Dear Parna Ganguli

Sometime if the particle size is high and heavy, during ultrasonication it settle down at the bottom.

To make suspension, do stirring the solution using magnetic stirrer.

Regards,

Bapi

Do you have to dry them? You may achieve better results through repeated cycles of centrifuging, removing the supernatant fluid, adding clean water and sonicating the still-wet sediment.

Drying nanoparticles is usually a guaranteed way to make them impossible to resuspend.

John Francis Miller Sir, thank you so much for responding. Actually, I need to prepare different concentrations of AgNP impregnated growth media to test their antibacterial efficacy. For this purpose, I was trying to resuspend the dried nanoparticles in water (so as to achieve various concentration gradients of x mg/ml, y mg/ml and so on) and then add them to the respective media. But, I'll avoid drying the nanoparticles from now on as you advised and I'll try to determine the particle concentration spectrophotometrically.

Parna Ganguli

There are no magic chemicals to de-aggregate (agglomerates are loose and can be disrupted by sonication) fine powders. The only true dispersing agent is energy - sonication in the wet. The steps in making a stable dispersion from a powder are:

• Wetting - this is the main function of a surfactant. If the solid wets in the liquid then a surfactant is not needed
• Separation - the key step. This can only be accomplished by energy input (usually sonication in the wet) and huge amounts may be needed akin to ultrasonically milling the material. An air jet mill reaches the comminution limit around 0.5 - 1 μm for most dry materials
• Stabilization - steric or charge (electrostatic) where the optimum amount of additive will prevent recombination of separated particles

The only way to correctly keep a system 'nano' (100% < 100 nm if you use the ASTM and ISO definitions) is to never dry it and preserve it in the wet state.. Your dry powder can be measured by laser diffraction, not DLS as it will be in the 0.5 - 1 μm minimum size. If it settles in water it's probably tens of microns in size (verify with a Stokes' Law calculation. Mandatory)

Keep your material in liquid 'suspension'. Never dry. This will prevent the irreversible aggregation of nanoparticles. You can get different concentrations by dilution with the 'mother liquor' (never in DI water).

See (registration needed):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Generalities on silver colloids are found in these webinars (registration needed):

Silver colloids and invisible ink

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W150902SilverColloidInvisibleInk.html

Metal colloids - their preparation, application and characterization

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W160218MetalColloids.html

Be aware that the surface of silver is not silver metal but silver in the +1 oxidation state (and it’s – slightly soluble - Ag+) that’s the bactericide not silver metal.

This question or similar has been posed many times on RG. See, for example:

https://www.researchgate.net/post/Best_characterization_methods_for_nanoparticles_synthesized_from_plants

https://www.researchgate.net/post/How_to_deagglomerate_the_dry_nano-particle_powder_for_DLS_measurements_How_to_use_a_suitable_surfactant_for_this_purpose

2 quotes from those greater than I...:

• 'I think dry nanotechnology is probably a dead-end' Rudy Rucker Transhumanity Magazine (August 2002)
• ‘If the particles are agglomerated and sub-micron it may be impossible to adequately disperse the particles……‘The energy barrier to redispersion is greater if the particles have been dried. Therefore the primary particles must remain dispersed in water….’ J H Adair, E. Suvaci, J Sindel, “Surface and Colloid Chemistry” Encyclopedia of materials: Science and Technology pp 8996 - 9006 Elsevier Science Ltd. 2001 ISBN 0-08-0431526

Chapter by a criminal:

Chapter Characterization of Nanomaterials

This can be obtained by request to the aforesaid criminal.

Alan F Rawle Sir, thank you so much for the detailed insight. Your answer was really helpful. I just have one more query, Sir. For SEM and XRD analysis we are always asked to submit the nanoparticle sample in their dry form. Does drying the nanoparticles for this purpose produce an undesired impact on the respective results in any way?

Parna Ganguli

Yes, sample preparation alters the material. It's obvious in hindsight...

XRD can be performed in the wet with a sample preparation device like a tube with a Mylar cover. Possible on the Empyrean system (Malvern Panalytical).

Note drying the system could change a small amorphous material to a denser, larger more crystalline material (and we usually look for crystal planes in XRD, although the extent of amorphous material can be gauged often). Note that you may need many hours of irradiation for reasonable peaks (especially if you are to apply Scherrer or Williamson-Hall; I had to use 24 hours for my 1 - 10 nm catalyst systems but this was back in the 70's: not my 70's - which I am yet to attain - but the 1970's..) In SEM sample preparation can be controlled to leave the particles well-dispersed - search on RG. You could absorb them onto an inorganic oxide (such as dry silica) and then dry. This may (via steric stabilization) keep the small nanoparticles separated. This was the route I took for my catalysts (5% Ag nominal plus other Pd, Pd-Au, Pd-Ag systems) during my Ph.D.

2 more quotes from those greater than I...

• “It cannot be over-emphasised (Br.) that great care must be taken in preparing any sample for size analysis. The objective of the investigation is to determine the size distribution of the sample. What is often determined, however, is the effectiveness or destructiveness of the disaggregating or dispersing technique” J S Galehouse Sedimentation Analysis in R E Carver (Ed.) Procedures in sedimentary petrology Wiley-Interscience Chapter 4 (1971) ISBN 471 13855X pp 69 – 92
• “Very little appears to be known about how the differences of behaviour (Br.) arising from the differences between the conditions of test and use, can be predicted, but there is no doubt that such information is necessary before the control of such materials can be adequately carried out” Dr. Rose in Discussion (page 143) following paper “Particle Shape, Size and Surface Area” in Powders In Industry SCI Monograph No 14 Society of Chemical Industry, London (1961)

Alan F Rawle Sir, thanks a lot again!

I am not very familiar with electron microscopy, wonder whether it would be possible to do freeze fracture analysis of concentrated suspensions?

Alan F Rawle - 'the only true dispersing agent is energy' is not fully correct. For particles fused together by chemical bonds, chemicals like NaOH, surfactants, phosphates, solvents can assist in breaking these bonds. We also see this, e.g., for carbon black, where at low dispersing intensity other solvents are best performing than at higher intensity, simply because the 'chemical' contribution is more important at low input of dispersing energy.

Titus Sobisch

Yes, you are correct. My generalization with energy as the only dispersion agent should have had one caveat. Stress corrosion milling (as this dissolution technique is called) can be used to dissolve chemical bonds between aggregates (I've used this for copper powder with nitric acid). I mention this in the 'Dispersion and Nanotechnology' webinar noted above. It's not always easy to apply and fairly specialist: not a general technique for every application. The conditions have to be very, very carefully controlled e.g. pH. The possible 'side effect' is the complete dissolution of the powder. I've also seen it applied with silica where NaOH or KOH can be used.