Size effect of silica nanoparticles on thermal decomposition of PMMA

The size effect of silica nanoparticles (SiO₂) on thermal decomposition of poly(methylmethacrylate) (PMMA) was investigated by the controlled rate thermogravimetry. Thermal degradation temperature of PMMA–SiO₂ composites depended on both fraction and size of SiO₂, the thermal degradation temperature of 23 nm (diameter) SiO₂–PMMA (6.1 wt%) was 13.5 °C higher than that of PMMA. The thermal stabilities of 17 nm SiO₂–PMMA (3.2 wt%) and 13 nm SiO₂–PMMA (4.8 wt%) were 21 and 23 °C, respectively, higher than that of PMMA without SiO₂. The degree of degradation improvement was increased linearly with the surface area of SiO₂. The number of surface hydroxyl group in unit volume of SiO₂ particle increased with increasing the specific surface area of SiO₂, and the interaction between hydroxide group of SiO₂ and carbonyl group of PMMA had an important role to improve the thermal stability of PMMA.     

Size effect on the fluorescence properties of dansyl-doped silica nanoparticles

We present here the study of the photophysical properties of new dye-doped silica nanoparticles (DDNs) bearing dansyl fluorescent derivatives covalently linked to the silica matrix. The described experimental evidences show how the different location of the chromophores induces great changes in their photophysical behavior, suggesting that fluorophores located near the surface of the nanoparticles have a very different behavior with respect to the internal molecules. These latter ones, in fact, are shielded from the solvent and have a strong blue emission, while those at the periphery interact with the solvent and show a weaker red-shifted emission. As a consequence, the fluorescence properties of these nanoparticles are an average between the characteristics of the two different families of dyes. The relative amount of fluorophores located in the two compartments can be controlled simply by changing the size since, from our results, the thickness of the solvent permeable layer is not relevantly affected by the diameter of the nanoparticles. It is noteworthy that the fluorophores located in the outer shell exhibit very peculiar features: they are sensitive and interact with small molecules such as solvent molecules but, at the same time, they are not accessible to big receptor species such as beta-cyclodextrins. Such results indicate that most of the solvent-sensitive dansyl moieties are located within pores large enough to only accommodate solvent but not big molecules as cyclodextrins, giving precious insight on the morphology of the nanoparticles.     

Silica coating of silver nanoparticles using a modified Stober method

Silver nanoparticles prepared through a borohydride-reduction method were directly coated with silica by means of a seeded polymerization technique based on the Stober method. Various amine catalysts were used for initialization of a sol-gel reaction of TEOS with no need for a prior surface modification. Use of dimethylamine (DMA) as a catalyst was found to be necessary to obtain a proper coating. The silica shell thickness was varied from 28 to 76 nm for TEOS concentrations of 1-15 mM at 11.1 M water and 0.8 M DMA. The optical spectra of the core-shell silver-silica composite particles show a qualitative agreement with predictions by Mie theory.     

Size control in the synthesis of 1-6 nm gold nanoparticles via solvent-controlled nucleation

We report a facile synthetic route for size-controlled preparation of gold nanoparticles. Nearly monodisperse gold nanoparticles with core diameters of 1-6 nm were obtained by reducing AuP(Phenyl)(3)Cl with tert-butylamine borane in the presence of dodecanethiol in the solvent mixture of benzene and CHCl(3). Mechanism studies have shown that the size control is achieved by the solvent-controlled nucleation in which the nuclei concentration increases with increasing the fraction of CHCl(3), leading to smaller particles. It was also found that, following the solvent-controlled nucleation, particle growth occurs via ligand replacement of PPh(3) on the nuclei by Au(I)thiolate generated by the digestive etching of small particles. This synthetic strategy was successfully demonstrated with other alkanethiols of different chain length with which size-controlled, monodisperse gold nanoparticles were prepared in remarkable yield without requiring any postsynthesis treatments.     

Size control of dendrimer-templated silica

One of the most significant challenges facing the biomimetic synthesis of materials is achieving the requisite level of dimensional and spatial control. Typical reaction conditions for biomimetic silica synthesis allow for continued growth and ripening leading to the formation of larger nanospheres on the order of 200-600 nm in diameter. Herein, we have used polyamidoamine and polypropylenimine dendrimers as templates to expand the reaction conditions of biogenic silica production to produce a more robust synthesis leading to size-selective precipitation of silica nanospheres. Through the use of defined concentrations of phosphate buffer and main group metal chloride salts, we have shown that the biomimetic silica growth process is controlled by cationic neutralization of the anionic silica nanosphere surface. Neutralization minimizes electrostatic repulsions, allowing for agglomerization and continued growth of nanospheres. By controlling these concentrations, we can selectively produce silica nanospheres of desired dimensions between 30 and 300 nm without adversely affecting the template's activity.     

Biochemically functionalized silica nanoparticles

In this report, we demonstrate the biochemical modification of silica based nanoparticles. Both pure and dye-doped silica nanoparticles were prepared, and their surfaces were modified with enzymes and biocompatible chemical reagents that allow them to function as biosensors and biomarkers. The nanoparticles produced in this work are uniform in size with a 1.6% relative standard deviation. They have a pure silica surface and can thus be modified easily with many biomolecules for added biochemical functionality. Specifically, we have modified the nanoparticle surfaces with enzyme molecules (glutamate dehydrogenase (GDH) and lactate dehydrogenase (LDH)) and a biocompatible reagent for cell membrane staining. Experimental results show that the silica nanoparticles are a good biocompatible solid support for enzyme immobilization. The immobilized enzyme molecules on the nanoparticle surface have shown excellent enzymatic activity in their respective enzymatic reactions. The nanoparticle surface biochemical functionalization demonstrates the feasibility of using nanoparticles for biosensing and biomarking applications.     

Encapsulation of silica nanoparticles by redox-initiated graft polymerization from the surface of silica nanoparticles

This study describes a facile and versatile method for preparing polymer-encapsulated silica particles by ‘grafting from’ polymerization initiated by a redox system comprising ceric ion (Ce⁴⁺) as an oxidant and an organic reductant immobilized on the surface of silica nanoparticles. The silica nanoparticles were firstly modified by 3-aminopropyltriethoxysilane, then reacted with poly(ethylene glycol) acrylate through the Michael addition reaction, so that hydroxyl-terminated poly(ethylene glycol) (PEG) were covalently attached onto the nanoparticle surface and worked as the reductant. Poly(methyl methacrylate) (PMMA), a common hydrophobic polymer, and poly(N-isopropylacrylamide) (PNIPAAm), a thermosensitive polymer, were successfully grafted onto the surface of silica nanoparticles by ‘grafting from’ polymerization initiated by the redox reaction of Ce⁴⁺ with PEG on the silica surface in acid aqueous solutions. The polymer-encapsulated silica nanoparticles (referred to as silica@PMMA and silica@PNIPAAm, respectively) were characterized by infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. On the contrary, graft polymerization did not occur on bare silica nanoparticles. In addition, during polymerization, sediments were observed for PMMA and for PNIPAAm at a polymerization temperature above its low critical solution temperature (LCST). But the silica@PNIPAAm particles obtained at a polymerization temperature below the LCST can suspend stably in water throughout the polymerization process.     

Effect of surface modification of colloidal silica nanoparticles on the rigid amorphous fraction and mechanical properties of amorphous polyurethane–urea–silica nanocomposites

Colloidal silica nanoparticles (NPs) modified with eight different silane coupling agents were incorporated into an amorphous poly(tetramethylene oxide)‐based polyurethane–urea copolymer matrix at a concentration of 10 wt % (4.4 vol %) in order to investigate the effect of their surface chemistry on the structure–property behavior of the resulting nanocomposites. The rigid amorphous fraction (RAF) of the nanocomposite matrix as determined by differential scanning calorimetry and dynamic mechanical analysis was confirmed to vary significantly with the surface chemistry of the NPs and to be strongly correlated with the bulk mechanical properties in simple tension. Hence, nanocomposites with an RAF of about 30 wt % showed a 120% increase in Young's modulus, a 25% increase in tensile strength, a 15% decrease in elongation at break with respect to the neat matrix, which had no detectable RAF, whereas nanocomposites with an RAF of less than 5% showed a 60% increase in Young's modulus, a 10% increase in tensile strength and a 5% decrease in the elongation at break. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2543–2556     

Synthesis and properties of water-soluble silica nanoparticles

Methods for synthesis and optimum conditions of the formation of stable water-soluble silica nanoparticles are presented. The silica nanoparticles were synthesized by the hydrolytic polycondensation of tetraethoxysilane using two methods: under alkaline conditions (Stöber´s method) or in an acetic acid medium followed by the modification by grafting triethylene oxide moieties on the particle surface. The structure of the modified silica nanoparticles was confirmed by the data of IR and NMR spectroscopy. Polydispersity was evaluated by gel permeation chromatography and dynamic light scattering. The formation and stability of Langmuir monolayers of the silica nanoparticles modified by triethylene oxide moieties were studied.     

Synthesis and characterization of amino-functionalized silica nanoparticles

Monodispersed raw silica nanoparticles (RSNPs) with the particle size of 40 nm were successfully fabricated by condensation reaction of tetraethylorthosilicate in methanol with high concentration ammonia (1.2 M). The RSNPs were treated with the coupling agent 3-aminopropyltrimethoxysilane (APTMS) for grafting amine groups on the surface to obtain the amino-functionalized silica nanoparticles (ASNPs). The chemical structure and surface morphology of RSNPs and ASNPs were characterized by Fourier-transform infrared spectra, solid-state NMR spectra and scanning electron microscopy. In addition, a method to quantify the grafted amine groups on the surface of ASNPs was developed by using the ninhydrin assay. The ninhydrin analysis showed that 60 mol % of the APTMS molecules were immobilized on the surface, that is, 4.4 amine groups per nm2 of surface area were bonded on nonporous ASNPs. The weight loss of particles obtained from thermogravimetry analysis indicated the amount of grafted amine groups and was used as a reference to compare with the value determined from ninhydrin method.     

Electroactive core-shell nanocluster films of heme proteins, polyelectrolytes, and silica nanoparticles

Novel protein core-shell nanocluster films were assembled layer by layer on solid surfaces. In the first step, positively charged heme protein hemoglobin (Hb) or myoglobin (Mb) and negatively charged poly(styrenesulfonate) (PSS) were alternately adsorbed on the surface of SiO2 nanoparticles, forming core-shell SiO2-(protein/PSS)m nanoclusters. In the second step, the SiO2-(protein/PSS)m nanoclusters and polycationic poly(ethylenimine) (PEI) were assembled layer by layer on various solid substrates, forming [[SiO2-(protein/PSS)m]/PEI]n films. Various techniques were used to characterize the nanoclusters and monitor the film growth. [[SiO2-(protein/PSS)m]/PEI]n films at pyrolytic graphite (PG) electrodes exhibited well-defined, chemically reversible cyclic voltammetric reduction-oxidation peaks characteristic of the heme Fe(III)/Fe(II) redox couples. The proteins in the films retained near native conformations in the medium pH range, and the films catalyzed electrochemical reduction of oxygen and hydrogen peroxide. Advantages of the nanocluster films over the simple [SiO2/protein]n layer-by-layer films include a larger fraction of electroactive protein and higher specific biocatalytic activity. Using this approach, biocatalytic activity can be tailored and controlled by varying the number of bilayers deposited on the nanoparticle cores and the number of nanocluster layers on electrodes.     

Dextran-coated silica nanoparticles for calcium-sensing

This article describes the synthesis and characterisation of fluorescent composite nanoparticles consisting of a silica core and a dextran shell. The silica core contains a rhodamine-based reference dye, which allows ratiometric measurements and the dextran shell is labelled with the Ca(2+)-sensitive dye Fluo-4. The nanoparticles have an average hydrodynamic diameter of 95 nm, good colloidal stability and show a 2.9-fold increase in fluorescence intensity upon binding to Ca(2+) ions. The apparent dissociation constant of K'(d) ≈ 520 nM is well suited for measurements in the physiological range.     

Metal-free highly luminescent silica nanoparticles

Stable, cost-effective, brightly luminescent, and metal-free organosilica nanoparticles (NPs) were prepared using the St?ber method without any thermal treatment above 318 K. The white-light photoluminescence results from a convolution of the emission originated in the NH(2) groups of the organosilane and oxygen defects in the silica network. The time-resolved emission spectra are red-shifted, relative to those acquired in the steady-state regime, pointing out that the NPs emission is governed by donor-acceptor (D-A) recombination mechanisms. Moreover, the increase of the corresponding lifetime values with the monitored wavelength further supports that the emission is governed by a recombination mechanism typical of a D-A pair attributed to an exceptionally broad inhomogeneous distribution of the emitting centers peculiar to silica-based NPs. These NPs exhibit the highest emission quantum yield value (0.15 ± 0.02) reported so far for organosilica biolabels without activator metals. Moreover, the emission spectra and the quantum yield values are quite stable over time showing no significant aging effects after exposure to the ambient environment for more than 1 year, stressing the potential of these NPs as metal-free biolabels.     

Synthesis of well-defined, polymer-grafted silica nanoparticles via reverse ATRP

The surface grafting onto ultrafine silica via reverse ATRP of methyl methacrylate initiated by peroxide groups introduced onto the surface and conventional ATRP of Styrene initiated by the hybrid nanoparticles were investigated. The introduction of peroxide groups onto the silica surface was achieved by the reaction of hydrogen peroxide with chlorosilyl groups, which were introduced by the treatment of silica with thionyl chloride. Well-defined polymer chains were grown from the nanoparticle surfaces to yield individual particles composed of a silica core and a well-defined, densely grafted outer polymer layer. The polymerization was closely controlled in solution at quite low temperature such as 70 °C. In both cases, linear kinetic plots, linear plots of molecular weight (M_n) versus conversion, in hydrodynamic diameter with increasing conversion, and narrow molecular weight distributions (M_w/M_n) for the grafted polymer samples were observed. Hydrolysis of silica cores by hydrofluoric acid treatment enabled characterization of cleaved polymer using GPC. Ultrathin films of hybrid nanoparticles were examined using TEM and AFM.     

Synthesis of nitric oxide-releasing silica nanoparticles

The synthesis and characterization of a new nitric oxide (NO)-releasing scaffold prepared from amine-functionalized silica nanoparticles are reported. Inorganic-organic hybrid silica was prepared via cocondensation of tetraethoxy- or tetramethoxysilane (TEOS or TMOS) and aminoalkoxysilane with appropriate amounts of ethanol (or methanol), water, and ammonia. The amine functional groups in the silica were converted to N-diazeniumdiolate NO donors via exposure to high pressures of NO (5 atm) under basic conditions. Control over both the structure and concentration of the silane precursors (i.e., tetraalkoxy- and aminoalkoxysilanes) and specific synthetic conditions allowed for the preparation of NO donor silica particles of widely varying sizes (d = 20-500 nm), NO payloads (50-1780 nmol.mg-1), maximum amounts of NO released (10-5500 ppb.mg-1), half-lives (0.1-12 h), and NO release durations (up to 30 h). The silica nanoparticles were characterized by solid-state 29Si nuclear magnetic resonance (NMR), atomic force microscopy (AFM), elemental analysis, and gas adsorption-desorption isotherms. The advantages of silica-derived NO storage/delivery systems over previously reported macromolecular NO donors include the ability to (1) store large quantities of NO, (2) modulate NO release kinetics, and (3) readily tune particle size based on the composition of the particle. In addition, a one-pot strategy for preparing the NO donor silica allows for straightforward, high-throughput synthesis and purification.     

Formation of silica nanoparticles in microemulsions

Silica nanoparticles for controlled release applications have been produced by the reaction of tetramethylorthosilicate (TMOS) inside the water droplets of a water-in-oil microemulsion, under both acidic (pH 1.05) and basic (pH 10.85) conditions. In-situ FTIR measurements show that the addition of TMOS to the microemulsion results in the formation of silica as TMOS, preferentially located in the oil phase, diffuses into the water droplets. Once in the hydrophilic domain, hydrolysis occurs rapidly as a result of the high local concentration of water. Varying the pH of the water droplets from 1.05 to 10.85, however, considerably slows the hydrolysis reaction of TMOS. The formation of a dense silica network occurs rapidly under basic conditions, with IR indicating the slower formation of more disordered silica in acid. SAXS analysis of the evolving particles shows that approximately 11 nm spheres are formed under basic conditions; these are stabilized by a water/surfactant layer on the particle surface during formation. Under acidic conditions, highly uniform approximately 5 nm spheres are formed, which appear to be retained within the water droplets (approximately 6 nm diameter) and form an ordered micelle nanoparticle structure that exhibits sufficient longer-range order to generate a peak in the scattering at q approximately equal to 0.05 A-1. Nitrogen adsorption analysis reveals that high surface area (510 m2/g) particles with an average pore size of 1 nm are formed at pH 1.05. In contrast, base synthesis results in low surface area particles with negligible internal porosity.     

Rapid one-step synthesis, characterization and functionalization of silica coated gold nanoparticles

This paper describes a rapid, simple and one-step method for preparing silica coated gold (Au@SiO₂) nanoparticles with fine tunable silica shell thickness and surface functionalization of the prepared particles with different groups. Monodispersed Au nanoparticles with a mean particle size of 16 nm were prepared by citrate reduction method. Silica coating was carried out by mixing the as prepared Au solution, tetraethoxysilane (TEOS) and ammonia followed by microwave (MW) irradiation. Although there are several ways of coating Au nanoparticles with silica in the literature, each of these needs pre-coating step as well as long reaction duration. The present method is especially useful for giving the opportunity to cover the colloidal Au particles with uniform silica shell within very short time and forgoes the use of a silane coupling agent or pre-coating step before silica coating. Au@SiO₂ nanoparticles with wide range of silica shell thickness (5-105 nm) were prepared within 5 min of MW irradiation by changing the concentration of TEOS only. The size uniformity and monodispersity were found to be better compared to the particles prepared by conventional methods, which were confirmed by dynamic light scattering and transmission electron microscopic techniques. The prepared Au@SiO₂ nanoparticles were further functionalized with amino, carboxylate, alkyl groups to facilitate the rapid translation of the nanoparticles to a wide range of end applications. The functional groups were identified by XPS, and zeta potential measurements.     

Solid state synthesis of water-dispersible silicon nanoparticles from silica nanoparticles

A solid state synthesis for obtaining nanocrystalline silicon was performed by high temperature reduction of commercial amorphous nanosilica with magnesium powder. The obtained silicon powder contains crystalline silicon phase with lattice spacings characteristic of diamond cubic structure (according to high resolution TEM), and an amorphous phase. In ²⁹Si CP MAS NMR a broad multicomponent peak corresponding to silicon is located at −61.28 to −69.45 ppm, i.e. between the peaks characteristic of amorphous and crystalline Si. The powder has displayed red luminescence while excited under UV illumination, due to quantum confinement within the nanocrystals. The silicon nanopowder was successfully dispersed in water containing poly(vinyl alcohol) as a stabilizing agent. The obtained dispersion was also characterized by red photoluminescence with a band maximum at 710 nm, thus enabling future functional coating applications.