Single-Molecule Detection
Research Interests

Detecting minute quantities of biologically important molecules in solution has important applications in many fields, including molecular biology, medical diagnosis, and forensic analysis. High-sensitivity techniques are needed, for example, to detect genetic disorders and the growth of tumors or to diagnose bacterial or viral infections. Current methods do not have the required sensitivity to adequately detect the small amounts of antibodies, antigens, or nucleic acids present in clinical samples. In the case of DNA, for example, specific nucleic acid sequences must be amplified by means of the polymerase chain reaction (PCR), which duplicates DNA. The use of many PCR cycles may introduce ambiguities arising from contamination and by mechanisms not yet fully understood. Our research group focuses on the development of laser-based techniques for the ultrasensitive detection and analysis of biological molecules. Applications to molecular biology and medical diagnosis are being explored.

Single-Copy Gene Detection

We have recently developed a new technique for the rapid detection of specific nucleic acid sequences in unamplified DNA samples. The method consists of using two nucleic-acid probes complementary to different sites on a target DNA sequence. The two probes are each labeled with different fluorescent dyes. When mixed with a sample containing the target DNA, the two probes hybridize to their respective binding sites on the same target DNA molecule. The sample is then analyzed by a laser-based ultrasensitive fluorescence system capable of detecting single fluorescent molecules at two different wavelength channels simultaneously. Since the probes are bound to the same target DNA molecule, their signals appear simultaneously. Thus, coincident detection of both dyes provides the necessary specificity to detect an unamplified, single-copy target DNA molecule in a homogeneous assay. If the target is not present, only uncorrelated events originating from free probes will be observed at either channel.

Single-Molecule Electrophoresis

The single-molecule electrophoresis technique consists in measuring the electrophoretic velocity of individual molecules-the velocity at which molecules move in solution under the influence of an electric field-and identifies them by comparing their measured velocity with the velocity characteristic of a particular molecular species. The electrophoretic velocity of a molecule is determined by its size, shape, and ionic charge and by the chemical environment of the solution in which it is contained. The electrophoretic velocity therefore provides a unique identification signature of each molecular species.

Ultrasensitive detection of nucleic acids in a microchip