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Engineers have developed a technique for trapping hard-to-detect molecules, using forests of carbon nanotubes.
The team, based at the Massachusetts Institute of Technology (MIT), modified a simple microfluidic channel with an array of vertically aligned carbon nanotubes – rolled lattices of atoms like tiny tubes of chicken wire.
The researchers had previously devised a method for standing carbon nanotubes on their ends, like trees in a forest. With this method, they created a three-dimensional array of permeable carbon nanotubes within a microfluidic device, through which fluid can flow.
Now, the researchers have given the nanotube array the ability to snag certain particles. To do this, the team coated the array, layer by layer, with polymers of alternating electric charge. “You can think of each nanotube in the forest as being concentrically coated with different layers of polymer,” says Brian Wardle, professor of aeronautics at MIT. “If you drew it in cross-section, it would be like rings on a tree.”
Depending on the number of layers deposited, the researchers can create thicker or thinner nanotubes and thereby tailor the porosity of the forest to capture larger or smaller particles of interest.
The nanotubes’ polymer coating may also be chemically manipulated to bind specific bioparticles. To test this idea, the researchers applied an established technique to treat the surface of the nanotubes with antibodies that bind to prostate specific antigen (PSA), a common experimental target. The polymer-coated arrays captured 40% more antigens than arrays lacking the coating.
The combination of carbon nanotubes and multilayer coatings may help to finely tune microfluidic devices to capture extremely small and rare particles, such as certain viruses and proteins, says Wardle. “There are smaller bioparticles that contain rich amounts of information that we don’t have the ability to access in medical testing devices such as microfluidic chips,” he says. “Carbon nanotube arrays could target that size of bioparticle.”
The team integrated a 3D array of carbon nanotubes into a microfluidic device by using chemical vapour deposition and photolithography to grow and pattern nanotubes onto silicon wafers. They then grouped the nanotubes into a cylinder-shaped forest, and centred the array within a 3mm-wide, 7mm-long microfluidic channel.
The researchers coated the nanotubes in successive layers of alternately charged polymer solutions to create distinct, binding layers around each nanotube. The researchers then demonstrated that the array could be primed to detect a given molecule, by treating it with antibodies that bind to PSA. They pumped in a solution containing small amounts of PSA and found that the array captured the antigen effectively, throughout the forest, rather than just on the outer surface of a typical microfluidic element.
The array is versatile, as the nanotubes can be manipulated mechanically, electrically and optically, while the polymer coatings can be chemically altered to capture a wide range of particles.
An immediate target may be biomarkers called exosomes, which can be important signals of a disease’s progression, says Wardle. “Science is picking up on how much information these particles contain, but they’re hard to find,” he says.
“This device would allow you to go after bioparticles such as exosomes and other things that truly are nanometre-scale.”