Acousto-Optics Detects Fluorescence
Michael D. Wheeler
CHARLOTTESVILLE, Va. -- Capillary electrophoresis is fast becoming the preferred method for the separation of DNA, proteins and other molecules -- especially as it is performed on multichannel microchips. To induce fluorescence in these samples, a laser beam is scanned quickly over the chip in a linear pattern, providing a fast, accurate means for sample analysis.
To form the linear patterns, researchers often use either translation stages or galvanometric scanners. The drawback to these devices is that both are mechanically controlled, making it difficult to achieve ultrafast scanning rates. Also, it is difficult to cancel out the jitter and wobble associated with defocusing and displacement of the laser beam during detection.
Researchers at the University of Virginia have pioneered an approach that they say overcomes these problems: acousto-optical deflection. They constructed a detection system with an acousto-optical device -- essentially a "sound box" -- whereby the laser beam passing through is deflected based on the acoustic waves traveling inside the box.
The speed of sound
"By rapidly changing the input frequency to the acousto-optical device, the laser beam can be scanned over an area occupied by a series of microchannels on the nanosecond timescale," explained Zhili Huang, one of the principal researchers working on the project under the direction of James P. Landers. The beam can be focused on any position in an area and dwell at an exact time at that position. "This provides a simple, rapid laser beam scanning and fluorescence detection method with no moving parts."
Multiple scanning modes
Another advantage of acousto-optical laser beam deflection is that it enables four scanning modes: linear, bidirectional, step and random addressing. In linear scanning, the laser spot is swept at a constant velocity over a fixed range in a repetitive manner. At the end of each linear sweep, the scanner reverses direction, returns to the starting position and repeats the scan. In some applications, raster scanning also can be performed in alternating directions -- i.e., the bidirectional mode. Galvanometric, translation stage and acousto-optical deflection-based scanning all have been used successfully for these two modes. In step scanning, where the laser beam is moved rapidly from one position to another, a galvanometer is ineffective. And with random addressing, where the laser beam rapidly moves from one position to another without a defined traveling track and no set dwell time, neither the translation stage nor the galvanometric scanner is effective.
This eight-channel microchip was extrapolated from single-channel to multichannel to enhance the electrophoresis anaylsis. The channel was 50 µm wide and 20 µm deep with center-to-center spacing of 150 µm. Two outside channels were used as alignment channels. Once the alignment was done, the laser beam could be addressed to each separation channel directly and dwelled any period of time for laser-induced fluorescence detection by the acousto-optic-deflection-based scanning.
Huang said that with the faster scanning rates and improved spatial and temporal control that acousto-optical deflection affords, this should be an important advance for multichannel scanning. The next step for the group is increasing the scanning range and system speed for controlling and sampling.
Details of the team's work appear in the Dec. 1, 1999, issue of Analytical Chemistry.
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