Fibre laser machining for glassy carbon master mould and soft lithography based two-step printing for Ag nanoparticle structures

Traditional manufacturing techniques widely used in semiconductor industries involve many processing steps that consume both time and material and lead to high cost. Soft Lithography (SL) offers a new way to print micro/nano structures, which is a fast and low cost alternative to the conventional route, although the high processing temperature of metals, semiconductors and ceramics limits the application SL techniques. In this paper we report the use of Ag nanoparticles as building blocks to make structures by combing the merits of SL, nanotechnology and laser engineering, which provide a simple additive route with low capital investment. Glassy carbon (GC) was chosen as the material for the rigid master mould, as no release coating is needed for replicating the polydimethylsiloxane (PDMS) mould. GC moulds were machined by a nanosecond-pulsed Yb fibre laser. The machined GC moulds were further cleaned by PDMS and the same fibre laser system to remove the process debris. The master mould was further replicated by PDMS. PDMS replicas with either positive or negative features from the master mould were attainable. A two-step strategy was used to print patterns using PDMS mould and Ag nanoparticle paste. Metal patterns were formed on various substrates, and the PDMS mould was left clean and ready for reuse. The resultant printed patterns were found to be uniform over millimetre range, with negligible residual layer, and the thickness of up to several micrometres. The thermal responses of Ag nanoparticles at various sintering temperatures were investigated. The factors affecting the resolution of printed structures were discussed.     

SERS‐based detection of barcoded gold nanoparticle assemblies from within animal tissue

We have explored the potential of deep Raman spectroscopy, specifically surface‐enhanced spatially offset Raman spectroscopy (SESORS), for non‐invasive detection from within animal tissue, by employing SERS‐barcoded nanoparticle (NP) assemblies as the diagnostic agent. This concept has been experimentally verified in a clinically‐relevant backscattered Raman system with an excitation line of 785 nm under ex vivo conditions. We have shown that our SORS system, with a fixed offset of 2–3 mm, offered sensitive probing of injected 2‐quinolinethiol‐barcoded NP assemblies through animal tissue containing both protein and lipid. In comparison with that of non‐aggregated SERS‐barcoded gold NPs, we have demonstrated that the tailored SERS‐barcoded aggregated NP assemblies have significantly higher detection sensitivity. We report that these NP assemblies can be readily detected at depths of 7–8 mm from within animal proteinaceous tissue with high signal‐to‐noise ratio. In addition, they could also be detected from beneath 1–2 mm of animal tissue with high lipid content, which generally poses a challenge because of high absorption of lipids in the near‐infrared region. We have also shown that the signal intensity and signal‐to‐noise ratio at a particular depth is a function of the SERS tag concentration used and that our SORS system has a 2‐quinolinethiol detection limit of 10⁻⁶ M. Higher detection depths may possibly be obtained with optimization of the NP assemblies, along with improvements in the instrumentation. Such NP assemblies offer prospects for in vivo, non‐invasive detection of tumours along with scope for incorporation of drugs and their targeted and controlled release at tumour sites. These diagnostic agents combined with drug delivery systems could serve as a ‘theranostic agent’, an integration of diagnostics and therapeutics into a single platform. Copyright © 2013 John Wiley & Sons, Ltd.     

Hydrodynamic cavitation: A feasible approach to intensify the emulsion cross-linking process for chitosan nanoparticle synthesis

Chitosan nanoparticles (NPs) exhibit great potential in drug-controlled release systems. A controlled hydrodynamic cavitation (HC) technique was developed to intensify the emulsion crosslinking process for the synthesis of chitosan NPs. Experiments were performed using a circular venturi and under varying operating conditions, i.e., types of oil, addition mode of glutaraldehyde (Glu) solution, inlet pressure (P_in), and rheological properties of chitosan solution. Palm oil was more appropriate for use as the oil phase for the HC-intensified process than the other oil types. The addition mode of water-in-oil (W/O) emulsion containing Glu (with Span 80) was more favorable than the other modes for obtaining a narrow distribution of chitosan NPs. The minimum size of NPs with polydispersity index of 0.342 was 286.5 nm, and the maximum production yield (P_y) could reach 47.26%. A positive correlation was found between the size of NPs and the droplet size of W/O emulsion containing chitosan at increasing P_in. Particle size, size distribution, and the formation of NPs were greatly dependent on the rheological properties of the chitosan solution. Fourier transform infrared spectroscopy (FTIR) analysis indicated that the molecular structure of palm oil was unaffected by HC-induced effects. Compared with ultrasonic horn, stirring-based, and conventional drop-by-drop processes, the application of HC to intensify the emulsion crosslinking process allowed the preparation of a finer and a narrower distribution of chitosan NPs in a more energy-efficient manner. The novel route developed in this work is a viable option for chitosan NP synthesis.     

Designable formation of metal nanoparticle array with the deposition of negatively charged nanoparticles

Ultrafine gold and platinum nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas and the NPs 2-15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). The spherical NPs obtained showed a narrower distribution of diameters of anionic NPs over cationic NPs. With this knowledge, the anionic NPs were deposited onto silicon substrates designed by electron beam lithography processing, and designable patterns of arrayed NPs were produced by removing the photoresist layer.     

Understanding the mechanism of aluminium nanoparticle oxidation

Aluminium nanoparticles have gained importance in the last decade because of their increased reactivity as compared with traditional micron-sized particle. The physics of burning of aluminium nanoparticle is expected to be different than that of micron-sized particles, and the current article is motivated by these differences. We have previously measured the size resolved reactivity of nanoaluminium by single-particle mass spectrometry, to which we now add transmission electron microscope (TEM) and an on-line density measurement. The latter two studies revealed the presence of hollow particles following oxidation of nanoaluminium and indicating the significance of diffusion of aluminium in the overall process. Based on experimental evidence, we believe that aluminium nanoparticle oxidation occurs in two regimes. Prior to melting of aluminium slow oxidation occurs through the diffusion of oxygen through the aluminium oxide shell. Above the melting point, we transition to a fast oxidation regime whereby both aluminium and oxygen diffuse through the oxide shell to enhance the oxidation rate.

We also develop a phenomenological model for nanoaluminium oxidation that accounts for the experimentally observed rates, the fact that both fuel and oxidizer are diffusing, and a new effect related to internal pressure gradients. The latter phenomen is based on molecular dynamic simulations suggesting that there are large pressure gradients present inside these particles, with the aluminium core under a positive pressure and the aluminium oxide shell under a negative pressure. We have considered the effect of these pressure gradients on the oxidation process. A power law relation was obtained (t ∝ r ^{1.6± 0.1}) between the time required for oxidation and particle radius.     

Plasma-activated core-shell gold nanoparticle films with enhanced catalytic properties

Catalytically active gold nanoparticle films have been prepared from core-shell nanoparticles by plasma-activation and characterized by high-resolution transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Methane can be selectively oxidized into formic acid with an O₂–H₂ mixture in a catalytic wall reactor functionalized with plasma-activated gold nanoparticle films containing well-defined Au particles of about 3.5 nm in diameter. No catalytic activity was recorded over gold nanoparticle films prepared by thermal decomposition of core-shell nanoparticles due to particle agglomeration.     

A Monte Carlo and continuum study of mechanical properties of nanoparticle based films

A combination Monte Carlo and equivalent-continuum simulation approach was used to investigate the structure-mechanical property relationships of titania nanoparticle deposits. Films of titania composed of nanoparticle aggregates were simulated using a Monte Carlo approach with diffusion-limited aggregation. Each aggregate in the simulation is fractal-like and random in structure. In the film structure, it is assumed that bond strength is a function of distance with two limiting values for the bond strengths: one representing the strong chemical bond between the particles at closest proximity in the aggregate and the other representing the weak van der Waals bond between particles from different aggregates. The Young’s modulus of the film is estimated using an equivalent-continuum modeling approach, and the influences of particle diameter (5–100 nm) and aggregate size (3–400 particles per aggregate) on predicted Young’s modulus are investigated. The Young’s modulus is observed to increase with a decrease in primary particle size and is independent of the size of the aggregates deposited. Decreasing porosity resulted in an increase in Young’s modulus as expected from results reported previously in the literature.     

Modeling of X-ray excited luminescence intensity dependence on the nanoparticle size

The thermalization length distribution of electrons over their kinetic energy in a conduction band is calculated on the basis of the data on the electron effective mass, density of states in conduction band, dielectric permittivity and energy of longitudinal optical phonons. The method of modeling of a recombinational luminescence intensity dependence on the nanoparticle size is proposed on the basis of the assumption that the contribution to a recombinational luminescence gives only those charge carriers which in the result of thermalization did not reach a near-surface layer of nanoparticles. Using such the approach the theoretical dependence of recombinational luminescence intensity on the nanoparticle size for LaPO₄ and LuPO₄ are calculated. The revealed correlation of experimental and theoretical dependences confirms that the commensurability of electron thermalization length with nanoparticle size is the main reason of the sharp decrease of X-ray excited luminescence intensity when the nanoparticle size decreases.     

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm·m for CH4 and 15 ppm·m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1 - 2 km. The sensor is suitable for natural gas leakage monitoring application.     

Novel photonic technique creates micrometer resolution protein arrays and provides a new approach to coupling of genes, peptide hormones and drugs to nanoparticle carriers

We demonstrate that ultraviolet light can be used to make sterically oriented covalent immobilization of a large variety of protein molecules onto either thiolated quartz, gold or silicon. The reaction mechanism behind the reported new technology involves light-induced breakage of disulphide bridges in proteins upon UV illumination of nearby aromatic amino acids, resulting in the formation of free, reactive thiol groups that will form covalent bonds with thiol reactive surfaces. In general, the protein molecules retain their function. The size of the immobilization spot is limited to the focal point of illumination being as small as a few micrometers. This new technology allows for dense packing of different bio-molecules on a surface, allowing the creation of multi-potent functionalised new materials, such as nano-biosensors. We have developed the necessary technology for preparing large protein arrays of enzymes and fragments of monoclonal antibodies. Dedicated image processing software has been developed for making quality assessment of the protein arrays. This novel technology is ideal to couple drugs and other bio-molecules to nanoparticles which can be used as carriers into cells for therapeutic purposes.     

An approach to gas sensors based on tunable diode laser incomplete saturated absorption spectra

A spectral profile reconstruction method that can be applied to incomplete saturated-absorption spectra is proposed and demonstrated. Through simulation and theoretical calculation, it is proved that compared with the traditional wholeprofile fitting method, this new method can increase the concentration detection upper limit of a single absorption line by about 8.7 times. High-concentration water vapor is measured using TDLAS technology, the total water vapor pressure and the self-broadened half-width coefficient of the spectrum were simultaneously measured from incomplete saturatedabsorption spectra and compared with high-precision pressure sensors and the HITRAN databases. Their maximum relative deviations were about 4.63% and 9.10%, respectively. These results show that the spectral profile reconstruction method has great application potential for expanding the dynamic range of single-line measurements to higher concentrations,especially for in-situ online measurements under complex conditions, such as over large temperature and concentration dynamic ranges.     

Probing Ag nanoparticle surface oxidation in contact with (in)organics: an X‐ray scattering and fluorescence yield approach

Characterizing interfacial reactions is a crucial part of understanding the behavior of nanoparticles in nature and for unlocking their functional potential. Here, an advanced nanostructure characterization approach to study the corrosion processes of silver nanoparticles (Ag‐Nps), currently the most highly produced nanoparticle for nanotechnology, is presented. Corrosion of Ag‐Nps under aqueous conditions, in particular in the presence of organic matter and halide species common to many natural environments, is of particular importance because the release of toxic Ag⁺ from oxidation/dissolution of Ag‐Nps may strongly impact ecosystems. In this context, Ag‐Nps capped with polyvinolpyrrolidone (PVP) in contact with a simple proxy of organic matter in natural waters [polyacrylic acid (PAA) and Cl^? in solution] has been investigated. A combination of synchrotron‐based X‐ray standing‐wave fluorescence yield‐ and X‐ray diffraction‐based experiments on a sample consisting of an approximately single‐particle layer of Ag‐Nps deposited on a silicon substrate and coated by a thin film of PAA containing Cl revealed the formation of a stable AgCl corrosion product despite the presence of potential surface stabilizers (PVP and PAA). Diffusion and precipitation processes at the Ag‐Nps–PAA interface were characterized with a high spatial resolution using this new approach.     

Droplet printing through bubble contact in the laser forward transfer of liquids

The deposition process of the laser-induced forward transfer of liquids at high laser fluences is analyzed through time-resolved imaging. It has been found that, at these conditions, sessile droplets are deposited due to the contact of a generated cavitation bubble with the receptor substrate, in contrast to the jet contact mechanism observed at low and moderate laser fluences. The bubble contact results in droplets with a larger diameter, a smaller contact angle and a lower uniformity than those of the jet mechanism. Therefore, in order to attain a high degree of resolution this mechanism should be prevented.     

Monte Carlo simulation of the compensation and critical behaviors of a ferrimagnetic core/shell nanoparticle Ising model

A Monte Carlo simulation has been used to study the magnetic properties and the critical behaviors of a single spherical nanoparticle, consisting of a ferromagnetic core of spins surrounded by a ferromagnetic shell of S=±1, 0 or , spins with antiferromagnetic interface coupling, located on a simple cubic lattice. A number of characteristic phenomena has been found. In particular, the effects of the shell coupling and the interface coupling on both the critical and compensation temperatures are investigated. We have found that, for appropriate values of the system parameters, two compensation temperatures may occur in the present system.     

Near-field calculation based on the T-matrix method with discrete sources

We propose a novel approach to compute the field scattered by a particle in the near-zone, in the framework of the transition matrix formulation. This method is based on the expansion of the total near-field in terms of discrete sources vector spherical wave functions and turns out to be particularly effective to model strongly elongated or flattened particles with high permittivity. The performances are evaluated on gold spheroids with several aspect ratios. This method results very useful to understand the complex behavior of enhanced plasmon fields in resonant metallic nanostructures.     

Rb Bose-Einstein condensate with tunable interaction: A quantum many body approach

We present a quantum many body approach with van der Waal type of interaction to achieve ⁸⁵Rb Bose-Einstein condensate with tunable interaction which has been produced by magnetic field induced Feshbach resonance in the JILA experiment.     

Turbulence modulation model for gas–particle flow based on probability density function approach

The paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function(PDF) approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data.     

The experimental determination of the qPs contact density in matter

The electron density at the positron is a relevant quantity for characterizing a positron-electron correlated pair in matter, since both the intrinsic decay rates in triplet and singlet state and the fine structure interval depend on it. Information about this quantity is generally obtained through magnetic quenching experiments. In the present work we discuss other techniques, based on the lifetime as well as on three-gamma spectroscopy, which do not involve the Zeeman effect in positronium.