Chemist Punit Kohli is making a variety of nanoscale structures for a variety of possible uses. They're exotic-sounding beasts: quantum dots, nano-liposomes, and nanotubes, to name a few.

Nanotubes, a key focus of many materials scientists, are hollow structures that look like tiny drinking straws. In the past, Perspectives has featured research by SIUC's Physics Department on carbon nanotubes, which can bind and store gases such as hydrogen. But nanotubes can be made of "almost any material," Kohli says, from silica to gold to proteins—even DNA.

To make nanotubes and similar solid structures called nanowires, Kohli's lab first "grows" a thin film of aluminum oxide to use as a template. Due to the way this material naturally organizes itself, it's full of uniform, parallel nanopores running vertically through the film. One square centimeter of such a film contains a staggering number of such pores—about 100 billion.

Next the researchers deposit a chemical substance—for example, gold molecules—on the sides of the pores. Finally, they dissolve the template, leaving behind billions of hollow gold nanotubes. Alternatively, the researchers can make solid nanowires by simply filling up the pores with chemicals.

Such tubes and wires "have applications everywhere," Kohli says—in electronics, information storage, sensors, medicine. He's heard that one company is even exploring the idea of tagging valuable merchandise with customized nanorods to foil counterfeiters. (Nanotubes and nanorods can be made long enough to be seen under a microscope; their absence would show that the product wasn't genuine.)

Kohli is focusing on making nanotubes that have "different geometries—more complex sizes and shapes," he says. His lab is among the first to make curved nanotubes, branching nanotubes, and nanotubes with right-angle bends. These tubes could be used, for example, to carry current in nanoelectronic devices.

"If you want to make a device with hundreds of thousands of nanotransistors, it's really hard to connect a complicated circuit with straight structures," Kohli says. It's much more efficient to have nanotubes geometrically tailored to your fabrication needs. Likewise, in chemical synthesis, branched nanotubes might be used to channel and combine different substances in precise ways.

Connecting the dots

Kohli's lab also is working with quantum dots: semiconductor nanoparticles that, when excited by light, are fluorescent. Quantum dots emit various colors depending on their size. For example, a particle 2 nanometers in diameter might glow green, while a 5-nanometer particle of the same material might glow orange.

Kohli and other scientists think that solutions containing quantum dots could replace fluorescent dyes, which are widely used for detecting substances at low concentrations. For example, says Kohli, "Biomedical researchers tag proteins and antibodies with fluorescent dyes so they can see where they're going" in tissue cultures or in the body. Fluorescent dyes aren't very stable, however, and each type of dye requires a different wavelength of laser light to trigger its light-emitting properties.

In contrast, says Kohli, quantum dots, regardless of their size or the color they emit, can all be triggered by a single light source. Plus, because quantum dots are more photo-stable than fluorescent dyes, they would have a longer useful life. These qualities open the door to developing more efficient, less expensive detection systems.

Much work remains to be done to make the use of quantum dots practicable in real life. For example, because they are made of heavy metals, they'd have to be coated for safety for any medical use that required putting them into the body. Joe Weaver, an undergraduate assistant in Kohli's lab, is working with him on this aspect of the research.

Artificial cells

Kohli is working to make improved biosensors by incorporating quantum dots into nano-sized sacs called liposomes. These structures are like hollow artificial cells, but much smaller and with a stronger membrane.

Kohli attaches various kinds of receptor molecules to the surface of the liposomes. Each receptor molecule will dock with a particular microbe, protein, or chemical the way a key fits a lock. If that type of microbe (or protein or chemical) is present in a sample of fluid, it will hook on to the liposome with the corresponding receptor. When it does, the interaction will slightly change the color and intensity of light emitted by the quantum dots in the liposome. An instrument called a fluorometer can detect those minute changes.

Shelton Matthews, a student in SIUC's materials science REU program (see sidebar), worked with Kohli this past summer on the early stages of this concept. If the idea is successful, Kohli says, a chip with an array of these customized liposomes could test for the presence of hundreds of substances in one pass. The location where the array lit up would tell you what substance was detected. Potential uses would be in medical testing and biosecurity screening, where extreme detection sensitivity is critical.

Another of Kohli's ideas for the liposomes is to use them to carry drug molecules or genes into targeted body tissues. The liposome membrane would open under certain environmental conditions, releasing its cargo. "That trigger could be temperature, it could be pH, heat, radiation," Kohli says.

If the target tissue had a different chemical environment than healthy tissue—was a little more acid, say—the trigger would be internal. In other cases, the trigger could be X rays or ultrasound focused on the diseased tissue.

Although much work remains to be done to test the liposomes' feasibility and biocompatibility, Kohli thinks they would have certain advantages over targeted drug delivery devices now in use.

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