Fluorogen-peptide conjugates with tunable aggregation-induced emission characteristics for bioprobe design.

Fluorogens with aggregation-induced emission (AIE) characteristics are attracting intense research interest, and an AIE-peptide conjugate strategy has been reported for developing turn-on probes based on hydrophilic peptide ligands. To build a model also suitable for hydrophobic ligands, we propose to fine-tune the AIE characteristics for probe design. In this work, an iconic AIE fluorogen tetraphenylethene (TPE) was designed to conjugate with peptide fragments containing different numbers of aspartic acid (D) units. Relationships between the numbers of D and the hydrophilicity, optical properties, and aggregate sizes and the AIE characteristics of TPE-peptide conjugates were investigated carefully. Five carboxyl groups were found to be the threshold to "turn off" the fluorescence of TPE. As a proof-of-concept, TPE-SS-D5 containing a cleavable disulfide bond was synthesized for thiol turn-on detection. The validated tunable AIE characteristic offers new opportunities to design fluorescence turn-on probes based on hydrophobic recognition elements and AIE fluorogens.