Determination of elastic-interaction parameters and prediction of the tunnel decay curves. a, Conducting AFM measurements of interaction force (blue points) and simultaneously acquired tunnel current (red points) for A-thymidine with a cantilever of spring constant 0.35 N/m. The inset shows how the extent of the current-distance curves is controlled by the stiffness of the cantilever (black curves, K=2 N/m, red curves, K=0.35 N/m). b, Adhesion force plotted vs. set-point current for a 2-amino-8-mercaptoadenine functionalized probe interacting with a thymidine monlayer (red points) and a 8-mercaptoadenine probe interacting with a thymidine monolayer (blue points). The black dots are control data obtained with thio-phenol functionalized probes. The solid lines are fits to Fad ∝ NHB ln(BISP + 1) + FNS (assuming all bonds are of equal strength) with B set to 2.83 nA−1. The coefficient of the log terms are 1.97±0.3 (AA-T) and 1.0±0.16 (A-T), so their ratio is 1.96 (+0.7, −0.5) a range that spans the expected value of 1.5 (NHB (2AA −T)/NHB (A − T) = 3/2). Representative STM decay curves (red) for an 8-mercaptoguanine probe interacting with a deoxycytidine monolayer (c) and an 8-mercaptoadenine probe interacting with a thymidine monolayer (d) for several set-points (each curve is the average of 26 raw data curves). The blue dashed-lines show how the current should decay from each set point if it follows the form of the 3nA data. The green lines are predicted by the elastic distortion model described in the text where the same two parameters are used to fit both the G-C and A-T data.
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