of Asymmetric Nano-/Microcolloidal Architectures Grown by ATRP from Janus Seeds.

The fabrication of colloids has witnessed significant progress during the last decade, however, fabrication of anisotropic colloidal particles with complex geometries still represents a challenging task. Here, we present nano-/micro-sized colloidal architectures which 'grow' directly from nanoparticle seeds by controlled radical polymerization, resembling the growth of plants from seeds in the natural world. Specifically, we use the atom transfer radical polymerization (ATRP) technique to grow colloidal architectures from snowman-shaped Janus nanoparticle seeds (JNPS). The key to this synthetic approach is the asymmetric placement of the ATRP initiators in the bulk of one JNPS lobe. By starting the polymerization, monomers continuously add to the initiator containing the JNPS lobe, which subsequently grows into a larger colloidal structure. By controlling growth conditions mainly through the interaction strength between the monomer and JNPS, a variety of colloidal architectures result, for example, dish-, basket, cocoon-, flower-, helmet- mushroom-, dumpling and pumpkin-like geometries. Furthermore, each of these grown architectures have different surface morphologies, including smooth-, island- and grouped island nanostructures. The present work provides an alternative method to the synthesis of anisotropic particles with complex geometries and tunable surface morphologies, thus enriching the toolbox for the colloid synthesis.

Evolution of Self-Organized Microcapsules with Variable Conductivities from Self-Assembled Nanoparticles at Interfaces.

Self-organization dramatically affects the surface properties of materials on a macroscopic scale, such as wettability and adhesion. Fundamentally, it is equally interesting when self-organization at the nanoscale affects the bulk properties and thus provides a means to engineer the optoelectronic properties of the materials on larger scales. In this work, we report the evolution of conductive self-organized polymer microcapsules from a monomer emulsion droplet stabilized by a monolayer of conductive Janus nanoparticles (JNPs) via a mechanism resembling morphogenesis. The wall of the resulting conductive microcapsule has a honeycomb-like structure with highly oriented JNPs occupying each hollow cell. The JNPs consist of an electrically conductive lobe and an insulating lobe; because of their orientation and presence in the honeycomb, the conductivity of the microcapsule is greatly enhanced as compared to that of each of the constituting materials. This method can be universally applied to induce self-organization in conductive polymers forming by oxidative addition.

pH-Responsive Pickering Foams Generated by Surfactant-Free Soft Hydrogel Particles.

Pickering foams are foams stabilized by particles and are generally known to have good stability. A special subclass of particle-stabilized foams includes stimuli-responsive Pickering foams that can be formed or deconstructed by applying an external stimuli or changing the environmental conditions; such intelligent particles could find use in many practical applications. Here, we synthesized surfactant-free biocompatible poly[2(diethylamino)ethyl methacrylate] (PDEAEMA) hydrogel particles (HGPs) by emulsion polymerization. The morphology, structure, and surface charge of the HGPs were characterized by TEM, DLS, and the zeta potential, respectively. We have observed that the pH values of the aqueous solution have a strong influence on the formation of the Pickering foams in the presence of PDEAEMA HGPs. Namely, at pH values ≤4.0 no Pickering foams were produced, while at pH values >4.0 stable Pickering foams were formed. Moreover, the height, size and bubble size distribution of Pickering foams are strongly influenced by the pH values of aqueous solution and PDEAEMA HGPs concentration. The formed Pickering foams in basic aqueous solution can all be conveniently deconstructed by changing the pH values to below 4.0. Interestingly, the dried lamellas of the Pickering foams were constituted by either monolayers or multilayers of PDEAEMA HGPs as demonstrated by SEM.

Contrasting Mechanisms of Spontaneous Adsorption at Liquid-Liquid Interfaces of Nanoparticles Constituted of and Grafted with pH-Responsive Polymers.

Elucidating the mechanisms responsible for spontaneous adsorption of nanoparticles (NPs) at interfaces is important for their application as emulsifiers, bubble stabilizers, or foaming agents. In order to investigate the key factors that control the spontaneous adsorption of NPs at liquid-liquid interfaces, we synthesized seven different types of NPs from pH-responsive polymers poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) and poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) via surfactant-free emulsion polymerization or via "grafting from" polystyrene (PS) NPs. The dynamic interfacial tension (IFT) measurements at the toluene-water (Tol-H2O) interface reveal that when PDEAEMA and PDMAEMA are grafted from the surface of PS NPs the solubility of the grafted pH-responsive polymers in toluene is the key factor determining the NPs' interfacial adsorption. Under acidic conditions (pH < 6.0), PDEAEMA and PDMAEMA are protonated and show no solubility in toluene, and as a result, the grafted NPs do not adsorb at the Tol-H2O interface. Oppositely, under basic conditions (pH > 7.0), PDMAEMA dissolves in toluene and therefore the PDMAEMA-grafted NPs can adsorb at the Tol-H2O interface. Interestingly, when NPs are constituted of PDEAEMA, they can adsorb spontaneously at the Tol-H2O interface under acidic conditions (pH < 6.0) but not under basic conditions (pH > 7.0). In this case, the key factor determining the NPs' spontaneous adsorption at the Tol-H2O interface is the degree of softness of the NPs rather than the solubility of PDEAEMA in toluene. Furthermore, we found that the adsorption of NPs constituted of PDEAEMA- (pH 2.0-6.0) and PDMAEMA-grafted PS NPs (pH 7.0-10.0) at the Tol-H2O interface is a combination of diffusion-controlled and energy-barrier-controlled. The opposite trends observed for the interfacial attachment Δ E and activation energies Ea for the "constituted of" and "grafted from" NPs with pH suggest an opposite mechanisms of adsorption at the Tol-H2O interface. Finally, the synthesized NPs prove to be effective emulsifiers, where the phase of the Pickering emulsions can be changed dynamically by pH adjustment.

Influence of Geometries on the Assembly of Snowman-Shaped Janus Nanoparticles.

The self-assembly of micro/nanoparticles into suprastructures is a promising way to develop reconfigurable materials and to gain insights into the fundamental question of how matter organizes itself. The geometry of particles, especially those deviating from perfectly spherical shapes, is of significant importance in colloidal assembly because it influences the particle "recognition", determines the particle packing, and ultimately dictates the formation of assembled suprastructures. In order to organize particles into desired structures, it is of vital importance to understand the relationship between the shape of the colloidal building blocks and the assembled suprastructures. This fundamental issue is an enduring topic in the assembly of molecular surfactants, but it remained elusive in colloidal assembly. To address this issue, we use snowman-shaped Janus nanoparticles (JNPs) as a model to systematically study the effect of colloidal geometries on their assembled suprastructures. Ten types of JNPs with identical chemical compositions but with different geometries were synthesized. Specifically, the synthesized JNPs differ in their lobe size ratios, phase separation degrees, and overall sizes. We show that by altering these parameters, both finite suprastructures, such as capsules with different curvatures, and nonfinite suprastructures, including free-standing single-layered or double-layered JNPs sheets, can be obtained via self-assembly. All these different types of suprastructures are constituted by highly oriented and hexagonally packed JNPs. These findings demonstrate the significance of geometries in colloidal assembly, such that slightly changing the building block geometries could result in a large variety of very different assembled structures, without altering the chemistry of the particles.

Self-Assembly of Janus Nanoparticles into Transformable Suprastructures.

One of the greatest challenges in colloidal self-assembly is to obtain multiple distinct but transformable suprastructures from the same particles in monophasic solvent. Here, we combined deformable and rigid lobes in snowman-shaped amphiphilic Janus nanoparticles (JNPs). These JNPs exhibited excellent ability to self-assemble into micelles, worms, mini-capsules, giant- and elongated-vesicles. This rich suprastructural diversity was obtained by kinetic manipulation of the self-assembly conditions. The suprastructures consist of four to thousands of highly oriented JNPs with dimensions ranging from 500-nanometer to 30-µm. Moreover, the suprastructures can be transformed into one another or dissembled into individual particles. These features make colloidal assembly highly comparable to that of amphiphilic molecules, however, key differences were discovered.

Modeling the Interfacial Energy of Surfactant-Free Amphiphilic Janus Nanoparticles from Phase Inversion in Pickering Emulsions.

Determining the interfacial energy of nanoparticles is very challenging via traditional methods that first require measuring the contact angle of several liquids of a sessile drop on pellets or capillary rise in powder beds. In this work, we propose an alternative way to model the interfacial energy of nanoparticles directly from emulsion phase inversion data in Pickering emulsions. This could establish itself as a universal and facile way to determine the polarity of nanoparticles relative to a series of standard particles without the need to measure contact angles. Pickering emulsions of several oils in water were generated with a series of snowman-like Janus nanoparticles (JNPs), whose polarity gradually increased with the size of the more polar lobe. Depending on the oil to water ratio and the JNPs lobe size, oil-in-water (o/w) or water-in-oil (w/o) Pickering emulsions were obtained and the affinity of the JNPs to either water or oil can be inferred from the evolution of the emulsion phase inversion curves with these parameters. We further demonstrate that by adopting a simple model for the work of adhesion of JNPs with the water and oil phases, one can quantitatively calculate the relative interfacial energy change of the JNPs with the liquid. In addition, a knowledge of the interfacial energy of nanoparticles is useful for employing these in suspension polymerization to create surface nanostructured materials. The o/w and w/o Pickering emulsions obtained from monomers, such as styrene, could be polymerized, resulting in colloidosomes or hollow-like materials. The hollow materials exhibited a rather high volume storage capacity for the aqueous phase for extended periods of time, which could be released upon microwaving, making them ideal for use in long-term storage applications of various water-soluble actives.