Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy.

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.

Observation of removal of an Fmoc protecting group by scanning tunneling microscopy.

We demonstrate here by imaging successive surface reactions in self-assembled monolayers (SAMs) on Au(111) at molecular scale with a scanning tunneling microscope (STM): (i) SAM matrices formation with 1-octanethiol on Au(111) in ethanol, (ii) insertion of N-Fmoc-aminooctanethiol into the SAM matrices in ethanol, and (iii) removal of the Fmoc protecting group with tris(2-aminoethyl)amine (TAEA). The total reaction is formation of SAMs containing a small amount of NH2 terminated molecules in the CH3 terminated SAM matrices. After the reaction of the protecting group with TAEA, STM imaging revealed the decrease in heights of the inserted molecules on average. We attributed this observation to removal of the protecting group by taking account of a convolution of electronic and topographic contributions to observed STM heights. Apparent areas of the terminal groups, however, became larger on removal. The increase in the areas was attributed to water adsorption to the NH2 terminal group under air.

Imaging stretched single DNA molecules by pulsed-force-mode atomic force microscopy.

The effect of a surface water layer on DNA strands deposited on a substrate was studied by atomic force microscopy (AFM). DNA molecules were deposited and stretched on chemically modified glass coverslips by a molecular combing method. Lambda bacteriophage DNA molecules were aligned on the organosilane-modified substrate surfaces by chemical and physical adsorption during the molecular combing. The combed DNA molecules were observed in humidity-controlled air and in aqueous solutions by pulsed-force-mode AFM (PFM-AFM). Chemical modification of cantilevers with an Au-coated tip by organothiol compounds was also applied to DNA observation. Mapping adhesive forces in aqueous media was useful to discriminate chemically the DNA strands from the substrate surface. The results suggest that PFM-AFM can be used widely to image the stretched DNA molecules on the silane-modified substrates.

Near-field fluorescence imaging and simultaneous observation of surface potentials.

We present a new detection method to measure simultaneously surface potential and fluorescence intensity distributions using a combined scanning near-field optical microscope-atomic force microscope (SNOM-AFM). A surface potential image of phospholipid monolayers was obtained in non-contact mode using the SNOM-AFM with a thin-step etched optical fibre probe. For applying this technique, a phospholipid of dipalmitoylphosphatidylethanolamine labelled at the head with a nitrobenzoxadiazole group was used as a fluorescent and single component Langmuir-Blodgett film. It is well known that aggregation of the lipid molecules and their fluorescence intensities are very sensitive to its environmental conditions such as humidity and temperature. We demonstrated for the first time the near-field optical imaging and simultaneous observation of surface potentials with Maxwell stress microscopy.