DNA strands connect the quantum dots.

Borrowing from biology, chemists have devised a new way to assemble semiconductor bits into potentially useful materials. The specks are quantum dots, sometimes thought of as artificial atoms.

DNA can connect tiny pieces of cadmium selenide into three-dimensional arrays, report Gregory E Mitchell, Chad A. Mirkin, and Robert L. Letsinger of Northwestern University in Evanston, Ill. The resulting composite of inorganic and organic materials could have applications in biological sensing and electronics.

The researchers create the composite by attaching lengths of single-stranded DNA to cadmium selenide particles suspended in water. Then, they add to the solution double-length DNA sequences that complement the strands bound to the quantum dots. Two DNA strands attached to dots hook to a free strand like "chemical Velcro," bringing the particles together, says Mirkin.

The Northwestern researchers and a separate group a the University of California, Berkeley had connected gold beads in a similar fashion (SN: 8/17/96, p. 100). The current study, however, is the first to extend the technique to quantum dots, which have useful electronic and optical properties, says Mirkin.

Each cadmium selenide dot, 3.2 nanometers in diameter, bears 5 to 10 DNA strands, in contrast to the 220 strands on the 13-nanometer gold beads.

The biggest challenge in attaching the DNA strands to cadmium selenide was getting the semiconductor bits to dissolve in water, says Mirkin. The researchers synthesize the quantum dots in organic solvents, which makes them "very greasy particles," he explains. Mirkin's group bound acid molecules to their surface and then removed protons to give the dots an electric charge.

Mirkin and his colleagues describe their findings in the Sept. 8 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

Because of its adaptability, DNA offers researchers more control over the architecture of a three-dimensional array, says A. Paul Alivisatos of Berkeley. "It would be more difficult to do that with some kind of non-information-bearing self-assembly. DNA contains more information to use to create spatial organization," he says. For example, Mirkin and his colleagues have also created a hybrid material consisting of both gold and semiconductor dots linked by DNA.

Quantum dots have sparked the interest of scientists because the particles fluoresce in a wide range of colors depending on their size. They glow much brighter and for a longer time than conventional organic dyes, making them especially good for marking calls to view with microscopy (SN: 10/24/98, p. 271).

If electrons can travel through the DNA strands (SN: 8/14/99, p. 104), the quantum-dot aggregates might have interesting electronic properties, Mirkin suggests. He and his group are now exploring ways to use DNA-linked quantum dots as photonic materials, catalysts, and biological sensors.