The contrast mechanisms used for photoacoustic tomography (PAT) and fluorescence imaging differ in subtle but significant ways. Design of contrast agents for each or both modalities requires an understanding of the spectral characteristics as well as intra- and intermolecular interactions that occur during formulation. We found that fluorescence quenching that occurs in the formulation of near infrared (NIR) fluorescent dyes in nanoparticles results in enhanced contrast for PAT. The ability of the new PAT method to utilize strongly absorbing chromophores for signal generation allowed us to convert a highly fluorescent dye into an exceptionally high PA contrast material. Spectroscopic characterization of the developed NIR dye-loaded perfluorocarbon-based nanoparticles for combined fluorescence and PA imaging revealed distinct dye-dependent photophysical behavior. We demonstrate that the enhanced contrast allows detection of regional lymph nodes of rats in vivo with time-domain optical and photoacoustic imaging methods. The results further show that the use of fluorescence lifetime (FLT) imaging, which is less dependent on fluorescence intensity, provides a strategic approach to bridge the disparate contrast reporting mechanisms of fluorescence and PA imaging methods.
Pyrrolopyrrole cyanine (PPCy) dyes are presented as a novel class of near-infrared (NIR) chromophores, which are synthesized in a condensation reaction of diketopyrrolopyrrole with heteroarylacetonitrile compounds. Their optical properties are marked by strong and narrow-band NIR absorptions. Complexation products with BF(2) and BPh(2) show strong NIR fluorescence and hardly any absorption in the visible range. We synthesized a series of new PPCys that differ only in the heterocyclic peripheral groups of the chromophore. With this strategy, the absorption spectra can be tuned between 684 and 864 nm, while high fluorescence quantum yields are maintained. The influence of the heterocycle on the optical properties of the dyes is discussed.
NIRly there: The chromophore of compounds with intramolecular H bonds have been rigidified by substitution of the protons of the NH⋅⋅⋅N bridges by BR2 groups (R=F, Ph). This stiffening and the polymethine character of the chromophores affords dyes with intense fluorescence at room temperature and high NIR absorption, but virtually no absorption in the visible range (see the picture for an example, —: absorption, ••••: fluorescence).
Very invisible: Bis(pyrrolopyrrole) cyanines are extended chromophores based on diketopyrrolopyrroles (general structure shown). The compounds are characterized by extremely high extinction coefficients, narrowband absorption in the near‐IR range, and strong fluorescence around 1 μm. Since absorption in the visible region is negligible, they approach the ideal of selective NIR absorbers.
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