The invention includes RNA complexes comprising at least three monomeric units of an RNA molecule, each monomeric unit comprising an RNA polymer having first and second helical domains that have respective first and second binding sites, wherein the first binding sites are adapted to binding to one another and are not adapted to bind to the second binding sites, and the second binding sites are adapted to binding to one another and are not adapted to bind to the first binding sites; such that the at least three monomeric units are adapted to self-assemble by forming pairs of cognate interactions and so as to form the RNA complex in a circular closed complex. The invention also includes derivatives of these complexes including aptamers, and analytical methods and devices using same.