A novel surface architecture was developed to generate biocompatible and stable photoswitchable quantum dots (psQDs). Photochromic diheteroarylethenes, which undergo thermally stable photoconversions between two forms with different spectral properties in organic solvents, were covalently linked to an amphiphilic polymer that self-assembles with the lipophilic chains surrounding commercial hydrophobic core-shell CdSe/ZnS QDs. This strategy creates a small (∼7 nm diameter) nanoparticle (NP) that is soluble in aqueous medium. The NP retains and even enhances the desirable properties of the original QD (broad excitation, narrow emission, photostability), but the brightness of its emission can be tailored by light. The modulation of emission monitored by steady-state and time-resolved fluorescence was 35-40%. The psQDs exhibit unprecedented photostability and fatigue resistance over at least 16 cycles of photoconversion.

Multifunctional nanoparticles, quantum dots (QDs) are being developed as uniquely sensitive tools for elucidating the (bio)chemical and (bio)physical molecular mechanisms underlying functional states, i.e. the molecular physiology, of biological cells and organisms. Here we present a group of strategies and examples for (i) controlling the spectroscopic properties of QDs via Fluorescence Resonance Energy Transfer (FRET); (ii) determining the emission spectra of individual QDs in a population with an imaging spectrograph (ASI SpectraCube); and (iii) employing such liganded QDs as nano-probes in cellular studies of signal transduction.

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.

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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.Here we present nanosensors and nanoactuators based on QDs in which the multivalency of the particle plays an essential role in determining the functionality and sensing characteristics of the nanodevices. Two examples are discussed: (i) luminescent pH nanosensing QDs calibrated and measured by FLIM (Fig. A). The underlying process is bidirectional (a novel feature) FRET between the QD and the conjugated sensor with pH-sensitive spectral properties 1 . (ii) nanoactuators consisting of strepatividin QD to which biotinylated alpha-synuclein (AS) has been conjugated (Fig. B). These constructs are very efficient intiators and sensors of AS aggregation in vitro and in live cells 2 .