An apparatus for filtering species in a fluid includes a body having a first side and a second side, a first set of nano-fingers positioned on the body near the first side, a second set of nano-fingers positioned on the body closer to the second side than the first set of nano-fingers, wherein the nano-fingers in the second set of nano-fingers are arranged on the body at a relatively more densely than the nano-fingers in the first set of nano-fingers, and a cover positioned over the first set of nano-fingers and the second set of nano-fingers to form a channel with the body within which the first and second sets of nano-fingers are positioned.