WG: [RASMB] nanoparticles

Thu Oct 26 02:30:58 PDT 2006

Greetings, everyone

Those are excellent points which Wendel makes, and Helmut's chapter in the
AUC Book is indeed a mountain of useful information.

It is I agree much better, if you are characterising a broad distribution,
to use interference optics. However, if the distribution is a 'narrow cut'
one (and Louise's original query referred to 30 nm particles) then you can
validly use absorption optics, even though no actual true 'chromophore' is
present. However, it needs also to be noted that speed of data acquisition
is of importance, especially when dealing with high s values, and the
interference detector wins hands down on that one. Although, as I calculate,
even the dear old slowly working Beckman scanning system is fast enough for
30 nm particles - although not for micron sized ones, for sure.

As regards the issue of 'enormous dilutions', I think there is a cultural
thing going on here. Protein chemists do much of the time work with their
solutes in the range of 0.1 to 1.0 mg/mL. So going down a bit further for
colloidal particles is no great thing. And again for narrow cut
distributions, high dilution limits the danger of excessively steep
gradients (about which, Svensson theory and all that, see my recent RASMB
contribution) causing distortion or even loss of information.

Regards to all

Arthur

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Dear Louise,

yes, we have characterized lots of these and other nanoparticles with the
AUC. There is an excellent overview on this topic by Helmut Cölfen in the
2005 AUC-book.

1) Since inorganic particles have rather high dn/dc, scattering starts at
small diameters. When measuring agglomerates, you may have to dilute
enormously to get light through to the interference detector. The
Interference detector is much more reliable. The absorption signal is not
proportional to mass fraction, but instead is weighted by turbidity. In
fact, time-scanning turbidity detectors at fixed radius are sometimes even
more preferrable: They capture the entire bunch of agglomerates up to µm,
and they see the primary particles down to ~15nm.
2) Because of polydispersity from association or agglomeration you may not
capture the entire distribution at a single speed. We measure in the same
cell successively at increasing speeds. You can log/log-plot the multiple
size distributions and thus construct the entire distribution.
3) Density is in general not known. Even inside the nanoparticle, many of
them do not reach bulk density, and additionally comes the effect of the
surface layers. We normally calculate with the bulk density anyway, knowing
from comparison with TEM that we tend to underestimate the diameter by some
10%.
4) I can only support Arthur that ls-g(s*) is the approriate fitting
algorithm. Diffusion is very weak, especially when you work with fast
speeds and the interference detector.

Beste Grüße, Wendel

Dr. Wendel Wohlleben
Polymer Physics

Phone: +49 621 60-95339, E-Mail: wendel.wohlleben at basf.com
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Ludwigshafen, Germany

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            Louise Creagh
            <alcreagh at chml.ub
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                                       [RASMB] nanoparticles
            24.10.2006 21:31

Hello,
Does anyone have experience in running samples containing nanoparticles
(30 nm diameter; copper, iron, silver or gold) in aqueous buffer
solutions on the XL-I?
Thank you.
Louise Creagh.
University of British Columbia.

--
Louise Creagh

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