Generally the silver nanoparticles easily aggregated. When the aggregation will occur the color will change from brown to black. During the aggregation the size of particles formed. Sometimes the structure also changed. To reduce the problem, the silver nanoparticles should be washed several times by water with the help of ultra centrifuge. After washing the silver particles may dry in the room temperature. Otherwise the strong capping agent like PVP can use in the synthesis protocol.

It may help you.

file:///C:/Users/USER/Downloads/B130233_505.pdf

Sudarsan Raj: Thanks for your comment. There is no ink for the file.Please add the ink.

The main reason is in the fact that during drying the structures formed from aggregated stabilized silver nanoparticles are different in shape and a bit porous. So, these characteristics allowed to the aggregates adsorb a light of different wave length resulted in black color. Redispersing dry AgNP powder drastically increases a istance between AgNP and plasmons start to work.

Probably its because of the dielectric function of the media ( It is related to the plasmon) and its affects the wavelength absorbtion of AgNP.

Generally the silver nanoparticles easily aggregated. When the aggregation will occur the color will change from brown to black. During the aggregation the size of particles formed. Sometimes the structure also changed. To reduce the problem, the silver nanoparticles should be washed several times by water with the help of ultra centrifuge. After washing the silver particles may dry in the room temperature. Otherwise the strong capping agent like PVP can use in the synthesis protocol.

I agree with the comment above from BALAMURUGAN Matheswaran.

Thanks to every one answered my question.

If the deep brown colour is the colour of AgNPs Plasmon what is the black colour for? i.e. what is the structure or the form of AgNPs when it is black?.

The black is the aggregates of AgNPs, whose size is too large and lose the property of urface plasmon resonance.

Thanks Dr. S. El.Aleem.

The attached photo shows amazing change in colour in each stage of preparation of CMS-AgNPs::

1- After synthesis (dark brown)

2- After ppt. using ethanol (brown)

3-After drying (black)

4- After re-dissolution (yellow).

Silver nps turn black due to aggregation on drying

Actually silver nanoparticles in solution due to SPR shows brownish/amber color, but during the process of drying the color transformed to blackish(actual color of SNP) this is due to t the absence of surface plasmon resonance in dried condition.

I beg to differ with my young RG colleagues as:

Blackening of Ag metal while exposing in atmosphere( wearing Ag ornaments/ or even the VOTE INK ) is known since long.

During the conduct of the experiment for the formation of Ag NP s, the following changes take place:

Large surface area of the Ag NP exposes its loosely held outermost electrons(4d^10 5s^2) which can take part in reacting with some impurity . Energy- dispersive spectroscopy is used to gain further insight into this feature of AgNPs. Some impurity peaks are always detected below 1 keV which correspond to a carbon peak (CK) and oxygen peak (OK) and are probably related to the presence of molecules in the extract which play a significant role as reducing agents for silver ions.

I may further submit that the role of pH in the synthesis and nature of Ag NP s is also very important . Compounds with different compositions having different colors may form. At pH ( appxo. 4.5-4.6), no color change occures; indicating strong acidic pH suppresses the NP s synthesis. As pH increases( mild acidic- appxo5.5-5.6), the yellow colour was forms and turns into brown colour which indicates formation of AgNPs. At pH 6.5-6.6( just acidic ) both show dark blackish brown color with precipitation which indicates completion of NP synthesis. At high pH 7.6 -8.6, the brown colour is maintained without precipitation which indicates stabilization of synthesized NPs. On long standing, it turns black either due to its changing to Ag2O or Ag2S because both O and S are strong chalcogens and are electron acceptors.

Again on keeping, the pH changes to about 5-6 and the solution becomes yellow.

Dear Dr. Sarmiza Stanca,

It is a pleasure for me to “vote you up” as your good self has given an expression of support for my views already quoted by me in reference to the answer for this question ( third above from your answer) as:

“On long standing, it turns black either due to its changing to Ag2O or Ag2S because both O and S are strong chalcogens and are electron acceptors”.

Thanks to every one participated in answering my question. I just wanted to remind you what I noticed that the color is gone yellow again after re-dissolution. Which means that the nanoparticles are not silver oxide or silver sulphide.

Though the color change in Ag NPs with pH change is a common phenomenon,but no where it is reported to turn black . I had always thought that I had successfully explained the black color .

But the turning of the black to yellow again is really an irritant.

May I , humbly, ask?

Was it yellowish white?

Has, at stage of the experiment, there was an evolution of CO2 or any CO3 (-2) ions present?

If it is in the affirmative, I dare explain as:

[A]In the acidic pH range(5-6):

Ag2O + 2 HNO3 → 2 Ag(+) + 2 NO3 (−) + H2O.

(Black)

[B] If there is an evolution of CO2 or any CO3 (-2) present:

2 Ag(+) + CO3(-2) → Ag2CO3 (Yellowish ppt having a faint white tinge).

Ksp (Ag2CO3, 25 C)= 8.45x10^−12.

Manohar Sehgal: Thanks for your comments. No, there was no CO2, the color was yellow not yellowish white, no ppt. Please have a look on the photo included in one of my comments (four tubes).

In addition to the aggregation of the NPs upon drying, is there any possibility that the issue is related to the” feeling of color” rather than the color itself? Meaning that: when the dark brown color is more concentrated by drying, (elimination of water of dilution) one gets the feeling that the color is black due to the high concentration of the color after elimination of water. By re-dissolution you re-dilute the color again and then it returns back to its origin. The more the water added the more the faint yellow color obtained and the less the water added the more the dark brown color obtained. I need a “color science” expert for explaining that.

Silver nanoparticles like silver metal are prone to oxidation under ambient conditions and also when dried in presence of air/oxygen. We indeed see the presence of Ag0 and Ag2O nanoparticles, when we generate them using biological methods as well as chemical methods under oxygenic environment. I certainly agree that aggregation also turns them black.

Attached publication from our team might be of some interest to you. You may also look at our publication on Ag0/Ag2O NPs that will appear in PLoS One on September 03, 2014.

To minimize oxidation ,capping agents should be used.Otherwise you have to keep them in colloidal state. Our attached paper may be helpful.

Sorry, the paper is attached now.

I totally agree that AgNPs could be oxidized to Ag2O and aggregation occurs upon drying.Both factors could convert AgNPs from brown to black. But why it goes to yellow upon re-dissolution. So far no one gave an answer or comment on this observation.

Rate of dissolution depends surface area -size of the particles. Size may change during this process. Have you measure intial and final size of the silver nanoparticles?

The yellow is an indication of size and shape. Mie theory can assist here.