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.
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.
Photochromic FRET (pcFRET), a member of the family of acceptor depletion FRET techniques (adFRET), embodies a general conceptual and experimental scheme based on a coupled system of a fluorescent donor and a photchromic acceptor. The procedure involves the reversible and cyclic spectroscopic depletion of the acceptor, and was initially conceived for the determination of FRET efficiency on a continuos, pixel-by-pixel basis in the microscopy of living cells. However, the modulation of donor fluorescence in pcFRET has implications for a wide range of applications. We present the formalism for quantitative interpretations of photostationary and kinetic data, from which the relevant kinetic rate constants and quantum uields for the cyclization and cycloreversion reactions of the photochromic acceptor can be derived. The scheme was applied to a model system consisting of a fluorescent donor (Lucifer Yellow) covalently bound to a diheteroarylethene acceptor. In a Perspectives section, we discuss photochromic probes, instrumentation issues, and the potential of pcFRET for analyizing chemical equilibria and kinetics, in the latter case with a new technique we have denoted Photochromic Relaxation Kinetics (pcRelKin).
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