1. Chemists from the University of Würzburg have made progress to achieving artificial photosynthesis Nanocapsules have been loaded with reactive molecules which convert UV light to visible light which varies based on the pH of the environment. The chemical conversion to light could lead to artificial photosynthesis and separately be used for tiny sensors for pH.
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.
The diameter of one nanocapsule is a mere 20 to 50 nanometers. Dr Xin Zhang, a visiting scientist from China, managed to fill the nanocapsules with other photoactive molecules.
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.
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.
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.
The viruses often used to carry genes into the body can endanger patients. Furthermore, the particles created in Langer’s lab now rival viruses’ efficiency at delivering their DNA payload.
This summer the nanoparticle-delivered gene therapy successfully suppressed ovarian tumor growth in mice.
One drawback to non-viral vectors is that they are not as efficient as viruses at integrating their DNA payload into the target cell’s genome, says Leaf Huang, professor in the School of Pharmacy at the University of North Carolina. However, in the past several years, advances by Langer and others have improved that efficiency by several orders of magnitude.
“Non-viral vectors are now comparable to viral vectors, in some cases,” says Huang, whose research focuses on delivering genes surrounded by a fatty membrane. “They have come a long way compared to 10 years ago.”
Both viral and non-viral methods could eventually prove useful and safe, says gene therapy researcher Katherine High, who is part of a team that recently used viral gene therapy to restore some sight to children suffering from a congenital retinal disease.
The ovarian cancer treatment developed at MIT and the Lankenau Institute has been successful in animal studies but is not yet ready for clinical trials. Such trials could get under way in a year or two, says Anderson. Meanwhile, he and others in Langer’s lab are exploring other uses for their nanoparticles. Last month, the researchers reported using the particles to boost stem cells’ ability to regenerate vascular tissue (such as blood vessels) by equipping them with genes that produce extra growth factors.
“We’ve had success with gene delivery using these nanoparticles, so we thought they might be a safer, temporary way to modify stem cells,” says Anderson.