Time-resolved second
harmonic generation (SHG) spectroscopy is
used to investigate acetaminophen (APAP)-induced changes in the adsorption
and transport properties of malachite green isothiocyanate (MGITC)
dye to the surface of unilamellar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes in an aqueous colloidal
suspension. The adsorption of MGITC to DOPC liposome nanoparticles
in water is driven by electrostatic and dipole–dipole interactions
between the positively charged MGITC molecules and the zwitterionic
phospholipid membranes. The SHG intensity increases as the added MGITC
dye concentration is increased, reaching a maximum as the MGITC adsorbate
at the DOPC bilayer interface approaches a saturation value. The experimental
adsorption isotherms are fit using the modified Langmuir model to
obtain the adsorption free energies, adsorption equilibrium constants,
and the adsorbate site densities to the DOPC liposomes both with and
without APAP. The addition of APAP is shown to increase MGITC adsorption
to the liposome interface, resulting in a larger adsorption equilibrium
constant and a higher adsorption site density. The MGITC transport
times are also measured, showing that APAP decreases the transport
rate across the DOPC liposome bilayer, especially at higher MGITC
concentrations. Studying molecular interactions at the colloidal liposome
interface using SHG spectroscopy provides a detailed foundation for
developing potential liposome-based drug-delivery systems.
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