This letter establishes the manner in which the electric double layer induced by the surface charges of the plasma membrane (PM) enhances the nonspecific adhesion (NSA) of a metal nanoparticle (NP) to stiffer PMs (i.e., PMs with larger bending moduli). The NSA is characterized by the physical attachment of the NP to the membrane and occurs when the decrease in the surface energy (or any other mechanism) associated with the attachment process provides the energy for bending the membrane. Such an attachment does not involve receptor-ligand interactions that characterize the specific membrane-NP adhesion. Here, we demonstrate that a significant decrease in the electrostatic energy caused by the NP-attachment-induced destruction of the charged-membrane-electrolyte interface is responsible for providing the additional energy needed for bending the membrane during the NP adhesion to stiffer membranes. A smaller salt concentration and a larger membrane charge density augment this effect, which can help to design drug delivery to cells with stiffer membranes due to pathological conditions, fabricate NPs with biomimetic cholesterol-rich lipid bilayer encapsulation, etc.
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Counterion release refers to the release of the bound counterions from a charged surface when a macroion or macromolecule binds to that surface.23 Such counterion release (ensuring that the counterions leave the “bounded” state) increases the counterions' mixing entropy and serves as the driving force for this binding process. In the present study, there will indeed be counterion release from the MEI as the NP binds to the membrane. However, given that this process is occurring in a large bulk concentration of the coions and counterions from the salt, this counterion release would only lead to a small change in the overall number of counterions and hence a small change in the overall mixing entropy of the ions. Therefore, the contribution of the counterion release has been neglected in Eq. (1).