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oxides for this reaction appears to correlate with basicity, with no dependence on variable cation valencies or lattice oxygen mobilities as appears to be significant for some other reactions, such as CO oxidation, on these materials.49 Accordingly, in a mechanistic study by Minachev et it was proposed that hydrogen dissociation can occur due to the polarizing action of a rare-earth cation and an oxygen ion and is the rate-limiting step in C2H4 hydrogenation on these materials. On the basis of a series of isotopic studies on Dy203 at 220 K, Minachev et al. further proposed that ethylene is associatively adsorbed on the oxide surface and that the reaction proceeds via a semihydrogenated C2H4 (adsorbed) complex. This catalytic activity of the rare-earth oxides toward C2H4 hydrogenation does not necessarily contradict our observation of the absence of molecular adsorption of C2H4 on oxidized Gd under UHV conditions, because the correlation between the strength of adsorption and hydrogenation activity is unknown.Accordingly, two areas for further investigation are indicated. First, studies of the catalytic activity of rare earth oxide surfaces toward acetylene hydrogenation are necessary for comparison with studies of ethylene hydrogenation. Second, investigations by other techniques, especially vibrational spectroscopy, of the structure of the C2H2 moiety adsorbed on oxidized rare-earth surfaces at low temperature will be valuable for comparison with our UPS results. These studies will provide a clearer understanding of the adsorption selectivity of these rare earth oxide surfaces to C2Hz and CzH4 and of the possible correlation between adsorption selectivity and hydrogenation activity. V. Summary and ConclusionsThe adsorption of C2H2 and C2H4 has been studied on both the metallic Gd(0001) surface6 and on the oxidized surface of Gd(0001). Adsorption of both species is dissociative on the metallic surface at 165 K, and there is no detectable adsorption selectivity for these species on the metallic surface. In contrast, the Gd surface after oxidation preferentially adsorbs acetylene over ethylene at a substrate temperature of 165 K. The adsorption probability for acetylene under these conditions is estimated as 0.02 by using measurements of the carbon Auger signal versus exposure. The adsorption probability for ethylene on the oxidized Gd surface at 165 K is, at the very least, a factor of 3 lower and is probably lower by as much as a factor of 100. On the basis of the available UPS data, it is suggested that acetylene adsorbs molecularly on the oxidized surface at low temperature. The molecularly adsorbed CzH2 decomposes below 350 K, leaving carbon on the oxidized Gd surface. The enhanced selectivity of the oxidized Gd surface toward acetylene over ethylene adsorption is unique. This work suggests that studies of the hydrogenation of C2H2 and C2H4 on gadolinium oxide would be useful to probe the possible extension of the selectivity for adsorption into selectivity for hydrogenation.Two new types of highly soluble n...

Highly sensitive (lo-* moUdm3) and selective voltammetric detection of dopamine can be carried out with a closely spaced planar interdigitated array (IDA) or vertically separated new IDA electrodes. The sensitivity of dopamine was improved up to mol/dm5 as a result of high collection efficiency and redox cycling, because twin electrodes with a very small gap make it possible to collect most of the active species oxidized at adjacent electrodes and then to return them to dopamine by reverse reduction before the chemical reaction has terminated. The influence of L-ascorbic acid, which interferes with the detection of catecholamines, may be removed by oxidizing it at one electrode, because the oxidized acid converts rapidly to an electrochemically inactive species and is not detected at the other electrode. Dopamine was detected quantitatively in the presence of 10-fold excess of L-ascorbic acid in the same solution. LNTRODUCTIONElectrochemical detection of catecholamines has attracted considerable interest for biomedical applications such as liquid chromatography and biosensors and also for in vivo measurements of neutrotransmitters [ 1-31, One of the dominant problems when measuring catecholamines in a biological fluid is the presence of L-ascorbic acid, which reduces the sensitivity and selectivity of these neurotransmitters. This is because the oxidation peak of Lascorbic acid is close to that of catecholamines and its concentration is much higher. Various studies have been made to achieve the selective detection of catecholamines while maintaining high sensitivity.Electrochemically pretreated electrodes made of carbon fiber [4-61, carbon paste [7], or glassy carbon [8] can be used to distinguish between the oxidation peaks of catecholamines and L-ascorbic acid because the overpotential for the oxidation of L-ascorbic acid is reduced.Chemically modified electrodes (CMEs) have been also studied with a view to using them for distinguishing between catecholamines and L-ascorbic acid. The concept of using CMEs for the selective detection of catecholamines is that only catecholamines diffuse into the modified layer and reach the electrode surface, and almost all the L-ascorbic acid molecules are excluded. Electrodes of ' To whom correspondence should be addressed 0 1991 VCH Publishers, Inc.various kinds, modified with Nalion film [9-111, stearic acid [ 12, 131, and cross-linked, water-soluble polymers [14], have been studied to achieve highly selective detection.The application of closely spaced twin microelectrodes such as twin-band [15] or interdigitated array (IDA) electrodes [16, 171 is another interesting method for the selective detection of redox species in the presence of interfering molecules. When the anode and cathode in the twin electrodes are potentiostated with potentials higher and lower, respectively, than the redox potential of the electroactive species, the electroactive species whose oxidized form is relatively stable can reach the cathode and be detected. Then, the steadystate concentrati...

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Langmuir-Blodgett (LB) films of two new highly soluble nickel phthalocyanines involving four octadecyl amides (AmPcl and AmPc2) have been studied. The films have been prepared by both the vertical dipping and horizontal lifting techniques. The assembling numbers in AmPcl and AmPc2, which strongly depend on the the preparation condition of the LB films, are evaluated as being ca. 5-14 for the AmPcl and 5 for the AmPc2, using a modified molecular exciton theory. In-plane dichroisms in the LB films prepared by either the vertical dipping or the horizontal lifting technique can be invariably seen in UV-vis and IR regions. The dichroic ratios are sensitive to the condition of the film preparation (film pressures, transfer rates of substrates, additives, and film-transfer techniques). Molecular arrangement and orientation in the AmPcl LB film prepared by the vertical dipping method have been characterized by means of the polarized UV-vis and IR spectra, force-area isotherm, and mechanical stylus method.