Fluorescence Chemosensory Determination of Cu2+ Using a New Rhodamine–Morpholine Conjugate

A new rhodamine-B carbonyl-morpholine derivative (denoted as RECM) was prepared by a two-step synthesis procedure. The employed method allowed a lactam ring development of rhodamine-B and ethylenediamine to demonstrate a facile amide bond formation. The obtained RECM was confirmed by 1H NMR, 13C NMR, and mass spectrometry analysis. RECM was formed to detect copper ion (Cu2+) due to its problematic toxicity features in aquatic ecosystems. It showed a high selectivity toward Cu2+ in comparison with some environmentally relevant alkali, alkaline earth, and transition metal cations at 50 µM in acetonitrile. Moreover, non-fluorescent RECM showed fluorescence intensity and UV-Vis absorbance increases in the presence of Cu2+ with high linear dependent coefficients (R2 = 0.964 and R2 = 0.982 respectively) as well as a color change from colorless to pink owing to the ring opening of the rhodamine spirolactam form. Binding capability experiments presented a clear 1:1 stoichiometry of RECM–Cu2+ complex with the binding constant (Ka) as 2.25 × 104 M−1. The calculation of limits of detection (LOD) was 0.21 µM based on the linear regression method, which is below the maximum contaminant level goal (MCLG) value of Cu2+ (1.3 ppm equals to 20.46 µM) in drinking water. These characteristics make the RECM a promising candidate for the real-time detection of toxic Cu2+ in environmental monitoring applications.