DiatomInspired Microfluidic Generation of Tunable Emulsions for Macroporous

Diatom-Inspired Microfluidic Generation of Tunable Emulsions for Macroporous Silica Frank J. Zendejas, Dr. Uthara Srinivasan Jay D. Keasling, and Roger T. Howe University of California at Berkeley Project RTH/JDK 3 © 2006 University of California Prepublication Data Spring 2006

Diatoms: • Unicellular, eukaryotic photosynthetic brown algae • Grows ornate silica shells using sea water under benign conditions – 5 m – 500 m size – Pore sizes = 10’s nm to 5 m Traditional route to porous silica: • Strongly acidic or alkaline conditions • High temperatures (T >400 ºC) • Non-ideal conditions for biological species Nature’s inspiration to porous silica Motivation for biomimetic paths to silica precipitation: • Near neutral p. H (p. H ~ 7) • Ambient temperature • Benign for incorporation of biological species © 2006 University of California Prepublication Data Spring 2006

Large interest in engineered porous materials: – On chip filters/bio-separators » Protein purification Porous Membrane Cells ~(10 -100) m Bacteria ~(1 -10) m Proteins (nm) Diatom-Inspired Membrane – High surface area catalysts » Enzyme immobilization – Built in synthesis devices » Self repairing devices © 2006 University of California Pacific Northwest National Laboratory http: //www. pnl. gov Prepublication Data Spring 2006 Mesoporous silica support

Biomimetic Synthesis • Traditional route to porous silica: – Strongly acidic or alkaline conditions – High temperatures (T >400 ºC) • Recently, biomimetic paths to silica precipitation have been discovered: – Near neutral p. H (p. H ~ 7) – Ambient temperature – Benign for incorporation of biological species © 2006 University of California Prepublication Data Spring 2006

Emulsions and Emulsion Templating: • Use microfluidic chip to create emulsion with tunable droplet size and uniformity • Confine the droplets for close-packing • Template silica around emulsion to make custom porous material Water phase Oil phase Silica Step 1. Use chip to form a monodispersed emulsion © 2006 University of California Step 2. Using synthetic catalyst and silicic acid form silica gel around the emulsion droplets Step 3. Remove emulsion droplets to form macroporous silica Prepublication Data Spring 2006

Process Flow 1. Silicon substrate 2. Front side lithographic patterning 3. 30 -120 m Deep Reactive Ion Etching (DRIE) 4. Strip front side mask 5. Strip backside mask and 6. Anodic bond cover slip to and pattern backside prepare substrate for cover substrate for through hole DRIE slip bonding by immersing in of fluidic ports (ports © 2006 University (4: 1) of H California Prepublication Data Spring 2006 2 SO 4 / H 2 O 2 located outside image)

Fabricated Chip Dimensions: – – Chip area = 1 cm 2 Channel depth = 30 - 120 m Orifice Width = 10 - 43. 5 m Orifice Length = 105 - 210 m An SEM of our Flow Focusing chip design adapted from [Anna]. © 2006 University of California Prepublication Data Spring 2006

Gelation Process Flow Silica Gelation d Occurs . 1. Collect oil-in-water monodispersed emulsion droplets in a vial Ethanol 3. Ethanol exchange of the oil droplets for 24 hrs 4. Remove gel from vial and section off a thin slice © 2006 University of California 2. Introduce silicic acid and biocatalyst into water phase and wait for silica gelation and then age at 50 ºC overnight 5. Place sectioned piece on silicon substrate and calcinate for 4 hrs at 400 ºC Prepublication Data Spring 2006

Emulsion-Templated Gel • The mixture gels in ~7 min at room temperature, neutral p. H (~7) • Templated gels show monodisperse pore size Macroporous silica gels templated around monodisperse chip-generated emulsions with average drop diameters: a) 120 µm, b) 48 µm, c) 34 µm, and d) 11 µm. © 2006 University of California Prepublication Data Spring 2006

Emulsion-Templated Silica • Calcining gels at 400°C for 8 h produces glassy macroporous silica • A densification of ~50 -70% is observed SEM micrographs of emulsion-templated porous silica: a) average pore diameter 17 µm, corresponding to ~70% shrinkage from the original droplet dimensions b) average pore diameter 5. 6 µm, corresponding to ~53% shrinkage. Close-ups of the pores are shown in the inset SEMs. © 2006 University of California Prepublication Data Spring 2006