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Earlier Melanoma Detection Spurred on by Gold; Alexander Graham Bell
Posted: August 16, 2010
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Photoacoustic images of biological tissue can be made without the use of contrast agents, particularly if tissues are pigmented by molecules like hemoglobin or melanin. Still, photoacoustic images of melanomas are fuzzy and vague around the edges. To improve the contrast between the malignant and normal tissue, Xia loads the malignant tissue with gold.
"Gold is much better at scattering and absorbing light than biological materials," Xia says. "One gold nanocage absorbs as much light as a million melanin molecules," says Xia. Xia's contrast agent consists of hollow gold cages, so tiny they can only be seen through the color they collectively lend to the liquid in which they float.
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.