Tuning the Chain Conformation of Pyrenyl-Pyridine Conjugated Oligomer Nanoprobe for Ultrasensitive Fluorescent Assaying of Fe and Ascorbic Acid.

The chain conformation of conjugated oligomers are critical for constructing ultrafine fluorescent nanoprobes with outstanding sensing performances owing to their suitable molecular flexibility and fine-tuned aggregation state. Herein, a series of pyrenyl-pyridine oligomers (OPPs) were designed and conveniently prepared by one-pot Sonogashira coupling of flexible 2,6-bis(ethynyl)pyridine with different rigid pyrene unit. Novel fluorescent nanoprobes (OPPNPs, ca. 2.50-25.4 nm in diameter) were hence readily fabricated through a microemulsion route, showcasing significantly distinct detecting properties due to their distinctive chain conformations. In particular, ultrasmall OPP1NPs (~ 3.75 nm) bearing zig-zag-shaped 1,6-substituted pyrenyl-pyridyl conjugated chain presented superior π-π stacking, strong excimer emission and best sensing performances for Fe via electron transfer (ET) and OPP1NPs aggregation-based fluorescence quenching. Additionally, ascorbic acid (AA) could act as an effective reducer and chelator, resulting in the fluorescence recovery of OPP1NPs. Under the optimal conditions, ultralow detection limits of OPP1NPs for Fe (LOD, 0.06 nM, S/N = 3) and AA (LOD, 8 nM) were achieved. Furthermore, small and biocompatible OPP1NPs enabled efficient fluorescence imaging of Fe and AA in live cells. Moreover, the conformation-regulated sensing strategy and ET mechanism are also supported by DFT calculations.