**6. Challenges of aqueous synthesis of group IIB-VIA QDs**

The process of synthesizing all the QDs of the mentioned semiconductors in aqueous media is a challenging task. Literature [88, 89] to some extent reveals that the anionic/chalcogenide portion of the semiconductors plays the key role in the synthesis. It is because the reduction of sulfur (S), selenium (Se) and tellurium (Te) is very much different from each other due to the difference in their reduction potential, i.e., −0.467, −0.924 and − 1.143 respectively. Reduction of S is relatively easier in comparison to Se and Te in water, because it easily dissociates into S<sup>−</sup><sup>2</sup> from its available metallic salt. But strong reducing reagents are required to reduce Se and Te in room temperature. Moreover, they are not stable in the solution and can readily form Se0 and Te0 once the effect of reducing agent is finished. Proper precaution has to be taken. In case of Te, reduction has to be carried out under inert atmosphere (nitrogen/argon atmosphere) as it readily converts to Te0 in non-inert condition [90]. During the addition to the metal precursor, one has to be delicate and calculated enough, or else, Te<sup>−</sup><sup>2</sup> will be readily converted to Te0 . Hence, performing the synthesis in the aqueous medium at room temperature is a challenging task; however, the successful completion of this challenge may lead the world into different class of environmentally benign, easy and cost effective synthesis route for QDs.
