Researchers constructed self-assembled, protein-based circuits that can execute rudimentary logic tasks in a proof-of-concept study. The research shows that it is possible to build stable digital circuits that take advantage of an electron’s quantum-scale features. The development of modular protein logic for regulating protein activity at the posttranscriptional level is a synthetic biology challenge.
One of the difficulties in building molecular circuits is that the circuits become faulty as the circuit size decreases. At the quantum scale, the electrons required to generate current act like waves rather than particles. For example, in a circuit with two wires one nanometer apart, the electron can “tunnel” through the two wires and effectively be in both places at the same time, making controlling the direction of the current difficulties. Molecular circuits can help to solve these issues, however, single-molecule junctions are either short-lived or low-yielding because to the difficulties in producing electrodes at that scale.
“Our goal was to attempt to develop a molecular circuit that leverages tunneling to our benefit rather than resisting it,” says Ryan Chiechi, associate professor of chemistry at North Carolina State University and co-corresponding author of a publication outlining the study.
Chiechi and co-corresponding author Xinkai Qiu of the University of Cambridge created the circuits by first putting two different types of fullerene cages on patterned gold substrates. They then immersed the structure in a solution of photosystem one (PSI), a typical chlorophyll protein complex.
Our goal was to attempt to develop a molecular circuit that leverages tunneling to our benefit rather than resisting it. Oriented PSI rectifies current, which means it only allows electrons to travel in one direction. We can control how charge flows through PSI ensembles by varying the net orientation
Ryan Chiechi
These circuits were used to build simple diode-based AND/OR logic gates, which were then incorporated into pulse modulators, which can encode information by switching one input signal on or off based on the voltage of another input. The PSI-based logic circuits switched a 3.3 kHz input signal, which, while not as rapid as modern logic circuits, is nevertheless one of the fastest molecular logic circuits documented to date. The proof of concept results are still being worked on. The study was published in Nature Communications. Chiechi and Qiu, co-authors, previously worked at the University of Groningen in the Netherlands.
The different fullerenes induced PSI proteins to self-assemble on the surface in specific orientations, creating diodes and resistors once top-contacts of the gallium-indium liquid metal eutectic, EGaIn, are printed on top. This process both addresses the drawbacks of single-molecule junctions and preserves molecular-electronic function.
“Where we needed resistors, we patterned one form of fullerene on the electrodes on which PSI self-assembles, and where we wanted diodes, we patterned another type,” Chiechi explains. “Oriented PSI rectifies current, which means it only allows electrons to travel in one direction. We can control how charge flows through PSI ensembles by varying the net orientation.”
The researchers connected the self-assembled protein ensembles to human-made electrodes and created basic logic circuits that employed electron tunneling behavior to regulate the current.
“These proteins scatter the electron wave function, mediating tunneling in ways that are still not completely understood,” Chiechi says. “The result is that despite being 10 nanometers thick, this circuit functions at the quantum level, operating in a tunneling regime. And because we are using a group of molecules, rather than single molecules, the structure is stable. We can actually print electrodes on top of these circuits and build devices.”
The researchers used these circuits to build simple diode-based AND/OR logic gates, which they then incorporated into pulse modulators, which can encode information by switching one input signal on or off dependent on the voltage of another input. The PSI-based logic circuits were able to switch a 3.3 kHz input signal, which, while not as fast as current logic circuits, is nevertheless one of the fastest molecular logic circuits documented to date.
“This is a proof-of-concept simple logic circuit that uses diodes and resistors,” Chiechi explains. “We’ve demonstrated here that you can use proteins to make robust, integrated circuits that operate at high frequencies.”
“In terms of immediate utility, these protein-based circuits could lead to the development of electronic devices that augment, replace, and/or expand the functionality of conventional semiconductors.”
