Is it possible to express the concentrations, in terms of molarity, of those molecules or compounds where we know their respective atomic/molecular (sometimes formula) weight, but what about nanoparticles? There are some publications where mM have been used for nanoparticles.
How to express the concentrations of nanorods, nanowires or quantum dots or nanospheres into molarity (for example 0.01 mM)?
The answer for spherical nanoparticles (Quantum Dots) is straightforward. If a certain weight of nanoparticles are dissolved using aqua regia, the concentration of the constituent elements can be determined using AAS or AES. If the diameter of the particles is then measured from transmission electron microscopy, the concentration of nanoparticles in a known volume can be determined.
Furthermore, if the optical absorption spectrum is measured on this solution with a known concentration, then the optical extinction coefficient can be calculated. There have been several good publications in this area, and for a range of semiconducting nanoparticles the extinction coefficient is well approximated by several empirical formulas arrived at in this way.
http://pubs.acs.org/doi/abs/10.1021/jp906827m
The answer for spherical nanoparticles (Quantum Dots) is straightforward. If a certain weight of nanoparticles are dissolved using aqua regia, the concentration of the constituent elements can be determined using AAS or AES. If the diameter of the particles is then measured from transmission electron microscopy, the concentration of nanoparticles in a known volume can be determined.
Furthermore, if the optical absorption spectrum is measured on this solution with a known concentration, then the optical extinction coefficient can be calculated. There have been several good publications in this area, and for a range of semiconducting nanoparticles the extinction coefficient is well approximated by several empirical formulas arrived at in this way.
http://pubs.acs.org/doi/abs/10.1021/jp906827m
if the molar mass of the material forming the nanoparticles is known, and if the nanoparticle concentration is known (either by absorption coefficient, or by whatever other technique, then sure you can also express the concentration in molar concentration terms.
I prefer not to. I think one should avoid using molarity for nanoparticles / colloidal dispersions. I give you an example:
I am working with Polyaniline, which is an organic nanometal (conductive polymer). Many scientists (including the Nobel prize winners Heeger, MacDiarmid and Shirakawa) believed that certain polyaniline (or other ICP) / solvent systems are true solutions, they ignored the results of lots of experiments which showed that these transparent and colourful liquids in fact are dispersions, containing polyaniline (or other ICPs) in form of nanoparticles having a size of between 10 - 30 nm (or even up to 100 nm, depending on dispersion capability).
Nevertheless, the scientific community still until today uses GPC to determine "molecular weight" of polyaniline, i.e. believing that these "solutions" can be compared with standard PS solutions, they ome to the conclusions polyaniline having a Mw of (let's say:) 40,000 (or 1,000,000)
However, as they had used dispersions, GPC can only tell how these nanoparticles behave in the column, but can not tell the Mw.
Therefore, any molarity information is totally misleading because in this case the Mw is even not known, but worse: has been measured using in inappropriate method.
When using MALDI-TOFF, we found much lower Mw values, i.e. Polyaniline only being an oligomer built from around 10 monomer units.
Hi Ramin,
As an alternative to AAS, you can obtain the molar mass and concentration of particles in a dispersion with the following information:
1) Particle diameter by TEM (or UV-vis, if using a known correlation)
2) Bulk density of the material (e.g., wurtzite CdSe)
3) Thermal gravimetric analysis (TGA) to determine the weight percent organic content in the sample. Take a known volume of your dispersion, bring it to dryness, and obtain the mass per unit volume. Then get thermal analysis of the powder.
Based on the above information you can calculate the molar mass per inorganic colloid (using diameter and density). Determine what weight percent of your material is due to the inorganic colloid (TGA). The remaining mass is due to a combination of free and bound ligands for sterically stabilized colloids, which must be taken into account when weighing out particles from a dry powder. Now you have mass per unit volume, inorganic content per unit mass, and moles of inorganic colloid per mass inorganic content. Do unit conversions to get to moles of inorganic colloids per unit volume.
I like this method because it uses reasonable assumptions and enables weighing known amounts of particles from dry powders. You can use this method for spheres or nanorods. We approximated nanorods as cylinders with hemispherical endcaps, but you can get as sophisticated as you like. Attached is a link to supporting informaiton showing this calculation (Fig. S-16).
As to expressing concentrations of particles, it is helpful to clearly state whether you are referring to moles of a material (i.e., Cd) or mole of particles (i.e., CdSe quantum dots) per unit volume.
http://pubs.acs.org/doi/suppl/10.1021/nn3019859/suppl_file/nn3019859_si_001.pdf
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