Mechanics of colloidal supraparticles under compression

Hydrogel-Assisted Robust Supraparticles Evolved from Droplet Evaporation

Supraparticles, formed through the self-assembly of nanoparticles, are promising contenders in catalysis, sensing, and drug delivery due to their exceptional specific surface area and porosity. However, their mechanical resilience, especially in dimensions spanning micrometers and beyond, is challenged by the inherently weak interactions among their constituent building blocks, significantly constraining their broad applicability. Here, we have exploited a robust supraparticle fabrication strategy by integrating hydrogel components into the assembly system and evaporating on the superamphiphobic surface. The resultant SiO2/SA (sodium alginate) supraparticles, achieved by evaporating a 15% volume fraction dispersion of SiO2 nanoparticles containing 18.46 mg/mL of sodium alginate and subsequently cross-linking with Ca2+, demonstrate mechanical robustness with a fracture force of 6.04 N, representing a mechanical strength enhancement of 60 times higher than that prior to the incorporation of the hydrogel component. The supraparticles maintain their original morphology after 30 min of ultrasonic treatment (200 W), demonstrating mechanical stability. This method exhibits generalizability, enabling the customization of supraparticles with various building blocks and hydrogel backbone materials. Based on such a methodology, we have synthesized enzyme-carrying supraparticles, further expanding the potential applications in intricate cascade reactions. The encapsulated glucose oxidase and horseradish peroxidase maintained their inherent reactivity, and such hydrogel-assisted robust supraparticles exhibited exceptional performance in accurate glucose assays, indicating great practical application in biocatalysis.

Controlled Nanopore Sizes in Supraparticle Supports for Enhanced Propane Dehydrogenation with GaPt SCALMS Catalysts

The efficient immobilization of GaPt liquid metal alloy droplets onto tailored supports improves catalytic performance by preventing coalescence and subsequent loss of active surface area. Herein, we use tailored supraparticle (SP) supports with controlled nanopores to systematically study the influence of pore sizes on the catalytic stability of GaPt supported catalytically active liquid metal solution (SCALMS) in propane dehydrogenation (PDH). Initially, GaPt droplets were prepared via an atom-efficient and scalable ultrasonication method with recycling loops to yield droplets <300 nm. Subsequently, these droplets were immobilized onto SiO2-based SPs with controlled pore sizes ranging from 45 to 320 nm. Catalytic evaluations in PDH revealed that GaPt immobilized on SPs with larger pores demonstrated superior stability over 15 h time-on-stream evidenced by reduced deactivation rates from 0.046 to 0.026 h-1. Nanocomputed tomography and identical location SEM confirmed the successful immobilization of GaPt droplets within the interstitial sites formed by the primary particles constituting the SPs. These remained unchanged before and after the catalytic reaction, demonstrating efficient coalescence prevention. Our findings underscore the importance of support pore size engineering for improving the stability of GaPt SCALMS catalysts and highlight, particularly, the high potential of using SPs in this context.

Fabrication of Spherical Colloidal Supraparticles via Membrane Emulsification

Colloidal supraparticles are micrometer-scale assemblies of primary particles. These supraparticles have potential application in photonic materials, catalysis, gas adsorption, and drug delivery. Thus, the synthesis of colloidal supraparticles with a narrow size distribution and high yield has become essential. Here, we demonstrate membrane emulsification as a high-throughput approach for fabricating spherical supraparticles with a narrow size distribution and control over particle size and crystallinity. Spherical supraparticles with well-ordered surface structures are synthesized by generating emulsion droplets of an aqueous colloidal dispersion in fluorocarbon oil using a Shirasu porous glass membrane followed by the consolidation of particles through water removal within the emulsion. We systematically investigate process parameters, including the flow rate of the particle dispersion, the particle concentration, and the average pore diameter of the membrane, on the mean size and size distribution of the supraparticles, revealing key factors governing supraparticle properties and production throughput. A comparative evaluation with commonly employed methods highlights the advantage of membrane emulsification, which combines well-defined internal structure and controlled supraparticle sizes with comparably high yields on the order of tens of grams per day. Importantly, in contrast to widely used droplet-based microfluidics, membrane emulsification allows fabrication of supraparticles in nonfluorinated oil. Overall, membrane emulsification offers a simple yet versatile method for fabricating colloidal supraparticles with high quality and yield and may serve as a bridge between existing high-precision techniques, such as droplet-based microfluidics, and high-throughput processes with less control, such as spray-drying.

Analytical solution of a microrobot-blood vessel interaction model

This study develops a dynamics model of a microrobot vibrating in a blood vessel aiming to detect potential cancer metastasis. We derive an analytical solution for microrobot's motion, considering interactions with the vessel walls modelled by a linear spring-dashpot and a constant damping value for blood viscosity. The model facilitates instantaneous state transitions of the microrobot, such as contact with the vessel wall and free motion within the fluid. Amplitudes and phase angles from the transient solutions of dynamics model of the microrobot are solved at arbitrary moments, providing insights into its transient dynamics. The analytical solution of the proposed system is validated by experimental data, serving as a benchmark to examine the influence of pertinent parameters on microrobot's dynamic response. It is found that the contact force transmitted to the vessel wall, assessed by system's transmissibility function dependent on damping and frequency ratios, decreases with increasing damping ratio and intensifies when the frequency ratio is below 2 . At the frequency ratio is equal to 1, resonance phenomenon is dominated by the magnification factor linked to the damping ratio, increasing the amplitude of resonance as damping decreases. Finally, different sets of system parameters, including excitation frequency and magnitude, fluid damping, vessel wall's stiffness and damping, reveal multi-periodic motions and fake collision of the microrobot with the vessel wall. Simulation results imply that these phenomena are minimally affected by vessel wall's stiffness but are significantly influenced by other parameters, such as fluid damping coefficient and damping coefficient of the blood vessel wall. This research provides a robust theoretical foundation for developing control strategies for microrobots aimed at detecting cancer metastasis.

Functional supraparticles produced by the evaporation of binary colloidal suspensions on superhydrophobic surfaces

Hierarchically structured supraparticles can be produced by drying droplets of colloidal suspensions. Using binary suspensions provides degrees of structural and functional control beyond those possible for single components, while remaining tractable for fundamental mechanistic studies. Here, we implement evaporative co-assembly of two distinct particle types - 'large' polystyrene microparticles and 'small' inorganic oxide nanoparticles (silica, titania, zirconia, or ceria) - dried on superhydrophobic surfaces to produce bowl-shaped supraparticles. We extend this method to raspberry colloid templating, in which the binary suspension consists of titania nanoparticles together with gold-decorated polystyrene colloids. Following removal of the polymer particles, we demonstrate catalytic oxidative coupling of methanol to methyl formate using the resulting mesoporous supraparticles, showcasing their practical application.

Engineering Porosity-Tuned Chitosan Beads: Balancing Porosity, Kinetics, and Mechanical Integrity

Chitosan, a cationic natural polysaccharide derived from the deacetylation of chitin, is known for its solubility in diluted acidic solutions, biodegradability, biocompatibility, and nontoxicity. This study introduces three innovative methods for preparing various types of porous chitosan beads: solvent extraction, surfactant extraction, and substance decomposition. These methods involve the integration and subsequent extraction or decomposition of materials during the synthesis process, eliminating the need for additional steps. We used state-of-the-art characterization techniques to analyze and evaluate the chemical and physical properties of the beads, such as Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and three-dimensional (3D) computed tomography (CT) scanning. The 3D CT scans visualized and measured the porosity of different bead types, ranging from 68.4% to 39.3%. This study also evaluated the mechanical properties of the particle beads under compressive forces in both wet and dry conditions, highlighting the influence of porosity on their mechanical integrity and compression pressure behavior. The adsorptive properties of these chitosan beads were studied using methylene blue as a model pollutant, emphasizing the importance of balancing porous structure, surface area, kinetics, and structural integrity. This study paves the way for the development of environmentally sustainable polymeric beads, highlighting the crucial need to balance porosity, surface area, and structural integrity to optimize their effectiveness in real-world applications.

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.

Machine Learning-Assisted High-Throughput Identification and Quantification of Protein Biomarkers with Printed Heterochains

Advanced in vitro diagnosis technologies are highly desirable in early detection, prognosis, and progression monitoring of diseases. Here, we engineer a multiplex protein biosensing strategy based on the tunable liquid confinement self-assembly of multi-material heterochains, which show improved sensitivity, throughput, and accuracy compared to standard ELISA kits. By controlling the material combination and the number of ligand nanoparticles (NPs), we observe robust near-field enhancement as well as both strong electromagnetic resonance in polymer-semiconductor heterochains. In particular, their optical signals show a linear response to the coordination number of the semiconductor NPs in a wide range. Accordingly, a visible nanophotonic biosensor is developed by functionalizing antibodies on central polymer chains that can identify target proteins attached to semiconductor NPs. This allows for the specific detection of multiple protein biomarkers from healthy people and pancreatic cancer patients in one step with an ultralow detection limit (1 pg/mL). Furthermore, rapid and high-throughput quantification of protein expression levels in diverse clinical samples such as buffer, urine, and serum is achieved by combining a neural network algorithm, with an average accuracy of 97.3%. This work demonstrates that the heterochain-based biosensor is an exemplary candidate for constructing next-generation diagnostic tools and suitable for many clinical settings.

Building respirable powder architectures: utilizing polysaccharides for precise control of particle morphology for enhanced pulmonary drug delivery

INTRODUCTION

Dry powder inhaler (DPI) formulations are gaining attention as universal formulations with applications in a diverse range of drug formulations. The practical application of DPIs to pulmonary drugs requires enhancing their delivery efficiency to the target sites for various treatment modalities. Previous reviews have not explored the relation between particle morphology and delivery to different pulmonary regions. This review introduces new approaches to improve targeted DPI delivery using novel particle design such as supraparticles and metal-organic frameworks based on cyclodextrin.

AREAS COVERED

This review focuses on the design of DPI formulations using polysaccharides, promising excipients not yet approved by regulatory agencies. These excipients can be used to design various particle morphologies by controlling their physicochemical properties and manufacturing methods.

EXPERT OPINION

Challenges associated with DPI formulations include poor access to the lungs and low delivery efficiency to target sites in the lung. The restricted applicability of typical excipients contributes to their limited use. However, new formulations based on polysaccharides are expected to establish a technological foundation for the development of DPIs capable of delivering modalities specific to different lung target sites, thereby enhancing drug delivery.

Benchmarking the Humidity-Dependent Mechanical Response of (Nano)fibrillated Cellulose and Dissolved Polysaccharides as Sustainable Sand Amendments

Soil quality is one of the main limiting factor in the development of the food sector in arid areas, mainly due to its poor mechanics and lack of water retention. Soil's organic carbon is nearly absent in arid soils, though it is important for water and nutrient transport, to soil mechanics, to prevent erosion, and as a long-term carbon sink. In this study, we evaluate the potential benefits that are brought to inert sand by the incorporation of a range of, mainly, cellulosic networks in their polymeric or structured (fiber) forms, analogously to those found in healthy soils. We explore the impact of a wide range of nonfood polysaccharide-based amendments, including pulp fibers, nanocellulose, cellulose derivatives, and other readily available polysaccharide structures derived from arthropods (chitosan) or fruit peels (pectin) residues. A practical methodology is presented to form sand-polymer composites, which are evaluated for their soil mechanics as a function of humidity and the dynamics of their response to water. The mechanics are correlated to the network of polymers formed within the pores of the sandy soil, as observed by electron microscopy. The response to water is correlated to both the features of the network and the individual polysaccharides' physicochemical features. We expect this work to provide a rapid and reproducible methodology to benchmark sustainable organic amendments for arid soils.

Multifunctional, Hybrid Materials Design via Spray-Drying: Much more than Just Drying

Spray-drying is a popular and well-known "drying tool" for engineers. This perspective highlights that, beyond this application, spray-drying is a very interesting and powerful tool for materials chemists to enable the design of multifunctional and hybrid materials. Upon spray-drying, the confined space of a liquid droplet is narrowed down, and its ingredients are forced together upon "falling dry." As  detailed in this article, this enables the following material formation strategies either individually or even in combination: nanoparticles and/or molecules can be assembled; precipitation reactions as well as chemical syntheses can be performed; and templated materials can be designed. Beyond this, fragile moieties can be processed, or "precursor materials" be prepared. Post-treatment of spray-dried objects eventually enables the next level in the design of complex materials. Using spray-drying to design (particulate) materials comes with many advantages-but also with many challenges-all of which are outlined here. It is believed that multifunctional, hybrid materials, made via spray-drying, enable very unique property combinations that are particularly highly promising in myriad applications-of which catalysis, diagnostics, purification, storage, and information are highlighted.

Tunable Mechanical Response of Self-Assembled Nanoparticle Superlattices

Self-assembled nanoparticle superlattices (NPSLs) are an emergent class of self-architected nanocomposite materials that possess promising properties arising from precise nanoparticle ordering. Their multiple coupled properties make them desirable as functional components in devices where mechanical robustness is critical. However, questions remain about NPSL mechanical properties and how shaping them affects their mechanical response. Here, we perform in situ nanomechanical experiments that evidence up to an 11-fold increase in stiffness (∼1.49 to 16.9 GPa) and a 5-fold increase in strength (∼88 to 426 MPa) because of surface stiffening/strengthening from shaping these nanomaterials via focused-ion-beam milling. To predict the mechanical properties of shaped NPSLs, we present discrete element method (DEM) simulations and an analytical core-shell model that capture the FIB-induced stiffening response. This work presents a route for tunable mechanical responses of self-architected NPSLs and provides two frameworks to predict their mechanical response and guide the design of future NPSL-containing devices.

Quantitative Optical and Structural Comparison of 3D and (2+1)D Colloidal Photonic Crystals

Colloidal crystals are excellent model systems to study self-assembly and structural coloration because their periodicities coincide with the wavelength range of visible light. Different assembly methods inherently introduce characteristic defects and irregularities, even with nearly monodisperse colloidal particles. Here, we investigate how these imperfections influence the structural coloration by comparing two techniques to obtain colloidal crystals. 3D colloidal crystals produced by convective assembly are well-ordered and periodically arranged but show microscopic cracks. (2+1)D colloidal crystals fabricated by stacking individual monolayers show a decreased hexagonal order and limited crystal registration between single monolayers in the z-direction. We investigate the optical properties of both systems by comparing identical numbers of layers using correlative microspectroscopy. These measurements show that the less ordered (2+1)D colloidal crystals exhibit higher reflected light intensities. Macroscopic reflection integrating all angles shows that the reflected light intensity levels out with an increasing number of layers, whereas incoherent scattering increases. Although both types of colloidal crystal show similar angle-dependent color shifts in specular reflection, the less-ordered structure of the (2+1)D colloidal crystal scatters light within a larger angular range under diffusive illumination. Our results suggest that structural coloration is surprisingly robust toward local defects and irregularities.

From Meso to Macro: Controlling Hierarchical Porosity in Supraparticle Powders

A drying droplet containing colloidal particles can consolidate into a spherical assembly called a supraparticle. Such supraparticles are inherently porous due to the spaces between the constituent primary particles. Here, the emergent, hierarchical porosity in spray-dried supraparticles is tailored via three distinct strategies acting at different length scales. First, mesopores (<10 nm) are introduced via the primary particles. Second, the interstitial pores are tuned from the meso- (35 nm) to the macro scale (250 nm) by controlling the primary particle size. Third, defined macropores (>100 nm) are introduced via templating polymer particles, which can be selectively removed by calcination. Combining all three strategies creates hierarchical supraparticles with fully tailored pore size distributions. Moreover, another level of the hierarchy is added by fabricating supra-supraparticles, using the supraparticles themselves as building blocks, which provide additional pores with micrometer dimensions. The interconnectivity of the pore networks within all supraparticle types is investigated via detailed textural and tomographic analysis. This work provides a versatile toolbox for designing porous materials with precisely tunable, hierarchical porosity from the meso- (3 nm) to the macroscale (≈10 µm) that can be utilized for applications in catalysis, chromatography, or adsorption.

Spray-Dried Photonic Balls with a Disordered/Ordered Hybrid Structure for Shear-Stress Indication

Optical microscale shear-stress indicator particles are of interest for the in situ recording of localized forces, e.g., during 3D printing or smart skins in robotic applications. Recently developed particle systems are based on optical responses enabled by integrated organic dyes. They thus suffer from potential chemical instability and cross-sensitivities toward humidity or temperature. These drawbacks can be circumvented using photonic balls as shear-stress indicator particles, which employ structural color as the element to record forces. Here, such photonic balls are prepared from silica and iron oxide nanoparticles via the scalable and fast spray-drying technique. Process parameters to create photonic balls with a disordered core and an ordered particle structure toward the exterior of the supraparticles are reported. This hybrid disordered-ordered structure is responsible for a color loss of the indicator particles during shear-stress application because of irreversible structural destruction. By adjusting the primary silica particle sizes, nearly all colors of the visible spectrum can be achieved and the sensitivity of the response to shear stress can be adjusted.

New Class of Polymer Materials-Quasi-Nematic Colloidal Particle Self-Assemblies: The Case of Assemblies of Prolate Spheroidal Poly(Styrene/Polyglycidol) Particles

Assemblies of colloidal polymer particles find various applications in many advanced technologies. However, for every type of application, assemblies with properly tailored properties are needed. Until now, attention has been concentrated on the assemblies composed of spherical particles arranged into so-called perfect colloidal crystals and on complex materials containing mixtures of crystal and disordered phases. However, new opportunities are opened by using assemblies of spheroidal particles. In such assemblies, the particles, in addition to the three positional have three angular degrees of freedom. Here, the preparation of 3D assemblies of reference microspheres and prolate spheroidal poly(styrene/polyglycidol) microparticles by deposition from water and water/ethanol media on silicon substrates is reported. The particles have the same polystyrene/polyglycidol composition and the same volumes but differ with respect to their aspect ratio (AR) ranged from 1 to 8.5. SEM microphotographs reveal that particles in the assembly top layers are arranged into the quasi-nematic structures and that the quality of their orientation in the same direction increase with increasing AR. Nano- and microindentation studies demonstrate that interactions of sharp and flat tips with arrays of spheroidal particles lead to different types of particle deformations.

Electro-Microfluidic Assembly Platform for Manipulating Colloidal Structures inside Water-in-Oil Emulsion Droplets

Colloidal assembly is a key strategy in nature and artificial device. Hereby, an electromicrofluidic assembly platform (eMAP) is proposed and validated to achieve 3D colloidal assembly and manipulation within water droplets. The water-in-oil emulsion droplets autoposition in the eMAP driven by dielectrophoresis, where the (di)electrowetting effect induces droplet deformation, facilitating quadratic growth of the electric field in water droplet to achieve "far-field" dielectrophoretic colloidal assembly. Reconfigurable 3D colloidal configurations are observed and dynamically programmed via applied electric fields, colloidal properties, and droplet size. Binary and ternary colloidal assemblies in one droplet allow designable chemical and physical anisotropies for functional materials and devices. Integration of eMAP in high throughput enables mass production of functional microcapsules, and programmable optoelectronic units for display devices. This eMAP is a valuable reference for expanding fundamental and practical exploration of colloidal systems.

Supraparticles with a Mechanically Triggerable Color-Change-Effect to Equip Coatings with the Ability to Report Damage

Small scratches and abrasion cause damage to packaging coatings. Albeit often invisible to the human eye, such small defects in the coating may ultimately have a strong negative impact on the whole system. For instance, gases may penetrate the coating and consequently the package barrier, thus leading to the degradation of sensitive goods. Herein, the indicators of mechanical damage in the form of particles are reported, which can readily be integrated into coatings. Shear stress-induced damage is indicated by the particles via a color change. The particles are designed as core-shell supraparticles. The supraparticle core is based on rhodamine B dye-doped silica nanoparticles, whereas the shell is made of alumina nanoparticles. The alumina surface is functionalized with a monolayer of a perylene dye. The resulting core-shell supraparticle system thus contains two colors, one in the core and one in the shell part of the architecture. Mechanical damage of this structure exposes the core from the shell, resulting in a color change. With particles integrated into a coating lacquer, mechanical damage of a coating can be monitored via a color change and even be related to the degree of oxygen penetration in a damaged coating.

Self-assembled hierarchical metal–polyphenol-coordinated hybrid 2D Co–CTA@g-C3N4 heterostructured nanosheets for efficient electrocatalytic oxygen reduction

We exquisitely designed the in situ growth of Co nanoparticles coated with tannin-carbon embedded on the hierarchical ultrathin g-C3N4 nanosheets for boosting electrocatalytic oxygen reduction through the MPNs strategy.

Influence of Surfactant-Mediated Interparticle Contacts on the Mechanical Stability of Supraparticles

Colloidal supraparticles are micron-scale spherical assemblies of uniform primary particles, which exhibit emergent properties of a colloidal crystal, yet exist as a dispersible powder. A prerequisite to utilize these emergent functionalities is that the supraparticles maintain their mechanical integrity upon the mechanical impacts that are likely to occur during processing. Understanding how the internal structure relates to the resultant mechanical properties of a supraparticle is therefore of general interest. Here, we take the example of supraparticles templated from water/fluorinated oil emulsions in droplet-based microfluidics and explore the effect of surfactants on their mechanical properties. Stable emulsions can be generated by nonionic block copolymers consisting of a hydrophilic and fluorophilic block and anionic fluorosurfactants widely available under the brand name Krytox. The supraparticles formed in the presence of both types of surfactants appear structurally similar, but differ greatly in their mechanical properties. While the nonionic surfactant induces superior mechanical stability and ductile fracture behavior, the anionic Krytox surfactant leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin light spectroscopy that is very sensitive to the interparticle contacts for subnanometer-thick adsorbed layers atop of the nanoparticle. While the anionic Krytox does not significantly affect the interparticle bonds, the amphiphilic nonionic surfactant drastically strengthens these bonds to the point that individual particle vibrations are not resolved in the experimental spectrum. Our results demonstrate that seemingly subtle changes in the physicochemical properties of supraparticles can drastically impact the resultant mechanical properties.