Gas nanosensors for health and safety applications in mining

The ever-increasing demand for accurate, miniaturized, and cost-effective gas sensing systems has eclipsed basic research across many disciplines. Along with the rapid progress in nanotechnology, the latest development in gas sensing technology is dominated by the incorporation of nanomaterials with different properties and structures. Such nanomaterials provide a variety of sensing interfaces operating on different principles ranging from chemiresistive and electrochemical to optical modules. Compared to thick film and bulk structures currently used for gas sensing, nanomaterials are advantageous in terms of surface-to-volume ratio, response time, and power consumption. However, designing nanostructured gas sensors for the marketplace requires understanding of key mechanisms in detecting certain gaseous analytes. Herein, we provide an overview of different sensing modules and nanomaterials under development for sensing critical gases in the mining industry, specifically for health and safety monitoring of mining workers. The interactions between target gas molecules and the sensing interface and strategies to tailor the gas sensing interfacial properties are highlighted throughout the review. Finally, challenges of existing nanomaterial-based sensing systems, directions for future studies, and conclusions are discussed.


Introduction
Gas sensors are an integral part of health and safety monitoring in many industrial sectors ranging from healthcare to manufacturing and defense. 1 The working environments in underground mines are known for their complexity and harshness with some hazards arising directly from various combinations of gaseous chemicals. 2The dynamic and tough mining environments pose serious challenges in maintaining occupational health and safety (OHS) of the workers in mines.A fatality rate of 2.9 per 100 000 was reported for Australia's mines in 2019. 3This number represents the fourth highest rate among various industrial sectors. 3Sustainable growth of the mining sector, which accounts for AUD$105 billion revenue for coal mines in Australia from 2012 to 2022 alone, 4 requires investment in OHS monitoring technology and infrastructure of underground mines.
Since the emergence and commercial usage of gas sensors, a variety of gas sensing modules, congurations, and sensor/ analyte interfaces have been introduced and their development remains at a fast pace. 5Advanced placeable, wearable, and implantable sensor technologies will play an increasingly important role in the future as envisioned by various health and regulatory organizations, employers, and researchers. 6iven the critical role of the sensing element in gas sensor performance, many of the efforts in gas sensor development are concentrated on exploring new sensing or transducer materials, for example, the metal-organic frameworks (MOFs). 7ommercial gas sensors generally contain bulk or thick-lm sensing materials with limitations on the size, weight, performance, and power consumption of the overall sensor device. 8eplacing traditional sensing materials with nanomaterials thus promises to signicantly reduce the size, weight, and power consumption in the next generation of nanomaterialenabled gas sensors.Gas sensor miniaturization will drive new consumer products in portable or wearable electronics, internet-of-things (IoTs), and multi-gas detection technology.Sensors with sensing elements at the nanometer scale, i.e., nanosensors, promise improved response time, sensitivity, and limit of detection (LOD). 1,9,10The recent strides in the design and implementation of nanomaterials have propelled signicant progress in nanosensor technology, owing to the emergence of diverse classes of nanostructured interfaces.Noteworthy examples include nanoarchitectonic materials, 11,12 few-layered 2D structures, 13,14 1D semiconductors, 15,16 silicon carbide, 17,18 and magnetic systems, 19 as well as mesoporous 20 and nanoporous 21 materials.These nanosensing interfaces have demonstrated remarkably enhanced performance.
3][24] Gas sensing performance of these nanosensors is optimized by the size and shape, 25 chemical composition, 26 and interfacial assembly 27 of the nanomaterials.
Despite the development of a wide range of nanomaterials for gas sensing, our understanding of the underlying gas detection mechanisms of nanomaterials is incomplete and therefore warrants further research.
The present review aims to provide a comprehensive overview of the current understanding of the operational principles and detection mechanisms of different gas sensing systems, along with a discussion of their advantages and disadvantages for their potential applications in the mining industry.To that end, we aim to capture the essential chemical environments in underground mines as well as current gas sensor technologies deployed for mine health and safety monitoring and unmet gas sensing needs of the mining industry.The role of nanomaterials in advancing such gas sensors toward fullling the needs of the mining sector for health and safety monitoring is highlighted in selective case studies.The interactive pathways between the nanomaterials incorporated into different sensing interfaces and the target gaseous analytes are examined to bring forth directions for developing nanosensors specic to the mining sector.Finally, gaps in the literature and motifs for future developments are provided.

Sensing modules of nanomaterialenabled gas sensors
Nanomaterials have been incorporated into gas sensing interfaces based on different sensing modules. 1,5Depending on the combinations of the nanosensor material type and the target gas molecules, different sensing technologies have been introduced. 28In this section, we provide a summary of common sensing modules and the related sensing mechanisms.

Resistive gas sensors
Resistive gas sensors are among the most well researched sensors. 29The operational principle of these sensors is based on a change in the electrical resistance of the sensing material upon exposure to the target gas molecules.The resistance change is induced by chemisorption or physisorption of gas molecules on the surface of the sensing material. 30,31Metal oxide semiconductors (MOSs) are the most common type of nanomaterials used in chemiresistive sensors owing to their unique electronic structures and high numbers of active sites. 29,32MOS-enabled nanosensors detect gases based on the adsorption and ionization of oxygen molecules in a typical temperature range of 200-400 °C. 33The oxygen ionization process leads to the removal of electrons from MOSs, forming an electron depletion layer (EDL) and a hole accumulation layer (HAL) in n-type and p-type semiconductors, respectively. 34epending on the type of target gas molecules that interact with such electronic structures, electrons are either injected into MOSs (in the case of reducing gases) or removed from MOSs (in the case of oxidizing gases).For instance, in an n-type MOS, exposure to reducing gases leads to a decrease in the width of the EDL and a reduction in resistance; meanwhile, the interaction between oxidizing gas molecules and an n-type MOS increases the width of the EDL and resistance.A reduction in the concentration of charge carriers and increase in resistance are observed aer the adsorption of reducing gas molecules on a p-type MOS.6][37] MOS nanosensors are promising for gas detection in harsh environments such as underground mines.Their drawbacks include high operating temperature and limited lifetime of use due to surface poisoning. 38

Gas sensors based on eld-effect transistors (FETs)
A typical FET device consists of a sensing layer placed between a drain and a source terminal.An input voltage is applied to the sensing interface through a dielectric layer and a third gate terminal.In the context of gas sensing, the drain-source current is measured upon the interaction of the sensing layer with the target gas molecules at a given gate voltage. 39The owing current can be modulated by adjusting the magnitude of the applied gate voltage, enabling a tuneable sensitivity for measuring different concentration levels of gas molecules. 40A wide variety of nanomaterials such as metallic 41 and organic 42 semiconductors, polymers, 43 graphene, 44 and carbon nanotubes (CNTs) 45 have been used to construct FET nanosensors. 46FETbased gas sensors have several advantages over others including cost-effectiveness, low power consumption, and facile fabrication and miniaturization. 47

Optical gas sensors
The operational mechanism of optical gas sensors is based on changes in the optical properties of the sensing nanomaterial aer its exposure to gas species of interest.In this case, the sensing signal can be reectivity, colorimetry, uorescence, surface plasmon resonance (SPR), Raman scattering, absorbance, or refractive index. 48Optical gas sensors operate at room temperature and offer high chemical selectivity and fast response.However, challenges in integrating optics into electronic devices and their moderate sensitivity have restricted the real-world applications of optical gas sensors. 49

Gas sensors based on quartz crystal microbalance (QCM)
Such sensors work based on the piezoelectric effect of quartz crystals where an interfacial mass adsorption perturbs the resonance frequency of the crystal.Therefore, the change in the resonance frequency upon exposure to target gases is recorded as the analytical signal. 50The sensing mechanism in QCM sensors is based on the adsorption and desorption of gaseous analytes, leading to a change in interfacial mass and resonance frequency.The integration of nanomaterials into QCM-based interfaces amplies the available active sites for adsorption and desorption processes. 502][53] Compared to other types of sensors, QCM sensors are compatible with a wider range of nanomaterials.These sensors operate at room temperature and require low power for operation.However, improvements in their sensing stability, reproducibility, and handling are required to meet the needs of the market. 53

Electrochemical gas sensors
Depending on the electrochemical activity of the target gas molecules, this type of gas sensor employs different electrochemical techniques for quantication, such as amperometry, potentiometry, and electrochemical impedance spectroscopy (EIS). 54In these sensors, gas molecules diffuse through a membrane into a solid or liquid electrolyte toward the surface of a working electrode (WE), where an electrical input is applied to track subsequent electrical output events.The output current, due to the redox reaction of a target gas at the WE, is recorded as the sensing signal.EIS is an ultrasensitive and a more universal electrochemical sensing module that responds to changes in the interfacial chemistry of the WE upon gas adsorption.Interactions between the dissolved gas species and the WE surface result in variations in the impedance component, which can be used as the sensing signal. 55While electrochemical gas sensors offer high selectivity and sensitivity, they suffer from temperature sensitivity and leakage of liquid electrolytes.To enhance the performance and durability of such sensors, nanomaterials have been exploited to modify the WE surface for increasing the electrochemically active interface. 56anostructures have also been integrated to such systems as solid electrolytes to improve their durability and robustness. 57part from the sensing modules discussed above, there are other types of gas sensors such as surface acoustic wave (SAW), 58 catalytic combustion, 59 and thermal conductivity 60 gas sensors.In brief, in SAW sensors, the properties (i.e., amplitude or Fig. 1 Schematic representation of a typical nanosensor interface, the gases of concern in coal mines, the common sensing methods utilized for mine monitoring, and the advantages of nanosensing interfaces.Nanoparticles and nanowires (grey objects) are predominantly used in nanosensor interfaces.
velocity) of an acoustic wave propagating through a material, are tracked upon the sensor interactions with the gas molecules. 58Catalytic combustion-based gas sensors use a sensing element with catalytic activity dispersed within a supporting matrix.Such sensors operate at an elevated temperature where the combustion of ammable gases, such as methane and hydrogen, further increases the temperature, resulting in a change in the sensor's resistance. 59Thermal conductivity gas sensors work based on measuring heat loss (change in resistance) upon the adsorption of gases with a lower thermal conductivity than air.The measurements are accomplished using a Wheatstone bridge circuit against a ow of a reference gas on a second sensing element. 60Fig. 1 outlines the major sensing methods used to detect critical gas molecules relevant to mining safety, as well as the contribution of nanomaterials in enhancing the performance and applicability of these sensors.

Gas sensing needs in underground mines
As shown in Fig. 2A, it is challenging to ensure health and safety of mining environments because of the size and shape of the underground roadways, typically with lengths of tens of kilometers and widths of several meters.Ideally many environmental factors, including the amount of gas, water, and dust, should be monitored continuously at many places throughout the tunnel, which demands high sampling density and number of sensor devices.Current mine environmental monitoring is typically conducted in a sparse and manual way due to the lack of advanced, reliable, and economical sensing techniques. 61he two major underground coal mining methods are the room-and-pillar (also referred to as the Bord-and-Pillar) method and the longwall method. 62In room-and-pillar mining, the coal is continuously cut and loaded onto a face transport vehicle (e.g., a shuttle car) by a miner.In longwall mining, a longwall shearer does the same job, cutting and loading the coal onto a face conveyor on which it rides.The more recent longwall mining technology accounts for one third of all underground coal production.It is a continuous process using a rotating shear on the mining machine to cut into a block of coal.The coal is then removed by a conveyor from the mine.Desirable requirements relevant to the sensor technology include (1) remote management of the entire monitoring system including communication and routing mechanisms under all conditions and (2) in situ interactions with stationary sensors deployed on the walls, poles, and oors as well as mobile sensors integrated into devices carried by the miners (Fig. 2B).
The application of gas detection technology is signicantly inuenced by the challenging operating environment of gas sensors in underground coal mines.This environment is characterized by several factors, including wide uctuations in atmospheric pressure, large temperature and relative humidity variations, high concentrations of dust particles, and strong electromagnetic interference.Additionally, there are other factors such as coal-rock collapse, mechanical vibrations, and unexpected impacts, which have varying degrees of impact on the gas detection devices.The production safety of underground coal mines is mainly dependent on the environmental conditions of the mines.The monitoring and maintenance system using traditional wire communication suffers from many shortcomings including high construction cost, damage of communication cables, high fault rate, inconvenient system maintenance, and others. 63As a result, a wireless sensor network (WSN) has emerged as an essential technology for continuous monitoring of the workplace environment in underground coal mines. 64Wireless operations impose strict requirements on power consumption of sensor nodes. 65,66A low degree of informationization and regular calibration requirements are the other main limitations in current coal production safety technologies. 67Therefore, it is of great signicance to develop low-cost, low power consumption, and maintenancefree gas sensors based on new technology to detect various poisonous and inammable gases.
The gases of relevance for coal mine explosion or re are methane (CH 4 ), carbon dioxide (CO 2 ), carbon monoxide (CO), and oxygen (O 2 ). 68The gaseous environments more relevant to iron and other metal mines can be found elsewhere in the literature. 69CH 4 , acetylene, hydrogen, and higher hydrocarbons are considered nontoxic but explosive.CO 2 , radon, and its daughter products are toxic.CO, sulfur dioxide, nitrogen oxides, and hydrogen sulde (H 2 S) are acutely poisonous.Other impurities of concern are coal dust and water vapor. 66nderground res can be caused by open ames, spontaneous combustion of coal, electricity, friction from cutting and drilling, welding, blasting, explosion, etc. 66,70 Spontaneous combustion of coal is the main cause of re in underground coal mines and ideally should be continuously monitored.Commonly used gas ratios and indices extracted from gas monitoring data to predict spontaneous combustion of coal are Graham's ratio, Young's ratio, and the oxides of carbon ratio, and the C/H ratio.Graham's ratio is the most widely used metric and is given by the ratio of CO produced to the oxygen consumed (DO 2 ) in the process of spontaneous combustion.Graham's ratio = (100 × CO)/DO 2 .Young's ratio is given by the ratio of CO 2 produced to O 2 consumed.Young's ratio = (100 × CO 2 )/DO 2 .An increase in Young's ratio and decrease in Graham's ratio as a result of CO burning indicates the progress of re from smouldering to open ame.The oxides of carbon ratio is dened as the ratio of the difference in the nal and initial concentrations of CO and CO 2 .CO/CO 2 ratio = (nal CO − initial CO)/(nal CO 2 − initial CO 2 ).The advantage of using this ratio is that it is uninuenced by the inow of air, nitrogen, or CH 4 .This ratio is a more sensitive indicator of re than Graham's ratio.The C/H ratio is used to predict the intensity of re along with O 2 deciency.C/H ratio = 6(CO 2 + CO + CH 4 + 2C 2 H 4 )/2(DO 2 − CO 2 + C 2 H 4 + CH 4 ) + H 2 -CO.For more information on these and other re gas indices and how they are used to predict underground coal res, refer to the excellent review by Muduli et al. 66 While smoke detectors are a mature technology, work by Gottuk et al. showed that combining conventional smoke detectors with CO sensors can reduce false alarms while increasing re detection sensitivity. 71he underground coal mine explosions are caused either by ignition of CH 4 or coal dust or a combination of them.The release of inammable gases from coal, CH 4 and other minor gases (redamp) can cause explosions.Real-time monitoring of CH 4 and O 2 is therefore critical for the detection and prevention of underground explosions.When CH 4 buildup in an underground coal mine reaches a certain concentration range, 5-15%, explosion can be initiated by the presence of a small heat source.The minimum concentration of CH 4 (in air) of this explosive concentration range is termed the lower ammability limit (LFL) (or the lower explosive limit (LEL)).The maximum concentration of this range is called the upper ammability limit (UFL) (or the upper explosive limit (UEL)).When CH 4 concentration falls below the LEL, the amount of CH 4 becomes too low to ignite.Similarly, the amount of O 2 becomes too low when the CH 4 concentration reaches above the UEL and no ignition occurs. 72Kundu et al. 73 reviewed and summarized the explosion concentration range at different temperatures and pressures as well as the inuence of various obstacles and geometries on explosions in an underground mine. 73CO and O 2 sensors at the inlet and outlet of a working panel together with temperature sensors placed at the pillar junctions will enable real-time monitoring of health and safety risks to miners.The difference between the ratio of CO and O 2 concentration at the outlet and inlet signal is used to detect res when the difference is above a preset threshold.This results in activation of the temperature sensor nodes to identify the exact re position. 62n addition to the major consequences caused by combustion and explosion of ammable and oxidizing gases, overexposure to certain gases in the mining environment could result in adverse effects on the health and safety of miners. 66,74herefore, real-time and selective monitoring of certain gases in complex mining environments will meet a critical demand to ensure safe work conditions for mine workers.In the following subsections, we provide an overview of different nanomaterialbased sensing modules under research and development for detecting critical mining gases.

Nanosensors for critical gases in the coal mines
Traditional mine sensors monitor parameters such as temperature, smoke particles, and color of the re to provide early warnings but with limited accuracy.Advances in gas sensor technologies have enabled the research and development of real-time, low-cost, and networkable gas sensors for mine and re safety. 75ommercial chemical sensors deployed for gas monitoring in mining environments are usually based on bulk or thick-lm materials. 76Such bulky devices require high power consumption and oen contain limited diffusive pathways for gas molecules to interact with the sensor.Replacing bulk materials with nanomaterials thus allows device miniaturization with signicant reduction in weight and power consumption.Nanosensors provide a larger surface-to-volume ratio of the sensing interface than traditional sensors leading to improved gas-detection sensitivity.Sensor miniaturization enables the fabrication of multilayered assemblies and interfaces or nanosensor arrays with tailored chemistry for enhanced gasdetection selectivity for a single gas as well as gas mixtures.Moreover, nanosensors offer faster response times owing to the improved diffusion of gas molecules and larger interfacial surface area. 1,5opular mining gas sensors include sensors based on thermal conductivity 60 and catalytic combustion, 77 tunable diode laser absorption spectroscopy (TDLAS) sensors, 2 and nondispersive infrared (NDIR), 78 and electrochemical sensors. 55ptical spectroscopic sensors such as TDLAS and NDIR sensors require large and expensive equipment and are challenging to integrate into portable or wearable optoelectronics.Thermal conductivity and catalytic combustion sensors are widely used for CH 4 detection, but their performance is limited by the bulk nature of the sensing interfaces.Incorporating nanomaterials thus offers a solution to address unmet needs in gas monitoring in mining operations.Electrochemical sensors are typically used to monitor O 2 and CO, 55 but their real-world applications are limited by short lifetime and electrolyte leakage.Nanomaterials comprising solid electrolytes and molecularly structured ionic liquids have emerged as promising candidates for improving electrochemical sensing systems. 79,80Table 1 provides an overview of the sensors commonly utilized for mine monitoring and their advantages and disadvantages.It also outlines the benets and limitations associated with the integration of nanomaterials in each sensor.
As discussed above, existing sensing technologies for monitoring critical gases in mining environments fall short of the requirements for efficient, sensitive, and real-time sensing.Nanotechnology holds the potential to address the limitations of current systems and enable the development of next generation nanosensors specically designed for complex mining environments and operations.In the following sections, an overview of the nanomaterial-incorporated sensors under development for the mining sector is provided.The gas and sensor interaction mechanisms are described wherever relevant throughout the review.Strategies for boosting the performance of nanosensors are also discussed.

Methane sensors
Resistive sensors.Most methane sensors studied so far are based on MOS-type resistive sensors.Such methane sensors mainly exploit an n-type semiconductor sensing layer where the exposure to reducing methane molecules results in an increase in the concentration of charge carriers at the interface and a decrease in resistance. 82Among different n-type sensing elements, tin oxide (SnO 2 ) is by far the most studied functional material for methane [93][94][95][96][97][98] and several successful commercial methane sensors have been developed based on SnO 2 . 99Further improvements in the performance of SnO 2 -based methane sensors have been accomplished through making composites, forming heterojunctions, and structural doping. 100,101enerally, doping MOS structures, i.e., SnO 2 or indium oxide (In 2 O 3 ), with noble metals such as platinum (Pt) and palladium (Pd) improves gas sensing performance through the formation a This technology, yet to be implemented in coal mines, is included here based on its potential applicability.b In NDIR and TDLS, gas molecules interact with electromagnetic waves rather than nanomaterials.
of a wider EDL by enhancing interfacial oxygen chemisorption and ionization. 1024][105] In addition to chemical sensitization, the presence of noble metal dopants in the MOS structure may lead to the formation of a "Schottky barrier" leading to electronic transmissions and charge separation at the metal/MOS interface, which is called "electron sensitization" (Fig. 3A). 106,107ue to known catalytic activity of Pd in the oxidation of hydrocarbons, Pd is one of the most researched dopants for MOS-based sensors. 108Depending on the operating conditions, in some cases oxidation of doped Pd to palladium oxide (PdO) at high temperatures is observed.This results in direct combustion of methane and reformation of non-oxidized Pd sites. 109In one example, Pd-and antimony (Sb)-doped-SnO 2 interfaces exhibited excellent methane sensing performance in terms of sensitivity, response time, and reproducibility. 108The improvement in sensing properties of SnO 2 upon doping was attributed to the catalytic effect of Pd on the dissociation of oxygen molecules and formation of oxidizing species as well as compensation of Sb 5+ substitution in the SnO 2 lattice leading to a reduction in the baseline resistance of the MOS sensor. 108In another study, pure SnO 2 thin lms (control) and lms doped with different elements such as nickel (Ni), osmium (Os), Pd, and Pt were employed for methane sensing. 110The performance of the SnO 2 thin lm sensor was optimized by leveraging the results obtained from employing different dopants.Among the utilized dopants, Os appeared to improve methane sensing performance and reduce the working temperature of the sensor.Os was substituted into the SnO 2 lattice as Os 3+ with an

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Nanoscale Advances unpaired d electron to catalyze rst-step fragmentation of methane into hydrogen and CH 3 radicals. 110In a similar study, ) with a response time of 3.9 s at 350 °C working temperature and 1% methane.The improved gas sensing performance was explained by the surface area increase using doped MOS particles and generation of holes upon Cr incorporation, which leads to a decrease in electron concentration, increasing the width of the depletion layer.The gas sensing mechanism, shown in Fig. 3B, consists of several steps: (1) successive oxidation of methane to CO and CO 2 and formation of reactive oxygen species (ROS) at the MOS interface, (2) desorption of ROS, (3) interfacial release of electrons, (4) formation of a receded depletion layer, and (5) an increase in the electrical conductivity. 1113][114][115] In a study by Vuong et al., nickel oxide (Ni 2 O 3 )-decorated SnO 2 composite lms reduced the working temperature and enhanced methane sensing performance due to the synergistic effect of the composite material. 116In this case, a p-n heterojunction between Ni 2 O 3 and SnO 2 is formed.In addition, Ni 2 O 3 depletes the electrons from an n-type MOS more than chemisorbed O 2 .
Other studies have employed carbonaceous nanomaterials for methane sensing. 103Kooti et al. employed a hybrid material consisting of SnO 2 nanorods and nanoporous graphene. 117heir results show a substantial reduction in the operating temperature down to 150 °C and 600% increase in gas response compared to that of pure SnO 2 .This signicant improvement was attributed to a higher surface-to-volume ratio and rapid charge carrier transport through SnO 2 sites uniformly dispersed on conductive graphene (Fig. 3C).
Using similar strategies, other n-type MOS sensors based on In 2 O 3 , 106,118-123 zinc oxide (ZnO), [124][125][126][127][128][129][130][131][132] tungsten trioxide (WO 3 ), 133,134 iron borate (Fe 3 BO 6 ), 135 molybdenum disulde (MoS 2 ), 136 and titanium oxide (TiO 2 ) 137,138 have been developed for methane sensing.In a study by Lu et al., Pd-In 2 O 3 was utilized for methane sensing and its cross-sensitivity to several interfering gases was observed. 139To maintain the selectivity for methane, they constructed a multilayer sensor consisting of a catalytic lm on top of the sensing element.Catalytic lters of Pt-TiO 2 , Pt-cerium oxide (CeO 2 ) and Pt-zirconium oxide (ZrO 2 ), printed on the Pd-In 2 O 3 layer, were effective in removing the background interference from CO, NO 2 , and ethanol.The basic sensing mechanism of a noble metal-doped MOS did not change aer incorporating the catalytic lters.
There are some reports on p-type semiconductors for methane detection.Holes are the main charge carrier in the ptype materials.The chemisorption of O 2 at high temperature removes the electrons from the conduction band of the semiconductor, thus increasing the hole charge carrier concentration and broadening the HAL.This results in a decrease in the resistance of the sensor.Exposure to a reducing gas such as methane releases the electrons back to the conduction band leading to the recombination of electrons and holes and an increase in the resistance of the sensor.So far, p-type interfaces based on tricobalt tetraoxide (Co 3 O 4 ), 140 lead sulde (PbS), 141,142 vanadium dioxide (VO 2 ), 143 iron oxide (Fe 2 O 3 ), 144 Co 3 O 4 /dicobalt tetraoxide (Co 2 O 4 ), 145 and copper(I) oxide (Cu 2 O) 146 have been exploited for resistive methane sensing.
Other emerging nanomaterials applied to methane detection include MXenes, 147,148 metallic complexes, 149 and metal organic frameworks (MOFs). 150Lee et al. demonstrated roomtemperature methane sensing by using 2D vanadium carbide MXene (Fig. 3D) reaching a limit of detection (LOD) of ∼9 ppm. 151Oxygen-containing surface functional groups of the MXene presumably provide necessary affinity for methane adsorption.In a study on metallic complexes for methane sensing, a composite of single-walled carbon nanotubes (SWCNTs) and Pt polyoxometalate (Pt-POM) showed selectivity and ppm-level sensitivity for methane detection. 149Sensing was carried out at room temperature and the sensitivity for methane was attributed to the catalytic activity of the composite and redox cycling of Pt (Fig. 3E).
Catalytic combustion sensors.For methane detection, a common sensor type employed in coal mines is the catalytic combustion type of methane sensors. 152,153The heat generated by methane combustion on a catalytic material is converted into an electrical signal in this type of sensor.A typical conguration consists of a catalyst embedded into an aluminum oxide (Al 2 O 3 ) lm mounted on a pellistor. 154Pt, Pd, Rh, and rare-earth perovskites are among the most commonly used catalysts in methane combustion sensors. 59,77,155,156In a study by Wang et al., different catalytic systems of Pt-Pd/Al 2 O 3 , Pt-Pd/n-Al 2 O 3 , and Pt-Pd/n-Ce-Al 2 O 3 were tested for combustion-based methane sensing. 81Their results show that doping the nanostructures with Ce presented anti-sulfur ability, lowered the reaction temperature and enhanced the catalytic activity owing to the redox properties of Ce. 81 Methane combustion sensors are cost-effective, simple, easyto-fabricate, and selective.However, they suffer from catalyst poisoning, saturation upon exposure to high concentration of gases, inaccuracy in small enthalpy changes, and high power consumption. 46To reduce the power consumption and improve the sensitivity of such methane sensors several strategies have been put forth, including application of a pulsed voltage to the bridge circuit, 157 miniaturization, 158 and exploitation of dual catalysts on hot and cold terminals. 155lectrochemical sensors.Electrochemical sensing of methane is mainly accomplished through its oxidation reaction and resultant current changes based on gas concentration.Different electrode materials, catalysts, and electrolytes are being explored to improve the performance of methane electrochemical sensors.The earliest reports on methane electrochemical sensing were based on methane oxidation on a Pt electrode in liquid electrolytes. 159,160Due to the limited diffusion of gas molecules in liquid electrolytes as well as electrolyte leakage and evaporation, other types of electrolytes including ionic liquids (ILs) 80 and solid-state electrolytes 161 were explored.In the case of IL electrolytes, negligible vapor pressure, thermal stability, and a wide potential window improve the performance and lifetime of the sensors.In a study by Wang et al., a pyrrolidinium-based IL electrolyte was used for simultaneous sensing of methane and oxygen.The sensing mechanism, shown in Fig. 4A, was based on the incomplete oxidation of methane to CO, followed by CO oxidation to CO 2 by active oxygen species generated from the oxygen reduction reaction.The in situ produced CO 2 was used as an internal standard enabling crossvalidation and measurement error reduction. 162s a leakage-free and thermally stable class of electrolytes, solid-state electrolytes appear to be a good alternative to conventional liquid electrolytes in methane electrochemical sensors. 163,164Fig. 4B shows a solid-state methane sensor developed by Gross et al. 161 In this conguration, Naon was used as a solid-state electrolyte, which conducts the protons produced during the redox reaction of methane between the WE and the counter electrode.
Other types of methane sensors.Although most methane sensors are of the resistive and combustion types, other nanomaterial-enabled sensing modules have been reported for methane sensing.In the category of optical sensors, Mishra et al. utilized graphene-CNT/poly (methyl methacrylate) for SPRbased ber optic sensing of methane. 165The shi in the resonance wavelength upon exposure to the gas was correlated to the methane concentration in the range of 10-100 ppm.Other optical sensing methods based on midinfrared light emitting diodes, 166 refractive index-modulated optical ber systems, 167 photoacoustic spectroscopy, 168 and photoelectrochemical detection 169 have been demonstrated for methane quantication.In addition to optical sensing, methane detectors based on QCM 170 and SAW 171 have been developed, but their complexity in device design and user training has so far limited their applications in the mining sector.

Carbon dioxide sensors
Resistive gas sensors.Similar to methane sensing, most of the resistive CO 2 sensors are based on MOS materials.ZnO is one of the most popular MOS materials for CO 2 sensing.ZnO is an n-type semiconductor and CO 2 is an oxidizing gas.The chemisorption of CO 2 on the surface of ZnO results in EDL widening and increased resistance.Structural doping, 172,173 UV illumination, 174 and heterojunction formation 175,176 in a ZnObased MOS have been studied to improve its CO 2 sensing performance.In a study by Joshi et al., a heterostructure of ZnOcalcium oxide (CaO) was shown to achieve sensitive (26-91%) and selective CO 2 sensing at 150 °C in the range of 100-1000 ppm. 177The heterojunction was synthesized through chemical conversion of zinc hydroxide carbonate to ZnO by using calcium hydroxide, which enabled the formation of an nn type nanointerface with extensive modulation of the potential barrier.The improved selectivity and sensitivity were attributed to higher CO 2 adsorption on CaO due to the basicity of the Ca ion and improved charge-transfer reversibility.In another similar study, an Ag-doped ZnO-CuO heterojunction was utilized for room-temperature, sensitive CO 2 detection within the range of 150-1000 ppm. 178The sensing mechanism, shown in Fig. 5A, was explained by the formation of a p-n heterojunction at the ZnO-CuO interface, which results in the movement of the electrons and holes (due to the difference in work functions of ZnO and CuO) and an increase in the number of free electrons near the surface.This is followed by chemisorption and ionization of oxygen and water molecules as well as a reduction in the HAL and an increase in the resistance of the sensor upon exposure to CO 2 .The dopant (Ag) improved the sensing performance due to the formation of a Schottky barrier, increased carrier mobility, and chemical sensitization.
Other sensing materials including SnO 2 , 179 TiO 2 , 180 CeO 2 , 181 CuO, 182,183 In 2 O 3 , 184 rare-earth oxycarbonates and oxides, 185 bismuth oxide (Bi 2 O 3 ), 186 and tungsten disulde (WS 2 ) 187 have been explored for CO 2 sensing.In a study by Zito et al., yolkshell CeO 2 nanoparticles with high surface area and enhanced gas diffusion were explored for CO 2 sensing. 181Their sensor showed fast response, stability, and high sensitivity to CO 2 at 100 °C owing to the high adsorption capacity of the yolk-shell nanoparticles.In another study, quantum dots (QDs) of Rudecorated WS 2 were applied for room-temperature CO 2 sensing within the concentration range of 500-5000 ppm. 187In Fig. 4 (A) Electrochemical methane sensing using IL-based electrolytes.CO 2 generated by successive methane oxidation on the WE, and active oxygen species were used as an internal standard. 162Adapted with permission from ref. 162

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Nanoscale Advances the case of WS 2 , which is a p-type semiconductor, the sensing mechanism (Fig. 5B) was explained by CO 2 chemisorption and breakage into CO and O 2 leading to electron donation to the sensing interface, a decrease in the concentration of holes, and an increase in the resistance.Ru was assumed to have a catalytic role enabling fast CO 2 reduction.Moreover, the presence of Ru led to a reduction in the ohmic loss and rectication at the interface of Ru-WS 2 /Au electrodes (an electrode on the sensor substrate).
In addition to conventional oxides based on single metallic elements, high-entropy metal oxide nanoparticles have been used for room-temperature and wide-range CO 2 sensing (250-10,000 ppm).Gd 0.2 La 0.2 Y 0.2 Hf 0.2 Zr 0.2 O 2 (Y-HEC) was explored for CO 2 sensing using three different electrodes including Ag, Au, and indium tin oxide (ITO). 188The results show that Y-HEC made a perfect ohmic contact with the Ag electrode where the total resistance of the sensor was only controlled by the channel resistance of the sensing material without any contribution from the contact resistance at the metal-semiconductor interface.On the other side, a Schottky contact was formed at the Y-HEC/ITO and Y-HEC/Au interface with a lower Schottky barrier height (SBH) in the case of ITO.The highest response was obtained at the ITO interface, which revealed the vital role of the Schottky contact in gas sensor performance.Exposure to CO 2 and release of electrons at the interface result in a downward shi in the level of the conduction band, which is followed by Schottky barrier modulation (SBM) and a reduction in the SBH.In gas sensors based on SBM, the presence of an optimized SBH leads to improved sensor performance.The lower sensitivity in Y-HEC/Au, compared to ITO one, was attributed to a large SBH, which prevented effective charge transfer. 188lectrochemical sensors.Electrochemical sensors based on potentiometric, amperometric, and impedance measurements have been employed for CO 2 sensing.Among them, potentiometric sensors suffer from limited sensitivity as they respond to changes in EMF (electromotive force) against concentration on a logarithmic scale. 189Amperometric sensors are the most common module for electrochemical sensing of CO 2 owing to their sensitivity, selectivity, ease of operation, and facile data interpretation.
Different types of solid and liquid electrolytes have been exploited for CO 2 electrochemical sensing.IL-based CO 2 sensors have been developed to take advantage of the tunable composition of ILs to enhance CO 2 solubility and detection selectivity. 190In a study by Fapyane et al., a mixture of IL 1-ethyl-3-methylimidazolium dicyanamide (EMIMDCA) and Nanoscale Advances Review dimethylformamide (DMF) was used for the amperometric sensing of CO 2 via its reduction on the Ag WE. 191 The addition of DMF reduced the response time and overpotential of CO 2 reduction due to a decrease in stability of the CO 2 -EMIMDCA complex.The IL mixture enabled quantitative measurements of CO 2 in the range of 0-4.62 kPa with a LOD of 0.5 kPa.In another study, Sridhar et al. developed an IL-based CO 2 sensor operated based on the impedance readout. 192They investigated their gas sensing setup at different temperatures using a Pt black electrode.A decrease in the real component of the resistance was observed upon CO 2 exposure, which was attributed to CO 2 inclusion and disruption of the cation-anion interaction in molecularly structured IL lm formed at the interface (Fig. 5C).At lower operating temperatures, increased viscosity of ILs and formation of a dense lm made of ionic charges appear to increase the sensitivity of the sensor.
In addition to ILs, solid electrolytes are another group of popular electrolytic media widely employed for CO 2 electrochemical sensing. 193Yttria-stabilized zirconia (YSZ) is oen utilized as such an electrolyte, which has shown promise for amperometric measurements of CO 2 up to the concentration level of 10% within the temperature range of 600-750 °C. 194In a study on the development of solid electrolytes for CO 2 sensing, Ma et al. introduced Y-doped La 9.66 Si 5.3 B 0.7 O 26.14 (Y-LSBO) as the electrolyte, which was coated with a working electrode lm made of a Li 2 CeO 3 -Au-Li 2 CO 3 composite. 195In this layered assembly, Li 2 CeO 3 functions as an ionic bridge between the solid electrolyte with O 2− conductivity and Li 2 CO 3 as the Li + conductor.The sensor was operated based on the EMF readout and showed a Nernstian behavior for CO 2 measurements within the range of 400-4000 ppm at 400 °C.
Apart from the reports replying only on the electrochemical input, a nanocomposite of ZnO/MoS 2 /reduced graphene oxide (rGO) was reported to allow sensitive photoelectrochemical sensing of CO 2 .In this case, a heterojunction was formed at the ZnO-MoS 2 interface, and rGO functioned as a conducive bridge to facilitate the electron transfer.This assembly enabled CO 2 detection in the range of 10-7820 ppm with a LOD of 10 ppm and response time of 10 s. 196 Optical sensors.A variety of nanomaterial-incorporated optical CO 2 sensors based on SPR, 197 colorimetry, 198 and infrared spectroscopy 199 have been developed.In SPR-based CO 2 sensors, CNTs are the most utilized plasmonic materials owing to their high affinity to CO 2 .However, poor selectivity and the existence of interfering excitation regions limit the application of CNT sensors in challenging mining environments. 200olorimetric CO 2 sensing is usually carried out by using semiconductor QD nanocrystals where a change in the emission intensity and/or blue or red shi can occur upon CO 2 adsorption. 198,200In such sensors, although colorimetric measurements allow CO 2 sensing in a cost-effective and facile manner, the semi-quantitative readout and poor long-term stability of QDs make them a less popular choice for the mining industry.
In CO 2 sensors based on IR spectroscopy, nanomaterials are incorporated either into the light emitting source or the photodetector part. 200Such sensors normally have a chamber conguration, where certain IR wavelengths are absorbed by CO 2 molecules.Having the CO 2 IR spectrum as the readout, these sensors offer high accuracy, fast response, and durability.Their drawbacks include high cost, device complexity, and difficulty to scale up.
Other types of sensors.Based on our literature review, CO 2 sensing is dominated by electrochemical and MOS-type resistive sensors.Other less common CO 2 nanosensors rely on SAW, 201 QCM, 202 and FET. 203An advantageous version of such sensors with potential applications in mining, was introduced by Ersoez et al. 203 Their sensor was based on a new concept of an electrolyte-gated transistor (EGT).As shown in Fig. 5D, they used an In 2 O 3 lm as a channel-forming layer in a FET and [EMIM][BF 4 ] IL as an electrolyte separating the gate from the channel.Exposure to CO 2 caused O 2 depletion at the MOS interface resulting in an increased conductivity of the sensor.In this case, the IL provides a medium for dissolution of the gaseous reactants and modulates the charge carrier distribution in a MOS via the formation of an electrical double layer.This conguration enabled CO 2 quantication in the range of 400-4000 ppm with a sensitivity of 0.1%/ppm and a recovery time of 20 s.

Carbon monoxide sensors
Resistive sensors.Functional nanomaterials utilized in CO sensors include n-type 204 and p-type 205 MOSs as well as polymers. 206Considering CO as a reducing gas, CO interactions with an n-type MOS remove the ionized oxygen species, inject electrons back to the MOS, and decrease the overall electrical resistance of the sensor.A variety of nanostructures based on ntype MOSs of SnO, ZnO, In 2 O 3 , TiO 2 , WO 3 , and CeO 2 have been employed for CO sensing. 207In a recent study, the CO sensing mechanism of a composite of SnO 2 /SiO 2 -PdO x was studied in dry and humid air using DRIFT (diffuse reectance infrared Fourier transform spectroscopy) analysis. 204The spectra revealed the contribution of PdO x to CO oxidation and the role of SiO 2 in preservation of the bridge oxygen atoms on SnO 2 as well as the prevention of carbonate poisoning by decreasing the basicity of the sensing interface. 204In addition to n-type semiconductors, p-type MOSs have also been employed for CO sensing due to their higher catalytic activity and less temperature dependency of their conduction at elevated temperatures. 205,207CuO 205 and Co 3 O 4 208 are among popular p-type MOSs for CO sensing.Regardless of the MOS type, doping, 209 heterojunction assembly, 210 and nanocomposite formation 211 have been used to enhance CO sensing performance of MOS-based sensors.In a recent study by Yuan et al. porous nanoplates of n-ZnO/p-Co 3 O 4 (Fig. 6A), derived from the zeolitic imidazolate framework, were used for selective and sensitive CO sensing. 212his nanomaterial exhibited a large surface area and high level of oxygen vacancy in the crystal structure, thus allowing strong chemisorption of CO molecules and high sensitivity with a response value of 35.4.According to the results, inclusion of Zn-based components appeared to be essential for antiinterference, i.e., selectivity against interferents such as CH 4 , H 2 S, nitric oxide (NO), ammonia (NH 3 ), and H 2 .
Electrochemical sensors.Electrochemistry is one of the best developed detection methods for CO sensing.Among different electrochemical modules, amperometry is the most utilized technique, where the current produced upon oxidation of CO to CO 2 is tracked against time. 92In addition to amperometric sensors, there are a few demonstrations of potentiometric 213,214 and EIS-based 215 systems for CO sensing.
CO electrochemical sensor development mainly explores WE materials and electrolytic media.For the WE materials, metallic 216 and metal oxide-based 217 nanostructures as well as CNTs 218 have been shown to provide effective sensing interfaces for CO electrochemical sensing.A Pt microdisk electrode modied with multi-walled CNTs (MWCNTs) was found to have a catalytic effect on CO oxidation with a reduced overpotential.The incorporation of MWCNTs allowed CO sensing within the range of 0.72-52 mg ml −1 with a LOD of 0.60 mg ml −1 . 219Similarly, a nanocomposite of Pt-Ni alloy deposited on polyaniline-MWCNTs exhibited a bifunctional catalytic activity toward CO oxidation while neighboring Ni removed the reaction intermediates.A linear sensing response was obtained within the range of 1.0-50 mM with a LOD of 0.5 mM. 220The last two mentioned studies both used a liquid electrolyte of perchloric acid for sensing.To fabricate robust and durable sensors specically suited for mining environments, electrolytic media other than aquatic solutions are required.To this end, electrochemical CO sensors based on solid electrolytes and ILs have been reported. 221,222Both inorganic and polymeric solid electrolytes 223 have shown promises for CO sensing.In one of the earliest reports, a semipermeable and proton-conductive Naon membrane was used to cover the surface of all three electrodes (sputtered Pt lms served as the working and counter electrodes and a sputtered Au lm was used as the reference electrode) required for electrochemical sensing. 223The sensor showed excellent durability of >2 year lifetime, a working range of 0-2000 ppm, and a response time of 30 s for CO sensing.The remarkable sensing performance was attributed to CO permeability of Naon and the higher oxidation rate of CO.In a study of inorganic solid electrolyte-based CO sensors, Phawachalotorn et al. used a Fe-doped La 0.8 Sr 0.2 GaO 3 solid electrolyte in combination with electrocatalysts Au 10 wt%-In 1.9 Sn 0.1 O 3 (ITO955) and RuO 2 -La 0.6 Sr 0.4 CoO 3 (LSC64) for amperometric CO sensing. 224This type of sensor operated within a temperature range of 300-500 °C and showed a sensitive (8.83 mA per decade) and selective (over CH 4 , CO 2 , and H 2 ) response to CO.
Other types of sensors.In addition to the traditional CO sensors developed for mining applications such as the chemiresistive and electrochemical types, other less developed CO sensors include QCM, 225 FET, 226 SAW, 227 and optical (e.g., SPR, 228 reectometry, 229 and uorescence, 230 ) sensors.In particular, FET-based CO sensors are promising for eld applications.Singh et al. demonstrated room-temperature CO sensing by using Zn-doped In 2 O 3 nanowires (NWs) in an FET conguration. 231Zn doping enhanced the sensor response and enabled CO sensing within the range of 1-5 ppm with a selective response over NO and nitrogen dioxide (NO 2 ).

Oxygen sensors
Electrochemical sensors.Electrochemical O 2 sensors based on solid electrolytes are the most studied commercial sensors for O 2 measurements in the gas phase.Traditionally, O 2 sensing is carried out at high temperature in a planar conguration using YSZ as a solid electrolyte with oxygen conduction.As can

Nanoscale Advances Review
be seen in Fig. 6B, the sensor is constructed in a multilayered conguration with two Pt electrodes exposed to the test gas stream and the reference gas.The electromotive force, resulting from the oxygen pressure disparity between the two Pt electrodes, serves as the measured signal readout.Upon exposure to the target gas stream, the molecular oxygen adsorbs on the Pt sites, which is followed by O 2 dissociation to atomic oxygen and its ionization/reduction at the electrode-electrolyte-gas boundary, referred to as the triple phase boundary (TPB).As YSZ has a high level of oxygen conduction, the chemical potential of reduced oxygen species is not changed in the solid electrolyte media.Thus, the difference in chemical potential of O 2 exists at the test stream, and the reference stream generates an EMF for potentiometric O 2 sensing. 79,91Advances in such O 2 sensors involve the replacement of the reference Pt/gas interface, with metal/metal oxide interfaces to simplify the sensor conguration and improve its applicability.Several metal/metal oxide interfaces based on Sn, In, Ni, and Ru have been explored as the reference, and they can maintain a desirable oxygen partial pressure at a given temperature (<500 °C). 232part from commercial potentiometric oxygen sensors, amperometric modules have been employed to remove the logarithmic dependency of concentration to the readout and allow O 2 sensing within a wider concentration window.The amperometric sensors record the current generated from oxygen reduction and have been used for sensing of O 2 dissolved in liquid electrolytes 233 and O 2 in the gas phase at the interface of a solid electrolyte and an electrode. 232In the case of solid electrolytes, in addition to the conventional YSZ, 79 samarium (Sm)doped CeO 2 has high oxygen ion conductivity enabling amperometric O 2 sensing within the range of 100-500 ppm at 550 °C. 234he limiting current in these sensors depends on the applied direct current (DC) potential utilized to pump molecular oxygen to the working electrode surface.Depending on the target test stream, different potentials may be required to achieve a steadystate current.To compensate for this dependency and improve the reliability of O 2 amperometric sensors, a combined amperometric-potentiometric sensing technique has been introduced.Fig. 6C shows a typical design consisting of two electrochemical chambers of an amperometric and a potentiometric cell, respectively.In the amperometric chamber, O 2 is pumped and measured based on the limiting current, and the potentiometric chamber records the EMF value and provides additional information on sensor performance and the analyte concentration. 79,232esistive sensors.O 2 resistive sensors usually operate based on O 2 chemisorption on MOS materials.Among a variety of MOSbased O 2 sensors, Ti-, Ga-, and Ce-based semiconductors are the most common.Semiconductors of TiO 2 , SrTiO 3 , Ga 2 O 3 , CeO 2 , and Nb 2 O 5 with n-type characteristics have shown sensitivity toward O 2 molecules. 79,235The sensitivity is obtained through a sequence of events including the formation of oxygen adsorbents, occupation of oxygen vacancies in the n-type MOS, a reduction in the concentration of electrons as charge carriers, and an increase in the resistance of the sensor.So far, several O 2 resistive sensors based on TiO 2 thick lms have made it into the market. 235New advances have focused on the exploitation of nanostructures and thin lms of TiO 2 (either in a pristine or composite form) to further enhance O 2 sensing properties. 236Thin lms of TiO 2 with a particle size of ∼34 nm were found to have a high response to O 2 at low operating temperatures within 150-300 °C. 237Strontium titanate (SrTiO 3 ) is another popular Ti-based semiconductor with a perovskite structure widely employed for high temperature O 2 sensing.At a low oxygen partial pressure, oxygen vacancies in SrTiO 3 result in an n-type behavior while at a high oxygen partial pressure (>1 Pa), Sr vacancies dominate the semiconductor structure giving rise to a p-type behavior of the sensing material. 79n this case, doping the structure with donor or acceptor elements (i.e., La or Fe, respectively) brings about a shi in the p-n transition and a monotonic signal change versus the O 2 concentration in the donor-doped structures. 238mong the other n-type MOS interfaces reported, CeO 2 is a well-researched material for O 2 sensing. 239The Ce atoms inside the CeO 2 crystalline lattice possess variable oxidation states of Ce 3+ /Ce 4+ , which leads to oxygen storage capability and fast oxygen vacancy diffusion in CeO 2 .The latter feature was attributed to the signicant reduction in the response time of the O 2 sensor. 79,235Films of CeO 2 have been reported to have response times within 5-10 ms. 197Moreover, the addition of Zr to Ce forms a mixed oxide phase and further increases the charge carrier mobility resulting in a response time within the range of 1-20 ms. 240everal p-type semiconductor interfaces exhibiting temperature-independent resistivity have also been explored for O 2 sensing.Examples of such temperature-independent interfaces include lanthanum cuprate (La 2 CuO 4+d ), SrTi 1−x Fe x O 3 (STF), and BaFe 1−y Ta y O 3 . 235The temperature independency of resistance in STF was explained by the compensation of temperatureinduced formation of charge carriers due to strong temperaturedependent mobility of holes and a decrease in the bandgap upon the incorporation of Fe electronic bands into SrTiO 3 . 241ther types of sensors.Other types of O 2 sensors utilizing nanomaterials, less suitable for harsh mining environments, include FET 242 and photoluminescence sensors. 243Fan et al. incorporated single-crystal ZnO NWs into the FET conguration for O 2 sensing, 244 and reported thinner NWs to exhibit higher sensitivity enabling O 2 measurements in the range up to 50 ppm.
An amine-functionalized silver-chalcogenolate-cluster-based MOF was employed as a dual uorescence-phosphorescence probe for O 2 sensing.Ratiometric sensing was conceived as O 2 -induced phosphorescence quenching relative to an O 2independent uorescence signal as the readout.It was shown that a second functionalization with methyl moieties can interfere with the quenching process providing a wider sensing concentration range of 0.5-20 ppm.The sensor displayed a response time of 0.3 s. 245

Direct electrodeposition of charge-transfer complex-based nanosensors
Charge-transfer complexes (CTCs) refer to a group of organic and organometallic conductors and semiconductors with

Review
Nanoscale Advances unique electrocrystallization properties.7][248] Fig. 7A shows the molecular packing structure in tetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ), one of the most studied CTCs. 249In such a molecular assembly, electrons are delocalized along the stacks of electron acceptor/donor molecules, and conductive crystals are grown along the c-axis leading to a 1D columnar structure. 250These 1D semiconducting crystals offer numerous synthetic chemistry variations for scalable manufacturing of gas nanosensors.We have previously shown the possibility of controlled electrocrystallization of 1D CTCs through seed mediation 251 (Fig. 7B) and substrate patterning 252 (Fig. 7C).These strategies offer a versatile approach for precise electrodeposition of CTC nanosensors directly on sensor circuitry.Direct electrocrystallization of such nanowire sensors will enable low-cost production of high-quality crystalline nanomaterials as an effective additive manufacturing strategy to overcome a major challenge in nanosensor commercialization. 253TC electrocrystallization has shown promises to create gas sensitive interfaces owing to CTC's semiconducting characteristics and tunable chemistry.Various compositions of CTCs can be exploited using electrochemistry (Table 2).Variations in molecular stacking, counterions, or stoichiometry of CTCs lead to different physical and chemical properties, which can be adjusted to achieve selective interactions with target gas molecules.255 There are several literature reports on CTCs being used for detecting both reducing 256 and oxidizing 257 gases.Our group has demonstrated the sensing performance of tetrathiafulvalene bromide (TTFBr 0.76 ) nanowires electrodeposited directly on patterned electrodes for ammonia measurements.256 According to our observations, depending on the CTC stoichiometry, different sensor readouts of ammonia were accomplished.In insulating TTBr 1.0 , exposure to reducing ammonia increases the concentration of charge carriers and reduces the resistance.In the case of conductive TTFBr 0.76 , electron injection neutralizes TTF + and obscures intermolecular donor-acceptor interactions resulting in an increased resistance.256 In another research study on gas sensing capability of CTCs, Wang et al. applied the TTF-TCNQ complex for measuring and differentiating alkyl amines and aromatic amines.258 In both cases, aer exposure to the amines, the electrical current readout over TTF-TCNQ decreased due to donor-acceptor interactions between amines and TCNQ, which competes with that of TTF-TCNQ.Given the higher basicity of alkylamines than that of aromatic amines, they form a stronger bond with TCNQ, and an irreversible signal was observed in this case, while in the case of aromatic amines, the sensor signal was recovered aer a few seconds.Hence, the recovery behaviour of the sensor was used as a criterion for distinguishing between alkyl amines and aromatic ones.258 Another research study has demonstrated the sensitivity of the TTF-TCNQ complex to oxidizing gas species, such as CO 2 , O 2 ,

Nanoscale Advances Review
and NO 2 , through alteration of the charge transfer in the complex resulting in a reduction in conduction. 259aken all together, CTCs present a potential application for gas monitoring in mines.Compared to MOS-based systems, the sensing interfaces based on CTCs offer a versatile manufacturing technology, possess tailorable chemistry, and operate at room temperature.CTC nanosensors could respond to reducing and oxidizing gas molecules of CH 4 and CO, respectively, as demonstrated in a limited number of studies on similar molecules so far. 258,259The possibility of fabricating aligned nanowires through a simple substrate-directed electrochemical route offers a means for constructing nanosensor arrays capable of multiplexed measurements in a single wearable or portable and networkable device.

Conclusions and future remarks
The present review provides an overview of the various sensing methodologies and nanomaterials employed for the detection of critical gases of interest in the mining industry.The sensing mechanisms and the interactions between the target gas molecules and the sensing nanomaterial are discussed for each sensing module and gas type.Based on our review of literature, it is evident that the emergence and extensive application of nanomaterials have advanced the gas sensor eld rapidly and signicantly.So far, a large variety of nanomaterials with certain characteristics, depending on the sensing method, have been incorporated into gas sensors for different gases.However, despite the broad development of a diverse array of nanomaterials, new understanding of the interactions between the adsorbed gas and the sensing interface has been lacking in the literature.In this regard, most literature reports rely on generally accepted mechanisms and theories for describing their sensing systems.Mechanistic investigations through actual experiments to provide deeper understanding of the sensing pathways and the role of each component in a typical composite material have been lacking.In addition to this, the majority of gas sensors developed to date (with the exception of certain optical and electrochemical systems) exhibit cross-sensitivity to both the target gas and background gases.Selectivity in gas sensors remains to be improved.To complement the development of nanomaterials with inherent gas selectivity, it is essential to incorporate surface functionalization, external ltering, and various data optimization methods to achieve the ultimate selectivity, enabling applications in highly variable mining environments.
The integration of nanostructures into gas sensing interfaces holds great promise for fabricating miniaturized, efficient, and accurate portable and wearable devices.Nanoscale structures provide a larger responsive interface over a given sensor area, leading to faster response times and higher sensitivity.The utilization of these portable devices enables early warnings over a wide area, making them easily networkable and applicable in the mining sector.
While the incorporation of nanomaterials into gas sensing platforms has greatly improved their applicability and performance, there still remain challenges in terms of developing facile, cost-effective, reproducible, and scalable fabrication techniques for integrating nanostructures into sensing interfaces.The direct electrocrystallization of nanostructures on patterned substrates, as discussed in the preceding section, offers a promising approach to overcome challenges in the integration of nanomaterials into sensing interfaces.Efforts aimed at upscaling laboratory nanosensor production, guided by new science and engineering principles, are crucial to propel scientic discoveries towards commercialization and widespread industry adoption.

Nanoscale Advances
Review

Fig. 2 (
Fig. 2 (A) Illustration of a typical underground coal mine.(B) Typical devices carried by miners.Reprinted with permission from ref. 61.

Fig. 3 (
Fig. 3 (A) Scheme of the chemical sensitization effect of noble metals incorporated into the sensing interface to improve the sensitivity of MOS sensors. 106Reproduced with permission from ref. 106 Copyright 2022 Elsevier.(B) Methane sensing mechanism of Cr-doped SnO 2 structures based on successive methane oxidation and a decrease in the width of the depletion layer. 111Adapted with permission from ref. 111 Copyright 2019 Elsevier.(C) Charge carrier transport across dispersed SnO 2 sites and nonporous graphene with a high surface area, enabling sensitive methane detection. 117Reprinted with permission from ref. 117 Copyright 2019 Elsevier.(D) Application of 2D vanadium carbide MXene for resistive methane sensing through the interaction of physisorbed methane with the surface functional groups of MXenes. 151Reproduced with permission from ref. 151 Copyright 2019 American Chemical Society.(E) Methane sensing achieved by the catalytic function and redox cycling of Pt sites in the SWCNTs/Pt-POM composite. 149Reproduced with permission from ref. 149 Copyright 2020 Proceedings of the National Academy of Sciences.
Fig.4(A) Electrochemical methane sensing using IL-based electrolytes.CO 2 generated by successive methane oxidation on the WE, and active oxygen species were used as an internal standard.162Adapted with permission from ref.162Copyright 2014 Royal Society of Chemistry.(B) Scheme showing a multilayered electrochemical device based on solid-state electrolyte Nafion for methane sensing.161Reprinted with permission from ref.161Copyright 2018 American Chemical Society.

Fig. 5 (
Fig. 5 (A) Room-temperature CO 2 sensing by a Ag-doped ZnO-CuO (p-n) heterojunction where the charge carrier movement provides more electrons for oxygen chemisorption.This is followed by a reduction in the HAL upon exposure to CO 2 . 178Reprinted with permission from ref. 178 Copyright 2023 Elsevier.(B) Illustration showing the CO 2 sensing mechanism using Ru-WS 2 /Au electrodes. 187Adapted with permission from ref. 187 Copyright 2020 Institute of Physics.(C) EIS-based CO 2 sensing via disruption of the IL assembly at the electrode interface due to CO 2 inclusion. 192Reproduced with permission from ref. 192 Copyright 2023 American Chemical Society.(D) A FET sensor developed for CO 2 sensing based on an electrolyte-gated mode using an IL and In 2 O 3 as the electrolyte and channel forming layer, respectively. 203Reprinted with permission from ref. 203 Copyright 2020 Elsevier.

Fig. 6 (
Fig. 6 (A) Zeolitic imidazolate framework-derived n-ZnO/p-Co 3 O 4 nanomaterials for CO sensing.The sensing nanomaterial presents many oxygen vacancies and enables strong chemisorption. 212Reprinted with permission from ref. 212 Copyright 2023 Elsevier.(B) Schematic of a potentiometric O 2 sensor based on YSZ solid electrolyte. 79Adapted with permission from ref. 79 Copyright 2003 Springer.(C) A typical configuration of a mixed mode potentiometric-amperometric O 2 sensor. 232Reprinted with permission from ref. 232 Copyright 2022, The Authors, under Creative Commons Attribution (CC-BY) license, published by Multidisciplinary Digital Publishing Institute.

Table 1
Technologies for monitoring critical gases in coal mines and their advantages and shortcomings.Advantages and shortcomings of nanomaterial incorporation are highlighted for each type of sensor Bunpang et al. prepared chromium (Cr)-doped SnO 2 nanoparticles through substitutional incorporation of Cr (in the form of Cr 3+ ) into the MOS lattice. 111Cr-doped SnO 2 showed remarkable sensitivity (sensing response, which is dened by the ratio of initial sensor resistance to its resistance aer exposure to methane, = 1268.6)and selectivity (evaluated against H 2 , C 2 H 2 , NO 2 , NO, N 2 O, CO, NH 3 , SO 2 , C 2 H 5 OH, C 3 H 6 O, and H 2 O