Synthesis and functionalization of poly(ethylene glycol) microparticles as soft colloidal probes for adhesion energy measurements

We report on the synthesis and operation of new soft colloidal probes (SCPs) as sensors for adhesion energy measurements. The measurements involve determination of the thermodynamic work of adhesion using the Johnson-Kendall-Roberts (JKR) approach. To maximize the sensitivity as well as the specificity of adhesion measurements in aqueous media we use highly compliant poly(ethylene glycol) (PEG) microparticles as SCPs. The chemical inertness of PEG offers advantages as probe material, but at the same time complicates the integration of functional groups. Consequently, we focus on the development of a straightforward yet variable surface modification procedure involving radical surface chemistry using benzophenone as the photoinitiator. With this highly versatile method we are able to introduce various functionalities like carboxy or amine groups directly to the PEG network. These functionalized SCPs can then be further modified with more complex structures such as dendritic oligo(amidoamines). With a first set of SCPs, adhesion energies were measured on model surfaces revealing the contributions due to acid-base, electrostatic and hydrophobic interactions in water. We successfully showed that the developed PEG probes allow for the study of contact behaviour without expensive instrumentation and with high sensitivity suitable to detect very weak (biological) interactions.