November 27, 2006
New Method Enables Ultrasensitive Cellular Imaging
Investigators at Harvard Medical School have developed a new imaging technique that uses gold or silver nanoparticles to image single cells with a resolution sufficient to obtain molecular information from within the cell. This technique, the investigators note in a paper published in the Proceedings of the National Academy of Sciences USA, could enable researchers to detect and study individual biomolecules in their native environment within a cell.
Katrin Kneipp, Ph.D., led this effort to develop surface-enhanced hyper-Raman scattering (SEHRS) as a means of characterizing the local biochemical environment within single cells. Surface-enhanced Raman spectroscopy is a well-utilized technique that uses a laser to excite gold or silver surfaces, which in turn transfer energy to nearby molecules. These molecules then give off light energy of their own that can be detected and used to identify these molecules. Hyper-Raman scattering is a related technique that can, in theory, provide such molecularly detailed information but over very small volumes. However, hyper-Raman scattering has not proven applicable to biological samples because it requires the use of lasers of very high energy, which destroy such samples.
To develop SEHRS, the investigators took advantage of the unique optical properties of gold and silver nanoparticles and the sensitivity of those properties to the exact physical environment surrounding these particles when small numbers of them clump together to form nanoscale aggregates. The researchers found that when the nanoaggregates are irradiated with certain wavelengths of light from low-power lasers, these nanoparticles will trigger a strong hyper-Raman scattering effect from molecules within very small volumes surrounding the nanoparticles. The resulting light emissions can be used to identify the molecules within that small volume.
In their paper, the investigators go on to show how this technique can be used to pinpoint specific individual molecules within a cell. This application requires making gold nanoparticles labeled with a targeting agent, such as an antibody or aptamer, and a reporter molecule. When injected into a cell, the nanoparticles will bind to their target, and when irradiated with a laser, the nanoparticles will trigger an SEHRS emission characteristic of the reporter molecule.
This work is detailed in a paper titled, “Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering.” An abstract of this paper is available through PubMed.