Optical impedance spectroscopy with single-mode electro-active-integrated optical waveguides

Impedance Analysis of Electrochemical Systems

Interpretation of impedance spectroscopy data requires both a description of the chemistry and physics that govern the system and an assessment of the error structure of the measurement. The approach presented here includes use of graphical methods to guide model development, use of a measurement model analysis to assess the presence of stochastic and bias errors, and a systematic development of interpretation models in terms of the proposed reaction mechanism and physical description. Application to corrosion, batteries, and biological systems is discussed, and emerging trends in interpretation and implementation of impedance spectroscopy are presented.

Adsorption Properties and Electron-transfer Rates of a Redox Probe at Different Interfaces of an Immunoassay Assembled on an Electro-active Photonic Platform

Physical and chemical properties of a redox protein adsorbed to different interfaces of a multilayer immunoassay assembly were studied using a single-mode, electro-active, integrated optical waveguide (SM-EA-IOW) platform. For each interface of the immunoassay assembly (indium tin oxide, 3-aminopropyl triethoxysilane, recombinant protein G, antibody, and bovine serum albumin) the surface density, the adsorption kinetics, and the electron-transfer rate of bound species of the redox-active cytochrome c (Cyt-C) protein were accurately quantified at very low surface concentrations of redox species (from 0.4 to 4% of a full monolayer) using a highly sensitive optical impedance spectroscopy (OIS) technique based on measurements obtained with the SM-EA-IOW platform. The technique is shown here to provide quantitative insights into an important immunoassay assembly for characterization and understanding of the mechanisms of electron transfer rate, the affinity strength of molecular binding, and the associated bio-selectivity. Such methodology and acquired knowledge are crucial for the development of novel and advanced immuno-biosensors.

Potential-Modulated Total Internal Reflection Fluorescence for Measurement of the Electron Transfer Kinetics of Submonolayers on Optically Transparent Electrodes

An electroreflectance method to determine the electron transfer rate constant of a film of redox-active chromophores immobilized on an optically transparent electrode when the surface coverage of the film is very low (<0.1 monolayer) is described herein. The method, potential-modulated total internal reflection fluorescence (PM-TIRF) spectroscopy, is a fluorescence version of potential-modulated attenuated total reflection (PM-ATR) spectroscopy that is applicable when the immobilized chromophores are luminescent. The method was tested using perylene diimide (PDI) molecules functionalized with p-phenylene phosphonic acid (PA) moieties that bind strongly to indium-tin oxide (ITO). Conditions to prepare PDI-phenyl-PA films that exhibit absorbance and fluorescence spectra characteristic of monomeric (i.e., nonaggregated) molecules were identified; the electrochemical surface coverage was approximately 0.03 monolayer. The tilt angle of the long axis of the PDI molecular plane is 58° relative to the ITO surface normal, 25° greater than the tilt angle of aggregated PDI-phenyl-PA films, which have a surface coverage of approximately one monolayer. The more in-plane orientation of monomeric films is likely due to the absence of cofacial π-π interactions present in aggregated films and possibly a difference in PA-ITO binding modes. The electron transfer rate constant (k_s,opt) of monomeric PDI-phenyl-PA films was determined using PM-TIRF and compared with PM-ATR results obtained for aggregated films. For PDI monomers, k_s,opt = 3.8 × 10³ s^-1, which is about 3.7-fold less than k_s,opt for aggregated films. The slower kinetics are attributed to the absence of electron self-exchange between monomeric PDI molecules. Differences in the electroactivity of the binding sites on the ITO electrode surface also may play a role. This is the first demonstration of PM-TIRF for determining electron transfer rate constants at an electrode/organic film interface.

Detection of influenza virus by electrochemical surface plasmon resonance under potential modulation

In this study we report the development of a novel viral pathogen immunosensor technology based on the electrochemical modulation of the optical signal from a surface plasmon wave interacting with a redox dye reporter. The device is formed by incorporating a sandwich immunoassay onto the surface of a plasmonic device mounted in a micro-electrochemical flow cell, where it is functionalized with a monoclonal antibody aimed to a specific target pathogen antigen. Once the target antigen is bound to the surface, it promotes the capturing of a secondary polyclonal antibody that has been conjugated with a redox-active methylene blue dye. The methylene blue displays a reversible change in the complex refractive index throughout a reduction-oxidation transition, which generates an optical signal that can be electrochemically modulated and detected at high sensitivity. For proof-of-principle measurements, we have targeted the hemagglutinin protein from the H5N1 avian influenza A virus to demonstrate the capabilities of our device for detection and quantification of a critical influenza antigen. Our experimental results of the EC-SPR-based immunosensor under potential modulation showed a 300 pM limit of detection for the H5N1 antigen.

Electron-Transfer Rate in Potential-Modulated Redox Reactions with Electro-Active Optical Waveguides

A novel methodology has been developed to determine electron-transfer rate in electrically driven redox reactions. Based on a widely adopted electrical circuit describing faradaic processes in an electrochemical cell, the approach uses a combination of impedance data from optical and electrical measurements that are simultaneously acquired in a spectroelectrochemical experiment. Once the consistency of our methodology was experimentally corroborated, it was put to practice for investigating electron-transfer rate of cytochrome c adsorbates at very low concentrations on an indium tin oxide electrode by using a highly sensitive, single-mode, electro-active, integrated optical waveguide platform. Different surface densities of redox species on the electrode interface and different ionic strengths in the electrolyte solution were studied. Higher surface densities and higher ionic strengths are shown to slow down the electron-transfer process between the redox molecules and the working electrode.

Influenza virus immunosensor with an electro-active optical waveguide under potential modulation

Here we report the development of a novel immunosensor-based strategy for label-free detection of viral pathogens by incorporating a sandwich bioassay onto a single-mode, electro-active, integrated optical waveguide (EA-IOW). Our strategy begins with the functionalization of the electro-active waveguide surface with a capture antibody aimed at a specific virus antigen. Once the target antigen is bound to the photonic interface, it promotes the binding of a secondary antibody that has been labeled with a methylene blue (MB) dye. The MB is a redox-active probe whose optical absorption can be electrically modulated and interrogated with high sensitivity by a propagating waveguide mode. In this effort, we have targeted the hemagglutinin (HA) protein from the H5N1 avian influenza A virus to demonstrate the capabilities of the EA-IOW device for detection and quantification of an important antigen. Our initial results for the HA H5N1 influenza virus show a remarkable limit of detection in the pico-molar range.

Spectroelectrochemical Properties of Ultra-Thin Indium Tin Oxide Films under Electric Potential Modulation

In this work, the spectroscopic properties of ultra-thin ITO films are characterized under an applied electric potential modulation. To detect minute spectroscopic features, the ultra-thin ITO film was coated over an extremely sensitive single-mode integrated optical waveguide, which provided a long pathlength with more than adequate sensitivity for optical interrogation of the ultra-thin film. Experimental configurations with broadband light and several laser lines at different modulation schemes of an applied electric potential were utilized to elucidate the nature of intrinsic changes. The imaginary component of the refractive index (absorption coefficient) of the ultra-thin ITO film is unequivocally shown to have a dependence on the applied potential and the profile of this dependence changes substantially even for wavelengths inside a small spectral window (500-600 nm). The characterization technique and the data reported here can be crucial to several applications of the ITO material as a transparent conductive electrode, as for example in spectroelectrochemical investigations of surface-confined redox species.