Optical Trapping, Raman Spectroscopy Measure Live-cell Interaction

Using multiple laser beams and Raman spectroscopy, researchers at the Universities of Nottingham and Glasgow have designed and built a new instrument that could help scientists learn more about how infections take hold and how antibiotic-resistant bacterial biofilms are formed. The instrument uses holographic optical traps to form connections between multiple human immune cells. Changes in the cell interactions are measured over time using Raman spectroscopy.

To enable optical trapping and Raman spectroscopy to be used simultaneously at many sample points, the research team combined a liquidcrystal spatial light modulator (LCSLM) with a digital micromirror device (DMD) to create reflective virtual pinholes that could be customized for each sampling point and rapidly controlled with a computer.

Researcher Faris Sinjab at Nottingham said, “The multi-point optical trapping and Raman spectroscopy can be controlled interactively and in real-time using the software developed by Miles Padgett’s group at the University of Glasgow. This software allows completely automated experiments, which could be useful for carrying out complex or large systematically repeated experiments.”

After demonstrating that the performance of their Raman instrument was comparable to a single-beam Raman microscope, the researchers used it to move multiple polystyrene particles around with the optical traps, while simultaneously acquiring Raman spectra at 40 spectra per second.

Next, the researchers showed they could control the power in each laser beam and avoid damaging trapped cells with the laser. To demonstrate the capability of the instrument for cell biology applications, they brought multiple live T cells into contact with a dendritic cell to initiate the formation of immunological synapse junctions where these immune cells met. Measuring Raman spectra at multiple points over time revealed molecular differences among the junctions formed.

“This type of experiment would not previously have been possible because spectra could not be acquired from such rapidly changing locations,” said Sinjab.

The researchers are now working to further automate portions of the Raman spectroscopy so that non-expert users can use the instrument to carry out experiments. They are also exploring how to miniaturize the instrument by incorporating a custom microscope and a spectrometer with a more compact high-power laser.

The researchers believe that their instrument could provide a new way to investigate how immune cells communicate in the body.

“Many techniques in biology measure a large number of cells at once, or require added labels or invasive techniques to look at the single cell level,” said Nottingham professor Ioan Notingher. "Our technique is non-invasive — meaning that it doesn't disturb or destroy the biological sample — and requires no labeling, which is more desirable for studying individual cells.

“The instrument we have created is quite robust and sensitive and can be used in many types of experiments on cells,” said Notingher. “In addition to biological investigation the instrument could also be used to study polymers, nanomaterials, and various chemical processes. It could also be combined with other microscopy techniques to obtain even more information.”

The research was published in Optics Express, a publication of OSA, The Optical Society (doi:10.1364/OE.26.025211).

Researchers have developed a holographic optical trap to analyze in real time how cells communicate. Courtesy of the University of Nottingham.