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Visualization, Ultrasensitive and Recyclable Dual-Channel Fluorescence Sensors for Chemical Warfare Agents Based on the State Dehybridization of Hybrid Locally-excited and Charge Transfer Materials Xiaobai Li, Yining Lv, Siyu Chang, Huaqian Liu, Wanqi Mo, Hongwei Ma, Changjiang Zhou, Shitong Zhang, and Bing Yang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02085 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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Analytical Chemistry

Visualization, Ultrasensitive and Recyclable Dual-Channel Fluorescence Sensors for Chemical Warfare Agents Based on the State De-hybridization of Hybrid Locally-excited and Charge Transfer Materials Xiaobai Li, †,ǁ Yining Lv,† Siyu Chang,† Huaqian Liu,† Wanqi Mo,† Hongwei Ma,*,† Changjiang Zhou,‡ Shitong Zhang,*,‡,§ and Bing Yang*,‡ †Department

of Chemistry and Chemical Engineering, College of Science, Northeast Forestry University, Harbin 150040, P. R. China ‡State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China §Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China ǁNational Key Laboratory of Science and Technology on Advanced Composites, Harbin Institute of Technology, Harbin 150001, P. R. China Supporting Information ABSTRACT: Simple and fast detection of chemical warfare agents vapor is necessary and urgent to fight against the uncertain terrorist attacks and wars. In this contribution, inspiring by the design of hybrid locally excited and charge transfer (HLCT) excited state, two fast and highly sensitive visualization and fluorescence probes for DCP detection with relative small interstate coupling (J) TPA-2AC and TPA-9AC are reported. Upon exposure to saturated DCP vapor, the TPA-9AC test strips exhibited a rapid fluorescent response in no more than 1 s, accompanied by a change of the color from green to red. The detection limit of the test strips can be estimated as sensitive as 0.15 ppb, which is far superior to the “harmless” level (7 ppb) of human response to acute sarin exposure. More impressively, the fluorescent intensity of the test strips can be quickly restored when exposed to ammonia vapor for cyclic utilization, demonstrating an application prospect in the real-time detection of chemical warfare agents.

Nerve agents have been utilized as chemical warfare agents (CWAs) since the past century. They can rapidly destroy the nerve conduction in human and lead to death or painful sequelae.1 Although nerve agents have been strictly prohibited by the Chemical Weapons Convention of 1997, however, they are still used in uncertain terrorist attacks, leaving threaten to the public security 2 (Scheme 1a). Therefore, for the material scientists, fastrespond, real-time detection of nerve agents is becoming an urgent topic, 3 and certainly, it is a difficult task, due to the following two main problems. First, nerve agents are toxic and dangerous, so that the access to the nerve agents are always limited because of ethical issues, thus, proper replacements should be developed for general research. An example is that the diethyl chlorophosphate (DCP) is used as the simulant of (RS)-OIsopropyl methylphosphonofluoridate (known as the Sarin) for its lower toxicity and similar chemical activity. 4 The more important another problem is that the existing detection methods for nerve agents, for example, mass spectroscopy, 5 ion mobility spectrometry 6 and electrochemical method, 7 are usually high-

cost, using large instruments with complicated operation, which is not portable and not suitable for fast and real-time detection.

Scheme 1. (a) Chemical structures of typical nerve agents Sarin, Soman and Tabun, and their simulant diethyl chlorophosphate (DCP); (b) The detection mechanism for DCP vapor based on dehybridized HLCT excited states; (c) Preparation of test strips and the recyclable fluorescence and color response to DCP vapor (DCP, 60 ppm; NH3, 8743 ppm). Fluorescence detection of DCP has been gaining great attention because of its low-cost, high sensitivity, good portability and rapid response. 8 For example, Cheng et al described a concise and efficient fluorescence probe via intramolecular charge transfer (ICT) mechanism for DCP vapor detection, and the detection limit could reach 2.6 ppb. 9 Yoon et al reported a dual-channel fluorescence sensor for the discrimination between phosgene and DCP in the solution or gas phase. 10 Yang et al developed a

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“covalent-assembly” approach for sarin detection and the fluorescence sensors exhibited a colorimetric and a turn-on fluorimetric signal in sarin solution. 11 Obviously, great efforts have been made to develop fluorescence probes for DCP detection, but there are still some remaining problems that severely limited the on-site detection till now, such as long response time, poor sensitivity and repeatability. 12-14 As far as we know, most fluorescence probes for DCP detection are based on the mechanisms of photoinduced electron transfer (PET) or fluorescence resonance energy transfer (FRET), 15 they both rely on an newly generated energy trap state. However, it is only a necessary but not sufficient condition for effective energy transition and quenching. 16 To rationally design the fast response fluorescence probes, the essence of their excited state should be throughly reconsidered. In recent years, Ma and co-workers 17-19 reported a series of highly-efficient donor-acceptor (D-A) fluorescent materials for organic light emitting diode (OLED), utilizing the hybrid locally excited and charge transfer (HLCT) excited state. The excited states of these materials are consisted of two initial states of the locally excited (LE) state and the charge-transfer (CT) state, in which the former determains the photo luminenscent quantum yield (PLQY) and the essentially non-emissive latter is responsible for the utilization of electro-triplet excitions in OLED. Recently, Yang and co-workers also investigated the statehybridization of a series of acridines based D-A materials, and found that the D-A materials with better state coupling can greatly promote the limited PLQY of bare acridine, and the 9-substituted D-A material TPA-9AC is state-dehybridized, which is of relative low PLQY. 20,21 However, this work inspired us that in the occasion of DCP detection, the de-hybridized HLCT excited state should be in turn more expected to be sensitive to the external stimulation (Scheme 1b). Specifically, the N-containing aromatic hydrocarbon detection of DCP, 22 since the sp2-hybridized N atom can be efficiently protonated via a catalytic hydrolysis of DCP, and make it a more active acceptor unit for the formation of totally non-emissive CT excited state, which is expected to greatly help optimize the detection limit and shorten the response time. At the same time, the newly separated CT excited state can also offer an obvious color changing effect for visible detection without expensive instruments such as the fluorescence spectrophotometer (Scheme 1c).

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to DCP than TPA-2AC, which can be assigned to its dehybridization property as is reported. Meanwhile, the solution color changed from yellow to red, and the absorption spectrum of TPA-9AC and TPA-2AC demonstrated a newly generated wide absorption peak at 525 nm comparing to the bare TPA-9AC and TPA-2AC absorption spectrum (Figure 1a, Figure S2). Different from the TPA→AC ICT absorption at around 400 nm, 20 this n→π*-charactered peak represented a new ICT process arisen by DCP, which is closely related to the detection performance of TPA-9AC and TPA-2AC. In order to confirm the above excited state structure-property relationship and the detection mechanism between TPA-9AC and DCP, the titration experiment of 1H-NMR and 31P-NMR spectra were carried out. As is shown in Figure 1b, Upon addition of DCP, some proton signals of TPA-9AC shifted to the downfield, and other resonance signals demonstrate nearly no shifts. These results could be attributed to the protonation of the N atom in acridine group, which is consistent with previous reports. 25 Therefore, we can conclude a possible mechanism for the DCP detection of TPA-9AC by combining the results of 31P-NMR spectra (Figure S3): the N atom in the acridine group initially undergoes a nucleophilic substitution reaction with DCP, successively the substituted complex undergoes a hydrolysis reaction with H2O in the air, and the protonated TPA-9AC acted as the final quenched complex (Figure 1c). This two-step mechanism can act very fast properties of the two probes are investigated by Time-Dependent Density Functional Theory (TDDFT) calculations 26 in terms of the comparison between the lowest singlet excited states (or the S1 excited states) of the bare probes and those of the protonated probes (Figure 2). Firstly, the two probes both show great energy decreasing (Figure S4) and satisfied quenching in their oscillator strengths (f), and TPA-9AC demonstrates a more obvious quenching, which is in agreement with the exact performances (both color variation and quenching) of the two probes. This quenching can also be revealed by the NTO analysis of the S1 excited states. Before protonating, the S1 transition of both the two probes are LE-dominated, specifically, the LE transition of TPA-substituted acridine, thus they are emissive. However, after protonation, the vertical electron effect of acridine is forbidden, therefore ICT transition can more possibly take place in the protonated probes (which also matches

Figure 1. (a) The absorption spectrum of TPA-9AC before (black line) and after addition of DCP (red line); (b) 1H-NMR of TPA9AC before and after the addition of DCP (10 µM) in CDCl3; (c) The possible detection mechanism of TPA-9AC to DCP. Firstly, we investigated the fluorescent responses upon DCP stimulation of two reported HLCT materials TPA-9AC and TPA2AC. 17, 20, 23, 24 As is shown in Figure S1, upon addition of DCP into the solution, the TPA-9AC exhibited more obvious sensitivity

Figure 2. TDDFT calculations and the NTO calculations of the two probes. Herein, protonating is adopted to simulate the DCP substrate stimulation according to the previous researches.


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with the absorption spectrum in Figure 1a), resulting in the great energy decreasing and satisfied quenching of the oscillator strengths for effective DCP detection. More importantly, we can also understand that the essential distinction of their excited states decides the difference in the performance of DCP detection. In our previous work, we have demonstrated that TPA-2AC is of larger interstate coupling (J) between the S1 and the S2 excited states due to their different twisted angle, 24 so that the protonated TPA-9AC can even demonstrate a completely quenched f to an absolute zero due to its less hybridized excited states. This result indicates that TPA-9AC can be a more effective quenching probe for DCP. At the same time, combined with the experimental results (Figure S1) and theoretical calculations, we found that despite the calculated emissive energy of TPA-9AC (1.2457 eV), is even higher than TPA-2AC (0.9880 eV) (Figure S4), TPA-9AC demonstrates higher sensitivity than that of TPA-2AC, which is another evidence that its sensitive detection to DCP should be assigned to the state de-hybridization, rather than the traditional energy trap mechanism.

Figure 3. (a) Quenching efficiency of TPA-9AC test strips under various concentration of DCP vapor; Inset: the liner fluorescence

response of test strips to DCP vapor. (b) Fluorescence response of test strips to DCP and interferences vapor. Their concentrations are: DCP (1 ppm), hydrochloric acid (HCl, 21 ppm), acetic acid (HAC, 12 ppm), aniline (66 ppm), trifluoroacetic acid (TFA, 452 ppm ), ethyl acetate (EA, 1258 ppm), diethylcyanophosphate (DCNP, 208 ppm), pinacolyl methylphosphonate (PAMP, 1.7 ppm), pyridine (1271 ppm), benzene (4290 ppm), toluene (Tol, 677 ppm), dimethyl methylphosphonate (DMMP, 100 ppm) and triethyl phosphate (TEP, 101 ppm); (c) The recovery test of test strips (Solid arrows: quenching process; Dashed arrows: recovery process). Based on the state de-hybridization mechanism, the TPA-9AC test strips exhibited high sensitivity to DCP vapor with a wide linear range from 0.001-0.1 ppm (Figure 3a). Regarding to this result, the limit of detection (LOD) of this test strip can be estimated as excellent as 0.15 ppb. (Figure S5). This result is standout from the previously reported works (Table S1) as far as we know. Also, the LOD of 0.15 ppb can no doubt well meet the IDLH (Immediately Dangerous to Life or Health) concentration demand of sarin (7 ppb), since sarin is more reactive than DCP in this detection system, even for the more practical on-site detection use. 22 Moreover, the fluorescence response of test strips to the possible interferences were investigated, such as the volatile organic compounds (VOCs) and other nerve agent simulants. As is shown in Figure 3b, low concentration of DCP vapor could lead to significant fluorescence quenching of test strips, meanwhile, the test strips demonstrated almost no quenching to all the listed interferences even under very high concentrations. Notably, as we stressed in Figure 1c, the DCP detection mechanism of TPA-9AC is inferred to be undergoing a two-step fast reaction, and phosphorylation intermediate is only present in DCP detection, which will be the main reason for the high selectivity of TAP-9AC to DCP compared to HCl, HAC and TFA. 9,27,28 In addition, the binding force of the Brønsted acids and their protons could be strong in the organic acids or the gaseous, unionized inorganic strong acid. The last but not the least, the reversibility of fluorescence sensors is also a key factor in the continuous detection proces. Since the protonation of acridine groups can lead to fluorescence quenching and color change of test strips, simple deprotonation is a hopeful method for the recycling of test strips. As is shown in Figure 3c, after exposing to the saturated NH3 vapor for 30 s, the fluorescent intensity of the

Figure 4. The sketch of the quantitative colorimetric card of TPA9AC-DCP detection test strips: (a) for naked eyes (b) for fluorescent probes.



initially quenched test strips can be nearly fully recovered. More importantly, after enduring several continuous quenchingrecovery cycles, the detection ability of the test strips was not affected at all. Finally, for the practical applications, we hope to achieve a portable sensing approach for DCP vapor detection using TPA9AC. Regarding of this purpose, we calibrated a quantitative colorimetric card (Figure 4) by testing a series of certain amount of DCP vapor samples. In this contribution, firstly, without the aid of external tools, only by naked eyes we can clearly observe the color change from yellow to red with the increase of DCP concentration. Secondly, under a hand-held UV lamp (365 nm), a more precise green to purple color gradation was achieved, which indicated that an even better detection result can be realized by combining the TPA-9AC test strips to the commercial handheld. In conclusion, we mainly reported a visualization and ultrasensitive fluorescence sensor TPA-9AC for DCP vapor fast and accurate detection. TPA-9AC was more easily to realize a “on-off” effect based on the state hybridization and dehybridization of HLCT excited state than TPA-2AC, and showed better detection ability than TPA-2AC. We then achieved a highly effective DCP test strip based on TPA-9AC. This test strip exhibited high sensitivity to vapor with an LOD of 0.15 ppb, which is far superior to the “harmless” level (7 ppb) of human response to acute sarin exposure. Moreover, this test strip demonstrated high selectivity and excellent reversibility. Finally, for practical applications, the test strips as the color and fluorescence probes were used for DCP vapor detection, with clear quantitative color gradations. These excellent detection performances indicated that the new design strategy of HLCT fluorescence probes is hopeful and potential for the real-time, onsite and highly efficient DCP detection.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Videos of test strips were used for fluorescence and color response to DCP vapor. Detailed methods, characterizations and supplementary figures (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]; *[email protected]

ORCID Hongwei Ma: 0000-0003-0128-0828 Shitong Zhang: 0000-0003-0177-2929 Bing Yang: 0000-0003-4827-0926

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We are grateful for the financial supported by the Fundamental Research Funds for the Central Universities (2572018BC09), the Natural Science Foundation of Heilongjiang Province (YQ2019E003), China Postdoctoral Science Foundation Funded Project (2018M630331, 2018M641824 and 2017M620108), Heilongjiang Postdoctoral Fund (LBH-Z18010), the National

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Natural Science Foundation of China (51873077, 51803071 and 51673083), the Postdoctoral Innovation Talent Support Project (BX20170097).

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A decent fluorescence probe (TPA-9AC) was reported and its test strips for visualization, ultrasensitive and recyclable detection of the nerve agent simulant diethyl chlorophosphate (DCP), based on a new detection mechanism of the dehybridization of hybrid locally excited and charge transfer (HLCT) excited state with extraordinary detection abilities and applications.



A decent fluorescence probe (TPA-9AC) was reported and its test strips for visualization, ultrasensitive and recyclable detec-tion of the nerve agent simulant diethyl chlorophosphate (DCP), based on a new detection mechanism of the de-hybridization of hybrid locally excited and charge transfer (HLCT) excited state with extraordinary detection abilities and applications. 164x83mm (150 x 150 DPI)


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Scheme 1. (a) Chemical structures of typical nerve agents Sarin, Soman and Tabun, and their simulant diethyl chlorophosphate (DCP); (b) The detection mechanism for DCP vapor based on de-hybridized HLCT excited states; (c) Preparation of test strips and the recyclable fluorescence and color response to DCP vapor (DCP, 60 ppm; NH3, 8743 ppm). 280x264mm (150 x 150 DPI)



Figure 1. (a) The absorption spectrum of TPA-9AC before (black line) and after addition of DCP (red line); (b) 1H-NMR of TPA-9AC before and after the addition of DCP (10 µM) in CDCl3; (c) The possible detection mechanism of TPA-9AC to DCP. 258x160mm (150 x 150 DPI)


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Figure 2. TDDFT calculations and the NTO calculations of the two probes. Herein, protonating is adopted to simulate the DCP substrate stimulation according to the previous researches. 239x276mm (92 x 92 DPI)



Figure 3. (a) Quenching efficiency of TPA-9AC test strips under various concentration of DCP vapor; Inset: the liner fluorescence response of test strips to DCP vapor. (b) Fluorescence response of test strips to DCP and interferences vapor. Their concentrations are: DCP (1 ppm), hydrochloric acid (HCl, 21 ppm), acetic acid (HAC, 12 ppm), aniline (66 ppm), trifluoroacetic acid (TFA, 452 ppm ), ethyl acetate (EA, 1258 ppm), diethylcyanophosphate (DCNP, 208 ppm), pinacolyl methylphosphonate (PAMP, 1.7 ppm), pyridine (1271 ppm), benzene (4290 ppm), toluene (Tol, 677 ppm), dimethyl methylphosphonate (DMMP, 100 ppm) and triethyl phosphate (TEP, 101 ppm); (c) The recovery test of test strips (Solid arrows: quenching process; Dashed arrows: recovery process). 110x233mm (150 x 150 DPI)


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Figure 4. The sketch of the quantitative colorimetric card of TPA-9AC-DCP detection test strips: (a) for naked eyes (b) for fluorescent probes. 192x160mm (150 x 150 DPI)


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