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Aggregation State Reactivity Activation of Intramolecular Charge Transfer Type Fluorescent Probe and Application in Trace Vapor Detection of Sarin Mimics Wei Xu,†,‡ Yanyan Fu,† Junjun Yao,†,‡ Tianchi Fan,†,‡ Yixun Gao,†,‡ Qingguo He,*,† Defeng Zhu,† Huimin Cao,† and Jiangong Cheng*,† †

State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Changning Road 865, Shanghai 200050, China ‡ University of the Chinese Academy of Sciences, Yuquan Road 19, Beijing,100039, China S Supporting Information *

ABSTRACT: The reactivity of most Intramolecular Charge Transfer (ICT) based probes in the film state is much poorer than that in solution, due to the serious solid state aggregation of the large polarity molecules. In this contribution, an efficient method for activating the aggregation state reactivity of ICT based probes has been developed. Multiple hydrogen bonds formed by the oxime group, together with the phenol anion, could activate the aggregation state reactivity of the oxime group. By enhancing frontier orbital energy level, and constructing porous film structure, the probe becomes more compatible for highly efficient vapor phase reaction. In application, the TOP-I film can distinguish different organic phosphates with significant fluorescence change. The detection limit for diethyl chloro phosphate (DCP) is 1.2 ppb, lower than the Immediately Dangerous to Life and Health (IDLH) level of Sarin. Such a reactivity activating strategy can be extended to detect other harmful vapors by inducing suitable functional groups as the acceptor of the ICT system. Furthermore, with the increasing importance of green chemistry, the method may be beneficial for applications in solvent-free reactions. KEYWORDS: aggregation state reactivity, intramolecular charge transfer, multiple hydrogen bonds, warfare agents, fluorescence detection

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works of Rebek and Anslyn.19,20 With both hydrogen-bonding donor and acceptor, oxime group shows high reactivity to CWAs in solution case;21−23 however, it shows poor reactivity in film just as other ICT type molecules. Hence, a small molecule ICT probe (trialdoxime phenol, TOP) for CWAs detection was designed and synthesized, referring to our previous structure.24 A further reaction of TOP with diethyl amine produced the ionized probe (TOP-I). The ionized probe demonstrated a different aggregation state in the micro/ nanoscale and molecular scale and much better sensing performance than that of TOP in vapor phase detection for Sarin mimic diethyl chloro phosphate (DCP). The aggregation activation is a potential way to adapt the ICT probes for highly efficient trace harmful vapor detection. Scheme 1 demonstrated the reaction process of TOP-I. In the ICT system, three aldehyde oxime units acted as both sensing units and electron acceptors while the phenol was

ntramolecular Charge Transfer (ICT) based compounds contain both electron-donating groups and electron-accepting groups in the same conjugated system.1,2 They have been utilized in many areas such as chemical detection3,4 and electroluminescence.5,6 Taking advantage of the long Stokes shift and high polarity, ICT probes are efficient in solution detection of polar analytes such as ions and biomolecules.7−9 However, due to the high polarity of the ICT probes, molecules also suffer from very heavy aggregation in film state, which cause the energy level variation, reduce fluorescence efficiency, decrease the vapor penetration, and hence greatly influence the sensing reactivity of the probes.10−13 Therefore, such probes are not suitable for the trace detection of vapor analytes such as chemical warfare agents (CWAs). Although various kinds of efficient nucleophilic groups have been developed for CWAs detection14,15 and much progress has been achieved in solution detection,16−18 the reactivity of the probes in aggregation state is much poorer than that in solution. Oxime group has been successfully utilized in detection of warfare agent mimics in solution, according to the pioneering © 2016 American Chemical Society

Received: June 2, 2016 Accepted: July 27, 2016 Published: July 27, 2016 1054

DOI: 10.1021/acssensors.6b00366 ACS Sens. 2016, 1, 1054−1059

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ACS Sensors

salicylaldoxime structure in reference to a related theory about salicylamide.25 As compared with that in the film state, in THF solution, TOP-I was also reactive just like common ICT compounds, but not as active as its aggregation state. Different from the detection of fluorescence quenching in vapor, the fluorescence increased immediately upon the addition of DCP (Figure 3) in

Scheme 1. Reaction Process of TOP-I in Film State and Solution

adopted as the electron donor. This activated ICT system was both efficient in film state and in solution.

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CHARACTERIZATIONS As shown in Figure 1, in the film state, the maximum emission wavelength of TOP-I film was 470 nm while the emission

Figure 3. Fluorescence emission curves of 10−4 M TOP-I/THF solution in different concentrations of DCP.

solution. The measurement limit of the reactive TOP-I solution is only 40 μM. The reaction process was proven by NMR, GCMS, and FTIR spectra (Figures S-3 to S-5). Cyclic product was produced in solution as expected, in agreement with the results in ref 20. In contrast, a faint fluorescent acyclic compound formed in film instead of the strong fluorescent cyclic product (Scheme 1). The hydroxyl unit of oxime could react with DCP to form a P−O bond, the introduced units are electrondeficient, which results in increased intramolecular charge transfer and narrower bandgap, and so the emission wavelength red-shifted. Therefore, the intermolecular interaction (such as multiple hydrogen-bonding structures) in the solid state can prevent the formation of unfavorable cyclic byproducts in the reaction.

Figure 1. Emission spectra of TOP and TOP-I films before and after exposure in DCP.

intensity of TOP film was too weak to be accurately measured, so TOP could not be used for fluorescence sensing. Upon exposure to DCP, the emission band of TOP-I at 470 nm disappeared with the appearance of a very weak emission band peak at 530 nm. A significant difference in reactivity efficiency was observed by monitoring the fluorescence intensity changes with time of both TOP and TOP-I upon exposure to DCP vapor as shown in Figure 2. 76% fluorescence of TOP-I was quenched in the first 30 s while TOP showed almost no response, suggesting a distinct activation effect by the ionization of TOP. It is mentionable that organic semiconductors often suffer from serious photobleaching, but the TOP-I film is highly photostable in air under photoexcitation, possibly caused by its

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EXPERIMENTAL SECTION

General Methods. The solvent is tetrahydrofuran (THF) and the concentration for the film fabrication is 1 mg/mL (3.3 × 10−6 M). For the fabrication of sensing film, the solution was deposited on filter paper and dried with an air blower. For the SEM samples, the solution was deposited on quartz slides in a spin coater at the rotation speed of 2000 r/min. Then the films were dried under vacuum for 20 min and the UV−vis absorption and fluorescence analysis of the films was conducted. All solvents and reagents were purchased from commercial sources and used as received. 1H NMR and 13C NMR spectra were measured with Bruker DRX500 instrument, using tetramethylsilane (TMS) as the internal standard substance. Mass spectra were measured with Bruker Daltonics Inc. APEXII FT-ICR Mass Spectrometer and an Agilent 7890A/5975C Gas Chromatograph Mass Spectrometer (GCMASS). Fluorescence spectra and time-course curves were measured with HORIBA Fluoro-Max 4 spectrometer while UV−vis absorption spectra were measured with Jasco V-670 spectrophotometer. AFM photos were measured with Bruker Bioscope Catalyst. TFP: 6.9 g (65 mmol) phenol, 70 mL trifluoroacetic acid (TFA), and 20.1 g (143 mmol) hexamethylenetetramine (HMTA) was added into a flask and the mixed solvent was refluxed at 120 °C for 20 h. The solvent was heated to 150 °C for 3 h and cooled back to 120 °C. For initiating the hydrolysis reaction, 100 mL 3 N hydrochloric acid was added into the solvent and the solvent was heated at 102 °C for 30 min, followed by natural cooling, filtering, washing, and drying processes. The product was recrystallized in DMF and the yield was 4.28 g (37.3%).

Figure 2. Fluorescence quenching time courses of TOP and TOP-I films in air and in DCP vapor. 1055

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ACS Sensors H NMR (500 MHz, DMSO-d6) δ 10.31 (s, 2H), 10.00 (s, 1H), 8.52 (s, 2H). TOP: 178 mg (1 mmol) TFP, 250 mg (3.6 mmol) hydroxylamine hydrochloride and 144 mg (3.6 mmol) sodium hydroxide was dispersed in 20 mL methanol. After completely reacting in room temperature for 2 h, the residue was filtered and washed with distilled water. Then 190 mg (yield, 85.2%) TOP was obtained. 1H NMR (500 MHz, DMSO-d6) δ 11.57 (s, 2H), 11.07 (s, 1H), 11.02 (s, 1H), 8.40 (s, 2H), 8.09 (s, 1H), 7.77 (s, 2H). 13C NMR (126 MHz, DMSO) δ 154.99, 147.02, 146.61, 126.55, 124.87, 119.38. ESI-MS: M+H+ 224.06654 (Cal. C9H10N3O4, 224.0666). TOP-I: (Scheme 2) 20.6 mg TOP was dispersed in 5 mL THF. After adding 2 mL diethylamine (Et2NH), the TOP sample dissolved. 1

Figure 5a,b presented the SEM images of TOP and TOP-I film samples deposited on quartz slides by a spin coating

Scheme 2. Synthetic Route of TOP-I

The excess amine in the solution was evaporated in vacuum to afford 27.2 mg TOP-I (yield, 99.5%). Then the solution of TOP-I was dropcast on quartz slides and dropped in filter papers, and the sensing performance was tested in several kinds of analytes, such as water, dichloromethane, ethanol, THF, and peroxide. 1 H NMR (500 MHz, DMSO-d6) δ 8.39 (d, J = 1.4 Hz, 2H), 8.08 (d, J = 2.0 Hz, 1H), 7.76 (d, J = 1.8 Hz, 1H), 2.83 (d, J = 7.2 Hz, 1H), 1.12 (t, J = 7.2 Hz, 2H), 0.96 (s, 2H). 13C NMR (126 MHz, DMSO) δ 155.61, 147.13, 146.61, 126.46, 124.46, 119.54, 47.40, 41.59. ESI MS: M− 222.05194 (Cal. C9H8N3O4, 222.0520).

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DISCUSSION We suppose that the aggregation state reactivity of oxime group was mainly activated by the higher energy level of the frontier orbitals and the irregular multiple hydrogen bonding aggregation structure. To understand the activation mechanism, the energy levels of the frontier orbitals were calculated with DMol3 while the molecule stacking of TOP-I was calculated with Forcite Plus by Materials Studio 8 software. Frontier Orbital Energy Level. From TOP to TOP-I molecule, the simulated HOMO level elevated from −5.16 to −0.77 eV (Figure 4), which could also be supported by cyclic

Figure 5. SEM photos of the aggregation state probes on quartz slides: (a) TOP film; (b) TOP-I film; (c) TOP-I film reacted with DCP vapor.

process. The film of TOP is composed of regular sticks with a relatively smooth surface and showed poor fluorescence. In contrast, TOP-I were converted to rough ellipsoids with strong fluorescence, suggesting an obvious aggregation change in micro/nanoscale including size, shape, and surface character after simply tuning the phenol group to its anion. The structure change could also be found by AFM in Figure S-1. Both films of TOP and TOP-I demonstrated amorphous character supported by XRD results as shown in Figure S7a. In addition, the fluorescence lifetimes of both films were measured; TOP-I film showed a shorter fluorescence time than that of TOP film, as shown in Figure S6. The reactivity of the film changed significantly with the aggregation structure change. TOP film was inactive to DCP while both color and fluorescence of TOP-I film changed immediately. Upon reaction with DCP vapor, the pale yellow color of TOP-I film became colorless and the light green fluorescence vanished. The rough ellipsoid structure further changed to irregular and partly fused particles (Figure 5c). The color and aggregation state change directly indicated a quick reaction process between TOP-I and DCP vapor. GIXRD spectra in Figure S-7b indicated that the TOP-I film has an extra small peak (d = 3.1726) compared with the TOP film. This extra peak proved the different and disordered packing mode in TOP-I film in the molecular scale. For further understanding of the aggregation in the molecular scale, we performed the calculation for both TOP and TOP-I by putting

Figure 4. Frontier orbitals of TOP and TOP-I calculated with MS 8.0.

voltammetry (CV) data in Figure S-2. The oxidizing potential declined from 1.21 to 0.1 V, so that TOP-I was easier to oxidize and more active than TOP in film state. Aggregation State. In addition, the aggregation state also played a vital role in the reactivity, proven by both experimental observations and simulations from the micro/nanoscale and molecular scale, respectively. 1056

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dichloromethane, hydrogen peroxide, and even triethyl phosphite (TEP).

seven molecules in one cell. As Figure 6a,b shows, after a 500 ps dynamic process calculation, the nearest intermolecular

Figure 7. Fluorescence quenching rates of TOP-I film in different vapors in 300 s.

We checked the quenching mechanism of the interferent organic phosphates. As Figure 8 shows, the fluorescence of

Figure 6. Self-aggregation state of (a) TOP and (b) TOP-I calculated with MS 8.0.

distances of the phenyl units are 3.527 and 4.348 Å, respectively. TOP-I molecules stacked less tightly than TOP molecules. Moreover, the energy changes have been systematically analyzed as shown in Figures S-14 to S-18. From TOP to TOP-I, the electrostatic energy increased from −79.91 to −37.73 kcal/mol. The number of hydrogen bonds between aromatic units declined from 23 to 15, together with the increase of hydrogen bonding energy from −45.85 to −35.94 kcal/mol. The nonbonding energy, including electrostatic, van der Waals, and hydrogen bonding energy, totally increased from 26.350 to 89.823 kcal/mol. The higher nonbonding energy indicated the destabilization effect of the phenol anions. The increased hydrogen bonds and stronger π−π stacking endow TOP molecules with tighter stacking in the solid state. Electrostatic repellent among the phenol anions prevented TOP-I from stacking as tightly as TOP aggregation. With enlarged intermolecular distance, decreased intermolecular interaction and the porous ellipsoid-shaped aggregates were positive for the adsorption of DCP vapor. Then the reaction will be more efficient between the DCP vapor molecules and oxime groups. In addition, at the hydrogen bonding nodes and the reactive sites, the oxime groups were restricted by the intermolecular multiple hydrogen bonds, so that they could be protected against the interferents, i.e., the probe will have nice selectivity. As shown in Figure 7, TOP-I film showed no response to common interferent vapors including air, water, THF, ethanol,

Figure 8. Fluorescence emission curves of TOP-I films after sensing with different vapors for 300 s.

TOP-I film was fiercely quenched in DCP, slightly quenched in methyl dimethyl phosphate (DMMP) and acephatemet (MAP), and hardly quenched in TEP (Scheme 3). These Scheme 3. Chemical Structures of the Phosphate Compounds

different organic phosphates could be easily distinguished by the fluorescence change. The chemical reaction led to the great quenching efficiency of TOP-I in DCP vapor; the emission wavelength of the sensing film changed from 470 to 530 nm, different from the spectra in other phosphate vapors. The slight emission intensity change of TOP-I films in DMMP and MAP mainly comes from the entrance of the strong polarity phosphate and formation of hydrogen bonding interaction between NOH and −PO. The quenching effects just agreed with the intermolecular interaction simulations in Figure 9. The TOP-I molecule formed two 1057

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ACS Sensors hydrogen bonds with MAP at the −PO site and the −NH2 site, one hydrogen bond with DMMP at the −PO site, and no hydrogen bonds with TEP.

detection limit for DCP is 1.2 ppb, lower than the IDLH concentration of Sarin. The reactivity-activated multiple hydrogen bonding structure can be extended to detect other harmful vapor phase analytes by inducing other functional groups as the electron-acceptor of the ICT system. Furthermore, with the increasing importance of green chemistry, the method may be beneficial for applications in solvent-free reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.6b00366. AFM image, CV data, NMR, MS spectra, fluorescence lifetime, XRD, simulation parameters for the calculation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 9. Intermolecular interactions of single TOP-I molecule with DMMP, MAP, and TEP.

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ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology (Grant No.: 2016YFA0200800), NSFC (Grant Nos. 61325001, 21273267, 61321492, and 51473182), and Youth Innovation Promotion Association CAS (2015190). We would also like to express our thanks to Mr. Pengcheng Xu and Mr. Hui Zhao for their help in measurements and discussions.

Figure 10 presented the relationship of fluorescence quenching efficiency and the concentration of DCP vapor, by

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Figure 10. Quenching rates of TOP-I films and linear fit of detection limit.

a linear fitting; considering the Signal Noise Ratio (SNR) as 1%, the detection limit to DCP could be obtained as low as 1.2 ppb, which is below the 7 ppb IDLH level of Sarin.26,27 In contrast with the previous work for DCP detection,28 the detection limit (the considering the SNR as 1‰) has been greatly improved from 2.6 ppb to 5 ppt. It is noted that Sarin has much higher reactivity, and hence we expect the sensitivity to Sarin to be better than that of DCP.

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CONCLUSION We have developed a new method for activating the aggregation state reactivity of ICT type fluorescent probes. The multiple hydrogen bonds, together with the ionized phenol structure of TOP-I, activated the aggregation state reactivity of oxime group with higher frontier orbital energy level, and a porous structure compatible with efficient and selective Sarin mimics vapor phase reaction. In application, different organic phosphates were easily distinguished by the fluorescence change of TOP-I film, and the 1058

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