Eskil Sahlin

Micromachined electrophoresis chips with thick-film electrochemical detectors

A capillary electrophoresis (CE) microsystem, based on the combination of microphotolithographically fabricated separation chips and thick-film electrochemical detector strips, is described. The microsystem consists of a planar screen-printed carbon line electrode mounted perpendicular to the flow direction. Such coupling obviates the need for permanent attachment of the detector, to allow easy and fast replacement of the working electrode. Variables influencing the separation efficiency and amperometric response, including the channel-electrode spacing, separation voltage, or detection potential, are assessed and optimized. The versatility, simplicity, and low-cost advantages of the new design are coupled to an attractive performance, with submicromolar detection limits, and good precision. Applicability for assays of mixtures of nitroaromatic explosives or catecholamines is demonstrated. Such use of screen-printed detectors should also benefit conventional CE systems, particularly in applications requiring a frequent replacement of the working electrode.

Renewable pencil electrodes for highly sensitive stripping potentiometric measurements of DNA and RNA

Renewable graphite pencil electrodes are demonstrated to be excellent materials for adsorptive stripping measurements of trace nucleic acids. While displaying an attractive stripping performance, comparable to that of conventional carbon paste electrodes, the pencil electrode offers a convenient (mechanical) renewal, with each stripping potentiogram recorded at a fresh surface. Various pencil lead materials and lengths have been examined and experimental variables of the pretreatment and measurement procedures have been explored and optimized. The extremely low detection limits (e.g., 3 micrograms l-1 tRNA with 10 min accumulation) are coupled to a good surface-to-surface reproducibility (RSD of 6.4% for 14 repetitive measurements of 1 mg l-1 ssDNA).

Integrated electrophoresis chips/amperometric detection with sputtered gold working electrodes

An on-chip electrochemical detector for micromachined capillary electrophoresis (CE) systems, based on sputtering a gold working electrode directly onto the capillary outlet, is described. The new on-chip detector preparation requires no microfabrication or alignment procedures nor a decoupling mechanism. The attractive performance of the integrated electrophoresis chips/amperometric detection was demonstrated for the anodic detection of neurotransmitters. The response for dopamine was linear from 20 to 200 μM, with a LOD of 1.0 μM and a sensitivity of 52 pA/μM. Such intimate coupling of capillary electrophoresis chips and electrochemical detection facilitates the realization of complete integrated microanalytical devices.

Mercury nucleation on glassy carbon electrodes

Abstract The amount of mercury, deposited on a glassy carbon electrode at various potentials during the first 200 ms of electrolysis, has been determined by constant current coulometric stripping potentiometry. Eight microliter samples, containing approximately 4 M chloride, were placed on top of a combined glassy carbon, Ag counter and Ag AgCl reference electrode positioned up-side-down under a stereo microscope. It was found that, if the electrode was conditioned for 200 ms at −1.80 V. mercury was reversibly deposited during the following electrolysis stripping cycle. If, however, the conditioning cycle was omitted overpotentials of the order of magnitudes 0.50 V were needed in order to initiate mercury nucleation. If the electrode was vigorously anodised at 1.80 V for one minute, 8–10 conditioning pulses were required to restore the electrode sensitivity. Under the microscope it was observed that, provided that the electrode was potentiostatically conditioned, mercury was deposited homogeneously on the glassy carbon surface as shown by a monochromatic sky blue colour of the electrode surface. If not conditioned, the electrode became multi-coloured indicating an inhomogeneous deposition of mercury droplets. Electrode conditioning was found to have a profound effect on the coulometric stripping potentiometry determination of copper(II). On a conditioned electrode, copper(II) could be determined in a five decades concentration range, 1–10000 μM, with a relative accuracy varying only between 96 and 103%. If not conditioned, the accuracy varied between 70–90%. Mechanisms for the deposition and stripping of mercury and copper on glassy carbon electrodes are proposed.

Apparent Formation of an Oxidant by Electrochemical Reduction in the Mercury(0,I,II) Chloride System

The paradoxical appearance of a cathodic reaction sometimes observed in anodic stripping voltammetry and stripping potentiometry when using mercury film electrodes in chloride media containing mercury(II) has been investigated by systematically varying relevant chemical and electrochemical parameters and comparing the results with thermodynamic equilibrium calculations. Microscopic observations of morphological changes on the electrode surface caused by potential variations were made possible by using a novel electrode design. Three conditions have to be fulfilled for the cathodic reaction to occur:  (a) formation of calomel by reaction between elemental mercury on the electrode surface and mercury(II) in solution, (b) subsequent reduction of mercury(II) to elemental mercury on the calomelized electrode surface, and (c) a chloride concentration in the range 0.001-3.5 M. Different ways of avoiding the interference from the cathodic reaction in stripping voltammetry and stripping potentiometry are experimentally demonstrated, and a mechanism for the appearance of the cathodic reaction is proposed.

Capillary zone electrophoresis in laboratory-made fluorinated ethylene propylene capillaries.

Capillaries made of fluorinated ethylene propylene (FEP) with an inner diameter of 50 microm have been employed in capillary zone electrophoresis with UV-Vis absorbance detection. The capillaries were made in the laboratory with a recently developed technique using fluoropolymer heat shrink/melt tubing and a tungsten wire as a template for the channel. An electroosmotic flow was obtained in the channels and it is shown that an FEP capillary is more effective for a cationic test solute than a fused-silica capillary. The compatibility of FEP capillaries with optical detection is evaluated briefly.