Additional Diffusion Barrier Research Articles

Characterization of Tib2/TISI2 Bilayer Structure Deposited by Sputtering

AbstractBilayer of TiB2/TiSi2 was deposited by magnetron co-sputtering on silicon and alumina substrates, and this structure was investigated for structural and electrical properties. Substrate bias and annealing in vacuum have been applied to vary the film properties. X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM) were used to characterize the structure, and chemical composition was characterized by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Resistivity was measured by four probe method. Diffusion barrier properties were studied by AES. As deposited films are amorphous with resistivities of about 40 μΩcm. Post deposition annealing in vacuum shows that the amorphous titanium boride film is very stable. Crystallization starts above 1000°C as seen by XRD, and the crystallization temperature depends on the thickness of TiB2. TiSi2 C54 forms in the temperature range 586°C - 922°C, when TiB2 still remains in amorphous form. The TiSi2 sublayer serves as an additional effective diffusion barrier, preventing outdiffusion of boron from TiB2 into the Si substrate.

Performance of integrated glucose and lactate thin-film microbiosensors for clinical analysers

Integrated thin-film glucose and lactate sensors were produced using thin photo-patternable multi-enzyme membranes. The glucose and lactate sensors are based on the measurement of H 2O 2 on a Pt electrode produced by the enzymes glucose oxidase (GOD) and lactate oxidase (LOD), respectively, which were entrapped in photo-patterned pHEMA hydrogel membranes. To prevent electrochemical interferences and fouling of the Pt electrode an electropolymerized semipermeable membrane was used. A second enzyme membrane layer containing the enzyme catalase was immobilized above the oxidase enzymes. To separate the two enzyme layers and to introduce an additional diffusion barrier a third pHEMA membrane without enzyme was placed between the oxidase and catalase enzyme membranes. The intended use of this device is the measurement of undiluted biological fluids in a clinical analyser. The sensors were tested in buffer solutions and undiluted serum and exhibited excellent performances.

Glucose biosensors based on thick film technology

Various techniques for the fabrication of thick film biosensors are described. With electrodes printed on conventional thick film substrates, glucose is measured via the formation of hydrogen peroxide using glucose oxidase. Additional diffusion barriers extend the linear range of the sensor up to 15 m m. Using a new, flexible, unfired substrate material sensors with a three-dimensional structure can be made. The resulting cavities can be filled with carbon paste. Hence, the mediator technique and the use of dehydrogenases was applied to the fabrication of thick film biosensors. The linear range, sensitivity and stability of these sensors is strongly dependent on preparation methods such as the immobilization technique (mediated electrode) and the amount of cofactor and enzyme. Without any diffusion barrier the linear range is limited to about 1 m m for the sensor based on the dehydrogenase.

Energization-induced redistribution of charge carriers near membranes

The electric field arising from proton pumping across a topologically closed biological membrane causes accumulation close to the membrane of ionic charges equivalent to the charge of the pumped protons, positive on the side towards which protons are pumped, negative on the other side. We shall call this the 'active surface charge'. We here use the Poisson-Boltzmann equation to evaluate the effects of zwitterionic buffer molecules and uncharged proteins in the aqueous phase bordering the membrane on the magnitude and ionic composition of the active surface charge. For the positive side of the membrane, the main results are: (1) If the membrane is freely accessible to bulk phase ions, pumped protons exchange with these ions, such that the active surface charge consists of salt cations. (2) If a significant fraction of the ions in bulk solution consists of buffer molecules, then some of the pumped protons will remain close to the membrane and constitute a major fraction of the active surface charge. (3) If a protein layer borders the membrane, a significant part of the transmembrane electric potential difference exists within that protein layer and protons inside this layer dominate the active surface charge. (4) On the negative side of the membrane the corresponding phenomena would occur. (5) All these effects are strictly dependent on the transmembrane electric potential difference arising from proton pumping and would come in addition to the well known effects of buffers and electrically charged proteins on the retention of scalar protons. (6) No additional proton diffusion barrier may be required to account for a deficit in number of protons observed in the aqueous bulk phase upon aeration-induced proton pumping.

Gasometric measurement of sulfite oxidation

AbstractIn the commonly used sulfite method the consumption of sulfite is determined by iodometry. Since however, the addition of organic substances may interfere with iodometry (e.g. due to chemical reactions with iodine) the gasometric measurement of sulfite oxidation has been developed for analysis of how different culture media may influence the oxygen transfer rate. The striking decrease of sulfite oxidation rate due to addition of culture media to the sulfite solution suggests that adsorption of orgnic components in the gas liquid interface may account for an additional diffusion barrier and thus for a decrease of the oxygen transfer coefficient which in addition gives an explanation for differences between values found by the sulfite method and by aerobic cultivations. Consequently identical values of oxygen transfer rate have been obtained for both systems whenever the sulfite system has been properly adjusted to the aerobic cultivation conditions. In so far, the gasometric sulfite method proved to be a unique tool for rapid determination of factors influencing oxygen transfer rate in fermentation processes which may give rise to a reappraisal as to the relevance of the sulfite method for oxygen transfer optimization.

Relationship between thermal treatment of milk and hysteresis of the milk protein system. II. Studies on casein micelles (author's transl)

The association of colloidal calcium phosphate with the casein micelles is the principal reason for hysteresis of the milk protein system. Because of this association the secondary calcium phosphate cannot be titrated at its pK = 7.2, but only at lower pH values depending on the previous thermal treatment of the milk. However after overcoming certain energy barriers it can be titrated (between pH 5.8 and 5.2), but then calcium phosphate is separated irreversibly from the micelles. Therefore in the subsequent base titration other molecular conditions are passed through than in acid titration. The consequence is a hysteresislike titration curve. By heating, the surface of the micelles are covered successively with serum proteins. Due to this coating additional diffusion barriers are constructed. The diffusion of the protons into the interior of the micelles is measurably retarded. The expansion of the hysteresis loop can be determined quantitatively.

Unusual features of the active hexose uptake system of Chlorella vulgaris

1. 1. No indication for a co-transport of hexoses and Na + or K + in Chlorella vulgaris has been obtained. 2. 2. Uncoupling agents added during the steady state of 3- O-methylglucose uptake do not cause any efflux, although net influx and steady-state influx of the sugar are strongly inhibited at the same poison concentration. 3. 3. The rapid exchange efflux initiated by the addition of non-radioactive sugars to cells in the steady-state of 3- O-methyl [ 14C]glucose uptake can be completely inhibited by uncouplers. 4. 4. Net efflux achieved by high dilution is a very slow process. The rate of efflux is increased by a factor of 30–80 when the dilution medium contains hexoses (trans-stimulation). 5. 5. Carbonyl cyanide p-trifluoromethoxyphenylbydrazone (FCCP) inhibits uptake also below concentration equilibration. 6. 6. The Q 10 values of net influx, steady-state efflux and influx are 2.8, 2.4 and 2.8, respectively; for net efflux considerably higher values were observed. 7. 7. Although a very high transmembrane effect for efflux and only a small one for influx was observed, no indication for an additional diffusion barrier was obtained. 8. 8. Based on the results in this and the subsequent paper a transport model is proposed. Metabolism is thought to accelerate the translocation both of the carrier-substrate complex into and of free carrier out of the cells.

Unusual features of the active hexose uptake system of Chlorella vulgaris.

1. 1. No indication for a co-transport of hexoses and Na + or K + in Chlorella vulgaris has been obtained. 2. 2. Uncoupling agents added during the steady state of 3- O -methylglucose uptake do not cause any efflux, although net influx and steady-state influx of the sugar are strongly inhibited at the same poison concentration. 3. 3. The rapid exchange efflux initiated by the addition of non-radioactive sugars to cells in the steady-state of 3- O -methyl [ 14 C]glucose uptake can be completely inhibited by uncouplers. 4. 4. Net efflux achieved by high dilution is a very slow process. The rate of efflux is increased by a factor of 30–80 when the dilution medium contains hexoses (trans-stimulation). 5. 5. Carbonyl cyanide p -trifluoromethoxyphenylbydrazone (FCCP) inhibits uptake also below concentration equilibration. 6. 6. The Q 10 values of net influx, steady-state efflux and influx are 2.8, 2.4 and 2.8, respectively; for net efflux considerably higher values were observed. 7. 7. Although a very high transmembrane effect for efflux and only a small one for influx was observed, no indication for an additional diffusion barrier was obtained. 8. 8. Based on the results in this and the subsequent paper a transport model is proposed. Metabolism is thought to accelerate the translocation both of the carrier-substrate complex into and of free carrier out of the cells.