Magnetic AC mode atomic force microscopy (MAC Mode AFM) was used to characterize the process of adsorption of DNA on a highly oriented pyrolytic graphite (HOPG) electrode surface using different concentrations of DNA and adsorption procedures. AFM of DNA immobilized on the HOPG showed that both single-stranded DNA and double-stranded DNA molecules have the tendency to spontaneously selfassemble from solution onto the solid support and the process was very fast. DNA condensed on the substrate in a tight and well-spread two-dimensional lattice covering the entire surface uniformly. The interaction of DNA with the hydrophobic HOPG surface induced DNA superposition, overlapping, and intra-and intermolecular interactions. The application of a positive potential of 300 mV (vs Ag wire) to the HOPG electrode during adsorption was studied. The applied potential considerably enhanced the robustness and stability to mechanical stress of the DNA films, through multiple electrostatic interactions between the negatively charged hydrophilic sugar-phosphate backbone and the positively charged carbon surface. The characteristics of the DNA films and the apparent height of the network wires were dependent on the DNA concentration and the immobilization procedure. The DNA lattices were held together on the substrate surface only by noncovalent interactions such as hydrogen bonding, base stacking, electrostatic, van der Waals, and hydrophobic interactions.10.
Electrode surface characteristics represent an important aspect on the construction of sensitive DNA electrochemical biosensors for rapid detection of DNA interaction and damage. Two different immobilization procedures of double-stranded DNA (dsDNA) at the surface of a HOPG electrode were evaluated by MAC mode AFM performed in air. A thin dsDNA adsorbed film forming a network structure with holes exposing the electrode surface and a thick dsDNA film completely covering the electrode surface, presenting a much rougher structure, were investigated. The DNA surface characteristics and structure are discussed with respect to the degree of surface coverage.
Single-stranded and double-stranded DNA electrochemical biosensors prepared by adsorption during 3 min on HOPG, with or without an applied potential, at pH 5.3 and 7.0, were characterised by MAC mode AFM. During adsorption DNA condenses on the substrate forming complex network films with pores exposing the HOPG surface. The thin films formed in pH 5.3 acetate buffer always presented a better coverage of the HOPG surface with DNA molecules than films formed in pH 7.0 phosphate buffer. The application of a positive potential of 300 mV during adsorption enhanced the robustness and stability of the DNA films with the formation of bigger network holes and a more condensed and compact self-assembled DNA lattice. The knowledge of the morphology of adsorbed DNA on electrode surfaces explains non-specific adsorption on the electrode surface and can be used to improve and develop DNA-electrochemical biosensors.
Guanine adsorbed onto a highly oriented pyrolytic graphite electrode was studied by MAC-Mode Atomic Force Microscopy (AFM), and the electrochemical behaviour of the guanine layer was investigated with Electrochemical AFM. Guanine adsorbs spontaneously, without forming a well-packed structure, into nucleation spots, which are stable with time and cover the surface uniformly and almost completely. The process of guanine adsorption and nucleation can be controlled and the effect of altering the exposure time and varying the potential was investigated. D
Adriamycin adsorbs strongly and irreversibly onto surfaces and this enabled electrochemical detection of in situ adriamycin oxidative damage to DNA. The adsorption of adriamycin onto glassy carbon and highly oriented pyrolytic graphite (HOPG) electrodes was studied by voltammetry and mode atomic force microscopy (MAC). At a glassy carbon electrode (GCE), the adsorbate has similar voltammetric behaviour to adriamycin in solution, which enabled the cyclic, differential pulse and square wave voltammetric study of the electron transfer reaction. The total surface concentration of adriamycin adsorbed onto GCE, from a 50 nM adriamycin solution during 3 min, was calculated to be 2.57)/10 (12 mol cm (2 . The oxidation of adsorbed adriamycin is pHdependent and corresponds to a two electron/two proton mechanism, and the detection limit for adriamycin adsorbed onto the GCE was 3.33)/10 (10 M. In situ AFM images show quick and spontaneous adsorption of the adriamycin onto a HOPG surface. Adriamycin forms a stable monolayer when adsorbed from different concentrations of adriamycin solutions and for short adsorption times. The strong and irreversible chemisorption of adriamycin onto carbon electrodes enables detection limits of the order of picomolar, which is much lower than the detection limits attainable by voltammetric methods for most organic compounds. #
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