A series of silsesquioxane nanoparticles containing reactive internal organic functionalities throughout the entire particle body have been synthesized using a surfactant-free method with organosilanes as the sole precursors and a base catalyst. The organic functional groups incorporated are vinyl, allyl, mercapto, cyanoethyl, and cyanopropyl groups. The sizes and morphologies of the particles were characterized using SEM and nitrogen adsorption, while the compositions were confirmed using TGA, FT-IR, solid state NMR, and elemental analysis. The accessibility and reactivity of the functional groups inside the particles were demonstrated by performing bromination and reduction reactions in the interior of the particles.

Much research has been done using polymer and silica particles as support materials for catalytically active noble metal nanoparticles, but these materials have limited stability in organic solvents or under extreme reaction conditions such as high pH. Here we present a robust and versatile composite polymer-diamond support for ultrasmall noble metal nanoparticles combining chemical and mechanical stability of diamond with the chemical versatility of a polymer. By exploiting the rich surface chemistry of nanodiamond and incorporating a reactive thiol–ene polymer, a thinly coated polymer-diamond composite was formed. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) confirmed the presence of the polymer. High resolution scanning transmission electron microscopy (S/TEM) analysis showed that in situ growth of gold, platinum and palladium nanoparticles produced high density coverage at the polymer-diamond support surface. Energy dispersive spectroscopy mapping and S/TEM imaging indicated spatial alignment of nanoparticles with chemical groups present in the polymer used for particle tethering. The polymer-diamond supported nanoparticles catalyze the NaBH4 reduction of para-nitrophenol to para-aminophenol and possess better stability than silica supports which dissolve at high pH resulting in nanoparticle aggregation. With the high robustness of the diamond and the ability to tailor the monomer combinations, this polymer-diamond support system may be expanded to a wide range of nanoparticle compositions suitable for various reaction conditions.Keywords: catalysis; diamond; nanoparticles; noble metals; polymers

This article summarizes a recently developed approach for the preparation of membrane materials by the self-assembly of inorganic, polymeric or hybrid nanoparticles, with the focus on functional membranes possessing permselectivity. Two types of such membranes are discussed, those possessing size and charge selectivity suitable for ultra- and nanofiltration and chemoselective separation, and those possessing proton or lithium transport properties suitable for fuel cell and lithium battery applications, respectively. This article describes the preparation methods of nanoparticle membranes, as well as their mechanical, molecular, and ionic transport properties.

We report in vitro and in vivo evaluation of a newly designed trifunctional theranostic agent for targeting solid tumors. This agent combines a dendritic wedge with high boron content for boron neutron capture therapy or boron MRI, a monomethine cyanine dye for visible-light fluorescent imaging, and an integrin ligand for efficient tumor targeting. We report photophysical properties of the new agent, its cellular uptake and in vitro targeting properties. Using live animal imaging and intravital microscopy (IVM) techniques, we observed a rapid accumulation of the agent and its retention for a prolonged period of time (up to 7 days) in fully established animal models of human melanoma and murine mammary adenocarcinoma. This macromolecular theranostic agent can be used for targeted delivery of high boron load into solid tumors for future applications in boron neutron capture therapy.

Nanoporous membranes are important for the study of the transport of small molecules and macromolecules through confined spaces and in applications ranging from separation of biomacromolecules and pharmaceuticals to sensing and controlled release of drugs. For many of these applications, chemists need to gate the ionic and molecular flux through the nanopores, which in turn depends on the ability to control the nanopore geometry and surface chemistry. Most commonly used nanoporous membrane materials are based on polymers. However, the nanostructure of polymeric membranes is not well-defined, and their surface is hard to modify. Inorganic nanoporous materials are attractive alternatives for polymers in the preparation of nanoporous membranes.In this Account, we describe the preparation and surface modification of inorganic nanoporous films and membranes self-assembled from silica colloidal spheres. These spheres form colloidal crystals with close-packed face centered cubic lattices upon vertical deposition from colloidal solutions. Silica colloidal crystals contain ordered arrays of interconnected three dimensional voids, which function as nanopores. We can prepare silica colloidal crystals as supported thin films on various flat solid surfaces or obtain free-standing silica colloidal membranes by sintering the colloidal crystals above 1000 °C. Unmodified silica colloidal membranes are capable of size-selective separation of macromolecules, and we can surface-modify them in a well-defined and controlled manner with small molecules and polymers. For the surface modification with small molecules, we use silanol chemistry. We grow polymer brushes with narrow molecular weight distribution and controlled length on the colloidal nanopore surface using atom transfer radical polymerization or ring-opening polymerization.We can control the flux in the resulting surface-modified nanoporous films and membranes by pH and ionic strength, temperature, light, and small molecule binding. When we modify the surface of the colloidal nanopores with ionizable moieties, they can generate an electric field inside the nanopores, which repels ions of the same charge and attracts ions of the opposite charge. This allows us to electrostatically gate the ionic flux through colloidal nanopores, controlled by pH and ionic strength of the solution when surface amines or sulfonic acids are present or by irradiation with light in the case of surface spiropyran moieties. When we modify the surface of the colloidal nanopores with chiral moieties capable of stereoselective binding of enantiomers, we generate colloidal films with chiral permselectivity. By filling the colloidal nanopores with polymer brushes attached to the pore surface, we can control the ionic flux through the corresponding films and membranes electrostatically using reversibly ionizable polymer brushes. By filling the colloidal nanopores with polymer brushes whose conformation reversibly changes in response to pH, ionic strength, temperature, or small molecule binding, we can control the molecular flux sterically.There are various potential applications for surface-modified silica colloidal films and membranes. Due to their ordered nanoporous structure and mechanical durability, they are beneficial in nanofluidics, nanofiltration, separations, and fuel cells and as catalyst supports. Reversible gating of flux by external stimuli may be useful in drug release, in size-, charge-, and structure-selective separations, and in microfluidic and sensing devices.

We describe a one-pot synthesis of novel large-pore mesoporous silica nanoparticles using a nonsurfactant template, tannic acid, and protein immobilization on these particles. The tannic-acid-templated mesoporous silica nanoparticles (TA-MSNPs) possess tunable large pores (6–13 nm), unique pore morphology, and a uniform diameter of ca. 200 nm. TA-MSNPs show high protein adsorption capacity of 77.1 mg/g for lysozyme, 396.5 mg/g for bovine hemoglobin, and 130.0 mg/g for bovine serum albumin and rapid protein uptake. They can also adsorb a large amount of m-MDH (421 ± 13 mg/g) and protect this enzyme from the environment, as demonstrated by its high activity when encapsulated inside the mesopores of TA-MSNPs.

We prepared mesoporous silica colloidal membranes pore-filled with polymer brushes with different degrees of sulfonation. In these membranes, the assembly of silica colloidal spheres serves as a rigid matrix containing a continuous network of interconnected mesopores and providing mechanical and thermal stability, non-swelling and water retaining properties, while sulfonic acid group-containing polymer brushes grown on the surface of silica provide proton conductivity. We studied the proton conductivity of these membranes as well as open circuit voltage and linear polarization of the fuel cells prepared using these membranes as a function of sulfonic acid group content in the pore-filling polymer brushes. We found a sigmoidal dependence of the proton conductivity on the amount of sulfonic acid groups. We showed that the proton conductivity of the membrane does not increase significantly after reaching ca. 75% sulfonic acid group content. The fuel cell performance, on the other hand, decreased after reaching ca. 65–70% sulfonic acid group content, which was attributed to the increased methanol permeability of the membranes at higher sulfonic acid group content.

We prepared robust nanoporous membranes with controlled area and uniform thickness by pressing silica colloidal spheres into disks followed by sintering. Three different diameters of silica particles, 390, 220, and 70 nm, were used to prepare the membranes with different pore size. In order to evaluate their size-selectivity, we measured the diffusion of polystyrene particles through these membranes. Although pressed silica colloidal membranes do not possess visible order or uniform pore size, they showed size-selective transport. We also demonstrated that pressed silica colloidal membranes can be functionalized via pore-filling. Sulfonated polymer brushes were grown inside the pores via surface-initiated atom transfer radical polymerization, which resulted in a material with high proton conductivity suitable for fuel cell applications.Keywords: nanoporous membranes; pore-filled membranes; silica colloidal spheres; size-selective diffusion;

Membranes with 1–100 nm nanopores are widely used in water purification and in biotechnology, but are prone to blockage and fouling. Reversibly assembled nanoporous membranes may be advantageous due to recyclability, cleaning, and retentate recovery, as well as the ability to tune the pore size. We report the preparation and characterization of size-selective nanoporous membranes with controlled thickness, area, and pore size via reversible assembly of polymer brush-grafted (“hairy”) silica nanoparticles. We describe membranes reversibly assembled from silica particles grafted with (1) polymer brushes carrying acidic and basic groups, and (2) polymer brushes carrying neutral groups. The former are stable in most organic solvents and easily disassemble in water, whereas the latter are water-stable and disassemble in organic solvents.Keywords: membrane; nanoporous; polymer-grafted particles; reversible assembly; ultrafiltration

A new class of surface-modified silica nanoparticles has been developed for potential applications in boron neutron capture therapy. Sub-50 nm silica particles were synthesized using a modified Stöber method and used in surface-initiated atom transfer radical polymerization of two biocompatible polymers, poly(2-(hydroxyethyl)methacrylate) and poly(2-(methacryloyloxy)ethyl succinate). The carboxylic acid and hydroxyl functionalities of the polymeric side chains were functionalized with carboranyl clusters in high yields. The resulting particles were characterized using DLS, TEM, solution 1H NMR, solid state 11B NMR and thermogravimetric analysis. The particles contain between 13 and 18 % of boron atoms by weight, which would provide a high amount of 10B nuclides for BNCT, while the polymer chains are suitable for further modification with cell targeting ligands.

We prepared silica nanospheres 360 nm in diameter surface-modified with p-tert-butylthiacalix[4]arenes containing amine, carboxyl, and guanidinium groups. We found that these silica nanoparticles selectively adsorb model oligonucleotides and proteins. The particles modified with the macrocycle containing guanidinium fragments selectively adsorbed long-chain oligonucleotides and those modified with the macrocycle containing amine groups adsorbed BSA and hemoglobin with pH-dependent selectivity. We compared this behavior with that of silica nanoparticles carrying amine and carboxyl groups, and concluded that both electrostatic interactions and specific binding are responsible for the observed selectivity.

We prepared colloidal crystals by self-assembly of gold-coated silica nanospheres, and free-standing nanoporous membranes by sintering these colloidal crystals. We modified the nanopore surface with ionizable functional groups, by forming a monolayer of l-cysteine or by surface-initiated polymerization of methacrylic acid. Diffusion experiments for the cationic dye Rhodamine B through l-cysteine-modified membranes showed a decrease in flux upon addition of an acid due to the nanopore surface becoming positively charged. Diffusion experiments for the neutral dye, ferrocenecarboxaldehyde, through the PMAA-modified membranes showed a 13-fold increase in flux upon addition of an acid resulting from the protonated polymer collapsing onto the nanopore surface leading to larger pore size. Our results demonstrate that SiO2@Au core–shell nanospheres can self-assemble into colloidal crystals and that transport through the corresponding surface-modified Au-coated colloidal membranes can be controlled by pH.

This article describes recent developments in the design, fabrication and investigation of reversible nanovalves prepared in inorganic nanoporous materials such as glass, silicon nitride, mesoporous silica, anodized alumina and silica colloidal crystals. Their surfaces have been modified with pH-, temperature- and light-responsive molecules, as well as molecules that can undergo redox reactions and bind small molecules. This surface modification allows reversibly opening and closing of the nanovalves to the transport of ions based on electrostatic interactions, and to the transport of molecules based on mechanically gating the nanopores.

We developed a method to obtain water-dispersible boron nanoparticles (BNPs) as a potential boron delivery agent for cancer treatment. The 40 nm nanoparticles are prepared by mechanical milling in undecylenic acid followed by ligand exchange with dopamine. These particles can be rendered fluorescent by the attachment of a fluorophore to the primary amine of the dopamine, which could be used to locate the particles in a cell media. Dopamine-modified boron nanoparticles did not show signs of toxicity toward murine macrophage cells.

We prepared silica colloidal membranes suspended in glass openings and containing no major mechanical defects. The surface of these colloidal membranes was modified with amine groups. The diffusion rate of Fe(bpy)32+ through the suspended amine-modified colloidal membranes was attenuated by adding acid to the solution. The amine-modified colloidal membranes displayed an average selectivity (the ratio of diffusion rates in the absense and presence of the acid) of 2.6 for Fe(bpy)32+. This selectivity is believed to result from the electrostatic repulsion between the protonated amine-modified membrane surface and positively charged Fe(bpy)32+ and was confirmed by observing no change in (1) the diffusion rate of Fe(bpy)32+ through an unmodified suspended colloidal membrane, and (2) the diffusion rate of a neutral molecule through the amine-modified colloidal membrane with and without the acid present in solution.

The effects of precursor structure and polycondensation conditions on the properties of hybrid nanoparticles synthesized from organo-trimethoxysilanes were studied. Hybrid nanoparticles containing groups capable of forming hydrogen bonds were synthesized from functional derivatives of 3-aminopropyltrimethoxysilane. For the synthesis of phenylurea-functionalized organosilica nanoparticles different approaches to nanoparticle preparation were used. It was shown that the nature of the functional groups (proton-donor or proton-acceptor) affects the aggregation of silica nanoparticles. Also, the difference in behavior of nanoparticles prepared using surface modification and polycondensation was demonstrated for different pH, ionic strength and solvent polarity. As a result, by changing the pH of the solutions, it is possible to shift the aggregation pattern of these nanoparticles, such as the size of the initially formed aggregates.

We report the preparation of 20 and 65 nm radii glass nanopores whose surface is modified with DNA aptamers controlling the molecular transport through the nanopores in response to small molecule binding.

The surface of nanopores in colloidal films assembled from 200 nm silica spheres was modified with thiacalix[4]arene moieties and transport of two redox-active species, ferrocene dimethanol and iron tris(bipyridyl) hexafluorophosphate, through the films has been studied using cyclic voltammetry. Evidence for two different molecular transport mechanisms was observed. Ferrocene dimethanol is transported via a simple diffusive mechanism while iron tris(bipyridyl) is transported via a surface-hopping mechanism. The difference in transport mechanisms is believed to result from different interaction strength between the diffusing species and the surface-bound organic moieties, and sheds light on transport mechanisms in nanoporous systems.

The transport of positively charged redox active species through spiropyran-modified nanopores in silica colloidal films was controlled using light. The silica colloidal films were comprised of 18 layers of face centered cubic (fcc)-packed 170 nm silica spheres. The surface of the films was first modified with amines, which were then used to attach the spiropyran moiety to the surface. The limiting current of a positively charged redox active species through the spiropyran-modified nanopore decreased after irradiation with UV light at pH 6.6. When the silica colloidal film was subsequently irradiated with visible light the initial limiting current was restored.

The interior of 237 nm spherical vinylsilsesquioxane nanoparticles has been covalently modified and their surface functionalized under mild conditions to yield a novel type of hybrid silsesquioxane nanoparticles. Data obtained from thermogravimetric and elemental analysis show that the vinyl groups inside the nanoparticles can be easily brominated or hydroborated, leading to the nanoparticles containing 59.9 wt % of bromine or 3.6 wt % of boron, respectively. Our results demonstrate that the vinyl groups inside the nanoparticles are highly accessible, which may lead to the preparation of a host of hybrid organosilica nanoparticles with complex structures. We also show that the surface of the brominated and boronated nanoparticles is unhindered for further amination.

The surfaces of nanopores in colloidal films assembled from 255 nm silica spheres were modified with poly(L-alanine) brushes using surface-initiated polycondensation. Polymer length was controlled by the polymerization time. The molecular transport through the colloidal films was studied using cyclic voltammetry as a function of polymer brush thickness, temperature, and pH. Transport rates through the colloidal films were found to be dependent on temperature and pH, with the magnitude of the effect dependent on the length of the polypeptide chain.

We prepared robust free-standing 200 μm-thick colloidal membranes (nanofrits) with a relatively large area and no mechanical defects by sintering silica colloidal films. The silica spheres used to prepare the nanofrits were 338, 300, or 251 nm in diameter, leading to 25, 22.5, and 19 nm nanopore sizes, respectively. The room-temperature diffusional flux through these membranes is of the order of 3.6 × 10−10 mol s−1 cm−2 for a Fe(bpy)32+ ion in acetonitrile test solution in the absence of applied pressure and is in good agreement with the calculated diffusional flux for colloidal crystals of the same thickness. To evaluate the feasibility of nanofrit surface modification, we treated them with 3-aminopropyltriethoxysilane after rehydroxylation. We found, by measuring the surface coverage for dansyl amide on the surface, that the number of the amines on the nanofrit surface is lower as compared to that observed for colloidal films not treated with heat. As a result, the selectivity for the transport of Fe(bpy)32+ through the aminated nanofrits in the presence of acid is lower than the selectivity observed for amine-modified colloidal films.

We studied the proton transport in free-standing assemblies of surface-sulfonated silica nanospheres, either randomly packed or self-assembled into a close-packed arrangement. The transport studies were performed in the absence of solvent, under either dry or high humidity conditions. We demonstrated that colloidal assemblies prepared using surface-sulfonated silica nanospheres possess proton conductivity that depends on the ordering of the material, temperature and relative humidity. Based on the comparison between the close-packed and disordered assemblies made of the same spheres we conclude that the increase in structural organization of the self-assembled colloidal materials leads to increased proton conductivity and better water retention.

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown using surface-initiated atom transfer radical polymerization on the nanopore surface inside the colloidal films assembled from 255 nm silica spheres. The molecular transport through PDMAEMA-modified colloidal nanopores was studied as a function of pH and ionic strength by measuring the flux of neutral and positively charged redox-active species across the colloidal films using cyclic voltammetry. Nanopores modified with PDMAEMA brushes exhibited pH- and ion-dependent behavior as follows. The diffusion rates decreased with decreasing pH as a result of electrostatic interactions and steric hindrance. At low pH (in the protonated state) the diffusion rates increased with increasing salt concentration because of the charge screening. We also quaternized the surface-grafted PDMAEMA, which led to the formation of strong polyelectrolyte brushes that hindered the diffusion of neutral molecules through the nanopores due to steric effects and the diffusion of positively charged species due to both electrostatic and steric effects.

The chiral permselectivity in thin opal films modified on the silica surface with chiral selector moieties was studied as a function of opal film geometry, supporting electrolyte concentration, solvent polarity, and chiral selector and linker structure. While opal film thickness, supporting electrolyte concentration and linker length and structure did not have a significant influence on the chiral permselectivity, the nanopore size, solvent polarity and selector structure had a pronounced effect. These observations are in agreement with the chiral selectivity mechanism in the opal films in which the permeating enantiomers are transported with different rates through the surface utilizing non-covalent interactions between the chiral permeant molecules and surface-bound chiral selectors. The chiral selectivity (transport rate ratio for S and R enantiomers) achieved in the present study was 4.5, which is one of the highest reported for chiral membranes.

This article describes recent developments in the design, fabrication and investigation of reversible nanovalves prepared in inorganic nanoporous materials such as glass, silicon nitride, mesoporous silica, anodized alumina and silica colloidal crystals. Their surfaces have been modified with pH-, temperature- and light-responsive molecules, as well as molecules that can undergo redox reactions and bind small molecules. This surface modification allows reversibly opening and closing of the nanovalves to the transport of ions based on electrostatic interactions, and to the transport of molecules based on mechanically gating the nanopores.

We report the preparation of 20 and 65 nm radii glass nanopores whose surface is modified with DNA aptamers controlling the molecular transport through the nanopores in response to small molecule binding.

We prepared mesoporous silica colloidal membranes pore-filled with polymer brushes with different degrees of sulfonation. In these membranes, the assembly of silica colloidal spheres serves as a rigid matrix containing a continuous network of interconnected mesopores and providing mechanical and thermal stability, non-swelling and water retaining properties, while sulfonic acid group-containing polymer brushes grown on the surface of silica provide proton conductivity. We studied the proton conductivity of these membranes as well as open circuit voltage and linear polarization of the fuel cells prepared using these membranes as a function of sulfonic acid group content in the pore-filling polymer brushes. We found a sigmoidal dependence of the proton conductivity on the amount of sulfonic acid groups. We showed that the proton conductivity of the membrane does not increase significantly after reaching ca. 75% sulfonic acid group content. The fuel cell performance, on the other hand, decreased after reaching ca. 65–70% sulfonic acid group content, which was attributed to the increased methanol permeability of the membranes at higher sulfonic acid group content.

The chiral permselectivity in thin opal films modified on the silica surface with chiral selector moieties was studied as a function of opal film geometry, supporting electrolyte concentration, solvent polarity, and chiral selector and linker structure. While opal film thickness, supporting electrolyte concentration and linker length and structure did not have a significant influence on the chiral permselectivity, the nanopore size, solvent polarity and selector structure had a pronounced effect. These observations are in agreement with the chiral selectivity mechanism in the opal films in which the permeating enantiomers are transported with different rates through the surface utilizing non-covalent interactions between the chiral permeant molecules and surface-bound chiral selectors. The chiral selectivity (transport rate ratio for S and R enantiomers) achieved in the present study was 4.5, which is one of the highest reported for chiral membranes.

We studied the proton transport in free-standing assemblies of surface-sulfonated silica nanospheres, either randomly packed or self-assembled into a close-packed arrangement. The transport studies were performed in the absence of solvent, under either dry or high humidity conditions. We demonstrated that colloidal assemblies prepared using surface-sulfonated silica nanospheres possess proton conductivity that depends on the ordering of the material, temperature and relative humidity. Based on the comparison between the close-packed and disordered assemblies made of the same spheres we conclude that the increase in structural organization of the self-assembled colloidal materials leads to increased proton conductivity and better water retention.

We developed a method to obtain water-dispersible boron nanoparticles (BNPs) as a potential boron delivery agent for cancer treatment. The 40 nm nanoparticles are prepared by mechanical milling in undecylenic acid followed by ligand exchange with dopamine. These particles can be rendered fluorescent by the attachment of a fluorophore to the primary amine of the dopamine, which could be used to locate the particles in a cell media. Dopamine-modified boron nanoparticles did not show signs of toxicity toward murine macrophage cells.

This article summarizes a recently developed approach for the preparation of membrane materials by the self-assembly of inorganic, polymeric or hybrid nanoparticles, with the focus on functional membranes possessing permselectivity. Two types of such membranes are discussed, those possessing size and charge selectivity suitable for ultra- and nanofiltration and chemoselective separation, and those possessing proton or lithium transport properties suitable for fuel cell and lithium battery applications, respectively. This article describes the preparation methods of nanoparticle membranes, as well as their mechanical, molecular, and ionic transport properties.