This is closely depending on the size of the metal nanoparticles. However, by performing a centrifugation for 30' at a speed of ten thousand revolutions per minute, the complete collapse of the metal particles is usually obtained. 

Thank you sir for the useful information

Dear Researcher, I worked on Solid gold Nanoparticles and as per my experience it totally depend on its size. Example for 50 nm particle size it is 8000 nm for 30 min, while for 20 nm it is 14000 nm for 30 min.

Dear sir thank you for your information on rotation depends on size of the nanoparticles in colloids

Welcome Sibashish. Good luck for your research.

Any smaller particle is difficult to agglomerate if the zeta potential is high enough- and gold is such a case. In our lab we simply freeze the liquid and let the particles agglomerate during the time ice melts!! This is however not a universal formula...... but for gold it works well. The concept is to disturb the charge double layer (that can also be disturbed by putting in excess salt in the colloid)

You will get 70-80% of pellets if you do centrifugaion at 13000-14000 rpm.

i have prepared the Au nanoparfticle by citrate reduction the size is around 10nm i tried centrifuge with 9000 RPM for 30 min but i noticed the particle aggregates (the color of the particle change from red to violet). joydeep sir can u give any solution for this problem, and please explain about your freezing technique.

First of all you have to consider the centrifugal acceleration not the rotation speed. It also is determined by geometry of the device.

Than it is quite straight forward - use the Stokes equation. As long as particle concentration is not much higher than 1 vol % you may neglect hindrance between particles. If particles are sufficiently stabilized they will not agglomerate in the sediment. If in the range of 100 nm or smaller they will redisperse by diffusion over longer durations.

Prof. Rakesh Kumar Tekade  could please tell me if there is an equation to know the required rpm to separate nano-particles from a colloid  depending on their size  

as written Stokes equation gives you the relation between particle velocity (distance/settling time) and particle size. First determine centrifugal acceleration from rpm.

to use Stoke's law we need the density of the nanoparticles. Anyone does know how to obtain the anisotropic bimetallic nanoparticles' density?or is there another method for calculation the minimum centrifuge speed?

Do a worst case assumption - i.e. use literature value of density of the heavier metal.

For 10 nm it is usually 15000 for 20 min.

use Stokes for estimation - acceleration is needed not rpm

https://www.nanopartz.com/gold-nanoparticles-properties-centrifuge-speeds.asp

Titus Sobisch When I use the Stokes equation, the required speed for gentrification, is obtained much larger than reported number for similar nanoparticle in literature.

besides other possible reasons it might be simply a calculation error. It is easy to overlook the unities, e.g. 1 mPas*s is 0.001 kg/(m*s).

Dear Dr Dutta,

If you have nanoparticles of size 10-30 nm, please centrifuge them at 14000 RPM for 30 minutes. If the size is 50 + then centrifuge them at 10000 RPM for 15 minutes.

Repeat the centrifugation washing 2-3 times untill you get a clear supernatant. The pellet should be the pure NP.

Hope this helps

Regards

Rakesh

effective acceleration is needed not rpm if centrifuge is not built identically.

I used to use 18,000 rpm in Sorvall™ WX Floor model at 10 oC for 30 min for ~40 nm sized gold nanoparticles. But centrifugation can cause irreversible aggregation of nanoparticle.

I find the equation to be unreliable and counter productive. The variables like heterogeneity of the particle size, viscosity of the solution based on reactants used in synthesis of the particles and the length of the centrifuge tube make the equation impractical to use. Furthermore, sedimentation rate of a homogeneous set of nanomolecule in a solution of known viscosity all at the same distance from the center of the spinning axis is affected (more) by what you added onto its surface than its size. Overestimating your rpm (acceleration) can alter the nanoparticles.

irreversible aggregation of nanoparticles by centrifugation is a myth, though if separated sediment might be hard to be redispersed.

Also separating the adsorbed/adhering layers on their surface is highly unlikely.

Molecules are nano anyway - so no need to call them nanomolecule - suspect nanoparticle is meant, which is always composed of several atoms/molecules.

https://www.nature.com/articles/s41598-018-23064-4

This article discusses the variables that determine the size of a nanoparticle formed in electrostatic interactions between TPP and Chitosan. It identifies the concentration of Chitosan and its relative concentration to TPP as the most important factors determining nanoparticle size. This raises the theoretical possibility that a centrifuge tube is in fact a reaction tube carrying out the final step in making larger entities than originally intended. The constraints and difficult to control/estimate variables of the equation can be overcome using a variable speed centrifuge and patience. However, even though I agree that any clump can be broken up with enough time and force, the integrity of a finicky nanoparticle that can open up and release its contents with a simple change in pH should be handled with great care.

What about the effet of colloidal solutuon volume?

minimum centrifugal acceleration required does not change with volume to be treated, however, problem is development of larger devices which can cope with stress on construction material.

It is associated with the preparation methods such as use of the stabilizers or the reduction agents. So, there is no exact rpm value for precipitation of the every silver nanoparticles.