Effect of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and phospholipase A2 (PLA2) on surface properties of silica materials

The effect of phospholipid DPPC monolayer on surface properties of powdered quartz (SiO2/DPPCML) and silicon wafer (Si/DPPCML) before and after phospholipase A(2) (PLA(2)) action was determined. DPPC on quartz particles was deposited from chloroform solution and the monolayer on silicon surface by using the Langmuir-Blodgett technique. The surface free energy and its components of powdered quartz before and after contact with PLA(2) in NaCl solution were determined by imbibition-drainage method (I-DM), which is based on the capillary rise method. The probe liquids (water, formamide and diiodomethane) wick the powder bed. The DPPC monolayer supported on silicon after the contact with NaCl or Tris solution (without and with enzyme PLA(2)) was monitored by contact angle measurements of the probe liquids. In order to visualize the changes in the DPPC monolayer structure, its topography images were obtained from AFM. The obtained results show that most evident changes in wettability, i.e. the apparent surface free energy of DPPC monolayer supported on silicon (Si/DPPCML) and treated with PLA(2) occur within the first 15 min after initiation of the hydrolysis. The analysis of the topography images of the phospholipid monolayer clearly reveals that degradation of monolayer occurs. The modification of quartz powder with DPPC resulted in decreased apparent surface free energy, which however increased after the PLA(2) action, up to the value obtained for bare quartz surface. This again revealed the hydrolysis reaction. The influence of PLA(2) on the surface free energy changes of DPPC monolayer deposited on silicon correlated with those observed for quartz powder. These results pointed out a complete hydrolysis of DPPC by PLA(2) enzyme. The AFM images shed more light on the relation between DPPC monolayer topography and the apparent surface free energy changes taking place during the hydrolysis catalyzed by PLA(2). (C) 2015 Elsevier B.V. All rights reserved.