— A University at Buffalo research team has invented
a new way to synthesize quantum dots -- luminescent
nanocrystals made from semiconductor material.
Sometimes called artificial atoms, quantum dots have
the potential to be used to build exciting new devices
for biological and environmental sensing, quantum
computing, lasers and telecommunications, among other
The new technique developed by a team led by T.J.
Mountziaris, Ph.D., professor of chemical and biological
engineering in the UB School of Engineering and Applied
Sciences, enables precise control of particle size
by using a microemulsion template formed by "self-assembly."
The process involves the direct mixing of a nonpolar
substance (heptane), a polar substance (formamide)
and an amphiphilic substance or surfactant (a block
copolymer) to form a uniform dispersion of heptane
droplets in formamide, stabilized by the surfactant.
A patent is pending on the technique, which was described
in a recent issue of the journal Langmuir. Mountziaris'
co-researchers are Paschalis Alexandridis, Ph.D.,
UB professor of chemical and biological engineering;
Athos Petrou, Ph.D., professor of physics in the UB
College of Arts and Sciences; Georgios Karanikolos,
a graduate student in the UB Department of Chemical
and Biological Engineering; and Grigorios Itskos,
a graduate student in the UB Department of Physics.
Using the technique, the UB researchers demonstrated
the controlled synthesis of zinc selenide (ZnSe) quantum
dots that exhibit size-dependent luminescence. When
excited by ultraviolet light, quantum dots emit a
particular fluorescent color and brightness, depending
on the dot's size. The problem for scientists has
been devising simple techniques to control the size
of quantum dots, which would give them the ability
to control a quantum dot's color properties. Such
control is a critical factor in the quantum dot's
The ZnSe quantum dots have potential for use in clinical
and therapeutic diagnostics and for DNA analysis.
The dots may be used, for example, as biological tags,
attaching themselves to diseased cells, tumors or
particular genes, alerting scientists to their presence
in the body or in biological samples.
"The luminescent properties of quantum dots make
them ideal for such applications," Mountziaris
The technique developed by Mountziaris and co-researchers
gives them the ability to precisely control the size
(and luminescence wavelength) of the ZnSe dots in
one step. The researchers were able grow ZnSe dots
inside "nanoreactors" formed by the heptane
nanodroplets of the emulsion. By reacting hydrogen
selenide gas with diethyl-zinc (DEZn) dissolved in
the heptane, a single quantum dot is grown in each
nanoreactor, allowing precise control of particle
size by simply controlling the initial concentration
of DEZn in the heptane. Small clusters of ZnSe nucleate
in each heptane nanodroplet and fuse into one particle
by a process called coalescence. The researchers run
the process at room temperature, but still obtain
"Since we run the process at room temperature,
we were expecting amorphous particles or crystalline
particles with many defects. To our surprise, we obtained
almost perfect crystals," Mountziaris says. "We
believe that the localized energy release during cluster-cluster
coalescence is the key to forming single crystalline
"The energy released anneals the particles and
leads to perfect crystals," he adds.
ZnSe quantum dots created by this technique have maintained
their luminescent properties for more than a year.
To make quantum dots useful for practical applications,
functional molecules must be attached to their surface
after they are synthesized, Mountziaris explains.
"Researchers are creating biological tags of
certain colors based on quantum dots by decorating
their surface with functional molecules that selectively
attach to a specific biological molecule," he
says. "This gives the molecules something like
a tail light, and you could follow them in the body
by exciting their luminescence with ultraviolet light."
Mountziaris' group is collaborating with UB bioengineers
to use quantum dots in DNA analysis.
"The challenge of quantum-dot technology has
been how to make dots of a precise size, how to functionalize
the surface and also scale up the process for commercial
applications," Mountziaris says. "Our technique
can be scaled up very easily because it is based on
self-assembly and does not depend on mixing efficiency
or process time to control the size of the dots. We
have demonstrated 'dial-a-size' capability."
"One nanoreactor makes one quantum dot,"
he adds. "My colleague, Paschalis Alexandridis,
and our student, George Karanikolos, have developed
a very stable microemulsion that has very slow droplet-droplet
interactions. This prevents agglomeration of the nanocrystals
after they are formed, which can adversely affect
their properties. It is also responsible for the remarkable
stability of the quantum dot loaded emulsion."
Mountziaris and co-researchers are at work synthesizing
additional compounds, such as cadmium selenide and
lead selenide to cover a wide spectrum of luminescence
wavelengths. They also are developing functional water-soluble
caps for the quantum dots that would enable their
use as biological tags, without diminishing the dots'
Multicolor quantum dots could be used to create "optical
bar codes" from a sequence of joined quantum
dots possessing different luminescent properties,
Mountziaris says. "This would be very useful
in multiplexed experiments by assigning a different
function to different groups of dots and tracking
them as they attach to different biomolecules,"
The University at Buffalo is a premier research-intensive
public university, the largest and most comprehensive
campus in the State University of New York. UB's more
than 27,000 students pursue their academic interests
through more than 300 undergraduate, graduate, and
professional degree programs.