(550c) Performance Analysis of Morpholino Monolayers: Platforms for Label-Free Detection of DNA Molecules

Surface hybridization, a reaction in which nucleic acid molecules in solution react with nucleic acid partners immobilized on a surface, is widely practiced in life science research. In these applications the immobilized partner, or "probe", is typically single-stranded DNA. Because DNA is strongly charged, high salt conditions are required to enable analyte nucleic acids ("targets") in solution to bind with the densely-immobilized DNA probes. High salt, unfortunately, compromises prospects for label-free monitoring or control of the surface hybridization reaction through surface electric fields. In this work Morpholinos, a class of uncharged DNA analogues, are introduced for surface-hybridization applications. Monolayers of Morpholino probes on gold supports can be fabricated with methods similar to those employed with DNA, and are shown to hybridize efficiently and sequence-specifically with target strands. The impact of hybridization on layer organization, as monitored through the surface capacitance, is greatly enhanced for Morpholino relative to DNA probe layers. The molecular mechanisms underlying the diagnostic contrast are identified and further interpreted through comparison to predictions from numerical Poisson-Boltzmann calculations. Interestingly, positive as well as negative capacitive responses (contrast inversion) to hybridization are possible, depending on surface populations of mobile ions as set by the read-out potential. Response sensitivities of Morpholino layers when hybridizing DNA targets are analyzed under conditions spanning an experimentally-relevant range of probe coverage and ionic strength. Quantitative correlation of surface capacitance with target coverage (targets/area) demonstrates sensitivities comparable to or exceeding those of label-free methods such as surface plasmon resonance.