Chemical lift-off lithography (CLL) is a subtractive soft-lithographic technique used to pattern self-assembled monolayers (SAMs) containing functional alkanethiolates on Au surfaces. Alkanthiolate molecules, along with a single monolayer of Au, are removed in this process by oxygen-plasma activated polydimethysiloxane (PDMS) stamps. Monolayers patterned via CLL are robust, in contrast to conventional soft lithography, enabling high-fidelity patterning over a dynamic range of feature sizes (millimeter to nanometer). Recently, I have advanced CLL to pattern additional surfaces, including other coinage metals (e.g., Pt, Pd, Ag, Cu), reactive and transition metals (e.g., Ni, Ti, Al), and semiconductors (e.g., Ge). This process has a wide range of applications, including patterning of bioactive molecules and supported metal monolayers, as well as the simple, convenient fabrication of field-effect transistors (FETs).
I have designed and developed FET biosensors coupled to rationally designed and chemically synthesized oligonucleotide sequences with molecular recognition capabilities, termed aptamers, for the detection of a diverse range of biologically important small-molecule targets (e.g., neurotransmitters, amino acids, sugars, lipids). Upon target capture, aptamers undergo conformational changes that redistribute charge densities on FET surfaces resulting in measurable changes in transconductance. These aptamer-FET sensors detected targets in full ionic strength biological fluids, with high specificity and selectivity, over large concentration ranges with low detection limits.
Using this platform, I have developed sensors for the direct detection of the amino acid phenylalanine for potential point-of-care use for patients with phenylketonuria (PKU), a common genetic disorder that results in elevated and potentially harmful levels of phenylalanine in the blood and brain. Phenylalanine aptamer-FETs showed high sensitivity and selectivity towards phenylalanine versus similarly structured molecules, and were able to differentiate physiological phenylalanine concentrations in serum from mice treated with a phenylalanine hydroxylase inhibitor (i.e., a PKU mouse model). In parallel, I have extended this FET biosensor platform for the detection and discrimination of single base-pair mismatches in oligonucleotides towards electronic single-nucleotide polymorphism genotyping. I demonstrated the ability of DNA-functionalized FETs to distinguish hybridization between fully complementary DNA sequences, and analogous sequences with different types and locations of mismatches. Performing FET measurements under physiologically relevant conditions and elucidating sensing mechanisms is necessary for ultimate use in disease diagnostics and clinical point-of-care applications.