The exact mechanism is not fully understood yet. But it is often proposed that as nanoparticles have high surface area and high surface energy, biomolecules get adsorbed and thereby they lose their normal physiological functioning and die. The reference may shed some light on your query.
My foreposter Abhishek Das tried to explain the principal mechanism underlying autophagy of NP's. Unfortunately the cited article for most of us is PPV (pay per view).
At least you can see the abstract of the Morones et al 2005 paper, published in Nanotechnology 16 pp.2346ff (doi:10.1088/0957-4484/16/10/059): < The bactericidal effect of silver nanoparticles>. Unfortunately (if you don't have a subscription to the Journal) you'll have to pay for downloading the article...(PPV). I would suggest you do a Google search, search phrase (as an example):
332.000 results>
You asked about ? I guess mechanism of intake of NP'S depends (varies a bit) also on (with) the nature of the particles used. And there are many types/classes of NP's....
As of my kowledge, autophagy induction by nanoparticles can be "considered a kind of an attempt to degrade what is perceived by the cell as foreign or aberrant" (cf. http://www.particleandfibretoxicology.com/content/9/1/20 :
REVIEW: Autophagy and lysosomal dysfunction as emerging mechanisms of nanomaterial toxicity, by Stephan T Stern et al, Particle and Fibre Toxicology, 2012, 9:20 (OPEN ACCESS). or another one:
(Citation) (End of citation) from H-L Liu et al., Cell Death and Disease (2011) 2, e159; doi:10.1038/cddis.2011.27 Published online 19 May 2011: "A functionalized single-walled carbon nanotube-induced autophagic cell death in human lung cells through Akt–TSC2-mTOR signaling", downloadable for free (at least for me) at: http://www.nature.com/cddis/journal/v2/n5/full/cddis201127a.htm.
Best of luck (I am sure you will find multiple and "free" answers/downloads yet in this , special field of cell biology ).