C.J. Brinker - Publications and Proceedings

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Modulus–density scaling behaviour and framework architecture of nanoporous self-assembled silicas

Hongyou Fan, Christopher Hartshorn, Thomas Buchheit, David Tallant, Roger Assink, Regina Simpson, Dave J. Kissel, Daniel J. Lacks, Salvatore Torquato, and C. Jeffrey Brinker

Nature Materials, June 2007, vol. 6, p.418-423 [PDF]

Natural porous materials such as bone, wood and pith evolved to maximize modulus for a given density1. For these threedimensional cellular solids, modulus scales quadratically with relative density2,3. But can nanostructuring improve on Nature’s designs? Here, we report modulus–density scaling relationships for cubic (C), hexagonal (H) and worm-like disordered (D) nanoporous silicas prepared by surfactantdirected self-assembly.Over the relative density range, 0.5 to 0.65, Young’s modulus scales as (density)n where n(C) <n(H) <n(D) <2, indicating that nanostructured porous silicas exhibit a structurespecific hierarchy of modulus values D < H < C. Scaling exponents less than 2 emphasize that the moduli are less sensitive to porosity than those of natural cellular solids, which possess extremal moduli based on linear elasticity theory4. Using molecular modelling and Raman and NMR spectroscopy, we show that uniform nanoscale confinement causes the silica framework of self-assembled silica to contain a higher portion of small, stiff rings than found in other forms of amorphous silica. The nanostructure-specific hierarchy and systematic increase in framework modulus we observe, when decreasing the silica framework thickness below 2 nm, provides a new ability to maximize mechanical properties at a given density needed for nanoporous materials integration5.

Logarithm of Young’s modulus versus logarithm of bulk density for self-assembled thin-film nanostructures C, H and D determined by nanoindentation. The power-law relationships of C, H and D films are E∼ ρ0.6 , E∼ ρ1.0 and E∼ ρ1.9, respectively. Modulus values were determined at a constant depth (less than 1/10 of the film thickness) where the modulus versus depth values were constant. This procedure should ensure modulus values comparable to bulk values18. Young’s modulus was calculated from the nanoindentation modulus according to Er = E(1−ν2 ), where E is Young’s modulus and ν is Poisson’s ratio. The standard deviation was determined from the mean often measurements.

Cell-Directed Assembly of Lipid-Silica Nanostructures Providing Extended Cell Viability.

Helen K. Baca, Carlee Ashley, Eric Carnes, Deanna Lopez, Jeb Flemming, Darren Dunphy, Seema Singh, Zhu Chen, Nanguo Liu, Hongyou Fan, Gabriel P. Lopez, Susan M Brozik, Magaret Werner-Washburne, C. Jeffrey Brinker

Science, Jul 21, 2006; vol. 313, p. 337-341[ PDF ] [ Supplementary Info ]

Living cells combine molecular recognition, amplification, and signal transduction in an extremely small ‘package’, making them ideally suited for miniaturized, standalone, environmental or physiological sensors. However, cellular integration into devices is problematic. Cells require functional bio/inorganic interfaces, benign synthesis conditions (1-3), and external fluidic support systems or immersion in buffer to avoid dehydration. Furthermore, as recently noted by Zhang (4), it is necessary to move beyond two-dimensional (2D) adhesion in dishes to 3D architectures that better represent the extracellular matrix, enabling cells to be surrounded by other cells, maintaining fluidic accessibility, and allowing development of 3D molecular or chemical gradients.

Fig. 1. S. cerevisiae organize a lipid-rich shell that interfaces coherently with the surrounding nanostructured silica host. (A) Confocal fluorescence image of immobilized cells with 1% substitution of the fluorescently labeled lipid analog, 1-hexanoyl-2-{6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]hexanoyl}-snglycero-3-phosphocholine (diC6PC-NBD). Brighter areas indicate preferential concentration of lipid around cells compared to the surrounding lipid/ silica host matrix. (B and C) TEM images of cell immobilized within nanostructured lipid/silica matrix by spin-coating directly on holey carboncoatedcopper grid. (D and E) SEM of cells immobilized in silica host prepared with (D) and without (E) lipids. The dark region around the cells in (D) corresponds to an area of high carbon/phosphorus concentration consistent with the presence of lipids (fig S2). In (E), the dark region is a crevice. Cells are firmly immobilized only when the lipid interface is present. In the absence of lipid, cell washout occurs and film cracking is prevalent.

Drying transition of confined water.

Seema Singh, Jack Houston, Frank van Swol, C. Jeffrey Brinker

Nature, Aug 3, 2006; vol. 442 p. 526 [ PDF ] [ Supplementary Info ]

Long-range hydrophobic interactions operating underwater are important in the mediation of many natural and synthetic phenomena, such as protein folding, adhesion and colloid stability. Here we show that rough hydrophobic surfaces can experience attractive forces over distances more than 30 times greater than any reported previously, owing to the spontaneous evaporation of the intervening, confined water. Our finding highlights the importance of surface roughness in the interaction of extended structures in water, which has so far been largely overlooked. The existence of ‘long-range’ hydrophobic interactions has been debated for more than 25 years1–5, because their reported range of 1–100 nanometres exceeds that of van der Waals forces and cannot be explained by water restructuring. However, investigations have been limited to smooth, flat model surfaces4, even though most surfaces are rough. Roughness strongly influences wetting — as evidenced by the high water-contact angles (0 > 160) and rolling of water droplets on lotus leaves6.

Figure 1 | Cavitation between superhydrophobic surfaces. a, Plot of force versus displacement for the underwater approach of a tip towards a flat surface. The experiment starts at zero micrometres relative displacement (arbitrarily defined). The sudden development of a negative (attractive) force at about 0.4 m relative displacement corresponds to cavitation. Contact with the surface is indicated by the inflection at about 2.2 m (arrow). The distance between the onset of adhesion and contact (1.8 m) is the distance over which cavitation occurs for this sample. The inset shows the cavitation geometry: pγ(1/rw 1/rm), where p is the pressure difference across the interface, γ is the liquid– vapour interfacial tension, rm is the radius of the meniscus, rw is the radius of its waist, and rc is the contact radius of the cavity on the tip surface. D is the critical separation below which cavitation is thermodynamically favoured. See also supplementary information. b–e, Optical images of cavitation: b, position of superhydrophobic tip and substrate just before cavitation; c, cavitation occurs about 33 ms later; d, cavity meniscus, as seen during tip retraction, one frame before its unstable collapse; e, a cavity ‘bubble’ is left behind on both the tip and substrate. These bubbles, attributed to air supplied from water and the porous superhydrophobic surface, are unstable and are readsorbed in about 6 seconds. In all frames, the circular image at the bottom is the reflection of the spherical 150-m-diameter tip in the flat superhydrophobic surface.