Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces
The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments,platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM).In this work, we performed systematic analysis with SAMs having various terminal groups(–OEG, –OH, –COOH, –NH2, and –CH3). The results of the protein adsorption experiment bythe quartz crystal microbalance (QCM) method suggested that having one EG unit and theneutrality of total charges of the terminal groups are essential for protein-resistance. In particular,QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM viaa very weak interaction compared with other SAMs. Contrary to the protein resistance, atleast three EG units as well as the charge neutrality of the SAM are found to be required foranti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate,our force measurements revealed that only the OEG-SAMs possessing more than two EG unitsshowed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs withother terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs wasalways observed independent of solution conditions [NaCl concentration (between 0 and 1 M)and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disruptsthe three-dimensional network of the water molecules. We therefore concluded that the repulsionoriginated from structured interfacial water molecules. Considering the correlation between theabove results, we propose that the layer of the structured interfacial water with a thickness of2 to 3 nm (half of the range of the repulsion observed in the surface force measurements)plays an important role in deterring proteins and platelets from adsorption or adhesion.
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