There are many papers published using plants extraction to assist synthesis of silver nanoparticles. The mechanism of the role of plants in the process of synthesis has been discussed. The following is some of them

https://www.sciencedirect.com/science/article/pii/S2090123215000314

https://www.semanticscholar.org/paper/Synthesis-of-gold-nanotriangles-and-silver-using-Chandran-Chaudhary/944983ec252f3f4c52fd20f61969f7271aa430e4

https://link.springer.com/article/10.1007/s40097-018-0291-4

Hope it helps.

than you..but i have already read these papers

B R Siddharth

Simple - silver salts are reduced to the metal via an appropriate reducing agent.

• Ag+ + e- → Ag E0 = + 0.799 V
• Ag(NH3)2 + e- → Ag + 2 NH3 E0 = + 0.373 V

More control with the latter (ammoniacal) route and this is usually preferred.

The traditional reductants include hydrogen gas, sodium citrate, formaldehyde, hydrazine hydrate, sodium borohydride etc. I am not a fan of borohydride reduction as it introduces a significant amount of boron into the metal (silver in this case).

It seems to be the trendy fashion to use un-characterized plant extracts under the guise that this is a 'green' synthesis. An inorganic salt precursor still needs to be used and these are usually toxic and not at all green - for example, ammoniacal silver nitrate.

Also be aware in the presence of air or oxygen (atoms or molecules) you'll not have silver in the zero oxidation state on the surface. The surface (5 - 10 atomic layers) will have the silver totally in the +1 oxidation state (i.e. Ag2O and not Ag0)

See:

Silver colloids and invisible ink

https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22

Good answer from Alan F Rawle, but some details must be added.

At the first, plant extracts are polyfunctional substances, i.e. they plays role both of reducing agent and stabilizer.

And the second moment is that if nanoparticles are in solution (silver sols) that contains stabilizer (plant extract, for example), the sentence about 5-10 oxidized atomic layers is some controversial. For example, we obtained small Ag nanoclusters both in ethylene glycol solutions (d ~ 2 nm, ca 300 atoms, i.e. particles possess ~3-4 fully completed shells) and in sodium polyacrylate (d ~ 1.5 - 4 nm). That is why investigations of interaction of Ag-surface and different functional groups are still actual despite of the huge number of references.

Good luck!

Liliya

P. S. Dear B R Siddharth, additionally to my previous answer, some more details concerned on small Ag nanoparticles are presented in our last papers (https://doi.org/10.1002/kin.21249 and https://doi.org/10.1007/s00396-019-04488-4) and on my page.

Liliya Bazylyak Yes, the 3 stages in making a stable dispersion (of powder, in particular) are wetting, separation (the key step), and stabilization. For metal colloids then they're produced in a bottom-up process and stabilization is vital here. This can be steric (polymer - PEG or PEI say of 25 - 50 kDa) or charge (sometimes called electrostatic). In all my early studies, in the 1970's, ESCA/XPS showed in air, water, on oxide supports that the silver (produced by hydrazine hydrate reaction) was in the +1 oxidation state. I can believe in your system, protected by EG, that the 0 oxidation state could be preserved but I would like to see evidence - for example, without Ag+ then the material can't act as a bactericide.

Dear Alan F Rawle ,

Thank You for discussion, but, if I am not mistaken, the mechanism of antimicrobial and/or antiviral activity of AgNPs is not cleared yet. There are many references that present contraversional results. In any case, this is "headache" for biologists, but I'm chemist :)

In any case, it is nice to chat with You ;)

With kind regards,

Liliya

To investigate the mechanism of the reaction is possible only with the individual substance from the extract as we did.

Article Investigation of interaction between silver ions and rutin i...

In addition, you can see the reduction of silver ions tea extract.

Article Preparation and bactericidal properties of silver nanopartic...

Cations and anions are reducted in different ways.

Article Kinetics and mechanism of the interaction between HAuCl4 and rutin

Liliya Bazylyak 2 points in response:

• XRD measures the bulk phase and cannot look at the surface composition. My experience in the 1970's with ESCA/XPS was that the surface layers in Ag containing preparations were always oxidized whenever O-containing species were present (water or O from oxide supports). Plus there was always a significant carbon content. More information in this webinar:
• Silver colloids and invisible ink

https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22

I believe that the mechanism of Ag+ as a bactericide has been known since the early 1900's. The problem is those people that believe that Ag/Ag+ will cure everything from Ebola to AIDS and hence the FDA's short note in 1999 ban on claims for such materials. As we know Ag metal is not soluble in water so cannot act in such a manner.

Liliya Bazylyak

'At the first, plant extracts are polyfunctional substances, i.e. they plays role both of reducing agent and stabilizer' I agree with this statement, but...

For me, the key is the metal precursor as that's where the nanoparticles will actually come from. It's this precursor that prevents the process being 'green', IMHO. All these precursors tend to be toxic to a greater or lesser extent - AgNO3 (usually in ammoniacal form), HAuCl4.xH2O, Ni salts, and so on.

I see little point in using poorly characterized plant extracts if the goal is a metal colloid.

Dear Alan F Rawle ,

It is clear that surface of silver will be oxidized in the presence of oxygen. BUT, as You could see, it was written: "the sentence about 5-10 oxidized atomic layers is some controversial". Let us consider. For simplification we will use the "magic number concept" (good description You can found in DOI: 10.1007/s11244-005-9261-4). Accordingly to "magic number concept" let us calculate some characteristics of AgNPs for different sizes.

1 layer, sum of atoms - 13; percentage of surface atoms - 92%; d=0.6 nm

2 layer, sum of atoms - 55; percentage of surface atoms - 76%; d=1.1 nm

3 layer, sum of atoms - 147; percentage of surface atoms - 63%; d=1.7 nm

4 layer, sum of atoms - 309; percentage of surface atoms - 52%; d=2.3 nm

5 layer, sum of atoms - 561; percentage of surface atoms - 44%; d=2.9 nm

...

20 layer, sum of atoms - 29000; percentage of surface atoms - 14%; d=11.5 nm

Here (in percentage) we considered ONLY 1-st surface layer. THEN, if even 5 layers for 20-layered cluster (~11 nm) are oxidized, that is mean that obtained product consist of 56 mass % of Ag2O.

And this is nonsense, because such quantity of Ag2O will be well observed on XRD. Even using of DRON-3.0 ;)

That is, the controversial is the NUMBER of layers, but not the fact that surface can be oxidized.

As to mechanism, Dr. Yuri Mirgorod is completely right - the all is depend on the nature of reducer and any individual substance must be studied separately.

Alan F Rawle

"For me, the key is the metal precursor as that's where the nanoparticles will actually come from. It's this precursor that prevents the process being 'green', IMHO. All these precursors tend to be toxic to a greater or lesser extent - AgNO3 (usually in ammoniacal form), HAuCl4.xH2O, Ni salts, and so on."

Then, let us use silver acetate! :)

Liliya Bazylyak OK, understood; I get your point and agree. Yes, it's all about the number of layers.

The Ag2O can be amorphous though - XRD won't show this but XPS will.....

Silver acetate - see MSDS. Classified as hazardous in the US.

Liliya Bazylyak

One question as your papers are not easily retrieved. Perhaps you can post them.

So, how long did you need to irradiate your 0.6 nm 'particles' to get a XRD pattern? I assume that these particles were in colloidal suspension and not dry? I had issues (in the 1970's it has to be said) in getting good diffraction patterns from 5% Ag on silica where the average size of the silver was around 10 nm by number. The sample(s) were irradiated for 24 hours in a standard Debye-Scherrer camera.

Liliya Bazylyak

You: 'As to mechanism, Dr. Yuri Mirgorod is completely right - the all is depend on the nature of reducer and any individual substance must be studied separately'

Me: 'silver salts are reduced to the metal via an appropriate reducing agent'

Pretty much the same, I think....

Alan F Rawle

We did not investigated 0.6 nm particles - the parameters above were presented as an example.

But in references there are many XRD patterns for small silver clusters. At the same time, in our 2015 paper ( https://doi.org/10.1002/kin.20913) it is presented XRD pattern for ca 15 nm nanoparticles and crystallite size is in good agreement with TEM. IMHO, Your problems with XRD was caused by low amount of silver, and contrary, we obtained XRD from the powdered and dried Ag nanoparticles using standard procedure.

In any case, thank You very much for interesting discussion!

With kind regards,

Liliya

Liliya Bazylyak Thank you - I know where the issues were in the XRD of my Ag/SiO2 catalysts were - the relatively low mass loading (as you say) and the small size (~ 10 nm) of the particles broadening the XRD pattern. I would have assumed that in about 50 years since then that the technique has improved markedly and lower irradiation times and better interpretation would be around.

Your paper requires payment for access.

Once dried and powdered your particles will have become a fused mass of sub- and post-micron aggregates and agglomerates. Fine for XRD but what was the SSA after drying?

Alan F Rawle

After drying the samples was not interesting for us due to the main goal was to study the kinetics. AgNPs were dried especially for XRD and, using both GSAS and Scherrer equation, crystallite sizes were calculated. The values were ~13 nm (dried, XRD) and ~15 nm (TEM, plated from the solution).

You may do request for me via RG and I will send the paper for Your private use.

Alan F Rawle

"Silver acetate - see MSDS. Classified as hazardous in the US."

But what about CH3COO(-)? ;) Ag(+) will transform to Ag(0), solution will contain only CH3COO(-).

Or, maybe, in USA vinegar also is considered as toxic :) ;)

Thanks to all of you people for answering the question