Organic and Biological Molecular Layers on Functionalised Sensor Surfaces Studies with Atomic Force Microscopy

Biological and organic molecular layers related to two specific sensor surfaces have been studied with Atomic Force Microscopy (AFM). Besides AFM imaging, scratching experiments, where parts of a molecular layer are physically removed by the AFM tip have been performed. The AFM measurements have been supplemented by electrochemistry, fluorescence- and radioactive-labelling experiments. The two sensors are a cantilever-based sensor developed in the BioProbe project at MIC, and a blood-gas sensor developed at Radiometer Medical A/S.In the cantilever-based sensor system short, single stranded DNA (ssDNA) molecules have been immobilized on ultra-flat gold surfaces, whereafter mercaptohexanol (MCH) and the complementary ssDNA strands have been introduced to the surface to obtain hybridization. The gold surfaces used are not the actual gold surfaces used in the sensor, but have been fabricated by evaporating gold onto mica to meet the demands of the AFM measurements.AFM measurements indicate that MCH can release unspecific bonds between thessDNA molecules and the gold surface, resulting in a molecular layer where all molecules bind to the gold surface through their thiol-group. When the complementary ssDNA strand is introduced, changes in the appearance of the surface indicate that hybridization has occurred. The occurrence of hybridization is supported by an increase in the thickness of the molecular layer.AFM measurements on polymer surfaces related to the blood-gas sensor indicate that blood proteins adsorb strongly to the polymer surfaces, and that the adsorbed layer can reduce the adsorption of blood cells and platelets to the surfaces.During AFM measurements of gold surfaces immersed in clean water, spontaneously formed nanobubbles have been observed on both contact and tapping mode AFM images. A model of the forces between a nanobubble and an AFM tip shows that if the tip is sharp and hydrophilic, nanobubbles can be imaged as protrusions on AFM images, even if the tip has penetrated the surface of the bubbles. By combining imaging with force measurements, the real dimensions of the bubbles have been estimated.