Optical Properties of Micro-patterned Silver Nanoparticle Substrates

In this paper, we describe a novel technique for depositing metal nanoparticles (NPs) on a planar substrate whereby the NPs are micro-patterned on the surface by a simple stamp-printing procedure. The method exploits the attractive force between negatively charged colloidal metal NPs and positively-charged polyelectrolyte layers which have been selectively deposited on the surface. Using this technique, large uniform areas of patterned metal NPs, with different plasmonic properties, were achieved by optimisation of the stamping process. We report the observation of unusual fluorescence emission from these structures. The emission was measured using epifluorescence microscopy. Fluorescence lifetime behaviour was also measured. Furthermore, the μ-patterned NPs exhibited blinking behaviour under 469 nm excitation and the fluorescence spectrum was multi-peaked. It has been established that the fluorescence is independent of the plasmon resonance properties of the NPs. As well as optimising the novel NP μ-patterning technique, this work discusses the origin and characteristics of the anomalous fluorescence behaviour in order to characterise and minimise this unwanted background contribution in the use of metal NPs for plasmonic enhancement of fluorescence for optical biochip applications.     

Permanent magnetism in phosphine- and chlorine-capped gold: from clusters to nanoparticles

Magnetometry results have shown that gold NPs (∼2 nm in size) protected with phosphine and chlorine ligands exhibit permanent magnetism. When the NPs size decreases down to the subnanometric size range, e.g. undecagold atom clusters, the permanent magnetism disappears. The near edge structure of the X-ray absorption spectroscopy data points out that charge transfer between gold and the capping system occurs in both cases. These results strongly suggest that nearly metallic Au bonds are also required for the induction of a magnetic response. Electron paramagnetic resonance observations indicate that the contribution to magnetism from eventual iron impurities can be disregarded.     

Charge‐dependent adsorption of rhodamine 6G on gold nanoparticle surfaces: fluorescence and Raman study

We study the adsorption behaviors of rhodamine dyes on gold nanoparticles (Au NPs) depending on their surface charges. Rhodamine 6G (Rh6G) dye is tested comparatively for positively and negatively charged Au NPs prepared by the reduction of chitosan and citric acid, respectively. The adsorption of Rh6G is found to be weaker on the positively charged Au NPs, whereas more substantial aggregation is found on negatively charged Au NPs. An increase in the concentration of Au NPs enhances the surface‐enhanced Raman scattering (SERS) intensities only for the Au(−) NPs, whereas the Au(+) NPs do not exhibit any strong SERS signals. Our findings suggest that SERS and reciprocal fluorescence measurements of Rh6G can be used to estimate the surface charges and atomic percentages of Au NPs less than ∼5 ppm. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Laser nano-fabrication of large-area plasmonic structures and surface plasmon resonance tuning by thermal effect

A simple and flexible technique aimed to generate large-area periodic nano-dot array features on metal thin films by laser interference lithography (LIL) has been demonstrated. In this paper, gold nano-dot arrays with a period of ∼450 nm and a dot diameter of ∼100 nm on quartz substrates coated with a gold film of 50 nm thick were fabricated. Multiple enhanced transmission peaks were observed in this patterned film. In addition to the characteristic peak of the gold surface plasmon resonance around 500 nm, multiple shoulder peaks that range from 550 to 700 nm were also observed in the nano-chain array structures. These shoulder peaks disappeared after thermal annealing. It was found that the nano-dots became smaller and well-separated nano-balls under the high temperature annealing process. These nano-structures have potential applications in solar cell, nano-lithography and biosensing.     

Ultrasound-assisted surface treatment of ZrO2 with BSA and incorporating in PVC to improve the properties of the obtained nanocomposites: Fabrication and characterization

This paper describes the preparation of poly(vinyl chloride) (PVC) nanocomposites (NCs) reinforced with modified zirconia (ZrO₂) nanoparticles (NPs). The ZrO₂ NPs were defined as efficient filler for PVC NCs. For achieving the best dispersion and improvement of properties, the surface of ZrO₂ NPs was modified by Bovine Serum Albumin (BSA). Carboxylic acids and amines are important functional groups of BSA which handle the grafting BSA on the surface of ZrO₂ NPs. The PVC/ZrO₂-BSA NCs were fabricated by incorporation of various amounts of the ZrO₂-BSA NPs (3, 6 and 9 wt%) into PVC matrix. All the above processes were accomplished by ultrasonication as a green and environmentally-friendly method. Also, the magnetic and mechanical stirrer was used for the preparation of samples but the results are not suitable and the aggregation was observed which indicated the use of ultrasonic irradiation is the best method for the preparation of NC. The products were characterized by Fourier transform infrared spectroscopy, Transmission electron microscopy, Field emission scanning electron microscopy, X-ray diffraction, Thermogravimetric analysis, Ultraviolet–visible spectroscopy, photoluminescence spectroscopy, energy dispersive X-ray spectroscopy, wettability, and mechanical tests. The achieved PVC/ZrO₂-BSA NCs showed high thermal stability, good mechanical, optical and wettability properties compared to the pure PVC. In addition, among the obtained NCs, the PVC/ZrO₂-BSA NC 6 wt% showed the best improvement.     

Synthesis of quantum dots via microreaction: structure optimization for microreactor system

Microreactor systems existed as a powerful tool for the continuous synthesis of quantum dots. However, the lack of structure optimization for the discrete units led to empirical determination of the length scale, and the properties of the formed products varied in different cases. In this article, the optimizations for the micromixer volume and capillary diameter were presented based on the synthesis of CdSe nanocrystals (NCs). Spectra investigation revealed that the application of a small convective mixer of 36 μL led to 1/3 increase of CdSe concentration in the crude solution. The enhanced mixing of the precursors in this case was also demonstrated favorable to achieve CdSe NCs with narrow PL width. Fast heating and uniform reaction condition achieved in a narrow channel favored the preparation of high quality CdSe NCs under short residence time. However, the application of wide channel did not necessarily result in CdSe NCs with poor quality. Here, we demonstrated that high-quality CdSe NCs with narrow full width at half maximum (FWHM) as 32 nm and high quantum yield (QY) 34.7% could be prepared using an 844 μm inner diameter capillary. Based on the obtained results, the scaled-up synthesis of CdSe NCs was demonstrated, and a high quantity of 0.8 g dry CdSe NCs powder (3.5 nm, σ ~ 8.2%) was obtained within 1 h.     

Characterization of a dielectric microdroplet thermal interface material with dispersed nanoparticles

Gold nanoparticles (NPs) with a size close to 1.5?nm, coated with organic ligands bearing Si(OEt)₃ groups, were synthesized and used to obtain self-standing films by a sol?Cgel process catalyzed by formic acid. Using FESEM images, FTIR, and UV?Cvisible spectra, it was observed that very small gold NPs self-assembled by Si?CO?CSi covalent bonds forming crosslinked clusters with sizes up to about 50?nm in which NPs preserve their individuality. The possibility of fixing very small gold NPs in a crosslinked film opens a variety of potential applications based on the specific properties of small-size particles. As an example, we illustrated the way in which one can take advantage of the low melting temperature of these NPs to generate tiny gold crystals partially embedded at the surface, a process that might be used for the development of catalysts or sensors. Besides, the shift and change in the intensity of the plasmon band produced by heating to 100?°C may be employed to develop an irreversible sensor of undesirable temperature excursions during the life-time of a specific product.     

High-density attachment of FePt nanoparticles on carbon nanotubes by a facile microwave-assisted polyol method

High-density attachment and one-dimensional (1D) array of FePt nanoparticles (NPs) along carbon nanotubes (CNTs) surface to generate FePt/CNT nanocomposites were successfully obtained via a facile CNT-mediated microwave polyol method. The as-prepared 1D FePt/CNTs is about 10–20 nm in diameter and up to μm scale in length. By adjusting the solvents, the ratio of Fe/Pt and the attached density of FePt NPs on the surface of CNTs could be well controlled. The structures, composition, and magnetic properties of the FePt/CNTs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and magnetic measurements. The possible growth mechanism has also been proposed.     

Manganese–gold nanoparticles as an MRI positive contrast agent in mesenchymal stem cell labeling

We report a straightforward approach to prepare multifunctional manganese–gold nanoparticles by attaching Mn(II) ions onto the surface of 20 nm citrate-capped gold nanoparticles. In vitro MRI measurements made in agarose gel phantoms exhibited high relaxivity (18.26 ± 1.04 mmol⁻¹ s⁻¹). Controlled incubation of the nanoparticles with mesenchymal stem cells (MSCs) was used to study cellular uptake of these particles and this process appeared to be controlled by the size of the nanoparticle aggregates in the extracellular solution. SEM images of live MSCs showed an increased concentration of particles near the cell membrane and a distribution of the size of particles within the cells. Survivability for MSCs in contact with Mn–Au NPs was greater than 97% over the 3-day period and up to the 1 mM Mn used in this study. The high relaxivity and low cell mortality are suggestive of an enhanced positive contrast agent for in vitro or in vivo applications.     

Solvent effects on the linear and nonlinear optical response of silver nanoparticles

Linear and nonlinear (NL) optical properties of silver colloids stabilized with poly(N-vinylpyrrolidone) (PVP) in water, acetone, methanol, and ethylene glycol were studied. Images obtained by transmission electron microscopy reveal narrow size distributions of silver nanoparticles (NPs) with diameters centered at ≈ 6.3 nm (aqueous colloid) and in the 4.3–4.9 nm range for the other colloids. The behavior of the surface plasmon resonance band associated with the NPs was monitored through the linear absorption spectrum, and its dependence on the linear refraction index and the electric dipole moment (EDM) of the solvent molecules was analyzed. The phenomenological parameter, A, obtained from the linear absorption spectra, includes contributions due to the surface effects and the solvent. The third order susceptibility of the colloid was measured using the Z-scan technique at 532 nm, and the NL optical susceptibility of the NPs was determined using the Maxwell–Garnett model. The results indicate that the NL response of the colloids is largely influenced by the molecules adsorbed on the NPs surfaces and the EDM of the solvent molecules. PACS  42.65.-k; 42.65.An; 73.20.Mf; 78.67.-n; 78.67.Bf; 82.70.Dd     

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.     

Synthesis of highly fluorescent and thio-linkers stabilize gold quantum dots and nano clusters in DMF for bio-labeling

This study demonstrates a one versus two-step synthesis of fluorescent gold quantum dots (F-AuQDs) and nano clusters (F-AuNCs) functionalized with thiolated organic linkers using reduction of gold precursor in N,N′-dimethylformamide in 1 h of reaction. The F-AuQDs and F-AuNCs show fluorescence emission at 425 ± 5 nm upon excitation at 345 ± 5 nm of wavelength, with good water solubility and stability. Five different thiolated organic binary linkers consisting of various functional groups including: carboxylic acid, hydroxyl, and aromatic amine, were conjugated with the F-AuQDs and F-AuNCs. The formation mechanism and functionalization of the F-AuQDs and F-AuNCs was characterized using UV–vis absorption spectra, UV–vis light, fluorescent emission spectra, pH, TEM, and FTIR. The fluorescence emission of the F-AuQDs and F-AuNCs is greatly dependent on the thio-linker. This novel one-step approach provides facile and fast synthesis of F-AuQDs and F-AuNCs over the two-step method, with less than 5 h of reaction and workup compared to more than 28 h of reaction for the two-step approach. These thio-linker functionalized F-AuQDs and F-AuNCs have a wide application in fluorescent labeling of biomolecules, optical devices, imaging, energy transfer, and biosensing.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Ultrasonic-assisted fabrication and characterization of PVC-SiO2 nanocomposites having bovine serum albumin as a bio coupling agent

In this work, SiO₂ nanoparticles (NPs) were modified with bovine serum albumin (BSA) under ultrasound irradiations as a green and fast route to achieve their good dispersion. Subsequently, different weight percentages of the modified NPs (3, 6, and 9 wt%) were incorporated in poly(vinyl chloride) (PVC) as the matrix. Thermogravimetric analysis of the SiO₂-BSA NPs indicated that 12 wt% of the modifier was loaded on the surface of SiO₂ NPs. Encapsulation of the SiO₂-BSA resulted in a meaningful improvement in the optical, mechanical and thermal characteristics of the prepared PVC nanocomposites (NCs). X-ray diffraction (XRD) patterns for the PVC/SiO₂-BSA NCs showed a crystalline behavior for the NC with 6 wt% of the SiO₂-BSA originated from the phosphate buffer on the NPs. Water contact angle of the PVC/SiO₂-BSA NCs showed that the hydrophilicity enhanced with increasing of the NPs contents.     

Nanostructures of gold coated iron core-shell nanoparticles and the nanobands assembled under magnetic field

Pure metal iron nanoparticles are unstable in the air. By a coating iron on nanoparticle surface with a stable noble metal, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming self-assembling nanoarrays. Gold-coated iron core-shell structure nanoparticles (Fe/Au) synthesized using reverse micelles were characterized by transmission electron microscopy (TEM). The average nanoparticle size of the core-shell structure is about 8 nm, with about 6 nm diameter core and 1∼2 nm shell. Since the gold shell is not epitaxial growth related to the iron core, the morié pattern can be seen from the overlapping of iron core and gold shell. However, the gold shell lattice can be seen by changing the defocus of TEM. An energy dispersive X-ray spectrum (EDS) also shows the nanoparticles are air-stable. The magnetic measurement of the nanoparticles also proved successful synthesis of gold coated iron core-shell structure. The nanoparticles were then assembled under 0.5 T magnetic field and formed parallel nanobands with about 10 μm long. Assembling two dimensional ordered nanoarrays are still under going. Received 29 November 2000     

Dielectrophoretically Modulated Optical Spectroscopy of Colloidal Nanoparticle Solutions in Microfluidic Channels

Optical spectroscopic techniques (e.g., extinction, scattering, and fluorescence spectroscopies) are important for the analysis of colloidal solutions of nanoparticles (NPs). They are routinely applied to plasmonic and quantum-dot NP samples assuming that these contain a single population of particles with modest size and shape dispersity. However, these spectroscopic techniques become less effective when the sample is a mixture of particles with different sizes, shapes, or composition. Here, an original microfluidic method is proposed for the optical spectroscopic analysis of colloidal NP solutions that combines periodic trapping of NPs by dielectrophoresis (DEP) with in situ optical extinction spectroscopy. The periodic trapping leads to modulation of the continuously monitored optical spectrum depending on the DEP properties of the NPs. DEP-modulated spectroscopy is demonstrated using colloidal gold NPs as small as 40 nm diameter. It is found that the DEP modulation is significantly enhanced when employing suitable microfluidic flow over a multielectrode array. Finally, it is shown that the method can identify and characterize the NP species simultaneously present in a mixture of 40 and 80 nm gold NPs, opening the way toward optical spectroscopic analysis of higher complexity NP mixtures through the combination of the DEP-modulated spectroscopy with chemometric methods.     

Plasmonic resonance enhancement of single gold nanorod in two-photon photopolymerization for fabrication of polymer/metal nanocomposites

We investigated the plasmonic resonance enhanced two-photon photopolymerization (PETPP) using the isolated chemical synthesized gold nanorods for fabrication of polymer/metal nanocomposites. The isolated gold nanorods with the plasmonic resonance band around 750 nm covered by photoresist were irradiated by a femtosecond laser with the wavelength of 780 nm. The PETPP trigged by the plasmonic resonance enhancement of gold nanorods was localized only in the distance smaller than 30 nm from the surface of gold nanorods, which matched the distance of plasmonic resonant enhanced field of the gold nanorod. The shapes of obtained polymer/gold nanocomposites were changed from the “dumbbell” to the “ellipsoid” with the increase of laser irradiating intensity used for PETPP. This study would provide a potential method for fabricating the plasmonic nanomaterials and nanostructures of polymer/metal nanocomposites, which could be expected to be applied in the emerging fields such as nanophotonics, nanobiosensor, nanolithography.     

Structural phase transformations in annealed cubic ZnS nanocrystals

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn²⁺ pulse annealed in vacuum and in air, up to 500 °C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid–liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn²⁺(I) centers. No spectra from Mn²⁺ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 °C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 °C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn²⁺-type centers in a hexagonal ZnO structure at T ~ 300 °C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.     

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.