Dear Cesar,

from a chemist's point of view, an ICP (inductively coupled plasma) measurement would be the best. Don't forget to wash (centrifuge-decant-redisperse-repeat) them well, to remove any traces of your platinum precursor salt.

From an electrochemist's point of view, you could also try simple cyclic voltammetry, depositing a known amount of NPs (let a droplet of a known volume of the dispersion dry) on an inert electrode (gold, carbon). The hydrogen ad/desorption region in acidic media could then be used as a good measure of both the surface cleanliness of your NPs and their total surface area. Then, if you are sure they are 4nm, you will have matching numbers. If not - maybe your assumption about their diameter is off.

You can then also do more accurate stripping voltammetries (e.g. CO stripping) to get the best possible measure of the surface area of your catalyst, assuming these are meant to become one. 

Good luck!

Dear lendon, the best method to accurately calculate the concentration of Pt nanoparticles is to do either ICPMS or AAS.

By saying the concentration of Pt nanoparticles, do you mean the number of nanoparticles in the solution? 

We used to calculate out the total Pt atom numbers and the atom numbers of a single Pt nanoparticle (D=mean size), and get the total number.

Yes, I will explain, my problem is that I synthesized the Pt nanoparticles by using NaBH4 method added dropwise, once I finish the synthesis based on number of atoms and amount of Pt I added and the mean size of the nanoparticle the calculated concentration is 1 micromolar, however when I start doing dilutions to get into a picomolar range I don't get adequate response instead when I used a nanomolar concentration it gives the right response.

In what solution do you disperse your Pt NPs, and what stabilizer do you use? Maybe you need to sonicate it further to get a nice Pt dispersion.

I used to have PVP-Pt NPs, they would take quite a long time to redisperse in EtOH and water.

You must make sure your suspension is homogeneous.

I believe that the accurate method to calculate concentration of nanoparticle is ICP.

Dear Cesar,

from a chemist's point of view, an ICP (inductively coupled plasma) measurement would be the best. Don't forget to wash (centrifuge-decant-redisperse-repeat) them well, to remove any traces of your platinum precursor salt.

From an electrochemist's point of view, you could also try simple cyclic voltammetry, depositing a known amount of NPs (let a droplet of a known volume of the dispersion dry) on an inert electrode (gold, carbon). The hydrogen ad/desorption region in acidic media could then be used as a good measure of both the surface cleanliness of your NPs and their total surface area. Then, if you are sure they are 4nm, you will have matching numbers. If not - maybe your assumption about their diameter is off.

You can then also do more accurate stripping voltammetries (e.g. CO stripping) to get the best possible measure of the surface area of your catalyst, assuming these are meant to become one. 

Good luck!

use Atomic Forced spectrometer

You could use either AAS or ICP.

Capping agent is citrate they are in water apparently they are not very well disperse becuase the theoretical concentration doesn't match the experimental one, it is about 10 times more diluted. I'm guessing is because they are not very well disperse is there a way to reduce that aggregation and disperse them after the synthesis.

Thanks