Changing the coating alters quantum dot color without affecting size
Researchers led by
Takashi Jin at Hokkaido University in Sapporo, Japan, have discovered that coating
quantum dots with calixarene alters their optical properties without changing their
“From the point of view of biological application,
this technique is very useful to prepare multicolored quantum dots with similar
particle sizes for fluorescent labeling probes to use in Förster resonance
energy transfer [FRET] experiments,” Jin said.
Scientists prefer to use similar size
fluorophores in such experiments because the FRET efficiency depends on the distance
between the centers of the fluorophores used. Larger fluorophores have a greater
distance from their center to their outer valence.
In addition, Jin said that the size
of the quantum dots — less than 10 nm — may allow them to permeate the
cell membrane more easily and make them less likely to interfere with normal biological
processes than larger quantum dots coated with amphiphilic polymers.
Using cadmium-selenide quantum dots
with a zinc-sulfide shell, the researchers coated the quantum dots with calixarene
carboxylic acids, with four, six or eight arenes. They added potassium tert-butoxide
to deprotonate the carboxylic acids, which made the quantum dots water-soluble.
In the July 26 issue of The Journal of the
American Chemical Society, the scientists reported that calixarene coating resulted
in redshifts, depending on the position of the arene group. For example, a quantum
dot with a 530-nm emission peak had redshifts of 8, 21 and 35 nm for calixarenes
with four, six and eight arenes, respectively. The effect of the calixarene coat
on the redshift decreased with increasing particle size.
In addition, the quantum yield of the
coated particles was 0.1 to 0.34, which the researchers said was high. The photostability
remained unchanged for more than one month in aqueous solution. This is important
because modifying quantum dots to make them water-soluble typically results in decreased
photostability and quantum yield.
Despite the diminished effect of calixarene
coating with increasing particle size, the researchers said that their technique
is useful for changing the optical properties of quantum dots without altering their
Jin said that coating the quantum dots
with calixarenes that have another chemical embedded in them can be useful for detecting
metal ions and small organic compounds.
The research team already has tested
the utility of this method for detecting acetylcholine.
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