Simple and Efficient Chromophoric-Fluorogenic Probes for

Jul 30, 2018 - In this work, we developed two small-molecule probes for real-time and onsite detecting of diethylchlorophosphate (DCP) vapor by ...
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Simple and efficient chromophoric-fluorogenic probes for diethylchlorophosphate vapor Yanyan Fu, Jinping Yu, Kaixia Wang, Huan Liu, Yaguo Yu, Ao Liu, Xin Peng, Qingguo He, Huimin Cao, and Jiangong Cheng ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00313 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Simple and efficient chromophoric-fluorogenic probes for diethylchlorophosphate vapor Yanyan Fu*a, Jinping Yu a,b, Kaixia Wanga,b, Huan Liua,b, Yaguo Yua,b, Ao Liu a,b, Xin Peng a,b, Qingguo He a, Huimin Cao a, and Jiangong Cheng*a

a

State Key Lab of Transducer Technology, Shanghai Institute of Microsystem andInformation Technology, Chinese Academy of Sciences, Changning Road 865, Shanghai 200050, China. b

University of the Chinese Academy of Sciences, Yuquan Road 19, Beijing,100039, China

ABSTRACT: In this work, we developed two small-molecule probes for real-time and onsite detecting of diethylchlorophosphate (DCP) vapor by incorporating amine groups into Schiff base skeletons. Both probes can be easily synthesized with high yield through one-step and low-cost synthesis. They can detect DCP vapor in the chromophoric-fluorogenic dual mode, which combines both the advantages of the visualization of color sensing and the high sensitivity of the fluorescence sensing. Furthermore, its sensing is based on the "turn-on" mode which can avoid the interference arising from photobleaching or fluorescence quenching agents based on “turn-off” mode. The detection limit was quantified to be as low as 0.14 ppb.

KEYWORDS:

chromophoric-fluorogenic;

probe;

Detection of trace chemical warfare agents (CWAs) continues to receive particular attentions in recent years due to the frequent occurrences of terrorist incidents around the world. In particular, the nerve agents (e.g., Sarin, Soman and Tabun) which inhibit enzyme acetyl cholinesterase can cause 1-3 deathful threats to public security . For instance, the notorious colorless sarin gases were used on the Tokyo subway by 4 terrorists in 1995 leading to 13 deaths and over 5000 injuries . Although some traditional detection methods such as ion 5 6 mobility spectroscopy , mass spectrometry , enzyme-based 7 8 biosensors , surface acoustic waves and electrochemical 9-13 methods have been applied for the sensing of nerve agents, a rapid, sensitive and reliable detection method is urgently needed. Chromo-fluorogenic sensing is an ideal detection method for nerve agent due to its features of visualization, fast response time, high sensitivity and portable 14-18 testing instrument.

diethylchlorophosphate;

22,23

vapor;

24-28

synergistic

29,30

effect

and pyridine are usually oxime , aliphatic hydroxyl designed in the sensing materials for their good nucleophilicity towards electrophilic phosphorus atoms. However, the design of a good fluorescent probe for DCP still faces the following challenges: (1) Lots of existing fluorescent probes have complex structures, requiring multi-step and high-cost synthesis; (2) Many probes focus only on the detection in solution. However, the detection of gaseous DCP is less involved which is more valuable in practical applications; (3) Most of these probes are based on a “turn-off” mode which is prone to disturbances by photobleaching or conventional fluorescent quenchers (e.g., halogen ions, heavy metal ions, oxygen molecules, nitro compounds). On the contrary, a “turn-on” fluorescent probe from zero background is more attractive. For these reasons, a simple and “turn-on” probe for DCP vapor sensing is highly desired.

Because of their lethal toxicities, sarin and other nerve agents have been strictly controlled. DCP was broadly used as mimics of Sarin in library due to its similar reactivity with 19-21 nerve agent and low toxicity . So far, quite a few smart fluorescent probes have been developed for DCP sensing. Among them, several typical structures such as hydroxyl

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weak fluorescence in solutions (Fig. 1b). Upon the addition of DCP, a fluorescence emission peak appeared at 477 nm corresponding to strong fluorescence of yellow-green. The bind2 -1 ing constant was determined to be 1.30×10 M according to the empirical formula for calculating the complex constant 35 based on fluorescence enhancement (Fig. S1 and Fig. S2) .

Scheme 1 Structures of probes 1，2 pounds M1, M2.

and model com-

Schiff base is a good skeleton framework to construct fluorescent probes due to its easy-to-synthesize and easy derivation. So far, most of the fluorescent probes based on Schiff 31,32 33 bases are used to detect metal ions and anions .They are rarely used to detect small molecule. This is mainly due to the lack of appropriate recognition groups for molecule analyte. Taraba et al previously reported some fluorescent probes containing amine groups to discriminate chemical 34 warfare agents . In their study, the fluorescence of these probes was quenched in some extension by a photoinduced electron transfer from free amine group. Subsequent acylation or phosphorylation of the amine group therefore increased fluorescence of these compounds. Amine group is thus proven a good binding unit. Inspired by this, we propose to combine the Schiff base structure with the amino functional group. Owing to the good nucleophilic ability of amine, it can interact with the phosphorus in DCP which may change the optical properties of the molecule. Herein, we design two simple probes for DCP sensing through onestep synthesis. Compound 1, 2 are conveniently synthesized through Schiff-Base reaction with high yields by reacting corresponding aldehydes with hydrazine hydrate (Scheme 1). The molar -1 -1 -1 -1 extinction coefficients are 43200 M cm and 63600 M cm for 1 and 2, respectively. Obviously, compound 2 has a higher molar extinction coefficient than compound 1 indicating the possible higher sensitivity for DCP detection using colorimetric technique.

-5

Figure 2. (a) The UV/Vis response of 2 (THF, 1×10 M) upon addition of DCP (0-550eq.） (b) The fluorescence re-5 sponse of 2 (THF, 1×10 M) upon addition of DCP（0-550eq. ） The low binding constant between compound 1 and DCP suggested the poor interactions between them. Increasing the number of binding functional groups may improve the binding constant. Compound 2 was thus designed which contains two recognition sites in one molecule. As shown in Fig 2a, the maximum absorption peak of 2 was located at 384 nm. With the addition of DCP, the absorption peak at 384 nm decreased and a new absorption peak appeared at 470 nm. The change of fluorescence spectra (Figure 2b) upon addition of DCP to compound 2 was similar to those obtained with 1. With the addition of DCP, the solution of compound 2 changed from non-fluorescence to orange fluorescence with maximum emission peak at 570 nm. The binding constant between 2 and DCP was determined to be 4 -1 7.40×10 M which was 569 times higher than that of 1· DCP. Obviously, adding complexation site in molecule increases the contact probability of DCP and compound 2. Correspondingly, the intermolecular complexation constant is improved.

-5

Figure 1. (a) The UV/Vis responses of 1 (THF, 2×10 M) upon addition of DCP (0-750eq.) (b) The fluorescence respons-5 es of 1 (THF, 2×10 M) upon addition of DCP (0-750eq.) Firstly, the absorption and emission spectra of 1 in solutions along with their responses to DCP were investigated. As shown in Fig. 1a, the maximum absorption peaks of 1 were initially located at 370 nm. With gradual addition of DCP to the solution of 1, the absorption peak of compound 1 at 370 nm decreased and a new absorption peak appeared at 440 nm. At the same time, the color of the solution changed from light yellow to yellow (Fig. 1a). Compound 1 showed very

Figure 3 (a) Fluorescent spectra of the film 1 before (blue) and after (red) exposure to saturated DCP vapor for 300 s at room temperature. (b) Time-resolved fluorescent intensity of film 1 at 570 nm upon consecutive exposure to saturated DCP vapor. (c) Fluorescent spectra of the film 2 on a quartz plate before (blue) and after (red) exposure to saturated DCP va-

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ACS Sensors por for 300 s at room temperature. (d) Time-resolved fluorescent intensity of 2 film at 605 nm upon consecutive exposure to saturated DCP vapor.

Compared with the detection in the solution phase, it is more valuable to detect gas analytes in situ as a real-time assay. In order to deeply investigate the possibility of vapor sensing, we further studied the responses of film 1 and 2 to DCP vapor. Films of compounds 1 and 2 on quartz substrates were prepared respectively using spin-coating methods. In comparison with those in the solution, the maximum absorption peaks of film 1 and film 2 showed a little blue shift to 354 nm and 370 nm, respectively. （Fig. S3 and Fig. S4） Films 1 and 2 both showed weak fluorescence in solid states. When film 1 and 2 were exposed to DCP vapor, a strong fluoresce of orange and red occurred, respectively, with the maximum emission wavelengths at 570 nm and 605 nm, respectively (Fig 3a, 3c). In order to investigate the effect of films made by different concentrations on the fluorescence enhancement, we prepared different films made from 1 mg/ml, 2 mg/mL and 4 mg/mL solutions by concentration control. Their time-resolved fluorescent intensities upon consecutive exposures to saturated DCP vapor were investigated. When the films were made by relatively dilute solutions such as 1 mg/mL and 2 mg/mL, the fluorescent intensities of the films in DCP vapor increased to a certain extent and then quenched (Fig S5 and Fig S6). When the film was made by nearly saturated solutions (e.g., 4 mg/mL), the fluorescent intensities first increased to a certain extent and then kept stable. Because dilute solutions produce thin films. Accordingly, the photo bleaching effect of thin film will be significant, leading to unstable fluorescence. Thus, unless otherwise mentioned, 4 mg/mL was selected as the optimized condition to prepare films in this study. Upon exposure in saturated DCP vapor for 300 seconds, the fluorescent intensities of the films 1 and 2 increased by 25 times and 40 times respectively (Fig 3b, Fig 3d), indicating its good response to DCP vapor. The quantum efficiencies of the 1, 1·DCP, 2 and 2·DCP in the film are 0.23%, 2.85%, 0.13 % and 2.48% respectively, measured by the calibrated integrating sphere. Obviously, the quantum efficiencies of film1 and film 2 have been greatly improved after sensing. This is very consistent with the experimental phenomena we observe. To evaluate the sensitivity of two probes for DCP vapor, we tested the fluorescent changes of film 1 and film 2 in DCP gas with different concentrations. Different concentrations of DCP vapor were obtained by stepwise dilution of saturated DCP vapor (132 ppm). When the concentrations of DCP vapor were 13.20 ppm, 6.60 ppm, 2.64 ppm, 1.32 ppm, 0.66 ppm, and 0.33 ppm, respectively, the fluorescence enhancement of film 1 was 578%, 392%, 218%, 88%, 63%, and 33%, respectively. The changing data of fluorescence is well-fitted to the Langmuir equation. The detection limit for film 1 was thus extrapolated according to their fitted plots. The detection limit of film 1 was de-

termined as 4.0 ppb based on S/N=3 (Fig S7.) We studied the response of film 2 to DCP in the same way. The fluorescence enhancement of film 2 was 696%, 336%, 187%, 130%, 82.1%, and 67.5%, respectively, as the concentrations of DCP vapor were 28.20 ppm, 6.60 ppm, 2.64 ppm, 1.32 ppm, 0.66 ppm, and 0.33 ppm, respectively. The results revealed that the detection limit of probe 2 for DCP vapor was 0.14 ppb (Fig S8). Obviously film 2 is more sensitive than film1 1. Both values are all below the IDLH (Immediately Dangerous to Life or Health) concentration of Sarin at 7 ppb36 indicating their good potential application. As a comparison, we summarized the properties of these two probes and other representative chromophoric/fluorogenic probes for DCP reported in the past two years (Table S1).31, 32, 37-40 The literatures [37]- [40] investigated in depth the application of DCP sensing materials in solutions. However, gas-phase sensing research were not involved too much. Our group reported two “turn-off” sensitive probes TBPY-TPA and HPFP for DCP vapor in 2016 and 2017. 29, 30 (Scheme 2). The detection limit of Probe 2 (0.14ppb) is an order of magnitude lower than that of TBPY-TPA (2.4ppb). This indicates that a simple molecule can also achieve excellent detection effect by smart design. Compared to the detection limit of HPFP (0.064ppb), the detection limits of probes 1 and 2 are slightly higher. However, the advantages of low cost, easy synthesis and visualization make probe 1 and probe 2 more versatile

Scheme 2 Structures of TBPY-TPA and HPFP In order to explore the influence of possible interferents on sensing effect, we studied the fluorescent changes of film 1 and film 2 upon exposure in air and saturated vapor of DCP, dimethyl methylphosphonate (DMMP), pinacolyl methylphosphonate (PAMP), triethyl phosphate (TEP), diethylcyanophosphate(DCNP), respectively. As shown in Figure 4, the film 1 changed from nonfluorescence to strong orange fluorescence after 100 s exposure in DCP vapor. While in presence of other possible interferents including DMMP, PAMP, TEP and DCNP, no

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obvious fluorescent changes were observed indicating its high selectivity to DCP. Similar results were also obtained for thin film 2 with the red fluorescence in DCP vapor (Fig. S9). Along with the fluorescent changes, both film 1 and film 2 also demonstrated obvious color change in DCP vapor which was easily distinguished by naked eyes (Fig. S10). Diisopropylfluorophosphate (DFP) is also a common simulant. Unfortunately, the probes were not tested against DFP due to institutional restrictions on use of this hazardous chemical. In addition, the effects of common volatile solvents on the detection of DCP were also investigated. In common organic solvents including dichloromethane, tetrahydrofuran, acetone, ethanol and ethyl acetate, the fluorescence intensity of film 1 was almost unchanged (less than 5%, Fig. S11). Film 2 also showed the same excellent selectivity towards DCP vapor (Fig S12). Considering that volatile acid, such as hydrochloric acid (HCl), might lead to false positive result. We also tested the fluorescence response of film 1 and 2 with HCl (Fig S13, Fig S14). In HCl vapor, the color of film 1 and film 2 all changed to red. The emission maximum of film1 and film 2 red-shifted to 585nm and 619 nm, respectively. In DCP vapor, the fluorescence emission of film 1 and film 2 is 570 nm and 605 nm, respectively. This means that we can distinguish the HCL and DCP by the position of the fluorescence emission peak after the reaction.

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solution is at 280 nm with a shoulder peak at 329 nm. As shown in Fig. S17, the maximum absorption peak at 280 nm increases as DCP (0-700 eq) is gradually added to the M2 (2×10 −5 M) solution. At the same time, the shoulder peak at 329 nm is gradually shifted to 312 nm. When the film of M2 is exposed to DCP vapor, its fluorescence can be partially quenched (53%). Obviously, only one kind of nitrogen atom is not enough to bring about a color response. From these experimental phenomena, it seems that nitrogen in dimethylamine group is more likely responsible for the sensing signal. At the same time, Schiff base skeleton plays a role like a chromophore, making the signal change more pronounced. In order to verify the sensing mechanism, we also studied the 1 H NMR changes of 2 before and after exposure to DCP vapor (Fig. 5). Due to the symmetry of compound 2, it contains four hydrogen protons of a, b, c, d. After interacting with DCP, the chemical shifts of a, b, and d shift to low field in different degrees. However, the chemical shifts of c which is far from two kinds of nitrogen atoms are hardly changed. This shows that both nitrogen atoms interact with DCP. There is a competition complex between them.

Figure 4. Fluorescence of film 1 excited by UV lamp 365 nm after 100 s exposure in air and several possible interferents (1 air; 2 DCP; 3 DMMP 4 PAMP; 5TEP; 6 DCNP)

Probe 1 and probe 2 contain two kinds of nitrogen atoms: nitrogen atom in dimethylamine group and nitrogen atom in Schiff base. Both nitrogen atoms are likely to be complex sites. To investigate the sensing mechanism and find out which nitrogen atom is the site of action, we synthesize model compounds M1 and M2 and study their sensing behavior with DCP (Fig. S15). Model compound 1 just contains nitrogen atom in Schiff base. It is light yellow solid with weak fluorescence in solution and solid. In both solution and film, there was no significant change in color and fluorescence signal before and after interaction with DCP. As shown in Fig. S16, with gradual addition of DCP (0-700eq) to the solution of M1 (2×10-5 M), its maximum absorption peaks located at 285 nm scarcely changes. Model compound 2 just contains nitrogen atom in diethylamine group. It is light yellow solid with strong fluorescence in both solution and solid. When M2 interacts with the DCP in the solution, its color does not change very much but the fluorescence is obviously quenched. The maximum UV absorption peak of M2 in

1

Figure 5. H NMR spectra of 2 before and after exposure to DCP vapour. In summary, by incorporating amine groups into Schiff base skeletons, we developed two small-molecule chromophoric-fluorogenic probes which can detect DCP in solution/film with excellent selectivity, short response time and high sensitivity (as low as 0.14 ppb). During the sensing process, the nitrogen atom in dimethylamine is responsible for the sensing signal, and the nitrogen atom in the Schiff base acts as a chromophore. The synergy of the two kinds of nitrogen atom together gives the probe 1 and probe 2 excellent sensing properties. This work offers an interesting strategy for the design of simple yet highly efficient fluorescent probes for vapor anylates.

ASSOCIATED CONTENT SUPPORTING INFORMATION

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ACS Sensors Additional figures as noted in text. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected] ORCID Yanyan Fu：0000-0003-2521-3267

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was supported by the research programs from the Ministry of Science and Technology (Grant No. 2016YFA0200800), the National Natural Science Foundation of China (Grant No. 61771460, 51473182 and 61731016) ，a grant from the Youth Innovation Promotion Association CAS (2015190). We also thank Prof. Yuanping Yi, Prof. Tao Li and Dr. Jian Chen for their helpful suggestions. REFERENCES (1) Sidell, F. R.; Borak, J. Chemical warfare agents: II. Nerve agents. Ann. Emerg. Med., 1992, 21, 865-871 (2) Marrs, T. C. Organophosphate poisoning. Pharmacol. Ther., 1993, 58, 51-56 (3) Tuovinen, K. Organophosphate-induced convulsions and prevention of neuropathological damages. Toxicology, 2004, 196, 31-39 (4) Vale, A. What lessons can we learn from the Japanese sarin attacks? Przegl. Lek., 2005, 62, 528-532 (5) Utriainen, M.; Karpanoja, E.; Paakkanen,H. Combining miniaturized ion mobility spectrometer and metal oxide gas sensor for the fast detection of toxic chemical vapors. Sens. Actuators, B, 2003, 93,17-24 (6) Black, R. M.; Clarke, R. J.; Read, R. W.; Reid, M. T. J. Application of gas chromatography-mass spectrometry and gas chromatography-tandem mass spectrometry to the analysis of chemical warfare samples, found to contain residues of the nerve agent sarin, sulphur mustard and their degradation products. J. Chromatogr. A, 1994, 662, 301-321 (7) Russell, R. J. ; Pishko, M. V.; Simonian, A. L.; Wild, J. R. Poly(ethylene glycol) hydrogel-encapsulated fluorophoreenzyme conjugates for direct detection of organophosphorus neurotoxins. Anal. Chem., 1999, 71, 4909-4912 (8) Ngeh-Ngwainbi, J.; Foley, P. H.; Kuan, S. S. ; Guilbault, G. G. Parathion antibodies on piezoelectric crystals. J. Am. Chem. Soc., 1986, 108, 5444-5448 (9) Khan, M. A. K.; Kerman, K.; Petryk, M.; Kraatz, H. B. Noncovalent modification of carbon nanotubes with ferroceneamino acid conjugates for electrochemical sensing of chemical warfare agent mimics. Anal. Chem. 2008, 80, 2574-2582 (10) Diakowski, P. M.; Xiao, Y.; Petryk, M. W. P.; Kraatz, H. B. Impedance based detection of chemical warfare agent mimics using ferrocene-lysine modified carbon nanotubes. Anal. Chem. 2010, 82, 3191-3197 (11) Wang, J.; Pumera, M.; Collins, G. E. ; Mulchandani, A. Measurements of chemical warfare agent degradation products using an electrophoresis microchip with contactless conductivity detector. Anal. Chem. 2002, 74, 6121-6125

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Two small-molecule chromophoric-fluorogenic probes for real-time and onsite detecting of DCP vapor were reported by incorporating amine groups into Schiff base skeletons

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