By Sean Henahan, Access Excellence

EVANSTON, Ill. (August 14, '96) The discovery of a method for assembling nanoparticles (particles with a diameter 1 billionth of a meter) into materials using artificial DNA chould make it possible to tailor the optical, electrical, mechanical, and structural properties of new materials much more precisely, report researchers at Northwestern University

The new procedure takes advantage of the ability of DNA to recognize and complex to artificial or naturally occurring sequences of nucleotides to form stable structures.

The researchers attached sequences of artificial DNA to Au nanoparticles, and then added a complementary DNA sequence to the solution of nanoparticles to trigger a reaction that assembles the nanoparticles into ordered solids.

When the complementary DNA is introduced into the solution, the color changes immediately from red to purple, which means that the technology could also be used as a diagnostic tool for pathogenic DNA -- visibly changing color when a particular DNA sequence of a disease is encountered.

"We are using the exquisite molecular recognition properties of DNA to assemble these particles into ordered, solid materials," said Chad Mirkin, associate professor of chemistry and leader of the research team. "The fact that the gold particles change color when the appropriate DNA sequence is present makes this particularly promising for application in the biosensor arena."

The findings are an outgrowth of a collaboration between Mirkin, who specializes in surface and materials chemistry, and Robert Letsinger, professor emeritus of chemistry and senior research associate. Letsinger is known as the "father of the gene machine" for his pioneering work on the fabrication of artificial DNA.

When the DNA strands used in this study are heated to over 42 degrees, the two strands separate. When they are cooled again, the complementary sequences find each other and bind together once again. In the current Northwestern research, the materials can be assembled and disassembled by simply cooling and heating.

The first step in this research was to produce tiny particles of gold by mixing a gold salt with a reducing agent, sodium citrate. This results in uniform gold particles measuring just 13 nanometers in diameter. Modified artificial DNA strands consisting of sequences of eight bases are then attached to one group of the particles, in solution. A second set of DNA strands that are not complementary to the first are attached to a separate batch of particles .

Then two kinds of DNA strands consisting of 20 segments each are added to the solution. The two strands are complementary for 12 segments, so they automatically align with each other. The remaining eight segments align to each of the two different sequences attached to the gold particles, and the particles are assembled into a solid.

As soon as the linking strands are added to the solution, the color changes from red to purple. After several hours, the solution becomes clear and a pinkish-gray precipitate settles to the bottom. The change in color is due to change in shape of the particles, as they are linked together, changing their optical properties.

The researchers say this technology can be used to "tailor the optical, electrical and structural properties of these novel DNA/colloidal biomaterials by controlling the choice of particle size, shape and composition and the oligonucleotide sequence and length."

Mirkin believes this system can be used to make materials useful in a wide range of applications, from chemical sensors to nanofabrication schemes and microimaging techniques. Perhaps the most promising short-term applications, he said, will be in medical diagnostics. Since the change in color is so immediate and noticeable when the DNA sequences are joined, it should be possible to design sequences that would immediately react to the DNA of viruses and infectious diseases, making possible a point of site diagnosis.

The research is reported in the Aug. 15 issue of the journal Nature.

Related information on the Internet

Dr. Mirkin's Home Page, with Graphics.

Nanoworld