SEPTEMBER 10, 2008 -- A team involving American and German researchers (from Carnegie Mellon University in Pittsburgh, Hunter College in New York, and the RWTH Aachen University in Germany) has developed a new method for environmentally friendly production of nanoscopic gold rods, which are useful for such biomedical applications as tumor treatment. The team has avoided the use of cytotoxic additives, which are necessary for other production methods, and used an ionic liquid, a "liquid salt" for synthesis.
Gold nanoparticles, which absorb light very strongly in the near infrared -- a spectral region that is barely absorbed by tissue -- can be functionalized to bind to tumor cells. Because the cancer cells are relatively temperature-sensitive, IR light can excite and overheat the gold particles, and thus kill the cells.
But only rod-shaped particles efficiently convert the light energy into heat -- and the additives typically needed to crystallize the rod-shaped particles from aqueous solutions are cytotoxic.
The team, headed by Michael R. Bockstaller, is pursuing a new strategy: instead of aqueous solution, they chose to use an ionic liquid as their medium of crystallization. Ionic liquids are "liquid salts", organic compounds that exist as oppositely charged ions, but in the liquid state. In this way, the researchers have been able to produce gold nanorods without the use of any cytotoxic additives.
In the first step, seed crystals are produced in the form of tiny spherical gold particles. These crystals are added to a "secondary growth solution" containing monovalent gold ions, silver ions, and the weak reducing agent ascorbic acid. The solvent is an imidazolium-based ionic liquid. In this medium, the crystals don't continue to grow into spheres; instead they form rods with the round crystallization nuclei as "heads". The mechanism is presumed to involve the various, energetically inequivalent surfaces of the crystal lattice: the aromatic, nitrogen-containing five-membered rings of the ionic liquid prefer to accumulate at the highly energetic facets of gold surfaces. They thus stabilize crystal shapes that have fewer low-energy facets than the normal spherical equilibrium form. This results in long rods.