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  • Dual-beam fiber trap improves Raman microspectroscopy

Aug 2006
Lauren I. Rugani

Raman spectroscopy is employed in many biomedical applications; however, it has several drawbacks. The low intensity of the emitted signal combined with the possibility of recording signals from the surrounding environment makes it difficult to distinguish desired signal from background noise. Longer acquisition times may increase the signal but also could damage live cells. Optical trapping has been combined with Raman spectroscopy to reduce integration time, yet this limits the ability to move large cells as desired.

To eliminate these and other obstacles, researchers at the University of St. Andrews in the UK realized the use of a dual-beam fiber optic trap for holding and maneuvering large cells and recording localized Raman signals. The team wanted to decouple Raman excitation from the primary trapping mechanism. “This would allow for very specific control over where we examine, and opens up the opportunity for three-dimensional mapping of the cell or any object under consideration,” explained Phillip R.T. Jess, a member of the investigative team.

As seen in this demonstration of a dual-beam fiber trap, an object is held away from the surface while Raman excitation is employed to collect spectra, providing sensitive localized information with minimal damage to the object.

The dual-beam fiber optic trap was controlled by an ytterbium fiber laser from IPG Photonics, operating at 1070 nm and placed above a Raman microspectroscopy setup centered on a Nikon TE 2000-U microscope. A beam from a Sanyo temperature-stabilized diode laser, operating at 785 nm, was transmitted to the sample, where the Raman signal was reflected and imaged onto a spectrograph equipped with a CCD camera, both from Jobin Yvon, to record Raman spectra.

In one localized Raman experiment, the researchers trapped a 30-μm primary human keratinocyte cell with 40 mW of power from each fiber and a fiber face separation of 85 μm. After integrating the Raman signal in the sample for two minutes using a 20-mW beam, they recorded spectra from the membrane, cytoplasm and nucleus of the cell. By varying the laser power in the fibers and/or adjusting the fiber positions, the researchers were able to move the cell around in the trap, and the cell’s upward attraction to the light beams held it away from any interfering surface.

Although it is generally difficult to load a trap with a cell, the problem was easily solved by using a microfluidic channel to confine cells and force them between the fibers where they could be trapped, Jess noted. Unlike single-beam traps, the dual-beam fiber trap is highly stable and less likely to affect intracellular dynamics because of its low power density.

The ability to obtain localized information while holding large cells away from interfering surfaces holds promise for trapping and analyzing cells in microfluidic systems. To demonstrate this potential, the researchers developed such a system using the dual-beam fiber trap. With 80 mW of power from each fiber and an acquisition time of 60 seconds, they recorded Raman spectra from a human leukemia cell line flowing through the system without damaging the cells.

Future research may include the development of a prefabricated lab-on-a-chip instrument and the investigation of various biological applications. “This technique could open up the possibility of studying single-cell dynamics in an isolated environment,” Jess said. Possible applications include monitoring the effect of drugs on individual cellular components and observing communication between two trapped cells when one cell is stressed.

Optics Express, June 12, 2006, pp. 5779-5791.

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