Atomic Force Microscopy Characterization of a Microcontact Printed, Self‐Assembled Thiol Monolayer for Use in Biosensors

Abstract

This paper describes friction experiments and pull‐off force measurements using atomic force microscopy (AFM), between a nonfunctionalized silicon probe and a 2.5 µm 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) · 10⁻⁹ N, (2.68±1.09) · 10⁻⁸ N, and (4.60±0.24) · 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.

Keywords:

• Microcontact printing
• self‐assembled monolayer
• atomic force microscopy

This work has been supported by the Spanish government MEC (Ministerio de Educación y Ciencia) through the projects PCI 76/03‐04, HF2005‐0055, NAN2004‐09415‐C05‐01 and TEC2005‐07996‐C02‐021MIC. The authors wish to acknowledge Dr. E. Martínez from Nanotechnology Platform of Barcelona Science Park for her helpful and insightful comments concerning this subject.