Synthesis of 50 ppm Silver Nanoparticle Suspension (Colloidal Silver) using Trisodium Citrate and Polyvinylpyrrolidone

Here I will explain a detailed procedure to synthesize a very stable 50 ppm Silver Nanoparticle suspension, also known as Colloidal Silver. In my previous post I have explained a procedure of synthesizing 50 ppm nano suspension using trisodium citrate as reducing as well as capping agent. Now in order to make the suspension more stable, we will use PVP (polyvinylpyrrolidone) as an additional capping agent or as a stabilizer. I have observed that using PVP makes the suspension more stable for a longer period. 

So, let us see below how it goes.

MATERIALS REQUIRED FOR 1 LITER

1. Silver Nitrate :  72 mg
2. Trisodium Citrate:  1000 mg
3. Polyvinylpyrrolidone:  1000 mg 
4. Distilled water

EQUIPMENT

1. 1 or 2 Liter glass beaker.
2. Magnetic Stirrer with heating.
3. Weighing balance which can measure weights around 50mg accurately.
4. 50 or 100 ml glass beakers for preparing Trisodium Citrate and PVP solutions.

PROCEDURE

1. Take 900 ml distilled water in the glass beaker and put it on the magnetic stirrer. Its better to cover the beaker with an aluminum foil. 
2. Weigh 72mg Silver Nitrate and add it to the glass beaker containing 900 ml distilled water. Start the stirrer and the heater. Put a thermometer in the glass beaker to note down the temperature.
3. As the silver nitrate solution gets heated up, in the meanwhile prepare both the solutions of trisodium citrate and PVP, each in 50ml of distilled water.
4. As the temperature of the silver nitrate solution reaches 90°C, stop heating as the temperature will still rise little higher. 
5. Start dropwise addition of the prepared Trisodium Citrate solution. The addition must be done evenly in about 10 minutes. As you will reach the completion of addition of Trisodium Citrate solution, you will notice a change in color of the solution. The color is now slightly yellowish. I strongly suggest keeping a laser pointer handy to observe the generation of nanoparticles. As you pass the beam of laser horizontally through the solution you will see the change gradually with time. Initially the laser beam is not visible in the solution as there are no particles to diffract the light, but as reduction of silver ions happen with time, you will first see very few illuminating particles, then a very faint beam and then the beam keeps on becoming brighter due to synthesis of nanoparticles. Its really a fun to watch that!
6. Please do not stop the stirring at any point of time. 
7. The temperature must be maintained between 90°C to 95°C all the time during synthesis.
8. After 10 minutes of completion of Trisodium Citrate addition, add the PVP solution at once under stirring. At this point the appearance of solution must be light yellow and if you are using a laser pointer then a very faint line should appear. If not, then wait for another 3-5 minutes before adding the PVP solution. 
9. Keep maintaining the temperature between 90°C to 95°C and continue stirring for another 10 minutes. 
10.  Now the appearance must be golden yellow and laser beam should be very clearly visible and strong. 
11.  Now stop the stirrer and remove the beaker from the hot plate. 
12.  Keep the beaker in a water bath to cool it down fast. You can add ice to the water bath to speed up the cooling. 
13. When the suspension has cooled down to 40-50°C, transfer the suspension in a preferably amber color or non-transparent bottle and keep it in refrigerator for further cooling.
14. Now, your 50 ppm Silver Nanoparticle suspension is ready! 
15. The appearance of this 50 ppm Silver Nanoparticle suspension must be golden yellow when seen across in a clear glass container. This color shows that you have made it perfectly.

As per my experimentation, the average size of nanoparticle synthesized using this method is between 10-15 nanometer. The wavelength of max absorbance when scanned in a UV-Vis spectrophotometer is between 410-420 nm which also clearly indicates the formation of silver nanoparticles of the mentioned size range. This phenomenon is called surface plasmon resonance which is a characteristic property of nanoparticles and due to this the color of suspension appears golden yellow. 

Watch this video showing the sysnthesis of silver nanoparticles.

This Silver Nanoparticle suspension is stable for 2-3 months at least, if kept in a refrigerator. 
I would be more than happy to assist you in synthesizing your own Silver Nanoparticle suspension. You may get in touch with me through agrawal.achal@gmail.com.

Thanks for reading this article!