Raman joins the dots

Nanoparticles can boost Raman signals sufficiently to allow the simultaneous detection of several biomarkers even deep within tissues of a living animal. The discovery could help in disease diagnostics, biomedical research, and potentially cancer treatment.

In 2008, US researchers developed a method for detecting single-walled carbon nanotubes or commercially available silica-coated gold nanoparticles as SERS contrast agents in animal studies.

Now, the team based in the Molecular Imaging Program, at the Department of Radiology and Bio-X Program, Stanford University School of Medicine, California, and Oxonica Materials, Inc., in Mountain View, California, have used nanoparticles designed specifically by Oxonica to produce a bright Raman spectroscopic signal to show that they can track as many as ten different optical tags at the same time in a living animal.

Principal investigator of the Stanford CCNE, Sanjiv Sam Gambhir, says the work is the first to use surface-enhanced Raman spectroscopy (SERS) for multiplexed imaging in a living animal. He and his colleagues report details in the Proceedings of the National Academy of Sciences (USA). They suggest that the research has potential for imaging multiple cancer biomarkers.

Gambhir and colleagues, Cristina Zavaleta and Bryan Smith, and Oxonica's Ian Walton, William Doering, Glenn Davis, Borzoyeh Shojaei, and Michael Natan, have used a set of ten different silica-coated gold nanoparticles to create a multiplexed in vivo imaging technique with picomolar sensitivity. They explain that each of the SERS nanotags generates a unique Raman spectrum when irradiated with near-infrared light. They then use spectral analysis software to separate the signals for each nanotag and so can determine the concentration of each nanotag.

The team demonstrated proof of principle by injecting the nanotags individually into a mouse and then obtaining a map of the location of each nanotag in its body by scanning in 750 micrometre steps at 1 second per frame. They next selected just the five brightest nanotags with the least overlap for injection, which allowed the researchers to quantify the concentrations of those nanotags as they accumulated in the mouse liver.

"These results show great potential for multiplexed imaging in living subjects in cases in which several targeted SERS probes could offer better detection of multiple biomarkers associated with a specific disease," the researchers say.

This concept feeds into recent research through which the biomedical community has begun to recognize that no single targeting agent can adequately address all aspects of a particular disease process. A growing body of work is emerging looking at ways to discover multiple biomarkers and targeting ligands for improved, earlier detection and management of different diseases. SERS using nanoparticles addresses this need but side-steps some of the toxicity issues associated with quantum dots and their limited response in the near-infrared range.

The team is now working on optimizing the Raman microscope to maximize the depth within the body at which they can accurately measure the Raman signal from their nanotags. The chemical structure of the nanotags will also facilitate the attachment of specific binding agents for particular biomarkers, in cancer, for example, as biomedical researchers uncover new markers.