A scanning probe microscopy study of the physisorption and chemisorption of protein molecules onto carboxylate terminated self-assembled monolayers

Scanning probe microscopy offers the possibility of investigating biomolecular structure and function. However, successful imaging is technically limited by interactions between the probe and the sample. A strong attachment of the biomolecule to the substrate is often required. Here, we investigate the binding of the protein catalase to gold surfaces modified by self-assembled monolayers (SAMs). The chemical and physical adsorption of the protein molecules onto SAMs of 3-mercaptopropanoic acid (3-MPA), 11-mercaptoundecanoic acid (11-MUA) and a mixture of the two acid thiols (Mixed) was investigated utilizing tapping mode atomic force microscopy (AFM), scanning tunneling microscopy (STM) and surface plasmon resonance (SPR). The surface concentration of catalase adsorbed on the SAMs decreased in the order: Mixed > 11-MUA > 3-MPA. Utilizing the terminal carboxylic acid functionalities, catalase was immobilized with a water soluble carbodiimide and N-hydroxysuccinimide (NHS). Immobilization resulted in increased coverage of the protein. SPR studies on silver surfaces modified by these SAMs indicate immobilization of carbodiimide and NHS decreased in the order: Mixed > 11-MUA > 3-MPA.