Amperometric detection of DNA hybridization on a gold surface depends on the orientation of oligonucleotide chains

We tested the possibility of amperometric detection of DNA hybridization on a gold surface influenced by the immobilization of oligonucleotide giving different orientations of single stranded DNA relative to the gold surface. The DNA sensor was fabricated by chemisorption of 18-mer oligonucleotide m odified by a phosphorothioate group either at its 3 ' or both 3 ' and 5 ' t erminal. After immobilization of oligonucleotide to the gold support, the s ensor was immersed in 11-mercaptoundecanoic acid (MUA) solution. Further ch emisorption of MUA resulted in approximately 10-fold increase of resistance of the organic layer. Addition of complementary oligonucteotide resulted i n an increase of conductivity for DNA sensor oriented perpendicular to the gold support (DNA with one thiol group), while the conductance decreased fo r DNA sensor with single stranded DNA oriented parallel to the gold support (with DNA modified by thiol groups at both 3 ' and 5 ' terminals). Additio n of non-complementary chain resulted a slight decrease or no change of sen sor conductivity. The hybridization process at both types of DNA orientatio ns is not cooperative and can be described by Langmuir isotherms. The hybri dization event on gold support has been confirmed by mass detection using t he quartz crystal microbalance technique. (C) 2001 Elsevier Science B.V. Al l rights reserved.