I would make a particle size (and size / number distribution) analysis using standard methods, check whether the size distribution is normal, take the average, and calculate their volume

Would you please elaborate, how to do particle size analysis?

I think the best way to do so is SEM or TEM. Hope your particleas are not very sensitive thou.

no, don't use SEM or TEM, you can never get precise enough information about particle size distribution, you can only see a very limited number of particles and you don't know how representative those you see are.

The best is to use Laser Doppler (the dispersion should be transparent, not opaque)

Please tell me whether we can use Dynamic Light Scattering (Zeta Sizer) for that or not?

DLS can give you an idea about the particle size distribution but it is not accurate.

yes, you can also use DLS

yes, you can also use DLS

I think, that for this purposen  using DLS will be good method. Each methods have owen accuracy. May be your accuracy will be no high, but you can estimate your volume concentration. I think, that you will use avarage particle size.

Thank You all for your replies.

Sorry to drop in on the conversation.

Izon sell a fantastic piece of equipment that can characterise individual nanoparticles:

http://www.izon.com/products/qnano-2/

A new and promising technique that is growing traction from Prof. Richard Compton's group is electrochemical collision detection (termed nano-impacts), at a recent conference held by my supervisor Prof. Frank Marken,  the group presented results showing that the are beginning to characterise the shape of the nanoparticles too.

Example paper:

http://onlinelibrary.wiley.com/doi/10.1002/open.201402161/epdf

1. If you know the density of your nanoparticles (rho-np) and the density of the solvent (rho-s), then measure the density of your dispersion (see #4 below), rho-d. Let phi be the unknown volume fraction of your nanoparticles. Your answer can be obtained by solving the following for phi:

rho-d = rho-s x (1-phi) + rho-np x phi

2. An alternative approach is to measure weight fractions and then convert to volume fractions. Let w be the weight fraction in dispersion of your nanoparticles; this can generally be measured accurately gravimetrically by drying a weight amount of dispersion in a drying pan.

1/rho-d = (1-w)/rho-s + w/rho-np

Typically you know rho-s and you measure rho-d, and then you measure w. You can then solve for rho-np.

3. If you have access to an ultracentrifuge and you can create a density gradient on it that exceeds the density of your nanoparticles, you spin until your nanoparticles have equilibrated at their density. Typically you remove fluid and measure its density very accurately (preferably with a u-tube density meter) and simultaneously measure for particle content (turbidity or UV/Vis absorption), and this approach is probably the most accurate, as it includes surface effects ignored in #1 and #2 above (such as effects of strongly bound solvent or effects of strongly bound ions and counter-ions).

4. In your question you use the term "nanofluids." Dilute to concentrated dispersions of nanoparticles have been used in industry for over 100 years (at least in the nearly extinct photography industry) and for milennia in inks. The term nanofluids has been used to denote dilute dispersions of common inorganics that purportedly have significant heat transfer properties. I believe the existence of such significant effects are marginal, at best, and this nanofluids term is used to hype an otherwise disappointingly small physical effect. What you call nanofluids are simply dispersions (of nanoparticles in solvent) or suspensions.

Thank you John for your elaborate answer and I find it very interesting and useful. I agree with your views on 'Nanofluids'. Addition to your points, I want to say that earlier days we were only focusing on preparing nanopartcles  from the suspensions but now the importance of these suspensions (nanofluids) are also noticed in different applications. Although as u said, it has been used in ink and photography industries as well.