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Watching energy transport through biomimetic nanotubes

Scientists from the University of Groningen (the Netherlands) and the University of Würzburg (Germany) have investigated a simple biomimetic light-harvesting system using advanced spectroscopy combined with a microfluidic platform. The double-walled nanotubes work very efficiently at low light intensities, while they are able to get rid of excess energy at high intensities. These properties are useful in the design of novel materials for the harvesting and transport of photon energy. The results were published in the journal Nature Communications on 10 October.

The remarkable ability of natural photosynthetic complexes to efficiently harness sunlight — even in dark environments — has sparked widespread interest in deciphering their functionality. Understanding energy transport on the nanoscale is key for a range of potential applications in the field of (opto)electronics. The overwhelming complexity of natural photosynthetic systems, consisting of many hierarchically arranged sub-units, led scientists to turn their attention to biomimetic analogs, which are structured like their natural counterparts but can be more easily controlled.

Ligh-harvesting molecules

The Optical Condensed Matter Science group and the Theory of Condensed Matter group (both at the Zernike Institute for Advanced Materials, University of Groningen) have joined forces with colleagues from the University of Würzburg (Germany) to gain a comprehensive picture of energy transport in an artificial light-harvesting complex. They used a new spectroscopic lab-on-a-chip approach, which combines advanced time-resolved multidimensional spectroscopy, microfluidics, and extensive theoretical modeling.

The scientists investigated an artificial light-harvesting device, inspired by the multi-walled tubular antenna network of photosynthetic bacteria found in nature. The biomimetic device consists of nanotubes made out of light-harvesting molecules, self-assembled into a double-walled nanotube. ‘However, even this system is rather complex,’ explains Maxim Pshenichnikov, professor of ultrafast spectroscopy at the University of Groningen. His group devised a microfluidic system, in which the outer wall of the tube can be selectively dissolved and, thus, switched off. ‘This is not stable, but in the flow system, it can be studied.’ In this way, the scientists could study both the inner tube and the complete system.

Adapting

At low light intensity, the system absorbs photons in both walls, creating excitations or excitons. ‘Due to the different sizes of the walls, they absorb photons of different wavelengths,’ Pshenichnikov explains. ‘This increases the efficiency.’ At high light intensity, a large number of photons are absorbed, creating a huge number of excitons. ‘We observed that, when two excitons meet, one of them actually ceases to exist.’ This effect acts as a kind of safety valve, as high numbers of excitons could damage the nanotubes.

Thus, the scientists also demonstrated that the double-walled molecular nanotube is capable of adapting to changing illumination conditions. They mimic the essential functional elements of nature’s design toolbox at low light conditions by acting as highly sensitive antennas but get rid of excess energy at high intensities when there is too much light — a situation that would not normally occur in nature. Both these properties pave the way for better control of the transport of energy through complex molecular materials.

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Scientists construct energy production unit for a synthetic cell

Scientists at the University of Groningen have constructed synthetic vesicles in which ATP, the main energy carrier in living cells, is produced. The vesicles use the ATP to maintain their volume and their ionic strength homeostasis. This metabolic network will eventually be used in the creation of synthetic cells — but it can already be used to study ATP-dependent processes. The researchers described the synthetic system in an article that was published in Nature Communications on 18 September.

‘Our aim is the bottom-up construction of a synthetic cell that can sustain itself and that can grow and divide,’ explains University of Groningen Professor of Biochemistry Bert Poolman. He is part of a Dutch consortium that obtained a Gravitation grant in 2017 from the Netherlands Organisation for Scientific Research to realize this ambition. Different groups of scientists are producing different modules for the cell and Poolman’s group was tasked with energy production.

Equilibrium

All living cells produce ATP as an energy carrier but achieving sustainable production of ATP in a test tube is not a small task. ‘In known synthetic systems, all components for the reaction were included inside a vesicle. However, after about half an hour, the reaction reached equilibrium and ATP production declined,’ Poolman explains. ‘We wanted our system to stay away from equilibrium, just like in living systems.’

It took three Ph.D. students in his group nearly four years to construct such a system. A lipid vesicle was fitted out with a transport protein that could import the substrate arginine and export the product ornithine. Inside the vesicle, enzymes were present that broke down the arginine into ornithine. The free energy that this reaction provided was used to link phosphate to ADP, forming ATP. Ammonium and carbon dioxide were produced as waste products that diffused through the membrane. ‘The export of ornithine produced inside the vesicle drives the import of arginine, which keeps the system running for as long as the vesicles are provided with arginine,’ explains Poolman.

Transport protein

To create an out-of-equilibrium system, the ATP is used to maintain ionic strength inside the vesicle. A biological sensor measures ionic strength and if this becomes too high, it activates a transport protein that imports a substance called glycine betaine. This increases the cell volume and consequently reduces the ionic strength. ‘The transport protein is powered by ATP, so we have both production and use of ATP inside the vesicle.’

The system was left to run for 16 hours in the longest experiment that the scientists have performed. ‘This is quite long — some bacteria can divide after just 20 minutes,’ says Poolman. ‘The current system should suffice for a synthetic cell that divides once every few hours.’ Eventually, different modules like this one will be combined to create a synthetic cell that will function autonomously by synthesizing its own proteins from a synthetic genome.

Artificial chromosome

The current system is based on biochemical components. However, Poolman’s colleagues at Wageningen University & Research are busy collecting the genes needed for the production of enzymes used by the system and incorporating them into an artificial chromosome. Others are working on lipid and protein synthesis, for example, or cell division. The final synthetic cell should contain DNA for all these modules and operate them autonomously like a living cell, but in this case, engineered from the bottom-up and including new properties. However, this is many years away. ‘In the meantime, we are already using our ATP-producing system to study ATP-dependent processes and advance the field of membrane transport,’ says Poolman.

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Scientists create fully electronic 2-dimensional spin transistors

Physicists from the University of Groningen constructed a two-dimensional spin transistor, in which spin currents were generated by an electric current through graphene. A monolayer of a transition metal dichalcogenide (TMD) was placed on top of graphene to induce charge-to-spin conversion in the graphene. This experimental observation was described in the issue of the journal Nano Letters published on 11 September 2019.

Spintronics is an attractive alternative way of creating low-power electronic devices. It is not based on a charge current but on a current of electron spins. Spin is a quantum mechanical property of an electron, a magnetic moment that could be used to transfer or store information.

Heterostructure

Graphene, a 2D form of carbon, is an excellent spin transporter. However, in order to create or manipulate spins, interaction of its electrons with the atomic nuclei is needed: spin-orbit coupling. This interaction is very weak in carbon, making it difficult to generate or manipulate spin currents in graphene. However, it has been shown that spin-orbit coupling in graphene will increase when a monolayer of a material with heavier atoms (such as a TMD) is placed on top, creating a Van der Waals heterostructure.

In the Physics of Nanodevices group, led by Professor Bart van Wees at the University of Groningen, Ph.D. student Talieh Ghiasi and postdoctoral researcher Alexey Kaverzin created such a heterostructure. Using gold electrodes, they were able to send a pure charge current through the graphene and generate a spin current, referred to as the Rashba-Edelstein effect. This happens due to the interaction with the heavy atoms of the TMD monolayer (in this case, tungsten disulfide). This well-known effect was observed for the first time in graphene that was in proximity to other 2D materials.

Symmetries

‘The charge current induces a spin current in the graphene, which we could measure with spin-selective ferromagnetic cobalt electrodes,’ says Ghiasi. This charge-to-spin conversion makes it possible to build all-electrical spin circuits with graphene. Previously, the spins had to be injected through a ferromagnet. ‘We have also shown that the efficiency of the generation of the spin accumulation can be tuned by the application of an electric field,’ adds Ghiasi. This means that they have built a spin transistor in which the spin current can be switched on and off.

The Rashba-Edelstein effect is not the only effect that produces a spin current. The study shows that the Spin-Hall effect does the same, but that these spins are oriented differently. ‘When we apply a magnetic field, we make the spins rotate in the field. Different symmetries of the spin signals generated by the two effects in interaction with the magnetic field help us to disentangle the contribution of each effect in one system,’ explains Ghiasi. It was also the first time that both types of charge-to-spin conversion mechanisms were observed in the same system. ‘This will help us to gain more fundamental insights into the nature of spin-orbit coupling in these heterostructures.’

Graphene Flagship

Apart from the fundamental insights that the study can provide, building an all-electrical 2D spin transistor (without ferromagnets) has considerable significance for spintronic applications, which is also a goal of the EU Graphene Flagship. ‘This is especially true because we were able to see the effect at room temperature. The spin signal decreased with increasing temperature but was still very much present under ambient conditions.’

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