Researchers at Kyushu University have developed water-soluble, metal-free luminescent organic radicals that emit deep-red fluorescence and enhance magnetic resonance imaging contrast in test images. Published in Advanced Science, the proof-of-concept molecules offer a potential alternative to conventional gadolinium-based contrast agents.
Bioimaging relies heavily on two distinct technologies: fluorescence imaging, which visualizes cells at high resolution but lacks imaging depth, and magnetic resonance imaging, which captures deep bodily structures without single-cell clarity. Combining these modalities in a single molecule has long proved difficult because their functional mechanisms differ entirely. Fluorescence depends on the absorption and reemission of light wavelengths, while magnetic resonance imaging detects the molecular spin of hydrogen atoms in body water. Exogenous contrast materials generally improve MRI clarity by shortening the relaxation times of water.
Conventional MRI contrast agents depend on paramagnetic metals such as gadolinium. While effective, these metal-based agents carry the potential for adverse side effects. Addressing this limitation, a research team led by associate professor Ken Albrecht at Kyushu University’s Institute for Materials Chemistry and Engineering has synthesized water-soluble, metal-free molecules capable of functioning in both fluorescence imaging and magnetic resonance imaging.
Optimizing Luminescent Organic Radicals for Water Solubility
The research focused on a class of compounds known as luminescent organic radicals, specifically tris(2,4,6-trichlorophenyl) methyl (TTM), which naturally produces fluorescence and molecular spin. However, earlier iterations of these organic radicals were hydrophobic, which severely restricted their utility in biological environments. Early attempts by the Kyushu University team yielded first-generation molecules that exhibited strong MRI activity but exceptionally weak fluorescence.
To overcome these performance hurdles, the team optimized the molecular architecture. They developed structures bound in three directions to water-soluble groups. This chemical modification successfully produced deep-red fluorescence in water while retaining the magnetic spin properties required for diagnostic scanning.
Performance and Dual-Modal Testing of TTM-trisTP3B and TTM-trisTP9B
The optimization efforts resulted in two distinct molecules: TTM-trisTP3B and TTM-trisTP9B. Laboratory tests demonstrated that both compounds enhanced the contrast of MRI phantom images approximately 2.5 to 4.5 times more than previously reported water-soluble TTM radicals
, according to Phys.org reporting based on the Advanced Science study.
Further biological testing evaluated TTM-trisTP3B in living cells. The molecule emitted fluorescence concentrated around the cell nucleus and maintained its luminescent activity for at least 24 hours. While these results establish a clear proof of concept for metal-free fluorescence and MRI dual-modal imaging, researchers note that animal studies remain necessary to fully examine the viability of the molecules for broader medical diagnostics and quantum sensing technologies.
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