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FIGURE 6.31 Single-molecule detector for individual fl uorescent molecules in
solution.

molecules spend in the laser beam; therefore each molecule is reex-
cited many times and yields many fluorescent photons. The signature
of the passing molecule is the burst of photons that occurs when the
molecule is passing the laser beam (Fig. 6.31).
A lens, or microscope objective, and a slit are arranged to image the
photons from a small region around the laser beam waist onto a micro-
channel plate photomultiplier (MCPP) that counts individual photons.
The intense excitation light from the laser is blocked from reaching the
MCPP by a bandpass spectral filter, which is centered near the peak
fluorescent wavelength. The excitation light consists of extremely short
pulses, each about 70 trillionths of a second. The dye molecule does not
emit light until a billionth of a second after excitation, so the flash of
laser light fades before the feeble molecular glow occurs. For reliable
identification of individual molecules, the technique maximizes the
number of the detected photons and minimizes the number of back-
ground photons. Although some background remains, the technique
registers over 85 percent of the fluorescent molecules.
Developed to aid in sequencing chromosomes for the Human
Genome Project, the sensor detector’s high activity would allow DNA
sequencing rates hundreds of times faster than those obtainable with
present techniques. It would also eliminate the need for radioactive
materials, gels, and electrophoresis solutions, which often create dis-
posal problems, and it is expected to help make DNA sequencing a
routine diagnostic and clinical tool. One eventual benefit of the DNA
research may be rapid screening for any of 3500 known genetic dis-
eases such as diabetes, cystic fibrosis, and Alzheimer’s disease.
The ultrasensitive detector can be used to find and quantify min-
ute amounts of chemicals, enzymes, and viruses in the blood and
monitor the dispersal of extremely low concentrations of environ-
mental pollutants. The device may also be useful in studying the
interaction of viruses and their binding sites. It may be possible to