Which is the smallest particle that has been actually seen by the scientists? When I say "actually seen", (may be using some ultra advanced microscope or any other man made eye, using any wavelength or phenomena) I really mean it; just like we have seen the red blood cells.
Davidmh's answer is pretty much in line with what I am asking
Best Answer
Taking your question literally, you can see a single barium ion:
They are in traps like this one:
Also, Warren Nagourney from Washingtong University took a picture of a single Barium atom scattering light from a laser:
Only two minutes exposure time, so probably in a dark enough room, someone with good sensitivity could actually see it.