Chemists at the University of California, San Diego have developed minute grains of silicon that spontaneously assemble, orient and sense their local environment, a first step toward the development of robots the size of sand grains that could be used in medicine, bioterrorism surveillance and pollution monitoring.
In a paper to be published in September in the Proceedings of the National Academy of Sciences, which will appear in the journal’s early on-line edition this week, Michael Sailor, a professor of chemistry and biochemistry at UCSD, and Jamie Link, a graduate student in his laboratory, report the design and synthesis of tiny silicon chips, or “smart dust,” which consist of two colored mirrors, green on one side and red on the other. Each mirrored surface is modified to find and stick to a desired target, and to adjust its color slightly to let the observer know what it has found.
This is a key development in what we hope will one day make possible the development of robots the size of a grain of sand,” Sailor explains. “The vision is to build miniature devices that can move with ease through a tiny environment, such as a vein or an artery, to specific targets, then locate and detect chemical or biological compounds and report this information to the outside world. Such devices could be used to monitor the purity of drinking or sea water, to detect hazardous chemical or biological agents in the air or even to locate and destroy tumor cells in the body.”
To create the smart dust, the researchers use chemicals to etch one side of a silicon chip, similar to the chips used in computers, generating a colored mirrored surface with tiny pores. They make this porous surface water repellent, or hydrophobic, by allowing a chemical that is hydrophobic to bind to it. They then etch the other side of the chip to create a porous reflective surface of a different color and expose the surface to air so that it becomes hydrophilic, or attractive to water.
Using vibrations, they can break the chip into tiny pieces, each about the size of the diameter of a human hair. Each piece is now a tiny sensor with opposite surfaces that are different colors, with one attracted to water and one repelled by water and attracted to oily substances.
When added to water, the “dust” will align with the hydrophilic side facing the surface of the water and the hydrophobic side facing toward the air. If a drop of an oily substance is added to the water, the dust surrounds the drop with the hydrophobic side facing inward. In addition to this alignment, which will occur in the presence of any substance that is insoluble in water, a slight color change occurs in the hydrophobic mirror. The degree of this color change depends on the identity of the insoluble substance. The color change occurs as some of the oily liquid enters the tiny pores on the hydrophobic side of the silicon particle.
“As the particle comes in contact with the oil drop, some of the liquid from the target is absorbed into it,” Sailor explains. “The liquid only wicks into the regions of the particle that have been modified chemically. The presence of the liquid in the pores causes a predictable change in the color code, signaling to the outside observer that the correct target has been located.”
The hydrophilic side of the chip behaves in a similar way; it changes color according to the identity of the hydrophilic liquid it contacts. While each individual particle is too small to observe the color code, the collective behavior of the particles facilitates the detection of the signal. This research effort, funded by the National Science Foundation and the Air Force Office of Scientific Research, builds on previous work by the Sailor group to develop various types of sensing devices from silicon chips. A year ago, the group reported the development of silicon particles with a single sensing surface.
Link, the first author on the paper, says the dual-sided particles have the additional benefit of being able to collect at a target and then self-assemble into a larger, more visible reflector that can be seen from a distance. “The collective signal from this aggregate of hundreds or thousands of tiny mirrors is much stronger and more easily detected than that from a single mirror,” she points out. “The tendency of these particles to clump together will therefore enable us to use this technology for remote sensing applications.”