WHAT IS A MICROELECTRODE ?

1. WHAT IS A MICROELECTRODE ? • An electrode with at least one dimensionsmall enough • (typically smaller than 50 μm) so that some of its properties • (e.g. mass transport) depend on its size. • These properties include: • High rates of mass transfer of the reactants/products to/from the electrode due to edge diffusion effects. • Small charging currents (background) due to small surface area. • Small ohmic losses (IR drop) due to small currents.

2. Diffusion fields for some electrode geometries Planar macro-electrode Microsphereelectrode Microdisc electrode • Non-planar diffusion at microelectrodes

3. Schematic top- and side view + optical micrograph of microdisc Microwire sealed in glass capillary by melting glass Microwire sealed in glass capillary with epoxy resin • Microelectrode types and fabrication • Microdiscs / microfibres

4. Gold microdisc array by photoresist lithography (S.Papadimitriou, D.P.Casey, J.F.Rohan and S.Sotiropoulos, ISE 55th Annual Meeting, 2004) Carbon microdisc array by laser lithography (Ecosse Sensor Ltd) • Microelectrode types and fabrication • Microdisc Arrays

5. Gold microband electrode by film technologies (D.Williams et al, Harwell Laboratory, 1986) • Microelectrode types and fabrication • Microband electrodes

6. Increased sensitivity ( i / C ) with where r is the microdisc radius • Short response time (to concentration and potential steps) • ADVANTAGES OF MICROELECTRODE SENSORS • High mass transfer rates • Response is independent of natural convenction or moderate flow rates.

7. ADVANTAGES OF MICROELECTRODE SENSORS • Low ohmic (IR) losses • Electrochemistry in low or ambient electrolyte level media. • Two-terminal simple electrochemical setup is adequate. • Small physical size • Low electronic noise and charging current background. • Small sample volume suffices. • Implantation to tissues with minimum damage.

8. APPLICATIONS OF MICROELECTRODE SENSORS • In electrochemical detectors of FIA, HPLC and CE analysis/separation systems (detector miniaturization, low noise). • In environmental field analysisand monitoring of natural waters and atmosphere (low cost, fast response, operational under ambient electrolyte and mass transfer conditions). • In foodstuffs and pharmaceuticals product control line analysis (low cost, fast response, operational in product conditions). • In biological systems research for intracellular and extracellular analysis both in vitro and in vivo (small size, operational under physiological conditions).

9. Solution inlet (from chromatograph) Teflon block with embedded microelectrodes Solution outlet Hollow spacer HPLC electrochromatogram of aminoacids Carbon microdisc electrode at different preparation stages Thin channel flow cell electrochemical detector P.Agrafiotou and S.Sotiropoulos , Analytica Chimica Acta, 497 (2003) 175. • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • High Pressure Liquid Chromatography (HPLC) with Electrochemical Detection (ECD)

10. Oxygen reduction voltammogram and potential pulse response. Pt microdisc sealed in glass and wrapped in an Al foil counter electrode Dissolved O2calibration curve in tap water D.Pletcher and S.Sotiropoulos, Anal.Chim.Acta, 322(½)(1996)83 • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • Dissolved Oxygen determination in waters

11. Solid Polymer Electrolyte film-covered Pt microdisc with a Ag ring counter electrode. Current response to O2concentration steps. Voltammograms in nitrogen and air gas streams. K.Wallgren and S.Sotiropoulos, Analytica Chimica Acta, 388 (1999) 51 • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • Oxygen determination in the gas phase

12. Co-planar electrode solid polymer electrolyte-based device with effectively microband electrodes . Voltammograms in nitrogen, 8% v/v O2and air gas streams. Gaseous O2calibration plot. K.Wallgren and S.Sotiropoulos, Sensors and Actuators B, 60 (1999) 174. • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • Oxygen determination in the gas phase

13. Ascorbic acid in an apple • Paracetamol of drug tablets Paracetamol oxidation at a Aumicroband electrode in a tablet dissolved in 0.1 M NaClO4 solution Ascorbic acid oxidation at a Aumicroband electrode attached / inserted into an apple. (D.Williams et al, Harwell Laboratory, 1986) • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS

14. Schematic diagram of experimental setup and micrograph of tip-cell aproach . Photograph of electrode-mammalian macrophage cell contact and current response. Principle of Cholesterol detection at an enzyme-modified Pt microelectrode. James D. Burgess and coworkers JACS 2007, 129, 11352 • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • Extracellular Measurements:Electrochemical Determination of Membrane Cholesterol

15. Schematic diagram of experimental setup for electrode tip cell intrusion. Dopamine oxidation voltammogram and current response upon electrode intrusion into the giant neuron of Phanorbis corneus slug. D. Sulzer, M. Lindau and coworkers J Neurosci 2003, 23, 5835 • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • Intracellular Measurements: dopamine inside invertebrate cells

16. release uptake Electrical stimulus Photograph of mapped rat brain and micrograph of carbon microfiber inserted in various brain areas. Dopamine oxidation voltammogram and current response upon electrical stimulation of rat brain. R.M.Wightman et alNeurosci 1988, 25, 513 • EXAMPLESOF MICROELECTRODE SENSOR APPLICATIONS • In vivoMeasurements: dopamine release/uptake inside rat brain