St Andrews Researchers Discover Two New Molecules

Have you ever held a torch up to your hand to see it shine through your skin? Or maybe shone a torch into your mouth while telling a scary story to make your face glow red? It seems pretty childish looking back, but scientists and surgeons capitalise on the same idea (called fluorescent imaging) to find out what goes on inside living tissues. By injecting a dye into a patient and shining a laser, doctors can measure the re-emitted light to locate blood vessels and even tumours. 

Unsurprisingly, firing lasers at people comes with some side effects. Namely, it can damage cells in the light path, causing irritation, swelling and pain. Two new molecules discovered in part by the Zysman-Colman group at St Andrews, however, are promising for new techniques that could improve the accuracy, depth, and patient experience of medical imaging.

The two newly reported molecules have two properties that make them useful for fluorescent imaging: circularly polarised luminescence and two-photon excitation. 

Luminescence is the property of molecules giving off light when excited in any way besides heat. A good example is those little glow-in-the-dark stars you had on your ceiling as a kid, except in this case, the light is invisible to humans. One problem with fluorescent imaging is that some molecules in the human body are luminescent on their own, making the image noisy and the dye hard to see. These new molecules emit circularly polarised light (meaning it spins as it travels), which is quite rare, especially in organic systems, allowing it to be used as a fingerprint to identify the dye. This quality makes the images much more accurate and much clearer, helping to map blood vessels.

The other property, two-photon excitation, is more important for attenuating the side effects and increasing applicability. For most molecules, to emit one photon (particle) of light, they must first be hit by one with enough energy. Typically, this requires high-energy photons, which can damage your cells in a similar way to a sunburn, but these new molecules can instead absorb two photons of lower energy before emitting one. 

The first advantage of two-photon excitation is that it causes less damage. If one photon is sent in one direction and another from a different direction, the interposing cells receive less energy. Only where they meet is the energy high enough to cause an emission of light. Another advantage is that the excitation allows for imaging deeper into the human body. This works because lower-energy photons have a longer wavelength, allowing them to push through more material without being as absorbed. The light still has the same amount of trouble coming out, but with less loss going in, allowing researchers to image up to ten times deeper than they could with single-photon methods.

Fluorescent imaging is widely used to help brain surgeons find the edges of tumours during surgery, to trace the lymph nodes where cancers spread, and to aid eye exams. It can also be used to check whether grafts or transplants are getting adequate blood flow — these molecules may have even more far-reaching applications in optical data storage and photonics. It may be a while before the molecules reach a hospital, but with their unique qualities, they could have huge impacts on medicine, biology, computing, and communication, all while advancing hardware and saving lives in the process.

Image from Wikimedia Commons