Vesselin N. Paunov

Fabrication of asymmetrically coated colloid particles by microcontact printing techniques

We developed a novel method for preparation of asymmetrically coated colloid particles by using a microcontact printing technique. Films of water-insoluble ionic surfactants deposited on PDMS stamps were printed onto latex particle monolayers of opposite surface charge in order to produce spherical latex particles of dipolar surface charge distribution. We studied the effects of salt on the aggregation of such dipolar particles in aqueous suspensions. Upon addition of salt, dipolar colloid particles were found to give “linear” aggregates. We then extended this microcontact printing technique to the directed assembly of colloidal particles. Microcontact printing of one colloidal monolayer over a latex particle monolayer of opposite charge was used to fabricate particles of complex internal structure. We demonstrated that if the two colloid monolayers consist of particles of comparable sizes, this method allows fabrication of particle doublets. When the particle monolayer was stamped with another colloid monolayer of much smaller particle size complex structures as half-coated “raspberry”-like particles were obtained. Possible applications of these asymmetrically coated colloids include photonic crystals with novel symmetries, colloidal substitutes for liquid crystals and water-based electrorheological fluids.

Capillary meniscus interaction between colloidal particles attached to a liquid—fluid interface

Abstract General expressions for the energy of capillary meniscus forces acting between particles attached to a liquid—fluid interface are derived. These expressions are specified for the cases of two vertical cylinders and two similar spheres partially immersed in a liquid layer on a horizontal solid substrate. The shape of the meniscus around the particles is determined from the Laplace equation by using the method of the matched asymptotic expansions. The derived asymptotic expressions are valid with a very good accuracy when the contact line radius and the interparticle distance are smaller than 100 μm (liquid—gas interface). The range of validity can be wider for emulsion-type interfaces. The capillary meniscus forces turn out to be attractive and very long-ranged. The results can be important for interpreting the surface coagulation phenomena accompanying flotation processes as well as two-dimensional ordering of colloidal particles and protein molecules.

Long-Term Stabilization of Foams and Emulsions with In-Situ Formed Microparticles from Hydrophobic Cellulose

We report a simple method to produce foams and emulsions of extraordinary stability by using hydrophobic cellulose microparticles, which are formed in situ by a liquid-liquid dispersion technique. The hydrophobic cellulose derivative, hypromellose phthalate (HP), was initially dissolved in water-miscible solvents such as acetone and ethanol/water mixtures. As these HP stock solutions were sheared in aqueous media, micron sized cellulose particles formed by the solvent attrition. We also designed and investigated an effective and simple process for making HP particles without any organic solvents, where both the solvent and antisolvent were aqueous buffer solutions at different pH. Consequently, the HP particles adsorbed onto the water/air or water/oil interfaces created during shear blending, resulting in highly stable foams or foam/emulsions. The formation of HP particles and their ability for short-term and long-term stabilization of interfaces strongly depended on the HP concentration in stock solutions, as well as the solvent chemistry of both stock solutions and continuous phase media. Some foams and emulsion samples formed in the presence of ca. 1 wt% HP were stable for months. This new class of nontoxic inexpensive cellulose-based particle stabilizers has the potential to substitute conventional synthetic surfactants, especially in edible, pharmaceutical and biodegradable products.

An environmentally benign antimicrobial nanoparticle based on a silver-infused lignin core

Silver nanoparticles have antibacterial properties, but their use has been a cause for concern because they persist in the environment. Here, we show that lignin nanoparticles infused with silver ions and coated with a cationic polyelectrolyte layer form a biodegradable and green alternative to silver nanoparticles. The polyelectrolyte layer promotes the adhesion of the particles to bacterial cell membranes and, together with silver ions, can kill a broad spectrum of bacteria, including Escherichia coli, Pseudomonas aeruginosa and quaternary-amine-resistant Ralstonia sp. Ion depletion studies have shown that the bioactivity of these nanoparticles is time-limited because of the desorption of silver ions. High-throughput bioactivity screening did not reveal increased toxicity of the particles when compared to an equivalent mass of metallic silver nanoparticles or silver nitrate solution. Our results demonstrate that the application of green chemistry principles may allow the synthesis of nanoparticles with biodegradable cores that have higher antimicrobial activity and smaller environmental impact than metallic silver nanoparticles.

Fabrication of novel colloidosome microcapsules with gelled aqueous cores

Novel colloidosome microcapsules with aqueous gel cores and integral shells of polymeric colloid particles have been prepared and characterized. Our method is based on templating water-in-oil emulsions stabilized by polystyrene (PS) latex particles, followed by gelling of the aqueous phase with a suitable hydrocolloid. The obtained microcapsules were transferred into water after dissolving the oil phase in ethanol and multiple centrifugation–washing with ethanol and water. The presence of an aqueous gel core was found to be crucial for the structural integrity and mechanical stability of the obtained colloidosome microcapsules. The effect of the oil type on the final structure of the colloidosome membrane was also studied. It was demonstrated that by using an appropriate oil, the particles within the colloidosome monolayer can be partially or completely swollen which allows direct control over the membrane pore size and its permeability. Such colloidosome microcapsules may find applications as delivery vehicles for controlled release of drugs and cosmetic or food supplements.

Fabrication of environmentally biodegradable lignin nanoparticles.

We developed a method for the fabrication of novel biodegradable nanoparticles (NPs) from lignin which are apparently non-toxic for microalgae and yeast. We compare two alternative methods for the synthesis of lignin NPs which result in particles of very different stability upon change of pH. The first method is based on precipitation of low-sulfonated lignin from an ethylene glycol solution by using diluted acidic aqueous solutions, which yields lignin NPs that are stable over a wide range of pH. The second approach is based on the acidic precipitation of lignin from a high-pH aqueous solution which produces NPs stable only at low pH. Our study reveals that lignin NPs from the ethylene glycol-based precipitation contain densely packed lignin domains which explain the stability of the NPs even at high pH. We characterised the properties of the produced lignin NPs and determined their loading capacities with hydrophilic actives. The results suggest that these NPs are highly porous and consist of smaller lignin domains. Tests with microalgae like Chlamydomonas reinhardtii and yeast incubated in lignin NP dispersions indicated that these NPs lack measurable effect on the viability of these microorganisms. Such biodegradable and environmentally compatible NPs can find applications as drug delivery vehicles, stabilisers of cosmetic and pharmaceutical formulations, or in other areas where they may replace more expensive and potentially toxic nanomaterials.

A direct technique for preparation of magnetically functionalised living yeast cells

A direct technique for preparation of magnetically functionalised yeast cells by using polyelectrolyte mediated deposition of magnetite nanoparticles is reported. We demonstrate that the cells preserve their viability after the magnetite deposition and show that the magnetic nanoparticles form a multilayered coating on the outer side of the yeast cell’s wall. We applied our technique to produce magnetically functionalised yeast cells expressing green fluorescent protein (GFP) under the control of RAD54-GFP reporter and demonstrated that their fluorescence emission is not influenced by the presence of magnetite-polyelectrolyte composite coating. We show that the individual cells can be successfully manipulated by an external magnetic field which can be used for their deposition, holding and subsequent removal from microfluidic devices for genotoxicity and cytotoxicity biosensor applications. Our technique for direct magnetization of cells can find many other biotech applications including biosensors, bioreactors and bioseparation.

Novel anisotropic materials from functionalised colloidal cellulose and cellulose derivatives

This feature article describes selected examples of the properties and the methods of preparation of cellulose micro and nano crystallites (whiskers) and derivatives, with aspects related to fabrication of various anisotropic materials. Nanometre sized cellulose crystallites have a variety of novel anisotropic properties markedly different from those of common forms of cellulose. They can be obtained from a variety of native cellulose sources through partial hydrolysis with strong acids or via mechanical defibrillation. We discuss different fabrication techniques and surface modifications of cellulose whiskers which determine their wettability, surface charge and range of applications. Concentrated suspensions of cellulose whiskers of high aspect ratio can form chiral nematic liquid crystalline phases which retain their structure upon evaporation, producing iridescent films. At present, the bulk of the research on cellulose whiskers is focused on creation of composite materials in which they enhance mechanical properties and improve their biodegradability. The high strength of the cellulose nanocrystals has also been utilised in the fabrication of reinforced composite films with applications for anisotropic microcapsule preparation. Microrods and multifunctional microampules from hydrophobised cellulose have recently been recognised as being able to produce super-stable foams with long shelf life and allow the foam structural elements to encapsulate a range of liquid and solid additives.

Functionalization of whole-cell bacterial reporters with magnetic nanoparticles

We developed a biocompatible and highly efficient approach for functionalization of bacterial cell wall with magnetic nanoparticles (MNPs). Three Acinetobacter baylyi ADP1 chromosomally based bioreporters, which were genetically engineered to express bioluminescence in response to salicylate, toluene/xylene and alkanes, were functionalized with 18 ± 3 nm iron oxide MNPs to acquire magnetic function. The efficiency of MNPs functionalization of Acinetobacter bioreporters was 99.96 ± 0.01%. The MNPs‐functionalized bioreporters (MFBs) can be remotely controlled and collected by an external magnetic field. The MFBs were all viable and functional as good as the native cells in terms of sensitivity, specificity and quantitative response. More importantly, we demonstrated that salicylate sensing MFBs can be applied to sediments and garden soils, and semi‐quantitatively detect salicylate in those samples by discriminably recovering MFBs with a permanent magnet. The magnetically functionalized cells are especially useful to complex environments in which the indigenous cells, particles and impurities may interfere with direct measurement of bioreporter cells and conventional filtration is not applicable to distinguish and harvest bioreporters. The approach described here provides a powerful tool to remotely control and selectively manipulate MNPs‐functionalized cells in water and soils. It would have a potential in the application of environmental microbiology, such as bioremediation enhancement and environment monitoring and assessment.

Fabrication of novel anisotropic magnetic microparticles

We report a novel technique for fabrication of magnetically anisotropic microparticles based on “arresting” of the alignment of oleic acid coated magnetite nanoparticles (OCMNs) dispersed within the oil drops of a polymerisable oil-in-water emulsion. This was achieved by polymerising the oil drops after gelling the continuous aqueous phase in the presence of an external magnetic field. This approach allowed us to produce magnetic Janus particles with anisotropic optical and magnetic properties which form unusual zig-zag chains and structures when exposed to an external magnetic field. We studied the magnetic properties of these novel microparticles and showed that they retained remanence magnetisation with high coercivity values indicative of ferromagnetic behaviour. This indicates that the composite polymeric Janus microparticles posses a net magnetic dipole and behave like micromagnets due to the “arrested” orientation of the OCMNs in their polymeric matrix. Utilizing the same technique, magnetic Janus microparticles have been prepared based on emulsions stabilised only by OCMNs without the use of surfactants, and the effect of pH of continuous aqueous phase on the morphology of these microparticles has been investigated.