One singular electrochemical sensation.

There's no such thing as being too sensitive when it comes to analytical chemistry. Available techniques can readily detect trace elements in concentrations as low as parts per trillion, but picking out a lone molecule in the presence of many others is a much more difficult challenge.

Last year, Fu-Ren F. Fan and Allen J. Bard of the University of Texas at Austin demonstrated a method of detecting the tiny electrochemical current produced by a single molecule in solution. Now, along with their collaborator Juhyoun Kwak of the Korea Advanced Institute of Science and Technology in Taejon, the researchers have measured that current even more precisely. The group's findings appear in the Oct. 9 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

Improved sensitivity is the goal of much research in analytical chemistry. "The ultimate detection in analytical chemistry is the single molecule," says Maryanne M. Collinson of Kansas State University in Manhattan. She regards the approach of Fan and his colleagues as valuable. Such electrochemical techniques, for example, could be used someday to detect extremely low concentrations of trace contaminants in water.

Moreover, isolated molecules may have different physical and chemical properties from ensembles of the same molecule and show greater variation in behavior, Fan says. Scientists may obtain a clearer understanding of those properties by looking at individual molecules instead of many molecules in a solution.

Fan and his colleagues conducted their experiments with a solution of an iron compound that can undergo an electrochemical reaction. Since a small molecule requires a comparably small sensor, the team used a platinum-iridium electrode with a diameter of only 15 nanometers to measure the current.

A sheath of insulating wax surrounded the electrode tip, so it trapped a minuscule volume of solution--one-millionth of a trillionth of a milliliter--when it was lowered onto a conducting disk of indium-tin oxide. The researchers adjusted the concentration of the solution so that the volume trapped beneath the electrode would most likely contain just a single molecule of the iron compound.

As this molecule shuttled back and forth between the electrode tip and the conductive disk, the researchers detected a current of less than a trillionth of an ampere. The technique is about 60 times more sensitive than conventional electrochemical detection methods, Fan says.

Next, the group would like to try detecting the luminescence of a single molecule. Last year, Collinson and R. Mark Wightman of the University of North Carolina at Chapel Hill were able to detect the light given off by individual molecules undergoing chemical reactions. "If we could combine those experiments, that would be very interesting," Fan says. "Photodetection is usually much faster."