Highly selective turn-on detection of (strept)avidin based on self-assembled near-infrared fluorescent probes

Abstract

Selective detection and visualization of specific proteins are important in clinical diagnostics and biological research. For protein sensing, small-molecule-based fluorescent turn-on probes are preferable because of their high sensitivity, simplicity and detection with high-throughput. Herein we demonstrated a small molecular fluorescent dye (SQ-Biotin) which can self-assemble into a non-fluorescent probe in aqueous solution for near infrared turn-on detection of avidin protein. This probe consisting of a hydrophobic squaraine (SQ) as a fluorophore and a specific and strong protein ligand (biotin) formed self-assembled aggregates in aqueous solution (fluorescence off), and the aggregates of the probe disassembled in response to the target protein (avidin) through the specific protein–ligand interaction (fluorescence on). The conversion of the aggregation of SQ-Biotin was confirmed by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The fluorescence intensities at 665 nm were linearly proportional to the concentration of avidin over the range of 0.76–1.46 μM. The detection limit was calculated to be about 70 nM. SQ-Biotin showed good selectivity to avidin over other proteins, enabling turn-on fluorescent detection of avidin in the near infrared region. The strategy demonstrated the great potential applicability of the self-assembled small-molecule-based fluorophores for protein sensing in clinical diagnosis.