Different approaches for increasing the sensitivity, selectivity and stability of a voltammetric probe for mercury ions were probed. These involve the application of adsorbed monolayers of a host mol., i.e., 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (Kryptofix-222) and thin polymeric films. The stability, reproducibility and durability of the sensors in the anal. of Hg2+ from different sources were examd. and compared. A monolayer of the macrocyclic ligand, which is capable of forming a strong and selective complex with the mercury, is superior to polymeric films in terms of sensitivity and ease of regeneration. However, the stability and durability of polymeric films, in which Kryptofix-222 is incorporated, will eventually dictate their use as preferred interfaces for repetitive analyses esp. in flow anal. systems. An application of a polymer-Kryptofix-222 modified electrode for monitoring mercury in a bioremediation pilot was examd. using an automatic sequential-injection stripping anal. (SISA) system. [on SciFinder(R)]

A review is given on scanning electrochem. microscopy (SECM) as a tool for surface modification. It is distinguished between the direct mode and the feedback mode of the SECM. In the direct mode, the substrate serves as the auxiliary electrode, while, in the feedback mode, the substrate is unibiased, and a mediator is used that shuttles between the ultramicroelectrode and the surface. The etching of semiconductors, the metal deposition and etching, the deposition of conducting polymers, and the patterning of org. and biomols. are treated for both the direct- and the feedback mode technique of the SECM. The achievable resoln. and the patterning rate are addressed. [on SciFinder(R)]

A novel approach for micropatterning of surfaces with organic and biological microstructures using the scanning electrochemical microscope (SECM) is described. The approach is based on the introduction of the spatial resolution by local deposition of gold particles followed by monolayer formation and functionalization. Specifically, gold patterns were deposited locally on silicon wafers with the SECM as a result of the controlled anodic dissolution of a gold microelectrode. The gold patterns were further used as microsubstrates for assembling cystamine monolayers to which either fluoresceine isothiocyanate (FIT) or glucose oxidase (GOD) were covalently attached. Characterization of the organic monolayers, as well as the biological activity of the enzyme patterns, was carried out by fluorescence microscopy and the SECM, respectively.[on SciFinder (R)]

The presence of a disorganized octadecylsilane, C18, monolayer on an indium-tin oxide (ITO) electrode has a profound effect on the electrochem. of hexacyanoferrate, Fe(CN)63-/4-. The alkylsilane monolayer forms a hydrophobic matrix, which affects the heterogeneous rate of electron transfer of the metallic complex, depending on the nature of the counter cation presence in the soln. A model is suggested which accounts for these effects as well as for the changes in the formal redox potential that are obsd. [on SciFinder(R)]

The self-assembled monolayers (SAMs) of the three isomers of aminothiophenol, ATP, have been studied by means of electrochem., in-situ STM and XPS. We find that the isomers are organized differently. The organization is presumably governed by their intermol. and surface interactions. Moreover, the chem. and phys. characteristics of the interface are significantly and differently affected by the isomers comprising the SAMs. The electrochem. of the modified surface as well as the bare gold surface in contact with the thiol-contg. electrolyte, copper deposition and palladium(II) extn. have been studied for the 2-, 3- and 4-ATP monolayers. The organization of the layers has been investigated by STM. In essence, we find that the different isomers are likely to adopt an orientation that is perpendicular to the surface. While 4-ATP forms the densest monolayer, 2-ATP probably interacts with the gold surface with both the amino and the mercapto groups. Moreover, STM images indicate the partial formation of more than a monolayer. This is supported by XPS measurements and electrochem. and can be attributed to the SAM modification soln., i.e. sulfuric acid, which allows the formation of sulfate-bridged ATP bilayers. The chem. and phys. characteristics of the interfaces are governed by the different organization as well as by the individual chem. properties of each isomer. [on SciFinder(R)]

Diaphorase was hydrophobically immobilized onto a self-assembled monolayer of alkanethiol at an interdigitated Au microarray. The diaphorase activity at the array substrates was imaged with the scanning electrochem. microscopy (SECM) in the presence of NADH and ferrocenemethanol as a redox mediator. The images demonstrate that diaphorase was immobilized onto the SAM layer to form a line-and-space micropattern which reflected the pattern of the Au microarray. [on SciFinder(R)]

A novel approach for micropatterning of surfaces with org. and biol. microstructures using the scanning electrochem. microscope (SECM) is described. The approach is based on the introduction of the spatial resoln. by local deposition of Au particles followed by monolayer formation and functionalization. Specifically, Au patterns were deposited locally on Si wafers with the SECM as a result of the controlled anodic dissoln. of a Au microelectrode. The Au patterns were further used as microsubstrates for assembling cystamine monolayers to which either fluoresceine isothiocyanate (FIT) or glucose oxidase (GOD) were covalently attached. Characterization of the org. monolayers, as well as the biol. activity of the enzyme patterns, was carried out by fluorescence microscopy and the SECM, resp. [on SciFinder(R)]

The feedback mode of the scanning electrochem. microscope (SECM) was used to induce and measure charge-transfer coupling (CTC) between two ion-transfer (IT) processes across liq./liq. interfaces. Results are interpreted using a general model for coupling processes under steady-state conditions that is also applicable to other modes of coupling such as IT-ET and ET-ET (ET = electron transfer). The measured feedback curves demonstrate the effect of mass-transfer limitations in these processes on the potential across the interface, shedding new light on the dynamics of bioenergetics and phase-transfer catalysis systems as well as on previous SECM studies of liq./liq. interfaces. [on SciFinder(R)]

This contribution represents the first comprehensive attempt to treat complex geometry configurations of the scanning electrochem. microscope (SECM) using the alternating direction implicit finite difference method (ADIFDM). Specifically, ADIFDM is used to simulate the steady-state as well as the transient (chronoamperometric) behavior of a hemispherical ultramicroelectrode (UME) tip of the SECM. The feedback effect in this configuration is less pronounced as compared with a disk-shaped UME system. The differences between the two systems are discussed. Anal. approxns. for the steady-state behavior and for characteristic features of the transient behavior are suggested. Finally, exptl. feedback currents measured above a conductor and an insulator are in excellent agreement with the theory. [on SciFinder(R)]

The authors described fast electrochem.-spectroscopic "writ- ing-reading-erasing" system based on a scanning probe technique. The fast "reading" and "erasing" processes are due to the fact that there is no phase transition between the two states and that the ingression and egression of ions is very fast. In principle, the resoln. of patterning depends on the size of the microelectrodes, which currently can be reduced to the nanometer range. The uniqueness of this system is that the propagation of the lateral charge inside the thin film can be detected independently of its formation. This allows the study of lateral charge transport and enables the development of better systems in terms of resoln. and information storage capabilities. [on SciFinder(R)]

The electrochem. of functionalized self-assembled monolayers on Hg is studied by cyclic voltammetry and AC voltammetry. The results suggest that thiols can be adsorbed either chem. or phys. depending on the Hg potential. The transformation between these two states occurs via an oxidn.-redn. process. While a multi-layer of short chemisorbed thiols is formed under pos. potentials than the oxidn.-redn. process, a dense monolayer is self-assembled under more neg. potentials. [on SciFinder(R)]

An approach to sol-gel film formation is described, involving electrochem. control of pH near a conductive surface, in which methyltrimethoxysilane (MeTMOS) is deposited onto indium tin oxide (ITO) and Au surfaces. The method is based on a 2-step sol-gel prepn. procedure in which MeTMOS is first pre-hydrolyzed and then polycondensed. A const. neg. potential is applied to the electrode surface, increasing the concn. of hydroxyl ions, which act as a catalyst in the condensation process. This results in greater control over the deposition process and thus over film properties. [on SciFinder(R)]

The feedback mode of the scanning electrochem. microscope (SECM) was used for studying a surface-catalyzed electron-transfer reaction. As a model system the authors examd. the redn. of protons by reduced N,N'-dimethyl-4,4'-bipyridinium, methylviologen radical cation, MV+•, catalyzed by a platinum surface. The exptl. configuration comprises a biased amalgamated Au ultramicroelectrode (UME) that approaches an unbiased Pt UME substrate of the same radius. MV+• that is electrochem. generated at the amalgamated UME is oxidized at the Pt surface as a result of reducing protons. This novel configuration permits the simultaneous measurement of the catalytic current and the mixed potential (open-circuit potential) attained at the Pt surface. The results are interpreted by an electrochem. model which was previously suggested; however, the authors' approach makes it possible to measure, for the 1st time, the current-potential relation predicted by the theory. The authors' results indicate that methylviologen behaves reversibly under all the exptl. conditions, and that the rate const. of hydrogen evolution, that is calcd., is in perfect agreement with previous reports. The significant advantages of the authors' approach are discussed as well as its applicability for studying other electrochem. systems. [on SciFinder(R)]

The alternating direction implicit finite difference method (ADIFDM) was used to simulate the steady-state as well as the transient (chronoamperometric) behavior of sphere-cap microelectrodes (SCMs). The necessary steps to implement the ADIFDM for solving the rather complex geometry involved are described. The results for the steady-state behavior are in excellent agreement with a previously suggested anal. soln. making the ADIFDM an excellent tool to develop a comprehensive theory for SCMs. [on SciFinder(R)]

Electrochem. polymn. of 2',5'-bis(1-methylpyrrol-2-yl)thiophene (NSN) was carried out by scanning electrochem. microscopy, using Ru(bpy)33+ as oxidant for the polymn. process and for the ultramicroelectrode surface-distance probe. While this led to deposition of flat layers of polymer on a PMMA substrate, use of the weak oxidant IrCl62- gave sets of thin needles. Micrographs of the various patterns are shown (with resolns. of ≈ 15 μm) and the mechanism of polymn. is discussed. [on SciFinder(R)]

The formation of self-assembled monolayers (SAMs) of ω-mercaptoalkanoic acids on a mercury surface under potential control was studied. Cyclic voltammetry and a.c. voltammetry were used for studying the different phases and transitions that the homologous series of these thiols form and undergo. The thiols are either physisorbed or chemisorbed depending on the applied potential and the transition between these two states occurs through a faradaic process. Also, the chemisorption of the thiols results in multilayer deposition when the thiol monolayer does not block electron transfer. Finally, the implications of this study to the formation and manipulation of SAMs of ω-functionalized alkanethiols on Hg are discussed. [on SciFinder(R)]

The visualization and endurance of fingerprints on cartridge cases after the firing process have been examined. Cartridges of M16, AK-47 (Kalashnikov) and Parabellum have been tested. Despite difficulties in visualizing these fingerprints, it was found that in some cartridge cases under laboratory conditions--for instance, on M16 brass cartridges--substantial parts of the fingerprints remain intact after shooting. The careful use of illumination after metal vapor deposition enabled visualization. Different possible mechanisms responsible for the partial destruction of the fingerprints are discussed.[on SciFinder (R)]

Fe redox chem. was studied in Lake Kinneret (Sea of Galilee), a mesotrophic, monomictic lake in the central part of the Jordan Rift Valley, Israel. Fe2+ and total Fe concns. in the epilimnion and hypolimnion were measured; relationships between Fe2+ and other parameters (e.g., light, pH) were examd. In addn., lab. expts. were conducted where filtered (biota-free) Lake Kinneret water, sterile unfiltered Lake Kinneret water, and distd. water were spiked with various Fe3+ and Fe2+ concns. Fe3+ and Fe2+ concns. were measured as a function of time in water samples under a variety of pH, O2, and radiation conditions. Fe2+ concns. in the epilimnion were below detection limits (0.04 μM) during nighttime, whereas in daytime, Fe2+ concns. were always above detection limits and changed significantly throughout the year (0.05-0.15 μM). Fe2+:Fe(tot) ratios measured in the lake (3-99%) were higher than expected values for a high pH (pH ∼8), low ionic strength (∼10 mM) aquatic system. In addn. to photo-induced redn. of Fe3+, there was strong evidence that Fe2+ is stabilized in O2-satd. lake water, since Fe2+ concns. were detected at a depth of 10 m, where <10% of light penetrates. Results of oxidn. and photoredn. expts. suggested obsd. rates of Fe redox reactions in sterile lake water were consistent with known chem. (abiotic) rates, but that these rates cannot account for obsd. Fe2+ concns. in epilimnetic water. Therefore, it was proposed that the photo-induced Fe redox cycle in the epilimnion of Lake Kinneret is largely controlled by biol. activity and that abiotic photoredn. of Fe accounts for only a small fraction of Fe2+ in the epilimnion. [on SciFinder(R)]

A two-dimensional conducting polyaniline (PAN) monolayer has been formed on an elec. insulating monolayer. The approach is based on the electrochem. polymn. of surface-confined anilinium ions that were electrostatically attached to a neg. charged self-assembled monolayer of ω-mercaptodecanesulfonate (MDS), HS(CH2)10SO3-, on a gold surface. The formation and characterization of the two-dimensional film and the MDS monolayer have been examd. by cyclic voltammetry, Fourier transform IR spectroscopy, XPS, wettability, and scanning electrochem. microscope. The formation of a capacitor-like assembly, in which electron transfer was blocked between PAN and the gold surface, was accomplished by electrochem. incorporating hexadecanethiol (C16) into the MDS monolayer. The PAN monolayer exhibits properties similar to those of a thin polymer film. [on SciFinder(R)]

We have developed a selective electrode for chromium(VI), based on a self-assembled monolayer of 4-(mercapto-n-alkyl)pyridinium on gold surfaces, which exhibits unique speciation capabilities. Cr(VI) levels as low as 1 parts per trillion can be detected using a 4-(mercaptoethyl)pyridinium monolayer. The different parameters that govern the analytical performance of these electrodes have been studied in detail and optimized. In addition, the organization of the monolayers has been examined by a variety of surface techniques such as XPS, FT-IR, and electrochemistry. Our results show that structuring and understanding the solid-liquid interface at the molecular level are essential for designing probes with superior analytical characteristics.[on SciFinder (R)]