Abstract
Shortwave infrared (SWIR) fluorescence detection gradually becomes a pivotal real-time imaging modality, allowing one to elucidate biological complexity in deep tissues with subcellular resolution. The key challenge for the further growth of this imaging modality is the design of new brighter biocompatible fluorescent probes. This review summarizes the recent progress in the development of organic-based nanomaterials with an emphasis on new strategies that extend the fluorescence wavelength from the near-infrared to the SWIR spectral range and amplify the fluorescence brightness. We first introduce the most representative molecular design strategies to obtain near-infrared-SWIR wavelength fluorescence emission from small organic molecules. We then discuss how the formation of nanoparticles based on small organic molecules contributes to the improvement of fluorescence brightness and the shift of fluorescence to SWIR, with a special emphasis on the excited-state engineering of molecular probes in an aggregate state and spatial packing of the molecules in nanoparticles. We build our discussion based on a historical perspective on the photophysics of molecular aggregates. We extend this discussion to nanoparticles made of conjugated polymers and discuss how fluorescence characteristics could be improved by molecular design and chain conformation of the polymer molecules in nanoparticles. We conclude the article with future directions necessary to expand this imaging modality to wider bioimaging applications including single-particle deep tissue imaging. Issues related to the characterization of SWIR fluorophores, including fluorescence quantum yield unification, are also mentioned.
Highlights
Fluorescence microscopy has been developing faster than any other imaging modality
The organic Shortwave infrared (SWIR) fluorophores that we introduced in the previous section have a small size and mitigated toxicity, which are suitable for many imaging applications
Unlike J-aggregates assembled from benzothiadiazole-based charge-transfer complexes, a V-shaped D-A-D system composed of benzothiadiazole and two electron-donating thiophene units formed a dark state associated with an intermediate aggregate form, which was responsible for the observed fluorescence quenching.[127]
Summary
Fluorescence microscopy has been developing faster than any other imaging modality. Through recent innovations of fluorescence imaging techniques, including laser scanning confocal, multiphoton, light sheet, and superresolution microscopy, it has been realized that their applicability and future improvements depend heavily on the development of new fluorochromes used for labeling the targeted structures. Several studies on inorganic semiconductor-based luminophores indicated a pharmacokinetics or acute or chronic toxicity of these probes in vivo.[1−8] The progress of the development of SWIR emitting probes based on inorganic luminophores can be found in other excellent review articles.[9−11] Organic probes (e.g., small-molecule dyes, polymers, and organic nanoparticles) have been used for a wide range of applications, including drug delivery systems,[12] antiviral therapeutics,[13] photoactivatable enzymes,[14] enzyme monitoring systems,[15,16] neurotransmitter sensing,[17] cancer phototherapeutics,[18] glucose level monitoring systems,[19] imaging of intracellular traffics,[20] etc Their applicability often goes beyond the scope of bioimaging. This review is intended for both an expert and a graduate student who work on the development of new SWIR probes from a photophysics standpoint, in particular, those who work on SWIR probes based on molecular aggregates and nanoparticles
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