Measurement of particle concentration can be done by any of the following methods Nanoparticle Tracking Analysis (NTA), Resonant Mass Measurement (RMM), Transmission electron microscopy (TEM) or UV-Vis spectroscopy. Preparation of nanoparticles for a diverse range of applications is currently a very vibrant area of research. Many groups are working on these aspects and quite a good number of literature is available

Measurement of particle concentration can be done by any of the following methods Nanoparticle Tracking Analysis (NTA), Resonant Mass Measurement (RMM), Transmission electron microscopy (TEM) or UV-Vis spectroscopy. Preparation of nanoparticles for a diverse range of applications is currently a very vibrant area of research. Many groups are working on these aspects and quite a good number of literature is available

I add ICP-MS/OES/... to the answers before :)

All the above are number counters of course - numbers of particles per unit volume.  Lots of techniques for this.   Not sure how TEM gets a particle concentration though..The concentration in moles/liter (or wt % or ppm) can be measured by standard chemical routes depending on the type and chemistry of the nanoparticles.  For example the concentration of Pd on a Pd/C catalyst can easily estimated by acid digestion and appropriate wet chemical technique from AAS, or ICP to UV-VIS and other colorimetric techniques

General method to calculate the concentration of nanoparticles in the solution is as follows;

a) You know the weight of NPs you have synthesized ( W)

b) Density of NPs = gcm-3 (d)

c) Now you calculate the volume of NPs taken = W/d

d) Calculate the average size of nanoparticles using TEM (make sure nanoparticles are monodispersed) (2R)

e) Calculate the volume of a single nanoparticle (assuming sphere) = 4x(pi x R^3)/3 = V

f) You can calculate the number of particles in colloidal solution (N) = (W/d) / V

g) Calculate the conc of NP = N / Final volume of colloidal solution

Thank you all for kindly helping me out.