Hansen et al. Nano Lett. 2005; 5 (10), 1937 – 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 Common example: gold nanoparticles absorb different wavelengths of light depending on particle size 12nm 18nm 30nm 40nm 72nm Gold particle solutions with diameters of:

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 +, Au 3+ ) 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

Typical Nanoparticle Syntheses Dilute solution of ions (<10 -4 M): Gold: tetrachloroauric acid (HAuCl 4 ) Silver: silver nitrate (AgNO 3 ) Platinum: hydrogen hexachloroplatinate (H 2 PtCl 6 ) Stabilizer is added, followed by reductant Typical stabilizers: surfactants (SDS, CTAB, sodium 3- mercaptopropionate) Typical reductants: NaBH 4, N 2 H 4, 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.

Some results… 50 nm

Other Examples “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) 20 nm