Many natural proteins self-assemble in complex ways, either to fulfill their biological function or introduce particular properties, such as high strength and toughness. We report the morphological transition in water from a spherical to rod-like shape of Bombyx mori silk fibroin by reducing the pH. Transmission electron microscopy, scanning electron microscopy, and dynamic light scattering were used to characterize the dilute solutions of silk fibroin in an aqueous environment, and provide direct visualization of the transformation of spherical micelles at pH 6.8 to nanofibrils at pH 4.8. This change in morphology occurred as a result of the stretching entropy due to the formation of β-sheets, which was analyzed using circular dichroism spectroscopy. This study demonstrates the selfassembly of silk fibroin as a function of pH.

The hydrogen-bonding induced alternating multilayer thin films of dendrimers and block copolymer micelles were demonstrated. The block copolymer micelles derived from amphiphilic poly(2-ethyl-2-oxazoline)-block-poly(ε-carprolactone) (PEtOz-PCL) in aqueous phase have a core-shell structure with a mean hydrodynamic diameter of 26 nm. The hydrogen bonding between the PEtOz outer shell of micelle and the carboxyl unit of poly(amidoamine) dendrimer of generation 4.5 (PAMAM-4.5G) at pH 3 was utilized as a driving force for the layerby-layer alternating deposition. The multilayer thin film was fabricated on the poly(methyl methacrylate) (PMMA) thin film spin-coated on silicon wafer or glass substrate by the alternate dipping of PEtOz-PCL micelles and PAMAM dendrimers in aqueous solution at pH 3. The formation of multilayer thin film was characterized by using ellipsometry, UV-vis spectroscopy, and atomic force microscopy. The PEtOz outer shell of PEtOz-PCL micelle provided the pH-responsive hydrogen bonding sites with peripheral carboxylic acids of PAMAM dendrimer. The multilayer thin film was reversibly removed after dipping in aqueous solution at pH ≥ 5.6 due to dissociation of the hydrogen bonding between PEtOz shell of PEtOz-PCL micelle and peripheral carboxyl units of PAMAM dendrimer.

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