You may look into core-shell quantum dots or the newer class of rare earth luminescent nano particles in case you do not want to work with Cadmium Selenide particles. they do not photobleach, have excellent optical cross-section and quantum yields and can be excited over a wide range of wavelengths. There are also various surface chemistries available for the tags.

Regards

Marko

thanks Marko. what types of rare earths do you mean when you say 'rare earth luminescent nano particles'?

you can make particles from europium doped YVO4 or other lanthanides.

http://www.nanomaterialstore.com/nano-phosphor.php

http://www.nanomaterialstore.com/pdf/20101004%20Phosphor%20Dot%20Brochure.pdf

Unless you need to avoid Cadmium I'd just just use normal CdSe-particles.

Thanks Marko, will review the URL links.