Coffee on the nanoscale: Graphene oxide membrane removes half the caffeine while retaining key compounds

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The humble coffee filter has the relatively simple job of letting the coffee through and leaving the grounds behind. But researchers from the ARC Center of Excellence for Carbon Science and Innovation (COE-CSI) are exploring whether a filter operating on the molecular scale can do something much more difficult: separate the caffeine from the coffee itself.

It's no ordinary challenge. A cup of coffee isn't simply water and caffeine; it's a mixture of various chemical compounds, many of which contribute to its flavor, aroma and other characteristics. Removing caffeine while leaving the things that make coffee taste like coffee means distinguishing between molecules at an extraordinarily small scale.

For UNSW Team Graphene master's research student Yihan Tian, the challenge had plenty of appeal. Not only is she a huge fan of coffee, but the science itself also excites her. "What attracted me most is that this research connects fundamental membrane science with a very practical, everyday product," she said.

Tongxi Lin and COE-CSI research fellow Dr. Xiaojun Ren, Tian developed a membrane using graphene oxide—an atomically thin, sheet-like carbon material that can be assembled in layers. The research is published in the Journal of Membrane Science.

Tiny channels between those layers allow water and other molecules to pass through the membrane and, by incorporating sodium alginate and crosslinking it with calcium ions, the researchers were able to change those channels and influence which molecules could make their way through. In effect, they created an extraordinarily fine sieve. Where an ordinary coffee filter separates relatively large particles of ground coffee from liquid, the graphene-oxide membrane attempts to separate molecules already dissolved in the coffee.

Of course, removing caffeine from coffee is nothing new; conventional decaffeination generally takes place before the coffee beans have even been roasted. But separating caffeine from coffee beans—without also adding or extracting other compounds—has always been part of the challenge, because some of them contribute directly or indirectly to the flavor of the end product. Rather than removing caffeine from the beans, Tian and Ren approached the problem from the other end.