These self-propelled microscopic carbon-capturing motors may reduce carbon-dioxide levels in oceans

September 25, 2015

Nanoengineers have invented tiny tube-shaped micromotors that zoom around in water and efficiently remove carbon dioxide. The surfaces of the micromotors are functionalized with the enzyme carbonic anhydrase, which enables the motors to help rapidly convert carbon dioxide to calcium carbonate. (credit: Laboratory for Nanobioelectronics, UC San Diego Jacobs School of Engineering)

Nanoengineers at the University of California, San Diego have designed enzyme-functionalized micromotors the size of red blood cells that rapidly zoom around in water, remove carbon dioxide, and convert it into a usable solid form.

The proof-of-concept study represents a promising route to mitigate the buildup of carbon dioxide, a major greenhouse gas in the environment, said the researchers.

The team, led by distinguished nanoengineering professor and chair Joseph Wang, published the work this month in the journal Angewandte Chemie.

In their experiments, the nanoengineers demonstrated that the micromotors rapidly decarbonated water solutions that were saturated with carbon dioxide. Within five minutes, the micromotors removed 90 percent of the carbon dioxide from a solution of deionized water.

The micromotors were just as effective in a sea-water solution and removed 88 percent of the carbon dioxide in the same time frame.

“In the future, we could potentially use these micromotors as part of a water treatment system, like a water decarbonation plant,” said Kevin Kaufmann, an undergraduate researcher in Wang’s lab and a co-author of the study.

The micromotors are essentially six-micrometer-long tubes that help rapidly convert carbon dioxide into calcium carbonate, a solid mineral found in eggshells, the shells of various marine organisms, calcium supplements and cement. The micromotors have an outer polymer surface that holds the enzyme carbonic anhydrase, which speeds up the reaction between carbon dioxide and water to form bicarbonate. Calcium chloride, which is added to the water solutions, helps convert bicarbonate to calcium carbonate.

The fast, continuous motion of the micromotors in solution makes the micromotors extremely efficient at removing carbon dioxide from water, according to the researchers. The team explained that the micromotors’ autonomous movement induces efficient solution mixing, leading to faster carbon dioxide conversion.

Self-propulsion from oxygen gas bubbles

To fuel the micromotors in water, researchers added hydrogen peroxide, which reacts with the inner platinum surface of the micromotors to generate a stream of oxygen gas bubbles that propel the micromotors around. When released in water solutions containing as little as two to four percent hydrogen peroxide, the micromotors reached speeds of more than 100 micrometers per second.

Vdeo frames showing the movement of a micromotor in sea water (credit: Laboratory for Nanobioelectronics, UC San Diego Jacobs School of Engineering)

However, the use of hydrogen peroxide as the micromotor fuel is a drawback because it is an extra additive and requires the use of expensive platinum materials to build the micromotors. As a next step, researchers are planning to make carbon-capturing micromotors that can be propelled by water.

“If the micromotors can use the environment as fuel, they will be more scalable, environmentally friendly and less expensive,” said Kaufmann.

Abstract of Micromotor-Based Biomimetic Carbon Dioxide Sequestration: Towards Mobile Microscrubbers

We describe a mobile CO2 scrubbing platform that offers a greatly accelerated biomimetic sequestration based on a self-propelled carbonic anhydrase (CA) functionalized micromotor. The CO2 hydration capability of CA is coupled with the rapid movement of catalytic micromotors, and along with the corresponding fluid dynamics, results in a highly efficient mobile CO2 scrubbing microsystem. The continuous movement of CA and enhanced mass transport of the CO2 substrate lead to significant improvements in the sequestration efficiency and speed over stationary immobilized or free CA platforms. This system is a promising approach to rapid and enhanced CO2 sequestration platforms for addressing growing concerns over the buildup of greenhouse gas.