Near-infrared emitting probes for biological imaging: Organic fluorophores, quantum dots, fluorescent proteins, lanthanide(III) complexes and nanomaterials

Lanthanide DO3A‐Tropone Complexes: Efficient Dual MR/NIR Imaging Probes in Aqueous Medium

“…Gd 3+ - and Eu 3+ -complexes are well established as MR imaging contrast agents [1] while several other Ln 3+ have been explored for their luminescent properties in the visible or near-infrared (NIR) range. [2] Ln 3+ -based luminescent probes have several favorable properties for bio applications that include narrow emission bands, large difference between excitation and emission wavelengths and strong resistance towards photobleaching. [2a–c] Current research is focusing on NIR probes because of the potential deep tissue penetration of NIR photons and low autofluorescence of biological tissues in this range.…”

“…[2] Ln 3+ -based luminescent probes have several favorable properties for bio applications that include narrow emission bands, large difference between excitation and emission wavelengths and strong resistance towards photobleaching. [2a–c] Current research is focusing on NIR probes because of the potential deep tissue penetration of NIR photons and low autofluorescence of biological tissues in this range. [2c–f] Lanthanide ions share similar coordination chemistry, thus one can explore the entire lanthanide series with the same ligand.…”

“…[2a–c] Current research is focusing on NIR probes because of the potential deep tissue penetration of NIR photons and low autofluorescence of biological tissues in this range. [2c–f] Lanthanide ions share similar coordination chemistry, thus one can explore the entire lanthanide series with the same ligand. This versatility makes it possible to construct agents for combined or dual MR/optical imaging that contain Gd 3+ for MRI and another lanthanide, such as Nd 3+ or Yb 3+ for NIR optical detection.…”

“…This last point is particularly important since most of f-f- transitions of Ln 3+ are Laporte-forbidden and exhibit very low molar absorption coefficients making their direct excitation difficult. [2,3] However, this problem can be overcome if the organic ligand includes an appropriate chromophoric moiety that can absorb excitation energy and transfer it to the corresponding Ln 3+ . [2,3] Efficient relaxation of bulk water protons by Gd 3+ requires at least one rapidly exchanging inner sphere water molecule, [1] but this requirement appears to be incompatible with the design of optical agents, especially those emitting in the NIR range, because the overtones of O-H vibrations can quench the Ln 3+ excited states through non-radiative deactivation.…”

“…[2,3] However, this problem can be overcome if the organic ligand includes an appropriate chromophoric moiety that can absorb excitation energy and transfer it to the corresponding Ln 3+ . [2,3] Efficient relaxation of bulk water protons by Gd 3+ requires at least one rapidly exchanging inner sphere water molecule, [1] but this requirement appears to be incompatible with the design of optical agents, especially those emitting in the NIR range, because the overtones of O-H vibrations can quench the Ln 3+ excited states through non-radiative deactivation. [5] It has been shown, however, that in some Ln 3+ -based complexes, the NIR emission can be efficiently sensitized despite the presence of several coordinated water molecules.…”