You may look at this article

http://pubs.acs.org/doi/pdf/10.1021/la202177s

You can try following method if your particles are visible using SEM (or you can try to modify it and use TEM or AFM). Use enough diluted solution (you must try optimal one) and let it equally evaporate from silicon wafer (make form with exact surface e.g. 1x1 cm). Than make several random SEM shots with same area and magnification, use image analysis to evaluate the number and size on this surface and recalculate it to used surface and used volume and dilution. This is, in my opinion, sufficiently demonstrable method of real np concentration determination.

 You may look at this article

http://pubs.acs.org/doi/pdf/10.1021/la202177s

Lukas Richtera: Sir, the method you wrote is a good method for determining size (i also uses it), but will it be suitable for all the particles in the solution at a time?

You can calculate using UV Vis

Ref:http://www.ncbi.nlm.nih.gov/pubmed/17458937

It can be done from UV-Vis Light spectra

Check the following reference: http://pubs.rsc.org/en/content/articlelanding/2014/an/c4an00978a/#!divAbstract

what kind of nanoparticles are you using?

may be, for organic nanoparticles you can use uv-vis, making a calibration curve.

for metallic nanoparticles, you can use a picnometer and determin the mass with the density of solution

You can use  inductively coupled plasma (ICP). FIrst, you have to diggest the inorganic particles in aqua regia and then measure the atomic concentration by ICP.

You can also estimate the amount of NPs by weighting 1 ml of the liquid (without particles) and 1 ml of the soution, the difference in weight is the NPs mass. 

Regards,

Check the reference

http://pubs.acs.org/doi/abs/10.1021/ac0702084

One simple method is also to dry one small part of your solution and therefore determine the massic concentration of your solution

Down is the link of an excellent review concerning your inquiry:

Tutorial Review: Nanoparticle counting: towards accurate determination of the molar concentration

http://pubs.rsc.org/en/Content/ArticleLanding/2014/CS/C4CS00128A#!divAbstract

General processes are 1.) measuring the osmotic preasure [1] - I have never seen it in an article about NPs, 2.) using pores. There are several equipment for that, izon is one example for the manufacturer [2]. It works only for relatively large particles. 

3.) As it was mentioned by Thomas Lécuyer, measuring the weight of dried sample and measuring also the size distribution allow you to estimate the molar concentration of your sample. This is the most common method as I see

Of course there are several processes specialized for some kind of nanoparticles

Omar Ayyad cited a very interesting article. Thank you for that. Maybe that is the right answer for your question

[1]: http://www.anaesthesiamcq.com/FluidBook/fl2_4.php

[2]: http://www.izon.com/trps-technology/concentration/