Tumour cell membranes often have an abnormally high number of receptor sites to capture molecules of folic acid (folate), a form of vitamin B that many tumour cells crave.
The researchers attached folate to the gold nanorods, which enabled them to target the receptors and attach to the tumour cell membranes. "The cells are then illuminated with light in the near-infrared range. This light can easily pass through tissue but is absorbed by the nanorods and converted rapidly into heat, leading to miniature explosions on the cell surface," said Ji-Xin Cheng, an assistant professor in Purdue's Weldon School of Biomedical Engineering.
Scientists have recently determined that gold nanorods and other nanostructures can be used to target and destroy tumour cells. Contrary to the assumption that cell death occurs due to the high heat produced by the light-absorbing nanoparticles, the researchers have now discovered that a more complex biochemical scenario is responsible for killing the cells.
"We have found that rather than cooking the cells to death, the nanorods first punch holes in the membrane, and cell death is then chemically induced, in this case by an influx of calcium," said Alexander Wei, an associate professor of chemistry at Purdue. The researchers have revealed that the gold rods are less than 15 nanometers wide and 50 nanometers long, or roughly 200 times smaller than a red blood cell.
According to them, the small size of gold rods is critical for the technology's potential medical applications because the human immune system quickly clears away particles larger than 100 nanometers, whereas smaller nanoparticles can remain in the bloodstream far longer. Shining light on the gold nanorods causes them to become extremely hot, ionizing the molecules around them.
"This generates a plasma bubble that lasts for about a microsecond, in a process known as cavitation. Every cavitation event is like a tiny bomb. Then suddenly, you have a gaping hole where the nanorod was," Wei said. During the experiments with tumour cells in laboratory cultures, the nanorods attached to the cell membranes and were eventually taken up into the cells. The researchers found that it could take far less power to injure cells by exposing the nanorods to near-infrared light while they are still on the membrane surface instead of waiting until the nanorods are internalised.
"This means that if you wait until the nanorods are inside the cell, then you really have to pump up the laser power, so localizing the nanorods on the cell membrane strongly influences their ability to inflict cell damage," Cheng said. The findings suggest an optimal window of opportunity for applying near-infrared light to the nanorods for cancer treatment. "We like to believe this opens the possibility of using nanorods for biomedical imaging as well as for therapeutic purposes," Cheng said.
The researchers, however, admit that it is too early to determine when this technique may come to be used in clinics. The study has been published in the journal Advanced Materials.