Won Bae Han

Effect of temperature and humidity on coarsening behavior of Au nanoparticles embedded in liquid crystalline lipid membrane.

Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP) membrane was investigated by subjecting the nanoparticle-embedded DOTAP membrane to two different annealing conditions (at 100 °C under no humidity and at 20 °C and 80% relative humidity). Although the coarsening rate was relatively slow because of the low temperature (from 5.6 nm in the as-deposited state to ~7 nm after 30 h), it was identified that at 100 °C without humidity the Au nanoparticles resulted in shape refinement whereas the high humidity at 20 °C induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules from the nanoparticle array and forced the nanoparticles into a tighter area. In the case of the high-humidity sample, the lipid segregation eventually led to extensive coalescence of the Au nanoparticles.

Structure of solid-supported lipid membrane probed by noble metal nanoparticle deposition.

Direct deposition of a noble metal layer onto a solid-supported membrane was proposed as an indirect microscopy tool to visually observe different lipid phases that may develop in the lipid membrane. The method relied on the different permeability of the lipid membrane towards the incident atoms during deposition. Liquid state or structural defects such as phase boundaries, step ledges in a multi-lamellar stack, and pores permitted the metal atoms to penetrate and nucleate inside the membrane whereas rigid gel state was relatively impermeable to the incident atoms, thus enabling visualization of liquid phase or structural defects inside the gel state. Based on the proposed method, we demonstrated the phase states resulting from thermotropic transitions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dioleoylphosphatidylethanolamine (DOPE)/DPPC mixture, and 1,2-dioleoyl-3-trimethylammonium propane (DOTAP). Although the proposed method does not allow in-situ observation of equilibrium states, the method should be an excellent complementary tool for visualizing the lipid phases as the method can resolve fine structural details (up to tens of nanometer scale) as seen in the DPPC membrane while providing macroscopic images (up to several micrometers).

Structure and magnetic properties of low-temperature annealed Ni-Mn-Al alloys

Off-stoichiometric Ni2Mn2−xAlx (x = 0.5–1.2) and Ni2+y(Mn1.3Al0.7)1−y/2, (y = 0.04–0.12) alloys were prepared to locate the composition range which is likely to form the ferromagnetic ordered L21 phase through low-temperature thermal annealing. In the Ni2Mn2−xAlx (x = 0.5–1.2) series, the alloy partially transformed to the ferromagnetic phase in the narrow composition range of x = 0.8–1.1. For the Ni-enriched composition series, Ni2+y(Mn1.3Al0.7)1−y/2, (y = 0.04–0.12), magnetic ordering was not observed and the magnetic susceptibility proportionally decreased with the Ni concentration. It was concluded that moving away from stoichiometric composition would likely increase the disordering of Mn and Al site occupation and is not conducive to achieving ferromagnetism in Ni2MnAl.

Molecular dynamics simulation of interlayer water embedded in phospholipid bilayer.

1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayer with a thin layer of water molecules inserted in the hydrophobic region was simulated at 300K to observe the pore structure formation during escape of the water molecules from the hydrophobic region. The transformation of the water slab into a cylindrical droplet in the hydrophobic region, which locally deformed the lipid monolayer, was prerequisite to the pore formation. If the thickness of the interlayer water was increased beyond a critical value, the local deformation was suppressed as such deformation would rupture the lipid bilayer. Hence, it was demonstrated that the pore structure formation or local permeability of the lipid membrane is closely related to the rigidity of the lipid membrane.

Stabilization of Solid-Supported Phospholipid Multilayer against Water by Gramicidin Addition

It was demonstrated that hydrophobicity of solid supported planar dipalmitoyl phosphatidylcholine (DPPC) phospholipid multilayer can be greatly increased by incorporating a transmembrane protein, gramicidin, into the DPPC membrane. The contact angle of deionized water droplet on the gramicidin-modified DPPC membrane increased from 0° (complete wetting) without gramicidin to 55° after adding 15 mol % gramicidin. The increased hydrophobicity of the gramicidin-modified DPPC membrane allowed the membrane to remain stable at the air/water interface as well as underwater. The Au nanoparticles deposited on the gramicidin-modified DPPC membrane reproduced the characteristic surface plasmon resonance peak after being kept underwater or in phosphate-buffered saline solution for 5 days, attesting to the membrane stability in an aqueous environment. The enhanced underwater stability of the lipid multilayer substantially broadens the potential application of the lipid multilayer which includes biosensing, enzymatic fuel cell, surface enhanced Raman spectroscopy substrate.

Deposition of Metal Nanoparticles on Phospholipid Multilayer Membranes Modified by Gramicidin

A planar dipalmitoyl phosphatidylcholine (DPPC) multilayer phospholipid membrane was structurally modified by introducing a transmembrane protein, gramicidin (up to 25 mol %), to study its effect on the metal nanoparticles deposited on the membrane. Without gramicidin, when 3-nm-thick Ag, Sn, Al, and Au were deposited, the nanoparticles hardly nucleated on the DPPC membrane in rigid gel state (except for Au); however, the gramicidin addition dramatically enhanced the DPPC membrane surfaces affinity for metal atoms so that a dense array of metal (Ag, Sn, and Au) or metal-oxide (Al-oxide) nanoparticles was produced on the membrane surface. The particle sizes ranged from 3 to 15 nm depending on the metal and gramicidin concentration, whereas the particle density was strongly dictated by the gramicidin concentration. The proposed method provides a convenient, generally applicable synthesis route for preparing different metal or metal-oxide nanoparticles on a relatively robust biocompatible membrane.

Effect of plasma etching on photoluminescence of SnOx/Sn nanoparticles deposited on DOPC lipid membrane

The photoluminescence characteristic of the SnO(x)/Sn nanoparticles deposited on a solid supported liquid-crystalline phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine) membrane was probed after plasma etching the nanoparticle monolayer. It was shown that the plasma etching of the nanoparticle surface greatly altered the particle morphology and enhanced the PL effect, especially when the particle size was below 10 nm in spite of strong presence of surrounding carbon. The enhancement mainly stemmed from the growth of a new PL peak due to the additional defect states produced on the nanoparticle surface by the plasma etching. It was also shown that hydrating the SnO(x)/Sn nanoparticles similarly improved the PL response of the nanoparticles as the hydration produced an additional oxygen-rich oxide layer on the particle surface.

Effect of in-situ application of ultrasonic waves during formation of silver nanoparticles embedded in phospholipid membrane

Effect of in-situ application of ultrasonic waves (up to 1 MHz) on the Ag nanoparticles spontaneously produced inside the 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) membrane by thermal evaporation of Ag was studied by placing the membrane on a ultrasonic transducer during the metal deposition. Application of the ultrasonic vibration promoted spatial ordering of the deposited nanoparticles due to the induced phase transition from Lα to HII for DOPE. Arising from the agitation effect, particle size refinement, which depended on the amplitude of the ultrasonic vibration, was observed. It was also shown that a stiff gel state 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane can be made locally permeable to incident Ag atoms by introducing DOPE molecules into the DPPC membrane as the Ag nanoparticles preferentially nucleated in the DOPE-rich region. Application of ultrasonic vibration with increasingly higher amplitude or frequency made the Ag nanoparticles uniformly distributed in the DPP...

Coarsening of Au nanoparticles embedded in liquid crystalline phospholipid membrane

Au nanoparticles embedded in a liquid phospholipid membrane underwent significant morphological changes during heat treatment at a relatively low temperature (below 100 °C) and high humidity conditions. It was shown that the nanoparticles quickly coarsened in a high humidity environment due to the high lateral mobility of the lipid molecules.