Multimodal Olfactory Sensor Array with QCMs

The sensor array in this artificial olfactory system consists of four QCMs placed inside a small chamber. QCM sensors allow quick and easy testing of different sensing materials just by depositing the sensing materials on the surface. The sensing material absorbs the gas and the resonant frequency of the QCM changes with the absorbed mass. The measurements of the QCM resonance characteristics are made by measuring a conductance curve (conductance vs frequency) then numerically optimizing parameters in a theoretical equivalent electric circuit, as shown in Figure 2a [11 (link)]. This calculation was made by MATLAB software.
This equivalent circuit is described by Equation (1), and it is a common way to characterize the QCMs in which R in the electrical circuit represents the loss in the physical device, L represents the mass loading of the QCM, and the series resonant frequency is given by Equation (2) [11 (link)]. Figure 2b shows two different measured frequency characteristics of one QCM before (blue solid line) and after (red dashed line) gas exposure.
$G=\frac{R}{R^2+{\left(2\pi fL-\frac{1}{2\pi fC}\right)}^2}$
$f=\frac{1}{2\pi \sqrt{LC}}$
The conductance curves were measured independently by four vector network analyzers (VNAs) (DG8SAQ VNWA v3), and each spectrum was measured every 2 s. VNAs work by running a proprietary software (DG8SAQ Version 36.7.6) in the computer that does the control and data recording. This software can communicate with other external software in diverse ways; in this work, they work by continuously measuring and dumping the data to text files. This method was more stable and generated faster measurements than interrogating or controlling the VNA software from external software.
Commercial QCMs (SEIKO EG & G, AT-cut) with a resonant frequency of 9 MHz were used to build the sensor array. According to the Sauerbrey equation (Equation (3))—where f₀ is the resonant frequency, Δf is the frequency shift, Δm is the mass change, A is the electrode area, ρ_q is the density of quartz, and μ_q is the shear modulus of quartz—the frequency shift depends on the square of the resonant frequency, and because the frequency is relatively easy to measure, high-frequency QCMs are preferred.
$\Delta f=\frac{2f_0^2}{A \sqrt{{\rho }_q{\mu }_q}}\Delta m$
The QCMs were coated with different room temperature ionic liquids (RTILs) by dip coating [12 (link)]. Before coating each QCM, its resonant frequency was recorded. Then the QCMs were submerged in a solution of the RTIL and a solvent (chloroform or acetone). The pullout speed was controlled by the dip coater and the final resonant frequency shift caused by the sensing layer (ΔF_s) was recorded. The characteristics of the different sensors are described in Table 1.
The RTILs used in this work were 1-Methyl-3-n-octylimidazolium Bis(trifluoromethanesulfonyl)imide (abbreviated here as [MOIM][TFSI]), 1-Methyl-3-n-octylimidazolium Hexafluorophosphate ([MOIM][PF₆]), 1-Butyl-3-methylimidazolium Chloride ([BMIM][Cl]), and 1-Butyl-3-methylimidazolium Bromide ([BMIM][Br]), as described in Table 1.
In addition to the mass loading and its influence on resonant frequency shift, the responses of QCMs coated with RTILs can also be characterized by the changes in RTIL viscosity. Unlike the more common approach of interrogating QCMs just by the resonant frequency with a frequency counter, the VNAs make it possible to simultaneously measure both changes on the series resonant frequency and resistance. This allows the measurement of viscosity effects, and although a QCM resonator does not work well under heavy viscous damping, VNA works even in that situation [12 (link)].