In their experiments, Catherine Murphy and colleagues started by creating a set of gold nanorods with electrostatically trapped rhodamine 6G molecules on their surfaces. These molecules were used as model “drugs”. The researchers then wrapped successive layers of charged polymers – negatively charged poly(acrylic acid, sodium salt) and positively charged poly(allylamine hydrochloride) – around the nanorods. These polymers alter the surface charge of the rods and help trap the rhodamine 6G molecules.

Next, the team split the samples into two batches: the first batch was irradiated with a 785 nm diode laser for up to one hour. The second batch was not irradiated at all, and served as a control. Laser light at 785 nm was chosen because this is the wavelength at which the rods absorb the most light, thanks to their aspect ratio of 3.6.

The following step in the process consisted of separating out the nanorods from the supernatants using a centrifuge at different time intervals, and then quantifying – by fluorescence – the concentration of free rhodamine in the supernatants compared with the controls that had not been illuminated. This gave the number of light-emitting molecules trapped inside the nanorods that were released upon illumination, compared with just those that had diffused out passively.

The researchers found that the number of molecules that could be released from the rods was related to the number of polymer layers wrapped around the rods and that the number of molecules released could be tuned 100-fold.

Promising drug delivery vehicles

The small size of the gold nanorods (around 10–100 nm) coupled with the fact that they can easily be functionalized means that these structures are promising as drug-delivery vehicles. Near-infrared laser irradiation-triggered drug release is especially attractive because biological tissue is transparent to light at these wavelengths (around 800–1200 nm) and gold nanorods can easily be made to absorb in this region of the electromagnetic spectrum too, says Murphy.

Gold nanorods also generate a fair amount of heat when they absorb near-infrared light and this heat can be used to locally destroy cancer cells. Indeed, the researchers also undertook thermal control experiments in a water bath and found that the effective temperature change was around 10 °C for an hour’s worth of irradiation with light at 785 nm.

The results will have implications for light-initiated drug-delivery applications, says Murphy, because the release of the drug could be controlled by carefully choosing the number of polymer wrapping layers.

The team is now working on using pulsed lasers to better measure the temperature near the nanorods. “We also want to repeat our experiments in more physiologically relevant conditions,” Murphy told nanotechweb.org, “and explore other thermally activated molecular processes in cells.”

The current work is detailed in Nano Letters.