Alexey I. Chizhik

Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling.

Inorganic carbon nanomaterials, also called carbon nanodots, exhibit a strong photoluminescence with unusual properties and, thus, have been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. Here, we present a single particle comprehensive study of carbon nanodot photoluminescence, which combines emission and lifetime spectroscopy, defocused emission dipole imaging, azimuthally polarized excitation dipole scanning, nanocavity-based quantum yield measurements, high resolution transmission electron microscopy, and atomic force microscopy. We find that photoluminescent carbon nanodots behave as electric dipoles, both in absorption and emission, and that their emission originates from the recombination of photogenerated charges on defect centers involving a strong coupling between the electronic transition and collective vibrations of the lattice structure.

Super-Resolution Optical Fluctuation Bio-Imaging with Dual-Color Carbon Nanodots

Success in super-resolution imaging relies on a proper choice of fluorescent probes. Here, we suggest novel easily produced and biocompatible nanoparticles-carbon nanodots-for super-resolution optical fluctuation bioimaging (SOFI). The particles revealed an intrinsic dual-color fluorescence, which corresponds to two subpopulations of particles of different electric charges. The neutral nanoparticles localize to cellular nuclei suggesting their potential use as an inexpensive, easily produced nucleus-specific label. The single particle study revealed that the carbon nanodots possess a unique hybrid combination of fluorescence properties exhibiting characteristics of both dye molecules and semiconductor nanocrystals. The results suggest that charge trapping and redistribution on the surface of the particles triggers their transitions between emissive and dark states. These findings open up new possibilities for the utilization of carbon nanodots in the various super-resolution microscopy methods based on stochastic optical switching.

Excitation Isotropy of Single CdSe/ZnS Nanocrystals

We study the dimensionality of the excitation transition dipole moment for single CdSe/ZnS core-shell nanocrystals using azimuthally and radially polarized laser modes. The comparison of measured and simulated single nanocrystal excitation patterns shows that single CdSe/ZnS quantum dots possess a spherically degenerated excitation transition dipole. We show that the dimensionality of the excitation transition dipole moment distribution is the same for all individual CdSe/ZnS nanocrystals, disregarding the difference in core size and irrespective of variations in the local environment. In contrast to the emission transition dipole moment, which is oriented in one plane, the excitation transition dipole moment of a single CdSe/ZnS quantum dots possesses an isotropy in three dimensions.

Probing the Radiative Transition of Single Molecules with a Tunable Microresonator

Using a tunable optical microresonator with subwavelength spacing, we demonstrate controlled modulation of the radiative transition rate of a single molecule, which is measured by monitoring its fluorescence lifetime. Variation of the cavity length changes the local mode structure of the electromagnetic field, which modifies the radiative coupling of an emitting molecule to that field. By comparing the experimental data with a theoretical model, we extract both the pure radiative transition rate as well as the quantum yield of individual molecules. We observe a broad scattering of quantum yield values from molecule to molecule, which reflects the strong variation of the local interaction of the observed molecules with their host environment.

Enhanced dimerization drives ligand-independent activity of mutant epidermal growth factor receptor in lung cancer

Epidermal growth factor receptor kinase mutations drive oncogenesis, but the molecular mechanism of pathological signal initiation is poorly understood. Using high-resolution microscopy methods, the authors reveal that these kinase mutations induce structural changes in the receptor ectodomain that lead to enhanced, ligand-independent dimerization.

Nanocavity‐Based Determination of Absolute Values of Photoluminescence Quantum Yields

We present a new method for determining absolute values of quantum yield of luminescent emitters, which is based on the modification of the radiative transition of emitters within a tunable metallic nanocavity. The method presented is easy to set up and works without any calibration. It will thus be useful for all applications where absolute and calibration-free measurements of luminescence quantum yields are needed. Moreover, it requires only a minute amount of low-concentration fluorophore solution. We give a detailed description of the theory and data evaluation of the nanocavity measurements, and report experimental results for several common dyes in aqueous solution.

Three-Dimensional Orientation of Single Molecules in a Tunable Optical lambda/2 Microresonator

A tightly focused radially polarized laser beam forms an unusual bimodal field distribution in an optical lambda/2-microresonator. We use a single-molecule dipole to probe the vector properties of this field distribution by tuning the resonator length with nanometer precision. Comparing calculated and experimental excitation patterns provides the three-dimensional orientation of the single-molecule dipole in the microresonator.

Single-Molecule Metal-Induced Energy Transfer (smMIET): Resolving Nanometer Distances at the Single-Molecule Level

We present a new concept for measuring distance values of single molecules from a surface with nanometer accuracy using the energy transfer from the excited molecule to surface plasmons of a metal film. We measure the fluorescence lifetime of individual dye molecules deposited on a dielectric spacer as a function of a spacer thickness. By using our theoretical model, we convert the lifetime values into the axial distance of individual molecules. Similar to Förster resonance energy transfer (FRET), this allows emitters to be localized with nanometer accuracy, but in contrast to FRET the distance range at which efficient energy transfer takes place is an order of magnitude larger. Our technique can be potentially used as a tool for measuring intramolecular distances of biomolecules and complexes.

Quantum yield measurement in a multicolor chromophore solution using a nanocavity.

Using our nanocavity-based method for precise determination of the absolute quantum yield values, we measure the quantum yield of each of the three different chromophores types with strongly overlapping absorption spectra in a multicolor solution, which is impossible to do by any other existing technique. By measuring the modification of the radiative rate of the chromophores at different cavity lengths, we extract the quantum yield of CdSe/ZnS semiconductor nanocrystals emitting at three different wavelengths and excited by a single wavelength in the multicolor solution. Using several detection channels, we show that only one measurement is needed for determining the quantum yield values of all the types of chromophores in the sample. The obtained results show a very good agreement with the values commonly observed using other techniques. The nanocavity-based method opens up new opportunities of determining fluorescence quantum yields in complex systems and can be used for a big variety of fluorescence studies where multichromophore samples are involved.

Microcavities: tailoring the optical properties of single quantum emitters

AbstractWe present a general review of different microresonator structures and how they can be used in future device applications in modern analytical methods by tailoring the optical properties of single quantum emitters. The main emphasis is on the tunable λ/2-Fabry–Perot-type microresonator which we used to obtain the results presented in this article. By varying the mirror distance the local mode structure of the electromagnetic field is altered and thus the radiative coupling of fluorescent single quantum emitters embedded inside the resonator to that field is changed, too. As a result a modification of the optical properties of these quantum emitters can be observed. We present experimental as well as theoretical results illustrating this effect. Furthermore, the developed resonator can be used to determine the longitudinal position of embedded emitters with an accuracy of λ/60 by analyzing the excitation patterns of nano-sized fluorescent polymer spheres after excitation with a radially polarized doughnut mode laser beam. Finally, we will apply this resonator to a biological system and demonstrate the modification of Förster resonant energy transfer (FRET) efficiency by inhibiting the excited state energy transfer from the donor to the acceptor chromophore of a single DsRed protein. FigureEffect of a microresonator on single quantum emitters (from left to right): PI molecule or DsRed protein invesitigated in a microresonator with resulting exciation patterns of the PI molecule after exciation with a radially polarized laser beam or the cavity-controlled emisison spectrum of DSRed in comparison with its free space spectrum (hatched). The background shows the Newton rings of the microrsonator.