Chemiluminescence gets a boost from microwaves
Lynn M. Savage
Chemiluminescence-based methods of
detecting biological processes at the cellular level work well because most samples
do not generate unwanted luminescence that must be filtered out. Using these techniques,
however, means that one is dependent upon the quantum efficiency of the chemical
reaction and the depletion of the reactants over time.
Researchers at the University of Maryland Biotechnology
Institute (UMBI) in Baltimore recently reported that placing samples on substrates
immobilized with deposits of silver nanoparticles enhances chemiluminescence signatures.
They reported that plasmons generated by chemiluminescence molecules in proximity
to the surface boost the luminescence intensity of the sample particles. They named
this effect “metal-enhanced chemiluminescence.”
Now these researchers, led by Chris
D. Geddes, professor and director of UMBI’s Institute of Fluorescence, have
added low-power microwave energy to this technique, reporting additional increases
in luminescence intensity and faster reaction times. Furthermore, they developed
the method using common materials that one can purchase at a discount retail store.
For chemiluminescent reactants, they
used red, blue and green glow sticks — plastic tubes that contain a phenyloxalate
ester, a fluorescent probe and a capsule containing hydrogen peroxide. Fluorescence
occurs when the tube is bent, the capsule snapped and the chemicals shaken together.
The investigators activated the glow sticks, placing ∋10 μl of the fluid
on silver island films — slides covered with silver dots that were 200 Å
in diameter by 40 Å high. They also used unadorned glass coverslips as a control.
Researchers exposed chemiluminescent dyes to microwaves, thereby enhancing the luminescence
intensity of the materials. Dyes were tested on either glass slides (left) or on
silver island films (right), with those on the silvered substrates exhibiting the
more pronounced enhancements. Mw = addition of 140 W of microwave energy; a.u. =
arbitrary units. Reproduced with permission of the Journal of the American Chemical
One hundred fifty seconds after luminescence
initiation, they placed the slides inside a General Electric Co. consumer microwave
oven (700 W of maximum power), set the power to 20 percent (140 W), and measured
the spectra of the solutions before and after a single 10-s exposure of microwaves.
They decided on the 140-W setting because
they determined it to be optimal. “The more power you put in, the faster the
reaction,” Geddes said. “Too fast, though, and you saturate the detector.
Too slow, and you might as well use another technique.” In addition, too high
a setting could denature proteins, which they are investigating.
To obtain the spectra, the scientists
used a spectrometer connected to a 1-mm-diameter fiber optic probe with a numerical
aperture of 0.22 made by Ocean Optics Inc. of Dunedin, Fla. They used a consumer-model
3.2-megapixel digital camera made by Olympus Imaging America Inc. to acquire real-color
photographs of the reagents.
According to Geddes, although the institute
has tens of thousands of dollars’ worth of equipment, they used a commonly
available microwave cavity and digital camera because they wanted to create a simple
and inexpensive system that would be easy to recreate.
They found that the microwave energy
by itself enhanced the chemiluminescence of the reagents, whether glass or silver
island films were used: from 22x for blue dye on glass slides to 85x for green dye
on a silvered substrate. The differences in intensity among dyes on glass that were
not irradiated and dyes on silver island film that were irradiated were more pronounced,
ranging from 54x to 125x, depending on the dye color.
Microwaving also increased the reaction
speeds, providing measurable responses in as little as 10 s, compared with up to
5 min for chemiluminescence reactions induced without microwaves.
The researchers are using the technique
to analyze particular proteins — for example, ricin, a deadly toxin that must
quickly be measured, Geddes said, down to the range of femtograms per milliliter
of whole blood.
Journal of the American Chemical Society, Oct. 18, 2006, pp. 13372-13373.
MORE FROM PHOTONICS MEDIA