A novel fluorescent peptidyl probe for highly sensitive and selective ratiometric detection of Cd(ii) in aqueous and bio-samples via metal ion-mediated self-assembly

Abstract

It is still a significant challenge to synthesize ratiometric fluorescent probes for Cd²⁺ ions in aqueous solutions and biosamples. In the present study, we synthesized a fluorescent peptidyl probe (1, pyrene-Cys-Gly-Pro-Cys-OH) based on a Cd²⁺-triggered self-assembling process for the ratiometric fluorescent detection of Cd²⁺ in aqueous solution, urine, and live cells. The peptidyl probe (1) consisting of a pyrene fluorophore and a tetrapeptide receptor did not self-assemble in aqueous solutions, resulting in significant pyrene monomer emission at 395 nm. Upon addition of Cd²⁺, the probes self-assembled into nanoparticles, showing significant enhancement of the excimer emission at 475 nm and a significant decrease in monomer emission at 395 nm, as revealed by fluorescence, dynamic light scattering, and transmission electron microscopy studies. Among 16 metal ions tested, only Cd²⁺ induced a sensitive ratiometric response. The ratiometric response showed an excellent linearity to Cd²⁺ ions, ranging from nanomolar to micromolar concentrations in aqueous solutions and urine samples. The detection limit for Cd²⁺ was 22 nM (R² = 0.984) and the dissociation constant was measured to be 130 pM (R² = 0.997) in aqueous solutions. Two thiol groups and a C-terminal carboxylic acid group of the peptide receptor of 1 play a critical role in binding with Cd²⁺. Furthermore, the probe penetrated live cells and detected intracellular Cd²⁺ by ratiometric responses. The specific metal ion-induced self-assembly of peptidyl probes could be a novel platform for the development of fluorescent detection methods for metal ions in aqueous samples and biosamples.