AmericanChemicalSociety.com

1. Project Progress

With gradual addition of water into the DMF solution of PS-b-QP2VP-b-PE, typical opalescence appeared, indicating the formation of aggregates. After dialysis to completely remove DMF, the size of micellar aggregates was determined by dynamic light scattering (DLS), which showed an average hydrodynamic diameter of 41 nm (Chart 1a). The structure of aggregates was further visualized by transmission electronic microscopy (TEM) as shown in Chart 1b. Because of the hydrophobic driving force in the aqueous solution, hydrophobic PS formed the core with hydrophilic quaternized QP2VP and PEO as corona. The concentration of the micelle solution was adjusted to 1.0 mg/mL for further complexation with ssDNA.

Chart 1.  (a) Hydrodynamic diameter distributions of quaternized PS-b-P2VP-b-PEO micelles aqueous solutions; (b) TEM image of typical micelles from PS-b-QP2VP-b-PEO.

With addition of 1.0 mL ssDNA solution (0.1 mg/mL) into 0.5 mL triblock copolymer micelle solution (1.0 mg/mL) by syringe pump over 24 h, the suspension started to precipitate out. The TEM image in Chart 2a clearly showed the formation of ultrathin films. Moreover, at high amplification (Chart 2b), no obvious phase separation was observed.

Chart 2.  TEM images of typical unltrathin films obtained after adding the ssDNA solution into the micelle solution.

To understand the mechanism for the formation of ultrathin films, the intermediate sample at two hours was investigated. Clearly, cylindrical aggregates comprised of several fused spherical micelles are seen in Chart 3a. The length of them was above 100 nm. After stained by RuO₄, the superstructure network appeared more clearly with the arrowed in Chart 3b showing that two micelles merged together to form a new aggregate. The further proof of a coarse-grained film, in which the part of polystyrene still maintains as micelles in the film, quaternized P2VP and PEO chains merge together to form the framework of the ultrathin film,  was also provided by TEM image (Chart 3c and d). Moreover, some parts of PS in the film showed the transition from the spherical shape to elliptical and to cylindrical finally.

Thus, we speculate that after addition of ssDNA into the PS-b-QP2PVP-b-PEO solution, one ssDNA with negative charges could permeate through the PEO layer to complex with QP2VP with positive charges. However, the contour length of ssDNA (~8.6 nm) is much longer than that of QP2VP (~ 1.5 nm) and similar to the length of PEO block (~9 nm). Then, this ssDNA chain may have two possibilities. One is to further complex with another QP2VP chain in the same micelle and condense onto it. The other is to extend its chain outward to further complex with another micelle. Once the second possibility happens, two micelles link together via ssDNA, merge and reorganize into cylindrical micelles (Chart 3b). With further aggregation, these cylindrical micelles will merge and reorganize into the ultrathin films (Chart 3c and d).

Chart 3.  TEM images for the samples obtained at 2 h (a) without and (b) with staining using RuO₄; (c) an ultrathin film composed of the triblock copolymer and ssDNA at 24 h. (d) is a high magnification image.

As the volume ratio of ssDNA solution to the triblock copolymer reached 3:1, the TEM observation shows that the ultrathin film disappeared gradually and vesicular structure appeared as shown in Chart 4a. Although still some irregular films were observed, the spherical structure became the main population of the aggregates. Interestingly, by further increasing the volume ratio of the ssDNA solution to the triblock copolymer to 5:1, we found by TEM that there are only spherical aggregates with the diameter about 100 nm and no ultrathin film left (Chart 4b). This is consistence with the dialysis of DLS for the aggregates, which showed 87 nm for the diameter of aggregates (Chart 4d). A further atomic force microscopy (AFM) experiment given (Chart 4c). reveals that the aggregates are circular and the diameter is around 100~150 nm. This size is a little larger than the result given from DLS, indicating that the aggregates collapse on the surface of glass. The height of the aggregates is only 25 nm, suggesting their vesicular structure. It is noteworthy that the contour length of the triblock copolymer is 14 nm (3.5 nm for PS chain, 1.5 nm for QP2VP chain, and 9 nm for PEO chain). Thus, based on the AFM result, we proposed that the vesicles have a bilayer structure, where hydrophobic PS blocks interdigitated to form the inner core of the ultrathin film and hydrophilic QP2VP complexed with ssDNA and PEO forming the outside.

Chart 4.  The aggregates formed at different ratios of ssDNA:PS-b-QP2VP-b-PEO after adding the DNA solution into the triblock copolymer solution over 24 h. (a and b) TEM images and (c) AFM image; (d) Hydrodynamic diameter distributions in PS-b-QP2VP-b-PEO/ssDNA vesicle suspension.

Based on the results above, we supposed the whole self-assembly process in Chart 5. In the initial stage, upon addition of ssDNA into the triblock copolymer solution, micelles begin to link together by ssDNA to form cylindrical aggregates, then further aggregate to form a bilayer thin film and finally form vesicular structure as the result of the increase in volume fraction of the hydrophilic parts of the complex.

Chart 5.  The illustration of the formation of ultrathin films and vesicles from the complexation of PS-b-QP2VP-b-PEO micelles with ssDNA.

2. Program Impact

This project has initiated a new direction of the PI's research in the fields of supramolecular self-assembly of polymeric particles and gene transfection, which may be viable for future grant applications to NSF and NIH. The postdoctoral fellow has been trained to broaden his area from organic synthesis to bio-related science and engineering, and this is expected to be beneficial to his future professional career.

High School Student Research:  During the last summer, a high school student, Ms. Bethany Qiang from Western Reserve Acedamy, joint my group to carry out two-week research.  In this experience, she worked with Dr. Bing Guan on TEM morphology study of the PS-b-QP2VP-b-PEO/ssDNA complexes.