Gel Trapping Technique Research Articles

Emission Properties of Individual AIE‐Luminogens in Solution at Room Temperature†

Comprehensive SummaryHere, we combined the photon antibunching analysis, fluorescence correlation spectroscopy, and time‐domain fluorescence lifetime imaging microscopy (TD‐FLIM) to study the emission properties of a representative AIE‐luminogen—4,4’‐(benzo[c][1,2,5] thiadiazole‐4,7‐diyl)bis(N,N‐diphenylaniline) (TPA‐BT) at the single emitter level in a tetrahydrofuran (THF)/water solution where water is a non‐solvent for TPA‐BT. Our findings suggest that, at a constant water fraction in the solution, the size of TPA‐BT aggregates increases with the TPA‐BT concentration; TPA‐BT aggregates are not a quantum emitter at room temperature in the solution. Moreover, utilizing TD‐FLIM and a gel trapping technique allowed us to study the fluorescence lifetime of individual TPA‐BT aggregates. Adding a polar solvent like water does not result in an overall decrease in fluorescence lifetime. Rather, it causes the fluorescence lifetime distribution to become wider, and only some molecules experience a decrease in their fluorescence lifetime. These results could represent a step forward in further understanding the photophysics of AIE‐luminogens.

Pickering Emulsions Stabilized by Polystyrene Particles Possessing Different Surface Groups.

Colloidal polystyrene (PS) latex particles in water can undergo interesting charge reversal in the presence of particular electrolytes. It is worth exploring the effect of charge reversal on the properties of Pickering emulsions they stabilize. Herein, emulsions stabilized by PS latex particles possessing different surface groups (sulfate, amidine, or carboxyl) were prepared in the presence of tetrapentylammonium bromide (TPeAB) or sodium thiocyanate (NaSCN) electrolytes. The effect of salt concentration on the charge of the particles and their colloid stability was measured. Emulsions were prepared from aqueous dispersions, and their type and stability were determined. The three-phase contact angle of particles at the planar oil–water interface was also measured using a gel trapping technique. It was found that the type of emulsion stabilized by latex particles is dominated by the hydrophobic PS portion on particle surfaces, although their surface charge is strongly affected by electrolyte addition. Preferred emulsions were always water-in-oil with dodecane, and charge reversal had little influence on the emulsion type and stability. However, transitional phase inversion of emulsions stabilized by carboxyl latex particles occurred on adding salt when the oil was a low-viscosity polydimethylsiloxane.

Gel Trapping Enables Optical Spectroscopy of Single Solvated Conjugated Polymers in Equilibrium.

Single-molecule studies have provided a wealth of insight into the photophysics of conjugated polymers in the solid and desolvated state. Desolvating conjugated chains, e.g., by their embedding in inert solid matrices, invariably leads to chain collapse and the formation of intermolecular aggregates, which have a pronounced effect on their properties. By contrast, the luminescent properties of individual semiconducting polymers in their solvated and thermodynamic state remain largely unexplored. In this paper, we demonstrate a versatile gel trapping technique that enables the chemistry-free immobilization and interrogation of individual conjugated macromolecules, which retain a fully equilibrated conformation by contrast to conventional solid-state immobilization methods. We show how the technique can be used to record full luminescence spectra of single chains, to evaluate their time-resolved fluorescence, and to probe their photodynamics. Finally, we explore how the photophysics of different conjugated polymers is strongly affected by desolvation and chain collapse.

Gastrointestinal digestion and stability of submicron-sized emulsions stabilized using waxy maize starch crystals

We prepared Pickering emulsions using tricaprylin (TC) and waxy maize starch nanocrystals (SNCs). The SNCs were obtained using an acid hydrolysis method and subsequently modified with octenyl succinic anhydride (OSA), to stabilize the water/TC interface. The particle size (PS) of fabricated OSA-SNCs was ∼42 nm, and contact angle of OSA-SNCs at the interface was determined as ∼90° using a gel trapping technique. Only 0.15 wt% OSA-SNCs in the continuous phase of the emulsion effectively saturated the surface of the TC droplets (5 wt%) to achieve a stable Pickering emulsions (PS, ∼0.5 μm). Theoretically, a TC droplet was covered with ∼450 OSA-SNCs at the saturation point. The excess amount (≥0.15 wt%) of OSA-SNCs used to prepare the Pickering emulsions did not affect the digestion of the emulsions during the in vitro study in simulated gastrointestinal fluids and the stability during the 15-days storage at various conditions (pH: 4, 7, and 11; temperature: 4, 25, and 37 °C). Based on the storage results, the Pickering emulsions stabilized with OSA-SNCs were good to be stored in neutral/basic conditions under refrigeration. This study may serve as a basis for further research that aims for development of foods, cosmetics, and drugs to incorporate lipophilic bioactives.

Novel aqueous spongy foams made of three-dimensionally dispersed wood-fiber: entrapment and stabilization with NFC/MFC within capillary foams

This article describes the preparation of novel aqueous spongy foams that are composed of three-dimensionally distributed wood-fiber networks stabilized with nanofibrillate cellulose (NFC) and/or microfibrillated cellulose (MFC). The free standing aqueous spongy foams were prepared with the entrapment of NFC and/or MFC—stabilized air-in-water (A/W) capillary foams using “gel trapping technique”. The stability of spongy foams could be controlled by manipulating the volume fraction of NFC and/or MFC and a secondary liquid immiscible with the continuous phase of the NFC and/or MFC suspension. Possible morphology and mechanical distribution of NFC and/or MFC within spongy foams were verified with optical microscope, SEM, and functional load-bearing method. Owing to three-dimensionally dispersed wood-fiber structure, ultra-lightweight (0.01–0.06 g/cm3), high porosity (>90%), and microporous (10–80 μm), the NFC and/or MFC reinforced spongy foams, improved compressional strength-vertical direction obviously, from 0.0 to more than 13.78 kPa.

Contact angle and detachment energy of shape anisotropic particles at fluid-fluid interfaces

The three phase contact angle of particles, a measure of its wettability, is an important factor that greatly influences their behaviour at interfaces. It is one of the principal design parameters for potential applications of particles as emulsion/foam stabilizers, functional coatings and other novel materials. In the present work, the effect of size, shape and surface chemistry of particles on their contact angle is investigated using the gel trapping technique, which facilitates the direct visualization of the equilibrium position of particles at interfaces. The contact angle of hematite particles of spherocylindrical, peanut and cuboidal shapes, hematite-silica core-shell and silica shells is reported at a single particle level. The spherocylindrical and peanut shaped particles are always positioned with their major axis parallel to the interface. However, for cuboidal particles at air-water as well as decane-water interfaces, different orientations namely - face-up, edge-up and the vertex-up - are observed. The influence of gravity on the equilibrium position of the colloidal particles at the interface is studied using the hematite-silica core-shell particles and the silica shells. The measured contact angle values are utilized in the calculations of the detachment and surface energies of the hematite particles adsorbed at the interface.

Particle detachment from fluid interfaces: theory vs. experiments.

Microparticle adsorption and self-assembly at fluid interfaces are strongly affected by the particle three-phase contact angle θ. On the single-particle level, θ can be determined by several techniques, including colloidal-probe AFM, the gel-trapping technique (GTT) and the freeze-fracture shadow-casting (FreSCa) method. While GTT and FreSCa provide contact angle distributions measured over many particles, colloidal-probe AFM measures the wettability of an individual (specified) particle attached onto an AFM cantilever. In this paper, we extract θ for smooth microparticles through the analysis of force-distance curves upon particle approach and retraction from the fluid interface. From each retraction curve, we determine: (i) the maximal force, Fmax; (ii) the detachment distance, Dmax; and (iii) the work for quasistatic detachment, W. To relate Fmax, Dmax and W to θ, we developed a detailed theoretical model based on the capillary theory of flotation. The model was validated in three different ways. First, the contact angles, evaluated from Fmax, Dmax and W, are all close in value and were used to calculate the entire force-distance curves upon particle retraction without any adjustable parameters. Second, the model was successfully applied to predict the experimental force-distance curve of a truncated sphere, whose cut is positioned below the point of particle detachment from the interface. Third, our theory was confirmed by the excellent agreement between the particle contact angles obtained from the colloidal-probe AFM data and the ensemble-average contact angles measured by both GTT and FreSCa. Additionally, we devised a very accurate closed-form expression for W (representing the energy barrier for particle detachment), thus extending previous results in the literature.

Self-assembled polyoxometalates nanoparticles as pickering emulsion stabilizers.

We easily produced a series of polyoxometalate (POM) nanoparticles by taking benefit from electrostatic attractions between various POMs and alkylammonium cations. These self-assembled supramolecular nanoparticles are fully characterized in terms of shape, nanostructure, and physicochemical properties. The nanoparticle differences are discussed on the basis of the chemical composition of the initial POM. Moreover, such particles have the ability to stabilize water-in-oil Pickering emulsions. Using a gel-trapping technique coupled to atomic force microscopy (AFM) observations, we determined their affinity toward oil by the contact angle of adsorbed nanoparticles. We show that the emulsion droplet size and stability can be directly linked to the nanoparticle hydrophobicity, which is tuned by the charge localization and molecular packing of POMs with the ammonium cations. Such particles are of special interest as they open large possibilities for Pickering interfacial catalysis.

Visualization of the equilibrium position of colloidal particles at fluid-water interfaces by deposition of nanoparticles.

We present a general yet simple method to measure the contact angle of colloidal particles at fluid-water interfaces. In this method, the particles are spread at the required fluid-water interface as a monolayer. In the water phase a chemical reaction involving reduction of a metal salt such as aurochloric acid is initiated. The metal grows as a thin film or islands of nanoparticles on the particle surface exposed to the water side of the interface. Analyzing the images of particles by high resolution scanning microscopy (HRSEM), we trace the three phase contact line up to which deposition of the metal film occurs. From geometrical relations, the three phase contact angle is then calculated. We report the measurements of the contact angle of silica and polystyrene (PS) particles at different interfaces such as air-water, decane-water and octanol-water. We have also applied this method to measure the contact angle of surfactant treated polystyrene particles at the air-water interface, and we find a non-monotonic change of the contact angle with the concentration of the surfactant. Our results are compared with the well-known gel trapping technique and we find good comparison with previous measurements.

Fabrication of transparent SERS platform via interface self-assembly of gold nanorods and gel trapping technique for on-site real time detection.

A transparent SERS platform was fabricated via the gel-trapping method coupled with a liquid/liquid interface self-assembly technique. We employed gold nanorods as the building blocks for interface self-assembly because of their strong localized surface plasmons upon excitation by infrared radiation. Based on a "top cover" configuration, this transparent SERS platform endows high signal reproducibility for directly detecting liquid samples by confining the sample droplet into a regular shape. The Au NR PDMS platform was able to directly detect crystal violet in aqueous solutions down to 10 ppb level with high enhancement factor (0.87 × 10(5)) and signal uniformity (RSD = 3.9%). Furthermore, SERS-based anti-fungal agent detection on a fish scale was demonstrated by simply covering the fish scale with a tailored GNRs PDMS film. The experimental results clearly show that the Au NR PDMS SERS platform has great potential for on-site real time detection of contaminants in water as well as on curved surfaces.

Pickering emulsions: what are the main parameters determining the emulsion type and interfacial properties?

We synthesized surface-active lipophilic core-hydrophilic shell latex particles, and we probed their efficiency as emulsion stabilizers. The relative weight percentage of the shell, RS/P, was varied to trigger the balance between lipophilicity and hydrophilicity of the particles. Particle wettability could concomitantly be tuned by the pH of the aqueous phase determining the surface charge. Emulsions covering a wide range of RS/P and pH values were fabricated, and their type, oil-in-water (O/W) or water-in-oil (W/O), and kinetic stability were systematically assessed. By adapting the particle gel trapping technique to pH-variable systems and by exploiting the limited coalescence process, we were able to determine the proportion of oil/water interfacial area, C, covered by the particles as well as their contact angle, θ. All of these data were gathered into a single generic diagram showing good correlation between the emulsion type and the particle contact angle (O/W for θ < 90° and W/O for θ > 90°) in agreement with the empirical Finkle rule. Interestingly, no stable emulsion could be obtained when the wettability was nearly balanced and a "bipolar"-like behavior was observed, with the particles adopting two different contact angles whose average value was close to 90°. For particles such that θ < 90°, O/W emulsions were obtained, and, depending on the pH of the continuous phase, the same type of particles and the same emulsification process led to emulsions characterized either by large drops densely covered by the particles or by small droplets that were weakly covered. The two metastable states were also accessible to emulsions stabilized by particles of variable origins and morphologies, thus proving the generality of our findings.

Adsorption of shape-anisotropic and porous particles at the air–water and the decane–water interface studied by the gel trapping technique

We have studied the attachment and orientation of anisotropic and porous microparticles at liquid surfaces by using the gel trapping technique (GTT). This technique involves spreading of the microparticles of interest at the liquid interface, subsequent setting of the aqueous phase to a hydrogel thus “arresting” the particle positions at the liquid surface, and further replication of the hydrogel surface with curable polydymethilsiloxane (PDMS). The advantage of the GTT comes from the possibility to look at the PDMS replica with scanning electron microscopy (SEM) or atomic force microscopy (AFM), which allows even sub-micrometer particles to be studied at the air–water and the oil–water interface. Here we report our results on the adsorption of non-spherical anisotropic particles at liquid surfaces using the GTT. Although the GTT was originally designed to measure three-phase contact angles of spherical colloid particles, here we used this technique to reveal the orientation of a variety of shape-anisotropic and porous microparticles of practical interest at both the air–water and decane–water interfaces. We show results on typical attachment and orientation of needle-like (aragonite), rhombohedra-like (calcite) microcrystals, ethyl cellulose micro-rods, as well as highly porous hydrophilic and hydrophobic silica microparticles at these liquid interfaces. The results are important for understanding the adsorption behaviour of shape-anisotropic particles as well as porous microparticles which are used in industrial formulations as fillers, foam stabilisers and emulsifiers.

Adsorption of carboxylic modified latex particles at liquid interfaces studied by the gel trapping technique.

We have studied how carboxylic modified latex (CML) microparticles adsorb at liquid surfaces and the preferred type of emulsion they can stabilise depending on the particle size and the surface density of carboxylic groups. We measured the particle contact angle by using the gel trapping technique (GTT) for CML particles adsorbed at air-water and oil-water interfaces. Using this method we obtained scanning electron microscopy (SEM) micrographs of polydimethylsiloxane (PDMS) replicas of the liquid interface with the particles, where the PDMS replicates the non-polar phase and measured the particle contact angle. We discovered that the particle wettability correlates well with the surface density of the carboxylic groups but is not very sensitive to the presence of electrolyte in the aqueous phase and the value of the particle zeta potential. We demonstrated that CML microparticles with a high surface density of COOH groups stabilise oil-in-water (O/W) emulsions while those with the lowest coverage of COOH groups favour the formation of water-in-oil (W/O) emulsions. We found that this corresponds to a change of the CML particle contact angle from lower than 90° to higher than 90° upon decrease of the surface density of COOH groups. The findings confirm that the surface density of polar groups has a much bigger effect on the particle wettability and the preferred emulsion than the particle surface charge and zeta potential. Our results on the type of stabilised Pickering emulsion agree with other experimental studies with different particle materials. We propose an alternative explanation for the link between the particle contact angle and the type of stabilised Pickering emulsion.

Understanding the multiple orientations of isolated superellipsoidal hematite particles at the oil–water interface

Non-spherical particles have the potential to adopt multiple orientations once adhered to a liquid–liquid interface. In this work we combine simulations and experiments to investigate the behaviour of an isolated microscopic hematite particle of superellipsoidal shape. We show that this microparticle can adopt one of three orientations when adhered to a hexadecane–water interface. Two of the orientations, and estimates for their relative populations, could be assigned to two thermodynamic minima on the energy landscape as generated through both free-energy minimization and particle trajectory simulations. The third orientation was found to correspond to a kinetically trapped state, existing on certain particle trajectories in a region of a negligible gradient in free energy. To underpin the simulations the individual orientation of a set of 100 isolated particles was explored by means of scanning electron microscopy (SEM) using the gel trapping technique as a tool. Atomic force microscopy (AFM) was additionally used to support the experimental findings. This is the first example of such a kinetic metastable state being observed for particles at liquid–liquid interfaces.

Adsorption of Sterically Stabilized Latex Particles at Liquid Surfaces: Effects of Steric Stabilizer Surface Coverage, Particle Size, and Chain Length on Particle Wettability

A series of five near-monodisperse sterically stabilized polystyrene (PS) latexes were synthesized using three well-defined poly(glycerol monomethacrylate) (PGMA) macromonomers with mean degrees of polymerization (DP) of 30, 50, or 70. The surface coverage and grafting density of the PGMA chains on the particle surface were determined using XPS and (1)H NMR spectroscopy, respectively. The wettability of individual latex particles adsorbed at the air-water and n-dodecane-water interfaces was studied using both the gel trapping technique and the film calliper method. The particle equilibrium contact angle at both interfaces is relatively insensitive to the mean DP of the PGMA stabilizer chains. For a fixed stabilizer DP of 30, particle contact angles were only weakly dependent on the particle size. The results are consistent with a model of compact hydrated layers of PGMA stabilizer chains at the particle surface over a wide range of grafting densities. Our approach could be utilized for studying the adsorption behavior of a broader range of sterically stabilized inorganic and polymeric particles of practical importance.

Influence of particle surface roughness on the behaviour of Janus particles at interfaces

We demonstrate that it is of critical importance to prepare Janus particles with a high degree of control over the surface roughness. To elucidate information regarding the behaviour of Janus particles at an interface, we have used a gel-trapping technique coupled with metal evaporation to prepare Janus particles. Since one face is covered in gold, direct observation of the different faces at air–water and water–oil interfaces can be carried out. For particles prepared by this gel-trapping technique, the interface between hemispheres is rough and this leads to the observed orientation being random. We show that for these particles the surface roughness can dominate over the surface chemistry with regards to particle orientation. This has important implications for many of the synthetic routes towards Janus particles suggesting that, for optimum performance, it is necessary to prepare particles where the surface roughness is kept to a minimum.

Emulsions stabilised by food colloid particles: Role of particle adsorption and wettability at the liquid interface

We study the effect of the particle wettability on the preferred type of emulsion stabilised solely by food colloid particles. We present results obtained with the recently developed gel trapping technique (GTT) for characterisation of wettability and surface structuring of individual food colloid particles adsorbed at air–water and oil–water interfaces. This method allows us to replicate a particle monolayer onto the surface of polydimethylsiloxane (PDMS) without altering the position of the particles. By observing the polymer surface with scanning electron microscopy (SEM), we are able to determine the contact angle of the individual particles at the initial liquid interface. We demonstrate that the GTT can be applied to fat crystal particles, calcium carbonate particles coated with stearic acid and spray-dried soy protein/calcium phosphate particles at air–water and oil–water interfaces. Subsequently, we prepare emulsions of decane and water stabilised by the same food colloid particles and correlate the wettability data obtained for these particles to the preferred type of emulsions they stabilise.

Contact Angles of Colloid Silica and Gold Particles at Air−Water and Oil−Water Interfaces Determined with the Gel Trapping Technique

We have used the recently developed gel trapping technique (GTT) to determine the three-phase contact angles of submicrometer silica particles partially coated with octadecyl groups. The particles were spread at air-water and decane-water surfaces, and the aqueous phase was subsequently gelled with a nonadsorbing polysaccharide. The particles trapped at the surface of the aqueous gel were lifted by molding with curable poly(dimethylsiloxane) and imaged with scanning electron microscopy (SEM) to determine the particle contact line diameter which allows their contact angle at the original air-water or oil-water interface to be estimated. We report for the first time the use of the GTT for characterizing the contact angle of individual submicrometer particles adsorbed at liquid interfaces. The SEM images also reveal the structure of the particle monolayer at the interface and the structure of adsorbed particle aggregates. We have also determined the contact angles of agglomerated gold powder microparticles at the air-water and the decane-water interfaces. It was found that agglomerated gold particles demonstrate considerably higher contact angles than those on flat gold-coated surfaces.

Supraparticles and “Janus” Particles Fabricated by Replication of Particle Monolayers at Liquid Surfaces Using a Gel Trapping Technique

Fabrication of composite supraparticles and “Janus” particles via a novel, generic, gel trapping technique is presented. The method is based on templating particle monolayers at air–water or oil–water interfaces, followed by lifting of the particles with poly(dimethylsiloxane) and gold deposition. Replicating ordered colloid monolayers of repulsive particles at liquid surfaces allows fabrication of microporous surfaces and composite supraparticles (see Figure).

Fabrication of microlens arrays by gel trapping of self-assembled particle monolayers at the decane–water interface

A new technique for producing ordered microlens arrays has been developed which is based entirely on self-assembly of charged latex particles spread at an oil–water interface. It includes using a gel trapping technique and replication of the ordered particle monolayers by casting with PDMS. Microlens arrays were fabricated by taking an inverse replica of the PDMS template with a photopolymer.