We report design and synthesis of a series of activatable "OFF/ON" CdS:Mn/ZnS
quantum dot (Qdot) based sensing probes. The Qdot "OFF" state represent the "quenched
state" where the Qdot fluorescence is quenched by ligands attached to Qdot surface.
Fluorescence quenching is likely due to ligand assisted electron transfer process. Qdot
fluorescence is restored when the electron transfer process is stopped. Using this
activatable Qdots, we have successfully demonstrated usefulness of these Qdot probes for
reliable detection of toxic cadmium ions in solution, selective detection of glutathione
and sensitive detection of intracellular cancer drug release event. In this paper, we will
discuss a simple but robust method of making water-soluble CdS:Mn/ZnS Qdots at the
room-temperature. Two different water-soluble biomolecules, the N-acetyl cysteine
(NAC) and the glutathione (GSH) were used as surface coating ligands. This is a singlestep,
one-pot synthesis where the Qdot nanocrystals were grown in the presence of the
biomolecules. These Qdots were characterized by fluorescence spectroscopy. Stability of
the GSH coated Qdots and the NAC coated Qdots were studied by treating with
ethylenediaminetetraacetic acid (EDTA, a strong chelating agent for Zn and Cd ions).
Our results show that fluorescence properties of Qdots are affected by the type of surface coated ligands. In comparison to the GSH coated Qdots, the NAC coated Qdots show
broad but strong emission towards near infra-red region. When treated with EDTA,
fluorescence property of the GSH coated Qdot was affected less than the NAC coated
Qdots. This preliminary study shows that NAC coated Qdots could potentially be used to
develop activatable ("OFF/ON") probes for potential deep-tissue imaging applications.
Similarly, the GSH coated Qdots could be applied for probing desired analytes or for
bioimaging purposes in environmentally harsh conditions.