Gold(Au) Nanoparticles (and silver)

1. Gold(Au) Nanoparticles(and silver)

2. Outline • Motivation • History • Colloids • Nucleation • Growth • Coagulation • Procedure • Viewing the particles

3. Motivation • Some quacks still assert profound medicinal properties • Spherical nanoparticles can serve as biological tags for tracking purposes • The red color in stained glass windows is due to colloidal gold • Can be used for a new ultrasensitive and selective detection scheme for DNA • Gold nanorods can be bar-coded Au/Pt

4. History • Alchemists believed Au sols might be the “elixir of life” • Faraday researched many of the properties of colloidal gold in the 1850’s. • Mie’s theory of light scattering was developed to explain the color of colloidal gold. • Medical applications were developed that diagnosed certain diseases based on the interaction of colloidal gold and spinal fluids • The first comprehensive investigation using the electron microscope began in 1948 at Princeton University and RCA Labs

5. The Optics of Gold and Silver Nanoparticles • The color of the sol arises from a combination of absorption and scattering of light and depends on particle size • More specifically it is due to a resonance of the free electrons in the metal particle. The light’s electromagnetic field causes them to slosh back and forth (plasmon oscillations). • At a characteristic frequency which depends of the size and the metal, the sloshing is the most intense. This is the frequency where plasmon oscillations are excited. • The plasmon resonance is easily seen in the extinction spectrum of the sol.

6. Colloids • A colloid is a homogeneous dispersion of particles in a solution which are so small as to not settle out easily • A sol is a specific type of colloid characterized as a solid dispersed in a liquid • The colloid is stabilized by electric charges on its surface due to adsorbed ions. The charge causes the particles to repell each other. The both the Gold and Silver sols used here have a negative charge. • The particles experience the constant buffeting of Brownian motion which also helps to keep them in suspension. • Formulation of Au nanoparticles is a three step process: nucleation, growth, and coagulation

7. Nucleation • Nucleation is the creation of nuclei upon which growth can occur • This is a redox reaction: oxidation of the citrate ion produces the necessary reducing reagent for the gold: acetone dicarboxylic acid • The acetone dicarboxylic acid is the limiting reagent for nucleation • The formation of this molecule in the solution creates an induction period before which no product can be seen • The nature of the nucleation curve is evidence of an autocatalytic reaction • That is to say it has a rapid growth after the induction period followed by a linear portion and then decay

8. Nucleation (cont.) • A type of polymerization (complexation) occurs in which the gold ions coordinate with acetone dicarboxylic acid and join together • When the “polymer,” or complex, reaches a critical mass that is just greater its thermodynamic stability, reduction to metallic gold occurs, yielding the nuclei • Reduction is the rate determining step in the kinetics of the reaction • The less citrate in the mixture, the larger the particles will be in size

9. Growth • Growth is the addition of more gold particles to the existing nuclei • The process of growth stops when all of the gold is used • The rate of growth is a first order in the gold nuclei size • Having the equation dD/dt = kD, where k is a constant whose value is independent of particle size

10. Coagulation • Creation of the larger gold particles, such as 20 nm, requires a coagulation of multiple (smaller) twins of various shapes • A conglomeration of multiple nuclei into particles can be large enough to disturb the stability and fall out of the colloid • Control of the coagulation process during preparation determines the size, structure, and size distribution of the particle • Once the preparation of the gold nanoparticles is complete, the absence of coagulation insures its stability

11. Procedure • Bring to a boil 50 mL of 2.5×10-4 M chlorauric acid solution • Add 0.16 to 1.0 mL of 34 mM sodium citrate solution to the boiling solution while stirring • After a minute will be faint blue and then darkening over 5 min to a brilliant red • The size of the gold nanoparticles can be controlled by varying the amount of sodium citrate solution. The above procedure can grow controlled sizes from 147 nm (0.16 ml) down to 16 nm (1.0 ml).

12. Further Reading • John Turkevich, Gold Bulletin,18(3), 86 (1985). • John Turkevich, Gold Bulletin,18(4), 125 (1985). • Turkevich, Stevenson, Hillier, Discussions of the Faraday Society,11, 58 (1951). • G. Frens, Nature Physical Science,241, 20 (1971).