In the ongoing quest to devise faster, lower-cost methods for sequencing the human genome, scientists at University of Illinois at Urbana–Champaign have developed a novel approach: DNA molecules are sensed by passing them through a layer of constricted graphene embedded in a solid-state membrane containing a nanopore (a small hole with a roughly 1 nm internal diameter), located in a graphene nanoribbon (GNR). A critical feature of the new paradigm is that graphene's electrical properties allow the layer to be tuned in several distinct ways – namely, altering the shape of its edge, carrier concentration and nanopore location – thereby modulating both electrical conductance and external charge sensitivity. The researchers found that their novel technique can detect the DNA strand's rotational and positional conformation, and demonstrated that a graphene membrane with quantum point contact geometry exhibits greater electrical sensitivity than on with so-called uniform armchair geometry. The team has proposed a graphene-based field-effect transistor-like device for DNA sensing.
Prof. Jean-Pierre Leburton briefed Phys.org on the research he and his colleagues – Anuj Girdhar, Chaitanya Sathe and Klaus Schulten – conducted. "Simulations are presently leading experimental efforts on this specific topic – but transport models based on density functional theory cannot handle a large number of atoms due to limited computational resources," Leburton tells Phys.org, recounting some of the challenges the scientists faced. Density functional theory, or DFT, is a quantum mechanical modeling method used in physics and chemistry to investigate the electronic structure of many-body systems.
"In addition," Leburton continues, "these models are restricted to solid-state systems, while we're dealing with a hybrid solid-liquid system. For this reason, very simplistic and idealistic physical conditions are assumed on graphene nanoribbons." Such assumptions include uniform GNR widths with perfect armchair or zigzag edges, the nanopore being placed in the center of the graphene nanoribbon, and an absence of electrostatic perturbations from either the electrolytic solution or the dielectric supporting the graphene nanoribbon."In our approach, we use a multiorbital tight-binding (TB) technique that can handle a much larger number of atoms than DFT to account for the non-uniform GNR width, its irregular edges, and various sizes and positions of the nanopore," Leburton explains. The TB technique uses a superposition of wave functions of isolated atoms located at each atomic site to calculate the electronic band structure of solids.
"The electronic spectrum obtained from the tight binding model is then fed into a transport model based on a non-equilibrium Green function technique to compute the electrical conductance in general GNR configurations." A non-equilibrium Green (aka Green's) function, or NEGF, can be used to solve an inhomogeneous differential equation with boundary conditions in a way that is roughly analogous to the use of Fourier series in the solution of ordinary differential equations. Over the last decade, NEGF techniques have become widely used in corporate, engineering, government, and academic laboratories for modeling high-bias, quantum electron and hole transport in a wide variety of materials and devices.