Study of the Optical Properties of CdZnSe/ZnS-Quantum Dot–Au-Nanoparticle Complexes

The interaction of gold nanoparticles (NPs) and semiconductor alloyed CdZnSe/ZnS quantum dots (QDs) in colloidal solutions is studied. It is shown that the photoluminescence intensity of QDs in a mixture decreases compared to that in the initial QD solution, which is caused by resonance nonradiative energy transfer from QDs to Au NPs in spontaneously formed aggregates. To control the formation of pairs of interacting QDs and Au NPs, we proposed have a method for creating QD–Au NP complexes bound by special molecules—ligands. It is shown that the morphology and optical properties of the samples obtained depend on the method of their preparation, in particular, on the chemical environment of QDs. It is found that the complexes form in the case of addition of hydrophilic Au NPs to hydrophobic QDs and that this almost does not change the optical properties of the latter compared to those of quasi-isolated QDs in colloidal solution.     

SHINERS and plasmonic properties of Au Core SiO2 shell nanoparticles with optimal core size and shell thickness

Shell‐isolated nanoparticles (NPs)‐enhanced Raman spectroscopy (SHINERS) can be potentially applied to virtually any substrate type and morphology. How to take a step forward to prepare SHINERS NPs (SHINs) with superior performance is critical for the practical applications of surface‐enhanced Raman scattering (SERS) in the breadth and depth. Here, we present a method to obtain 120 nm diameter gold NPs coated with ultrathin silica shells (1–4 nm). The silica shell can be controlled growth through carefully tuning a series of parameters, such as amount of 3‐aminopropyl triethoxysilane used, pH, reaction time, and reaction temperature. We compare the enhancement factor of the obtained 120 nm Au with a 4 nm silica shell NPs to the 55 nm Au with a 4 nm silica shell NPs, and the activity of a 120 nm SHINs is nearly 24 times that the 55 nm SHIN from a single particle view. We also compare the enhancement factor of 1 nm silica shell Au@SiO₂ NPs with the bare Au NPs. The enhancement factor of 1 nm silica shell Au@SiO₂ NPs was found to be about twice that of the bare particles. For a deeper understanding of the source of the giant enhanced electrical field of the 1 nm silica shell Au@SiO₂ NPs, we study the plasmonic property of single 1 nm silica shell Au@SiO₂ NP on a gold film substrate through correlation of the structure of single NP using SEM with its SPR spectroscopy. We find that the multipolar interaction between the single Au@SiO₂ NP and gold film substrate is important for the SERS. Our studies on the performance of 120 nm SHINs and the plasmonic property of these particles can significantly expand the applications of SHINERS technique and improve the understanding of physical nature of SHINs. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Deliberate Introduction of Particle Anisotropy in Helical Gold Nanoparticle Superstructures

Helical nanoparticle (NP) superstructures are an important class of chiral NP assemblies. The nature of the constituent NPs (size and shape) within these assemblies dictates their optical properties. However, the construction of helical NP superstructures consisting of various anisotropic NPs remains challenging. Here, a set of cetyltrimethylammonium bromide derivatives is employed to transform constituent spherical gold NPs (≈3 nm) within a chiral single‐helical assembly into gold nanoprisms (edge length ? 10 nm). Careful optimization of this strategy may lead to designed chiral NP architectures with tunable optical properties.     

In situ studies on free-standing synthesis of nanocatalysts via acoustic levitation coupled with pulsed laser irradiation

Acoustic levitation is a distinctive and versatile tool for levitating and processing free-standing single droplets and particles. Liquid droplets suspended in an acoustic standing wave provide container-free environments for understanding chemical reactions by avoiding boundary effects and solid surfaces. We attempted to use this strategy for the production of well-dispersed uniform catalytic nanomaterials in an ultraclean confined area without the addition of external reducing agents or surfactants. In this study, we report on the synthesis of gold and silver nanoparticles (NPs) via acoustic levitation coupled with pulsed laser irradiation (PLI). In situ UV–Visible and Raman spectroscopic techniques were performed to monitor the formation and growth of gold and silver NPs. The PLI was used for the photoreduction of targeted metal ions present in the levitated droplets to generate metal NPs. Additionally, the cavitation effect and bubble movement accelerate the nucleation and decrease the size of NPs. The synthesized Au NPs with ∼ 5 nm size showed excellent catalytic behavior towards the conversion of 4-nitrophenol to 4-aminophenol. This study may open a new door for synthesizing various functional nanocatalysts and for achieving new chemical reactions in suspended droplets.     

不同形貌金纳米粒子的制备及其表面增强拉曼散射效应的研究

不同形状的金纳米粒子在表面增强拉曼散射(surface enhanced Raman scattering,SERS)中有不同的增强效果,多面体金纳米粒子具有多角结构,显示出比金纳米板更为明显的增强效果,近年来对其合成和性质的研究备受关注。该研究探究了十二面体,二十面体,三角板,球形四种形状的金纳米粒子在SERS中不同的增强效果。分别采用硼氢化钠还原法和以N,N-二甲基甲酰胺(DMF)为还原剂制备金三角纳米片和二十面体金纳米粒子,又以二十面体金纳米粒子为种子制备出十二面体金纳米粒子,并分别以以上三种不同形貌的金纳米粒子及球形金溶胶为基底,4-巯基吡啶,对巯基苯甲酸为探针分子检测了其在不同激发波长下的增强效果。透射电子显微镜结果表明金三角纳米板的平均边长为130nm,二十面体和十二面体金纳米粒子的粒径分别为100和120nm。三者的紫外可见吸收峰分别在589,598和544nm处。表面增强拉曼散射结果表明金多面体比金三角纳米板表现出更好的增强效果。     

Colorimetric quantitative sensing of alkali metal ions based on reversible assembly and disassembly of gold nanoparticles

A novel and simple method for the colorimetric quantitative sensing of individual alkali metal ions (Li⁺, Na⁺, K⁺, and Rb⁺) based on the reversible properties of self-assembled aggregates and individual gold nanoparticles (Au NPs) is described. This paper demonstrates reversible self-assembly processes where the degree of assembly and disassembly is dependent on the individual alkali metal ion concentration, nanoparticle size, and alkali metal ionic radii. The color changes of the colloidal Au NPs with metal ion concentrations in colloidal NP solutions occur reversibly. Below a certain concentration of alkali metal ions, the aggregates of Au NPs are redispersed. As the Au NP diameters and the alkali metal ionic radii increase, the critical concentration decreases.     

Stabilizer specific interaction of gold nanoparticles with a thermosensitive polymer hydrogel

We report the experimental results on temperature-dependent studies of interactions between a novel biocompatible thermosensitive polymer hydrogel and different stabilizing agent capped gold nanoparticles (Au NPs) with particle size ranging from 5 to 20 nm. Stabilizing agents such as thioglycolic acid, tryptophan, and phenylalanine have been used as capping agents for Au NPs. The poly-N-isopropyl acrylamide-co-acrylic acid (pNIPAm-AAc) with 3.0 ± 0.7 μm in size was synthesized by radical polymerization of a selected mixture of N-isopropyl acrylamide (NIPAm), methylene-bis-acrylamide and acrylic acid (AAc). The capped Au NPs were mixed with a solution of pNIPAm-AAc hydrogel. The temperature-dependent properties of the mixture were studied by UV–vis spectroscopy, dynamic light scattering based particle size analysis, and transmission electron microscopy (TEM). The observations indicated change in the lower critical solution temperature (LCST) depending on the nature of the stabilizer, with hydrophobic ones lowering the value while hydrophilic stabilizers increasing the same. Also, the optical absorption due to Au NPs, when stabilized with hydrophobic groups, reduced significantly at above LCST along with significant blue shift of wavelength maximum.     

Lithography-free fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles

We report simple and efficient fabrication of large-area gold nanostructures using solution-processible gold nanoparticles, where lithography and vacuum evaporation techniques are not involved in the fabrication processes. These gold nanoisland structures exhibit strong particle plasmon resonance that is characterized by optical extinction spectroscopy in the visible spectral range. The tunability of the optical response is realized by controlling the annealing temperature and by changing the concentration of the colloidal solutions of gold nanoparticles. This enables a low-cost route for exploiting new photonic devices, biosensors, and optoelectronic devices with localized field-enhancement.     

Ultrathin silver‐coated gold nanoparticles as suitable substrate for surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) is an extremely powerful tool for the analysis of the composition of bimetallic nanoparticle (BNP) surfaces because of the different adsorption schemes adopted by several molecules on different metals, such as Au and Ag. The preparation of BNPs normally implies a change in the plasmonic properties of the core metal. However, for technological applications it could be interesting to synthesize core–shell structures preserving these original plasmonic properties. In this work, we present a facile method for coating colloidal gold nanoparticles (NPs) in solution with a very thin shell of silver. The resulting bimetallic Au@Ag system maintains the optical properties of gold but shows the chemical surface affinity of silver. The effectiveness of the coating method, as well as the progressive silver enrichment of the outermost part of the Au NPs, has been monitored through the SERS spectra of several species (chloride, luteolin, thiophenol and lucigenin), which show different behaviors on gold and silver surfaces. A growth mechanism of the Ag shell is proposed on the basis of the spectroscopic and microscopic data consisting in the formation and deposit of Ag clusters on the Au NP surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Lock‐In Thermography to Analyze Plasmonic Nanoparticle Dispersions

The physicochemical properties of nanoparticles (NPs) strongly rely on their colloidal stability, and any given dispersion can display remarkably different features, depending on whether it contains single particles or clusters. Thus, developing efficient experimental methods that are able to provide accurate and reproducible measures of the NP properties is a considerable challenge for both research and industrial development. By analyzing different NPs, through size and concentration, it is demonstrated that lock‐in thermography, based on light absorption and heat generation, is able to detect and differentiate the distinct aggregation and re‐dispersion behavior of plasmonic NPs, e.g., gold and silver. Most importantly, the approach is nonintrusive and potentially highly cost‐effective compared to standard analytical techniques.     

Brownian diffusion of gold nanoparticles in an optical trap studied by fluorescence correlation spectroscopy

The effect of thermal-induced Brownian motion on gold nanoparticles (Au NPs) in optical traps is studied by fluorescence correlation spectroscopy (FCS) method. The Brownian motion and optical trapping potential are investigated by the decay time of the FCS curve and the laser power. It is shown that that the probability of finding a gold nanoparticle in the trap depends on the ratio of the optical energy of the particle to its thermal energy. A power threshold is observed by the decay time as a function of laser power. The experimental studies show that the temperature rise does not seriously affect the average number of particles in the focal spot, but the average residence time is more sensitively affected by the temperature.     

Synthesis and characterization of superparamagnetic nanoparticles coated with fluorescent gold nanoclusters

Multifunctional nanoparticles (NPs) combining the superparamagnetism of Mn−Zn ferrite and the fluorescence property of gold nanoclusters (NCs) have been prepared by wet chemistry. Magnetic NPs synthesized by co-precipitation method were coated several times with oppositely charged polyelectrolytes (PEs) using the layer-by-layer technique. Common techniques (Fourier transform infrared spectroscopy, electron microscopy, zeta potential, etc.) indicated the monodispersity and the stability of the coated NPs providing a positive charged surface. Fluorescent gold NCs bound to a standard protein bovine serum albumin were adsorbed on the surface of the magnetic NPs. Structural investigations proved the presence of small gold clusters (~2 nm) in a shell surrounding the magnetic nanomaterial. The stable nanocomposite kept the original fluorescence property of the metal clusters with 211-fold increase of the red emission (λ = 690 nm) compared to the uncoated NPs. These NPs can be moved with a permanent magnet despite a 72-wt% increase of the non-magnetic fraction due to the PE coating and the protein adsorption.     

Experimental study using optoacoustic spectroscopy (OAS) on spherical gold nanoparticles

In this paper a spectroscopic characterisation method based on the optoacoustic technique has been used to investigate the optical properties of two separate spherical gold nanoparticle (SGNP) solutions where an absorption peak located at 520 nm has been observed. This analysis has been carried out over the visible wavelength range from 410 to 650 nm using a Q-switched Nd:YAG pumped optical parametric oscillator (OPO). To verify the reliability of optoacoustic spectroscopy (OAS), the results obtained have been compared to those from more standard and limited spectrophotometer and reference collimated optical transmission schemes, where good agreement is shown. The experimental procedure presented here demonstrates the potential of this technique for integration along with optoacoustic imaging methods to identify physiological information for non-destructive in-vivo applications.     

Synthesis and photo physical properties of Au @ Ag (core @ shell) nanoparticles disperse in poly vinyl alcohol matrix

Synthesis of core @ shell (Au @ Ag) nanoparticle with varying silver composition has been carried out in aqueous poly vinyl alcohol (PVA) matrix. Core gold nanoparticle (~15 nm) has been synthesized through seed-mediated growth process. Synthesis of silver shell with increasing thickness (~1–5 nm) has been done by reducing Ag⁺ over the gold sol in the presence of mild reducing ascorbic acid. Characterization of Au @ Ag nanoparticles has been done by UV–Vis, High resolution transmission electron microscope (HRTEM) and energy dispersive X-ray (EDX) spectroscopic study. The blue shift of surface plasmon resonance (SPR) band with increasing mole fraction of silver has been interpreted due to dampening of core, i.e. Au SPR by Ag. The dependence of nonlinear optical response of spherical core @ shell nanoparticles has been investigated as a function of relative composition of each metal. Simulation of SPR extinction spectra based on quasi-static theory is done. A comparison of our experimental and the simulated extinction spectra using quasi-static theory of nanoshell suggests that our synthesized bimetallic particles have core @ shell structure rather than bimetallic alloy particles.     

Fabrication Techniques and Characteristics of Ultra-Thin Optical Polarizing Films Using Gold Island Films

Fabrication techniques of ultra-thin optical polarizing films using gold island films have been investigated for the near-infrared region. The polarizing films are fabricated by stretching the periodic multilayers consisting of gold island layers and glass layers. We have experimentally investigated the optimum fabricating conditions necessary to induce large optical anisotropy into the gold island layers and have formed submicrometer-thick polarizing films with an extinction ratio of ≥20 dB in a near-infrared region longer than 800 nm. These polarizing films could be useful micro-polarizers for fiber-embedded in-line optical devices, micro-optics, and hybrid integrated optics.     

Laser optoacoustic spectroscopy of gold nanorods within a highly scattering medium

We describe a spectroscopic comparative analysis based on the optoacoustic technique over the wavelength range from 410nm to 1000nm using a Q-switched Nd:YAG pumped optical parametric oscillator tunable source on a gold nanostructure solution located within a highly scattering medium. The advantages of this method over standard spectroscopy techniques are the possibility to localize and monitor the spectroscopic response of absorbing materials located within turbid media. The operation is confirmed using a comparative analysis with the spectroscopic results obtained from a reference measurement scheme, based on a highly sensitive collimated optical transmission setup in parallel and under the same experimental conditions as the optoacoustic technique.     

Studies on multivalent interactions of quantum dots–protein self-assemble using fluorescence coupled capillary electrophoresis

A method is presented for using a single laser pulse to transform polymorphic polycrystalline gold nanoparticles (Au NPs) into crystalline nanospheres. Transmission electron microscope analysis of the nominally 60-nm-diameter Au NP starting material and the product suspension confirms the production of slightly smaller particles that exhibit greater crystallinity and feature reduced caliper diameter aspect ratios and anisotropy. Centrifugation followed by ultrasonic resuspension into citrate solution separates the nanospheres from the <15 nm ablation fragments, providing a route to monodisperse spherical crystalline Au NPs.     

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.     

Optical,structural, and antibacterial properties of biosynthesized Ag nanoparticles at room temperature using Azadirachta indica leaf extract

We report an enhancement of antibacterial properties of Ag nanoparticles (NPs) synthesized at room temperature using leaf extract of Azadirachta indica (Neem) following green synthesis route. To study such antibacterial properties Ag NPs of sizes within 9 nm to 17 nm were synthesized by varying the concentration of Neam leaf extract (NLE). The NP size and size distribution were seen to increase and decrease, respectively, with increase in NLE concentration. Also Ag NPs having a fixed size (~26 nm) was also synthesized by varying the precursor (AgNO₃) concentration. It is noticed that concentration of NLE has significant effects on the control of NP size as well as size distribution whereas there is almost no role of precursor concentration of the NP size. All the Ag NPs are found to have face-centred-cubic crystal structure with preferential growth along (111) plane which is stable one. The peak of X-ray diffraction at ~32.4° (2θ value), which is prominent for low concentrations of NLE and precursor, is identified as (101) plane of Ag crystal. The generation and growth of Ag NPs had also been confirmed using electron microscopic studies. These Ag NPs show prominent surface plasmon resonance (SPR) absorption at ~ 420 nm confirming again the genesis of Ag NPs. The SPR peak shifts towards longer wavelength (redshift) with a corresponding reduction in full width at half maximum with increase in NP size. All of the samples containing Ag NPs show a broad blue photoluminescence (PL) emission at ~ 471 nm. Emission peak is seen to redshift with increase in NP size and is consistent with the optical absorption data. Such PL emission is argued as due to interband transition or plasmon luminescence. Being biocompatible of the green synthesis process, antibacterial properties of these Ag NPs were studies in details considering all the samples (with varied NP size for one set and with fixed NP size for other set of samples). As per our knowledge this is the first report of size related total study of Ag NPs, showing increased antibacterial effect as size decreased and equal antibacterial effect as size equals. It is found that smaller Ag NPs has enhanced antibacterial effects due to large surface area to volume ratio in comparison with bigger sized Ag NPs.