Chemotherapy drugs can slow or halt the progression of cancer, but the damage they do to healthy cells often initiates life-threatening side effects. Now, researchers at the Victoria University of Wellington in New Zealand1 have developed a new method to synthesize biocompatible quantum dots that may be able to selectively target and destroy cancerous growths and tumors.

Fig. 1: Tin-sulfide quantum dots (left, colored suspension in water; right, a transmission electron micrograph) may be able to selectively eradicate cancer cells.

Quantum dots are made by carefully solidifying semiconductors into tiny nanoscale crystals. At these dimensions, quantum confinement alters the internal electronic structure of the crystal, unleashing new and unique optical behavior. Tin-sulfide (SnS) semiconductor nanocrystals are very good at absorbing and emitting infrared radiation — a property that has attracted intense interest for biomedical hyperthermia treatment.

“During hyperthermia treatment, nanoparticles are targeted to tumor cells,” says Richard Tilley, a member of the research group. “Once at the tumor, they are heated with a laser and become very hot to kill the surrounding cancerous growths.”

Ensuring biological stability, however, requires a technique to make SnS nanoparticles soluble in water — a difficult proposition since most synthetic procedures must rigorously avoid water or oxygen. The researchers solved this problem by using water-compatible materials called ethanolamines — tripod-shaped compounds containing nitrogen and either one, two or three ethanol molecules — to control crystallization when tin and sulfide ions were mixed together in ethylene glycol, a syrupy alcohol commonly found in antifreeze.

Transmission electron microscopy revealed that this new procedure produced SnS nanoparticles of 3–5 nm in size. The nanocrystal dimensions could be controllably adjusted through the ethanolamine reagent, with the tri-ethanol compound yielding the smallest, most uniform nanocrystals. Intriguingly, optical measurements demonstrated that when the SnS nanocrystal size fell below 5 nm, they took on new infrared absorbance properties that differed sharply from bulk material behavior.

“These nanoparticles are so small,” says Tilley, “that we can tune absorbance properties through the quantum confinement of electrons.”

In addition to targeted cancer treatments, water-soluble nanocrystals have other potential applications, including as light-harvesting components in polymer solar cells.