Simeon D. Stoyanov

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.

Surface Rheology of Saponin Adsorption Layers

Extracts of the Quillaja saponaria tree contain natural surfactant molecules called saponins that very efficiently stabilize foams and emulsions. Therefore, such extracts are widely used in several technologies. In addition, saponins have demonstrated nontrivial bioactivity and are currently used as essential ingredients in vaccines, food supplements, and other health products. Previous preliminary studies showed that saponins have some peculiar surface properties, such as a very high surface modulus, that may have an important impact on the mechanisms of foam and emulsion stabilization. Here we present a detailed characterization of the main surface properties of highly purified aqueous extracts of Quillaja saponins. Surface tension isotherms showed that the purified Quillaja saponins behave as nonionic surfactants with a relatively high cmc (0.025 wt %). The saponin adsorption isotherm is described well by the Volmer equation, with an area per molecule of close to 1 nm(2). By comparing this area to the molecular dimensions, we deduce that the hydrophobic triterpenoid rings of the saponin molecules lie parallel to the air-water interface, with the hydrophilic glucoside tails protruding into the aqueous phase. Upon small deformation, the saponin adsorption layers exhibit a very high surface dilatational elasticity (280 ± 30 mN/m), a much lower shear elasticity (26 ± 15 mN/m), and a negligible true dilatational surface viscosity. The measured dilatational elasticity is in very good agreement with the theoretical predictions of the Volmer adsorption model (260 mN/m). The measured characteristic adsorption time of the saponin molecules is 4 to 5 orders of magnitude longer than that predicted theoretically for diffusion-controlled adsorption, which means that the saponin adsorption is barrier-controlled around and above the cmc. The perturbed saponin layers relax toward equilibrium in a complex manner, with several relaxation times, the longest of them being around 3 min. Molecular interpretations of the observed trends are proposed when possible. Surprisingly, in the course of our study we found experimentally that the drop shape analysis method (DSA method) shows a systematically lower surface elasticity, in comparison with the other two methods used: Langmuir trough and capillary pressure tensiometry with spherical drops. The possible reasons for the observed discrepancy are discussed, and the final conclusion is that the DSA method has specific problems and may give incorrect results when applied to study the dynamic properties of systems with high surface elasticity, such as adsorption layers of saponins, lipids, fatty acids, solid particles, and some proteins. The last conclusion is particularly important because the DSA method recently became the preferred method for the characterization of fluid interfaces because of its convenience.

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.

Unique Properties of Bubbles and Foam Films Stabilized by HFBII Hydrophobin

The HFBII hydrophobin is an amphiphilic protein that can irreversibly adsorb at the air/water interface. The formed protein monolayers can reach a state of two-dimensional elastic solid that exhibits a high mechanical strength as compared to adsorption layers of typical amphiphilic proteins. Bubbles formed in HFBII solutions preserve the nonspherical shape they had at the moment of solidification of their surfaces. The stirring of HFBII solutions leads to the formation of many bubbles of micrometer size. Measuring the electrophoretic mobility of such bubbles, the ζ-potential was determined. Upon compression, the HFBII monolayers form periodic wrinkles of wavelength 11.5 μm, which corresponds to bending elasticity k(c) = 1.1 × 10(-19) J. The wrinkled hydrophobin monolayers are close to a tension-free state, which prevents the Ostwald ripening and provides bubble longevity in HFBII stabilized foams. Films formed between two bubbles are studied by experiments in a capillary cell. In the absence of added electrolyte, the films are electrostatically stabilized. The appearance of protein aggregates is enhanced with the increase of the HFBII and electrolyte concentrations and at pH close to the isoelectric point. When the aggregate concentration is not too high (to block the film thinning), the films reach a state with 12 nm uniform thickness, which corresponds to two surface monolayers plus HFBII tetramers sandwiched between them. In water, the HFBII molecules can stick to each other not only by their hydrophobic moieties but also by their hydrophilic parts. The latter leads to the attachment of HFBII aggregates such as dimers, tetramers, and bigger ones to the interfacial adsorption monolayers, which provides additional stabilization of the liquid films.

Super stable foams stabilized by colloidal ethyl cellulose particles

Here we report the preparation of super stable liquid foams with various bubble sizes stabilized by colloidal ethyl cellulose (EC) particles. What is novel and different in this particle stabilized foam is that both the initial material (EC) and processes used are in principle food grade, thus it may offer scope in food applications. The particles were prepared using a conventional anti-solvent precipitation method, involving the dissolution of EC polymer into acetone, followed by fast mixing with water (anti-solvent), leading to the precipitation of EC particles, then followed by the rotary evaporation of acetone. The interfacial tension of the resulting dispersion is 36 mN m−1, indicating that particles co-exist with surface active and water soluble components, which is most likely a low molecular weight EC fraction. The average particle diameter is 0.13 μm and their zeta potential is −50mV at pH = 6, increasing to −5mV at pH = 3. This negative surface potential is attributed to adsorption of hydroxyl ions, known to occur on many hydrophobic surfaces, including oil–water, air–water and hydrophobic particle–water. As a result, there is strong electrostatic repulsion between EC particles at neutral and low ionic strength, which stabilizes EC dispersion and also significantly increases the adsorption barrier of EC particle at the air–water interface. Due to their similar origin, both inter-particle repulsion and adsorption barrier can be controlled by pH and/or ionic strength, which leads to dispersion destabilization and at the same time good foamability and extreme foam stability at acidic conditions (pH 20 mM). Foam coarsening shows an initial stage with coarsening time of approximately 1 week, followed by a plateau, where the coarsening has been arrested for a period of months. By using cryo scanning electron microscopy, we reveal that these EC foams are Pickering stabilized, where EC particles are closely packed at the air–water interface forming a single or multi-layers. We also show that super stable EC foams can be prepared using various aeration techniques, allowing us to vary the bubble diameter from a few microns to hundreds of microns.

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.

Interfacial layers from the protein HFBII hydrophobin: Dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.

Sporopollenin micro-reactors for in-situ preparation, encapsulation and targeted delivery of active components

We report a simple and robust technique for loading of sporopollenin microcapsules from Lycopodium clavatum with a range of inorganic and organic nanomaterials based on in-situ preparation of nanoparticles by a chemical reaction in the sporopollenin interior.

Anisotropic nano-papier mache microcapsules

We report the fabrication of anisotropic microcapsules produced by using alternating layers of nano-cotton fibres and oppositely charged polyelectrolytes over anisotropic inorganic templates with needle-like and rhombohedral morphology.

Scalable fabrication of anisotropic micro-rods from food-grade materials using an in shear flow dispersion–solvent attrition technique

We report a scalable fabrication of novel, food-grade anisotropic micro-rods from shellac and ethyl cellulose with controllable morphology which was studied using scanning electron microscopy and optical microscopy. The aqueous suspensions of these anisotropic materials show great potential for preparation of super-stable food-grade foams.