# Nanotechnology: ferrofluids to liquid crystals

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## Part i. synthesis of gold nanoparticles (demo)

Chemicals

1.0 mM ${\text{HAuCl}}_{4}$ (stored in amber bottle)

• 1% trisodium citrate dihydrate solution

## Nanoparticle synthesis

• Add 20 mL of 1.0 mM ${\text{HAuCl}}_{4}$ to a 50 mL Erlenmeyer flask on a stirring hot plate. Add a magnetic stir bar and bring the solution to a boil
The TA’s need to be told to add the citrate as soon as the reaction boils or the solution turns colorless.
.
• Add 2 mL of 1% trisodium citrate dihydrate solution and observe the properties.

## Chemicals

1 M ${\text{FeCl}}_{3}$ in 2 M HCl Solution:

2 M ${\text{FeCl}}_{2}$ in 2 M HCl Solution

• 25% tetramethylammonium hydroxide in water

1.0 M ${\text{NH}}_{3}$ Solution: Dilute at least 200 mL of concentrated ammonium hydroxide with water to 3.0 L) Open containers of ammonia are odorous and their concentration will decrease over periods of time.

CAUTION: Ferrofluids can be messy. This particular ferrofluid will permanently stain almost all fabrics. Also DO NOT LET THE MAGNETITES TOUCH THE SURFACE OF THE MAGNET DIRECTLY.

## Magnetite synthesis

• In a hood, place 4.0 mL of 1M ${\text{FeCl}}_{3}$ and 1.0 mL of 2M ${\text{FeCl}}_{2}$ solution into a 100 mL beaker. Stir on a magnetic stir plate.
• While stirring, slowly add 50 mL of 1.0 M aqueous ${\text{NH}}_{3}$ solution over a 5 minute period using a buret. Initially a brown precipitate will form followed by a black precipitate, which is magnetite.
• CAUTION: Even though 1M NH3 is fairly dilute, ${\text{NH}}_{3}$ is a strong base.
• Remove from stirring and immediately use a strong magnet to work the stir bar up the walls of the beaker. Remove the stir bar with a gloved hand being careful not to let it touch the magnet.
• Allow the magnetite to settle, then decant off the clear liquid into a waste beaker without losing a large amount of precipitate. The settling process can be expedited by placing a strong magnet below the beaker.
• Transfer the solid to a plastic weighing boat. Rinse out the beaker with a wash bottle.
• Use a strong magnet to attract the ferrofluid to the bottom of the weigh boat. Carefully decant as much clear liquid as possible into the waste beaker. Rinse again with water from the wash bottle and decant. Repeat the rinsing process a third time. What are you removing by rinsing?
• Add 1-2 mL of 25% tetramethylammonium hydroxide. Gently stir the solution with a glass stir rod for at least a minute to suspend the solid in the liquid. Use a strong magnet to attract the ferrofluid to the bottom of the weigh boat. Pour off and discard the dark liquid. Move the strong magnet around and again pour off any liquid. If the ferrofluid does not produce spikes, continue to move the strong magnet around, pouring off any liquid.

## Ferrrofluid properties

• Hold a magnet underneath the weigh boat that contains the ferrofluid. Move the magnet around the underside of the weigh boat. Move the magnet close to and far from the weigh boat. Record your observations
• Add a couple of drops of ferrofluid to a small piece of clean paper. Let the solution dry. Once it is dry, bring a strong magnet close to the paper. What happens?
• Use several different magnets to observe the properties of the ferrofluid and record your observations in your notebook.

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