Detection of some amino acids with modulation-doped and surface-nanoengineered GaAs Schottky P-I-N diodes

Most current techniques for analyzing amino acids require substantial instrumentation and significant sample preprocessing. In this study, we designed, fabricated, and tested a scalable diode-based microdevice that allows for direct sensing of amino acids. The device is based on modulation-doped GaAs heterostructure with a Schottky contact on one side. The relatively high mobility and relatively small dielectric constant of GaAs are naturally helpful in this problem. We also paid attention to a proper etching procedure allowing for substantial modification of the surface properties, thereby further boosting the sensing performance. Transport data (I-V, differential conductance) are presented for three qualitatively different classes of amino acids (i.e., nonpolar with aliphatic R-group, polar uncharged R-group, and charged R-group) with glycine, cysteine, and histidine as specific examples, respectively. The conductance for the GaAs-amino acid interface measured using a scanning tunneling microscope (STM) was previously reported to have distinct spectral features. In this paper, we show that measuring the differential conductance of a GaAs diode, whose surface is in direct contact with an aqueous solution of amino acid, is a simple methodology to access useful information, previously available only through sophisticated and equipment-demanding STM and molecular electronics approaches. Density functional theory calculations were used to examine which adsorption processes were likely responsible for the observed surface conductance modification. Last, in future and ongoing work, we illustrate how it might be possible to employ standard multivariate data analysis techniques to reliably identify distinct (95$%$⁠) single amino acid specific features in near-ambient differential conductance data.