Size and Cross-Linking Dependent Gold Nanoparticle Interactions with Dynamic Pectin Model Plant Cell Walls

A lack of understanding of the properties that determine nanomaterial-plant cell wall interactions limits the advancement of nanoenabled agriculture. Herein, we integrated experimental and computational approaches to elucidate the role of nanoparticle size and Ca²⁺ cross-linking on the interactions between model plant cell walls composed of pectin and branched polyethylenimine-coated (bPEI) gold nanoparticles (AuNP 6.1, 36.1, and 75.5 nm). Quartz crystal microbalance with dissipation monitoring (QCM-D) and X-ray photoelectron spectroscopy (XPS) analyses demonstrated a size and Ca²⁺-induced cross-linking dependence during the attachment to and passive penetration into model cell walls due to pore size. In the absence of electrolytes, AuNP penetration into the pectin layer occurred with simultaneous expulsion of water for all particle sizes. In the presence of Ca²⁺, pectin cross-linking reduced porosity and blocked penetration of the 75.5 nm AuNPs while unexpectedly increasing the uptake of 6.1 and 36.1 nm AuNPs. Computational simulations showed a reduction in pectin pore size due to Ca²⁺-induced cross-linking that resulted in a high free energy cost of pore expansion. In the presence of Ca²⁺, significant overtone spreading was observed by QCM-D for the 6.1 and 36.1 nm AuNPs that was not observed without electrolytes, indicating that the smaller particles still penetrated through the cross-linked pectin layer, but the process was slower. Upon replacing Ca²⁺ with Mg²⁺, the pectin film was permeable to all three sizes of AuNPs, and less overtone spreading was observed, indicating that cross-linking effects are exclusive to Ca²⁺. Interestingly, a size-dependent induction time not reported before was observed for AuNP attachment to pectin model surfaces. This study elucidated size- and Ca²⁺ cross-linking-dependent physicochemical interactions of nanomaterials with plant cell wall model biosurfaces through both experimental and computational approaches. Nanomaterial-plant model cell wall interaction studies can guide the rational design of nanomaterials for more efficient delivery strategies in agriculture and elucidating their impact on the environment.