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Metal ion linked multilayers offer a means of controlling interfacial energy and electron transfer for a range of applications including solar energy conversion, catalysis, sensing, and more. Despite the importance of structure to these interlayer transfer processes, little is known about the distance and orientation between the molecules/surface of these multilayer films. Here we gain structural insights into these assemblies using a combination of UV-Vis polarized visible attenuated total reflectance (p-ATR) and Förster Resonance Energy Transfer (FRET) measurements. The bilayer of interest is composed of a metal oxide surface, phosphonated anthracene molecule, Zn(II) linking ion, and a platinum porphyrin with one (P1), two (P2), or three (P3) phenylene spacers between the chromophoric core and the metal ion binding carboxylate group. As observed by both time-resolved emission and transient absorption, the FRET rate and efficiency decreases with an increasing number of phenylene spacers (P1 > P2 > P3). However, from p-ATR measurements we observe a change in orientation of porphyrins in the bilayer, which inhibits a uniform determination of the orientation factor (κ2) across the series. Instead, we narrow the scope of viable structures by determining the best agreement between experimental and calculated FRET efficiencies. Additionally, we provide evidence that suggests, for the first time, that the bilayer structure is similar on both planar and mesoporous substrates. [ABSTRACT FROM AUTHOR]

We present a simple chip-based refractometer with a central organic light-emitting diode (OLED) light source and two opposed organic photovoltaic (OPV) detectors on an internal reflection element (IRE) substrate, creating a true dual-beam sensor platform. For first-generation platforms, we demonstrate the use of a single heterojunction OLED based on electroluminescence from an [Alq.sub.3]/TPD heterojunction (tris-(8-hydroxyquinoline)-aluminum/N,N'-bis(3-methylphenyl)-N,N'-diphenyl- benzidine) and light detection with planar heterojunction pentacene/[C.sub.60] OPVs. The sensor utilizes the considerable fraction of emitted light from conventional thin-film OLEDs that is coupled into guided modes in the IRE, instead of into the forward (display) direction. A ray-optics description is used to describe light throughput and efficiency-limiting factors for light coupling from the OLED into the substrate modes, light traversing through the IRE substrate, and light coupling into the OPV detectors. The arrangement of the OLED at the center of the chip provides for two sensing regions: a 'sample' channel and a 'reference' channel, with detection of light by independent OPV detectors. This configuration allows for normalization of the sensor response against fluctuations in OLED light output, stability, and local fluctuations (temperature) that might influence sensor response. The dual-beam configuration permits significantly enhanced sensitivity to refractive index changes, relative to single-beam protocols, and is easily integrated into a field-portable instrumentation package. Changes in refractive index ([DELTA]RI) between [10.sup.-2] and [10.sup.-3] RI units could be detected for single beam operation, with sensitivity increased to [DELTA]RI [approximately] [10.sup.-4] RI units when the dual-beam configuration is employed. 10.1021/ac9026109

The structural and functional properties of ultrathin (< 5 nm) poly(aniline) (PANI) films deposited on indium tin oxide (ITO) have been investigated using electrochemical and attenuated total reflection (ATR) spectroscopy methods. Layer-by-layer (LbL) self-assembly was used to form films composed of one and two bilayers of PANI and poly(acrylic acid) (PAA), as well as single PANI layers of approximately monolayer thickness. PANI deposited on an ITO electrode is electroactive at neutral pH, both with and without codeposition of an acid dopant such as PAA. In the absence of PAA, it is hypothesized that the acidic surface groups on ITO can function as the counterion. The pH response of PANI single layer, [(PANI/PAA).sub.1], and [(PANI/PAA).sub.2] films was examined using both potentiometry and ATR spectroscopy. Near-Nernstian potentiometric responses over pH 3-9 were observed for all three types of films, consistent with the weak acid--base behavior expected of polymers assembled in a LbL film. The ATR spectral sensitivity to pH increases as the number of layers in the film increases, with the highest sensitivity achieved by monitoring the absorbance at 800 nm (predominately due to the emeraldine salt form) of [(PANI/ PAA).sub.2] films. Codeposition of PANI and PAA appears to produce a wide distribution of strengths of acidic and basic sites in the film and thus a large linear dynamic range, up to six pH units. The water contact angle of [(PANI/PAA).sub.2] is ~16[degrees], which is considerably more hydrophilic than either the PANI single layer or [(PANI/ PAA).sub.1] films (~40[degrees]). This film is shown to be a suitable substrate for deposition of a planar supported phospholipid bilayer. The supported membrane is highly impermeable to protons, which makes this architecture useful for monitoring transmembrane charge transport.

The study describes a method to determine the anisotropic optical and surface coverage of molecular films using polarized attempted total reflectance (ATR) absorbance measurements. Anisotropic values of the imaginary part of the refractive index (k) allow for the determination of the surface coverage and one order parameter of the film.

A technique to determine the angular orientation of a molecular assembly bound to the surface of a planar optical waveguide of arbitrary structure is described. The approach is based on measuring the absorption dichroic ratio by using the waveguide evanescent fields with orthogonal polarizations (TE, TM) and the same mode order to probe two molecular assemblies, (i) a reference sample composed of an isotrepic orientation distribution of dipoles and (ii) a sample of interest. The isotropic sample is used to characterize the waveguide structure, which then allows the orientation parameters of a molecular assembly under investigation to be determined from a measured dichroic ratio. The method developed here is particularly important for applications in gradient-index and multilayer planar waveguide platforms because in those cases the extension of previously reported approaches would require a full experimental characterization of the guiding structure, which would be problematic and may yield inaccurate results. OCIS codes: 240.6490, 240.6690, 130.2790, 130.3120.

A single-mode, electroactive waveguiding platform capable of measuring spectroelectrochemical responses of surfaceadsorbed redox-active molecules over a broad spectral bandwidth has been created. This new planar waveguide spectrometer is a combination of the previously developed electroactive integrated optical waveguide (EA-IOW; Dunphy, D. R.; Mendes, S. B.; Saavedra, S. S.; Armstrong, N. R. Anal Chem. 1997, 69, 3086-3094) with a recently reported simplified approach to broadband coupling (Bradshaw, J. T.; Mendes, S. B.; Saavedra, S. S. Anal Chem. 2002, 74, 1751-1759). With the use of a commercially available prism as an incoupling element, the EA-IOW can now guide visible light from at least 500 to 700 nm, improving upon its previously demonstrated monochromatic nature. Coupling profiles of various laser lines along with transmission spectra of narrow band-pass filters at various potentials are used to demonstrate the optical characteristics of this broadband EA-IOW and to compare its response to that of a conventional transmission instrument. Assessment of spectral resolution, performed by measuring the fwhm of various laser lines, ranges from 0.6 to 0.8. To demonstrate the capabilities of this technology, we show the acquisition of absorbance spectra of two different adsorbates, cytochrome c and ferrocenedicarboxylic acid, as a function of applied potential. Subtleties in the redox chemistries of adsorbed molecules, which were difficult to monitor with a monochromatic waveguide, are readily apparent when using the broadband coupling scheme.

The quantitative measurements of the complex binding process between rgp 120 and GSL receptors at a model lipid membrane surface are detailed. The total internal reflection fluorescence (TIRF) results demonstrates that a carbohydrate on ceramide is a requirement for rgp120 recognition, and there is little dependence on the chemical composition of the fatty acid chain of ceramide in terms of both hydroxylation and fatty acid chain length.

Research demonstrates the synthesis and characterization of new class of porphyrazines containing 'polyetherol' groups, which bind metal ions and aid in pophyrazines solubility. Absorbance and fluorescence spectral data on the modified porphyrazines are provided and their usage in chemical sensors is discussed.

A water vapor sensor based on a combination of sol-gel processing and planar optical waveguide technologies has been developed. The indicator erythrosin B was entrapped in a thin sol-gel film (thickness [approximately] 100 nm) prepared from methyltriethoxysilane, dimethyldiethoxysilane, and tetraethoxysilane. This dye exhibits an increase in absorbance in the presence of liquid or gaseous water. The sol-gel layer containing the dye was deposited onto a sol-gel-derived, single-mode planar waveguide. Outcoupled light intensity measurements (at 514.5 nm) over a range of water vapor concentrations (in a nitrogen gas stream) yielded a response over a wide range of relative humidity (< 1 - [approximately] 70%) at room temperature. Response and reversal times were less than 1 min, which may make this sensor attractive for real-time monitoring applications.

Avidin and streptavidin are widely used tetrameric proteins, acting as binding mediators in many bioanalytical applications. There is much interest in the structure and bioactivity of multilayer films that contain such proteins. Molecular orientation in asymmetric protein bilayers on planar glass substrates has been investigated, with a multilayer architecture comprising a biotin capped phospholipid bilayer, streptavidin and yeast cytochrome c biotinylated at cysteine 102. Orientation distributions were almost identical, indicating that lateral interactions are not need to form macroscopically oriented protein films.

Highly sensitive spectroelectrochemistry of adsorbed films on ITO is demonstrated with the electroactive integrated optical waveguide (EA-IOW). The EA-IOW, a single-mode planar waveguide coated with an ITO layer, is [approximately][10.sup.4]-fold more sensitive to changes in absorbance occurring during electrochemical events versus a single-pass transmission spectroelectrochemical experiment, as demonstrated by reduction of surface-adsorbed methylene blue. Furthermore, the EA-IOW is selective to near-surface events, as it is relatively insensitive to absorbance by solutions of dissolved chromophores at < 1 mM. The EA-IOW is also used to monitor the formation of Prussian Blue during the reduction of ferricyanide, an event that is not easily followed using current-detected cyclic voltammetry, due to interfering faradaic and non-faradaic electrochemical events. The optical background of the EA-IOW is potential-dependent and is explained by ion diffusion into the ITO and by voltage-dependent changes in optical constants for the material. Finally, the high sensitivity of the EA-IOW (relative to other evanescent-field-based spectroelectrochemical techniques) is discussed in terms of its design.

The molecular orientation in hydrated cytochrome c (cty c) films produced by adsorption to substrates of varying surface chemistry was examined. Findings indicated that a macroscopically arranged film of adsorbed cyt c molecules is formed when a single, high-affinity type of noncovalent binding occurs between the surface of the protein and the substrate surface. For example, electrostatic absorption of the positively charged protein to the negatively charged head groups of a Langmuir-Blodgett film of arachidic acid produces a narrow orientation distribution.

The molecular orientation in films of yeast cytochrome c immobilized through disulfide bonding between cysteine 102 and the thiol tail groups of self-assembled monolayers (SAMs) coated on planar glass substrates was studied. Findings indicated that the molecular architecture does not produce a highly oriented protein film. A significant portion of the immobilized cytochrome c is nonspecifically absorbed to the SAM surface, which produces a relatively broad distribution of heme orientations.

A novel sensor for gaseous iodine has been developed using a combination of sol-gel processing and planar integrated optical waveguiding technologies. The sensing principle is based on the detection of a charge transfer complex formed between iodine and phenyl groups that have been incorporated into a porous, methylated glass film. The glass film was prepared from siloxane precursors by the sol - gel method. Sensors were fabricated by coating the film over a single-mode planar waveguide. Light was coupled into and out of the laminate structure using integral grating couplers, and formation of the charge transfer complex was monitored as attenuated total reflection of the guided wave. The sensor exhibits a linear response to [I.sub.2] in the range of 100 ppb to 15 ppm with response and recovery times less than 15 s. The response is selective to 4 ppm iodine in the presence of 10 ppm chlorine and is stable for at least 3 months.