Nanostars rival fluorophores for imaging

18 Nov 2009

Nanostars are just as good as the more traditional organic fluorophores for many imaging applications. So say researchers at the University of Maine in the US who have compared the optical signals emitted by active surface-coated gold nanostars, tagged with a Raman probe, with common single fluorescent molecules. The results also suggest that the nanoparticles may even be better for long or repeated imaging experiments than their fluorescent counterparts.

Fluorescent molecules are currently the most widely employed materials for imaging studies. However, in recent years scientists have found that nanoprobes based on surface enhanced Raman scattering (SERS) of light from metallic nanoparticles are promising alternatives. This is because the nanoparticles are more stable than many fluorescent molecules and do not photobleach, blink or saturate. The surfaces of the nanoparticles can also be readily modified, which allows them to be used in chemical and biological environments. Indeed, SERS-based Raman probes have already been used for live cell imaging, detecting biomarkers and tumours in vivo, imaging small animals and much more.

Recently, researchers have developed a new class of star-shaped gold nanoparticles, dubbed nanostars, which show higher optical signals than other nanoparticles. This is because they have rough surfaces (the points of the "star"), which result in a large SERS per molecule. What's more, by adding a coating to the nanostars, the SERS signal is protected from its surroundings, even under harsh conditions.

Now, Michael Mason and colleagues have imaged star-shaped SERS probes and the Alexa Fluor 633 dye and compared the two. After imaging, the researchers employed a custom Matlab script to locate all single molecule/single probe features in the images and compute the total count rate from each feature. Next, they computed the mean count rate at each power for the fluorescent dye and the Raman nanoprobes.

"This technique allowed us to compare both systems," team member Edward Allgeyer told nanotechweb.org. "We also imaged the same region of our single molecule/single probe samples to compare how the count rate behaved over several image acquisitions." The results show that the Raman probes are a good choice for applications where multiple images over a long period are needed, because the nanostars do not bleach.

Mason and colleagues say that their work will continue to make Raman imaging more mainstream and routine. "We hope that it will lend further credibility to Raman imaging as a viable and worthwhile technology," said Allgeyer.

The work was published in Nano Letters.