Investigators at Boston College in Chestnut Hill, Mass., suggest that gold nanoparticles are attractive substitutes for fluorophores or semiconductor nanoparticles as labels in the imaging of microscopic biological structures. In a study published in Nano Letters online May 18, they report that multiphoton-absorption-induced luminescence in 2.5- to 125-nm-diameter gold nanoparticles offers comparable emission intensities to and higher quantum efficiencies than fluorophores and better short- and long-term photostability than semiconductor nanoparticles.

To measure the response of the gold nanoparticles, the researchers excited thin films of uncoated and silica-coated samples using a homebuilt Ti:sapphire laser that produced sub-100-fs pulses of 790-nm radiation at a repetition rate of 76 MHz. An inverted microscope with a 1.3 NA, 40× objective, both from Carl Zeiss GmbH of Jena, Germany, served as the optical setup. Single-photon-counting avalanche photodiodes and a fiber optic spectrometer collected luminescence data and emission spectra, respectively.

Logarithmic plots of the emission intensity versus laser power showed that the excitation is a three-photon process, enabling strong emission from relatively weak excitation pulses. Employing synthesis techniques to produce gold nanoparticles with the desired asymmetries to exhibit electric field enhancements in the near-IR, the metal structures promise to enable in vivo imaging at the single-molecule level, the researchers suggest.