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Bio-inspired artificial cells for energy storage

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People have long been looking for ways to replicate biological machinery with non-biological components and artificially-created cells that fulfil a key biological function of transforming light into chemical energy.

Now (2019), scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have designed cell-like hollow capsule structures through spontaneous self-assembly of hybrid gold-silver nanorods held together by weak interactions. They wrapped the walls of these capsules with a light-sensitive membrane protein called bacteriorhodopsin and were thus able to channel protons unidirectionally from the interior of the artificial cells to the external environment.

The key to the research came from coupling the group of artificial cells that were generating protons to a second group of artificial cells. These cells contained molecular motor machinery that used the proton gradient to generate a molecule called ATP, which is well known as the fundamental unit of energy currency in biological systems. 

By using protons as exchangeable ions, the scientists were able to construct a pathway by which two different groups of artificial cells could communicate. ​“By using a minimalistic artificial cell, we’ve stripped away pretty much all the major cellular functions, leaving behind only the basic function needed for communication and synthesis of energy-rich carrier molecules,” they explained.

Scientists at Argonne have been carrying out research on transforming light into chemical energy for many years. In 2013, they were able to extend the light reactivity of titanium dioxide nanoparticles to the visible part of the electromagnetic spectrum. In their efforts to increase the efficiency of the reaction, the scientists conducted a series of experiments to establish ways in which titanium dioxide could react with the visible and not only the UV light. This is when the team turned to the protein bacteriorhodopsin. It has the ability to utilize sunlight in order to maintain the process of transferring protons from inside the cell to the extracellular space.

In 2017, scientists at Argonne created synthetic purple membranes which contained tiny discs of organic compounds called lipids, man-made proteins and semiconducting nanoparticles that, when taken together, could transform sunlight into hydrogen fuel. To design the synthetic version of the membrane protein, the researchers used a minimum of key cell elements: the nanodiscs, synthetic DNA that encoded the protein; other biological components needed for protein manufacturing, including amino acids; isolated ribosome-protein manufacturing machinery. This led to the successful expression of synthetic bacteriorhodopsin across the nanodiscs. Once prepared, the synthetic purple membranes were assembled with nanoparticles of titanium dioxide for hydrogen evolution under visible light.

The new design has several advantages: the synthetic protocell incorporates an important intrinsic property of noble metal colloidal particles: plasmonic resonance. The near‐field coupling between adjacent metal nanoparticles is responsible for strongly localized electric fields and a broad absorption in the whole visible spectra, which in turn enhances the flux of photons to bacteriorhodopsin and accelerates the proton-pumping kinetics. The cell‐like potential of this design is further demonstrated by leveraging the outward pumped protons as “chemical signals” for triggering ATP biosynthesis in a coexistent synthetic protocell population. One additional benefit of the inorganic support is that metallic nanoparticles act like a lens focusing and amplifying the incoming light near the light-harvesting protein, leading to enhanced biological function.

Researching cellular communication with bio-inspired materials is of particular interest to scientists as they attempt to reproduce more complex biofunctional systems. ​Some of the ideas for future experiments involve extending the capsule structures with the hybrid gold-silver nanorods so that they resemble artificial neurons. The main goal of this research was to replicate nature, yet use inanimate materials to probe how cells accomplish their biological tasks.