Chemical – Electronic Coupling and the InAs Two-Dimensional Electron Gas (2DEG): Highly Sensitive and Selective Sensing of Surface-Based Biomolecule Interactions

April S. Brown
Dept. of Electrical and Computer Engineering
Dept. of Biomedical Engineering
Pratt School of Engineering
Duke University

Abstract
The origin of surface donors yielding the InAs surface-potential confined two-dimensional electron gas (2DEG) remains unknown, but is believed to be defect-based and therefore related to surface reconstruction, strain, and/or native defects associated with surface processes such as oxidation. We show that the surface oxide (either engineered or “native”) can be used as a reagent in surface-based chemical interactions modifying surface molecular attachment, both conformation and coverage, in a concentration-dependent fashion. These molecular interactions can be, in turn, “sensed” through changes in the 2DEG density and mobility.

Using x-ray photoelectron spectroscopy (XPS) and Hall measurements, we show that changes in the oxide chemistry resulting from surface DNA and protein interactions are sensitively reflected in changes in the 2DEG density and mobility. As an electronic sensor, biomolecule (DNA/protein) adsorption can be measured down to fM concentrations with dynamic range of greater than four orders of magnitude. DNA hybridization can also be electronically sensed and differentiated from non-specific binding in the fM concentration range. These results, realized with a planar device implementation, show performance equal to that realized in a nanowire-based devices.

At higher concentrations (μM), the average conformation and coverage of adsorbed single-stranded DNA (ssDNA) are highly dependent on the evolving oxide chemistry that changes from In-oxide (In2O3)- to As-oxide (As2O3/As2O5)-rich or lean as a result of specific DNA ligand interactions. These changes are readily “read-out” in 2DEG density and mobility changes. The 2DEG electron density changes in relation to the degree of binding of the phosphate backbone to In oxide and the mobility changes in relation to surface coverage. We find that amine groups bind to the As/As oxide and that As oxide-richness can lead to the creation of dense ssDNA brushes associated with changes in the hydrogen bonding network as determined with FTIR.

Bio Sketch
April S. Brown is the John Cocke Professor of Electrical and Computer Engineering at Duke University. Her research focuses on the synthesis of compound semiconductors for device applications. She is a Fellow of the IEEE and the APS.