Sol-Gel Chemistry Chimie douce

diatoms are making silica glasses from solute silica in water Chemists at the school of diatoms diatoms are making silica glasses at room temperature from solute silica in water Chimie douce

Silica from the soils is dissolved by water and goes to the sea Si(OH)4 Fe2O3 SiO2 + 2H2O Si(OH)4 silicic acid Si(OH)4 Si OH HO ≈ mg/l

Silicic acid is a weak acid Proton exchange between Si(OH)4 and the aqueous solvant protonation or deprotonation depending on pH Silicic acid is a weak acid Si OH HO

- Silicic acid is a weak acid OH OH OH Si Si Si O HO O Proton exchange between Si(OH)4 and the aqueous solvant protonation or deprotonation depending on pH Silicic acid is a weak acid pH ≈ 3 OH- - 2- Si OH O Si OH O Si OH HO deprotonation

- Silicic acid is a weak acid + OH Si H2O HO O Proton exchange between Si(OH)4 and the aqueous solvant protonation or deprotonation depending on pH H+ pH ≈ 3 OH- Si OH HO O H2O - 2- + Point of Zero Charge SiO2 precipitation of silica Silica is soluble at high pH silicates

Precipitation of silica [Si(OH)3(OH2)]+ [Si(OH)4]0 [SiO(OH)3]- [SiO2(OH)2]2- 2 9,9 13 pH Precipitation of silica Si(OH) 4 SiO(OH) 3 - SiO 2 (OH) 2- 6 8 10 12 pH 20 40 60 80 100%

Precipitation of silica via the acidification Na2O.SiO2 water glass H+ Precipitation of silica via the acidification of an aqueous solution of silicate Precipitated silica Industrial product : charge, chromatography, …. Silica gardens

Aqueous solution of Na2SiO3 Silicate gardens Aqueous solution of Na2SiO3 pH ≈ 12 Metal salt CuSO4 FeCl3 Ni(NO3)2

10 Magic Rocks

Biogenic synthesis of silica by diatoms Solute silica = Si(OH)4 silicic acid OH HO Si ≈ mg/l Biogenic synthesis of silica by diatoms Si(OH)4 SiO2 + 2H2O Si(OH)4 7 Si(OH)4 SiO2 + 2H2O Condensation Si - OH + HO - Si Si - O - Si + H2O

Synthesis of silica by chemists R = CH3, C2H5, ... Synthesis of silica by chemists Silicon alkoxide = Si(OR)4 OR RO Si Molecular precursor Si - OR + HO-H Si-OH + ROH Hydrolysis alkoxide water OH HO Si Condensation Si(OR)4 + 2H2O Si(OH)4 + 4ROH Si - OH + HO - Si Si - O - Si + H2O

Polycondensation ≠ precipitation monomer dimer trimer tetramer particle Si-OH + HO-Si Si-O-Si + H2O M+ + X- MX

inorganic polymerization silicate silica [SiO4] SiO2 inorganic polymerization drying molecules oligomers colloïds powder mm nm 10 nm Colloidal silica particles

Two basic reactions Hydrolysis Condensation Si - OR + HOH Si - OH + ROH Condensation Si - OH + HO - Si Si - O - Si + H2O Si - OR + HO - Si Si - O - Si + ROH

Hydrolysis of silicon alkoxides Si(OR)4 Si - OR + HO - H Si - OH + ROH Nucleophilic substitution SN2 coordination 5 O d- Si OR RO d+ H OH + ROH

Polycondensation of silicon alkoxides Si(OR)4 Si - OR + HO - Si Si - O - Si + ROH Nucleophilic substitution SN2 Si O OH d- OR d+ coordination 5 + ROH H

The chemical reactivity of silicon alkoxides is very low Si(OR)4 + 2H2O Si(OH)4 + 4ROH SiO2 + 2H2O hydrolysis condensation small positive charge d+ of the cation base catalysis acid catalysis PZC electronegativity coordination expansion difficult acid and base catalysis V depends on the pH of water

Silica gel formation no catalyst ≈ 103 hours acid or base ≈ 102 hours 20 Silica gel formation Si(OEt)4 + 2H2O SiO2 + 4EtOH Catalyst pH Tg (h) nothing 7 1000 HF 2 12 HCl 0 92 AcOH 3,7 72 NH3 10 107 no catalyst ≈ 103 hours acid or base ≈ 102 hours bio-silicification ≈ 1 hour c ≈ 1 mole/l c ≈ 10-3 mole/l

Acid catalysis (pH < 3) RO - Si - OR OR ROH d- protonation of Si-OH or Si-OR that become better leaving groups H+ > H2O Base catalysis (pH > 3) RO - Si - OR OR d+ OH- Si-O- OH- and Si-O- better nucleophile than H2O or Si-OH catalysis does not only speed up the reactions it also controls the shape of the silica particles

Acid catalysis pH < 3 Partial charges dSi dOH A +0.50 -0.06 HO OH A C B H+ Partial charges dSi dOH A +0.50 -0.06 B +0.58 +0.06 C +0.54 0.00 chain polymers H+ toward the most negative Si-OHd-

Base catalysis pH > 3 Partial charges OH- dSi dOH A +0.50 -0.06 HO OH A C B Partial charges OH- dSi dOH A +0.50 -0.06 B +0.58 +0.06 C +0.54 0.00 OH- branched polymers OH- toward the most positive Sid+ - OH

Catalysis catalysis speeds up the reactions And controls the shape of the silica particles RO - Si - O - Si - O - Si - OR O Si OR RO H+ SiO- Acid catalysis Base catalysis end groups midle Si chain polymers branched polymers

nanoparticles fibres

Catalysis controls the shape of silica particles Si(OR)4 + 4H2O Si(OH)4 + 4ROH SiO2 + 2H2O hydrolysis condensation Acid catalysis (pH < 3) Sol (1-2 nm) Gel pH > 3 (10-100 nm) pH < 3 fast hydrolysis chain polymers microporous gels (pores < 20Å) Base catalysis (pH >3) fast condensation spherical particles (Stöber silica) mesoporous gels (pores > 20Å)

Colloidal silica in diatoms Girdle bands Raphe 20 m 500 nm 150 nm pH ≈ 5 Silica walls are build up from ca. 5nm particles to give ca. 40nm diameter particles that are organised within the frustule.

Stöber silica monodispersed silica colloids

hydrated silica - SiO2,nH2O 30 hydrated silica - SiO2,nH2O Si(OH)4 SiO2 + 2H2O Si-OH

water- silica interface 1. Adsorption - dissociation - Si - O - Si - O - Si - H H O H 2. Acid ionisation Si - OH + H2O Si - O- + H3O+

Some definitions Colloid = small solid particle (diameter < 0,1 mm) Sol or colloidal solution = suspension of colloidal particles in a solvent gravity Brownian motion Brownian motion > gravity

interactions between particles increase with concentration Sols and gels interactions between particles increase with concentration Percolation sol-gel transition Sol = solid colloidal particles dispersed in a solvent Gel = solvent trapped within a particleframework

colloidal solutions are not stable Small particles tend to aggregate collision aggregation flocculation

≠ water- silica interface - 3. Electrostatic stabilisation the surface is negatively charged - ≠

Stabilisation by surface charges = peptisation Stabilisation of sols + Stabilisation by surface charges = peptisation + H+ Electrostatic repulsion Stabilisation by steric hindrance Grafted polymers

Transition metal alkoxides are highly reactive toward hydrolysis and condensation Si Ti electronegativity

Coordination expansion is easy Si(OPri)4 Si4+ = 0,40 Å c = 1,74 Ti(OPri)4 Ti4+ = 0,64Å c = 1,32 Ti Si SiO2 TiO2 [SiO4] [TiO6] SiO2 gelation takes several days Fast precipitation of TiO2 Speed up gelation via catalysis Slow down the reaction via complexation

coordination saturation slowly hydrolyzable complexing ligands

Biogenic Silica Questions and Answers Genetics 40 Biogenic Silica Questions and Answers Genetics which genes are involved in the formation of bio-silica ? Biology which proteins control the formation of silica and how ? Chemistry can we mimic nature and make silica in similar conditions ?

The biologist approach 1. Extraction and characterisation of proteins associated with biosilica Diatom frustules Silaffins N. Kröger et M. Sumper : Regensburg -Germany Glycine Lysine Proline Serine …. Sponge spicules Silicatein D. Morse - Santa Barbara - USA 2. Check their activity toward the condensation of silica

Diatom frustules Silaffins Manfred Sumper N. Kröger et M. Sumper : Regensburg -Germany Manfred Sumper Silaffins

cationic polypeptides interactions with negatively charged silica Bio-synthesis of silica by diatoms silaffin dissolution of silica frustules in HF Silaffins Proteins involved in the formation of the silica shell cationic polypeptides interactions with negatively charged silica N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066

Silaffin = cationic polypeptide two ‘lysine’ groups linked to long chain polyamines Catalytic activity due to these lysines

Precipitation of silica with silaffins N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066 Coprecipitation of silaffins with silica SiO2/ silaffin ≈ 12 1A1 pH = 6,4 silaffins catalysts for the condensation of silicic acid 1A2 Spherical nanoparticles

Sponge spicules D. Morse - G. Stucky - Santa Barbara - USA Silicatein

around organic filaments that behave as templates and catalysts 1. Sponges spicules D. Morse et al. PNAS 95 (1998) 6234 Spicules are formed around organic filaments that behave as templates and catalysts Spicules of Tehya aurantia HF

Silicatein Strong relation of amino-acid sequence between disulfur bridges serine histidine active site Strong relation of amino-acid sequence between Silicatein and Cathepsin (hydrolase)

2 of the 3 amino-acids of the active site are the same Silicatein and Cathepsine L 2 of the 3 amino-acids of the active site are the same Serine-26 and Histidin-165

Formation of silica from TEOS after G. Stucky, D. Morse, PNAS 96 (1999) 361 Silicatein filament before precipitation of silica Cellulose filament No reaction with TEOS 50

nucleophilic activation Catalytic mechanism Role of the serine-histidine couple Serine-26 nucleophilic substitution Histidine-165 nucleophilic activation pentavalent Si

-O-Si- The chemist approach catalytic role of amino-acids silica [Si(OH)4]0 + [SiO(OH)3]- (HO)3Si-O-Si(OH)3 + OH- Species in aqueous solutions at pH ≈ 7 amino acids -OOC NH3+ Interactions between silica species and amino-acids Electrostatic interactions Hydrogen bonds -NH3+ -O-Si- -COO- HO-Si-

dilute aqueous solution Precipitation of silica in the presence of amino-acids and peptides dilute aqueous solution of silica pH ≈ 7 peptides + H 3 N COO - NH Lysine Arginine 2 HO Serine Chemical titration number of Si(OH)4 monomers

pH 5.4 6.3 7.2 8.3

Nanoparticles of silica precipitated from silicic acid in the presence of polyamines M. Sumper et al. Nano Letters 2 (2002) 91 penta propylene hexamine

Amino-acids and peptides + H 3 N COO - NH Lysine Arginine 2 HO Serine Chain length Side groups Small effect Precipitation speed increases with ‘n’ Poly-Lysine

-O-Si-(OH)3 Silica condensation in the presence of peptides -COO- + H 3 N COO - NH Lysine -COO- -NH3+ -O-Si-(OH)3 HO-Si(OH)3 [Si(OH)4]0 and [SiO(OH)3]- Silica precursors are attracted by the peptide chain They come close together and can react