Neutron reflectivity study of substrate surface chemistry effects on supported phospholipid bilayer formation on [formula omitted] sapphire

Abstract

Oxide-supported phospholipid bilayers (SPBs) used as biomimetic membranes are significant for a broad range of applications including improvement of biomedical devices and biosensors, and in understanding biomineralization processes and the possible role of mineral surfaces in the evolution of pre-biotic membranes. Continuous-coverage and/or stacked SPBs retain properties (e.g., fluidity) more similar to native biological membranes, which is desirable for most applications. Using neutron reflectivity, we examined the role of oxide surface charge (by varying pH and ionic strength) and of divalent Ca2+ in controlling surface coverage and potential stacking of dipalmitoylphosphatidylcholine (DPPC) bilayers on the (112¯0) face of sapphire (α-Al2O3). Nearly full bilayers were formed at low to neutral pH, when the sapphire surface is positively charged, and at low ionic strength (I=15mM NaCl). Coverage decreased at higher pH, close to the isoelectric point of sapphire, and also at high I⩾210mM, or with addition of 2mM Ca2+. The latter two effects are not additive, suggesting that Ca2+ mitigates the effect of higher I. These trends agree with previous results for phospholipid adsorption on α-Al2O3 particles determined by adsorption isotherms and on single-crystal (101¯0) sapphire by atomic force microscopy, suggesting consistency of oxide surface chemistry-dependent effects across experimental techniques.