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 an choring group on the assembly of presynthesized oligonucleotides at a gold surface. The work seeks to advance fundamental insight into issues that imp act the structure and behavior of surface-immobilized DNA layers, as in, fo r instance, DNA microarray and biosensor devices. The present study contras ts 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 th e thiol anchoring group strongly enhances oligonucleotide immobilization, b ut 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 o rganize 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 nota bly with probe length. The decrease is consistent with a less ordered arran gement of the DNA chains, presumably reflecting increasingly polymeric beha vior.