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NANOSCALE SYNTHESIS

Synthesis of CdSe nanocrystals | Synthesis of Au colloids

Synthesis of Au colloids

This experiment demonstrates the size-dependent optical properties of gold nanoparticles. Nano-sized gold particles will be synthesized and then used as a salt sensor. This procedure is adapted from "Color My Nanoworld." (McFarland, Adam D.; Haynes, Christy L.; Mirkin, Chad A.; Van Duyne, Richard P.; Godwin, Hilary A. J. Chem. Ed. 2004, 81).

BACKGROUND:

All physical and chemical properties are size-dependent over a certain size regime specific to the material and property of interest. When materials, like silver or gold metal, are similar in size to the wavelengths of visible light (400-750 nm), they interact with light in interesting ways. Accordingly, the apparent color of a solution of silver or gold nanoparticles depends strongly on the size and shape of the constituent nanoparticles. The volume (and shape) of a nanoparticle determines how it interacts with light, and the color observed from the nanoparticles. For example, while bulk gold is yellow, a solution of nano-sized particles of gold can appear to be many different colors, depending on the size and shapes of the nanoparticles.

MATERIALS:

• HAuCl4
• Sodium Citrate (Na₃C₆H₅O₇)
• NaCl
• Beaker, measuring flask, spatula, glass vials, stir/hot plate, stir bar

PROCEDURE: (Click on pictures to view the videos)

	Prepare 1 mM HAuCl4 solution by dissolving of HAuCl4 in 500mL of distilled water and pour into a beaker.
	Prepare a 38.8 mM sodium citrate by dissolving the solid in 50 ml of distilled water.
	Pour 20 ml of 1 mM HAuCl4 into a 50 ml beaker.
	Add a magnetic stir bar and heat the solution to boiling on a stir/hot plate while stirring.
	After the solution begins to boil, add 2 ml of 38.8 mM Na3C6H5O7.
	Continue to boil and stir the solution until it is deep red color (about 10 min). As the solution boils, add distilled water to keep the total solution volume near 22 ml.
	When the solution is a deep red color, turn off the hot plate and stirrer and let the solution cool to room temperature.
	In each of the three glass vials, place 3 mL of the gold nanoparticle solution and add 3 mL of distilled water to each vial.
	With a dropper, add 5-10 drops, one at a time of the salt solution to one of the vials.
	With a dropper, add 5-10 drops, one at a time of the sugar solution to one of the vials.

QUESTIONS:

1. Based on the fact that the citrate anions covers the surface of each nanoparticle, explain what keeps the nanoparticles from sticking together (aggregating) in the original solution.
2. Why does adding the salt solution produce a different result from adding the sugar solution?
3. How could the effect in part B be used to detect the binding of biomolecules, such as DNA or antibodies, that stick to one another or to other molecules? How could these molecules be used to cause aggregation of the nanoparticles?