Self-Assembly of Lubricin (PRG-4) Brushes on Graphene Oxide Affords Stable 2D-Nanosheets in Concentrated Electrolytes and Complex Fluids.

Graphene oxide (GO) and other 2D-nanosheet materials exhibit a range of useful physical, chemical, electrical, and optical properties that may usher in the next generation of biomedical, bioimaging, or sensing technologies. One limitation of GO is its poor stability in concentrated electrolytes and other complex fluids that requires steric stabilizers, including surface grafted polymers, to overcome attractive van der Waals interactions. Here, we describe a simple, rapid, and highly effective method of modifying GO and other 2D-nanosheets with thick, grafted (bio)polymer brushes via the solution self-assembly of lubricin (LUB; a.k.a. PRG4), an antiadhesive glycoprotein. Atomic force microscopy (AFM) imaging and force measurements were used to characterize the morphology and nanomechanical response of these LUB–GO, 2D-nanosheet complexes (2D-NSC). These characterization studies reveal a strong correlation between the GO surface area and the thickness (i.e., molecular extension) of the grafted LUB brush caused by edge free volume effects. Likewise, this edge free volume influences the extension of the LUB brush structure more than 300 nm away from the edge, resulting in a transition region of increasing brush extension before reaching a fully extended state within the central regions of the 2D-NSC. Fitting AFM normal force measurements using an adapted Alexander–de Gennes polymer brush model also indicate that the edge free volume leads to a mechanical softening of the LUB brush due to the lateral spreading and/or deflection of LUB molecules under compression. Finally, the stability studies of 2D-NSCs dispersed in concentrated electrolyte solutions demonstrate the effectiveness of the grafted LUB brushes at inhibiting aggregation even in the harshest environments. 2D-NSCs thus represent a simple solution to modifying nanosheets with thick, multifunctional brushes with promising application in biosensing, bioimaging, catalysis, and biolubrication, where nanosheets must perform in concentrated electrolytes or complex fluids. [ABSTRACT FROM AUTHOR]