H-aggregation of heptamethine cyanine dye induced by Smiles rearrangement for fluorescence sensing of biological species: A near-infrared ratiometric fluorescent assay for esterase

Heptamethine cyanine dyes have been extensively employed as the probe skeleton, as a result of the great potential of near-infrared (NIR) emission. To improve the working performance of a probe, several measures have been developed, and the reduced fluorescence overlapping between the receptor and product could make a huge contribution. Herein, a novel NIR probe Cy-NE for esterase was elaborated based on the cyanine framework, with a large fluorescence change of ca. 160 nm upon the sensing procedure leading to a well-behaved detection accuracy. This excellent working performance was achieved by judicious introduction of N-alkyl-ethyl ester at the C4′-position of a cyanine dye, capable of undergoing Smiles rearrangement spontaneously upon the specific hydrolysis by esterase at the ester site, and the subsequent H-aggregation of the product was responsible for the red emission. This proposed strategy has been applied for visualizing esterase activity in cells, slices and mice, and the ratiometric pattern also contributes to the good accuracy. This sensing scheme has been fully clarified by HR-MS, DLS, inhibition experiment and spectral comparison. To the best of our knowledge, this is the first attempt to employ the Smiles rearrangement of a C4′-N-alkyl substituted cyanine in analysis design. It is discovered that Smiles rearrangement of this kind cyanine dyes may serve as an alternative solution to improve sensing accuracy, and this work may provide a reference for design.