Amide-decorated carbon dots as sensitive and selective probes for fluorescence enhancement detection of cadmium ion

As highly toxic metal ions, cadmium ions (Cd²⁺) are prevalent in varying concentrations around the world. The establishment of an accurate and effective method for Cd²⁺ determination with high sensitivity and selectivity is of particular concern. The present work fabricated a fluorescence chemosensor for the detection of Cd²⁺ based on functionalized carbon dots (CDs), which were hydrothermally prepared using amidated hyperbranched-polyethyleneimine (HPEI). As investigated by FTIR, NMR, and XPS, the stably grafted amide groups endowed the CDs with thermosensitivity and high cloud point due to the change in hydrophilic-hydrophobic behaviors. The CDs chemosensor with optimal amidation degree exhibited high sensitivity, selectivity, and stability in the determination of Cd²⁺ from various water environments. Notably, the fluorescence intensity enhanced with the increase of Cd²⁺ concentration, originating from the improved structure rigidity caused by the interactions between grafted amides and Cd²⁺. These impressive features made the CDs not only sensitive to detecting Cd²⁺ in low-concentration solutions with a limit of detection of 3.41 nM (the lowest known value for Cd²⁺ detection) but also accurate for the quantification in high-concentration solutions with a detectable Cd²⁺ concentration of 6.0 × 10^-2 M. Owing to the broad detection range, the CDs developed in present work show great potential applications in various scenarios.