Time-resolved second harmonic generation (SHG) is used to investigate molecular kinetics at colloidal nanoparticle surfaces in water for potential applications in drug delivery. In one study, molecular adsorption and transport kinetics of the positively charged molecular dye, malachite green (MG), at the surface of various liposomes under different buffer and salt concentrations are monitored in real time using SHG measurements. MG is observed to transport more rapidly in 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPG and DOPS, respectively) liposomes in citrate buffer solutions with lower ionic strengths due to ion pair formation. Corresponding adsorption isotherms obtained from these SHG measurements show that MG adsorbs at higher densities and with lower free energies of adsorption under higher buffer salt concentrations. However, no adsorption and no transport is observed for MG interacting with trimethyl quinone-1,2-dioleoyl-sn-glycero-3-phosphoethanolamine and 1,2-dioleoyl-sn-glycero-3-phosphocholine (QPADOPE and DOPC, respectively) liposomes. These results highlight the complexity of interactions between molecules, ions, and lipid bilayers. A second set of studies are focused on drug delivery using colloidal plasmonic nanoparticles. MicroRNA is functionalized through a thermally-cleavable linker to spherical gold-silver-gold core-shell-shell nanoparticles which have a plasmon resonance in the near-infrared accompanied by very high photothermal efficiencies. The photothermal cleaving dynamics are determined using time-dependent SHG measurements under power-dependent near-infrared irradiation. In a final study, the growth dynamics of gold nanoparticles are monitored in real time using in-situ SHG. Seed-mediated growth of gold nanoparticles by chemical reduction of gold chloride by hydroquinone and citrate show a rapid increase of SHG upon initiation of the reaction followed by a slow decay with concentration-dependent lifetimes and signal intensities. These SHG time traces are compared to corresponding characterizations from electron microscopy and extinction spectroscopy in order to determine and control the underlying mechanisms involved in these seed-mediated nanoparticle growth dynamics. Overall, time-dependent SHG is shown to be a powerful tool for investigating molecular kinetics at colloidal nanoparticle surfaces for advancing potential drug-delivery applications.

Second harmonic generation studies of nanoparticle-based drug-delivery systems

255th National Meeting (2018)

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