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.     

Preparation of Silica Microspheres Containing Ag Nanoparticles

Two sol-gel fabrication processes were investigated to make silica spheres containing Ag nanoparticles: (1) a modified Stöber method for silica spheres below 1 m size, and (2) a SiO₂-film formation method on spheres of 3–;7 m size. The spheres were designed to incorporate silver nanoparticles of high ⁽³⁾ in a spherical optical cavity structure for the resonance effect. For the incorporation, interaction between [Ag(NH₃)₂]⁺ ion and Si-OH was important. In the Stöber method, the size of the silica spheres was determined by a charge balance of plus and minus ions on the silica surface. In the film formation method, the capture of Ag complex ion on the silica surface depended on whether the surface was covered with OH groups or not. After doping [Ag(NH₃)₂]⁺ into silica particles or SiO₂ films on the spheres, these ions w ere reduced by NaBH₄ to form silver nanoparticles. From plasma absorption at around 420 nm wavelength and TEM photographs of nanometer-sized silver particles, their formation inside the spherical cavity structures was confirmed.     

Au@Y2O3:Eu3+ rare earth oxide hollow sub-microspheres with encapsulated gold nanoparticles and their optical properties

A facile method to synthesize novel Au@Y₂O₃:Eu³⁺ hollow sub-microspheres encapsulated with moveable gold nanoparticle core and Y₂O₃:Eu³⁺ as shell via two-step coating processes and a succeeding calcination process has been developed. Silica coating on citrate-stabilized gold nanoparticles with a size of 25 nm can be obtained through a slightly modified Stöber process. Gold particles coated with double shell silica and Eu doped Y(OH)₃ can be obtained by coating on the Au@SiO₂ spheres through simply adding Y(NO₃)₃, Eu(NO₃)₃ and an appropriate quantity of NH₃·H₂O. Au@Y₂O₃:Eu³⁺ hollow sub-microspheres with moveable individual Au nanoparticle as core can be obtained after calcination of Au@Y₂O₃:Eu³⁺ particles at 600 °C for 2 h. These new core–shell structures with encapsulated gold nanoparticles have combined optical properties of both the Au nanoparticles and the Y₂O₃:Eu³⁺ phosphor materials which might have potential applications.     

Deposition of gold nanoparticles on silica spheres by electroless metal plating technique

A previously proposed method for metal deposition with silver [Kobayashi et al., Chem. Mater. 13 (2001) 1630] was extended to uniform deposition of gold nanoparticles on submicrometer-sized silica spheres. The present method consisted of three steps: (1) the adsorption of Sn(2+) ions took place on surface of silica particles, (2) Ag(+) ions added were reduced and simultaneously adsorbed to the surface, while Sn(2+) was oxidized to Sn(4+), and (3) Au(+) ions added were reduced and deposited on the Ag surface. TEM observation, X-ray diffractometry, and UV-vis absorption spectroscopy revealed that gold metal nanoparticles with an average particle size of 13 nm and a crystal size of 5.1 nm were formed on the silica spheres with a size of 273 nm at an Au concentration of 0.77 M.     

Y2O3:Eu (5 mol%) with Ag nanoparticles prepared by citrate precursor

Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) and Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) containing 1 mol% of Ag nanoparticles were prepared by heat treatment of a viscous resin obtained via citrate precursor. TEM and EDS analyses showed that Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) is formed by nanoparticles with an average size of 12 nm, which increases to 30 nm when Ag is present because the effect of metal induced crystallization occurs. Ag nanoparticles with a size of 9 nm dispersed in Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) were obtained and the surface plasmon effect on Ag nanoparticles was observed. The emission around 612 nm assigned to the Eu³⁺ (⁵D₀→⁷F₂) transition enhanced when the Ag nanoparticles were present in the Y₂O₃:Eu³⁺ luminescent material.     

Synthesis of Au/SnO2 core-shell structure nanoparticles by a microwave-assisted method and their optical properties

Au/SnO₂ core-shell structure nanoparticles were synthesized using the microwave hydrothermal method. The optical and morphological properties of these particles were examined and compared with those obtained by the conventional hydrothermal method. In microwave preparation, the peak position of the UV-visible plasmon absorption band of Au nanoparticles was red-shifted from 520 to 543 nm, due to the formation of an SnO₂ shell. An SnO₂ shell formation was complete within 5 min. The thickness of the SnO₂ shell was 10-12 nm, and the primary particle size of SnO₂ crystallites was 3-5 nm. For the core-shell particles prepared by a conventional hydrothermal method, the shell formed over the entire synthesis period and was not as crystalline as those produced, using the microwave method. The relationship between the morphological and spectroscopic properties and the crystallinity of the SnO₂ shell are discussed.     

Hematite template route to hollow-type silica spheres

Hollow-type silica spheres with controlled cavity size were prepared from Fe₂O₃-SiO₂ core-shell composite particles by selective leaching of the iron oxide core materials using acidic solution. The spherical Fe₂O₃ core particles with a diameter range of 20-400 nm were first prepared by the hydrolysis reaction of iron salts. Next, the Fe₂O₃-SiO₂ core-shell particles were prepared by the deposition of a SiO₂ layer onto the surface of Fe₂O₃ particles using a two-step coating process, consisting of a primary coating with sodium silicate solution and a subsequent coating by controlled hydrolysis of tetraethoxysilicate (TEOS). The Fe₂O₃ core was then removed by dissolving with acidic solution, giving rise to hollow-type silica particles. Scanning electron microscopy clearly revealed that the cavity size was closely related to the initial size of the core Fe₂O₃ particle. According to the cross-sectional view obtained by transmission electron microscopy, the silica shell thickness was about 10 nm. The porous texture of the hollow-type silica particles was further characterized by nitrogen adsorption-desorption isotherm measurements.     

Preparation of strongly fluorescent silica nanoparticles of polyelectrolyte-protected cadmium telluride quantum dots and their application to cell toxicity and imaging

Based on the polyelectrolyte-protected CdTe quantum dots (QDs), which were prepared by self-assembling of QDs and poly-diallyldimethylammonium chloride (PDADMAC) in the help of electrostatic attraction, the strong fluorescence silica nanoparticles (QDs-PDADMAC@SiO₂) have been prepared via a water-in-oil reverse microemulsion method. Transmission electron microscopy and Zeta potential analysis were used to characterize the as-prepared nanoparticles. All of the particles were almost spherical and there is a uniform distribution of the particle size with the average diameter about 25 nm. There is a large Zeta potential of −35.07 mV which is necessary for good monodispersity of nanoparticles solution. As compared with the QDs coated by SiO₂ (QDs@SiO₂), the QDs-PDADMAC@SiO₂ nanoparticles have much stronger fluorescence, and their fluorescence stability could be obviously improved. Moreover, QDs-PDADMAC@SiO₂ exhibits good biological compatibility which promotes their application in cellular imaging.     

Preparation of a new core-shell Ag@SiO2 nanocomposite and its application for fluorescence enhancement

Ag@SiO₂ nanoparticles with different shell thicknesses were synthesized via modified Stöber method. Rhodamine B isothiocyanate was covalently bound onto the surface of Ag@SiO₂ nanoparticles to form fluorescent core-shell Ag@SiO₂ nanocomposites. Effects of shell thickness on the fluorescence enhancement were examined using the corresponding nanobubbles prepared by cyanide etching as a control. The result showed that the fluorescence enhanced as the shell thickness increased till the distance between fluorophore and metal core reached about 75 nm with the optimal enhancement factor of ∼5-folds. Further increasing of fluorophore-metal distance caused a decrease in the enhancement factor.     

Preparation of silica-PS composite particles and their application in PET

To investigate how the superfine particles disperse in the polymers, the paper presented the preparation of monodisperse silica particles by Stöber method, and then grafted by γ-methacrylic propyl trimethoxysilane (MPS) as a coupling agent. Using these modified particles, the more stable silica-PS superfine composite particles with higher monodispersity than these of previous reports are prepared and reported through dispersion polymerization (DP) method, whose morphology is investigated with transmission electron microscope (TEM). Their high stability is provided from the bonding of CC groups of MPS to the silanol groups on the surface of silica particles from FTIR.Using this DP process, the influence of different size grafted silica particles on the morphology, polystyrene (PS) encapsulation behavior and the distribution in these composite particles have been investigated. When the grafted silica size is in nanoscale or less than 54 nm, the spherical shape of neither silica particles nor their composite particles is regular, but they can homogeneously disperse in polystyrene. As the size (d_n) of grafted silica particles increase to submicrometer (or 100 nm < d_n < 1000 nm), their coefficient variance of size distribution (C_v) ranges from only 9.0% to 1.5%. These obtained particles are completely encapsulated by PS with more regular shape, and have their C_v below 7%. When the size of silica particles reaches 380 nm, their C_v obviously reduces to 2.5%, and specially, the number of grafted silica particles approaches to one in each of the composite particles. But, when the silica size reaches 602 nm, PS can hardly encapsulate grafted silica particles and free silica particles appear in reactive system.Furthermore, using the silica particles of 380 nm, a series of core-shell structured superfine composite particles of 640-1100 nm with C_v lower than 11% are obtained. Under the set conditions, the preparing factors on these composite particles using 380 nm grafted silica particles is discussed, and the best reaction condition for the well-dispersed and regular periphery silica-PS composite particles is optimized as, the additions amounts of PVP, styrene, AIBN, grafted SiO₂ and H₂O are 0.23 mmol L⁻¹, 0.60 mol L⁻¹, 6.10 mmol L⁻¹, 0.10 mol L⁻¹ and 5.50 mL, respectively. Under this case, the composite particles can be prepared with C_v below 8%.At last, these composite particles are mixed with poly(ethylene terephthalate) (PET) to investigate their nucleation effect. Results show that all different size particles can promote PET’s crystallization and enhance the crystallization rate, and PET’s crystallization temperature (T_mc) is obviously enhanced from 193 to 205 °C through differential scanning calorimetry (DSC). It is strongly suggested that different silica size level all play nucleation role in PET, and thus explain the nucleation effect of multiscale inorganic particles.     

Comparative toxicity study of Ag, Au, and Ag–Au bimetallic nanoparticles on Daphnia magna

A comparative assessment of the 48-h acute toxicity of aqueous nanoparticles synthesized using the same methodology, including Au, Ag, and Ag–Au bimetallic nanoparticles, was conducted to determine their ecological effect in freshwater environments through the use of Daphnia magna, using their mortality as a toxicological endpoint. D. magna are one of the standard organisms used for ecotoxicity studies due to their sensitivity to chemical toxicants. Particle suspensions used in toxicity testing were well-characterized through a combination of absorbance measurements, atomic force or electron microscopy, flame atomic absorption spectrometry, and dynamic light scattering to determine composition, aggregation state, and particle size. The toxicity of all nanoparticles tested was found to be dose and composition dependent. The concentration of Au nanoparticles that killed 50% of the test organisms (LC₅₀) ranged from 65–75 mg/L. In addition, three different sized Ag nanoparticles (diameters = 36, 52, and 66 nm) were studied to analyze the toxicological effects of particle size on D. magna; however, it was found that toxicity was not a function of size and ranged from 3–4 μg/L for all three sets of Ag nanoparticles tested. This was possibly due to the large degree of aggregation when these nanoparticles were suspended in standard synthetic freshwater. Moreover, the LC₅₀ values for Ag–Au bimetallic nanoparticles were found to be between that of Ag and Au but much closer to that of Ag. The bimetallic particles containing 80% Ag and 20% Au were found to have a significantly lower toxicity to Daphnia (LC₅₀ of 15 μg/L) compared to Ag nanoparticles, while the toxicity of the nanoparticles containing 20% Ag and 80% Au was greater than expected at 12 μg/L. The comparison results confirm that Ag nanoparticles were much more toxic than Au nanoparticles, and that the introduction of gold into silver nanoparticles may lower their environmental impact by lowering the amount of Ag which is bioavailable.     

Preparation of highly uniform Ag/TiO2 and Au/TiO2 supported nanoparticle catalysts by photodeposition

Photodeposition of Ag nanoparticles on commercial TiO2 particles and nanoparticles was performed in order to provide direct visualization of the spatial distribution of photoactive sites on sub-micrometer-scale and nanoscale TiO2 particle surfaces and to create materials for potential catalytic applications. HRTEM (high-resolution transmission electron microscopy) and HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) were used to characterize these materials. The size and spatial distributions of the Ag nanoparticles on the commercial TiO2 were not uniform; the concentration of Ag was higher on grain boundaries and at the edges of these submicrometer particles. In the case of TiO2 nanoparticles, the size distribution of the Ag nanoparticles deposited was relatively uniform and independent of irradiation time and photon energy. The amount of Ag deposited on TiO2 nanoparticles was at least 6 times higher than that on the commercial samples for comparable irradiation conditions. Compared to the case of Ag photodeposition, the difference in the amount of Au photodeposited on TiO2 particles and nanoparticles was even greater, especially at low precursor concentrations. Photodeposition on TiO2 nanoparticles is suggested as a potential method for the preparation of Au/TiO2 catalysts, as loadings in excess of 10 wt % of uniform 1 nm metal particles were achieved in this work.     

Synthesis and characterization of monodisperse, mesoporous, and magnetic sub-micron particles doped with a near-infrared fluorescent dye

Recently, multifunctional silica nanoparticles have been investigated extensively for their potential use in biomedical applications. We have prepared sub-micron monodisperse and stable multifunctional mesoporous silica particles with a high level of magnetization and fluorescence in the near infrared region using an one-pot synthesis technique. Commercial magnetite nanocrystals and a conjugated-NIR-dye were incorporated inside the particles during the silica condensation reaction. The particles were then coated with polyethyleneglycol to stop aggregation. X-ray diffraction, N₂ adsorption analysis, TEM, fluorescence and absorbance measurements were used to structurally characterize the particles. These mesoporous silica spheres have a large surface area (1978 m²/g) with 3.40 nm pore diameter and a high fluorescence in the near infrared region at λ=700 nm. To explore the potential of these particles for drug delivery applications, the pore accessibility to hydrophobic drugs was simulated by successfully trapping a hydrophobic ruthenium dye complex inside the particle with an estimated concentration of 3 wt%. Fluorescence imaging confirmed the presence of both NIR dye and the post-grafted ruthenium dye complex inside the particles. These particles moved at approximately 150 μm/s under the influence of a magnetic field, hence demonstrating the multifunctionality and potential for biomedical applications in targeting and imaging.     

Synthesis and characteristic of the Fe3O4@SiO2@Eu(DBM)3·2H2O/SiO2 luminomagnetic microspheres with core-shell structure

The core-shell structured luminomagnetic microsphere composed of a Fe₃O₄ magnetic core and a continuous SiO₂ nanoshell doped with Eu(DBM)₃·2H₂O fluorescent molecules was fabricated by a modified Stöber method combined with a layer-by-layer assembly technique. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), confocal microscopy, photoluminescence (PL), and superconducting quantum interface device (SQUID) were employed to characterize the Fe₃O₄@SiO₂@Eu(DBM)₃·2H₂O/SiO₂ microspheres. The experimental results show that the microshpere has a typical diameter of ca. 500 nm consisting of the magnetic core with about 340 nm in diameter and silica shell doped with europium complex with an average thickness of about 80 nm. It possesses magnetism with a saturation magnetization of 25.84 emu/g and negligible coercivity and remanence at room temperature and exhibits strong red emission peak originating from electric-dipole transition ⁵D₀ → ⁷F₂ (611 nm) of Eu³⁺ ions. The luminomagnetic microspheres can be uptaken by HeLa cells and there is no adverse cell reaction. These results suggest that the luminomagnetic microspheres with magnetic resonance response and fluorescence probe property may be useful in biomedical imaging and diagnostic applications.     

Size sorting of Au and Pt nanoparticles from arbitrary particle size distributions

A K₂BSPP (dipotassium bis(p-sulfonatophenyl)phenylphosphane dihydrate)-based method to sort and size refine Au and Pt nanoparticles has been developed. It makes use of K₂BSPP to impart graduated stability to the nanoparticles in a number of NaCl solutions. The method offers a systematic approach to preparing metal nanoparticles of small diameters and a narrow size distribution from an arbitrary particle size distribution. TEM investigations confirmed the size refinement efficacy of this treatment method: in a typical experiment, the mean particle size and relative standard deviation of Au nanoparticles after three successive treatments were 5.18 and 0.055 nm, respectively, down from the corresponding values of 8.22 and 0.26 nm from the initial untreated nanoparticle solution. The K₂BSPP-based size refinement procedure is a simple alternative to more complex chemical procedures and stringent process control that are currently required for the preparation of mono-dispersed systems.     

Electrospinning nanosuspensions loaded with passivated Au nanoparticles

Aqueous polyethylene oxide (PEO) solutions (2 MDa, 2-5 wt %) with or without citrate passivated Au nanoparticles (5.7×10⁻⁷ wt %) have been electrospun, producing fibres with diameters from 290 μm to 55 nm. The incorporation of nanoparticles suppresses the diameter of the fibres and increases the degree of crystallinity. Such nanocomposite fibres are of interest as self-assembled templates for bottom-up fabrication methodologies.     

A novel nonenzymatic fluorescent sensor for glucose based on silica nanoparticles doped with europium coordination compound

Amino-functionalized luminescent silica nanoparticles (LSNPs) doped with the europium(III) mixed complex, Eu(TTA)₃phen with 2-thenoyltrifluoroacetone (TTA) and 1,10-phenanthroline(phen) were synthesized successfully using an revised Stöber method. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR), and fluorescence spectroscopy were performed for characterizing the synthesized nanoparticles. In the presence of glucose, the fluorescence intensity of the amino-functionalized LSNPs was enhanced due to the enhanced fluorescence resonance energy transfer. Based on fluorescence-enhancing effect, a simple and sensitive method for the determination of glucose was proposed. Under the optimized experimental conditions, the enhanced fluorescence intensity ratio (ΔF/F₀) was linear with the concentration of glucose (c) in the range of 0.0-180 μg ml⁻¹ with a detection limit of 0.8 μg ml⁻¹ (S/N = 3). The R.S.D. values were 0.33% and 0.37% at the levels of 22.5 and 100 μg ml⁻¹, respectively. The proposed method was applied to the determination of glucose in synthetic samples with satisfactory results. The proposed method was also performed to the analysis of blood glucose in human serum samples and the results were in good agreement with clinical data provided by the hospital, which indicates that the method presented here is not only simple, sensitive, but also reliable and suitable for practical applications.     

Preparation and characterization of stable monodisperse silver nanoparticles via photoreduction

Silver nanoparticles were synthesized by UV irradiation of [Ag(NH₃)₂]⁺ aqueous solution using poly(N-vinyl-2-pyrrolidone) (PVP) as both reducing and stabilizing agents. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption maxima around 420 nm. It was found that the formation rate of silver nanoparticles from Ag₂O was much quicker than that from AgNO₃, and the absorption intensity increased with PVP concentration as well as irradiation time. The maximum absorption wavelength (λ_max) was blue shift with increasing PVP content until 8 times concentration of [Ag(NH₃)₂]⁺ (wt%). The transmission electron microscopy (TEM) showed the resultant particles were 4–6 nm in size, monodisperse and uniform particle size distribution. X-ray diffraction (XRD) demonstrated that the colloidal nanoparticles were the pure silver. In addition, the silver nanoparticles prepared by the method were stable in aqueous solution over a period of 6 months at room temperature (25 °C).     

Synthesis of Ag/SiO2 nanocomposite material by adsorption phase nanoreactor technique

Ag/SiO₂ nanocomposite was synthesized in a nanoreactor formed by adsorption layer on silica surface. Ag nanoparticles were prepared by the reduction of Ag ion with ethanol at alkaline condition. By using TEM and XRD, the effects of NaOH concentration, water and temperature on the appearance and grain size of Ag particles were analyzed, respectively. The adsorption curve of NaOH was measured by electrical conductivity meter. The experiment result revealed that Ag grain size decreased while increasing NaOH concentration or while increasing water in our system. Ag grain size increased with the increase of temperature. And Ag aggregated seriously when temperature is up to 60 °C. Finally, after exploring the optimum conditions of reaction, we successfully obtained the well-distributed Ag nanoparticles on surface of silica, and average grain size of Ag nanoparticles reached 5 nm.     

Synthesis of silica-polystyrene core-shell nanoparticles via surface thiol-lactam initiated radical polymerization

This paper reports the synthesis of structurally well-defined silica-polystyrene (SiO₂@PS) hybrid nanoparticles using a thiol-lactam-initiated, radical-polymerization technique. The surface of silica particles, 80 nm in size, were functionalized with (3-mercaptopropyl) trimethoxysilane and used as seeds in the polymerization of styrene in the presence of butyrolactam. ¹H nuclear magnetic resonance and X-ray photoelectron spectroscopy showed that the thiol groups on the SiO₂ surface could initiate polymerization with the aid of butyrolactam. Transmission electron microscopy showed that the hybrid particles had uniform core-shell morphologies. The molecular weight of grafted PS increased with increasing polymerization time.