Researchers create self-assembled protein-based logic circuits

The proof-of-concept study, described in Nature Communications, demonstrates that it is possible to create stable digital circuits that take advantage of an electron’s properties at quantum scales, researchers said.

A challenge in creating molecular circuits is that as the circuit size decreases, the circuits become unreliable as the electrons needed to create current behave like waves, not particles, at the quantum scale.

For example, on a circuit with two wires that are one nanometre apart, the electron can ‘tunnel’ between the two wires and effectively be in both places simultaneously, making it difficult to control the direction of the current.

Molecular circuits can mitigate these problems, but single-molecule junctions are short-lived or low-yielding due to challenges associated with fabricating electrons at that scale.

Ryan Chiechi, associate professor of chemistry at North Carolina State University and co-corresponding author of the research paper said the team’s goal was to create a molecular circuit using tunnelling as an advantage, rather than fighting against it.

Chiechi and co-corresponding author Xinkai Qiu, Cambridge University, said they built the circuits by first placing two different types of fullerene cages on patterned gold substrates. They then submerged the structure into a solution of photosystem one (PSI), a commonly used chlorophyll protein complex.

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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, EGaln, are printed on top. This process addresses drawbacks of single-molecule junctions and preserves molecular-electronic function, the team said.

“Where we wanted resistors we patterned one type of fullerene on the electrodes upon which PSI self-assembles, and where we wanted diodes we patterned another type,” Chiechi said in a statement. “Oriented PSI rectifies current – meaning it only allows electrons to flow in one direction. By controlling the net orientation in ensembles of PSI, we can dictate how charge flows through them.”

The team coupled the self-assembled protein ensembles with human-made electrodes and made simple logic circuits that used electron tunnelling behaviour to modulate the current.

Chiechi said that the proteins scatter the electron wave function, mediating tunnelling in ways that are still not completely understood.

“The result is that despite being ten nanometres thick, this circuit functions at the quantum level, operating in a tunneling regime,” Chiechi added. “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.”

According to researchers, they created simple diode-based AND/OR logic gates from the circuits and incorporated them into pulse modulators, which can encode information by switching one input signal on or off depending on another input’s voltage.

The PSI-based logic circuits were reportedly able to switch to a 3.3kHz input signal which, while not comparable in speed to modern logic circuits, is one of the fastest molecular logic circuits yet reported.

“This is a proof-of-concept rudimentary logic circuit that relies on both diodes and resistors,” Chiechi said. “We’ve shown here that you can build robust, integrated circuits that work at high frequencies with proteins.

Chiechi said that the circuits could lead to development of electronic devices that enhance, supplant and/or extend the functionality of classical semiconductors.