Precise quantification of nanoparticle surface free energy via colloidal probe atomic force microscopy

Interfacial interactions of nanoparticles (NPs) in colloids are greatly influenced by the NP surface free energy (SFE). Due to the intrinsic physical and chemical heterogeneity of the NP surface, measuring SFE is nontrivial. The use of direct force measurement methods, such as colloidal probe atomic force microscopy (CP-AFM), have been proven to be effective for the determination of SFE on relatively smooth surfaces, but fail to provide reliable measurements for rough surfaces generated by NPs. Here, we developed a reliable approach to determine the SFE of NPs by adopting Persson's contact theory to include the effect of surface roughness on the measurements in CP-AFM experiments. We obtain the SFE for a range of materials varying in surface roughness and surface chemistry. The reliability of the proposed method is verified by the SFE determination of polystyrene. Subsequently, the SFE of bare and functionalized silica, graphene oxide, and reduced graphene oxide were quantified and validity of the results was demonstrated. The presented method unlocks the potential of CP-AFM as a robust and reliable method of the SFE determination of nanoparticles with a heterogeneous surface, which is challenging to obtain with conventionally implemented experimental techniques.