Electrostatic assembly of zwitterionic and amphiphilic supraparticles

Catalyst Supraparticles: Tuning the Structure of Spray-Dried Pt/SiO2 Supraparticles via Salt-Based Colloidal Manipulation to Control their Catalytic Performance

The structure of supraparticles (SPs) is a key parameter for achieving advanced functionalities arising from the combination of different nanoparticle (NP) types in one hierarchical entity. However, whenever a droplet-assisted forced assembly approach is used, e.g., spray-drying, the achievable structure is limited by the inherent drying phenomena of the method. In particular, mixed NP dispersions of differently sized colloids are heavily affected by segregation during the assembly. Herein, the influence of the colloidal arrangement of Pt and SiO2 NPs within a single supraparticulate entity is investigated. A salt-based electrostatic manipulation approach of the utilized NPs is proposed to customize the structure of spray-dried Pt/SiO2 SPs. By this, size-dependent separation phenomena of NPs during solvent evaporation, that limit the catalytic performance in the reduction of 4-nitrophenol, are overcome by achieving even Pt NP distribution. Additionally, the textural properties (pore size and distribution) of the SiO2 pore framework are altered to improve the mass transfer within the material leading to increased catalytic activity. The suggested strategy demonstrates a powerful, material-independent, and universally applicable approach to deliberately customize the structure and functionality of multi-component SP systems. This opens up new ways of colloidal material combinations and structural designs in droplet-assisted forced assembly approaches like spray-drying.

From binary AB to ternary ABC supraparticles

Control over the assembly and morphology of nanoscale functional building blocks is of great importance to hybrid and porous nanomaterials. In this paper, by combining different types of spherical nanoparticles with different size ratios in a hierarchical assembly process which allows us to control the final structure of multi-component assemblies, we discuss self-assembly of an extensive range of supraparticles, labelled as AB particles, and an extension to novel ternary particles, labelled as ABC particles. For supraparticles, the organization of small nanoparticles is known to be inherently related to the size ratio of building blocks. Therefore, we studied the formation of supraparticles prepared by colloidal self-assembly using small silica nanoparticles (SiO₂ NPs) attached on the surface of large polystyrene latex nanoparticles (PSL NPs) with a wide size ratio range for complete and partial coverage, by controlling the electrostatic interactions between the organic and inorganic nanoparticles and their concentrations. In this way hierarchically ordered, stable supraparticles, either fully covered or partially covered, were realized. The partially covered, stable AB supraparticles offer the option to create ABC supraparticles of which the fully covered shell contains two different types of nanoparticles. This has been experimentally confirmed using iron oxide (Fe₃O₄) nanoparticles together with silica nanoparticles as shell particles on polystyrene core particles. Cryo-electron tomography was used to visualize the AB binary and ABC ternary supraparticles and to determine the three-dimensional structural characteristics of supraparticles formed under different conditions.

Assembly of partially covered strawberry supracolloids in dilute and concentrate aqueous dispersions

HYPOTHESIS

Core-corona supracolloids can be assembled in aqueous dispersions by controlling the physical interactions between the corona and core colloidal particles. A raspberry corona configuration with full surface coverage of the core can be reached by inducing strong attractive interactions between the individual particles. A controlled partial surface coverage of the core, i.e. strawberry configuration, is however, more difficult to achieve. Supracolloids with different surface coverage ratio exhibit unique and multifunctional surface properties.

EXPERIMENTS

By counterbalancing the multiple physical interactions playing a role during the assembly, the configuration and stability of the assemblies could be fine-tuned over a wide range of concentrations. Supracolloids consisting of polyethylene glycol (PEO)-grafted polymer particles covered by silica nanoparticles were assembled with different configurations, by adjusting the pH and ionic strength of the dispersion, the PEO grafting density and the particles concentration. The self-assembly process and resulting configurations were monitored via cryogenic transmission electron microscopy (Cryo-TEM) and light scattering.

FINDINGS

The suitable conditions to assemble supracolloids with partial corona coverage have been established. Stable strawberry supracolloids could be prepared, both for diluted (1 wt%) and concentrated (12 wt%) dispersions. These hybrid supracolloids with well-defined configuration are highly relevant to developing advanced water-borne paints and inks, food dispersions, cosmetic and healthcare products.

Development of Submicrocapsules Based on Co-Assembled Like-Charged Silica Nanoparticles and Detonation Nanodiamonds and Polyelectrolyte Layers

Capsules with shells based on nanoparticles of different nature co-assembled at the interface of liquid phases of emulsion are promising carriers of lipophilic drugs. To obtain such capsules, theoretically using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and experimentally using dynamic light-scattering (DLS) and transmission electron microscopy (TEM) methods, the interaction of like-charged silica nanoparticles and detonation nanodiamonds in an aqueous solution was studied and their ratios selected for the formation of submicron-sized colloidosomes. The resulting colloidosomes were modified with additional layers of nanoparticles and polyelectrolytes, applying LbL technology. As a model anti-cancer drug, thymoquinone was loaded into the developed capsules, demonstrating a significant delay of the release as a result of colloidosome surface modification. Fluorescence flow cytometry and confocal laser scanning microscopy showed efficient internalization of the capsules by MCF7 cancer cells. The obtained results demonstrated a high potential for nanomedicine application in the field of the drug-delivery system development.

Supraparticles with silica protection for redispersible, calcined nanoparticles

Calcination of nanoparticles is always accompanied by undesired sintering. A calcination route preventing hard-agglomeration to bulk lumps, which is transferable to almost any kind of metal oxide nanoparticle, is developed by surrounding targeted nanoparticles by silica nanoparticles within a nanostructured microparticle. After calcination, the desired nanoparticles are regained as a monodisperse sol via silica dissolution.

Reconfigurable assembly of charged polymer-modified Janus and non-Janus particles: from half-raspberries to colloidal clusters and chains

Understanding the dynamic and reversible assembly of colloids and particles into complex constructs, inspired by natural phenomena, is of fundamental significance for the fabrication of multi-scale responsive and reconfigurable materials. In this work, we investigate the pH-triggered and reconfigurable assembly of structures composed of binary mixtures of oppositely charged polyacrylic acid (PAA)-modified non-Janus and poly(2-dimethylamino)ethyl methacrylate (PDMAEMA)/poly(N-isopropylacrylamide) (PNIPAM)-modified Janus particles driven by electrostatic interactions. Three different target structures are visible both in dispersions and in dry state: half-raspberry structures, colloidal clusters and colloidal chains depending on the mass, numerical and particle size ratio. All formed structures are well-defined and stable in a certain pH range. Half-raspberry-like structures are obtained at pH 6 and numerical ratios N JP/PAA-HP of 1 : 500 (for 200-PAA-HP), 1 : 44 (for 450-PAA-HP) and 1 : 15 (for 650-PAA-HP), respectively, due to electrostatic interactions between the central JP and the excessive PAA-HP. Colloidal chains and cluster-like structures are generated at numerical ratios N JP/PAA-HP of 4 : 5 (for 200-PAA-HP), 4 : 3 (for 450-PAA-HP), and 4 : 1 (for 650-PAA-HP). Moreover, the smaller the size of a "connecting" PAA colloid, the larger is the average length of a colloidal chain. Depending on the particle size ratio S JP/PAA-HP, some of the observed structures can be disassembled on demand by changing the pH value either close to the IEP of the PDMAEMA (for half-raspberries) or PAA (for colloidal clusters and chains) and then reassembled into new stable structures many times. The obtained results open a pathway to pH-controlled reconfigurable assembly of a binary mixture composed of polymeric-modified non-Janus and Janus particles, which allow the reuse of particle building blocks.

Emulsions stabilized with mixed SiO2 and Fe3O4 nanoparticles: mechanisms of stabilization and long-term stability

Using SiO₂ and Fe₃O₄ nanoparticles as stabilizers makes it possible to obtain Pickering emulsions with long-term stability to coalescence and creaming.

Block copolymer-assisted synthesis of monodisperse colloidal patchy nanoparticles

Amphiphilic block copolymers are able to assemble into spherical micelles in an aqueous solution. Spherical micelles are expected to adsorb on the surface of colloidal nanoparticles (NPs) through hydrogen-bonding interaction. Hence, it should be possible to guide the area-selective deposition of precursors onto the exposed surface of colloidal seeds, where no micelles are adsorbed. Using colloidal silica and polydopamine nanospheres as seeds, block copolymer F127 and P123 are used as surface modifiers to guide the controlled solution-phase deposition of precursors on a selectively exposed surface of seed NPs, leading to the formation of patchy NPs. Effects of the addition amount of tetraethoxysilane (TEOS), types of block copolymers, and the volume fraction of miscible organic solvents on the size and morphology of patchy silica NPs are investigated systematically through electron microscopic imaging. Block copolymer micelles adsorption model for the formation of colloidal patchy NPs is first proposed. Our study suggests that the shape and size of patchy silica NPs are determined by the amount of TEOS and dielectric constant of solution.

Supraparticles: Functionality from Uniform Structural Motifs

Under the right process conditions, nanoparticles can cluster together to form defined, dispersed structures, which can be termed supraparticles. Controlling the size, shape, and morphology of such entities is a central step in various fields of science and technology, ranging from colloid chemistry and soft matter physics to powder technology and pharmaceutical and food sciences. These diverse scientific communities have been investigating formation processes and structure/property relations of such supraparticles under completely different boundary conditions. On the fundamental side, the field is driven by the desire to gain maximum control of the assembly structures using very defined and tailored colloidal building blocks, whereas more applied disciplines focus on optimizing the functional properties from rather ill-defined starting materials. With this review article, we aim to provide a connecting perspective by outlining fundamental principles that govern the formation and functionality of supraparticles. We discuss the formation of supraparticles as a result of colloidal properties interplaying with external process parameters. We then outline how the structure of the supraparticles gives rise to diverse functional properties. They can be a result of the structure itself (emergent properties), of the colocalization of different, functional building blocks, or of coupling between individual particles in close proximity. Taken together, we aim to establish structure-property and process-structure relationships that provide unifying guidelines for the rational design of functional supraparticles with optimized properties. Finally, we aspire to connect the different disciplines by providing a categorized overview of the existing, diverging nomenclature of seemingly similar supraparticle structures.

Aggregation and Stabilization of Colloidal Spheroids by Oppositely Charged Spherical Nanoparticles

Heteroaggregation of colloids is an important yet complex physical process involving colloidal/nanosized particles and is relevant in river delta formation, paper-making, water treatment, blood flocculation, and so on. Despite the earlier studies on oppositely charged spherical colloids, heteroaggregation of colloids of different shapes is less explored. In this regard, we report an experimental study to investigate the colloidal stability of mixture of positively charged spheroidal hematite and negatively charged spherical silica nanoparticles. In this study, pH and surface area ratio (silica to hematite, S_S-H) are varied to tune the colloidal stability/instability of the suspension. At pH 6.5 and low S_S-H, the silica particles adsorb onto the hematite particles and reduce the effective charge of the latter, leading to aggregation and resulting in unstable dispersions. At higher S_S-H, adsorption of silica on hematite leads to overcharging and charge reversal, which leads to a stable dispersion. Similar experiments were performed at pH 2.4 and 3.5, and the crossover from unstable to stable dispersion is observed as a function of S_S-H. Calculation of Derjaguin, Landau, Verwey, and Overbeek (DLVO) interaction between particles in the binary mixture, as a function of pH and S_S-H, based on the aggregate size and zeta potential, explains the transition from unstable to stable dispersion. The size and zeta potential of heteroaggregates in the dispersion were analyzed by dynamic light scattering (DLS) technique. Adsorption of silica nanoparticles on hematite particles was visualized by scanning electron microscopy (SEM). The study provides a framework based on DLVO interactions to stabilize or destabilize a colloidal dispersion of nonspherical particles by controlled addition of oppositely charged spherical colloids, which is a feat that is not possible with simple salt. The stability ratio ( W) calculated from DLVO interactions demark the unstable-stable dispersion regions, which is found to be in agreement with the experimental results.

Engineering nanomaterials to overcome the mucosal barrier by modulating surface properties

Although nanotechnology has been investigated during recent years to increase the bioavailability and therapeutic effects of mucosal administrated drugs, numerous barriers (e.g., pH environment, enzymes and mucus) still limit the delivery efficiency. And the epithelium would also affect the systemic mucosal drug delivery. Amongst all the barriers, the protective mucus has drawn more and more attention, which strongly hinders the accessibility of nanovehicles to epithelium. Therefore, trials to conquer the mucus barrier have been designed using two controversial strategies: mucoadhesion and mucus-penetration. This review summarizes the influence of mucus layer on nanomaterials and introduces the modification strategies by modulating surface properties (i.e., hydrophilicity/hydrophobicity and surface charge) to overcome mucus barriers. Furthermore, it also reviews advanced modification methods to meet the different surface requirements of nanovehicles to overcome mucus and epithelium barriers in systemic mucosal delivery.