Eco-Friendly Fungal Chitosan-Silica Dual-Shell Microcapsules with Tailored Mechanical and Barrier Properties for Potential Consumer Product Applications.

Commercial perfume microcapsules are becoming popular across the globe to fulfill consumers' demands. However, most of microcapsules rely on synthetic polymers and/or animal-sourced ingredients to form the shells. Therefore, replacement of the shell materials is imperative to minimize environmental microplastic pollution, as well as to meeting peoples' needs, religious beliefs, and lifestyles. Herein, we report a methodology to fabricate environmentally benign dual-shell (fungal chitosan-SiO2) microcapsules laden with fragrance oil (hexyl salicylate). Anionically stabilized oil droplets were coated with fungal chitosan via interfacial electrostatic interactions at pH 2, which were then covered by an inorganic coating of SiO2 produced via external alkaline mineralization of sodium silicate. Core-shell microcapsules with a spherical morphology were achieved. Under compression, dual-shell chitosan-SiO2 microcapsules yielded a mean nominal rupture stress of 3.0 ± 0.2 MPa, which was significantly higher than that of single-shell microcapsules (1.7 ± 0.2 MPa). After 20 days in neutral pH water, only ∼2.5% of the oil was released from dual-shell microcapsules, while single-shell microcapsules cumulatively released more than 10%. These findings showed that the additional SiO2 coating significantly enhanced both mechanical and barrier properties of microcapsules, which may be appealing for multiple commercial applications, including cosmetics and detergents.

Effective delivery and selective insecticidal activity of double-stranded RNA via complexation with diblock copolymer varies with polymer block composition.

BACKGROUND: Chemical insecticides are an important tool to control damaging pest infestations. However, lack of species specificity, the rise of resistance and the demand for biological alternatives with improved ecotoxicity profiles means that chemicals with new modes of action are required. RNA interference (RNAi)-based strategies using double-stranded RNA (dsRNA) as a species-specific bio-insecticide offer an exquisite solution that addresses these issues. Many species, such as the fruit pest Drosophila suzukii, do not exhibit RNAi when dsRNA is orally administered due to degradation by gut nucleases and slow cellular uptake pathways. Thus, delivery vehicles that protect and deliver dsRNA are highly desirable. RESULTS: In this work, we demonstrate the complexation of D. suzukii-specific dsRNA for degradation of vha26 mRNA with bespoke diblock copolymers. We study the ex vivo protection of dsRNA against enzymatic degradation by gut enzymes, which demonstrates the efficiency of this system. Flow cytometry then investigates the cellular uptake of Cy3-labelled dsRNA, showing a 10-fold increase in the mean fluorescence intensity of cells treated with polyplexes. The polymer/dsRNA polyplexes induced a significant 87% decrease in the odds of survival of D. suzukii larvae following oral feeding only when formed with a diblock copolymer containing a long neutral block length (1:2 cationic block/neutral block). However, there was no toxicity when fed to the closely related Drosophila melanogaster. CONCLUSION: We provide evidence that dsRNA complexation with diblock copolymers is a promising strategy for RNAi-based species-specific pest control, but optimisation of polymer composition is essential for RNAi success. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Controlling adsorption density of polymer-stabilised metal nanoparticles at the oil-water interface.

Emulsion droplets offer an alternative to solid supports as templates for the deposition of metallic nanoparticles. An emulsion interface provides the opportunity to exploit both sides of the nanoparticles and to utilise the liquid core as a microreactor in addition to forming a scaffold for encapsulation. However, despite the extensive literature studying a very broad range of factors influencing the characteristics of particle-stabilised (Pickering) emulsions, most reports focus on particles of diameters >100 nm and a very small proportions consider particles of diameters <10 nm. For catalytic purposes of course, the latter species are of utmost interest. Here, we report the synthesis of poly(vinyl pyrrolidone) (PVP) stabilised platinum nanoparticles, where the platinum core ranges between 3 and 5 nm in diameter and their subsequent use as emulsifiers for the oil-water interface where they form a densely packed layer. The nanoparticle density at the interface is quantified by both measuring the remaining concentration of nanoparticles in the aqueous phase after adsorption and also directly at the oil-water interface via cryo-TEM. The effect of electrolyte concentration and of addition of excess PVP in the bulk aqueous nanoparticle dispersion prior to emulsification on the resulting nanoparticle density at the oil-water interface is also determined.

Pectin-based microgels for rheological modification in the dilute to concentrated regimes.

HYPOTHESIS: A novel range of microgel particles of different internal cross-linking densities can be created by covalently cross-linking sugar beet pectin (SBP) with the enzyme laccase and mechanically breaking down the subsequent parent hydrogels to sugar beet pectin microgels (SBPMG) via shearing. The bulk rheological properties of suspensions of the different SBPMG are expected to depend on the microgel morphology, elasticity (crosslinking density) and volume fraction respectively. EXPERIMENTS: The rheology of both dilute and concentrated dispersions of SBPMG were studied in detail via capillary viscometry and shear rheometry, supplemented by information on particle size and shape from static light scattering, confocal microscopy and electron microscopy. FINDINGS: For dilute suspensions of SBPMG, data for viscosity versus effective volume fraction (ɸeff) falls on a 'master' curve for all 3 types of SBPMG. In the more concentrated regime, the softer microgels allow greater packing and interpenetration and give lower viscosities at the same ɸeff, but all 3 types of microgel give much higher viscosities than the equivalent concentration of 'non-microgelled' pectin. The firmer microgels can be concentrated to achieve elasticities equivalent to the original parent hydrogel. All SBPMG suspensions were extremely shear thinning but showed virtually no time-dependence.

Protection of Double-Stranded RNA via Complexation with Double Hydrophilic Block Copolymers: Influence of Neutral Block Length in Biologically Relevant Environments.

Interaction between the anionic phosphodiester backbone of DNA/RNA and polycations can be exploited as a means of delivering genetic material for therapeutic and agrochemical applications. In this work, quaternized poly(2-(dimethylamino)ethyl methacrylate)-block-poly(N,N-dimethylacrylamide) (PQDMAEMA-b-PDMAm) double hydrophilic block copolymers (DHBCs) were synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization as nonviral delivery vehicles for double-stranded RNA. The assembly of DHBCs and dsRNA forms distinct polyplexes that were thoroughly characterized to establish a relationship between the length of the uncharged poly(N,N-dimethylacrylamide) (PDMA) block and the polyplex size, complexation efficiency, and colloidal stability. Dynamic light scattering reveals the formation of smaller polyplexes with increasing PDMA lengths, while gel electrophoresis confirms that these polyplexes require higher N/P ratio for full complexation. DHBC polyplexes exhibit enhanced stability in low ionic strength environments in comparison to homopolymer-based polyplexes. In vitro enzymatic degradation assays demonstrate that both homopolymer and DHBC polymers efficiently protect dsRNA from degradation by RNase A enzyme.

Incorporation of nanogels within calcite single crystals for the storage, protection and controlled release of active compounds.

Nanocarriers have tremendous potential for the encapsulation, storage and delivery of active compounds. However, current formulations often employ open structures that achieve efficient loading of active agents, but that suffer undesired leakage and instability of the payloads over time. Here, a straightforward strategy that overcomes these issues is presented, in which protein nanogels are encapsulated within single crystals of calcite (CaCO3). Demonstrating our approach with bovine serum albumin (BSA) nanogels loaded with (bio)active compounds, including doxorubicin (a chemotherapeutic drug) and lysozyme (an antibacterial enzyme), we show that these nanogels can be occluded within calcite host crystals at levels of up to 45 vol%. Encapsulated within the dense mineral, the active compounds are stable against harsh conditions such as high temperature and pH, and controlled release can be triggered by a simple reduction of the pH. Comparisons with analogous systems - amorphous calcium carbonate, mesoporous vaterite (CaCO3) polycrystals, and calcite crystals containing polymer vesicles - demonstrate the superior encapsulation performance of the nanogel/calcite system. This opens the door to encapsulating a broad range of existing nanocarrier systems within single crystal hosts for the efficient storage, transport and controlled release of various active guest species.

Aqueous Phase Behavior of a NaLAS-Polycarboxylate Polymer System.

Linear alkylbenzene sulfonate (NaLAS) surfactant is often combined with polycarboxylate polymers in detergent formulations. However, the behavior of these aqueous surfactant-polymer systems in the absence of an added electrolyte is unreported. This work investigates the behavior of such systems using polarized light microscopy, small-angle X-ray scattering (SAXS), centrifugation, and 2H NMR techniques. A phase diagram at 50 °C is reported for 0-50 wt % NaLAS concentrations and 0-10 wt % polycarboxylate concentrations. The NaLAS-water system is micellar at concentrations <35 wt %, and a 2-phase micellar-lamellar system is seen at higher NaLAS levels, consistent with that reported by previous studies. As polymers are added at low surfactant concentrations (∼10 to 20 wt % NaLAS), a second optically isotropic phase is formed; this is thought to be a polymer-rich phase. Further addition of polycarboxylate leads to the formation of a lamellar phase. At high surfactant concentrations (>20 wt % NaLAS), the addition of a polymer induces a second lamellar phase. These observed behaviors are thought to arise as a result of depletion flocculation and salting-out effects. The observed lamellar phases adopt colloidal multilamellar vesicle (MLV) structures, and the average MLV radii were estimated using 2H NMR by probing the diffusion and anisotropy of D2O within the bilayers of the vesicles. The NMR results show that as the polymer concentration was increased from 0 to 10 wt %, an increase in the average multilamellar vesicle size from ∼200 to ∼500 nm was observed. This increase in the calculated average MLV radius likely results from depletion flocculation-induced MLV fusion.

Evaluation of Anionic Eco-Friendly Flocculants Prepared from Eucalyptus Pulps with Diverse Lignin Contents for Application in Effluent Treatment.

Modification of cellulosic-rich materials for the production of cellulose-based polyelectrolytes (PELs) can bring several benefits, such as high biodegradability and low or no toxicity, for numerous applications, when compared with the use of traditional, synthetic PELs. Moreover, cellulose-based PELs originating from wood wastes, contribute to the valorisation of such wastes. In this work, Eucalyptus pulps with diverse lignin contents, extracted from Eucalyptus wood wastes, were anionized by a two-step reaction procedure (periodate oxidation followed by sulfonation). Applying different reaction times (24-144 h) in the sulfonation step allowed for producing a range of cellulose-based anionic polyelectrolytes with different characteristics. PELs obtained after 24 and 72 h of sulfonation were thoroughly characterized (Fourier transform infrared and 1H nuclear magnetic resonance spectroscopies, zeta potential and degree of substitution (elemental analysis) and hydrodynamic diameter (dynamic light scattering)) and subsequently evaluated as flocculants in decolouration processes of model effluents (Methylene Blue and Crystal Violet) and an industrial effluent from a textile industry. Furthermore, possible flocculation mechanisms induced by the use of the various PELs are discussed. Results are compared with those obtained with a commonly applied, synthetic flocculant (polyacrylamide). It is demonstrated that it was possible to obtain water-soluble lignocellulosic PELs starting from raw materials with different degrees of purity and that those PELs are promising eco-friendly alternative flocculation agents for the decolouration of effluents.

A facile method for generating worm-like micelles with controlled lengths and narrow polydispersity.

This work shows that highly uniform worm micelles formed by polymerisation induced self-assembly can be obtained via simple post-synthesis sonication. Importantly, this straightforward and versatile strategy yields exceptionally monodisperse worms with tunable aspect ratios ranging from 7.2 to 17.6 by simply changing the sonication time.

Evaluation of Anionic and Cationic Pulp-Based Flocculants With Diverse Lignin Contents for Application in Effluent Treatment From the Textile Industry: Flocculation Monitoring.

In wastewater treatment, flocculation is a widely used solid/liquid separation technique, which typically employs a charged polymer, a polyelectrolyte (PEL). Polyelectrolytes features, such as charge type, charge density and molecular weight, are essential parameters affecting the mechanism of flocculation and subsequent floc sedimentation. The effectiveness of the process is also influenced by the characteristics of the system (e.g., type, size, and available surface area of suspended particles, pH of the medium, charge of suspended particles). Thus, a good understanding of the flocculation kinetics, involved mechanisms and flocs structure is essential in identifying the most adequate treatment conditions, having also into consideration possible subsequent treatments. In this study, Eucalyptus bleached pulp and a cellulosic pulp with high lignin content (~4.5 wt%) obtained from Eucalyptus wood waste were used for bio-PELs production. Firstly, a pre-treatment with sodium periodate increased the pulps reactivity. To produce cationic cellulose the oxidation step was followed by the introduction of cationic groups in the cellulose chains, through reaction with Girard's reagent T. Applying different molar ratios (0.975 and 3.9) of Girard's reagent T to aldehyde groups led to cationic PELs with diverse charge density. On the other hand, to obtain anionic cellulose a sulfonation reaction with sodium metabisulfite was applied to the intermediate dialdehyde cellulose-based products, during 24 or 72 h, and anionic-PELs with diverse features were obtained. The developed water soluble, anionic and cationic bio-PELs were characterized and tested as flocculation agents for a textile industry effluent treatment. Initially, jar-tests were used to tune the most effective flocculation procedure (pH, flocculant dosage, etc.). Flocculation using these conditions was then monitored continuously, over time, using laser diffraction spectroscopy (LDS). Due to the small size of the dyes molecules, a dual system with an inorganic complexation agent (bentonite) was essential for effective decolouration of the effluent. Performance in the treatment was monitored first by turbidity removal evaluation (75-88% with cationic-PELs, 75-81% with anionic-PELs) and COD reduction evaluation (79-81% with cationic-PELs, 63-77% with anionic-PELs) in the jar tests. Additionally, the evolution of flocs characteristics (structure and size) during their growth and the flocculation kinetics, were studied using the LDS technique, applying the different PELs produced and for a range of PEL concentration. The results obtained through this monitoring procedure allowed to discuss the possible flocculation mechanisms involved in the process. The results obtained with the bio-PELs were compared with those obtained using synthetic PELs, commonly applied in effluents treatment, polyacrylamides. The developed bio-PELs can be competitive, eco-friendly flocculation agents for effluents treatment from several industries, when compared to traditional synthetic flocculants with a significant environmental footprint. Moreover, LDS proved to be a feasible technique to monitor flocculation processes, even when a real industrial effluent is being tested.

Metal-shell nanocapsules for the delivery of cancer drugs.

Cytotoxic drugs tend to have substantial side effects on healthy tissues leading to systemic toxicity, limited tolerated doses and reduced drug efficacy. A prominent research area focuses on encapsulating cytotoxic drugs for targeted delivery to cancer tissues. However, existing carriers suffer from low drug loading levels and high drug leaching both when circulating systemically and when accumulating in non-target organs. These challenges mean that only few encapsulation technologies for delivery of cytotoxic drugs have been adopted for clinical use. Recently, we have demonstrated efficient manufacture of impermeable metal-shell/liquid core microcapsules that permit localised delivery by triggering release with ultrasound. This method has the potential to improve on existing methods for localised drug delivery because it:We demonstrate here the further miniaturization of both the emulsion droplet template and the thickness of the surrounding metal shell to the nanoscale in an attempt to take advantage of the EPR effect and the excretion of nanoparticles by the hepatobiliary system.

Ultrasound-triggered release from metal shell microcapsules.

Metal shell microcapsules have been shown to completely retain their core until its release is triggered, making them a promising candidate for use as a controllable drug delivery vehicle due to their superior retention properties as compared to polymer shell microcapsules. Focused ultrasound (FUS) has been successfully utilised to trigger release of lipophilic drugs from polymer microcapsules, and in this work the response of gold shell microcapsules with and without an inner polymeric shell, to FUS and standard ultrasound is explored. The results show that gold shell microcapsules with an inner polymer shell rupture when exposed to standard ultrasound and that there is a linear correlation between the gold shell thickness and the extent of shell rupture. When FUS is applied to these microcapsules, powers as low as 0.16 W delivered in bursts of 10 ms/s over a period of 120 s are sufficient to cause rupture of 53 nm gold shell microcapsules. Additional findings suggest that gold shell microcapsules without the polymer layer dispersed in a hydrogel matrix, as opposed to aqueous media, rupture more efficiently when exposed to FUS, and that thicker gold shells are more responsive to ultrasound-triggered rupture regardless of the external environment. Release of dye from all successfully ruptured capsules was sustained over a period of between 7 and 35 days. These findings suggest that emulsion-templated gold shell microcapsules embedded in a hydrogel matrix would be suitable for use as an implantable drug delivery vehicle with FUS used to externally trigger release.

Encapsulation of Emulsion Droplets with Metal Shells for Subsequent Remote, Triggered Release.

A two-step method to encapsulate an oil core with an impermeable shell has been developed. A thin metallic shell is deposited on the surface of emulsion droplets stabilized by metal nanoparticles. This thin shell is shown to prevent diffusion of the oil from within the core of the metal-shell microcapsules when placed in a continuous phase that fully dissolves the oil. The stabilizing nanoparticles are sterically stabilized by poly(vinyl pyrrolidone) chains and are here used as a catalyst/nucleation site at the oil-water interface to grow a secondary metal shell on the emulsion droplets via an electroless deposition process. This method provides the simplest scalable route yet to synthesize impermeable microcapsules with the added benefit that the final structure allows for drastically improving the overall volume of the encapsulated core to, in this case, >99% of the total volume. This method also allows for very good control over the microcapsule properties, and here we demonstrate our ability to tailor the final microcapsule density, capsule diameter, and secondary metal film thickness. Importantly, we also demonstrate that such impermeable microcapsule metal shells can be remotely fractured using ultrasound-based devices that are commensurate with technologies currently used in medical applications, which demonstrate the possibility to adapt these microcapsules for the delivery of cytotoxic drugs.

Cationization of Eucalyptus wood waste pulps with diverse lignin contents for potential application in colored wastewater treatment.

Modification of cellulosic-rich materials such as Eucalyptus wood waste and production of cellulose-based polyelectrolytes (PELs) presents several advantages for a variety of applications, when compared to the utilization of synthetic PELs, due to the nature, availability, high biodegradability and low or no toxicity of cellulosic materials. Moreover, valorization of the cellulosic waste itself to provide end products with higher added value is also an important aspect. In the present work, the objective was to evaluate the possibility of cationizing more complex and heterogeneous chemical pulps, obtained from Eucalyptus wood waste, with different cellulose purity and a relatively high lignin content (up to 4.5%). A two-step reaction (with sodium periodate and Girard's reagent T) was employed and a range of cellulose-based cationic polyelectrolytes were produced with different degrees of substitution. The final products were characterized by several analytical techniques and the bio-PELs with the highest and the lowest substitution degree by cationic groups were evaluated in a new application, as flocculants in the decoloration of model effluents, bentonite having been used as an inorganic aid. Also, possible mechanisms of flocculation were discussed and the results compared with those of a synthetic flocculant, often used in these treatments, cationic polyacrylamide. Lignocellulosic-PELs proved to be very favorable eco-friendly flocculation agents for the decoloration of dye-containing waters with potential application in several industries.

pH-responsive polymer microcapsules for targeted delivery of biomaterials to the midgut of Drosophila suzukii.

Drosophila suzukii or spotted wing Drosophila is an economically important pest which can have a devastating impact on soft and stone fruit industries. Biological pesticides are being sought as alternatives to synthetic chemicals to control this invasive pest, but many are subject to degradation either in the environment or in the insect gut and as a result require protection. In this study we identified a sharp change in pH of the adult midgut from neutral to acidic (pH <3), which we then exploited to develop poly(2-vinylpyridine) (P2VP) microcapsules that respond to the change in midgut pH by dissolution and release of their cargo for uptake into the insect. First, we used labelled solid poly(methyl methacrylate) (PMMA) particles to show that microcapsules with a diameter less than 15 µm are readily ingested by the adult insect. To encapsulate water-soluble biological species in an aqueous continuous phase, a multiple emulsion template was used as a precursor for the synthesis of pH-responsive P2VP microcapsules with a fluorescent (FITC-dextran) cargo. The water-soluble agent was initially separated from the aqueous continuous phase by an oil barrier, which was subsequently polymerised. The P2VP microcapsules were stable at pH > 6, but underwent rapid dissolution at pH < 4.2. In vivo studies showed that the natural acidity of the midgut of D. suzukii also induced the breakdown of the responsive P2VP microcapsules to release FITC-dextran which was taken up into the body of the insect and accumulated in the renal tubules.

Influence of pH-Responsive Monomer Content on the Behavior of Di-Block Copolymers in Solution and as Stabilizers of Pickering Latex Particle Emulsifiers.

In this study, diblock copolymers poly(methyl methacrylate)-block-poly (2-dimethylaminoethyl methacrylate) (pMMA-b-pDMAEMA) are investigated for the steric stabilization of latex particles and the subsequent use of these latex particles as Pickering emulsifiers. Solution properties of the diblock copolymers highlight that the pDMAEMA block length influences the critical micelle concentration (CMC) and micelle hydrodynamic diameter in response to changes in pH and the pKa. The block length can also be used as a way to control the particle size of sterically stabilized polystyrene latex particles prepared via emulsion polymerization. The suspension properties of these latex particles are also presented. Emulsion studies using these latex particles as emulsifiers show that both continuous phase pH and electrolyte concentration affect emulsion stability to coalescence. At high pH, stable emulsions are formed due to the affinity of the particles to the interface. At low pH, protonation of the amine groups reduces the affinity and thus droplet coalescence is observed. Increasing the electrolyte concentration improves emulsion stability, but causes an increase in droplet size due to adsorption of flocculated/aggregated particles. Finally, it is shown that these latex particles can be used in conjunction with membrane emulsification techniques to produce emulsions with low polydispersity.

The rheology of polyvinylpyrrolidone-coated silica nanoparticles positioned at an air-aqueous interface.

Particle-stabilized emulsions and foams are widely encountered, as such there remains a concerted effort to better understand the relationship between the particle network structure surrounding droplets and bubbles, and the rheology of the particle-stabilized interface. Poly(vinylpyrrolidone)-coated silica nanoparticles were used to stabilize foams. The shear rheology of planar particle-laden interfaces were measured using an interfacial shear rheometer and the rheological properties measured as a function of the sub-phase electrolyte concentration and surface pressure. All particle-laden interfaces exhibited a liquid-like to solid-like transition with increasing surface pressure. The surface pressure-dependent interfacial rheology was then correlated to the formed micron-scale structures of the particle-laden interfaces which were imaged using a Brewster angle microscope. With the baseline knowledge established, foams were prepared using the same composite particles and the particle network structure imaged using cryo-SEM. An attempt has been made to correlate the two structures observed at a planar interface and that surrounding a bubble to elucidate the likely rheology of the bubble stabilizing particle network. Independent of the sub-phase electrolyte concentration, the resulting rheology of the bubble stabilizing particle network was strongly elastic and appeared to be in a compression state at the region of the L-S phase transition.

A two-step synthesis for preparing metal microcapsules with a biodegradable polymer substrate.

Metal microcapsules have recently received attention and are being developed as improved carrier materials when compared to polymer microcapsules. In this work we have developed a novel, simplified method by which polymeric microcapsules can be synthesised using a combination of poly(vinyl pyrrolidone)-stabilised platinum nanoparticles (PVP-Pt) and poly(vinyl pyrrolidone) (PVP) as stabilisers, to allow for a secondary metal shell to be grown. We investigate the relationship between the molar ratio of reducing agent to platinum salt and the size of the resulting NPs and seek to develop further fundamental understanding of the factors governing the secondary metal shell thickness and quality, to allow production of cost-effective metal microcapsules without sacrificing core retention efficiency. We found that the size of the nanoparticles had no significant effect on secondary shell thickness, but did affect the quality of the resulting gold shells. Gold salt concentration was found to be a limiting factor in the electroless deposition of the metal shell. In this work, we have successfully produced PLGA microcapsules with more cost-effective gold shells, as thin as 56 ± 13 nm and capable of complete core retention of volatile actives.

Adsorption of Catalytic Nanoparticles onto Polymer Substrates for Controlled Deposition of Microcapsule Metal Shells.

Efficient encapsulation of small chemical molecules and their controlled targeted delivery provides a very important challenge to be overcome for a wide range of industrial applications. Typically rapid diffusion of these actives across capsule walls has so far prevented the development of a versatile widely applicable solution. In an earlier publication, we have shown that thin metal shells are able to permanently retain small molecules. The critical step in the microcapsule synthesis is the formation of a strongly adsorbed, dense monolayer of catalytic nanoparticles on the surface as this affects the secondary metal film quality. Control over Pt-nanoparticle adsorption density and a clear understanding of Pt-nanoparticle adsorption kinetics is therefore paramount. Maximising the density of heterogeneous catalysts on surfaces is generally of interest to a broad range of applications. In this work, transmission electron microscopy (TEM) and quartz crystal microbalance (QCM) are used to demonstrate that the concentration of nanoparticle polymer stabilizer used during particle synthesis and nanoparticle suspension concentration can be used to control nanoparticle surface adsorption density. We demonstrate that excess polymer, which is often used in nanoparticle synthesis but rarely discussed as an important parameter in the literature, can compete with and thus drastically affect the adsorption of the Pt-nanoparticles.

Foaming Behavior of Polymer-Coated Colloids: The Need for Thick Liquid Films.

The current study examined the foaming behavior of poly(vinylpyrrolidone) (PVP)-silica composite nanoparticles. Individually, the two components, PVP and silica nanoparticles, exhibited very little potential to partition at the air-water interface, and as such, stable foams could not be generated. In contrast, combining the two components to form silica-PVP core-shell nanocomposites led to good "foamability" and long-term foam stability. Addition of an electrolyte (Na2SO4) was shown to have a marked effect on the foam stability. By varying the concentration of electrolyte between 0 and 0.55 M, three regions of foam stability were observed: rapid foam collapse at low electrolyte concentrations, delayed foam collapse at intermediate concentrations, and long-term stability (∼10 days) at the highest electrolyte concentration. The observed transitions in foam stability were better understood by studying the microstructure and physical and mechanical properties of the particle-laden interface. For rapidly collapsing foams the nanocomposite particles were weakly retained at the air-water interface. The interfaces in this case were characterized as being "liquid-like" and the foams collapsed within 100 min. At an intermediate electrolyte concentration (0.1 M), delayed foam collapse over ∼16 h was observed. The particle-laden interface was shown to be pseudo-solid-like as measured under shear and compression. The increased interfacial rigidity was attributed to adhesion between interpenetrating polymer layers. For the most stable foam (prepared in 0.55 M Na2SO4), the ratio of the viscoelastic moduli, G'/G″, was found to be equal to ∼3, confirming a strongly elastic interfacial layer. Using optical microscopy, enhanced foam stability was assessed and attributed to a change in the mechanism of foam collapse. Bubble-bubble coalescence was found to be significantly retarded by the aggregation of nanocomposite particles, with the long-term destabilization being recognized to result from bubble coarsening. For rapidly destabilizing foams, the contribution from bubble-bubble coalescence was shown to be more significant.