A paper-based fluorometric sensor using L-Cys-CdTe quantum dots coupled with smartphone readout for automated...

A paper-based fluorometric sensor based on L-cysteine modified CdTe quantum dots (L-Cys-CdTe QDs) was developed for simple, cost-effective, and on-site detection of copper(II) ions (Cu(II)). The sensing platform was integrated with a smartphone-based colorimetric application, enabling automated sample filtration, pretreatment, and sequential fluorometric detection of Cu(II) concentrations. The L-Cys-CdTe QDs were pre-deposited onto designated triplicate sensing zones patterned on a paper substrate. In the presence of Cu(II), a color change occurs due to the quenching of L-Cys-CdTe QDs, transitioning from yellow to colorless. This change could be easily observed under UV-black light with the naked eye and quantitatively analyzed using a smartphone camera coupled with a colorimetric application. The sensor exhibited a detection range of 0.005-7.5 ppm for water samples and 0.025-7.5 ppm for synthetic urine samples. The limits of detection (LOD) were 0.005 ppm and 0.025 ppm by the naked eye and 0.003 ppm and 0.016 ppm using smartphone analysis for water and synthetic urine, respectively. The proposed platform was successfully applied to environmental water samples and synthetic urine as a model biological matrix, and the results showed good agreement with those obtained by atomic absorption spectrometry (AAS) (t-test, p = 0.89). This developed sensor provides a promising tool for on-site monitoring of Cu(II) ions.