A molecular tool and a label-free method useful for detecting a target nucleic acid in a sample are disclosed, the molecular tool comprising a substrate comprising or consisting of an optical material; a first polymeric molecule, selected from nucleic acids or nucleic acid analogues, coupled with the substrate; and a second nucleic acid analogue coupled with the first polymeric molecule by means of nucleobase complementarity. The molecular tools and methods of the invention are configured for the optical detection of oligonucleotide binding events for diagnostic, point-of-care, drug screening applications using for instance a carbon nanotube optical signal. Circulating nucleic acids that have diagnostic and prognostic value for cancer, metabolic disease, organ rejection, foetal health and infectious disease can be therefore identified and characterized.