High Resolution DNA Imaging by Dynamic Atomic Force Microscopy: The Effect of the Substrate and Sample Preparation
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High Resolution DNA Imaging by Dynamic Atomic Force Microscopy: The Effect of the Substrate and Sample Preparation Tzu-Chieh Tang, † Carlo A. Amadei, † Matteo Chiesa †* † Laboratory for Energy and NanoScience (LENS), Institute Center for Future Energy (iFES), Masdar Institute of Science and Technology, Abu Dhabi, UAE ABSTRACT Adsorption of charged biomolecules onto atomically flat mica substrates is facilitated by the deposition of metal ions. Despite successfully acting as preferential anchoring sites, the presence of ions on the mica surface also changes its physicochemical characteristics something that is rarely quantified from a nanoscale point of view. In this study the nanoscale physicochemical properties of nickel-functionalized Muscovite mica are investigated by reconstructing the conservative force profile between an atomic force microscopy (AFM) tip and the surface. Various nickel ion concentrations (i.e. 1.0 mM to 20.0 mM) along with different incubation times (30 seconds and 5 minutes) are directly analyzed. Details in the spatial and temporal variations in surface properties due to the ion mediated adsorption of water are presented in details and in light of the binding efficiency of the metal ions. This insight benefits our understanding in the behavior of ion distribution that plays a crucial role in biomolecule imaging using AFM. KEYWORDS: Ion Exchange, Adsorption, Wetting, Sample Preparation INTRODUCTION Freshly cleaved sheets of muscovite mica are frequently employed as the substrate on which biological samples are investigated during atomic force microscopy (AFM) studies in both liquid and air1. Although the atomically flat mica surface has a negative net charge that allows for natural adsorption of charged biomolecules by electrostatic interaction, this is often not sufficient for satisfactory adsorption of biomolecules2. Researchers have developed ion-exchange based strategies to functionalize the mica surface with multivalent cations and enhance the adsorption strength3,4,5. This enhanced adsorption characteristics due to the presence of the cations come with a subtle alteration of the nanoscale physicochemical properties of the mica surfaces and, despite the widespread use of the nickel/mica system by the AFM community, knowledge of its nanoscale physicochemistry remains limited6. Early investigations of the interaction between two cleaved mica surfaces immersed in various metal-ions solutions was carefully carried out with the aid of the surface force apparatus (SFA)7,8,9. The combination of the effect of the shorter range van der Waals attraction and the electrostatic repulsion due to the electrical double layer of counterions in the solution is well described in terms of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. More recently, the original SFA-based work was extended to the investigation of single DNA molecules bound onto mica3,10. Due to the superior lateral resolution, the AFM was complementary to the SFA and the results obtained were successfully interpreted by means of a gene
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