Portland, Ore. - Researchers have shown recently that magnetic silicon nanoparticles-smart dust-can act as chaperones, surrounding and herding rare sample molecules so that the samples navigate channels to microfluidic sensors without leaving any residue. Someday such chaperones might surround cancer cells and "escort" them to the exit.
"When you start talking about samples on the molecular scale, your surface-to-volume ratio is so high that you can't let any [part of the sample] stick to microfluidic channels," said professor Michael Sailor at the University of California at San Diego.
Previously, Sailor used his method to fabricate porous silicon nanoparticles with a selective response, and demonstrated that his smart dust could seek out, surround and read out the composition of a sample by changing colors (www.eet.com/article/showArticle.jhtml?articleId=18307615). A designed reflective layer is laid down on the surface of each type of nanoparticle with the use of a process called a rugate filter. Each reflects only a narrow spectral band. By varying the reflective bands, researchers can use a 20-bit code to read out the composition of samples, simply looking up their index number in a catalog. Each chemical will change the reflectivity of a cloud of particles in a unique manner, creating a unique signature that can be detected from a distance with lasers.
Now Sailor has magnetized the nanoparticles. "By infusing iron into our silicon nanoparticles, making them magnetic, we can direct their movement through a microfluidic device without leaving any residue," he said.
When the system is fully developed, solders with special goggles will be able to spot the chemical composition of a material through its unique signature at up to 1 kilometer away, Sailor said.
Jason Dorvee, one of Sailor's graduate students, discovered that a magnetic iron oxide added to Sailor's smart dust enables them to be easily moved about with a handheld magnet. The smart dust was developed by Sailor and Jamie Link, a graduate researcher in Sailor's laboratory. Someday they hope to surround cancer cells with it, and remove them from the body magnetically.
Sailor's group encodes entire wafers with the same reflective-color selectivity by virtue of a galvanostatic anodic etch of the crystalline silicon. The process creates an optically uniform layer of porous silicon, the thickness and porosity of which are controlled during the electrochemical process through changes in current density during the etch, the duration of the etch and the composition of the etchant solution.
In addition to being magnetic, one side of the smart dust is hydrophobic, or water repelling, and the other side is hydrophilic, or water loving. This enables them to spontaneously assemble around the samples to be tested-the smart dust tries to keep one end inside the hollow core, because the hydrophobic end prefers the interior to be filled with anything but water. As each molecule of the sample to be tested gets surrounded by nanoparticles, they start touching each other, then spontaneously merge, mixing their contents. Eventually they all merge with the entire sample to be tested, become one giant lab in-a-drop. Small changes in the color of the nanoparticles reveal the spectra. This enables remote identification of the specific chemicals encased by the smart dust.
For the future, the team is working with medical researchers at UCSD to surround and precisely direct the motion of cancer cells, bacteria and other miniscule objects within a tiny drop of liquid.
Bioengineering associate professor Sangeeta Bhatia and Austin Derfus, a graduate student working in her lab at UCSD's Jacobs School of Engineering, also contributed to the development of Sailor's smart dust. Funding came from the Air Force Office of Scientific Research and the National Cancer Institute.