the present disclosure provides methods and compositions for conducting an assay to detect a polynucleotide . in particular , ruthenium complexes having reduction potentials that do not coincide with the reduction potential of molecular oxygen are disclosed and amperometric techniques for their use are described . in preferred embodiments , the ruthenium complex is ruthenium pentaamine pyridine and the polynucleotide that is detected is dna . further , techniques for enhancing detectable contrast between hybridized and unhybridized nucleic acids are disclosed . in particular , the use of elongated target strands as well as the use of uncharged probe strands are discussed .