Photons are the individual particles that make up light. In two-photon fluorescence, two photons hit the nanorod at the same time.
Because of the two-photon effect, the method might enable scientists to develop advanced "non-linear optical techniques" that provide better contrast than conventional technologies. Cheng is applying the two-photon fluorescence and other non-linear optical effects to develop imaging methods that provide improved contrast over conventional technologies.
The researchers injected the nanorods into mice and then took images of the tiny structures as the nanorods flowed through blood vessels in the animals' ears. Individual nanorods proved to be 58 times brighter than the two-photon fluorescence from a single rhodamine molecule. Within a half hour after being injected, the nanorods could no longer be observed in the animals' blood, presumably because the rods had been filtered out of the blood by the kidneys.
"To be able to detect cells at an early stage of disease such as cancer, it is important to have a reliable technique that has sensitivity at the single-particle level," Wei said. "The gold nanorods demonstrate that Cheng's nonlinear imaging methods are capable of this level of detection."
The research has been funded by the National Institutes of Health and is affiliated with the Birck Nanotechnology Center and the Bindley Bioscience Center, both in Purdue's Discovery Park, the university's hub for interdisciplinary research.
The research paper was written by Haifeng Wang, a postdoctoral research associate in biomedical engineering; Terry B. Huff, a graduate teaching assistant in chemistry; Daniel A. Zweifel, a graduate student in chemistry; Wei He, a graduate research assistant in chemistry; Low; Wei; and Cheng.