Experimental and theoretical studies of the optimisation of fluorescence from near-infrared dye-doped silica nanoparticles

There is substantial interest in the development of near-infrared dye-doped nanoparticles (NPs) for a range of applications including immunocytochemistry, immunosorbent assays, flow cytometry, and DNA/protein microarray analysis. The main motivation for this work is the significant increase in NP fluorescence that may be obtained compared with a single dye label, for example Cy5. Dye-doped NPs were synthesised and a reduction in fluorescence as a function of dye concentration was correlated with the occurrence of homo-Förster resonance energy transfer (HFRET) in the NP. Using standard analytical expressions describing HFRET, we modelled the fluorescence of NPs as a function of dye loading. The results confirmed the occurrence of HFRET which arises from the small Stokes shift of near-infrared dyes and provided a simple method for predicting the optimum dye loading in NPs for maximum fluorescence. We used the inverse micelle method to prepare monodispersed silica NPs. The NPs were characterised using dynamic light scattering, UV spectroscopy, and transmission electron microscopy (TEM). The quantum efficiency of the dye inside the NPs, as a function of dye loading, was also determined. The fluorescent NPs were measured to be approximately 165 times brighter than the free dye, at an optimal loading of 2% (w/w). These experimental results were in good agreement with model predictions.

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Figure The change in nanoparticle fluorescence versus increased dye loading modelled using homo-Förster resonance energy transfer.

     

Mesoporous silica reinforced by silica nanoparticles to enhance mechanical performance

Silica nanoparticle/mesoporous silica composite films were prepared by direct mixing with mechanical stirring and thermal imidization. The structural morphology was elucidated by scanning electron microscopy and the surface of the film was imaged by atomic force microscopy. The functional groups and desorption process of the films were elucidated by Fourier transform infrared spectroscopy and thermal desorption spectroscopy. The mechanical properties were investigated using a nanoindenter system. The gel matrix and the filler are very compatible because they have similar molecular content. The composite films had a higher mechanical strength than pure porous silica film. Their strength is related to the silica nanoparticle content. The interfacial compatibility, dispersion effect, and interfacial strength also affect the mechanical strength of composite films.     

Controlling the growth of particle size and size distribution of silica nanoparticles by the thin film structure

Nanostructured silicondioxide thin films were prepared by sol–gel spin coating technique. The SiO₂ films were made using a conventional mixture of tetraethoxysilane (TEOS), deionized water and ethanol with various NH₃/TEOS ratios. The nanostructured silica films were made using a mixture of the SiO₂ sol and regular SiO₂ sol to control the enlargement of the particles inside the films. The structural, morphological and optical characterizations of the as-deposited and annealed films were carried out using X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy, NKD spectrophotometer and ultraviolet–visible (UV–vis) spectroscopy. The transmittance data of the infrared spectra of the films were recorded using an FT-IR Spectrometer. The XRD studies showed that as-deposited films were amorphous and the formation of the alfa-cristobalite phase of the silica film was investigated at annealing temperature close to 1,100 °C. Optical properties of the transmittance spectra in the s and p-polarization modes were collected. Refractive indices and extinction coefficients were determined with respect to the NH₃/TEOS ratios in the compositions of the films. Optical cut-off wavelength values were investigated from the extrapolation of the absorbance spectra which was estimated from the UV–vis spectroscopy measurements. A red shift in the absorption threshold indicated that the size of silica nanoparticles was increased by an increase in the NH₃/TEOS volume ratio from 1:64 to 1:8.     

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.     

Experimental design applied to the determination of several contaminants in Duero River by solid-phase microextraction

Several samples taken from Duero River source in Spain have been analysed to evaluate the potential risk of uncontrolled discharges from wood industries located in the surrounding area. Toluene, dodecane, tetradecane, naphthalene, 1-tetradecene, 1-hexadecene, BHT, benzophenone, diisobutylphthalate (DiBP), dibenzofuran and fluoranthene have been selected as representative compounds from solvents and other components of varnishes or coatings, the most likely contamination origin. Solid-phase microextraction (SPME) with further GC-MS has been selected as analytical technique by both its versatility, high process speed, low cost and sensitivity. In order to reach the maximum overall performance, three fibres with different polarity: 100 μm polydimethylsiloxane (PDMS), 85 μm polyacrylate, and 65 μm Carbowax-divinylbenzene have been evaluated. In addition, and due to the high number of involved variables, a two-level full factorial experiment design has been applied for optimisation being sampling time (5-20 min), sorption temperature (room: 50 °C), desorption temperature (minimum recommended +10%, maximum recommended −10%) and salt concentration (NaCl, 0-1 M) as ionic strength modifier the variables under study. After statistical evaluation of experimental design sampling time proved to be the most significant variable, and a more detailed kinetic study has been carried out. The 85 μm polyacrylate fibre was shown to be the most efficient one. Optimum conditions as well as quantitative values are shown and discussed. Toluene, dodecane, tetradecane, benzophenone, BHT and DiBP were found in the concentration range from 2 to 141 ng ml⁻¹.     

Spherical ensembles of gold nanoparticles on silica: electrostatic and size effects

Core-shell ensembles of citrate-stabilized gold nanoparticles (20-80 nm) on submicron silica cores (330-550 nm) have been prepared by electrostatic self-assembly with shell packing densities as high as phi = 0.55.     

Luminescent nanoparticles prepared by encapsulating lanthanide chelates to silica sphere

The luminescent nanoparticles were prepared by encapsulating the [LnL₄]^? (Ln = Eu, Tb; L = BTFA, HFAA, TTFA, TFAA) complexes anion into the silicon framework. We firstly synthesized a series of novel siloxy-bearing lanthanide complex precursor, and then encapsulated them into the silica sphere by a modified Stöber process. As a result, four europium and two terbium tetrakis β-diketonate complexes functionalized silica sphere nanoparticles were obtained and characterized in detail using Fourier transform infrared spectra, X-ray diffraction, scanning electronic microscope, thermogravimetric analysis, luminescence excitation and emission spectroscopy, luminescence lifetime measurements, and diffuse reflectance UV–Vis spectroscopy. The result shows that these luminescent nanoparticles maintain the distinctive luminescence character of lanthanide chelate including broad excitation spectra, line-like emission spectra, high quantum efficiency, and long luminescent lifetime, which makes them great potential application in the synthesis of luminescent nanoparticle.     

Encapsulation of silica nanoparticles by miniemulsion polymerization

Hybrid silica/polystyrene nanoparticles were synthesized by miniemulsion polymerization. With the objective to prepare core‐shell hybrid nanoparticles having narrow particle size distributions (PSDs) as well as a high degree of silica encapsulation, the effect of adding surface modifiers, the size of silica nanoparticles, the ratio styrene/silica, the surfactant concentration, and the presence of ethanol in the reaction mixture were studied. A synergistic effect was observed using oleic acid (OA) together with 3‐(trimethoxysilyl)propyl methacrylate (TPM) in the compatibilization step between the organic phase (monomer) and inorganic nanoparticles (silica). Mono and multinuclear eccentric core‐shell hybrid nanoparticles were obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 935–948, 2009     

Experimental design applied to the analysis of volatile compounds in apple juice by headspace solid-phase microextraction

A simple and fast method based on solid-phase microextraction (SPME) followed by fast gas chromatography (Fast GC) has been developed for the analysis of volatile compounds in Asturian apple juices employed in the cider production. Three different fiber coatings have been checked (PDMS, PDMS-DVB and CAR-PDMS) and PDMS-DVB has been presented to be the most suitable one. Experimental design has been employed in the optimization of extraction factors and robustness assessment. The use of Fast GC allowed the separation of 14 compounds (esters, aldehydes and alcohols) in approximately 4 min, clearly reducing the analysis time when compared to conventional GC. Good linearity, recoveries and repeatability of the solid-phase microextraction were obtained with r(2) values, recoveries and relative standard deviations ranging from 0.9822 to 0.9998, 83.2 to 109.8% and 0.5 to 11.7%, respectively, using standard solution.     

Polymer–silica hybrids obtained by microemulsion polymerization

The styrene (St), butyl acrylate (BuA), and methyl methacrylate (MMA) polymerization in microemulsion in the presence of sodium dodecylsulfate is studied. This process is conducted in the presence of some comonomers having groups that can participate in sol–gel processes: 3(trimethyloxysilyl) propyl methacrylate (MPTS), triethoxy vinylsilane (VTES), and a comonomer with a sulfate group, styrene sodium sulfonate (StSO₃Na). It has been observed that stabile latexes are obtained by radical polymerization at pH = 7, followed by a sol–gel process in the presence of ammonia. Latex particles sizes and zeta potential grow with MTPS concentration and in StSO₃Na presence. VTES effect depends on its reactivity in St, MMA, and BuA copolymerization. Glass transition temperature and thermal decomposing temperature are influences by functional comonomer concentration and chemical structure. The Fourier transform infrared spectrum and inorganic residue growth after organic part thermal decomposition shows the presence of silica in obtained latexes.     

Properties of tetrabutylammonium fluoride applied to silica gel

Conclusions The effect of the surface concentration of Bu₄NF applied to silica gel and Aerosil on its catalytic capacity in condensation of benzaldehyde with fluorene was studied. The capacity of the Bu₄NF applied to the support to pass into DMF solution was determined by the ionometric method. In the case of monolayer application of Bu₄NF on silica gel, catalysis of the model reaction is heterogeneous in character.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 375–378, February, 1988.     

New basic mesoporous silica catalyst obtained by ammonia grafting

NH3-treated mesoporous silica (FSM-16) contains SiNH2 sites which exhibit basic catalytic activity for Knoevenagel condensation; SiNH2 and SiOH pair sites formed at lower NH3-treatment temperatures exhibit higher turnover frequencies (TFs) in comparison with SiNH2 single sites.     

Synthesis of phthalazine compounds using heterogeneous base catalyst based on silica nanoparticles obtained from rice husk

Research on Chemical Intermediates - In this study, the nanoparticles of amorphous silica were easily extracted from low-cost rice husk ash. They were functionalized with 3-(chloropropyl)...     

Functional silica nanoparticles synthesized by water-in-oil microemulsion processes

Water-in-oil (W/O) microemulsion is a well-suitable confined reacting medium for the synthesis of structured functional nanoparticles of controlled size and shape. During the last decade, it allowed the synthesis of multi-functional silica nanoparticles with morphologies as various as core–shell, homogenous dispersion or both together. The morphology and properties of the different intermediates and final materials obtained through this route are discussed in the light of UV–Vis–NIR spectroscopy, dynamic light scattering (DLS) and X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and magnetometer SQUID analysis.     

Aggregation of silica nanoparticles directed by adsorption of lysozyme

The interaction of the globular protein lysozyme with silica nanoparticles of diameter 20 nm was studied in a pH range between the isoelectric points (IEPs) of silica and the protein (pH 3-11). The adsorption affinity and capacity of lysozyme on the silica particles is increasing progressively with pH, and the adsorbed protein induces bridging aggregation of the silica particles. Structural properties of the aggregates were studied as a function of pH at a fixed protein-to-silica concentration ratio which corresponds to a surface concentration of protein well below a complete monolayer in the complete-binding regime at pH > 6. Sedimentation studies indicate the presence of compact aggregates at pH 4-6 and a loose flocculated network at pH 7-9, followed by a sharp decrease of aggregate size near the IEP of lysozyme. The structure of the bridged silica aggregates was studied by cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The structure factor S(q) derived from the scattering profiles displays characteristic features of particles interacting by a short-range attractive potential and can be represented by the square-well Percus-Yevick potential model, with a potential depth not exceeding 3k(B)T.     

Thinning down of polymer matrix by entrapping silica nanoparticles

The simple and effective approach for preparation of highly porous poly(TRIM)/MCM-41 composite material has been presented. The structural properties of the spherically shaped particles of the composite were investigated by positron annihilation lifetime spectroscopy (PALS) and low temperature adsorption of nitrogen. The parameters characterizing the porosity of investigated materials derived from two different techniques have been compared. PALS provides a convenient tool for characterization of polymer-based composite materials.     

Agglomeration of alumina submicronparticles by silica nanoparticles: application to processing spheres by colloidal route

In aqueous media, heterocoagulation between submicronic alumina (400 nm) and nanometric silica (25 nm) leads to the adsorption of silica on the alumina surface. By controlling the coverage rate of alumina particles, this adsorption destabilizes the suspension that leads to a very porous network of agglomerated particles. This work shows that the structure is all the more open as the density of charge carried by the two oxides is high and the ionic strength in the suspension low. From such a flocculated suspension, a new colloidal process to fabricate ceramic spheres is proposed which is based on a size increase of agglomerates. Under a controlled rotation of the vessel, electrostatic attraction between the surface charges of opposite polarity induces a size increase of agglomerates until the formation of spheres occurs. It has been shown that the mechanism of growth is poisoned by species adsorbed such as ions. Nevertheless, this new process proves very promising because it leads to a narrow size distribution of spheres by colloidal way, which can be subsequently consolidated by sintering, with a smooth surface.