## 1 Introduction  
THz radiation remains of interest for biomolecular sensing because, in principle, it can interact resonantly with collective vibrations that involve hundreds or more atomic constituents. It can therefore be more specific to the secondary structure of the biomolecules, compared to the interatomic vibrations or primary structure sensed by traditional Fourier transform IR (FTIR) spectroscopy. In addition, THz is inherently non-ionizing and, like IR spectroscopy, can detect \"label free\" (i.e., no stains or dyes are required as in fluorescence spectroscopy). Sub-THz vibrational modes in nucleic acid DNA and RNA have been investigated for several decades, going back to seminal theoretical work by [PERSON] [1] and experimental work by [PERSON] [2], [PERSON] [3] (Raman spectroscopy), [PERSON] [4] (FTIR), and others. These early papers emphasized the likely existence of sub-THz optically active phonon-like modes in DNA via the hydrogen bonds distributed between the base pairs. Later, the importance of localized vibrational modes was emphasized [5; 6; 7], and THz signatures in nucleic acids became a contentious topic in experimental studies, especially the difference between solid-state and aqueous samples. The relatively new technique of time-domain spectroscopy (TDS) provided a tool with unprecedented sensitivity in the THz region, but both supportive [8; 9] and contradictory [10] results were published. Importantly, sample presentation, concentration, and the role of liquid water became more emphasized. Precise numerical modelling tools, such as molecular dynamics, were exercised to justify the existence of optically active phonon-like modes, but without accounting for the effect of water [11]. However, many researchers continued to believe that the aqueous state would dampen the nucleic acid THz vibrations to the point of poor- or non-observability.  
Fortunately, more recent theoretical and experimental research has suggested otherwise. For example, the utilization of ultrafast optical Kerr-effect spectroscopy has indicated that the temporal dynamic effect of water is reduced significantly in aqueous DNA, by roughly two orders of magnitude compared to conventional estimation [12]. The explanation is rather complex, which is not surprising, since it has long been known that the interaction of biomolecules with liquid water is a highly correlated \"dance\" [13] involving a thin layer of bound water on the surface of the biomolecule in dynamic interaction with the free (or bulk) water around it. Furthermore, the nano-mechanical properties of the double-stranded nucleic acids have become better understood [14]. A key parameter in this regard is the persistence length--the distance over which the molecules behave like rigid structures subject to classical elastic mechanics, and not dominated by thermal fluctuations and collisions. We will return to this topic later in the Discussion section.  
The primary aim of this paper is to present experimental evidence that sharp (i.e., strong and narrow) THz absorptive spectral signatures do exist in aqueous nucleic acid samples. We utilize nanofluidic channel technology, which allows the nucleic acid samples to be investigated in very thin (\(\approx\)500 nm) and small volume profile, thereby reducing the bulk water absorption to a very low level. Our original work on this topic started over ten years ago, but on silicon-based nanofluidic chips [15]. Here, we report on fused quartz-based chips, which have provided better data on aqueous double-stranded nucleic acids in the range of \(\sim\)17 base pairs (si-RNA) to 48.5 kbp (\(\lambda\)-DNA). The smaller molecule samples in the range of \(\sim\)20 to \(\sim\)500 bp show sharp (strong and narrow) absorption signatures, whereas somewhat longer samples (\(\sim\)1 kbp) display weaker and broader signatures, but are reproducible, nonetheless. The longest DNA molecule tested (48.5 kbp) shows no prominent signatures at all.