Chitosan nanobead encapsulation of quantum dots (QDs) produces watersoluble, stable fluorescent markers for biomedical imaging. As shown in the Figure, derivatization of QDs with 3-mercaptopropionic acid (3MPA) produces a negatively charged surface to which positively charged chitosan is electrostatically attracted. MRI contrast agents such as Gd-DTPA can also be incorporated into the nanobeads, which are small enough to be incorporated into individual cells.