Simple and efficient rhodamine-derived VO2+ and Cu2+-responsive colorimetric and reversible fluorescent chemosensors toward the design of multifunctional materials

Abstract

A rhodamine-based derivative, (Z)-2-((3,5-dibromo-2-hydroxybenzylidene)amino)-3′,6′-bis(diethylamino)spiro-[isoindoline-1,9′-xanthen]-3-one (RhDBS), has been chosen as a representative dual chemosensor for VO²⁺ and Cu²⁺ among twenty-five Schiff-base receptors. It has been synthesized by Schiff-base condensation and characterized by standard techniques including single crystal X-ray crystallography. RhDBS can hypersensitively detect VO²⁺via fluorescence “off–on” type switching with a high quantum yield. Meanwhile, the chemosensor exhibits Cu²⁺-selective chromogenic behavior and Cu²⁺ ions can be detected by the naked-eye. The detection limits for VO²⁺ and Cu²⁺ have been determined to be 3.65 nM and 0.84 μM respectively, which are much lower than the allowable maxima set by the WHO, demonstrating the reliability and high efficiency of this chemosensor. Job's plot and mass spectral analysis show 1 : 1 stoichiometry between RhDBS and metal ions. RhDBS has also shown distinct responses to VO²⁺ through electrochemical (CV) signals. Additionally, the RhDBS·Cu²⁺ and RhDBS·VO²⁺ adducts can act as two secondary colorimetric and fluorescent sensors for pyrophosphate (PPi) anions. Advanced molecular memory devices have been constructed through these properties and interpreted. Notably, the analytical applicability of RhDBS can be further achieved by using test paper strips and silica gel plates. More excitingly, RhDBS can be resoundingly employed to trace intracellular VO²⁺ ions in living cells and it is the first reported smart chemosensor capable of detecting VO²⁺in vitro. In the solid state, RhDBS displays distinguished VO²⁺ and Cu²⁺ responsiveness along with obvious colour changes and different emission wavelengths which lead to large red shifts of the CIE-diagram. More importantly, some meaningful regular patterns are discovered for the first time through theoretical and experimental studies toward the design of multifunctional materials.