Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 5. Simulation of sensor response for different excitation potential waveforms

Evanescent Wave Cavity Ring-Down Spectroscopy in a Thin-Layer Electrochemical Cell

The application of evanescent wave cavity ring-down spectroscopy (EW-CRDS) in monitoring electrogenerated species within a thin-layer electrochemical cell is demonstrated. In the proof-of-concept experiments described, ferricyanide, Fe(CN)6(3-), was produced by the transport-limited oxidation of ferrocyanide, Fe(CN)6(4-), in a thin-layer solution cell (25-250 microm) formed between an electrode and the hypotenuse of a fused-silica prism. The prism constituted one element of a high-finesse optical cavity arranged in a triangular ring geometry with light being totally internally reflected at the silica/solution interface. The cavity was pumped with the output (approximately 417 nm) of a single-mode external cavity diode laser, which was continuously scanned across the cavity modes. The presence of electrogenerated ferricyanide within the resulting evanescent field, beyond the optical interface, was detected by the enhanced loss of light trapped within the cavity, as measured by the characteristic cavity ring down. In this way, the EW-CRDS technique is sensitive to absorption in only the first few hundred nanometers of solution above the silica surface. The cavity ring-down response accompanying both cyclic voltammetric and step potential chronoamperometry experiments at a variety of electrode-surface distances is presented, and the results are shown to be well reproduced in modeling by finite element methods. The studies herein thus provide a foundation for further applications of EW-CRDS combined with electrochemistry.

Further investigations on a poly(vinyl alcohol)- polyelectrolyte chemically selective optical film

The ionomer poly(vinylbenzyltrimethylammonium chloride) has been blended with cross-linked poly(vinyl alcohol) to form optically clear composites that can be covalently linked as thin films to oxide surfaces. Films are characterized using spectroscopic ellipsometry with refractive index (n) and extinction coefficient (k) data presented for wavelengths 300 to 1100 nm. A refractive index of 1.54 and average thickness of 709 nm are typical of an air dry film prepared by spin-coating. Dynamic in situ ellipsometry results for films exposed to 0.1 M KNO(3) and 1.0 mM Fe(CN)(6)(-3) are presented. Upon initial exposure to 0.1 M KNO(3), an air dry film expands by about 160% and stabilizes in size and refractive index at about 18 hours. Ion exchange of film cationic groups with ferricyanide is marked by slight film contraction, presumably due to electrostatic cross-linking by the multivalent anion. These films are useful in the spectroelectrochemical sensor with our newly developed fluorescence detection mode, as demonstrated by results of the reversible incorporation of the fluorescent anion fluorescein.

Spectroelectrochemical Sensing Based on Multimode Selectivity Simultaneously Achievable in a Single Device. 16. Sensing by Fluorescence

A fluorescence spectroelectrochemical sensor capable of detecting very low concentrations of metal complexes is described. The sensor is based on a novel spectroelectrochemical sensor that incorporates multiple internal reflection spectroscopy at an optically transparent electrode (OTE) coated with a selective film to enhance detection limits by preconcentrating the analyte at the OTE surface. Nafion was used as the selective cation exchange film for detecting Ru(bpy)(3)(2+), the model analyte, which fluoresces at 605 nm when excited with a 441.6-nm HeCd laser. The unoptimized linear dynamic range of the sensor for Ru(bpy)(3)(2+) is between 1 x 10(-)(11) and 1 x 10(-)(7) M with a calculated 2 x 10(-)(13) M detection limit. The sensor employs extremely thin films ( approximately 12 nm) without significantly sacrificing its sensitivity. The sensor response is demonstrated with varying film thicknesses. A state-of-the-art flow cell design allows variable cell volumes as low as approximately 4 microL. Fluorescence of the sample can be controlled by electromodulation between 0.7 and 1.3 V. Sensor operation is not reversible for the chosen model film (Nafion) and sample (Ru(bpy)(3)(2+)) but it can be regenerated with ethanol for multiple uses.

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 13. Detection of aqueous iron by in situ complexation with 2,2'-bipyridine

A spectroelectrochemical sensor with attenuated total reflectance at an indium-doped tin oxide (ITO) optically transparent electrode coated with a thin film of Nafion has been demonstrated for the determination of aqueous iron ion. The novelty of this sensor stems from its ability to take up colorless iron ion (Fe2+) from solution and complex it with an organic ligand, 2,2'-bipyridine (bipy), that has been previously loaded in the optically transparent charge-selective Nafion film coating the electrode. The resulting complex ion, tris(2,2'-bipyridyl)iron(II), Fe(bipy)3(2+), absorbs strongly, making it easily detectable via optical spectroscopy. Fe(bipy)3(2+) loaded into the selective film is oxidized to colorless Fe(bipy)3(3+), which gives rise to an absorbance change for quantifying iron. This paper maps the development of this sensor, from the spectroelectrochemical characterization of the complex ion at an ITO optically transparent electrode to an analysis of the uptake, retention, and optical response of the complex ion in the Nafion film. Finally, an evaluation of the uptake of aqueous Fe2+ by the bipy-loaded Nafion film is reported. These data include preliminary results illustrating the dependence of the sensor response on differing concentrations of Fe2+ in solution.

Easily constructed spectroelectrochemical cell for batch and flow injection analyses

The design and performance of an easily constructed spectroelectrochemical cell suitable for batch and flow injection measurements are described. The cell is fabricated from a commercially available 5-mm quartz cuvette and employs 60 ppi reticulated vitreous carbon as the working electrode, resulting in a reasonable compromise between optical sensitivity and thin-layer electrochemical behavior. The spectroelectrochemical traits of the cell in both batch and flow modes were evaluated using aqueous ferricyanide and compare favorably to those reported previously for similar cells.