Synthesis and characterization of a rhodamine derivative as a selective switch-on fluorescent sensor for Cu2+ ions in aqueous PBS buffer and living cells

Abstract

A novel rhodamine-based receptor molecule (L₁) was synthesized via the reaction between rhodamine hydrazide and acetylacetone. Several spectroscopic methods, viz., FTIR spectroscopy, ¹H NMR and ¹³C NMR, and ESI-MS, were used to characterize the receptor molecule (L₁). The L₁ demonstrated turn-on response towards Cu²⁺ ions out of 18 different metal ions studied in an aqueous buffer at pH 7.4 with a minimum detection limit of 3.58 × 10⁻⁸ M. The binding interaction between L₁ and Cu²⁺ ions was shown by absorbance studies, wherein isosbestic points were observed at 286 and 329 nm. Job's plots indicate the formation of a 1 : 1 complex between L₁ and Cu²⁺ ions. Additionally, DFT simulations show a strong interaction between the Cu²⁺ ions and the receptor L₁, and TD-DFT investigations show an optical absorption shift in favor of a longer wavelength in the {L₁ + Cu²⁺} complex. Further, the biological applicability of L₁ for the in vitro detection of Cu²⁺ ions in L929 and HeLa living cells was demonstrated. These findings illustrated that L₁ could be used to detect Cu²⁺ ions in environmental and biologically relevant media.