Luminescent porous organic polymer nanotubes for highly selective sensing of H₂S

Abstract

In this study, we synthesize a luminescent porous organic polymer nanotube (PNT-1) through copolymerization of the monomers 1,3,5-tris-(3-bromo-phenyl)-[1,3,5]triazine (TBT) and 2,6-dibromopyridine (2,6-DP) by Ni-catalyzed Yamamoto reaction. Results indicate that PNT-1 exhibits high luminescence intensity, stability and reusability, and it can be used as an efficient luminescent probe for sensitively and selectively sensing H₂S. Furthermore, we also use UV-absorption as well as EDS and XPS spectra to further explore the luminescence quenching mechanism of PNT-1 as a luminescent probe for sensing H₂S. All the results reveal that the formation of N–S bond between PNT-1 and H₂S makes the electron in PNT-1 more stable and therefore leads to the luminescence quenching effect of PNT-1 for sensing H₂S. Moreover, we also find that PNT-1 as a luminescent probe is still applicable for sensing H₂S in vitro assay system simulated using PBS buffer solution.

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1. Introduction

As is well known, hydrogen sulfide (H₂S) gas is a toxic gas with a rotten egg smell and is fatally harmful to humans. Recently, H₂S as a signal transmission molecule in human body,^1–3 same as nitric oxide (NO) and carbon monoxide (CO), has attracted increasing attention. Generally, H₂S presents in the form of HS⁻ and S²⁻ to achieve equilibration in biological environments, and it plays an important role in physiological and pathological processes,^4–6 such as in nervous, respiratory, cardiovascular, digestive and endocrine systems. It is very crucial to detect H₂S in living cells for human health.

Currently, several methods have been used to detect H₂S, for example, time-resolved laser-induced breakdown spectroscopy,⁷ electrochemical method,⁸ gas chromatography,⁹ and high performance liquid chromatography.¹⁰ Although these techniques can detect H₂S, they also have some drawbacks, such as high cost and complex operations. Recently, luminescence-based method has emerged owing to its high sensibility and selectivity, short response time and high practicability.^11–17 Therefore, it is important to develop luminescent materials-based chemical sensors for detecting H₂S, and in fact, Na₂S has always been used as an H₂S substitute for study in the area of luminescence sensing.^18,19

A lot of luminescent materials have been reported to detect H₂S, in which Na₂S was used as the analyte.^20–22 Xiong et al.²³ reported a type of Au–Ag core–shell nanoparticles (NPs) for highly sensitive detection of H₂S in live cells. Zhang et al.²⁴ reported a UiO-66-(COOH)₂ MOF modified with Eu³⁺ and Cu²⁺ for sensing H₂S. Nagano et al.²⁵ synthesized a novel fluorescence probe HSip1 for sensing H₂S. Tropiano et al.²⁶ reported a lanthanide based sensor (Eu complex) for highly sensitive and selective detection of H₂S. Yu et al.²⁷ synthesized a carbon-dot-based ratiometric luminescent sensor for the detection of H₂S in aqueous media. Liu et al.²⁸ designed a lanthanide coordination polymer for sensing H₂S in biological systems. Ma et al.²⁹ reported quinoline group-modified Fe₃O₄@SiO₂ luminescent NPs for detection of Zn²⁺ and H₂S in aqueous solutions. Chen et al.³⁰ reported a luminescent probe constructed by combining coumarin and merocyanine, and it can be used for specific ratiometric sensing of H₂S. Although these metal-containing materials can act as luminescent probes for effectively sensing H₂S, some disadvantages still exist, such as low solvent thermal stability and the possibility of secondary pollution.

Covalent organic materials (COMs)^31–33 are widely studied as luminescent probes because of their excellent luminescence property and high hydrothermal stability. For example, Jiang's group³⁴ reported a luminescent azine-linked covalent organic framework (COF) for the detection of 2,4,6-trinitrophenol explosive. Liu et al.³⁵ reported an azine-linked COF-JLU3 for detecting Cu²⁺. Recently, porous covalent organic polymer (COP),³⁶ a subclass of COM materials, has also been widely used as a luminescent probe for sensing nitro explosives, small organic molecules and metal ions.^37–39 Xiang et al.⁴⁰ reported three types of luminescent COP materials with clear-cut layer texture structure for sensing nitro explosives and small organic molecules. Guo et al.⁴¹ synthesized a series of color-tunable COP materials with spherical structure for sensing PA and Fe³⁺. Apparently, these COP luminescent probes show excellent sensing properties for the detection of nitro explosives, metal ions and small organic molecules, but they have not been applied in the detection of H₂S yet.

In this work, we use 1,3,5-tris-(3-bromo-phenyl)-[1,3,5]triazine (TBT) and 2,6-dibromopyridine (2,6-DP) as double monomers to synthesize a porous organic polymer nanotube (PNT-1) via Ni-catalyzed Yamamoto reaction, and the synthesis process is presented in Scheme 1. Then, we further investigate its potential luminescence property for sensing H₂S. Moreover, we also explore its feasibility as a luminescent probe for detection of H₂S in vitro assay system by simulating the human environment. Finally, some discussion is addressed.

Scheme 1 Synthetic route of PNT-1 using 2,4,6-tris-(4-bromo-phenyl)-[1,3,5]-triazine (TBT) and 2,6-dibromopyridine (2,6-DP) as double monomers via the nickel-catalyzed Yamamoto-type Ullmann cross-coupling reaction.

2. Experimental

2.1 Synthesis of polymer nanotube (PNT-1)

The synthesized process of PNT-1 was divided into two parts: the reaction process in glove box and the post-treatment process in draught cupboard. First, 1,5-cyclooctadiene (cod, 0.50 mL, 3.96 mmol), bis(1,5-cyclooctadiene)nickel(0) ([Ni(cod)₂], 1.125 g, 4.09 mmol), and 2,2′-bipyridyl (0.640 g, 4.09 mmol) were added into dry dimethylformamide (DMF) (65 mL) and stirred until the mixed solution turned purple and completely dissolved. Then, 1,3,5-tris-(3-bromo-phenyl)-[1,3,5]triazine (TBT, 0.122 g, 0.314 mmol) and 2,6-dibromopyridine (2,6-DP, 0.112, 0.471 mmol) were subsequently added to the resulting purple solution and heated at 80 °C overnight under an argon atmosphere. After cooling to room temperature, the reaction vessel was put into the draught cupboard for further post-treatment process. The concentrated HCl was added to the deep purple suspension continuously, and stirred until the solution changed into aqua transparent solution. After filtration, the residue was washed by chloroform (CHCl₃, 5 × 15 mL), tetrahydrofuran (THF, 5 × 15 mL) and H₂O (5 × 15 mL), respectively, and dried in a vacuum oven. The dried sample was placed in containers and stored in a desiccator.

2.2. Characterization of PNT-1 materials

FTIR spectroscopy was performed on a Nicolet 8700 instrument with the wave range of 4000–400 cm⁻¹. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4700 SEM instrument. EDS measurements were performed on an energy dispersive spectrometer (Oxford INCA Energy 350 ADD), which is an accessory of SEM instrument, using Mn Kα radiation. Thermogravimetric analysis (TGA) data were obtained on a STA449C (NETZSCH) instrument, with a heating rate of 10 °C min⁻¹ under flowing N₂. Ultraviolet absorption spectra were recorded using TU-1901 spectrophotometers. N₂ adsorption/desorption isotherms at 77 K were measured by a Micromeritics ASAP 2020 system. Pore size distribution data were calculated from the N₂ sorption isotherms based on the NLDFT model in the Micromeritics ASAP 2020 software package. The XPS data were recorded with a ThermoFisher ESCALAB 250 X-ray photoelectron spectrometer equipped with a twin anode Mg-Kα X-ray source. The luminescent spectra were measured by Hitachi F-7000 Fluorescence Spectrophotometer with a PMT voltage of 420 V and a scan speed of 120 nm min⁻¹, and the width of the excitation slit is 5 nm and that of emission slit is 10 nm for the PNT-1 material.

2.3 Luminescence measurements for sensing anions

The sodium salt analytes used for the luminescent sensing experiments are Na₂S, NaF, Na₂SO₄, NaCl, NaBr, NaI, NaIO₃, NaSCN, NaHSO₃, Na₃PO₄, NaNO₃, and Na₂CO₃. The dried powder of PNT-1 was dispersed in methanol (CH₃OH, 2 mg/10 mL) solution for ultra-sonication for about an hour, and the resulting PNT-1–CH₃OH suspension was used for the subsequent experiment. The photoluminescence (PL) sensing properties of the synthesized PNT-1 material in CH₃OH solutions were investigated at room temperature. To explore the luminescence properties of PNT-1 for sensing anions, the fluorescence spectra of PNT-1–CH₃OH suspension were recorded by successive addition of aliquots of Na_xM (M^x− = S²⁻, F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻, respectively). The width of the excitation slit is 5 nm and that of the emission slit is 10 nm for PNT-1.

3. Results and discussion

Several basic characterizations have been performed to determine the inherent property of the PNT-1 material. The success of the phenyl–phenyl coupling in PNT-1 can be confirmed by Fourier transform infrared (FTIR) analysis (see Fig. 1a). The characteristic absorption bands for carbon–bromine bonds are highlighted in the pink region (see inset of Fig. 1a), in which the lack of bromine peak in the PNT-1 indicates completion of copolymerization. As shown in Fig. 1b, the SEM image clearly shows the hollow tubular structure of PNT-1. We also further measured N₂ adsorption isotherm of PNT-1 at 77 K (Fig. 1c). The pore size distribution calculated by N₂ uptake is shown in Fig. S1 (ESI†). The BET specific surface area of PNT-1 is 238.5 m² g⁻¹ and its pore size has a wide range from 1.8 to 12 nm, indicating that it contains micropores (<2 nm) and mesopores (2–50 nm). Thermogravimetric analysis under N₂ (Fig. 1d, black line) suggests that PNT-1 shows very high thermal stability with only 5% weight loss at 620 °C.

Fig. 1 (a) FTIR spectra of the PNT-1 (black line), 2,6-DP (red line) and TBT (blue line) from 400–4000 cm⁻¹ and 400–800 cm⁻¹ (the inset). The characteristic absorption bands for carbon–bromine are highlighted via pink region (see the bottom panel), clearly showing the lack of bromine in the PNT-1 and indicating the formation of the polymeric PNT-1 structure; (b) SEM image of PNT-1; (c) N₂ adsorption isotherms of PNT-1 at T = 77 K. Black and red lines represent adsorption and desorption, respectively. (d) TGA curves of PNT-1 and PNT-1a.

The luminescence spectra of solid PNT-1 and the two monomers (TBT and 2,6-DP) were measured and are shown in Fig. 2. Compared to the monomers, the luminescence intensity of PNT-1 increased significantly, owing to the increase in the number of π–π conjugate bonds produced by coupling reaction, and the fact that PNT-1 are more stable and reduce the energy consumed by vibration, similar to the AIE effect.^42,43 The high luminescence intensity of the PNT-1 material suggests that it is a good candidate as a luminescent probe.

Fig. 2 PL emission spectra of solid materials, PNT-1 (black line), TBT (red line) and 2,6-DP (black line).

To study the potential property of luminescent PNT-1 for sensing H₂S, without loss of generality, we first considered the sensing of PNT-1 for its anion formation, as reported in previous literature.^20,21 The solid PNT-1 material was dispersed in CH₃OH solution and highly dispersed PNT-1 suspension (2 mg/10 mL) was obtained. Subsequently, quantitatively different methanol solutions of Na_x(M) (M^x− = S²⁻, F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻) were added to the suspension for measurements. As shown in Fig. 3a, after adding S²⁻, the color of PNT-1 suspension suddenly became deeper and precipitation appeared rapidly, while this special phenomenon did not occur for other anions. The luminescent picture of PNT-1 suspension in Fig. 3b before and after adding S²⁻ can be easily identified by the naked eyes under a UV light (λ_ex = 365 nm). The corresponding luminescence spectra of PNT-1 suspension before and after adding anions were measured and are shown in Fig. S2 (ESI†), in which the peak position of the emission spectrum does not change. Intuitively, the luminescence intensities of PNT-1 suspensions before and after adding different anions are shown in Fig. 3c. Compared to PNT-1 suspension matrix, the luminescence intensity of PNT-1 after adding anions is changed in different degrees. In particular, after adding S²⁻, the luminescence of PNT-1 is almost quenched completely, suggesting that PNT-1 may be a potential luminescent probe for sensing H₂S.

Fig. 3 (a) The photographs of PNT-1 before and after adding S²⁻ in methanol solutions under sunlight; (b) PL quenching photograph by adding S²⁻ into PNT-1 suspensions in methanol solutions, where PL is excited under λ_ex = 365 nm using a portable UV lamp. (c) The luminescence intensity of PNT-1 interacting with different anions in 1 × 10⁻³ mol L⁻¹ methanol solution of Na_x(M) (M^x− = S²⁻, F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻), where the width of the excitation slit is 5 nm and that of the emission slit is 10 nm for PNT-1.

To confirm the high selectivity of PNT-1 as a candidate for sensing S²⁻, we performed a series of competition experiments to explore the effects of other anions (F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻) on the sensing property of PNT-1 for detecting S²⁻. In these competition experiments, we measured the luminescence intensities of PNT-1 in the presence of other anions before and after adding S²⁻ in CH₃OH solution. The luminescence spectra of PNT-1 in multi-component mixtures (with S²⁻ and without S²⁻) of other anions were measured and are shown in Fig. 4 and Fig. S3–S12 (ESI†). As shown in Fig. 4, the luminescence intensity of PNT-1 almost has no change after adding anion mixture A (A = SO₄²⁻ + IO₃⁻ + CO₃²⁻ + F⁻ + Cl⁻ + PO₄³⁻ + Br⁻ + HSO₃⁻ + NO₃⁻ + I⁻ + SCN⁻) to the solution, while the luminescence almost quenches completely after adding S²⁻ to the above solutions. Similar results were obtained under sixty-five different conditions of multi-component mixtures, while the components of these mixtures were contained in A (A = SO₄²⁻ + IO₃⁻ + CO₃²⁻ + F⁻ + Cl⁻ + PO₄³⁻ + Br⁻ + HSO₃⁻ + NO₃⁻ + I⁻ + SCN⁻) and made up of different combinations in the form of 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9-, and 10-component mixtures. We then measured the luminescence intensity changes of PNT-1 after adding the mixtures in methanol solutions, as shown in Fig. S3–S12 (ESI†). Clearly, the luminescence intensity of PNT-1 showed no obvious change after adding these sixty different types of anion mixtures to PNT-1 methanol solutions, while the luminescence intensity of PNT-1 almost quenched after adding S²⁻ to the abovementioned solutions. These results indicate that PNT-1 exhibits extremely high selectivity for sensing S²⁻ even in the existence of other anions.

Fig. 4 (a) The luminescence emission spectra of PNT-1 before (black line) and after adding A (red line) and A′ (blue line) in methanol solution, (A′ = A + S²⁻, A = SO₄²⁻ + IO₃⁻ + CO₃²⁻ + F⁻ + Cl⁻ + PO₄³⁻ + Br⁻ + HSO₃⁻+ NO₃⁻ + I⁻ + SCN⁻), where the widths of the excitation slit are 5 nm and emission slit is 10 nm for the COP. (b) The luminescence intensity of PNT-1 before and after adding A and A′ in methanol solution, (c) the PL quenching photograph by before and after adding anions into PNT-1 suspensions in methanol solutions, where PL is excited under λ_ex = 365 nm using a portable UV lamp.

Moreover, the luminescence sensitivity of PNT-1 for detecting H₂S was investigated via real-time luminescence response. The methanol solutions of Na₂S with different concentrations were gradually added to the PNT-1 suspension, and the corresponding luminescence spectra were measured; the peak position of the emission spectra did not change upon the addition of different concentrations of Na₂S solutions (see Fig. 5a). At 30 ppm S²⁻ concentration, the quenching degree reaches about 56.2%, and when the concentration of S²⁻ is 60 ppm, the quenching degree hits about 77%. Then, we calculated the quenching coefficient by Stern–Volmer equation: I₀/I = 1 + K_sv [M], where I₀ is the luminescence intensity of the luminescent material, and I is the luminescence intensity of the luminescent material after adding the analyte of concentration [M], and K_sv is the quenching coefficient. There is a good linear Stern–Volmer relationship of PNT-1 for sensing S²⁻, and the K_sv of PNT-1 for detecting S²⁻ reaches 6.79 × 10⁴ (see Fig. 5b), which means that PNT-1 also possesses high sensitivity for S²⁻ detection. We also used the LOD (limited detection concentration) equation: LOD = 3σ/K to calculate the LOD, where σ is standard deviation of the blank luminescent materials (see Fig. S13, ESI†), and K is the K_sv constant of the materials for sensing analytes. The results indicate that the LOD of PNT-1 for detecting S²⁻ reaches 0.053 ppm, which is lower than that of other luminescent probes for sensing S²⁻, as listed in Table S1 (ESI†).

Fig. 5 (a) The luminescence emission spectra of PNT-1 after adding different concentrations of S²⁻, where the width of the excitation slit is 5 nm and that of the emission slit is 10 nm for PNT-1; (b) Stern–Volmer plots of PNT-1 for sensing S²⁻.

Essentially, the luminescence detection mechanism was also studied by a series of experiments, including UV absorption spectra, XPS, EDS and TGA. The UV absorption spectra of the PNT-1 suspension before and after adding different anions were measured and are shown in Fig. 6. Compared to PNT-1 suspension matrix, the absorption peak and the shape of the spectra of PNT-1 suspension after adding these anions of F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻ (Fig. 6b–l) have no significant change except the change of the absorption intensity. However, the absorption spectra of PNT-1 suspension after adding S²⁻ disappear (see Fig. 6a), indicating the appearance of new matter or occurrence of a chemical reaction between PNT-1 and S²⁻. To prove this observation, we measured the EDS spectra and XPS spectra of PNT-1 before and after adding S²⁻. After a series of filtration, washing and drying, the obtained solid PNT-1–S²⁻ materials were marked as PNT-1a. EDS data (see Fig. 7) indicate that S²⁻ definitely exists in some special form in PNT-1a after adding S²⁻ to PNT-1, because an obvious peak S²⁻ appears in the PNT-1a EDS spectrum while it is absent in the PNT-1 EDS spectrum. We also further measured the XPS spectra of PNT-1 and PNT-1a, shown in Fig. S14 (ESI†), in which the N1s, C1s and S1s peaks appear in PNT-1a XPS while S1s peak is absent in PNT-1 XPS. Compared to that of PNT-1, the C1s spectrum (see Fig. S15, ESI†) of PNT-1a has no significant change. High-resolution N1s XPS spectra details are shown in Fig. 8. Clearly, PNT-1 only has a peak at the position of 398.6 eV (Fig. 8a), which is in good agreement with the type of nitrogen of PNT-1, i.e., pyridinic N. However, the N1s peak of PNT-1a shows a new peak at 401.8 eV (Fig. 8b), indicating the formation of N–S bond. Therefore, we believed that the pyridinic N in PNT-1 forms an N–S bond with S²⁻ and makes the electron in PNT-1 more stable and not easy to transfer, thus causing the luminescence quenching effect. Moreover, TGA curve of PNT-1a was obtained and is shown in Fig. 1d. Compared to PNT-1, PNT-1a starts to collapse rapidly at 400 °C, and when the temperature reaches 620 °C, about 40% weight is lost, indicating the decrease in its stability. All the UV-spectra, EDS, XPS and TGA results indicate that N–S bond was formed between PNT-1 and S²⁻, resulting in the luminescence quenching effect of PNT-1 as luminescent probes for sensing S²⁻.

Fig. 6 Absorption spectra of PNT-1 before (black line) and after (red line) adding different anions. (a–l) are before and after adding S²⁻, F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, and CO₃²⁻, respectively.

Fig. 7 EDS energy spectra of PNT-1 (black line) and PNT-1a (red line).

Fig. 8 High-resolution N1s XPS spectra of (a) PNT-1 and (b) PNT-1a.

We also investigated the luminescence stability of PNT-1 when dispersed in solutions. The PL spectra of PNT-1 dispersed in a solvent were measured at different times. As shown in Fig. S16 (ESI†), the PL spectra and luminescence intensity almost do not change with time, indicating that PNT-1 have excellent stability in solvents. Furthermore, the reusability of PNT-1 as a luminescent probe for detecting S²⁻ was also studied. As shown in Fig. S17 (ESI†), the original luminescence intensity of PNT-1 after a series of washing and filtration shows basically no change, and the quenching efficiency for sensing S²⁻ also remains almost the same after repeated usage, suggesting excellent reusability of PNT-1.

In order to confirm the applicability of the PNT-1 material in the actual environment, we assembled PNT-1 into a luminescent test paper. Subsequently, we exposed the luminescent sensing properties of the test paper for different gas atmospheres (e.g. H₂S, SO₂, NO₂, NH₃, N₂, CO₂, CO, H₃PO₄, H₂SO₄, ethylic acid, etc.). As shown in Fig. 9, H₂S gas can apparently quench the luminescence of the PNT-1 test paper, while other gases (e.g. NO₂, NH₃, N₂, CO₂, H₃PO₄, H₂SO₄, ethylic acid, etc.) cannot, indicating that the assembled PNT-1 luminescent test paper can be used for selective detection of H₂S gas in realistic atmosphere. It is worth mentioning that the process of test paper for sensing H₂S gas is in real-time, and the response can be identified by the naked eye under a UV lamp.

Fig. 9 Luminescence quenching of COP test paper for selectively sensing H₂S gas under a 365 nm UV lamp. The final concentrations of all gases were 1 ppm, while for H₃PO₄, H₂SO₄ and ethylic acid, experiments were performed in their saturated vapors.

To extend its practical application in living cells, we also studied the luminescent sensing property of PNT-1 for detecting S²⁻ in simulated living cells, in which we used PBS buffer solution as the simulated environment of the living cells, and performed a series of measurements about the luminescent sensing property of PNT-1 for detecting S²⁻in vitro assay. The specific experimental process is presented in the ESI† (Section S1). As shown in Fig. 10, the luminescence intensity of PNT-1 has no significant change after adding the analytes (F⁻, SO₄²⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, SCN⁻, HSO₃⁻, PO₄³⁻, NO₃⁻, CO₃²⁻, Cys and Vc), while the luminescent quenching still occurs after adding S²⁻, which suggests that the PNT-1 as a luminescent probe is still applicable for sensing S²⁻ in simulated living cells. The observation greatly extends the possibility of PNT-1 as efficient luminescent sensors for sensing H₂S in practical applications.

Fig. 10 The luminescence intensity of PNT-1 interacting with different analytes in 1 × 10⁻³ mol L⁻¹ PBS solution, where the width of the excitation slit is 5 nm and that of emission slit is 10 nm for PNT-1.

4. Conclusions

In summary, we have synthesized a luminescent porous organic polymer nanotubes (PNT-1) material through copolymerization of the monomers of TBT and 2,6-DP. Luminescence measurement suggested that PNT-1 can be used as a luminescent probe for selective sensing of H₂S. Furthermore, we used a series of characterizations to study the luminescence quenching mechanism. All the results indicated that the formation of N–S bond between PNT-1 and H₂S makes the electron in PNT-1 more stable and therefore leads to the luminescence quenching effect of PNT-1 for sensing H₂S. Moreover, we found that the PNT-1 as a luminescent probe is still applicable for sensing H₂S in vitro assay system simulated using PBS buffer solution, and the PNT-1-based test paper can definitely achieve the selective sensing of H₂S gas in real-time.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Science Fund for Distinguished Young Scholars (No. 21625601) and the Major Project of NSF of China (No. 91334203) and Outstanding Talent Fund from BUCT.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00423k

‡ Equally contributed to this work.

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