Self-assembled Monolayers on III-V Semiconductor and Silicon Surfaces

Continuous advancement in bio-recognition is an important goal that can improve our everyday lives. From detecting contamination, to identifying and managing diseases, to better understanding our environment, biosensors provide the means to quantify the living world. Developing new sensors for the analytical detection of biological analytes involves creating devices that are either better than current testing methods or provide novel analyte detection. The goal of the research presented in this dissertation is to develop new electrical biosensors through the direct chemical functionalization of semiconductor surfaces with self-assembled monolayers (SAMs).The monolayers serve as a linker that attracts a target analyte, connecting biology with technology.

I first studied the functionalization of gallium nitride, a promising biocompatable material, with carboxylic acid and silane self-assembled monolayers. Although functionalization was successful, I discovered that the native oxide, to which the monolayers are covalently bound, is soluble in aqueous solutions. The solubility of the oxide was explored for both powdered Ga₂O₃ and solid GaN with its native Ga₂O₃ layer. The powder was found to be soluble in aqueous solutions at around 1-3 ppm.For 1 cm² solid GaN wafers soaked in 5 ml of liquid, the Ga concentration was found to be 0.264 ± 0.05 ppb/mm² for 18 MΩH₂O and 0.100 ± 0.015 ppb/mm² for pH = 7 buffer.

The second portion of my research focused on the modification of aminopropyltrithoxysilane (APTES) monolayers on silicon and III-V nitride multilayered semiconductors for high electron mobility transistors (HEMT's). The APTES monolayers were modified via reductive amination with 3 different aldehydes: 5-bromo-2-hydroxy-3-methoxybenzaldehyde, for APTES binding confirmation, 4-formylbenzo-15-crown-5, to create a cation binding monolayer, and dipicolylaldehyde, to create dipicolylamine (DPA) ligands that coordinate Zn²⁺ and create an anion binding monolayer. Although proving ion capture was unsuccessful, the DPA ligands do coordinate zinc. This creates a positively charged surface that I used for attracting negatively charged DNA origami.