Decrease In Surface Concentration Research Articles

Numerical analysis of particle radial distribution during spray pyrolysis of dual-solute precursor

This study investigates the droplet-to-particle formation process in spray pyrolysis, focusing on interactions between low-solubility precursor A (KNO3) and high-solubility precursor B (NaNO3). A numerical model simulates evaporation, multi-component transport, and precipitation in droplets, and is validated with experimental data. The numerical results reveal that the preferential precipitation of KNO3 near the surface hinders NaNO3, lowering its surface molar concentration. The radial distribution of NaNO3 varies with parameters, exhibiting monotonic or non-monotonic trends. The presence of a small amount of low-solubility KNO3 decreases NaNO3 near the surface, while a higher KNO3 proportion forms a shell layer, increasing NaNO3 from the center to the surface. The results also show that environmental factors influence particle size and component structure. Adjusting the precipitation rate of KNO3, along with reduced temperature and droplet size, leads to smaller particles, enhancing NaNO3 accumulation inside and decreasing surface concentration. This work deepens understanding of pyrolysis droplet evolution, supporting customizable particle design for improved droplet-based technologies.

Effect of solid support and membrane tension on adsorption and lateral interaction of amphipathic peptides.

A wide class of antimicrobial amphipathic peptides is aimed to selectively form through pores in bacterial membranes. The partial incorporation of the peptides into the lipid monolayer leads to elastic deformation of the membrane. The deformation influences both the adsorption of the peptides and their lateral interaction. Detailed study of pore formation mechanisms requires an accurate determination of the surface concentration of the peptides at their given bulk concentration. Widely used methods to register the adsorption are atomic force microscopy (AFM), surface plasmon resonance refractometry (SPRR), and inner field compensation (IFC). AFM and SPRR utilize membranes deposited onto a solid support, while IFC operates with model membranes under substantial lateral tension. Here, we theoretically studied the effect of the solid support and lateral tension on the elastic deformations of the membrane induced by partially incorporated amphipathic peptides and thus on the peptide adsorption energy and lateral interaction. We demonstrated that, under conditions typical for AFM, SPRR, and IFC, the adsorption energy can increase by up to 1.5 kBT per peptide leading to about 4 times decreased surface concentration as compared to free-standing tensionless membranes. In addition, the effective lateral size of the peptide molecule increases by about 10%, which can have an impact on the quantitative description of the adsorption isotherms. Our results allow estimating the effects of the solid support and lateral tension on the adsorption and interaction of amphipathic peptides at the membrane surface and taking them into account in interpretation of experimental observations.

Agar-stabilized sulfidated microscale zero-valent iron: Its stability and performance in chromate reduction

Sulfidated microscale zero-valent iron (SmZVI) attracts much attention recently in remediation of contaminated groundwater, but whether polymer coating on SmZVI would impact on its reactivity and capacity is yet to be understood. In this work, SmZVI was prepared by milling mZVI with elemental sulfur, and its stability in agar solution was evaluated. The impact of polymer coating on SmZVI grains’ capacity and reactivity for chromate reduction was then examined. Experimental results indicated that SmZVI having the best overall performance was attained by grinding mZVI with elemental sulfur at 0.05 S/Fe molar ratio for 10 h. SmZVI’s stability can be substantially improved if dispersed in 2.0 g/L agar solution. Existence of agar films on the SmZVI grain (A-SmZVI) lowered the material’s capacity for chromate reduction by 56%, and the associated reaction kinetics by 70.4%, as estimated by pseudo first-order reaction model using the early-stage experimental data. Analysis of XPS spectra of A-SmZVI post reaction with chromate indicated that multiple reductive species including Fe0, Fe(II), FeS, and S(-II) may have jointly participated in the redox reaction taking place on the A-SmZVI-water interface. Fitting of XPS data supported that S(-II) was oxidized to SO42-, S2O32-, and S0, in order of decreasing surface concentration.

Quantitative estimation of meteorological impacts and the COVID-19 lockdown reductions on NO2 and PM2.5 over the Beijing area using Generalized Additive Models (GAM)

Unprecedented travel restrictions due to the COVID-19 pandemic caused remarkable reductions in anthropogenic emissions, however, the Beijing area still experienced extreme haze pollution even under the strict COVID-19 controls. Generalized Additive Models (GAM) were developed with respect to inter-annual variations, seasonal cycles, holiday effects, diurnal profile, and the non-linear influences of meteorological factors to quantitatively differentiate the lockdown effects and meteorology impacts on concentrations of nitrogen dioxide (NO2) and fine particulate matters (PM2.5) at 34 sites in the Beijing area. The results revealed that lockdown measures caused large reductions while meteorology offset a large fraction of the decrease in surface concentrations. GAM estimates showed that in February, the control measures led to average NO2 reductions of 19 μg/m3 and average PM2.5 reductions of 12 μg/m3. At the same time, meteorology was estimated to contribute about 12 μg/m3 increase in NO2, thereby offsetting most of the reductions as well as an increase of 30 μg/m3 in PM2.5, thereby resulting in concentrations higher than the average PM2.5 concentrations during the lockdown. At the beginning of the lockdown period, the boundary layer height was the dominant factor contributing to a 17% increase in NO2 while humid condition was the dominant factor for PM2.5 concentrations leading to an increase of 65% relative to the baseline level. Estimated NO2 emissions declined by 42% at the start of the lockdown, after which the emissions gradually increased with the increase of traffic volumes. The diurnal patterns from the models showed that the peak of vehicular traffic occurred from about 12pm to 5pm daily during the strictest control periods. This study provides insights for quantifying the changes in air quality due to the lockdowns by accounting for meteorological variability and providing a reference in evaluating the effectiveness of control measures, thereby contributing to air quality mitigation policies.

EXPERIMENTAL STUDIES OF ACCELERATED CHLORIDE TRANSPORT IN CONCRETE

This contribution deals with the efficiency of electromigration of chlorides used as a repair method for reinforced concrete structures. Experimental studies of accelerated chloride transport tests were performed on samples of concrete without chlorides and with admixed sodium chloride during concreting. Two concrete types from Portland cement characterized with normal and low compressive strengths were studied. The electromigration was applied to penetrate chlorides into the chloride-free sample and for extraction of chlorides from the sample. The effectiveness of the chloride extraction process for rehabilitation of reinforced concrete in terms of lowering the chloride concentration in different concrete types and surface concentration was observed. Electrical extraction was found to be effective for lowering of initial chloride concentration by 15-20% after 24 hours. The decrease in surface concentrations was found in the range of 40-50%. The extraction process was found to be feasible and effective for both concrete types.

Enhanced RDX Detection Studies on Various Types of Substrates via Tunable Quantum Cascade Laser Spectrometer Coupled with Grazing Angle Probe

Owing to scientific advances in the field of materials sciences and engineering, researchers have developed new energy sources used for spectroscopic applications and measurements of properties resulting from the interaction of matter and electromagnetic radiation in the mid-infrared (MIR) region. MIR lasers, such as quantum cascade lasers (QCLs), used for spectroscopy have quickly found numerous applications in a wide cadre of IR techniques. This provides the opportunity to study properties of highly energetic materials (HEM), among many other applications. MIR laser spectroscopy based detection experiments of HEMs were carried out using a QCL optically coupled to compact grazing angle probe mount (QCL-GAP) enabling reflection-absorption infrared spectroscopy (RAIRS) measurements of thin films of HEMs. A saturated solution of RDX in acetone was prepared, and aliquots of subsequent dilutions of the stock solutions were transferred to test surfaces for QCL-GAP back-reflectance measurements. RDX reflectance signals were monitored as function as the decreasing surface concentration until the signal/noise was ∼ 3. Stainless steel (SS) plates were used as reflective substrates, and anodized aluminum (AN-Al), cardboard, and Teflon were used as non-reflective (matte) substrates. Using generated calibration curves a low limit of detection (LOD) of 1.7 ng/cm2 for RDX/SS and 95 μg/cm2 for RDX/AN-Al were found. Based on the area of laser spot (0.3 cm2) we conclude the minimum masses detected were 490 pg (RDX/SS) and 28 μg (RDX/AN-Al).

Impact of surface dilation rate on dynamic surface tension

In past studies, estimating the dynamic surface tension in processes involving significant surface deformation relied heavily on empirical relations. Despite its importance, the detailed dynamics of surfactant adsorption onto a dilating surface have not been investigated extensively. In this study, we modified the pendant bubble tensiometer to provide direct observation on the dynamics of surfactant adsorption during rapid surface dilation. The relaxation of dynamic surface tension for bubbles rapidly expanding in the solution of four different bulk concentrations of Triton-X 100 was studied. It was found that surface tension rose with increasing surface area due to the decrease in surface concentration. Moreover, when surface dilational rate was increased, it was found that the relation between surface tension and surface area came to resemble the equation of state of two dimensional surfactant phase when dilational rates were larger than a certain threshold value, indicating that the adsorption of surfactant became negligible. This threshold value was found to increase with surfactant concentration. The results showed that the procedure established in this study could serve as a simple guideline to determine whether adsorption can be neglected in a process involving dilating surfaces. In addition, the detailed dynamical data reported in this work can serve as an experimental reference for multiphase flow simulations involving species that adsorb on interfaces.

Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

The compositions in the (100 − x)La0.6Sr0.4CoO3±δ-xCeO2 (LSCC) system with 5 ≤ x ≤ 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (Rc) and modified ceria with fluorite structure (Fmm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3±δ (LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 °C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (Rc) to cubic (Pmm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grain's surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x = 8–37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57–76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5–76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (δ) above certain values: δ > 0.08 in LSC, δ > 0.13 in LSCC02, and LSCC with x = 5–76. The LSCCx with x = 5–37 and with x = 57–76 show slightly different reduction behaviour under a(o2) = 7.4 × 10−5. In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57–76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.

Effect of pH on calcite growth at constant [formula omitted] ratio and supersaturation

In situ Atomic Force Microscopy (AFM) was used to study the growth of calcite at a constant supersaturation ( Ω = 6.5) and solution stoichiometry ( a Ca 2 + / a CO 3 2 - = 1 ) in the pH range 7.5–12. The calcite growth rate decreased with increasing pH in the studied range. The results can be successfully explained by the surface complexation model for calcite growth and by considering the effect of OH − ions on solute hydration. At pH below 8.5, growth occurs mainly by CaCO 3 0 incorporation at >CaHCO 3 0 surface sites. CaCO 3 0 should be more easily incorporated than free Ca 2+ ions, as water exchange is usually faster if water molecules in the ion hydration shells are substituted by other ligands, as in CaCO 3 0. However, at pH above 9, Ca 2+ incorporation at >CaHCO 3 0 sites also contributes to calcite growth as a result of the increased frequency of water exchange in calcium hydration shells due to the presence of strongly hydrated OH −. This also leads to an increase in the solid–liquid interfacial tension at high pH, which seems to reduce the nucleation rate and increase the average size of crystals precipitated in macroscopic, non-seeded experiments. The reduction of calcite growth rate is a consequence of decreasing surface concentration of active growth sites (i.e., >CaHCO 3 0) with increasing pH under our experimental conditions. Changes in two-dimensional island morphology were observed at high pH (12), possibly due to the stabilization of polar scalenohedral faces by the presence of OH − ions. This work may help to improve our understanding of the effects of carbonate–solution reactions resulting from potential changes in the pH of the oceans and surface waters in response to variations in atmospheric CO 2, as well as in understanding calcite precipitation in highly alkaline environments both natural (e.g., alkali lakes) and artificial (e.g., cement carbonation).

Desorption kinetics during capillary flow

A model for coupling desorption and convective diffusion processes during capillary flow is considered. The desorption leads to the decrease in surface concentration, i.e., to surface cleaning. The model is specified with respect to the cleaning of long, narrow tubes of endoscopes, specifically for the case when desorption plays a larger role than hydrodynamic detachment. During the final stage of cleaning, coverage of the tube surface by contaminants (adsorbate) is low. Consideration of contaminants as isolated adsorbed particles (molecules) with no lateral interaction with each other on a uniform tube surface enables one to use a rather general mixed-desorption model called the kinetic-diffusion controlled model. Our model predicts that after cleaning, the tube remains slightly contaminated near its exit even though there is almost complete cleaning at the entrance. This is caused by regularities of the coupling between desorption and convective diffusion processes. Since this prediction agrees with our observations, a practical recommendation for improving tube exit cleaning is formulated. Modeling of this system became possible due to the simple velocity distribution of Poiseuille's flow in this geometry, which allows us to apply the Levich theory of convective diffusion in our modeling.

Concentration of gold nanoparticles in the QCM sensitive film as a factor of adsorption properties controlling

The adsorption properties of sensitive films formed by gold nanoparticles, dropped from solutions with different nanoparticles content are investigated using mass-sensitive (QCM) sensors. The dependence of surface adsorption capacity on nanoparticles concentration is shown. The decrease of surface concentration of nanoparticles leads to the increase of sensors sensitivity to different alcohols and water. The adsorption character of water (response amplitude and kinetics) substantially differs form that of alcohols. The sensitivity of gold nanoparticles layers to water is more then one order higher then to alcohols. The kinetics of water adsorption runs in two stages.

Optimal Conditions for Protein Array Deposition Using Continuous Flow

Optimal conditions for depositing protein microarrays using a continuous-flow microfluidic device, the continuous-flow microspotter (CFM), have been determined using a design of experiments approach. The amount of protein deposited on the surface depends on the rates of convective and diffusive transport to the surface and binding at the surface. These rates depend on parameters such as the flow rate, time, and capture mechanism at the surface. The process parameters were optimized, and uniform protein spots were obtained at a protein concentration of 10 microg/mL and even at 0.4 microg/mL. A 150-fold dilution in protein concentration in the sample solution decreased surface concentration by a factor of only 16. If the capture mechanism of the protein on the substrate is nonspecific, optimal deposition is obtained at higher flow rates for short periods of time. If the capture mechanism is specific, such as biotin-avidin, deposition is optimal at medium flow rates with little advantage beyond 30 min. The CFM can be used to deposit protein arrays with good spot morphology, spot-to-spot uniformity and enhanced surface concentration. The CFM was used to deposit an array of various antibodies, and their interactions with an antigen were studied using surface plasmon resonance (SPR). Affinity values were obtained at low antibody concentrations (5 microg/mL) with low coefficients of variation. Thus, the CFM can be used to effectively capture proteins and antibodies from dilute samples while depositing multiple spots, thereby increasing the quality of spots in protein microarrays and especially improving screening throughput of SPR.

Stationary phases for hydrophilic interaction chromatography, their characterization and implementation into multidimensional chromatography concepts

Hydrophilic interaction chromatography (HILIC) is becoming increasingly popular for separation of polar samples on polar columns in aqueous-organic mobile phases rich in organic solvents (usually ACN). Silica gel with decreased surface concentration of silanol groups, or with chemically bonded amino-, amido-, cyano-, carbamate-, diol-, polyol-, or zwitterionic sulfobetaine ligands are used as the stationary phases for HILIC separations, in addition to the original poly(2-sulphoethyl aspartamide) strong cation-exchange HILIC material. The type of the stationary and the composition of the mobile phase play important roles in the mixed-mode HILIC retention mechanism and can be flexibly tuned to suit specific separation problems. Because of excellent mobile phase compatibility and complementary selectivity to RP chromatography, HILIC is ideally suited for highly orthogonal 2-D LC-LC separations of complex samples containing polar compounds, such as peptides, proteins, oligosaccharides, drugs, metabolites and natural compounds. This review attempts to present an overview of the HILIC separation systems, possibilities for their characterization and emerging HILIC applications in 2-D off-line and on-line LC-LC separations of various samples, in combination with RP and other separation modes.

Influence of a dust storm on carbon monoxide and water vapor over the Indo‐Gangetic Plains

Dust storms are meteorological phenomena that produce air quality hazards over specific regions lasting from a few hours to many days. They are common in the western part of India during the months of April–June, particularly over Delhi and the surrounding Indo‐Gangetic (IG) plains. In this paper, a dust storm event over Delhi, Kanpur and Varanasi during 1–11 April 2005 was used to study the vertical changes in the atmosphere. We studied data from the Measurement of Pollution in the Atmosphere (MOPITT) instrument onboard the Terra satellite, daytime vertical carbon monoxide (CO) mixing ratio and the Atmospheric Infra‐red Sounder (AIRS) onboard the Aqua satellite water vapor mass mixing ratio associated with dust storm over three locations in the IG plains. Evidence of vertical transport of CO to upper troposphere (UT) is observed from stability indices derived from radiosonde data, NCEP reanalysis wind and HYSPLIT model over Delhi. The strong upward convection during dust storms reduces CO in the lower troposphere and increases CO at around 350 hPa pressure level. Water vapor mass mixing ratio shows an increase at 700–850 hPa pressure level during the dust storm event over all locations. The changes of water vapor mass mixing ratio and CO during the dust storm are found to be more pronounced over Delhi and Kanpur while they are comparatively less over Varanasi, due to low intensity during transport of dust from west to east in the IG plains. The increased concentration of CO and water vapor mass mixing ratio at different pressure levels 350 hPa and 700–850 hPa respectively with corresponding decrease in surface concentration (at pressure level 700–1000 hPa and 925–1000 hPa respectively) have been investigated during a major dust storm period. The changes in CO and water vapor mass mixing ratio are found to be consistent with the observed changes in vertical stability of the atmosphere.

Direct observation of the kinetically relevant site of CO hydrogenation on supported Ru catalyst at 700 K by time-resolved FT-IR spectroscopy

Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on the millisecond time scale at 700 K using pulsed release of CO and a continuous flow of H(2)-N(2) (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm(-1)), water (1900-1300 cm(-1)), and carbon dioxide (2348 cm(-1)). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +/- 0.4 s(-1) and coincides with the rate of carbon dioxide growth (3.4 +/- 0.6 s(-1)), thus indicating that CH(4) and CO(2) originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm(-1) at early times (36 ms) that shifts gradually to 1960 cm(-1) over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm(-1) decays with a rate constant of 2.9 +/- 0.1 s(-1), which coincides with the rise constant of CH(4). This demonstrates that the linear CO site at 2026 cm(-1) is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.

Suppression and enhancement of non-native molecules within biological systems

With the aim of evaluating the potential of SIMS to provide molecular information from small molecules within biological systems, here we investigate the effect of different biological compounds as they act as matrices. The results highlight the fact that the chemical environment of a molecule can have a significant effect on its limit of detection. This has implications for the imaging of drugs and xenobiotics in tissue sections and other biological matrices. A 1:1 mixture of the organic acid 2,4,6-trihydroxyacetophenone and the dipeptide valine–valine demonstrates that almost complete suppression of the [M + H] + ion of one compound can be caused by the presence of a compound of higher proton affinity. The significance of this is highlighted when two similar drug molecules, atropine (a neutral molecule) and ipratropium bromide (a quaternary nitrogen containing salt) are mixed with brain homogenate. The atropine [M + H] + ion shows significant suppression whilst the [M − Br] + of ipratopium bromide is detected at an intensity that can be rationalised by its decreased surface concentration. By investigating the effect of two abundant tissue lipids, cholesterol and dipalmitoylphosphatidyl choline (DPPC), on the atropine [M + H] + signal detected in mixtures with these lipids we see that the DPPC has a strong suppressing effect, which may be attributed to gas phase proton transfer.

Surface and catalytic investigations of CuO–Cr 2O 3/Al 2O 3 system

A series of CuO–Cr 2O 3/Al 2O 3 solids were prepared by the incipient wetness impregnation using Al(OH) 3 solid and chromium and copper nitrates. The amounts of copper and chromium oxides were varied between 12.2 and 13.5 wt.% CuO and 3.1 and 8.8 wt.% Cr 2O 3. The prepared solids were calcined at 400–1000 °C. The solid–solid interactions between the different constituents of the prepared solids were investigated using XRD and XPS analyses. The surface and catalytic properties of various solids were studied using nitrogen adsorption at −196 °C and CO-oxidation by O 2 at 50–250 °C. The results showed that the investigated adsorbents are mesoporous solids and measure specific surface areas within the range 73–356 m 2/g depending on the amount of dopant added and precalcination temperature. Cr 2O 3-treatment of the system investigated brought about a significant decrease in its BET-surface area which was found to decrease by increasing the precalcination temperature of the various solids. However, this treatment resulted in a progressive increase in the activation energy of sintering of different solids subjected to heat treatment at 400–800 °C. The concentration of copper and chromium species on top surface layers of the samples precalcined at 400 and 600 °C are greater than those present in the bulk of the solids. The doping process effected an increase in the surface concentration of copper species. The increase in precalcination temperature of pure and doped solids from 400 to 600 °C effected an inward diffusion of copper and chromium species towards their interior leading to a decrease in surface concentration of these species. Doping CuO/Al 2O 3 system with Cr 2O 3 decreased the crystallinity of CuO phase and enhanced its interaction with Al 2O 3 at 800 °C yielding CuAl 2O 4. XPS measurements showed also the possibility of formation of CuCrO 2 on the top surface layers of the heavily doped sample precalcined at 600 °C. The catalytic activity and durability of CuO/Al 2O 3 system increased by treatment with Cr 2O 3. Doping of this system did not modify the mechanism of the catalytic reaction but increased the concentrating of catalytically active sites without changing their energetic nature.

Viscoelastic Modeling of Template-Directed DNA Synthesis

In the present study, we have used the QCM-D technology to study the replication of surface attached oligonucleotide template strands using Escherichia coli DNA polymerase I (Klenow fragment, KF). Changes in resonance frequency (F) and energy dissipation (D) for DNA hybridization and polymerization were recorded at multiple harmonics. Formation of the polymerase/DNA complex led to a significant decrease in energy dissipation, which is consistent with a conformational change induced upon enzyme binding. This interpretation was further strengthened by a data analysis using a Voigt-based viscoelastic model. The analysis revealed a significant increase in shear viscosity and shear modulus during KF binding, whereas the viscoelastic properties of single- and double-stranded templates were almost identical. During the actual DNA synthesis, an initial increase in rigidity (shear viscosity) was followed by a gradual decrease that has two components corresponding to the release of enzyme and to the presence of the catalytically active enzyme/substrate complex. The corresponding decrease in surface concentration was found to underestimate the rate of enzyme release due to viscously coupled water that compensates for the loss in enzyme mass. Furthermore, the modeling elucidates that significant changes in both F and D originate from variations in the viscoelastic properties, which means that changes in F alone should be used with care for estimations of coupled mass and kinetics. Therefore, the modeled temporal variation in effective thickness, being proportional to coupled mass and, thus, independent of structural changes, was used to estimate the catalytic constants of the polymerization reaction. The reported work is the first example providing this type of structural information for the catalytic action of an enzyme, thereby demonstrating the potential of the technique for advanced analysis of complex biological reactions, including proper analysis of enzyme kinetics.

The effect of recasting on corrosion of a silver–palladium alloy

Objectives. Noble and semi-noble dental casting alloys are often reused by recasting. The aim of this study was to determine if repeated casting has a detrimental effect on the corrosion behavior of a silver-based dental alloy. Methods. A commercial Ag–Pd–Cu–Au alloy was cast and four times recast. Electrochemical tests were performed to determine the effects of recasting on the polarization parameters and the corrosion current density in artificial saliva. Changes in hardness, chemical composition and microstructure were determined. Results. Recasting had little effect on the shape of the polarization curves, but the corrosion current density dramatically increased after the fifth cast. Hardness increased with recasting and the microstructure showed grain growth. Increased subgrain formation was found for the fifth cast. The elemental map showed decreased surface concentration of Au for the fifth cast. The bulk concentrations of Au, Ag and Pd remained constant, but the copper and zinc contents slightly decreased and the content of impurities increased. Significance. Recasting of an Ag–Pd–Cu–Au alloy initially had little effect on the corrosion susceptibility in artificial saliva, but a severe degradation of properties was observed after the fifth cast. The results indicate that a dental alloy of this type can be safely recast only a small number of times.

Quantification of ligand surface concentration of bulk-modified biomimetic hydrogels

This study describes a method for the quantification of active ligand surface concentration for bulk-modified hydrogels. Two poly(propylene fumarate-co-ethylene glycol) (P(PF-co-EG)) block copolymers were synthesized with terminal poly(ethylene glycol) (PEG) chains of number average molecular weight 1960 and 5190 g/mol. Hydrogels were synthesized with bulk-modified biotin as a model ligand, making use of a PEG spacer arm with a molecular weight of 3400 g/mol. Bulk concentration of biotin was calculated from the initial concentration of biotin, sol fraction, equilibrium water content, and relative incorporation of the polymers to the hydrogel. Surface concentration of biotin bulk-modified hydrogels was quantified with an enzyme linked immunosorbent assay using mouse monoclonal anti-biotin antibody (IgG), horseradish peroxidase-conjugated anti-mouse IgG, and a chemiluminescent substrate. The larger size of the IgG relative to the mesh size of the hydrogels allowed for the quantification of the active biotin at the surface of the hydrogels. Luminescent imaging was used to qualitatively show the isolation of the horseradish peroxidase-conjugated antibodies to the surface of the bulk-modified hydrogel. The active biotin ligands at the surface of hydrogels synthesized with terminal PEG chains of 1960 g/mol were at the top 7.2 nm while for those synthesized with terminal PEG chains of 5190 g/mol were at the top 4.4 nm of the bulk-modified hydrogel. The relationship between bulk ligand concentration and the active ligand concentration at the surface was dependent on the hydrogel composition. The relative magnitude of the PEG spacer arm of the ligand compared to the PEG block length of the copolymer affected the surface availability of the ligand. The results suggest that steric hindrances caused by mobile PEG chains of the copolymer of molecular weight greater than that of the PEG spacer arm contributed to the decreased surface concentration of ligand. This work relates the bulk concentration of a ligand to its surface concentration, an important parameter for the adhesion, migration, and function of anchorage dependent cells.