If I may, I'd like to amplify John's comments -- this is the area our 
lab is working in. Not so much quantum dots, as labels for correlated 
light microscopy and electron microscopy, and multiple labels for EM. 
So naturally, we've looked into QDs.
They're mostly made of cadmium-selenide and similar alloys. The 
composition isn't secret, but QD is (reasonably) close about how they 
make the dots. It's not particularly easy to make quantum dots 
(unlike colloidal gold and other metals), and  involves organic 
reagents and non-aqueous solvents. Which can make the conjugation 
difficult (as opposed to gold, which is easy).
A second problem is that the elements used -- e.g., CdSe -- are 
toxic, which causes problems with living systems obviously, and with 
the antibodies or other ligands people wish to stick onto the dots. I 
believe QD's dots aren't so much conjugated to proteins or whatever 
in the sense that gold particles are, but are covalently bound with 
small intermediate molecules. This enlarges the final particle + 
macromolecule size and so reduces the spatial resolution useful in 
EM, but probably is irrelevant in LM.

Phil

>Quantum dots are nanocrystals of a semiconductor
>(exact chemical identity seems to be trade secret).
>They may have a future in a new generation of
>tiny computers.
>
>Quantum dots fluoresce in a wide range of colours
>and can be conjugated to antibodies, avidin,
>phalloidin etc. You can buy conjugated reagents
>or kits to conjugate your own.
>The Quantum Dot Corporation's web site
>http://www.qdots.com/live/index.asp
>shows some pretty pictures of labelled cells.
>
>There is a recent paper about quantum dots in the
>J. Histochem. Cytochem. Here's the bibliographic
>information and abstract, from
>http://www.jhc.org/cgi/content/abstract/52/1/13
>If your institution subscribes to the journal you'll
>be able to get the full text on line.
>
>---------
>Rozalia Nisman, Graham Dellaire, Ying Ren, Ren Li and David P.
>Bazett–Jones
>(2004)
>Application of Quantum Dots as Probes for Correlative Fluorescence,
>Conventional, and Energy-filtered Transmission Electron Microscopy
>J Histochem Cystochem 52:13–18
>
>Luminescent semiconductor quantum dots (QDs) are a new class of
>fluorescent label with wide-ranging applications for cell imaging. The
>electron density and elemental composition of these materials permit the
>extension of their use as probes in conventional electron microscopy
>(TEM) and energy-filtered TEM (EFTEM). Here we illustrate the
>feasibility of using streptavidin-conjugated QDs as TEM tags by labeling
>a nuclear protein on cell sections and obtaining correlative
>fluorescence and TEM data. We also show that QD probes can be employed
>in conjunction with immunogold for co-localization of proteins at the
>ultrastructural level. Furthermore, by obtaining cadmium elemental maps
>of CdSe/ZnS QDs distributed on a nuclear structure, we demonstrate the
>potential of QDs for co-localization of multiple proteins when used in
>combination with EFTEM.
>---------
>
>A Google search for quantum dots brings up thousands of hits.
>Searching for "quantum dots" and histochemistry and cytochemistry
>brought up 60 hits, at least half of them in English and
>relevant.
>
>John Kiernan
>London, Canada.
>-----------------------------------------------
>Patsy Ruegg wrote:
>>
>>  What are "quantum dots"?
>>
>>  -----Original Message-----
>>  From: histonet-bounces@lists.utsouthwestern.edu
>>  [mailto:histonet-bounces@lists.utsouthwestern.edu] On Behalf Of Anna
>>  Elisse Beaudin
>>  Sent: Wednesday, October 13, 2004 7:55 AM
>>  To: histonet@lists.utsouthwestern.edu
>>  Subject: [Histonet] IHC withquantum dots
>>
>>  Hello,
>>
>>    Has anyone ever done immunohistochemistry on tissue sections using
>>  primary (or secondary) antibodies conjugated to quantum dots?  if so, do
>>  you think it would be possible to do multiple labeling using different
>>  colored quantum dots?  thanks in advance for your help!
>>
>>  Anna Beaudin
>>  Division of Nutritional Sciences
>>  Cornell University
>-------------------------------------------------------
>
>_______________________________________________
>Histonet mailing list
>Histonet@lists.utsouthwestern.edu
>http://lists.utsouthwestern.edu/mailman/listinfo/histonet

-- 
Philip Oshel
Supervisor, BBPIC microscopy facility
Department of Animal Sciences
University of Wisconsin
1675 Observatory Drive
Madison,  WI  53706
voice: (608) 263-4162
fax: (608) 262-5157 (dept. fax)

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