Combining materials ideas from two unrelated organisms—lotus leaves and mussels—researchers have made a new kind of water-repellent microparticle (J. Am. Chem. Soc., DOI: 10.1021/ja303037j). The scientists coated particles with a polymer based on mussel adhesive and then chemically modified the coated particles to mimic the structure of lotus leaves.
Previous methods of creating highly water-repellent surfaces have worked only with particular types of materials. “We needed a versatile method to make many kinds of surfaces hydrophobic,” says coauthor Ning Zhao, of the Beijing National Laboratory for Molecular Sciences. Materials with water-repellent properties have many potential industrial uses, he says, including as anti-fouling coatings for equipment used in marine environments.
The researchers looked to the adhesive protein that mussels use to stick themselves to surfaces in wet environments. They thought the protein was a good starting point because the adhesive can stick to many different types of materials.
Zhao and his team coated a variety of particles using dopamine, a neurotransmitter with many biological functions. They chose it because it is structurally similar to L-3,4-dihydroxyphenylalanine, an amino acid found in high levels in mussel adhesive protein. At high pH, the dopamine spontaneously reacts to form a polymer around the core of the particles. In 2007, Phillip Messersmith and colleagues at Northwestern University had shown that polydopamine can deposit on almost any type of organic or inorganic material—even on so-called nonstick materials like Teflon (Science, DOI: 10.1126/science.1147241).
The researchers then modified the surface of the coated particles to mimic that of a lotus leaf. Lotus leaves’ superhydrophobicity is due to their morphology and chemical composition, Zhao says. The rough, bumpy surface of a lotus leaf is dotted with nanosized wax particles, which make water bead up and roll off.
To mimic the bumps, the researchers first deposited silver nanoparticles onto the polydopamine-coated particles. Then they added a layer of thiol fluoroalkanes to make the surfaces slippery. The resulting particles, they found, are highly water repellent, with a contact angle between particle and water of 170 degrees. Researchers consider surfaces to be superhydrophobic when water beads up with contact angles of at least 150 degrees.
To demonstrate a potential application, Zhao and his colleagues made superhydrophobic magnetic particles with iron as the core material. When they added the particles to a mixture of oil and water and touched a magnet to the side of the container, the particles moved toward the magnet, drawing the oil with them. When the researchers used the magnet to pull the mass under the water’s surface, the particles surrounded the oil, forming a sphere. Magnetic particles could help clean up oil spills, Zhao says.
Messersmith applauds the study. “The method is simple,” he says, “and can be applied to a variety of particles.”