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Gold
Quantum Dots: Fluorescing "Artificial Atoms"
Could Have Applications in Biological Labeling,
Nanoscale Optoelectronics
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Fluorescence
is shown from solutions of small gold nanoclusters
dissolved in water. These nanoclusters behave like
multielectron artificial atoms, emitting at discrete
wavelengths in the visible and IR with the wavelength
increasing with the size of the cluster. Shown from
left to right are emission from solutions of Au5,
Au8, and Au13 clusters. |
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A
new class of water-soluble quantum dots made from
small numbers of gold atoms could be the basis for
a new biological labeling system with narrower excitation
spectra, smaller particle size and fluorescence comparable
to systems based on semiconductor quantum dots.
Providing
the “missing link” between atomic and nanoparticle
behavior in noble metals, these multi-electron “artificial
atoms” could also serve as light-emitting sources
in nanoscale optoelectronics and in energy transfer
pairs.
“We have discovered a new class of quantum dots that
are water soluble, strongly fluorescent, and display
discrete excitation and emission spectra that make
them potentially very useful for biological labeling,”
said Robert Dickson, associate professor in the School
of Chemistry and Biochemistry at the Georgia Institute
of Technology. “Their potential applications are really
complementary to those of semiconductor quantum dots.”
The gold nanodots are made up of 5, 8, 13, 23 or 31
atoms, each size fluorescing at a different wavelength
to produce ultraviolet, blue, green, red and infrared
emissions, respectively. The fluorescence energy varies
according to the radius of the quantum dot, with the
smallest structures the most efficient at light emission.
In contrast, quantum dots made from semiconductors
such as cadmium selenide are much larger, containing
hundreds or thousands of atoms. Semiconductor quantum
dots obey different size scaling under confinement,
producing weaker emissions.
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Diagram
shows arrangement of gold atoms in an eight-atom
quantum dot (left) while fluorescence is shown from
a solution containing those quantum dots (right).ensitive
that it can resolve details smaller than atoms.
Frank DiMeo/Cornell University
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The
gold quantum dots were reported August 13 in the journal
Physical Review Letters, and highlighted on the journal
cover. Additional information on the work was presented
August 23rd at the 228th national meeting of the American
Chemical Society in Philadelphia, PA. The work was sponsored
by the National Science Foundation, National Institutes
of Health, Sloan and Dreyfus Foundations, Blanchard
and Vassar Woolley Endowments and the Georgia Tech Center
for Advanced Research in Optical Microscopy.
In addition to Dickson, the research team includes Professor
Yih Ling Tzeng of Emory University; Jie Zheng, Lynn
Capadona and Caiwei Zhang of Georgia Tech, and Jeffrey
Petty of Furman University.
Because of their narrow excitation spectra and small
physical size, the gold quantum dots could be particularly
useful in fluorescence resonance energy transfer (FRET)
systems, in which emission from one nanodot would be
used to excite another as a means of measuring proximity.
The broad excitation spectra of semiconductor quantum
dots and their larger size make them more difficult
to use in FRET-based research, Dickson noted.
By using poly-amidoamine (PAMAM) dendrimers to encapsulate
their gold clusters, the researchers produced quantum
dots with very clean mass spectra. The 8-atom cluster,
for instance, produces bright blue emission and fluorescence
quantum yields of 42 percent in an aqueous solution.
The researchers produce the nearly spectrally pure,
size-tunable gold nanodots through a slow reduction
of gold salts (HAuCl4 or AuBr3) within aqueous PAMAM
solutions, followed by centrifugation to remove large
nanoparticles. By controlling the relative concentration
of gold to PAMAM and the generation of the dendrimers,
the researchers can control nanocluster size – and therefore
the emission wavelengths.
“Nanodots encapsulated through PAMAM exhibit higher
fluorescence quantum yields than do clusters encapsulated
by other matrices, suggesting an important role for
amines in gold nanodot creation,” Dickson noted.
The nanodot solutions are stable, lasting for months
either in solution or as dried powders. Solutions from
re-dissolved nanodot powders have the same properties
as when they were originally created.
Dickson’s research group has been working with fluorescent
and electroluminescent silver nanoclusters for several
years, evaluating their use in optical computing and
other applications. While silver quantum dots offer
promise because of their strong emission, their narrower
size range (2-8 atoms) makes them difficult to separate
to create solutions with distinct emission spectra.
“Silver fluoresces very strongly and it has awesome
optical properties, even better than gold because it
has very short lifetimes and high quantum yields,” Dickson
said. “But it is more difficult for us to separate them
to get high concentrations of pure samples. Right now
the scalings are much clearer and more easily understood
in gold, so we will take what we’ve learned there and
ultimately apply it to silver.”
Before these gold quantum dots can be useful in biological
labeling, however, the researchers must develop a mechanism
for attaching them to proteins that scientists wish
to track in cells.
“We are continuing to investigate these quantum dots,
to probe their fundamental photophysical and spectroscopic
properties, and to develop different chemistries for
functionalizing the scaffolding that encapsulates the
nanoclusters so we can attach them to other molecules,”
Dickson noted.
Much of that work will be done with newly obtained support
from the National Institutes of Health, which has funded
a Roadmap Initiative Center in High Resolution Cellular
Imaging to a team of Georgia Tech chemists and Prof.
Tzeng at Emory University.
“We will need to determine ways to functionalize these
quantum dots so they will get across cell membranes,
seek out specific proteins inside a cell and label those
proteins,” explained Dickson. “We are basically developing
the tools for in-vivo, single-molecule sensitivity and
labeling in living systems in the presence of very high
backgrounds. We expect to produce a new set of probes
that will be size-tunable, non-toxic and very bright.”
Beyond the potential applications, studying the gold
clusters provides basic information about the properties
of small clusters of noble metals, how they share conduction
electrons, and how they fluoresce under quantum confinement.
“They can help us understand the very small size scale
that is really not well understood for noble metals,”
Dickson added. “They can provide the ‘missing link’
between atomic and nanoparticle behavior in these metals.”
RESEARCH NEWS & PUBLICATIONS OFFICE
Georgia Institute of Technology
75 Fifth Street, N.W., Suite 100
Atlanta, Georgia 30308 USA
MEDIA RELATIONS CONTACTS: John Toon (404-894-6986);
E-mail: (john.toon@edi.gatech.edu); Fax: (404-894-4545)
or Jane Sanders (404-894-2214); E-mail: (jane.sanders@edi.gatech.edu).
TECHNICAL CONTACT: Robert Dickson (404-894-4007); E-mail:
(robert.dickson@chemistry.gatech.edu).
WRITER: John Toon
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www.nano-tsunami.com
This
story has been adapted from a news release -
Diese Meldung basiert auf einer Pressemitteilung
-
Deze
tekst is gebaseerd op een nieuwsbericht -
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