<p>The first part of this study details the formation and characterization of octadecanethiol (ODT) self-assembled monolayers (SAMs) on GaAs (100) substrates from solution and vapor phases. The liquid-phase-deposited monolayers were prepared by immersing the substrate in an ethanolic solution, while vapor-deposited monolayers were prepared by the vapor phase transport of ODT in an ultrahigh vacuum (UHV) environment. The structural and optical properties of the resulting SAMs were examined with contact angle (CA) analysis, photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), high-resolution x-ray photoelectron spectroscopy (HRXPS), and spectroscopic ellipsometry. Although well-ordered films were formed by both preparation routes, PL, AFM, CA analysis, HRXPS, and ellipsometry measurements revealed that the overall quality, structure, and durability of the mono layers depend on the deposition technique. Collectively, the results suggested that more robust monolayers exhibiting greater surface coverage and therefore, increased passivation and stability characteristics are assembled from vapor phase. </P> <P> The second part of this work describes the development of a hybrid GaAsaptamer biosensor for the label-free detection of analytes. The implemented sensmg strategy relies on the use of functional alkanethiol SAMs as biorecognition elements as well as the sensitivity of the GaAs PL emission to the local environment at its surface. Specifically, GaAs substrates were modified with thiol-derivatized aptamers and exposed to the target biomolecules. The resulting modification in the PL intensity is attributed to a specific aptamer-target biorecognition interaction and the accompanying ligand-induced structural change in the aptamer conformation. Modeling the performance data by means of Poisson-Boltzmann statistics in combination with the dead layer model indicates a good correlation between the structural conformation of the aptamers and the GaAs PL yield. The results demonstrate the potential of the prospective luminescence-based GaAs-SAM biosensor in real-time sensing assays requiring a simple and effective means of direct analytical detection. </p> / Thesis / Doctor of Philosophy (PhD)