Dual-beam fiber trap improves Raman microspectroscopy
Lauren I. Rugani
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
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
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
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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