I have synthesized pure Ag, pure Au and AuAg alloy ( for e.g. Au25Ag75) nanoparticles (NPs). Solvent used is milli-Q water. The NPs are presently in colloidal form. I have observed a color change within 24 hours for the samples containing Ag. The samples are currently stored in a beaker, mouth covered using Al foil at room temperature. What should be a better method for storing the NPs and what is the reason for the color change?
The materials need to be made under nitrogen and stored under nitrogen. Your silver will become silver oxide (certainly on the surface) in the presence of oxygen (and water, unfortunately). It can't be avoided.
MilliQ water is not a good stabilization mechanism for any small particles. You need to look at steric or charge (electrostatic) stabilization and that involves ions in solution, use of a polymer (50KDa PEG or PEI) or absorption onto a substrate (this is often used for supported metal catalysts). For more on silver, see:
September 2nd, 2015 Silver colloids and invisible ink
https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22
The color change is due to changes in size (aggregation and agglomeration) caused by inadequate stabilization plus the chemical change from metal to (surface) oxide.
Thank you for the answer sir. The NPs are synthesized under bubbling N2 in a beaker but stored in a glass vial at atmospheric conditions. We have used Adenosine Triphosphate (ATP) as a stibilization mechanism.
Can vacuum dessicator or a glove box be sufficient to avoid O2 contamination of the samples?
You can't prevent oxygen getting in the system be it from water or present in AgNO3 the usual silver precursor prior to reduction. ESCA/XPS always shows the Ag to be in the +1 oxidation state on the surface (top 5 - 10 atomic layers) never as Ag0.
You also need to look at your stabilization mechanism to prevent van der Waals attraction causing aggregation and agglomeration.
I would suggest using some falcon tubes or brown glass, in which can imagine to have an effect on your sample Is through conversion of residual silver ions to elemental silver by UV-light due to that color change may happen. Otherwise, agglomeration, precipitation, and even chemical–physical decomposition are commonly observed when aqueous suspension of nanoparticles is stored forcing to use only freshly prepared nanoparticles, which limits their application in the real world.
See the below links:
https://www.researchgate.net/post/should_silver_nanoparticle_colloid_solution_be_stored_in_plastic_falcon_tubes_or_glass_bottle_for_stability
https://www.researchgate.net/post/How_to_minimize_the_aggregation_of_nanoparticles2
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