Concentration and pH dependence of colloidal scale solute clustering within aqueous solutions of small organic molecule

Aqueous solutions of well soluble molecules are usually assumed to be essentially homogenous systems with some degree of local structuring due to specific interactions on the sub-nanometre scale, involving the creation of, e.g. molecular clusters, hydration shells, etc., usually not exceeding several solute molecules. These small molecular structures (molecular clusters) are commonly observed (experimentally and theoretically) in aqueous solutions of many organic and inorganic systems. The presence of colloidal scale (mesoscale) structures has been well established in aqueous solutions of various small molecules [1] and these clusters have been proposed to be involved in non-classical crystal nucleation mechanisms [2,3]. Clusters in solutions of amino acids have been reported to be thermodynamically stable and can be present in both undersaturated and supersaturated solutions with respect to the solid-liquid equilibrium [4,5]. We investigated concentration and pH dependence of mesoscale clustering in aqueous solutions of small organic molecules, including amino acids (such as glycine) and amines (such as triethylenetetramine), which are well soluble in water and have a range of charged states that can be adjusted by changing solution pH. We used Dynamic Light Scattering (DLS) and Brownian Microscopy/ Nanoparticles Tracking Analysis (NTA) in order to measure size distributions and number concentrations as well as scattered intensities and optical contrast of mesoscale clusters. Mesoscale clusters were present in undersaturated solutions at solute concentrations well below the solid-liquid equilibrium (saturation) concentration at a given temperature, with mean diameters within 200-400 nm. Scattering intensities and number concentrations increased with increasing solute concentrations, while the mean size of mesoscale clusters remained approximately constant. When pH was varied in amino acid solutions, values away from the isoelectric point resulted in a decrease of the number concentration of mesoscale clusters. While the mean size of mesoscale clusters remained approximately independent of pH, scattering intensities decreased sharply as solution pH moved more than 3 units away from the isoelectric point in either direction. This change was closely correlated with the speciation of charged species, whereby the zwitterions predominate near the isoelectric point and positively and negatively charged species predominate at lower and higher pH, respectively. We note that the mesospecies are not to be seen as a separate phase and the system is better described as a thermodynamically stable mesostructured liquid containing solute-rich domains (mesoscale clusters) dispersed within bulk solute solution [6]. At a given temperature, solute molecules in such a mesostructured liquid phase are subject to equilibrium distribution between solute-rich mesospecies and the surrounding bulk solution

Rheological properties of bioinks for printing optimisation

3D bio-printing is a rapidly growing technique that aims to combine engineering principles and life sciences for the fabrication of in-vitro tissue, organs, and other biological constructs by utilising biomaterial-based scaffolds and living cells, in a layer-by-layer process. The fabrication of scaffold-based in-vitro models requires an architectural platform like the native extracellular matrix that provides structural support and aids in cellular proliferation and migration in the form of a bioink. Fabrication of bioinks for extrusion-based 3D bio-printing is challenging in achieving complex tissue structures. The rheological properties of bioinks determine the printability, structural fidelity, in turn depicting the viability of the cells during the printing process. Ideal bioinks should possess non-Newtonian characteristics such as shear-thinning during printing followed by quick structural recovery. Hydrogels are an appealing scaffold because they inherit natural biomimicry properties similar to the extracellular matrix of many tissues. Here, quantitative rheological assessments studying the relationship between stress (stimuli) and strain (response) of Alginate and Gelatin were explored to determine the flow behaviour and deformation to establish printing parameters. The results display low shear induced flow-behaviour properties with ideal viscosities that are desirable in preventing shear stress-caused damage to cells. This rheological study provides a predictive tool to understand the relationship between the material and its attributes displayed to optimise printing parameters which in turn can easily aid in the development of bioinks

Utilising DOE to optimise hydrogel composition for tissue engineering

printability, biocompatibility, ideal mechanical properties, and batch-to-batch consistency. Currently there is no standardised protocol to design a functional bioink. Here, a methodology was designed to optimise hydrogel composition of Gelatin (GEL),Hyaluronic Acid (HA) and Dextran-40 (DEX), utilising the ‘Design of Experiment’ (DOE) statistical tool and rheological behaviour of each composition

Water desorption from an oxygen covered Pt(111) surface: multi-channel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample

Water desorption from an oxygen covered Pt(111) surface: multi-channel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample