▎ 摘　　要

NOVELTY - Molecular tool comprises (a) a substrate comprising an optical material, (b) a first polymeric molecule chosen from nucleic acids or nucleic acid analog, coupled with the substrate, and (c) a second nucleic acid analog coupled with the first polymeric molecule by nucleobase complementarity. USE - The molecular tool is useful in system for detecting target nucleic acid in sample. The target nucleic acid is cell-free tumor DNA, circulating mRNA, or circulating microRNA (miRNA), preferably miRNA. The sample is a biological sample chosen from blood, plasma, saliva, urine, biopsies, organs, tissues or cell samples. The sample is a native, unprocessed biological sample (all claimed). ADVANTAGE - The molecular tool ensures label-free detection of target nucleic acid in sample. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following: a label-free method for detecting a target nucleic acid in a sample involves (a) contacting a sample with the molecular tool under conditions allowing the coupling by nucleobase complementarity between the second nucleic acid analog of the molecular tool and the target nucleic acid in the sample, (b) providing an electromagnetic radiation to the sample comprising the molecular tool under conditions allowing the interaction of the electromagnetic radiation with the optical material of the molecular tool, and (c) collecting and analyzing an optical signal deriving from the optical material of the molecular tool, where optical signal is indicative of the coupling between the second nucleic acid analog and the target nucleic acid in the sample; a kit comprising at least one molecular tool, at least one container and instructions for use; and a system comprising the molecular tool, a source of electromagnetic radiation, and an electromagnetic radiation detector for collecting and analyzing the optical signal deriving from the optical material of the molecular tool.