Functional group imaging by chemical force microscopy.

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

Molecular self-assembly of two-terminal, voltammetric microsensors with internal references.

Self-assembly of a ferrocenyl thiol and a quinone thiol onto Au microelectrodes forms the basis for a new microsensor concept: a two-terminal, voltammetric microsensor with reference and sensor functions on the same electrode. The detection is based on measurement of the potential difference of current peaks for oxidation and reduction of the reference (ferrocene) and indicator (quinone) in aqueous electrolyte in a two-terminal, linear sweep voltammogram in which a counterelectrode of relatively large surface area is used. The quinone has a half-wave potential, E((1/2)), that is pH-sensitive and can be used as a pH indicator; the ferrocene center has an E(1/2) that is a pH-insensitive reference. The key advantages are that such sensors require no separate reference electrode and function as long as current peaks can be located for reference and indicator molecules.

Orthogonal self-assembled monolayers: alkanethiols on gold and alkane carboxylic acids on alumina.

This work demonstrates the practicality of forming two self-assembled monolayers (SAMs), independently but simultaneously, by adsorption of two different adsorbates from a common solution onto a substrate exposing two different materials at its surface. The experimental procedure and the degree of independence achieved in the resulting SAMs are illustrated by examination of monolayers obtained by adsorption of alkanethiols on gold and alkane carboxylic acids on alumina. This procedure provides a method for modifying the surface characteristics of microlithographically generated patterns and offers a versatile technique for controlling solid-vapor and solid-liquid interfacial properties in systems having patterns with dimensions of the order of 1 micrometer.

Time and spatial dependence of the concentration of less than 105 microelectrode-generated molecules.

The time and spatial dependence of the concentration of as few as 40,000 electrogenerated, redox-active molecules has been determined. The distance between generator and detector microelectrodes in an array used in the study could be varied from 0.8 to 28 micrometers. Measurements of a sufficiently small ensemble of molecules allowed the experimental results to be compared with a quantitative simulation of the random movement of each member of the ensemble. The transit time of an electrogenerated species from the generator to a collector microelectrode was measured as a function of viscosity, diffusivity, and distance.

Surface functionalization of electrodes with molecular reagents.

The use of molecular reagents to manipulate the properties of electrode surfaces has broad application in areas such as electrochemical synthesis, energy conversion and storage, displays, sensors, and new kinds of microelectronic devices. Surface modification of electrodes has contributed to a revival of interest in basic and applied research in electrochemistry and electrochemical devices. This article is focused on specific examples of systems modified electrodes where basic developments provide promising opportunities for applications stemming from the properties of molecules attached to an electrode surface.

Electrochemical reduction of horse heart ferricytochrome c at chemically derivatized electrodes.

Platinum or gold electrodes derivatized with an N,N'-dialkyl-4,4'-bipyridinium reagent can be used to reduce horse heart ferricytochrome c, whereas reduction doses does not occur at the "naked" electrodes. From 3 to 17.7 millimoles per liter, the reduction of ferricytochrome c is mass transport-limited at electrode potentials more negative than about -0.6 volt against a saturated calomel reference electrode. Data for the photoreduction of ferricytochrome c at derivatized p-type silicon photocathodes show directly that the rate of reduction is mass transport-limited. Use of derivatized electrodes may allow convenient manipulation and analysis of biological molecules that do not ordinarily respond at conventional electrodes.

Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes.

p-Si photocathodes functionalized first with an N,N'-dialkyl-4,4'-bipyridinium redox reagent, (PQ(2+/+-))(surf), and then with a Pt precursor, PtCl(6) (2-), give significant efficiency (up to 5%) for photoelectrochemical H(2) generation with 632.8-nm light. Naked p-Si photocathodes give nearly zero efficiency, owing to poor H(2) evolution kinetics that are improved by the (PQ(2+/+-))(surf)/Pt modification. The mechanism of H(2) evolution from p-Si/(PQ(2+/+-))(surf)/Pt is first photoexcitation of electrons to the conduction band of Si followed by (PQ(2+))(surf) --> (PQ(+-))(surf) reduction. The dispersion of Pt then catalyzes H(2)O reduction to give H(2) and regeneration of (PQ(2+))(surf). The overall energy conversion efficiency rivals the best direct optical to chemical conversion systems reported to date.

n-Type Si-based photoelectrochemical cell: New liquid junction photocell using a nonaqueous ferricenium/ferrocene electrolyte.

n-Type Si has been shown to serve as a stable photoanode in a cell for the conversion of light to electricity. The other components of the cell are a Pt cathode and an electrolyte consisting of an ethanol solution of [n-Bu(4)N]ClO(4) with a redox couple of ferricenium/ferrocene. Data from a two-compartment cell show that ferrocene is oxidized to ferricenium with 100 +/- 2% current efficiency at the Si photoanode. Furthermore, prolonged irradiation of the Si in a one-compartment cell yields constant photocurrent and output characteristics. The maximum open-circuit photopotential is approximately 700 mV, and the short-circuit quantum yield for electron flow at low light intensity exceeds 0.5. Conversion of monochromatic 632.8-nm light to electricity with approximately 2% power efficiency at an output voltage of approximately 200 mV has been sustained. These results represent a stable n-type Si-based photoelectrochemical cell.

Photoassisted electrolysis of water by irradiation of a titanium dioxide electrode.

Ultraviolet irradiation (351, 364 nm) of the n-type semiconductor TiO(2) as the single crystal electrode of an aqueous electrochemical cell evolves O(2) at the TiO(2) electrode and H(2) at the Pt electrode. The gases are typically evolved in a two: one (H(2):O(2)) volume ratio. The photoassisted reaction seems to require applied voltages, but values as low as 0.25 V do allow the photoassisted electrolysis to proceed. Prolonged irradiation in either acid or base evolves the gaseous products in amounts which clearly demonstrate that the reaction is catalytic with respect to the TiO(2). The wavelength response of the TiO(2) and the correlation of product yield and current are reported. The results support the claim that TiO(2) is a true photoassistance agent for the electrolysis of water. Minimum optical storage efficiencies of the order of 1% can be achieved by the production of H(2).