Highlights

Nanofluidics transport in lab-on-chip devices requires nanochannels that are difficult to fabricate since they require challenging top-down technological approaches. We present a bottom-up, scalable, low-cost and robust alternative to construct large areas of mesofluidic Pillared Planar Nanochannels (PPNs). Microscopy images displayed in Fig. 1 are representative of the whole system, and show the typical PPN layers made of dense and mesoporous materials organized into vertical nanopillars supporting a continuous roof. Such conformation is homogeneously present all over the coated substrate. They have been obtained upon combining self-assembly of block copolymer, nanostructured sol-gel coatings and highly controlled liquid deposition processing. The mesoporous structure contains two types of porosity. The first one is the fully open and accessible inter-pillar porosity with characteristic dimensions adjustable between 20 and 400 nm, which is necessary to allow facile diffusion despite the double layer effect. The second one is present inside the pillars and the roof, and is composed of less than 10 nm pores, which aim at increasing the surface area. Both layer structures have been characterised by Grazing Incidence Small Angle X-ray Scattering. Pillar ordering can be assessed by GISAXS analysis as shown in Fig. 2 for F127-templated titanosilicate systems. The intense Bragg diffraction signal at q_y = 0.14 nm^-1, together with its first harmonic at q_y = 0.28 nm^-1, is associated to both (01)p and (02)p diffractions of the highly ordered 2D lateral hexagonal structure formed by the pillar array with a corresponding inter-pillar distance of 51 nm (d(01) = 44 nm). The latter dimensions can be found on TEM and SEM images whatever the type of inorganic material used for impregnation. The fact that the (01)p diffraction has a vertical tail (diffraction rod) extending only in the z direction, suggests a high degree of vertical alignment of the pillars. For the F127-templated titanosilicate layers, the GISAXS diagram (Fig. 2) exhibits, together with the previous (01)p diffractions of the pillars at low q, the characteristic (110)m, (101)m and (1-10)m diffraction points of the Im3m mesostructure, revealing that the nanoporous network is composed of a contracted body centre cubic arrangement of pores with extended domains having the [110] direction normal to the surface.

The lateral periodicity is found to be 14.5 nm, which corresponds to the structure obtained for plain continuous films, for which pores sizes were deduced from ellipsometry porosimetry to be around 6 nm. The high order structure is clearly observed in SEM and TEM, see the corresponding images in Fig.s 1c)-f). Interestingly, the SEM image c) reveals that the pillars do not have the same texture as the roof. The TEM image of Fig.s 1f, 1g, shows that the pillars are actually hollow cylinder (tubes) below the roof.
These Pillared Planar Nanochannel showed the ability to vehicle fluids in the inter-pillars porosity through natural capillary forces, for which the classical Washburn model of diffusion is verified. In addiction PPNs are compatible with lithography techniques, such as deep X-ray lithography, for the production of complex designs and thus demonstrating to be ideal candidates for micro/nano fluidic applications.


Figure 1.  Pillared Planar Nanochannels layers with various chemical compositions, morphologies, and pillar and roof inner structures. a) and b) are SEM (profile cut) and TEM (top view) images respectively of a dense TiO₂ PPN. Images c) to f) were taken from a similar system for which the material is composed of mesoporous 10%SiO₂-90%TiO₂. SEM images c) to e) display profile cuts, where one can easily observe the pillars supporting a roof of different thicknesses (obtained with various sol-gel deposition condition), but all bearing the same ordered Im3m mesoporosity induced by the F127.  TEM images f) and g) are top views of the layer and reveal a clear emptiness of the pillars. The latter are organised into hexagonal organisation. Scale bars = 50 nm for a), b), and f); 100 nm for c), d), and e); and 5 nm for g).

Figure 2. GISAXS pattern of Im3m mesoporous (F127 templated) 10%SiO2-90%TiO2 mesofluidic Pillared Planar Nanochannels.

Retrieve article

Bottom-up approach toward titanosilicate mesoporous pillared planar nanochannels for nanofluidic applications;
M. Faustini, M. Vayer, B. Marmiroli, M. Hillmyer, C. Sinturel, H. Amenitsch and D. Grosso;
Chem. Mater. 22, 5687 (2010).
10.1021/cm101502n