Sodium Borohydride Synthesis of Silver Nanoparticles

The synthesis procedure shown here was adapted by Steve Ng and Chris Johnson from a procedure developed by S.D. Solomon, M. Bahadory, A.V. Jeyarajasingam, S.A. Rutkowsky, C. Boritz, and L. Mulfinger, Journal of Chemical Education, 84, 322-325, (2007).

The formation of silver nanoparticles can be observed by a change in color since small nanoparticles of silver are yellow. A layer of absorbed borohydride anions on the surface of the nanoparticles keep the nanoparticles separated until sodium chloride is added.

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/nanolab/silver/index.html) shows a movie of that step.

Procedure

Wear eye protection

Never look directly into a laser or shine a laser at another person.

Add 30 mL of 0.002M sodium borohydride (NaBH₄) to an Erlenmeyer flask. Add a magnetic stir bar and place the flask in an ice bath on a stir plate. Stir and cool the liquid for about 20 minutes.

Drip 2 mL of 0.001M silver nitrate (AgNO₃) into the stirring NaBH₄ solution at approximately 1 drop per second. Stop stirring as soon as all of the AgNO₃ is added.

The presence of metal nanoparticles can be detected by their interaction with a beam of light since the oscillating electric field causes quantized light emission from the particles. Can you see a laser beam as it passes through the solution? The light from a laser pointer may be polarized with the electric field oscillation in only in one plane. Is your laser pointer polarized? In so the plasmon emission would occur only in one plane. Shine the laser through the solution and rotate the laser. What fraction of a full rotation separates the maximum and minimum observed brightness?

Record the visible spectrum of the solution. You will need data from 375 nm and longer wavelengths. What is the peak width at half height?

Transfer a small portion of the solution to a test tube. The addition of a few drops of 1.5 M sodium chloride (NaCl) solution causes the suspension to turn darker yellow, then gray as the nanoparticles aggregate.

Materials

Stock Solutions for 8 batches

• 0.001M AgNO₃: Dissolve 0.017 g of AgNO₃ into 100 mL distilled water. This solution can be kept for later usage.
• 0.002M NaBH₄: Dissolve 0.0189 g of NaBH₄ into 250 mL distilled water. This solution must be made fresh before the experiment.

Equipment
• Small Erlenmeyer flask
• 50 mL graduated cylinder (NaBH₄)
• Large dish of ice
• Magnetic stirrer
• 1" stir bar
• 2 mL dropper (AgNO₃)
• Laser pointer, polarizing filter

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