Time-resolved identification of individual mononucleotide molecules in aqueous solution with pulsed semiconductor lasers

We applied a short-pulse diode laser emitting at 640 nm with a repetition rate of 56 MHz in combination with a confocal microscope to study bursts of fluorescence photons from individual differently labeled mononucleotide molecules in water. Two newly synthesized dyes, an oxazine dye (MR121) and a rhodamine dye (JA53), and two commercially available dyes, a carbocyanine dye (Cy5) and a bora-diaza-indacene dye (Bodipy630/650), were used as fluorescent labels. The time-resolved fluorescence signals of individual mononucleotiode molecules in water were analyzed and identified by a maximum likelihood estimator (MLE). Taking only those single molecule transits which contain more than 30 collected photoelectrons, the two labeled mononucleotide molecules, Cy5-dCTP and Bodipy-dUTP, can be identified by time-resolved fluorescence spectroscopy with a probability of correct classification of greater than 99%. Our results show that at least three differently labeled mononucleotide molecules can be identified in a common aqueous solution. We obtain an overall classification probability of 90% for the time-resolved identification of Cy5-dCTP, MR121-dUTP and Bodipy-dUTP molecules via their characteristic fluorescence lifetimes of 1.05 ± 0.33 ns (Cy5-dCTP), 2.07 ± 0.59 ns (MR121-dUTP) and 3.88 ± 1.71 ns (Bodipy-dUTP).