**3.3 Study III**

In a study, an array of four micro beams, S1-S4 are attached to a common shuttle mass, SM for the detection and identification of multiple analytes [19, 38]. Geometrical asymmetry in the micro beams (length mismatch) assured sufficient separation of individual resonant peaks (as seen in **Figure 8(a)**) at the corresponding eigenmode frequencies in the output response. An output response of the fabricated prototype along with mode shapes are shown in **Figure 8(a)**. A capillary tube containing the specific polymer solutions was interfaced to one or all of the micro beams to functionalize them for vapor detection. Specifically, toluene and methanol, and toluene/methanol mixtures were used with the polymers to prepare analyte concentration for the functionalization of the surface of the microbeam/s in an

#### **Figure 8.**

*Frequency response of the single input single output scheme in an array of coupled micro beams. (a) Composite response indicated by B and the individual mode-frequencies (M1-M4) of the mode-localized micro beams upon mass absorption due to the added analyte concentration. Also shown are the resonant mode shapes of the system. (b) Resonant frequency shifts of the four individual modes as a function of added mass concentration of the analyte. Slopes of the curve determine the sensitivity to the analyte, also making it possible to identify particular vapor concentration. Reprinted from [38] with the permission of AIP Publishing.*

#### *Ultra-Precise MEMS Based Bio-Sensors DOI: http://dx.doi.org/10.5772/intechopen.93931*

array. The functionalized prototype was excited to motion by the piezoelectric actuator operating at a pressure level of 200 *Torr*. As shown in **Figure 8(b)**, a single output signal (resonant frequency shifts of all micro beams as a function of analyte concentration in %) was measured optically from the SM to obtain the composite response due to the mode-localization. Detection process is as follows: i) measure the resonance frequencies, M1-M4 (without added mass) in the pure nitrogen gas, ii) introduce an analyte and wait for the absorption to reach steady state, iii) measure the resonance frequencies once more, and determining the resulting frequency shifts. In this experiment, the frequency shift of each localized microbeam mode (M1-M4) was determined for various analyte concentrations (**Figure 8(b)**). As observed from **Figure 8(b)**, M3 shows the highest sensitivity to the toluene, whereas, M4 exhibits highest sensitivity to the added mass concentration of the methanol. Knowing the shifts of the two most sensitive microbeam modes M3 and M4 and their sensitivities from **Figure 8(b)**, the concentrations of methanol and toluene in the vapor were estimated to be 2.8% and 2.0%, respectively, and found in good agreement with the actual concentrations of 2.3% methanol and 2.3% toluene.
