Polymer composite coated multifiber microelectrodes as voltammetric microsensors for the detection of dopamine

For the investigation of biochemical communication mechanisms in brain and the understanding of brain diseases like Parkinson’s or Alzheimer as well as for pharmacological screening studies it is necessary to determine and to monitor neurotransmitter concentrations in brain tissue with high local resolution. In the majority of cases voltammetric detection with carbon fiber microelectrodes (CFMEs) is used for this purpose [2].

An alternative method for the fabrication of a miniaturised dopamine sensor with integrated polymermodified working electrode, reference electrode and counter electrode is presented here.

For the construction of a new dopamine sensor we used platinum multifiber microelectrodes manufactured by Thomas RECORDING (Giessen, Germany) with active electrode surface diameters of 10-20 µm [1]. These tip-shaped electrodes consist of up to 7 platinum/tungsten wires which are insulated from each other by quartz glass and each of which can be addressed independently. They have a constant outer diameter of 100 µm over a length of more than ten centimeters und thus are equally well suited as are carbon fiber microelectrodes as minimally invasive probes for the voltammetric determination of biogenic compounds in brain slices or implanted in living brain.

Unlike carbon electrodes, noble metal electrodes are not commonly used in this field, due to electrode fouling [2, 3] and the relatively small potential window. However, these disadvantages can be overcome by polymer coating [2]. An integrated microsensor with two Pt counter electrodes, an electrochemically deposited Ag/AgCl reference electrode and a dopamine sensing working electrode covered with an antifouling film of polypyrrole deposited by electropolymerization has been realised (Fig. 1) [4].

Fig. 1.  Integrated multifiber microelectrode sensor. GE: counter electrode, RE: Ag/AgCl reference electrode, AE: Working electrode coated with polypyrrole.

Currently PEDOT/ CNT/ surfactant-modified Pt microelectrode surfaces are tested. Thin films of PEDOT and PEDOT/ CNTs were formed by potentiodynamic electropolymerization in aqueous solution. By addition of surfactants [5] adherent and stable films with a high CNT content could be obtained [Fig. 2].
Another beneficial effect is improved permselectivity for cationic molecules like dopamine as opposed to anionic species like ascorbate, the main interferent present in high concentrations in the brain.

Fig. 2.  SEM image of Pt microelectrodes surfaces  after electropolymerization of PEDOT in aqueous solution (top). SEM image of Pt microelectrodes surfaces  after electropolymerization of PEDOT/ CNTs /surfactant composite from aqueous solution (bottom). White bars: 3 µm. 

Using platinum microelectrodes coated with this PEDOT/CNT/Additives-composite we were able to measure dopamine concentrations in phosphate buffer solution pH 7,4/ Argon of less than 500 nM with differential pulse voltammetry (DPV) [Fig. 3].

Fig. 3.  Baseline corrected DPV current measurements of different dopamine concentrations in PBS/ argon.

Electrodes coated solely with PEDOT showed lower sensitivity. Stability could be verified with microelectrodes which were stored for three weeks under ambient conditions. Even though the current sensitivity decreased to less than 50 % of its original value, the detection limit was still below 500 nM [Fig. 4].

Fig. 4.  Peak current vs. concentration calibration curves, a) of a freshly prepared PEDOT/ CNT/ surfactant modified electrode, and b) of the same electrode, 3 weeks later.

The electrochemical potentiodynamic coating method of the specially manufactured multifiber microelectrodes allows for a high degree of reproducibility. This procedure can be easily adapted to inexpensive and non-polluting large scale production as there are no special or intricate requirements.

By modification with different functional coatings of every individual platinum electrode surface and/or by applying different voltammetric protocols, it will be possible to sample several parameters at the same time. This versatile concept, already established in planar electrode array fabrication, seems not to be realised yet for tip-shaped microsensors.

References:

[1]       Thomas RECORDING GmbH, Gießen, Germany, Metal Microelectrodes, 2008. [online]. Available: http://www.thomasrecording.com/en/cms/front_content.php?idcatart=61&lang=1&client=1 [accessed: Dec. 2, 2008].

[2]       D. L. Robinson, A. Hermans, A. T. Seipel, and R. M. Wightman, "Monitoring rapid chemical communication in the brain," Chemical Reviews, vol. 108, pp. 2554-2584, 2008.

[3]       R. F. Lane and A. T. Hubbard, "Differential double pulse voltammetry at chemically modified platinum electrodes for in vivo determination of catechol amines," Anal. Chem., vol. 48, pp. 1287-1293, 1976.

[4]       U. Thomas, K. Bauer, and J. Heinze, "Microsensor composed of multi-fiber microelectrodes." Application DE102004060742A, Juli 6, 2006

[5]       N. Sakmeche, E. A. Bazzaoui, M. Fall, S. Aeiyach, M. Jouini, J. C. Lacroix, J. J. Aaron, and P. C. Lacaze, "Application of sodium dodecylsulfate (SDS) micellar solution as an organized medium for electropolymerization of thiophene derivatives in water," Synthetic Metals International Conference on Science and Technology of Synthetic Metals, vol. 84, pp. 191-192, 1997.