The new dye enables visualization of deep tissues, enhancing the diagnosis and treatment of cancer.

Using a dye from the family of bile pigments as a foundation, they developed a unique ring structure capable of binding rhodium and iridium. Measurements and modeling indicated strong absorption in the second near-infrared (NIR) range and exceptional photostability. The second waves of the near-infrared range easily penetrate human tissues; the new dye may find applications in the therapy and visualization of deep tissues.

The second near-infrared region of the electromagnetic spectrum (1000-1700 nanometers) is a potentially significant range of wavelengths for medicine. In this range, light is not as strongly scattered or absorbed by biological tissues. This transparency makes it ideal for delivering energy to deeper parts of the body, both for visualization and treatment. An important example of such therapy is photoacoustic imaging in the diagnosis and treatment of cancer. When light hits a contrast agent introduced into the body, it emits heat, creating tiny ultrasound bursts that can be detected for imaging or used to damage cancer cells.

The effectiveness of this approach relies on the availability of stable contrast agents that can effectively absorb light at these wavelengths. However, most contrast agents are more sensitive in the first near-infrared range (700-1000 nanometers), where scattering effects are stronger, and energy delivery is less efficient.

Now, a group of researchers led by Associate Professor Masatoshi Ichida from Tokyo Metropolitan University has developed a new chemical compound that eliminates this "Achilles' heel." Using a dye from the family of bile pigments called bilatriene as a base, they employed a method known as N-confusion chemistry to modify the ring structure of bilatriene so that it could bind metal ions. In their latest work, they successfully attached rhodium and indium ions to the ring through nitrogen atoms.

The new dye exhibited the strongest light absorption at a wavelength of 1600 nanometers under normal conditions, which falls within the second near-infrared range. It also proved to be highly photostable, meaning it does not degrade when exposed to light. Detailed measurements of the molecule's response to magnetic fields and numerical calculations using density functional theory (DFT) showed that the unique distribution of electrons in the cloud surrounding the entire complex structure of the metal-binding molecule (also known as a π-radicaloid) leads to absorption that is unattainable in existing similar compounds.

Since the second near-infrared range is not as strongly absorbed by tissues, areas sensitized by the dye will be more exposed to light, allowing for clearer images and better heat delivery for treatment. The team hopes that their molecule will open doors to new approaches in deep tissue medicine as well as more general applications in chemical catalysis.