Formation of organized adsorption layers by amphiphilic dendrimers

Adsorption and Aggregation Properties of Gemini-Type Amphiphilic Dendrimers

Gemini-type amphiphilic dendrimers featuring two dodecyl chains and two poly(amidoamine) dendrons (2C₁₂-2denGn, where n is the dendron (den) generation (G) number (1, 2, 3, 4, or 5)) were synthesized using ethylenediamine, alkyl bromide, and methyl acrylate. These gemini-type dendrimers were characterized by surface tension, pyrene fluorescence, static light scattering, and small-angle X-ray scattering. The results showed clear breakpoints in the surface tension versus concentration plots, which indicated adsorption at the air/water interface and micelle formation in solution despite the bulky dendron structure (e.g., generations 3 to 5), contrasting the behavior of conventional surfactants. The 2C₁₂-2denGn dendrimers could be densely packed at the air/water interface owing to enhanced interaction between the dendrons and between the alkyl chains. Furthermore, these dendrimers formed spherical micelles at a concentration of 5.0 mmol dm^-3 in solution (pH 9); the overall micelle size was not dependent on the generation number of the dendron, while as the generation number increased, the core radius of the micelle decreased, and the shell thickness of the micelle increased.

Tweaking dendrimers and dendritic nanoparticles for controlled nano-bio interactions: potential nanocarriers for improved cancer targeting

Nanoparticles have shown great promise in the treatment of cancer, with a demonstrated potential in targeted drug delivery. Among a myriad of nanocarriers that have been recently developed, dendrimers have attracted a great deal of scientific interests due to their unique chemical and structural properties that allow for precise engineering of their characteristics. Despite this, the clinical translation of dendrimers has been hindered due to their drawbacks, such as scale-up issues, rapid systemic elimination, inefficient tumor accumulation and limited drug loading. In order to overcome these limitations, a series of reengineered dendrimers have been recently introduced using various approaches, including: (i) modifications of structure and surfaces; (ii) integration with linear polymers and (iii) hybridization with other types of nanocarriers. Chemical modifications and surface engineering have tailored dendrimers to improve their pharmacokinetics and tissue permeation. Copolymerization of dendritic polymers with linear polymers has resulted in various amphiphilic copolymers with self-assembly capabilities and improved drug loading efficiencies. Hybridization with other nanocarriers integrates advantageous characteristics of both systems, which includes prolonged plasma circulation times and enhanced tumor targeting. This review provides a comprehensive summary of the newly emerging drug delivery systems that involve reengineering of dendrimers in an effort to precisely control their nano-bio interactions, mitigating their inherent weaknesses.

Supramolecular Self-Organization of “Janus-like” Diblock Codendrimers: Synthesis, Thermal Behavior, and Phase Structure Modeling

We report on the design and synthesis of three series of segmented amphiphilic block codendrimers, and on their self-organizing behavior in liquid-crystalline mesophases. Connecting two prefunctionalized monodendrons, each differing in their chemical constitution and generation number, yielded these diblock supermolecules. One wedge of the codendrimer was made hydrophobic, and is based on a branched poly(benzyl ether) monodendron functionalized at the periphery by lipophilic aliphatic fragments (also known as Percec dendrons). The other segment was made hydrophilic by the grafting of hydroxyl-containing moieties onto the focal functions of the former dendrons. Both types of dendrons were prepared independently by convergent methods and then joined in the ultimate stage of the synthetic procedure by cross-coupling reactions. In this way, the proportion of the dendritic blocks was varied independently to allow control of the hydrophilic/hydrophobic balance (HHB), the hydrogen-bonding ability, and consequently the capacity to tune the mesomorphic properties of the resulting "superamphiphiles" was anticipated. Essentially all the dendritic compounds display a thermotropic mesomorphism directly at or near room temperature as determined by using X-ray diffraction, polarized optical microscopy, and differential scanning calorimetry. The nature and the supramolecular organization of the mesophases, namely columnar and cubic phases, are correlated to the size of the respective block monodendrons and the chemical structures of the dendromesogens. The molecular organization within the cubic phases can be geometrically described and well understood by the space-filling polyhedron model.

Fabrication of metal nanoparticle monolayers on amphiphilic poly(amido amine) dendrimer Langmuir films

A newly designed 1.5th generation poly(amido amine) dendrimer with an azacrown core, hexylene spacers, and octyl terminals was spread on gold nanoparticle (Au-NP) suspension. The surface pressure-area isothermal curves indicated that the molecular area of dendrimer on Au-NP suspension was significantly smaller than that on water, indicating the formation of dendrimer/Au-NP composites. The dendrimer Langmuir films on the Au-NP suspension were transferred to copper grids at various surface pressures and observed by transmission electron microscopy. The transferred films consisted of a fractal-like network of nanoparticles at low surface pressure and of a defect-rich monolayer of nanoparticles at high surface pressure. From these results, it was suggested that the dendrimers bind Au-NPs, and dendrimer/Au-NP composites formed networks or monolayers at the interface. From the intensity decrease of the Au plasmon band of Au-NP suspension after the formation of composite, it was estimated that some (approximately 14) dendrimer molecules bind to one Au-NP. Furthermore, neutron reflectivity at the air/suspension interface and X-ray reflectivity of the film transferred on a silicon substrate revealed that the dendrimer molecules are localized on the upper-half surface of Au-NP. Metal affinity of azacrown, flexibility of hexylene spacer, and amphiphilicity of dendrimer with octyl terminals played important roles for the formation of dendrimer/Au-NP hybrid films. The present investigation proposed a new method to fabricate the self-assembled functional polymer/nanoparticle hybrid film.

Adsorption behaviors of poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers on solid substrates

Adsorption behaviors of functional poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers were investigated on gold and self-assembled monolayer (SAM) by means of time course attenuated total reflection-surface enhanced infrared absorption and surface plasmon resonance spectroscopies. While 1.5th and 2.5th generation (G1.5 and G2.5) ester-terminated dendrimers were slightly adsorbed on all substrates examined, the adsorption of G2 amine-terminated dendrimer increased in the order dodecanethiol SAM<bare gold<3-mercaptopropionic acid (MPA) SAM. The time course results also revealed that the G2 dendrimer displayed the steep increase of adsorption on the bare gold, followed by the slight progress of adsorption. G2 dendrimers adsorbed on MPA SAM involved their subsequent conformational rearrangement, while the adsorption equilibrium was quickly attained for G3 dendrimer on the MPA SAM. It was supported that G3 dendrimer was adsorbed with the perpendicularly extended structure on the MPA SAM substrate, and G2 dendrimer varied its conformation from the globular structure to the extended one perpendicular to the substrate.

Analytical investigation of two-step adsorption kinetics on surfaces

Analytical equations of two-step adsorption kinetics on surface have been derived. Moreover, computer simulations have been carried out to interpret various experimental adsorption kinetics previously reported. In the first case, molecules are further adsorbed from a solution onto a layer consisting of previously adsorbed molecules. This model was applied to the adsorption kinetics of hexadecyltrimethylammonium chloride (C16TAC) on a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (T. Imae, H. Torii, J. Phys. Chem. B 104 (2000) 9218). The second case is that some of the initially adsorbed molecules are released from the adlayer with further time course. The adsorption of C16TAC on 1-dodecanethiol SAM (T. Imae, T. Takeshita, K. Yahagi, Stud. Surf. Sci. Catal. 132 (2001) 477) agrees with this mechanism. The strict mathematical developments presented in this work are demanded to specify the physical meaning of observed non-Langmuir adsorption kinetics, consisting of the two exponential terms.

Direct force measurements between adlayers consisting of poly(amidoamine) dendrimers with primary amino groups or quaternary ammonium groups

Adsorption of poly(amidoamine) generation 3 (PAMAM G3) dendrimer with surface amino groups or PAMAM G0 dendrimer with quaternary ammonium groups (C8qbG0) onto glass has been studied by colloidal probe atomic force microscopy. The adlayer-adlayer interactions for these adsorbates are quite different despite the fact that they are almost equal in the hydrodynamic radius. In aqueous PAMAM G3 dendrimer solutions the electrostatic repulsion is predominant. The conformation of the adsorbed layer is flat and the protrusion of the individual dendrimers is negligible. On the other hand, C8qbG0 behaves as a surfactant and the layered structure of C8qbG0 is expected to be a patchy bilayer. Dispersion stability of silica suspensions with the adsorption of these dendrimers can be correlated with the force data obtained.