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)

•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

•NucleationSparingly soluble species, high supersaturationPrecipitation of dilute metal cations from solution•Use reducing agent to reduce ions (eg. Ag+, Au3+) to neutral species

•GrowthAggregation of smaller particles (Ostwald ripening) to make larger particles•Can occur during any stage of synthesis

•TerminationStabilization (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 (<10-4M): 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) ratioIncreasing the ratio decreases the avg. particle sizeCan 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