Interfacial assembly of a phospholipid Langmuir monolayer by electrodeposited silver colloids

Two-dimensional nanostructured silver films were electrodeposited at the surface of a silver nitrate subphase coated by a negatively charged dimyristoylphosphatidylglycerol (DMPG) Langmuir monolayer. The modifications of the phospholipid interfacial organization generated by the growing colloidal silver film were investigated using surface pressure-time isotherms and grazing incidence X-ray diffraction experiments (GIXD). A decrease in the initial surface pressure of the DMPG monolayer is observed outside of the growing silver film, followed by a stabilization of the surface pressure when the radius of the metallic layer reaches its plateau value. This behavior is attributed to the compression of the DMPG molecules above the silver film and to the correlated relaxation and expansion of those outside the silver film area as recorded by a Wilhelmy pressure sensor. GIXD experiments further evidenced the contraction of the phospholipid monolayer above the electrochemically growing films. Indeed, the diffraction spectra show a shift in the peak position toward higher values of the in-plane component of the wave-vector transfer, indicating a closer packing of the DMPG alkyl chains. This is also in agreement with the observed loss of the chain tilt angle, suggesting that the colloidal silver film induces interfacial structuring of the DMPG monolayer.