Building a Remote Refractive Index Sensor with Quantum Dots

Hank Hogan

Whispering works best if done correctly and up close. That’s one explanation for a new type of remote refractive index sensor developed by a group from Texas A&M University in College Station. The researchers embedded quantum dots into polystyrene microspheres and, when excited, the quantum dots acted as a local source for optical resonances, creating structures sensitive to the nearby environment.

Quantum dot-embedded microspheres make a label-free molecular sensor. In a sample layout, the microsphere is immobilized on a polyethyleneimine (PEI) film, with the liquid to be sensed surrounding it (top). Excitation light enters, and the resulting spectra exit through the objective (OBJ). On the bottom left is a bright-field image of a quantum dot-embedded microsphere. On the bottom right is a dark-field image of a quantum dot-embedded microsphere with excitation at the lower point on the equatorial plane. The intensity maximum is at the upper point on the equatorial plane. Reprinted with permission from Applied Physics Letters.

Team leader Kenith E. Meissner, an assistant professor of biomedical engineering, noted that the surface of the microspheres could be functionalized with analytes, making the sensors respond to specific chemicals in the local environment. That could be valuable in a variety of areas, he said. “We believe this type of sensor will be useful in biomedical applications within tissue as well as in ex vivo and in vitro environments.”

The sensors exploit what are known as whispering gallery modes that arise because of total internal reflection in the microspheres. As a result of these modes, light travels around the beads in an optical resonance. The resonances shift because of local refractive index changes driven by the environment of the microspheres. These micro-resonators, therefore, can detect molecules near the surface without the need for labeling and can serve as the basis for a sensor.

Although microspheres have been used in this fashion before, one barrier to remote operation has been how to get the light into them. Typically, that has required contact or near contact along with exacting spectral resolution and control of the source.

The Texas A&M group got around this coupling problem by using quantum dots, which are nanometer-size and which emit at narrow, specific, size-related wavelengths when excited.

In effect, Meissner said, the quantum dots acted as a translator or coupler between an external, remote excitation source and the microresonator.

In a demonstration, the investigators started with 9.6-μm-diameter polystyrene microspheres from Molecular Probes Inc. of Eugene, Ore. They prepared cadmium selenide/zinc sulfide core/shell quantum dots with an emission peak at 550 nm and embedded them in the microspheres through a passive diffusion process. After this was completed, the quantum dots had penetrated less than 100 nm into the microspheres.

The scientists then immobilized the quantum dot-embedded microspheres and exposed them to a liquid while interrogating them with two-photon excitation at 800 nm from a Coherent laser. Meissner noted that single-photon excitation would have worked as well. They measured the response using an Acton spectrograph and a thermoelectrically cooled Princeton Instruments CCD camera.

They tested the sensors using a mixture of ethanol in water, shifting the refractive index up in steps from 1.333 to 1.362 by adding ethanol. With this change, the whispering gallery mode spectra redshifted and broadened. The researchers found the shift to be about 160 nm per refractive index unit, more than five times greater than a simple model of the microresonators predicted. This discrepancy probably occurred because the quantum dots formed a thin, high-refractive-index layer near the microsphere surface, the scientists concluded.

Work in this area continues, with the ultimate goal being the creation of sensors that measure the local refractive index and that target specific analytes. That targeting could enable some innovative label-free sensors, Meissner said. “We believe that we can leverage the unique properties of quantum dots to create spectrally multiplexed, multianalyte systems.

Applied Physics Letters, June 3, 2008, 221108.