Chemists at MIT have developed a new family of glowing red dyes that could make biomedical imaging much clearer.
These new fluorescent materials, based on a special form of boron known as a borenium ion, can shine in the red and near-infrared range—ideal for looking deep inside the body.
The discovery, published in Nature Chemistry, is exciting because near-infrared light passes through tissue far better than blue or green light.
This makes it especially useful for imaging tumors and other hidden structures.
But until now, red dyes have been notoriously unstable and dim, producing weak signals that limit their usefulness in medical applications.
“Red and near-IR light penetrates tissue much more effectively than other wavelengths,” said Robert Gilliard, the Novartis Professor of Chemistry at MIT and senior author of the study.
“The challenge has always been making dyes in that range that are both stable and bright. That’s what we set out to solve.”
Fluorescent dyes are already widely used in biology and medicine, but most give off blue or green light.
Inside the body, that creates problems: tissues produce faint natural blue-green fluorescence that interferes with the signal, and the light scatters quickly. Red dyes could solve those issues—if only they worked reliably.
Borenium ions, which contain a positively charged boron atom, were first identified in the 1980s. Chemists immediately saw that they could emit red light, but they were so unstable they had to be handled in airtight containers. Over the years, researchers discovered that attaching stabilizing molecules, called ligands, could protect these ions.
In 2019, Gilliard’s lab found that borenium ions could even change color with temperature. But they were still too reactive to handle outside of special sealed gloveboxes. Two years ago, the team developed a better stabilizing strategy using ligands called carbodicarbenes, or CDCs. These CDC-borenium compounds could finally survive in normal lab conditions, opening the door to more detailed studies.
In their latest work, the MIT researchers looked closely at how the negatively charged parts of these compounds—called anions—interacted with the borenium cations. They discovered that these interactions produced a phenomenon called exciton coupling, which pushed the molecules’ light emission further into the red and near-infrared spectrum. Even better, the dyes became much brighter.
“Not only are we in the right color range,” said Gilliard, “but the molecules are also impressively efficient. We’re seeing quantum yields—basically, light output efficiency—up to 30 percent in the red, which is very high for this part of the spectrum.”
The researchers were able to make the dyes in several forms, including powders, films, crystals, and colloidal suspensions. That versatility could allow them to be tailored for different applications.
For biomedical use, the team envisions packaging the dyes in polymers so they could be injected safely into the body as imaging agents. As a first step, they plan to test how the dyes perform inside living cells, working with colleagues at MIT and the Broad Institute.
Because the dyes also respond to temperature, they could serve as “molecular thermometers” to track whether medicines like vaccines are kept within safe ranges during shipping. In other forms, the compounds might be used in flexible electronic displays, as organic light-emitting diodes (OLEDs), or even as anti-counterfeiting materials.
“The very high efficiency and stability make these dyes extremely interesting,” said Frieder Jaekle, a Rutgers University chemistry professor not involved in the study. “They have enormous potential for both bioimaging and advanced materials.”
Looking ahead, Gilliard’s lab is experimenting with ways to extend the dyes’ glow even deeper into the near-infrared region by adding more boron atoms. That could make them even more powerful—but also more unstable. To address that, the researchers are designing new carbodicarbene ligands that may hold the molecules steady.
If successful, these bright, stable red dyes could one day allow doctors to see tumors more clearly, monitor temperature-sensitive medicines, and even power next-generation screens—all thanks to a once-fragile ion of boron.
Source:MIT.
