Debbie, here are a couple of links for literature with discussions specifically on this problem: http://goo.gl/lA1SlB and http://goo.gl/G3lFHe .  Here is a previous thread in RG on this very topic: http://goo.gl/KChR2J   Let me know if you have difficulty in accessing any of these.

I would be tempted to add electrophoretic and magnetophoretic techniques as well -- but without knowing the type of NP you are looking at, it would be speculative.

Use fixed volume of solution, let's say 1 ml, weigh it by a precise scale. Wait the solvent to evaporates and weigh it again . In this way you will have the mass of NP in 1 ml and it will be very easy to calculate the concentration.

Debbie, here are a couple of links for literature with discussions specifically on this problem: http://goo.gl/lA1SlB and http://goo.gl/G3lFHe .  Here is a previous thread in RG on this very topic: http://goo.gl/KChR2J   Let me know if you have difficulty in accessing any of these.

I would be tempted to add electrophoretic and magnetophoretic techniques as well -- but without knowing the type of NP you are looking at, it would be speculative.

With this equipment it is also possible - NanoSight 

ex: http://www.malvern.com/en/products/product-range/nanosight-range/nanosight-ns300/

It largely depends on the type of nanoparticles you are working with. AAS works for metals, or metal-containing compounds such as oxides. The NTA from Nanosight is an interesting option, but your particles have to scatter significantly more light that their matrix (so it becomes a problem if you have, for instance, small silica particles in a protein-containing medium). Fluorescent lables on your particles circumvent this problem. The dispersity of the samples is also an issue, as having polydisperse particles results in some difficulty with the illumination settings when using NTA for quantification purposes and the quantification takes a while (so it is not the fastest technique). 

Could you be more specific about the nanoparticles you are interested in?

Hi, 

I think Dynamic Light Scattering (DLS) could also be a feasible way to determine the concentration of the nanoparticles (e.g., Ag) with an information about the distribution of their size. A similar method is proposed in "Kolloidnyi Zhurnal, 2009, Vol. 71, No. 6, pp. 728–733". 

Use UV-visible absorption spectroscopy. From the absorbance value A, A = abc, get absorption coefficient a value from literature. Chad MIrkin's group has done this, followed by numerous update from his & other groups, taking into account the ligand effect etc. Hope it helps. Thank you.

Dear Despoina

  Can try UV-Vis, AAS and ICP-MS.