Material witness: quantum writ large.

430 nature materials | VOL 7 | JUNE 2008 | www.nature.com/naturematerials salts — the charged material causes the reduction of Cu2+ ions to Cu metal and the latter is deposited on the electrode (Fig. 1b). By analysis of the product, the charge density is estimated to be similar to that found in the previous experiment, around 7 × 1014 cm–2. Further electrochemical reactions were possible — including the reduction of Fe(CN)6 to Fe(CN)6 and the generation of chemiluminescence — all of which indicate the electron is the charge carrier for the single-electrode system. As is the case with any new process in science, single-electrode electrochemistry raises new questions. For instance, is there a spatial distribution of electrons on the surface chains, or does a certain electronic distribution exist inside the dielectric material? And, is it possible to use electronic conduction on the dielectric to produce cooperative electrodeposition, nucleation and growth of metals or gases? In future studies, the technique could be applied to other dielectric materials of various sizes and shapes, and different media including solutions, gases, gels and protoplasm. Both negative and positive electrodes should be possible by contacting two insulators and then using them as individual electrodes in different cells. Also, by rubbing together different materials, it may be possible to obtain a wide range of electrode potentials. This could culminate in a list of electrode potentials — similar to normal electrode potentials and the triboelectric series1 — for electrostatically charged insulators. This would allow a scientist to choose, at a glance, the most suitable dielectric material to generate chemicals, collect and identify pollutants from soil and water, obtain metallic patterns of dielectric materials and stimulate biological processes in living cells. Further questions remain about the dielectric nature of individual polymeric chains, and whether the storage of electrons followed by reduction in solution induce, as in the case of conducting polymers6, conformational movements of the chains. The system requires deeper study to determine the potential gradient across the electrical double layer at the dielectric–solution interface. In addition, evolution of hydrogen eliminates protons from the solution and induces solution basification (OH– concentration increases); if the single-electrode discharges other cations, for example, Cu2+ in the copper electrodeposition experiment, an equivalent number of anions (for example, SO4) should remain in solution — how these surplus charges are balanced requires clarification. Finally, the efficiency of the process and the possibility of deterioration of the single dielectric electrodes, as is observed with the long-term use of metallic electrodes, need to be investigated. Its practical simplicity makes this one-electrode experiment attractive in the clarification of any complex electrochemical concept. As a result, this single-electrode technique is not limited to electrochemists — biologists, clinicians, physicists and engineers could easily use it to develop products and understand biological processes.