Aptamer-based fluorescence "off-on" sensor for EGFR-positive lung cancer CTCs using streptavidin magnetic beads and N-CQDs@Ag⁺.

Circulating tumor cells (CTCs) are rare but critical biomarkers for lung cancer diagnosis and metastasis monitoring. We report an aptamer-based fluorescence "off-on" sensor for the sensitive and specific detection of epidermal growth factor receptor (EGFR)-positive lung cancer CTCs. The sensor integrates streptavidin magnetic beads (MBs) for target capture and nitrogen-doped carbon quantum dots complexed with silver ions (N-CQDs@Ag⁺) for signal readout. In the absence of target cells, the system remains in a fluorescence "off" state due to efficient quenching by Ag⁺. Upon recognition of EGFR-positive CTCs by the biotinylated aptamer, a complementary S2 strand containing a polyC sequence is released and competitively binds Ag⁺, restoring the fluorescence of N-CQDs ("on" state). Under optimal conditions, the sensor achieves a detection limit of 50 cells/mL in buffer and 96 cells/mL in human serum, with excellent selectivity. This aptamer-based off-on sensor provides a simple and reliable platform for lung cancer CTC detection.