A europium metal-organic framework for dual Fe3+ ion and pH sensing.

Metal-organic frameworks (MOFs) with ratiometric sensing properties are desirable for many applications due to their intrinsic self-calibration. We report the re-assessment of the sensing properties of a MOF, originally reported as containing europium(III) and 2-hydroxyterephtalic acid, and having fluorescent ratiometric iron(III) sensing properties. Synchrotron single-crystal X-ray diffraction and proton nuclear magnetic resonance (1H NMR) spectroscopy revealed that the MOF is composed of 2-methoxyterephthalate, not 2-hydroxyterephthalate as originally reported. We found that the MOF exhibits a sensor turn-off response towards Fe3+ ion concentrations in the range 0.5-3.7 ppm (band 425 nm), and a turn-on response towards a decrease of pH from 5.4 to 3.0 (band 375 nm), both resulting from the addition of acidic Fe3+ salt solution to a MOF suspension. Thus, the ratiometric sensing properties and the originally proposed mechanism no longer apply; our work reveals a dynamic quenching mechanism for the fluorescence turn-off response due to the presence of Fe3+ ions, and a ligand protonation mechanism for the turn-on response to a decrease in pH. Our work highlights the importance of a thorough investigation of the structure of any newly synthesized MOF, and, in the case of potential sensors, their selectivity and any environmental effects on their sensing behavior.

ATR FTIR Study of the Interaction of TiO2 Nanoparticle Films with ß-Lactoglobulin and Bile Salts.

The technique of in situ particle film attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) has been used to probe the adsorption and coadsorption (sequential) of a common food protein (ß-lactoglobulin, BLG) and two representative bile salts (taurocholic acid and glycocholic acid, abbreviated as TCA and GCA) onto the surface of titanium dioxide (TiO2) nanoparticles. Evaluating of binding interactions between commonly used (historically now, in some countries) food additives and food components, as well as the body's own digestion chemicals, is a critical step in understanding the role of colloidal phenomena in digestion and bioavailability. TCA is found to adsorb onto TiO2 but without any significant ability to be retained when it is not present in the aqueous phase. GCA is also found to adsorb via two distinct binding mechanisms, with one type of adsorbed species being resistant to removal. BLG adsorbs, is irreversibly bound, and has altered conformation when adsorbed at pH 2 (stomach conditions) to the conformation when adsorbed at pH 6.5 (small intestine conditions). This altered conformation is not interface-dependent and is mirrored in the solution spectra of BLG. Sequential coadsorption studies indicate that TCA and GCA adsorb onto TiO2 nanoparticle surfaces and display similar degrees of reversibility and binding in the presence or absence of preadsorbed BLG.

Dynamic wetting of imidazolium-based ionic liquids on gold and glass.

Many of the applications of ionic liquids rely on their bulk properties or their solvation abilities. However, it is their interactions with solid surfaces that underpin many of their potential applications in advanced technologies. Whether it is as lubricants for wind turbines or as electrolytes in supercapacitors, there are many areas where ionic liquids can provide an improvement in performance relative to more commonplace liquids. However, there are some barriers to their implementation in such applications. Foremost of these is the lack of systematic studies of their interactions with solid surfaces as well as neglecting the effect of the absorbed water on wetting. The present study explores the dynamic wetting of three ionic liquids (with a different length of hydrocarbon chain in the cation) on gold and glass substrates, both of which are relevant for nano- and micromechanical machine applications, under well-controlled environmental conditions. The form of data capture (Wilhelmy plate) allows for a direct analysis using analytical expressions for the two dominant approaches for dynamic wetting: the hydrodynamic and molecular kinetic models. All ionic liquids yield data that are described best by the molecular kinetic model. Substrate-ionic liquid and water-ionic liquid interactions contribute to the mechanisms involved in the wetting process.

Dynamics of capillary-driven liquid-liquid displacement in open microchannels.

The dynamics of the spontaneous spreading of a liquid droplet along an open hydrophilic microchannel filled with another immiscible liquid is primarily determined by the competition between the capillary driving force and the viscous drag. While the former force depends on the channel cross-section and dimensions, interfacial tension between two liquids and the contact angle formed between the channel's wall and the two liquids, the latter arises from the motion of fluid molecules in the two bulk liquids. This paper focuses on the influence of the outer (displaced) phase viscosity. In general, as the viscosity of the displaced phase increases relative to the viscosity of the displacing phase, the velocity of the liquid-liquid meniscus decreases. The experiments were interpreted by extending a previously established correlation for liquid-vapour systems (J. Phys. Chem. C, 2011, 115(38), 18761-18769) in open microchannels of the same geometry. The relationship between the liquid-liquid flow dynamics and the properties of the liquids (e.g. viscosities) is still unclear. Nonetheless, by taking a self-consistent empirical approach to estimate the influence of the viscosities on the flow kinetics for a given system, it is possible to obtain a reasonable theoretical description for the experimental system over a specific range of viscosity ratios.

Air at hydrophobic surfaces and kinetics of three phase contact formation.

This review focuses on the importance of air presence at hydrophobic solid surfaces for wetting film rupture and kinetics of three phase contact formation. Affinity to air is a typical feature of hydrophobic surfaces, but it has been often either overlooked or not taken into consideration. When the hydrophobic surface, contacted earlier with air, is immersed into water then air can stay attached to the surface. The origin of long range hydrophobic forces and data showing that these interactions were due to the bridging of nanobubbles attached to the hydrophobic surfaces are discussed. A major part of the review is devoted to the description and analysis of data showing that air (nano-, micro-bubbles and/or air film) present at a hydrophobic surface facilitated rupture of the liquid film and three phase contact formation during bubble collisions with flat Teflon plates of different surface roughness. Although all Teflon plates were highly hydrophobic (contact angles ca. 100 degrees -130 degrees ) the time of the three phase contact (TPC) formation and attachment of the colliding bubble was strongly affected by the plate surface roughness. The time of the TPC formation was shortened from over 80 down to 2-3 ms when the roughness was increased from below 1 microm to over 50 microm. Higher surface roughness means that larger amounts of air was entrapped during the Teflon plates' immersion in water. Additional experimental evidence is given, showing that facilitation of the TPC formation and the bubble attachment was due to air presence and re-distribution over the Teflon surfaces: i) prolonging the plate immersion time resulted in quicker attachment; ii) irregular and disappearing air pockets were recorded at a Teflon surface; iii) a satellite bubble left at a Teflon surface during the first collision facilitated the attachment; iv) attachment always occurred during the first collision in the case of a very rough "Teflon V" surface, but in highly concentrated n-octanol and n-heptanol solutions there was bouncing and attachment occurred during the second collision, moreover; v) the degree of bubble kinetic energy transferred into surface energy was significantly smaller during collisions with hydrophobic (Teflon) surfaces than with the hydrophilic ones. The mechanism of air entrapment and redistribution over Teflon plates immersed in water is presented.

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements.

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.

Wetting films in attachment of the colliding bubble.

The importance of wetting films in three phase contact formation and attachment of the bubble colliding with different solid surfaces is described The paper reviews main factors determining stability, drainage, rupture, three phase contact (TPC) formation and expansion of the TPC perimeter under dynamic conditions. There are shortly reviewed specific forces of interactions (DLVO and non-DLVO), kinetics of drainage and mechanisms of the wetting film rupture, as well as the TPC formation and expansion. The review is focused on the role of hydrophobic/hydrophilic properties, surface roughness and heterogeneity of the solid substrates for the wetting film stability and rupture under dynamic conditions. Phenomena occurring during collisions of the rising bubble with solid plates of different surface properties are discussed in relation to the kinetics of the wetting films drainage and TPC formation. It is showed that stability and drainage kinetics of the wetting films are decisive for the TPC formation and attachment of the colliding bubble.

Air-facilitated three-phase contact formation at hydrophobic solid surfaces under dynamic conditions.

The paper presents results documenting the mechanism of facilitation of the three-phase contact (TPC) formation due to gas entrapped during immersion of hydrophobic (Teflon) plates into distilled water and n-octanol solutions. Collisions, bouncing, the time scale of the TPC formation, and bubble attachment to Teflon plates of different surface roughness were studied using a high-speed camera. Processes occurring during the microscopic wetting film formation at the Teflon plates were monitored using the microinterferometric method (Scheludko-Exerowa cell). A strong relation between the time necessary to form a stable TPC and the roughness of the Teflon was observed. The higher the Teflon roughness was the shorter the time for the TPC formation. This effect can be attributed to two factors: (i) local differences in the thickness of the thinning intervening liquid layer (quicker attainment of rupture thickness at pillars of rough surface) and/or (ii) the presence of gas at the hydrophobic surface. Experimental findings, that (i) prolongation of the plate immersion time resulted in quicker TPC formation, (ii) white irregular and disappearing spots (air pockets) were recorded during the wetting film formation, and (iii) high n-octanol concentration caused prolongation of the time of the TPC formation, show that attachment (TPC formation) of the colliding bubble to hydrophobic surfaces was facilitated by air entrapped at the Teflon plates (and re-distributed) during their immersion into water phase. Thus, on collision instead of solid/gas wetting liquid film a thin gas/liquid/gas foam film was formed which facilitated the TPC formation.

Influence of surface active substances on bubble motion and collision with various interfaces.

Bubble motion as a function of distance from a point of its detachment and phenomena occurring during the bubble approach and collision with liquid/gas and liquid/solid interfaces in pure water and solutions of various surface active substances are described and discussed. It is showed that presence of surface active substance has a profound influence on values of the terminal velocity and profiles of the local velocity. At low solutions concentrations there are three distinct stages in the bubble motion: (i) a rapid acceleration, (ii) a maximum velocity value followed by its monotonic decrease, and (iii) attainment of the terminal velocity, while at high concentrations (and in pure water) there are only stages (i) and (iii). It is showed that the bubble terminal velocity decreases rapidly at low surfactant concentration, but there can be found some characteristic concentrations (adsorption coverage's) above which the velocity almost stopped to decrease. Immobilization of the bubble surface resulting from adsorption of the surface active substances (surface tension gradients inducement) causes over twofold lowering of the bubble velocity. Presence of the maximum on the local velocity profiles is an indication that a stationary non-uniform distribution of adsorption coverage (needed for immobilization the bubble interface) was not established there. When the rising bubble arrives at liquid/gas interface or liquid/solid interface there can be formed either foam or wetting film or three-phase contact (TPC). It is showed that prior to the foam and/or wetting film formation the bubble colliding with the interfaces can bounce backward and simultaneously its shape pulsates rapidly with a frequency over 1000 Hz. It is rather unexpected that even in the case of the free surface the bubble's shape and consequently its surface area can vary so rapidly. It shows straightforward that on such a rapidly distorted interface the adsorption coverage can be very different from that at equilibrium. This fact should be taken into account more appropriately in the discussion of the mechanism of formation and stabilization of various dispersed systems (e.g. foams, emulsions). Bubble collision with solids and formation of the three-phase contact is a necessary condition for flotation separation. It is rather common understanding that immediate attachment should occur in the case of hydrophobic surface, while there should be no attachment in the case of the hydrophilic ones. It is reported that even in the case of such hydrophobic solid surface as Teflon, the bubble attachment did not need to occur at first collision and in distilled water the bubble can bounce a few times without attachment. Presence of frother facilitates the bubble attachment to hydrophobic solid surface. Time scale of the TPC formation is very short, of an order of single ms. It was observed that presence of a micro-bubble at the solid surface facilitated drastically an attachment of the colliding bubble. Roughness of Teflon surface increases probability of the bubble attachment-most probably-as a result of higher probability of micro- and/or nano-bubbles presence at the solid surface.

Applaying of high pressure to yoghurt preservation.

Effects of the high pressure treatment in the range of 200 - 1000 MPa/15 min., at the room temperature, on survivability of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus was investigated. Sensorial characteristics of yoghurt was conducted by a panel of experts. Influence of yoghurt acidity on the effect of pressurization was also determined. In the experiment, we also attempted to extend shelf-life of yoghurt, subjecting it to the pressure of 550 MPa/15 min. Pressure treated yoghurt was stored for 4-weeks at room and refrigerated temperature; every seven days microbial number, acidity and sensory attributes of the beverage was determined. Addition of fruit jam on the effect of pressurization was also checked. Survivability of microorganisms depends on the value of pressure used in the experiment. The research displayed that yoghurt acidity did not influence the effect of pressurization. During 4-weeks of storage, yoghurt processed at 550 MPa maintained its beneficial characteristics longer compared to the non-pressurized one. Addition of fruit jam beneficially affected consistency of the pressure treated yoghurt. The conducted studies showed that there was possibility of preserving yoghurt by the method of high pressures. Thus, to preserve yoghurt, the complete inactivation of microflora is not necessary.

Structure-function correlation in transient amacrine cells of goldfish retina: basic and multifractal analyses of dendritic trees in distinct synaptic layers.

Amacrine cells generating light-evoked transient ON-OFF responses were stained by intracellular injection of horseradish peroxidase after determining their input-output (voltage response vs. light intensity) profiles. Ten cells specifically having bistratified dendritic trees were analyzed. The cross-sectional area of the dendrites in each sublamina (a and b) of the inner plexiform layer was initially measured. Although some variability was observed, there was no statistically significant overall difference in the cross-sectional areas of the dendritic trees in sublaminae a and b. Also, the amplitudes of the ON and OFF responses, generated by a midrange criterion stimulus, could not be correlated with the cross-sectional areas or the number of branches of the dendrites in sublaminae b and a, respectively. On the other hand, determination of the generalized fractal spectra revealed that the negative (up to -3) and zero-order fractal dimensions of the dendritic trees in sublamina a were consistently higher than those for sublamina b. Furthermore, there was a positive correlation between response amplitude and some part of the generalized fractal dimension in the respective parts of the dendritic trees. It is concluded that dendritic tree characteristics differ in the two halves of the inner plexiform layer and that these can be related to the cells' light-evoked response amplitudes. Furthermore, generalized fractal analysis appears to be a useful method for correlating structure and function in retinal amacrine cells with complex dendritic trees.

Effect of high pressure on some properties of lactic acid bacteria present in kefir.

The aim of the work was to determine the effect of pressurization under 700 and 1000 MPa on the following: biochemical and antibacterial activity of individual strains of streptococci and lactobacilli, isolated from kefir inoculum, and of kefir microflora. After pressurization under 1000 MPa, the studied microorganisms lost their ability to inhibit the growth of the test strains, preserving the aroma-producing ability at the same time. The pressure of 1000 MPa caused also the changes in conformation of lactic acid, produced by the bacteria in skim milk culture. The changes in metabolism of the isolated bacteria, as being the result of pressurization under 700 MPa were dependent on the strain of microorganisms.

Influence of temperature and acidity of milk on the degree of transfer of coagulating preparations from milk to the cheese grain.

The effect of temperature, milk acidity and time of curd holding on the degree of passage of enzymic coagulating preparations, was studied. It was found that the degree of the passage of the examined preparations was increasing together with the rise of temperature and milk acidity. Any significant effect of the curd's holding on the quantity of the preparation in the cheese grain was not found.