Optical Impedance Spectroscopy Research Articles

Electrochemically modulated surface plasmon waves for characterization and interrogation of DNA-based sensors.

This work reports on a comparative analysis of electrical and optical measurements for structural characterization and for assessing signal transduction performance of a redox-labeled DNA-based sensing platform. We conducted complementary investigations employing conventional electrochemical techniques with electric current measurements in cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and confronted those results with optical measurements using surface plasmon waves while the redox assembly was undergoing similar electrochemical modulation as in the electrical CV and EIS measurements. The specific sensor configuration deployed here was composed of a methylene blue (MB)-modified single-stranded DNA (ssDNA) signaling probe and an unlabeled capture ssDNA probe that complements the signaling probe. Two types of signaling probes were employed: one with MB attached to the 3' end, which positions the redox marker closer to the electrode surface upon hybridization with the capture probe, and the other with MB attached to the 5' end, which places the redox marker farther from the electrode surface. For each molecular assembly and for each electrochemical modulation protocol, both the electrical and optical experimental data were quantitatively analyzed to determine the surface density of electro-active species and the rate of electron transfer between the redox marker and the electrode surface. Our experimental results highlight the consistency of the confronted methodologies and indicate that optical impedance spectroscopy utilizing electrochemically modulated surface plasmon waves, which is a transduction protocol immune from non-faradaic interferents that invariably are present in the electrical methodology, can provide a powerful route for developing a redox-labeled DNA-hybridization biosensing strategy.

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.

Electro-optical measurements of lithium intercalation/de-intercalation at graphite anode surfaces.

Graphite anode behaviour is of great interest for the optimization of Lithium-ion batteries. The improvement of battery performance depends on an understanding of lithium intercalation/de-intercalation in graphite anodes, especially in regards to energy and power density.In this study we present a new approach for investigating graphite anode behaviour under equilbirum and nonequlibirum conditions. It is based on reflectance change measurements from a graphite anode surface, relative to state of charge (SOC). We have introduced an innovative optical test cell and used a photodiode.A reflectance change hysteresis occurs between lithium intercalation and de-intercalation, under equilbrium conditions. This was ascribed to specific lithium-carbon bonds at the graphite particle edge region. Charging and discharging have a unique reflectance change characteristic. This was assigned to SOC nonequilibrium in the anode and serves as clear evidence that limited lithium mobility in the porous microstructure is a signficant loss factor. We then performed a more detailed analysis of the anode loss processes by correlating the anode reflectance change with dynamic electrical excitation. This method, Optical Impedance Spectroscopy (OIS), provided detailed information about the frequency range of the loss processes. It also confirmed our assigning the low frequency range (1mHz-3Hz) to solid state diffusion and lithium transport in the electrolyte filled pores. The results are in excellent agreement with previous EIS studies. We conclude that OIS is applicable for validating physicaly-based graphite anode models and determining model parameters, in combination with EIS.

Optical Impedance Spectroscopy with Single-Mode Electro-Active-Integrated Optical Waveguides

An optical impedance spectroscopy (OIS) technique based on a single-mode electro-active-integrated optical waveguide (EA-IOW) was developed to investigate electron-transfer processes of redox adsorbates. A highly sensitive single-mode EA-IOW device was used to optically follow the time-dependent faradaic current originated from a submonolayer of cytochrome c undergoing redox exchanges driven by a harmonic modulation of the electric potential at several dc bias potentials and at several frequencies. To properly retrieve the faradaic current density from the ac-modulated optical signal, we introduce here a mathematical formalism that (i) accounts for intrinsic changes that invariably occur in the optical baseline of the EA-IOW device during potential modulation and (ii) provides accurate results for the electro-chemical parameters. We are able to optically reconstruct the faradaic current density profile against the dc bias potential in the working electrode, identify the formal potential, and determine the energy-width of the electron-transfer process. In addition, by combining the optically reconstructed faradaic signal with simple electrical measurements of impedance across the whole electrochemical cell and the capacitance of the electric double-layer, we are able to determine the time-constant connected to the redox reaction of the adsorbed protein assembly. For cytochrome c directly immobilized onto the indium tin oxide (ITO) surface, we measured a reaction rate constant of 26.5 s–1. Finally, we calculate the charge-transfer resistance and pseudocapacitance associated with the electron-transfer process and show that the frequency dependence of the redox reaction of the protein submonolayer follows as expected the electrical equivalent of an RC-series admittance diagram. Above all, we show here that OIS with single-mode EA-IOW’s provide strong analytical signals that can be readily monitored even for small surface-densities of species involved in the redox process (e.g., fmol/cm2, 0.1% of a full protein monolayer). This experimental approach, when combined with the analytical formalism described here, brings additional sensitivity, accuracy, and simplicity to electro-chemical analysis and is expected to become a useful tool in investigations of redox processes.

Optical Impedance Spectroscopy as a New Characterization Method for Electrochromic Windows

not Available.

GaN light emitting diode as a photoreflectance pump source

A GaN light emitting diode is used as a photoreflectance pump source to acquire optical impedance spectroscopy data. Such a pump source has the advantage of having a controllable pump wave form (intensity, modulation depth, and shape) over a broad frequency range. Given the ready availability of light emitting diodes, many different wavelengths are potentially available.

Photolytic spectroscopy of simple molecules. V. Prompt and delayed photolysis of Cs2 excited at yellow wavelengths

This report belongs to a series concerned with the correlation of photolysis bands observed in electronic transition spectra of simple molecules with their dissociation products. In this work a time-delayed, double resonance application of optical impedance spectroscopy was used for the study of the state selective photolysis of Cs2 excited in the yellow range of visible wavelengths. Two independently tunable dye lasers were synchronously pulsed, the first being used to dissociate the constituents of the sample along the various possible channels and the second, delayed pulse being used to identify the products by exciting them selectively into easily detected Rydberg states. The time between photolysis and detection could be varied, starting from a value too short to permit collisional mixing or radiative cascading of the products. Particular attention was paid to the photolytic production of the fine structure components of the first and second electronically excited state of atomic cesium and to the subsequent processes which tended to degrade the selectivity produced in the initial distributions of product populations among the available states. Data were fit to a quantitative model from which rate coefficients could be extracted for various mixing processes. Reported here is what appears to be a first value for the fine-structure mixing cross section (5 2D5/2→5 2D3/2) of 17 Å2±60%. In addition a delayed process for the selective production of Cs(5 2D) atoms was found to result from the excitation of a state having a lifetime less than 100 ns that is tentatively identified as a molecular state correlated with Cs(5 2D), possibly 5d 3Δu.