Compression of polyethylene glycol chains grafted onto silicon nitride surface as measured by scanning force microscopy

Abstract

Monomethoxypolyethylene glycol (PEG) with a molecular weight of 2000 Da has been grafted to a silicon nitride surface that has been silanized with 3-aminopropyltriethoxysilane (APS). A scanning force microscope (SFM) was used to measure a force—distance profile between the PEG-modified surface and the unmodified silicon nitride SFM tip. In 0.1 M KNO 3 solution the force increased monotonically with distance, showing no adhesion between the SFM probe and the PEG-modified surface. The monotonically increasing force is interpreted as a steric repulsion force. Increasing solution concentration of MgSO 4 caused the appearance of a weak attractive force between the SFM probe and the PEG-modified surface. The magnitude of the attractive force increased with increasing MgSO 4 solution concentration. The addition of MgSO 4 generated poor solvent conditions for grafted PEG chains and eliminated the steric repulsion force, probably through a collapse of the tethered chains. The interpretation of the MgSO 4 effect on the tethered PEG chains is hindered by the fact that surfaces treated with APS alone caused a cantilever instability at a separation distance of approximately 25 nm. An attractive force at this large separation distance suggested that APS treatment resulted in a surface coating thicker than a monolayer.