A novel dual-switch fluorescent probe for Cr(iii) ion based on PET–FRET processes

Abstract

Two different strategies for photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET) have been designed and combined into one sensing system. The novel probe NNRhB was developed based on 1,8-naphthalimide and rhodamine moieties, in which two fluorophores are sensitive to the presence of Cr³⁺ in different chromium ion concentration regimens. Therefore, the proposed sensing system represents dual-switch states and segmented detection behavior, with the fluorescence emission color spans from green to orange over an increasing Cr³⁺ concentration gradient. When excited in the visible region, the initial emission band at 537 nm was enhanced. That was attributed to the suppression of the PET process, which arose from Cr³⁺-coordination with a 1,8-naphthalimide derivative. At a sufficiently high concentration of Cr³⁺ (over 9 μM), the spirolactam rhodamine component in NNRhB converted to the opened form as a result of Cr³⁺ coordination, which turned the emission color from green to orange via FRET. The fluorescence phenomena of the compound 1 and compound 2 split from compound NNRhB confirm our hypothesis of the spectral response mechanisms. Moreover, compared with a single fluorescent response in compound 1 or compound 2, the dual-switch fluorescent probe NNRhB shows a more sensitive and distinct visual detection ability for Cr³⁺ ions. This probe affords a high selectivity and sensitivity to Cr³⁺ from 30 nM to 80 μM; the detection limit was 0.14 nM. The results of practical application experiments suggest that the Cr³⁺-selective ligand prepared here may provide an effective strategy for detection of Cr³⁺ in environmental and biological applications.