SAW sensor system with micro-fluidic channels to detect DNA molecules

We developed a surface acoustic wave (SAW) sensor system for detection of the immobilization and hybridization of DNA (deoxyribonucleic acid) molecules. The experiments of DNA immobilization and hybridization were performed with 15-mer oligonucleotides (probe and complementary target DNA). The sensor consists of twin SAW delay line oscillators (sensing channel and reference channel) fabricated on 36° rotated Y-cut X-propagation LiTaO ₃piezoelectric single crystals. Center frequency of the SAW oscillators is 200 MHz. The relative change in the frequency of the two oscillators was monitored to detect the immobilization of a probe DNA with thiol group on an Au coated delay line of the SAW device and the hybridization between a target DNA and the immobilized probe DNA in pH 7.4 PBS (phosphate buffered saline) solution. The target solution was continuously flowing over the probe DNA layer via micro-fluidic channels made of polyimide tubes. The exact amount of the flow was controlled by digital pumps operated by LabView® software. The sensitivity of the was as high as 136 pg/ml/Hz. It has been normally considered that a SAW sensor is inferior in sensitivity to optical systems in detecting DNA molecules. However, in addition to the inherent advantage of being label-free, the SAW DNA sensor was confirmed by the results in this paper to have promising practical applicability in terms of the sensitivity.