Introduction:: Surface Acoustic Wave (SAW) devices are MEMS devices that are widely used in the telecommunication industry. These devices are often used as RF filters in wireless communication, making the critical infrastructure needed for their fabrication, readily available. SAW sensors are now being explored for Biological-MEMS (BioMEMS) applications due to their high sensitivity, potentially low cost, real-time response, and low attenuation when analyzing liquid samples. Biosensors that use SAW technology, incorporates one or more acoustic transducer that generates or receives a wave whose propagation velocity changes with mass or viscosity loading. The change in the wave velocity of surface acoustic waves at the sensor-media interface to monitor changes in the biochemical system under the observation field. Our SAW biosensor is capable of detecting perturbation at the solid-liquid interface induced by the binding of a receptor to a complementary ligand in liquid samples. The resulting perturbation is caused by a change in the physical properties of the sensor surface due to the interaction of the targeted specie. The change in the physical property will alter the local viscosity and therefore the elasticity modulus of the layer just above the surface in the region where the SAW propagates. Minute changes in the viscoelastic properties are measured and recorded as a function of time. The effects of the changes in these physical properties are recorded as changes in SAW amplitude attenuation and SAW wave velocity. We can then apply these biosensors to rapidly detect viral, bacterial, or fungal pathogens in complex biological matrices.
Materials and Methods:: Experimentally, we determined that the standard deviation of the baseline measurement (the blank) for the SH-SAW was 0.49o. Based on this standard deviation in the signal, we could calculate the limit of detection (LOD) for the device, as described in Eqn. 1 below.
Eqn. 1 LOD = 3.3*(Standard deviation)/Sm
Where Sm is the experimentally determined overall mass sensitivity of the system. This was determined by measuring the slope of the acoustic response plotted versus the square root of the mass-viscosity product. If we then multiply the mass sensitivity by the area of the sensor, 1.01 mm2, we get the mass limit of the system. The lowest mass detectable by the system was therefore 2 Femtograms (Fg). The LOD may difficult to achieve if the instrument has a high electronic jitter or other parasitic electrical noises that cause the signal to fluctuate (although not observed). For this reason, we also calculated the Limit of quantification (LOQ) as a more practical measurement of the limit for mass detection on the SH-SAW device. The LOQ is given in Eqn. 2 below:
Eqn. 2 LOQ = 10*(Standard deviation)/Sm
The LOQ was calculated to be 6.0 Fg. This suggests that the device can precisely make measurements for analytes in increments of 6.0 Fg. Similarly, we wanted to get a practical measure of the concentration of analytes that could be evaluated. Since we have a sample reservoir of 5 μl, then the device will be able to measure concentrations of 1.2 Pg ml-1.
Results, Conclusions, and Discussions:: We observed the effect of fluid viscous loading using a 250 MHz quartz SH-SAW biosensor while monitoring different concentrations of aqueous glycerol solutions. The sensitivity of the biosensor was determined by fitting the data to models derived from perturbation theory. All measurements were made using phosphate-buffered saline at pH 7.4 (1x PBS) as the reference solvent. The slope resultant of the plot of the acoustic response versus the square root of the mass-viscosity product, which gives the sensitivity of the sensor was S = 3,333 om2s Kg-1. The mass sensitivity, for our 250 MHz quartz SH-SAW sensors was calculated, using the average 0.49o for the baseline signal, which was Sm = 1.70 x 1012m2 Kg-1. This is a significantly higher mass sensitivity than previously reported values for similar quartz SAW sensors. This results in a LOD = 2 x10-18Kg or 2 Fg. We calculated a limit of quantitation (LOQ) of 6 Fg was also calculated. These limits are for liquids with viscosities ranging from the estimated range of 0 – 2.5 mPa*s. These values correspond to a LOD concentration of 0.2 picograms (Pg)/ml and a LOQ concentration of 1.2 Pg/ml if you consider the area of the sensor.
Acknowledgements (Optional): : We acknowledge TST Biomedical Electric Company Limited for providing us with the iprotin reader and 250 MHz SH-SAW quartz sensors.
