Detecting what's small and dark

Hank Hogan

Cats are legendary for their ability to see in dim light, leading to the myth that they can see in the dark. However, they have nothing on researchers from ETH Zurich in Switzerland. Exploiting an interferometric technique they first used to image nonfluorescent virus particles, they have imaged a single dark quantum dot, successfully measuring the absorption of an object a few nanometers in size.

The technique could have biological applications. For example, the method could make it possible to see organic dyes that are not emitting. Researcher Vahid Sandoghdar, head of the nano-optics group at ETH Zurich, noted that even relatively bright dye might be too dim in some circumstances, such as when in certain solvents. “We believe absorption detection will make it possible to image and monitor such emitters.”

The investigators detected dark quantum dots by optimizing the interference between light coherently scattered by the nanodot and the reflection of the incident laser beam. To do this, they used two photodiodes from Thorlabs Inc. of Newton, N.J., a laser from World Star Technologies Inc. of North Star, Ontario, Canada, and a homemade high-numerical-aperture microscope.

One photodiode monitored the intensity of the 532-nm-wavelength laser output, with about half the light sent to this detector. The other captured the sample response at the excitation wavelength. By normalizing and subtracting the two signals, the researchers effectively reduced laser intensity fluctuations to about 0.01 percent, which enabled them to detect absorption changes of about the same order.

They also used an avalanche photodiode from PerkinElmer Inc. of Waltham, Mass., in photon-counting mode to monitor sample fluorescence. To do this, they first removed the excitation light with a dichroic mirror and a long-wave pass filter, ensuring the purity of the fluorescence signal.

With this setup, they experimented with detecting quantum dots, semiconductor nanocrystals with specific emission peaks. They placed the quantum dots on thin mica sheets at a concentration that ensured a single quantum dot per field of view, then measured the absorption. Their results for the individual quantum nanodots agreed with those estimated from measurements of masses of these nanocrystals.

A single CdSe quantum dot exhibits fluorescence blinking (a). A simultaneously recorded absorption – or dark – image of the same quantum dot is shown (b). Reprinted with permission from Nano Letters.

Besides providing a measurement of the absorption for dark objects, this technique could offer insight into what happens on the nanoscale. The group’s results, for example, show that a photobleached quantum dot has a smaller extinction cross section than one that is not photobleached, but that a fluorescently blinking nanodot suffers no such change.

This data, Sandoghdar noted, could not have been obtained either from group measurements or from fluorescence studies of single dots. Explanations for the phenomena are rooted in the nature of the quantum dot itself.

As for the future, the researchers are not yet done seeing what is small and dark. “We have hopes of detecting single proteins, but that requires some more work,” Sandoghdar said.

Nano Letters, published online Aug. 1.