Fluorescence switching via competitive ESIPT and spirolactam ring opening in a multifunctional rhodamine B probe for selective detection of Cu2+ and OCl−: theoretical insights with anticancer and biosensor activity

Abstract

A multifunctional ESIPT-based rhodamine-derived probe (BHS) was synthesized and developed as a colorimetric and fluorometric sensor for the selective detection of copper (Cu²⁺) and hypochlorite (OCl⁻) in aqueous solutions. Initially, BHS exhibits intense whitish blue fluorescence due to the active excited-state intramolecular proton transfer (ESIPT) mechanism within the molecule. However, upon interaction with Cu²⁺ and OCl⁻, noticeable changes in absorption and fluorescence occur, attributed to the inhibition of ESIPT resulting from analyte binding with BHS, leading to spirolactam ring opening. Furthermore, significant Stokes shifts in absorption (Δλ = 34 nm and 170 nm for Cu²⁺, and 163 nm for OCl⁻) and emission (Δλ = 67 nm for both Cu²⁺ and OCl⁻) further confirm this transformation. The spirolactam ring opening is induced by Cu²⁺ coordination, whereas for OCl⁻, it is triggered by oxidative cleavage. To explore potential biological applications, fluorescence titration experiments were conducted to study the interactions of the BHS–Cu²⁺ complex with ct-DNA and the transport protein bovine serum albumin (BSA). Additionally, molecular docking studies were performed to assess these interactions, while DFT calculations were employed to optimize the structures of BHS and its Cu²⁺ complex. The fluorescence changes of BHS in the presence of Cu²⁺ and OCl⁻ in biological samples have been examined by the anticancer and biosensor activity of BHS in HCT-116 colorectal cancer cells.