Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 17. Improvement in detection limits using ultrathin perfluorosulfonated ionomer films in conjunction with continuous sample flow.

We report herein an attenuated total reflectance (ATR) absorbance-based spectroelectrochemical sensor for tris(2,2'-bipyridyl)ruthenium(II) ion [Ru(bpy)(3)(2+)] that employs ultrathin (24-50 nm) Nafion films as the charge-selective layer. This film serves to sequester and preconcentrate the analyte at the optically transparent electrode surface such that it can be efficiently detected optically via electrochemical modulation. Our studies indicate that use of ultrathin films in tandem with continuous flow of sample solution through the cell compartment leads to a 100-500-fold enhancement in detection limit (10 nM) compared to earlier absorbance-based spectroelectrochemical sensors ( approximately 1-5 microM); markedly shorter analysis times also result. We report the dependence of the measured absorbance on sample flow rate and Nafion film thickness, and also provide calibration curves that illustrate the linear range and detection limits of the sensor using a 24 nm film at a constant sample flow rate of 0.07 mL/min.

Spectroelectrochemical sensing based on attenuated total internal reflectance stripping voltammetry. 2. Determination of mercury and lead.

Detection of lead and mercury by attenuated total internal reflectance spectroscopy coupled to stripping voltammetry is demonstrated. Changes in attenuation of light passing through an indium tin oxide optically transparent electrode (ITO-OTE) accompany the electrodeposition and stripping of lead and mercury on the electrode surface. The change in absorbance during stripping of electrodeposited metal constitutes the analytical response that enables detection over a range of 2.5 x 10(-7)-5 x 10(-5) and 5 x 10(-8)-5 x 10(-5) M for mercury and lead, respectively. The spectroelectrochemical responses of mercury and lead on the ITO surface are characterized and optimized with respect to solution conditions, the potential excitation signals used for deposition and stripping, and wavelength for detection. The deposited metals were examined by environmental scanning electron miscroscopy, and the electrodeposition pattern of lead and mercury was found to influence the optical response.

Spectroelectrochemical sensing based on attenuated total internal reflectance stripping voltammetry. 3. Determination of cadmium and copper.

The optical and electrochemical properties of metallic films on ITO surfaces resulting from deposition of copper and cadmium were monitored by stripping voltammetry-attenuated internal reflectance spectroscopy. The voltammetric or optical responses of both metals were examined with respect to solution conditions such as pH and presence of dissolved oxygen. The morphologies of these films were also examined using environmental scanning electron microscopy, and the microscopic electrodeposition patterns were found to influence the optical response. The wavelength dependence of the optical response of deposited copper was determined and compared with calculations; optimal performance was at 400 nm for copper. A linear calibration curve was obtained over a range of 1 x 10(-7)-1 x 10(-4) M for copper and compared with that of cadmium. The simultaneous determination of cadmium and copper was demonstrated, and the mechanism of co-deposition is discussed.

Spectroelectrochemical sensing based on attenuated total internal reflectance stripping voltammetry. 1. Determination of lead and cadmium.

The electrodeposition and subsequent stripping of lead and cadmium on an indium tin oxide (ITO) optically transparent electrode (OTE) were monitored by attenuated total internal reflectance. Light passing through the ITO-OTE is attenuated proportionally to the concentration of metal ion and deposition time. The wavelength dependence of the optical responses of deposited Pb and Cd was determined; optimal performance based on maximum sensor absorbance was at 750 nm for Pb and at 400 nm for Cd. Calibration curves were obtained over a range of 5 x 10(-8) to 5 x 10(-5) M and 1 x 10(-9) to 1 x 10(-5) M for Pb and Cd, respectively, using change in absorbance that accompanied deposition and subsequent stripping of the electrodeposited metal from the ITO.

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.