Capsules Mock Photosynthesis

German chemists report progress toward achieving artificial photosynthesis by packing thousands of similar molecules together to create a tiny capsule, then using a different kind of molecule as a light absorbing and emitting “filling.”

Scientists worldwide are working to achieve in the lab what a simple blade of grass can do: harness energy from sunlight to convert the harmful gas carbon dioxide into organic compounds. Because photosynthesis changes it into beneficial carbohydrates and oxygen, artificial photosynthesis is seen as a possible way to reduce greenhouse gases in the atmosphere and create valuable raw materials such as sugar, starch and methane gas.

This illustration of the University of Würzburg nanocapsule shows thousands of similar molecules packed together to create a capsule that is filled with molecules of a different kind. (Image: Institute of Organic Chemistry, University of Würzburg)

The photosynthesis-emulating capsule, with a diameter of 20 to 50 nm, or about one ten-thousandth of a pinhead, was created in the organic chemistry laboratory at the University of Würzburg. The chemists report that their nanocapsule is extraordinarily elaborate and possesses an ability important to plant photosynthesis that other chemically synthesized molecules cannot replicate: The molecules inside their capsule absorb light energy and emit some of it again in the form of fluorescent light. The rest is transmitted by energy transfer to the capsule molecules, which then also cast fluorescent light.

In principle, the nanocapsules should make suitable components for an artificial photosynthesis device, the chemists say.

“They would even use the light far more efficiently than plants because their synthetic bilayer membranes would be composed entirely of photoactive material,” professor Frank Würthner said.

The nanocapsules are composed of a unique material developed by Würthner’s working group on the basis of so-called amphiphilic perylene bisimides. If the base material, which can be isolated as a powder, is placed in water, its molecules automatically form unstable vesicles. Through photopolymerization with light, they become robust nanocapsules that are stable in an aqueous solution, regardless of its pH value.

It was the visiting scientist from China, Dr. Xin Zhang, who managed to fill the nancapsules with other photoactive molecules. A fellow of the Humboldt Foundation, he is currently a member of Würthner’s group.

Zhang stuffed bispyrene molecules, which conveniently change shape to suit their environment, into the nanocapsules. In an acidic environment with a low pH value, they assume an elongated form. If they are then excited with ultraviolet light, they emit blue fluorescent light.

If the pH value rises, the molecules fold. In this shape, they emit green fluorescent light. In this state, the bispyrenes excite the capsule shell energetically, which reacts to this with red fluorescence.

At a pH value of 9, they emit white fluorescent light, “a so-far-unique effect in the field of chemical sensing, which might be groundbreaking for the design of fluorescence probes for life sciences,” Würthner said.

The chemists have access to an extremely sensitive nanoprobe: The pH value of an aqueous solution can be determined with nanoscale spatial resolution over the wavelength of the fluorescent light emitted by the nanocapsules.

This means that nanocapsules are not just an option for artificial photosynthesis, but can also be used for diagnostic applications. For example, they could be equipped with special surface structures that purposefully dock to tumor cells and then make these visible by means of fluorescence.

Both possible applications are the subject of further research by Würthner and his team. The work described here was funded by the German Research Foundation.

The research is featured on the cover of the November issue of the journal Nature Chemistry.

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