To be suitable bio-labeling agents, quantum dots need to be water soluble. Researchers in this field have developed various strategies for solubilizing quantum dots in water. The main methods involved are coating the quantum dots with amphiphilic compounds. Bifunctional mercaptocarbonic acids [HS-(CH2)n-COOH] have been widely used. The carboxylic acid group, linked to quantum dots via the thiol group, gives the quantum dots water solubility. When amphiphilic block copolymers are used as coating materials, one segment of the block chain is designed for linking the quantum dots.

We developed a facile route to solubilize quantum dots in water. In brief, we designed a polymerization, in which Mn-doped ZnS quantum dots (called ZnS:Mn2+) initiate acrylic acid polymerization at the surface, to convert the quantum dots to water-soluble ones for biological chromophores. The reason for choosing ZnS:Mn2+ quantum dots is that they are free of toxic heavy-metal elements and, thus, possible toxicology problems should be eliminated.

The results show that our technique directly converts the quantum-dot suspension to a transparent solution. The quantum dots in the solution are isolated with each other and the size is about 4 nm. When the prepared solution is stored for several months, no precipitation is observed, which shows a long-term stability. The prepared water-soluble ZnS:Mn2+ quantum dots have a quantum efficiency of 18.8%, which is qualified for the biological applications. According to results from Raman studies, the high efficiency is attributed to the interaction as a result of the novel quantum dot-initiated polymerization.

Our work opens up the possibility of achieving water-soluble quantum dots by using quantum dots as initiators of polymerization. This method may also be applied to solubilizing other kinds of quantum dots if the surface interaction is properly controlled.