Size dependent ratiometric detection of Pb (II) ions in aqueous solution by light emitting biogenic CdS NPs

Highlights: • CdS nanoparticle was prepared at room temperature using aqueous pomegranate peel extract. • Effect of precursor, pH, and solvent on the fluorescence of nanoparticle is investigated. • The nanoparticles have shown size dependent sensitive and selection detection towards Pb (II) ions in aqueous solution. • The nanoparticles exhibited a large Stokes shift in the presence of Pb (II) ions which enables the nano molar detection by naked eye. - Abstract: We have synthesized water soluble CdS quantum dots (QDs) in different sizes (~ 3, 10 and 25 nm) via a green rout using pomegranate peel extract as a reducing and stabilizing agent. The particles with the size of ~ 3, 10 and 25 nm respectively exhibit blue, green and yellowish red emission under UV light. We have utilized the NPs for the discrimination of metal ions in aqueous solution. Interestingly, the fluorescence from the NPs undergoes a ratiometric variation only in presence of Pb²⁺ ions, indicating that the system can act as a potential Pb²⁺ ion sensor. There was a remarkable size dependent stock shift (~ 270 nm) upon the addition of Pb²⁺ ions, which enables the detection by naked eye. DLS study indicates that Pb²⁺ ions induce aggregation in the system owing to the complex formation between the capping agent and the metal ion. The aggregation, in turn, generates inter plasmon coupling and results in emission color variation. The naked eye detection limit of Pb²⁺ ions in solution was found to be 10⁻⁹ M. This is the first study of Pb²⁺ ion sensing using CdS quantum systems and the detection procedure is easy-to-make, less time consuming and highly re-producible. The present method enables direct application of the system for detecting Pb²⁺ ions due to the presence of ratiometric fluorescence, leading to a large Stokes shift and associated color change in the visible region.