Characterization of pigment particle absorption efficiencies using frequency domain photon migration

Time-dependent measurements of multiply scattered light were made using frequency domain photon migration (FDPM) techniques in polystyrene latex as a function of ppm pigment concentration (by weight) in order to determine the wavelength-dependent absorption efficiencies for three different pigment particles. The results demonstrate that the absorption spectra of pigment particles within their dispersing vehicles concur with the complementary color chart. FDPM offers a first-principles method for assessing optical characteristics of pigments within their dispersing vehicles and without the need to resort to conventional measurement of diffuse reflectance from coatings and data analysis using phenomenological theory.     

High rectifying efficiencies of microtubule motility on kinesin-coated gold nanostructures

We demonstrate highly efficient rectification of microtubule motility on gold nanofabricated structures. First, we present a novel nanofabrication process for the creation of gold tracks for microtubule motility recessed in silicon oxide. This approach is particularly useful because it enables the use of the well-understood PEG-silane chemistry on SiO2 for the blocking of kinesin, whereas the gold tracks allow possible electrical control. We demonstrate excellent confinement of microtubule motility to the gold nanostructures and that microtubules move on the gold with speeds comparable to that on glass. Second, we present designs of three advanced rectifier geometries. We analyze the microtubule pathways through the geometries, and we demonstrate highly efficient rectification with up to 92% efficiency. As a result, we find that up to 97% of the microtubules move unidirectionally.     

Exchange coupling controlled ferrite with dual magnetic resonance and broad frequency bandwidth in microwave absorption

Ti-doped barium ferrite powders BaFe_12−xTi_xO₁₉ (x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were synthesized by the sol–gel method. The phase structure and morphology were analyzed by x-ray diffraction (XRD) and scanning electron microscopy, respectively. The powders were also studied for their magnetic properties and microwave absorption. Results show that the Ti-doped barium ferrites (BFTO) exist in single phase and exhibit hexagonal plate-like structure. The anisotropy field H_a of the BFTO decreases almost linearly with the increase in Ti concentration, which leads to a shift of the natural resonance peak toward low frequency. Two natural resonance peaks appear, which can be assigned to the double values of the Landé factor g that are found to be ∼2.0 and ∼2.3 in the system and can be essentially attributed to the existence of Fe³⁺ ions and the exchange coupling effect between Fe³⁺ and Fe²⁺ ions, respectively. Such a dual resonance effect contributes a broad magnetic loss peak and thus a high attenuation constant, and leads to a dual reflection loss (RL) peak over the frequency range between 26.5 and 40 GHz. The high attenuation constants are between 350 and 500 at peak position. The optimal RL reaches around −45 dB and the practicable frequency bandwidth is beyond 11 GHz. This suggests that the BFTO powders could be used as microwave absorbing materials with extraordinary properties.     

Optical trapping near resonance absorption

Expressions for radiation-induced forces are presented for the case of a Rayleigh particle near the focus of a Gaussian laser beam at near-resonant conditions. Classical electromagnetic theory was used to obtain the dependence of the scattering and gradient forces on the incident laser frequency, the beam convergence angle, and the spatial position of the particle with respect to the focus. Approximative numerical analysis performed for particles with a single resonant absorption peak demonstrates the occurrence of up to 50-fold enhanced trapping forces at near-resonant frequencies. The use of this technique of gradient force enhancement may provide optical tweezers with enhanced trapping strengths and a degree of specificity.     

Light absorption by black sand dust

Light absorption by fine and coarse aerosols derived from Hawaiian black sand was determined by light transmission. The real part of the refractive index of this material was measured directly, and the imaginary part was estimated. The measured light absorption coefficients (B(ap)) were adjusted for multiple scattering artifacts by use of absorption-to-extinction ratios estimated with Mie theory. The best agreement between calculated and measured fine and coarse B(ap) was achieved with a value of 0.014 for the imaginary part of the refractive index. The corresponding absorption efficiencies for the fine and coarse black sand aerosols were 0.22 and 0.09 m(2)/g, respectively.     

Electronic coupling between azurin and gold at different protein/substrate orientations

By means of constrained classical molecular dynamics simulations, we have computed the structure of azurin deposited on a Au(111) surface at different possible orientations and the azimuthal forces acting on the protein at each sampled conformation. We have then evaluated the effect of the angular variation on the speed of electron tunneling between the protein redox site and the metal surface. We find that the azurin/gold electronic coupling has a strong dependence on the molecular orientation and is greatly enhanced by inclining the protein to lie as flat as possible on the surface. We discuss the implications of our results for scanning probe microscopy experiments in which tunneling currents are measured while the protein is subjected to mechanical forces exerted by the tip of the instrument.     

Energy absorption of gold nanoshells in hyperthermia therapy

The unique optical characteristics of a gold nanoshell motivate the application of nanoshell-based hyperthermia in drug delivery and cancer treatment. However, most of our understanding on energy absorption and heat transfer is still focused on individual particles, which may not be accurate for nanoshell aggregates in a real application due to the strong optical interaction of nanoshells. This paper investigates the relationship between the optical interaction and the interparticle distance in the visible and near-infrared regions by means of a finite-difference time-domain (FDTD) method. The objective is to explore the energy transportation mechanism, which is critical for hyperthermia therapy. From the numerical simulation results of different forms of nanoshell aggregates, including individual nanoshells, 1-D chains, 2-D arrays, and 3-D clusters, it was found that the interparticle distance plays a crucial role from the maximal absorption point of view. The interparticle distance affects both field enhancement and surface plasmon resonance position. The accurate prediction of energy absorption also helps the way nanoshells are populated in the tumor cell so as to prevent heat damage to healthy tissues in clinic applications. In the case of 3-D clusters, the laser energy decays exponentially along the wave propagation, and the penetration depth greatly depends on the interparticle distance. The closer the nanoshells are placed, the shorter the penetration depth is. The maximal total length for the laser penetration through the shell of gold nanoparticles is about a few hundred to several nanometers. The actual penetration depth primarily depends not only on the interparticle distance, but also on the size of the nanoshells as well as other factors. Since the absorption energy is concentrated on the surface clusters of nanoparticles, heat transfer mechanisms in metal-nanoparticles-based hyperthermia will differ from that in other hyperthermia. The information obtained from this paper will serve as a basis for further study of heat transfer in metal-nanoparticles-based hyperthermia.     

Surface plasmon resonance scattering and absorption of anti-EGFR antibody conjugated gold nanoparticles in cancer diagnostics: applications in oral cancer

Gold nanoparticles with unique optical properties may be useful as biosensors in living whole cells. Using a simple and inexpensive technique, we recorded surface plasmon resonance (SPR) scattering images and SPR absorption spectra from both colloidal gold nanoparticles and from gold nanoparticles conjugated to monoclonal anti-epidermal growth factor receptor (anti-EGFR) antibodies after incubation in cell cultures with a nonmalignant epithelial cell line (HaCaT) and two malignant oral epithelial cell lines (HOC 313 clone 8 and HSC 3). Colloidal gold nanoparticles are found in dispersed and aggregated forms within the cell cytoplasm and provide anatomic labeling information, but their uptake is nonspecific for malignant cells. The anti-EGFR antibody conjugated nanoparticles specifically and homogeneously bind to the surface of the cancer type cells with 600% greater affinity than to the noncancerous cells. This specific and homogeneous binding is found to give a relatively sharper SPR absorption band with a red shifted maximum compared to that observed when added to the noncancerous cells. These results suggest that SPR scattering imaging or SPR absorption spectroscopy generated from antibody conjugated gold nanoparticles can be useful in molecular biosensor techniques for the diagnosis and investigation of oral epithelial living cancer cells in vivo and in vitro.     

Light absorption in coated ellipsoidal quantum lenses

In the framework of adiabatic approximation the energy states of electron and direct light absorption in coated ellipsoidal quantum lens is investigated. Analytical expressions for particle energy spectrum is obtained taking into account that electron effective masses are different in the coat and in quantum lens. Obtained results were applied for the case of rectangular quantum well of finite height. The investigation of energy level dependence on geometrical parameters of quantum lens was performed. In particular, it has been shown, that the particle energy is equidistant for both ellipsoidal and spherical segment cases, and dependence of energy on geometrical parameters has root character. Influence of the presence of a coating at a quantum lens on direct light absorption is investigated. Dependence of edge of absorption on geometrical parameters of a quantum lens and coating is obtained. The selection rules as in case of presence so and at absence of a coating for both ellipsoidal and spherical segment cases were derived. This paper was originally presented as part of the Special Issue: Selection of Papers from the 2007 Semiconducting and Insulating Materials Conference (SIMC-XIV).     

X-ray absorption of gold nanoparticles with thin silica shell

Silica-coated gold (Au) nanoparticles were prepared and their morphological and X-ray absorption properties were investigated. These core-shell type nanoparticles are very stable in aqueous media and may be suitable for an X-ray contrast agent in biological systems. Transmission electron micrographs confirmed well-separated and relatively homogeneous morphology of the nanoparticles in highly concentrated colloids. Peak position for Au plasmon resonance was red-shifted with increasing shell thickness. X-ray absorption by the colloids of silica-coated Au nanoparticles was stronger than that by those of silica-coated Agl nanoparticles, a recently investigated X-ray contrast agent, at similar experimental conditions.     

Fabrication of organic shell-covered gold nanospheres with near-infrared absorption

A simple protocol is developed for the fabrication of stable organic shell-covered gold nanospheres with near-infrared absorption characteristics. The designed strategy mainly concentrates on two steps: (i) the polymerization of gold nanoparticles (AuNPs) induced by 1,2-ethanedithiol in water/ethanol; (ii) conjugation of the polymerized AuNPs with marocyclic compound for the formation of the organic shell. In the first step, the 1,2-ethanedithiol containing two thiols towards both ends of the chain enables the successful polymerization of AuNPs due to the forceful appetency of thiols to AuNPs. In the second step, the polymerized AuNPs are covered by the marocyclic compound attributing to the hydrogel-bonding effect between NH and SH. Because of the organic shell, the stability and dispersibility of the obtained nanospheres are improved. The optical properties of the fabricated gold nanospheres (400 nm average diameter) are well investigated by the UV-vis absorption spectroscopy which exhibit intense near-infrared absorption at 972 nm in acetone. The strategy developed in this study is promising in that excellent stability and dispersibility of gold nanospheres can be achieved in a facile and economic way.     

Plasmon absorption of gold nanoparticles in linear polymer solutions

Gold nanoparticles (GNPs) are stable in a number of linear polymer solutions, including poly(vinyl alcohol), poly(N-vinyl-2-pyrrolidone), hydroxyethyl cellulose, and poly(ethylene oxide) (PEO). Of these, PEO, with a wide range of molecular mass (from 0.3 to 8 MDa), is particularly attractive for exploring the interaction between the GNPs and polymer molecules. We have found that the colors of the GNPs are significantly different in PEO solutions at concentrations below and above entanglement threshold concentration (Phi*), which allows one to determine Phi* values for different sizes of PEO from the inflection points of the plots of the absorbance at 600 nm against the concentration of PEO. The Phi* values are close to those obtained by the viscosity measurements, showing the usefulness of this simple method. Transmission electron microscopy images have confirmed that the change in the absorbance is due to the aggregation and/or agglomeration of the GNPs in PEO solutions.     

Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles

Surface plasmon resonance measurements and magnetic characterization studies have been carried out for two types of thiol-capped gold nanoparticles (NPs) with similar diameters between 2.0 and 2.5?nm and different organic molecules linked to the sulfur atom: dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammonium bromide have also been included in the investigation since such capping molecules weakly interact with the gold surface atoms and, therefore, this system can be used as a model for naked gold NPs; such particles presented a bimodal size distribution with diameters around 1.5 and 5?nm. The plasmon resonance is non-existent for tiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered with dodecanethiol molecules and a bulk-like feature is measured for NPs capped with tetralkyl ammonium salts. These differences would indicate that the modification of the surface electronic structure of the Au NPs depends on the geometry and self-assembling capabilities of the capping molecules and on the electric charge transferred between Au and S atoms. Regarding the magnetization, dodecanethiol-capped NPs have a ferromagnetic-like behaviour, while the NPs capped with tiopronin exhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs are diamagnetic across the studied temperature range; straight chains with a well-defined symmetry axis can induce orbital momentum on surface electrons close to the binding atoms. The orbital momentum not only contributes to the magnetization but also to the local anisotropy, giving rise to permanent magnetism. Due to the domain structure of the adsorbed molecules, orbital momentum is not induced for tiopronin-capped NPs and the charge transfer only induces a paramagnetic spin component.     

Controlled AFM detachments and movement of nanoparticles: gold clusters on HOPG at different temperatures

The effect of temperature on the onset of movement of gold nanoclusters (diameter 27 nm) deposited on highly oriented pyrolytic graphite (HOPG) has been studied by atomic force microscopy (AFM) techniques. Using the AFM with amplitude modulation (tapping mode AFM) we have stimulated and controlled the movement of individual clusters. We show how, at room temperature, controlled detachments and smooth movements can be obtained for clusters having dimensions comparable to or smaller than the tip radius. Displacement is practically visible in real time and it can be started and stopped easily by adjusting only one parameter, the tip amplitude oscillation. Analysing the energy dissipation signal at the onset of nanocluster sliding we evaluated a detachment threshold energy as a function of temperature in the range 300-413 K. We also analysed single cluster thermal induced displacement and combining this delicate procedure with AFM forced movement behaviour we conclude that detachment threshold energy is directly related to the activation energy of nanocluster diffusion and it scales linearly with temperature as expected for a single-particle thermally activated process.     

The resonance frequency of laminated piezoelectric rectangular plates

Based on the theory of elasticity and piezoelectricity, a dynamic model of laminated elastic-piezoelectric rectangular plates is considered. The bending equations are established and solved, taking the nonlinear behavior of the piezoelectric material into account. The resonance frequency of a laminated piezoelectric rectangular plate with four kinds of different boundary conditions is then investigated. The present results agree very well with the experimental findings and can be extended to practical applications, such as considering the effect of an epoxy package on the resonance of piezoceramic plate.     

Light polarization and color from guided-mode resonance filter

The characteristics of reflected light of a 1-D guided-mode resonance filter (GMRF) are studied in this paper. A triple-layer GMRF is designed by using the finite difference time domain method under non-polarized light illumination. Numerical results show that the reflectance spectra of TE and TM polarizations can be changed by altering the fill factor f of the GMRF. Moreover, by calculating the color of the reflected light with the chromaticity theory, we find that the color of reflected light becomes pure when f is 0.9. The results show that the color and polarization degree of the reflected light of a GMRF are tunable by altering the fill factor.     

Diode-laser frequency stabilization by two-frequency Doppler-broadened absorption spectroscopy

We demonstrate a robust method for frequency stabilization of a diode laser by two Doppler-broadened absorption spectra of the cesium D2 line. This technique employs an acousto-optical modulator to generate another frequency component from a diode laser to perform the spectroscopy. The 852-nm diode laser with frequency stabilization at the zero crossing of the error signal showed a peak-to-peak fluctuation of 800 kHz compared with a frequency-stabilized femtosecond laser over a 2-h period. This frequency-locking method is free of modulation and can be applied to frequency discriminators as well as to atomic resonances.     

Atomic absorption determination of mercury in solid samples of placer gold

The possibility of atomic absorption determination of mercury in solid samples of placer gold is shown. Mercury-gold compounds with a variable composition are assumed to be found in the process of thermal atomization. The conditions that contribute to an increase in the degree of atomization of mercury in the process of heating of a solid sample in the range of concentrations above 10⁻⁷ wt % are selected.     

Ultrafast pump-probe surface plasmon resonance spectroscopy of thin gold films

A time-resolved reflection pump-probe method was combined with a surface plasmon resonance technique in Kretschmann geometry for the investigation of ultrafast light-induced processes in thin films. Transient changes in the gold layer's reflectivity were observed when the layer was excited by 3 ps duration pulses with photon energy exceeding the interband transition and by probing with photon energy close to the interband transition. Comparison of the experimental and modeling results has shown that the imaginary part of the dielectric function of gold increases linearly during excitation, whereas the real part remains unchanged. The decay of the light-induced changes has two components. The first component is faster than the pulse duration, and the second is much longer than 1.5 ns; they are related to cooling of the electron plasma and lattice, respectively.