The characterization of acoustic cavitation bubbles - an overview

Acoustic cavitation, in simple terms, is the growth and collapse of preexisting microbubbles under the influence of an ultrasonic field in liquids. The cavitation bubbles can be characterized by the dynamics of oscillations and the maximum temperatures and pressures reached when they collapse. These aspects can be studied both experimentally and theoretically for a single bubble system. However, in a multibubble system, the formation of bubble streamers and clusters makes it difficult to characterize the cumulative properties of these bubbles. In this overview, some recently developed experimental procedures for the characterization of acoustic cavitation bubbles have been discussed.     

Effects of ultrasound on the thermal and structural characteristics of proteins in reconstituted whey protein concentrate

The sonication-induced changes in the structural and thermal properties of proteins in reconstituted whey protein concentrate (WPC) solutions were examined. Differential scanning calorimetry, UV-vis, fluorescence and circular dichroism spectroscopic techniques were used to determine the thermal properties of proteins, measure thiol groups and monitor changes to protein hydrophobicity and secondary structure, respectively. The enthalpy of denaturation decreased when WPC solutions were sonicated for up to 5 min. Prolonged sonication increased the enthalpy of denaturation due to protein aggregation. Sonication did not alter the thiol content but resulted in minor changes to the secondary structure and hydrophobicity of the protein. Overall, the sonication process had little effect on the structure of proteins in WPC solutions which is critical to preserving functional properties during the ultrasonic processing of whey protein based dairy products.     

The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions

In this study, it was found that a hybrid technique, sonophotocatalysis, is able to degrade a parent organic pollutant (methyl orange) as well as its by-products. The analysis of products formed during the whole degradation has demonstrated that the pH or the selection of oxidation process (sonolysis/photocatalysis/sonophotocatalysis) is able to control the degradation pathway. It was shown in the by-products analysis that the solution pH can alter the amount of each product generated during the sonophotocatalytic degradation. It was revealed that the different degradation rates of methyl orange and its products result from the solution pH and the nature of the organic products. Furthermore, a comparison of the data obtained from the oxidation processes on the degradation of the reaction intermediates identified the advantages of the combined system. It is concluded that sonophotocatalysis is capable of yielding a more complete and faster mineralization of organic pollutants than the individual processes. However, as in the degradation of the parent compound, the overall mineralization is lower than an additive effect (negative synergistic effect).     

Novel molecularly imprinted polymeric microspheres for preconcentration and preservation of polycyclic aromatic hydrocarbons from environmental samples

Molecularly imprinted polymer (MIP) microspheres with diameters in the range 60–500 μm were synthesized in a continuous segmented flow microfluidic reactor and used as packing material for microtraps for the selective separation of benzo[a]pyrene (BAP) from environmental aqueous samples. The synthesis involved the pumping of monodisperse droplets of acetonitrile containing methacrylic acid as the functional monomer, BAP as a template, and ethylene glycol dimethacrylate as the cross-linking monomer into the microchannels of the microfluidic reactor. The microspheres showed high adsorption capacity and selectivity for BAP in aqueous solutions; both are important for the environmental monitoring and analysis of BAP. The adsorption capacity for BAP of the smallest MIP microspheres (size range 60–80 μm), prepared as part of this study, was 75 mg g^-1 in aqueous solutions; furthermore, this adsorption capacity was close to 300 % higher than that of commercially used activated carbon. Microtraps packed with MIP retained BAP intact for at least 30 days, whereas microtraps packed with activated carbon for BAP showed 40 % reduction in BAP concentration for the same period. This study has demonstrated that MIP microtraps have significant potential for the selective enrichment and preservation of targeted polycyclic aromatic hydrocarbons from complex environmental samples.     

Sonochemical synthesis of graphene oxide supported Pt–Pd alloy nanocrystals as efficient electrocatalysts for methanol oxidation

Pt–Pd bimetallic nanoparticles supported on graphene oxide (GO) nanosheets were prepared by a sonochemical reduction method in the presence of polyethylene glycol as a stabilizing agent. The synthetic method allowed for a fine tuning of the particle composition without significant changes in their size and degree of aggregation. Detailed characterization of GO-supported Pt–Pd catalysts was carried out by transmission electron microscopy (TEM), AFM, XPS, and electrochemical techniques. Uniform deposition of Pt–Pd nanoparticles with an average diameter of 3 nm was achieved on graphene nanosheets using a novel dual-frequency sonication approach. GO-supported bimetallic catalyst showed significant electrocatalytic activity for methanol oxidation. The influence of different molar compositions of Pt and Pd (1:1, 2:1, and 3:1) on the methanol oxidation efficiency was also evaluated. Among the different Pt/Pd ratios, the 1:1 ratio material showed the lowest onset potential and generated the highest peak current density. The effect of catalyst loading on carbon paper (working electrode) was also studied. Increasing the catalyst loading beyond a certain amount lowered the catalytic activity due to the aggregation of metal particle-loaded GO nanosheets.     

Influence of the Morphology of Lysozyme‐Shelled Microparticles on the Cellular Association,Uptake, and Degradation in Human Breast Adenocarcinoma Cells

The ultrasound‐assisted self‐assembly and cross‐linking of lysozyme at the water–air and water–perfluorohexane interfaces are shown to produce lysozyme‐shelled hollow microbubbles (LSMBs) and microcapsules (LSMC), respectively. The arrangement of lysozyme at the air–liquid or oil–liquid interfaces is accompanied by changes in the bioactivity and conformational state of the protein. The interaction of LSMB and LSMC with human breast adenocarcinoma cells (SKBR3) is studied. LSMB and LSMC are phagocyted by cells within 2 h without exerting a cytotoxic activity. The cellular internalization kinetics of LSMB and LSMC and the effects on cell cycle are evaluated using flow cytometry. Evidence for the internalization of microparticles and degradation within the cell are also monitored by confocal and scanning electron microscopic analyses. The integrity of cell membrane and cell cycle is not affected by LSMBs and LSMCs uptake. These studies show that the positively charged LSMB and LSMC are not cytotoxic and can be readily internalized and degraded by the SKBR3 cells. LSMBs and LSMCs show a different uptake kinetics and intracellular degradation pattern due to differences in the arrangement of the protein at the air–liquid or oil–liquid interfaces.     

Cobalt(III), nickel(II) and ruthenium(II) complexes of 1,10-phenanthroline family of ligands: DNA binding and photocleavage studies

DNA binding and photocleavage characteristics of a series of mixed-ligand complexes of the type [M(phen)₂LL]ⁿ⁺ (where M = Co(III), Ni(II) or Ru(II), LL = 1,10-phenanthroline (phen), phenanthroline-dione (phen-dione) or dipyridophenazine (dppz) andn = 3 or 2) have been investigated in detail. Various physico-chemical and biochemical techniques including UV/Visible, fluorescence and viscometric titration, thermal denaturation, and differential pulse voltammetry have been employed to probe the details of DNA binding by these complexes; intrinsic binding constants (K _b) have been estimated under a similar set of experimental conditions. Analysis of the results suggests that intercalative ability of the coordinated ligands varies as dppz>phen>phen-dione in this series of complexes. While the Co(II) and Ru(II) complexes investigated in this study effect photocleavage of the supercoiled pBR 322 DNA, the corresponding Ni(II) complexes are found to be inactive under similar experimental conditions. Results of detailed investigations carried out inquiring into the mechanistic aspects of DNA photocleavage by [Co(phen)₂(dppz)]³⁺ have also been reported.     

Effect of alcohols on the initial growth of multibubble sonoluminescence

The effect of alcohols on the initial growth of the multibubble sonoluminescence (MBSL) intensity in aqueous solutions has been investigated. With increasing concentrations of the alcohols, the number of pulses required to grow the MBSL intensity to a steady state (N(crit)) increases (relative to that of water) initially to a maximum for all the alcohols used in this study, followed by a decrease for methanol and ethanol. The cause of the initial increase in N(crit) is attributed to the inhibition of bubble coalescence in the system. This inhibition in bubble coalescence results in a population of bubbles with a smaller size range and thus a larger number of pulses is required to grow the bubbles to their sonoluminescing size range. It is suggested that the decrease in the N(crit) at higher alcohol concentrations may be caused by an increase in the bubble growth by rectified diffusion.     

Sensitivity enhancement in membrane separation flow injection analysis by ultrasound

The effect of ultrasound on gas-diffusion and pervaporation flow injection separation was investigated. Ammonia and three aliphatic amines (propylamine, tri-ethylamine and di-n-butylamine) with different volatility and surface activity were used as model analytes. Under the experimental conditions used, sonication did not enhance gas-diffusion separation efficiency and resulted in up to 62% improvement in pervaporation mass transfer. Based on these findings and taking into account the surface activity of the analytes studied which decreased with their molecular mass it was postulated that ultrasound-induced surface rippling was primarily responsible for the enhanced evaporation in the donor chamber of the pervaporation cell. The results reported in this paper suggest that ultrasonic pervaporation separation could extend the applicability of this on-line flow injection separation technique to the direct determination of higher molecular mass volatile and semi-volatile analytes in 'dirty' samples.     

Sonochemical synthesis of gold nanoparticles: effects of ultrasound frequency

The rate of sonochemical reduction of Au(III) to produce Au nanoparticles in aqueous solutions containing 1-propanol has been found to be strongly dependent upon the ultrasound frequency. The size and distribution of the Au nanoparticles produced can also be correlated with the rate of Au(III) reduction, which in turn is influenced by the applied frequency. Our results suggest that the rate of Au(III) reduction as well as the size distribution of Au particles are governed by the chemical effects of cavitation and are not significantly affected by the physical effects accompanying ultrasound-induced cavitation.