Some of the nanostructures fabricated by the novel technique, made with various different materials, including diamond, gold and polystyrene. Image: Carnegie Mellon University.
Some of the nanostructures fabricated by the novel technique, made with various different materials, including diamond, gold and polystyrene. Image: Carnegie Mellon University.

Yongxin (Leon) Zhao from Carnegie Mellon University and Shih-Chi Chen from the Chinese University of Hong Kong have a big idea for manufacturing nanostructures.

Zhao’s Biophotonics Lab develops novel techniques for studying biological and pathological processes in cells and tissues. Through a process called expansion microscopy, the lab works to advance techniques for proportionally enlarging microscopic samples embedded in a hydrogel, allowing researchers to be able to view fine details without upgrading their microscopes.

In 2019, an inspiring conversation with Shih-Chi Chen, a professor in the Chinese University of Hong Kong’s Department of Mechanical and Automation Engineering who was visiting Carnegie Mellon as an invited speaker, sparked a collaboration between the two researchers. They thought they could use their combined expertise to find novel solutions to a long-standing challenge in microfabrication: developing ways to reduce the size of printable nanostructures to as small as tens of nanometers or several atoms thick.

Their solution is the opposite of expansion microscopy: create the 3D pattern of a material in a hydrogel and then shrink it down to the nanoscale.

“Shih-Chi is known for inventing the ultrafast two-photon lithography system,” said Zhao, an associate professor of biological sciences. “We met during his visit to Carnegie Mellon and decided to combine our techniques and expertise to pursue this radical idea.” The results of their collaboration, reported in a paper in Science, open new doors for designing sophisticated nanostructures.

Conventional 3D nanoscale printers focus a laser point to serially process materials, and take a long time to complete a design. Chen’s invention changes the width of the laser’s pulse to form patterned light sheets, allowing a whole image containing hundreds of thousands of pixels (voxels) to be printed at once, without compromising the axial resolution.

This novel fabrication technique is called femtosecond project two-photon lithography (FP-TPL). It is up to 1000 times faster than previous nanoprinting techniques and could lead to cost-effective large scale nanoprinting for use in in biotechnology, photonics or nanodevices.

For the process, researchers use the femtosecond two-photon laser to modify the network structure and pore size of a hydrogel, which creates boundaries for water-dispersible materials. The hydrogel is then immersed in water that contains nanoparticles made from materials such as metal, alloys, diamond, molecular crystals, polymers or fountain pen ink.

“Through fortuitous happenstance, the nanomaterials we tried were all attracted automatically to the printed pattern in hydrogel and assembled beautifully,” Zhao said. “As the gel shrinks and dehydrates, the materials become even more densely packed and connect to each other.”

For example, if a printed hydrogel is placed into a silver nanoparticle solution, the silver nanoparticles self-assemble in the gel along the laser-printed pattern. As the gel dries out, it can shrink to up to 13 times its original size, making the silver dense enough to form a nano silver wire that can conduct electricity.

Because the gels are three-dimensional, the printed patterns can be as well.

As a demonstration of the technique’s use for encrypted optical storage — which is how CDs and DVDs are written and read with a laser — the team designed and built a seven-layer 3D nanostructure that read ‘SCIENCE’ after it was optically decrypted.

Each layer contained a 200x200-pixel hologram of a letter. After shrinking the sample, the entire structure appears as a translucent rectangle under an optical microscope. The correct details about how much to expand the sample and where to shine a light through would be needed to read the information.

“Based on our result, the technique can pack 5 petabits worth of information in a tiny cubic centimeter of space,” Zhao said. “That’s roughly 2.5 times of all US academic research libraries combined.”

In the future, the researchers’ goal is to build functional nanostructures with multiple materials.

“In the end we would like to use the new technology to fabricate functional nanodevices, like nanocircuits, nanobiosensors, or even nanorobots for different applications,” Zhao said. “We are only limited by our imagination.”

This story is adapted from material from Carnegie Mellon University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.