It is common to measure the peak absorbance of the plasmonic nanoparticles in solution and then get the following:

1. Molarity (moles/liter)
2. Number of particles per ml
3. Weight concentration (ug/ml)

I wrote a small python function to extract these parameters.  The following inputs are needed for the function

1. d_nm :   diameter of the particle in nanoparticle
2. od :  peak OD measured from the absorbance plot
3. path_length_cm :  cuvette length in centimeter
4. density_g_per_cm3 :  for gold it is 19.28, for silver it is 10.49
5. molar_Extinction_perM_percm :  Molar extinction coefficient extracted from here.
6. This can extracted from measuring absorbance at different concentrations and then fitting a linear line. the slope will be molar extinction coefficient

Here is the code.