Applied Science and Convergence Technology 2023; 32(4): 82-88
Published online July 30, 2023
Copyright © The Korean Vacuum Society.
aCritical Diseases Diagnostics Convergence Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon 34141, Republic of Korea
bKRIBB School, Korea University of Science and Technology (UST), Daejeon 34113, Republic of Korea
cDepartment of Biochemistry, Chungnam National University, Daejeon 34134, Republic of Korea
†These authors contributed equally to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
Ethylene (C2H4) is a crucial plant hormone that regulates fruit ripening and aging. As the only gaseous plant hormone, ethylene has been identified as a significant contributor to produce spoilage, causing economic losses of billions of dollars annually. To address this issue, the development of a biosensor that can accurately and specifically monitor ethylene levels as an indicator of production loss is of paramount importance to the fruit, flower, and vegetable industries, owing to its potential to mitigate losses and increase profits. Consequently, there is an increasing demand for affordable, reliable, and improved methods for rapid and accurate detection and quantification of ethylene. Consequently, the selection of appropriate sensor solutions should specify the requirements and contexts of individual applications. In this review, we explore the recent progress in chemical sensing technologies for ethylene detection and highlight the current research trends and future challenges in this field.
Keywords: Gas sensor, Ethylene, Gaseous hormone, Biosensor
Ethylene (C2H4), a gaseous hormone with a simple chemical structure, plays a vital role in numerous physiological and developmental processes throughout the lifespan of plants, including seed germination, growth, organ senescence, fruit ripening, abscission, and responses to various environmental stressors [1,2]. Ethylene induces accelerated softening of fruits and senescence of flowers during transport and storage. It also damages fresh produce, including fruits, herbs, vegetables, and flowers [2,3]. Therefore, the detection and precise quantification of ethylene emissions are of utmost importance for monitoring ripening and aging processes. For fruit cultivators, the optimal harvesting time can be determined based on the amount of ethylene emitted by the fruit. In addition, ethylene emission levels must be checked during the storage and transportation of fruits to assess the aging process. Ethylene gas is directly released by plants and fresh produce and is effective at very low concentrations ranging from 10 ppm to 10 ppb [4,5]. Furthermore, under uncontrolled growth conditions, the presence of ethylene often leads to faster degradation of agricultural produce after harvesting, storage, and transportation, resulting in significant economic losses . Considering the fundamental role of ethylene in the lifecycle of plants, the detection and precise quantification of this hormone has become one of the most significant challenges in the crop, fruit, and horticultural industry . Currently, ethylene gas sensors commercialized by various companies, including the ethylene gas detector (electrochemical gas sensor) from Forensics Detectors Inc., Ethylene Optima Plus [infrared (IR) gas sensor] from Honeywell Inc., ethylene gas detector (electrochemical gas sensor) from Macurco Inc., and others, are being introduced into the market.
The immediate precursor of ethylene in plants is 1-aminocyclopropane- 1-carboxylic acid (ACC), which is synthesized from Sadenosylmethionine by ACC synthase. ACC is subsequently degraded by ACC oxidase to release ethylene [8–10]. Endogenous ethylene production and emission in plants can be modified by various biotic and abiotic factors such as pathogen attack, herbivorous predation [11,12], and circadian rhythms . In Arabidopsis, ethylene is perceived by a family of five receptor proteins (ETR1, ETR2, ERS1, ERS2, and EIN4) [14–16]. Proteins involved in ethylene signaling in plants can be used as capture agents for ethylene sensing through appropriate protein design.
Apart from its influence on agriculture, ethylene has far-reaching effects on other industries. As a precursor pollutant, it contributes to the formation of ground-level ozone, which poses health risks to humans. Ethylene concentrations in the air vary from 5 mg/m3 in rural areas to 1,000 mg/m3 in urban areas with heavy traffic . Furthermore, ethylene gas released from histopathological tissues, such as in acute myocardial infarction, chronic asthma, peritonitis, ultraviolet (UV) radiation damage to human skin, and lipid peroxidation in the lung epithelium [18,19], has emerged as an intriguing biomarker for medical diagnosis. With the increasing demand for ethylene detection in various fields, particularly agriculture and medical diagnosis, numerous devices have been developed to detect and control ethylene concentrations. These devices must meet several requirements, including the ability to measure ethylene at low concentrations (< 1 ppm), low cost, small size, and robustness. In this review, we summarize gas sensor-based techniques for detecting ethylene and analyze the advantages and disadvantages of the various types of sensor devices used for ethylene detection.
Typically, a chemical sensor comprises three main components: a sensing layer, a transducer, and a signal processing system. The transducer converts the detected signals into measurable electrical, optical, and thermal signals to identify the target analyte. In this review, we categorize ethylene sensors into various classes based on the transducer type, including gas chromatography (GC), optical gas sensors, piezoelectric gas sensors, and electrochemical gas sensors.
GC is an analytical chemistry technique commonly used to separate and analyze chemical vapors without decomposing them. Following the first recorded application of GC for the identification of ethylene in apples in 1959 , it has been widely regarded as one of the most effective methods for characterizing the separation, detection, and quantification of volatile compounds in both laboratory and industrial settings [21–23]. GC separates the various gases present in a complex mixture by passing a sample carried by a carrier gas through a column containing a solid or liquid matrix that interacts with the analytical gas. The mobile phase typically consists of inert reactive gases such as helium, argon, and nitrogen, whereas the stationary phase is a microscopic layer of liquid or polymer on inert solid support located inside the column. A detector is used to measure the resulting signal at the end of the column, which produces a series of peaks that form a chromatogram. Each compound is identified based on its retention time in the detector.
GC can be broadly classified into three different sample injection methods . 1) Manual injection: Ethylene is sampled from the headspace of a closed bottle containing the plant and manually injected into the GC column using an airtight syringe. 2) Auto-injection: Multiple samples are automatically injected into the GC column using an automatic sampler equipped with a concentric rotary valve. 3) Adsorption/desorption: An adsorption/desorption device capable of concentrating ethylene is coupled to a GC system, enabling highly sensitive sampling. Commonly used GC detectors include a thermal conductivity detector, which has a lower sensitivity but is non-destructive; a flame ionization detector, which is sensitive to hydrocarbons; and a photoionization detector, which is sensitive to aromatic and olefin hydrocarbons. GC systems have advanced significantly, allowing for fully automated and continuous monitoring of ethylene emissions with a detection limit of a few tens of nL/L and a repetition rate of less than 10 min. Additionally, the miniaturization of GCs has made them compact, robust, and low-power-consuming, which has expanded their potential applications. This has led to the development of integrated GCs through various fabrication processes [25,26] that can measure a range of volatile compounds using a single device, making GC a widely used tool in the plant research community.
An ethylene sensor can measure other volatile organic compounds (VOCs) in a sample gas. Many fruits and vegetables produce a variety of VOCs during ripening and senescence, which can be analyzed using GC to provide a more comprehensive understanding of the postharvest physiology of the plant material. GC has some limitations, including the need for a pre-concentration step, large and heavy instrumentation, and high cost for the highest-performing systems. Ethylene is a highly volatile compound, and its concentration can change rapidly. Therefore, it is important to ensure representative sampling and minimize any loss or alteration of ethylene during sample collection and preparation.
Optical gas sensors are electronic detectors that convert light, or a change in light, into an electronic signal. When light interacts with the ethylene molecules, it is absorbed, emitted, and scattered. Optical sensors essentially involve the exposure of a gas sample to a light source in the infrared (IR) region, typically a lamp or laser, within a chamber. Similar to other volatile compounds, ethylene has its own absorption characteristics, which are strongest in the mid-IR region. The molecular ethylene concentration can be quantified by understanding the absorption strength of ethylene in a specific IR region .
Non-dispersive IR (NDIR) sensors utilize optical filters to select specific narrow bands of IR light corresponding to the absorption wavelengths of the target gas. These selected wavelengths are not dispersed or separated but are detected simultaneously . This is in contrast to dispersive IR (DIR) sensors, in which a source of IR radiation emits a broad spectrum of IR light, sequentially measuring different wavelengths of light and enabling individual measurements of the absorption of specific wavelengths.
NDIR sensors rely on the absorption of IR radiation by ethylene molecules to detect their presence. However, other gases with similar absorption characteristics in the IR region, such as methane and carbon dioxide, can interfere with ethylene measurements. In addition, NDIR sensors are sensitive to particles or contaminants in the gas stream that can interfere with the optical detection mechanism. Dust, oil residue, and other contaminants can be deposited on the optical surfaces of the sensor, leading to signal attenuation or distorted readings, necessitating regular cleaning and maintenance of the sensor to ensure accurate ethylene measurements.
Another type of optical sensor relies on the color change, which is usually irreversible, of the constituent materials in response to the presence of target compounds. Colorimetric sensors usually utilize various organic compounds to capture target molecules, including dyes, fetal organic complexes, and polymers [32,33]. Although various functional materials have been investigated as colorimetric sensors, most of them are limited to the detection of ionic or molecular substances in solution and have difficulties in the effective detection of gaseous hazardous and toxic substances. Colorimetric sensors that measure absorbance or fluorescence spectral shifts for substance detection represent highly promising approaches in the field of gas sensing.
Lang and Hübert  introduced a molybdenum (Mo)-based colorimetric gas sensor, in which the chromophores undergo a color spectrum transition from white/light yellow to blue upon exposure to ethylene owing to the partial reduction of Mo(VI) to Mo(V). The sensitivity of Mo color change reactions could be modulated by manipulating the composition and pH values (pH 1.4–1.5) of the ammonium molybdate solution. In another study, Cabanillas-Galan
One of the biggest challenges of colorimetric sensors for ethylene detection is achieving high selectivity in the presence of potentially interfering gases or compounds. Achieving selectivity requires careful design and optimization of the colorimetric sensor, including the selection of indicator materials with a high affinity and specificity for ethylene. The presence of complex sample matrices, such as fruit or vegetable emissions, may introduce additional challenges. These matrices contain various volatile compounds that may interact with the indicator material, leading to color changes unrelated to ethylene.
Chemical luminescence sensors can detect marks that are visible only under UV light. This is achieved using fluorescent substances present on the mark, which convert UV light into visible light. The luminescence sensor receives and evaluates the reflected light beam. An interesting approach to detecting ethylene was introduced based on the emission changes in luminescent organic polymers or metalorganic frameworks functionalized with copper or silver ions . This type of sensor takes advantage of the high affinity between olefins and transition metals, such as gold, silver, and copper. The interaction between the two was proven to be based on the superimposition of the π orbital of ethylene with an empty metal orbital.
The luminescent materials used in chemical sensors must exhibit good stability and longevity to ensure reliable and consistent performance over time. Therefore, factors such as photochemical degradation, temperature variations, and chemical interactions can impact the stability and longevity of the sensor, leading to drift or changes in response.
Piezoelectric sensors, such as quartz crystal microbalance (QCM), are mass sensors based on the principle of gravimetry . Mass spectrometry techniques for gaseous substances can provide efficient and reproducible approaches for detecting VOCs. However, owing to their limitations in sensitivity, piezoelectric sensors have seldom been applied for the detection of ethylene, which has a low molecular weight. Nevertheless, several studies have explored the application of piezoelectric gas sensors for ethylene detection.
The sensing element of a piezoelectric sensor is typically a thin film or quartz crystal resonator. The sensing element material should have a high affinity for the target gas (i.e., ethylene) to ensure efficient adsorption. The structure and morphology of a sensing element significantly affect its performance. Achieving a large surface-area-tovolume ratio is important for maximizing the gas-sensing area. In addition, maintaining mechanical stability and minimizing unwanted effects, such as mechanical noise, can be challenging.
The electrochemical gas sensor is a type of gas detector that determines the concentration of a specific gas based on the current measured at the electrode through oxidation or reduction reactions [46, 47]. These sensors generate an electrical signal that is directly proportional to the gas concentration based on the reaction between the target gas and sensor. Typically, electrochemical sensors are fabricated using various materials such as sensing electrodes, electrolyte compositions, and hydrophilic porous barriers . The application of electrochemical sensors has expanded to include the detection of VOCs such as ethylene .
Amperometric sensors generate current by applying a potential difference between two electrodes . The first report on ethylene detection using an amperometric sensor was published by Larry and Jordan  in 1997. The sensing electrode was fabricated by depositing gold on a Nafion substrate using a 0.5 M sulfuric acid electrolyte. These electrodes exhibited a low detection limit of 40 ppb for ethylene, with a signal-to-noise ratio of 3. As shown in Fig. 3, Zevenbergen
In amperometric ethylene gas sensors, the measurable current is associated with the oxidation reaction. Therefore, platinum (Pt), iridium (Ir), and rhodium (Rh) are excellent electrocatalysts for ethylene oxidation at relatively high temperatures . These metals allow the amperometric detection of ethylene at room temperature because the formation of metal oxides begins at a potential higher than the potential required for ethylene oxidation in strongly acidic solutions. This enables the amperometric detection of ethylene under ambient conditions . The working electrode of an amperometric gas sensor is typically made of a material that promotes an electrochemical reaction with ethylene. Commonly used materials include metal oxides , conducting polymers , and carbon-based materials, such as graphene or carbon nanotubes (CNTs) [56,57]. In the amperometric detection of ethylene, oxygen is essential for maintaining the current generated when ethylene is present and reacts with the gas. Therefore, there may be limitations in the operation of this type of sensor under low-oxygen storage conditions after harvest in the horticultural industry.
Potentiometric gas sensors operate based on the measurement of the potential difference between working and reference electrodes [58, 59]. When ethylene gas interacts with the working electrode, it undergoes electrochemical reactions that generate a measurable potential difference proportional to the concentration of ethylene gas, allowing quantitative detection [60,61]. Potentiometric ethylene gas sensors utilize various materials and designs to achieve optimal performance. Working electrodes are typically composed of nanostructured powders or films, such as metal oxides or composites, which exhibit high catalytic activity towards ethylene gas. The reference electrode was selected based on its stability and compatibility with the working electrode material. Conducting electrolytes, often in the solid state, facilitate ion conduction between the electrodes and enhance the sensor response. Several strategies have been employed to improve the sensitivity and selectivity of potentiometric ethylene sensors. Functionalization of the working electrode surface with nanoparticles such as Ni, Ti, or Al enhances the catalytic activity towards ethylene, while reducing interference from other gases such as CO [62,63]. Nanoparticle decoration promotes surface reactions, leading to improved sensor response and selectivity. Additionally, the use of advanced materials such as reduced graphene oxide (rGO)–copper (Cu) nanocomposites can enhance electron transport and facilitate faster response times. Sekhar
Chemoresistive sensors are materials that change their electrical resistance in response to changes in the surrounding chemical environment . These sensors typically consist of a sensing material that connects two electrodes; the presence of an analyte can be detected by measuring the resistance between the two electrodes. Materials used in chemoresistive sensors for ethylene detection include metal oxide semiconductors [66,67], CNTs [68,69], and assemblies of metal oxide nanoparticles [70,71]. Among these, SnO2 is the most popular material for chemoresistive sensors [72,73]. In 1962, Taguchi filed a Japanese patent for gas sensors based on porous SnO2 ceramic materials, marking the inception of SnO2 gas sensors . After extensive research and development, SnO2-based gas sensors were introduced in 1968. The most widely used chemoresistive sensors for ethylene detection are based on n-type metal oxide semiconductors such as WO3 [75,76]. The combinations of these metal oxides provide interesting insights. Pimtong-Ngam
Gas sensors based on metal-oxide semiconductors have low manufacturing costs, are easy to produce, and have fast response times, making them useful not only in agriculture but also in other industries, such as environmental monitoring and medical diagnostics. Ethylene is a small and highly reactive molecule that can interact with various gases and VOCs present in the environment. This can result in crosssensitivity. Thus, low specificity and high-temperature operation are limiting factors for large-scale practical applications of these sensors.
FET gas sensors utilize the changes in capacitance and conductivity induced by the target gases. The dielectric layer and its dielectric constant play crucial roles in modulating the electrical properties of the semiconductor channels [80,81]. The principle of FET gas sensors involves modulation of the conductance or threshold voltage of the semiconductor channel by the adsorption or reaction of gas molecules on its surface. This modulation is achieved using a gate electrode sensitive to the target gas. When the gas interacts with the surface of the semiconductor material, it alters the charge carrier density, and consequently, the electrical properties of the channel. This type of sensor offers significant advantages, including low power consumption and fast response time, and is capable of responding within a few minutes. Nevertheless, these sensors can also exhibit strong responses to other compounds, such as ethanol, acetic acid, ammonia, acetone, and humidity variations, which necessitates the consideration of selectivity issues for practical applications [82,83].
In a recent study, Besar
More recently, Hasegawa
This review briefly discusses gas-sensor-based techniques for the detection of ethylene. Examples of ethylene gas sensors are presented in Table I. With increasing demand from various fields, including agriculture, environmental protection, and medical diagnosis, several types of sensors with low limits of detection and costs have been developed in recent years. Nanostructured materials, with their high surfaceto- volume ratios and easily tunable electrical and mechanical properties, have enabled remarkable progress in gas sensing, particularly in ethylene detection. An ideal sensor for ethylene monitoring should be rapid, sensitive, highly selective, practical, and inexpensive. However, in practice, no ideal sensor exists, and different solutions should be chosen based on specific applications. For example, laser-based sensors are an excellent choice for applications requiring high sensitivity and real-time analysis, whereas electrochemical sensors should be considered when selectivity is a higher priority than sensitivity. GC is a better choice for the measurement of gas mixtures, despite being an older technique. Nevertheless, much work remains to be done to develop a device that simultaneously satisfies the ideal requirements, which are essential for widespread utilization in fresh produce monitoring.
This work was supported by the National Research Council of Science and Technology (NST) grant from the Korean Government (CRC22021-300), the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Ministry of Science & ICT (2021m3A9I5021439), and the Korea Research Institute of Bioscience and Biotechnology (KRIBB) Initiative Research Program (KGM9952314).
The authors declare no conflict of interest.