Tritoroidal particle rings formation in open microfluidics induced by standing surface acoustic waves

In this paper, the particle movements in a sessile droplet induced by standing surface acoustic waves (SSAWs) are studied. Tritoroidal particle rings are formed under the interaction of acoustic field and electric field. The experimental results demonstrate that the electric field plays an important role in patterning nanoparticles. The electric field can define the droplet shape due to electrowetting. When the droplet approximates a hemisphere, the acoustic radiation force induced by SSAWs drives the particles to form tritoroidal particle rings. When the droplet approximates a convex plate, the drag force induced by acoustic steaming drives the particle to move. The results will be useful for better understanding the nanoparticle movements in a sessile droplet, which is important to explain the mechanism that SSAWs enhance reaction and crystallization in droplet.     

Localized surface plasmon resonance spectroscopy near molecular resonances

The peak location of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, new phenomena are revealed by exploring the influence of interacting molecular resonances and nanoparticle resonances. The LSPR peak shift and line shape induced by a resonant molecule vary with wavelength. In most instances, the oscillatory dependence of the peak shift on wavelength tracks with the wavelength dependence of the real part of the refractive index, as determined by a Kramers-Kronig transformation of the molecular resonance absorption spectrum. A quantitative assessment of this shift based on discrete dipole approximation calculations shows that the Kramers-Kronig index must be scaled in order to match experiment.     

Localized surface plasmon resonances of anisotropic semiconductor nanocrystals

We demonstrate that anisotropic semiconductor nanocrystals display localized surface plasmon resonances that are dependent on the nanocrystal shape and cover a broad spectral region in the near-IR wavelengths. In-plane and out-of-plane dipolar resonances were observed for colloidal dispersions of Cu(2-x)S nanodisks, and the wavelengths of these resonances are in good agreement with calculations carried out in the electrostatic limit. The wavelength, line shape, and relative intensities of these plasmon bands can be tuned during the synthetic process by controlling the geometric aspect ratio of the disk or using a postsynthetic thermal-processing step to increase the free carrier densities.     

A SERS-active microfluidic device with tunable surface plasmon resonances

A surface-enhanced Raman scattering (SERS)-active microfluidic device with tunable surface plasmon resonances is presented here. It is constructed by silver grating substrates prepared by two-beam laser interference of photoresists and subsequent metal evaporation coating, as well as PDMS microchannel derived from soft lithography. By varying the period of gratings from 200 to 550 nm, surface plasmon resonances (SPRs) from the metal gratings could be tuned in a certain range. When the SPRs match with the Raman excitation line, the highest enhancement factor of 2×10(7) is achieved in the SERS detection. The SERS-active microchannel with tunable SPRs exhibits both high enhancement factor and reproducibility of SERS signals, and thus holds great promise for applications of on-chip SERS detection.     

Investigation of surface plasma resonances in small sodium clusters

Photoabsorption cross sections of small neutral sodium clusters are measured at wavelengths from 452 to 604 nm. We find that surface plasma oscillations of the valence electrons dominate the photoabsorption spectra. The experimental results are consistent with predictions based on an extended Clemenger-Nilsson model, general sum rules, and the experimental static electric polarizabilities. For Na₈ a single broad resonance is observed, the peak wavelength is in excellent agreement with the model. The data for Na₉ and Na₁₀ support the prediction of double resonances.     

Tunable localized surface plasmon resonances in tungsten oxide nanocrystals

Transition-metal oxide nanocrystals are interesting candidates for localized surface plasmon resonance hosts because they exhibit fascinating properties arising from the unique character of their outer-d valence electrons. WO(3-δ) nanoparticles are known to have intense visible and near-IR absorption, but the origin of the optical absorption has remained unclear. Here we demonstrate that metallic phases of WO(3-δ) nanoparticles exhibit a strong and tunable localized surface plasmon resonance, which opens up the possibility of rationally designing plasmonic tungsten oxide nanoparticles for light harvesting, bioimaging, and sensing.     

An ab initio potential energy surface and predissociative resonances of HArF

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF --> H + Ar + F) channel and a barrier of 1.017 eV in the dissociation (HArF --> Ar + HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.     

Breaking the mode degeneracy of surface plasmon resonances in a triangular system

In this paper, we present a systematic investigation of symmetry-breaking in the plasmonic modes of triangular gold nanoprisms. Their geometrical C(3v) symmetry is one of the simplest possible that allows degeneracy in the particle's mode spectrum. It is reduced to the nondegenerate symmetries C(v) or E by positioning additional, smaller gold nanoprisms in close proximity, either in a lateral or a vertical configuration. Corresponding to the lower symmetry of the system, its eigenmodes also feature lower symmetries (C(v)), or preserve only the identity (E) as symmetry. We discuss how breaking the symmetry of the plasmonic system not only breaks the degeneracy of some lower order modes, but also how it alters the damping and eigenenergies of the observed Fano-type resonances.     

Water induced hydrophobic surface

A polyurethane coating is described that has hydrophilic wetting behavior when dry and hydrophobic when wet. A difference of approximately 25 degrees in advancing contact angles for dry (83 degrees ) and wet (108 degrees ) states is found by sessile drop and dynamic methods. The term "contraphilic" is suggested for this reversible change opposite customary amphiphilic behavior. Contraphilic behavior results from a soft block containing semifluorinated and 5,5-dimethyhydantoin segmers. Amide inter/intramolecular hydrogen bonding is proposed for the hydrophilic (dry) state, while surface-confined, amide-water hydrogen bonding "releases"semifluorinated groups, giving the hydrophobic state. Water-induced hydrophobic surfaces may lead to applications for easily switched wetting, such as in microfluidics.     

Interference of surface plasmon resonances causes enhanced depolarized light scattering from metal nanoparticles

We show that the strongly depolarized light scattering from noble metal particles is a result of interference of two surface plasmon resonances on the same particle. The maximum depolarization occurs between two resonances. Under favorable conditions the anisotropy of the scattering light can be much lower than what is possible for dielectric particles. This explanation is discussed in relation to earlier published experimental measurements. Comparison of experimental results with theoretical calculations provides information on the shape distribution of metallic particles in the suspension.     

Plasmon resonances and plasmon‐induced charge transport in linear atomic chains

In linear hydrogen atomic chains, plasmon resonances and plasmon‐induced charge transport are studied by time‐dependent density functional theory. For the large linear chain, it is a general phenomenon that, in the longitudinal excitation, there are high‐energy resonances and a large low‐energy resonance. The energy of the large low‐energy resonance conforms to the results calculated by the classical Drude model. In order to explain the formation mechanism of the high‐energy resonances, we present a simple harmonic oscillator model. This model may reasonably account for the relationship between low‐energy and high‐energy resonances, and has a certain degree of universality. As the interatomic distance decreases, the current shows a gradual transition from insulator to metal. The current enhancement mainly depends on the local field enhancement associated with plasmon excitation, and the enhanced electron delocalization effect as a result of the decrease of the interatomic distance. © 2013 Wiley Periodicals, Inc.     

Solvent shifts induced by dimethylsulphoxide in α-proton magnetic resonances of carbonyl compounds

The chemical shifts of α- and β-proton resonances of a number of carbonyl compounds in carbon tetrachloride and dimethylsulphoxide have been determined. The magnitude of the effect of the solvent change on the α-proton resonances seems to vary directly with its lability.     

Focusing surface acoustic waves assisted electrochemical detector in microfluidics

This article demonstrates a novel electrochemical detection device. The device is composed by two focusing interdigital transducers for exciting focused surface acoustic waves by applying an AC signal, a three-electrode system for electrochemical measurement, and a liquid pool for holding liquid on a LiNbO₃ wafer. The amperometry current of ferrocenecarboxylic acid and potassium phosphate buffer solution is used to characterize the detection sensitivity. Two experiments are carried out to optimize the device design. The result shows that the two focusing interdigital transducers with arc degree 30° and distance 5 mm can remarkably enhance the liquid mixing rate. Under this condition, the oxidation current is about 27 times larger than that without surface acoustic wave stirring.     

Transportation and mixing of droplets by surface acoustic wave

Unit operations for complicated biochemical analysis cannot usually be integrated into one substrate. A possible solution to solve this problem is to integrate multi-unit operations into two or more substrates. In this case, transporting droplets from one substrate to another is essential. In this work, a new method to transport droplets from a hydrophobic glass substrate to a piezoelectric substrate is proposed. An interdigitated transducer (IDT) and reflectors were fabricated on an optic grade 128° YX-cut lithium niobate (LiNbO₃) substrate, and its working surface between the IDT and a reflector was modified to be hydrophobic. Droplets to be transported were first pipetted onto a glass substrate. Adjust the glass substrate so that the droplets could contact the working surface of the piezoelectric substrate, and then was moved down. These droplets could be successfully transported from the glass surface to the piezoelectric substrate because of their “adhesion work” difference. By using this mechanism, water and red dye droplets were successfully transported from glass substrate to piezoelectric substrate. As an application, droplets mixing process was demonstrated in the piezoelectric substrate by using surface acoustic wave after they have been transported from the glass substrate.     

Rapid and fine tailoring longitudinal surface plasmon resonances of gold nanorods by end-selective oxidation

Facile achievement of gold nanorods (AuNRs) with controllable longitudinal surface plasmon resonance (LSPR) is of great importance for their applications in various fields. The LSPR of AuNRs is sensitive to their aspect ratio, which is still hard to be precisely tuned by direct synthesis. In this work, we report a simple approach for end-selective etching of AuNRs by a rapid oxidation process with Au(III) in cetyltrimethylammonium bromide (CTAB) solution at a mild temperature. The LSPR wavelength and the length of AuNRs blue shifted linearly as a function of the amount of Au(III), while the diameter of AuNRs remained nearly constant. The oxidative rate is temperature dependent, and the oxidative process for a desired LSPR can be accomplished within 15 min at 60 °C. Further investigations indicated that Br^– determine the occurrence of the oxidation between AuNRs and Au(III), and a small amount of surfactant chain (CTA⁺) is crucial for stabilizing AuNRs. This method presents a quick but robust strategy for acquiring AuNRs with an arbitrary intermediate LSPR wavelength using the same starting AuNRs, and can be a powerful tool for subsequent applications.     

Uniform mixing in paper-based microfluidic systems using surface acoustic waves

Paper-based microfluidics has recently received considerable interest due to their ease and low cost, making them extremely attractive as point-of-care diagnostic devices. The incorporation of basic fluid actuation and manipulation schemes on paper substrates, however, afford the possibility to extend the functionality of this simple technology to a much wider range of typical lab-on-a-chip operations, given its considerable advantages in terms of cost, size and integrability over conventional microfluidic substrates. We present a convective actuation mechanism in a simple paper-based microfluidic device using surface acoustic waves to drive mixing. Employing a Y-channel structure patterned onto paper, the mixing induced by the 30 MHz acoustic waves is shown to be consistent and rapid, overcoming several limitations associated with its capillary-driven passive mixing counterpart wherein irreproducibilities and nonuniformities are often encountered in the mixing along the channel--capillary-driven passive mixing offers only poor control, is strongly dependent on the paper's texture and fibre alignment, and permits backflow, all due to the scale of the fibres being significant in comparison to the length scales of the features in a microfluidic system. Using a novel hue-based colourimetric technique, the mixing speed and efficiency is compared between the two methods, and used to assess the effects of changing the input power, channel tortuousity and fibre/flow alignment for the acoustically-driven mixing. The hue-based technique offers several advantages over grayscale pixel intensity analysis techniques in facilitating quantification without limitations on the colour contrast of the samples, and can be used, for example, for quantification in on-chip immunochromatographic assays.     

Water sorption isotherms on aminopropyltriethoxysilane coated surface acoustic wave sensors

Water sorption isotherms were obtained on surface acoustic wave sensors (SAWS) coated with aminopropyltriethyoxysilane (APTES), and on uncoated SAWS of which the substrate material was polished ST-quartz. The isotherms were obtained at 25 degrees , 30 degrees and 40 degrees over the range 1-80% relative humidity (RH). The isotherms exhibit BDDT type III characteristics typical of weak gas-solid interaction. The isotherms showed good fit to quadratic equations relating frequency change on exposure to humid air with relative humidity. There was no significant hysteresis in the isotherms when the SAWS was taken through a cycle of relative humidity at any of the three temperatures employed. These results are similar to those obtained in earlier work on FPOL and polyvinylpyrollidone coated SAWS. They demonstrate that a correction algorithm based on a quadratic equation should be possible to overcome water vapour response of coated SAWS.     

Integrated immunoassay using tuneable surface acoustic waves and lensfree detection

The diagnosis of infectious diseases in the Developing World is technologically challenging requiring complex biological assays with a high analytical performance, at minimal cost. By using an opto-acoustic immunoassay technology, integrating components commonly used in mobile phone technologies, including surface acoustic wave (SAW) transducers to provide pressure driven flow and a CMOS camera to enable lensfree detection technique, we demonstrate the potential to produce such an assay. To achieve this, antibody functionalised microparticles were manipulated on a low-cost disposable cartridge using the surface acoustic waves and were then detected optically. Our results show that the biomarker, interferon-γ, used for the diagnosis of diseases such as latent tuberculosis, can be detected at pM concentrations, within a few minutes (giving high sensitivity at a minimal cost).     

Synthesis of Au@Ag core-shell nanocubes containing varying shaped cores and their localized surface plasmon resonances

We have synthesized Au@Ag core-shell nanocubes containing Au cores with varying shapes and sizes through modified seed-mediated methods. Bromide ions are found to be crucial in the epitaxial growth of Ag atoms onto Au cores and in the formation of the shell's cubic shape. The Au@Ag core-shell nanocubes exhibit very abundant and distinct localized surface plasmon resonance (LSPR) properties, which are core-shape and size-dependent. With the help of theoretical calculation, the physical origin and the resonance mode profile of each LSPR peak are identified and studied. The core-shell nanocrystals with varying shaped cores offer a new rich category for LSPR control through the plasmonic coupling effect between core and shell materials.