Atomic force microscopy characterization of a microcontact printed, self-assembled thiol monolayer for use in biosensors

This paper describes friction experiments and pull-off force measurements using atomic force microscopy (AFM), between a nonfunctionalized silicon probe and a 2.5 mu m diameter CH3 and COOH terminated thiol self-assembled monolayer pattern. The pattern is microcontact printed onto a gold-coated silicon wafer, in air, at room temperature, with a relative humidity around 30%, and used to examine probe-monolayer interactions. Atomic force microscopy imaging reveals that the patterns have been successfully reproduced on the substrate surface. We obtained force values of (8.67 +/- 2.60) center dot 10(-9) N, (2.68 +/- 1.09) center dot 10(-8) N, and (4.60 +/- 0.24) center dot 10(-8) N for CH3 terminated alkyl-thiol, COOH terminated thiol, and gold substrate respectively. Normalizing these values with the tip radius we obtained (0.87 +/- 0.27) N/m for CH3 terminated alkyl-thiol, (2.68 +/- 1.10) N/m for COOH terminated thiol, and (4.60 +/- 2.50) N/m for bare gold. These interactions are discussed in terms of the chemical affinity between the probe and the substrate.