1 Introduction

Sulfur dioxide (SO2) constitutes a significant proportion of harmful air pollutants, and the main sources of SO2 gas include the combustion of sulfur-containing fuels in the manufacturing and construction sectors and volcanic eruptions and forest fires [1, 2]. Additionally, sulfur dioxide not only contributes to acid rain but also has detrimental effects on both human and plant growth [3]. Research has found that the sensitivity to SO2 gas varies significantly among different plants, sensitive plants exposed to an atmosphere with 0.5 ppm of SO2 exhibit chloroplast damage and photosynthesis stagnation after 8 h. When exposed to an atmosphere with 1–4 ppm of SO2, damage occurs within 3 h [4].

Sulfur dioxide gas sensors are wildly applied in many fields, such as comprehensive treatment of environmental pollution [5], monitoring the quality of the healthy environment inspection [6, 7] and improving plant productivity [8]. Conventionally, SO2 concentrations are measured using two optical tracking technologies, infrared radiation spectroscopy or UV absorbance spectroscopy. In the quest to achieve continuous monitoring of harmful pollution, solid-state resistive gas sensors employing semiconducting have been the widest spread in gas-sensing application owing to their compactness and versatility. In particular, the room temperature SO2 sensors based on two-dimensional materials have gradually evolved into the development trend on account of their low power consumption (compared to required high working temperature for most oxide–semiconductor sensors) and small sizes (no integrated limitation for microheater and sensing layer) [9,10,11]. Given lacking supplied energy at high operation temperature to activate the sensing-layer, the performances of room-temperature SO2 sensors are more dependent on the intrinsic activity of sensing materials. Compared with mental oxides semiconductor (MOS) sensors that work at high temperatures, molybdenum disulfide has been extensively studied for its excellent gas-sensing property at the room temperature. For example, previous study has demonstrated that do** transition metal to inner MoS2 supports can markedly improve the SO2 sensing property [12]. Therefore, engineering highly active sensitive materials to meet the requirements of high sensitivity and low limit of detection (LOD) for room-temperature SO2 detection is crucial, yet challenging.

In recent years, single-atom catalysts (SACs) have been considered as potential candidates for high-performance gas sensors due to their maximum utilization efficiency of metal atoms and uniform unsaturated configuration of active centers [13,14,15,16,17]. Generally, the single metal centers (especially for Pd, Pt, Au and so on) are considered as highly active adsorption and reactive sites during the gas-sensing process [18,19,20,21,23, 24]. On the other hand, the metal center can also induce more electron transfer with the target gas molecules to improve the sensing response [25]. However, this sensitization action may result in a major electron concentration localized at these single-metal sites [13, 16, 26, 27], while the participation of support materials is often ignored. Considering the extremely low single metal loadings, activating the main support materials are thus considered as a crucial strategy to further enhance the gas-sensing performance. Meanwhile, the interactive status between single-atom centers and the whole activated supports may trigger the linkage effect to synergistically enhance the gas adsorption and electronic transform process. This route of reaction mechanism is thus apparently different from that of traditional SACs. However, engineering active support-associated SACs and deeply exploring the gas-sensing action are still rarely studies.

Here, we present an ingenious feedback-regulation system by changing the interactional mode between single Pt sites and MoS2 supports for high-efficiency room-temperature SO2 sensing. Firstly, the introduced single Pt atoms can activate the whole S species on supports to form S vacancy-assisted single Pt sites (Pt-Vs) at reductive atmosphere (Fig. 1-①). Reversely, the activated S species can provide a feedback role in tailoring the antibonding-orbital electronic occupancy state of Pt atoms (Fig. 1-②). Detailedly, the assistant sulfur vacancy can elevate the d-band center position of the single Pt sites and reduce the occupation state of the Pt–S antibonding orbital, thus increasing the strength of the Pt–S bond and further improving the SO2 adsorption. Gas adsorption/desorption experiments and in situ gas adsorption breakthrough experiments demonstrate sulfur vacancy-aided Pt1-MoS2 catalysts (Pt1-MoS2-def) exhibit the strongest SO2 gas adsorption capacity. Ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy further experimentally confirm that the intact feedback-regulation system can expand the electron transfer path from single Pt sites to whole Pt1-MoS2 supports in SO2 gas atmosphere. This thus endows Pt1-MoS2-def sensors extremely low limit of detection (500 ppb) and high response sensitivity (3.14% to 500 ppb SO2) at room temperature. At the same time, equipped with Bluetooth intelligent monitoring modules, the Pt1-MoS2-def sensors array can further realize real-time monitoring of SO2 levels and cloud-data transmission/storage for plant growth.

Fig. 1
figure 1

The schematic of the feedback-regulation system in the Pt1-MoS2-def

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2 Experimental Section

2.1 Materials

Chloroplatinic acid (H2PtCl6), ascorbic acid (AA), ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), aniline, l-Cysteine, hydrochloric acid (HCl) and ethanol (C2H5OH) were analytical grade and purchased from Shanghai Chemical Corp. All reagents in the experiment were used without any further purification. All solutions were prepared in deionized water. Gases including hydrogen sulfide (H2S), nitrous oxide (N2O), xylene (C8H10), nitrogen dioxide (NO2), nitric oxide (NO), hydrogen (H2), ammonia (NH3) and sulfur dioxide (SO2) were obtained from Shanghai Shenkai Gases Technology Co., Ltd.

2.2 Preparation of Trimolybdate, MoS2, MoS2-def, Pt1-MoS2, Pt1-MoS2-def and Pt NPs-MoS2

2.2.1 Preparation of Aniline Trimolybdate (MoO 3 -ANE)

The MoO3-ANE was synthesized following a previous report [28] with minor modification. First, 2.48 g of (NH4)6Mo7O24·4H2O was dissolved in 50 mL of deionized water and 3.27 g of aniline was added. Then, 1 M HCl aqueous solution was added dropwise with magnetic stirring at room temperature until a white precipitate appeared (pH 4–5). After a reaction at 50 °C for 2 h, the product was filtered and thoroughly washed with ethanol, and then dried at 70 °C.

2.2.2 Preparation of Flower-Shaped MoS 2

The obtained MoO3-ANE (0.1 g) was dispersed in a homogenous solution containing deionized water (15 mL) and l-Cysteine (0.2836 g). The dispersion solution was subsequently transferred to 50 mL Teflon-lined stainless-steel autoclaves and maintained at 200 °C for 14 h. The obtained samples were rinsed with ethanol and water and oven-dried at 70 °C for one day. Then, as-prepared powder was calcined in a tube furnace at 400 °C for 1 h at the heating rate of 2 °C min−1 under N2 atmosphere.

2.2.3 Synthesis of MoS 2 -def

The obtained black powder of MoS2 (200 mg) was treated in a tube furnace at 100 °C for 1 h under the 5% H2/Ar2 atmosphere at the heating rate of 2 °C min−1. The final product MoS2-def was collected.

2.2.4 Synthesis of Pt 1 -MoS 2 , Pt 1 -MoS 2 -def and Pt NPs-MoS 2

The obtained black powder of MoS2 (200 mg) was dispersed in aqueous solution of deionized water (25 mL) and ethanol (25 mL). An aqueous solution of chloroplatinic acid (10 mg mL−1, 0.2 mL) was subsequently added into the dispersion and stirred for 24 h at room temperature. The precipitate was separated, washed and dried under vacuum at 70 °C for 24 h. The as-above precipitate was treated in a tube furnace at 100 °C for 1 h under the 5% H2/Ar atmosphere at the heating rate of 2 °C min−1. The final product Pt1-MoS2-def was collected. Pt1-MoS2 was prepared with the same synthesis procedure of Pt1-MoS2-def except the tube furnace was not treated. The decoration of Pt (0.1 wt%) NPs was achieved through in situ reduction method. 200 mg as-prepared MoS2 powder were subsequently dispersed in 25 mL ethanol and 25 mL deionized water. An aqueous solution of chloroplatinic acid (10 mg mL−1, 30 μL) and 1 mL AA solution (0.1 M) were subsequently added into the dispersion and stirred for 24 h at room temperature. The product was precipitated by centrifugation and dried at 70 °C for overnight.

2.3 Characterizations

X-ray diffraction patterns (XRDs) were collected by using powder XRD (Rigaku D/MAX-2500 X-ray diffractometer, with Cu Kα1 radiation λ = 0.154056 nm 40 kV and 40 mA, scanning from 10° to 70°). The microtopography of the synthesized materials was recorded with scanning electron microscopy (SEM, JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F, Japan). The high-resolution TEM and high-angle annular dark-field scanning TEM (HAADF-STEM) images were recorded on an FEI Tecnai G2 F20 S-Twin high-resolution transmission electron microscope (HRTEM; Hillsboro, OR, United States) set at 200 kV, and a JEOL JEM-ARM300F TEM/STEM (Tokyo, Japan), respectively, with a spherical aberration corrector worked at 300 kV. Through-focal HAADF series were acquired at nanometer intervals, with the first image under-focused (beyond the beam exit surface) and the final image over-focused (before the beam entrance surface). Then, the images were aligned manually to remove the sample drift effects. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker CW Elexsys E500 spectrometer by applying an X-band (9.4 GHz) microwave with swee** magnetic field at room temperature.

3 Results and Discussion

3.1 Construction and Characterization of Pt1-MoS2 and Pt1-MoS2-def

The MoS2 were firstly synthesized by an anion-exchange strategy by using MoO3-ANE (aniline-functional MoO3) as precursors (Fig. S1). The SEM, TEM and HRTEM images in Fig. S2 reveal a uniform hierarchical nanosheet configuration with a well-ordered lattice fringe spacing of 0.27 nm, which agrees well with the (100) facet of hexagonal molybdenum disulfide. Subsequently, the Pt species were introduced through a deposition–precipitation process. As seen in Fig. S3, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image shows the atomic dispersion of Pt species over the Pt1-MoS2 surface and no notable Pt clusters are observed. Driven by thermal treatment of Pt1-MoS2 in a reductive atmosphere, Pt1-MoS2-def catalyst was synthesized. No remarkable architectonic changes and Pt nanoparticles are found (Fig. S4a, b). Meanwhile, the energy-dispersive X-ray spectroscopy (EDS) map** (Fig. S4c) further demonstrates the homogeneous distribution of Pt, Mo and S elements. Furthermore, HAADF-STEM (Fig. 2a) clearly shows that the individual Pt atoms atomically disperse over the whole support surface without agglomerate, which is further confirmed by the extractive X–Y intensity profiles and atom-overlap** Gaussian-function fitting images of selective area scan (Fig. 2b–d). The simulating configuration (Fig. 2f) clearly exhibits the distribution of single dispersed Pt atoms, it is in accordance with the STEM image (Fig. 2e). For comparison, Pt nanoparticle modified MoS2 (Pt NPs-MoS2) was also synthesized through the ascorbic acid reduction in Fig. S5. More representative SEM, TEM, XRD and BET images of Pt1-MoS2, Pt1-MoS2-def, Pt NPs-MoS2 are displayed in Figs. S6-S9. Moreover, the Pt loading contents of Pt1-MoS2-def and Pt NPs-MoS2 are measured as 0.1% by inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S1).

Fig. 2
figure 2

Characterization of Pt1-MoS2 and Pt1-MoS2-def. a HAADF-STEM image of Pt1-MoS2-def. b The intensity profiles obtained in regions X–Y in (a). c, d The atom-overlap** Gaussian-function fitting images of selective area scan (measured from white frame of (a). e, f HAADF-STEM image of Pt1-MoS2-def and its simulated image. g Pt L-edge XANES spectra. h FT-EXAFS spectra and i corresponding fitting curve of Pt L3 edge. j Wavelet transform patterns of Pt foil, PtS2, Pt1-MoS2 and Pt1-MoS2-def

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Element-selective X-ray absorption fine structure (XAFS) was further performed to explore the coordination environment of Pt species. As seen in Fig. 2g, the X-ray absorption near edge structure (XANES) spectra reveal the white line intensities of Pt1-MoS2-def and Pt1-MoS2 peaks are located between the PtS2 and Pt foil, suggesting the oxidation state of Ptδ+ (0 < δ < 4). The Fourier transferred extended X-ray absorption fine structure (FT-EXAFS) in Fig. 2h displays two notable peaks at ~ 1.90 Å, which are contributed by the Pt–S coordination path. No Pt–Pt characteristic peak (at ~ 2.57 Å) is detected for both Pt1-MoS2-def and Pt1-MoS2 samples, confirming the atomic dispersion of Pt on the supports. Moreover, the first-shell EXAFS fitting curves (Fig. 2i) and corresponding fitting results (Fig. S10 and Table S2) validate that the central Pt atom in both Pt1-MoS2 and Pt1-MoS2-def samples directly coordinates with three S atoms with the average bond lengths of 2.32 and 2.33 Å, respectively. Moreover, the wavelet transforms spectra of Pt EXAFS oscillations in Fig. 2j display only one peak at 7 Å−1 for Pt1-MoS2-def and Pt1-MoS2 samples, regarded as the Pt–S coordination contribution, indicating the high-distributed status of single Pt species.

3.2 Feedback-regulation System of Pt1-MoS2-def

The first-principal calculations were further conducted to verify the Pt-site configuration on the support. Figure 3a simulates the diffusion path of the Pt atom on the MoS2 surface, including ①Pt on the S site, ②Pt on the hollow site and ③Pt on the Mo site. The results of the DFT calculation (Fig. S11) show that the Pt atom can stabilize on the top of the Mo site with the lowest binding energy of − 3.265 eV in contrast to − 2.764 eV (Pt on the S site) and 6.024 eV (Pt on the hollow site). Also, this configuration of Pt atom bonding with three S atoms is consistent with the XAFS fitting results. After further treating the Pt1-MoS2 in a reduced atmosphere, the S atoms will spontaneously escape from the MoS2 surface to generate sulfur vacancy sites (Pt1-MoS2-def). This process will induce five potential configurations in Fig. S12. Considering the adjudication of experimental investigations (detailed descriptions in supporting information), the defect 2 model (adjacent non-bonding S defect) was selected as the optimal configuration with the lowest vacancy formation energy of − 1.876 eV. This spontaneous reaction process (Fig. S13) indicates that the introduced single Pt atom can promote the evaporation of adjacent S species. Importantly, this three-coordinated structure of Pt1-MoS2-def is also well conforming by XAFS results. Furthermore, the theoretical deduction can be verified by the experimental electron paramagnetic resonance (EPR) analysis. As shown in (Fig. 3b), the signal intensity of sulfur vacancies (at g = 2.003) of Pt1-MoS2-def is significantly higher than that of MoS2-def [29] (more synthetic process showed in supporting information), demonstrating single Pt atoms can induce the formation of sulfur vacancies and thus activate the whole S plane.

Fig. 3
figure 3

Structural characterizations of Pt1-MoS2-def and reference materials. a The diffusion path of Pt atoms on MoS2-def surface and diagram of S vacancy formation energy. b Electron-paramagnetic resonance images of MoS2, MoS2-def, Pt1-MoS2 and Pt1-MoS2-def. c Mo 3d XPS of MoS2, Pt1-MoS2 and Pt1-MoS2-def. d S 2p XPS of MoS2, Pt1-MoS2 and Pt1-MoS2-def. e Pt 4f XPS of Pt1-MoS2 and Pt1-MoS2-def. f The planar-averaged electron density difference ∆ρ(z) of Pt1-MoS2 and Pt1-MoS2-def

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The XPS measurements are employed to unveil the chemical environment of Mo (Fig. 3c), S (Fig. 3d) and Pt (Fig. 3e) species. As shown in Fig. 3c, the peaks at 231.9 eV (Mo 3d3/2) and 228.7 eV (Mo 3d1/2) are attributed to Mo4+ and the peaks at 233.9 eV (Mo 3d3/2) and 230.7 eV (Mo 3d1/2) are assigned to Mo5+ [30], respectively. After introducing the Pt species, the doublet peaks of both Mo4+ (232.1 and 228.9 eV) and S2− (161.9 and 163.2 eV) in the Pt1-MoS2 shift to higher binding energy (0.2 and 0.3 eV left shifts, respectively) compared with pure MoS2. Meanwhile, the binding energy of Pt 4f peak is higher than Pt0 (71.2 eV, Fig. S14) but lower than that of Pt4+ (74.0 eV) [26] according to the XAFS results. For Pt1-MoS2-def catalysts, the double peaks of Mo5+ (233.9 and 230.7 eV) and the high valence Sn+ peaks (164.5 and 169.2 eV) gradually disappear. The Pt peaks in Pt1-MoS2-def (72.8 eV) shift toward lower binding energy (0.3 eV) in contrast to the Pt1-MoS2 sample. The theoretical planar-averaged electron density difference ∆ρ(z) and Bader charge transfer in Fig. 3f further verify that the Pt atom in the as-established Pt1-MoS2-def model can reduce the electronic loss toward supports, according well to experimental XPS investigation. Both experimental and theoretical results reveal that activated S species will provide a feedback role in tailoring the electronic states of Pt atoms. In conclusion, the single Pt sites on the MoS2 surface can induce easier volatilization of adjacent S species and the activated S species can inversely impact the electronic configuration of Pt atoms, thus leading an intact feedback-regulation system.

3.3 Sensing Properties and Mechanism of MoS2, Pt1-MoS2 and Pt1-MoS2-def

The sensing properties of the MoS2, Pt1-MoS2 and Pt1-MoS2-def MEMS sensors were investigated at room temperature (RT). The dynamic response transitions curves of the sensors along with SO2 concentrations in the range of 0.5–40 ppm are exhibited in Fig. 4a, b. Obviously, the Pt1-MoS2-def sensor displays the highest response values toward different concentrations of SO2 compared with Pt1-MoS2 and MoS2 sensors. Some baseline drift of the three sensors at room temperature could be attributed to the incomplete desorption of the SO2 molecule [31]. Especially, the Pt1-MoS2-def sensors (25% to 5 ppm SO2) exhibit an enhanced response approximately four times and five times higher than that of Pt1-MoS2 (7% to 5 ppm SO2) sensors and MoS2 sensors (5% to 5 ppm SO2), respectively. Figure 4c shows the linear relationships between responses and SO2 concentrations within 0.5–40 ppm for three sensors. Impressively, the Pt1-MoS2-def sensors exhibit an exceptional linear section with increased SO2 concentrations, while the MoS2 sensors endure the erratic response-concentration relationship. Additionally, the experimental limit of detection (LOD) of the Pt1-MoS2-def sensor can reach as low as 500 ppb, whereas the Pt1-MoS2 sensors and MoS2 sensors cannot perceive SO2 gas below 1 ppm. Such a distinguished response performance exceeds that of Pt NPs-MoS2 sensors (Fig. S15) and most reported SO2 sensors (Fig. 4d and Table S4). More dynamic response-recovery resistance curves of MoS2, Pt1-MoS2 and Pt1-MoS2-def sensors are shown in Fig. S16. As shown in Fig. 4e, eight kinds of typical vaporous molecules including sulfur dioxide (SO2), ammonia (NH3), hydrogen (H2), sulfuretted hydrogen (H2S), nitric oxide (NO), xylene (C8H10), nitrogen dioxide (NO2) and nitrous oxide (N2O) were detected at the same concentration of 5 ppm at RT. It is found that the responses of interfering gases are significantly negligible. Furthermore, the calculated selectivity coefficients (K) of all sensors (Fig. S17) reveal relatively high K values, indicating excellent SO2 selectivity for gas-sensing performances. The adsorption (kads) and desorption (kdes) rate constants of all sensors (Fig. 4f) were further calculated by fitting the response versus time curves (Fig. S18) using the equations below:

$$R\left(t\right) \text{ for} \, {\text{SO}}_{2} \, {\text{adsorption}} \text{ = }{R}_{{\text{max}}\bullet }\frac{{C}_{a}K}{1+{KC}_{a}}\left(1-{e}^{\left[-\frac{1-{KC}_{g}}{K}\bullet {k}_{ads}t\right]}\right)$$
(1)
$$R\left(t\right) \text{ for} \, {\text{SO}}_{2} \, \text{desorption } = {R}_{0}{e}^{\left[-{k}_{des}t\right]}$$
(2)

where Rmax is the maximum response value, R0 is the response in air, Ca is the concentration of SO2 gas, t is the time and K is the equilibrium constant (kads/kdes) [32]. The results indicate that the kinetic adsorption and desorption of SO2 molecules are promoted by Pt species and sulfur vacancies. In addition, the cycle stability (Fig. 4g) and long-term durability measurements (Fig. S19) show that the Pt1-MoS2-def sensor can remain almost unchanged responses during continuous eight-cyclical exposure of SO2 and superior stability for up to 35 days. The effects of operating temperature for Pt1-MoS2-def sensor were conducted in Fig. S21, in which the sensors can realize optimal gas-sensing performance at room-temperature conditions. Besides, the resistances of the Pt1-MoS2-def sensor may be affected by the humidity environment (Fig. S22), which is mainly because water molecules might react with SO2 molecules. A water filtration membrane to filter out water molecules was added on the device (Fig. S23a). As shown in Fig. S23b, c, after adjusting the device, the response of the material to 25 ppm SO2 under different humidity conditions were tested. It was observed that under 90% humidity, the sensor’s response decreased by no more than 9% (before the improvement, the sensor’s response decreased by about 28%), effectively improving the normal operation of the device in high humidity environments.

Fig. 4
figure 4

Gas-sensing performance of MoS2, Pt1-MoS2 and Pt1-MoS2-def. a, b Time-related dynamic responses in the concentration range of 0.5–5 and 20–40 ppm. c The responses of MoS2, Pt1-MoS2 and Pt1-MoS2-def in different SO2 concentrations. d RT operation state-of-the-art SO2 chemiresistors for response. e The selectivity of MoS2, Pt1-MoS2 and Pt1-MoS2-def in different gases. f Adsorption and desorption rate constant Kads and Kdes values. g stability tests using sensors upon 8 cyclic exposures to 25 ppm SO2. (All the sensing tests were conducted at RT in (30%RH) condition)

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To further clear the design superiority of the Pt1-MoS2-def catalysts in the gas-sensing reaction, SO2 adsorption and desorption tests were implemented (experimental details are shown in Methods and Table S3). As Fig. 5a–c shows, the Pt1-MoS2-def reveals the maximum adsorption capacity (2.34 wt%) and the fastest average adsorption rate (0.52 wt% s−1) for SO2 compared with MoS2 (1.32 wt%, 0.18 wt% s−1), Pt1-MoS2 (1.62 wt%, 0.41 wt% s−1) and Pt NPs-MoS2 (1.75 wt%, 0.44 wt% s−1) at room temperature, indicating the S vacancy-assisted single Pt sites (Pt-Vs) can induce superior adsorption and excitation ability for sulfur dioxide. Ex situ XPS spectra were further employed to investigate the reaction mechanism of different catalysts. As seen in Fig. 5d, the peaks of Pt4+ in Pt1-MoS2 suddenly appear in the SO2 atmosphere while no obvious new peaks are found in the Mo and S species, confirming the electron transfer path is from the single Pt site to adsorbed SO2 molecule. However, a small number of surface-adsorbed SO2 molecules are not distinguished by the XPS detection. Same results can also be found in MoS2-def catalysts in Fig. S24. Considering the resistances of MoS2-based sensors significantly reduce in the oxidizing SO2 atmosphere (Fig. S25), the sensing type can be identified as p-type semiconductors. Detailly, the SO2 species can capture electrons from the conduction band of p-type semiconductors and induce the hole generation, resulting in decreased resistances (Fig. S26). For the Pt1-MoS2-def catalyst (Fig. 5e), two new higher-valence S peaks (168.83 and 170.1 eV, S6+) and Pt4+ peaks are discovered after SO2 treatment while there is a slight high binding energy shift in the Mo 3d spectrum, revealing that both Pt and MoS2 supports lose electrons during the gas-sensing process. The in situ Raman spectra display the 150 s exposure periods of Pt1-MoS2-def, Pt1-MoS2 (Fig. 5f) and MoS2-def (Fig. S27) after treating with 1000 ppm SO2. Clearly, the A1g peak (at 402.25 cm−1) of the Pt1-MoS2-def sample shifts to a higher wavenumber after SO2 exposure for 60 s, while no peak changes for the Pt1-MoS2 and MoS2-def samples. Therefore, this unequivocally corroborates that the electronic transform process in Pt1-MoS2-def involves the adsorbed SO2 molecules and the whole supports. In conclusion, experimental results demonstrate there are two potential reaction paths dominant to the SO2 sensing performance for Pt1-MoS2-def and Pt1-MoS2 sensors. For Pt1-MoS2 (Fig. 5g, type I), the electrons mainly locate around the single Pt site and the electron transfer only occurs within Pt–S (SO2). In comparison, the Pt1-MoS2-def sensors contain synergistically single Pt sites and activated inert S plane, which prompts the whole supporting surface to participate in the SO2 sensing process (Fig. 5g, type II). Meanwhile, the rapid electron transfer between Pt1-MoS2-def and the oxidizing SO2 guarantees high response and low LOD. In addition, the in situ SO2 adsorption breakthrough curves (Fig. 5h) were implemented to quantitatively measure the absorption of SO2, in which the adsorbed SO2 amounts on Pt1-MoS2-def is 281 μmol g−1 more than that on Pt1-MoS2, and there is 292 μmol g−1 more than that on MoS2-def at 25 °C, according well with the SO2 adsorption and desorption tests in Fig. 5a–c.

Fig. 5
figure 5

The experimental investigation of the mechanism for the gas-sensing property of Pt1-MoS2-def. a The SO2 adsorption curves, b desorption curves and c SO2 adsorption rate of MoS2, Pt1-MoS2, Pt NPs-MoS2 and Pt1-MoS2-def. The ex situ XPS spectra of d Pt1-MoS2 and e Pt1-MoS2-def. f In situ Raman spectra of the Pt1-MoS2 and Pt1-MoS2-def. g Two different path types of Pt1-MoS2 and Pt1-MoS2-def with SO2. h In-situ SO2 adsorption breakthrough curves of MoS2, Pt1-MoS2 and Pt1-MoS2-def at 25 °C

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3.4 Theoretical Investigations

DFT calculations were conducted to explore the structure–activity relationships between single Pt sites and gas-sensing performance. As seen in Fig. 6a, the Pt 4d partial DOS (pDOS) results show that the d-band center in Pt1-MoS2-def (-2.58 eV) obviously upshifts compared with the Pt1-MoS2 (− 4.10 eV). The corresponding DOS images are displayed in Fig. S28. When interacting with SO2 molecule, the Pt–S (SO2) projects crystal orbital Hamilton population (pCOHP) is calculated in Fig. 6b, where the negative –pCOHP represents the antibonding contribution and the positive –COHP stands for bonding contribution [33,34,35]. Obviously, the antibonding contribution gradually moves upward (compared to Ef) from Pt1-MoS2-def to Pt1-MoS2, proving a decreased occupation state and a strengthening Pt–S bond. The –IpCOHP (the integral of –COHP up to the Fermi level) values are further calculated in Fig. 6c. The antibonding orbital occupation state of the Pt–S bond in Pt1-MoS2-def is smaller than that in Pt1-MoS2, indicating more stable adsorption of SO2 for Pt1-MoS2-def, which is conformed to the d-band center theory. In addition, the density of states (DOS) analysis in Fig. 6d displays that a new peak around Fermi energy level could be observed in SO2/Pt1-MoS2-def, suggesting more electrons transfer between SO2 and Pt1-MoS2-def compared with SO2/Pt1-MoS2 and SO2/MoS2 models. This can be further verified by the differential charge density comparisons in Fig. 6e, f. Also, the Pt1-MoS2-def reveals larger the adsorption energy (Eads) of -2.40 eV than that of the Pt1-MoS2 (− 1.17 eV) and MoS2 (− 0.035 eV) upon interaction with SO2 molecules (Fig. S29), according well with gas-sensing performance. Overall, utilizing Pt species to promote adjacent S evaporation, the electronic state of S vacancy-assisted single Pt atom pronouncedly changes and thus induces a gradually decreased antibonding contribution state of Pt–S and a stronger SO2 adsorption. Hence, the establish of feedback-regulation system is crucial for SO2-sensing material with high response and low LOD (Fig. 6g).

Fig. 6
figure 6

The theoretical investigation of the mechanism for the gas-sensing property of Pt1-MoS2-def. a PDOS for Pt 4d orbital and the d band center of Pt1-MoS2 and Pt1-MoS2-def. b pCOHP and c ICOHP of terminal Pt–S bonds of Pt1-MoS2 and Pt1-MoS2-def. d The partial density of states (pDOS) analysis and adsorption energy of SO2/Pt1-MoS2-def, SO2/Pt1-MoS2 and SO2/MoS2 models. The electronic transfer between SO2 and e Pt1-MoS2-def and f Pt1-MoS2 sample. The yellow and blue lobes represent the accumulation and depletion of charge. g Mechanism for tailoring the level of antibonding-orbital occupancy state to unidirectionally determined the gas-sensing property of SO2

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3.5 Bluetooth-Based Devices for Monitoring the Environment of Plant Growth

As we all known, the real-time monitor, track and prediction of stressful circumstances (such as SO2) is of vital importance to optimize and adjust plant growth [8]. For instance, an appropriate SO2 environment can promote seed germination and stomatal movement, while excessive SO2 concentrations will disturb the physiological and biochemical metabolism [36,37,38]. Therefore, engineering multifunctional SO2 sensors array to timely collect and transmit the gas/temperature/humidity signals is desired for plant cultivation process (Fig. 7a). Based on the superior SO2-sensing materials of Pt1-MoS2-def, a miniaturized, integrated and wireless sensing SO2 sensors device is further developed in Fig. 7b, c. The disk-shaped device is composed of Bluetooth, Active Front End (AFE) and Micro Controller Unit (MCU) on one side, while a MEMS array (containing six sensors) and temperature/humidity modules are embedded at the back. Wireless data transmission from devices to smartphones can be achieved through the Bluetooth system. The corresponding block diagram is presented in Fig. 7d, including the power delivery, data conditioning and data transmission pathways. Impressively, synchronous six sensing signals can be received and revealed on the screen to calibrate the errors. More details of printed circuit board and the main interfaces of the custom mobile application device are shown in Figs. S30–S33. Furthermore, the portable sensing device displays constant responses for 5 and 10 ppm SO2 gases (Fig. 7e) in a realistic greenhouse environment. More gas-sensing performance tests are shown in Fig. S34. Notably, the current design still stays in a proof-of-concept stage, it may require more optimizing process including signal handling, data integration, intellectual detection and so on in future work.

Fig. 7
figure 7

Pt1-MoS2-def-based SO2 gas sensors for monitoring the plants growth. a Schematic of real-time detection of SO2 gas concentration in a greenhouse. b Photo of the device in a simulated greenhouse with plants, with a smartphone and Bluetooth and data transmission. c Corresponding block modules. d Block diagram of the whole system. e Data measured by this device

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4 Conclusions

In summary, by designing the feedback-regulation system in Pt1-MoS2-def sensing materials, the superior SO2 wireless detection sensors can be realized. When introducing the single Pt atom to MoS2 surface, the adjacent S species is inclined to evaporate to form S vacancy-assisted single Pt sites. Reversely, the electronic state of Pt atom pronouncedly changes by the activated S planes. After treating with SO2 gas, the antibonding contribution states of Pt–S gradually decreased and thus induce a stronger SO2 adsorption. This result can further be confirmed by the gas adsorption/desorption experiments and in situ gas adsorption breakthrough experiments. The final Pt1-MoS2-def sensors show extremely high performance for low-concentration SO2 gas sensing at room temperature (3.14% to 500 ppb). Combined with a Bluetooth transmission system, the Pt1-MoS2-def sensor arrays can further achieve real-time SO2 monitoring and data transmission under real conditions for plant growth. Our work is thus expected to broaden the rational design of highly effective gas sensors.