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  1. (Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, Korea )
  2. (Advanced Institute of Convergence Technology, Seoul National University, Suwon, 16229, Korea)

Food quality monitoring, in-situ monitoring, chemoresistive gas sensor, nano-structure, semiconductor


Foods that provide nutrients essential for human survival have the disadvantage of being easily spoiled. Protein and moisture in food provide an optimal environment for microorganisms to reproduce, and the reproduced microorganisms degenerate food cells [1-4]. In addition, it is difficult to control the circumstance of the entire supply chain or storage period, and since there are many variables, food quality monitoring is essential to keep the threat away from the table [5]. The World Health Organization reported that 600 million suffer from foodborne diseases worldwide, with 420,000 death in 2020 [6].

The freshness of the food was evaluated by methods including gas chromatography (GC) and mass spectroscopy (MS) [7]. However, the existing methods are ex-situ methods in which sample collection and measurement are done separately, and they have disadvantages such as requiring the preservation of the collected sample, reducing accuracy, and consuming a lot of time and money. In-situ food quality monitoring systems are being actively developed to compensate for these limitations. Among them, gas sensing food quality monitoring, which determines whether food is spoiled through the gas generated when it decays, has been one of the great candidates.

In particular, due to the rise of the Internet of Things (IoT), sensor technologies such as imaging, pressure sensing [8], voice recognition, electronic tongues, and gas monitoring are regarded as irreplaceable technology that can deliver the surrounding environment to a device and are showing a tendency to grow rapidly. Various types of gas sensors have been studied according to their operating principles, namely optical gas sensor [9,10], cantilever gas sensor [11], and solid-state electrochemical gas sensor [12-14]. Among them, chemoresistive gas sensors have been suggested as suitable for simple structure, facile synthesis process, and low cost [15-18]. In addition, strong physical resistance and easy miniaturization, which are essential properties for food quality monitoring, are accelerating research.

In this review, we introduce various semiconductor materials for food quality monitoring and strategies to enhance gas sensing performance. To the best of the author’s knowledge, there has been no organized review of the chemoresistive gas sensors based on various semiconductor materials, including metal oxides, metal sulfides, carbon nanomaterials, polymers, and their composites. We are quite sure that this review can provide a fresh perspective on the chemoresistive gas sensors to observe food freshness in the supply chain.


1. Gas Sensors in Food Quality Monitoring

The most important part of evaluating food quality through gas sensors is which type of gas sensor is used because a type of gas sensor that is not suitable for the operation circumstance is difficult to commercialize no matter how good the performance is. Among the previous classifications of gas sensors, chemoresistive gas sensors have advantages in many aspects. Optical gas sensors are challenging to miniaturize since they are composed of optical and electrical components [9,10]. Cantilever gas sensors are vulnerable to physical impact, so their performance depends on the operating environment [11]. Solid-state electrochemical gas sensors have trouble retaining the performance for a long time and covering a wide range of performance temperatures [12]. Chemoresistive gas sensors based on semiconductors are suitable for food quality monitoring due to their low mass production cost, simple electrical circuit-based device structure, and excellent adaptability to the operating environment [15,16].

2. Operation Mechanism of Semiconductor Chemoresistive Gas Sensor

Various materials including metal, metal oxide, and polymer have been used for the sensing materials of chemoresistive gas sensors [19-30]. The most commonly considered sensing mechanism is sensing through a change in resistance due to the semiconducting material [31]. When the sensing material is exposed to air, the chemical adsorption of oxygen molecules on the surface of sensing materials creates the surface receptor states (O$^{2-}$, O$^{-}$, O$_{2}$$^{-}$). Adsorbed oxygen captures electrons at the surface, resulting in a larger electron depletion layer (EDL) [31-35]. When the reactant gas approaches the surface of the reactant, a reaction involving oxygen in adsorbed state occurs, and the electron density near the surface of the reactant changes and the thickness of the EDL also changes. According to the major charge carriers of the semiconductor device material, the resistance increases or lowers due to the oxidation-reduction tendency of the n and p types and the reaction gas as shown in Fig. 1 [36]. In the case of n-type, since the main charge carriers are electrons, as the amount of adsorbed oxygen increases, the thickness of the EDL increases and the density of the main charge carrier decreases, increasing the resistance. When the adsorbed oxygen reacts with the reducing gas, the resistance drops, and changes in the surrounding gas environment are detected.

Fig. 1. Schematic illustrations of gas sensing mechanism of chemoresistive gas sensors.

Three basic factors describe the performance of gas sensors based on semiconductor materials: receptor function, transducer function, and utility factor [33]. First, the receptor function influences the reaction between the grain of the sensing material and gas and determines how sensitive and selective sensing is. The receptor function can be improved by attaching a catalyst to the surface to improve gas adsorption and reaction. The transducer function occurs at the contact between grains or particles of the sensing material and indicates whether the particle's reaction determines the resistance of the entire device. The double Schottky barrier between particles is determined according to the change of EDL generated on the surface. Lastly, the utility factor is a factor determined by gas diffusion and refers to how widely the reactant gas contacts the surface of the sensor. The more porous the structure of the reactant, the greater the resistance change can be induced because the thickness can be fully utilized.


1. Gas Sensors for Fruit Quality Monitoring

The Volatile Organic Compounds (VOCs) are released in the process of ripening and rotting fruits. The VOCs can give information on fruits' quality, such as freshness [37-40]. In the past, the Gas Chromatography-Mass Spectrometry (GC-MS) technique was conventionally used. By using this method, it is possible to infer the molecular weight of the analysis component enabling the accurate qualitative analysis of the sample. However, it is hard to be used broadly because the equipment is expensive and requires a long time to analyze a single sample [41-43]. Fruits release VOCs as secondary metabolites during biological interactions and non-biological stress reactions [44]. As an alternative to this conventional method, semiconductor-based chemo-resistive gas sensors are widely used due to their fast response, small volume, and high sensitivity [45-47]. Jeong et al. investigated ethylene sensors based on SnO$_{2}$ coated with nanoscale catalytic Cr$_{2}$O$_{3}$ overlayer [48]. Ethylene is one of the most emitted VOCs when the fruits are in the process of ripening or rotting. Therefore, chemoresistive gas sensors that can effectively detect low concentrations of ethylene are essential for fruit quality monitoring. The gas sensing mechanism is based on a surface reaction between the target gas and the ionized surface oxygen. As shown in Fig. 2(a), the SnO$_{2}$ sensor was fabricated by the screen-printing method. Subsequently, a Cr$_{2}$O$_{3}$ catalytic overlayer was coated on the SnO$_{2}$ sensor using an electron-beam (e-beam) evaporator (Fig. 2(a)). They prepared 0.05-, 0.3-, and 0.6-um-thick Cr$_{2}$O$_{3}$ overlayer on the SnO$_{2}$ sensing film. Then, they figured out 0.3 Cr$_{2}$O$_{3}$- SnO$_{2}$ specimen is the most suitable for the ethylene sensing material by using Scanning Electron Microscope (SEM), Electron Probe X-ray Micro Analyzer (EPMA), Transmission Electron Microscope (TEM), and X-ray diffraction (XRD). Fig. 2(b) shows the gas sensing properties of SnO$_{2}$ with a thickness of 9 um (left), SnO$_{2}$ with a thickness of 22 um (middle), and 0.3 Cr$_{2}$O$_{3}$- SnO$_{2}$ with a thickness of 21 um (right), respectively. As shown in the right-sided graph in Fig. 2(b), the 0.3 Cr$_{2}$O$_{3}$- SnO$_{2}$ sensor shows the highest selectivity for ethylene compared to other sensors. The sensing transients of the sensor to 0.1-2.5 ppm ethylene at 375 $^{\circ}$C were measured to show high reactivity, as shown in Fig. 2(c). Also, the sensor exhibited high reliability upon exposure to 2.5 ppm ethylene (Fig. 2(c)). Meanwhile, the detection limit of ethylene was measured to be 24 ppb. Also, the sensor exhibited outstanding long-term stability over 15 days when exposed to 2.5 ppm ethylene. The fruits were placed inside the open chamber at room temperature (20 $^{\circ}$C~ 25 $^{\circ}$C), where the relative humidity was 35-55%. The response to ethylene emitted from various fruits was monitored for 15 days. The ethylene response of the sensor was increased as the ripening proceeded, and these experimental results show that the 0.3 Cr$_{2}$O$_{3}$- SnO$_{2}$ sensor can be used to estimate the freshness and ripening degree of fruits. In particular, there was a noticeable difference in the sensor reactivity on the first day and the ninth day (Fig. 2(d)). Consequently, the ethylene gas sensor comprised of a SnO$_{2}$ sensing layer coated by a Cr$_{2}$O$_{3}$ overlayer shows great gas sensing performances, which can be efficiently used in estimating the ripeness of fruits.

Fig. 2. (a) Process used to fabricate sensors; (b) Gas-sensing properties of the thin SnO$_{2}$ sensor (thickness: ~9 um, left graph), thick SnO$_{2}$ sensor (thickness: ~22 um, middle graph), 0.3 Cr$_{2}$O$_{3}$-SnO$_{2}$ sensor (thickness: ~21 um, right graph); (c) Dynamic gas-sensing transients of the 0.3 Cr$_{2}$O$_{3}$-SnO$_{2}$ sensor (right graph), six repeated measurements of sensing properties of the sensor to 2.5 ppm ethylene at 375 $^{\circ}$C); (d) Gas sensing characteristics of the 0.3 Cr$_{2}$O$_{3}$-SnO$_{2}$ sensor for five different fruits[45].

Esser et al. reported a chemoresistive gas sensor based on a mixture of single-walled carbon nanotubes (SWNTs) and copper complex 1 [49]. The copper complex 1 is shown in Fig. 3(a). The SWNTs and copper complex 1 were drop-casted between the gold electrodes, and the resistance change of the sensor to ethylene was measured. The gas sensing mechanism of SWNTs largely relies on their electronic surroundings that affect adsorption of gas molecules and subsequently charge transfer to the molecules. The SWNTs show high sensitivity, and the copper (1) complex shows high selectivity to ethylene. To obtain both high sensitivity and high selectivity to ethylene, these two materials were mixed. The ratio of copper 1 complex to SWNT carbon atoms was almost 1:6. According to the mixing ratio, the mixture of SWNTs and copper complex 1 was named 1-SWNT. The 1-SWNT sensor showed high sensitivity to ethylene, as shown in Fig. 3(b). The graph on the top of Fig. 3(b) shows the relative response of 1-SWNT sensors to 0.5, 1, 2, 5, 20, and 50 ppm ethylene diluted with nitrogen gas. Compared to the response of pristine SWNT, which is not mixed with copper, the 1-SWNT sensor exhibited 40 times higher sensitivity to 20 ppm ethylene. The graph at the bottom of Fig. 3(b) displays the average responses of three different sensors. Despite a slight error in the 1-SWNT sensor, the error was within an acceptable range, and the sensitivity of the sensor was proportional to the ethylene concentration. Then, the 1-SWNT sensor was used to compare the ethylene emission from five common fruits such as banana, avocado, apple, pear, and orange. The responses of 1-SWNT sensors to five different fruits are shown at the top of Fig. 3(c). The intensities are shown in relation to the response to 20 ppm ethylene and were normalized to 100 g of each fruit. The ethylene concentration emitted from banana, avocado, apple, and pear was over 20 ppm, while the concentration was below 20 ppm for oranges. Especially, the banana showed a high concentration of ethylene emission, which was more than 8 times higher than 20 ppm ethylene. During the ripening process, the ethylene emission tended to decrease gradually, as shown at the bottom of Fig. 3(c). It is expected that the ripeness of fruits can be determined by measuring the amount of ethylene using the 1-SWNT sensor.

Fig. 3. (a) A mixture of single-walled carbon nanotubes (SWNTs) and copper complex 1 is drop-cast between gold electrodes. The change in resistance to ethylene exposure is measured; (b) Relative responses of 1-SWNT sensor to 0.5, 1, 2, 5, 20, and 50 ppm ethylene diluted with nitrogen gas and pristine SWNT to 20 ppm ethylene (top), average responses of 3 different sensors (bottom); (c) Responses of 1-SWNT sensor to 100 g of 5 different fruit relative to 20 ppm ethylene (top), ethylene emission of 5 different fruits over several weeks[46].

2. Gas Sensors for Meat Quality Monitoring

Due to the changes in lifestyle in modern society, the proportion of online purchases of meat is increasing. Since most of the meat quality is determined in the delivery process, it is important to efficiently manage meat spoilage that occurs in the process of delivering meat [50-52]. In this context, many studies have been conducted in that the meat quality monitoring system based on chemoresistive gas sensors can monitor changes in meat quality in real-time during delivery and selectively detect gases mainly emitted from meat [53-55]. During the meat spoilage process, Volatile Basic Nitrogen (VBN) gases are mainly emitted, and the degree of meat spoilage can be determined by measuring the concentration of VBN with chemoresistive gas sensors [56,57]. Spoilage micro-organisms and natural enzymes in the meat break down proteins and produce VBN compounds [58]. Liu et al. reported naphthyl end-capped terthiophene-based chemoresistive gas sensors to monitor meat spoilage [59]. They compared 5-(naphthalen-1-yl)-2,2’:5’,2’’-terthiophene (NA-3T) and 5,5’’-di(naphthalen-1-yl)-2,2’:5’,2’’-terthiophene (NA-3T-NA), which are suitable materials for detecting Biogenic Amines (BAs). There are plenty of shallow charge traps below (above) the charge conduction level for electrons (holes). These traps allow the thermal activation and hopping of charge carriers, resulting in charge transportation. NA-3T-NA showed high sensitivity to trimethylamine (TMA) compared to NA-3T and for aromatic BAs such as dopamine, histamine, tryptamine, and tyramine. Especially, the NA-3T-NA sensor showed good sensing properties to TMA, including high sensitivity, low detection limit ({\textless} 22 ppm), and short recovery time (23 s). The response of the NA-3T-NA based sensor to various aromatic BAs is shown in Fig. 4(a), which includes dopamine (R=2.3), histamine (R=2.8), tryptamine (R=2.6), and tyramine (R=3.4). Also, the response to TMA was significantly high (R=4.1). On the other hand, the response to aliphatic BAs such as cadaverine, putrescine, hexanediamine, spermidine, and spermine was marginal. Fig. 4(b) shows that NA-3T-NA based sensors have a short recovery time, which is within 23 s to the level of 95 % resistance change. The reactivity of the NA-3T-NA based sensor changes linearly with TMA concentration, as shown in Fig. 4(c). The intensity of the sensor went up with an increasing concentration of TMA with a linear relationship over a concentration range from 2110 to 16880 ppm, as shown in the inset of Fig. 4(d). Based on the outstanding gas selectivity to TMA, the real-time meat quality monitoring test was conducted. Fig. 4(e) shows the response to the vapors emitted from various raw meat samples, such as pork, chicken, and fish which were stored at different temperatures for 0-5 days. Compared to 4 ℃ samples, 25 ℃ samples show a high response. As time passed from 0 days to 5 days, the responses of the sensor tended to increase in all samples. The NA-3T-NA sensor can be used for an in-situ evaluation of meat freshness by monitoring the concentration of relevant volatile BAs.

Fig. 4. (a) Responses of the NA-3T-NA based sensor to various gas; (b) Response and recovery time of NA-3T-Na based sensor to TMA; (c) A reusability of the sensor to TMA at 25 ℃; (d) The signal intensity of the NA-3T-NA based sensor at different TMA concentrations. Inset: a plot of the differences in the response versus the concentrations of TMA; (e) Responses of the sensor to the vapors above various meat samples. Reproduced with permission from[56]Copyright © 2019 John Wiley & Sons.

Liu et al. reported the chemoresistive gas sensor based on single-walled carbon nanotube (SWCNT)/metallo-porphyrin composites [60]. The magnitude of the sensor response could be improved through the changes in the oxidation state of the metal and the electron-withdrawing characteristic of the porphyrinato ligand. SWCNTs can be functionalized covalently or noncovalently with other materials to pass on sensitivity or selectivity for the desired analyte. Porphyrins are attractive compounds for functionalizing SWCNTs because their aromatic core can noncovalently bind to the SWCNTs' walls. Fig. 5(a) shows the response of the sensor composed of various materials with exposure to NH$_{3}$. By using the Co(tpp)ClO$_{4}$-SWCNT which exhibited high selectivity to NH$_{3}$, the meat spoilage monitoring was conducted for 4 days, as shown in Fig. 5(b). The measurements were made with 4 types of meats such as cod, salmon, chicken, and pork at both 4 ℃ and 22 ℃. For the samples measured at 4 ℃, the sensor showed no increase in response for 4 days. However, for the samples measured at 22 ℃, the response of the sensors increased over time gradually. Overall, the gas sensor based on SWCNT-metalloporphyrin composites can be an excellent candidate for monitoring meat spoilage by measuring the concentration of volatile BAs.

Fig. 5. (a) Responses of the sensors based on porphyrin-SWCNT composites to various concentration of NH$_{3}$ for 30 s; (b) Responses of the sensor to 30 s exposures of emitted vapors from various meat samples stored at RT (22 ℃) and 4 ℃. Reproduced with permission from[57]Copyright © 2015 John Wiley & Sons.

3. Gas Sensors for Fish Quality Monitoring

Fish is the most representative food whose freshness directly determines the taste. For that reason, freshness maintenance of the fish is a major challenge, and several studies have proceeded to improve the maintenance [61-65]. After the death of fish, the microorganisms on the fish surface increase exponentially and permeate into the tissues. The microorganisms spoiled fish and emitted various kinds of gas such as trimethylamine (TMA), dimethylamine (DMA), ammonia, and CO [66,67]. Mostly, TMA gas was formed by bacteria such as Shewanella putrifaciens, Aeromonas spp., psychrotolerant Enterobacteriacceae, P.phosphoreum, and Vibrio spp. Fish use trimethylamine oxide (TMAO) as osmoregulant to avoid dehydration in marine environments. In such an environment, the bacteria produced TMA gas to obtain energy by reducing TMAO [65].

Zhu et al. reported the TMA gas sensor synthesized by the amphiphilic perylene diimide derivative (1,6,7,12-tetra-chlorinated perylene-N-(2-hydroxyethyl)-N-hexyl amine-3,4,9,10-tetracarboxylic bisimide, TC-PDI) and CdS [68]. As the heterojunction of organic-inorganic compounds formed, the gas sensitivity and selectivity were enhanced. The crystal structures of CdS were either cubic or hexagonal so that it could simplify the decoration of the microstructure and construct the stable heterostructures. Fig. 6(a) is the SEM image of TC-PDI/CdS, which has micro belts structure with 1${\mu}$m width and over a hundred micrometers in length. The heterostructures were formed by the Cd$^{2+}$ that built the Cd${-}$O formation between the Cd$^{2+}$center and the hydroxyethyl groups of TC-PDI. The Cd${-}$O coordination bonds were balanced with hydrophobic interactions between side chains and induced the belt-like gas was injected into the TC-PDI/CdS, the resistance was increased significantly. In case the various concentration of TMA was exposed from 1 to 100 ppm, the resistance curve exhibited a good linear relationship. By following Dua’s method, TC-PDI/CdS sensor could detect 200 ppb of TMA. The long-term stability of TC-PDI/CdS was also excellent. After storing for over 30 days in the air at room temperature, there was no significant difference in the gas sensing properties. Fig. 6(b) shows the application to detect a hairtail for 72 hours. The response was linearly increased with the storage time. Fig. 6(c) depicts the selectivity of the TC-PDI/CdS to 20 ppm of TMA and 100 ppm of other gases, including acetone, benzene, ammonia, ethanol, and NO$_{2}$. TC-PDI/CdS exhibited the highest response to TMA among the gases. Considering the application in a real situation, the fishes were stored in a high relative humidity atmosphere Fig. 6(d) shows the response to 60 ppm of TMA in various RH atmospheres from 30 to 100% RH. The response to TMA was maintained even in high humid atmosphere verifying the capability in practical applications. These results showed that the TC-PDI/CdS is suitable for real-time fish freshness monitoring.

Fig. 6. (a) SEM images of TC-PDI/CdS microbelts; (b) Responses of the TC-PDI/CdS sensor to hairtail storage time at room temperature; (c) Response to TC-PDI/CdS sensor to 20 ppm TMA and 100 ppm of various gases at room temperatures; (d) The responses of TC-PDI/CdS sensor to 60 ppm TMA under different humidity. Reproduced with permission from[66]Copyright © 2019 ACS Publications.

Tonezzer et al. reported NH$_{3}$ sensor using tin oxide nanowire and analyzed the quantitative assessment of trout fish spoilage [69]. Fig. 7(a) is the SEM image of SnO$_{2}$nanowires obtained by chemical vapor deposition (CVD). The average diameter of SnO$_{2}$nanowires was 40-80 nm. The thin morphology and potential barrier in the nanowires led to the improvement of gas sensing performance. Fig. 7(b) shows the dynamic resistance of the sensor at three different temperatures (200, 250, and 300 ℃). The resistance of the SnO$_{2}$nanowires sensor was constant in the air at three temperature conditions. When NH$_{3}$ gas was flushed into the chamber, the resistance dropped sharply. The resistance was rapidly returned to the initial state when the pure air was injected. The resistance variation was linearly changed depending on the NH$_{3}$ concentration. When the working temperature was increased, the SnO$_{2}$nanowires also showed a higher response to NH$_{3}$. NH$_{3}$ gas is a reducing gas that reduces the resistance of n-type semiconductors. When the SnO$_{2}$nanowires were exposed to air, oxygen molecules were adsorbed and changed to O$^{-}$or O$^{2-}$. The adsorbed oxygen dragged the electrons from the SnO$_{2}$nanowires. When NH$_{3}$ molecules were exposed to SnO$_{2}$ nanowires, they reacted with the adsorbed oxygen and induced the oxygen molecules to release electrons into the SnO$_{2}$nanowires. As the number of electrons in the sensor increased, the resistance of the SnO$_{2}$nanowires diminished. Fig. 7(c) shows the sensor response and bacterial total viable count (TVC) in fish at room temperature over 60 hours. The response was increased at all temperatures gradually. The TVC also increased similarly so that it could be considered for measuring the freshness of fish. The dashed horizontal green line indicates the threshold that the shelf life of fish was ended. The threshold of shelf life was about 22 h and 40 min of storage at room temperature. Fig. 7(d) shows the sensor response as a function of the logarithm of TVC. The response showed the linear property at all temperatures with Pearson’s correlation coefficients over 0.99 in all cases. The graph shows that the sensor response is suitable to measure the TVC. Fig. 7(e) is a principal component analysis (PCA) that could maximize the interpretability of the sensing properties. The PCA graph was divided by a group of the TVC value. Each group was arranged in a zigzag line that goes down to 6-7, then goes up to 8, and goes down to 10 repeatedly. The PCA results demonstrated that the SnO$_{2}$ nanowire sensor was suitable for analyzing the freshness of fish in various stages of the degradation process.

Fig. 7. (a) SEM images of the SnO$_{2}$ nanowires grown by CVD; (b) Dynamic resistance at three temperature values during the injection of different concentrations of ammonia; (c) Sensor response (solid symbols, left scale) and bacterial population (green open circles, right scale) in fresh trout fish kept at room temperature (25 ℃) over a period of 60 hours; (d) Double-blind measurements of the sensor response as a function of the total viable count in rainbow trout samples; (e) PCA plot of random samples of rainbow trout[67].

4. Others

Besides fruits, meats, and fish, the chemoresistive gas sensors can be used to detect various foods like dairy [70], eggs [71], wines [72], oils [73], and spices [74]. For example, milk, one of the dairy products, cannot be digested by most adults in natural conditions. Thus, it is necessary to add adulterations to milk. However, various side effects depend on the amount or type of adulterations. Accordingly, Pirsa et al. reported the gas sensor to detect adulteration in milk by sensing the volatile compounds using a poly-pyrrole-ZnO (PPy-ZnO) fiber gas sensor [75]. The PPy-ZnO was synthesized by doping ZnO nanoparticles on a polymer via the chemical polymerization method. The adulterants with NaClO, H$_{2}$CO$_{3}$, citric acid, and NaHCO$_{3}$were detected by the PPy-ZnO. As the volatile compounds were released from adulterants in milk, they were extracted from milk via the headspace method. After that, the volatile compounds gases were injected into the PPy-ZnO gas sensor and analyzed by response surface methodology (RSM). The PPy-ZnO sensor showed a linear response to NaClO, H$_{2}$CO$_{3}$, citric acid, and NaHCO$_{3}$. By using a statistical model based on the adulterant, the PPy-ZnO gas sensor could discriminate the type or the number of adulterants based on the positive and negative peaks. Hence, the PPy-ZnO fiber could help to classify the various kinds of milk adulterants.

Eggs have sulfur-containing amino acids producing ppb level of H$_{2}$S gas under the action of microorganisms. Guo et al. reported the H$_{2}$S gas sensor based on SnSe$_{2}$/WO$_{3}$ composite for evaluating the spoilage of eggs at room temperature [71]. The WO$_{3}$was decorated on the SnSe$_{2}$nanosheet via a one-step ultrasound method. While metal oxide semiconductor gas sensors are vulnerable to high humidity, the SnSe$_{2}$/WO$_{3}$gas sensor showed high endurance to high RH. Furthermore, The H$_{2}$S response was enhanced at high RH levels because of the water-activated electron transfer. For the experimental process, the glass bottles contained fresh eggs, the spoiled eggs were prepared, and the SnSe$_{2}$/WO$_{3}$ gas sensor was put in each bottle. The resistance of the sensor in all bottles was decreased, but the variation of resistance in the bottle of the spoiled egg was much higher. The SnSe$_{2}$/WO$_{3}$ gas sensor could be used for the portable and non-olfactory contact sensor to monitor the quality of eggs.

Wine is one of the alcoholic drinks which is consumed all over the world. For the taste of wine, it is crucial to monitor the ripening process elaborately. While the wine is ripened, volatile sulfur compounds (VSCs) are released such as H$_{2}$S and methyl mercaptan (MM). Lim et al. reported the CuO/CuFe$_{2}$O$_{4}$ nanopattern chemoresistitve gas sensor that could detect H$_{2}$S and MM dually. The nanopattern was composed of aligned 1D CuO/CuFe$_{2}$O$_{4}$ using direct-write near field electrospinning (NFES). The response of the CuO/CuFe$_{2}$O$_{4}$nanopattern structure was much higher than the thin-film or nanofiber structure. The CuO/CuFe$_{2}$O$_{4}$gas sensor showed high selectivity to H$_{2}$S at 200 ℃ and MM at 400 ℃. At 200 ℃, the gas response to H$_{2}$S and MM of pure Fe$_{2}$O$_{3}$was low, but the Cu$_{4}$Fe showed a higher response to H$_{2}$S. This phenomenon was associated with the phase transformation of p-type CuO to metallic CuS when the sensor was exposed to H$_{2}$S. By the H$_{2}$S gas, CuS formed the nanoscale p-n junction and led to the enhancement of gas response. On the contrary, the mechanism of gas sensing was changed significantly at 400 ℃. The high temperature caused CuS to be oxidized to CuO. The Cu$_{4}$Fe sensors showed higher responses to MM. It can be seen that the CuO/CuFe$_{2}$O$_{4}$ junction formation is an effective strategy for the highly selective detection of MM. The difference in the degree of oxidation shows high selectivity to H$_{2}$S and MM. Overall, the dual-mode gas sensing in this study is expected to facilitate the real-time monitoring of the wine ripening.


Chemoresistive gas sensors have proven to be highly successful in monitoring food quality by sensitive and selective gas detection. However, chemoresistive gas sensors continue to face significant problems such as humidity resistance, which is one of the most important properties for monitoring food quality. Poor humidity resistance is a flaw that originated from the sensing mechanism of chemoresistive gas sensors. Since chemoresistive gas sensors are the devices whose operating principle is that the resistance changes through the change of oxygen adsorption on the surface, when the surrounding humidity is high, oxygen adsorption is interrupted by H$_{2}$O molecules and lowering sensitivity and stability. A method for preventing the absorption of water molecules without interfering with the adsorption of oxygen on the surface is required to improve humidity resistance. A typical method is to apply a humidity-resistant material, including PDMS, to the surface to prevent water molecule adhesion. Also, heterojunction between sensing material and humidity-resistant material can be considered as an expected effect. The study of new humidity-resisting materials and their combination with existing sensing materials remains an important challenge in securing the humidity stability of chemoresistive gas sensors.

Another issue of food quality monitoring based on chemoresistive gas sensors is high operation temperature due to relatively low sensitivity and selectivity. According to the sensing mechanism of chemoresistive gas sensors, the more oxygen is adsorbed, and the more electrons are taken away, the larger the area of EDL generated on the sensing material surface. In the case of semiconductor materials, since the mobility of the main charge carriers increases as the temperature increases, more electrons can participate in the reaction, which is usually operated at a high temperature (100-400 $^{\circ}$C) to promote EDL generation and amplify resistance changes. However, since food quality monitoring has to be conducted in a distribution environment rather than a high temperature, or even at a low temperature of 0 $^{\circ}$C or less, the low mobility of charge carriers stands out. In order to overcome the inherent limitations of semiconductor materials, methods to improve the three basic factors introduced above are used. To improve the utility factor, the sensing material is nanostructured to increase the surface area and allow more oxygen to reach the surface, leading to wider EDL generation. The receptor function can be improved by inducing more oxygen adsorption and gas molecule adsorption through catalyst integration or heterojunction. This enables easy food quality monitoring through chemoresistive gas sensors with high reactivity and selectivity without limiting the operating environment.


In this article, we review the chemoresistive gas sensors based on semiconductor materials for food quality monitoring. As online food purchases increased in the pandemic era, the food quality monitoring system, which detects gas occurring in the food distribution process, has become particularly important. The Gas Chromatography-Mass Spectrometry (GC-MS) technique, which has been mainly used in the past, has several drawbacks, such as high-priced equipment and a time-consuming measurement system. However, the chemoresistive gas sensors have advantages such as fast response, small size, and high reactivity compared to conventional techniques. Various materials such as metal oxide, metal sulfide, carbon nanomaterials, polymers, and their composites are widely used to detect the gases emitted during the food ripening process. In this work, foods were categorized into 4 groups: fruit, meat, fish, and others (milk, egg, and wine). Different types of gases are emitted from fruits (ethylene), meats (TMA), and fishes (TMA, DMA, and NH$_{3}$). Moreover, diverse gases such as VOCs, H$_{2}$S, and VSCs also appear in milk, egg, and wine. Although there are some challenges, such as poor humidity stability and high operation temperature, many researches are being conducted to overcome these limitations and improve the gas sensing performance. Through overcoming the drawbacks, chemoresistive gas sensors are highly anticipated to be developed into an innovative technology for the next generation of food quality monitoring.


This work was carried out with the support of the “Cooperative Research Program for Agriculture Science and Technology Development (Project No. PJ017067022022)” Rural Development Administration, Republic of Korea. The Inter-University Semiconductor Research Center and Institute of Engineering Research at Seoul National University provided research facilities for this work.


Gram L., Ravn L., Rasch M., Bruhn J.B., Christensen A.B., Givskov M., 2002, Food spoilage—interactions between food spoilage bacteria, International Journal of Food Microbiology, Vol. 78, pp. 79-97DOI
Scott W.J., 1957, Water relations of food spoilage microorganisms, Advances in food research, Elsevier, pp. 83-127DOI
Brito‐Ravicini A., Calle‐Vallejo F., 2022, Interplaying coordination and ligand effects to break or make adsorption‐energy scaling relations, Exploration, Vol. 2, pp. 20210062DOI
Lee S.A., Yang J.W., Choi S., Jang H.W., 2021, Nanoscale electrodeposition: Dimension control and 3D conformality, Exploration, Wiley Online Library, pp. 20210012DOI
Zhong R., Xu X., Wang L., Food supply chain management: systems, implementations, and future research, Industrial Management & Data Systems, Vol. 2017DOI
WHO , 2020, Food safety, in: WHO (Ed.)Google Search
Chan S.T., Yao M.W., Wong Y., Wong T., Mok C., Sin D.W., 2006, Evaluation of chemical indicators for monitoring freshness of food and determination of volatile amines in fish by headspace solid-phase microextraction and gas chromatography-mass spectrometry, European Food Research and Technology, Vol. 224, pp. 67-74DOI
Chen Y., Gao Z., Zhang F., Wen Z., Sun X., 2022, Recent progress in self‐powered multifunctional e‐skin for advanced applications, Exploration, Vol. 2, pp. 20210112DOI
Demon S.Z.N., Kamisan A.I., Abdullah N., Noor S.A.M., Khim O.K., Kasim N.A.M., Yahya M.Z.A., Manaf N.A.A., Azmi A.F.M., Halim N.A., 2020, Graphene-based materials in gas sensor applications: A review, Sens. Mater, Vol. 32, pp. 759-777DOI
Rakow N.A., Suslick K.S., 2000, A colorimetric sensor array for odour visualization, Nature, Vol. 406, pp. 710-713DOI
Bhushan B., Bhushan B., Baumann , Springer handbook of nanotechnology, Springer2007Google Search
Liu X., Cheng S., Liu H., Hu S., Zhang D., Ning H., 2012, A survey on gas sensing technology, sensors, Vol. 12, pp. 9635-9665Google Search
Nisar A., Khan M.A., Hussain Z., 2022, Synthesis and characterization of PANI/MOF-199/Ag nanocom-posite and its potential application as non-enzymatic electrochemical sensing of dopamine, Journal of the Korean Ceramic Society, pp. 1-11DOI
Naim N., Abdullah H., Hamid A., 2019, Influence of Ag and Pd contents on the properties of PANI–Ag–Pd nanocomposite thin films and its performance as electrochemical sensor for E. coli detection, Electronic Materials Letters, Vol. 15, pp. 70-79DOI
Bruce J., Bosnick K., Heidari E.K., 2022, Pd-decorated ZnO nanoflowers as a promising gas sensor for the detection of meat spoilage, Sensors and Actuators B: Chemical, Vol. 355, pp. 131316DOI
Tonezzer M., 2021, Single Nanowire Gas Sensor Able to Distinguish Fish and Meat and Evaluate Their Degree of Freshness, Chemosensors, Vol. 9, pp. 249DOI
Jang J.S., Jung H.J., Chong S., Kim D.H., Kim J., Kim S.O., Kim I.D., 2020, 2D materials decorated with ultrathin and porous graphene oxide for high stability and selective surface activity, Advanced Materials, Vol. 32, pp. 2002723DOI
Jang J.-S., Winter L.R., Kim C., Fortner J.D., Elimelech M., 2021, Selective and sensitive environmental gas sensors enabled by membrane overlayers, Trends in Chemistry, Vol. 3, pp. 547-560DOI
Eom T.H., Cho S.H., Suh J.M., Kim T., Yang J.W., Lee T.H., Jun S.E., Kim S.J., Lee J., Hong S.H., 2022, Visible Light Driven Ultrasensitive and Selective NO2 Detection in Tin Oxide Nanoparticles with Sulfur Doping Assisted by l‐Cysteine, Small, pp. 2106613DOI
Kim T.H., Hasani A., Kim Y., Park S.Y., Lee M.G., Sohn W., Nguyen T.P., Choi K.S., Kim S.Y., Jang H.W., 2019, NO2 sensing properties of porous Au-incorporated tungsten oxide thin films prepared by solution process, Sensors and Actuators B: Chemical, Vol. 286, pp. 512-520DOI
Kim Y., Kwon K.C., Kang S., Kim C., Kim T.H., Hong S.-P., Park S.Y., Suh J.M., Choi M.-J., Han S., 2019, Two-dimensional NbS2 gas sensors for selective and reversible NO2 detection at room temperature, ACS sensors, Vol. 4, pp. 2395-2402DOI
Park S.-W., Jeong S.-Y., Moon Y.K., Kim K., Yoon J.-W., Lee J.-H., 2022, Highly Selective and Sensitive Detection of Breath Isoprene by Tailored Gas Reforming: A Synergistic Combination of Macroporous WO3 Spheres and Au Catalysts, ACS Applied Materials & Interfaces, Vol. 14, pp. 11587-11596DOI
Hencz L., Chen H., Wu Z., Qian S., Chen S., Gu X., Liu X., Yan C., Zhang S., 2022, Highly branched amylopectin binder for sulfur cathodes with enhanced performance and longevity, Exploration, pp. 20210131DOI
Lee J., Liao H., Wang Q., Han J., Han J.H., Shin H.E., Ge M., Park W., Li F., 2022, Exploration of nanozymes in viral diagnosis and therapy, Exploration, pp. e20210086DOI
Bulemo P.M., Kim I.-D., 2020, Recent advances in ABO3 perovskites: their gas-sensing performance as resistive-type gas sensors, Journal of the Korean Ceramic Society, Vol. 57, pp. 24-39DOI
Kim T.-H., Yoon J.-W., Lee J.-H., 2016, A Volatile Organic Compound Sensor Using Porous Co 3 O 4 Spheres, Journal of the Korean Ceramic Society, Vol. 53, pp. 134-138DOI
Andrysiewicz W., Krzeminski J., Skarżynski K., Marszalek K., Sloma M., Rydosz A., 2020, Flexible gas sensor printed on a polymer substrate for sub-ppm acetone detection, Electronic Materials Letters, Vol. 16, pp. 146-155DOI
Cho S.H., Suh J.M., Eom T.H., Kim T., Jang H.W., 2021, Colorimetric sensors for toxic and hazardous gas detection: A review, Electronic Materials Letters, Vol. 17, pp. 1-17DOI
Eom T.H., Cho S.H., Suh J.M., Kim T., Lee T.H., Jun S.E., Yang J.W., Lee J., Hong S.-H., Jang H.W., 2021, Substantially improved room temperature NO 2 sensing in 2-dimensional SnS 2 nanoflowers enabled by visible light illumination, Journal of Materials Chemistry A, Vol. 9, pp. 11168-11178DOI
Lee C.W., Suh J.M., Choi S., Jun S.E., Lee T.H., Yang J.W., Lee S.A., Lee B.R., Yoo D., Kim S.Y., 2021, Surface-tailored graphene channels, npj 2D Materials and Applications, Vol. 5, pp. 1-13DOI
Wang C., Yin L., Zhang L., Xiang D., Gao R., 2010, Metal oxide gas sensors: sensitivity and influencing factors, sensors, Vol. 10, pp. 2088-2106DOI
Barsan N., Koziej D., Weimar U., 2007, Metal oxide-based gas sensor research: How to?, Sensors and Actuators B: Chemical, Vol. 121, pp. 18-35DOI
Matsunaga N., Sakai G., Shimanoe K., Yamazoe N., 2002, Diffusion equation-based study of thin film semiconductor gas sensor-response transient, Sensors and Actuators B: Chemical, Vol. 83, pp. 216-221DOI
Cui X., Zhang Z., Yang Y., Li S., Lee C.S., 2022, Organic radical materials in biomedical applications: State of the art and perspectives, Exploration, pp. 20210264DOI
Park S.Y., Kim Y., Kim T., Eom T.H., Kim S.Y., Jang H.W., 2019, Chemoresistive materials for electronic nose: Progress, perspectives, and challenges, InfoMat, Vol. 1, pp. 289-316DOI
Choopun S., Hongsith N., Wongrat E., 2012, Metal-oxide nanowires for gas sensors, Nanowires-Recent Advances, pp. 3-24DOI
John A.T., Murugappan K., Nisbet D.R., Tricoli A., 2021, An Outlook of Recent Advances in Chemiresistive Sensor-Based Electronic Nose Systems for Food Quality and Environmental Monitoring, Sensors (Basel), Vol. 21DOI
Love C., Nazemi H., El-Masri E., Ambrose K., Freund M.S., Emadi A., 2021, A Review on Advanced Sensing Materials for Agricultural Gas Sensors, Sensors (Basel), Vol. 21DOI
Matindoust S., Farzi G., Nejad M.B., Shahrokhabadi M.H., 2021, Polymer-based gas sensors to detect meat spoilage: A review, Reactive and Functional Polymers, Vol. 165DOI
Peris M., Escuder-Gilabert L., 2009, A 21st century technique for food control: electronic noses, Anal Chim Acta, Vol. 638, pp. 1-15DOI
Chauhan A., 2014, GC-MS Technique and its Analytical Applications in Science and Technology, Journal of Analytical & Bioanalytical Techniques, Vol. 5Google Search
Griffiths W.J., Wang Y., 2009, Analysis of neurosterols by GC-MS and LC-MS/MS, J Chromatogr B Analyt Technol Biomed Life Sci, Vol. 877, pp. 2778-2805DOI
Ruiz-Matute A.I., Hernandez-Hernandez O., Rodriguez-Sanchez S., Sanz M.L., Martinez-Castro I., 2011, Derivatization of carbohydrates for GC and GC-MS analyses, J Chromatogr B Analyt Technol Biomed Life Sci, Vol. 879, pp. 1226-1240DOI
Rodríguez A., Alquézar B., Peña L., 2013, Fruit aromas in mature fleshy fruits as signals of readiness for predation and seed dispersal, New Phytologist, Vol. 197, pp. 36-48DOI
Li C., Krewer G.W., Ji P., Scherm H., Kays S.J., 2010, Gas sensor array for blueberry fruit disease detection and classification, Postharvest Biology and Technology, Vol. 55, pp. 144-149DOI
Paul V., Pandey R., Srivastava G.C., 2012, The fading distinctions between classical patterns of ripening in climacteric and non-climacteric fruit and the ubiquity of ethylene-An overview, J Food Sci Technol, Vol. 49, pp. 1-21DOI
Zhao Q., Duan Z., Yuan Z., Li X., Si W., Liu B., Zhang Y., Jiang Y., Tai H., 2020, High performance ethylene sensor based on palladium-loaded tin oxide: Application in fruit quality detection, Chinese Chemical Letters, Vol. 31, pp. 2045-2049DOI
Jeong S.Y., Moon Y.K., Kim T.H., Park S.W., Kim K.B., Kang Y.C., Lee J.H., 2020, A New Strategy for Detecting Plant Hormone Ethylene Using Oxide Semiconductor Chemiresistors: Exceptional Gas Selectivity and Response Tailored by Nanoscale Cr2O3 Catalytic Overlayer, Adv Sci (Weinh), Vol. 7, pp. 1903093DOI
Esser B., Schnorr J.M., Swager T.M., 2012, Selective detection of ethylene gas using carbon nanotube-based devices: utility in determination of fruit ripeness, Angew Chem Int Ed Engl, Vol. 51, pp. 5752-5756DOI
Andre R.S., Ngo Q.P., Fugikawa-Santos L., Correa D.S., Swager T.M., 2021, Wireless Tags with Hybrid Nanomaterials for Volatile Amine Detection, ACS Sens, Vol. 6, pp. 2457-2464DOI
Ghasemi-Varnamkhasti M., Mohtasebi S.S., Siadat M., Balasubramanian S., 2009, Meat quality assessment by electronic nose (machine olfaction technology), Sensors (Basel), Vol. 9, pp. 6058-6083DOI
Matindoust S., Farzi A., Baghaei Nejad M., Shahrokh Abadi M.H., Zou Z., Zheng L.-R., 2017, Ammonia gas sensor based on flexible polyaniline films for rapid detection of spoilage in protein-rich foods, Journal of Materials Science: Materials in Electronics, Vol. 28, pp. 7760-7768DOI
Li S., Chen S., Zhuo B., Li Q., Liu W., Guo X., 2017, Flexible Ammonia Sensor Based on PEDOT:PSS/Silver Nanowire Composite Film for Meat Freshness Monitoring, IEEE Electron Device Letters, Vol. 38, pp. 975-978DOI
Ma Z., Chen P., Cheng W., Yan K., Pan L., Shi Y., Yu G., 2018, Highly Sensitive, Printable Nanostructured Conductive Polymer Wireless Sensor for Food Spoilage Detection, Nano Lett, Vol. 18, pp. 4570-4575DOI
Tonezzer M., 2021, Single Nanowire Gas Sensor Able to Distinguish Fish and Meat and Evaluate Their Degree of Freshness, Chemosensors, Vol. 9DOI
Bruce J., Bosnick K., Kamali Heidari E., 2022, Pd-decorated ZnO nanoflowers as a promising gas sensor for the detection of meat spoilage, Sensors and Actuators B: Chemical, Vol. 355DOI
Tang X., Yu Z., 2020, Rapid evaluation of chicken meat freshness using gas sensor array and signal analysis considering total volatile basic nitrogen, International Journal of Food Properties, Vol. 23, pp. 297-305DOI
Sujiwo J., Kim D., Jang A., 2018, Relation among quality traits of chicken breast meat during cold storage: correlations between freshness traits and torrymeter values, Poultry science, Vol. 97, pp. 2887-2894DOI
Liu Q., Mukherjee S., Huang R., Liu K., Liu T., Liu K., Miao R., Peng H., Fang Y., 2019, Naphthyl End-Capped Terthiophene-Based Chemiresistive Sensors for Biogenic Amine Detection and Meat Spoilage Monitoring, Chem Asian J, Vol. 14, pp. 2751-2758DOI
Liu S.F., Petty A.R., Sazama G.T., Swager T.M., 2015, Single-walled carbon nanotube/metalloporphyrin composites for the chemiresistive detection of amines and meat spoilage, Angew Chem Int Ed Engl, Vol. 54, pp. 6554-6557DOI
Zhang W.-H., Zhang W.-D., 2008, Fabrication of SnO2–ZnO nanocomposite sensor for selective sensing of trimethylamine and the freshness of fishes, Sensors and Actuators B: Chemical, Vol. 134, pp. 403-408DOI
Wu K., Zhang W., Zheng Z., Debliquy M., Zhang C., 2022, Room-temperature gas sensors based on titanium dioxide quantum dots for highly sensitive and selective H2S detection, Applied Surface Science, pp. 152744DOI
Park S.H., Kim B.-Y., Jo Y.K., Dai Z., Lee J.-H., 2020, Chemiresistive trimethylamine sensor using monolayer SnO2 inverse opals decorated with Cr2O3 nanoclusters, Sensors and Actuators B: Chemical, Vol. 309, pp. 127805DOI
Sovizi M.R., Mirzakhani S., 2020, A chemiresistor sensor modified with lanthanum oxide nanoparticles as a highly sensitive and selective sensor for dimethylamine at room temperature, New Journal of Chemistry, Vol. 44, pp. 4927-4934DOI
Kwak C.-H., Woo H.-S., Lee J.-H., 2014, Selective trimethylamine sensors using Cr2O3-decorated SnO2 nanowires, Sensors and Actuators B: Chemical, Vol. 204, pp. 231-238DOI
Ólafsdóttir E.M. G., Jónsson E. H., 1997, Rapid Gas Sensor Measurements To Determine Spoilage of Capelin (Mallotus villosus), J. Agric. Food Chem., pp. 2654-2659DOI
Olafsdóttir G., Martinsdóttir E., Oehlenschläger J., Dalgaard P., Jensen B., Undeland I., Mackie I.M., Henehan G., Nielsen J., Nilsen H., 1997, Methods to evaluate fish freshness in research and industry, Trends in Food Science & Technology, Vol. 8, pp. 258-265DOI
Zhu P., Wang Y., Ma P., Li S., Fan F., Cui K., Ge S., Zhang Y., Yu J., 2019, Low-Power and High-Performance Trimethylamine Gas Sensor Based on n-n Heterojunction Microbelts of Perylene Diimide/CdS, Analytical Chemistry, Vol. 91, pp. 5591-5598DOI
Tonezzer M., Thai N.X., Gasperi F., Van Duy N., Biasioli F., 2021, Quantitative Assessment of Trout Fish Spoilage with a Single Nanowire Gas Sensor in a Thermal Gradient, Nanomaterials, Vol. 11, pp. 1604DOI
Shantini D., Nainggolan I., Nasution T.I., Derman M.N., Mustaffa R., Abd Wahab N.Z., 2016, Hexanal Gas Detection Using Chitosan Biopolymer as Sensing Material at Room Temperature, Journal of Sensors, Vol. 2016, pp. 1-7DOI
Guo X., Ding Y., Liang C., Du B., Zhao C., Tan Y., Shi Y., Zhang P., Yang X., He Y., 2022, Humidity-activated H2S sensor based on SnSe2/WO3 composite for evaluating the spoilage of eggs at room temperature, Sensors and Actuators B: Chemical, Vol. 357DOI
Lozano J., Santos J.P., Gutiérrez J., Horrillo M.C., 2007, Comparative study of sampling systems combined with gas sensors for wine discrimination, Sensors and Actuators B: Chemical, Vol. 126, pp. 616-623DOI
Lerma-García M.J., Cerretani L., Cevoli C., Simó-Alfonso E.F., Bendini A., Toschi T.G., 2010, Use of electronic nose to determine defect percentage in oils. Comparison with sensory panel results, Sensors and Actuators B: Chemical, Vol. 147, pp. 283-289DOI
Ghasemi-Varnamkhasti M., Amiri Z.S., Tohidi M., Dowlati M., Mohtasebi S.S., Silva A.C., Fernandes D.D.S., Araujo M.C.U., 2018, Differentiation of cumin seeds using a metal-oxide based gas sensor array in tandem with chemometric tools, Talanta, Vol. 176, pp. 221-226DOI
Pirsa S., Tağı Ş., Rezaei M., 2021, Detection of Authentication of Milk by Nanostructure Conducting Polypyrrole-ZnO, Journal of Electronic Materials, Vol. 50, pp. 3406-3414DOI
Seonyong Lee

Seonyong Lee is a Ph.D. candidate in the Department of Materials Science and Engineering in Seoul National University. His research focuses on the synthesis of metal oxide materials and their applications in chemoresistive gas sensors.

Seungsoo Kim

Seungsoo Kim is a Ph.D. candidate in the Department of Materials Science and Engineering in Seoul National University. His research focuses on the fabrication of chemoresistive gas sensors based on two-dimensional materials and the demonstration of electronic nose.

Gi Baek Nam

Gi Baek Nam is a Ph.D. candidate in the Department of Materials Science and Engineering in Seoul National University. His research focuses on visible light activated chemoresistive gas sensor based on metal sulfides and array configuration of electronic nose.

Tae Hoon Eom

Tae Hoon Eom is Postdoctoral Associate at Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University. He earned his PhD from the Department of Materials Science and Engineering, Seoul National University in February, 2022. His research focuses on visible light activated chemoresistive gas sensor based on 2D materials and metal oxides.

Ho Won Jang

Ho Won Jang is a Full Professor in the Department of Materials Science and Engineering at Seoul National University. He earned his Ph.D. from the Department of Materials Science and Engineering in Pohang University of Science and Technology in 2004. He worked as a Research Associate at the University of Wisconsin-Madison from 2006 to 2009. Before he joined Seoul National University in 2012, he worked in Korea Institute of Science and Technology as a Senior Research Scientist. His research interests include the synthesis of oxides, 2D materials, and halide perovskites, and their applications in nanoelectronics, solar water splitting cells, chemical sensors, plasmonics, metal–insulator transition, and ferroelectricity.