English

• 0.   Introduction

  Improper handling of meat/seafood dur ing storage and transportation can easily lead to spoilage and deterioration, which is characterized by the production of various volatile NH₃/amine compounds. Therefore, these compounds can be utilized as biomarkers for food decay, and their levels can be measured to assess food freshness^[1-2]. In comparison to traditional methods for NH₃/amine detection^[3-5], luminescence detection offers numerous advantages such as rapid detection, simplified operation, low cost, high sensitivity, and visibility to the naked eye^[6]. Consequently, it has attracted widespread attention.

  Recently a new type of sensor known as luminescent coordination polymers (LCPs) formed by metal ions and organic ligands, has attracted attention due to their structural diversity and excellent luminescent performance^[7-9]. So far many CP - based sensors have been reported for various detection^[10-12], and some have been used for NH₃/amine detection^[13-15]. However, most of the sensors are only effective for solution detection rather than vapor detection, and they can typically only detect one or two types of NH₃/amine. Sensors capable of sensing a wide range of NH₃/amine vapors are rare. Therefore, constructing CP - based sensors capable of rapid and wide - range detection of various NH₃/amine vapors remains challenging. On the other hand, due to the low cost and environmental friendliness of Bi³⁺ ion, its high affinity for multi-dentate ligands containing O/N atoms, and different coordination modes, Bi(Ⅲ)-based CPs have attracted widespread attention in recent years^[16]. Some Bi - CPs have been employed for detecting biological thiol and explosives owing to their excellent and sensitive luminescence properties^[16]. However, to our knowledge, there have been no reports on their use for bioamine detection so far.

  Herein a new LCP, (TBA) [Bi(bp4do)Br₄] (1), where bp4do=4, 4'-bipyridine-N, N'-dioxide and TBA⁺= tetrabutylammonium, with a 1D chain structure was prepared. A solid - state thin film sensor (1/PVP) with high luminescence stability was created by combining 1 with polyvinyl pyrrolidone (PVP). 1/PVP shows excellent wide - range sensing capabilities for quickly sensing 11 NH₃/amine vapors due to the collapse of 1's framework. Additionally, 1/PVP has been successfully used to monitor the freshness of food products such as seafood and fresh meat.

  1.   Experimental

  1.1   Materials and methods

  All commercially available reagents and solvents were used as received without further purification. IR spectra were recorded on a Nicolet 6700 spectrometer in a range of 4 000-650 cm^-1. The UV -Vis absorption spectra were collected on a Hitachi U-3900 spectrophotometer. Thermogravimetric analyses (TGA) were performed under a nitrogen atmosphere with a heating rate of 10 ℃·min^-1 using a TA -Q50 thermogravimetric analyzer. Elemental analyses of C, H, and N were determined on a Vario EL Ⅲ elemental analyzer. Powder X - ray diffraction (PXRD) patterns were collected on a D/MAX -2400 X -ray Diffractometer with Cu Kα radiation (λ =0.154 060 nm) at a scan rate of 10 (°)·min^-1 (voltage: 40 kV, current: 25 mA, scan range: 5° - 50°). Luminescent spectra were acquired at ambient temperature by using a Hitachi F -7000 fluorescence spectrophotometer. The lifetime of the sample was recorded on an Edinburgh Instruments FLS1000 fluorescence spectrophotometer (Analysis and Testing Center of Dalian University of Technology). Photoluminescence graphs were taken using a Canon camera (ixus 230 HS) under UV lamp irradiation.

  1.2   Synthesis of CP 1

  bp4do (24.4 mg, 0.13 mmol), BiBr₃ (57.1 mg, 0.13 mmol) and (TBA)Br (41.0 mg, 0.13 mmol) were dissolved in the minimum amount of DMSO. After stirring for 10 min, the solution was filtered and the filtrate was slowly evaporated. A few weeks later, orange block - shaped crystals were filtered out, washed with ethyl acetate, and dried in the air (82.1 mg, 67% yield). Element analysis Calcd. for C₂₆H₄₄BiBr₄N₃O₂(%): C, 32.55; H, 4.62; N, 4.38. Found(%): C, 32.60; H, 4.50; N, 4.29. IR (cm^-1): 3 097 (w), 2 975 (m), 1 621 (m), 1 543 (w), 1 469 (s), 1 418 (m), 1 311 (w), 1 211 (s), 1 174 (m), 1 024 (m), 831 (s), 736 (w), 698 (m), 547 (m), 466 (w).

  1.3   Preparation of 1/PVP film

  A 15.0 mg ground sample of CP 1 was added into 1.5 mL DMSO solution of PVP (10 mg·mL^-1), and the mixture was sonicated for 20 min to form a homogeneous suspension. Then, the suspension was dropped onto a 4.0 cm×2.5 cm glass plate, which was heated at 85 ℃ for 5 h to prepare the 1/PVP film.

  1.4   Structure determination

  Intensity data from a single crystal of CP 1 was measured at 150 K on a Bruker SMART APEX Ⅱ CCD area detector system with graphite-monochromated Mo Kα (λ=0.071 073 nm) radiation. Data reduction and unit cell refinement were performed with Smart - CCD software. The structure was solved by direct method using SHELXS-2014 and was refined by full-matrix least squares method using SHELXL-2014^[17]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms related to C atoms were generated geometrically. A summary of structure refinement data is given in Table 1. Selected bond lengths and angles are given in Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystal data collection and structure refinement parameters for CP 1

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  Parameter	1		Parameter	1
  Formula	C26H44BiBr4N3O2		Dc / (g·cm-3)	1.878
  Formula weight	959.26		μ / mm-1	9.932
  Crystal system	Monoclinic		F(000)	1 840
  Space group	P21/n		θ range /(°)	2.792-24.998
  a / nm	1.21016(14)		Reflection collected, unique, observed	69 598, 5 976, 4 836
  b / nm	1.853 1(2)		Rint	0.066 3
  c / nm	1.547 01(19)		GOF on F2	1.025
  β/(°)	102.027(4)		R1a, wR2b [I > 2σ(I)]	0.027 7, 0.058 9
  V / nm3	3.393 0(7)		R1a, wR2b (all)	0.043 1, 0.062 9
  Z	4		Max/mean shift in the final cycle	0.002/0.000
  a R=∑||Fo|-|Fc||/∑|Fo|; b wR=[∑w(Fo2-Fc2)/∑w(Fo2)2]0.5, w=[(σFo)2+(aP)2+bP]-1, where P=(Fo2+2Fc2)/3, a=0.025 6, b=7.002 5.

  2.   Results and discussion

  2.1   Crystal structure of CP 1

  Single crystal X - ray structural analysis reveals that CP 1 exhibits a 1D chain structure. As depicted in Fig. 1, the structure contains only one independent Bi³⁺ ion, which has a distorted {O₂Br₄} octahedral coordination polyhedron and is coordinated with four Br^- ions and two oxygen atoms from two bp4do ligands. Each bp4do ligand coordinates to two Bi³⁺ ions to form a 1D zigzag chain structure running along the b - axis direction. Furthermore, C—H···Br interactions are formed between the pyridine groups and Br^- ions from a neighboring chain. The corresponding H···Br separations fall in a range of 0.286-0.291 nm (Fig.S1). These interactions connect the chains into 2D anionic layers which are separated by the TBA⁺ cation.

  Figure 1

  

  Figure 1.  Structure of CP 1: (a) 1D zigzag chain running along the b-axis direction; (b) packing diagram

  The TBA⁺ cations are represented in space-filling mode; H atoms have been omitted for clarity.

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  2.2   Characterization of CP 1 and the 1/PVP film

  The PXRD pattern of as - synthesized CP 1 matched well with the simulated one, confirming its phase purity (Fig.S2a). TGA experiment reveals that 1 was stable before 220 ℃ (Fig.S3). It showed a striking weight loss in a temperature range of 220-440 ℃, indicating the complete collapse of the framework. The final product was Bi₂O₃ (Obsd. 23.27%, Calcd. 24.29%). IR analysis of the ligand showed that it had a series of absorption peaks at 3 150-3 020 cm^-1, which can be attributed to the C—H stretching vibration of the aromatic ring. In addition, the peaks at 1 465, 1 178, and 1 020 cm^-1 indicate the presence of pyridine rings. After the formation of the CP, the emerging peaks at 2 975-2 845 cm^-1 can be attributed to the C—H stretching vibration of TBA⁺ cation. At the same time, the peak of the N→O bond also redshifted from about 1 234 cm^-1 of the free ligand to about 1 211 cm^-1, indicating the formation of the coordination bond.

  The luminescence properties of bp4do and CP 1 were investigated (Fig. 2 and S4). Upon excitation, bp4do had an emission peak centered at 465 nm. Under the same excitation, 1 bore nearly identical prompt and delayed spectra (Fig. 2a) with two emission peaks located at 615 nm (120 µs) and 650 nm (122 µs), which can be attributed to phosphorescent emission. 1 had a high quantum yield of 69%. The luminescence colors of bp4do and 1 under a 365 nm UV lamp are blue and red, respectively (Fig. 2b), which is consistent with the results of luminescence spectra.

  Figure 2

  

  Figure 2.  Luminescence properties of bp4do and CP 1: (a) emission spectra upon excitation at 380 nm; (b) photographs of the solid samples under sunlight and 365 nm UV lamp

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  By using the drop-coating method, CP 1 and PVP were mixed to prepare a 1/PVP film. PXRD analysis shows that its pattern matched well with that of 1, confirming the successful synthesis of the film (Fig. S2b). The SEM images (Fig.S2c) reveal a homogeneous distribution of crystals on the surface of the film. Furthermore, the membrane exhibited a thickness of about 48 µm. The emission spectrum of the film exhibited two peaks at ca. 640 and 680 nm, which were slightly redshifted compared to those of 1 (Fig.S5a). The film displayed brownish - yellow and red colors under sunlight and a 365nm UV lamp, respectively. The very uniform luminescence indicates a good dispersion of 1 in PVP.

  The luminescence stability of 1/PVP film was studied under different temperatures, humidity levels, and environmental conditions (Fig. S5b-S5d). The results showed that temperature changes (25 - 120 ℃) and long- term storage (up to two months) had almost no effect on the luminescence of the film. 1/PVP also exhibited good stability over a wide range of relative humidity (Fig.S5e), with red luminescence still observable by the naked eye even at very high relative humidity (70% - 100%). These experimental results confirm that 1/PVP film has excellent luminescence stability, making it beneficial for sensing applications.

  2.3   Sensing of NH₃/amine vapors by 1/PVP sensor

  Then 1/PVP was used to sense various NH₃/amine vapors, including NH₃, n - butylamine (n - BuNH₂), n - propylamine (n-PrNH₂), ethylenediamine (EDA), ethanolamine (MEA), di - n - propylamine (DPA), dimethylamine (DMA), aniline (AN), trimethylamine (TMA), triethylamine (TEA) and 1, 4-butanediamine (1, 4-BDA). After exposure to different saturated vapors at 25 ℃, all vapors could induce color changes in the sensor (Fig. 3a and S6- S15). The sensor changed from brown-yellow to white under sunlight and from red to blue under a UV lamp, providing a strong luminescence color contrast for visual observation. The emission spectra were tested over time, showing that all vapors quenched the red emission of 1/PVP. After complete quenching, a new blue emission peak attributed to the ligand appeared near 460 nm and strengthened with increasing contact time. The time for 1/PVP to reach response equilibrium varied for different vapors (Fig. 3b), likely due to differences in their saturation vapor pressure. NH₃ has the highest saturated vapor pressure, causing 1/PVP to reach the end of the luminescence change in only 1.5 min. In contrast, as amine saturation vapor pressure decreases (especially in MEA, AN, and 1, 4 - BDA), the time required to reach response equilibrium gradually increases.

  Figure 3

  

  Figure 3.  (a) Time-dependent luminescent spectra of 1/PVP when exposed to NH₃ vapor at 25 ℃; Histograms of response equilibrium time (b) and I_{640 nm} complete quenching time (c) when 1/PVP was exposed to NH₃/amine vapors at 25 ℃; (d)Photographs of 1/PVP after exposure to NH₃/amine vapors, with 330 min of exposure for MEA, AN and 1, 4-BDA, and 30 min for other amines

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  The sensing results also indicate that while the response equilibrium required a significant amount of time (1.5-15 min for the vapors except MEA, AN, 1, 4-BDA), the quenching of red emission (640 nm) occurred relatively quickly (Fig. 3c). Specifically, 8 out of 11 NH₃/amine vapors only required 1 - 2 min to quench the emission. Therefore, the sensing ability of 1/PVP to different NH₃/amine vapors can also be judged by the I_{640 nm} complete quenching time. In addition, controlled experiments also showed a range of volatile organic solvents, including MeCN, CH₂Cl₂, MeOH, EtOH, THF, HAc, ethyl acetate (EA), and acetone (AC), can not cause significant luminescence changes of 1/PVP (Fig. S16). Therefore, 1/PVP has a strong anti - interference ability for NH₃/amine vapors sensing.

  2.4   Mechanism study

  NH₃ was used as an example for the mechanism study. The PXRD result showed that the sample after NH₃ fumigation (1 -NH₃) was amorphous, indicating that the fumigation can cause framework collapse (Fig. S17). Further study also showed that 1-NH₃ exhibited a blue emission peaked at ca. 430 nm, which is close to the emission of ligand. In contrast, as shown in Fig. 2b, the EtOH-washed sample (1-NH₃/EtOH) was non-luminescent (bp4do is soluble in EtOH). These results further prove that the fumigation leads to the structural decomposition of 1 and the release of ligand, thus the sensor shows red emission quenching and blue emission enhancement during the sensing process.

  There was a large overlap between the solid UV - Vis absorption spectrum of CP 1 and the emission spectrum of bp4do (Fig.S18a), indicating that there is resonance energy transfer between bp4do and 1. This can explain why blue emission is enhanced only when red emission is completely quenched during the sensing process. A controlled experiment on the luminescence response of the suspension of bp4do to NH₃ was performed (Fig. S18b). The results show that with the increase of NH₃ concentration, the emission of bp4do increased accordingly, indicating that the presence of NH₃ promotes the emission of bp4do. This can be used to explain the enhancement of blue emission during the later stages of the sensing process.

  The sensor's exceptionally wide - range sensing performance towards NH₃/amine vapors can be attributed to the high sensitivity of Bi—O←N bonds of CP 1 to these substances. The fumigation leads to the breakage of the bonds and subsequent collapse of the framework. On the other hand, the products obtained from the combination of Bi³⁺ with bp4do ligand and its derivatives usually exhibit strong red luminescence^[18]. Therefore, there is a significant difference in luminescence color before and after sensing NH₃/amine vapors, which facilitates naked - eye observation. Last but not least, compared with other commonly used transition metal ions, Bi³⁺ ion has the characteristics of low price and low toxicity, which meets the requirements of green environmental protection^[16]. These factors indicate that 1 has certain advantages in the detection of NH₃/amine vapors.

  2.5   Meat/fishery products freshness monitoring

  A series of experiments were conducted to moni tor the freshness of meat/aquatic products using 1/PVP (Fig. S19). Initially, 1/PVP and the products were sealed in an airtight container and stored in a -4 ℃ refrigerator for 12 h to simulate the food storage conditions. As food decay is very slow at -4 ℃ and the release of bioamines is negligible, no significant change in the luminescence color of the film was observed after 12 h. Then the container was removed from the refrigerator and left at room temperature (25 ℃). It was found that the red emission of 1/PVP gradually decreased with storage time, and almost completely quenched within 3-6 h, indicating its potential utility for monitoring food freshness. Table S2 lists several reported CP - based sensors for monitoring food freshness, but they are hindered by long response times, lack of convenience for naked - eye observation, and reusability issues. In contrast, the 1/PVP thin film sensor in this study has the advantages of rapid response and easy naked-eye observation. More importantly, the framework of 1 collapsed after sensing and could not be regenerated, preventing merchants from cheating and ensuring food safety.

  3.   Conclusions

  In summary, a Bi³⁺ - based CP 1 was synthesized and used to prepare a solid - state thin - film sensor (1/PVP) with excellent luminescence stability under normal conditions. Interestingly, the sensor can sense 11 NH₃/amine vapors and the mechanism is framework collapse owing to the high sensitivity of Bi—O←N bonds of 1 to base. The sensor has the advantages of easy naked-eye observation (luminescence color change from red to blue), rapid response (less than 15 min response equilibrium time for eight NH₃/amine vapors), wide-range sensing, strong anti-interference ability and non- reusable (prevent forgery), which makes it successfully applied to monitoring food freshness of meat/ aquatic products. This work provides a good example for the study of bioamine solid film sensors.

  
  Acknowledgments: This research is supported by the National Natural Science Foundation of China (Grant No. 21871038). Special thanks are due to the instrumental/data analysis from the Instrumental Analysis Center of Dalian University of Technology. Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  Structure of CP 1: (a) 1D zigzag chain running along the b-axis direction; (b) packing diagram

  The TBA⁺ cations are represented in space-filling mode; H atoms have been omitted for clarity.

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  Figure 2  Luminescence properties of bp4do and CP 1: (a) emission spectra upon excitation at 380 nm; (b) photographs of the solid samples under sunlight and 365 nm UV lamp

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  Figure 3  (a) Time-dependent luminescent spectra of 1/PVP when exposed to NH₃ vapor at 25 ℃; Histograms of response equilibrium time (b) and I_{640 nm} complete quenching time (c) when 1/PVP was exposed to NH₃/amine vapors at 25 ℃; (d)Photographs of 1/PVP after exposure to NH₃/amine vapors, with 330 min of exposure for MEA, AN and 1, 4-BDA, and 30 min for other amines

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  Table 1.  Crystal data collection and structure refinement parameters for CP 1

  Parameter	1		Parameter	1
  Formula	C26H44BiBr4N3O2		Dc / (g·cm-3)	1.878
  Formula weight	959.26		μ / mm-1	9.932
  Crystal system	Monoclinic		F(000)	1 840
  Space group	P21/n		θ range /(°)	2.792-24.998
  a / nm	1.21016(14)		Reflection collected, unique, observed	69 598, 5 976, 4 836
  b / nm	1.853 1(2)		Rint	0.066 3
  c / nm	1.547 01(19)		GOF on F2	1.025
  β/(°)	102.027(4)		R1a, wR2b [I > 2σ(I)]	0.027 7, 0.058 9
  V / nm3	3.393 0(7)		R1a, wR2b (all)	0.043 1, 0.062 9
  Z	4		Max/mean shift in the final cycle	0.002/0.000
  a R=∑||Fo|-|Fc||/∑|Fo|; b wR=[∑w(Fo2-Fc2)/∑w(Fo2)2]0.5, w=[(σFo)2+(aP)2+bP]-1, where P=(Fo2+2Fc2)/3, a=0.025 6, b=7.002 5.

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