The synthesis procedure shown here was adapted by Troy Dassler from Angshuman Pal, Sunil Shah, and Surekha Devi, ” Microwave-assisted synthesis of silver nanoparticles using ethanol as a reducing agent“, Materials Chemistry and Physics, 114(2-3), 530-532 (15 April 2009).
Silver nanoparticles are made by a chemical reduction of a silver salt in the presence of a stabilizing agent. Their formation can be observed by a change in color since small nanoparticles of silver are yellow. In this synthesis ethanol is the reducing agent, polyvinylpyrrolidone prevents aggregation, and rapid microwave heating and agitation gives monodispersed particles.
Procedure
- Add 10 mL of 1.0% polyvinylpyrrolidone (PVP) in 100% ethanol to a small Erlenmeyer flask. Add 200 µL 0.10 M AgNO3.
- Cover loosely.
- Microwave for 5 seconds at 100% power. It may work better to set the microwave timer for a longer period and then stop the microwave after 5 seconds. The solution may be quite hot.
- Wait several minutes to see if the solution turns yellow. The color will continue to darken with time.
Optional: Record the visible absorbance spectrum as a function of time. What is the peak width at half height? The presence of a colloidal suspension can be detected by the reflection of a laser beam from the particles. The light from a laser pointer may be polarized. When polarized light causes plasmon emission the beam may disappear at some angles. When the beam from the laser is visible, is it invisible in a view perpendicular to the first?
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.
The solution can be used to test the antibacterial properties of silver nanoparticles.
Materials
Stock Solutions for many batches
- 1.0% Polyvinylpyrrolidone (PVP) MW=10,000 in 100% ethanol: Dissolve 0.5 g of PVP into 50 mL of 100% ethanol. The ethanol should be anhydrous.
- 0.10 M AgNO3: Dissolve 0.17 g of AgNO3 into 10 mL distilled water. This solution should be stored in the dark.
Equipment
- Small Erlenmeyer flask. In some ovens a 10 mL flask works better and in other ovens a 25 mL flask works better.
- 10 mL syringe (PVP), 200 µL syringe (AgNO3)
- Microwave oven (Heating for 5 seconds works for an 800W oven; other power ovens can be used but the heating time will need to be adjusted.)
- Laser pointer, polarizing filter