It is helpful — even life-saving — to have a warning sign before a structural system fails, but when the system is only a few nanometers in size, having a sign that is easy to read is a challenge. Now, however, a newly developed molecular design can provide such a warning in the form of a simple color change. In a study conducted by Daniel Hammer of the University of Pennsylvania and his colleagues there and at Duke University in Durham, N.C., the researchers worked with two molecules: porphyrins, a class of naturally occurring pigments, and polymersomes, artificially engineered capsules that can carry a molecular payload in their hollow interiors. In this case, they hypothesized that polymersomes could be used as stress sensors if their membranes were embedded with a certain type of light-emitting porphyrins. An enhanced image of a polymersome changing color under stress. (Image: Neha Kamat, University of Pennsylvania) The group designed polymersomes studded with the light-emitting porphyrin molecules. When light is shone on these labeled polymersomes, the porphyrins absorb the light and then release it at a specific near-IR wavelength. The porphyrins play a critical role in using the polymersomes as stress sensors because their configuration and concentration controls the release of light. “When you package these porphyrins in a confined environment, such as a polymersome membrane, you can modulate the light emission from the molecules,” Hammer said. “If you put a stress on the confined environment, you change the porphyrin’s configuration, and, because their optical release is tied to their configuration, you can use the optical release as a direct measure of the stress in the environment.” The labeled polymersomes could be injected into the bloodstream, for example, and serve as a proxy for neighboring red blood cells. As both the cells and polymersomes travel through an arterial blockage, scientists would be able to better understand what happens to the blood cell membranes by making inferences from the stress label measurements. Their work was published in Proceedings of the National Academy of Sciences. For more information, visit: www.upenn.edu