it is not because of nano particles, it may be because of d level transitions in Ag ions. You can be sure about it by taking UV-Vis spectra of your silver salt solution and rule out the doubt. Auric acid solution also shows peak in 200-300 nm due to same reason.

d-d transitions are usually very weak, because they are forbidden! What is the absorbtion coefficient of that transition? The rod like shows two transitions because of the different lengths. Short lenght, short energy but 220 nm that`s UV..... What is your capping agent? Solvent?

This may be due to the formation of byproducts. Nanorods show two peaks but in the visible.

I would also suspect this to arise from other components of the mixture. Check the UV-Vis extinction from all pure reactants and whatever side-products are formed after the reduction.

Pooja Devi has right, repeat the experiment and confirm that. maybe you have a sample mixed.

It maybe the handling error ..you may repeat the experiment. Otherwise it maybe the chemical molecules reaction.

Hi, around 380-420 nm is the peak of silver nanoparticles..

you said that you use a sonochemistry, so you used a solution, maybe:

You are no doing you blank with all you added to your solution (buffers, deionized water..)

if you are using biomass this peak could be part of proteins, if not its an impurity

but..

You have to check wich peak its more significant,

if the peak of silver nanoparticles its very obvious, do the uv analysis again

if the peak its also there, check your synthesis method

=)

I've seen this in some silver nanoparticle suspensions that were known to contain a large amount of PVP. While we haven't definitively assigned this peak yet, we suspect it is from the PVP.

I agree with Dr. Ralph...d-d transitions are weak. But still we can see peaks of salt solution in UV region. However, the reason given by Dr. Robert may be also one. So, I will suggest to take UV-Vis spectra of all your reactants. If still your doubt is not cleared then repeat the experiment to make sure it is not handling error.

Hi, thanks for your answers! I checked the reagents and none of them shows any peak around 220 nm. I used chitosan (treated before with NaOH) as reducing and stabilizing agent. The signal persists in different samples.

Not by the moment, but I can do that tomorrow. Thank you!

I think it would be AQC as well.

You cannot discard yet that this peak can be attributed to byproducts of the oxidation of chitosan after the synthesis of Ag NPs. Double bonds normally show absorption in the UV. Check the presence of C=C by FTIR.

Rafael, I made an analysis of my samples following the same protocol I used for the synthesis of nanoparticles but without adding the AgNO3. The UV-vis spectrum of this solution does not show any peak around the area. By FTIR I analyzed all my samples and the chitosan solution and they show the same two peaks (including a peak around 1635 which could correspond to C=C) at almost the same intensity and wavelength.

Also I want to apologize since the peak is not around 220 nm, instead it is around 260 nm. I repeated several times the experiment and always this peak appears. Here I show you the UV-vis spectra. Both curves correspond to the same solution but the lower one is a dilution 1:6 and the upper one 1:3.

It is stable your solution?, the peak change with time,? Do you have spectra for different times after the preparation, for the same sample?

Very interesting!, another stable equilibrium?

by the way what is the temperature of your process?, do you stop to stirring after you finish to add the AgNO3 ?

My solution is relatively stable (in some experiments after several days a little dark precipitate appears but it is not significant, after that it keeps as a colloid for months). I just noticed this phenomena so I haven't taken UV-vis in time in that range. The last year (when I established the method) last year I just identified the peak around 300-450 nm. My probe got damaged so until now I retook the experiments (I just bought another probe).

Thanks Alejandro! I will check the 400 nm peak in these days. Also, I will try to measure if there is any fluorescence.

I would suggest to remove the Ag nanoparticles by membrane filtration and make another UV-Vis spectrum of the remaining Ag nanoparticle-free solution. Then you can determine whether or not this absorption may come from the Ag nanoparticles or from some organic by -products (which I would assume).

To make sure there is no silver any more in the remaining solution, you would need to a) take a very fine membrane filter (I think they are available down to 10 nm or so)

b) run AAS on the filtered solution

c) make an extract of any organic (by extracting with suitable water-immiscible organic solvents) and run UV-Vis of this organic extract

sorry, I sent too early:

d) after a), you should run a particle size measurement by laser soppler or such to make sure there are no colloidal particles any more, only a true solution

I like the idea of Bernhard, if is a population of smaller nanoparticles should stay after filtration, then a singular peak around 260 nm,

by the way where you can buy this membrane?

I forgot where we bbought them, it is years ago, but it should be easy to find them after some search in the web.

btw, I saw some typo in my former comment, it should read " particle size measurement by laser doppler "

I never did that, how you can measure the size with laser doppler?, you have a reference?

I will try to find those membranes. Thank you for all your answers! If anyone has another idea or comments I will really appreciate them.

Hi Suman, I'm not using PVP. I'm using chitosan.

What are your starting materials and what are the expected products?

No peaks from rods are at much higher frequency. I would tip for chitosan or so.