The Discoveries are in the Details

Naomi Halas, plenary speaker at SPIE Photonics Asia 2019, discusses her research on nanoparticles and its broad-reaching applications
25 September 2019
By Daneet Steffens
solar concentrator
Concentrating the sunlight on tiny spots on the heat-generating membrane exploits an inherent and previously unrecognized nonlinear relationship between photothermal heating and vapor pressure.

More than 15 years ago, Naomi Halas's nanophotonics group at Rice University demonstrated that if they manipulated a noble metal nanoparticle's geometry, they could control the color of the light that it absorbed. "This is a fundamental concept," says Halas, who will be delivering a plenary at 2019 SPIE Photonics Asia in October. "But at the time the world of nanoscience revolved around quantum dots, not metallic nanoparticles with plasmon resonances. No one was thinking about nanoparticles as objects by which you could control light at the nanoscale and beyond."

Halas recalls, "At that time people were also just beginning to learn to exploit the light-penetrating properties of tissue, in particular the spectral regions of increased transparency, known as ‘water windows'. We realized that we could design and create nanoparticles that could absorb light in those wavelength regions, which would induce a photothermal heating effect. That resulted in demonstrating—first in cells, then in mice—photothermal cancer therapy: the nanoparticles reside in the tumor, you shine light into it, and the nanoparticles heat up and destroy the cancer."

A clinical trial on prostate cancer utilizing photothermal cancer therapy has been ongoing for the past year and a half, with a 94 percent success rate. "It's so promising," says Halas. "People have had this treatment without side effects. It has been a huge improvement for the better."

A more recent breakthrough for Halas' team has been enhancing the efficiency of their solar-powered desalination system by over 50 percent, simply by adding arrays of lenses that concentrate sunlight into "hot spots." Improving their nanophotonics-enabled solar membrane-distillation (NESMD) system also emerged from Halas' focus on understanding the fundamental science behind the vaporization process central to distillation. The NESMD technology, originally created five years ago to create a mobile, off-grid version of more conventional, much larger, energy-consuming facilities, utilizes light-absorbing nanoparticles to transform the membrane—a small, sheet-like-device that has hot, salty water flowing across one side while cool, filtered water flows across the other—into a solar-driven heating element.

The physical process of the system, says Halas, depends on the saturation vapor pressure of water which has a temperature dependence that is exponential. "Focusing light increases the temperature of the system in localized regions. Instead of trying to expand our equipment to capture more light, we just intensified the light we were already getting, and the efficiency jumped substantially."

Halas feels a strong sense of responsibility of science to society, and part of that means continuing to explore her own scientific discoveries. "The truth is that scientists are always curious. They're always going to stop and ask—even if they're doing something as mundane as building a membrane distillation apparatus to take outdoors—"Well, gee guys, do we really understand how this thing really works?" My business is nanoparticles and light, and the question always remains: how can we use that to develop innovative methods to solve our biggest challenges?"

-Daneet Steffens is the PR Manager for SPIE.

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