An Organic Nanofibrous Polymeric Composite for Ratiometric Detection of Diethyl Chlorophosphate (DCP) in Solution and Vapor

Diethyl Chlorophosphate (DCP) is an important nerve agent mimic. Its misused use by terrorists and accidental release from industries creates severe panic among the general public. So, nowadays DCP sensing is of great interest to researchers. Herein, polymeric nanofibers in conjunction with 3‐(benzothiazol)‐5‐bromo‐2‐hydroxybenzaldehyde oxime (BBHO) was prepared as a specific sensing device for nerve gas simulant DCP. Scanning electron microscopic study revealed

Keywords: DCP quantification; organic nano fiber; Ratiometric fluorescence; Sensors; Supramolecular Chemistry

Polymeric nanofibers in conjunction with 3‐(benzothiazol)‐5‐bromo‐2‐hydroxybenzaldehyde oxime (BBHO) were prepared as a specific ratiometric sensing device for nerve gas simulant DCP. The electrospun nanofiber mat displayed distinct naked eye as well as fluorescence color change upon exposure to gaseous DCP.

Introduction

A chemical warfare agent (CWA) is a chemical substance, envisioned to be used in military purpose to kill, injure or incapacitate humans or animals through its toxicological effects. Nerve gases are the toxic, volatile organophosphorous reagents, widely used as a chemical warfare reagent.[[1], [3]] Nerve agents such as Tabun (GA), Sarin (GB) and Soman (GD) are the basis of chemical weapons and may enter in the body through inhalation or skin absorption. The reactive phosphate unit of these nerve agents irreversibly reacts with the hydroxyl group of the acetylcholinesterase (AChE), one of the neurotransmitters in the mammalian body, thereby disrupting the nerve impulses which is accompanied with neurological imbalance in the cholinergic synapse, organ failure and even a sudden death.[[4]] Not only to AChE, nerve agents also inhibit butyryl cholinesterase (plasma cholinesterase) and erythrocyte cholinesterase. Erythrocyte cholinesterase is a subtle indicator of nerve agent poisonousness and endorses experience to nerve agent.[6] Thus the development of a facile, reliable and selective method for detection of nerve agents is important for environmental and national safety. Due to loftier toxicity and unavailability of the real nerve agent, similar reactive nerve agent mimics are normally used for scientific research. Many methods and innovations have been developed for detection of that distortion on the surface of the nanofibers was due to DCP responsive chemical reaction with BBHO. The fibers were used to notice the degradation of DCP with time when the former was applied to different soil samples spiked with DCP. This probe can quantify DCP in different water sample. Most importantly, the limit of detection (LOD) of DCP in both solution and solid protocols are much lower than the safety level of DCP concentration to human exposure. these toxic species including enzymatic assays,[[7], [9], [11], [13]] surface acoustic wave (SAW) devices,[[14]] interferometry,[16] electrochemistry,[[17], [19]] microcantilever[20] and gas chromatography mass spectrometry.[21] But, those detection procedures are suffering from various limitations such as non‐portability, a slow response time, lack of specificity and sensitivity and especially, inconvenience in real‐time application. As an alternative, small organic fluorescent probes are very much attractive due to its operational simplicity, high sensitivity, low cost and a real‐time response. Those small organic fluorescent molecules displayed chemical and physical changes in presence of external stimuli like volatile toxic gases which led to observable changes in the absorption and emission wave lengths effectively. However, in the past decade, lots of fluorescent sensors for the detection of nerve agents have been reported.[[23], [25]] The general design strategy relies on photoinduced electron transfer (PET) process through nucleophlic attack of the probe to the electrophilic OP, thereby disrupting the PET process thus generating a turn‐on fluorescence signal. Thus, most of the fluorescent sensors are composed of amine or alcohol functional groups, which are responsible for phosphorylation reaction followed by intramolecular N‐alkylation to form quaternary amine salt.[[26], [28], [30]] An important limitation of the conventional PET sensors using the hydroxyl or amine functionality is the possible interference of acetylating, oxidizing agents, acids and phosphorylating substances, which leads to false response.[[31], [33]]

Dale et al. have reported several hydroxyl oximes for detection of nerve gases.[25] The oxime hydroxyl unit first reacts with OP nerve agents which then undergoes intramolecular cyclization with the ortho hydroxyl group to form different aryl isoxazoles. Although lots of turn‐on fluorescent chemosensors for nerve agent have been reported so far, only limited examples are available in the literature revealing about ratiometric sensors. A ratiometric sensor is much promising as it reduces the background interference thereby reducing the human as well as instrumental error. In this aid, our group made a cyclization induced emission enhancement (CIEE) based fluorescent chemodosimeter for ratiometric detection of nerve gas simulant DCP. Xuan et al. have made an effort to prepare a Förster resonance energy transfer (FRET) based ratiometric sensor for selective detection of DCP.[35] However, the vital limitation for most of these fluorescent sensors is related to the slow response rates and real time application.

Thus, to overcome all the previous problems, here in, we have introduced a 3‐(benzothiazol)‐5‐bromo‐2‐hydroxybenzaldehyde oxime (BBHO) riding on excited state intramolecular proton transfer (ESIPT) which could serve as selective ratiometric sensor for nerve agent simulant DCP. Being a 'super nucleophile', oximes are used as good sensor for nerve agent as it is capable of attacking the phosphorus center of the OP nerve agent thereby reducing the nerve impulse ability of AChE.[[36], [38]] But the main drawback in probe designing hampered the ratiometric nature of that probe. Thus, it is anticipated to build a fluorescence sensor for DCP to enhance the performance thereby improving the recognition unit which upon reactions with DCP, may deliver intensified fluorescence. Hence, we have designed the probe in such a way so that it would behave as a good ratiometric sensor for DCP. In this context, we have introduced a benzothiazole moiety ortho to the phenyl hydroxyl group, responsible for ESIPT process. Again, introduction of an oxime reactive unit to another ortho position of the phenyl hydroxyl group enhances the PET process. Upon reaction with DCP, an isoxazole compound is formed (scheme ), PET process was inhibited, which enhanced the ESIPT process and thus a bathochromic shift occurred in the emission spectra and a ratiometric fluorescence response was achieved.

In recent years a small organic molecule doped in a polymeric system is an essential ondemand protocol for detection of toxic gases.[[39], [41], [43]] Thus, in present days we are able to fabricate (BBHO) doped PVDF (polyvinylidene‐difluoride) electrospun fiber for rapid recognition of lethal DCP gas through naked eye conception of change in color of the covered organic nanomats. In this context, we are also able to show that response time of probe immersed filter papers to DCP is slower than that of the probe doped nano mats. Importantly, for the first time, we have checked the evaporation rate of DCP from different soil samples by optical method.

Results and Discussion

Compound BBHO was synthesized in following synthetic method (Scheme ). The spectral data of all relevant compounds are given the supplementary information.

Inspection of the spectroscopic responses towards DCP

At first, the optical properties of the probe were investigated by UV‐Vis absorption spectroscopy with various nerve gas agents. Absorption profile of the probe displayed in Figure a. DMF solution of the probe (1.0 μΜ in DMF) showed an intense band at 432 nm assigned to the n–π* transition. Upon concomitant addition of DCP, the absorbance intensity at 432 nm diminished with the advent of new band at 360 nm. Eventually, colour of the solution changed intensely from yellow to colourless, which allowed the direct visual detection of DCP (inset: Figure a). A well defined isosbestic point at 385 nm was observed, representative of the formation of a new compound. As shown in scheme , the probe upon reaction with DCP a phosphate ester was formed which then undergoes intramolecular cyclization to form a benz‐isoxazole derivative.

Next, we attempt for fluorescence behavior of the probe (1.0 μM) in presence and absence of DCP. Initially, the probe emitted green fluorescence (λex= 385 nm) in the region of 496 nm (Figure a). Upon gradual addition of DCP (1.8 equiv.) a new emission band at 554 nm in the Yellowish orange area appeared and progressively increased with respect to the green band upon addition of 1.8 equivalents of DCP (Figure S1).

A ratiometric fluorescence response was achieved. A clear isoemissive point at 514 nm supports the formation of a new complex which is nothing but the isoxazole derivative. Next, we attempt for fluorescence behavior of the probe (1.0 μM) in presence and absence of DCP. Initially, the probe emitted green fluorescence (λex= 385 nm) in the region of 496 nm (Figure a). Upon gradual addition of DCP (1.8 equiv.) a new emission band at 554 nm in the Yellowish orange area appeared and progressively increased with respect to the green band upon addition of 1.8 equivalents of DCP (Figure S1, Supplementary Information). A ratiometric fluorescence response was achieved. A clear isoemissive point at 514 nm supports the formation of a new complex which is nothing but the isoxazole derivative.

Initial low intense green fluorescence of the probe was due to combined effect of photoinduced electron transfer (PET) from =N‐OH group to the benzothiazole moiety and ESIPT of phenolic –OH group with benzthiazole moiety. After reaction with DCP as an isoxazole was formed the PET process was diminished. Chen et al. have reported that the cyano group may affect an enhancement of acidity of the phenolic –OH group and inhibit the ESIPT process and obtained a turn‐on response upon addition of the nerve agent (DCNP).[45] This is where we have changed the design strategy by putting a bromo substituent para to the –OH group. Incorporation of halogen reduces the acidity of the –OH group thereby enhancing the ESIPT process and producing less energetic keto form in dynamic keto ↔ enol tautomerisation process. Also, the bromo substituent reduce the acidity of the proton of isoxazole thereby restrict the formation of nitrile compound which in turn enhance the ESIPT process. This occurrence is accountable to the ratiometric fluorescence behavior of the sensor to DCP sensing.

According to Figure S2. Supplementary Information, a linear relationship that was established between I554/I496 with the concentration of DCP in the range 0.1‐0.8 μM at with a correlation coefficient of R2=0.9965. Based on 3σ/k (where σ is the standard deviation of the blank solution and k is the slope of the calibration plot), the detection limit of DCP was calculated to be 33.5 nM. This indicates appreciable sensitivity when compared to recently reported phosgene probes.

Selectivity studies for sensing of DCP: To clarify the selectivity of the probe to DCP, absorbance and fluorescence studies were carried out with different similar reactive analytes. The spectra signifies (Figure b & 2b) that BBHO is selective to DCP only. Also, a competitive fluorescence intensity ratio at 496 and 554 nm (I496/I554) as a function of the analytes added are shown in Figure S3 (Supplementary Information) which clearly indicates excellent selectivity to DCP.

The DMF solutions of the probe were inspect under a 385 nm handheld UV lamp after the addition of a variety of analytes and a noticeable fluorescence color change from green to Yellowish orange was observed in the presence of DCP (Figure ). It is significant to examine the detecting aptitude of the probe towards DCNP and other nerve gas agents as some of previously reported DCP probes experienced meddling from these agents.[46] Our results show no other substances instigated seeming changes in absorbance or fluorescence.

Response rate of BBHO to DCP in solution

Reaction rate is an imperative stricture to be a perfect sensor to detection of toxic gas. Figure a, displayed the time dependent fluorescence intensity of BBHO upon addition of DCP. It was noticed that, addition of DCP (1.8 equiv.) fluorescence intensity ratio (I554/I496) got saturated within 20 sec. Commencing the kinetic sketch for reactions of BBHO (1.0 μM in DMF) with DCP (10 equiv.), the pseudo first‐order rate constant was determined to be 0.2458 sec−1 (Figure b.).The rapid enhancement in fluorescence intensity is due to the DCP triggered quick chemical transformation. Importantly, such quick response and extraordinary sensitivity of BBHO to DCP, makes the probe a suitable candidate for inspecting DCP for its real time applications.

Reaction mechanism

The reaction between BBHO and DCP begins with nucleophilic attack of oxime hydroxyl group to the phosphorous centre of DCP to form oxime‐N−O‐ethyl phosphate (IM). The pehenolic hydroxyl group then undergo intramolecular cyclization[47] with IM to form the corresponding isoxazole derivative (scheme ). 1H NMR study revealed the evidence of formation of isoxazole compound. Partial 1H NMR spectra shows that oxime‐OH (Hb) and Phenyl–OH(Ha) protons of are resonating at 12.3 and 11.9 ppm respectively (Figure ).

Whereas, in isoxazole, those two characteristic protons are disappeared. This result suggests that both oxime‐OH and Phenyl –OH groups are taking part in the reaction and proved the formation of isoxazole. Also, ESI‐MS proves the formation of benz‐isoxazole. The spectrum found for the isoxazole compound has a major peak at m/z 329.9469 (calcd 329.9462 for C14H9BrN2O2S) (Figure S14, Supplementary Information).

DFT study

To distinguish the optical change due to the change in electronic properties of BBHO, before and after addition of DCP, density functional theory (DFT) calculations were carried out. To evaluate the DFT/TDDFT calculations B3LYP exchange functional retaining with 6–31+G(d,p) basis sets using Gaussian 09 programs were accomplished. Energy minimization structures of BBHO and compound 4 are displayed in Figure . Favoured electronic transition of BBHO and compound 4 are listed in Table S1 (Supplementary Information). The perpendicular key transitions projected by TDDFT (Table S1, Supplementary Information) were parallel with the experimentally perceived UV−vis spectra. HOMO→LUMO major transition (▵E=2.87 eV, f= 0.5176) observed at 430 nm is comparable with the practically observed band at 432 nm (in DMF solvent). Again, HOMO →LUMO energy difference of compound 4 is 3.47 eV and the corresponding band observed at 357 nm is also comparable to those of the practical value of 360 nm. This enhancement in energy from BBHO to the nitrile compound is accountable for the blue shift in absorbance spectra.

Also, DFT/TDDFT calculation reveals the fluorescence properties of BBHO and compound 4. A photo induced electron transfer (PET) inhibited ESIPT process was responsible for the initial green fluorescence of BBHO. The PET process was diminished in nitrile compound and thus a d‐PET compound 4 enhanced the ESIPT process is responsible for Yellowish orange fluorescent. This phenomenon can be proved by the DFT/TDDFT method. Initially, the PET process was continued from oxime‐phenyl group to benzthiazole moiety. DFT/TDDFT revealed that energy of HOMO of thiazole unit remained at −0.2371 a.u. and LUMO at −0.03644 a.u.. Again, energy of HOMO of oxime‐phenyl group remained at −0.21701 a.u. and that of LUMO remained at −0.03427 a.u.. In the excited state, radiative migration of the electron back to the HOMO of benzthiazole moiety is inhibited due to PET (Nonradiative) from HOMO of oxime‐phenyl group. This is due to the relative positioning of the HOMO and LUMO of benzthiazole and oxime‐phenyl in terms of energy. This electronic distribution inhibit the emission of BBHO. Upon reaction with DCP as an isoxazole compound was formed, HOMO energy level got reduced to −0.24611 a.u. and thus the PET process was inhibited. Thus, strong ESIPT process was facilitated in isoxazole compound which is responsible for bathochromic shift in emission spectra. So, a ratiometric fluorescence characteristic was achieved.

Response to gaseous DCP of BBHO in solid protocols

As conferred previously, recognition of DCP in gas stage is much significant for agitation of environmental safety, synthesis of a modest device as of nano and microscale constructions via electrospining is the most persuasive tactic. The PVDF elastomer was favored as the polymer matrix due to its commendable properties, as it is low cost polymer, easily obtainable, optically transparent, biocompatible, registant to UV irradiation and possess good mechanical steadiness. Most importantly, this elastomer is very much potent to fabricate by electrospinning. Preparation procedure of nanofiber by electrospinig is inscribed in experimental section.

An average diameter of 350±30 nm fibers were acquired by using 12% wt of PVDF with respect to 0.1 wt of BBHO. Prepared fibers have wire like architecture (Figure a & 7b) as confirmed by scanning electron microscopic study (SEM images).Upon exposure of DCP(2 min) the uniform wire like architecture of composite PVDF become uneven and surface of the fiber become rough (Figure c).

It is expected that reaction of BBHO in PVDF polymer matrixes with DCP yield the consistent benz‐isoxazole derivatives on the surface of nanofibers and thus a partial deformation in the surface of the nano matrix was achieved.

The polymeric fiber having BBHO, exhibitions same fluorometric and colorimetric change as was in solution upon experience to DCP (Figure  top). Examination of the fluorescence images of BBHO/PVDF merged nanofiber shows distinct color change from green to orange upon acquaintance to DCP. The result designates that this easy technique can be employed to quick detection of this toxic gas. The reaction time for BBHO/PVDF composite nanofiber to 3.0 ppm DCP is ∼25 sec (Figure S4, supplementary information) is lesser than that of formerly conveyed DCP sensors (Table S4, supplementary information). Furthermore, we analyses the inferior level of DCP gas that can be perceived in nano fiber mats to be 0.72 ppm (Figure S7, supplementary information) (upon 2.0 minute exposure to the gas). Thus, 0.72 ppm is much lower (7 ppm) than the IDLH (Immediately Dangerous to Life or Health) recommended safety level of sarin (sarin is the most reactive nerve gas).[48] Thus, our polymeric composite nanofiber device delivered as a potent sensory system for selective detection of a health hazardous toxic DCP in very quick response time.

Eventually, we did the same experiments for BBHO embedded filter paper for the detection of DCP gas, as an another sensory device. Figure  (bottom) revealed that, fluorescence colour of the test strips changed green to orange rapidly upon exposure to DCP. Response time of DCP exposure to the strips is ∼ 45 sec (Figure S5, Supplementary information). The limit of detection of the gas is about 1.25 ppm (Figure S8, Supplementary information). Notably, both response time and LOD that have calculated by using the strips have higher value than the corresponding polymer matrix. Thus, more sensitive device like polymeric nano matrix is required for rapid and selective detection of toxic gas. In this context, it is not worthy to mention that nanofibrous composite exemplifies a greater specific surface to volume ratio as compare to probe immersed filter paper of the same material, permits a more proficient and homogeneous adsorption of gas thus heighten the interaction between the gas molecules.

Fluorescence imaging of BBHO/PVDF nano fiber upon exposure to DCP

Fluorescence imaging of the BBHO/PVDF composite nanofiber before and after exposure to DCP are shown in Figure , Initially, the BBHO/PVDF nanofiber was green colour (Figure d) which upon exposure to DCP vapour turned into orange (Figure  9 h). An intense orange fluorescence is responsible for DCP promoted chemical reaction of BBHO to form the corresponding isoxazole derivatives. Thus, fluorescence imaging of the BBHO/PVDF nano composite serve as a good colorimetric and ratiometric fluorescent device for detection toxic nerve gas like DCP.

Detection of DCP in different water sample

It is well known that the halogen in the sarin is highly labile and undergo nuleophilic substitution in presence of H2O, OH−, amines etc. Thus, sarin gets readily hydrolyse in aqueous solution but depending upon the pH of the solution. The hydrolysis product of sarin is methylphosphonic acid (MPA). Again, sarin with this residual acid degrades a period of several months. Thus, it is very much essential to detect the sarin toxicity in different water sample. In most of the reported methods are relying on the separation of hydrolysis product to calculate the reaming percentage of sarin in water samples.[49] But those procures are very much complicated and not easy to handle. Thus, here in our case, we have collected different water samples and added a known amount of DCP to it (a sarin mimic). Fluorescence measurement with the probe reveals that our probe can quantify the amount of sarin remains after different time intervals. The result suggested that, only a 10% of sarin remains intact after 2 days (Figure S9, Table S2, Supplementary information). Again, the recovery rate is highly pH responsive of the water sample. River water (The Ganga, India) being a slightly lower in pH (6.20) shown to be highest recovery sample than the rest. Thus the results suggest that, our sensory system delivered as a unique protocol for quantification of DCP in different water sample with an easy and low cost way.

Detection of DCP in various soil samples. It still remained as a problem for extraction of nerve agent from complex matrixes, e. g. soils. Soil is known to be a sampling medium for authentication of alleged use of chemical warfare reagent on the field. Even, lots of use in battle field, it is obvious to detect the toxicity level of nerve agent in the soil. The volatility potential of V type nerve agent (VX) is 3.0 x 10−11 mm Hg/mg/kg, indicate low evaporation rate in to air.[[50], [52]] Thus, they may moderately persistent on bare ground. Persistent possibility depends on carbon content in the soil, moisture and temperature. Here, in our case, we have used three different soil samples for quantification of remaining DCP at different time intervals. The results of the experiment is listed in (Figure S10, Table S3, Supplementary information). The results indicate that, moisture contain clay material able to recover 44% of DCP after one day. Whereas, DCP was recovered fairly efficiently from other two soil (dry sandy materials) with typically in the range of 49 to 51%. Thus, it is not worthy to mention that only using a solid protocol of fluorescent material is the easiest way to detect the level of soil toxicity created by the harmful nerve agent.

Conclusion

In summary, in this report, we have prepared a 3‐(benzothiazol)‐5‐bromo‐2‐hydroxybenzaldehyde oxime (BBHO) derivative for selective detection of DCP. At first, inherent oxime sensitive unit of BBHO reacts with DCP. The probe displayed ratiometric emission spectral response upon reaction with DCP to from oxime N−O‐diethylphosphate which then undergo intramolecular cyclization with the phenyl hydroxyl group to form an isoxazole derivatives. This chemical transformation is responsible for distinct color and fluorescence change in naked eye as well as in the electrospun nanofiber mat upon exposure to gaseous DCP. Having a greater surface area, the fiber showed quick response than the probe immersed filter paper towards DCP. The extensive penetrating capacity of gaseous DCP to the surface of the nano mats delivered very low limit of detection of 0.72 ppm. The fiber displayed same visual change as in the solution state. SEM images of fluorescent microscopic study manifest that partial deformation on the surface of the nano material was due to DCP triggered chemical reaction with BBHO. The nano composite was utilized for detection of DCP in different solid samples. We are able to detect fraction of DCP present in different water sample at different time intervals. Thus, this efforts conveyed to establish a cost effective way to detect a nerve agent simulant e. g. DCP in different soil and water samples.

Supporting Information Summary

Synthesis procedures for the probe and its reaction product have been described in the Electronic Supporting Information along with characterization of the compounds synthesized, including 1H, 13C & ESI‐MS Spectra.

Acknowledgements

We are grateful to DST‐SERB [File No. EMR/2016/006640] for financial support. SSA wish to acknowledge UGC‐MANF (MANF‐WES‐67786) for the financial and research support

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

GRAPH: Supplementary

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By Uday Narayan Guria; Kalipada Maiti; Syed Samim Ali; Ankita Gangopadhyay; Sandip Kumar Samanta; Krittish Roy; Dipankar Mandal and Ajit K. Mahapatra

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