A low electrode-electrolyte impedance interface is critical in the design of electrodes for biomedical applications. To design low-impedance interfaces a complete understanding of the physical processes contributing to the impedance is required. In this work a model describing these physical processes is validated and extended to quantify the effect of organic coatings and incubation time. Electrochemical impedance spectroscopy has been used to electrically characterize the interface for various electrode materials: platinum, platinum black, and titanium nitride; and varying electrode sizes: 1 cm2, and 900 microm2. An equivalent circuit model comprising an interface capacitance, shunted by a charge transfer resistance, in series with the solution resistance has been fitted to the experimental results. Theoretical equations have been used to calculate the interface capacitance impedance and the solution resistance, yielding results that correspond well with the fitted parameter values, thereby confirming the validity of the equations. The effect of incubation time, and two organic cell-adhesion promoting coatings, poly-L-lysine and laminin, on the interface impedance has been quantified using the model. This demonstrates the benefits of using this model in developing better understanding of the physical processes occurring at the interface in more complex, biomedically relevant situations.

Volta potential mapping of AA2024-T3 on surfaces was performed with an atomic force microscope. A linear relation was found between the Volta potential measured in air and the corrosion potential in aqueous solution for a range of pure metal samples, indicating that this potential is a measurement of the practical nobility of the surface. Large differences in the Volta potential of intermetallic particles in AA2024-T3 and the matrix phase resulted in a potential map with high contrast that clearly identifies the location of the particles. All intermetallic particles, including the Mg-containing S-phase particles, had a Volta potential noble to that of the matrix. Surface films on the particles and the matrix were found to have strong effects on the potential, and probably explain the noble nature of the Mg-containing particles, which have been reported to be active to the matrix in solution. The effect of these surface films was examined by refreshing the sample surface using different techniques. Lateral heterogeneities in certain intermetallic particles were also revealed.

The localized corrosion of AA2024-T3, and the behavior of intermetallic particles in particular, were studied using different capabilities of the atomic force microscope (AFM). The role of intermetallic particles in determining the locations and rates of localized corrosion was determined using scanning Kelvin probe force microscopy in air after exposure to chloride solutions. Al-Cu-Mg particles, which have a noble Volta potential in air because of an altered surface film, are actively dissolved in chloride solution after a certain induction time. Al-Cu-(Fe, Mn) particles are heterogeneous in nature and exhibit nonuniform dissolution in chloride solution as well as trenching of the matrix around the particles. Light scratching of the surface by rastering with the AFM tip in contact mode in chloride solution results in accelerated dissolution of both pure Al and alloy 2024-T3. The abrasion associated with contact AFM in situ resulted in the immediate dissolution of the Al-Cu-Mg particles because of a destabilization of the surface film.

Heat shock resulted in rapid accumulation of large amounts of trehalose in Saccharomyces cerevisiae. In cultures growing exponentially on glucose, the trehalose content of the cells increased from 0.01 to 1 g/g of protein within 1 h after the incubation temperature was shifted from 27 to 40°C. When the temperature was readjusted to 27°C, the accumulated trehalose was rapidly degraded. In parallel, the activity of the trehalose-phosphate synthase, the key enzyme of trehalose biosynthesis, increased about sixfold during the heat shock and declined to the normal level after readjustment of the temperature. Surprisingly, the activity of neutral trehalase, the key enzyme of trehalose degradation, also increased about threefold during the heat shock and remained almost constant during recovery of the ceils at 27°C. In pulse-labeling experiments with [}4C]glucose, trehalose was found to be turned over rapidly in heat-shocked cells, indicating that both anabolic and catabolic enzymes of trehalose metabolism were active in vivo. Possible functions of the heat-induced accumulation of trehalose and its rapid turnover in an apparently futile cycle during heat shock are discussed.The nonreducing disaccharide trehalose (a-D-glucopyranosyl-1,1-a*-D-glucopyranoside) is ubiquitously found in fungi, in which it is supposed to function as a reserve carbohydrate (24). In Saccharomyces cerevisiae, trehalose accumulates during periods of reduced growth, for example during starvation for nitrogen, phosphorus, or sulfur (13,19).Trehalose is also abundant in chemostat cultures maintained at low dilution rates (11,12) and in batch cultures during adaptation to new carbon sources or transition to the stationary phase (13,18). Under some of these conditions, trehalose can account for up to 23% of the dry weight of the cells (13).Trehalose biosynthesis in yeast proceeds in two steps. First, trehalose-6-phosphate (P)-synthase (UDP-glucose:Dglucose-6-P-1-glucosyltransferase, EC 2.4.1.15) condenses UDPG and glucose-6-P to yield trehalose-6-P. Second, a specific phosphatase (trehalose-6-P phosphohydrolase, EC 3.1.3.12) cleaves off phosphate from trehalose-6-P (3). Trehalose degradation is mediated by trehalase (trehalose 1-glucohydrolase, EC 3.2.1.28) (24). Two trehalases with different pH optima have been found in S. cerevisiae (15). One of them, neutral trehalase, has attracted much attention because its activity is regulated by cyclic AMP (cAMP)-dependent phosphorylation (14,(25)(26)(27). Current research has therefore focused on trehalose catabolism, whereas the study of anabolism has been neglected.It has been reported that yeast cells growing at 37°C continuously contain appreciable amounts of trehalose (7)

A variety of interfaces relevant to corrosion processes were examined by the scanning Kelvin probe force microscopy ͑SKPFM͒ technique in order to study the influences of various parameters on the measured potential. SKPFM measurements performed on AA2024-T3 after solution exposure showed that surface composition is not the only parameter that controls the Volta potential difference, which is measured by SKPFM. The influence of surface oxide structure and adsorption at the oxide surface can be probed by SKPFM and lateral potential gradients can be observed in the absence of significant differences in oxide composition. The influence of tip-sample separation distance on the measured Volta potential difference was studied for different pure oxidecovered metals. SKPFM measurements were made in air on pure Ni and Pt samples withdrawn from solution at open circuit or under potential control. The Volta potential difference was found to be composed of a transient component that slowly discharged and a more permanent component associated with the charge of adsorbed species. The Volta potential difference transients measured on the samples emersed under potential control decayed much slower than the open-circuit potential transient measured in solution upon release of the potential control. These different measurements validate the use of SKPFM for the prediction of local corrosion sites and the study of surface modification during solution exposure. Scanning Kelvin probe force microscopy ͑SKPFM͒ is a powerful technique to characterize the corrosion processes associated with local inhomogeneities on passive surfaces. [1][2][3][4] The topography and potential distribution of a surface can be simultaneously mapped with submicrometers sensitivity by this technique. It is a scanning probe microscopy ͑SPM͒ method that is a modification of atomic force microscopy ͑AFM͒ and associated techniques. This technique senses the electrostatic field near an interface using a different approach than that of a standard Kelvin probe. Also, the probe tip operates at a much closer distance from the surface, which enables the enhanced spatial resolution relative to a standard Kelvin probe. SKPFM was developed and first used to study photoresist-covered Si wafers 5 and semiconductor dopant profiles. 6 This technique is quite new [7][8][9][10][11][12] and the number of investigations using it has increased in the past two years. [13][14][15][16] The ability to map the potential on a submicrometers scale is extremely useful in studies of localized corrosion of certain Al alloys, which contain heterogeneities on that scale and larger.1-4 Furthermore, it may be considered that the distribution of potential across a surface is even more relevant to the corrosion process than surface composition, which can be determined by a range of surface analytical techniques.The classical Kelvin probe technique has been used for some time to measure the potentials of various metal surfaces by nulling the current flowing between the sample and a closely position...

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