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Moataz Mekawy, Farahdiana Wan Yunus, [Edhuan Ismail](https://orcid.org/0000-0003-1031-6562), [Jin Kawakita](https://orcid.org/0000-0002-4821-4150), [Izumi Ichinose](https://orcid.org/0000-0002-2236-0942)

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[Temperature-controlled direct detection of volatile fatty acids by a membrane-covered moisture sensor](https://mdr.nims.go.jp/datasets/d20ead41-720b-43ff-ad87-fb6ebaab991a)

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Direct Monitoring of Ammonia and Volatile Fatty Acids Using a Temperature-Controlled Moisture Sensor Chip Farahdiana Wan Yunus, Moataz Mekawy, Edhuan Ismail, Jin Kawakita,* and Izumi Ichinose*Research Center for Macromolecules and Biomaterials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Japan*Corresponding authors: KAWAKITA.Jin@nims.go.jp, ICHINOSE.Izumi@nims.go.jpAbstractThe detection of ammonia (NH3) and volatile fatty acids (VFAs) during anaerobic processes plays an important role in the management of livestock and their waste, as well as in biogas production. A newly developed moisture sensor chip (MSC) was used to monitor these gasses volatized from aqueous solutions. The concentrations of NH3, acetic acid (AA), propionic acid (PA), and butyric acid (BA) were in the range 60–150 mM. To substantially improve the detection sensitivity, the sensor chip was cooled from 24 to 19 °C. Because the temperature was decreased, volatized water and solute condensed on the surface of the MSC and the formed microdroplets and the detection current increased to several thousand picoamperes. The detection current also increased with increasing concentration of VFAs and was highly sensitive to the pH conditions and the partial pressure of VFAs in the air phase. The amount of water droplets formed on the surface of MSC increased with increasing pH of the aqueous solution in the order NH3 > water > VFAs. Droplet formation preferentially occurred in the order AA > PA > BA, indicating that a shorter alkyl chain of VFA was preferred. The proposed temperature-controlled MSC is applicable for the detection of NH3 under basic conditions and for the detection of VFAs under acidic conditions.Keywords: Ammonia (NH3), volatile fatty acids (VFAs), acetic acid (AA), propionic acid (PA), butyric acid (BA), moisture sensor chip (MSC)1. IntroductionAmmonia (NH3) is a critical material in many industrial processes, and it is being considered as a future fuel in power plants [1] and a precursor of hydrogen fuel [2]. NH3 is also produced widely via anaerobic processes in landfills [3], sewage treatment plants [4], and agricultural activities [5–7]. However, NH3 concentrations that exceed the safe range can cause numerous human health and environmental problems. Access to safe water is one of the 17 sustainable development goals (SDGs) outlined by the United Nations. When NH3 is dissolved in water, nitrogen-rich wastewater will cause an algae bloom, which harms aquatic ecosystems. Groundwater polluted with NH3 is not potable, and long-term exposure to NH3 gas causes health issues such as eye irritation, dizziness, and damage to the lung, liver, and kidney [8]. Currently available methods for monitoring NH3 levels in water require a liquid sampler to form gaseous samples suitable for gas chromatography [9,10]. Chemical indicators and electrochemical probes for NH3 are available. However, the sensitivity of chemical sensors decreases with the consumption of chemicals. One difficulty in NH3 detection is that various nitrogen compounds coexist in the aquatic environment and their concentrations vary depending on other chemicals such as alcohols, active hydrogen compounds, and volatile fatty acids VFAs. Wang et al. pointed out that water pollution is a complex issue and stressed the need for a simpler and widespread monitoring device that can provide real time data for nitrogen-based compounds [11]. Japan's national Science and Technology Research Partnership for Sustainable Development (SATREPS) is also actively collaborating with developing countries to control the levels of nitrogen-based water pollutants [12].The same anaerobic processes that produce NH3 also produce VFAs [13,14]. In particular, large amounts of VFAs are produced in the digestion system of ruminants, where microorganisms decompose the organic materials into various carboxyl compounds and alcohols [15]. Like NH3, VFAs are important intermediates or products in industries such as biogas production [16], food additives [17], and chemical manufacturing [18]. In ruminants, which represent a vital food source for people, VFAs are metabolized to provide energy [19]. Giardi et al. [20] and Dinsdale et al. [21] have noted that VFAs serve as an important indicator for ruminants’ health and for dairy and meat production. Among ruminants, cows produce the largest amount of VFAs [22]. When the VFA concentration becomes too high, the resultant low-pH condition leads to subacute ruminal acidosis, abscess of the liver, and inflammation of tissue above the hoof [23–25]. For these reasons, the development of an NH3 and VFA sensing system is critical for the livestock and agricultural sectors [26]. Monitoring the actual amounts of NH3 and VFAs produced in microbial digestion systems is therefore important. Simple monitoring of pH cannot estimate the activity of a microbiome. The production of NH3 under high pH conditions and that of VFAs under low pH conditions need to be monitored.Conventionally, NH3 has been measured using microdiffusion adsorption, ion-selective electrodes, and titration [27]. As demonstrated in previous studies, the concentration of NH3 can be measured using the colorimetric method or microdiffusion. Meanwhile, the total or individual VFA concentration has been measured using chromatography techniques [28–31]. A study based on electrochemistry, as reported by Korent et al., achieved the detection of NH3 in a biological fluid using an Au electrode coated with polyaniline that undergoes a change in oxidation state depending on the concentration of NH3 [32]. Minami et al. demonstrated the strong potential of a nanomechanical sensor array that can distinguish silage gas samples containing different concentrations of VFAs [33]. These samples contained acetic acid (AA), propionic acid (PA), butyric acid (BA), and valeric acid at parts-per-million concentrations. Lamb et al. have developed an organic dye-based sensor that is promising for detecting low concentrations of VFAs via a color change [34]. To the best of our knowledge, the literature contains no studies on the simultaneous detection of NH3 and VFAs using a single sensor chip. Herein, using the recently developed moisture sensor chip (MSC) [35], we studied the sensing behaviors of humid gases volatized from aqueous solutions of NH3, VFAs, and their mixtures. MSC can detect adsorbed water molecules with high sensitivity and monitor the condensation behavior [36]. NH3 and VFA vapors co-adsorb with water molecules onto the MSC surface. Because the sensor chip is not directly exposed to aqueous samples, this detection method can prevent the corrosion of electrodes and maintain high sensitivity [37]. The aim of the present work is to monitor the adsorption and condensation behaviors of aqueous solutions containing different concentrations of NH3 and VFAs. To the best of our knowledge, their detection using a single sensor chip is reported here for the first time. The condensation behaviors were studied alongside microscopic observations. Careful analysis of the behaviors revealed that, in addition to the concentrations of volatile solutes (i.e., NH3 and VFAs), the change in surface wettability also determines the response currents. This newly developed microsensor can be used for monitoring microbial activities under a wide range of anaerobic conditions.2. Materials and method2.1. MaterialsPA (> 99.5% purity) and BA (>99.0% purity) were purchased from TCI, and AA (99.9% purity) and 28 wt% NH3 solution were obtained from FUJIFILM Wako Chemicals.2.2. Experimental section 2.2.1. Sensor configurationThe fabrication of the MSC has been explained elsewhere [38]. Briefly, a silicon substrate with a 200 nm silicon oxide insulating layer was prepared using an additional chemical vapor deposition process of silicon oxide. Interdigitated electrodes with a 1300 µm length, 2 µm width, and 0.2 µm height were prepared on the silicon substrate by sputter deposition using a photolithography process. The interdigitated electrodes were made of Au and Al, and the electrode gap was 0.5 µm. Schematics of the sensor configurations are shown in Figure 1a and 1b. Sensor response is measured as galvanic current between two electrodes of Al and Au by using a tailored module with a current-input A/D converter (DDC118, Texas Instruments). Note that Al electrode is not very stable for strong acidic or strong basic conditions. Concentrated NH3 or VFA gases are not available.2.2.2. Observation setupA schematic of our experimental setup is shown in Figure 1c, and photos of the actual setup are shown in Figure 1d. The sensor was placed inside a small chamber in which the temperature could be controlled in the range 24–19 °C. A Peltier device was attached to the bottom of the sensor (MSC) for temperature adjustment. This small chamber and buffer chamber were placed inside a large container in which the temperature was maintained at 25 °C. A Keyence VHX digital microscope was used for the microscopic observation of the MSC surface. An NRP 3000-EYELA pump was used to circulate the air containing NH3 or VFAs into the buffer and then into the sensor chamber.Fig. 1. (a) Schematic of the sensor chip configuration. (b) Cross-sectional configuration. (c) Experimental setup. (d) Photos of the actual setup.2.2.3. Measurement proceduresA pump (NRP 3000-EYELA) was used to circulate 50 mL of test solution at 80 rpm. The temperature was decreased in a stepwise manner from 24 to 19 °C. The setup in Fig. 1 was designed to circulate a humid gas sample in a closed system. Once the pump starts, bubbles form in the sample solution, causing vaporization of the NH3 or VFAs with water. These vapors are then transferred to the buffer chamber and to the sensor chamber. The vapors slowly condense into liquid on the MSC surface when its temperature is adjusted to be lower than the chamber temperature. Further explanations are given in supporting information sections SI1. 3. Results and discussion3.1. Condensation behavior of NH3 and VFA solutions Fig. 2a shows the response currents of the MSC exposed to vapor generated from 150 mM NH3, 150 mM PA, 150 mM AA, and 150 mM BA solutions and water as a reference. Fig. 2b shows the bar chart replotted from Fig. 2a by averaging the initial and final current values at each temperature and was repeated as shown in Figure S1a. Fig. 2c–g represent the formation of condensed vapor observed using an optical microscope.The response current for 150 mM NH3 increased from a few picoamperes to 1380 pA at 24 °C over a period of 16 min. When the temperature was lowered to 23 °C, the response current increased to 5610 pA. A further decrease of the temperature to 20 °C led to an increase in the current to 15,040 pA. At 19 °C, the response current showed the maximum value of 17,500 pA, which is the detection limit of our sensor. In the case of 150 mM PA, at 24 °C, the current increased from several picoamperes to ~200 pA; the current then continuously increased as the temperature of the sensor was decreased to 20 °C. The current values increased to 360 pA at 23 °C and 1180 pA at 20 °C. At the final temperature (19 °C), the current for the 150 mM PA solution was 2250 pA. This response current corresponds to 87% lower than that of the response current of the 150 mM NH3 solution at the same temperature. The response current for water increased to 990 pA at 21 °C, to 1970 pA at 20 °C, and to 5340 pA at 19 °C. From Fig. 2a, we conclude that the 150 mM NH3 solution gave far higher response currents than water. The response currents of PA are higher than those of water at temperatures between 24 and 22 °C. However, the response currents become lower than those of water at 21–19 °C. We speculate that the accumulation of water molecules accelerated at 21 °C and that the rate was higher than that of the 150 mM PA solution. These water molecules started to form water droplets at 20 °C, as discussed later. Fig. 2. (a) The response currents of the MSC to vapors generated from water and from NH3 and VFA solutions, plotted as a function of time. The temperature was varied with time from 24 to 19 °C. (b) Conversion of the log plots in (a) to linear plots. (c–g) Microscopic images of the surface of the MSC at temperatures from 22 to 19 °C for the vapors generated from 150 mM NH3, 150 mM PA, 150 mM AA, and 150 mM BA solutions and from water.The 150 mM AA solution gave response currents that gradually increased with decreasing temperature. The values were 120 pA and 4710 pA at 24 °C and 19 °C, respectively. As evident in Fig. 2a, at 23 °C and 21°C, the response current of AA was 1.4 and 2.6 times greater than that of water. However, at the final temperature (19 °C), the AA response current was ~12% lower than that of water. There are two effects of AA. The first effect is the increase in conductivity of the adsorbed water layer. At temperatures lower than 23 °C, AA generates ionic species in the thin water layer. The second effect is the increase in hydrophobicity of the MSC surface. This effect prevents the accumulation of water molecules at the surface. At 19 °C, the amount of accumulated water molecules from an AA solution becomes smaller than that from water. As a result, the response current of an AA solution becomes lower than that of water. The 150 mM BA solution showed the lowest response current among the VFA solutions. The currents increased from 62 pA to 1610 pA as the temperature decreased from 24 °C to 19 °C. The strong hydrophobic effect of BA must prevent the accumulation of water even at low temperatures [39]. In conclusion, the response currents increase in the order BA < AA < water < PA < NH3 at 24 °C, in the order BA < PA ≈ water < AA < NH3 at 21–20 °C, and in the order BA < PA < AA < water < NH3 at 19 °C.The microscopic images reveal that no water droplets were present at 22 °C, indicating that NH3 and VFA molecules are in a thin layer of adsorbed water in the temperature range 24–22 °C (Figure 2c–g (i)). At 21 °C, in the NH3 and VFAs cases, water droplets started to be observed on the MSC surface (Fig. 2c–d (ii) and 2f–g (ii)). By contrast, for water, condensation started at 20 °C (Fig. 2e (iii)). Droplet coverage for NH3 solutions increased from 1% (21 °C) to 49% (19 °C). Droplet coverage for PA solutions increased from 5% to 30% at these temperatures. Water gave 3% and 35% coverages at 21 °C and 19 °C, respectively. Among the three investigated VFAs, AA showed the largest droplet coverage of 34% at 19 °C. This coverage decreased with increasing molecular weight of the VFAs (30% for PA and 5% for BA). The details of the values are also described in Table 1, and the droplet coverage traced values are shown in Figure S2.Table 1. Response currents and droplet coverage at 24 to 19 °C Solution Response currents (pA)and droplet coverage (%)  24 °C 23 °C 22 °C 21 °C 20 °C 19 °C  pA % pA % pA % pA % pA % pA % 150 mM NH3 1380 0 5610 0 8770 0 13,060 1 15,040 5 17,500 49 150 mM PA 200 0 360 0 530 0 730 5 1180 9 2250 30 Water 130 0 270 0 500 0 990 0 1970 3 5340 35 150 mM AA 120 0 370 0 940 0 2580 0.2 3840 3 4710 34 150 mM BA 62 0 81 0 120 0 230 1 530 3 1610 53.2. Concentration-dependent response currentsThe concentration dependency of the response currents was studied at three temperatures that correspond to the initial (24 °C) and middle (22 °C) stages of the formation of a water layer with a uniform thickness and the final stage of droplet accumulation (19 °C). The results are summarized in Fig. 3a–c; the raw data are shown in Figure S1c–e. As the concentrations of the NH3 and VFAs were increased from 60 mM to 90 mM, 120 mM, and 150 mM, the initial-stage (24 °C) response currents of the NH3 solutions increased substantially from 1400 pA (60 mM) to 3160 pA (150 mM). The VFAs also showed a concentration dependency at this stage. The response current of the PA solutions increased from 63 pA (60 mM) to 130 pA (150 mM). AA exhibited smaller response currents of 24 pA (60 mM) and 30 pA (150 mM), and the BA response currents slightly increased from 22 pA (60 mM) to 24 pA (150 mM). Compared with the response current of 43 pA for pure water, that of NH3 is very high; however, those of AA and BA are small. During the middle stage (22 °C), the water response current increased to 320 pA. In sharp contrast, NH3 solutions gave large response currents of 4250 pA (60 mM) to 10,120 pA (150 mM), which was 13–32 times the response current of water. The VFAs showed small increases of the response currents with increasing concentration. However, the BA solutions showed lower values than water. Specifically, the response currents were 360 pA (60 mM) to 460 pA (150 mM) for PA, 63 pA (60 mM) to 700 pA (150 mM) for AA, and 73 pA (60 mM) to 85 pA (150 mM) for BA. At this temperature, only NH3 and AA showed a clear concentration dependency, as indicated by the log-scale curves in Fig. 3b. At the final stage (19 °C), the NH3 solutions reached the maximum current of our sensor system, giving a value of 17,500 pA for all concentrations. Unlike NH3, PA showed a decrease in response current as the concentration was increased from 60 mM (3750 pA) to 150 mM (1910 pA). This tendency was also observed for BA, for which the response current decreased from 2950 pA (60 mM) to 1250 pA (150 mM). By contrast, AA showed an increase of the response currents, where the values ranged from 3240 pA (60 mM) to 5030 pA (150 mM). The difference in VFAs response currents indicates that the degree of wettability of the MSC surface toward these aqueous solutions differs. All of these experimental data are summarized in Table 2. Fig. 3d–h show optical microscopic images of droplets on the MSC surface at the final stage (19 °C). The high coverage of the droplets observed (except for the BA solutions) indicates that condensation fully proceeded. The droplet images of the NH3 solution and water are similar, showing droplets widely spread on the MSC surface. Compared with the droplet size for water (Figure 3d), the droplet sizes for the NH3 solutions are slightly larger (Fig. 3e). Compared with NH3 and water, the VFAs showed smaller droplets. Among the VFAs, PA exhibited the largest droplet size of approximately 136–149 m in our investigated concentration range (Fig. 3f). The AA droplets in Fig. 3g show a clear increase in droplet size as the concentration increased. The droplets increased from 70 m (60 mM) to 134 m (150 mM), indicating an increase in hydrophilicity of the MSC surface. In contrast to the droplet size of AA, that of BA decreased substantially with increasing concentration and no droplets larger than 8 m were obtained at a concentration of 120 mM or more (Fig. 3h). This result indicates that the surface becomes hydrophobic with increasing BA concentration [39,40].The large expansion of the NH3 solution droplets is due to the deprotonation of silanol units on the silicon dioxide (SiO2) surface between the Au and Al electrodes of the MSC. The high vapor pressure of NH3 enables it to easily accumulate on the MSC surface with water vapor; however, the transport rate of water vapor might slightly decrease in the presence of 150 mM NH3 solution (Fig. 3e). By contrast, the droplet size increased with increasing AA concentration (Fig. 3g), which can be explained by the increase in hydrophilicity of the MSC surface upon protonation of the SiO2 surface by AA. The coordination of AA to the SiO2 surface leads to the observed increase in hydrophobicity; however, this effect appears to be weak. The effect of protonation is consistent with the observation that the response current increased with increasing concentration of AA (Fig. 3b). The droplets formed from PA solutions were approximately the same size irrespective of the PA concentration (Fig. 3f). We speculate that the protonation effect (i.e., an increase in hydrophilicity) and the coordination effect (i.e., an increase in hydrophobicity) are almost compensated [41]. Droplets generated from BA solutions decreased in size with increasing BA concentration (Fig. 3h). The MSC surface becomes more hydrophobic with increasing BA concentration because of its longer alkyl chain [42,43]. Our results demonstrate that the order in which aqueous solutions accumulate on the MSC surface is NH3 > water > AA > PA > BA. These findings were confirmed by contact angle measurements and can be explained by the formation of a cationic or anionic MSC surface, as described later. Fig. 3. Concentration-dependent currents for NH3 and VFA solutions at (a) 24 °C (initial stage), (b) 22 °C (middle stage), and (c) 19 °C (final stage). Optical microscopic images of the droplets of (d) water and (e) NH3, (f) PA, (g) AA, and (h) BA solutions formed on the MSC surface at 19 °C. Droplet size was monitored at concentrations of 60–150 mM. Photos were obtained at 90 min experimental time in Fig. 2a.Table 2. Response current for 60 mM, 90 mM, 120 mM, and 150 mM aqueous solutions at 24 °C, 22 °C, and 19 °C. Concentration (mM) Stages Solutions and response current (pA)   NH3 PA AA BA 0 Initial (24 °C) 43  Middle (22 °C) 320  Final (19 °C) 5130 60 Initial (24 °C) 1,400 63 24 22  Middle (22 °C) 4250 360 62 73  Final (19 °C) 17,500 3750 3240 2950 90 Initial (24 °C) 1,800 82 25 23  Middle (22 °C) 4950 390 110 74  Final (19 °C) 17,500 3440 4230 2460 120 Initial (24 °C) 1,910 110 27 24  Middle (22 °C) 6250 400 220 80  Final (19 °C) 17,500 2330 4820 1400 150 Initial (24 °C) 3,160 130 30 24  Middle (22 °C) 10,120 460 700 85  Final (19 °C) 17,500 1910 5030 12503.3. MSC surface wettabilityWe checked the surface wettability of MSC surface by the coverage of the condensed droplets. Coverage is described as percentages in Fig. S3f and is summarized in Table S2; the ImageJ raw data are shown in Figure S4. The results were repeated (Figure S1b). Details are given in section SI2 in supporting information. MSC surface wettability is one of the most important factors for the detection of NH3 and VFAs in water. Therefore, we carefully measured the contact angles of the MSC surface, changing the temperature of these solutions. As evident in Fig. 4a, the contact angles increased by approximately 6–8° for all of the solutions when the MSC temperature was lowered from 24 to 19 °C. The measured values were 61°–69° (NH3), 67°–75° (PA), 65°–73° (water and AA), and 70°–76° (BA). Details of the contact angle measurements are presented in Figure S5. The order of the contact angles is NH3 < water ≈ AA < PA < BA. The surface tension of an aqueous solution is known to increase with decreasing temperature [44]. Our observations reveal a similar tendency. Interestingly, BA, which has a longer alkyl chain than the other investigated VFAs [45], showed the largest contact angle. In general, hydrophobic molecules lower the surface tension of water, like surfactant molecules, and lead to a decrease in the contact angle. The fact that BA exhibited the highest contact angle strongly suggests that the hydrophobicity of the MSC surface increased substantially and that this effect was more significant than the decrease in the surface tension of the BA solution. PA showed a similar effect (i.e., it coordinates to the SiO2 surface and makes the surface hydrophobic). NH3 did not decrease the surface tension of the aqueous solutions. However, we observed contact angles lower than that of water. This result indicates that the MSC surface becomes more hydrophilic because of deprotonation of the silanol groups. AA has a contact angle similar to that of water, indicating that the protonation effect and the decrease in the surface tension are compensated. As evident in Figure 4b, the response currents are correlated to the contact angle values. The NH3 solutions had the smallest contact angles among the investigated solutions and exhibited the highest response currents of 1000–16,000 pA. BA exhibited the highest contact angle, which led to it exhibiting the lowest response currents of 30–1100 pA. PA showed a similar tendency as BA, exhibiting relatively high contact angles and lower response currents of 100–1800 pA. The contact angles and response currents (60–4000 pA) of water and AA show similar tendencies. Figure 4c shows a plot of the contact angle as a function of pH. The values at pH 10.9 correspond to a 150 mM NH3 solution. The contact angles near pH 2.7 correspond to 150 mM VFA solutions. The color changes indicate the temperature in the range from 24 °C (red) to 19 °C (blue). As the temperature decreased, the contact angles of solutions increased because of the reduced surface tension. These changes can be discussed on the basis of the point of zero charge (PZC) of the SiO2 surface. SiO2 has a PZC value of pH 2.8 [46, 47]. At this pH, SiO2 has no overall surface charge, which is denoted by (ii) in Figure 4c. VFA solutions with a pH near 2.7 may slightly protonate the SiO2 surface, forming Si–OH2+ and thereby imparting a slight positive charge. This range is denoted by (i) in Figure 4c. Conversely, water and NH3 (pH 10.9) solutions, which have pH levels far greater than the PZC of the SiO2 surface, will enhance deprotonation, forming Si–O− on the surface and thereby increasing the amount of negative charge, as shown by range (iii) in Fig. 4c. As reported by Virga, the solution wettability increases with increasing amounts of ions on the metal oxide surfaces [58]. Notably, the BA, PA, and AA solutions exhibit different contact angles despite their pH levels being similar to the PZC value of SiO2. These different contact angles indicate that the coordination effect is far more significant than the increase in positive charge.Fig. 4. (a) Changes in contact angles as a function of temperature. The concentrations of NH3 and VFAs are 150 mM. (b) Changes of response currents as a function of contact angle. (c) Changes of the contact angle at different pH levels and temperatures. PZC: point of zero charge (for SiO2). (i) acidic range, (ii) PZC pH, and (iii) basic range.4. ConclusionsA newly developed MSC was applied to the detection of NH3 and VFA vapors for the first time. The results revealed that, among the investigated analytes, NH3 was detectable with the highest sensitivity. The large response current for NH3 is due to the deprotonation of the silanol groups on the MSC surface. This reaction increases the ion concentration and conductivity of the thin water layer, resulting in a large increase of the MSC surface wettability and in a response current of a few thousand picoamperes. VFAs were also detectable when an appropriate temperature was selected for the sensor chip. The length of VFAs' alkyl chain changes the hydrophobicity of the MSC surface, leading to response currents that decrease in the order AA > PA > BA. BA exhibits the smallest response current because the sensor chip cannot form the water droplet even at 19 °C. That is, the concentration of BA is determined by the decrease in the current, especially at low temperatures. The MSC is a promising sensor with a chip size of 0.25 cm2 and a high sensitivity of thousands of picoamperes. The rapid detection rate of the sensor is expected to enhance the sensitive monitoring of NH3 under basic conditions and VFAs under acidic conditions. CRediT authorship contribution statement F. W. Yunus: Methodology, Validation, Investigation and Formal analysis, Data curation, Writing – original draft, M. Mekawy: Validation, Investigation and Formal analysis, Writing - review & editing. E. Ismail: Investigation and Formal analysis, Writing - review & editing. J. Kawakita: Conceptualization, Methodology. I. 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