Enhanced Fluorescence Emission and Magnetic Alignment Control of Biphasic Functionalized Composite Janus Particles

Janus particles, particles that have two distinct aspects on their surface or interiors, have attracted much attention due to their potential for application. For the application of Janus particles to high‐resolution displays, and as light sources for optical circuits and fluorescent probes, the Janus particles should be nanosize to ensure high‐resolution display and analysis, responsive to external stimuli, and highly fluorescent. However, it is still a challenging issue to develop such highly fluorescent nanoscale Janus particles and control their alignment. Magnetoresponsive Janus particles, of which the orientation can be controlled by an external magnetic field, are prepared by the simple introduction of polymer‐coated magnetic nanoparticles (NPs) into the hemispheres of Janus particles. If these magnetoresponsive Janus particles can be combined with a strong fluorescence system, then they could be ideal candidates as components of the previously mentioned applications. In the present study, Janus particles are prepared with a fluorescent dye and gold nanoparticles (Au NPs) on one side. The optical properties of the resulting particles are assessed and discussed. Furthermore, the response of composite Janus particles containing dyes, Au NPs, and iron oxide NPs to an external magnetic field is discussed.     

Laser-Scanning Microscopy for Electrophoretic Mobility Characterization of Single Nanoparticles

To enable detailed studies of interactions between nanoparticles and their environment and the correlations between various nanoparticle properties, one must go beyond ensemble averages and toward single-particle measurements. However, current methodologies for the single-nanoparticle analysis of charge and size either lack the flexibility to study dynamic processes on the single-particle level or are highly specific and require complex microfluidic devices. In addition, accurate measurements of the electrophoretic mobility (or zeta-potential) based on the optical detection of single nanoparticles remain challenging due to the low photon budget, the required sampling frequency, and the fact that electroosmosis in typical microfluidic devices must be analyzed carefully. In this study, a method is investigated to accurately characterize the electrophoretic mobility of individual nanoparticles and estimate their size by simultaneously analyzing the electrokinetic- and Brownian motion in a simple microfluidic channel. Fast laser scanning excitation and sensitive detection of fluorescent photons enable single-nanoparticle velocimetry experiments in an oscillating electric field at high frame rates.     

Probing the acoustic vibrations of single metal nanoparticles by ultrashort laser pulses

The strong interaction of metal nanoparticles with light makes it possible to detect individual particles by far‐field optical methods. In this article, the interaction of a metal nanoparticle with a short laser pulse is discussed, with the emphasis on the coherent excitation of mechanical (acoustic) modes and the optical detection of these vibrations. The literature on acoustic vibrations of single metal nanoparticles of different shapes (spheres, dumbbells, rods, cubes, wires, prisms) is reviewed, and the modes that have been excited and detected in these particles are discussed. Finally, the insights and potential applications enabled by these studies are summarized.     

Secondary electron emission from highly charged carbon grains

Surfaces in contact with a plasma can influence its characteristics and, on the other hand, the impact of plasma particles can change surface properties of materials immersed in a plasma. Carbon is often present in plasma systems either as a building material or a product of technological processes, thus its behavior is an important factor of these applications. The paper deals with investigations of secondary emission of 1–6 μm spherical grains from amorphous carbon under the electric field of the order of 10⁸ V/m. We have found that the secondary emission yield increases with the electric field at the sample surface nearly linearly and does not depend on the grain diameter. Long-lasting (hours) electron irradiation of the sample surface leads to a significant decrease of the yield that was attributed to the removal of an absorbed layer from the grain surface. This conclusion is supported by the fact that a similar effect was achieved after several minutes of simultaneous electron and ion treatments.     

Scattering behavior of nonabsorbing metallic nanoparticles

The optical property of nanosized metallic particles is unique and size-dependent, which cause color variation. In this work, the relationship between diameter and refractive index of nonabsorbing metallic nanoparticles and their scattering properties is studied by using Mie theory. Obtained results indicate that the optical scattering of metallic nanoparticle depends on their refractive index and diameter. The effect of refractive index on optical scattering depends on the nanoparticle diameter. So that, for very fine nanoparticle (10 nm diameter) the effect of refractive index on scattering is not significant. But the effect of refractive index of large size nanoparticle (700–900 nm diameter) on their optical scattering is higher than fine and medium size nanoparticles. The wavelength with maximum scattering depends on refractive index and nanoparticles diameter. In addition, the colorimetric study indicates that the color of nanoparticle depends on their size and refractive index. So that, the lightness, hue, and colorfulness of nanoparticles is changed by changing size and refractive index.     

Manipulation of Nanoparticles Using Dark-Field-Illumination Optical Tweezers with Compensating Spherical Aberration

Based on our previous investigation of optical tweezers with dark field illumination [Chin. Phys. Left. 25（2008）329] nanoparticles at large trap depth are better viewed in wide field and real time for a long time, but with poor forces. Here we present the mismatched tube length to compensate for spherical aberration of an oil-immersion objective in a glass-water interface in an optical tweezers system for manipulating nanoparticles. In this way, the critical power of stable trapping particles is measured at different trap depths. It is found that trap depth is enlarged for trapping nanoparticles and trapping forces are enhanced at large trap depth. According to the measurement, 70-nm particles are manipulated in three dimensions and observed clearly at large appropriate depth. This will expand applications of optical tweezers in a nanometre-scale colloidal system.     

Nonadiabatic nondegenerate excitation by optical near-field and its application to optical pulse-shape measurement

Using DCM dye grains and light of different wavelengths generated by two CW laser diodes that oscillate in the near-infrared wavelength region, visible light emission from dye grains due to near-infrared excitation based on a nonadiabatic, nondegenerate excitation process was observed for the first time. Unlike sum-frequency generation with nonlinear polarization, the difference in polarization angles of the two beams did not affect the emitted light intensity. Optical sampling based on this nonadiabatic, nondegenerate excitation principle was demonstrated for the first time. The optical pulse shape in the wavelength band of λ=1250–1350 nm, which is close to the wavelength range used for optical fiber communications, was measured with a temporal resolution of 0.8–1.1 ps.     

Tailor-made metal nanoparticles as SERS substrates

In this contribution we summarize recent experiments with the objective to generate optimized substrates for surface-enhanced Raman spectroscopy (SERS). For this purpose, the well-established laser-assisted growth technique has been applied, which relies on a precise control of the growth kinetics of supported metal nanoparticles. With this method reproducible and stable SERS substrates with tailor-made optical properties possing best field enhancements were produced for specific excitation wavelengths and detection ranges. Optimization of the SERS substrates has been achieved by stabilizing the localized surface plasmon polariton resonance (SPR) of gold nanoparticles in the vicinity of the laser wavelength of λ=647 nm and λ=785 nm used for SERS excitation. After nanoparticle preparation, SERS spectra of pyrene were obtained using naturally grown nanoparticles and nanoparticles prepared by laser-assisted growth. The most important result is that the optimized substrates prepared by laser-assisted growth exhibit a significantly higher signal-to-noise ratio as compared to naturally grown nanoparticles. They are even better than substrates whose SPR has been tuned to the excitation wavelength by an elevated temperature during preparation. Another important observation is that all SERS spectra exhibit excellent reproducibility and the substrates do not show degradation during the measurements. Finally, the SERS enhancement factors due to the optimized substrates have been estimated and are on the order of 10⁵ to 10⁶.     

Nanophotonics of isolated spherical particles

The problem of extreme focusing of an optical beam into the spatial region with wavelength dimensions is considered with the use of the special features of radiation interaction with isolated spherical particles. Results of numerical computations of the optical field intensity at the surface of silver particles of different radii upon exposure to laser radiation with different wavelengths are presented. It is demonstrated that the relative intensity of the plasmon optical field on the nanoparticle surface increases and the field focusing region decreases with increasing particle radius. Results of numerical computations illustrating the influence of the shell of composite nanoparticles comprising a dielectric core and a metal shell on the optical field intensity in the vicinity of the particle are presented. The problem of local optical foci of a transparent microparticle (photonic nanojets) is investigated. It is established that variation of the micron particle size, its optical properties, and laser radiation parameters allows the amplitude and spatial characteristics of the photonic nanojet region to be controlled efficiently.     

Nonlinear optical properties of nanometer-size silver coated polydiacetylene composite vesicles and resulting Langmuir–Blodgett films

Noble metal-coated PDA composite vesicles were expected to increase the effective third-order nonlinear optical susceptibility χ ⁽³⁾(ω), due to the enhancement of the optical electric field induced by localized surface plasmon resonance. Different size (20, 50 and 80 nm) Ag colloidal nanoparticles were coated on the outer surface of polydiacetylene (PDA) vesicles to form PDA/Ag nanocomposite vesicles and the size-dependent effect of Ag colloidal nanoparticles on NLO properties enhancement has been explored. The explanation based on the competition of a size-dependent light-confinement effect and a size-dependent dielectric constant of Ag particles had been presented. Furthermore, these PDA/Ag composite vesicles were successfully immobilized onto the solid substrate by the Langmuir–Blodgett (LB) method and their linear and nonlinear optical properties were characterized, respectively. Obviously, PDA/Ag composite vesicles Langmuir–Blodgett (LB) films promoted the enhancement of the third-order NLO properties.     

Nanoparticle Tracking Analysis: Enhanced Detection of Transparent Materials

This study proposes a novel and simple in-house design of a nanoparticle tracking analysis (NTA) device for the online characterization of nanoparticles in an aqueous solution. The particle size distribution of two sets of model nanoparticles, for example, transparent (SiO₂) and opaque (TiO₂) materials with respect to water as a dispersion medium could be successfully analyzed. Experiments are conducted using two different laser wavelengths of 632.8 (red) and 510 nm (green) and a range of concentrations. The accuracy of the green laser is larger compared to the red laser for all particle concentrations used. The measured average diameter using the presented in-house NTA setup is in the acceptable range compared to the electron microscopy data. The average diameter of the transparent (SiO₂) and opaque (TiO₂) samples is calculated as 36.29 and 27.26 nm using NTA, 36.44 and 27.8 nm analyzing field emission scanning electron microscopy images, and 23.97 and 19.7 nm analyzing transmission electron microscopy images. In the new viewing sample holder, nanoparticles undergo mere Brownian motion with no bulk drift velocity. The effect of solid concentration and wavelength of the laser light on the performance of the NTA sensor is investigated, and the optimal concentration range for model particles is reported.     

Structural and optical properties of the nanopowder of the Eu3+ doped LiLaP4O12 produced by sol gel route

Structural and optical properties of nanopowders of Eu³⁺-doped LiLaP₄O₁₂ (LiLaPO) were studied. The samples were synthetized via polyvinyl alcohol assisted sol gel method. Powder X-ray diffraction (XRD) results showed that the samples have the LiLaPO crystalline phase and scanning electron microscopy (SEM) showed that the particles are in the nanometer scale, mostly as isolated particles with same aggregates. Photoluminescence (PL) properties were evaluated via excitation and emission spectra. The excitation spectra, in the range 2.6–13.8 eV, allowed the identification of the charge transfer band, 4f → 4f5d excitation, exciton formation and the interband excitation of the Eu³⁺ ions in the matrix. The emission spectra revealed that Eu³⁺ is located in two different sites in the samples, one of them being at the bulk and the other close to the surface of the nanoparticles, result confirmed by the CT excitation band.     

Effect of silver nanoparticles with different shapes on luminescence of samarium complex at two different excitation wavelengths

The luminescence properties of Sm(TTFA)₃ complex in presence of the silver (Ag) nanoparticles with size ranged from 80 nm to 120 nm and different shapes (nanorod, cube, tetrahedron, and nanowire) were investigated at two different excitation wavelengths of 397 nm and 350 nm, which was resonant and off-resonant excitation, respectively. The luminescence enhancement for the resonant excitation was much greater than that for the off-resonant excitation. The electric and magnetic dipole transitions were affected by the Ag nanoparticles and the results showed that their emission enhancement depended on the balance of the overlap between the emission wavelengths and the localized surface plasmon resonant of nanoparticles and their sensibility to the variation of local environments. The enhancement and quenching of the luminescence were both observed at the resonant excitation.     

表面结构对SnO2半导体纳米粒子荧光性质的影响

本文利用荧光光谱对ＳｎＯ２纳米粒子水溶胶及其有机溶胶的光学性质进行了研究。发现粒子的表面结构对其光学性质具有极大的影响。水溶胶的荧光发射是由氧空位控制的,其发射强度工;有机溶胶由于表面活性剂的作用,改变粒子的表面结构,得到较强的荧光发射。     

Long‐Term Colloidal and Chemical Stability in Aqueous Media of NaYF4‐Type Upconversion Nanoparticles Modified by Ligand‐Exchange

Surface capping is an essential component of nanoparticles as it provides access to their outstanding properties in the real world. Upconversion nanoparticles are predominantly interesting for use in biological environments, due to their excellent optical properties such as the conversion of near‐infrared excitation light into emissions in the visible or UV range of the spectrum, high photostability, and the absence of any intermittence. One of the most efficient upconversion nanoparticles, consisting of lanthanide doped NaYF₄, suffers from limited stability in aqueous media. This study investigates a set of five types of surface coatings, ranging from small ligands to polymers of different charge and different coordinating groups, on monodisperse 28 ± 0.9 nm sized NaYF₄(Yb,Er) nanoparticles modified by a two‐step ligand exchange mediated by NOBF₄. Information on the long‐term chemical and colloidal stability for highly diluted aqueous dispersions of these particles is acquired by transmission electron microscopy, dynamic light scattering, and luminescence spectroscopy. The findings are of importance for the development of probes and labels based on upconversion nanoparticles for biological applications.     

Multicomponent periodic nanoparticle superlattices

In this article, we review the state-of-the-art in the preparation and characterization of multicomponent self-assembled superlattices of colloidal nanoparticles with core sizes in the range of 2–20 nm and interparticle spacing less than 2 nm down to intimate contact stemming from sintering. Several aspects of the field are discussed, including: structural organization, the role of particle size distribution, key interparticle forces at play, and methods of investigation of the structures. Contrary to the extensively studied colloidal crystals composed of microscale particles, the nanoparticles possess unique size-dependent properties, such as electronic, optical, or magnetic, which when combined into periodic structures can potentially lead to new collective states stemming from precise positioning of the nanocolloids. As such, we examine a number of emerging applications of this new class of metamaterials. Finally, we speculate on the potential impact of these materials, the new directions, and the challenges for the researchers.     

Optical activity of helical quantum-dot supercrystals

The size of chiral nanoparticles is much smaller than the optical wavelength. As a result, the difference in interaction of enantiomers with circularly polarized light of different handedness is practically unobservable. Due to the large mismatch in scale, the problem of enhancement of enantioselectivity of optical properties of nanoparticles is particularly important for modern photonics. In this work, we show that ordering of achiral nanoparticles into a chiral supercrystal with dimensions comparable to the wavelength of light allows achieving nearly total dissymmetry of optical absorption and demonstrate this using a helical super-crystal made of semiconductor quantum dots as an example. The proposed approach may find numerous applications in various optical and analytical methods used in biomedicine, chemistry, and pharmacology.     

The nanoparticle shape's effect on the light scattering cross-section

In the framework of kinetic approach we develop a theory for light scattering by ellipsoidal metallic nanoparticles whose dimensions are less than those of a free electron path. In this case, the surface of the particle starts to play a dominant role in electron scattering. When the size of the particle decreases below the free electron path at least in one direction, the optical conductivity becomes a tensor quantity, and the diagonal components of this tensor define the half-widths of the plasmon resonances peaks. Thus, the effect of the particles' shape both on the frequencies of plasmon resonances and on their half-widths is considered. Additionally, the expression for a cross-section of the light scattering by a collection of chaotically oriented spheroidal nanoparticles is obtained and averaged over different directions of particles in the collection. Our results highlight the plasmonic properties of metallic nanospheroids, notably, the spectrum of the light scattering has two peaks at the frequencies of plasmon resonances even if there is no preferential direction in the collection of particles.     

Electromagnetic interaction of fluorophores with designed two-dimensional silver nanoparticle arrays

We study the fluorescence enhancement of dye molecules adsorbed on regular two-dimensional arrays of designed silver nanoparticles. The silver particles show two orthogonal optical resonances at different wavelengths because of their elongated shape. The short-wavelength resonance was designed to fit to the absorption maximum of the fluorophore. When the excitation light drives the short-wavelength resonance, the measured fluorescence intensity is strongly enhanced compared to that for the orthogonal particle orientation. This shows directly a strong electromagnetic coupling between the nanoparticles and the fluorophore. Additionally enhanced photochemical bleaching is observed due to the interaction of fluorophores with the particles. Using a rate model describing the fluorescence enhancement and the bleaching enhancement, an average value for the particle-induced increase in the radiative fluorescence rate is obtained, together with a lower limit for the averaged particle-induced field intensity enhancement factor. Received: 3 July 2001 / Revised version: 3 September 2001 / Published online: 15 October 2001     

Shifting of Fluorescence Peak in CdS Nanoparticles by Excitation Wavelength Change

CdS nanoparticles with different size are prepared by chemical bath deposition method. These particles show strong fluorescence at emission wavelength of 507 nm. It has been observed that this emission peak changes through a range of 147 nm, by varying the excitation wavelengths through 370–480 nm.The emission peak can thus be tuned by varying the excitation wavelengths. This peak emission wavelength shift is due to the selective excitation of vibronic levels in the surface state of the CdS nanoparticles.