Interplay of multivalency and optical properties of quantum dots: implications for sensing and actuation in living cells

Quantum dots (QDs) are unique probes due to their special properties (brightness, photostability, narrowband emission and broadband absorption), and excellent bio(chemical)compatibility for imaging structures and functions of living cells. When functionalized with ligands, they enable the recognition of specific targets and the tracking of dynamic processes for extended periods of time, detecting biomolecules with a sensitivity extending to the single molecule level. Thus, devices and probes based on such nanoparticles are very powerful tools for studying essential processes underlying the functions and regulation of living cells. Here we present nanosensors and nanoactuators based on QDs in which the multivalency of these particles plays an essential role in the functionality and sensing characteristics of the nanodevices. Two examples are discussed, the first being pH nanosensors based on the interplay of the multivalency and energy transfer between the nanoparticles and small molecules on their surface, and the second nanoactuators in which a controlled number of molecules of the amyloid protein α-synuclein (AS) specifically regulate the aggregation of fluorescently labeled bulk AS protein both in vitro and in live cells.