Highly selective Cu²⁺ detection with a naphthalimide-functionalised pillar[5]arene fluorescent chemosensor

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Analytical Chemistry

Background:

• Pillar[5]arene macrocycles offer versatile platforms for molecular recognition.
• Naphthalimide derivatives are known for their fluorescent properties.
• Developing selective chemosensors for metal ions is crucial for environmental and biological monitoring.

Purpose of the Study:

• To synthesize and characterize a novel pillar[5]arene-based fluorescent chemosensor for metal ion detection.
• To investigate the sensing mechanism and selectivity of the designed ligand towards various metal ions.
• To evaluate the performance of the chemosensor for Cu²⁺ detection, including its binding stoichiometry, affinity, and limit of detection.

Main Methods:

• Synthesis of Ligand 1, a pillar[5]arene functionalized with naphthalimide groups via click chemistry.
• UV-vis and fluorescence spectroscopy to study the binding properties with 15 different metal ions.
• ¹H NMR titration, high-resolution mass spectrometry, and density functional theory (DFT) calculations to elucidate the coordination mechanism.

Main Results:

• Ligand 1 displayed dual emission (monomer and excimer) characteristic of naphthalimide moieties.
• The chemosensor exhibited exceptional selectivity for Cu²⁺ ions over other tested metal ions.
• Upon Cu²⁺ binding, a ratiometric change in fluorescence was observed: excimer emission decreased while monomer emission increased.
• A 1:1 binding stoichiometry and a high association constant (3.39 ± 0.40 × 10⁵ M^-1) were determined for the 1·Cu²⁺ complex.
• A low limit of detection (LOD) of 185 ± 7 nM for Cu²⁺ was achieved.

Conclusions:

• Ligand 1 is a highly effective ratiometric fluorescent chemosensor for Cu²⁺.
• The synergistic coordination between Cu²⁺ and the triazole groups of Ligand 1 is responsible for the sensing mechanism.
• The developed sensor shows significant potential for sensitive and selective detection of copper ions in various applications.