Templated synthesis of cubic crystalline single networks

“Single networks with large lattice parameters (>240 nm) were synthesized by cross-linking inorganic precursors within the open network of the polymer cubosomes.”

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In 2014, we published our first study on the solution self-assembly of dendritic-linear block copolymers (BCPs) into inverse bicontinuous cubic phases. At that time, these highly crystalline BCPs structures, identical to those of cubic lipid membranes, had rarely been studied. Since our first publication, we have focused on the structural analysis and functionalization of colloidal particles (polymer cubosomes) and monoliths of inverse bicontinuous cubic mesophases of BCP. The mesophases of BCP bilayers internalize two non-intersecting networks of water channels arranged in the cubic crystalline order. We realized that cubic BCP mesophases possess an identical crystalline symmetry to those of cubic lipid mesophases and complex biological membranes. In our journey to these complex but symmetrical structures, we came across the fact that biophotonic crystals such as butterfly wing scales and beetle cuticles are single network skeletal structures of chitin, whose synthesis is templated by ordered smooth endoplasmic reticulum (OSER), a complex biological membrane internalizing two non-intersecting cubic networks. We began new experiments to determine how biophotonic single cubic networks are created using the OSER as a template, which could lead to a new synthetic route to photonic crystals and metamaterials composed of a single diamond network, by simple templated synthesis with polymer cubosomes.

We expected that our synthetic OSER analogues could serve as templates for the formation of cubic crystalline single networks, and likewise, the biosynthesis of photonic crystals, based on the similarity in structure. We had previously synthesized polymer cubosomes (PCs) with a range of lattice parameters (40–100 nm) smaller than those of biophotonic crystals. Since the periodicity and pore size of PCs can be increased by increasing the molecular weights of polymer blocks , we recently synthesized branched-linear diblock copolymers composed of tri-arm poly(ethylene glycol) and a polystyrene of a different length.

First, we synthesized PEG5503-PSn (Mn = 16–18 kDa) for the self-assembly of PCs with cubic crystalline structures. By comparing SEM images of the interface and the internal mesophases of the PC, we inferred that only one network remained open, leaving the other channel network closed to the surroundings. As we surmised , TEM tomography of PC showed that only one channel of the non-intersecting double networks was connected to the surface pores. Based on these results, we investigated the accessibility of the open network of the PCs by backfilling them with the inorganic precursor, to replicate the internal channel network using the sol-gel reaction. After cross-linking, we confirmed the single network morphology of the SiO2 replicas by SEM, TEM, and SAXS experiments.

Secondly, we synthesized a high molecular weight block copolymer, with a molecular weight exceeding 200 kDa, for the formation of single networks with a large open-space lattice to exhibit a photonic bandgap. In this case, PCs were not formed by solution self-assembly due to the high molecular weight. Instead, we used the ‘solvent diffusion-evaporation mediated self-assembly (SDEME)’ method which allows slow diffusion of water into a dioxane solution of BCPs in a humidity chamber. The single networks with large lattice parameters (>240 nm) and structural colours were synthesized using the resulting PCs as templates. The SiO2 replica of the PCs had a blue hue and a UV-vis reflectance peak centered at 450 nm, while the TiO2 replica had an optical bandgap centered at 525 nm.

Finally, we fused polymersomes encapsulating fluorescein and PCs in order to study the fusion induced topological inversion at the interface. The fusion experiment showed that the open channel of PCs closed, and the closed channel opened only at the interface. As a result, the encapsulated fluorescein molecules diffused into the closed channel of the PCs. Our result complements the pre-folding mechanism of the plasma membrane and OSER, which allows the influx of chitin into only one exposed water channel network of the OSER template.

For more details, please read our paper published in Nature Communications.

Kyoung Taek Kim

Associate Professor, Seoul National University