Nanocomposite ZnO/Au formation by pulsed laser irradiation

The ZnO/Au nanocomposite formation involves synthesis of Au and ZnO colloidal solutions by 532 nm pulse laser ablation of metal targets in deionized water followed by laser irradiation of the mixed colloidal solution. The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images show evolution of spherical particles into ZnO/Au nanonetworks with irradiation time. The formation mechanism of the nanonetwork can be explained on the basis of near resonance absorption of 532 nm irradiation by gold nanoparticles which can cause selective melting and fusion of gold nanoparticles to form network. The ZnO/Au nanocomposites show blue shift in the ZnO exciton absorption and red shift in the Au plasmon resonance absorption due to interfacial charge transfer.     

Formation of gold nanoparticles in heat-treated reactive co-sputtered Au-SiO2 thin films

In this work, formation of gold nanoparticles in radio frequency (RF) reactive magnetron co-sputtered Au-SiO₂ thin films post annealed at different temperatures in Ar + H₂ atmosphere has been investigated. Optical, surface topography, chemical state and crystalline properties of the prepared films were analyzed by using UV-visible spectrophotometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD) techniques, respectively. Optical absorption spectrum of the Au-SiO₂ thin films annealed at 800 °C showed one surface plasmon resonance (SPR) absorption peak located at 520 nm relating to gold nanoparticles. According to XPS analysis, it was found that the gold nanoparticles had a tendency to accumulate on surface of the heat-treated films in the metallic state. AFM images showed that the nanoparticles were uniformly distributed on the film surface with grain size of about 30 nm. Using XRD analysis average crystalline size of the Au particles was estimated to about 20 nm.     

Size-controlled synthesis of superparamagnetic iron oxide nanoparticles and their surface coating by gold for biomedical applications

The size mono-dispersity, saturation magnetization, and surface chemistry of magnetic nanoparticles (NPs) are recognized as critical factors for efficient biomedical applications. Here, we performed modified water-in-oil inverse nano-emulsion procedure for preparation of stable colloidal superparamagnetic iron oxide NPs (SPIONs) with high saturation magnetization. To achieve mono-dispersed SPIONs, optimization process was probed on several important factors including molar ratio of iron salts [Fe³⁺ and Fe²⁺], the concentration of ammonium hydroxide as reducing agent, and molar ratio of water to surfactant. The biocompatibility of the obtained NPs, at various concentrations, was evaluated via MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay and the results showed that the NPs were non-toxic at concentrations <0.1 mg/mL. Surface functionalization was performed by conformal coating of the NPs with a thin shell of gold (∼4 nm) through chemical reduction of attached gold salts at the surface of the SPIONs. The Fe₃O₄ core/Au shell particles demonstrate strong plasmon resonance absorption and can be separated from solution using an external magnetic field. Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, phase components, core–shell surface composition, and magnetic properties have confirmed the formation of the mono-dispersed core–shell nanostructure.     

On 308 nm photofragmentation of the silver nanoparticles

Silver nanoparticles (Ag NPs) were prepared by different chemical methods possessing different sizes 3 ± 2, 8 ± 2, and 20 ± 5 nm. The influence the size of Ag NPs was demonstrated by the absorption and fluorescence spectra, the maximum absorption of Ag NPs increases as the particle size increases. When Ag NPs irradiated with 308 nm excimer laser; the maximum absorption and the full width at half maximum decreased as the number of pulses increased up to 100,000 pulse; due to the size reduction. The fluorescence spectra of Ag NPs and irradiated Ag NPs with 308 nm excimer laser were recorded after excitation at 441.5 nm He-Cd laser, showing a red shift increasing as the particle size is increased.     

Optical features of the gold nanoparticles deposited on ITO substrates

We have performed firstly studies of the photoinduced second order susceptibilities in the Au nanoparticles (NP) A, B and C under simultaneous influence of the bicolor 1064 nm and bicolor laser treatment (1064 nm 10 ns pulsed laser with pulse power densities 532 nm 10 ns laser treatment and the cw 300 mW 532 nm SHG coherent laser beams. We have studied three types of samples possessing irregular and different dense parameters of the Au NP deposited on the ITO substrate. We have found that the maximal bicolor (1064 nm and 532 nm) stimulated optical second harmonic generation for the 10 ns pulse duration was observed for the samples possessing irregular Au NP deposited on the ITO. We have performed studies of the photoinduced second order susceptibilities in the Au NP under simultaneous influence of the bicolor 1064 nm and bicolor laser treatment (1064 nm 10 ns pulsed laser with pulse power densities 532 nm 10 ns laser treatment and the cw 300 mW 532 nm SHG coherent laser beams). We have found that during the 15-20 min of the cw treatment there occur the principal changes in the absorption maxima. These maxima indicate on the occurrence of the additional absorption nearby the 308 nm and 310 nm and 345 nm spectral bands. The later are caused by the occurrence of the trapping levels in the border between the ITO substrate and the Au nanoparticles.     

rf reactive co-sputtered Au-Ag alloy nanoparticles in SiO2 thin films

We have studied formation of Au-Ag alloy nanoparticles in sputtered SiO₂ thin films. Silica thin films containing Au-Ag nanoparticles were deposited on quartz substrates using rf reactive magnetron co-sputtering technique. The films heat-treated in reducing Ar + H₂ atmosphere at different temperatures. They were analyzed by using UV-vis spectrophotometry, atomic force microscopy and X-ray photoelectron spectroscopy (XPS) methods for their optical, surface morphological as well as structural and chemical properties. The optical absorption of the Au-Ag alloy nanoparticles illustrated one plasmon resonance absorption peak located at 450 nm between the absorption bands of pure Au and Ag nanoparticles at 400 and 520 nm, respectively, for the thin films annealed at 800 °C. XPS results showed that the alloys were in metallic state, and they had a greater tendency to lose electrons as compared to their corresponding monometallic state. Using lateral force microscopy analysis, we have found that the alloy particles were uniformly distributed on the surface with grain size of about 20 nm.     

Magnetic field heating study of Fe-doped Au nanoparticles

Fe-doped Au nanoparticles are ideal for biological applications over magnetic oxides due to their conjugation chemistry, optical properties, and surface chemistry. We present an AC magnetic field heating study of 8 nm Fe-doped Au nanoparticles which exhibit magnetic behavior. Magnetic heating experiments were performed on stable aqueous solutions of the nanoparticles at room temperature. The nanoparticles exhibit magnetic field heating, with a specific absorption rate (SAR) of 1.84 W/g at 40 MHz and H=100 A/m. The frequency dependence of the heating follows general trends predicted by power loss equations and is similar to traditional materials.     

Modulation of residual stress in diamond like carbon films with incorporation of nanocrystalline gold

Residual stress modulation in the diamond-like carbon coatings with incorporation of gold nanoparticles was studied critically. The films were deposited on glass and Si (1 0 0) substrates by using capacitatively coupled plasma chemical vapor deposition. Stresses in the films were determined from the broadening of the optical absorption tail and were found to decrease from 2.3 GPa to 0.48 GPa with increasing gold content (2-7 at.% Au) in the DLC matrix. Gold incorporation also made the films harder than the corresponding DLC coatings. Modulation of stress with nanocrystalline gold content in the DLC matrix was related to the relative amount of sp²/sp³ content in the DLC films.     

Observation of simultaneous reverse saturation absorption and saturation absorption in silver nanoparticles incorporated into europium oxide thin film

In this work, nano silver clusters incorporated into europium oxide thin films at a level of 3.8% and 12.5% have been prepared by a vacuum evaporation method on glass and silicon substrates. Samples were investigated by X-ray fluorescence, X-ray diffraction, and linear and nonlinear optical absorption methods. The X-ray diffraction reveals that the Eu oxide of these samples remains amorphous after pre-annealing at 400 °C. The linear optical absorption of the samples shows surface plasmon resonance (SPR) phenomena, which varies with the Ag content of the samples. The optical nonlinear absorption properties of the prepared films were investigated using an open Z-scan technique with cw laser at wavelengths 476 nm and 514 nm. A changeover from reverse saturation absorption (RSA) to saturation absorption (SA) was observed. RSA is attributed to interband transition via two photon absorption. SA is attributed to plasmon bleach.     

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.     

Chemical sensing based on the plasmonic response of nanoparticle aggregation: Anion sensing in nanoparticles stabilized by amino-functional ionic liquid

We report the synthesis, characterization and modellization of optical anion sensors based on gold nanoparticles (Au NPs) stabilized by amino-functional imidazolium ionic liquids (AFIL). The addition of different salts results in anion exchange of the imidazolium ionic liquid stabilizer leading to a change in the optical response of the original nanoparticle aqueous solution. In all cases except with dodecylbenzenesulfonic acid sodium salt, a sufficient amount of salt concentration (5 times larger than equimolar) leads to the appearance of an absorption band between 600 and 700 nm in the ultravioletvisible (UV-vis) spectrum. The presence of the salt produces aggregation of the particles that localise the optical response and produce a large spectral red shift. Transmission electron microscopy images demonstrated that this optical change was due to the aggregation of the nanoparticles. We simulate the optical response of both situations, before and after salt addition, and interpret the experimental observations in terms of the different response of metallic single nanoparticles and nanoparticle aggregates. Theoretical calculations for single nanoparticle and single nanoparticle dimers demonstrate that the colour change is not due to the enlargement or structural changes of the Au NPs, but due to the formation of NP aggregation. These results show the potential of nanoparticle plasmonics to perform effective chemical sensing.     

Optical properties and electric conductivity of gold nanoparticle-containing, hydrogel-based thin layer composite films obtained by photopolymerization

Poly(acrylamide) [poly(AAm)] and poly(N-isopropyl-acrylamide) [poly(NIPAAm)] based gel films containing Au nanoparticles (d = 14 ± 2.5 nm) were synthesized. Monomers and cross-linker were added to a gold nanodispersion, and after the addition of the initiator, polymer films were prepared on the surface of an interdigital microelectrode by photopolymerization. In the course of the syntheses the gold concentration of the films was constant (10.8 μg/cm²) and the volume fraction of Au nanoparticles (?_Au) in the polymer gel films varied in the range of 0.58-85.3%. Poly(AAm)-based films swell when the temperature increases: due to a temperature shift of 15 °C the Au plasmon absorption maximum at λ = ∼532 nm was shifted towards shorter wavelengths by 16.6 nm (blue shift) through the swelling of the polymer gel film. In the case of poly(NIPAAm) the temperature-induced shrinking resulted in a red shift, namely the maximum was shifted by 18.07 nm by a temperature shift of 15 °C. In the case of both composites, the electric conductivity of the samples was shown to increase with increasing Au particle concentration. In the case of the poly(AAm)-based composite containing ?_Au = 0.85 gold the resistance of the film spread on the surface of the electrode was 0.16 MΩ at 25 °C and 0.66 MΩ at 50 °C, i.e. the conductivity of the sample decreased with increasing temperature. The opposite effect is observed in the case of the poly(NIPAAm)-based composite: as temperature is raised, the resistance of the composite abruptly drops at the point of collapse of the NIPAAm gel (it is 0.28 MΩ at 32 °C and only 0.021 MΩ at 35 °C). This thermosensitive effect was detectable only at sufficiently high Au contents (?_Au = 0.85) in both gels.     

The effect of preadsorbed K on the size distribution of Au nanoparticles on TiO2(1 1 0) surface

The effect of K on the morphology of Au nanoparticles deposited on TiO₂(1 1 0) surface is investigated by STM-STS and AES methods. For comparison, the enhanced concentration of oxygen defect sites generated by Ar⁺ bombardment was also studied. It was found that both the K additive and the oxygen defect sites induce a pronounced decrease in the average size of the Au nanoparticles evolved at 320 K. On the clean TiO₂(1 1 0) the average size of Au particles is 4.3 nm at approximately monolayer coverage of gold, while in the presence of K or oxygen vacancies this value decreased to 2.5 nm. In spite of the reduced average diameter detected at room temperature, the mean size of the Au nanoparticles increased significantly from 2.5 nm up to 7 nm on the effect of annealing at 500-700 K for K precoverages of 0.3-1 ML. For the clean and the Ar⁺ pretreated TiO₂(1 1 0) surfaces the mean size of the Au particles changed only slightly on the effect of the same thermal treatments.     

pH dependent interaction of biofunctionalized CdS nanoparticles with nucleobases and nucleotides: A fluorimetric study

The interaction of DNA bases and corresponding nucleotides with CdS nanoparticles (NPs), biofunctionalized by cysteine, has been investigated by absorption and fluorescence spectroscopy. Unique enhancement effect of adenine, in contrast to other nucleobases, on the luminescence of cysteine capped CdS (cys-CdS) NPs at both pH 7.5 and 10.5 was found, the extent of enhancement being much higher at pH 10.5. At the latter pH, the difference optical absorption spectra show development of new peak at 278 nm with corresponding decrease in the absorption of adenine at 260 nm, which is attributed to binding of adenine anion to the CdS surface through N7 of the purine ring. Appearance of a new band at 478 cm⁻¹ and concomitant shift in the C₈-N₇ vibrations to 1610 cm⁻¹ in the FTIR spectra of cys-CdS NPs with adenine also suggest Cd-N7 binding on the particle surface. Amongst various nucleotides, ATP exhibited maximum luminescence enhancement on CdS NPs for a given change in concentration in the micro-molar range at physiological pH. A quantitative correlation between ATP concentration and PL enhancement of CdS NPs has been established, a step which in future might assist in developing new protocols for fluorescence sensing of adenine nucleotides under certain pathological conditions.     

Effects of post-thermal annealing temperature on the optical and structural properties of gold particles on silicon suboxide films

In this work, silicon suboxide (SiO_x) thin films were deposited using a RF magnetron sputtering system. A thin layer of gold (Au) with a thickness of about 10 nm was sputtered onto the surface of the deposited SiO_x films prior to the thermal annealing process at 400 °C, 600 °C, 800 °C and 1000 °C. The optical and structural properties of the samples were studied using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and optical transmission and reflection spectroscopy. SEM analyses demonstrated that the samples annealed at different temperatures produced different Au particle sizes and shapes. SiO_x nanowires were found in the sample annealed at 1000 °C. Au particles induce the crystallinity of SiO_x thin films in the post-thermal annealing process at different temperatures. These annealed samples produced silicon nanocrystallites with sizes of less than 4 nm, and the Au nanocrystallite sizes were in the range of 7-23 nm. With increased annealing temperature, the bond angle of the Si-O bond increased and the optical energy gap of the thin films decreased. The appearance of broad surface plasmon resonance absorption peaks in the region of 590-740 nm was observed due to the inclusion of Au particles in the samples. The results show that the position and intensity of the surface plasmon resonance peaks can be greatly influenced by the size, shape and distribution of Au particles.     

Dual plasmonic‐enhanced bulk‐heterojunction solar cell incorporating gold nanoparticles into solution‐processed anode buffer layer and active layer

A dual plasmonic resonance effect on the performance of poly(3‐hexylthiophene) (P3HT):phenyl C61‐butyricacid methyl ester (PC61BM) based polymer solar cells (PSCs) has been demonstrated by selectively incorporating 25 nm colloidal gold nanoparticles (Au NPs) in a solution‐processed molybdenum oxide (MoO₃) anode buffer layer and 5 nm colloidal Au NPs in the active P3HT:PCBM layer. The devices exhibit up to ～20% improvement in power conversion efficiency which is attributed to the dual effect of localized surface plasmon resonance (LSPR) of Au NPs with enhanced light absorption and exciton generation. Our report shows a guideline on the usage of dual LSPR effect for the solution‐processed polymer solar cells to achieve high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Plasmonics: Absorption and scattering of light of non-straight ordered array of Au rings

In this paper, Au nanorings in a SiO₂ substrate have been utilized to design a plasmonic waveguide with lower losses and perfect energy coupling. Our recommended structure consists of a chain of dozen Au nanorings with a given intercenter space between them going on resonance, if a beam with specific wavelength is launched in the input area of the waveguide. Nanoring has an extra degree of freedom in its geometry and have a preferable tunability in comparison to other shapes of nanoparticles (e.g. nanospheres). It is shown that a modified plasmon waveguide structure can be utilized at optical communication band (λ = 1550 nm), in optical integrated devices. Red-shifted of localized surface plasmon resonance (LSPR) has been considered as a fundamental condition to have a maximum of optical response at λ = 1550 nm. Cross-sectional depictions of field propagation through the structure are displayed in order to show the absorption and scattering of light by particles. Ultimately, transmitted power ratio is computed for the structure to clear-cutting its characteristics.     

Synthesis of PMA stabilized silver nanoparticles by chemical reduction process under a two-step UV irradiation

Poly(methacrylic acid) (PMA) stabilized silver nanoparticles (Ag NPs), also used in the surface modification of clothing fibers, were fabricated via chemical reduction processes under UV irradiation. To obtain an uniform size distribution it has been designed a new “two-step” process which employs two different UV radiation densities in order to control the kinetics of NPs nucleation. The as produced nanoparticles were characterized by UV-vis absorption spectroscopy and TEM microscopy. The results show the reduction of the Ag ions and the nanoparticles nucleation in the first step. In the second step, the final Ag NPs size distribution is controlled through a quick cross-linking of the PMA that freezes out any further modification. A narrow size distribution with more than 80% NPs smaller than 10 nm and none larger than 25 nm was obtained and the long-term stability (one month) of the colloidal solution was verified.     

A novel method for the synthesis of monodisperse gold-coated silica nanoparticles

Monodisperse silica nanoparticles were synthesised by the well-known Stober protocol, then dispersed in acetonitrile (ACN) and subsequently added to a bisacetonitrile gold(I) coordination complex ([Au(MeCN)₂]⁺) in ACN. The silica hydroxyl groups were deprotonated in the presence of ACN, generating a formal negative charge on the siloxy groups. This allowed the [Au(MeCN)₂]⁺ complex to undergo ligand exchange with the silica nanoparticles and form a surface coordination complex with reduction to metallic gold (Au⁰) proceeding by an inner sphere mechanism. The residual [Au(MeCN)₂]⁺ complex was allowed to react with water, disproportionating into Au⁰ and Au(III), respectively, with the Au⁰ adding to the reduced gold already bound on the silica surface. The so-formed metallic gold seed surface was found to be suitable for the conventional reduction of Au(III) to Au⁰ by ascorbic acid (ASC). This process generated a thin and uniform gold coating on the silica nanoparticles. The silica NPs batches synthesised were in a size range from 45 to 460 nm. Of these silica NP batches, the size range from 400 to 480 nm were used for the gold-coating experiments.     

Nanoindentation of gold nanoparticles functionalized with proteins

The hardness and Young's modulus of 10 and 20 nm gold nanoparticles (Au NPs) modified with bovine serum albumin and streptavidin were measured using a nanoindenter. The Au NPs were immobilized on a semiconductor surface through organic self-assembled monolayers. Changes in mechanical properties occurred when the Au NPs were immobilized on the surface. The hardness and Young's modulus were dependent on the size of the NPs, and the proteins on the particles showed highly plastic and elastic behavior compared to flat surfaces modified with self-assembled monolayers.