UV induced synthesis of starch capped CdSe quantum dots: Functionalization with thiourea and application in sensing heavy metals ions in aqueous solution

Abstract

Herein, a highly facile, rapid and one-pot approach has been applied for the synthesis of water soluble starch capped CdSe quantum dots (QDs) by using UV irradiation in the aqueous solution containing cadmium sulphate, sodium selenosulfate, acetone and 2-propanol. No external reducing agents were added to the solution, as the radicals generated in situ upon photoirradiation i.e., 2-hydroxy-2-propyl radicals, (CH3)2C·OH could reduce the precursor ions for the synthesis of these QDs. These QDs were characterised by various techniques such as UV–Vis absorption, XRD, Raman, FTIR, TEM and SEM measurements. The presence of strong quantum confinement effects could be realized from their very small size i.e., ∼3 nm as revealed by TEM studies. Besides, these QDs were found to exhibit photoluminescence (PL) in the longer wavelength region. The PL intensity as well as the charge carrier lifetime values could be conveniently tuned by simply varying the Cd to Se precursor ratio during the synthesis. Furthermore, a relatively novel approach has been adopted to extract these QDs from the colloidal solution by freezing it to 0 °C followed by de-freezing to room temperature. The extracted QDs were functionalized with thiourea in order to increase the PL quantum yield and the stability of the QDs. The effects of pH and temperature on the PL intensity of functionalized CdSe QDs were also investigated to explore their applicability in sensing of heavy metal ions. Interestingly, the QDs displayed highly selective PL quenching in the presence of Cu2+, Cr6+ and Hg2+ metal ions. The limits of detection for these metal ions have been determined and a probable PL quenching mechanism is postulated. Keywords: CdSe quantum dot, UV radiation, Starch, Surface functionalization, Sensor