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Hansen et al. Nano Lett. 2005; 5 (10), 1937 – 1942.

Colloids. Colloids can be classified as particles in the size range of about one nanometre to a micron in size Dispersed in a different phase (aqueous or non-aqueous)

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Hansen et al. Nano Lett. 2005; 5 (10), 1937 – 1942.

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  1. Colloids • Colloids can be classified as particles in the size range of about one nanometre to a micron in size • Dispersed in a different phase (aqueous or non-aqueous) • Recent interest: reducing size of the particles (<100 nanometres) the particles start to exhibit unique properties – magnetic, optical, catalytic Gold particle solutions with diameters of: • Common example: gold nanoparticles absorb different wavelengths of light depending on particle size 12nm 18nm 30nm 40nm 72nm Hansen et al. Nano Lett.2005;5 (10), 1937 – 1942.

  2. Synthesis of Metallic Nanoparticles: Overview • Nucleation • Sparingly soluble species, high supersaturation • Precipitation of dilute metal cations from solution • Use reducing agent to reduce ions (eg. Ag+, Au3+) to neutral species • Growth • Aggregation of smaller particles (Ostwald ripening) to make larger particles • Can occur during any stage of synthesis • Termination • Stabilization (repulsion) of particles by: • Capping ligands (steric repulsion of organic groups) • Electrostatic repulsion, from H+ or OH- on surface • Reducing agent may act as stabilizer

  3. Typical Nanoparticle Syntheses • Dilute solution of ions (<10-4M): • Gold: tetrachloroauric acid (HAuCl4) • Silver: silver nitrate (AgNO3) • Platinum: hydrogen hexachloroplatinate (H2PtCl6) • Stabilizer is added, followed by reductant • Typical stabilizers: surfactants (SDS, CTAB, sodium 3-mercaptopropionate) • Typical reductants: NaBH4, N2H4, sodium citrate • To ensure very small, homogeneous particles, reductant and stabilizer can be added together • Size can be varied by changing the Stabilizer/Metal (S/M) ratio • Increasing the ratio decreases the avg. particle size • Can achieve 3.3 nm dia. Au particles with S/M = 5.0* • Nanoparticles can remain stable in solution for several months *Kimizuka et al. Langmuir.2000; 16, 5218 – 5220.

  4. Some results… 50 nm http://nanotech.sc.mahidol.ac.th/jhodak/page_1.html

  5. Other Examples 20 nm • “Green” Ag nanoparticles: use glucose as reductant, starch as stabilizing agent (Wallen et al., J. Am. Chem. Soc. 2003, 125, 13940) • Silica-Encapsulated Gold Nanoparticles: make Au nanoparticles then add 3-aminopropyltrimethoxysilane followed by sodium silicate solution (Meisel et al. J. Am. Chem. Soc. 2002, 124, 2312) • Core-shell: gold core, silver shell: make Au nanoparticles, then add NaAg(CN)2, solutions were gamma-irradiated under nitrous oxide (Hodak et al, J. Phys. Chem. B. 2000,104, 11708)

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