Electrochemical Studies of Morpholino-DNA Surface Hybridization.

Surface hybridization, in which nucleic acids from solution bind to complementary "probe" strands immobilized on a solid support, is widely used to analyze composition of nucleic acid mixtures. Most often, detection is accomplished with fluorescent techniques whose sensitivity can be extended down to individual molecules. Applications, however, benefit as much if not more from convenience, accuracy, and affordability of the diagnostic test. By eliminating the need for fluorescent labeling and more complex sample workup, label-free electrochemical assays have significant advantages provided transduction remains sufficiently sensitive for applications. To this end, we have been exploring morpholinos, which are uncharged DNA analogues, as the immobilized probe species in surface hybridization assays based on measurement of interfacial capacitance. Through comparison of experimental trends with those predicted from basic physical models, the origins of diagnostic contrast in capacitive sensing are reviewed for assays based on morpholino as well as on DNA probes.

Immobilization of nucleic acids at solid surfaces: effect of oligonucleotide length on layer assembly.

This report investigates the effect of DNA length and the presence of an anchoring group on the assembly of presynthesized oligonucleotides at a gold surface. The work seeks to advance fundamental insight into issues that impact the structure and behavior of surface-immobilized DNA layers, as in, for instance, DNA microarray and biosensor devices. The present study contrasts immobilization of single-stranded DNA (ssDNA) containing a terminal, 5' hexanethiol anchoring group with that of unfunctionalized oligonucleotides for lengths from 8 to 48 bases. Qualitatively, the results indicate that the thiol anchoring group strongly enhances oligonucleotide immobilization, but that the enhancement is reduced for longer strand lengths. Interestingly, examination of the probe coverage as a function of strand length suggests that adsorbed thiol-ssDNA oligonucleotides shorter than 24 bases tend to organize in end-tethered, highly extended configurations for which the long-term surface coverage is largely independent of oligonucleotide length. For strands longer than 24 bases, the surface coverage begins to decrease notably with probe length. The decrease is consistent with a less ordered arrangement of the DNA chains, presumably reflecting increasingly polymeric behavior.

Selectively Swollen Films of Triblock/Diblock Copolymer Blends: Dependence of Swollen Film Structure on Blend Composition.

Compositional variation in blends of triblock and diblock copolymer films can be used to adjust the film response to a selective solvent. We investigated the relationship between blend composition and film structure in ordered films containing poly(styrene-b-2-vinylpyridine) (PS-P2VP) diblocks and PS-P2VP-PS triblocks. The study focuses on films possessing a lamellar morphology. Methanol, a strongly selective solvent for P2VP, is used to swell the films. Since methanol solvates P2VP but not PS, periodic multilayer structures result in which solvent-rich P2VP domains are separated by undissolved PS domains. The film structure is characterized in the dry and swollen states with neutron reflectivity. Although the dry state morphology dimensions are practically identical for all samples, in the swollen state films richer in triblock swell less due to higher density of bridges interconnecting the PS domains. Furthermore, in swollen triblock-containing samples, polymer concentration variations in P2VP domains are suppressed and the PS domains are better aligned with respect to the substrate.