The use of peptides as gatekeepers for payloads of mesoporous silica nanoparticles would allow triggering the release of guests by various biological stimuli. We investigated the effect of peptide conformation on their gatekeeping capability by employing two model peptides with a turn or a random structure. The conformation-dependent gatekeeping properties provided an opportunity to utilize the conformational conversion of peptides as a valuable motif for stimuli-responsive gatekeepers. Based on that investigation, we demonstrated that Fmoc-CGGC-SS-Si, which exhibited a zero-release property without any stimuli due to a turn-like conformation induced by the intramolecular disulfide bond, can be triggered to release guests by converting its conformation to a random structure, induced by reduction of the disulfide bond upon addition of glutathione. We further demonstrated that the conformational conversion of Fmoc-CGGC by Zn(II) ion can also be utilized as a triggering motif.

Lead is one of the most toxic substances. However, there are few ratiometric fluorescent probes for sensing Pb²⁺ in aqueous solution as well as living cells because specific ligands for Pb²⁺ ions have not been well characterized. Considering the interactions between Pb²⁺ and peptides, we developed ratiometric fluorescent probes for Pb²⁺ based on the peptide receptor in two steps. First, we synthesized fluorescent probes (<b>1</b>-<b>3</b>) based on the tetrapeptide receptor (ECEE-NH₂) containing hard and soft ligands by conjugation with diverse fluorophores that showed excimer emission when they aggregated. After investigation of fluorescent responses to metal ions, benzothiazolyl-cyanovinylene was evaluated as an appropriate fluorophore for ratiometric detection of Pb²⁺. Next, we modified the peptide receptor to decrease the number of hard ligands and/or to replace Cys with disulfide bond and methylated Cys for improving selectivity and cell permeability. From this process, we developed two fluorescent probes (<b>3</b> and <b>8</b>) among the probes (<b>1</b>-<b>8</b>) that exhibited remarkable ratiometric sensing properties for Pb²⁺ including high water solubility (≤2% DMF), visible light excitation, high sensitivity, selectivity for Pb²⁺, low detection limits (<10 nM), and fast response (<6 min). The binding mode study revealed that specific Pb²⁺-peptide interactions of the probes caused nanosized aggregates in which the fluorophores of the probes came close each other, exhibiting excimer emission. In particular, <b>8</b> based on tetrapeptide bearing a disulfide bond and two carboxyl groups with a good permeability successfully quantified intracellular uptake of Pb²⁺ in live cells through ratiometric fluorescent signals. The ratiometric sensing system based on specific metal-peptide interactions and excimer emission process could provide a valuable tool to quantify Pb²⁺ in live cells and pure aqueous solutions.