Infrared Diode Laser Excites Visible Fluorophores

Brent D. Johnson, Senior News Editor

Many diagnostic and biological tests use antibodies or DNA probes, called markers, that bond to specific biological structures and fluoresce when excited by a specific optical wavelength. Typically, the fluorophores emit at a frequency longer than the excitation wavelength; that is, toward the red spectrum. However, if the excitation and emission signals are proximate in wavelength, separating them at the detector can sometimes be difficult.

Infrared light may not excite fluorescence in biological tissue, but it does excite nanospheres synthesized from rare-earth materials appearing here with their color enhanced. Used as biological markers, the nanospheres up-convert infrared light from a multimode diode laser module and deliver a visible fluorescent signal easily distinguishable from source background. Courtesy of OraSure Technologies Inc.

Ceramic nanospheres, such as those used in OraSure Technologies' UPlink system, work in reverse. Like biological markers, they can preferentially bind to the surface of specific antibodies or DNA probes and fluoresce when optically excited. However, the nanospheres are synthesized from rare-earth metals that up-convert infrared light to a visible spectrum; that is, toward the blue spectrum.

Because infrared light does not excite visible fluorescence in biological materials, the optical signals emitted by the nanospheres are easily distinguished from background radiation from the excitation source. The particles can be formulated to emit discrete wavelengths and can be detected using a multiplexed configuration.

Despite the edge that up-conversion provides the UPlink system, selecting the right light source was important. It needed to emit in narrow bands, especially at 980 nm. The company had experimented with halogen pulsed lamps, which produced good illumination in the infrared, but laser diodes produced the best light intensity.

The engineers at OraSure sampled a fiber-coupled module equipped with a single-mode diode from Power Technology, but they found that the module didn't meet the UPlink's extreme operating temperature range requirements (5 to 40 °C). In response, Power Technology accelerated its development of a more thermally stable fiber-coupled module, the IQ1C. Once completed, the company incorporated into it a multimode diode laser with a Schott RG850 filter at the laser's output. The filter blocks the normally negligible amount of visible light emitted by the diode and minimizes the potential for error signals that would invalidate the UPlink's test results. The IQ1C laser module enabled OraSure to develop a portable system that can be used in a variety of temperatures and environments.

Samples are collected on a swab from a subject's mouth and inserted into a plastic disposable cartridge, which breaks a vial of buffer solution. The contents of the swab are mixed into solution and adsorbed by nanospheres arranged in up to 10 test lines. The laser then illuminates the test strips, and a detector quantifies the fluorescence from the analyte of interest, such as a drug or microbe.

The system has stirred interest as a test for drivers, for on-site analysis of infectious disease markers, and for detecting chemical and biological warfare agents.