Engineered flower-like core–double-shell upconversion nanoarchitectures for ultrasensitive activatable FRET imaging of cathepsin B in vitro and in vivo

초록

Multidimensional core–double-shell upconversion nanostructures offer exceptional luminescence brightness and efficient energy transfer, yet their use for cathepsin B (CTSB)-responsive imaging remains largely unexplored. Herein, we report a peptide-functionalized upconversion nanoflower platform, UCNF@SiO2@TAMRA–pep, for sensitive CTSB detection in buffer, living cells, and tumor-bearing mice. The probe was constructed via seed-mediated epitaxial growth of core–double-shell nanoflowers, followed by silica encapsulation and covalent conjugation of a TAMRA-labeled CTSB-cleavable peptide (CLLGPLGDDDK). In the intact probe, TAMRA quenches the green upconversion emission through fluorescence resonance energy transfer (FRET). CTSB-mediated peptide cleavage releases TAMRA, restoring green emission and enabling activatable FRET imaging. The probe shows a rapid, concentration-dependent response over 0.01–20 μg mL−1, with excellent linearity (R2 = 0.991) and a detection limit of 11.4 ng mL−1. This performance arises from synergy between the multidimensional nanoflower morphology and the multilayered core–double-shell architecture, which jointly enhance brightness, FRET efficiency, and peptide loading. Comparative studies across core, core–shell, and core–double-shell nanoflower structures confirm progressively improved luminescence and FRET performance. Control experiments with heat-inactivated CTSB verify that signal recovery is enzyme-specific. The probe exhibits negligible cytotoxicity and enables sensitive monitoring of intracellular CTSB during cisplatin-induced apoptosis. Cisplatin-treated HeLa cells show roughly fourfold higher luminescence recovery than MDA-MB-231 cells, reflecting elevated CTSB activation, and the signal correlates linearly with HeLa cell number from 5 × 103 to 1 × 106. In vivo imaging further demonstrates sensitive CTSB detection in tumor-bearing mice without observable toxicity, establishing this platform as a promising tool for activatable enzyme-responsive cancer imaging and therapeutic evaluation. © 2026 Elsevier B.V.

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