Abstract
DNA-water interactions have revealed as very important actor in DNA mechanics, from the molecular to the macroscopic scale. Given the particularly useful properties of DNA molecules to engineer novel materials through self-assembly and by bridging organic and inorganic materials, the interest in understanding DNA elasticity has crossed the boundaries of life science to reach also materials science and engineering. Here we show that thin films of DNA constructed through the self-assembly of sulfur tethered ssDNA strands demonstrate a Young’s modulus tuning range of about 10 GPa by simply varying the environment relative humidity from 0% up to 70%. We observe that the highest tuning range occurs for ssDNA grafting densities of about 3.5 × 1013molecules/cm2, where the distance between the molecules maximizes the water mediated interactions between the strands. Upon hybridization with the complementary strand, the DNA self-assembled monolayers significantly soften by one order of magnitude and their Young’s modulus dependency on the hydration state drastically decreases. The experimental observations are in agreement with molecular dynamics simulations.
Highlights
In order to determine the number of molecules at the cantilever surface we have performed a quantitative characterization of the DNA film by X-ray photoelectron spectroscopy (XPS)
The presence of nitrogen atoms is typically used as the experimental indicator of adsorbed DNA; since the used buffer in the immobilization is an unspecific source of nitrogen we have chosen the phosphorous as signature indicator
It is possible to calculate the actual thickness of the DNA layer by using the clean Au4f spectrum as reference
Summary
In order to determine the number of molecules at the cantilever surface we have performed a quantitative characterization of the DNA film by X-ray photoelectron spectroscopy (XPS). The signal coming from the gold 4 f peak is attenuated as the immobilization time for the ssDNA is increased. From this attenuation, it is possible to calculate the actual thickness of the DNA layer by using the clean Au4f spectrum as reference. The calculated thicknesses are used to correct the measured XPS peak ratios of the N and P atoms for attenuation. In order to do this, we have to calculate the practical effective attenuation length (PEAL, LAu) for electrons in the film using a reference film, whose thickness we have measured by atomic force microscopy. The relationship between the intensity of the XPS peak, IAu, and the thickness, t, is given by IAu = IAu0exp(−t/LAu
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