Electrochemical Impedance Spectroscopy in the Characterisation and Application of Modified Electrodes for Electrochemical Sensors and Biosensors

Electrochemical impedance spectroscopy is finding increasing use in electrochemical sensors and biosensors, both in their characterisation, including during successive phases of sensor construction, and in application as a quantitative determination technique. Much of the published work continues to make little use of all the information that can be furnished by full physical modelling and analysis of the impedance spectra, and thus does not throw more than a superficial light on the processes occurring. Analysis is often restricted to estimating values of charge transfer resistances without interpretation and ignoring other electrical equivalent circuit components. In this article, the important basics of electrochemical impedance for electrochemical sensors and biosensors are presented, focussing on the necessary electrical circuit elements. This is followed by examples of its use in characterisation and in electroanalytical applications, at the same time demonstrating how fuller use can be made of the information obtained from complete modelling and analysis of the data in the spectra, the values of the circuit components and their physical meaning. The future outlook for electrochemical impedance in the sensing field is discussed.     

Caminoside A,an antimicrobial glycolipid isolated from the marine sponge Caminus sphaeroconia

Extracts of the marine sponge Caminus sphaeroconia showed potent activity in a screen for bacterial type III secretion inhibitors. Bioassay guided fractionation of the extract led to the isolation of the novel antimicrobial glycolipid caminoside A (1). The structure of caminoside A was elucidated by analysis of spectroscopic data and chemical degradation.[structure: see text]     

An impedance study of the adsorption of nucleic acid bases at glassy carbon electrodes

Polyamidoamine (PAMAM) dendrimers (generation 3.5 and 4) interaction with bovine serum albumin (BSA) was studied. The intensity of intrinsic fluorescence of two tryptophan residues and a shift in wavelength of their emission maxima were chosen as indicators of protein conformational changes. It is shown that the generation 4 has a greater impact on spectral properties of serum albumin than generation 3.5.     

Electrochemical studies and square wave adsorptive stripping voltammetry of the antidepressant fluoxetine

The electrochemical reduction of the antidepressant drug fluoxetine was investigated by cyclic, linear sweep, differential pulse and square wave voltammetry using a hanging mercury drop electrode in alkaline buffer solution in water and in a water/acetonitrile mixed solvent. Cyclic voltammograms in aqueous solution showed very strong adsorption of fluoxetine on the electrode with formation of a compact film. The effect of addition of different percentages of acetonitrile on the voltammetric response was evaluated. It is shown that acetonitrile protects the electrode surface, thus preventing the adsorption of fluoxetine as a compact film, although reduction occurs at more negative potentials. Adsorption was used to accumulate the drug onto the electrode surface. The adsorbed species were measured voltammetrically by reduction at -1.3 V in an aqueous 0.05 M Ringer buffer, pH 12, 20% acetonitrile v/v. Linear calibration graphs were obtained in the range 0.52-5.2 M. The quantification of fluoxetine in pharmacological formulations existing in the market was performed using adsorptive square wave cathodic stripping voltammetry. and compared with data from UV spectrophotometry. The method is simple and not time-consuming. A comparative high performance liquid chromatography assay with UV detection was performed. Recovery data for both methods are reported.     

Spectral- and frequency-encoded fluorescence imaging

A method for obtaining fluorescence images with a high number of resolvable points by using spectral and frequency encoding is presented. Broadband excitation light is encoded with a wavelength-dependent frequency modulation and dispersed onto the sample with a grating to simultaneously illuminate an entire image line. The Fourier transform of the frequency-encoded fluorescence emission provides one line of the image. Mechanical scanning along a direction orthogonal to the wavelength-encoded axis allows creation of the two-dimensional fluorescent image. This method is applicable for developing submillimeter diameter endoscopes. The principles of the technique are validated by imaging indocyanine green fluorescence in microfluidic channels.     

Wavepacket propagation using time-sliced semiclassical initial value methods

A new semiclassical initial value representation (SC-IVR) propagator and a SC-IVR propagator originally introduced by Kay [J. Chem. Phys. 100, 4432 (1994)], are investigated for use in the split-operator method for solving the time-dependent Schrodinger equation. It is shown that the SC-IVR propagators can be derived from a procedure involving modified Filinov filtering of the Van Vleck expression for the semiclassical propagator. The two SC-IVR propagators have been selected for investigation because they avoid the need to perform a coherent state basis set expansion that is necessary in other time-slicing propagation schemes. An efficient scheme for solving the propagators is introduced and can be considered to be a semiclassical form of the effective propagators of Makri [Chem. Phys. Lett. 159, 489 (1989)]. Results from applications to a one-dimensional, two-dimensional, and three-dimensional Hamiltonian for a double-well potential are presented.     

The Brumer-Stark conjecture in some families of extensions of specified degree

As a starting point, an important link is established between Brumer's conjecture and the Brumer-Stark conjecture which allows one to translate recent progress on the former into new results on the latter. For example, if is an abelian extension of relative degree , an odd prime, we prove the -part of the Brumer-Stark conjecture for all odd primes with belonging to a wide class of base fields. In the same setting, we study the -part and -part of Brumer-Stark with no special restriction on and are left with only two well-defined specific classes of extensions that elude proof. Extensive computations were carried out within these two classes and a complete numerical proof of the Brumer-Stark conjecture was obtained in all cases.

     

In vivo human retinal imaging by ultrahigh-speed spectral domain optical coherence tomography

An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented that achieves acquisition rates of 29,300 depth profiles/s. The sensitivity of SD-OCT and time domain OCT (TD-OCT) are experimentally compared, demonstrating a 21.7-dB improvement of SD-OCT over TD-OCT. In vivo images of the human retina are presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW.     

Low-pressure gradient micro-ion chromatography with ultra-short monolithic anion exchange column

A 1.0 x 0.4 cm silica based monolithic anion exchanger has been produced and evaluated for use within a miniature gradient suppressed ion chromatography system based upon low-pressure micro-scale peristaltic pumps.