Coating of quantum dots (QDs), derived from both aqueous synthesis and organometallic routes, with a crosslinked peptide sheath via mimicking phytochelatin-coated heavy metal nanoclusters in plant cells is reported. This coating greatly improves the stability of QDs, while maintaining their optical properties. Coated QDs contain carboxylate and amine groups for facile conjugation with various bioprobes. They are biocompatible and have an overall size as small as 6 nm.