Preparation and characterization of spherical monodisperse silica dispersions in nonaqueous solvents

Direct measurements & simplified models of colloidal interactions & diffusion with adsorbed macromolecules

We report total internal reflection microscopy measurements of 3D trajectories of ensembles of micron sized colloidal particles near interfaces with and without adsorbed macromolecules. Evanescent wave scattering reveals nanometer scale motion normal to planar surfaces and sub-diffraction limit lateral motion is resolved via image analysis. Equilibrium and non-equilibrium analyses of particle trajectories reveal self-consistent position dependent energies (energy landscapes) and position dependent diffusivities (diffusivity landscapes) both perpendicular and parallel to interfaces. For bare colloids and surfaces, electrostatic and hydrodynamic interactions are accurately quantified with established analytical theories. For colloids and surfaces with adsorbed macromolecules, conservative forces are accurately quantified with models for interactions between brush layers, whereas directly measured position dependent diffusivities require novel models of spatially varying permeability within adsorbed layers. Agreement between spatially resolved interactions and diffusivities and rigorous simplified models provide a basis to consistently interpret, predict, and design colloidal transport in the presence of adsorbed macromolecules for diverse applications.

Spearheading a new era in complex colloid synthesis with TPM and other silanes

Colloid science has recently grown substantially owing to the innovative use of silane coupling agents (SCAs), especially 3-trimethoxysilylpropyl methacrylate (TPM). SCAs were previously used mainly as modifying agents, but their ability to form droplets and condense onto pre-existing structures has enabled their use as a versatile and powerful tool to create novel anisotropic colloids with increasing complexity. In this Review, we highlight the advances in complex colloid synthesis facilitated by the use of TPM and show how this has driven remarkable new applications. The focus is on TPM as the current state-of-the-art in colloid science, but we also discuss other silanes and their potential to make an impact. We outline the remarkable properties of TPM colloids and their synthesis strategies, and discuss areas of soft matter science that have benefited from TPM and other SCAs.

Thermoreversible gels of hollow silica nanorod dispersions

Colloidal suspensions of anisotropic particles are ubiquitous in particle-based industries. Consequently, there is a need to quantify the effects of particle shape on equilibrium phases and kinetic state transitions, particularly at lower aspect ratios (L/D ≈ 1-10). We present a new, colloidal system comprised of hollow, octadecyl-coated silica rods with 40 nm diameter with controlled aspect ratio and thermoreversible short-range attractions. Rheology and dynamic light scattering measurements on suspensions of these hollow adhesive hard rods with nominal aspect ratio ≈3 suspended in tetradecane exhibit thermoreversible gelation without complicating effects of gravitational settling. Small angle neutron scattering measurements of the microstructure are analyzed to determine the effective strength of attraction in the form of Baxter sticky parameter. Quantitative agreement is found with simulation predictions of the thermoreversible gel transition as a function of volume fraction, further validating a universal state diagram and providing guidance for the effects of aspect ratio on gelation.

Toughening colloidal gels using rough building blocks

Colloidal gels, commonly used as mesoporous intermediates or functional materials, suffer from brittleness, often showing small yield strains on the order of 1% or less for gelled colloidal suspensions. The short-range adhesive forces in most such gels are central forces-combined with the smooth morphology of particles, the resistance to yielding and shear-induced restructuring is limited. In this study, we propose an innovative approach to improve colloidal gels by introducing surface roughness to the particles to change the yield strain, giving rise to non-central interactions. To elucidate the effects of particle roughness on gel properties, we prepared thermoreversible gels made from rough or smooth silica particles using a reliable click-like-chemistry-based surface grafting technique. Rheological and optical characterization revealed that rough particle gels exhibit enhanced toughness and self-healing properties. These remarkable properties can be utilized in various applications, such as xerogel fabrication and high-fidelity extrusion 3D-printing, as we demonstrate in this study.

Tailoring the Morphology of Monodisperse Mesoporous Silica Particles Using Different Alkoxysilanes as Silica Precursors

The hard template method for the preparation of monodisperse mesoporous silica microspheres (MPSMs) has been established in recent years. In this process, in situ-generated silica nanoparticles (SNPs) enter the porous organic template and control the size and pore parameters of the final MPSMs. Here, the sizes of the deposited SNPs are determined by the hydrolysis and condensation rates of different alkoxysilanes in a base catalyzed sol-gel process. Thus, tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), tetrapropyl orthosilicate (TPOS) and tetrabutyl orthosilicate (TBOS) were sol-gel processed in the presence of amino-functionalized poly (glycidyl methacrylate-co-ethylene glycol dimethacrylate) (p(GMA-co-EDMA)) templates. The size of the final MPSMs covers a broad range of 0.5-7.3 µm and a median pore size distribution from 4.0 to 24.9 nm. Moreover, the specific surface area can be adjusted between 271 and 637 m² g^-1. Also, the properties and morphology of the MPSMs differ according to the SNPs. Furthermore, the combination of different alkoxysilanes allows the individual design of the morphology and pore parameters of the silica particles. Selected MPSMs were packed into columns and successfully applied as stationary phases in high-performance liquid chromatography (HPLC) in the separation of various water-soluble vitamins.

Hierarchically Structured Deformation-Sensing Mechanochromic Pigments

Mechanochromic materials alter their color in response to mechanical force and are useful for both fundamental studies and practical applications. Several approaches are used to render polymers mechanochromic, but they generally suffer from limitations in sensing range, capacity to provide quantitative information, and their capability to enable broad and simple implementation. Here, is it reported that these problems can be overcome by combining photonic structures, which alter their reflection upon deformation, with covalent mechanophores, whose spectral properties change upon mechanically induced bond scission, in hierarchically structured mechanochromic pigments. This is achieved by synthesizing microspheres consisting of an elastic polymer with spiropyran-based cross-links and non-close-packed silica nanoparticles. A strain of less than 1% can be detected in a shift of the reflection band from the photonic structure, while the onset strain for the conversion of the spiropyran into fluorescent merocyanine ranges from 30% to 70%, creating a broad strain detection range. The two responses are tailorable and synergistic, permitting the activation strain for the mechanophore response to be tuned. The mechano-sensing photonic pigments are demonstrated to be readily incorporated into different polymeric materials of interest and quantitatively probe spatially heterogeneous deformations over a large strain range.

Stöber method and its nuances over the years

Some characteristics of silica-based materials, such as the control/adjustment of their physical and chemical properties, compatibility, and friendly-use synthesis methods, have held the attention of several scientific groups over the years. This condition of prominence becomes even more evident when we seek these characteristics at the micro- and/or nanoscale. Among existing methods to obtain these micro/nanomaterials, the Stöber method is the focus of this review. This method is known to enable the production of silica micro- or nanoparticles from reagents of medium-easy manipulation under mild conditions using equipment that is common in most laboratories. However, this method has many nuances that must be considered to guarantee accurate results, either in size or distribution, and to ensure result reproducibility. Thus, in this review, we discuss the effects of the primary components used in the synthesis of these materials (i.e., TEOS, ammonia, and water), as well as those of other reaction conditions, such as solvent, temperature, and ionic strength. Therefore, we discuss studies involving the synthesis and characterization of micro- and nanoparticles over the years to establish discussions between their experimental observations and proposed models. This review provides experimental observations about the synthesis of these materials, as well as discussions according to complementary and/or contradictory evidence found over the years. This review seeks to help those who intend to work with this method and provide certain key points that, in our experience, can be important to obtain desired results.

Revisiting carboxylic group functionalization of silica sol-gel materials

The carboxylic chemical group is a ubiquitous moiety present in amino acids, a ligand for transition metals, a colloidal stabilizer, and a weak acidic ion-exchanger in polymeric resins and given this property, it is attractive for responsive materials or nanopore-based gating applications. As the number of uses increases, subtle requirements are imposed on this molecular group when anchored to various platforms for the functioning of an integrated chemical system. In this context, silica stands as an inert and multipurpose platform that enables the anchoring of multiple chemical entities combined through several orthogonal synthesis methods on the interface. Surface chemical modification relies on the use of organoalkoxysilanes that must meet the demand of tuned chemical properties; this, in turn, urges for innovative approaches for having an improved, but simple, organic toolbox. Starting from commonly available molecular precursors, several approaches have emerged: hydrosilylation, click thiol-ene additions, the use of carbodiimides or the reaction between cyclic anhydrides and anchored amines. In this review, we analyze the importance of the COOH groups in the area of materials science and the commercial availability of COOH-based silanes and present new approaches for obtaining COOH-based organoalkoxide precursors. Undoubtedly, this will attract widespread interest for the ultimate design of highly integrated chemical platforms.

Blood Protein Exclusion from Polymer Brushes

We report interactions between adsorbed copolymers of poly(ethylene glycol) (PEG) in the presence of two abundant blood proteins, serum albumin and an immunoglobulin G, up to physiological blood concentrations. We directly and nonintrusively measure interactions between PEG triblock copolymers (PEG-PPO-PEG) adsorbed to hydrophobic colloids and surfaces using Total Internal Reflection Microscopy, which provides kT- and nanometer-scale resolution of interaction potentials (energy vs separation). In the absence of protein, adsorbed PEG copolymer repulsion is consistent with dimensions and architectures of PEG brushes on both colloids and surfaces. In the presence of proteins, we observe concentration dependent depletion attraction and no change to brush repulsion, indicating protein exclusion from PEG brushes. Because positive and negative protein adsorption are mutually exclusive, our observations of concentration dependent depletion attraction with no change to brush repulsion unambiguously indicate the absence of protein coronas at physiological protein concentrations. These findings demonstrate a direct sensitive approach to determine interactions between proteins and particle/surface coatings important to diverse biotechnology applications.

Hollow Silica Nano and Micro Spheres with Polystyrene Templating: A Mini-Review

Synthesis of monodisperse hollow silica nanospheres, especially using a hard template route, has been shown to be successful, but a high yield is needed for this strategy to be used on an industrial scale. On the other hand, there is a research gap in the synthesis of hollow silica microspheres due to the popularity and easiness of the synthesis of silica nanospheres despite the larger spheres being beneficial in some fields. In this review, current trends in producing hollow silica nanospheres using hard templates, especially polystyrene, are briefly presented. Soft templates have also been used to make highly polydisperse hollow silica spheres, and complex designs have improved polydispersity. The effect of the main parameters on the coating is presented here to provide a basic understanding of the interactions between the silica and template surface in the absence or presence of surfactants. Surface charge, surface modification, parameters in the sol-gel method and interaction between the silica and templates need to be further improved to have a uniform coating and better control over the size, dispersity, wall thickness and porosity. As larger organic templates will have lower surface energy, the efficiency of the micro sphere synthesis needs to be improved. Control over the physical structure of hollow silica spheres will open up many opportunities for them to be extensively used in fields ranging from waste removal to energy storage.

The interfacial and assembly properties of in situ producing silica nanoparticle at oil-water interface

In multiphase materials, structured fluid-fluid interfaces can provide mechanical resistance against destabilization, applicable for conformance control, Pickering emulsion, liquid 3D printing and molding, etc. Currently all research prepare the particle-ladened fluid-fluid interfaces by dispersing ex situ acquired particles to the immiscible interface, which limits their application in the harsh environment, such as oil reservoir which can impair particle stability and transport ability. Here, we investigated the interfacial and assembly properties of the interface where SiO2 nanoparticles (NPs) were in situ produced. The experimental results show that ammonia as catalyst could accelerate the processes of silica NPs formation as well as the interfacial tension (IFT) evolution. High temperature could not accelerate the reaction processes to achieve the lowest equilibrium IFT, but it induced the sine-wave IFT evolution curves regardless of the presence of ammonia. The equilibrium IFTs corresponded to the saturation states of interfaces trapping with SiO2 NPs, while the sine-wave fluctuating patterns of IFT were attributed to the alternating transition between interfacial jammed and unjammed states changing along with the reaction process. Silica NPs diffusing into aqueous phase with high salinity also showed good stability, due to the abundant surface decoration with in situ anchored organic species.

Highly sensitive and selective detection of glutathione using ultrasonic aided synthesis of graphene quantum dots embedded over amine-functionalized silica nanoparticles

Glutathione (GSH) is the most abundant antioxidant in the majority of cells and tissues; and its use as a biomarker has been known for decades. In this study, a facile electrochemical method was developed for glutathione sensing using voltammetry and amperometry analyses. In this study, a novel glassy carbon electrode composed of graphene quantum dots (GQDs) embedded on amine-functionalized silica nanoparticles (SiNPs) was synthesized. GQDs embedded on amine-functionalized SiNPs were physical-chemically characterized by different techniques that included high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction spectroscopy (XRD), UV-visible spectroscopy, Fourier-transform infrared spectroscopy(FTIR), and Raman spectroscopy. The newly developed electrode exhibits a good response to glutathione with a wide linear range (0.5-7 µM) and a low detection limit (0.5 µM) with high sensitivity(2.64 µA µM-1). The fabricated GQDs-SiNPs/GC electrode shows highly attractive electrocatalytic activity towards glutathione detection in the neutral media at low potential due to a synergistic surface effect caused by the incorporation of GQDs over SiNPs. It leads to higher surface area and conductivity, improving electron transfer and promoting redox reactions. Besides, it provides outstanding selectivity, reproducibility, long-term stability, and can be used in the presence of interferences typically found in real sample analysis.

Monolayer Support Control and Precise Colloidal Nanocrystals Demonstrate Metal-Support Interactions in Heterogeneous Catalysts

Electronic and geometric interactions between active and support phases are critical in determining the activity of heterogeneous catalysts, but metal-support interactions are challenging to study. Here, it is demonstrated how the combination of the monolayer-controlled formation using atomic layer deposition (ALD) and colloidal nanocrystal synthesis methods leads to catalysts with sub-nanometer precision of active and support phases, thus allowing for the study of the metal-support interactions in detail. The use of this approach in developing a fundamental understanding of support effects in Pd-catalyzed methane combustion is demonstrated. Uniform Pd nanocrystals are deposited onto Al₂ O₃ /SiO₂ spherical supports prepared with control over morphology and Al₂ O₃ layer thicknesses ranging from sub-monolayer to a ≈4 nm thick uniform coating. Dramatic changes in catalytic activity depending on the coverage and structure of Al₂ O₃ situated at the Pd/Al₂ O₃ interface are observed, with even a single monolayer of alumina contributing an order of magnitude increase in reaction rate. By building the Pd/Al₂ O₃ interface up layer-by-layer and using uniform Pd nanocrystals, this work demonstrates the importance of controlled and tunable materials in determining metal-support interactions and catalyst activity.

Phase stability of colloidal mixtures of spheres and rods

We determined the phase boundaries of aqueous mixtures containing colloidal rod-like fd-viruses and polystyrene spheres using diffusing-wave spectroscopy and compared the results with free volume theory predictions. Excluded volume interactions in mixtures of colloidal rods and spheres lead to mediated depletion interactions. The strength and range of this attractive interaction depend on the concentrations of the particles, the length L and diameter D of the rods, and the radius R of the spheres. At strong enough attraction, this depletion interaction leads to phase separation. We experimentally determined the rod and sphere concentrations where these phase transitions occur by systematically varying the size ratios L/R and D/R and the aspect ratio L/D. This was done by using spheres with different radii and modifying the effective diameter of the rods through either the ionic strength of the buffer or anchoring a polymeric brush to the surface of the rods. The observed phase transitions were from a binary fluid to a colloidal gas/liquid phase coexistence that occurred already at very low concentrations due to the depletion efficiency of highly anisotropic rods. The experimentally measured phase transitions were compared to phase boundaries obtained using free volume theory (FVT), a well established theory for calculating the phase behavior of colloidal particles mixed with depletants. We find good correspondence between the experimental phase transitions and the theoretical FVT model where the excluded volume of the rod-like depletants was explicitly accounted for in both the reservoir and the system.

Structural and rheological aging in model attraction-driven glasses by Rheo-SANS

Aging in a model colloidal suspension comprised of particles with a thermoreversible attraction is studied using Rheo-SANS techniques in the attractive-driven glass state. Multiple thermal pathways lead to a common rheological and microstructural aging trajectory, as was observed previously for a thermoreversible gel. SANS measurements of the colloidal glass microstructure as a function of temperature and time during various quench protocols are quantitatively characterized in terms of an effective interaction strength that becomes an order parameter defining the microstructural state of the glass. Using previously validated concepts of a fictive temperature, a semi-empirical, quantitative relationship similar to an Avrami relationship is established between the mechanical aging (elastic modulus) and microstructural aging (order parameter) that is independent of thermal history for the thermal profiles studied herein at long times. Furthermore, shear rejuvenation is studied, and while shear may only partially reduce the degree of structure in the glass, aging upon flow cessation is found to follow a common trajectory when viewed in terms of the microstructural order parameter.

Silica Nanoparticles

Silica consists of one silicon atom and two oxygen atoms (SiO₂) and is commonly used in various aspects of daily life. For example, it has been used as glass, insulator, and so on. Nowadays, silica is used as core reagents for fabricating and encapsulating nanoparticles (NPs). In this chapter, the usage of silica in nanotechnology is described. Synthesis and surface modification of silica nanoparticles (SiNPs), including via the Stöber method, reverse microemulsion method, and modified sol-gel method, are illustrated. Then, various NPs with silica encapsulation are explained. At last, the biological applications of those mentioned NPs are described.

Nanostructured Silica with Anchoring Units: The 2D Solid Solvent for Molecules and Metal Ions

The ability to organize, separate and manipulate individual molecules and ions on a surface opens up almost unlimited opportunities. However, it often requires complex techniques and a proper support material. With this in mind, we show a new concept of 2D solid solvents and review a simple and efficient procedure which is based on nanostructured forms of silica with anchoring units. We describe silica supports, such as spherical nanoparticles and mesoporous silica structures, as well as review the methods for chemical modification of the surface of silica with the functional groups. Finally, we present a few particular examples of the immobilization of molecules and ions on the surface of 2D solid solvents along with the experimental investigation of the obtained materials.

Quantification of the Structure of Colloidal Gas-Liquid Interfaces

We have quantified the structure of the colloidal gas-liquid interface using synchrotron X-ray reflectivity measurements on a model colloid-polymer mixture. The interfacial width shows mean-field scaling with the colloid density difference, and the density profiles appear to be monotonic. Furthermore, our measurements allow us to distinguish between different theoretical polymer descriptions commonly used to model colloid-polymer mixtures. Our results highlight the importance of capturing the correct polymer physics in obtaining a quantitative theoretical description of the colloidal gas-liquid interface.

Nanomedicine and Early Cancer Diagnosis: Molecular Imaging using Fluorescence Nanoparticles

Incorporating nanotechnology into fluorescent imaging and magnetic resonance imaging (MRI) has shown promising potential for accurate diagnosis of cancer at an earlier stage than the conventional imaging modalities. Molecular imaging (MI) aims to quantitatively characterize, visualize, and measure the biological processes or living cells at molecular and genetic levels. MI modalities have been exploited in different applications including noninvasive determination and visualization of diseased tissues, cell trafficking visualization, early detection, treatment response monitoring, and in vivo visualization of living cells. High-affinity molecular probe and imaging modality to detect the probe are the two main requirements of MI. Recent advances in nanotechnology and allied modalities have facilitated the use of nanoparticles (NPs) as MI probes. Within the extensive group of NPs, fluorescent NPs play a prominent role in optical molecular imaging. The fluorescent NPs used in molecular and cellular imaging can be categorized into three main groups including quantum dots (QDs), upconversion, and dyedoped NPs. Fluorescent NPs have great potential in targeted theranostics including cancer imaging, immunoassay- based cells, proteins and bacteria detections, imaging-guided surgery, and therapy. Fluorescent NPs have shown promising potentials for drug and gene delivery, detection of the chromosomal abnormalities, labeling of DNA, and visualizing DNA replication dynamics. Multifunctional NPs have been successfully used in a single theranostic modality integrating diagnosis and therapy. The unique characteristics of multifunctional NPs make them potential theranostic agents that can be utilized concurrently for diagnosis and therapy. This review provides the state of the art of the applications of nanotechnologies in early cancer diagnosis focusing on fluorescent NPs, their synthesis methods, and perspectives in clinical theranostics.

Scattering from colloidal cubic silica shells: Part II, static structure factors and osmotic equation of state

HYPOTHESIS

The shape of colloidal particles affects the structure of colloidal dispersions. The effect of the cube shape on the thermodynamics of colloidal cube dispersions has not yet been studied experimentally. Static light scattering measurements on colloidal cubic silica shells at finite concentrations allows us to measure the structure factor of colloidal cube fluids and to test theoretical predictions for the equation of state of hard convex superballs.

EXPERIMENTS

Hollow silica nanocubes of varying concentrations in N,N,-dimethylformamide were studied with static light scattering. The structure factor was extracted from the scattering curves using experimental form factors. From this experimental structure factor, the specific density of the particles, and the osmotic compressibility were obtained. This osmotic compressibility was then compared to a theoretical equation of state of hard superballs.

FINDINGS

The first experimental structure factors of a stable cube fluid are presented. The osmotic compressibility of the cube fluid can be described by the equation of state of a hard superball fluid, showing that silica cubes in N,N,-dimethylformamide with LiCl effectively interact as hard particles.

Scattering from colloidal cubic silica shells: Part I, particle form factors and optical contrast variation

HYPOTHESIS

Colloidal cubic silica shells, prepared from cuprous oxide cubes, with a typical size of 100 nm are promising model particles for scattering studies on dilute, as well as concentrated fluids, of non-spherical colloids.

EXPERIMENTS

Small angle X-ray scattering, and static light scattering are employed to determine form factors of cubic silica shells and silica covered cuprous oxide cubes. Contrast variation experiments are performed to assess the refractive index and optical homogeneity of the cubic silica shells, which is important for the extension of the scattering study to concentrated dispersions of cubic shells in Part II (Dekker, submitted for publication).

RESULTS

The experimental form factors, which compare well to theoretical form factors, manifest cubic silica shells that are dispersed as single stable colloids with a shape intermediate between a sphere and a perfect cube. Contrast variation demonstrates that the silica shells are optically homogeneous, with a refractive index that is independent of the shell thickness. The results presented here open up the possibility to extract structure factors from light scattering measurements on concentrated cube dispersions in Part II.

Studies of silica sphere growth

Recipes are available for the preparation of monodisperse oxide powders and these materials are seeing increased use in the preparation of low flaw density ceramics. Despite the growing demand for these single sized powders, little is known about the physico-chemical processes which determine particle morphology. Here we report on studies of the mechanisms of particle growth.

As a model system we choose silica spheres prepared through the hydrolysis of silica alkoxides in basic alcoholic solutions. The rates of particle growth were studied by dipping carbon sputtered formvar coated electron microscope grids into the reaction mixture at different times. The grids were immediately placed on filler paper which rapidly wicked the fluid off the grid leaving the silica particles behind.

Self assembled monolayer of silica nanoparticles with improved order by drop casting

This paper reports on the formation of large area, self assembled, highly ordered monolayers of stearyl alcohol grafted silica nanospheres of ≈50 nm diameter on a silicon substrate based on the drop-casting method. Our novel approach to achieve improved order uses stearyl alcohol as an assistant by adding it to the colloidal NanoParticle (NP) dispersion from which the monolayers are formed. Additionally, a heat treatment step is added, to melt the stearyl alcohol in the monolayer and thereby give the particles more time to further self-assemble, leading to additional improvement in the monolayer quality. The formation of the monolayers is significantly affected by the concentration of the NPs and the stearyl alcohol, the volume of the drop as well as the time of the heat treatment. A high surface coverage and uniform monolayer film of SiO2 NPs is achieved by appropriate control of the above-mentioned preparation parameters. Structural characterization of the obtained SiO2 NP monolayer was done locally by Scanning Electron Microscopy (SEM), and globally by X-ray reflectivity (XRR) and grazing incidence small-angle X-ray scattering (GISAXS), where the data was reproduced by simulation within the Distorted Wave Born Approximation (DWBA). In conclusion, our modified drop-casting method is a simple, inexpensive method, which provides highly ordered self-assembled monolayers of silica particles, if combined with a compatible additive and a heat treatment step. This method might be more general and also applicable to different particles after finding an appropriate additive.

Simple Preparation of Hierarchically Porous Ce/TiO2/Graphitic Carbon Microspheres for the Reduction of CO2 with H2O under Simulated Solar Irradiation

Hierarchically porous Ce/TiO2/graphitic carbon microsphere composites (xCe/TiO2/GCM, where x = 0.2, 1.0, 2.0, 5.0 mmol·L-1) were prepared for the first time by using a simple colloidal crystal template and characterized by X-ray diffraction, nitrogen adsorption and desorption, scanning electron microscopy, and ultraviolet-visible diffuse reflectance spectra. In addition, the photocatalytic activity of CO2 reduction by H2O under simulated solar irradiation was studied. The results showed that the Ce/TiO2/GCM composite material was characterized by large porosity, high concentration of metal compounds and graphitized carbon matrix, and the content of acetone solvent having a great impact on its form. In terms of the photocatalytic CO2 reaction, the CH4 and CO productions were 4.587 and 357.851 μmol·g-1, respectively. The 2Ce/TiO2/GCM photocatalyst gave the highest production rate for three products. Under simulated solar irradiation, the Ce/TiO2/GCM has excellent photocatalytic activity in the photoreduction of CO2 from H2O, which was related to the special composition and the Ce/TiO2/GCM structure.

Effect of ethanol/TEOS ratios and amount of ammonia on the properties of copper-doped calcium silicate nanoceramics

Calcium magnesium silicate glasses could be suggested for the synthesis of scaffolds for hard tissue regeneration, as they present a high residual glassy phase, high hardness values and hydroxyapatite-forming ability. The use of trace elements in the human body, such as Cu, could improve the biological performance of such glasses, as Cu is known to play a significant role in angiogenesis. Nano-bioceramics are preferable compared to their micro-scale counterparts, because of their increased surface area, which improves both mechanical properties and apatite-forming ability due to the increased nucleation sites provided, their high diffusion rates, reduced sintering time or temperature, and high mechanical properties. The aim of the present work was the evaluation of the effect of different ratios of Ethanol/TEOS and total amount of the inserted ammonia to the particle size, morphology and bioactive, hemolytic and antibacterial behavior of nanoparticles in the quaternary system SiO₂-CaO-MgO-CuO. Different ratios of Ethanol/TEOS and ammonia amount affected the size and morphology of bioactive nanopowders. The optimum materials were synthesized with the highest ethanol/TEOS ratio and ammonia amount as verified by the enhanced apatite-forming ability and antibacterial and non-hemolytic properties.

Super-resolution microscopy on single particles at fluid interfaces reveals their wetting properties and interfacial deformations

Solid particles adsorbed at fluid interfaces are crucial for the mechanical stability of Pickering emulsions. The key parameter which determines the kinetic and thermodynamic properties of these colloids is the particle contact angle, θ. Several methods have recently been developed to measure the contact angle of individual particles adsorbed at liquid-liquid interfaces, as morphological and chemical heterogeneities at the particle surface can significantly affect θ. However, none of these techniques enables the simultaneous visualization of the nanoparticles and the reconstruction of the fluid interface to which they are adsorbed, in situ. To tackle this challenge, we utilize a newly developed super-resolution microscopy method, called iPAINT, which exploits non-covalent and continuous labelling of interfaces with photo-activatable fluorescent probes. Herewith, we resolve with nanometer accuracy both the position of individual nanoparticles at a water-octanol interface and the location of the interface itself. First, we determine single particle contact angles for both hydrophobic and hydrophilic spherical colloids. These experiments reveal a non-negligible dependence of θ on particle size, from which we infer an effective line tension, τ. Next, we image elliptical particles at a water-decane interface, showing that the corresponding interfacial deformations can be clearly captured by iPAINT microscopy.

Mesoporous silica-protected silver nanoparticle disinfectant with controlled Ag+ ion release, efficient magnetic separation, and effective antibacterial activity

Ag is the most effective metal disinfectant against pathogenic microorganisms and thus, various approaches have been exploited to enhance the dispersity and control the release of Ag+ ions from Ag nanoparticles. In this study, a superparamagnetic Fe3O4@SiO2@Ag@porous SiO2 disinfectant with a double-layer core-shell structure was developed. Its superparamagnetic Fe3O4 nanosphere core ensured its good dispersity in water and allowed its easy magnetic separation after treatment. Its dense SiO2 inner shell protected the Fe3O4 nanosphere core and allowed a good loading of Ag nanoparticles. Its mesoporous SiO2 outer layer effectively protected the Ag nanoparticles from detachment, and its mesoporous channels resulted in lower silver oxidation and dissolution for the controlled release of Ag+ ions. Thus, a highly efficient, silver-based disinfectant was developed, as demonstrated by its effective disinfection of Escherichia coli bacteria with good recycle performance, while the silver concentration in the treated water met the MCL of silver for drinking water.

Critical Features for Mesoporous Silica Nanoparticles Encapsulated into Erythrocytes

Mesoporous silica nanoparticles (MSNs) hold great potential as a versatile platform for biomedical applications, especially drug delivery. However, evidence shows that MSNs even when PEGylated are rapidly cleared from the bloodstream by the monocyte phagocytic system. Erythrocytes, also called red blood cells (RBCs), can serve as biocompatible carriers of various bioactive substances, including drugs, enzymes, and peptides. In this work, we synthesize a series of fluorescent PEGylated MSNs with different synthetic diameters ranging from 10 to 200 nm and investigate the size effect on their encapsulation in human RBCs (hRBCs) by a hypotonic dialysis-based method. According to fluorescence images and flow cytometry analyses, we demonstrated that a hydrodynamic diameter below 30 nm is critical for efficient MSN encapsulation. Confocal microscopy and scanning electron microscopy images further confirmed that PEGylated MSNs were successfully embedded inside RBC. PEGylation serves an important role not only for stabilizing MSNs in biological milieu but also for reducing significant hemolysis caused by bare MSNs and thus for successful encapsulation. In addition to PEGylation, we further introduce positively charged functional groups onto the MSNs to show that nanoparticle-encapsulated hRBCs could serve as depots for delivering biological molecules through electrostatic attraction or chemical conjugation with MSNs. Also, we verify the existence of CD47 membrane protein, a marker of self, on the nanoparticle-encapsulated hRBCs and assess its ability of circulation in the blood, which could act as a circulation reservoir for delivering pharmacological substances through an osmosis-based method with MSNs.

A New, General Strategy for Fabricating Highly Concentrated and Viscoplastic Suspensions Based on a Structural Approach To Modulate Interparticle Interaction

We report a general strategy to fabricate highly concentrated, viscoplastic and stable suspensions by designing the particle surface structure to control the interparticle attractive forces. Unlike conventional methods, where the choice of solvent is critical in balancing interparticle interactions, suspensions showing excellent stability and viscoplastic properties were made using various solvents. We demonstrated this approach using highly sparse agglomerates of carbon nanotubes (CNTs) as the particles. Our results revealed that the essential feature of the CNT agglomerate to fabricate these suspensions was high porosity with a spacing size much smaller than the overall size, which was only possible using long single-walled carbon nanotubes (SWNTs). In this way, the agglomerate surface was characterized by fine network of CNT bundles. These suspensions exhibited solid-like behavior at rest (characterized by a high yield stress of c.a. 100 Pa) and a liquid-like behavior when subjected to a stress (characterized by a significant drop of an apparent viscosity to 1 Pa·s at a shear rate of 1000 s^-1). Furthermore, in contrast to conventionally fabricated suspensions, these "CNT pastes" exhibited exceptional stability at rest, under flow, and at extremely high concentrations during the drying process, with only a weakly observable dependence on solvent type. As a result, highly uniform micrometer-thick SWNT films were successfully fabricated by dried blade-coated films of these pastes. Finally, we developed a simple, semiempirical model and clarified the importance of the CNT agglomerate microstructure (the ratio of spacing size/particle size and porosity) on tailoring the cohesive forces between particles to fabricate stable viscoplastic suspensions.

Influence of the initial chemical conditions on the rational design of silica particles

The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40 wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption.

Facile meltPEGylation of flame-made luminescent Tb3+-doped yttrium oxide particles: hemocompatibility, cellular uptake and comparison to silica

MeltPEGylation constitutes an elegant one-pot route for the efficient PEGylation of metal oxide nanoparticles with improved hemo- and cytocompatibility.

Dynamic Scaling of Colloidal Gel Formation at Intermediate Concentrations

We have examined the formation and dissolution of gels composed of intermediate volume-fraction nanoparticles with temperature-dependent short-range attractions using small-angle x-ray scattering, x-ray photon correlation spectroscopy, and rheology to obtain nanoscale and macroscale sensitivity to structure and dynamics. Gel formation after temperature quenches to the vicinity of the rheologically determined gel temperature, T_{gel}, was characterized via the slowdown of dynamics and changes in microstructure observed in the intensity autocorrelation functions and structure factor, respectively, as a function of quench depth (ΔT=T_{quench}-T_{gel}), wave vector, and formation time t_{f}. We find the wave-vector-dependent dynamics, microstructure, and rheology at a particular ΔT and t_{f} map to those at other ΔTs and t_{f}s via an effective scaling temperature, T_{s}. A single T_{s} applies to a broad range of ΔT and t_{f} but does depend on the particle size. The rate of formation implied by the scaling is a far stronger function of ΔT than expected from the attraction strength between colloids. We interpret this strong temperature dependence in terms of cooperative bonding required to form stable gels via energetically favored, local structures.

Locating Reactive Groups on Nanomaterials with Gold Nanoclusters: Toward a Surface Reactive Site Map

Nanoparticles (NPs) are often functionalized with reactive groups such as amines and thiols for the subsequent conjugation of further molecules, e.g., stabilizing polymers, drugs, and proteins for targeting cells or specific diseases. In addition to the quantitative estimation of the reactive conjugation sites, their molecular positioning and nanoscale arrangement on single nanoparticles become more and more important for the tailored engineering and design of functional nanomaterials. Here, we use maleimide or sulfo-succinimidyl ester-modified 1.4 nm gold nanoclusters (AuNCs) to specifically label reactive thiol and amine groups with sub-2-nm precision on metal oxide and polymeric nanostructures. We confirm the binding of AuNCs by measuring and modeling sedimentation properties using analytical centrifugation, imaging their surface distribution and surface distances by transmission electron microscopy (TEM), and comparing the results to ensemble measurements of numbers of reactive surface groups obtained by common photometric assays. We map thiol and amine groups introduced on silica NPs (SiNPs), titania stars (Ti), silica inverse opals (SiOps), and polystyrene NPs (PS NPs). We show that the method is suitable for mapping local, clustered inhomogeneities of the reactive sites on single SiNPs introduced by masking certain areas during surface functionalization. Mapping precise positions of reactive surface groups is essential to the design and tailored ligation of multifunctional nanomaterials.

Diffusing Colloidal Probes of kT-Scale Biomaterial-Cell Interactions

In the optimization of applied biomaterials, measurements of their interactions with cell surfaces are important to understand their influence on specific and nonspecific cell surface adhesion, internalization pathways, and toxicity. In this study, a novel approach using dark field video microscopy with combined real-time particle and cell tracking allows the trajectories of biomaterial-coated colloids to be monitored in relation to their distance from cell perimeters. Dynamic and statistical mechanical analyses enable direct measurement of colloid-cell surface association lifetimes and interaction potentials mediated by biomaterials. Our analyses of colloidal transport showed polyethylene glycol (PEG) and bovine serum albumin (BSA) lead to net repulsive interactions with cell surfaces, while dextran and hyaluronic acid (HA) lead to reversible and irreversible association to the cell surface, respectively. Our results demonstrate how diffusing colloidal probes can be used for nonobtrusive, sensitive measurements of biomaterial-cell surface interactions important to therapeutics, diagnostics, and tissue engineering.

Phase behaviour of colloids plus weakly adhesive polymers

The phase behaviour of a colloidal dispersion mediated by weakly adhesive polymers is considered. The polymers are depleted but are weakly adhesive and hence comprise a non-zero polymer concentration at the colloid's surface, in contrast to the classical assumption in depletion theories involving a zero polymer concentration at the surface. The theory is composed of a generalized free-volume theory for colloid-polymer mixtures and a self-consistent mean-field theory for polymers at surfaces. It is found that the weak adhesion of the polymers shifts the phase stability of the colloid-polymer mixtures to higher polymer concentrations as compared to assuming a full depletion effect. The predicted phase diagrams employing the new theory are consistent with experiments on mixtures of silica spheres coated with stearyl alcohol and polydimethylsiloxane in cyclohexane and with Monte Carlo simulation results.

Thermoreversible Gels Composed of Colloidal Silica Rods with Short-Range Attractions

Dynamic arrest transitions of colloidal suspensions containing nonspherical particles are of interest for the design and processing of various particle technologies. To better understand the effects of particle shape anisotropy and attraction strength on gel and glass formation, we present a colloidal model system of octadecyl-coated silica rods, termed as adhesive hard rods (AHR), which enables control of rod aspect ratio and temperature-dependent interactions. The aspect ratios of silica rods were controlled by varying the initial TEOS concentration following the work of Kuijk et al. (J. Am. Chem. Soc., 2011, 133, 2346-2349) and temperature-dependent attractions were introduced by coating the calcined silica rods with an octadecyl-brush and suspending in tetradecane. The rod length and aspect ratio were found to increase with TEOS concentration as expected, while other properties such as the rod diameter, coating coverage, density, and surface roughness were nearly independent of the aspect ratio. Ultrasmall angle X-ray scattering measurements revealed temperature-dependent attractions between octadecyl-coated silica rods in tetradecane, as characterized by a low-q upturn in the scattered intensity upon thermal quenching. Lastly, the rheology of a concentrated AHR suspension in tetradecane demonstrated thermoreversible gelation behavior, displaying a nearly 5 orders of magnitude change in the dynamic moduli as the temperature was cycled between 15 and 40 °C. The adhesive hard rod model system serves as a tunable platform to explore the combined influence of particle shape anisotropy and attraction strength on the dynamic arrest transitions in colloidal suspensions with thermoreversible, short-range attractions.