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Time-Resolved Optical Properties of Colloidal CdSe-CdS/ZnS Core-Multishell Quantum Dots in Bioimaging
KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Semiconductor quantum dots (QDs) have attracted great attention as a novel fluorescent material in the last twenty years. Their superior optical properties such as high brightness and photostability, broad absorption spectrum, narrow and size-tunable emission spectrum, enable them great application in bioimaging.

However, the fluorescence from single QDs shows irregular on (bright) and off (dark) switches under continuous irradiation which is known as blinking. QD blinking may lead to information loss in single particle tracking and lower brightness in other bioimaging applications. We studied the blinking behavior and its mechanism by using CdSe-core QDs with different shell thicknesses under different excitations. We observed two types of fluorescence behavior, blinking with apparent on and off states and flickering without distinguishable on and off states under low (1.8 and 3.9 W/cm2) and high (12.1 and 25 W/cm2) excitations, respectively. The transfer of photoexcited electron or hole from CdSe core to the QD surface is responsible for QD blinking under low excitations. And further intraband excitation of photoexcited electron and hole is responsible for QD flickering under high excitations.

Ca2+ serves as the second messenger in signal transduction. Monitoring Ca2+ concentration in live cell is a key technique in biological research especially in neuroscience. Most of the commercial Ca2+ indicators are organic dyes which are easy to be photobleached. In order to develop QD-based Ca2+ indicator, we investigated the effect of Ca2+ on the QD fluorescence. We found that the fluorescence intensity, lifetime, and on-state ratio in single QD fluorescence were all decreased by Ca2+ ion. Theoretical study shows that one free Ca2+ could attach stably to the surface of one QD, attracting the photogenerated electron and repel the photogenerated hole, suppressing the radiative recombination between them, and resulting in the reduction of fluorescence intensity, lifetime and on-state ratio.

Overexpression of vascular adhesion molecule-1 (VCAM-1) in endothelial cells is a hallmark of inflammation-induced activation of endothelium and may serve as a target for evaluation atherogenesis in early stages. We conjugated VCAM-1 binding peptide to amino-coated QDs and employed the functionalized QDs (VQDs) to specifically image activated endothelial cells. Upon the interaction between VQDs and endothelial cells, a blue-shift of about 30 nm in the QD fluorescence peak was observed. We anticipate that the VQDs and the blue-shift phenomenon could be very useful for VCAM-1 detection in vitro and in vivo.

Furthermore, we studied the fluorescence of QDs embedded in a porous alumina membrane which is widely used as biomolecule and cell filter for biological research. We found that the fluorescence spectrum has small peaks superimposed on the principle curve. Theoretical study identifies that this modulation is due to the photonic band structure introduced by the membrane pores. This work could supply information about the interaction between QD fluorescence and porous membrane structure which would be useful when applying QDs to image biomolecules or cells filtered by the porous alumina membrane.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xvi, 69 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:11
National Category
Physical Sciences
Research subject
Biological Physics
Identifiers
URN: urn:nbn:se:kth:diva-160939ISBN: 978-91-7595-473-8 (print)OAI: oai:DiVA.org:kth-160939DiVA: diva2:792977
Public defence
2015-03-27, Seminar room Air på SciLifeLab, Science for Life Laboratory, Tomtebodavägen 23A, Solna, 13:00 (English)
Opponent
Supervisors
Note

QC 20150306

Available from: 2015-03-06 Created: 2015-03-04 Last updated: 2015-03-06Bibliographically approved
List of papers
1. Blinking, Flickering, and Correlation in Fluorescence of Single Colloidal CdSe Quantum Dots with Different Shells under Different Excitations
Open this publication in new window or tab >>Blinking, Flickering, and Correlation in Fluorescence of Single Colloidal CdSe Quantum Dots with Different Shells under Different Excitations
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 9, 4844-4851 p.Article in journal (Refereed) Published
Abstract [en]

We studied the fluorescence spectra from single CdSe core-CdS/ZnS and CdS/CdZnS/ZnS shell colloidal quantum dots (QDs) (different shell thicknesses and different surface ligands) under different optical excitations. It was found that the single QD blinks (on and off) at low excitation and flickers (without distinguishable on and off) under high excitation. Theoretical analysis suggests that under low excitation the single QD blinks due to transfer of the photogenerated electron and hole between QD core states and surface states, and the transfer rate is inversely proportional to the shell thickness, as demonstrated experimentally. Under high excitation, the photogenerated electron and hole via interband excitation are further excited via intraband excitation, which will keep the electron and hole from relaxing to the ground exciton state, therefore significantly suppressing the radiative interband recombination, resulting in QD flickering. It was further observed experimentally that the fluorescence events from two neighboring QDs under high excitation are correlated. The correlation can be most possibly understood that one QD, which is kept at the excited state by the high excitation, radiatively recombines under the stimulation of the emission from its neighboring QD.

Keyword
Semiconductor Nanocrystals, Thickness, Photoluminescence, Cells
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-120287 (URN)10.1021/jp311975h (DOI)000315707600056 ()2-s2.0-84874838216 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130405

Available from: 2013-04-05 Created: 2013-04-04 Last updated: 2017-12-06Bibliographically approved
2. Modulated fluorescence of colloidal quantum dots embedded in a porous alumina membrane
Open this publication in new window or tab >>Modulated fluorescence of colloidal quantum dots embedded in a porous alumina membrane
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2013 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 177, no 45, 14151-14156 p.Article in journal (Refereed) Published
Abstract [en]

The fluorescence spectrum of CdSe core-CdS/ZnS shell colloidal quantum dots (QDs) embedded in porous alumina membrane was studied. Small peaks, superimposed on the principal QD fluorescence spectrum, were observed. Finite-difference time-domain simulation indicates that the QD point radiation emitting from within the membrane is strongly modulated by the photonic band structure introduced by the membrane pores, leading to the observed fine spectral features. Moreover, the principal QD fluorescence peak red-shifted when the optical excitation power was increased, which is attributed to QD material heating due to emitted phonons when the photoexcited electron and hole relax nonradiatively from high-energy states to the ground exciton state before fluorescence.

Keyword
Colloidal quantum dots, Finite difference time domain simulations, Fluorescence peak, Fluorescence spectra, Photoexcited electrons, Photonic band structures, Porous alumina membranes, Spectral feature
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-136255 (URN)10.1021/jp409132e (DOI)000327111200024 ()2-s2.0-84887899102 (Scopus ID)
Funder
Swedish Research Council, 621-2011-4381 B0460801Science for Life Laboratory - a national resource center for high-throughput molecular bioscience
Note

QC 20131217

Available from: 2013-12-04 Created: 2013-12-04 Last updated: 2017-12-06Bibliographically approved
3. Characterization of VCAM-1-Binding Peptide-Functionalized Quantum Dots for Molecular Imaging of Inflamed Endothelium
Open this publication in new window or tab >>Characterization of VCAM-1-Binding Peptide-Functionalized Quantum Dots for Molecular Imaging of Inflamed Endothelium
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2013 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 8, no 12, e83805- p.Article in journal (Refereed) Published
Abstract [en]

Inflammation-induced activation of endothelium constitutes one of the earliest changes during atherogenesis. New imaging techniques that allow detecting activated endothelial cells can improve the identification of persons at high cardiovascular risk in early stages. Quantum dots (QDs) have attractive optical properties such as bright fluorescence and high photostability, and have been increasingly studied and developed for bio-imaging and bio-targeting applications. We report here the development of vascular cell adhesion molecule-1 binding peptide (VCAM-1 binding peptide) functionalized QDs (VQDs) from amino QDs. It was found that the QD fluorescence signal in tumor necrosis factor alpha (TNF-alpha) treated endothelial cells in vitro was significantly higher when these cells were labeled with VQDs than amino QDs. The VQD labeling of TNF-alpha-treated endothelial cells was VCAM-1 specific since pre-incubation with recombinant VCAM-1 blocked cells' uptake of VQDs. Our ex vivo and in vivo experiments showed that in the inflamed endothelium, QD fluorescence signal from VQDs was also much stronger than that of amino QDs. Moreover, we observed that the QD fluorescence peak was significantly blue-shifted after VQDs interacted with aortic endothelial cells in vivo and in vitro. A similar blue-shift was observed after VQDs were incubated with recombinant VCAM-1 in tube. We anticipate that the specific interaction between VQDs and VCAM-1 and the blue-shift of the QD fluorescence peak can be very useful for VCAM-1 detection in vivo.

National Category
Biological Sciences
Identifiers
urn:nbn:se:kth:diva-141075 (URN)10.1371/journal.pone.0083805 (DOI)000329325200127 ()2-s2.0-84896730935 (Scopus ID)
Funder
Vinnova, P35914-1Swedish Foundation for Strategic Research Swedish Research Council, 621-2011-4381Science for Life Laboratory - a national resource center for high-throughput molecular bioscience
Note

QC 20140210

Available from: 2014-02-10 Created: 2014-02-07 Last updated: 2017-12-06Bibliographically approved
4. Reversible Modification of CdSe-CdS/ZnS Quantum Dot Fluorescence by Surrounding Ca2+ Ions
Open this publication in new window or tab >>Reversible Modification of CdSe-CdS/ZnS Quantum Dot Fluorescence by Surrounding Ca2+ Ions
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 19, 10424-10433 p.Article in journal (Refereed) Published
Abstract [en]

It has been known for a long time that the fluorescence intensity of colloidal quantum dots (QDs) becomes modified when free ions are added to the QD solution. The consequences of removing free ions from the QD solution, however, have not been closely investigated. In this work we studied fluorescence from 3-mercaptopropionic acid (3-MPA) coated CdSe-CdS/ZnS core-multishell QDs when free Ca2+ ions were added to and subsequently removed from the QD solution. It was found that QD fluorescence intensity was reduced when Ca2+ ions were added to the QD solution, while the wavelength of the QD fluorescence peak remained unchanged. QD fluorescence recovered when the concentration of free Ca2+ ions in the QD solution was reduced by adding Ca2+ chelator (ethylene glycol tetraacetic acid, EGTA). It was further observed that the time of single QD fluorescence at on-state and QD fluorescence lifetimes were also reduced after adding Ca2+ and then recovered when EGTA was added. Theoretical study shows that a free Ca2+ ion can attach stably to the system of [QD + surface ligand], attract the photoexcited electron, and repel the photoexcited hole inside the QD core, leading to the reduction of the radiative recombination between the electron and hole, thereafter decreasing the QD fluorescence intensity, on-state time, and fluorescence lifetimes, as observed experimentally. To the best of our knowledge, this is a first study to show that the changes of QD optical properties are reversible under the influence of Ca2+ ions. We further estimated the equilibrium association constant pK(a) of our QDs with Ca2+, which is much larger than QDs with Mg2+, Na+, and K+, indicating the feasibility of developing a QD-based Ca2+ sensor.

Keyword
PH Sensor, Blinking, Photoluminescence, Indicators, Mechanism
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-147041 (URN)10.1021/jp500853h (DOI)000336198900066 ()2-s2.0-84900821889 (Scopus ID)
Funder
Swedish Research Council, 621-2011-4381Swedish Foundation for Strategic Research Vinnova, P35914-1Science for Life Laboratory - a national resource center for high-throughput molecular bioscience
Note

QC 20140624

Available from: 2014-06-24 Created: 2014-06-23 Last updated: 2017-12-05Bibliographically approved

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