Investigating the electron transport properties of proteins is particularly interesting for those proteins that perform charge transfer tasks inside organisms. Here, electrical measurements necessitate the availability of electrical probes separated by a distance comparable to the size of proteins - a few nanometres. All-semiconductor electrodes can be fabricated in a symmetric manner, they offer much flexibility in tailoring their electronic properties and for integration in microelectronic technology.

Fabricating nanogap devices

An international team of researchers has fabricated silicon-only, vertical nanogap devices (VNDs) for electrical studies on the electron transfer protein cytochrome c. In nature, cytochrome C protein has a crucial part in cellular respiration, where it mediates charge transfer from complex III (coenzyme Q) to complex IV (cytochrome C oxidase). For this purpose, silicon-on-insulator (SOI) wafers are used, with highly p-doped top and bottom Si layers to minimize resistivity, and a buried oxide layer (BOX) of 4 – 8 nm thickness. Using standard semiconductor process technology, the top silicon layer is patterned into tower-like structures. Cytochrome c proteins are immobilized inside the recess-etched nanogap region from buffer solution.

Protein immobilization and transport measurement

Electrical measurements show that after protein immobilization, the junction current density at 1 V bias increases by up to seven orders of magnitude. Heating the samples above the protein denaturation temperature of 80°C results in a drop back to current levels before protein immobilization. Temperature-dependent measurements reveal two distinct electron transport regimes: for cryogenic temperatures only a weak temperature dependence is observed, while above 140 K temperature-activated electron transport with small activation energies (50 – 100 meV) is observed. The VND contact scheme can be applied to a variety of proteins (and other molecules) supporting efficient charge transfer, with prospects for integrated bioelectronics and sensing.