An Amino Acid‐Appended Naphthalene Bisimide‐Based Bolaamphiphile‐ Selective Sensing of Chaotropic Anions by Thixotropic Hydrogelations

An amino acid-based fluorophore probe, L-tryptophan appended naphthalene di-imide (NDI-W) forms stable hydrogels selectively in the presence of chaotropic anions (SCN^-, ClO₄ ^-, I^-, and NO₃ ^-), borderline anion (Br^-) and F^- in 50 mM phosphate buffer of pH 8. NDI-W did not form stable hydrogels in 50 mM phosphate buffer of pH 8 in the presence of kosmotropic anions (S₂O₃ ^2-, SO₄ ^2-, CH₃COO^-, and citrate) at the same conditions. However, gel-like assemblies for NDI-W in presence of S₂O₃ ^2-, SO₄ ^2-, CH₃COO^-, IO₃ ^-, HSO₃ ^-, HCO₃ ^-, and NO₂ ^- anions and precipitate formation in the presence of H₂PO₄ ^-, HSO₄ ^-, and citric acid in 50 mM phosphate buffer of pH 8 have been observed. Interestingly, NDI-W did not form hydrogel in 50 mM phosphate buffer of pH 8 at 4 times higher concentration of NDI-W also compared to the concentration used for the anions induced hydrogelations however, forms hydrogel when higher ionic concentration of HPO₄ ^2- is added into it. All these hydrogels are thixotropic in nature. Thus, NDI-W is selectively forming hydrogels with chaotropic and borderline anions which could be used to detect higher concentrations of salts in water and can be useful for corelating supramolecular complex viscoelastic properties at higher salt concentrations of biologically relevant systems.