Angewandte Chemie International Edition

Cover image for Vol. 56 Issue 50

Editor: Neville Compton; Editor Emeritus: Peter Gölitz

Online ISSN: 1521-3773

Associated Title(s): Angewandte Chemie, Chemistry - A European Journal, Chemistry – An Asian Journal, ChemistryOpen, ChemPhotoChem, ChemPlusChem, Zeitschrift für Chemie

For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (8), 1108-1110

Catcher in the Rye

Placeholders in a sort of "molecular grainfield"
help to build a new type of sensor

The development of sensors that can selectively fish a specific type of molecule out of a mixture is among the highest goals of many chemists. Vladimir M. Mirsky and his coworkers at the University of Regensburg have now come close to meeting this objective - with a clever coating for electrodes that recognizes molecules by their shape.

To accomplish this, they use a special method of "molecular imprinting", in which specific template molecules are brought together with an initially soft, malleable plastic. The shape of the template particle is pressed into the atomic mould. Once the material has hardened, the imprint remains, much like a handprint in fresh cement: If the template molecules are later brought in contact with the hardened plastic, they again fit into the indentation - they are "recognized" by the material.

However, Mirsky and his colleagues did not want to use a lump of plastic, but rather a metal surface to recognize molecules. They therefore coated the metal with long, rod-shaped molecules - in the presence of the substances to be recognized later, which are also deposited on the surface. Under a strong microscope, the result looks like a field of rye - where the rod-shaped molecules represent the stalks of grain - with gaps in the places where the template molecules have lain.

Unfortunately, this field loses its imprint very quickly, because once the template molecules are removed, the loosely attached rods wander around on the metal surface, erasing the applied pattern. However, Mirsky found a simple trick: He used longer "stalks" and also attached the template molecules to the electrode surface with a small anchor. These "placeholders" hinder the destruction of the pattern by the moving rod-shaped molecules; the longer "stalks" still reach beyond the placeholder - the imprint remains.

Electrodes coated in this manner can specifically fish molecules with which they have been imprinted out of solutions. The particles position themselves into the imprinted depressions like missing pieces in a puzzle. As a result, the electrical properties of the metal surface change, and the chemists can use a suitable instrument to determine if the desired molecules have docked.