Seed - Self-assembly of PEG Based Nanoparticle Assemblies Using Molecular Recognition
Jennifer Cha, IBM Almaden Research Center
With the many current techniques available for synthesizing well-defined inorganic nanostructures, there have been numerous efforts to build methodologies for assembling these nanomaterials over multiple length scales and dimensions. The predominant molecular interactions in biological systems are noncovalent in nature, examples including those that occur between amino acids in the active sites of enzymes or between two complementary strands of DNA. Cha and Frank have explored using similar noncovalent interactions such as directional hydrogen bonding to cooperatively assemble three-dimensional ordered nanoparticle structures starting from completely ionized polymers and nanomaterials that individually demonstrate no such macroscopic phase separation. Recently, they have synthesized a series of rigid-flexible A-B diblock copolypeptides, poly-(ethyleneglycol-L-lysine)n-block-poly(L-lysine), in order to investigate the role of secondary structure on polymer nanoparticle based assemblies. Since poly(ethylene glycol-L-lysine) is known to form stable alpha helices (Figure 3) and is nonionic, it can only affect the overall assembly of the final structure. Furthermore, because the interactions between the lysine groups and the nanoparticles are charge based and can be easily degraded in acidic environments, one of the motivations behind assembling these PEG based polymer nanoparticle composite structures is the possible use of such materials for targeted delivery applications, such as in vivo quantum dot labeling (Figure 4).
Figure 3. Circular dichroism spectra showing helical structures.
Figure 4. General scheme for quantum dot labeling.
Through dynamic light scattering measurements and TEM analyses, Cha and Frank discovered that the poly-(EG-L-lysine)n-lockb-(L-lysine)m diblock copolypeptides produced nanoparticle assemblies that were dramatically lower in size from those made when using poly-L-lysine alone. They furthermore demonstrated that a vesicle-to-spherical micelle transition occurs when moving from the homopolyelectrolyte to the diblock system. It was also discovered that increasing the ratio of the length of the poly(EG-L-lysine) block relative to that of the poly(L-lysine) caused a decrease in the dimensions of the polymer-nanoparticle micelles and ultimately nanoparticle micelles of both 60nm and 25nm in diameter were produced (Figure 5).
Figure 5. Bimodal population of nanoparticle micelles.
