Light induced assembly and self-sorting of silica microparticles.

To tailor the properties of colloidal materials, precise control over the self-assembly of their constituents is a prerequisite. Here, we govern the assembly of silica particles by functionalization with supramolecular moieties which interact with each other via directional and reversible hydrogen bonding. Through a generally applicable synthesis protocol, two different types of self-complementary hydrogen bonding moieties, BTA- and UPy-derivatives, are anchored to silica particles. Their self-assembly is initiated by the UV-induced removal of a photolabile protecting group, allowing the formation of hydrogen bonds between tethered molecules. The light-induced assembly of BTA- and UPy-decorated colloids in single-component dispersions and colloidal self-sorting in mixed dispersions is studied. Furthermore, we demonstrate that UPy-colloids can dissasemble upon addition of traces of a competitive binder (NaPy). This work provides further insight into the utility of supramolecular handles to orchestrate the assembly of micron-sized colloids via non-oligonucleotide hydrogen-bonding units.

Synthesis, Polymerization, and Assembly of Nanosilica Particles below the Isoelectric Point.

The particle growth of silica below the isoelectric point plays a key role in oil well cements, production of silica gels and production of nanosilica by dissolving silicates. In this article, we study the particle growth of silica below the isoelectric point using olivine, a silicate mineral, and sodium silicate solutions as silica sources in acid, where the particle size, soluble silica concentration, specific surface area and gelling time were measured. The size of the primary particles detected by laser light scattering is 3 nm in the experiments with sodium silicate solutions. These particles grow then by aggregation forming linear chains which in time will start to branch. The particle growth follows a quadratic polynomial function and particles as large as 100 and 500 nm are detected in the final stages of experiments using sodium silica solutions and olivine, respectively. Based on these findings, a comprehensive model describing the silica particle development below the isoelectric point is proposed. This model gives fundamental information about the growth mechanism and the properties of silica (e.g., particle size of the primary particles, size of the aggregates) at the different growth stages.

Temperature-Induced, Selective Assembly of Supramolecular Colloids in Water.

In this article, we report the synthesis and physical characterization of colloidal polystyrene particles that carry water-soluble supramolecular N,N',N″,-trialkyl-benzene-1,3,5-tricarboxamides (BTAs) on their surface. These molecules are known to assemble into one-dimensional supramolecular polymers via noncovalent interactions. By tethering the BTAs to charge-stabilized particles, the clustering behavior of the resulting colloids was dictated by a balance between interparticle electrostatic repulsion and the BTA-mediated attractions. Through careful tuning of the dispersing medium's ionic strength, a regime was found in which particle aggregation could be reversibly induced upon heating the dispersion. These findings clearly indicate that hydrophobic interactions, which become stronger upon heating, play an important role during the clustering process. Besides the thermoreversible nature of the generated hydrophobic interparticle attractions, we found the clustering to be selective, that is, the BTA-functionalized colloids do not interact with nonfunctionalized hydrophobic polystyrene particles. This selectivity in the association process can be rationalized by the preferred stacking of the surface-tethered BTAs. These selective intermolecular/particle bonds are likely stabilized by the formation of hydrogen bonds, as previously observed for analogous molecular BTA assemblies. The resulting driving force responsible for particle clustering is therefore dual in nature and depends on both hydrophobic attractions and hydrogen bonding.

Interaction of Artepillin C with model membranes.

Green propolis, a mixture of beeswax and resinous compounds processed by Apis mellifera, displays several pharmacological properties. Artepillin C, the major compound in green propolis, consists of two prenylated groups bound to a phenyl group. Several studies have focused on the therapeutic effects of Artepillin C, but there is no evidence that it interacts with amphiphilic aggregates to mimic cell membranes. We have experimentally and computationally examined the interaction between Artepillin C and model membranes composed of dimyristoylphosphatidylcholine (DMPC) because phosphatidylcholine (PC) is one of the most abundant phospholipids in eukaryotic cell membranes. PC is located in both outer and inner leaflets and has been used as a simplified membrane model and a non-specific target to study the action of amphiphilic molecules with therapeutic effects. Experimental results indicated that Artepillin C adsorbed onto the DMPC monolayers. Its presence in the lipid suspension pointed to an increased tendency toward unilamellar vesicles and to decreased bilayer thickness. Artepillin C caused point defects in the lipid structure, which eliminated the ripple phase and the pre-transition in thermotropic chain melting. According to molecular dynamics (MD) simulations, (1) Artepillin C aggregated in the aqueous phase before it entered the bilayer; (2) Artepillin C was oriented along the direction normal to the surface; (3) the negatively charged group on Artepillin C was accommodated in the polar region of the membrane; and (4) thinner regions emerged around the Artepillin C molecules. These results help an understanding of the molecular mechanisms underlying the biological action of propolis.

Synthesis and Characterization of Supramolecular Colloids.

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical properties for applications in photonics, drug delivery and coating technology. Here we present a new family of colloidal building blocks, coined supramolecular colloids, whose self-assembly is controlled through surface-functionalization with a benzene-1,3,5-tricarboxamide (BTA) derived supramolecular moiety. Such BTAs interact via directional, strong, yet reversible hydrogen-bonds with other identical BTAs. Herein, a protocol is presented that describes how to couple these BTAs to colloids and how to quantify the number of coupling sites, which determines the multivalency of the supramolecular colloids. Light scattering measurements show that the refractive index of the colloids is almost matched with that of the solvent, which strongly reduces the van der Waals forces between the colloids. Before photo-activation, the colloids remain well dispersed, as the BTAs are equipped with a photo-labile group that blocks the formation of hydrogen-bonds. Controlled deprotection with UV-light activates the short-range hydrogen-bonds between the BTAs, which triggers the colloidal self-assembly. The evolution from the dispersed state to the clustered state is monitored by confocal microscopy. These results are further quantified by image analysis with simple routines using ImageJ and Matlab. This merger of supramolecular chemistry and colloidal science offers a direct route towards light- and thermo-responsive colloidal assembly encoded in the surface-grafted monolayer.

Imaging Nanostructures by Single-Molecule Localization Microscopy in Organic Solvents.

The introduction of super-resolution fluorescence microscopy (SRM) opened an unprecedented vista into nanoscopic length scales, unveiling a new degree of complexity in biological systems in aqueous environments. Regrettably, supramolecular chemistry and material science benefited far less from these recent developments. Here we expand the scope of SRM to photoactivated localization microscopy (PALM) imaging of synthetic nanostructures that are highly dynamic in organic solvents. Furthermore, we characterize the photophysical properties of commonly used photoactivatable dyes in a wide range of solvents, which is made possible by the addition of a tiny amount of an alcohol. As proof-of-principle, we use PALM to image silica beads with radii close to Abbe's diffraction limit. Individual nanoparticles are readily identified and reliably sized in multicolor mixtures of large and small beads. We further use SRM to visualize nm-thin yet µm-long dynamic, supramolecular polymers, which are among the most challenging molecular systems to image.

Vitamin A Palmitate-ß-cyclodextrin inclusion complexes: characterization, protection and emulsification properties.

The interest in the production of foods enriched with vitamins, in order to prevent diseases related with their deficiency, has recently increased. However, the low stability and the low water solubility of certain vitamins make difficult their incorporation in foodstuff, especially in water-based formulations. This limitation is typically overcome by using encapsulating systems such as cyclodextrins. In this paper the formation of water-soluble inclusion complexes of Vitamin A Palmitate with ß-cyclodextrins, without the use of organic solvents, is described. The objective was to increase the water solubility of Vitamin A Palmitate and its stability against different external factors to eventually enrich aqueous-based products. The stability of Vitamin A Palmitate in the complexes towards temperature, oxygen and UV light was investigated. All results showed a notably increase of Vitamin A Palmitate water solubility and stability in front of those variables when encapsulated. The surface activity of the complex suggests its possible use as stabilizer in emulsion formulations.

Multiple emulsions as soft templates for the synthesis of multifunctional silicone porous particles.

Multiple emulsion templating is a versatile strategy for the synthesis of porous particles. The present work addresses the synthesis of multifunctional poly(dimethylsiloxane) porous particles using multiple water-in-oil-in-water emulsions as soft templates with an oil phase constituted by a crosslinkable poly(dimethylsiloxane) (PDMS) oil. Herewith, the impact of the viscosity of PDMS oil (i.e., molecular weight) on the properties of both the emulsion templates and the resulting particles was evaluated. The viscosity of PDMS oil has a strong effect on the size and polydispersity of the emulsion templates as well as on the mechanical properties of the derived particles. The elastic modulus can be tuned by mixing PDMS oils of different viscosities to form bimodal crosslinked networks. Iron oxide nanoparticles can be readily incorporated into the emulsion templates to provide additional functionalities to the silicone particles, such as magnetic separation or magnetic hyperthermia. The synthesized composite magnetic particles were found to be useful as recoverable absorbent materials (e.g., for oil spills) by taking advantage of their high buoyancy and high hydrophobicity.

In-situ formation of silver nanoparticles using nonionic surfactant reverse micelles as nanoreactors.

In this paper, we report the one-step synthesis of metallic silver nanoparticles (Ag NP) using nonionic surfactant reverse micelle as nanoreactors. Diglycerol monolaurate (C12G2) spontaneously self-assemble into spheroid reverse micelles having size 10-12 nm in cyclohexane under ambient conditions of temperature and pressure. The spheroid C12G2 reverse micelles swell with water. Swollen reverse micelles having size - 20 nm are formed upon incorporation of 1% water. We used C12G2 reverse micelles as nanoreactors for making ordered nanostructure of Ag-NP by replacing water with aqueous silver nitrate solution. The diglycerol moiety of the surfactant reduces silver ions into metallic silver and thereby stabilizes the generated Ag NP. We found that shape and size of the Ag NP is closely related to the structure of nanoreactor. Similar results have been observed in linear chain alkane n-octane. We found bigger Ag NP from the C12G2/octane reverse micelle system as the size of the micelle in this system is bigger than that of the C12G2/cyclohexane system. This simple approach based on in-situ reduction of metal ions (without the need of reducing agent) opens a new possibility for the development of controlled synthesis of nanostructured noble metallic nanoparticles.

Preparation of novel silicone multicompartment particles by multiple emulsion templating and their use as encapsulating systems.

Multicompartment poly(dimethylsiloxane) particles were produced for the first time using water-in-oil-in-water (W1/O/W2) emulsions as templates. Multiple silicone W1/O/W2 emulsions were successfully prepared by using silicone precursors with a low viscosity. Several formulation parameters were studied to determine their effect on the properties of emulsions and derived particles. It was observed that the mass fraction of the inner aqueous phase (φ(W1)) and the concentration of both the hydrophobic and hydrophilic surfactants played a crucial role in the morphology and stability of the emulsions. Thus, the derived silicone porous particles also showed different characteristics depending on the emulsion formulation because of the templating effect. At low φ(W1) or high concentrations of the hydrophobic surfactant, particles showed smaller pore sizes as a result of more stable inner droplets. On the other hand, high concentrations of the hydrophobic surfactant resulted in an increase in the size of the derived particles, whereas high concentrations of the hydrophilic surfactant caused the opposite effect. In addition, fluorescein was encapsulated into the hydrophobic particles during the synthesis process and released in a controlled manner. The possibility to encapsulate simultaneously but independently two different hydrophilic components inside the same globule was also tested. On the basis of these results, the obtained silicone porous particles are envisioned to have applications in several advanced fields, for instance, as hydrophobic delivery systems.

Fabrication of novel silicone capsules with tunable mechanical properties by microfluidic techniques.

A novel approach for the synthesis of silicone capsules using double W/O/W emulsions as templates is introduced. The low viscosity of the silicone precursors enables the use of microfluidic techniques to accurately control the size and morphology of the double emulsion droplets, which after cross-linking result in the desired monodisperse silicone capsules. Their shell thickness can be finely tuned, which in turn allows control over their permeability and mechanical properties; the latter are particularly important in a variety of practical applications where the capsules are subjected to large external forces. The potential of these capsules for controlled release is also demonstrated using a model hydrophilic substance.

A combination of hard and soft templating for the fabrication of silica hollow microcoils with nanostructured walls.

Hollow silica microcoils have been prepared by using functionalized carbon microcoils as hard templates and surfactant or amphiphilic dye aggregates as soft templates. The obtained materials have been characterized by electron and optical microscopy, nitrogen sorption and small angle X-ray scattering. The obtained hollow microcoils resemble the original hard templates in shape and size. Moreover, they have mesoporous walls (pore size ≈ 3 nm) with some domains where pores are ordered in a hexagonal array, originated from surfactant micelles. The obtained silica microcoils also show preferential adsorption of cationic fluorescent dyes. A mechanism for the formation of silica microcoils is proposed.