An "off-on" fluorescent aptasensor based on FRET and magnetic beads for detection of circulating tumor cells in lung cancer.

Circulating tumor cells (CTCs) serve as critical biomarkers for early diagnosis, prognosis monitoring, and treatment response evaluation in lung cancer. Herein, we report a novel "off-on" fluorescent aptasensor based on fluorescence resonance energy transfer (FRET) and streptavidin-modified magnetic beads (MBs) for the sensitive and specific detection of A549 CTCs in lung cancer. The sensing strategy employs a biotinylated EGFR-targeting aptamer (Apt) labeled with FAM (donor) and a complementary S1 strand modified with BHQ-1 (acceptor). In the absence of target cells, the aptamer hybridizes with S1, bringing FAM and BHQ into close proximity, resulting in efficient fluorescence quenching ("off" state). Upon recognition of EGFR-overexpressing A549 cells, the aptamer undergoes a conformational change or strand displacement, releasing S1-BHQ and restoring FAM fluorescence ("on" state). Under optimized conditions, the aptasensor exhibited a linear response to A549 cell concentrations ranging from 50 to 5 × 105 cells per mL, with a detection limit as low as 34 cells per mL. The assay also demonstrated excellent selectivity against non-target cancer cells (SW620, HGC-27, MCF-7) and performed well in human serum samples, achieving a limit of detection of 105 (1.05 × 102) cells per mL. This FRET-based magnetic platform offers a simple and sensitive approach for CTC detection and shows potential as a tool for lung cancer liquid biopsy pending further validation with clinical patient samples.