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nanoparticles for biological solutions

Nanoparticle quantification. When is a mole not a mole ?

The answer is when referring to the “molar” concentration of nanoparticles. There is some ambiguity in discussion groups, blogs and publications regarding the quantification of gold nanoparticles in cell uptake experiments. Some publications refer to gold nanoparticles in terms of molarity. Molarity actually refers to the amount material in a homogeneous solution of molecules  ( or atoms) as a fraction of  its molecular weight in a litre. A property of  1 Mole of material is that it contains 6.023 E23 molecules i.e. Avagadros’ number. Molarity cannot apply to nanoparticles because they are not molecules or homogenous in size. The stated size is a mean based on a spread of slightly different sizes.  If you consider 50 ug/ml of 10nm gold particles there would be about 5 E12 particles and a molarity of 0.25 mM with respect to elemental gold. A 50 ug/ml of 20 nm gold particles would be of the same molarity (0.25mM) but would contain about 0.6 E12 particles. So quoting molarity of gold has to refer to the amount of gold atoms present but without the mean particle size as well the molarity would be meaningless.  To confuse the issue even more  there are some publications where they refer to the Molar extinction coefficients of nanoparticles which appear to be based on Avagadros number rather than molecular weight as nanoparticles don’t have a molecular weight .  This is quite a useful was to express the number of particles used but the nomenclature leads to ambiguity.  A specific term such as the “particulate molarity” would distinguish  quantification based on numbers of particles rather than gold atoms. To summarise whatever method is used to state the quantity of gold particles used it should be clear that molarity  refers to elemental gold  and that quantity based on Avagadros’ number should be expressed in a more specific term than just molarity. Personally I just prefer to use g/l or particles/ml but also including the mean particle size.


18/11/2015 - Posted by | Quantification |

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