Green Synthesis of Sodium Alginate Capped -CuInS₂ Quantum Dots with Improved Fluorescence Properties

CuInS₂ (CIS) quantum dots (QDs) are known to be ideal fluorophores based on their low toxicity and tunable emission. However, due to low quantum yield (QY) and photostability, the surface is usually passivated by a higher bandgap shell (e.g. ZnS). This always resulted in a blue-shifted emission position which is not usually favourable for biological imaging. To address this problem, we herein report the passivation of green synthesized near infra-red emitting glutathione (GSH) capped CuInS₂ QDs using different concentration of sodium alginate (SA) at different temperatures. The as-synthesized QDs are small (~ 3.2 nm), highly crystalline and emitted in the near infra-red region. The optical results showed a 36% increase in photostability and a 2-fold increase in quantum yield at ratio 1:8 (SA: CIS) which is suitable for prolonged biological imaging applications. Transmission electron microscope and X-ray diffraction (XRD) analyses showed that the materials are highly crystalline without any change in shape and size after passivation with the biopolymer. Graphical Abstract.