Engineering Self-Calibrating Nanoprobes with Two-Photon-Activated Fluorescence Resonance Energy Transfer for Ratiometric Imaging of Biological Selenocysteine.

Selenocysteine (Sec) has proven to be the dominant active site of diverse selenoproteins that are directly linked with human health and disease. Thus, understanding the critical functions and dynamics of endogenous Sec at cellular and tissue levels is highly demanded. However, no method has been reported that is capable of providing reliable quantitative imaging analysis of Sec in living systems, especially in deep tissues, with low background signal and high sensitivity and imaging resolution simultaneously. To address this challenge, we herein report a novel class of engineered Sec-responsive fluorescent nanoprobes that combines two-photon excitation with Förster resonance energy transfer (FRET) mechanisms for direct, yet selective, sensing and imaging of biological Sec over abundant competing biothiols. Specifically, the two-photon excitation at the near-infrared window can minimize light scattering and background signals in tissues, thus offering improved spatial and temporal imaging of deep living tissues with reduced background interference. Moreover, a reasonable FRET donor-acceptor pair has further been designed and verified by theoretical calculation. The acceptor undergoes intramolecular rearrangement specifically in response to the nucleophilic attack of Sec, hence triggering remarkable FRET-mediated ratiometric fluorescence enhancement for sensitive and reliable quantification of Sec through self-calibration of two emission channels. These striking properties, along with good water solubility and biocompatibility, suggest that this strategy may serve as a valuable imaging tool for studying various Sec-related biological events in complex biological systems.