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Analytical Chemistry

Whispering microresonators detect absorption spectra of individual nanoparticles

New method expands range of materials for which ultrasensitive measurements can be made

by Celia Henry Arnaud
November 14, 2016 | APPEARED IN VOLUME 94, ISSUE 45

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Credit: Erik H. Horak
The resonance frequency of this microresonator (blue ring in this drawing) is sensitive to temperature. An adsorbed gold nanorod (white spot) heats in response to certain wavelengths in a scanning excitation laser (red). That heat dissipates into the microresonator, creating a measurable signal.
Credit: Erik H. Horak
The resonance frequency of this microresonator (blue ring in this drawing) is sensitive to temperature. An adsorbed gold nanorod (white spot) heats in response to certain wavelengths in a scanning excitation laser (red). That heat dissipates into the microresonator, creating a measurable signal.

Most optical measurements of single particles or single molecules are done using fluorescence techniques, which require targets to be emissive. An absorption measurement would expand the range of materials that could be detected at such low levels. David J. Masiello of the University of Washington, Randall H. Goldsmith of the University of Wisconsin, Madison, and coworkers have developed just such a single-particle absorption method using photonic devices called whispering gallery mode microresonators (Nat. Photonics 2016, DOI: 10.1038/npho​ton.2016.217). These devices act as ultrasensitive thermometers that measure the heat dissipated by individual nanoparticles that adsorb onto the microresonator surface. In a two-laser setup, the team uses one laser to optically excite an adsorbed gold nanorod, which leads to a temperature change that is proportional to the nanorod’s absorption cross section. That temperature change in turn causes a shift in the optical frequency of the microresonator’s whispering gallery mode that can be probed by a second laser. By scanning the excitation laser frequency, the team obtained absorption spectra for individual adsorbed nanorods. The team is working to lower the limit of detection enough to be able to collect absorption spectra of individual molecules, which have even smaller absorption cross sections than nanorods do.

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