Adsorption behavior of peptide nucleic acid (PNA) and DNA decamers (GTAGATCACT and the complementary sequence) on a mercury surface was studied by means of AC impedance measurements at a hanging mercury drop electrode. The nucleic acid was first attached to the electrode by adsorption from a 5-microliter drop of PNA (or DNA) solution, and the electrode with the adsorbed nucleic acid layer was then washed and immersed in the blank background electrolyte where the differential capacity C of the electrode double layer was measured as a function of the applied potential E. It was found that the adsorption behavior of the PNA with an electrically neutral backbone differs greatly from that of the DNA (with a negatively charged backbone), whereas the DNA-PNA hybrid shows intermediate behavior. At higher surface coverage PNA molecules associate at the surface, and the minimum value of C is shifted to negative potentials because of intermolecular interactions of PNA at the surface. Prolonged exposure of PNA to highly negative potentials does not result in PNA desorption, whereas almost all of the DNA is removed from the surface at these potentials. Adsorption of PNA decreases with increasing NaCl concentration in the range from 0 to 50 mM NaCl, in contrast to DNA, the adsorption of which increases under the same conditions.
Electrochemical nucleic acid biosensors have received considerable attention for the detection of biological agents. Control of the surface chemistry and coverage of the electrode transducer are essential for enhancing the performance of electrochemical DNA biosensors, and particularly for maximizing the hybridization efficiency and minimizing of nonspecific adsorption events. This review highlights recent advances and progress in the development of new surface chemistry approaches based on novel ternary DNA self-assembled monolayer (SAM)-interfaces using dithiols as new co-adsorbents/backfillers. The design and characterization of these powerful ternary SAM interfaces on different gold transducers are described, along with their implications to electrochemical biosensing of bioagents.
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