By altering the size and geometry of the particles, they can be tuned to absorb or scatter light over a wide range of wavelengths.
In this way the nanoparticles behave quite differently than bulk gold.
For photoacoustic imaging, Xia's team tunes the nanocages to absorb strongly at 780 nanometers, a wavelength that falls within a narrow window of tissue transparency in the near-infrared.
Light in this sweet spot can penetrate as deep as several inches in the body.
Once injected, the gold particles naturally tend to accumulate in tumors because the cells that line a tumor's blood vessels are disorganized and leaky.
But Xia has dramatically increased the uptake rate by decorating the nanoparticles with a hormone that binds to hormone receptors on the melanoma's cells.
The molecule is alpha-melanocyte-stimulating hormone, slightly altered to make it more stable in the body. This hormone normally stimulates the production and release of the brown pigment melanin in the skin and hair.
As is true in many types of cancers, this hormone seems to stimulate the growth of cancerous cells, which produce more hormone receptors than normal cells.
In experiments with mice, melanomas took up four times as many "functionalized" nanocages than nanocages coated with an inert chemical. With the contrast agent, the photoacoustic signal from the melanoma was 36 percent stronger.
But seeing is believing. Subcutaneous mouse melanomas barely visible to the unaided eye show up clearly in the photoacoustic images, their subterranean peninsulas and islands of malignancy starkly revealed.
|Contact: Diana Lutz|
Washington University in St. Louis