MIT Develops Stable Red-Emitting Dyes for Better Biomedical Imaging
Scientists at MIT have designed a new class of fluorescent molecules based on positively charged boron atoms, known as borenium cations, that glow in the red to near-infrared range. By stabilizing these ions with specially chosen ligands, the team has created materials that emit bright light suitable for imaging deep within biological tissues. This breakthrough could lead to clearer tumor images and other improved biomedical diagnostics.
The key advancement lies in attaching the unstable borenium ions to ligands called carbodicarbenes (CDCs). This stabilization enables practical handling outside of airtight gloveboxes and protects the dyes from photodegradation, a long-standing hurdle for red and near-infrared fluorophores. The approach was detailed in a study published in Nature Chemistry, with MIT chemist Robert Gilliard as the senior author.
“One of the reasons why we focus on red to near-IR is because those types of dyes penetrate the body and tissue much better than light in the UV and visible range. Stability and brightness of those red dyes are the challenges that we tried to overcome in this study,” Gilliard explains. Lead author Chun-Lin Deng and colleagues contributed to demonstrating the practicality of these stabilized borenium compounds in multiple states—films, powders, and crystals—each capable of absorbing and emitting light in the red to near-infrared region.
Why Red and Near-IR Light Matters
Imaging in the blue or green spectrum often encounters interference from natural autofluorescence and light scattering in tissue. Red and near-infrared (near-IR) light experiences less scattering and deeper penetration, allowing researchers to visualize structures that lie beneath the surface. The researchers report quantum yields in the thirties percent for red-emitting borenium dyes, a notable efficiency for near-IR fluorophores that frequently struggle with brightness.
Historically, borenium cations—the positively charged boron-containing molecules that can emit near-IR light—were considered lab curiosities due to their instability in air. The MIT team’s stabilization strategy, leveraging CDC ligands, makes these ions robust enough to handle in standard lab conditions while preserving their optical advantages. This stability is crucial for potential medical imaging applications where reliability and reproducibility are essential.
From Lab Chemistry to Biomedical Tools
The researchers explored how the negative counterions paired with the borenium cations influence light emission through a phenomenon called exciton coupling. By fine-tuning these interactions, they could shift emission and absorption toward the infrared and boost brightness. The team demonstrated that the same chemistry can produce solid crystals, films, powders, and colloidal suspensions, expanding possible deployment formats for biomedical use.
In terms of practical application, Gilliard envisions encapsulating these borenium-containing materials in polymers to enable injectable imaging dyes. Collaborations with MIT’s chemistry department and the Broad Institute aim to test these materials in cellular contexts, moving toward in vivo imaging. Beyond imaging, the materials’ temperature-responsive properties could render them useful as molecular thermometers for monitoring temperature exposure during drug or vaccine transport. In flexible electronics, the materials may find a role in organic light-emitting diodes (OLEDs) due to their strong near-IR emission and environmental stability.
Broader Impact and Next Steps
Experts unaffiliated with the study have highlighted the potential of these dyes for bioimaging, smart sensors, and advanced optoelectronics. Rutgers University’s Frieder Jaekle notes the combination of high near-IR quantum yield and stability makes this class of compounds particularly appealing for diverse technologies, including anti-counterfeiting and sensing applications. The MIT team is already exploring ways to extend emission further into the near-IR by incorporating additional boron atoms, while also developing robust carbodicarbene variants to maintain stability for these heavier targets.
The research received support from the Arnold and Mabel Beckman Foundation and the National Institutes of Health, reflecting the translational potential of these red-emitting dyes from fundamental chemistry to practical biomedical tools.
