## 4 Results and Discussion
of the suspensions in this frequency region were significantly different from each other. The experiment was repeated two times, and showed similar results. In cell detection experiment, we did not perform the measurement at frequency lower than 100 kHz, since the EDL will influence the measurement at low frequencies (approximately below 300 kHz) [12; 17]. In order to investigate the relationship between impedance value and cell concentration, we selected 1 MHz as the best representative frequency. Figure 6b illustrates the impedance responses of the sample containing different yeast cell concentrations and DI water at frequency measurement 1 MHz. The impedance of the solution was significantly increased from 207.63 k\(\Omega\) to 225.42 k\(\Omega\), 247.61 k\(\Omega\), 284.48 k\(\Omega\), 314.64 k\(\Omega\), and 348.51 k\(\Omega\) when the yeast concentration decreased from 10\({}^{9}\) cfu/mL to 10\({}^{8}\) cfu/mL, 10\({}^{7}\) cfu/mL, 10\({}^{6}\) cfu/mL, 10\({}^{5}\) cfu/mL, and 10\({}^{4}\) cfu/mL, respectively. After the cell concentrations were lower than 10\({}^{4}\) cfu/mL, impedance values showed no significant changes between each other or DI water. In addition, the pattern of the result shows a linear relationship between the impedance and the logarithmic value of the cell concentration at cell concentrations from 10\({}^{4}\) cfu/mL to 10\({}^{9}\) cfu/mL (see Figure 6b). The linear regression equation of this result is \(Z\) (k\(\Omega\)) = 58.3 log \(C\) (cells/mL) + 175.4 (Where \(Z\)-Impedance and \(C\)-Cell concentration), with \(R^{2}\) = 0.986. The detection limit was calculated to be \(1.2\times 10^{4}\) cfu/mL. Error bars are standard deviations of five measurement cycles. Based on this linear regression equation, the impedance of the yeast suspensions can be used to estimate the cell concentration in DI water suspensions. This device can be utilized to quantify cells in suspensions other than impedance microbiology and impedance biosensors for bacteria detection, since the detection limit of this method is comparable with other sensors. The reported sensor for detection of pathogenic bacteria are quartz crystal microbalance (QCM) immunosensors for detection of _Salmonella_ with detection limits of  
Figure 5: Image of the sensing area of the device. Figure 6: (**a**) Impedance spectra of yeast cells in water with cell concentrations ranging from 10\({}^{2}\) to 10\({}^{9}\) cfu/mL, along with DI water as control; (**b**) The linear relationship between the logarithmic value of the concentration of yeast cells and the impedance measured at 1 MHz. Error bars are standard deviations of 5 measurements. 9.9 \(\times\) 10\({}^{5}\) cfu/mL [28], surface plasmon resonance (SPR) sensor for the detection of _E. coli_ O157:H7 with a detection limit of 10\({}^{7}\) cfu/mL [29] and SPR immunosensors for the detection of _Salmonella enteritidis_ and _Listeria monocytogenes_ with detection limits of 10\({}^{6}\) cfu/mL [30]. As the result, we conclude that this device was capable of detecting the cell concentrations in solution medium at the high frequency range between 100 kHz and 5 MHz. In this experiment, we did not determine the detection capability at frequencies lower than 100 kHz. For future work, we will focus on the size and shape of microneedle [31; 32; 33], single cell detection [34], and will utilize non-polarizable electrodes (i.e., Ag/AgCl--to eliminate the EDL) in order to improve the performance of the device.