Cleaning and Polishing Voltammetric Electrodes
Polishing powders comprise micron and submicron sized particles which can be dangerous, especially if inhaled. Please follow your local safety guidelines and especially work in a well-ventilated environment.
Use a suitable polishing kit such as product number ET030 from eDAQ, that uses abrasive alumina polishing powders.
Local lapidary supply companies may have equivalent products. Diamond, silica, or silicon carbide powders of similar particle size can also be used. A polishing pad that has a pressure sensitive adhesive (PSA) backing can be affixed to a glass plate to ensure a flat polishing surface.
A dry cloth can scratch the electrode and can cause the cloth to tear, always wet a new cloth with abrasive powder suspension, or deionised water before use. Use each cloth only with a powder of single particle size. Rinse cloths with deionised water after each polishing cycle, but reserve them for use only with that particle size polish in the future. Rather than adding dry powder to a polishing pad, it is often better to use a prepared polishing suspension. If you have a source of dry powder suspend it in a wash bottle containing ethanol, and then add a small amount of the suspension (shake well before use) to the pad.
These procedures are recommended for eDAQ or other voltammetric disk electrodes.
Resist the temptation to over-polish the electrodes. In most cases the electrode remains clean after the experiment, or could be washed clean with suitable organic solvent to remove greasy organic materials, or dilute acid or base (0.1 mol/L HCl, HNO3, NaOH) to remove inorganics. Protein deposits can be hydrolysed with an appropriate commercial enzyme-based cleaner.
Mercury films on platinum or glassy carbon substrates can be removed by gently wiping the electrode tip with lab tissue (dispose of tissue in mercury waste container) any residual mercury can be removed by holding the electrode at an oxidizing potential (eg +1 V) in dilute acid solution (0.1 mol/L).
Only if there is an intractable coating electrodeposited on the electrode, or after extended use, will the electrode need any polishing at all. Polishing should only be done if there is visually obvious surface contamination (use a magnifying glass) or where a trial cyclic voltammogram in clean solvent indicates that there has been surface contamination. Standard maintenance polishing should use a 0.05 micron polishing powder, only a badly scratched electrode, or one with an extremely intractable coating would need to use coarser (larger particle size) powders. Some frequent users like to do a maintenance polish every day, however others may go many days without any surface degradation of the electrode.
Automatic polishing machines are useful if large numbers of electrodes have to be polished on a regular basis, however, manual polishing is more usual in a research or teaching lab.
When hand polishing, grip the electrode low down (near its tip) and hold it in a vertical position while making figure-8 motions on the polishing pad, which is moistened with deionized water and to which has been added a small amount of polishing powder suspension. After three or four of the figure-8 motions, turn the electrode through 90 degrees in your grip and repeat the figure-8 movement. The aim is to ensure even wear on the electrode tip so that a flat surface is maintained.
If you have previously used glass body electrodes then be aware that PEEK or Teflon body electrodes need significantly less downwards pressure to be applied while polishing. Also note that coarse powders (more than 0.05 micron) will cause extensive abrasion of the electrode surface which then takes a lot more polishing with 0.05 micron alumina powder to obtain a mirror smooth finish.
After polishing remove any adherent material with a soft cloth or tissue. Some users prefer immersion of the electrode tip in an ultrasonic bath for a few seconds to remove adherent polish. eDAQ PEEK body working electrodes will withstand sonication but only immerse the tips of the electrode. Do NOT immerse the electrical contacts.
- Only polish when necessary.
- Use only light downwards pressure when polishing. When polishing, grip the electrode at its base, ie the end where it is being polished.
- Only use 0.05 micron powder for routine polishing.
- Only use coarser powders if absolutely necessary for very contaminated electrodes.
SAM on Gold Electrodes
Biosensor electrodes that have a self assembled monolayer (SAM) on gold substrate electrode usually employ an anchoring sulfur atom which binds strongly to the gold surface. It is possible to gently remove the SAM by electrochemical desorption, usually by applying a sufficiently large anodic (oxidising) potential for a short period of time. See:
Formation and removal of alkylthiolate self-assembled monolayers on gold in aqueous solutions. Christie A. Canaria, Jonathan So, James R. Maloney, C. J. Yu, Jeffrey O. Smith, Michael L. Roukes, Scott E. Fraser and Rusty Lansford, Lab on Chip, 6, 289–295, 2006. doi:10.1039/b510661c. You can download this paper here.
If the SAM is on a traditional disk electrode then you can also remove the coating by polishing as above. You may need to reins the polishing pad an reapply the abrasive powered several time to was away any organosulfur compounds which have a strong affinity for gold surfaces.
Glassy Carbon Electrodes
Glassy carbon is a unique material often used for electrodes. In most cases the general cleaning procedure, above, will give satisfactory results. However, a variety of subsequent surface pretreatments have been reported to optimise electrode behaviour in specific instances. An IUPAC review article is available.
The tip of these reference electrodes can become fouled with use in which case it can be washed or lightly polished (like a working electrode) to remove intractable adhering material. Do not try to get a mirror finish, the surface will remain dull. In extreme cases you can even use a scalpel, or razor blade, to shave a thin slice from the tip, which exposes fresh polymer junction (no need to polish).