In Situ Measurements of Chemical Sensor Film Dynamics by Spectroscopic Ellipsometry. Partitioning of a Chromophore

Spectroscopic ellipsometry data are presented that describe the dynamics of the partitioning of a model chromophore (Ru(bpy)32+) into a thin porous solid film (Nafion). Backside optical interrogation permitted the study of the film at wavelengths where the bathing solution containing the chromophore significantly absorbed light. Data acquired in situ while the model chromophore Ru(bpy)32+ partitioned into the film were successfully interpreted using an optical multilayer layer model. This model was constructed by sequential experimental examination of the individual component layers of the system. A quantitative description of the dynamic film was achieved by assembling a model film layer that contained Tauc-Lorentz oscillators to describe both the real and imaginary parts of the refractive index using the Kramer-Kronig relationship. Subsequent analysis of experimental data using the model resulted in a quantitative description of how the optical constants and thickness of the film changed as chromophore Ru(bpy)32+ partitioned into it from solution. The film's thickness also changed during the course of the partitioning. On the first incorporation of the chromophore, the film contracted and then, as the film approached equilibrium with the bathing solution, re-expanded. Digital simulations of three different chromophore partitioning modes were presented. Analysis of Δ vs Ψ plots for the three simulated modes strongly supports a simple uniform optical layer model description of the partitioning process for the Ru(bpy)32+−Nafion film system under the prevailing experimental conditions. Overall, the results gave new insights into the acquisition of the data for and associated interpretation of the chromophore-film partitioning process

Dynamic In Situ Spectroscopic Ellipsometry of the Reaction of Aqueous Iron(II) with 2,2‘-Bipyridine in a Thin Nafion Film

Dynamic in situ spectroscopic ellipsometry studies of the chemical reaction between ferrous ion and 2,2‘-bipyridine (bpy) in a thin Nafion film are presented. A simple prototype system composed of a thin Nafion film on a glass substrate was used throughout the work. The reaction was detected by optically monitoring the formation of the strongly absorbing complex ion, Fe(bpy)32+ (ε520 = 7.70 × 103 M-1 cm-1 in 0.1 M NaCl). The changes in film optical constants, n and k, and the thickness upon exposure of it to various solutions were monitored in a flow cell with the film on the backside of the substrate relative to the interrogation by light. A “step-by-step” approach was used to isolate the component parts of the system in which the film was consecutively exposed to solutions in the following order:  supporting electrolyte, bpy, and, last, ferrous iron solution. The optical properties of the materials were quantitatively described before and during mass transport within the film by modeling using the appropriate multilayer optical models, i.e., the Cauchy equation for nonabsorbing media and the Urbach and Tauc−Lorentz (oscillator) functions for a film that absorbed. The experiments done allowed study of the diffusion in the film and the chemical reactions that are important in the sensing scheme for ferrous iron. Ligand (bpy) diffusion followed a two-stage diffusion mechanism described by a Berens−Hopfenberg model for incremental sorption (D25 = 7.04 × 10-13 cm2 s-1). The stabilities of the appropriate systems, i.e., Nafion film with bpy, iron, and iron complex, were studied by exposing equilibrated films to circulating supporting electrolyte solutions. The measurements gave important insights into a set of film chemical reactions and, in turn, selective film dynamics. This work exemplifies the usefulness of spectroscopic ellipsometry in monitoring the kinetics of a chemical reaction in situ, as well as the changes in the film physical properties under dynamic conditions

Cloud Point Extraction for Electroanalysis: Anodic Stripping Voltammetry of Cadmium

Cloud point extraction (CPE) is a well-established technique for the preconcentration of hydrophobic species from water without the use of organic solvents. Subsequent analysis is then typically performed via atomic absorption spectroscopy (AAS), UV–vis spectroscopy, or high performance liquid chromatography (HPLC). However, the suitability of CPE for electroanalytical methods such as stripping voltammetry has not been reported. We demonstrate the use of CPE for electroanalysis using the determination of cadmium (Cd²⁺) by anodic stripping voltammetry (ASV). Rather than using the chelating agents which are commonly used in CPE to form a hydrophobic, extractable metal complex, we used iodide and sulfuric acid to neutralize the charge on Cd²⁺ to form an extractable ion pair. This offers good selectivity for Cd²⁺ as no interferences were observed from other heavy metal ions. Triton X-114 was chosen as the surfactant for the extraction because its cloud point temperature is near room temperature (22–25 °C). Bare glassy carbon (GC), bismuth-coated glassy carbon (Bi-GC), and mercury-coated glassy carbon (Hg-GC) electrodes were compared for the CPE-ASV. A detection limit for Cd²⁺ of 1.7 nM (0.2 ppb) was obtained with the Hg-GC electrode. ASV with CPE gave a 20x decrease (4.0 ppb) in the detection limit compared to ASV without CPE. The suitability of this procedure for the analysis of tap and river water samples was demonstrated. This simple, versatile, environmentally friendly, and cost-effective extraction method is potentially applicable to a wide variety of transition metals and organic compounds that are amenable to detection by electroanalytical methods

Simplified Nitrate-Reductase-Based Nitrate Detection by a Hybrid Thin-Layer Controlled Potential Coulometry/Spectroscopy Technique

A novel method for the detection of nitrate was developed using simplified nitrate reductase (SNaR) that was produced by genetic recombination techniques. The SNaR consists of the fragments of the Mo–molybdopterin (MO–MPT) binding site and nitrate reduction active site and has high activity for nitrate reduction. The method is based on a unique combination of the enzyme-catalyzed reduction of nitrate to nitrite by thin-layer coulometry followed by spectroscopic measurement of the colored product generated from the reaction of nitrite with Griess reagents. Coulometric reduction of nitrate to nitrite used methyl viologen (MV²⁺) as the electron transfer mediator for SNaR and controlled potential coulometry in an indium tin oxide (ITO) thin-layer electrochemical cell. Absorbance at 540 nm was proportional to the concentration of nitrate in the sample with a linear range of 1–160 μM and a sensitivity of 8000 AU M^–1. The method required less than 60 μL of sample. Detection of nitrate could also be performed by measuring the charge associated with coulometry. However, the spectroscopic procedure gave superior performance because of interference from the large background charge associated with coulometry. Results for the determination of nitrate concentration in several natural water samples using this device with spectroscopic detection are in good agreement with analysis done with a standard method

Optically Transparent Carbon Nanotube Film Electrode for Thin Layer Spectroelectrochemistry

Carbon nanotube (CNT) film was evaluated as an optically transparent electrode (OTE) for thin layer spectroelectrochemistry. Chemically inert CNT arrays were synthesized by chemical vapor deposition (CVD) using thin films of Fe and Co as catalysts. Vertically aligned CNT arrays were drawn onto a quartz slide to form CNT films that constituted the OTE. Adequate conductivity and transparency make this material a good OTE for spectroelectrochemistry. These properties could be varied by the number of layers of CNTs used to form the OTE. Detection in the UV/near UV region down to 200 nm can be achieved using these transparent CNT films on quartz. The OTE was characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, UV–visible spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and thin layer spectroelectrochemistry. Ferricyanide, tris­(2,2′-bipyridine) ruthenium­(II) chloride, and cytochrome c were used as representative redox probes for thin layer spectroelectrochemistry using the CNT film OTE, and the results correlated well with their known properties. Direct electron transfer of cytochrome c was achieved on the CNT film electrode

Carbohydrate-Based Label-Free Detection of <i>Escherichia coli</i> ORN 178 Using Electrochemical Impedance Spectroscopy

A label-free biosensor for <i>Escherichia coli</i> (<i>E. coli</i>) ORN 178 based on faradaic electrochemical impedance spectroscopy (EIS) was developed. α-Mannoside or β-galactoside was immobilized on a gold disk electrode using a self-assembled monolayer (SAM) via a spacer terminated in a thiol functionality. Impedance measurements (Nyquist plot) showed shifts due to the binding of <i>E. coli</i> ORN 178, which is specific for α-mannoside. No significant change in impedance was observed for <i>E. coli</i> ORN 208, which does not bind to α-mannoside. With increasing concentrations of <i>E. coli</i> ORN 178, electron-transfer resistance (<i>R</i>_et) increases before the sensor is saturated. After the Nyquist plot of <i>E. coli</i>/mixed SAM/gold electrode was modeled, a linear relationship between normalized <i>R</i>_et and the logarithmic value of <i>E. coli</i> concentrations was found in a range of bacterial concentration from 10² to 10³ CFU/mL. The combination of robust carbohydrate ligands with EIS provides a label-free, sensitive, specific, user-friendly, robust, and portable biosensing system that could potentially be used in a point-of-care or continuous environmental monitoring setting

Carbon Nanotube-Loaded Nafion Film Electrochemical Sensor for Metal Ions: Europium

A Nafion film loaded with novel catalyst-free multiwalled carbon nanotubes (MWCNTs) was used to modify a glassy carbon (GC) electrode to detect trace concentrations of metal ions, with europium ion (Eu³⁺) as a model. The interaction between the sidewalls of MWCNTs and the hydrophobic backbone of Nafion allows the MWCNTs to be dispersed in Nafion, which was then coated as a thin film on the GC electrode surface. The electrochemical response to Eu³⁺ was found to be ∼10 times improved by MWCNT concentrations between 0.5 and 2 mg/mL, which effectively expanded the electrode surface into the Nafion film and thereby reduced the diffusion distance of Eu³⁺ to the electrode surface. At low MWCNT concentrations of 0.25 and 0.5 mg/mL, no significant improvement in signal was obtained compared with Nafion alone. Scanning electron microscopy and electrochemical impedance spectroscopy were used to characterize the structure of the MWCNT–Nafion film, followed by electrochemical characterization with Eu³⁺ via cyclic voltammetry and preconcentration voltammetry. Under the optimized conditions, a linear range of 1–100 nM with a calculated detection limit of 0.37 nM (signal/noise = 3) was obtained for determination of Eu³⁺ by Osteryoung square-wave voltammetry after a preconcentration time of 480 s

Electrospun Carbon Nanofiber Modified Electrodes for Stripping Voltammetry

Electrospun polyacrylonitrile (PAN) based carbon nanofibers (CNFs) have attracted intense attention due to their easy processing, high carbon yield, and robust mechanical properties. In this work, a CNF modified glassy carbon (GC) electrode that was coated with Nafion polymer was evaluated as a new electrode material for the simultaneous determination of trace levels of heavy metal ions by anodic stripping voltammetry (ASV). Pb²⁺ and Cd²⁺ were used as a representative system for this initial study. Well-defined stripping voltammograms were obtained when Pb²⁺ and Cd²⁺ were determined individually and then simultaneously in a mixture. Compared to a bare GC electrode, the CNF/Nafion modified GC (CNF/Nafion/GC) electrode improved the sensitivity for lead detection by 8-fold. The interface properties of the CNF/Nafion/GC were characterized by electrochemical impedance spectroscopy (EIS), which showed the importance of the ratio of CNF/Nafion on electrode performance. Under optimized conditions, the detection limits are 0.9 and 1.5 nM for Pb²⁺ and Cd²⁺, respectively

Bare and Polymer-Coated Indium Tin Oxide as Working Electrodes for Manganese Cathodic Stripping Voltammetry

Though an essential metal in the body, manganese (Mn) has a number of health implications when found in excess that are magnified by chronic exposure. These health complications include neurotoxicity, memory loss, infertility in males, and development of a neurologic psychiatric disorder, manganism. Thus, trace detection in environmental samples is increasingly important. Few electrode materials are able to reach the negative reductive potential of Mn required for anodic stripping voltammetry (ASV), so cathodic stripping voltammetry (CSV) has been shown to be a viable alternative. We demonstrate Mn CSV using an indium tin oxide (ITO) working electrode both bare and coated with a sulfonated charge selective polymer film, polystyrene-<i>block</i>-poly­(ethylene-<i>ran</i>-butylene)-<i>block</i>-polystyrene-sulfonate (SSEBS). ITO itself proved to be an excellent electrode material for Mn CSV, achieving a calculated detection limit of 5 nM (0.3 ppb) with a deposition time of 3 min. Coating the ITO with the SSEBS polymer was found to increase the sensitivity and lower the detection limit to 1 nM (0.06 ppb). This polymer modified electrode offers excellent selectivity for Mn as no interferences were observed from other metal ions tested (Zn²⁺, Cd²⁺, Pb²⁺, In³⁺, Sb³⁺, Al³⁺, Ba²⁺, Co²⁺, Cu²⁺, Ni³⁺, Bi³⁺, and Sn²⁺) except Fe²⁺, which was found to interfere with the analytical signal for Mn²⁺ at a ratio 20:1 (Fe²⁺/Mn²⁺). The applicability of this procedure to the analysis of tap, river, and pond water samples was demonstrated. This simple, sensitive analytical method using ITO and SSEBS-ITO could be applied to a number of electroactive transition metals detectable by CSV

Highly Oxidizing Excited States of Re and Tc Complexes

Like the Re analogue, the ligand-to-metal charge transfer (LMCT) excited-state of [Tc(dmpe)3]2+ (dmpe is bis-1,2-(dimethylphosphino)ethane) is luminescent in solution at room temperature. Surprisingly, both [M(dmpe)3]2+* species have extremely large excited-state potentials (ESPs) as oxidantsthe highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Re(Tc); E1/2* = +2.61(2.52) V versus SCE; λmax = 526(585) nm). Using a Rehm−Weller analysis with a series of aromatic hydrocarbons as electron-transfer quenchers, E1/2(Re2+*/Re+) has been determined to be 2.58 V, in good agreement with the calculated value. Both [M(dmpe)3]2+* species are quenched by chloride ion and both can function as excited-state oxidants in water solution