An ‘OFF–ON’ fluorescent chemosensor based on rhodamine 6G-2-chloronicotinaldehyde for the detection of Al³⁺ ions: Part II

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>A novel, optical rhodamine-2-chloronicotinaldehyde-type chemosensor (<B>R6CN</B>) was designed, synthesized and characterized as a reversible switch. <B>R6CN</B> displayed high selectivity toward Al<SUP>3+</SUP> from various metal ions, including Al<SUP>3+</SUP>, Li<SUP>+</SUP>, Na<SUP>+</SUP>, K<SUP>+</SUP>, Cs<SUP>+</SUP>, Mg<SUP>2+</SUP>, Ca<SUP>2+</SUP>, Fe<SUP>2+</SUP>, Co<SUP>2+</SUP>, Ni<SUP>2+</SUP>, Cu<SUP>2+</SUP>, Zn<SUP>2+</SUP>, Ag<SUP>+</SUP>, Cd<SUP>2+</SUP>, Hg<SUP>2+</SUP>, Pb<SUP>2+</SUP> and the resultant complex [<B>R6CN</B>-Al<SUP>3+</SUP>]. The ring-opening mechanism of the rhodamine spirolactam was induced by Al<SUP>3+</SUP> binding, and the 1:1 stoichiometric structure between <B>R6CN</B> and Al<SUP>3+</SUP> was adequately supported by the Job-plot evaluation, optical titration, FT-IR and <SUP>1</SUP>H NMR results. Theoretical calculations and modeling simulations were performed using <I>Material Studio 4.3 suite</I> (<I>VAMP</I>), and the results supported the formation of a 1:1 complex between <B>R6CN</B> and Al<SUP>3+</SUP>. The fluorescence quantum yield of <B>R6CN</B>-Al<SUP>3+</SUP> (<I>Φ</I> <SUB>f</SUB> =92.33%) was very high compared to that of the bare ligand. The detection limit for Al<SUP>3+</SUP> was 4.28×10<SUP>−9</SUP> M, and a significant color change from almost colorless to pale-pink occurred in the presence of Al<SUP>3+</SUP>. In turn, the <B>R6CN</B>-Al<SUP>3+</SUP> complex acted as a selective chemosensor toward N<SUB>3</SUB> <SUP>−</SUP> among various anions, including F<SUP>−</SUP>, Cl<SUP>−</SUP>, Br<SUP>−</SUP>, I<SUP>−</SUP>, NO<SUB>3</SUB> <SUP>−</SUP>, CH<SUB>3</SUB>COO<SUP>−</SUP>, ClO<SUB>4</SUB> <SUP>−</SUP>, CN<SUP>−</SUP>, SCN<SUP>−</SUP>, HSO<SUB>4</SUB> <SUP>−</SUP>, HPO<SUB>4</SUB> <SUP>−</SUP> and PF<SUB>6</SUB> <SUP>−</SUP>, in acetonitrile media. Moreover, the <B>R6CN</B>-Al<SUP>3+</SUP> complex also exhibited a high selectivity and sensitivity toward the azide anion upon the addition of Al<SUP>3+</SUP>, and the color reversed back to colorless when the two ions were present together in solution. At last, <B>R6CN</B> was productively applied to the PEGDMA polymer to sense Al<SUP>3+</SUP> ions, which was analyzed using FT-IR, fluorescence confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel rhodamine-2-chloronicotinaldehyde (<B>R6CN</B>) chemosensor was designed and synthesized in high yield. </LI> <LI> Rhodamine-2-chloronicotinaldehyde (<B>R6CN</B>) exhibits very high selectivity toward Al<SUP>3+</SUP> ions and reversible “turn-off” fluorescence was achieved using the azide (N<SUB>3</SUB> <SUP>−</SUP>) anion. </LI> <LI> The complex between the chemosensor <B>R6CN</B> and Al<SUP>3+</SUP> was found to have a 1:1 ratio by titration. </LI> <LI> The structure of the <B>R6CN</B>-Al<SUP>3+</SUP> complex was determined using <SUP>1</SUP>H NMR and FT-IR titration and molecular modeling studies. </LI> <LI> <B>R6CN</B> was mixed with PEGDMA polymer for chemosensing the Al<SUP>3+</SUP> ion with reversibility by N<SUB>3</SUB> <SUP>−</SUP> ion. </LI> </UL> </P>