Biomedical subjects

Leif Nyholm

Publications and source records attributed to Leif Nyholm.

6 recordsLinked to original sources

PubMed2003

Separation high voltage field driven on-chip amperometric detection in capillary electrophoresis.

A new potentiostatless detection scheme for amperometric detection in capillary electrophoresis is presented based on the use of microband array electrodes positioned in the capillary electrophoresis electric field. In the present study, the spatial potential difference in the CE separation high-voltage field was measured using two gold microband electrodes positioned in the proximity of the capillary outlet. The induced potential difference between the two electrodes was recorded as a function of the applied separation high voltage and the dependence of the electrochemically generated current on the high-voltage field, and the concentration of a redox couple (Fe(CN)6(4-)/Fe(CN)6(3-)) was investigated. The results show that plots of the generated current versus the CE separation voltage have the same shape as cyclic voltammograms obtained with the same electrodes in a traditional potentiostatic setup and that the current is proportional to the concentration of the redox couple. As a decoupling device is not needed, the described potentiostatless approach significantly simplifies the instrumental setup for amperometric detection. This approach consequently holds great promise for application in inexpensive portable chip-based CE devices.

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PubMed2003

Gold-coated fused-silica sheathless electrospray emitters based on vapor-deposited titanium adhesion layers.

Gold-coated fused-silica electrospray (ES) emitters based on vapor-deposited adhesion layers of titanium have been manufactured to investigate the possibilities of producing durable ES emitters applicable in chip-based analytical devices. The stabilities of the emitters were studied by both electrospray and electrochemical experiments and a marked increase in the emitter lifetime, compared to that for Cr/Au coated emitters, was found for the Ti/Au emitters in the ES durability tests. This indicates that Ti (rather than Cr) adhesion layers should be used in association with large-scale fabrication of ES emitters by vapor-deposition techniques. The lifetime of about 500-700 hours also allowed the Ti/Au-coated emitter to be used as an integrated part of a capillary liquid chromatography column coupled to a mass spectrometer in a series of LC/MS experiments. The Ti/Au coating was further studied by electrochemical techniques and scanning electron microscopy in conjunction with X-ray spectroscopy. It is shown that the eventual failure of the Ti/Au emitters in ES experiments was due to an almost complete detachment of the gold layer. Experimental evidence suggests that the detachment of the gold coating was due to a reduced adhesion to the titanium layer during oxidation in positive electrospray. Most likely, this was caused by the formation of an oxide layer on the titanium film. It is thus shown that unlimited emitter stabilities are not automatically obtained even if the metallic adhesion layer is stabilized by an oxide formation under positive electrospray conditions.

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PubMed2003

Electrochemically controlled solid-phase microextraction and preconcentration using polypyrrole coated microarray electrodes in a flow system.

Polypyrrole coated microarray electrodes have been used for electrochemically controlled solid-phase microextraction and preconcentration on individually addressable gold microband electrodes. In this study, a flow of analyte solution was maintained over the band electrodes by positioning a capillary in a vertical position over the electrode array during both the extraction and the detection of the desorbed compounds. This experimental set-up was used to evaluate the possibilities of using electrochemically controlled solid-phase microextraction with conducting polymers as a preconcentration step in miniaturised flow systems. The performance of the polymer, which was prepared by electrochemical polymerisation using a solution of 0.05 M pyrrole and 0.1 M LiClO4, was investigated using chloride as a model analyte employing different extraction times and analyte concentrations. It was found that significant preconcentration was possible using extraction times of only a few minutes and that a good linearity between the extraction time and detection response was present both for mM and microM chloride concentrations. Compared to a recent study (Liljegren et al., Analyst, 2002, 127, 591-597), using a more traditional solid-phase microextraction technique under electrochemical control, the preconcentration factor could be increased by a factor of about 210 by using the present flow system based approach. This increase in the preconcentration factor can be explained by the significant decrease in the desorption volume (i.e. reduced dilution of the desorbed analyte) associated with the use of the present flow system. With the present approach, the detection limit for the model analyte chloride could be decreased from 10 microM to 625 nM employing an extraction time of 180 s.

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PubMed2003

A comparison of the electrochemical stabilities of metal, polymer and graphite coated nanospray emitters.

Chronoamperometry (CA) and cyclic voltammetry (CV) were used to compare the electrochemical behavior of metal, polymer and graphite coated nanospray emitters. It is shown that electrochemical reactions occurring at the emitter surface limit the lifetime of the noble metal coated nanospray emitters while the graphite coated nanospray emitters show good electrochemical stabilities. Although the surface of the graphite coated emitters may be passivated at positive potentials, the conductive coating is not lost as for the noble metal coated nanospray emitters. The graphite coated nanospray emitters still produced a stable nanospray signal despite the presence of a passivated surface. The polymer (i.e. polyaniline) coated nanospray emitters showed very low electrochemical activity and could not be thoroughly tested by CA. The relative short lifetimes seen in the electrochemical tests are qualitatively comparable with those obtained in nanospray experiments, in which only the outmost tip of the emitter is electrochemically active. However, the electrochemical stress during CA far exceeds the stress during ESI, which implies that CA can be used to perform quick and simple estimates of emitter stabilities. To our knowledge, this is the first time the electrochemical behavior of metal, polymer and graphite coated nanospray emitters has been compared.

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PubMed2002

Deviceless decoupled electrochemical detection of catecholamines in capillary electrophoresis using gold microband array electrodes.

Samples containing microM concentrations of dopamine, (+/-)-isoproterenol, para-aminophenol and chlorogenic acid have been separated by capillary electrophoresis (CE) and detected using end-column amperometric detection based on a novel decoupling method. The present decoupling approach involves the use of an electrochemical detector chip containing an array of microband electrodes where the working and reference electrodes are positioned only 10 microm from each other. The short distance between the working and reference electrodes ensures that both electrodes are very similarly affected by the presence of the CE electric field. With this method, no shift in the detection potential was seen when the CE high voltage was applied. This eliminated the need for a reoptimization of the detection potential to compensate for the influence of the separation voltage on the detection. It is also demonstrated that catecholamines can be detected using gold microband electrodes by careful adjustment of the detection potential to avoid the formation of gold oxide. Such careful adjustments of the detection potential are straightforward using the present decoupling method.

Aminophenols↗

PubMed2002

Electrochemical solid-phase microextraction of anions and cations using polypyrrole coatings and an integrated three-electrode device.

A method for the extraction, transfer and desorption of anions and cations under controlled potential conditions employing a new integrated three-electrode device is described. The device, containing working, reference and counter electrodes, was prepared from tubes that could be moved vertically with respect to each other. In this way, a small amount of solvent, held by capillary force, remained between the electrodes when the device was lifted out of a solution after an extraction. This design allowed the potential control to be maintained at all times. With the new integrated device, it was possible to perform potential controlled desorption into vials containing as little as 200 microl of solution. The required ion exchange capacity was obtained by electrodeposition of a polypyrrole coating on the surface of the glassy carbon working electrode. Solid-phase microextractions of several cations or anions were performed simultaneously under potentiostatic control by doping the polypyrrole coating with different anions such as perchlorate and p-toluenesulfonate. The efficiency of the extractions, which could be altered by varying the potential of the working electrode, could be increased by 150 to 200% compared to extractions using normal solid-phase microextraction conditions under open circuit conditions. A constant potential of +1.0 V and -0.5 V with respect to the silver pseudo reference electrode, was found to be well-suited for the extraction of samples containing ppm concentrations of anions (chloride, nitrite, bromide, nitrate, sulfate and phosphate) and cations (cadmium, cobalt and zinc), respectively.

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