Single-Molecule Fluorescence Observed with Mercury Lamp Illumination

We demonstrate that it is possible to observe single fluorescent molecules using a standard fluorescence microscope with mercury lamp excitation and an inexpensive cooled charge-coupled device (CCD) camera. With this equipment, we have been able to observe single molecules of tetramethyl-rhodamine, rhodamine 6G, fluorescein isothiocyanate and green fluorescent protein. Immobilized molecules were observed both in air and in aqueous solution.