Laser-induced force for bubble-trapping in liquids

The laser-induced thermal force that traps a bubble in an absorbing liquid is studied theoretically and experimentally. The force origins from the thermal gradients of the surface tension induced by a laser beam over the bubble surface. It is shown that this force is proportional to the square of the bubble radius and to the thermal gradient of the surface tension. A simple experimental method based on comparison with the Archimedes' flotation force, is used for the measurement of this force. The experimental results, performed on ethanol solutions of iodine, confirm the theoretical predictions.     

Laser ablation of gadolinium targets in liquids for nanoparticle preparation

Tight focusing of a sub-picosecond laser pulse in a transparent dielectric provides a mean for localized deposition and plasma formation. A micro-explosion in a confined geometry results in a sub-micron cavity formation. Our numerical simulations show the cavity size is strongly dependent on the parameters of the equation of state such as the Grüneisen coefficient or the latent heat of sublimation. A comparison of numerical simulations with experimental data should allow a tuning of equations of state in the domain of extreme parameters     

Foam droplet separation for nanoparticle synthesis

A novel approach to nanoparticle synthesis was developed whereby foam bubble bursting produced aerosol droplets, an approach patterned after the marine foam aerosol cycle. The droplets were dried to remove solvent, leaving nanometer-sized particles composed of precursor material. Nanoparticles composed of sodium chloride (mean diameter, 100 nm), phosphotungstic acid ( 55 nm), and bovine insulin ( 5–30 nm) were synthesized. Foam droplet separation can be carried out at ambient temperature and pressure. The ‘soft’ nature of the process makes it compatible with a wide range of materials.     

Laser-induced breakdown spectroscopy: A tool for analysis of different types of liquids

Laser-induced breakdown spectroscopy (LIBS) was applied to detect elements in different liquids (water and oil). Spectra of plasmas generated by focusing the second harmonic 5=532 nm of a pulsed Nd:YAG laser on the liquids' surfaces at atmospheric pressure were recorded. Calibration curves for chromium in water and in oil were obtained, and detection limits were deduced.     

Laser-induced synthesis of compound semiconducting films

Compound semiconducting films are synthesized by laser irradiation of sandwich films of the constituants, under various conditions (time of irradiation, laser power, temperature, etc.). Experimental data give evidence of the roles of various parameters. After a presentation of the experimental results, the fundamental physico-chemical mechanisms of laser material interactions are analyzed. Particular emphasis is given to effects that need to be treated by using very-far-from-equilibrium concepts.     

液相等离子体及其在纳米材料制备中的应用

液相等离子体是冷等离子体的一个新分支,具有温度低、传质传热快、常压操作、反应活性高等特点。基于液相等离子体的过程强化技术在纳米材料制备、挥发性有机物降解、杀菌消毒、化学合成等领域有广泛的应用前景。以液相等离子体中纳米材料的制备为研究对象,介绍了反应体系可能存在的活性粒子、检测方法和反应机理;对常见的反应器结构进行归纳整理,按照放电是否在电解液内部进行将其分为非浸没式和浸没式液相等离子体两大类,并列举了几种典型的反应器结构;介绍了几类利用液相等离子体技术制备纳米材料的典例,并对该领域的研究现状做了总结;对该领域亟需解决的问题与发展方向进行讨论与展望。     

Microwave-assisted synthesis using ionic liquids

The research and application of green chemistry principles have led to the development of cleaner processes. In this sense, during the present century an ever-growing number of studies have been published describing the use of ionic liquids (ILs) as solvents, catalysts, or templates to develop more environmentally friendly and efficient chemical transformations for their use in both academia and industry. The conjugation of ILs and microwave irradiation as a non-conventional heating source has shown evident advantages when compared to conventional synthetic procedures for the generation of fast, efficient, and environmental friendly synthetic methodologies. This review focuses on the advances in the use of ILs in organic, polymers and materials syntheses under MW irradiation conditions.     

Laser induced synthesis of nanoparticles in liquids

The review of results on nanoparticles formation is presented under laser ablation of Ag, Au, and Cu-containing solid targets in liquid environments (H₂O, C₂H₅OH, C₂H₄Cl₂, etc.). X-ray diffractometry (XRD), UV-vis optical transmission spectrometry, and high resolution transmission electron microscopy (HRTEM) characterize the nanoparticles. The morphology of nanoparticles is studied as the function of both laser fluence and nature of the liquid. The possibility to control the shape of nanoparticles by ablation of an Au target by an interference pattern of two laser beams is demonstrated. Formation of alloyed Au-Ag and Ag-Cu nanoparticles is reported under laser exposure of a mixture of individual nanoparticles. The effect of internal segregation of brass nanoparticles is discussed due to their small lateral dimensions. The factors are discussed that determine the distribution function of particles size under laser ablation. The influence of laser parameters as well as the nature on the liquid on the properties of nanoparticles is elucidated.     

Impact and structure of literature on nanoparticle generation by laser ablation in liquids

The number of publications on laser ablation and nanoparticle generation in liquids increased by the factor of 15 in the last decade, with comparable high impact of the most cited articles in this field. A nearly unlimited variety of nanoparticle material, liquid matrix, and conjugative agent can be combined to a huge variety of colloids within a few minutes of laser processing. However, this diversification makes it hard to identify main research directions without a comprehensive literature overview. This investigation evaluates the impact and structure of the literature in this field tagging most prolific subjects and articles. Using an optimized search algorithm, the data sets derived from Science Citation Index (1998–2008) allow for statements on publication subject clusters, impact of articles and journals, as well as mapping global spots of activities.     

Ferritin as a photocatalyst and scaffold for gold nanoparticle synthesis

The ferrihydrite mineral core of ferritin is a semi-conductor capable of catalyzing oxidation/reduction reactions. This report shows that ferritin can photoreduce AuCl₄ ⁻ to form gold nanoparticles (AuNPs). An important goal was to identify innocent reaction conditions that prevented formation of AuNPs unless the sample was illuminated in the presence of ferritin. TRIS buffer satisfied this requirement and produced AuNPs with spherical morphology with diameters of 5.7 ± 1.6 nm and a surface plasmon resonance (SPR) peak at 530 nm. Size-exclusion chromatography of the AuNP–ferritin reaction mixture produced two fractions containing both ferritin and AuNPs. TEM analysis of the fraction close to where native ferritin normally elutes showed that AuNPs form inside ferritin. The other peak eluted at a volume indicating a particle size much larger than ferritin. TEM analysis revealed AuNPs adjacent to ferritin molecules suggesting that a dimeric ferritin–AuNP species forms. We propose that the ferritin protein shell acts as a nucleation site for AuNP formation leading to the AuNP-ferritin dimeric species. Ferrihydrite nanoparticles (~10 nm diameter) were unable to produce soluble AuNPs under identical conditions unless apo ferritin was present indicating that the ferritin protein shell was essential for stabilizing AuNPs in aqueous solution.     

Laser-induced synthesis of silver nanoparticles and their conjugation with protein

Partially oxidized spherical silver nanoparticles (AgNPs) of different size are prepared by pulsed laser ablation in water and directly conjugated to protein S-ovalbumin for the first time and characterized by various optical techniques. UV–Visible spectrum of AgNPs showed localized surface plasmon resonance (LSPR) peak at 396 nm which red shift after protein addition. Further the increased concentration of AgNPs resulted a decrease in intensity and broadening of S-ovalbumin peak (278 nm), which can be related to the formation of protein NPs complex caused by the partial adsorption of S-ovalbumin on the surface of AgNPs. The red shift in LSPR peak of AgNPs after mixing with S-ovalbumin and decrease in protein-characteristic peak with increased silver loading confirmed the formation of protein–AgNPs bioconjugates. The effect of laser fluence on the size of AgNPs and nanoparticle–protein conjugation in the size range 5–38 nm is systematically studied. Raman spectra reveal broken disulphide bonds in the conjugated protein and formation of Ag–S bonds on the nanoparticle surface. Fluorescence spectroscopy showed quenching in fluorescence emission intensity of tryptophan residue of S-ovalbumin due to energy transfer from tryptophan moieties of albumin to AgNPs. Besides this, small blue shift in emission peak is also noticed in presence of AgNPs, which might be due to complex formation between protein and nanoparticles. The binding constant (K) and the number of binding sites (n) between AgNPs and S-ovalbumin have been found to be 0.006 M^?1 and 7.11, respectively.     

Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors

Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)₅) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, ‘safe’ operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose.     

Laser-induced hybrid trap for micro-bubbles

Micro-bubbles have been stably trapped in liquid ethanol by a focused argon laser beam. Two equilibrium points where bubbles can be trapped are observed to be at the center and the rim of the laser beam. The light force is also able to push a bubble into the liquid to a position well below the liquid surface. Both the light-pressure force and the fluid force induced by the convection of the liquid medium are calculated. The results clearly show that these two kinds of forces account for the formation of trapping potential. Received: 3 February 2000 / Revised version: 6 July 2000 / Published online: 8 November 2000     

Laser-induced autofluorescence for medical diagnosis

The naturally occurring autofluorescence of cells and tissues is based on biomolecules containing intrinsic fluorophores, such as porphyrins, the amino acids tryptophan and tyrosine, and the coenzymes NADH, NADPH, and flavins. Coenzymes fluoresce in the blue/green spectral region (fluorecence lifetimes: 0.5–6 ns) and are highly sensitive indicators of metabolic function. Steadystate and time-resolved blue-green autofluorescence is, therefore, an appropriate measure of the function of the respiratory chain as well as of cellular and tissue damage. Autofluorescence in the yellow/red spectral region is based mainly on endogenous porphyrins and metalloporphyrins, such as coproporphyrin, protoporphyrin (fluorescence lifetime of porphyrin monomers: >10 ns), and Zn-protoporphyrin (2 ns). Various pathological microorganisms such asPropionibacterium acnes, Pseudomonas aeruginosa, Actinomyces odontolyticus, Bacteroides intermedius, andSaccharomyces cerevisiae are able to synthesize large amounts of these fluorophores and can therefore be located. This permits fluorescence-based detection of a variety of diseases, including early-stage dental caries, dental plaque, acne vulgaris, otitis externa, and squamous cell carcinoma. The sensitivity of noninvasive autofluorescence diagnostics can be enhanced by time-gated fluorescence measurements using an appropriate time delay between ultrashort laser excitation and detection. For example, videocameras with ultrafast shutters, in the nanosecond region, can be used to create caries images of the teeth. Alternatively, autofluorescence can be enhanced by stimulating protoporphyrin biosynthesis with the exogenously administered porphyrin precursor 5-aminolevulinic acid (ALA). The fluorophore protoporphyrin IX (PP IX) is photolabile and photodynamically active. Irradiation of PP IX-containing tissue results in cytotoxic reactions which correlate with modifications in fluorescence due to photobleaching and singlet oxygen-dependent photoproduct formation. Therefore, on-line autofluorescence measurements during the phototreatment can yield information on the efficiency of ALA-based photodynamic therapy.     

Barriers for transport in turbulent plasmas

The control of transport due to electrostatic turbulence is investigated using test-particle simulations. We show that a barrier for the transport, that is, a region where transport is reduced, can be generated through the randomization of phases of the turbulent field. This corresponds to the annihilation of coherent structures which are present at all scales, without actually suppressing turbulence. When the barrier is active, a flux of particles towards the center of the simulation box is present inside the region where the barrier is located.     

Iron oxide nanoparticle synthesis in aqueous and membrane systems for oxidative degradation of trichloroethylene from water

The potential for using hydroxyl radical (OH^?) reactions catalyzed by iron oxide nanoparticles (NPs) to remediate toxic organic compounds was investigated. Iron oxide NPs were synthesized by controlled oxidation of iron NPs prior to their use for contaminant oxidation (by H₂O₂ addition) at near-neutral pH values. Cross-linked polyacrylic acid (PAA) functionalized polyvinylidene fluoride (PVDF) microfiltration membranes were prepared by in situ polymerization of acrylic acid inside the membrane pores. Iron and iron oxide NPs (80–100 nm) were directly synthesized in the polymer matrix of PAA/PVDF membranes, which prevented the agglomeration of particles and controlled the particle size. The conversion of iron to iron oxide in aqueous solution with air oxidation was studied based on X-ray diffraction, Mössbauer spectroscopy and BET surface area test methods. Trichloroethylene (TCE) was selected as the model contaminant because of its environmental importance. Degradations of TCE and H₂O₂ by NP surface generated OH^? were investigated. Depending on the ratio of iron and H₂O₂, TCE conversions as high as 100 % (with about 91 % dechlorination) were obtained. TCE dechlorination was also achieved in real groundwater samples with the reactive membranes.     

Control of nanoparticle size and agglomeration through electric-field-enhanced flame synthesis

The isolated study of electrophoretic transport of nanoparticles (that are innately charged through thermionic emission), with no ionic wind, has been conducted under uniform electric fields. Titania nanoparticles are produced using a burner-supported low-pressure premixed flame in a stagnation-point geometry from corresponding organometallic vapor precursor. The material processing flow field is probed in-situ using laser-induced fluorescence (LIF) to map OH-radical concentrations and gas-phase temperatures. The experimental results of particle growth under different applied electric fields are compared with computations using monodisperse and sectional models. The results show that such electric field application can decrease aggregate particle size (e.g. from 40 to 18 nm), maintain metastable phases and particle crystallinity, and non-monotonically affect primary particle size (e.g. from 6 to 5 nm) and powder surface area. A specific surface area (SSA) for anatase titania nanopowder of 310 m²/g, when synthesized under an applied electric field of 125 V/cm, is reported. Results are also given for the synthesis of alumina nanoparticles.     

Scalable synthesis of palladium nanoparticle catalysts by atomic layer deposition

Potential applications in drug delivery from nanostructures composed of two oppositely charged polymethacrylates, eudragit^? L100 (EL) and eudragit^? EPO (EE), loaded with three model basic drugs (D), atenolol, propranolol, and metroclopramide were evaluated. The self-organized nanoparticles based on drug-interpolyelectrolyte complexes (DIPEC), (EL-D₅₀)?CEE_X, were obtained by mixing the aqueous dispersions of both polyelectrolytes at room temperature in an ultrasound bath. Dispersions of (EL-D₅₀) neutralized with increasing proportions of EE exhibited a rise of turbidity, particle sizes in the range of 150?C400?nm, and high negative zeta potential. The sign of zeta potential was shifted from negative to positive by changes in composition of DIPEC. Freeze dried DIPEC were easily redispersed in water yielding nearly the same parameters of fresh dispersions. In vitro release experiments using Franz cells showed that DIPEC systems behave as a drug reservoir that slowly releases the drug as water is placed in the receptor compartment. The release rate was raised by ionic exchange with counterions present in simulated physiological fluids placed in the receptor media. Delivery of D from DIPEC exhibited a remarkable robustness toward simulated physiological media of different pH. The DIPEC systems exhibit interesting properties to design nanoparticulate drug delivery systems for oral and/or topical routes.