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.

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 stabilizer can be added together 4 , sodium citrate •To ensure very small, homogeneous particles, reductant and •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 http://nanotech.sc.mahidol.ac.th/jhodak/page_1.html

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