Methods, systems, and devices are disclosed for capturing, concentrating, isolating, and detecting molecules. In one aspect, a molecular probe device includes a molecular probe having a complimentary base pair region initially zipped and structured to include a binding agent to chemically attach the molecular probe to an outer surface of a magnetic bead, and a binding molecule to chemically attach the molecular probe to a substrate of a microfluidic device, in which the complimentary base pair region is configured to hybridize to a complementary nucleic acid sequence of a DNA or RNA molecule.