Development of low-temperature SnO₂–Au gas sensors for H₂S detection in food freshness monitoring†

Kee-Ryung Park^ab, Jinhyeong Kwon*^ac and Hyeunseok Choi*^a
aResearch Institute of Sustainable Development Technology, Korea Institute of Industrial Technology (KITECH), Cheonan, 31056, Republic of Korea. E-mail: jhs0909k@kitech.re.kr; hchoi@kitech.re.kr
^bKorea National Institute of Rare Metals, Korea Institute of Industrial Technology (KITECH), Incheon, 21655, Republic of Korea
^cLaser-processed Nanomaterials Engineering Lab., Korea Institute of Industrial Technology (KITECH), Cheonan, 31056, Republic of Korea

First published on 27th August 2024

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Introduction

Maintaining a fresh food supply system is one of the essential factors for a sustainable society.¹ The United Nations Environment Program (UNEP) has reported that improper storage and inefficient logistics led to the wastage of approximately 17% of all globally produced food.² This wastage not only affects food availability for those experiencing starvation but also contributes to increased greenhouse gas emissions during transportation and production.³ Modern electronic technologies offer potential solutions to deal with the issue.⁴ For example, food status indicators/sensors could provide real-time information to ensure optimal consumption of food.^5,6 However, existing products encountered limitations such as complex manufacturing processes, sophisticated gas sensing system setups, expensive analytic instruments, and immobility. Furthermore, conventional gas sensor systems are not suitable for the different types of food species such as crops, fish, fruits, and meats.

Protein-containing foods emit various gases including ammonia (NH₃), trimethylamine (TMA), hydrogen sulfide (H₂S), among others, during decomposition.^7–9 These gases serve as important indicators of food freshness and quality. Among them, H₂S gas is particularly significant due to its harmful effects and distinct odor, making it a crucial target for food freshness detection.¹⁰ Consequently, researchers have focused extensively on developing gas sensor systems capable of detecting H₂S with high sensitivity and specificity.^11,12 Metal oxides such as CuO,¹³ Fe₂O₃,¹⁴ In₂O₃,¹⁵ SnO₂,^16,17 TiO₂,¹⁸ WO₃,^19,20 and ZnO^21–24 have been widely applied in gas sensor technology. These materials are favored due to their due to their cost-effectiveness, straightforward synthesis methods, fine gas-detecting capabilities, and long-term stability. However, a major limitation of metal oxide-based gas sensors is their high operating temperatures, often exceeding 300 °C, which are necessary to overcome activation energy barriers.²⁵ This requirement poses challenges for practical applications, especially in scenarios where low power consumption and safety are critical.

To address these challenges, it is essential to develop advanced gas sensors that can operate at lower temperatures while maintaining sensitive and selective detection of target gas molecules. Tin dioxide (SnO₂) is a promising candidate in this regard due to its desirable electrical properties, high chemical stability, reliability, and non-toxicity.^26,27 Recent studies have shown that SnO₂-based sensors can be engineered to achieve improved performance at lower temperatures, making them suitable for a wider range of applications, including food safety monitoring. For example, Liu et al. have fabricated an ethanol gas sensor using hydrothermally synthesized SnO₂–Au microspheres.²⁸ The ethanol sensor based on SnO₂–Au microspheres exhibits a three-fold higher response value, good reversibility, and fast on/off performance at 240 °C compared to a conventional SnO₂ gas sensor. Feng et al. also synthesize mesoporous SnO₂–Au nanospheres using non-toxic, renewable, and inexpensive plant polyphenols.²⁹ These nanospheres, with a uniform diameter of 120 nm and high surface areas, serve as the active material for a TMA gas sensor operating at 50 °C. Although these studies achieve significant gas detection performance, the researchers do not disclose the temperature and quantitative yield of the material synthesis.

In this study, we synthesize SnO₂–Au hollow nanofibers through a combination of electrospinning and calcination processes. The electrospinning technique enables the creation of Sn-based nanofibers with large-area, uniform structures in significant quantities. Additionally, the subsequent calcination process enhances the surface areas of the SnO₂-based nanofibers by generating hollow nanostructures. The optimized SnO₂–Au nanostructure, when used as an H₂S chemiresistive gas sensor, exhibits remarkable sensitivity, selectivity, and excellent repeatability at an operating temperature as low as 200 °C. Furthermore, we design and apply the sensor in a food freshness monitoring device to assess the freshness of beef. The device operates by measuring the concentration of H₂S, a key indicator of freshness, and seamlessly relays the results through a smartphone app. Through a series of experiments evaluating the freshness of beef, the developed tool demonstrates its efficacy in determining the freshness level with precision. We expect that the food freshness monitoring system developed in this study will contribute to improving the food supply chain and reducing food waste in the future.

Results and discussion

Fig. 1(a) illustrates the electrospinning process used to synthesize Sn/Au-containing nanofibers. The jetting solution consists of dimethylformamide (DMF), ethanol, deionized water, Sn/Au metal precursors, and polyvinylpyrrolidone (PVP) additive. The inset figures exhibit the smooth surface morphology and scalable productivity of the as-spun Sn/Au nanofibers, respectively. Subsequently, a calcination process is performed at 600 °C on the as-spun nanofibers, during which Sn⁴⁺ ions react with oxygen and Au³⁺ ions acquire electrons, resulting in the formation of SnO₂–Au nanofibers as shown in Fig. 1(b). Simultaneously, the inner structure of the nanofibers releases chloride species, PVP, and other components, resulting in SnO₂–Au hollow-structured nanofibers.³⁰ To obtain the proper surface morphologies of the SnO₂–Au nanofibers, the Au precursor ratio in the jetting solution is systematically controlled from 0, 0.1, 0.5, 1, 2, to 5 wt%. Fig. 1(c) exhibits the morphological evolution of SnO₂ nanofibers with increasing Au contents, synthesized via electrospinning, as visualized through SEM images: (i) SnO₂, (ii) SnO₂–Au 0.1 wt%, (iii) SnO₂–Au 0.5 wt%, (iv) SnO₂–Au 1 wt%, (v) SnO₂–Au 2 wt%, and (vi) SnO₂–Au 5 wt%. Image (i) depicts pristine SnO₂ nanofibers with a continuous fibrous structure and a diameter of approximately 200 nm. As Au content increases from 0.1 wt% to 5 wt%, notable structural changes are observed in images (ii) to (vi). In the image (ii) with 0.1 wt% Au, the nanofibers show the initial surface roughness and slight structural modifications. Image (iii) with 0.5 wt% Au reveals prominent hollow nanostructures with defined porous surfaces due to the Kirkendall effect. Image (iv) with 1 wt% Au shows further hollow structure development and slight aggregation. In the image (v) with 2 wt% Au, more significant aggregation and chunkier formations appear. Finally, image (vi) with 5 wt% Au shows distinct clusters, with nanofibers losing their fibrous nature and demonstrating significant morphological aggregation. This transition from smooth fibers to hollow and aggregated forms illustrates the impact of increasing Au content on SnO₂ nanofibers.

Further surface analysis reveals the inner structure, element distribution, and surface chemistry of the synthesized SnO₂–Au 0.5 wt% hollow nanofibers. A high-resolution TEM image in Fig. 2(a-i) displays porous structures with rough surfaces for the SnO₂–Au hollow nanofibers. This suggests that the SnO₂–Au 0.5 wt% hollow nanofibers provide large surface areas and sufficient gas interaction sites. Fig. 2(a-ii) shows the observed lattice distances of SnO₂ and Au, with the (101) plane of SnO₂ and the (111) plane of Au matching lattice distances of 2.63 nm and 2.36 nm, respectively.³¹ The selected area electron diffraction (SAED) pattern exhibits a polycrystalline structure for the SnO₂–Au 0.5 wt% hollow nanofibers in Fig. S1 (ESI†), displaying several significant ring patterns corresponding to the (101), (110), and (211) planes for SnO₂, and the (111) plane for Au. XRD analysis in Fig. 2(b) shows the crystal structure and phases of the SnO₂–Au hollow nanofibers, with all diffraction peaks in the spectrum corresponding to specific crystal planes of SnO₂ and Au. Specifically, the observed peaks at approximately 26.58°, 33.87°, 37.95°, and 51.77° are indexed to the (110), (101), (200), and (211) planes of the tetragonal crystal structure for SnO₂ (JCPDS card no. 01-071-0652). The two peaks at approximately 38.18° and 44.37° correspond to the (111) and (200) planes for the metallic cubic phase of gold (JCPDS card no. 01-089-3697). This result indicates that the SnO₂–Au hollow nanofibers have been properly synthesized through electrospinning and the subsequent calcination process without impurity phases.³² Fig. 2(c) presents the energy-dispersive X-ray spectroscopy (EDS) element mapping image of the SnO₂–Au 0.5 wt% hollow nanofibers, revealing the even dispersion of Sn, O, and Au elements within the hollow structured nanofibers, along with the uniform distribution of Au nanoparticles on the SnO₂ hollow nanofibers. Additionally, Fig. 2(d) displays the results of an X-ray photoelectron spectroscopy (XPS) survey, investigating the surface chemical state of each element in the SnO₂–Au 0.5 wt% hollow nanofibers. The Sn element exhibits two distinct peaks, Sn 3d_5/2 and Sn 3d_3/2, with binding energies of 486.18 eV and 494.63 eV, respectively, indicating the presence of Sn⁴⁺ in SnO₂.³³ The identified oxygen peak is located at 530.50 eV, consisting of two different inner peaks: 529.98 eV for the absorbed O⁻ species and 531.12 eV for the O²⁻ species. While existing literature suggests that the XPS peaks of Au are located at 83.53 eV and 87.23 eV, our study observes specific peaks for Au 4f_5/2 and Au 4f_7/2 at 83.43 eV and 87.08 eV, respectively.³⁴ These results imply that the synthesized SnO₂–Au 0.5 wt% hollow nanofibers exhibit proper chemical stability without inadequate chemical bindings.

The synthesized SnO₂–Au hollow nanofibers are adopted for gas sensor applications, utilizing their high surface areas, morphologies, and chemical stabilities. These characteristics enable the porous and hollow structured sensing materials to facilitate the diffusion of the target gas, ultimately enhancing the sensing properties while simultaneously reducing response and recovery times.^35,36 The synthesized SnO₂–Au hollow nanofibers can be used for chemiresistive gas sensor applications, leveraging their high surface areas, morphologies, and chemical stabilities. These characteristics enable the porous and hollow structured sensing materials to facilitate the diffusion of the target gas, ultimately enhancing the sensing properties while simultaneously reducing response and recovery times. Additionally, since SnO₂ is widely used for H₂S gas sensing, we aim to design a gas sensor specifically for H₂S detection. Fig. 3(a) illustrates the fabrication of a H₂S gas sensor module with predefined electrode patterns and a ceramic package using SnO₂–Au hollow nanostructures. The H₂S gas sensor modules are verified to confirm the optimal gas sensing response performances with varying compositions of the SnO₂–Au hollow nanostructures and different operating temperature conditions, as shown in Fig. 3(b). As the amount of Au added to the SnO₂ increases, the gas response value generally tends to increase. However, when over 1 wt% of Au is added, the response value decreases. This is likely because the nanostructures cluster together, resulting in a reduced surface area, as observed in the SEM images. Consequently, the most outstanding response value for H₂S gas is obtained with SnO₂–Au 0.5 wt% nanostructures. Fig. S2 (ESI†) exhibits the gas sensing properties with various compositions of the SnO₂–Au hollow nanostructures at different operating temperatures. The H₂S gas response properties of the SnO₂–Au 0.5 wt% gas sensor at various operating temperatures are shown in Fig. 3(c). As the operating temperature increases, the H₂S gas response steadily rises. However, above 200 °C, the gas response slightly declines. Fig. S3 (ESI†) shows gas response trend graphs at different operating temperatures from room temperature to 250 °C for the SnO₂–Au 0.5 wt% gas sensor. The declined sensing response beyond the optimum operating temperature in conventional metal oxide sensors can be attributed to two factors. Firstly, in the n-type material of SnO₂, the thickness of the depletion layer changes, which depends on the carrier concentration of adsorbed oxygen species on the surface.^37,38 Within the temperature range of 100 °C to 300 °C, O²⁻ species convert to O⁻ by consuming an electron: O²⁻ + e⁻ → 2O⁻. As the temperature exceeds 300 °C, O⁻ converts back to O²⁻ by consuming another electron: O⁻ + e⁻ → O²⁻, resulting in a rapid increase in resistance at higher temperatures.^39,40 Secondly, the adsorption/desorption reactions on the active sites are spontaneous and exothermic processes, characterized by negative Gibbs free energy.^39,40 However, if the sensing temperature exceeds the desired range, the desorption of adsorbate gas molecules accelerates without allowing sufficient surface reaction time.³⁹ While this kinetic process ensures faster recovery time, it leads to reduced responses to the target gas due to insufficient reaction time. Noble metal-added metal oxides facilitate easier absorption and diffusion of oxygen species on the surface.^41,42 As a result, an electron-depleted layer forms at the interface, referred to as the spillover effect. This phenomenon enables the rapid diffusion and adsorption of oxygen ions onto the metal oxide surface, significantly enhancing sensor sensitivity and reducing the operating temperature.^41,43 Nevertheless, it is also worth noting that an excessive amount of noble metal content in metal oxide gas sensors can lead to side effects. Excessive noble metals can chemically interact with gases, causing alterations in gas adsorption and reduction characteristics. These combined factors can impact the overall performance of the sensor, compromising its desired sensing properties.⁴⁴ Therefore, the optimum operating temperature for the SnO₂–Au 0.5 wt% gas sensor is 200 °C, at which it exhibits the highest response value of 270.231 (R_a/R_g) for 10 ppm of H₂S gas. The gas sensing mechanism, illustrated as a band diagram of the SnO₂–Au 0.5 wt% gas sensor at an operating temperature of 200 °C in both air and H₂S conditions, is shown in Fig. S4 (ESI†).⁴⁵ Fig. 3(d) indicates gas sensing properties of the SnO₂–Au 0.5 wt% gas sensor with various H₂S gas concentrations. The H₂S gas concentration is designed to gradually decrease from 10 ppm to 0.5 ppm, and it is confirmed that measurements can be accurately taken even at the minimum range of 0.5 ppm. Note that the theoretical limits of detection (LoD) for the SnO₂–Au 0.5 wt% gas sensor can be estimated as approximately 0.3 ppm.⁴⁶ Therefore, the counted LoD is close to its minimum H₂S detectable range of 0.5 ppm. The gas sensing repeatability of the SnO₂–Au 0.5 wt% gas sensor is verified in Fig. 3(e). The H₂S gas sensing properties show good stability during the 15-cycle gas sensing test with a 10 ppm H₂S gas concentration. Fig. S5 (ESI†) also demonstrates the stability of the measurement values. Various gas injection tests, including H₂S (10 ppm), C₂H₅OH (50 ppm), NH₃ (200 ppm), CH₃COCH₃ (20 ppm), and NO₂ (50 ppm), validate the gas selectivity of the SnO₂–Au 0.5 wt% gas sensor at 200 °C in Fig. 3(f). It demonstrates significant gas sensing selectivity of 270.231 (R_a/R_g) against H₂S gas, while other gases show weak activity. Additionally, mixed gas injection tests further indicate the excellent gas selectivity of the SnO₂–Au 0.5 wt% gas sensor in complex environments This test is designed to flow reference gases such as CH₃COCH₃, NO₂, NH₃, and C₂H₅OH in the first stage. Afterward, the gas sensor system injects H₂S gas into the sensor. At the final stage, the mixed gases, containing H₂S and other reference gas are being flowed simultaneously. As depicted in Fig. S6 (ESI†), there are no reactivity signals in the first stage. The gas reaction occurs when the H₂S gas is being injected. Obviously, a similar gas detection signal is observed during both H₂S and other reference gas injection conditions in the final stage. Therefore, the fabricated SnO₂–Au 0.5 wt% gas sensor exhibits outstanding gas sensitivity, selectivity, and repeatability for H₂S gas at 200 °C.

To validate the practical application of the SnO₂–Au 0.5 wt% gas sensor for monitoring food freshness, we implement the sensor in a handheld device. For our study, we use 10 grams of beef as a reference marker to observe and confirm the release trends of H₂S gas, which is indicative of food spoilage. The reference markers are stored separately under two different temperature conditions: room temperature (30 °C) and refrigerator temperature (4 °C) for 72 hours. Fig. 4(a) illustrates the visual states of the reference markers over time under these storage conditions. At 30 °C, the reference marker exhibits a noticeable dark brown discoloration and a dried surface within just 24 hours. In stark contrast, the reference marker stored at 4 °C shows minimal changes in appearance, even after 72 hours. Fig. 4(b) presents the measured H₂S gas responses from the two differently stored reference markers. Consistent with the visual observations, the reference marker kept at 30 °C demonstrates a gradual and steady increase in H₂S gas emission, signifying the progression of beef spoilage. In contrast, the sample stored at 4 °C maintains a stable and low level of H₂S gas, confirming its relatively fresh state. To establish a baseline for the H₂S gas levels, we set the maximum value observed from the sample stored in the refrigerator for 72 hours at approximately 1.05. This value serves as a reference point to differentiate between fresh and spoiled states in practical applications of the handheld device. By utilizing historical food spoilage data, including H₂S emission levels, temperature, and time, we further refine the accuracy of our freshness monitoring device. Through the analysis of this collected data, the device compares real-time H₂S gas readings with historical patterns, providing a more reliable assessment of food quality. Fig. 4(c) illustrates the assembly structure of the handheld device. It consists of the SnO₂–Au gas sensor module, a 3D-printed exterior case, a microcontroller unit (MCU), and other components. (Movie S1, ESI†) A detailed description of the circuit components and design of the handheld device is provided in the Experimental Section. Fig. 4(d) and Movie S2 (ESI†) demonstrate the working scenario of the handheld food freshness device, which detects whether the beef is good or bad. The handheld device, in conjunction with a smartphone application, promptly alerts the status of the food by displaying indicators for the good or bad condition of the target sample on the smartphone.

Conclusions

In this study, we successfully synthesize SnO₂–Au hollow nanofibers using electrospinning and calcination processes, demonstrating large surface areas, favorable morphologies, and high chemical stability. The chemiresistive gas sensing properties of these SnO₂ nanofibers, with various Au contents (0–5 wt%), are evaluated for H₂S detection. Among these, the SnO₂–Au 0.5 wt% hollow nanofiber is identified as the most effective material, exhibiting high sensitivity, selectivity, repeatability, and rapid response/recovery times at an operating temperature of 200 °C. The practical application of the SnO₂–Au 0.5 wt% gas sensor is demonstrated by integrating it into a handheld device designed for food freshness monitoring. This device successfully assesses the freshness of beef stored under different conditions by detecting H₂S emissions, providing real-time feedback through a smartphone app. By offering a reliable, non-invasive, and efficient method for detecting meat spoilage, this innovation could contribute to sustainable food management practices and the reduction of food waste in society.

Experimental section

2.1. Materials

SnCl₄·5H₂O, HAuCl₄·3H₂O, ethanol, N,N-dimethylformamide (DMF), and poly(vinylpyrrolidone) (PVP; M_w = 1300000) were purchased from Sigma-Aldrich, USA. All chemicals were used without further purification.

2.2. Preparations of SnO₂–Au hollow nanofibers

Firstly, we dissolved 0.45 g of Sn and different Au precursors (0, 0.1, 0.5, 1, 2, and 5 wt%) and PVP in 3 mL ethanol and 3 mL DMF with 1 g of DI water. The solution was then stirred at room temperature at 500 rpm for 4 h. Sn and Au precursor/PVP nanofibers synthesized with the solution by electrospinning a constant DC voltage of 19 kV applied between a stainless-steel collector and a syringe needle (27 gauge) with a feeding rate of 0.3 mL h⁻¹. The as-spun Sn and Au precursor/PVP nanofibers were calcined at 600 °C for 2 h at a ramping rate of 3 °C min⁻¹ in air ambient to obtain SnO₂–Au hollow nanofibers followed by Kirkendall effect.

2.3. Material characterizations

The synthesized materials were characterized using scanning electron microscopy (SEM, S-4800, Hitachi High-Technologies, Co., Tokyo, Japan), X-ray diffractometry (XRD, D/MAX-2500-PC, Rigaku International Co., Tokyo, Japan) with Cu Kα X-ray source (λ = 1.5418 Å) at 2θ, high resolution transmission electron microscopy (HR-TEM, JEM-2100 F, JEOL Ltd., Tokyo, Japan), high-angle annular dark-field scanning tunneling electron microscopy (HAADF-STEM), selected-area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (Theta Probe XPS, ThermoFisher Scientific, Waltham, MA, USA) with a base pressure of 4.8 × 10⁻⁹ mbar using a monochromatic Al Kα X-ray source (hν = 1486.6 eV).

2.4. Fabrication of gas sensor module

A 5 × 5 mm² SiO₂/Si wafer with pre-defined interdigitated electrodes (IDEs) was used as the substrate. Each electrode has a length of 3 mm and a width of 20 μm, with a gap distance of 20 μm between the electrodes. Synthesized SnO₂–Au hollow nanofibers were dispersed in ethanol, with 100 mg of nanofibers in 1 mL of ethanol. This suspension was drop-cast onto the electrode substrate, followed by drying at 60 °C to remove the ethanol. The assembled device, including a gas sensor module enclosed in a ceramic package, was then placed into a 3D-printed exterior case (Fig. 4(c)).

2.5. Chemiresistive gas sensor performance measurement

Firstly, 100 mg of SnO₂–Au nanofibers were dispersed in 1000 μL of ethanol. The 3 μL of SnO₂–Au dispersed solution was then drop casted on the prepared SiO₂ sensor substrate with Au electrodes followed by drying of the ethanol solvent. The gas sensing tests were carried out using a company (GMC 1200, ATOVAC, Yongin, Korea) gas sensor measurement system equipped with a data acquisition system (2450 Sourcemeter, Keithley, Solon, OH, USA). The resistance was calculated by the recorded current from applied constant DC voltage of 3 V. The gas concentration was controlled by changing the mixing ratio of nitrogen and H₂S (10 ppm in nitrogen) with fixed oxygen of 21% using mass flow controller (Model 5850E, Brooks Instrument, Hatfield, PA, USA) between 0.5 and 10 ppm. Nitrogen and oxygen were used as the carrier gas at a fixed flow rate of 500 sccm. The operating temperature was controlled from room temperature to 250 °C using a ceramic heater. The gas sensor performance was measured for 600 seconds with dried air and for 200 seconds with the target gas, respectively. A schematic of the gas sensor system is illustrated in Fig. S7 (ESI†). Various analyte gases (CH₃COCH₃, NO₂, C₂H₅OH, and NH₃) were exposed to envisage the gas selectivity of the sensor as a function of different concentration. The response was used to characterize the sensor performance using the equation response = R_a/R_g, where R_a and R_g are the electrical resistance of the sensor under dry air and the concentration of H₂S gas, respectively.

2.6. Preparation of food reference sample

We performed an experiment to assess the fresh beef. We placed two pieces of 10 g beef in a glass container and stored them under two different temperature conditions of 30 °C and 4 °C. We measured the sensing characteristics of each beef sample at four different time points: 0, 24, 48, and 72 hours after storage.

2.7. Hand-held device for food freshness detecting

A hand-held device for food freshness detecting was developed with the proposed H₂S gas sensor. This device consists of air suction fan unit, circuit board with Wi-Fi module (LOLIN D32 V1.0.0 ESP-32 WiFi-Bluetooth), smart phone apps and the gas sensor module. The device was designed with the SnO₂–Au the gas sensor module, ceramic heater, circuit board (with controller unit and commutation module), and a micro fan in its inside. In detail, inlet/outlet hole assemblies played the flow directions for the target gas molecules. Also, the sensor module is comprised of an alumina crucible measuring 1 × 1 × 1 cm³, which houses a circular ceramic heater with a diameter of 5 mm and a sensor element affixed to the ceramic heater. The device is capable of communicating with other devices via Wi-Fi, and can accurately assess the stability of beef within seconds using an app that is directly installed on Android phones. A lithium-ion battery was assembled to provide a power supply.

Author contributions

K. Park developed main idea of research and conducted the experiments. K. Park and J. Kwon wrote the manuscript in consultation with H. Choi. K. Park and J. Kwon revised the manuscript. H. Choi supervised the project and made the hand-held device as the application example of the sensor.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Big Issue Project (PEO24030) from Korea Institute of Industrial Technology and the Industrial Technology Innovation Program (no. 20023014) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea)

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Footnote

† Electronic supplementary information (ESI) available: SAED analysis results of the SnO2–Au 0.5 wt%. (Fig. S1); H2S gas sensing characteristics of SnO2–Au hollow nanostructures with different Au contents ratios. (Fig. S2) Gas sensing characteristics of SnO2–Au 0.5 wt% gas sensor to H2S gas in the range from 500 ppb to 10 ppm with different operating temperature. (Fig. S3) Band diagram of the SnO2–Au 0.5 wt% gas sensor. (Fig. S4); Sensitivity stability over 15 cycles of H2S measurement tests. (Fig. S5) Mixed gas sensing performance of the SnO2–Au 0.5 wt% gas sensor. (Fig. S6) Schematic of the gas sensing systems. (Fig. S7) Performance comparison data of the SnO2-based H2S gas sensors. (Table S1) A showcase of handheld device with its structure. (Movie S1) A working scenario of the handheld food freshness device. (Movie S2). See DOI: https://doi.org/10.1039/d4tc02901a

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