A biocompatible naphthalimide-functionalised polyaspartamide for fluorescence sensing of Fe²⁺ and Fe³⁺ ions

Abstract

A biocompatible fluorescent polymer for iron ion detection was developed through the functionalisation of polyaspartamide with naphthalimide chromophores. The synthetic strategy involved aminolysis of polysuccinimide with N-Boc-1,6-hexanediamine followed by sequential modification with 4-nitro-1,8-naphthalic anhydride and morpholine, yielding a water-compatible fluorescent material. The structure of the obtained polymer was confirmed by ¹H NMR spectroscopy. The sensing behaviour of the polymer toward Fe²⁺ and Fe³⁺ ions was investigated in aqueous solution by fluorescence spectroscopy. Addition of iron ions resulted in a concentration-dependent decrease in emission intensity attributed to fluorescence quenching. Analysis of the quenching process using Stern–Volmer relationships demonstrated efficient interaction of the polymer with both oxidation states of iron, while a more pronounced and rapid response was observed for Fe³⁺ ions. The performance of the material was further evaluated in thin solid films. Fluorescence microscopy revealed substantial emission suppression after exposure to iron ions, consistent with the behaviour observed in solution. Morphological changes induced by polymer–metal interactions were examined by atomic force microscopy, providing complementary information on film structure and surface characteristics. The combination of biocompatibility, aqueous-processability and fluorescence responsiveness highlights the potential of naphthalimide-functionalised polyaspartamides as versatile platforms for iron ion sensing in solution and solid-state environments.