"The cells are then illuminated with light in the near-infrared range", stated Ji-Xin Cheng (pronounced Gee-Shin), an assistant professor in Purdue's Weldon School of Biomedical Engineering. "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."
Scientists have recently determined that gold nanorods and other nanostructures can be used to target and destroy tumour cells, but it was generally assumed that cell death was due to the high heat produced by the light-absorbing nanoparticles. The Purdue team discovered, however, 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", stated Alexander Wei, an associate professor of chemistry at Purdue.
Findings are detailed in a research paper that appeared October 19 in the journal Advanced Materials. The paper, which also appeared on-line, was written by doctoral students Ling Tong, Yan Zhao, Terry B. Huff and Matthew N. Hansen, along with Alexander Wei and Ji-Xin Cheng.
The gold rods are less than 15 nanometers wide and 50 nanometers long, or roughly 200 times smaller than a red blood cell. Their small size is critical for the technology's potential medical applications: 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", Alexander Wei stated. "Every cavitation event is like a tiny bomb. Then suddenly, you have a gaping hole where the nanorod was."
The gold nanorods also are ideal for a type of optical imaging known as two-photon luminescence, used by Ji-Xin Cheng and his research group to monitor the position of nanorods in real time during tumour-cell targeting. The imaging technique provides higher contrast and brighter images than conventional fluorescent imaging methods.
In 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 internalized.
"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", Ji-Xin Cheng stated.
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", Ji-Xin Cheng stated.
The Purdue researchers observed that light-absorbing nanorods cause the formation of membrane "blebs", similar to severe blistering. These blisters, however, are not produced directly by the high heat generated by the nanorods. "The blebbing is triggered by the nanorods, but it's really caused through a complex biochemical pathway - a chemically induced process", Ji-Xin Cheng stated. "Extra calcium gets into the cell and triggers enzyme activity, which causes the infrastructure inside the cell to become loose, and that gives rise to the membrane blebs."
Researchers used a calcium-sensitive fluorescent dye to back up their argument that calcium influx caused the tumour cell death. When the nanorod-bearing tumour cells were maintained in a calcium-free nutrient medium, no blisters were formed if the nanorods were exposed to near-infrared light. But when the researchers added calcium to the medium, the blebbing took place immediately.
Although the technique offers promise for a new cancer treatment, it is too early to determine when it could be in clinical use, according to Alexander Wei, who is collaborating with the National Cancer Institute to determine the suitability of the functionalized gold nanorods for future clinical studies.