English

• Phosgene, diethyl chlorophosphate (DCP) and acyl chlorides are essential chemical intermediates, and have been widely used for manufacturing medicines, pesticides, dyes and polymers [1]. It is estimated that tens of thousands of tons of acyl chlorides are consumed globally each year. Whereas, phosgene, DCP and acyl chlorides are highly toxic volatile organic compounds [2]. Exposure to 20 ppm of phosgene can cause severe injury to lung and respiratory in a few minutes, and exposure to 90 ppm phosgene for 0.5 h can be lethal [3,4]. DCP can bind with acetylcholinesterase to inhibit conduction of nerve impulses, leading to organ failure and death within seconds [5-11]. Acetyl chloride can activates the flow of tears as little as 0.5 ppm, and high concentration may cause death or permanent damage [12]. Moreover, most of acyl chlorides can result in dermal and respiratory diseases [13,14-19]. Volatile acyl chlorides pose a serious threat to our public security once industrial leakage accidents happened or they were used as chemical warfare agents (CWAs) by terrorists. To safeguard our public security and human health, it is very important for the development of a simple and efficient tool for on-site high-throughput screening of phosgene, DCP and a diversity of acyl chlorides.

  In the past decades, some conventional approaches have been employed to determine phosgene and acyl chlorides, such as gas chromatography, liquid chromatography and electrochemical methods [20-23]. Those methods rely on expensive instruments, which are not suitable for fast security check in subway, airport, railway station and customs due to their poor portability and time-consuming. Fluorescent probes are versatile optical sensors that can generate changes in color and fluorescence signals through host-guest recognition patterns, and has the advantages of high selectivity and sensitivity, low cost and ease of operation [24-27]. A diversity of probes have been devised for sensing of phosgene, using different recognition sites, including ethylenediamine, 2-aminoethanol, aldehyde oxime, o-phenylenediamino and o-hydroxyaniline [28-36]. Likewise, tremendous fluorescent probes were developed for the detection of DCP by utilizing amino, hydroxyl and oxime groups as reaction sites, but some of them give false positive fluorescence signals [37-42]. Whereas, these sensors can only determine one analyte like phosgene or DCP, but neglect diphosgene, acetyl chloride and some other acyl chlorides because these fluorescent probe usually comprise a single luminescent sensor that detect a given analyte via the "lock-key "strategy. The array sensing technique inspired by mammalian olfactory/gustatory systems provides a promising solution to address this issue [43,44]. In an array sensing system, multiple independent sensors interact differentially with the target analytes, producing cross-reactive response signals [45]. A distinctive recognition pattern is then generated for each analyte from the signals by processing them through mathematical methods such as linear discriminant analysis (LDA) and principal component analysis, realizing the discrimination of different analytes [46-48]. Nevertheless, in comparison with single-sensor sensing, array sensing requires relatively time-consuming and laborious measurement processes to obtain signal data matrices from multiple sensors. Therefore, it is highly desirable to develop a single fluorescent sensor for high-throughput screening toxic phosgene, DCP and volatile acyl chlorides that are harmful for human health.

  Given that fluorescent probe with high-throughput detection performance is particularly attractive: (1) It is a simple but highly efficient manner, (2) more cost-efficient, and (3) it can avoid the spectral cross-talk of multiple sensors, we propose a strategy to sense multiple analytes by using a single fluorescence probe [49-54]. However, it is quite difficult to devise a sensor that can react with multiple analytes and generate different fluorescence signals to high-throughput detect volatile acyl chlorides. 1,2-Diaminocyclohexane (DCH) has two primary aliphatic NH₂ with pKa₁ and pKa₂ of 9.60 and 6.21, respectively. Nucleophilic substitution reaction between NH₂ and acyl chlorides would easily take place in a rapid and effective way. In particular, phosgene would couple with two NH₂ of DCH, which is quite different from some other acyl chlorides. Taking advantage of this feature, in this work, we employed DCH as a reaction site to prepare a "one-for-more" type probe BDP-CHD for high-thoughput detection of phosgene, DCP and some other acyl chlorides. DCH was anchored to the meso‑position of boron dipyrromethene (BODIPY) by nucleophilic substitution with 8-Cl-BODIPY, forming a BODIPY analogue with a C=N double bond. Due to the electron-donating properties of amino group in DCH, photo-induced electron transfer (PET) would happen, which will quench the intrinsic fluorescence of BODIPY core. The adjacent two amino groups in DCH would react with phosgene to form an octahydrobenzimidazolone and BODIPY moiety, giving rise to strong green fluorescence accompanied with chromogenic reaction. Upon exposure to acyl chlorides, the primary aliphatic NH₂ with higher pKa would react with acyl chloride or DCP to form amide/phosphamide, which would inhibit the PET course and boasted remarkable blue fluorescence enhancement. Therefore, BDP-CHD would realize high-throughput detection of phosgene and other acyl chlorides based on different fluorescence response manners (Scheme 1). Furthermore, a portable sensing platform was constructed by using BDP-CHD-loaded meltblown fabric for high-throughput detection of volatile organic pollutant.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the probe for high-throughput monitoring of phosgene, diphosgene, acyl chlorides and DCP in molecular level.

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  We first investigated the ultraviolet–visible (UV–vis) absorption and fluorescence spectral response of BDP-CHD to phosgene and some other acyl chlorides in chloroform. The sensing behavior of BDP-CHD to phosgene were assessed by using a non-volatile and low-toxicity counterpart, triphosgene, which can in situ generate phosgene in the presence of triethylamine. As shown in Fig. 1a, BDP-CHD displayed a powerful absorption band at 410 nm. After the addition of triphosgene (0–5.0 µmol/L) and triethylamine (TEA), the initial absorption peak (A₄₁₀) of BDP-CHD gradually decreased. Simultaneously, a new absorption peak (A₅₀₀) increased, and an obvious chromogenic reaction from colourless to yellow was observed (Fig. 1a). The isosbestic point was at 435 nm, indicating that BDP-CHD formed a single compound with phosgene. In addition, there was a good linear relationship between the absorbance of BDP-CHD and the concentrations of phosgene within the range of 0–2.0 µmol/L (R² = 0.991) (Fig. 1d). It is noteworthy that the initial absorption band (410 nm) was completely converted to another absorption band (500 nm) after addition of 1.0 equiv. phosgene, implying BDP-CHD has high reactivity and sensitivity for phosgene. Subsequently, we explored the fluorescence sensing properties of BDP-CHD towards phosgene. BDP-CHD exhibited a faint fluorescence band at 476 nm in chloroform (Φ_f = 0.12), which is due to the PET effect from the DCH to the fluorophore (Fig. 1b). After the addition of triphosgene/TEA to the BDP-CHD solution, a new fluorescence band at 524 nm (Φ_f = 0.46) emerged. Meanwhile, a clear fluorescence change from weak blue to bright green was observed, which might ascribed to the formation of octahydrobenzimidazolone and BODIPY (Fig. 1b). Furthermore, there was a good linear correlation between the fluorescence intensity of BDP-CHD and the concentrations of phosgene within the range of 0–3.0 µmol/L (R² = 0.991) (Fig. 1e), and an extremely low detection limit (LOD = 51.4 ppt) was obtained, which is sensitive enough to determine trace phosgene from the leakage of industry or accident CWAs terrorist attacks. These findings suggest that BDP-CHD could be employed as a ratiometric fluorescent and colormetric dual-mode probe for ultra-sensitive monitoring of phosgene.

  Figure 1

  

  Figure 1.  (a) UV–vis absorption and (b) fluorescence spectra responses of BDP-CHD (5.0 µmol/L) to incremental triphosgene (0–5.0 µmol/L)/TEA (100 µmol/L) in chloroform. (c) Fluorescence responses of BDP-CHD (5.0 µmol/L) to acetyl chloride (0–5.0 µmol/L) in chloroform. Linear relationship between the (d) absorbance and (e) fluorescence intensity of BDP-CHD (5.0 µmol/L) with the concentration of triphosgene/TEA or (f) AC (0–5.0 µmol/L). λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  We also investigated the spectral response of BDP-CHD to acetyl chloride (AC). Upon the addition of AC, the UV–vis absorption spectrum of BDP-CHD (5.0 µmol/L) remained silent (Fig. S2 in Supporting information), but a large fluorescence enhancement at 476 nm was observed (Φ_f = 0.62) (Fig. 1c). This observation might be due to the fact that the free aliphatic NH₂ in BDP-CHD has reacted with acetyl chloride and the PET process was interrupted. In addition, the fluorescence intensity of BDP-CHD had a good linear correlation (R² = 0.991) with the concentrations of acetyl chloride (Fig. 1f), and the LOD was calculated to be 132.9 ppt (LOD = 3σ/k). Based on the above findings, we can conclude that BDP-CHD is a highly sensitive fluorescence-enhanced probe that can be used to quantitatively detect acetyl chloride.

  Next, we investigated the spectral response of BDP-CHD to a wide range of toxic acyl chlorides, such as triphosgene, diphosgene, oxalyl chloride (OC), benzoyl chloride (BzCl), thionyl chloride (TsCl) and DCP, and found that all these acyl chlorides could boast a large fluorescence enhancement at 476 nm (Fig. S3 in Supporting information), which might be ascribed to the high reactivity of free aliphatic NH₂ in BDP-CHD. Hence, BDP-CHD can act as a high sensitive and general-purpose sensor for high-throughput screening of various acyl chlorides.

  For the detection of highly hazardous substances, the response speed of the fluorescent probe should be very fast to reduce the harm to human body. Herein, we explored the response rate of BDP-CHD towards phosgene and AC. As shown in Fig. 2a, the ratiometric fluorescence intensity (F₅₂₄/F₄₇₆) of BDP-CHD reached a plateau within 2 s after the addition of phosgene. Furthermore, we recorded the time-lapse fluorescence images by a live video, as shown in Fig. 2b. The BDP-CHD solution showed a faint blue fluorescence under the UV lamp. After the addition of phosgene, a distinct green fluorescence appeared immediately in a local area of the solution, indicating that BDP-CHD was able to react rapidly with phosgene. With the diffusion of the phosgene, the entire BDP-CHD solution showed an increasing green fluorescence and reached the plateau within 1.6 s. For AC, the fluorescence intensity of BDP-CHD at 476 nm reached a maximum within 3 s (Fig. 2c), and the BDP-CHD solution showed a distinct blue fluorescence within 2.4 s (Fig. 2d). From these observations, we can conclude that BDP-CHD can be used to rapidly detect phosgene and some other violate acyl chlorides.

  Figure 2

  

  Figure 2.  (a, c) Time-course fluorescence intensity and (b, d) images of BDP-CHD (5.0 µmol/L) in CHCl₃ containing triphosgene (5.0 µmol/L), TEA (100 µmol/L) or AC (5.0 µmol/L). λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  We then examined the fluorescence response of BDP-CHD to phosgene, various acyl chlorides and some potential interfering species, including diphosgene, dimethyl methylphosphonate (DMMP), DCP, TsCl, POCl₃, SOCl₂, HCl, BzCl, OC, formaldehyde (FA), methylglyoxal (MGO), acrolein, and triphosgene. As shown in Figs. 3a and b, BDP-CHD generated a remarkable ratiometric (F₅₂₄/F₄₇₆) fluorescence response to phosgene. Diphosgene, DCP and some other acyl chlorides stimulated BDP-CHD to produce large fluorescence enhancement at 476 nm, respectively. By contrast, BDP-CHD exhibited negligible fluorescence response to the potential interfering species such as hydrochloric acid, DMMP, formaldehyde, methylglyoxal and acrolein, suggesting that BDP-CHD has good selectivity for phosgene and acyl chlorides. Given that the different responses of BDP-CHD to the above analytes, we conducted a LDA statistical analysis which can identify the optimal linear combinations of features for different classes of analytes. The LDA statistical analysis results were projected on a two-dimensional (2D) space, as shown in Fig. 3c. BDP-CHD showed a remarkable green fluorescence after exposed to phosgene. Hence, phosgene itself was divided into a group. Aldehydes, acids, DMMP and BDP-CHD cannot lead to significant fluorescence changes, and thus they were divided into another group. Amazingly, acyl chlorides can be categorized into two groups in the LDA diagram according to different fluorescence intensity. These observations demonstrated that BDP-CHD can serve as a "one-to-more" probe for high-throughput detection of phosgene, acyl chloride, aldehydes and hydrochloric acid.

  Figure 3

  

  Figure 3.  Fluorescence intensity changes at (a) F₅₂₄/F₄₇₆ and (b) F₄₇₆ of BDP-CHD solution (5.0 µmol/L) upon exposure to various analytes (10.0 µmol/L). (1) Blank, (2) diphosgene, (3) DMMP, (4) DCP, (5) TsCl, (6) POCl₃, (7) SOCl₂, (8) HCl, (9) BzCl, (10) OC, (11) AC, (12) FA, (13) MGO, (14) acrolein, (15) triphosgene and (16) phosgene. (c) 2D LDA diagram of fluorescent arrays consisting of 16 types of analytes. λ_ex = 430 nm, slits: 2.5 nm/5.0 nm. Data are presented as mean ± standard deviation (SD) (n = 3).

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  Given that phosgene and acyl chlorides might co-exist in some cases, herein, we explored the capability of BDP-CHD for simultaneous detection of phosgene and AC. After the addition of phosgene and AC, BDP-CHD displayed two fluorescence bands at 476 and 524 nm, respectively (Fig. 4a). The fluorescence enhancement at 476 nm was caused by acetyl chloride, and thus can be used to determine acetyl chloride. Another fluorescence band at 524 nm was ascribed to phosgene, which can be utilized to determine phosgene (Fig. 4b). Therefore, BDP-CHD not only can discriminate phosgene and acyl chlorides from different emission channels, but also has a good capability for detection of phosgene and acyl chlorides simultaneously.

  Figure 4

  

  Figure 4.  (a) Fluorescence spectra of BDP-CHD solution (5.0 µmol/L) and TEA (100.0 µmol/L) upon the addition of phosgene and AC (0–5.0 µmol/L). (b) Fluorescence spectra of BDP-CHD solution (5.0 µmol/L) upon the addition of AC, phosgene or both of AC and phosgene. λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  To explore the working mechanism, we isolated the main product BDP-AC from the reaction mixtures of BDP-CHD and acetyl chloride, and tentatively cultured their single-crystals. As shown in Fig. 5a, the bond length between N003 and C007 is 1.322 Å in BDP-CHD, implying there is a C=N in BDP-CHD. Since BDP-CHD exists in the form of BODIPY isomer, it has much shorter UV–vis absorption wavelength and fluorescence wavelength in comparison with ordinary BODIPY core. Besides, BDP-CHD exhibited very weak blue fluorescence due to PeT course (Fig. 1). After reaction with acetyl chloride, BDP-CHD has transformed to BDP-AC, in which the molecular skeleton of BDP-CHD was maintained, but the primary amino group has converted to acetamide. This observation indicated that acetyl chloride has reacted with the primary amine of DCH to form acetamide, which might be reasoned from the fact that the aliphatic primary amine group has much stronger reactivity than tertiary amine (Figs. 5a and b). After reaction with acetyl chloride, the PeT course from amino group to BODIPY core was prohibited. As a result, BDP-CHD exhibited a large fluorescence enhancement (Fig. 1c).

  Figure 5

  

  Figure 5.  (a) The single-crystal structure of BDP-CHD, BDP-Phos and BDP-AC. (b) The reaction mechanism of BDP-CHD with phosgene and acetyl chloride, respectively.

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  We also investigated the working mechanism of BDP-CHD for phosgene, and obtained the single crystal of the final reaction product BDP-Phos (Fig. 5a). The single-crystal structure of BDP-Phos showed that phosgene has reacted with the primary amine and the tertiary amine of BDP-CHD to form octahydrobenzimidazolone. According to these observations, we proposed the possible working mechanism as following: the primary amine reacted with one chloride of phosgene to form an acetamide, then the adjacent tertiary amine attacked another chloride of phosgene in a nucleophilic addition-elimination reaction. As a result, the DCH group in BDP-CHD converted to octahydrobenzimidazolone, and a normal C—N single bond (1.411 Å) was formed, suggesting the probe has transformed to a normal BODIPY core (Fig. 5a). Thus, BDP-CHD displayed a remarkable chromogenic reaction along with high contrast green fluorescence response towards phosgene (Figs. 1a and b).

  Inspired by the above exciting properties, we would like to fabricate BDP-CHD into test strips for on-site screening phosgene and acyl chlorides vapors. Herein, melt-blown fabric was utilized as the matrix for test strips, which would improve the detection efficiency due to their high adsorption capacity. As shown in Fig. 6a, BDP-CHD test strips exhibit weak blue fluorescence. After exposure to phosgene (0.5–20 ppm), the original weak blue fluorescence of test strips changed to bright green fluorescence. To determine the accurate concentrations of phosgene and acyl chlorides, we fabricated a portable fluorescence sensing platform that can be used along with a smartphone with a color app to record RGB values of the strips (Fig. 6b). As shown in Fig. 6c, there was a good linear relationship between the ratios of green/blue (G/B) with the concentrations of phosgene in the range of 0–5.0 ppm (R² = 0.992), and an obvious fluorescence color change from blue to green was observed from the CIE chromaticity diagram (Fig. 6d). By contrast, acyl chlorides vapors boasted a remarkable blue fluorescence enhancement, and the B values exhibited a good linear correlation (R² = 0.994) with the concentrations of acetyl chloride (0–5.0 ppm). The CIE chromaticity coordinates also exhibited notable changes from weak blue to bright blue (Fig. 6e). Therefore, the BDP-CHD based sensing platform could be used as a portable detection tool to high-throughput determine the phosgene and volatile acyl chlorides. This portable high-throughput sensing platform is quite practical and superior to traditional instrumental analysis, because it did not rely on large stationary instruments and skilled operators.

  Figure 6

  

  Figure 6.  (a) Fluorescence response images of BDP-CHD test strips towards phosgene and acetyl chloride vapor (0–20 ppm). (b) Schematic illustration of fluorescent sensing platform integrated in a smartphone. (c) Linear correlation of the G/B values of strips with phosgene (0–5.0 ppm). Linear correlation of the B values of strips with acetyl chloride vapor (0–5.0 ppm). CIE1931 coordinates of BDP-CHD test strips towards incremental (d) phosgene or (e) acetyl chloride (0–20 ppm).

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  To examine the selectivity, BDP-CHD test strips were exposed to a variety of analytes vapors, including diphosgene, DMMP, DCP, TsCl, POCl₃, SOCl₂, HCl, BzCl, OC, AC, FA, MGO, acrolein, triphosgene and phosgene. As shown in Fig. 7a, DMMP, HCl, FA, MGO and acrolein did not cause any fluorescence changes. However, BDP-CHD test strips produced bright green fluorescence after exposure to phosgene. By contrast, the test strips exhibited strong blue fluorescence after exposure to DCP and some other acyl chlorides vapors. These significant fluorescence differences can be observed from the CIE chromaticity diagram, and can be used to discriminate phosgene, acyl chlorides and some other analytes (Fig. 7c). Furthermore, we used LDA to analyze the RGB values of BDP-CHD test strips after exposure to all these tested analytes, and found that phosgene, acyl chlorides and some other analytes located at different locations in the 2D LDA diagram (Fig. 7b). These findings indicate that BDP-CHD sensing platform can serve as a portable and practical tool for high-throughput screening phosgene and volatile acyl chlorides, and has great potential to be used for security screening in public.

  Figure 7

  

  Figure 7.  (a) Fluorescence photographs of BDP-CHD-loaded strips upon treatment with phosgene, acyl chlorides or various analytes (20 ppm): (1) blank, (2) diphosgene, (3) DMMP, (4) DCP, (5) TsCl, (6) POCl₃, (7) SOCl₂, (8) HCl, (9) BzCl, (10) OC, (11) AC, (12) FA, (13) MGO, (14) acrolein, (15) triphosgene, and (16) phosgene. (b) RGB values LDA diagram of BDP-CHD test strips for 16 analytes, respectively. (c) CIE1931 coordinates of BDP-CHD test strips in the presence of 16 analytes, respectively.

  DownLoad: Full-Size Img PowerPoint

  In summary, we have successfully developed a versatile fluorescence platform based on a single molecular probe BDP-CHD to realize multiple target analytes sensing and discrimination. BDP-CHD reacted with phosgene to form an octahydrobenzimidazolone and BODIPY moiety, giving rise to strong green fluorescence accompanied with chromogenic reaction. By contrast, BDP-CHD reacted with acyl chlorides or DCP to form amide/phosphamide, which would inhibit the PET course and boasted remarkable blue fluorescence enhancement. BDP-CHD has been fabricated to test strips that can be used together with smartphone to form a portable intelligent sensing platform. On-site quantitative discriminate detection of multiple analytes has been achieved based on the RGB values of the test strips recorded by smartphone. Thus, this work not only develops a portable fluorescence sensing platform for high-throughput detection of phosgene, DCP and volatile acyl chlorides, but also provides a good strategy to develop sensors for screening multi-analytes.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Beitong Zhu: Writing – original draft, Methodology, Investigation, Formal analysis. Xiaorui Yang: Investigation, Formal analysis. Lirong Jiang: Methodology, Investigation. Tianhong Chen: Writing – review & editing, Methodology, Conceptualization. Shuangfei Wang: Writing – review & editing. Lintao Zeng: Writing – review & editing, Supervision, Conceptualization.

  Acknowledgments

  Acknowledgment for the financial support of the National Natural Science Foundation of China (No. 22168009).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110222.

  
  
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  Scheme 1  Schematic illustration of the probe for high-throughput monitoring of phosgene, diphosgene, acyl chlorides and DCP in molecular level.

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  Figure 1  (a) UV–vis absorption and (b) fluorescence spectra responses of BDP-CHD (5.0 µmol/L) to incremental triphosgene (0–5.0 µmol/L)/TEA (100 µmol/L) in chloroform. (c) Fluorescence responses of BDP-CHD (5.0 µmol/L) to acetyl chloride (0–5.0 µmol/L) in chloroform. Linear relationship between the (d) absorbance and (e) fluorescence intensity of BDP-CHD (5.0 µmol/L) with the concentration of triphosgene/TEA or (f) AC (0–5.0 µmol/L). λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  Figure 2  (a, c) Time-course fluorescence intensity and (b, d) images of BDP-CHD (5.0 µmol/L) in CHCl₃ containing triphosgene (5.0 µmol/L), TEA (100 µmol/L) or AC (5.0 µmol/L). λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  Figure 3  Fluorescence intensity changes at (a) F₅₂₄/F₄₇₆ and (b) F₄₇₆ of BDP-CHD solution (5.0 µmol/L) upon exposure to various analytes (10.0 µmol/L). (1) Blank, (2) diphosgene, (3) DMMP, (4) DCP, (5) TsCl, (6) POCl₃, (7) SOCl₂, (8) HCl, (9) BzCl, (10) OC, (11) AC, (12) FA, (13) MGO, (14) acrolein, (15) triphosgene and (16) phosgene. (c) 2D LDA diagram of fluorescent arrays consisting of 16 types of analytes. λ_ex = 430 nm, slits: 2.5 nm/5.0 nm. Data are presented as mean ± standard deviation (SD) (n = 3).

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  Figure 4  (a) Fluorescence spectra of BDP-CHD solution (5.0 µmol/L) and TEA (100.0 µmol/L) upon the addition of phosgene and AC (0–5.0 µmol/L). (b) Fluorescence spectra of BDP-CHD solution (5.0 µmol/L) upon the addition of AC, phosgene or both of AC and phosgene. λ_ex = 430 nm, slits: 2.5 nm/5.0 nm.

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  Figure 5  (a) The single-crystal structure of BDP-CHD, BDP-Phos and BDP-AC. (b) The reaction mechanism of BDP-CHD with phosgene and acetyl chloride, respectively.

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  Figure 6  (a) Fluorescence response images of BDP-CHD test strips towards phosgene and acetyl chloride vapor (0–20 ppm). (b) Schematic illustration of fluorescent sensing platform integrated in a smartphone. (c) Linear correlation of the G/B values of strips with phosgene (0–5.0 ppm). Linear correlation of the B values of strips with acetyl chloride vapor (0–5.0 ppm). CIE1931 coordinates of BDP-CHD test strips towards incremental (d) phosgene or (e) acetyl chloride (0–20 ppm).

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  Figure 7  (a) Fluorescence photographs of BDP-CHD-loaded strips upon treatment with phosgene, acyl chlorides or various analytes (20 ppm): (1) blank, (2) diphosgene, (3) DMMP, (4) DCP, (5) TsCl, (6) POCl₃, (7) SOCl₂, (8) HCl, (9) BzCl, (10) OC, (11) AC, (12) FA, (13) MGO, (14) acrolein, (15) triphosgene, and (16) phosgene. (b) RGB values LDA diagram of BDP-CHD test strips for 16 analytes, respectively. (c) CIE1931 coordinates of BDP-CHD test strips in the presence of 16 analytes, respectively.

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