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

A new rhodamine based turn on florescent probe ()-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 (Cu 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 Cu 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 Cu 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 Cu 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 Cu ions was extended to real water samples such as tap, dam, and drinking water samples. These results demonstrated the effectiveness of RME in detecting Cu ions in complex matrices, making it a promising tool for environmental monitoring and water quality assessment.