Tiny self-assembling silicon chips that orient and sense their local environment could be used to detect disease, bioterrorism and pollution.
Michael Sailor, a professor of chemistry and biochemistry at the University of California, San Diego, and Jamie Link, a graduate student in his laboratory, designed and synthesized the tiny silicon chips.
The chips 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.
“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,” says Sailor.
“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,” he says.
Etched silicon
The researchers use chemicals to etch one side of a silicon chip and generate a colored mirrored surface with tiny pores that are designed to be water repellent, or hydrophobic.
They etch the other side of the chip to create a porous reflective surface of a different color and expose it to air so that it becomes water attractive, or hydrophilic.
Finally, they use vibrations to break the chips into tiny pieces the width of a human hair.
The chips become tiny sensors, with one side attracted to water and the other repelled by water and attracted to oily substances.
Added to water, the sensors align with the hydrophilic side facing the surface of the water and the hydrophobic side facing the air.
When an oily substance is added to the water, the sensors surround it with the hydrophobic side facing inwards.
“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.”
Collective behavior
While individual sensors are too small to observed with the naked eye, their collective behavior allows for the detection of a signal.
The researchers say that the dual-sided particles can 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,” says Link.
“The tendency of these particles to clump together will therefore enable us to use this technology for remote sensing applications,” she says.
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