Optically-active nanocrystals for inner filter effect-based fluorescence sensing: Achieving better spectral overlap

Among various photophysical processes for fluorescence modulation, inner filter effect (IFE) is well-known for its simplicity and no requirement of the distances between the fluorophores and the corresponding modulators. Theoretically, the key to maximize the sensitivity of IFE-based fluorescent assays is to enlarge the overlap between the absorption of the absorber and the excitation/emission of the fluorophores. However, it is difficult to achieve perfect spectral overlap for IFE in conventional organic fluorophores with constant excitation/emission. The emergence of various optically-active nanocrystals greatly revolutionize the IFE-based fluorescent sensing. Therefore in this critical review, we first made an introduction to IFE from the viewpoint of its photophysical process. Particularly, the similarities and differences of IFE and fluorescence resonance energy transfer for better understanding of IFE are discussed, and the general rules for verification of IFE are proposed. The use of various optically-active nanocrystals for enhanced IFE-based sensing is especially concerned in this review.