Facile preparation of a hydrophilic Eu-based ratiometric fluorescent nanosensor for Cu²⁺ ion detection and imaging in living cells

In this work, a hydrophilic Eu-based ratiometric fluorescent nanosensor (PAAC-Eu) was developed for Cu²⁺ ion detection in aqueous solutions and imaging in living cells. The sensor was prepared <i>via</i> a simple one-step reaction at room temperature, leveraging the synergistic coordination of commercially accessible polyacrylic acid (PAA) and coumarin-3-carboxylic acid (CCAH) with Eu³⁺ ions. PAAC-Eu was easy to disperse in aqueous media and exhibited two characteristic emission bands at 406 nm and 618 nm, respectively, upon excitation at 350 nm. Cu²⁺ ions could bind with the free carboxyl groups in PAAC-Eu within 10 min, leading to a decrease in fluorescence at 618 nm (<i>I</i>₆₁₈) and a negligible effect on fluorescence at 406 nm (<i>I</i>₄₀₆). Accordingly, a rapid, sensitive and selective method for detecting Cu²⁺ ions was established, which could complete Cu²⁺ ion assay within 30 min. A good linear relationship was observed between <i>I</i>₄₀₆/<i>I</i>₆₁₈ and Cu²⁺ ion concentration at 0-20.0 μM (0-1.28 mg L^-1) with a detection limit as low as 0.175 μM (11.2 μg L^-1). The proposed method was successfully applied to quantify Cu²⁺ ions in real water samples. Moreover, a portable paper-based sensor was developed by loading PAAC-Eu in filter paper, which could enable visual detection of Cu²⁺ ions without using large instruments. Finally, cell experiments demonstrated the low cytotoxicity and good cell permeability of PAAC-Eu, and ratiometric fluorescence imaging of Cu²⁺ ions in living cells was successfully performed.