A polymer that is custom designed to produce light that penetrates murky environ­ments has shown promise in bio­imaging trials, where it can detect nano-sized particles underneath the surface of realistic tissue models. Recent studies have demons­trated that fluorescent probes are particularly useful for bio­imaging when they radiate in the shortwave infrared (SWIR) region of the optical spectrum. Because this type of fluorescent light pene­trates deeper into biological objects without being absorbed or scattered, SWIR probes can be spotted farther into tissue than conven­tional emitters. These features have enabled SWIR probes to capture high-resolution images of structures located deep within the body, such as brain tissue, without the hazards of x-rays.

Satoshi Habuchi and his colleagues are working to improve fluorescent imaging by expanding the type of probes capable of producing SWIR radiation. Currently, most bright SWIR emitters are either semi­conductor quantum dots or rare-earth-doped nano­particles that are unsuitable for many specimens because of their toxic side-effects. On the other hand, materials that are more biocom­patible, such as organic dyes, are usually not intense enough to be seen inside tissue. To resolve this issue, KAUST researchers turned to polymers having donor-acceptor structures, a layout where electron-rich components alter­nate with electron-poor portions along a conductive molecular chain. “This distri­bution promotes charge transfer along the polymer backbone, which is a very effective way to obtain SWIR light,” explains Hubert Piwonski.

The team chose two donor-acceptor polymers with ideal charac­teristics for SWIR emission and then developed a preci­pitation procedure that fused the compounds into tiny polymer spheres, or dots, just a few nanometers wide. Optical characterizations revealed these materials had excep­tionally bright SWIR emissions that were easily spotted in biological tissue models. “Per volume, our particles have a brightness value larger than almost all other SWIR emitters reported so far,” says Habuchi. “This enabled detection of nano­meter-sized polymer dots in specimens one millimeter thick.”

In addition, the new polymer dots that fluoresce only for a nanosecond can produce low-noise images with single-molecule sensi­tivity due to high throughput detection of emitted fluorescence. The ability to visualize single probes at fast acqui­sition rates could benefit researchers looking to capture processes in tissues and organs as they happen. “There are huge opportunities for new probes and imaging moda­lities capable of addressing the dynamics of molecules in living systems, and our polymer dots are a big step toward single-particle tissue imaging,” says Piwonski. (Source: KAUST)

Reference: H. Piwonski et al.: Millimeter-Deep Detection of Single Shortwave-Infrared-Emitting Polymer Dots through Turbid Media, Nano Lett. 20, 8803 (2020); DOI: 10.1021/acs.nanolett.0c03675

Link: Biological and Environmental Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia