Noble-metal nanoparticles generate heat when illuminated with light. This heat can then be used to locally destroy cancer cells without harming surrounding healthy tissue. Gold nanoparticles are ideal candidates for photothermal therapy because their optical properties can be tuned in the near-infrared part of the electromagnetic spectrum. What's more, they are biocompatible and non-toxic, and their surfaces can be readily modified with antibodies to target specific receptors on tumour cells.

Until now, however, researchers have paid little attention to how nanoparticles should be designed to achieve the most efficient photothermal response. As team leader Teri Odom points out, nanoparticles have been tested as therapeutic agents based on availability instead of on optimized, tailor-made properties.

"Our work seeks to identify the most important structural parameters for designing noble-metal nanoparticles that can result in an optimized photothermal response," she told nanotechweb.org. "We evaluated how different sizes, shapes and shell thicknesses of pyramidal-shaped nanoparticles affected the photothermal response."

By comparing how much heat was generated from four types of gold pyramidal particles in solution when exposed to near-infrared light from a laser, the team discovered that particles with the thinnest shells and sharp tips produced the most heat. These particles would be the best for photothermal therapy. To ensure that only differences in geometry were responsible for the observed temperature increase, the researchers compared the absolute amount of gold present in each batch of particles.

"The results provide a simple platform to determine the most important structural features in designing nanoparticles for photothermal cancer therapy," explained Odom.

The Northwestern scientists are currently investigating the photothermal response of nanopyramids in vitro using breast cancer cells. They are doing this by first functionalizing the nanopyramids with antibodies that target the cancer cells and then irradiating them with a near-infrared laser to ablate the cells. "We are also investigating whether gold is the best material for photothermal therapy or if combinations of materials – such as other noble metals and dielectric materials – could exhibit a higher photothermal response than gold-only particles," revealed Odom.

The work was published in Nano Letters.