Nam S. Lee

Dynamic Cylindrical Assembly of Triblock Copolymers by a Hierarchical Process of Covalent and Supramolecular Interactions

We have developed a hierarchical process that combines linear triblock copolymers into concentric globular subunits through strong chemical bonds and is followed by their supramolecular assembly via weak noncovalent interactions to afford one-dimensionally assembled, dynamic cylindrical nanostructures. The molecular brush architecture forces triblock copolymers to adopt intramolecular interactions within confined frameworks and then drives their intermolecular interactions in the mixtures of organic solvent and water. In contrast, the triblock copolymers, when not preconnected into the molecular brush architectures, organize only into globular assemblies.

Multicompartment Polymer Nanostructures with Ratiometric Dual-Emission pH-Sensitivity

Pyrazine-labeled multicompartment nanostructures are shown to exhibit enhanced pH-responsive blue-shifted fluorescence emission intensities compared to their simpler core-shell spherical analogs. An amphiphilic linear triblock terpolymer of ethylene oxide, N-acryloxysuccinimide, and styrene, PEO(45)-b-PNAS(105)-b-PS(45), which lacks significant incompatibility for the hydrophobic block segments and undergoes gradual hydrolysis of the NAS units, underwent supramolecular assembly in mixtures of organic solvent and water to afford multicompartment micelles (MCMs) with a narrow size distribution. The assembly process was followed over time and found to evolve from individual polymer nanodroplets containing internally phase segregated domains, of increasing definition, and ultimately to dissociate into discrete micelles. Upon covalent cross-linking of the MCMs with pH-insensitive pyrazine-based diamino cross-linkers, pH-responsive, photonic multicompartment nanostructures (MCNs) were produced. These MCNs exhibited significant enhancement of overall structural stability, in comparison with the MCMs, and internal structural tunability through the cross-linking chemistry. Meanwhile, the complex compartmentalized morphology exerted unique pH-responsive fluorescence dual-emission properties, indicating promise in ratiometric pH-sensing applications.

Tuning core vs. shell dimensions to adjust the performance of nanoscopic containers for the loading and release of doxorubicin

Detailed studies were performed to probe the effects of the core and shell dimensions of amphiphilic, shell crosslinked, knedel-like polymer nanoparticles (SCKs) on the loading and release of doxorubicin (DOX), a widely-used chemotherapy agent, in aqueous buffer, as a function of the solution pH. Effects of the nanoparticle composition were held constant, by employing SCKs constructed from a single type of amphiphilic diblock copolymer, poly(acrylic acid)-b-polystyrene (PAA-b-PS). A series of four SCK nanoparticle samples, ranging in number-average hydrodynamic diameter from 14-30 nm, was prepared from four block copolymers having different relative block lengths and absolute degrees of polymerization. The ratios of acrylic acid to styrene block lengths ranged from 0.65 to 3.0, giving SCKs with ratios of shell to core volumes ranging from 0.44 to 2.1. Although the shell thicknesses were calculated to be similar (1.5-3.1 nm by transmission electron microscopy (TEM) calculations and 3.5-4.9 nm by small angle neutron scattering (SANS) analyses), two of the SCK nanoparticles had relatively large core diameters (19±2 and 20±2 nm by TEM; 17.4 and 15.3 nm by SANS), while two had similar, smaller core diameters (11±2 and 13±2 nm by TEM; 9.0 and 8.9 nm by SANS). The SCKs were capable of being loaded with 1500-9700 DOX molecules per each particle, with larger numbers of DOX molecules packaged within the larger core SCKs. Their shell-to-core volume ratio showed impact on the rates and extents of release of DOX, with the volume occupied by the poly(acrylic acid) shell relative to the volume occupied by the polystyrene core correlating inversely with the diffusion-based release of DOX. Given that the same amount of polymer was used to construct each SCK sample, SCKs having smaller cores and higher acrylic acid vs. styrene volume ratios were present at higher concentrations than were the larger core SCKs, and gave lower final extents of release., Higher final extents of release and faster rates of release were observed for all DOX-loaded particle samples at pH 5.0 vs. pH 7.4, respectively, ca. 60% vs. 40% at 60 h, suggesting promise for enhanced delivery within tumors and cells. By fitting the data to the Higuchi model, quantitative determination of the kinetics of release was made, giving rate constants ranging from 0.0431 to 0.0540 h⁻¹/² at pH 7.4 and 0.106 to 0.136 h⁻¹/² at pH 5.0. In comparison, the non-crosslinked polymer micelle analogs exhibited rate constants for release of DOX of 0.245 and 0.278 h⁻¹/² at pH 7.4 and 5.0, respectively. These studies point to future directions to craft sophisticated devices for controlled drug release.

Hierarchically Assembled Theranostic Nanostructures for siRNA Delivery and Imaging Applications

Dual functional hierarchically assembled nanostructures, with two unique functions of carrying therapeutic cargo electrostatically and maintaining radiolabeled imaging agents covalently within separate component building blocks, have been developed via the supramolecular assembly of several spherical cationic shell cross-linked nanoparticles clustered around a central anionic shell cross-linked cylinder. The shells of the cationic nanoparticles and the hydrophobic core domain of the anionic central cylindrical nanostructure of the assemblies were utilized to complex negatively charged nucleic acids (siRNA) and to undergo radiolabeling, respectively, for potential theranostic applications. The assemblies exhibited exceptional cell transfection and radiolabeling efficiencies, providing an overall advantage over the individual components, which could each facilitate only one or the other of the functions.

Influence of Nanostructure Morphology on Host Capacity and Kinetics of Guest Release

Nano-objects have gained wide interest as potential drugdelivery systems due to their unique properties that combine the internal capacity for loading cargo, an external surface area for the conjugation of ligands to direct their interactions with biological receptors, and dimensions that allow for extended biological circulation, together with their exceptional manipulability in size, shape, and the capacity to encapsulate hydrophobic or hydrophilic drugs. Today, there are several nanoscopic therapeutic systems clinically approved for usage and many more in various phases of clinical trials. [ 1 ] For example, SMANCS is a clinically approved polymer–protein conjugate between poly(styreneco -maleic acid) (PSco -MA) and neocarzinostatin (NCS). [ 2 ] Typically used as a formulation with Lipiodol, the therapeutic effect of the macromolecule relies on passive targeting to tumors by utilizing the enhanced permeability and retention (EPR) effect. DOXIL and Abraxane are also clinically approved, passive-targeting cancer chemotherapeutics that consists of PEGylated liposomes/doxorubicin (DOX) and albumin proteins loaded with paclitaxel, respectively. The former encapsulates water-soluble DOX within the interior of the liposome while the latter allows water-insoluble paclitaxel molecules to reside within the hydrophobic pockets of albumin. Both examples share two critical traits: the nanoscopic size of the drug–polymer composite and stealthy characteristics. The hydrodynamic diameter of DOXIL particles is ca. 90 nm and 110 nm for Abraxane particles (see Supporting Information (SI)). PEGylation provides the drug-encapsulating liposome a stealthy character in vivo and albumin is abundant in serum and, therefore, naturally uninteresting to the immune system. These simple, passivetargeting drug-delivery agents have found success in treating

A fundamental investigation of cross-linking efficiencies within discrete nanostructures, using the cross-linker as a reporting molecule

Various bi-functional pyrazine-based chromophores were used as cross-linkers to probe directly the efficiencies of their incorporation into the shell of block copolymer micelles. In addition, the block copolymer micelles were made to carry pre-installed reactive functionalities along the central block of an amphiphilic triblock copolymer. Unique photo-physical characteristics were observed, depending upon the type of pyrazine cross-linker, the conditions used for cross-linking and the stoichiometries applied.

Paclitaxel-loaded SCK nanoparticles: an investigation of loading capacity and cell killing abilities in vitro.

Block copolymer nanoparticles having two different hydrodynamic diameters (120 nm vs 50 nm) and core diameters (60 nm vs 20 nm) with variable paclitaxel loading (5 to 20 wt % with respect to polymer weight, 4.4 μg/mL to 21.7 μg/mL paclitaxel concentrations in ultrapure water) were prepared for their in vitro cytotoxicity evaluation. Empty nanoparticles did not show any inherent cytotoxicity even at their highest concentration, whereas paclitaxel-loaded nanoparticles resulted in IC50 values that were better than free paclitaxel at 2 h (0.021 μM vs 0.046 μM) incubation periods, and approximately equal to free paclitaxel at 72 h (0.004 μM vs 0.003 μM) continuous incubation. Confocal fluorescence microscopy images demonstrated that the drug-loaded nanoparticles internalized into KB cells within 2 h and released their payload, resulting in cytotoxicity as evident from the fragmented nuclei present. Functionalization of the nanoparticle surfaces with poly(ethylene oxide) (2 kDa PEO, 5 PEO per block copolymer chain) did not affect the loading of paclitaxel or cell kill ability. No free paclitaxel was found in these nanoparticle formulations indicated by analytical assays.

Tunable dual-emitting shell-crosslinked nano-objects as single-component ratiometric pH-sensing materials

Dual-emitting photonic nano-objects that can sense changes in the environmental pH are designed based on shell-crosslinked micelles assembled from amphiphilic block copolymers and crosslinked with pH-insensitive chromophores. The chromophoric crosslinkers are tetra-functionalized pyrazine molecules that bear a set of terminal aliphatic amine groups and a set of anilino amine groups, which demonstrate morphology-dependent reactivities towards the poly(acrylic acid) shell domain of the nano-objects. The extent to which the anilino amine groups react with the nano-object shell is shown to affect the hypsochromic shift (blue-shift). The ratio of fluorescence intensity at 496 nm over that of 560 nm is dependent upon the solution pH. We report, herein, observations on the pH-sensitive dual-emission photophysical properties of rod-shaped or spherical nano-objects, whose shell domains offer two distinct platforms for amidation reactions to occur-through formation of activated esters upon addition of carbodiimide or pre-installation of activated ester groups. We demonstrate that physical manipulations (changes in morphology or particle dimensions) or chemical manipulations of the crosslinking reaction (the order of installation of activated esters) lead to fine tuning of dual-emission over ca. 60 nm in a physiologically relevant pH range. Rod-shaped shell-crosslinked nanostructures with poly(p-hydroxystyrene) core show blue-shift as a function of increasing pH while spherical shell-crosslinked nanostructures with polystyrene core and poly(ethylene oxide) corona exhibit blue-shift as a function of decreasing pH.

Individual Nano-Objects Obtained via Hierarchical Assembly of Polymer Building Blocks

This chapter emphasizes synthetic methodological approaches toward complex nano-objects for which the term ‘complex’ is defined as ‘originating from several parts’. Those parts are polymer building blocks that are assembled hierarchically to achieve final nanoscopic structures that could not be constructed from small molecules in a single step. The state of the art has evolved from primitive methods that demonstrate control over the size of individual nanoscopic objects to elaborate schemes that exert fine-tuning of the composition, structure, and properties of multifunctional nanoscopic devices. The assembly, stabilization, and manipulation of polymers in solution, in the bulk, and in the presence of templates are discussed, with various examples being presented to demonstrate the roles of the polymer structure and the conditions applied to afford diverse types of nanostructured materials.