An atomic force microscopy study of ordered molecular assemblies and concentric ring patterns from evaporating droplets of collagen solutions

Ordered molecular assemblies spontaneously appeared from evaporating droplets of collagen solutions on glass substrates. Nanoscopic structures of the assemblies were examined using an atomic force microscope; well aligned collagen molecules oriented their ends toward the assembly surface and formed layered structures for concentrations >0.05 mg/mL, while the molecules, mostly curved, lay for concentrations <0.005 mg/mL. The layers exhibited a variety of morphological forms (e.g., regular, undulated, and rolled patterns) indicating their flexibility. For concentrations >5 mg/mL, concentric ring patterns additionally appeared in the assemblies; each ring consisted of multilayers of well-aligned tilting collagen molecules. The color of the rings alternatingly changed from blue to orange when using a color-sensitive plate. The ring width increased from 3 to 120 mu m with increasing collagen concentration and also increased toward the center of the patterns. The observations of temporal and spatial evolution of the concentric ring patterns with a polarizing-light microscope showed the repetitive appearance of the multilayered region which underwent a subsequent transition into a pair of blue and orange concentric rings near the droplet edge during evaporation. The formation time of one concentric ring was dependent on collagen concentration; it was 37-50 s at an early stage of the formation process.