Room-temp trace ammonia detection using SnS₂ nanosheets@PANI composite: a step towards ultrasensitive environment monitoring and non-invasive renal disease diagnosis

Abstract

Selective and rapid detection of ammonia (NH₃) gas over a wide concentration range is essential for applications such as early diagnosis of renal diseases and environmental safety. NH₃ in exhaled breath serves as a biomarker of kidney function, and its precise detection is vital for early renal disease diagnosis. This work reports a SnS₂/PANI heterojunction nanocomposite (SPA) sensor synthesized via a hydrothermal route followed by in situ oxidative polymerization. The layered SnS₂ nanosheets were surface-modified with sodium dodecyl sulfate (SDS) to promote Na⁺ ion intercalation, thereby enhancing exfoliation and reducing agglomeration, as confirmed by FESEM, TEM, Raman, and UV-VIS spectroscopy. Among all nanocompositions, the SPA75 sensor exhibits the best performance, detecting NH₃ from 0.5 to 150 ppm at 30 °C and 69% RH, with a maximum relative response of 213% at 150 ppm NH₃, a limit of detection of 0.5 ppm, and ultrafast response/recovery times of 18/64 s. The SPA75 sensor also delineates promising results towards healthy and simulated breath. The enhanced sensing performance is attributed to efficient charge transfer at the heterojunction interface. In addition, the large surface area and mesoporous nature of SPA75 (37.475 m² g⁻¹) facilitate gas adsorption and diffusion, further contributing to its high efficiency. These results establish SPA75 as a highly efficient material for ultrasensitive NH₃ detection in environmental monitoring and real-time exhaled breath analysis for renal disease diagnosis.

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1. Introduction

With the rapid pace of industrialization and urban development, there is an increasing demand for high-performance, portable gas sensors in diverse sectors such as environmental monitoring, industrial safety, healthcare, and public protection, especially for detecting hazardous gases like NH₃.¹ NH₃ is a colorless gas with a pungent odor and plays a critical role in the formation of particulate matter (PM2.5) in polluted environments. It is highly corrosive and poses significant health risks to the eyes, skin, respiratory system, liver, and kidneys at elevated concentrations. NH₃ is widely used across various industries, including fertilizer production, textiles (such as nylon manufacturing), petroleum refining (for neutralizing acidic by-products), rubber processing (to prevent latex coagulation), and the production of explosives like trinitrotoluene (TNT) and nitroglycerin. It also serves as a coolant in refrigeration and air conditioning systems. Due to its extensive industrial applications, accidental releases of NH₃ represent a serious safety and environmental hazard.² Consequently, the development of sensitive, selective, and reliable NH₃ sensors is crucial for early detection and effective risk mitigation in both industrial and environmental domains. NH₃ readily hydrolyzes into ammonium hydroxide upon absorbing moisture through the mucous membranes of the respiratory tract, leading to potential health hazards. Prolonged exposure to ammonia can cause chronic respiratory issues and damage to lung tissue.^3,4 According to the Occupational Safety and Health Administration (OSHA), the permissible exposure limit for NH₃ in workplace environments is below 50 ppm, with a long-term exposure limit set at 25 ppm for indoor settings. Given this low safety threshold, there is an urgent need for sensitive and selective sensors capable of detecting NH₃ across a broad concentration range.^5,6

Beyond its industrial relevance, NH₃ has recently gained significant attention as a noninvasive biomarker for diagnosing kidney- and liver-related disorders, such as hyperammonemia, hepatic encephalopathy, and chronic kidney disease (CKD). The global rise in CKD is further exacerbated by lifestyle factors, with sugar-sweetened beverages identified as a major contributor to kidney damage and increased mortality.⁷ It is found that globally, an estimated 850 million people are living with some form of kidney disease, primarily CKD. This figure represents over 10% of the world's population, which is a matter of great concern. It's a serious health issue, with a growing number of associated deaths. In this context, exhaled breath analysis has emerged as a promising diagnostic tool owing to its noninvasive, rapid, safe, and cost-effective nature, enabling real-time health assessment. In healthy individuals, the concentration of NH₃ in exhaled breath typically ranges from 0.25 to 1.8 ppm, whereas patients with renal dysfunction exhibit significantly elevated levels, reaching up to 4880 ppb in end-stage renal disease (ESRD).^8,9 Therefore, the development of reliable and highly responsive NH₃ sensors is of critical importance not only for industrial and environmental monitoring but also for medical diagnostics, particularly in the early detection of metabolic and organ-related diseases.

In recent years, nanocomposite-based sensors, including those utilizing conventional polymers, nanopolymers, and hybrid materials, have garnered significant attention due to their large surface area, tunable physicochemical properties, and improved gas adsorption capabilities. These characteristics enable nanocomposite sensors to achieve superior sensitivity and rapid response compared to bulk materials.¹⁰ Considerable progress has been made in developing nanoparticle-based gas sensors using a broad range of materials, such as metal oxides Metal oxides (e.g., MoO₃¹¹ SnO₂,¹² NiO, and WO₃¹³), conducting polymers such as polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), and PEDOT:PSS,^14–17 carbon-based nanomaterials including carbon nanotubes¹⁸ and graphene,¹⁹ as well as other two-dimensional materials such as MoS₂,²⁰ SnS₂,^21,22 WS₂,²³ MoSe₂²⁴ have been extensively investigated for sensing applications. Among these, metal oxide-based sensors have demonstrated excellent performance in detecting a broad range of gases with high sensitivity. However, their application is often limited by the need for high operating temperatures (200–450 °C) and, lack of selectivity.²⁵

In contrast, conducting polymers can operate efficiently at room temperature owing to their unique redox behavior, ease of functionalization, and superior mechanical flexibility.²⁶ However, intrinsic conducting polymers still suffer from limitations such as low sensitivity, slow response and recovery times, and inadequate long-term stability.²⁷ Although two-dimensional layered materials (2DLMs) such as MXenes, borophene, graphene, and phosphorene exhibit the key advantage of low-temperature gas sensing due to their tunable electronic structures, making them attractive for low-power and portable applications, their high synthesis cost and poor long-term stability under ambient conditions hinder their commercial translation.^28–30 Specifically, van der Waals heterojunctions in 2D-based gas sensors (whether n–n, p–n, or p–p configurations) significantly enhance performance, as both the intrinsic properties of the constituent materials and the characteristics of the depletion region at the interface can be precisely tuned. By strategically selecting 2D materials with different band structures, carrier concentrations, and work functions, the interfacial band alignment can be precisely engineered to improve selectivity and sensing efficiency.^31,32

Considering these advantages, recent studies have significantly broadened the range of 2D materials employed in NH₃ sensing applications, including polyaniline (PANI), polypyrrole (PPy), MXenes, borophene, and others. For instance, Liu et al. developed a PANI/MoS₂/SnO₂ (PMS) composite sensor, which exhibited a notable response of 10.9 to 100 ppm of NH₃ at room temperature, along with a low detection limit of 200 ppb.³³ Tian et al. developed a Pt/MoS₂/PANI flexible sensor that showed a significantly enhanced NH₃ response at room temperature, with a detection limit of 250 ppb. The improved performance was attributed to the p–n heterojunction and Schottky barrier formed between the components.³⁴ Li et al. fabricated an MXene/SnS₂ heterojunction sensor capable of detecting sub-ppm levels of NH₃ at room temperature, demonstrating a remarkable detection limit as low as 10 ppb for NH₃.³⁵ Tang et al. developed a 1D/2D heterostructure-based sensor WS₂/PANI composite, which exhibited excellent reproducibility and long-term stability. The sensor achieved a high response of 216.3% and rapid response/recovery times of 25 s/39 s for 100 ppm NH₃³⁶. Although these studies highlight promising strategies for NH₃ detection, several critical challenges still need to be addressed, such as cross-sensitivity, response/recovery time, stability, and resistance to humidity. In our work, these limitations have been mitigated by developing an easy-to-synthesize, cost-effective sensor with humidity-resilient stability and rapid response/recovery.

Herein, a surface modification technique was employed using as-synthesized PANI/SnS₂ nanocomposites, which contained varying weight ratios of layered SnS₂ nanosheets. These nanocomposites were synthesized through the in situ oxidative polymerization of aniline onto SnS₂ nanosheets. Prior to polymerization, SDS was used for the intercalation of Na⁺ ions into the SnS₂ layers, enhancing dispersion and reducing agglomeration. The resulting SnS₂/PANI nanocomposite selectively detects NH₃ within the concentration range of 0.5–150 ppm. Pure PANI and pristine SnS₂ nanosheets were also synthesized under similar conditions, and the performance of the PANI/SnS₂ composites was compared to that of pure PANI and pristine SnS₂. The enhanced sensing performance of the SnS₂/PANI composite underscores its potential for NH₃ detection in both environmental monitoring and non-invasive diagnosis of renal disease through exhaled breath analysis. Table 1 summarizes the NH₃ sensing performance of various chemiresistive sensors under ambient conditions.

Table 1 Comparison of the NH₃-sensing performance under ambient conditions across various chemiresistive sensors

Sensing materials	Testing condition (temp and %RH)	Conc. NH3 (ppm)	Response (%)	T res/Trec (s)	Ref.
Ti3C2Tx/SnS2	RT, 26%RH	10	42.9%	161/180	35
WS2/PANI	RT	100	216.3%	25/39	36
PANI/SnS2	RT, 50%RH	30	128%	58/421	37
PANI NF/WS2 composites	RT	200	81%	260/790	38
CuCo2O4	RT, 57% RH	400	7.9%	300/840	39
S-rGO/WS2	RT	10	250%	100/480	40
WS2/MoO3	RT	3	31.58%	57/—	41
SnS2/graphene	RT	100	62.36%	31/435	42
SPA75	RT, 69% RH	50	161.8%	13/59	This work

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2. Experimental

2.1 Materials

Tin(IV) chloride pentahydrate (SnCl₄·5H₂O, Sigma-Aldrich), thiourea (CH₄N₂S, Merck), ethanol (C₂H₅OH, Merck), sodium dodecyl sulfate (C₁₂H₂₅NaO₄S, SDS, Sigma-Aldrich), aniline (C₆H₇N, Merck), ammonium persulfate ((NH₄)₂S₂O₈, APS, Merck), and hydrochloric acid (HCl, Merck) were used as received without further purification. All chemicals were of analytical grade.

2.2 Synthesis of SnS₂ nanosheet and SnS₂/PANI composites (SPA25, SPA75, SPA100)

Step 1: hydrothermal synthesis of SnS₂ nanosheet (SnS₂_NS)

Tin(IV) chloride pentahydrate (SnCl₄·5H₂O) and thiourea were mixed in a molar ratio of 1 : 2.5 and dissolved in 50 mL of distilled water under continuous stirring to obtain a clear and homogeneous solution. The resulting mixture was transferred to a Teflon-lined stainless-steel autoclave for hydrothermal reaction at 200 °C for 12 h. After naturally cooling to room temperature, the yellow precipitate was collected by centrifugation, thoroughly washed with distilled water and ethanol to remove residual impurities, and subsequently dried in a vacuum oven at 70 °C overnight to yield SnS₂_NS.

Step 2: surface modification of SnS₂_NS

For surface modification, 0.868 mM of sodium dodecyl sulfate (SDS) was added to the as-prepared pure SnS₂ powder. The mixture was then sonicated for 30 min. After sonication, the mixture was aged overnight to enhance dispersion stability, washed with DI water to remove excess surfactant, and dried in a vacuum oven at 70 °C. The SDS surfactant facilitated the intercalation of Na⁺ ions into the interlayer spaces of the SnS₂_NS, effectively reducing agglomeration and improving their dispersion. This ion intercalation improved the interfacial interaction with PANI in the composite.

Step 3: preparation of SnS₂/PANI composites (SPA25, SPA75, SPA100)

SnS₂_NS of different masses (50, 75, and 100 mg) were dispersed in 100 mL DI water and sonicated for 10 min to ensure uniform exfoliation. The resulting solution was then continuously stirred, and HCl was added to each dispersion to protonate the aniline monomer, forming anilinium ions and enabling oxidative polymerization. Subsequently, 5 mmol aniline was added to each solution, followed by the gradual dropwise addition of APS solution as the oxidant. The polymerization reaction was carried out under continuous stirring in an ice bath for 6–7 h. The resulting composites were collected after centrifugation with water and ethanol, followed by drying in a vacuum oven at 60 °C overnight. The resultant composites were labeled as SPA50, SPA75, and SPA100 as per the SnS₂_NS weight ratios (schematic synthesis, Fig. 1).

Fig. 1 Schematic of the synthesis of SnS₂_NS and SnS₂/PANI composite.

2.3 Sensor fabrication and evaluation of gas sensing performance

The fabrication of the SnS₂/PANI sensor is initiated by dispersing the material in a small amount of ethanol through grinding to form a homogeneous mixture. This paste was then uniformly coated onto the surface of an alumina (Al₂O₃) microtube equipped with four platinum (Pt) electrodes, forming a sensing film. After coating, the substrate was stabilized at 60 °C overnight. A nickel–chromium (Ni–Cr) wire was inserted into the Al₂O₃ tube as a heating coil to enable precise temperature control of the sensor via adjustable heating voltage if needed, as reported in a previous study.¹⁴ However, this heating function was not employed in this study. The final sensor module, consisting of six electrodes (four Pt and two Ni–Cr wires), was then connected to a circular mesh substrate. Gas sensing performance was tested for various analytes using a digital multimeter. Relative humidity (RH) within the chamber was controlled between 32% and 80% using saturated salt solutions and monitored with a commercial humidity sensor described in Fig. S8. Different concentrations of the target gas were prepared using a standard liquid–gas equilibrium method. VOC solvents were injected into the sensing chamber using a microsyringe. The chamber was evacuated using a vacuum pump to eliminate any residual gases or contaminants. The required concentration of the target gas was calculated using the static liquid–gas equilibrium method, applying the following eqn (1).V_s is the volume of liquid NH₃ solution needed (μL), C is the desired gas concentration in ppm, ρ and M_s are the density (g mL⁻¹) and molar mass (g mol⁻¹) of NH₃ solution, and T is the temperature in Kelvin.

2.4 Material characterization

The crystal structure of the samples was examined using a PANalytical X’Pert PRO MPD X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54 Å), operated at 35 kV and 30 mA. Chemical bonding of nanocomposites was analyzed via Fourier transform infrared spectroscopy (FTIR) using a PerkinElmer IR SUBTECH SPECTRUM ASCII PEDS 4.00. Raman spectra were acquired with a Renishaw inVia Reflex confocal micro-Raman system using a 532 and 785 nm laser to confirm structural features and phase purity. Optical band gaps were estimated from UV-vis absorption spectra measured with a SHIMADZU UV-3600i Plus spectrophotometer. X-Ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250) was used to determine elemental composition and oxidation states. Specific surface area measurements were conducted using the Brunauer–Emmett–Teller (BET) method with a Quanta Chrome Autosorb-iQ-MP/XR. Surface morphology and elemental mapping were analyzed using FESEM (Carl Zeiss Ultra Plus) with EDX, while detailed microstructures and lattice fringes were observed via high-resolution transmission electron microscopy (HRTEM, Tecnai G2-F30 S-TWIN). Mott–Schottky (MS) measurements were performed using a Gamry Reference 3000 Potentiostat/galvanostat/ZRA to determine the semiconductor carrier type. Current–voltage (I–V) measurements were performed using an Agilent B2901A Precision Source/Measure Unit (SMU) to assess contact behavior. The gas sensing performance was evaluated at room temperature (30 °C) and 69% RH using a Keysight 34470A digital multimeter with GUI and data logging features. Relative humidity was controlled using saturated salt solutions and monitored with a commercial humidity meter.

3. Results and discussion

The XRD patterns of pristine SnS₂_NS, PANI, and SPA composites are shown in Fig. 2(a). The distinct diffraction peaks observed at 2θ values of 15.08°, 28.2°, 30.1°, 32.1°, 41.8°, 46.1°, 49.8°, 52.5°, 54.9°, and 58.3°corresponding to the (001), (100), (002), (101), (102), (003), (110), (111), (103), (200) confirms the formation of hexagonal-phase of SnS₂ (JCPDS card no. 23-0677). A broad diffraction peak between 15° and 30° is observed for PANI, indicating its semicrystalline nature.^37,43 All characteristic peaks of both SnS₂_NS and pure PANI are present in the SPA composites, confirming the successful integration of the two components.

Fig. 2 (a) XRD, (b) FTIR, spectra of SnS_{2_}NS, PANI, and SPA composites, RAMAN spectra of (c) pristine SnS₂_NS, (d) PANI and SPA composites.

The surface chemical characteristics and vibrational modes of chemical bonds in SnS₂_NS, PANI, and the SPA composites were further examined using FTIR spectroscopy within the wavenumber range 4000–400 cm⁻¹ in Fig. 2(b). In SnS₂_NS, the peak at 478 cm⁻¹ is attributed to the stretching vibration of Sn–S bending modes of layered SnS₂_NS.⁴⁴ The broad peak appearing around 3442 cm⁻¹ is typically attributed to the O–H stretching vibration of water molecules. Pure PANI exhibits characteristic FTIR peaks at approximately 1565 cm⁻¹, corresponding to the stretching vibration of the quinonoid structure (N=Q=N), and at 1468 cm⁻¹, which is attributed to the benzenoid ring vibration (N=B=N). Additional peaks are observed at 1295 cm⁻¹ (C–N stretching vibration), 1120 cm⁻¹ (C–H in-plane bending vibration), and 799 cm⁻¹ (C–H out-of-plane bending vibration).^45,46 For the SPA composite, the observed peaks in blue shift in the absorption spectrum suggest a strong interaction between SnS₂ and PANI. In the Raman spectrum shown in Fig. 2(c), the most intense peak for SnS₂_NS appears at 311 cm⁻¹, corresponding to the A_1g mode of hexagonal SnS₂. In Fig. 2(d), the Raman spectrum of the SPA composite also exhibits a prominent peak at 312 cm⁻¹, confirming the presence of SnS₂. Additionally, the Raman spectrum of PANI shows characteristic peaks at 1182, 1253, 1328, 1505, and 1579 cm⁻¹, which are attributed to the C–H stretching vibrations of benzenoid and quinoid rings, C–N stretching in the benzenoid unit, C=N stretching in the quinoid unit, and C=C (quinoid)/C–C (benzenoid) stretching vibrations, respectively.^47,48

The UV-vis-NIR absorption spectra of SnS₂_NS, PANI, and SPA composites in the wavelength range of 250–1200 nm are shown in Fig. 3(a). SnS₂_NS exhibits light absorption in the visible region, specifically between 350 nm and 500 nm. Consequently, the SPA composites exhibit absorption ability extended toward near-infrared (NIR) region due to the synergistic effect of PANI, with additional characteristic peaks observed at approximately 302 and 410 nm are attributed to π–π* transitions in the benzenoid rings of polyaniline polaron–π* transition.⁴⁹ The corresponding optical band gap of the synthesized nanocomposites was determined using Tauc plots, as illustrated in Fig. 3(b), by plotting the modified Kubelka–Munk function versus photon energy. The bandgap energy of SnS₂_NS and PANI is shown in Fig. S1(a) and (b). Notably, SnS₂ exhibited an increased band gap compared to its untreated counterpart, as revealed by the UV-vis absorbance spectra and Tauc bandgap plot, as shown in Fig. S1(c) and (d). The observed blue shift in the SnS₂_NS band gap, from 1.9 to 2.1 eV, is attributed to a combination of quantum confinement effects due to reduced particle size and Na⁺ ion intercalation within the layered SnS₂ structure.⁵⁰

Fig. 3 (a) UV-vis absorbance spectra, (b) Tauc plot for SPA composites, (c) M–S plot of SnS₂_NS and PANI (inset), (d) Band alignment of PANI and SnS₂_NS.

Fig. 3(c) shows the Mott–Schottky (M–S) plot of SnS₂_NS and PANI, used to determine the charge carrier concentration of the semiconductors. The characteristic curve of SnS₂_NS is positive, indicating n-type behavior, while PANI shows a negative slope, confirming its p-type behavior. The flat band potential (E_FB) of SnS₂ and PANI is –0.573 and 0.92 V vs. RHE, respectively. Using the relation E_CB/VB = E_FB ± 0.10 V and E_VB = E_CB + E_g, the conduction and valence band edge positions were estimated. These band edge positions are schematically illustrated in Fig. 3(d), reveal a favorable type-II (staggered gap) band alignment in the SnS₂/PANI binary composites. The conduction band (CB) of SnS₂ (−0.476 V) lies above the lowest unoccupied molecular orbital (LUMO) of PANI (−2.42 V), while its valence band (VB) (1.624 V) is also higher than the highest occupied molecular orbital (HOMO) of PANI (1.21 V). Such band offsets enable efficient charge separation and suppress recombination, which are crucial for enhancing gas sensing performance⁵¹.

The surface elemental composition and chemical valence states of SnS₂_NS and SPA75 were analyzed using XPS spectra, as shown in Fig. 4. The survey spectrum in Fig. 4(a) confirms the presence of Sn, S, C, and N elements in the SPA75 composite, whereas the survey spectrum of pristine SnS₂_NS is presented in Fig. S2(a). Fig. 4(b) exhibits the high-resolution S 2p spectrum, displaying two distinct peaks at binding energies of 161.4 eV and 162.3 eV, corresponding to S 2p_3/2 and S 2p_1/2, respectively. In comparison, the S 2p peaks in pristine SnS₂_NS appear at 161.2 eV and 162.4 eV (ref. Fig. S2(b)). In Fig. 4(c), the deconvoluted peaks of Sn 3d are observed at 487.1 and 495.6 eV, corresponding to Sn 3d_5/2 and Sn 3d_3/2, respectively. In comparison, pure SnS₂ (ref. Fig S2(c)) exhibits Sn 3d peaks at 486.6 and 495.6 eV. The observed small shifts in binding energies are attributed to interfacial interactions and charge transfer between SnS₂ and PANI.^52,53Fig. 4(d) shows the C 1s spectrum, which comprises three fitted peaks at 284.7, 286.1, and 288 eV, corresponding to C–C, C–N, and C=O bonds, respectively.⁵⁴ Similarly, Fig. 4(e) illustrates the N 1s spectrum, where peaks at 399.5 and 401.4 eV are attributed to –NH– and N⁺ groups, respectively.⁵⁵

Fig. 4 XPS spectra of the SPA75 nanocomposite, (a) survey, high resolution scans for (b) S 2p, (c) Sn 3d, (d) C 1s, (e) N 1s.

The morphology and microstructure of SnS₂_NS and SPA composites as revealed by FESEM and TEM images in Fig. 5. The SnS₂_NS exhibits reduced size and decreased agglomeration after the addition of SDS, as shown in the FESEM image in Fig. 5(a). In contrast, the pure SnS₂ before adding SDS, shown in Fig. S3(a), displays irregular and highly agglomerated nanosheets. Fig. 5(b)–(d) shows the FESEM images of the SPA composites, revealing the sheet-like morphology of SnS₂ intercalated with PANI. The FESEM image of pure PANI is shown in Fig. S3(b). The corresponding EDX spectra in Fig. 5(e) and (f) for bare SnS₂_NS and SPA75 confirm the presence of elements Sn and S for the SnS₂, and Sn, S, C, and N in the SPA75 composite. No additional peaks corresponding to impurities are observed. The EDX spectra of SPA50 and SPA100 are shown in Fig. S3(c) and (d), respectively. Fig. 5(g) and (h) shows low and high-magnification TEM images of SnS₂_NS, revealing a hexagonal, transparent, sheet-like morphology that confirms their highly crystalline nature. Fig. 5(i) shows the high-resolution TEM image with a lattice spacing of 0.314 nm, corresponding to the (100) crystal plane of hexagonal-phase SnS₂. Additionally, Fig. 5(j) presents the selected area electron diffraction (SAED) pattern, where the regular and bright diffraction spots confirm the well-crystallized nature of the SnS₂_NS. Fig. 5(k) and (l) show the low- and high-magnification TEM images of SPA75, illustrating the hexagonal SnS₂ encapsulated within the amorphous PANI matrix. The corresponding HRTEM image in Fig. 5(m) of SPA75 reveals a lattice spacing of 0.278 nm, consistent with the (101) plane of 2H-SnS₂, along with a diffraction ring in Fig. 5(n), indicating the presence of amorphous regions. The labeled diffraction rings correspond to the (100) and (102) planes, further confirming the 2H phase of SnS₂.

Fig. 5 FESEM images of (a) SnS_{2_}NS, (b) SPA50, (c) SPA75, (d) SPA100, (e) EDX Spectra of bare SnS₂, (f) EDX of SPA75, (g) low magnification TEM image of SnS₂_NS (h) high magnification of SnS2_NS, (i) HRTEM of SnS₂_NS, (j) SAED pattern of SnS₂_NS, (k) low magnification TEM image of SPA75, (l) high magnification TEM image of SPA75, (m) HRTEM image of SPA75, (n) SAED pattern of SPA75.

Fig. 6(a) and (b) shows the Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area (SSA) of SnS₂_NS and the SPA75 composites. The corresponding SSA data for SPA50 and SPA100 are provided in Fig. S4(a) and (b). The BET plot of 1/W[(P₀/P) − 1] versus P/P₀ yields a straight line within the relative pressure range of 0.05 ≤ P/P₀ ≤ 0.35. Among the nanocomposites, the SPA75 obtained the maximum specific surface area, measured at 37.475 m² g⁻¹. The specific surface areas of SnS₂, SPA50, and SPA100 were found to be 18.44, 26.225, and 29.511 m² g⁻¹, respectively. A distinct Type IV hysteresis loop, as shown in Fig. 6(c), was observed in the N₂ adsorption–desorption isotherm of SPA75 at relative pressures (P/P₀) between 0.30 and 0.95, confirming the mesoporous nature of the composite, which facilitates enhanced gas diffusion and improves the gas sensing properties.

Fig. 6 Multipoint BET plot of (a) SnS₂_NS and (b) SPA75, (c) N₂ adsorption–desorption isotherm for SPA75.

3.1 Gas sensing properties and electrical characterization of SnS₂ and SPA composites

All gas sensing studies were conducted at room temperature (∼30 °C) and 69% relative humidity (RH). The current–voltage (I–V) measurements for SPA and SnS₂_NS were carried out over a voltage range of –40 to 40 V at room temperature, as shown in Fig. 7(a). SnS₂_NS and SPA composites show linear ohmic behavior, indicating ideal electrical contact without significant rectifying properties. In contrast, PANI exhibits an asymmetric, nonlinear I–V characteristic, as shown in Fig. S5(a).^34,56

Fig. 7 (a) I–V curve of SnS₂_NS and SPA sensors, dynamic resistance of (b) SnS₂_NS, (c) SPA50, (d) SPA75, (e) SPA100. (f) Response vs. concentration plot of SPA sensors.

Fig. 7(b) and (e) shows the dynamic resistance curves of SnS₂_NS and SPA composites in response to NH₃ concentrations ranging from 500 ppb to 150 ppm. The SnS₂_NS sensor exhibits an n-type response to NH₃, whereas pure PANI demonstrates p-type sensing behavior, as shown in Fig. S5(b). The SPA composites exhibit a p-type response, indicating that the sensing behavior is predominantly governed by the charge transport characteristics of PANI. Among all the samples, the SPA75 composite exhibits the highest resistance change in response to NH₃. The resistance consistently increases with the NH₃ concentration up to 150 ppm, beyond which further increases in concentration result in no significant change in response. Therefore, the maximum relative sensor response is observed at 150 ppm. The sensor response (%S) is calculated from eqn (2)

where R_g and R_a represent the sensor resistance in the presence of NH₃ and the initial resistance in air, respectively.

Fig. 7(f) shows the response versus concentration curve of the SPA composites. The experimental data were fitted with an exponential growth function, indicating a nonlinear relationship between sensor response and NH₃ concentration over the range of 0.5 to 150 ppm. The response increases rapidly at lower concentrations and gradually approaches saturation as the concentration rises. The correlation coefficients (R²) for the SPA50, SPA75, and SPA100 sensors are 0.9563, 0.9355, and 0.9648, respectively. These results suggest that the SPA composites are highly suitable for trace NH₃ detection at room temperature.

Fig. 8(a) shows the dynamic response curve of the SPA75 sensor to NH₃ concentrations ranging from 0.5 to 150 ppm. The sensor exhibits a consistent and stable response as the concentration increases. For comparison, the dynamic response curves of pure PANI and SnS₂_NS are shown in Fig. S5(c) and (d), respectively, both of which display significantly lower responses than SPA75. The transient response–recovery curves of SnS₂_NS and SPA75 are presented in Fig. 8(b) and (c), respectively. Both sensors exhibit a fast response and stable recovery after exposure to 50 ppm NH₃ at high humidity conditions at 69% RH. The response times for SnS₂_NS and SPA75 are 14 s and 18 s, respectively, while the corresponding recovery times are 89 s and 64 s. Overall, the SPA75 sensor demonstrates superior response and recovery performance compared to SnS₂_NS. The transient response curves of the SPA50, SPA100, and PANI sensors are shown in Fig. S6(a–c). The PANI sensor exhibits a significantly longer recovery time of 202 s, which is attributed to the effects of high humidity and slow desorption kinetics of NH₃ from the polymer surface.⁵⁷ To evaluate the reproducibility of the SPA75 sensor, Fig. 8(d) illustrates its highly repeatable sensing response and complete recovery over three consecutive exposure cycles to NH₃ concentrations over 0.5 to 5 ppm.

Fig. 8 (a) Dynamic response curve of SPA75 to 0.5 to 150 ppm NH₃, (b) transient response curve of SnS₂_NS, (c) transient response of SPA75 sensor, (d) repeatability of SPA75 sensor to 0.5 to 5 ppm NH₃.

The effect of RH on the SPA75 sensor was evaluated under ambient conditions, as shown in Fig. 9(a). The sensor maintained stable performance across a wide range of humidity levels, with response values of 75.8%, 82.3%, 84.5%, 81.2%, 72.4%, and 69.7% at 32%, 44%, 54%, 69%, 82%, and 93% RH, respectively, for 5 ppm NH₃. Minimal variations in response were observed between 32% and 95% RH, indicating excellent humidity resistance. The moisture resistance of the SPA75 sensor is further illustrated in Fig. S7. The base resistance of the SPA75 sensor initially decreased as the RH increased from 32% to 54%, attributed to enhanced charge transport facilitated by the adsorption of water molecules. The presence of H₂O promotes proton hopping through the formation of H₃O⁺ ions, increasing the mobility of charge carriers and thereby improving conductivity. However, as the RH increased further from 54% to 95%, a slight increase in resistance was observed, indicating that excessive humidity may lead to the formation of a thick water layer that hinders charge mobility or partially blocks the active sites of the sensing layer.^37,58

Fig. 9 (a) Transient response curve of SPA75 to 5 ppm NH₃ under varying relative humidity levels ranging from 32% to 93% RH. (b) Selectivity of SPA75 toward various target gases. (c) Long-term stability of SPA75 in response to 10 ppm NH₃.

Selectivity is a critical parameter for the reliable detection of target gases in practical applications. As shown in Fig. 9(b), the SPA75 sensor exhibited the highest response to 10 ppm NH₃ at 69% RH, significantly outperforming its responses to other tested gases such as acetone (CH₃COCH₃), benzene (C₆H₆), ethanol (C₂H₅OH), formaldehyde (HCHO), xylene (C₆H₄(CH₂)₂), triethylamine (C₆H₁₅N), hydrogen sulfide (H₂S), and carbon monoxide (CO).

NH₃ exhibits superior selectivity due to its strong electron-donating ability via the lone pair on the nitrogen atom, enabling effective interaction with the SPA75 sensor. In contrast, the oxygen atoms in acetone (C=O), formaldehyde (–CHO), and ethanol (–OH) are more electronegative than nitrogen in NH₃, rendering these molecules less effective electron donors. The carbonyl and hydroxyl groups act as electron-withdrawing groups through the strong inductive effect (–I) of oxygen, which weakens their electron donation capability and, consequently, their interaction with the material. Although C₆H₁₅N is more nucleophilic than NH₃, its larger molecular size and steric hindrance likely reduce its interaction strength and selectivity toward the sensor surface. C₆H₆ and C₆H₄(CH₂)₂ are non-polar aromatic hydrocarbons with delocalized π-electrons, making them chemically stable and less reactive. H₂S and CO exhibit weaker responses than NH₃ due to their less effective electron donation and lower affinity for the sensor's active sites, leading to weaker interactions on the sensor surface.

The long-term stability of the SPA75 sensor to 10 ppm NH₃ at 69% RH over 50 days is shown in Fig. 9(c). A gradual increase in the base resistance of the sensor was observed, likely due to environmental factors. At lower temperatures, moisture accumulation on the sensor surface can block active sites, impeding charge carrier movement and causing an increase in resistance.

3.2 Sensing mechanism

The NH₃ sensing mechanism primarily depends on protonation and deprotonation processes of PANI triggered by the adsorption and desorption of NH₃ molecules, respectively. When the sensing material is exposed to NH₃, the NH₃ molecules interact with PANI by withdrawing protons or H⁺ from the protonated nitrogen sites (N⁺–H) of the PANI to form NH₄⁺ (eqn (3)). This deprotonation process converts PANI to conductive emeraldine salt form (PANI_ES) into its insulating emeraldine base form (PANI_EB), resulting in a reduced charge carrier (hole) concentration and an increase in electrical resistance. Additionally, this transformation shortens the conjugation length of the PANI chains and expands the depletion layer. Upon re-entering in air, the adsorbed NH₄⁺ dissociates to NH₃ and proton, which are released back to the PANI chain, regain conductivity, and re-doped PANI_EB to PANI_ES, resulting in a decrease in resistance,⁵⁹ as described by eqn (4) and (5).

NH₃ + PANI ↔ NH–PANI + NH₄⁺ (3)

NH₄⁺ ↔ NH₃ + H⁺ (4)

PANI_EB + H⁺ → PANI_ES (5)

Additionally, the enhanced NH₃ sensing performance of the SPA75 sensor is primarily attributed to the formation of a p–n heterojunction between p-PANI and n-SnS₂. The work functions of PANI and SnS₂ are approximately 4.7 eV and 5.18 eV, respectively.⁶⁰ Since the Fermi energy level of PANI is higher than that of SnS₂, electrons transfer from the PANI region to the SnS₂ region, while holes move in the opposite direction. The calculated band gap energies of SnS₂ and PANI are 2.1 eV and 3.6 eV, respectively. This charge redistribution continues until the Fermi levels equilibrate, resulting in the formation of a depletion region at the interface, accompanied by a band offset. Fig. 10(a) and (c) demonstrates the schematic structure and energy band diagram of the SPA75 sensor in air.

Fig. 10 (a), (c) Schematic diagram and energy band structure of PANI/SnS₂ sensor in air and (b), (d) in NH₃.

The formation of the heterojunction enhances the interaction between electrons in the conduction band of SnS₂ and oxygen molecules in air, as illustrated in eqn (6). Oxygen molecules adsorb onto the material surface and extract electrons to form oxygen ions (O₂⁻), which increases the hole concentration. Upon exposure to NH₃, the gas reacts with these adsorbed O₂⁻ species, releasing the trapped electrons back into the conduction band of SnS₂ as described in eqn (7). This increases the electron concentration and reduces the hole population, thereby decreasing the overall conductivity and resulting in an increased electrical resistance in the SPA composite. Fig. 10(b) and (d) illustrate schematic representations and energy band diagrams of the SPA75 sensor presence of NH₃.

O_2(ads) + e⁻ → O₂⁻(ads) (6)

4NH₃ + 7O₂⁻ ↔ 4NO₂ + 6H₂O + 7e⁻. (7)

3.3 Exhale and simulated breath study

The concentration of NH₃ in the exhaled breath of patients with last-stage renal disease (LSRD) shoots up to 15 ppm, with an average value of 4.88 ppm. In contrast, healthy individuals exhibit significantly lower levels, typically below 0.4 ppm.⁶¹ NH₃ of exhaled breath can serve as a non-invasive biomarker for detecting Helicobacter pylori infections or renal disorders.

For the simulated breath experiments, a 2.4 L desiccator was initially evacuated using a vacuum pump. Subsequently, NH₃ gas (either 0.5 ppm or 1 ppm) was injected into the desiccator using a microsyringe. The remaining volume was then filled with healthy exhaled breath (HB), which had been collected in a 1 L Tedlar bag and sealed properly, as illustrated in Fig. 11(a). For baseline HB measurements, the desiccator was filled solely with exhaled breath from the Tedlar bag, without the addition of NH₃, using the same procedure. This setup effectively simulated real exhaled breath conditions, enabling physiologically relevant testing of the sensor.⁶²

Fig. 11 (a) Stepwise procedure for simulated breath preparation, (b) response of the SPA75 sensor to 0.5 ppm NH₃ in air and exposures to HB samples (S1–S6), (c) comparative response of the SPA75 sensor to HB, simulated breath containing 0.5 and 1 ppm NH₃, and pristine NH₃ in air at the same concentrations. (d) Response of the SPA75 sensor to healthy (S1–S6) and simulated breath samples containing 0.5 and 1 ppm NH₃.

As shown in Fig. 11(b), the SPA75 sensor was tested by exposing it to exhaled breath samples from six healthy individuals (S1 to S6) and compared with its response to 0.5 ppm NH₃ in air. The sensor exhibited a significantly lower response to the healthy breath samples, which is consistent with the lower concentrations of NH₃ typically found in the exhaled breath of healthy individuals. Notably, the SPA75 sensor showed stable and repeatable performance during multiple exposure cycles, without any signs of degradation. These results suggest that the sensor can reliably distinguish between normal breath ammonia levels and elevated levels typically associated with renal disorders, offering potential for non-invasive breath-based diagnostics.

Fig. 11(c) illustrates a comparative analysis between healthy and simulated breath with 0.5 and 1 ppm NH₃, along with pure NH₃ in the air. The sensor demonstrates a clear ability to differentiate between healthy and simulated renal disease samples. Notably, the sensor response is slightly reduced in the simulated breath samples compared to NH₃ in ambient air, which can be attributed to the high relative humidity (85–95%) typically present in exhaled breath.

Fig. 11(d) demonstrates the reliability of the SPA75 sensor in diagnosing simulated renal breath by analyzing responses from six healthy individuals (S1 to S6) and comparing them with simulated renal breath samples containing 0.5 ppm and 1 ppm NH₃. These results confirm that SPA75 effectively detects varying NH₃ levels in exhaled breath, highlighting its potential for non-invasive renal disease diagnosis.

4. Conclusion

In summary, the SnS₂/PANI nanocomposite was successfully synthesized via in situ polymerization, followed by surface modification with layered SnS₂_NS. The resulting p–n heterojunction sensor (SPA75) demonstrates a stable and selective response to NH₃ across a broad concentration range, shown 0.5–150 ppm at room temperature. The SPA75 sensor exhibits a high relative response of 213% at 150 ppm NH₃, with a fast response of 18 s and a recovery time of 64 s, even at an elevated humidity of 69% RH. Moreover, the sensor maintains reliable performance even at 93% RH, exhibiting minimal resistance drift and stable recovery. The SPA75 sensor is capable of detecting varying concentrations of NH₃ both in exhaled and simulated breath, demonstrating its suitability for non-invasive diagnosis of renal disease. The excellent sensing performance of the SPA75 sensor is attributed to fast charge transfer through p–n heterojunction between SnS₂ and PANI. Overall, the sensor shows promising response characteristics for NH₃ detection in both environmental and exhaled breath samples.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the SI. The supplementary information file contains details of experimental characterization data, graphs, and supporting figures which referenced in the main manuscript. See DOI: https://doi.org/10.1039/d5tb01445j

Acknowledgements

This work is supported by the ASEAN-India Science & Technology Development Fund (Science and Engineering Research Board) (AISTDF(SERB)), Government of India, under ASEAN-India Collaborative R&D scheme [Sanction order no. CRD/2020/000238]. The authors sincerely acknowledge the AISTDF Fund by SERB, Government of India for financial support. The authors also express their sincere thanks to the Director, CSIR-Central Glass and Ceramic Research Institute (CGCRI), for providing the necessary infrastructural support.

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