Wim Noorduin/Harvard University
A new study details the process by which minerals can be made to assemble themselves into controlled yet complex structures.
CAMBRIDGE—The garden is marvelously lush, with hundreds of blossoming roses, tulips, lilies, and curvaceous, fungi-like plants. But these petals, twisting stems, and finely wrought leaves are invisible to the naked eye: Grown in the lab, this nano-landscape is best seen with an electron microscope.
The creation of Harvard researchers, the garden is a demonstration of how simple environmental changes, such as tweaking the temperature, can be used to precisely control the construction of tiny objects and devices—at a scale that is a fraction of a fraction of the width of a human hair.
Scientists toiling in this invisible realm are putting their new techniques to aesthetically pleasing purposes to show they work—and capture the public’s imagination. Others have recently made whimsical smiley faces by folding strands of DNA, or constructed Lego-like structures from DNA “bricks” that spell out the alphabet.
The ultimate goal, however, is to come up with industrial applications. Researchers envision a new generation of tiny medical sensors and microelectronics, or materials with novel properties, such as interesting ways to interact with light that could enable as-yet-uninvented technologies.
“In nature, you see many complex shapes and patterns,” said Wim Noorduin, a postdoctoral researcher at Harvard University who grew the flowers featured in the journal Science on Thursday. “There’s a huge interest to grow complex shapes at the microscale,” by harnessing nature’s ability to create detailed and intricate structures, such as those found in a coral reef or on a seashell.
On the ground floor of the Cambridge building where Noorduin works, researchers, wearing masks and protective suits to guard against dust and other contaminants, use sophisticated techniques to etch nanoscale structures. Noorduin is working at a similar scale, on structures of impressive complexity. But his work can be done in a simple glass beaker—he’s even done it in a coffee cup—using readily available ingredients found in most laboratories.
The technique is remarkably easy: fill a beaker with a solution that has a salt and a silicon compound dissolved in it. Put in a glass slide or a bit of metal to act as the soil on which the crystal “plants” will grow. Allow carbon dioxide from the air to diffuse into the solution, triggering a simple reaction that causes the dissolved chemicals to precipitate and form a crystal—one that is curvy, rather than jagged.
Noorduin often used whatever was at hand for his experiments, experimenting in one case on growing a surreal garden of undulating alien plants around the base of the Lincoln Memorial on a shiny penny.
Noorduin got his first glimpse of what he had grown three years ago: a black-and-white electron microscope image that has the crowded, slightly alien look of a fully-imagined Pixar world. Stems balloon into horn shapes, curvy stalks undulate. It took his breath away. Then, he buckled down to figure out how he had created the breathtaking image and how he could modify it. He later added dyes to the solution he was growing at different stages, but the electron microscope images have to be artificially colored.
Noorduin took his cues from nature: the structure of a shell changes in response to differences in the environment. By changing the acidity of the solution and the temperature, he discovered controlled ways to make his garden grow. He even accidentally discovered, when he put a cover on a beaker to keep out dust, that controlling the concentration of carbon dioxide could alter the thick of his flowers’ petals.
Working with materials science professor Joanna Aizenberg, Noorduin discovered that altering the acidity or alkalinity of a solution could cause crystal blossoms to grow outward into a bell shape, or to make them curl inward. Combining these kinds of techniques, they could create tendrils, the nested layers of petals in a rose, and the delicate cup of a tulip—which Noorduin felt especially obligated to grow, because he is Dutch. He was able to grow even more complex structures, such as a stem, a leaf, and a flower, all contained in a vase.
Juan Manuel Garcia-Ruiz, a research professor at CSIC-University of Granada in Spain, demonstrated a decade ago that crystals could grow in unexpected curves and spirals. For years, he said, no one had believed that the crystal forms he grew, which so closely resembled living forms, were really crystals—assuming instead that there was just biological contamination.
He said the new paper brings a finer level of control to the process, showing how it is possible to modify the shapes.
Researchers not involved in the work appreciated its beauty and the effort to find ways to control the mineralization process. But when asked why these studies showing how to manipulate matter at the smallest scale were so pleasing to look at, they all had a slightly different take.
Hendrik Dietz of the Laboratory for Biomolecular Nanotechnology at Technische Universität München in Germany, wrote in an e-mail that the choice to build something beautiful is only possible once it’s possible to control matter at the small scale. Thus, the intricate sculpture-like flowers are a way to judge the scientists’ level of control.
“Beautiful (or funny) things such as DNA smiley faces etc should therefore not be taken easily as child’s play,” Dietz wrote. “There is serious science ... that has enabled the authors to pull these things off.”
Shawn Douglas, an assistant professor at the University of California, San Francisco, said that an unexpected shape that clearly bears the imprint of human imagination, such as a smiley face that would never appear in nature, is a way of showing critics that the technique is not an accident.
“If you have to choose something to make, it almost seems obvious to choose a fun or interesting shape,” he wrote in an e-mail.