A rhodamine-conjugated fluorescent and colorimetric receptor for the detection of Cu²⁺ ions: environmental utility and smartphone integration

A new rhodamine based turn on florescent probe (<i>Z</i>)-3',6'-bis(ethylamino)-2-(((6-methoxy-2-oxo-1,2-dihydroquinolin-3-yl)methylene)amino)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one (RME) was efficiently synthesized through a simple condensation reaction of 2-amino-3',6'-bis(ethylamino)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one and 6-methoxy-2-oxo-1,2-dihydroquinoline-3-carbaldehyde. The receptor RME is highly non-fluorescent and when copper ions (Cu²⁺ ions) are added in DMF/water (1 : 2, v/v) medium, the receptor RME exhibits a specific "turn-on" colorimetric and fluorometric response. Moreover, RME binding with Cu²⁺ ions produced a remarkable color variation that was perceptible to the human eye, changing from colorless to pink. The binding stoichiometry was 1 : 1, confirmed through theoretical DFT, Job's plot, and mass spectral analysis. RME showed good sensing capability for Cu²⁺ ions, with LOD values of 11.25 nM for ratiometric colorimetry, 1.42 nM for fluorometry, and 107 nM for smartphone-based detection. These LOD values are significantly lower than WHO's acceptable threshold of 2 μM. Additionally, developing portable techniques based on paper-based test strips and smartphone assistance allowed for qualitative and quantitative detection of Cu²⁺ ions. These portable and economical methods eliminate the need for expensive laboratory apparatus, enabling precise and affordable on-site assessments. In addition, the developed sensing protocol for the detection of Cu²⁺ ions was extended to real water samples such as tap, dam, and drinking water samples. These results demonstrated the effectiveness of RME in detecting Cu²⁺ ions in complex matrices, making it a promising tool for environmental monitoring and water quality assessment.