How to detect nM levels of Copper(I) Oxide in blood?
The main analytical problem in your case of detecting Cu(I) oxide in blood is that you do not know the fate of the particles once they enter blood stream. As I said earlier, Cu(I) is thermodynamically not stable in any aqueous system. It oxidizes to Cu(II), which is water soluble. The non-destructive technique to detect species is X-ray photoelectron spectroscopy, but its detection limit is very poor. It may take a day to collect weak signals from such a low concentration of Cu(I) oxide. The sample has to be bone-dry as well! This is a species specific analysis, i.e. one can distinguish oxidation states of metals by deconvolution.

A practical suggestion is that you do a control test, i.e. measure copper levels before treatment in blood. As a result, all copper originates from your copper (I) oxide dispersion. You would not know the specific species of copper but you will get the total content. For that one can use common techniques such as graphite furnace atomic absorption analysis (only a few microliter of sample is required).