"Gold nanorods are promising as contrast agents for biological imaging because they have tunable absorption properties in the near-infrared," Alex Wei of Purdue told nanotechweb.org. "In addition, the biological inertness of gold is well known."
Wei and colleagues used dumbbell-shaped gold nanorods with an average length of about 49 nm and average midsection of roughly 16 nm. They imaged the nanorods in a scanning confocal microscope, using a Ti:sapphire laser beam with a wavelength of around 830 nm to generate TPL from the rods.
"Using this technique with labelled nanorods may allow us to detect diseases at an early stage of development," said Wei. "The high resolution of the TPL technique is also useful for investigating biological processes at the single-cell level."
The nanorods remained detectable for around half an hour after their injection into the mouse. Wei and colleagues reckon this was because the mouse's kidneys eventually filtered the nanorods out of the blood.
The scientists believe that plasmon resonance of the nanorods boosts the TPL signal. The longitudinal plasmon modes of gold nanorods are resonant at near-infrared wavelengths. These frequencies are ideal for biological imaging as water and biological molecules have relatively low absorption in the range. Experiments showed that the TPL excitation spectrum overlapped with the longitudinal plasmon band. Writing in a paper in PNAS, the researchers explain that this indicated that the TPL intensity was governed by the local field enhancement from the plasmon resonance.
"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," said Wei. "The gold nanorods demonstrate that [my colleague] Cheng's nonlinear imaging methods are capable of this level of detection."
According to the researchers, the nonlinear dependence of the TPL signal on the excitation intensity means it can be resolved in the axial direction. This enables spatial resolution in three dimensions. The signal intensity also varies with the excitation polarization (with a cos4 dependence), a feature that the scientists say may provide additional orientation information.
The researchers reported their work in PNAS.