Thermo- and pH-responsive hydrogel-coated gold nanoparticles prepared from rationally designed surface-confined initiators

Gold nanoparticles (~40 nm in diameter) were encapsulated by a hydrogel overlayer generated by the free radical polymerization of N-isopropylacrylamide-co-acrylic acid (NIPAM-co-AA), which was promoted by a specifically designed radical initiator covalently anchored to the surface of the gold core. The size and morphology of the shell/core nanoparticles were characterized by scanning electron microscopy and transmission electron microscopy. In addition, the optical properties of the nanoparticles were characterized by UV–Visible spectroscopy. Separately, the particle size was evaluated as a function of temperature and pH using dynamic light scattering. The shell/core hydrogel nanoparticles undergo reversible volume changes in water at a lower critical solution temperature (LCST) of ~34 °C as well as at pH values between 3 and 4. The unique composition and properties of these shell/core hydrogel nanoparticles make them attractive for use as nanoscale drug-delivery vehicles.     

Inkjet-printed gold nanoparticulate patterns for surface finish in electronic package

Gold (Au) pads for surface finish in electronic package were developed by the inkjet printing method. The Au ink for printing was prepared by Au nanoparticles (NPs) coated with capping molecules of dodecylamine (C₁₂H₂₅NH₂). The microstructures of the inkjet-printed Au films were characterized after sintering in various gas flows. The film sintered in air showed that bonding between NPs was not enough for further grain growth due to the incomplete decomposition of the capping layer. The film sintered under nitrogen (N₂) had NPs existing on the surface and the bottom which did not participate in sintering. When the film was sintered under N₂-bubbled through formic acid (FA/N₂), a large portion of the pores were observed to make a holey pancake-like structure of the film. The microstructures of the inkjet-printed Au film became denser with grain growth when Au NPs were sintered under mixed gas flows of FA/N₂ and N₂. The resistivity of film was 4.79 μΩ cm, about twice the bulk value. Organic analysis showed that about 0.43% of residual organics was left in the film. Therefore, this Au film was chosen for solder ball shear test because the microstructure was denser compared to the films sintered under other gasses such as N₂ or FA/N₂ and less organic residue was found from organic analyses. Even though the film sintered under N₂ showed the best electrical property (4.35 μΩ cm), it was not adopted in the shear test because NPs remaining on the bottom of the film could lead to the poor adhesion between the film and substrate and show low shear strength. The shear force was 8.04 newton (N) on average and the strength was 64 MPa. This shear strength is good enough to substitute the inkjet-printed Au nanoparticulate film for electroplating in electronic package.     

Simple method to prepare size‐controllable gold nanoparticles in solutions and their applications on surface‐enhanced Raman scattering

We report here, for the first time, a simple method to prepare size‐controllable Au nanoparticles (NPs) in aqueous solutions from bulk Au substrates. First, chitosan (Ch)‐capped Au‐containing complexes were prepared by electrochemical oxidation–reduction cycles in 0.1 N NaCl and 1 g/l Ch solutions. Then the solutions were heated from room temperature to boiling at different heating rates to synthesize size‐controllable Au NPs. The particle sizes of the prepared Au(111) NPs could be controlled from 5 to 30 nm with an increase of the heating rate during preparation. Experimental results indicate that the prepared Au(111) NPs with diameters ranging from 10 to 30 nm can serve as surface‐enhanced Raman scattering active probes for molecules of rhodamine 6G. Copyright © 2010 John Wiley & Sons, Ltd.     

Multifunctional core–shell nanoparticles: superparamagnetic,mesoporous, and thermosensitive

Multifunctional core–shell composite nanoparticles (NPs) have been developed by the combination of three functionalities into one entity, which is composed of a single Fe₃O₄ NP as the magnetic core, mesoporous silica (mSiO₂) with cavities as the sandwiched layer, and thermosensitive poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AAm)) copolymer as the outer shell. The mSiO₂-coated Fe₃O₄ NPs (Fe₃O₄@mSiO₂) are monodisperse and the particle sizes were varied from 25 to 95 nm by precisely controlling the thickness of mSiO₂-coating layer. The P(NIPAAm-co-AAm) were then grown onto surface-initiator-modified Fe₃O₄@mSiO₂ NPs through free radical polymerization. These core–shell composite NPs (designated as Fe₃O₄@mSiO₂@P(NIPAAm-co-AAm)) were found to be superparamagnetic with high r ₂ relaxivity. To manipulate the phase transition behavior of these thermosensitive polymer-coated NPs for future in vivo applications, the characteristic lower critical solution temperature (LCST) was subtly tuned by adjusting the composition of the monomers to be around the human body temperature (i.e. 37 °C), from ca. 34 to ca. 42 °C. The thermal response of the core–shell composite NPs to the external magnetic field was also demonstrated. Owing to their multiple functionality characteristics, these porous superparamagnetic and thermosensitive NPs may prove valuable for simultaneous magnetic resonance imaging (MRI), temperature-controlled drug release, and temperature-programed magnetic targeting and separation applications.     

Catalytic reduction of organic dyes at gold nanoparticles impregnated silica materials: influence of functional groups and surfactants

Gold nanoparticles (Au NPs) in three different silica based sol–gel matrixes with and without surfactants are prepared. They are characterized by UV–vis absorbance and transmission electron microscopic (TEM) studies. The size and shape of Au NPs varied with the organo-functional group present in the sol–gel matrix. In the presence of mercaptopropyl functionalized organo-silica, large sized (200–280 nm) spherical Au NPs are formed whereas in the presence of aminopropyl functionalized organo-silica small sized (5–15 nm) Au NPs are formed inside the tube like organo-silica. Further, it is found that Au NPs act as efficient catalyst for the reduction of organic dyes. The catalytic rate constant is evaluated from the decrease in absorbance of the dye molecules. Presence of cationic or anionic surfactants greatly influences the catalytic reaction. The other factors like hydrophobicity of the organic dyes, complex formation of the dyes with anionic surfactants, repulsion between dyes and cationic surfactant, adsorption of dyes on the Au NPs also play important role on the reaction rate.     

Synthesis of composite gold/tin-oxide nanoparticles by nano-soldering

Composite Au–SnO₂ nanoparticles (NPs) are synthesized by nano-soldering of pure Au and SnO₂ NPs. The multi-step process involves synthesis of pure Au and SnO₂ NPs separately by nanosecond pulse laser ablation of pure gold and pure tin targets in deionized water and post-ablation laser heating of mixed solution of Au colloidal and SnO₂ colloidal to form nanocomposite. Transmission Electron Microscopy (TEM) and High-Resolution Transmission Electron Microscopy (HRTEM) were used to study the effect of laser irradiation time on morphology of the composite Au–SnO₂ NPs. The spherical particles of 4 nm mean size were obtained for 5 min of post-laser heating. Increased mean size and elongated particles were observed on further laser heating. UV–vis spectra of Au–SnO₂ nanocomposites show red shift in the plasmon resonance absorption peak and line shape broadening with respect to pure Au NPs. The negative binding energy shift of Au 4f_7/2 peak observed in X-ray Photoelectron Spectra (XPS) indicates charge transfer in the nano-soldered Au–SnO₂ between gold and tin oxide and formation of soldered nanocomposite.     

Phosphorylcholine functionalized dendrimers for the formation of highly stable and reactive gold nanoparticles and their glucose conjugation for biosensing

Phosphorylcholine (PC)-functionalized poly(amido amine) (PAMAM) dendrimers were prepared and used as both reducing and stabilizing agents for synthesis of highly stable and reactive gold nanoparticles (Au NPs). Biomimetic PC-functionalized PAMAM dendrimers-stabilized gold nanoparticles (Au DSNPs) were formed by simply mixing the PC modified amine-terminated fifth-generation PAMAM dendrimers (G5-PC) with AuCl₄ ⁻ ions by controlling the pH, no additional reducing agents or other stabilizers were needed. The obtained Au DSNPs were shown to be spherical, with particle diameters ranging from 5 to 12 nm, the sizes and growth kinetics of Au DSNPs could be tuned by changing the pH and the initial molar ratio of dendrimers to gold as indicated by transmission electron microscopy (TEM) and UV–Vis data. The prepared Au DSNPs showed excellent stability including: (1) stable at wide pH (7–13) values; (2) stable at high salt concentrations up to 2 M NaCl; (3) non-specific protein adsorption resistance. More importantly, surface functionalization could be performed by introducing desired functional groups onto the remained reactive amine groups. This was exemplified by the glucose conjugation. The glucose conjugated Au DSNPs showed bio-specific interaction with Concanavalin A (Con A), which induced aggregation of the Au NPs. Colorimetric detection of Con A based on the plasmon resonance of the glucose conjugated Au DSNPs was realized. A limit of detection (LOD) for Con A was 0.6 μM, based on a signal-to-noise ratio (S/N) of 3. These findings demonstrated that the PC modified Au DSNPs could potentially serve as a versatile nano-platform for the biomedical applications.     

Solution templating of Au and Ag nanoparticles by linear poly[2-(diethylamino)ethyl methacrylate]

Abstract  

Linear poly[2-(diethylamino)ethyl methacrylate], poly(DEAMA), is an uncommon example of a homopolymer that can reduce salts of Au and Ag in solution to yield stable dispersions of nanoparticles (5–25 nm typical size). Poly(DEAMA)-stabilized Au and Ag nanoparticles were prepared in a mixture of water and 2-butoxyethanol, an amphiphilic organic solvent. The “loading ratio” (mole ratio of metal atoms to amines), a key parameter influencing particle size and clustering, was systematically varied. The size distribution and clustering of the nanoparticles were characterized by transmission electron microscopy and small-angle X–ray scattering. The maximum loading ratio achievable without inducing precipitation was approximately 0.3 for Au, but the maximum loading ratio for Ag was only about 0.04. The preparation of both Au and Ag nanoparticles in solution with a linear polymeric template illustrates that dendritic or hyperbranched architecture of the polymer is not a prerequisite for obtaining stable, non-aggregated dispersions. From a practical standpoint, poly(DEAMA) is an inexpensive template material that is readily immobilized on silica, which could facilitate development of novel, nanoparticle-based heterogeneous catalysts.     

Expandable photo-induced synthetic route to generate highly controlled noble metal nanoparticles

A photo-stimulation strategy was applied to synthesize colloidal noble-metal nanoparticles (NPs) with a highly controlling of size and morphology with high yield at room temperature. In this controlled synthesis, photoreduction of a mixture of the noble metal precursor and a chemical reducing agent under ultraviolet (UV) illumination was used to produce electrons that reduce metal ions (Au³⁺ and Ag⁺) in toluene. Prolonged UV irradiation at 365 nm at a power of 0.14 μmol S⁻¹ m⁻² induced ripening wherein the irradiation power, exposure time, and chemical interaction of the reducing and stabilizing agents were key factors in determining the nanoscale structure of the NPs. Under optimal irradiation and chemical conditions, size and shape deviations of <6% of the Au and Ag NPs were obtained.     

High stability of the crystalline configuration of Au nanoparticles embedded in silica under ion and electron irradiation

Au nanoparticles (NPs) with a size in the 2–12 nm range have been grown in silica by 2 MeV Au-ion implantation and a subsequent thermal annealing in air. The as-prepared Au NPs were irradiated with 10 MeV Si ions elongating some of them. From transmission electron microscopy in Z-contrast mode, we observed a narrow size distribution of the minor axis of the deformed NPs, which presents its higher frequency around 6–7 nm and have a saturation about 9 nm. This final result agrees well with the diameter of the track formed by Si ions of 10 MeV in silica, supporting the thermal spike model, which would explain the deformation of the NPs. In this model, the NP melts and creeps along the ion track. Our results show that the NP crystallization is in the fcc structure. On the other hand, a 200 keV electron irradiation provoked roundness on the previously elongated nanoparticles. This effect was observed in situ by high-resolution transmission electron microscopy, showing additionally that, during the roundness process, the fcc structure, as well as its crystalline orientation, remain unchanged. Thus, this study shows how Au NPs embedded in silica, within this size distribution, keep the fcc bulk structure under both ion and electron irradiations.     

Systematic Control of Size and Morphology in the Synthesis of Gold Nanoparticles

The synthesis of gold nanoparticles (Au NPs) capped by poly(1‐vinylpyrrolidin‐2‐one (PVP, average  = 10 000 kDa) yields moderately dispersed (6–8.5 nm) product with limited morphological control while larger NPs (15–20 nm) are reliably prepared using trisodium citrate (Na₃Cit) as a reductant/capping agent. Excellent size control in the intermediate 10 nm regime is achieved by hybridizing these methodologies, with highly monodisperse, polycrystalline Au NPs forming. For a Na₃Cit:PVP:Au ratio of 3.5:3.5:1, anisotropic NPs with an aspect ratio of 1.8:1 suggest the systematic agglomeration of NP pairs. Enhanced control of NP morphology is allowed by the 1,2‐tetradecanediol reduction of Au^III in the presence of straight chain, molecular anti‐agglomerants. Last, ligand substitution is used to controllably grow preformed Au seeds. In spite of the extended growth phase used, the replacement of phosphine by 1‐pentadecylamine affords highly monodisperse, cuboidal NPs containing a single clearly visible twinning plane. The allowance of particle growth parallel to this close‐packed plane explains the remarkable particle morphology.     

Influence of size and surface capping on photoluminescence and cytotoxicity of gold nanoparticles

Hydrophilic and homogeneous sub-10 nm blue light-emitting gold nanoparticles (NPs) functionalized with different capping agents have been prepared by simple chemical routes. Structure, average, size, and surface characteristics of these NPs have been widely studied, and the stability of colloidal NP solutions at different pH values has been evaluated. Au NPs show blue PL emission, particularly in the GSH capped NPs, in which the thiol-metal core transference transitions considerably enhance the fluorescent emission. The influence of capping agent and NP size on cytotoxicity and on the fluorescent emission are analyzed and discussed in order to obtain Au NPs with suitable features for biomedical applications. Cytotoxicity of different types of gold NPs has been determined using NPs at high concentrations in both tumor cell lines and primary cells. All NPs used show high biocompatibility with low cytotoxicity even at high concentration, while Au-GSH NPs decrease viability and proliferation of both a tumor cell line and primary lymphocytes.     

Kinetics and mechanism of the growth of gold nanoparticles by reduction of tetrachloroauric acid by hydrazine in Triton N-42 reverse micelles

The kinetics of the growth of gold nanoparticles during the reduction of tetrachloroauric acid by hydrazine in dispersed aqueous solution encapsulated by reverse micelles of Triton N-42 surfactant (with decane as dispersion medium) was studied by means of spectrophotometry. According to DLS data, at a set value of solubilization capacity V _s/V _o = 0.005 initial micelles have an aqueous core hydrodynamic diameter d _c = 3.6±0.2 nm. The final particles obtained after full reduction of Au^III have a metallic core of defect-free single-crystalline gold with a narrow size distribution and average core diameter d _Au = 7.7 ± 1.4 nm as shown by TEM. The rate of the particle growth is limited by the rate of gold reduction. The process kinetics corresponds to the model consisting of two stages of reduction Au^III → Au^I → Au⁰. The stages involve the formation and redox decay of the intermediate complexes Au(N₂H₄)Cl₃ and Au(N₂H₄)Cl, and each stage proceeds via two routes: (1) homogeneous in the dispersed aqueous phase, and (2) heterogeneous on the particle surfaces. Reactions taking route (2) are autocatalytic because they proceed with participation of the surface atoms of particles as the final products of Au^III reduction. The dependencies of observed rate constants on reagent concentrations, temperature, and solubilization capacity of the micellar solution are studied.     

Preparation and Characterization of Inorganic–Organic Composites with Highly Dense CdS Nanoparticles Using Poly(2-acetamidoacrylic acid) Hydrogels

We report the template-based synthesis of inorganic CdS nanoparticles (NPs) in the interior of a poly(2-acetamidoacrylic acid) (PAAA) hydrogel as a novel type of nanocomposite by ion exchange in an aqueous system. Transmission electron microscopy (TEM) confirmed that the semiconductor CdS NPs was dispersed homogeneously in the hydrogel without particle aggregation. The average crystallite sizes calculated from the X-ray powder diffraction (XRD) pattern using the Scherrer equation, the absorption threshold from the ultraviolet-visible (UV-Vis) spectra and TEM were 3.02, 5.16 and 5.2 nm, respectively. In contrast to the uncapped CdS NPs, which had precipitated from the solution immediately after the reaction, the CdS NPs capped by the PAAA hydrogels did not show any observable changes, even after 6 months. This was confirmed by the lack of a red-shift in the absorption edge of the PAAA-capped CdS NPs and by TEM. Thermogravimetric analysis (TGA) showed that the CdS-embedded PAAA gel had superior thermal stability to the pure PAAA gel. In addition, the CdS NPs content in the dry composite gel was more than 70 wt%, indicating that the PAAA hydrogels have a large holding capacity for CdS NPs.     

Determination of Paracetamol Based on its Quenching Effect on the Photoluminescence of CdTe Fluorescence Probes

L-Cysteine capped CdTe nanoparticles (NPs) were synthesized in aqueous medium, and their application as fluorescence probes in the determination of paracetamol was studied. The L-cysteine capped CdTe NPs were characterized by transmission electron microscopy, X-ray diffraction spectrometry, spectrofluorometry, ultraviolet-visible and Fourier transform infrared spectrometry. Based on the distinct fluorescence quenching of CdTe fluorescence probes in the presence of paracetamol, a simple, rapid and specific method for paracetamol determination was presented. Under optimum conditions, the relative fluorescence intensity of CdTe NPs was linearly proportional to paracetamol concentration from 1.0 × 10⁻⁸ mol/L to 1.6 × 10⁻⁷ mol/L with a detection limit of 4.2 × 10⁻⁹ mol/L. The proposed method was applied to detect paracetamol in commercial tablets with satisfactory results.     

Controlled release of ketorolac through nanocomposite films of hydrogel and LDH nanoparticles

A novel nanocomposite film for sustained release of anionic ophthalmic drugs through a double-control process has been examined in this study. The film, made as a drug-loaded contact lens, consists principally of a polymer hydrogel of 2-hydroxyethyl methacrylate (HEMA), in whose matrix MgAl-layered double hydroxide (MgAl-LDH) nanoparticles intercalated with the anionic drug are well dispersed. Such nanocomposite films (hydrogel-LDH-drug) contained 0.6–0.8 mg of MgAl-LDH and 0.08–0.09 mg of the ophthalmic drug (ketorolac) in 1.0 g of hydrogel. MgAl-drug-LDH nanoparticles were prepared with the hydrodynamic particle size of 40–200 nm. TEM images show that these nanoparticles are evenly dispersed in the hydrogel matrix. In vitro release tests of hydrogel-LDH-drug in pH 7.4 PBS solution at 32 °C indicate a sustained release profile of the loaded drug for 1 week. The drug release undergoes a rapid initial burst and then a monotonically decreasing rate up to 168 h. The initial burst release is determined by the film thickness and the polymerization conditions, but the following release rate is very similar, with the effective diffusion coefficient being nearly constant (3.0 × 10⁻¹² m²/s). The drug release from the films is mechanistically attributed to anionic exchange and the subsequent diffusion in the hydrogel matrix.     

Multiple switching behaviors of poly(N-isopropylacrylamide) hydrogel with spironaphthoxazine and D-π-A type dye

A multi-switchable poly(NIPAM-co-SPO-co-D-π-A dye) hydrogel with a photochromic spironaphthoxazine and an electron donor-π-conjugated-electron acceptor (D-π-A) type dye was prepared by typical radical copolymerization. The low critical solution temperature (LCST) behavior was investigated by UV–vis spectroscopy, which allows the measurement of the phase transition from 20 to 40 °C in aqueous solution. The fluorescence intensity of the poly(NIPAM-co-SPO-co-D-π-A dye) was temperature-dependent. Reversible modulation of fluorescence intensity was achieved using alternating irradiation with UV and visible light. Prepared polymer hydrogel also exhibited spectra change when not only used Cu²⁺ cation but also an acid unit.     

Fluorescence Investigations of “Smart” Microgel Systems

Fluorescence techniques, including lifetime, quenching, and time-resolved anisotropy measurements (TRAMS), were used to study microgel systems based upon N-isopropylacrylamide (NI-PAM) using pyrene as a fluorescent probe. These experiments have revealed that poly(N-isopropylacrylamide) (PNIPAM) nanoparticles undergo a phase transition at a lower critical solution temperature (LCST), of ca. 34°C, which involves collapse of the particles into compacted, hydrophobic spheres. A degree of control over the LCST has been achieved by copolymerization of NIPAM with varying amounts of dimethylacrylamide (DMAC). Incorporation of DMAC into the gel has the effect of changing the hydrophobic to hydrophilic balance and shifts the LCST to a higher temperature. Fluorescence methods indicate that the NIPAM/DMAC gels are of a more open, water-swollen nature above the LCST than that of their PNIPAM counterparts.     

Diamine-linked array of metal (Au,Ag) nanoparticles on glass substrates for reliable surface-enhanced Raman scattering (SERS) measurements

Metal (Au, Ag) nanoparticles (M NPs) (ca. 30–40 nm) prepared by citrate reduction method were arrayed on amine-terminated glass substrates using diamine linkers with different chain lengths. 1,4-diaminobutane (C-4 diamine) produced the uniform and densely-packed array of M NPs on glass substrates at appropriate concentration ranges, whereas diamine linkers with longer chain lengths (C-8 and C-12 diamines) produced more heterogeneous and aggregated array of M NPs. When compared to Ag NPs, Au NPs demonstrated more controllable and higher packing density due to their mono-dispersed size and higher affinity to diamine linkers. Uniformly arrayed M NPs (Au, Ag) on glass substrates exhibited high enhancement factors in SERS measurements of o-chlorothiophenol probes. Au NPs arrayed substrates exhibited an approximate power-law linearity of Raman intensity with probe concentrations (from 10⁻⁷ M to 10⁻⁴ M), demonstrating more reliable SERS substrates than Ag arrayed substrates with higher SERS activity.     

PHOBOS at RHIC: Some global observations

Particle production in Au+Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies for a large span of pseudorapidities, |η| < 5.4. Three empirical observations have emerged from this data set which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au + Au collisions, when expressed as dN/dη′ ( η′ ≡ – y_beam), becomes energy independent at high energy for a broad region of η′ around η′ = 0. This energy-independent region grows with energy, allowing only a limited region (if any) of longitudinal boost-invariance. Second, there is a striking similarity between particle production in e⁺e⁻and Au + Au collisions (scaled by the number of participating nucleon pairs). Both the total number of produced particles and the longitudinal distribution of produced particles are approximately the same in e⁺e⁻and in scaled Au + Au. This observation This presentation is based in large part on the PHOBOS summary talk by M Baker at the16th Int. Conf. on Ultrarelativistic Nucleus- Nucleus Collisions, Quark Matter 2002, Nantes, France was not predicted and has not been explained. Finally, particle production has been found to scale approximately with the number of participating nucleon pairs for (N _part ) > 65. This scaling occurs both for the total multiplicity and for highp _T particles (3 <p _T < 4.5 GeV/c). This presentation is based in large part on the PHOBOS summary talk by M Baker at the16th Int. Conf. on Ultrarelativistic Nucleus-Nucleus Collisions, Quark Matter 2002, Nantes, France