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In order to obtain information about Ag size nanoparticles and their aggregation you must to prepare these nanoparticles in polimer matrix. Then it is necessary to carry out the SEM measurements. Here we see SEM pattern for Ag nanoparticles in polymer matrix.

Nanoparticles synthesis completion time and when will u purify the nanoparticles time is the major source for the "no aggregates"

I mostly agree with Martin Rudolph except for that the forces which cause aggregation of nanoparticles are not covalent bonds, these are interfacial attraction forces, in general terms "van der Waal's forces", whatever that means. the effect of the agglomeration is that each particle which gets together another nanoparticle can reduce its surface tension (because then their "joint" interfacial tension is lower than the individual surface tension.

Which means, you can only get "isolated" nanoparticles if you disperse them = you force another medium to adsorb on your nanoparticles and create a lower interfacial tension.

This all is not trivial at all, it requires a deep theoretical understanding, and a lot of experimental work. I can refer you to some of my publications in my RG site:

https://www.researchgate.net/publication/202290104_Critical_Shear_Rate_-_the_Instability_Reason_for_the_Creation_of_Dissipative_Structures_in_Polymers?ev=srch_pub

and several more which you can find on my page.

Article Critical Shear Rate - the Instability Reason for the Creatio...

I suggest you to keep your NP in colloidal dispersion and functionalize the particles with molecules / polymers that can stabilize or cap their surface... but taking into the due account the application for which your NPs are intended (the application environment may de-stabilize particles favoring agglomeration during usage, that would not be so nice).

As an example, small Ag NPs, say around 10 nm diameter, in water are stabilized by just sodium citrate and may keep a stable state for ages if well stored. My direct experience is nearly 3 years stability, that is optical response fully reproduced, absence of any precipitate.

Also PMAA- or PVP- capped Ag nanoparticles are stable. n-thiole functionalized Ag or Au NPs are also stable in non-water solvent.... and much more. There is a plethora of possible ways/systems reported in literature. But I underline again: stability is quite important also (and perhaps mainly) in the application environment.

First try with TEM in order to know if you have big particles or aggregates.

Dear Giuseppe, to functionalize or to cap nanoparticle's surface with anything is a caveat if you investigate toxic effects of metallic nanoparticles as they are. In our experience with nAg, nAu and nCu a laser technique proved doing the trick. See for instance, Katsnelson, B.A.; Privalova, L.I., Gurvich, V.B. et al. Comparative in vivo assessment of some adverse bio-effects of equidimensional gold and silver nanoparticles and the attenuation of nanosilver’s effects with a complex of innocuous bioprotectors. International J. Molec. Sci., 2013,14,2449-2483 - the full text is available both online and in my ResGate profile.

Some citation from our paper I've just mentioned:

"For our study, we have prepared stable suspensions of silver and gold nanoparticles by the method of laser ablation in liquid including the following main phases: (a) laser ablation of the metal target in water, and, where necessary, (b) laser fragmentation for preventing particle aggregation, and (c) suspension concentrating. In our case, concentrating was necessary to enable the administration of sufficiently high NP doses to rats in minimal volumes of water.

A plate of silver or gold with a metal content of 99.99 % was placed on the bottom of a dish with deionised water. Metal ablation was performed using Fmark-20RL laser material processing system (by Laser Technology Center, Russia) based on ytterbium-doped pulsed fiber laser (pulse length 100 ns, repetition rate 21 kHz, wavelength 1064 nm). The energy density was 80 J/cm2. The target was irradiated in scanning mode with a rate of the laser ray 270 mm/s.

The concentration of the primary suspensions obtained by ablation was 0.10 mg/ml for gold, and 0.12 mg/ml for silver. Fragmentation was used for preventing the aggregation and for enhancing the stability of the nanogold suspension, involving after-treatment with a laser ray focused into the bulk of the suspension. An increase in concentration to 0.5 mg/mL was achieved by drying the suspensions, which was not accompanied by nanoparticle aggregation. Particle images were obtained after concentrating by scanning electron microscope (SEM), AURIGA CrossBeam Workstation (Carl Zeiss, Germany), which enabled us to identify their spherical form . Average NP diameters (±s.d.) obtained through statistical processing of hundreds of SEM images were: 50±20 nm for NG and 49±10nm for NS, the distribution being symmetrical. Nanogold particle size was also measured by the dynamic light scattering method by Zetasizer Nano ZS analyzer (Malvern Instruments, UK), and provided a sufficiently close result, 58±19 nm. No essential changes took place 30 days after the preparation of the suspensions in either zeta potential or the form and position of the plasmon resonance peak, providing evidence of their high stability."

Later on, we found that (a) the same good results were obtained with laser ablation of copper, and (b) if starting from a commercial nanopowder of the copper, a stable water suspension can be obtained directly by rhe laser fragmentation. Mind you, that no chemicals were added to the de-ionized water. In our earlier experiments with nanoparticles of supermagnetic Fe3O4 (see the publications in the same ResGate profile of mine) we used ultrasonication but stability of suspensions was not as good. In that case we also avoid any chemical intervention.

Dear Ari, is all this of any help to you?

pH, tempratuture, conc. of ions,reducing agent and capping agents plays vital roll in fabrication of metallic nano particles.

you may optimise nano particles synthesis by controlling these parameters also can try to use pure biomolecule (amino acid)

which can act as capping and reducing agent. this way you may able to reduce intensity of your issue

Deae Ari, I suggest you to use stabilizer or surfactant with simple techniques. You can use polymer(PEG, starch, ...), surfactant (AOT, SDS, CMC,...), TEOS and uder stabilizer.

If you want articles about the stability of of silver nanoparticles, I will send you.

Good luck.

Dear Boris,

I truly thank you for valuable info you provided.

Your question is how to keep your Ag apart from each other?

Have you heard of capping agents? they are normally organic acids, you may try citrate because it is commonly used in Au and Ag nano particle preparation. I don't know your you prepared your Ag nanoparticles (there are several ways by electrochemistry or reduction). There are plenty in the journals. you may assist with nonionic surfactants. If you were really producing Ag, you should be able to see silvery layer on your air-water surface.

I am agree with Misni Misran, capping agent (surfactant) are very effective means to avoid aggregation or agglomeration of nanoparticles.

But as a science worker I had found many times even you apply agents they had their own implications and further you have to work on those thing.

So few generic thing you can apply:

Selection of favorable solvent.

Storage of nanoparticles at different concentration so that you can analyze which concentration is good for storage. (concentration of solution also affect aggregation).

Storage temperature (please do not give temperature shock to solution try to place it at cold temperature about 4 to 5 )

Caping agent should be as per nature of nanoparticles.

With regards

zetapotential one of the factor for stability issue for stable particle(with out agglomerates) by keeping the high surface zetapotential i.e.either negative or positive above 30 .it prevents the agglomeration of your formulation.

to avoid aggregates in nanoparticles the electrostatic repulsion should be there between particles that is the surface charge which is indirectly the Zeta potential,by keeping the value of Zeta potential from -20 Mv to +20 Mv we can protects nanoparticles from aggregation. this can be done by coating nanoprticles with suitable Surfactant or by Probe sonication

sorry, aggregates (even if strong) do not have *covalent* bonds, there are no checmical bonds in the interface at all, only adhesive / cohesive forces are active forming agglomerates / aggregates.

After completion of synthesis work, keep the synthesized material in a stirrer, which will remove agglomeration / aggregation. Most of the bio-mas / bio-materials have stabilizing agent which also prevent agglomeration / aggregation.

Keeping them dispersed in a solvent rather than powder form, low concentration of dispersion, suitable stabilizer based on the type of nanoparticles (for example oleic acid, oleyl amine stabilized magnetic nanopartilces), mild reducing agent and the use of simple salt, use of ultrasonication to disperse are some of the common ways to avoid nanoparticles aggregation.

Nanoparticle Aggregation: Principles and Modeling, gives a brief overview in this area

http://link.springer.com/chapter/10.1007%2F978-94-017-8739-0_2

Aggregates can be process induced, think if you are activating amine functionalized particles with glutaraldehyde then you may get large aggregates. For more insight please refer to DOI:10.1039/C3AN01294H.