  THz technologies are a powerful tool for label-free detection of
biomolecules. However, significant reduction of the lower detection limit is
required to apply THz-sensors in biomedical diagnosis. This paper reports an
ultrasensitive THz-biosensor based on asymmetric double split ring resonators
(aDSRR) for the direct label- and PCR-free detection of DNA at physiologically
relevant concentrations. We introduce selective functionalization and localized
electric field concentration to enhance aDSRR sensitivity and specificity. The
sensor characteristics are demonstrated using the human tumor marker MIA in
cDNA samples produced from total RNA without PCR-amplification. Measurements of
DNA samples with concentrations as low as $1.55\,\times\,10^{-12}\,$mol/l are
presented.
