Imitating photosynthesis in plants? If we were to accomplish this, mankind
would have a little less to worry about. Chemists from the University
of Würzburg have now made progress on the road to achieving artificial
photosynthesis.
The structure that has been developed in the university's Organic Chemistry
laboratory is fascinatingly complex: thousands of similar molecules are packed
together to create a capsule that is filled with molecules of a different kind.
The diameter of one capsule is a mere 20 to 50 nanometers, which is one ten-thousandth
of a pinhead.
Structures that are so elaborate are far from the ordinary in chemistry. So,
it is hardly surprising that these Würzburg nanocapsules appear on the
front page of the November issue of the journal "Nature Chemistry".
What is more, they can also do something that has not been described before
for chemically synthesized molecules.
Encapsulated molecules transmit energy
Nanocapsules possess a property that is important in photosynthesis in plants:
the molecules inside the capsule absorb light energy and emit some of this again
in the form of fluorescent light. The rest of it, however, is transmitted by
energy transfer to the capsule molecules, which then also cast fluorescent light.
As far as photosynthesis is concerned nothing different happens, to put it simply:
molecules harness energy from sunlight and transmit it to other molecules in
a complex process, at the end of which the energy is bound chemically. The sun's
power then sits in valuable carbohydrates that plants, animals, and people use
to generate the energy they need to live.
In principle, therefore, the nanocapsules should make suitable components for
an artificial photosynthesis contraption. "They would even use the light
far more efficiently than plants because their synthetic bilayer membranes would
be composed entirely of photoactive material," says Professor Frank Würthner.
The value of artificial photosynthesis
Why conduct research into artificial photosynthesis? In photosynthesis, plants
consume the "climate killer" that is carbon dioxide. In view of global
warming, many scientists see artificial photosynthesis as a possible way of
reducing the volume of the greenhouse gas carbon dioxide in the atmosphere.
In addition, this process would also create valuable raw materials: sugar, starch,
and the gas methane.
Unique material for the capsule shell
The Würzburg nanocapsules are comprised of a unique material. This was
developed in Frank 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 so-called vesicles, though
these are not stable at that point. It is only through photopolymerization with
light that they become robust nanocapsules that are stable in an aqueous solution
- regardless of its pH value.
Bispyrenes as the filling inside the capsules
It was the visiting scientist from China, Dr. Xin Zhang, who managed to fill
the nanocapsules with other photoactive molecules. A fellow of the Humboldt
Foundation, he is currently a member of Professor Würthner's working group.
Zhang smuggled bispyrene molecules into the nanocapsules. The special thing
about these molecules is that they change their shape to suit their environment.
Where the pH value is low, in other words in an acidic environment, they assume
an elongated form. If they are then excited with UV 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.
Blue, green, and red. If the three primary colors overlap, this produces white
- as with a color television. It is the same with the nanocapsules: with a pH
value of 9, in other words just right of neutral, 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,"
explains Professor Würthner.
Nanoprobe for pH measurements
The Würzburg 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,
they 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 Frank Würthner
and his team. The work described here was funded by the German Research Foundation
(DFG).
Posted November 6th, 2009
