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1.
Nano Lett ; 19(3): 1695-1700, 2019 03 13.
Artigo em Inglês | MEDLINE | ID: mdl-30721068

RESUMO

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.

2.
J Chem Phys ; 148(20): 204201, 2018 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-29865842

RESUMO

Our paper presents the first theoretical and experimental study using single-molecule Metal-Induced Energy Transfer (smMIET) for localizing single fluorescent molecules in three dimensions. Metal-Induced Energy Transfer describes the resonant energy transfer from the excited state of a fluorescent emitter to surface plasmons in a metal nanostructure. This energy transfer is strongly distance-dependent and can be used to localize an emitter along one dimension. We have used Metal-Induced Energy Transfer in the past for localizing fluorescent emitters with nanometer accuracy along the optical axis of a microscope. The combination of smMIET with single-molecule localization based super-resolution microscopy that provides nanometer lateral localization accuracy offers the prospect of achieving isotropic nanometer localization accuracy in all three spatial dimensions. We give a thorough theoretical explanation and analysis of smMIET, describe its experimental requirements, also in its combination with lateral single-molecule localization techniques, and present first proof-of-principle experiments using dye molecules immobilized on top of a silica spacer, and of dye molecules embedded in thin polymer films.

3.
Mol Biol Cell ; 2018 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-29444956

RESUMO

We report a novel method, dual color axial nanometric localization by Metal Induced Energy Transfer (dcMIET), and combine it with Förster Resonant Energy Transfer (FRET) for resolving structural details in cells on the molecular level. We demonstrate the capability of this method on cytoskeletal elements and adhesions in human mesenchymal stem cells (hMSCs). Our approach is based on Fluorescence-Lifetime-Imaging Microscopy (FLIM), and allows for precise determination of the 3D architecture of stress fibers anchoring at focal adhesions, thus yielding crucial information to understand cell-matrix mechanics. In addition to resolving nanometric structural details along the z-axis, we use FRET to gain precise information on the distance between actin and vinculin at focal adhesions.

4.
J Phys Chem Lett ; 8(23): 5751-5757, 2017 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-29125299

RESUMO

This study focuses on the mechanism of fluorescence blinking of single carbon nanodots, which is one of their key but less understood properties. The results of our single-particle fluorescence study show that the mechanism of carbon nanodots blinking has remarkable similarities with that of semiconductor quantum dots. In particular, the temporal behavior of carbon nanodot blinking follows a power law both at room and at cryogenic temperatures. Our experimental data suggest that static quenching via Dexter-type electron transfer between surface groups of a nanoparticle plays a major role in the transition of carbon nanodots to off or gray states, whereas the transition back to on states is governed by an electron tunneling from the particle's core. These findings advance our understanding of the complex mechanism of carbon nanodots emission, which is one of the key steps for their application in fluorescence imaging.

5.
ACS Nano ; 11(12): 11839-11846, 2017 12 26.
Artigo em Inglês | MEDLINE | ID: mdl-28921961

RESUMO

The nuclear envelope, comprising the inner and the outer nuclear membrane, separates the nucleus from the cytoplasm and plays a key role in cellular functions. Nuclear pore complexes (NPCs), which are embedded in the nuclear envelope, control transport of macromolecules between the two compartments. Here, using dual-color metal-induced energy transfer (MIET), we determine the axial distance between Lap2ß and Nup358 as markers for the inner nuclear membrane and the cytoplasmic side of the NPC, respectively. Using MIET imaging, we reconstruct the 3D profile of the nuclear envelope over the whole basal area, with an axial resolution of a few nanometers. This result demonstrates that optical microscopy can achieve nanometer axial resolution in biological samples and without recourse to complex interferometric approaches.

6.
Nano Lett ; 16(1): 237-42, 2016 Jan 13.
Artigo em Inglês | MEDLINE | ID: mdl-26605640

RESUMO

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.


Assuntos
Carbono/química , Imagem Molecular , Nanopartículas/química , Núcleo Celular/ultraestrutura , Corantes Fluorescentes/química , Microtúbulos/ultraestrutura , Pontos Quânticos/química
7.
Phys Chem Chem Phys ; 17(22): 14994-5000, 2015 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-25986857

RESUMO

We present the results of a comprehensive photoluminescence study of defect centres in single SiO2 nanoparticles. We show that the photo-physical properties of the luminescent centres strongly resemble those of single dye molecules. However, these properties exhibit a large variability from particle to particle due to the different local chemical environment around each centre of each particle. This variability provides new insight into the complex photo-physics of single quantum emitters embedded into a random chemical environment. Moreover, a better understanding of the fundamental mechanism of the photoluminescence of defect centres in SiO2 structure is paramount for their application as white-light sources, non-toxic labels for bio-imaging, or for combining them with metallic and semiconductor nanostructures.

8.
Nano Lett ; 14(10): 5656-61, 2014 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-25247753

RESUMO

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.

9.
Chemphyschem ; 15(4): 705-11, 2014 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-24478241

RESUMO

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.

10.
Opt Express ; 20(3): 3200-8, 2012 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-22330557

RESUMO

We present a novel approach for convenient tuning of the local refractive index around nanostructures. We apply this technique to study the influence of the local refractive index on the radiative decay time of CdSe/ZnS quantum dots with three distinct emission wavelengths. The dependence of the luminescence decay time on the environment is well described by an effective medium approach. A critical distance of about 80 nm is found for the determination of the effective local index of refraction. An estimation for the emitting-state quantum efficiency can be extracted.


Assuntos
Algoritmos , Medições Luminescentes/métodos , Teste de Materiais/métodos , Pontos Quânticos , Refratometria/métodos , Semicondutores , Espectrometria de Fluorescência/métodos
11.
Phys Rev Lett ; 109(22): 223902, 2012 Nov 30.
Artigo em Inglês | MEDLINE | ID: mdl-23368120

RESUMO

Using a tunable optical subwavelength microcavity, we demonstrate controlled modification of the vibronic relaxation dynamics in a single SiO(2) nanoparticle. By varying the distance between the cavity mirrors we change the electromagnetic field mode structure around a single nanoparticle and the radiative transition probability from the lowest vibronic level of the electronically excited state to the progression of phonon levels in the electronic ground state. We demonstrate redistribution of the photoluminescence spectrum between zero-phonon and phonon-assisted bands and modification of the excited state lifetime of the same individual SiO(2) particle measured at different cavity lengths. By comparing the experimental data with a theoretical model, we extract the quantum yield of a single SiO(2) nanoparticle.

12.
Nano Lett ; 11(4): 1700-3, 2011 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-21410240

RESUMO

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.


Assuntos
Medições Luminescentes/instrumentação , Técnicas de Sonda Molecular/instrumentação , Dispositivos Ópticos , Transdutores , Desenho de Equipamento , Análise de Falha de Equipamento
13.
Phys Chem Chem Phys ; 13(5): 1722-33, 2011 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-21240406

RESUMO

Tautomerism process of single fluorescent molecules was studied by means of confocal microscopy in combination with azimuthally or radially polarized laser beams. During a tautomerism process the transition dipole moment (TDM) of a molecule changes its orientation which can be visualized by the fluorescence excitation image of the molecule. We present experimental and theoretical studies of two porphyrazine-type molecules and one type of porphyrin molecule: a symmetrically substituted metal-free phthalocyanine and porphyrin, and nonsymmetrically substituted porphyrazine. In the case of phthalocyanine the fluorescence excitation patterns show that the angle between the transition dipole moments of the two tautomeric forms is near 90°, in agreement with quantum chemical calculations. For porphyrazine we find that the orientation change of the TDM is less than 60° or larger than 120°, as theoretically predicted. Most of the porphyrin molecules show no photoinduced tautomerization, while for 7% of the total number of investigated molecules we observed excitation patterns of two different trans forms of the same single molecule. We demonstrate for the first time that a molecule, undergoing a tautomerism process stays in one tautomeric trans conformation during a time comparable with the acquisition time of one excitation pattern. This allowed us to visualize the existence of each of the two trans forms of one single porphyrin molecule, as well as the sudden switching between these tautomers.

14.
Nano Lett ; 11(3): 1131-5, 2011 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-21271740

RESUMO

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.

15.
Nano Lett ; 10(2): 504-8, 2010 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-20063862

RESUMO

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.


Assuntos
Nanotecnologia/métodos , Óptica e Fotônica , Algoritmos , Desenho de Equipamento , Luz , Teste de Materiais , Nanoestruturas/química , Dióxido de Silício/química , Espectrometria de Fluorescência/métodos
16.
Nano Lett ; 9(9): 3239-44, 2009 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-19653645

RESUMO

Silicon nanocrystals were synthesized by CO(2) laser pyrolysis of SiH(4). The fresh silicon nanopowder was oxidized in water to obtain SiO(2) nanoparticles (NPs) exhibiting strong red-orange photoluminescence. Samples of SiO(2) NPs embedded in low concentration in a thin polymer layer were prepared by spin-coating a dedicated solution on quartz cover slides. Using an argon ion laser at 488 nm with higher-order laser modes (azimuthally and radially polarized doughnut modes) for excitation, the three-dimensional orientation of the nanoparticles' transition dipole moment was investigated in a confocal microscope. The linear transition dipole moment was found to be rather stable and randomly oriented. However, dynamical effects such as fluorescence intermittency and transition dipole moment flipping could also be observed. The spectral analysis of single SiO(2) NPs revealed double-peak spectra consisting of a narrow zero-phonon line and a broader phonon band being associated with the excitation of longitudinal optical phonons in the SiO(2) NP.


Assuntos
Luminescência , Nanoestruturas/química , Dióxido de Silício/química , Argônio/química , Elétrons , Lasers , Teste de Materiais , Microscopia Confocal , Nanotecnologia , Tamanho da Partícula , Propriedades de Superfície
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