Formation of Silver Nanoparticles from Green Tea Reducing Equivalents

[R0WDY]

Hi everyone,

This is my first thread so I'll try to be as clear as possible and try to follow the rules.

My job is to synthesize silver nanoparticles, characterize them (TEM, DLS, EDAX, Z-POT), and then pass then on to the toxicologists. This is my thesis work to graduate.

A new thing I have been trying to synthesize is silver nanoparticles by using green tea extract. (-) epicatechin. (we have found that cells undergo oxidative stress per western blots, and using a biocompatible reducing equivalent will reverse toxicity) Toxicity assays we have performed support this hypothesis.

http://en.wikipedia.org/wiki/Catechin (slightly soluble in water, completely soluble at 70 degrees for the solutions I'm working with)

So far I have designed the experiment, such that I can produce nanoparticles of small size and with a poly-phenol coating. The issue at hand is inability to control variability in particle size and the presence of an insoluble compound. Here is the protocol I wrote:

Prepare .0025N Silver Nitrate solution (low concentration keeps ions in solution spread out to promote small particle size.

Prepare .005M (-) epicatechin solution (keep at 80-90 degrees C)

Place silver nitrate solution in ice bath with rapid stirring. (this will make excess reducing equivalent insoluble after quickly reacting)

Add hot reducing equivalent dropwise in 3:1 molar ratio (epi:Ag)

Allow to sit for 3 hours in icebath.

Sonicate reaction vessel, then transfer mixture to dialysis bags.

Dialysis at room temperature for 2 days (changing water every 8 hours)

Dialysis at 90 degrees C to remove Reducing equivalent for 4 days (changing every 8 hours)

Obtain nanomaterials from dialysis and run characterization.

Prepare a stock solution and determine concentration; pass to toxicologists.

I'd love to hear some ideas on how to better control my particle sizes. I'm getting a mean particle size of around 25nm with a standard deviation of 20nm, and its a major pain to keep rerunning the procedure. As far as the unknown insoluble organic material, I think it might be a byproduct of the epicatechin undergoing oxidation. Maybe there is a polymerization reaction happening that I'm unaware of?? According to EDAX indicates presence of C,O, and Ag. (H does not show up on our EDAX instrument)

Thanks everyone!
R0WDY

 

[Nemus]

It sounds like a quite messy system to begin with.
First, I'd like to go up in temperature to shorten reaction time. You want fast nucleation of many crystal cores and then a moderately fast growth period where the cores consume the remaining silver ions but don't have time to aggregate. The large spread of your particle size indicates that you are not seeing primary particles but rather aggregates. Don't be afraid of having an excess of your phenol present, it will protect the particles from interacting.
I would also want to add some water miscible silver-stable organic solvent to help solubilizing the oxidized phenol residues (probably everyting from monomeric quinone to polymers) you get after the reaction with silver ions. I suggest diglyme or triglyme. Maybe 20%. My guess is that this polymer is good for you because it will stick to the particle surfaces and protect them.
Third. Get rid of the dialysis sausages. Dialysis really sucks as a technique. Takes forever and is very irreproducible. Get a setup for laminar flow filtration (diafiltration). Very simple to build with a pump, some tubing and a manometer. You will have the particles cleaned up in a couple of hours. Less oxidation, less aggregation and more efficient. If you want to I can take a pic of my setup to show how it can be done.

 

[R0WDY]

Thanks for the very constructive response! :)

Transmission Electron Microscopy has been indicating individual particle sizes of 25 nm +/-20nm. A coating is clearly present on the particles. I chose epicatechin as the redicing agent, because it can act as a capping agent to prevent other silver ions from crystalyzing on the core. (I hope I am using the crystallography language correctly). I chose to use a cold temperature, because epicatechin comes out of solution below 80 degrees C as an insoluble organic compound. Adding hot epicatechin allows the silver to be reduced and capped quickly while excess reducing equivalent quickly becomes insoluble. I see your point on running the experiment hot all around, as it will generate many cores. I'll give it a shot.

You are correct with respect to the particle flocculation. Dynamic light scattering results indicate agglomerates in the 100nm-120nm range and zeta potential indicated low stability in water. (confirmed tendency to form aggregates)

I agree on eliminating dialysis completely. It is a major pain; and, it introduces enormous error to the whole procedure. I also hate doing it. If it is no problem, I would be very interested in having a look at your apparatus and making a suggestion to my advisor about giving it a shot. I'll look into using a stabilizing organic solvent.

Thanks again for the very helpful suggestions!



R0WDY