08 Sep 2015
A point-of-use solar distillation device that can clean up saltwater and wastewater without producing greenhouse gases has been constructed by a research team from King Abdullah University of Science and Technology (KAUST)1.
The key to the new technology is a floating membrane coated with a special light-absorbing polymer that repairs its hydrophobic “skin” when damaged.
For centuries, attempts have been made to use the sun’s heat to distill clean water from polluted sources. Simple solar stills, such as a glass plate placed over a water-filled box, are inexpensive to operate but are notoriously inefficient. This is because water is a poor light absorber, and any captured heat tends to distribute uniformly through the still instead of localizing at surfaces where evaporation occurs.
To combat these problems, researchers are developing floating “solar generator” materials that heat up quickly in sunlight and then trap heat at air–water interfaces for steam production.
A polypyrrole (PPy)-coated device that absorbs sunlight and releases it as heat can rapidly purify water through distillation Reproduced with permission © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
These devices are usually coated with water-repellant waxy molecules, such as fluorinated alkyl chains, for better floating. However, damage from ultraviolet rays and oxidative chemicals can degrade the hydrophobic layers, causing the generator to sink.
Inspired by the lotus flower, a plant that restores damage to its hydrophobic leaves through the migration of waxy molecules, KAUST Associate Professor Peng Wang and colleagues from the University’s Biological and Environmental Science and Engineering Division developed a self-healing solar generator.
The researchers coated a tightly woven stainless steel mesh with polypyrrole (PPy), a light-absorbing polymer with high photothermal conversion efficiency and bumpy surface microstructures. The team modified the PPy film with fluoroalkylsilane chains, enabling it to act as a reservoir that supplies additional hydrophobic chains to damaged regions through biomimetic self-migration.
The new device nearly tripled the output of freshwater from typical solar stills, thanks to a significant jump in temperature at the air–water interface and a conversion efficiency of close to 60 percent. It also exhibited remarkable damage resistance: after the team used a plasma source to oxidize the mesh and make it sink to the bottom of a beaker, they found a simple one-hour treatment in sunlight was sufficient to restore its self-floating capability.
The team’s first prototype — a transparent plastic condensing chamber and solar fan mounted on top of a PPy-coated mesh — floats lightly on the surface of seawater and distills a steady stream of water for more than 100 consecutive hours.
“Careful material selection allowed us to integrate two types of functions into one distillation device,” Wang said. “This has great potential to be employed in point-of-use potable water production.”
- Zhang, L., Tang, B., Wu, J., Li, R. & Wang, P. Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating. Advanced Materials advance online publication, 17 July 2015 doi: 10.1002/adma.201502362 | article
13 May 2015
Photoacoustic imaging is a ground-breaking technique for spotting tumors inside living cells with the help of light-absorbing compounds known as contrast agents. A*STAR researchers have now discovered a way to improve the targeting efficacy and optical activity of breast-cancer-specific contrast agents using conjugated polymer nanoparticles.
Generating photoacoustic signals requires an ultrafast laser pulse to irradiate a small area of tissue. This sets off a series of molecular vibrations that produce ultrasonic sound waves in the sample. By ‘listening’ to the pressure differences created by the acoustic waves, researchers can reconstruct and visualize the inner structures of complex objects such as the brain and cardiovascular systems.
Diagnosing cancer with photoacoustic imaging requires contrast agents that deeply penetrate tissue and selectively bind to malignant cells. In addition, they need a high optical response to near-infrared laser light, a spectral region that is particularly safe to biological materials. Traditional contrast agents have been based on gold and silver nanostructures, but the complex chemical procedures needed to optically tune these nanocompounds have left researchers looking for alternatives.
Photoacoustic imaging of model breast cancer cells in mice reveals that a polymer-based contrast agent can illuminate tumor sites within one hour. Credit: Dove Medical Press Limited
Malini Olivo and her colleagues from the A*STAR Singapore Bioimaging Consortium and the A*STAR Institute of Materials Research and Engineering investigated different contrast agents based on conjugated polymers. These organic macromolecules, which contain alternating double and single carbon bonds, have delocalized electrons in their frameworks that can produce useful optical properties such as photoluminescence. The researchers identified a conjugated polymer known as PFTTQ—a compound with multiple aromatic rings, alkyl chains, sulfur and nitrogen atoms—as a promising in vivo photoacoustic agent because of its biocompatible structure and light absorption that peaks in the near-infrared range.
To direct this contrast agent to cancer cells, the team synthesized ‘dot’-like nanostructures with an inner core of PFTTQ surrounded by water-soluble polyethylene glycol chains, terminated by an outer layer of folate molecules—a vitamin that specifically binds to folate receptor proteins commonly expressed by breast cancer tumors. Experiments with MCF-7 model breast cancer cells implanted in mice revealed the merits of this approach: in just one hour after administering the folate–conjugated polymer dots, strong photoacoustic signals emerged from the tumor positions. The folate functionality played a critical role in this bioimaging procedure, quadrupling the photoacoustic signals compared to unmodified PFTTQ dots.
“The folate–PFTTQ nanoparticles have great potential for diagnostic imaging and other biomedical applications,” says Olivo. “We are working to expand the library of biocompatible polymers to use as molecular photoacoustic contrast agents.”
More information: “Molecular photoacoustic imaging of breast cancer using an actively targeted conjugated polymer.” International Journal of Nanomedicine 10, 387–397 (2015). dx.doi.org/10.2147/IJN.S73558