Quantum dots (QDs) are inorganic nanoparticles that emit light at a specific wavelength when excited by light. Typically, they are synthesized using multi-step processes and hazardous organic solvents. These organic solvents must be removed prior to conjugating QDs with antibodies and other ligands for use in biological imaging applications. These multiple production steps make QDs expensive and limit their use.

To overcome these limitations and reduce the cost of QD manufacturing Drexel University’s researchers have developed an economic, direct synthetic method for producing water-soluble QDs that are ready for bioconjugation. Drexel’s methods can produce aqueous, highly luminescent metal sulfide (MS) QDs, (AQDs) with emission wavelengths varying from 400 nm to 700 nm. MS QDs are capped with thiol-containing charged molecules in a single step. The resultant AQDs exhibit no undesirable broadband emissions at higher wavelengths, a feature not attained by previous aqueous synthesis methods. AQDs range in size from 4 nm to 20 nm, and are stable in biological fluids over a long period of time. The metal in MS can be the traditionally used cadmium (Cd) or its non-toxic alternative, zinc (Zn). While the variations in composition change optical parameters, nevertheless, the non-toxic ZnS AQDs with good photoluminescence properties have been produced.

Further variations in the composition and methods of manufacturing resulted in production of near-infrared (NIR)-emitting AQDs. These included CdS-, ZnS-, CdSe-, ZnSe-, CdPbS- and SnS-based quantum dots capped with 3-mercaptopropionic acid (MPA), which stabilizes the AQDs in water. Such in situ stabilization of AQDs without the need for solvent exchange or ligand exchange makes Drexel AQDs ready for bioconjugation. Additional methods of AQDs manufacturing using different capping agents have been developed as well.