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.

The different approaches in which self-assembled monolayers (SAMs) have been applied in electroanalytical chemistry are reviewed. Three principal concepts have been used and can be categorized according to the mechanism of attachment of the monolayer to the electrode. These include the formation of covalent bonds between reactive organic molecules, such as chlorosilanes, with activated surfaces, the formation of lipid and bilayer lipid membranes that self-assemble on conductive surfaces and the adsorption of functionalized alkanes, e.g., alkanethiols, on well ordered surfaces. Systems for detecting inorganic, organic and biological species arc dcscribed and conclusions are drawn.

No abstract

Thin films with enantioselective properties for electrochemically active chiral probes were developed. Enantioselectivity was accomplished via molecular imprinting. The films were fabricated through the sol-gel technique and were spin-coated on ITO electrodes. The chiral selectivity recognition was detected using two enantiomer pairs: D- and L-3,4-dihydroxyphenylalanine (D- and L-dopa) and (R)- and (S)-N,N'-dimethylferrocenylethylamine [(R)-Fc and (S)-Fc]. A defined chiral cavity was obtained by selection of functional monomers that interact with the template molecule, followed by its removal. Chiral selection properties were measured by cyclic voltammetry and square wave voltammetry. For both template molecules, very good chiral recognition was revealed by electrochemical measurement. The nonspecific adsorption measured for reference nonimprinted films was negligible (less than 5%). Dopa imprinted films revealed both high sensitivity, by the detection of 1 nM (0.2 ppb) concentration, and excellent selectivity, when challenged with a series of catechol derivatives. Fc-imprinted films were able to detect ca. 2 ppm of the target molecule, with very good enantioselectivity and low nonspecific adsorption. To our knowledge, this is the first report of successful molecular imprinting of a ferrocene derivative.

A two-dimensional conducting polyaniline (PAN) monolayer has been formed on an electrically insulating monolayer. The approach is based on the electrochemical polymerization of surface-confined anilinium ions that were electrostatically attached to a negatively 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 examined by cyclic voltammetry, Fourier transform IR spectroscopy, X-ray photoelectron spectroscopy, wettability, and scanning electrochemical microscope. The formation of a capacitor-like assembly, in which electron transfer was blocked between PAN and the gold surface, was accomplished by electrochemically incorporating hexadecanethiol (C16) into the MDS monolayer. The PAN monolayer exhibits properties similar to those of a thin polymer film.

ExperimentalPreparation of Polymers 3 and 4: 3 and 4 were prepared by transition metal catalyzed ROP of [1]silaferrocenophane monomers according to wellknown procedures [5]. Molecular weights were estimated by gel permeation chromatography (GPC) calibrated to polystyrene standards. For 3: M w = 4.3´10 5 , Polydispersity Index (PDI) = 1.9. For 4: M w = 2.9´10 5 , PDI = 1.6.Preparation of Devices: Devices based on 3 and 4 were prepared by coating a 50 mm thick Mylar film with 1.5 mL of a 15 wt.-% solution of 3 or 4 in toluene. Film applications were accomplished with the use of a motor-driven blade applicator (designed and built in-house) using a stainless steel blade (Paul N. Gardiner Company) with a 0.25 mm slit opening driven at a constant linear velocity of 1 cm s ±1 . Mylar films (type A Mylar, Dupont) were purchased from Cadillac Plastics.Arc Discharge Simulations: Films of 3 and 4 supported on Mylar were placed underneath a copper mask possessing a 4.5 cm diameter circular aperture. The electron beam was generated using a modified SEM operating at variable filament currents. Discharge images were captured with a Tektronix model C1002 CCD video camera, which is fed into a Sony model EVO-9800 Hi8 videocassette recorder. Discharge current pulses were measured using a Tektronix model 72 500 transient digitizing oscilloscope.

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.

No abstract