A surface acoustic wave sensor modified from a wireless transmitter for the monitoring of the growth of bacteria

This work presents a novel surface acoustic wave (SAW) sensor for the measurement of the microbial count in a biological culture. The signal translation interface of the sensor was modified using a common commercial wireless SAW transmitter design. A pair of conductive electrodes was inserted in a 314.5 MHz SAW stabilized oscillator of the transmitter. Measurements were made by placing the electrodes within the culture solution of interest and measured the time required to identify readily detectable frequency changes (detection time, DT). The change of frequency was caused by the impedance change of the microbial metabolism. The calibration curve of detection times against density of Escherichia coli shows a linear correlation coefficient (R2 = 0.924) over the range of 102 to 107 cells/mL. A sample that contains 102 cells/mL of E. coli required a detection times (DT) of 7 h, shorter than was required using instruments based on conventional conductance methods. The proposed ultra high frequency SAW sensor (314.5 MHz) shows a large total frequency change and gives a sharp inflexion at the DT but possesses the same stability compared with that of low frequency serial piezoelectric quartz crystal (PQC) sensor for the measurement of bacteria concentration. This sensor platform enabled real time monitoring of bacterial growth within a sealed opaque container. This sensor is potentially applicable to a remote query wireless measurement in hazardous environments when a suitable antenna device is adapted.