Development of a Coherent Interferometric Biosensor

Extremely sensitive methods that also show a very high degree of specificity are needed todayin biological, medical, and environmental arenas for detecting biomolecular and molecular analytes atconcentrations below 10-12M. Because of the enormous variety of base sequences possible, the nucleicacids present a particular challenge. The goal of the proposed research is to design and build a devicethat can quickly detect a desired sequence of bases in minute quantities of nucleic acid fragments. Anovel biosensor will be constructed capable of determining when a DNA probe has correctly hybridizedto an oligonucleotide bound to the biosensor. The probe will be labeled with a fluorophore that reactsin the near infrared. The use of fiber optics to help form the biosensor will permit a small portion ofradiant energy to be refracted through the fiber core. This energy, known as an evanescent wave, willbe absorbed and cause emission from the fluorophore. An optical-fiber, laser-based Mach-Zehnderinterferometer, operating in the near-infrared region, will determine whether a phase shift has occurredbetween the laser beam in the test optical fiber compared to the same beam in a reference optical fiber.Optical shifts as small as 10-6 radians can be detected with this instrument configuration. Thebiosensor attached to the interferometer theoretically may be capable of detecting about 3 x 10-21 moleof analyte (molecular weight of 15,000 Daltons). The biosensor could also use ribosomal RNA (rRNA)to detect specific base sequences. rRNA sequences have already been used to determine phylogeneticrelationships among bacteria. The sensitivity of the proposed instrument may be sufficient to correctlyidentify a specific bacterial strain from the rRNA of just a single cell. The biosensor could also beimportant for DNA sequencing studies.