Photocount Probability Distributions for Single Fluorescent Molecules

Most recent work on the detection of single molecules in solution relies in some way on the large average number of fluorescence photons that can be emitted by the analyte molecule before photolytic decomposition. In one way or another, the signal burst from the analyte molecule must be distinguished from the noise when the analyte molecule is not present. The discrimination can be accomplished, for example, by establishing a threshold photocount level so that signals exceeding the threshold are attributed to analyte molecules and those below the threshold to null measurements. The average photon yield before photolysis has been considered to be the primary consideration in selecting a molecule for use as a fluorescent tag. The purpose of this note is to show that photocount distributions for molecules that are photolyzed by a single reaction are less favorable for signal-to-noise discrimination in most cases than are those for molecules with multiple fluorophores that photolyze independently, even though the total photon yields are the same.