Electrocatalytic oxidation of carbohydrates at copper(II) -modified electrodes and its application to flow-through detection

A handy detection cell for end-column electrochemical detection in capillary electrophoresis

A handy and simple detection cell was constructed using a mixing joint for end-column electrochemical detection in capillary electrophoresis (CE). The cell allows for positioning of the working electrode at the end of the separation capillary without the aid of micropositioners. The design facilitates the exchange of electrodes and capillaries without the need to refabricate the entire capillary-electrode setup. The cell can be assembled in a short period of time. Alignment with the joint screw proved to be reproducible for working electrodes of copper and gold. The advantages of reduced time and low cost make the device very attractive for the routine analysis of electroactive species, such as carbohydrates and their derivatives, purine bases and nucleosides, amino acids, and catecholamines etc. by CE with electrochemical detection.

Determination of electroactive organic acids by anion-exchange chromatography using a copper modified electrode

An ion-chromatographic method combined with electrochemical detection at a copper-based chemically modified glassy carbon electrode (Cu-GC) has been shown to provide a simple analytical approach for the determination of some common organic acids in alkaline medium. Under the optimized isocratic chromatographic conditions (i.e. 0.1 M NaOH plus 80 mM CH3COONa), organic acids such as gallic, ascorbic, gluconic, lactobionic, galacturonic and glucuronic acid could be separated in less than 20 min. Under constant potential amperometric detection (i.e. 0.55 V vs. Ag-AgCl) the Cu-GC modified electrode allowed detection limits between 2 and 5 pmol for all investigated organic acids while the linear dynamic range spanned generally over three orders of magnitude. Examples of applications included the separation and quantitation of some common organic acids in vinegar, honey and tea samples, are given.

Electrochemical determination of carbohydrates: enzyme electrodes and amperometric detection in liquid chromatography and capillary electrophoresis

In recent years, electrochemical detection (EC) methods have become increasingly important for the determination of carbohydrate compounds in a variety of biological and pharmaceutical samples. In this work, recent advances in the design and application of EC approaches are reviewed, with the goal of providing the non-electrochemist with a basic understanding of the most important EC approaches to carbohydrate detection and an overview of their current applications. Two specific EC detection strategies are considered in detail: enzyme electrodes and electrodes used for HPLC or capillary electrophoresis detection.

The electroanalysis of mannitol, xylose and lactulose at copper electrodes: voltammetric studies and bioanalysis in human urine by means of HPLC with electrochemical detection

The electrochemical behavior of mannitol, xylose and lactulose has been investigated at a copper working electrode. A sensitive, accurate and precise method employing HPLC with electrochemical detection in the d.c. amperometric mode, has been developed and validated for the determination of mannitol and lactulose in human urine. The ratio of these probe carbohydrates is altered in conditions that cause damage to the intestinal mucosal barrier. Systematic studies employing cyclic voltammetry indicate that the electrode reaction involves an electrocatalytic oxidation of each carbohydrate in a process yielding a single irreversible anodic wave that is dependent on the ionic strength of the sodium hydroxide supporting electrolyte solution. High performance liquid chromatography with electrochemical detection was performed using a thin-layer cell housing a custom manufactured copper working electrode. The optimized HPLC method can detect 72, 57 and 419 pg of mannitol, xylose and lactulose injected on column, respectively. The corresponding linear calibration ranges are 359 pg-2.24 microgram, 57.4 pg-896 ng and 419 pg-262 ng, respectively. Solid-phase extraction of human urine on polar sorbents, and direct injection after simple 1 + 99 dilution in 0.025 M NaOH were compared for bioanalysis. Direct injection was selected for further method developed as the technique proved robust and simple. The optimized method was validated for the determination of mannitol and lactulose in human urine over the concentration ranges predicted when assessing intestinal permeability (0.25-2.5 mg ml-1 mannitol and 0.05-1.0 mg ml-1 lactulose). Over these ranges intra- and inter-assay bias is < +/- 6.5%, and imprecision (coefficient of variation) is < 9% for each carbohydrate. The validated method provides a useful alternative to HPLC with pulsed-amperometric detection at gold electrodes.

Ultrasensitive voltammetric detection of underivatized oligonucleotides and DNA

Electrochemical detection of nucleotides, ssDNA, and dsDNA was accomplished by using sinusoidal voltammetric detection at copper microelectrodes. Generally, detection of these molecules utilizes the electroactive nature of adenine and guanine residues at most electrode surfaces. The detection approach used in this study is based on the electrocatalytic oxidation of sugars and amines at copper surfaces. All nucleotides and DNA molecules comprise a ribose sugar backbone and primary amines present on the different nucleobases. Consequently, the detection approach is universal to all types of nucleotides. As the number of sugar moieties increases with the length of an oligonucleotide, the detection sensitivity is enhanced for bigger oligonucleotides. Irreversible adsorption of these oligonucleotides and other biomacromolecules like dsDNA on the electrode surface was avoided with sinusoidal voltammetry since it is a scanning electrochemical technique. The sensitivity of the detection strategy is, however, still preserved due to the effective decoupling of the faradaic signal from the capacitive background currents in the frequency domain. The ssDNA and dsDNA were detected in the picomolar concentration range. The electrochemical signal due to dsDNA is actually higher than that due to ssDNA due to the larger number of easily accessible sugars on the outer perimeter of a dsDNA double helix compared to those on a ssDNA of the same size. This is in contrast to the existing electrochemical detections techniques based on the electroactivity of the nucleobase.