Rheological and interfacial properties of nucleic acid films studied by thickness-shear mode sensor and network analysis

Complete characterization of the thickness - shear mode (TSM) acoustic wave sensor coated with films of nucleic acid and subjected to conductive solution loading is achieved through network analysis of impedance measurements. The responses of coated sensors with respect to series resonant frequency can be separated into two regions. When 0 < sigma < 0.5 Omega(-1) m(-1) the frequencies for sensors with films of single strand (ss) DNA or polycytidylic (5') (Poly C) in place rise significantly with increasing conductivity. The motional resistances for sensors coated with both species peak at the same value of conductivity where the frequency has the most rapid shift. In the second region (0.5 < sigma < 12 Omega(-1) m(-1)), the frequency response for the ss DNA system exhibits a bell-shaped curve, with a minimum at sigma = 4 Omega(-1) m(-1). The motional resistances for both nucleic acid layers show reversible behaviour, whereas TSM sensors with bare PdO and Au electrodes yield little change. Models that consider only the bulk properties of solutions fail to explain the observed changes in frequency. On the other hand, an equivalent circuit model incorporating the rheological properties of nucleic acid layers on the sensor surface is successful in explaining the various responses.