1. Introduction

Most volatile organic compounds (VOCs) are deleterious to the liver and kidneys. Even short exposure to VOCs can damage the human eye, nose, and nervous system [1–3]. For example, falsified alcoholic beverages may be adulterated illegally with toxic methanol. Methanol ingestion of as little as 10 mL can cause blindness, and the ingestion of more than 30 mL can be seriously fatal [3]. Another popular example of VOC is acetone, which is a flammable and colorless substance that is widely used in industries. When exposure to acetone vapor concentration exceeds 173 ppm, the central nervous system can be damaged [1]. Acetone vapor with a volume concentration of approximately 2.5–12.8% mixed in ambient air is easily ignited by fire or any electrostatic discharges beyond the flash point (−20 °C) [2]. Therefore, using optical methods to identify hazardous organic vapors or distinguish similar VOCs (such as alcohol and methanol) in the surroundings is becoming an important task in food and security for quality control and public health.

Terahertz (THz) waves with an electromagnetic frequency of 0.1–10 THz have been developed for sensing gas molecules because certain low-frequency spectral lines caused by intermolecular vibration or rotation lie in this frequency range, distinct from those spectral properties in near- or middle-infrared rays. However, to probe gaseous molecules using THz spectroscopy, a long sample chamber to increase interaction length and a vacuum pump to manipulate gas samples are required to increase sensitivity [4–6]; this chamber is not flexible and is too bulky for a compact and portable THz spectroscopy system. The demonstrated compact THz sensors have strong resonance but lack sufficient interaction between gas molecules and THz field [7–9]. To solve the insufficient interaction length in a miniaturized sensor device, sponge-like active layers, such as the microporous polymer membranes, are incorporated into the THz spectroscopic system for gas sensing to significantly enhance detection sensitivity; this approach has the advantages of simple usage, low cost, and ppm-level sensitivity [10].

In this study, VOC detection sensitivity using a microporous polymer structure (MPS) composed of the multi-layer stacked meshes with different geometric parameters, including distinct pore sizes, layer numbers, and stacking configurations, are further investigated. The acetone vapor with a large dipole moment can be a standard volatile gas sample [10] for studying the effect of MPS geometries on vapor sensing performance. The sensitivity of MPS to identify different concentrations of acetone vapors is analyzed based on three sensing parameters, i.e., the 0.4 THz wave variations in transmittance, effective THz absorption, and refractive index variation of the unit pore. The mechanism and optimization of the structure-dependent sensitivity of MPS are also experimentally characterized based on the THz absorption and dispersion responsivity. The vapor mixtures of alcoholic aqueous solutions adulterated with different concentrations of toxic methanol are successfully recognized, employing the highest sensitivity of the MPS gas sensor.

2. Experimental setup and configuration of a MPS gas sensor

A MPS volatile gas sensor is constructed by multiple layers of polyethylene terephthalate (PET) mesh (SEFAR PET1000, SEFAR AG, Switzerland) and one microfluidic chamber made of Teflon material. A flexible PET mesh [Fig. 1(a)] consists of periodical square holes, which are two-dimensionally arranged in square arrays as shown in Fig. 1(b). Multiple layers of meshes are stacked and fixed by a rectangular acrylic holder to form a MPS device with large numbers of square air-holes (i.e. micropores) randomly distributed inside the composite [10]. The MPS is placed inside a microfluidic chamber for vapor sensing. Figure 1(c) shows the schematic diagram of the MPS-based THz volatile gas sensor, which consists of a MPS device and a Teflon sample chamber. The sensing module is compact and low loss for THz measurement and is externally connected with a flexible plastic tubing to the fluidic channel for easy manipulation of the vapor analyte. The inner volume of the chamber is larger than the MPS dimension and sealed to easily achieve the saturated pressure of vapor analyte; the inner chamber has a width of 21 mm (x-axis), a length of 60 mm (y-axis), and a height of 45 mm (z-axis). The Teflon chamber is machined for one fluidic channel (18 mm long in x-axis, 5 mm wide in y-axis, and 3 mm deep in z-axis) at the bottom as a container for loading the liquid analytes that are injected via the external tubing. The liquid sample naturally evaporates into vapors, diffusing into the MPS. In the volatile gas-sensing experiment, the sample loading and sensing processes are performed at room temperature and normal atmosphere without enforcing pump.

 

Fig. 1 (a) Photograph of a PET mesh. (b) Microscopic photograph of a PET mesh. White lines: PET grids. Black square holes: Micropores. (c) Configuration of a MPS volatile gas sensor. (d) Uniform and periodic configurations of the MPSs as sketched in the x–z plane.

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To study the structure-dependent sensing performance of MPS, four kinds of PET meshes with different thicknesses and square micropore sizes are utilized to stack into two configurations of MPSs, i.e., uniform and periodic structures [Fig. 1(d)]. The periodic MPS is formed by alternately stacking two kinds of PET meshes with different micropore sizes (i.e. different porosities or different effective refractive indexes of n₁ and n₂ in Fig. 1(d)); by contrast, the uniform MPS is made by stacking only one kind of PET mesh (i.e. single porosity or single effective refractive index). The square air holes (i.e. micropores) of the different layers of stacked PET meshes were not precisely aligned with each other but randomly placed layer by layer for both uniform and periodic MPSs. To investigate the pore size effect of one stacking configuration on sensitivity, large- and small-pore MPSs are prepared in both the uniform and periodic structures using the four kinds of PET meshes. In this presentation, large-pore periodic MPS is composed by the PET meshes of 90 and 249 µm pore widths, denoted as Periodic-90-249 MPS. Periodic-45-133 MPS represents the small-pore periodic MPS, with 45 and 133 µm pore widths of the PET meshes. Similarly, the small- and large-pore uniform MPSs are denoted as Uniform-45 and Uniform-90 MPSs, respectively, whose pore widths of the composed PET meshes are 45 and 90 µm, respectively. The specifications of the four types of MPS devices are summarized in Table 1, including the pore width and thickness of mesh, stacking layer number, device thickness, and effective porosity. To investigate the pore size effect of one stacking configuration on sensitivity, the pore number of the MPS gas sensor is fixed and only the pore width is changed. For example, the micropore number of the 23-layered Periodic-90-249 MPS is kept the same with that of the 6-layered Periodic-45-133 MPS, but the pore width of the former device is two times larger than the latter device for comparison of the pore-size-dependent sensitivity performance. Similarly, the 23-layered Uniform-90 MPS and 6-layered Uniform-45 MPS devices, possessed an identical pore number but different pore sizes, are also prepared for this purpose. Besides, the pore number effect on MPS sensitivity is also studied by changing the stacking layer numbers of Periodic-45-133 MPS and Uniform-45 MPS devices. As shown in Table 1, three different layer numbers of MPS devices with identical pore size are prepared for sensitivity comparison in both the Periodic-45-133 MPS and Uniform-45 MPS.

Table 1. MPS specification.

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The sensing performance of the MPS devices are characterized by a transmission-type THz time-domain spectroscopy (THz-TDS) with flexible optics along the x-axis, which optimally collimates various beam divergences out of different MPS thicknesses (0.30–3.46 mm). This feature ensures that the required beam size at the THz detector for THz wave transmission through different dielectric sample thicknesses can be matched. The intrinsic THz wave absorption of VOC is generally proportional to THz frequency [10, 11], but the highest absorption is at 0.4 THz within the allowable THz spectrum in this study. Therefore, the 0.4 THz wave is applied as the sensing wave with the practical signal-to-noise power ratio over 10⁴ for the free-space condition [Fig. 1(c)].

The sensitivity of different MPS is measured by the exposure of the microporous device to different amounts of VOC, and their linear THz responses in effective absorption and refractive index variation are related to the molecular dipole moment of the VOC [10]. Acetone vapor molecule is thus applied as the standard VOC in this study to calibrate the sensitivity performance of MPS because its high dipole moment (~2.88 Debye [12]) is easily perturbed by THz waves, performing obvious electromagnetic attenuation or dispersion. Different amounts of acetone vapor are prepared from different volume concentrations of acetone aqueous solutions: 2.5%, 5%, 10%, 20%, 40%, 60%, 80%, and 100%, which are individually loaded inside the microfluidic chamber to naturally evaporate into vapor phase under ambient atmosphere/room temperature until the saturated vapor pressures are approached. According to Raoult’s law [13], the vapor pressure inside the chamber is approximately proportional to the aqueous acetone concentration. When the MPS is exposed to the acetone vapor molecules, both of the vapor infiltration inside the micropores and physical adsorption on the hydrophilic pore-surface make the MPS sensitive to the presence of the vapor [10]. THz wave responses of transmittance and dispersion variation measured from the vapor-filled MPS are stronger than those in the vapor-filled free space because the porous medium increases the field–analyte interaction probability.

3. Experimental results of sensitivity characterization for different microporous structures

3.1 The sensing parameters of a MPS sensor

On the basis of the aforementioned sensing principle of the MPS [10], increasing the porous volume by adding the composed layers or higher average porosity can supposedly enhance THz wave responsivity because more molecules could be filled and adsorbed at a certain vapor pressure. Therefore, we first modify the MPS volume via stacking different layer-numbers of PET meshes in both periodic and uniform structures to study the pore-volume-dependent THz responsivity under the same porosity. The transmittance variation of 0.4 THz wave passing through one MPS structure with and without the acetone vapor infiltration is denoted as |ΔTr.| and shown in the Fig. 2. Figure 2(a) shows the responsivity analysis of Periodic-45-133 MPS with thicknesses of 6, 34, and 46 layers and an identical porosity of ~37.2%. The THz responsivity of MPS to vapor is represented by 0.4 THz wave transmittance variation (|ΔTr.|) under different concentrations of acetone aqueous solutions (σ) exposure, which are expressed in green, red, and black curves, respectively, for the three thickness of Periodic-45-133 MPSs. The transmittance variations increase within a low concentration range and approach saturation while the acetone solution concentrations are more than 20% σ for each thickness. Figure 2(a) reveals that the |ΔTr.| curve trend of Periodic-45-133 MPS is independent of the MPS volumes; however, the |ΔTr.| value reduces as the MPS volume increases. The |ΔTr.| sequence at each σ is 46-layered < 34-layered < 6-layered Periodic-45-133 MPS.

 

Fig. 2 Transmittance variations of 0.4 THz wave passing through different thicknesses of (a) Periodic-45-133 and (b) Uniform-45 MPSs with and without acetone vapor infiltration. The error bars are obtained from three repeated measurements of THz signal for each vapor density.

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In comparison with Periodic-45-133 MPS, the Uniform-45 MPS with a 29.6% porosity is observed for their thickness-dependent responsivity using 0.4 THz wave, as shown in Fig. 2(b). The |ΔTr.| values of 34-layered Uniform-45 are also lower than those of the 6-layered Uniform-45, and both are saturated above 20% σ, which are consistent with the THz responsivities for different thicknesses of Periodic-45-133 MPSs. The |ΔTr.| value of 12-layered Uniform-45 MPS is slightly higher than that of the 34-layered Uniform-45 MPS for each σ, which is expressed as the blue curve in Fig. 2(b). When the thickness of the Uniform-45 MPS is smaller than 6-layered thickness, the |ΔTr.| values are no longer increased and the responsivities, |ΔTr.| vs. σ, are similar to the 6-layered Uniform-45 MPS. The decrease of |ΔTr.| with the increased MPS volume reveals that the probabilities of acetone vapor molecules’ adsorption on hydrophilic surface and infiltration in micropore do not rise through MPS volume expansion.

The responsivity of |ΔTr.| presented in Fig. 2 would be influenced by the THz absorption from the composed PET meshes and different device thicknesses. To consider the 0.4 THz wave transmittance, contributed only from the acetone vapor molecules within the MPS micropores, and prevent the influence from material absorption of PET mesh and different device thickness, we utilize the effective absorption coefficient and refractive-index variation in a unit pore volume to assess the VOC sensing ability of MPS, which are denoted as α_eff, Δn_eff and expressed in Eqs. (1) and (2), respectively [10].

$P_{v+p}$, $P_p$, f, L, A, c, and $\upsilon$ respectively represent the THz wave transmission power through the MPS with and without vapor infiltration, porosity, thickness of MPS, THz wave beam spot area with a 1.5 mm beam diameter, light speed in vacuum, and 0.4 THz wave frequency. The two sensing parameters, α_eff and Δn_eff, are both normalized by device thicknesses and porosities of different MPSs that are only associated with the acetone gas molecules adsorbed on the hydrophilic surface and infiltrated in the micropores; however, the aforementioned transmittance variation (|ΔTr.|) is attributed from both MPS medium with different thicknesses and vapor molecules [Fig. 2].

3.2 Micropore size effect

The pore size-dependent sensitivity is discussed as follows. We fixed the micropore quantities within the illuminated THz spot for the periodic and uniform MPSs, and the micropore sizes of both structures are changed to compare their sensitivities in terms of α_eff and Δn_eff. For the periodic MPS, the small-pore structure (Periodic-45-133, average porosity ~37.2%), composed of 6-layered meshes and corresponding to a 0.45 mm thickness, has approximately 1,900 micropores covered by the THz beam. To maintain the same pore quantity in the structure of Periodic-90-249 (average porosity ~40.5%) under the same THz beam spot, the large-pore structural mesh should be stacked up to 23 layers, corresponding to 3.46 mm thickness. Similarly, for the uniform MPS, the micropore quantity covered by the THz beam spot in the 6-layered Uniform-45 MPS (porosity ~29.6% and thickness ~0.3 mm) is around 3,600. To keep the identical micropore quantity in the Uniform-90 MPS under the same THz beam spot, the composed meshes of Uniform-90 with 30.1% porosity should be stacked by 23 layers, corresponding to a 2.3 mm thickness.

Figures 3(a) and 3(b) respectively show α_eff and Δn_eff for the MPSs of Periodic-45-133 and Periodic-90-249, exposed to different acetone vapor densities, ρ. The vapor density ρ is estimated by substituting the vapor pressure generated from different concentrations of aqueous acetone solutions into the ideal gas formula, where the vapor pressures are obtained from the experimental database [14] under ambient condition instead of Raoult’s law. In this presentation, all of the error bars shown in α_eff- and Δn_eff-responsivities are obtained from three repeated measurements of THz signal for each vapor density. In Fig. 3(a), the absorption coefficients in the large (Periodic-90-249)- and small (Periodic-45-133)-pore MPSs both proportionally increase within a molecular density of about 6 nmol/mm³ and become saturated at high vapor density. The trace trends of refractive-index variations (Δn_eff) shown in Fig. 3(b) are similar to those of the absorption coefficients (α_eff) [Fig. 3(a)], except that the saturation phenomenon of Δn_eff for the small-pore structure, Periodic-45-133 MPS, occurs at a much lower vapor density (~2 nmol/mm³) compared with that of the large-pore structure, Periodic-90-249 MPS (~6 nmol/mm³).

 

Fig. 3 0.4 THz wave responsivity of (a) effective absorption coefficients and (b) refractive index variations per unit pore volume in the Periodic-45-133 and Periodic-90-249 MPSs with the same micropore quantity under the illuminated THz spot area.

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This consistent trace trend in both α_eff and Δn_eff implies that the increased vaporized acetone molecules not only significantly absorb the THz wave but also considerably contribute to phase retardation in the THz electric field oscillation. This performance shows that high vapor density (i.e., increasing vapor amounts inside the chamber) certainly increases both the infiltration of vapor molecules into the micropores and the molecular adsorption on the hydrophilic surface. In other words, the increased vapor densities can be confined in the micropores and adsorbed on the pore surface to cause the increments of α_eff and Δn_eff of the MPS. The comparison results in Fig. 3 show that the α_eff and Δn_eff of Periodic-45-133 MPSfor all vapor densities are much larger than those of Periodic-90-249 MPS. For example, the absorption coefficients of Periodic-90-249 and Periodic-45-133 MPSs [Fig. 3(a)] at the vapor molecular density of 13 nmol/mm³ are around 4 and 50 cm⁻¹, respectively. This result reveals that further shrinking the micropore volume under a certain vapor density exposure is able to increase the molecular occupied densities, which are confined in the micropore and adsorbed on the pore surface, to increase THz wave absorption and phase retardation. The enhancement in α_eff and Δn_eff of Periodic-45-133 MPS is attributed to its small pore size, whereas its pore quantity is identical to that of Periodic-90-249 MPS.

The relation between the absorption coefficients and vapor densities can be well fitted by Langmuir adsorption isotherms [15] with R² values greater than 96% for Periodic-90-249 and Periodic-45-133 MPSs, which are depicted as purple and pink curves, respectively, in Fig. 3(a). Langmuir fitting indicates that the monolayer adsorption of acetone vapor molecules on the hydrophilic microporous surface is mainly caused by physisorption [16]. For the molecular density, less than 6 nmol/mm³ (~350 ppm), the THz responsivities of the proportional relation between α_eff and ρ in Fig. 3(a) are linearly fitted as the black and red dash lines, which can be regarded as the sensitive region of the MPS vapor sensor. A comparison of the slopes of the linear fitting curves between the two periodic MPSs indicates that the α_eff of Periodic-45-133 MPS increases more rapidly than that of Periodic-90-249 MPS within the sensitive region (ρ < 350 ppm). The linear fitting slope of α_eff responsivity can be regarded as the sensitivity of the MPS for VOC detection.

The lowest concentration of acetone vapor prepared in the experiment for vapor sensing is 291 pmol/mm³, which is the first data point in the Fig. 3 and corresponds to 17 ppm. In addition, the detection limit of vapor concentration change can be estimated from the sensitivity and the measurement inaccuracy of α_eff, which are the slope of the linear fitting curve and the error bar in Fig. 3(a), respectively. Therefore, the minimum detectable concentration changes of acetone vapor for the Periodic-90-249 and Periodic-45-133 MPS sensors determined from Fig. 3 are estimated as less as 108 and 54 pmol/mm³, corresponding to 6.29 and 3.11 ppm, respectively. Obviously, the detection limit of Periodic-45-133 MPS is two times higher than that of Periodic-90-249 MPS, thereby indicating that a decrease in micropore volume (such as half of the pore width) obviously raises the detection sensitivity of the MPS gas sensor.

Figures 4(a) and 4(b) illustrate the vapor sensing results of the uniform MPSs, including Uniform-90 and Uniform-45 MPSs. Both the α_eff and Δn_eff are also linearly increased within the low vapor density region and become saturated at a high vapor density, similar to the trace trends of the periodic MPSs as shown in Fig. 3. However, the saturation vapor density for the two uniform MPSs, ~4 nmol/mm³ and corresponding to 200 ppm, is even lower than that of the periodic MPSs, ~6 nmol/mm³ or 350 ppm. The THz absorption coefficient curves ofUniform-90 and Uniform-45 MPSs are also well fitted by Langmuir adsorption isotherms with R² values greater than 93%, which are depicted as the green and cyan curves in Fig. 4(a), respectively. According to the slopes of the linear fitting curves [orange and blue dash lines in Fig. 4(a)] in the sensitive region (i.e., ρ < 200 ppm) and the measurement inaccuracy of the THz absorption coefficient, the minimum detectable concentration changes of acetone vapor using Uniform-90 and Uniform-45 MPS gas sensors can be estimated to be less than 46 and 31 pmol/mm³, corresponding to 2.68 and 1.83 ppm, respectively.

 

Fig. 4 0.4 THz wave responsivity of (a) effective absorption coefficients and (b) refractive index variations per unit pore volume in the Uniform-45 and Uniform-90 MPSs with the same micropore quantity under the illuminated THz spot area.

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The detection sensitivity of Uniform-45 MPS is apparently higher than that of Uniform-90 MPS, similar to the comparison result between Periodic-90-249 and Periodic-45-133 MPSs. The performance emphasizes again that half of the pore width, whether periodic or uniform configuration, facilitates the infiltration and adsorption of acetone vapor in the micropores and on the pore surface, leading to an enhanced vapor–field interaction to increase the sensitivity. Furthermore, the two uniform MPSs in Fig. 4 have apparently higher α_eff and Δn_eff than those of the periodic MPSs in Fig. 3 under the same vapor density exposure. For example, the largest THz absorption coefficient of Uniform-45 MPS is around 80 cm⁻¹, which is evidently larger than 50 cm⁻¹ of Periodic-45-133 MPS under the same 6-layered structural thickness. According to the responsivities of linear fitting slopes shown in Figs. 3 and 4, the two sensing parameters, α_eff and Δn_eff, of uniform MPSs are increased more rapidly within a narrower sensitive region compared with those of periodic MPSs. In other words, only fewer amounts of vapor molecules that infiltrate the uniform MPSs can drastically increase the THz absorption coefficient and refractive index variation until the desired chamber saturation is achieved. The average pore width of Uniform-90/-45 MPS is smaller than that of Periodic-90-249/-45-133 MPS under similar MPS thickness (i.e., the same stacked layer number of the PET microporous mesh). Therefore, the simple uniform MPS is particularly advantageous for minute vapor sensing with a detection limit of even lower ppm level compared with the periodic MPS.

3.3 Micropore number effect

In addition to the pore size, the effect of the micropore quantity on the detection sensitivity of the MPS gas sensor is discussed as follows. Figure 5 shows α_eff and Δn_eff for Periodic-45-133 MPS (i.e., small-pore periodic MPS) and Uniform-45 MPS (i.e., small-pore uniform MPS), wherein each type of MPS possesses three kinds of thicknesses formed by stacking different layer numbers of PET mesh as the conditions of Fig. 2. The α_eff and Δn_eff of Periodic-45-133 MPS with 6-layered thickness are evidently larger than those of 34- and 46-layered thicknesses for all vapor densities as expressed by the green, red, and black square dots in Figs. 5(a) and 5(b), respectively. Similarly, both α_eff and Δn_eff of Uniform-45 MPS areincreased with decreasing device thickness from 34 layers to 12 and 6 layers as represented by red, blue, and green square dots in Figs. 5(c) and 5(d), respectively. At a certain vapor density (ρ), the difference of α_eff (or Δn_eff) between 6- and 34-layered Uniform-45 MPSs is larger than that of Periodic-45-133 MPSs. In Fig. 5, the highest sensitivities for the periodic and uniform MPSs occur in their thinnest conditions (i.e., 6-layered structure). The experimental result indicates that decreasing the pore numbers also leads to increased THz wave responses in α_eff and Δn_eff in the micropores, thereby enhancing the vapor detection sensitivity. Under a constant amount of vapor exposure, increasing pore quantity or size is equivalent to expanding the pore volume of the microporous structure. The vapor density, congregated within the micropore and adsorbed on the hydrophilic surface, is thus diluted and eventually decreases the measured values in α_eff and Δn_eff. The experimental result of Fig. 5 is consistent with the result of transmittance variation (|ΔTr.|) in the acetone-filled MPSs as shown in Fig. 2, revealing that the vapor densities infiltrated and adsorbed in the MPS cannot significantly increase with a larger pore volume or surface area. In addition, the decreased α_eff-responsivities for the large layer number of MPS in Figs. 5(a) and (c) are not caused by their larger device-thickness-normalization based on Eq. (1) because the same situation can be verified in the pore-number-dependent THz absorption responsivities of Fig. 2, which are represented as |ΔTr.| and without being normalized with the device thickness. Figure 5 also illustrates that 6-layered Uniform-45 requires less acetone vapor amounts to saturate the responsivities of α_eff and Δn_eff because of its less micropore volume/number, comparing to those of 12- or 34-layered Uniform-45. For the 6-layered Periodic-45-133 MPS, the saturation effect also performs in Δn_eff. The vapor saturation density of 6-layered Uniform-45 MPS occurs at around 4 nmol/mm³ (i.e., 200ppm), indicating the dynamic range of linear responsivity is sufficiently wide for minute vapor detection.

 

Fig. 5 . 0.4 THz wave responsivity of (a), (c) effective absorption coefficients and (b), (d) refractive index variations per unit pore volume in the Periodic-45-133 and Uniform-45 MPSs with different thicknesses under the illuminated THz spot area.

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4. Sensing application for methanol adulterated in alcoholic solutions

As presented in Figs. 3–5, we summarize the micropore size dependent sensing performance of the four types of MPSs in Table 2, where the sensitivity corresponds to the slope of linear fit. The blank chamber represents the vapor sensing performance of the microfluidic chamber without the MPS device, and it is equivalent to THz vapor sensing in free space measured by traditional THz-TDS [10]. The Uniform-45 MPS with a 6-layered thickness has the highest sensitivity, detecting 1 ppm-level acetone vapor molecule. Its sensitivity is more excellent than that of the 23-layered Periodic-90-249 MPS and much higher than that of blank chamber, which were demonstrated in our previous publication [10]. We therefore applied 6-layered Uniform-45 MPS for identifying toxic methanol adulterated in alcoholic solutions. Different concentrations of adulterated alcoholic solutions are prepared by mixing different volume ratios of methanol with ethanol, including 1:0, 7:3, 5:5, 3:7, and 0:1 (ethanol/methanol). The adulterated alcoholic solution is injected into the microfluidic channel of the sealed Teflon chamber [Fig. 1(c)] and naturally evaporates into the vapor phase for THz wave sensing.

Table 2. Sensing performance of MPS for acetone vapor detection.

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Figure 6(a) illustrates the THz transmission spectrum of Uniform-45 MPS exposed to the vaporized mixtures that are generated from various concentrations of adulterated alcoholic solutions. The transmitted THz power apparently decreases within 0.25–0.45 THz as the volume ratio of methanol is increased. On the basis of Eq. (1), the THz absorption coefficient spectra for the different concentrations of adulterated alcoholic vapors can be estimated and shown in the inset of Fig. 6(a). The measured THz absorption coefficients for each concentration of alcoholic vapor are almost constant in the frequency range of 0.25–0.45 THz, and the relatively high absorption coefficients are resulted from the increment of the adulterated methanol concentration. The refractive index variation before and after exposure to different concentrations of alcoholic vapors can be calculated based on Eq. (2). Figure 6(b) plots the relations of the α_eff and Δn_eff at 0.4 THz against different concentrations of alcoholic aqueous solutions. α_eff and Δn_eff at 0.4 THz increase with the methanol concentration adulterated in the alcoholic solution, and the proportional relation of α_eff and ρ is linearly fitted as α_eff = 1.2 + 0.67ρ. The sensing result of Fig. 6 reveals that the colorless and high THz-absorbed alcoholic aqueous solutions with different concentrations of toxic methanol adulteration can be easily distinguished using the MPS gas sensor composed of 6-layered Uniform-45 MPS.

 

Fig. 6 (a) THz-wave transmission spectrum for different amounts of alcoholic vapor exposure. Inset shows their THz absorption coefficient spectra based on the 6-layered Unifrom-45 MPS. (b) Relations of 0.4 THz absorption coefficient and refractive index variation versus different concentrations of adulterated alcoholic aqueous solutions.

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5. Conclusion

We successfully demonstrate the feasibility of in situ and label-free detection for the VOC and high THz-absorbed volatile liquids using a simple multilayer-stacked MPS. The dependency of vapor sensing ability on the geometrical parameters of the MPS-based THz gas sensor is experimentally verified based on the analysis of THz absorption and refractive index variation before and after acetone vapor infiltration in the MPS structure. We have experimentally proven that the MPS with the smallest micropore volume can efficiently enhance THz absorptions both in the vapor-filled pore-space and on the vapor-adsorbed hydrophilic-pore-surface, resulting in the best sensitivity of approximately 1 ppm to detect a wide concentration range (<200 ppm) of acetone vapor. To our knowledge, the demonstrated sensitivity for VOC detection using the MPS is the highest compared among the resonator-type sensors in the THz regime. Different concentrations of toxic methanol adulterated in alcoholic aqueous solutions are successfully identified using the MPS with the best sensitivity. The MPS device is advantageous for THz wave gas sensing because of its room temperature operation, easy availability, low cost, low THz insertion loss, and label-free VOC sensing capability. The microporous THz gas sensor is highly promising for the detection of environmental pollutants, hazardous gas, and toxic adulterated beverages.