Fluorine in medical microbubbles – Methodologies implemented for engineering and investigating fluorocarbon-based microbubbles

Outersphere Approach to Increasing the Persistance of Oxygen-Sensitive Europium(II)-Containing Contrast Agents for Magnetic Resonance Imaging with Perfluorocarbon Nanoemulsions toward Imaging of Hypoxia

Radiographic mapping of hypoxia is needed to study a wide range of diseases. Complexes of Eu(II) are a promising class of molecules to fit this need, but they are generally limited by their rapid oxidation rates in vivo. Here, a perfluorocarbon-nanoemulsion perfused with N2 , forms an interface with aqueous layers to hinder oxidation of a new perfluorocarbon-soluble complex of Eu(II). Conversion of the perfluorocarbon solution of Eu(II) into nanoemulsions results in observable differences between reduced and oxidized forms by magnetic resonance imaging both in vitro and in vivo. Oxidation in vivo occurrs over a period of ≈30 min compared to <5 min for a comparable Eu(II)-containing complex without nanoparticle interfaces. These results represent a critical step toward delivery of Eu(II)-containing complexes in vivo for the study of hypoxia.

Hybrid Fluorocarbon-Hydrocarbon Surfactants: Synthesis and Colloidal Characterization

Four double-tailed hybrid fluorocarbon-hydrocarbon (F-H) surfactants with a poly(ethylene glycol) (PEG) polar headgroup were synthesized. The hydrophobic scaffold consists of an amino acid core, onto which were grafted both fluorocarbon and hydrocarbon chains of different lengths. The PEG polar head was connected to the hydrophobic scaffold through a copper(I)-mediated click reaction. The four derivatives exhibit aqueous solubility >100 g/L and self-assemble into micellar aggregates with micromolar critical micellar concentration (CMC) values, as demonstrated by isothermal titration calorimetry (ITC), surface tension (ST) measurements, and steady-state fluorescence spectroscopy. The CMC value decreased by a factor of ∼6 for each additional pair of CH₂ groups, whereas a decrease by a factor of ∼2.5 was observed when the size of the PEG polar head was reduced from 2000 to 750 g/mol. Dynamic light scattering (DLS) showed unimodal micelle populations with hydrodynamic diameters of 10-15 nm, in agreement with results obtained from size-exclusion chromatography (SEC). The aggregation number increased with the hydrocarbon chain length but decreased with increasing PEG chain lengths. The combination in one molecular design of both low CMC and high water solubility makes these new surfactants promising systems for novel drug-delivery systems.

Langmuir Films of Perfluorinated Fatty Alcohols: Evidence of Spontaneous Formation of Solid Aggregates at Zero Surface Pressure and Very Low Surface Density

In this work, Langmuir films of two highly fluorinated fatty alcohols, CF3(CF2)12CH2OH (F14OH) and CF3(CF2)16CH2OH (F18OH), were studied. Atomic Force Microscopy (AFM) images of the films transferred at zero surface pressure and low surface density onto the surface of silicon wafers by the Langmuir-Blodgett technique revealed, for the first time, the existence of solid-like domains with well-defined mostly hexagonal (starry) shapes in the case of F18OH, and with an entangled structure of threads in the case of F14OH. A (20:80) molar mixture of the two alcohols displayed a surprising combination of the two patterns: hexagonal domains surrounded by zigzagging threads, clearly demonstrating that the two alcohols segregate during the 2D crystallization process. Grazing Incidence X-Ray Diffraction (GIXD) measurements confirmed that the molecules of both alcohols organize in 2D hexagonal lattices. Atomistic Molecular Dynamics (MD) simulations provide a visualization of the structure of the domains and allow a molecular-level interpretation of the experimental observations. The simulation results clearly showed that perfluorinated alcohols have an intrinsic tendency to aggregate, even at very low surface density. The formed domains are highly organized compared to those of hydrogenated alcohols with similar chain length. Very probably, this tendency is a consequence of the characteristic stiffness of the perfluorinated chains. The diffraction spectrum calculated from the simulation trajectories compares favorably with the experimental spectra, fully validating the simulations and the proposed interpretation. The present results highlight for the first time an inherent tendency of perfluorinated chains to aggregate, even at very low surface density, forming highly organized 2D structures. We believe these findings are important to fully understand related phenomena, such as the formation of hemi-micelles of semifluorinated alkanes at the surface of water and the 2D segregation in mixed Langmuir films of hydrogenated and fluorinated fatty acids.

Influence of Semifluorinated Alkane Surface Domains on Phase Behavior and Linear and Nonlinear Viscoelasticity of Phospholipid Monolayers

Semifluorinated alkanes self-assemble into 30-40 nm-large surface domains (hemimicelles) at the air/water interface. They have been drawing increasing attention to stabilize microbubbles coated with lipids, which are used for enhancing the contrast in sonographic imaging. Although previous studies suggested that semifluorinated alkanes increase the stability of phospholipid membranes, little is known about how semifluorinated alkanes influence phase behaviors and mechanical properties of lipid-coated microbubbles. As a well-defined model of microbubble surfaces, we prepared monolayers consisting of a mixture of phospholipids and semifluorinated alkanes at the air/water interface and investigated the influence of hemimicelles of semifluorinated alkanes on the phase behavior and interfacial viscoelastic properties of phospholipid monolayers. Hemimicelles are phase-separated from phospholipids and accumulate at the phase boundary, which strongly modulates the correlation between solid phospholipid domains. Intringuingly, we found that the mixed monolayer of semifluorinated alkanes and phospholipids possesses linear and nonlinear viscoelastic properties comparable to those of phospholipid monolayers. Since the mixing of semifluorinated alkanes and phospholipids enables one to overcome the intrinsically low stability of pure semifluorinated alkanes against the change in the surface area of microbubbles through the partial dissolution of gas into the aqueous phase, this is a promising strategy for the stable coating of microbubbles in ultrasound diagnosis.

Adsorption and Desorption of a Phospholipid from Single Microbubbles under Pulsed Ultrasound Irradiation for Ultrasound-Triggered Drug Delivery

Microbubbles have potential for applications as drug and gene delivery systems, in which the release of a substance is triggered by an ultrasonic pulse. In this paper, we discuss the adsorption and desorption of a film of phospholipid on the surface of a single microbubble under ultrasound irradiation. Our optical observation system consisted of a high-speed camera, a laser Doppler vibrometer, and an ultrasound cell; 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was used as the surfactant. The adsorption of the DMPC molecules onto the surface of the bubble was evaluated by measuring the contact angle between the bubble and a glass plate. A decrease of the contact angle of the bubble indicates desorption of the DMPC molecules from the bubble surface into the surrounding aqueous solution. The amount of DMPC molecules adsorbed on the bubble's surface is shown to decrease over time after bubble generation. The type and intensity of the pulsed ultrasound waves were varied so as to mimic ultrasound-triggered drug release. Increasing the number of cycles and the amplitude of the sound pressure of the pulsed ultrasound yielded a greater increase of the contact angle. We also measured the radial vibrations of the microbubbles in the ultrasound field. The vibrational characteristics of the microbubbles and the desorption characteristics of the DMPC molecules showed the same variation; namely, a greater sound pressure amplitude induced greater vibrational displacement and a larger amount of molecular desorption under resonance conditions. These results support the possibility of controlling drug release with pulsed ultrasound in a microbubble-based drug delivery system.

Dynamics of the phospholipid shell of microbubbles: a fluorescence photoselection and spectral phasor approach

The lipid organization of microbubbles is important in many applications.

Self-Sorting and Coassembly of Fluorinated, Hydrogenated, and Hybrid Janus Dendrimers into Dendrimersomes

The modular synthesis of a library containing seven self-assembling amphiphilic Janus dendrimers is reported. Three of these molecules contain environmentally friendly chiral-racemic fluorinated dendrons in their hydrophobic part (RF), one contains achiral hydrogenated dendrons (RH), while one denoted hybrid Janus dendrimer, contains a combination of chiral-racemic fluorinated and achiral hydrogenated dendrons (RHF) in its hydrophobic part. Two Janus dendrimers contain either chiral-racemic fluorinated dendrons and a green fluorescent dye conjugated to its hydrophilic part (RF-NBD) or achiral hydrogenated and a red fluorescent dye in its hydrophilic part (RH-RhB). These RF, RH, and RHF Janus dendrimers self-assembled into unilamellar or onion-like soft vesicular dendrimersomes (DSs), with similar thicknesses to biological membranes by simple injection from ethanol solution into water or buffer. Since RF and RH dendrons are not miscible, RF-NBD and RH-RhB were employed to investigate by fluorescence microscopy the self-sorting and coassembly of RF and RH as well as of phospholipids into hybrid DSs mediated by the hybrid hydrogenated-fluorinated RHF Janus dendrimer. The hybrid RHF Janus dendrimer coassembled with both RF and RH. Three-component hybrid DSs containing RH, RF, and RHF were formed when the proportion of RHF was higher than 40%. With low concentration of RHF and in its absence, RH and RF self-sorted into individual RH or RF DSs. Phospholipids were also coassembled with hybrid RHF Janus dendrimers. The simple synthesis and self-assembly of DSs and hybrid DSs, their similar thickness with biological membranes and their imaging by fluorescence and (19)F-MRI make them important tools for synthetic biology.

Stable Small Composite Microbubbles Decorated with Magnetite Nanoparticles - A Synergistic Effect between Surfactant Molecules and Nanoparticles

Three approaches to preparing iron oxide nanoparticle-decorated microbubbles (NP-decoMBs) have been investigated. The size and stability characteristics of these microbubbles (MBs) were investigated by optical microscopy, laser light scattering and an acoustical method, and compared with those of non-decorated MBs. First, magnetite nanoparticles (Fe3O4NPs) grafted with dimyristoylphosphatidylcholine (DMPC) were synthesized and used to prepare MBs by brief sonication under an atmosphere of air saturated with perfluorohexane. These MBs had a rather large mean radius (r ~ 12 µm), and a moderate volume of encapsulated gas. Remarkably, a second approach that consisted of dispersing unbound DMPC molecules in the aqueous phase along with DMPC-grafted Fe3O4NPs prior to sonication was found to drastically change the situation, allowing the obtaining of monomodal populations of much smaller (r ~ 0.6 µm) NP-decoMBs. The latter were echogenic and stable for at least 10 days at room temperature, without significant variation of their size characteristics. In a third approach, NP-decoMBs were directly prepared from dispersions of naked Fe3O4NPs in the presence of DMPC. The resulting NP-decoMBs suspensions consisted of broadly distributed bubble populations mostly containing two populations (with r ~ 5 and ~ 15 µm). Control microbubbles made of DMPC only were small (r ~ 1.3 µm), although not as small as those formed from DMPC-grafted Fe3O4NPs in the presence of free DMPC, and were less stable, with a room temperature half-life of only ~1 day. These observations imply that there is a synergy between the Fe3O4NPs and the DMPC molecules in the air/water interfacial film stabilization process.