Nano-scale Morphological Gradients

In some applications, for example tribology, cell behavior, protein adsorption or the superhydrophobic effect, features in the nanometer range also play a crucial role. Morphological gradients with topographical features in the nanometer range can be generated via direct assembly of a nanoparticle density gradient by a simple dip-coating process.

Method

In this method, negatively charged silica particles are adsorbed onto a positively charged poly(ethylene imine) (PEI)-coated silicon wafer by means of electrostatic interactions. Since the adsorption is a kinetically controlled process, a particle gradient can be created by exposing different parts of the substrate for increasing times to a colloidal suspension. This is achieved by slowly immersing the substrate into a colloidal suspension of silica nanoparticles, leading to a gradual increase in particle density along the substrate surface (Fig. 1).

Fig. 1: AFM tapping mode images of different positions on particle-density gradient over 10mm. The different positions on the gradient are indicated in the sketch in the lower right corner of the ﬁgure. AFM images are 5 x 5 µm2 in size. — Fig. 1: AFM tapping mode images of different positions on particle-density gradient over 10mm. The different positions on the gradient are indicated in the sketch in the lower right corner of the ﬁgure. AFM images are 5 x 5 µm² in size.

In order to increase the mechanical stability of the particle array and to adjust their shape the particle gradients can be partially sintered into the substrate at temperatures between 1075 and 1200°C. At such temperatures all organic compounds are burnt out, leaving nothing behind but a bare SiO₂ nano-morphology gradient (Fig. 2).

Enlarged view: Fig. 2: SEM images of heat-treated silica particles (73 nm diameter) on a silicon wafer surface. Particles were heat treated at 1075°C (a), 1100°C (b), 1125°C (c), 1150°C (d), 1175°C (e) and 1200°C (f) for 2 hours. With increasing heat treatment temperature the particles start to form a neck where the particle is in contact with the surface. The scale bar is 50 nm. — Fig. 2: SEM images of heat-treated silica particles (73 nm diameter) on a silicon wafer surface. Particles were heat treated at 1075°C (a), 1100°C (b), 1125°C (c), 1150°C (d), 1175°C (e) and 1200°C (f) for 2 hours. With increasing heat treatment temperature the particles start to form a neck where the particle is in contact with the surface. The scale bar is 50 nm.

Cell Adhesion

Rat calvarial osteoblasts also adsorb gradually onto a nano-featured morphological gradient, as shown in Figure 3.

Enlarged view: Fig. 3: Fluorescence images of cell morphology at different positions on a particle-density gradient. With decreasing particle density, cells formed well-constituted focal adhesions (red) and a distinctive actin network (green). Scalebar is100 µm. — Fig. 3: Fluorescence images of cell morphology at different positions on a particle-density gradient. With decreasing particle density, cells formed well-constituted focal adhesions (red) and a distinctive actin network (green). Scalebar is100 µm.

References

Huwiler, C.; Kunzler, T. P.; Textor, M.; Vörös, J.; Spencer, N. D., external page Functionalizable Nano-Morphology Gradients via Colloidal Self-Assembly. Langmuir 2007, 23(11), 5929-593.
Kunzler, T. P.; Huwiler, C.; Drobek, T.; Vörös, J.; Spencer, N. D., external page Systematic Study of Osteoblast Response to Nanotopography by means of Nanoparticle-Density Gradients. Biomaterials 2007, 28(33), 5000-5006.