Single Molecule\nElectrochemical Detection in Aqueous\nSolutions and Ionic Liquids

Abstract

Single\nmolecule electrochemical detection (SMED) is an extremely\nchallenging aspect of electroanalytical chemistry, requiring unconventional\nelectrochemical cells and measurements. Here, SMED is reported using\na “quad-probe” (four-channel probe) pipet cell, fabricated\nby depositing carbon pyrolytically into two diagonally opposite barrels\nof a laser-pulled quartz quadruple-barreled pipet and filling the\nopen channels with electrolyte solution, and quasi-reference counter\nelectrodes. A meniscus forms at the end of the probe covering the\ntwo working electrodes and is brought into contact with a substrate\nworking electrode surface. In this way, a nanogap cell is produced\nwhereby the two carbon electrodes in the pipet can be used to promote\nredox cycling of an individual molecule with the substrate. Anticorrelated\ncurrents generated at the substrate and tip electrodes, at particular\ndistances (typically tens of nanometers), are consistent with the\ndetection of single molecules. The low background noise realized in\nthis droplet format opens up new opportunities in single molecule\nelectrochemistry, including the use of ionic liquids, as well as aqueous\nsolution, and the quantitative assessment and analysis of factors\ninfluencing redox cycling currents, due to a precisely known gap size.