Patent ID: 7572640

Claim:
A method for determining the presence or absence of a single molecule of a protein in a biological sample, comprising: a) labeling the molecule with a label, wherein the label comprises a fluorescent moiety that has low photobleaching and that is capable of emitting at least about 200 photons when stimulated by a laser emitting light at the excitation wavelength of the moiety; b) detecting the presence or absence of the label in a single molecule detector comprising a laser, wherein the label is detected by focusing the laser on a spot not less than about 5 microns in diameter that contains the moiety and directing no more than about 5 microJoules of total energy at the spot, passing a portion of the sample through an interrogation space comprising the spot, and detecting electromagnetic radiation emitted by the fluorescent moiety, wherein the electromagnetic radiation emitted by the fluorescent moiety is such that it allows detection of a labeled molecule at a limit of detection of less than about 100 femtomolar.