Researchers have been looking for ways to track the movement of nanoparticles in biological tissue to study their in vivo interactions. Typically, attaching fluorophore molecules has been a common method to monitor nanoparticle movement, but these fluorophores have a limited lifetime and so are not practical for longer observations. Now scientists from Rice University are reporting in the Proceedings of the National Academy of Sciences the use of lasers to reveal the displacement and orientation of gold nanoparticles.
Gold nanorods can be “lit up” at will. Lasers at particular wavelengths excite surface plasmons that absorb the energy and emit a heat signature that can be detected by a probe laser. Because plasmons are highly polarized along a nanorod’s length, reading the signal while turning the polarization of the laser tells researchers precisely how the rod is oriented.
An electron microscope photo from the new paper shows nanorods about 75 nanometers long and 25 nanometers wide on a glass slide at 90-degree angles to each other. An adjacent photothermal image shows them as pixilated smudges. The smudges are strongest when the laser polarization aligns lengthwise with the nanorods, but they disappear when the laser polarization and rods are 90 degrees out of phase.
“With plasmonics, you always have two properties: absorption and scattering,” Link said. “Depending on the size, one or the other dominates. What’s unique is that it’s now possible to do both on the same structure or do it individually — so we can only measure absorption or only measure scattering.”
Nanorods much smaller than 50 nanometers are not detectable by some scattering methods, Link said, but photothermal detection should work with metallic particles as small as five nanometers; this makes them useful for biological applications.
Image: The graph at left shows how nanorods photographed in an electron microscope at right appear and disappear, based on their orientation, when their photothermal signatures are detected with polarized lasers.
Nano imaging takes turn for the better …
Abstract in PNAS: Plasmonic nanorod absorbers as orientation sensors