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Fluorescent chemosensors with varying degrees of intramolecular charge transfer for detection of a nerve agent mimic in solutions and in vapor Yuan-Chao Cai, Chen Li, and Qin-Hua Song ACS Sens., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017
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ACS Sensors
Fluorescent Chemosensors with Varying Degrees of Intramolecular Charge Transfer for Detection of a Nerve Agent Mimic in Solutions and in Vapor Yuan-Chao Cai, Chen Li and Qin-Hua Song* Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. KEYWORDS fluorescent chemosensors, chemical warfare agents, nerve agents, DCP, quinoline
ABSTRACT: Nerve agents are highly toxic organophosphorus compounds, and their possible use in terrorist attacks has led to increasing interest in the development of reliable and accurate methods to detect these lethal chemicals. In this paper, we have prepared six 6-aminoquinolines with various N-substituents as chemosensors for a nerve-agent mimic DCP. The chemosensors with the nucleophilic pyridine-N atom as the active site detect DCP via a catalytic hydrolysis approach to form the protonated sensor. The nucleophilicity of the pyridine-N atom depends on the donating ability of the 6-amine group, which affects the intramolecular charge-transfer (ICT) character of sensors and the protonated sensors, leading to different fluorescence-response modes. The effects of the ICT character on the sensing property have been clarified. Among these charge transfer sensors, the sensor 3 displays ratiometric fluorescence response to DCP and a low limit of detection (8 nM). Furthermore, a facile testing strip with 3 has been fabricated with polyethylene oxide for real-time selective monitoring of DCP vapor.
Nerve agents are highly toxic chemical warfare agents (CWAs), such as Sarin, Soman and Tabun (Chart 1), which are organophosphorus (OP) compounds. Their reactive phosphate group are able to irreversibly react with the hydroxyl groups of acetylcholinesterase (AChE), thus blocking the decomposition of acetylcholine and resulting in the neurological imbalance in the cholinergic synapse, the paralysis of central nervous system, organ failure and rapid death.1-3 Possible uses both in terrorist attacks and in the war as CWAs are immediate threat to public safety and national security. Therefore, it is urgent to develop facile, selective and reliable methods for the detection of nerve agents. So far, a variety of detection methods for nerve agents have been developed,4 such as gas chromatograph/mass spectroscopy,5, 6 electrochemistry,7-9 enzymatic assay,10 interferometry.11 However, these methods suffer from limitations, such as slow response, lack of specificity, limited selectivity, low sensitivity, operational complexity, non-portability, difficulties in real-time monitoring, or false positive readings. As an alternative to these methods, fluorescent chemosensors have attracted considerable attention due to their low cost, portability, highly sensitively, and easy operation in recent years.12-14 Usually, diethylchlorophosphate (DCP) serves as a nerve-agent mimic due to its low toxicity but similar activity to Sarin (Chart 1). Nerve Agents
F
O P
O
Sarin
F
O P
O
Soman
The general design strategy is photoinduced electron transfer (PET) through nucleophilic attack of the sensor molecule on the electrophilic OP analyte, suppressing the PET process to result in a turn-on of the fluorescence. Most of them belong to one of two classes of sensors: (i) Phosphorylation reactions of amines15 or alcohols16 then are integrated into the structure of a fluorophore; (ii) After the phosphorylation of the primary alcohol, a rapid intramolecular N-alkylation leads to the formation of a cyclic quaternary ammonium salt.17 In addition, a novel sensing reaction via a "covalent-assembly" approach was developed by Yang and Lei, and results in colorimetric and fluorimetric signal from zero background.18 A cyclizationinduced emission enhancement (CIEE)-based ratiometric fluorogenic and chromogenic probe for DCP was reported by Mahapatra et al.19 However, one important limitation of these sensors is related with the usually slow rates of the phosphorylation reactions. Recently, a new design strategy was employed to construct fluorescent chemosensors of DCP with a pyridine unit as active site via a catalytic hydrolysis of DCP to form the protonated sensor, which exhibits a stronger intramolecular charge-transfer (ICT) process (Scheme 1). The sensors in solid as films or test strips exhibit rapid response to DCP vapor.20, 21 Scheme 1. The sensing mechanism of the chemosensor with a pyridine unit
Simulants O P O N
O P Cl O O
O P NC O O
Tabun
DCP
DCNP
NC
Cl Fluor
N
DCP
Chart 1. Structure of nerve agents and simulants
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Fluor
N
O P O O
+H2O -DHP
Fluor O
DHP = HO P O O
Cl NH
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In order to clearly understand the mechanism of catalytic hydrolysis and the ICT effects on the sensing property, we have prepared six 6-aminoquinoline derivatives with various N-substituents to detect DCP (Scheme 2). Their sensing processes reveal clearly the variation of ICT character with the donating ability of the amine group. Among them the sensor 3 displays a ratiometric fluorescence response, and a low limit of detection (LOD) (8 nM). Moreover, the testing membrane with 3 was fabricated for facile, selective and fast detection of DCP vapor (< 100 seconds). Scheme 2. Proposed sensing mechanism of probes to DCP 2 1
R
R N
2
weak ICT
DCP, H2O
1
R
R N
strong ICT
Cl N
N H
O + HO P O O
■ EXPERIMENTAL SECTION Materials and General Methods. All chemicals for synthesis were purchased from commercial suppliers and were used as received without further purification. 1H and 13C NMR spectra were measured with a Bruker AV spectrometer operating at 400 MHz and 100 MHz, respectively and chemical shifts were reported in ppm using tetramethylsilane (TMS) as the internal standard. Mass spectra were obtained with a Thermo LTQ Orbitrap mass spectrometer or MALDI-TOF. UV-vis absorption and fluorescence emission spectra were recorded with a Shimadzu UV-2450 UV/vis spectrometer and a Shimadzu RF-5301PC Luminescence Spectrometer, respectively. Synthesis of compound 2a.22 (Ac)2O (41 mg, 0.4 mmol) was added to a solution of 2-methyl-6-aminoquinoline (47 mg, 0.3 mmol) in pyridine (2 mL). The mixture reacted at room temperature for 3 h. The reaction was quenched by water (20 mL). Then the mixture was extracted with EtOAc (30 mL) for three times. The organic layer was dried with anhydrous Na2SO4 and concentrated to dryness under reduced pressure. The residue was purified by column chromatography (petroleum ether/EtOAc, 1/2, v/v) to afford compound 2a (45 mg, 75 %) as a light yellow solid. 1H NMR (400 MHz, DMSO-d6), δ: 10.23 (s, 1H, NH), 8.31 (d, J=2 Hz, 1H, Ar-H), 8.16 (d, J=8.4 Hz, 1H, Ar-H), 7.86 (d, J=9.2 Hz, 1H, Ar-H), 7.72 (dd, J1=2 Hz, J2=9.2 Hz, 1H, Ar-H), 7.35 (d, J=8.4 Hz, 1H, Ar-H), 2.62 (s, 3H, CH3), 2.12 (s, 3H, CH3). Synthesis of compound 2b.23 (Boc)2O (145 mg, 0.7 mmol) was dissolved in 1,4-dioxane (5 mL), and then added to the solution of 2-methyl-6-aminoquinoline (100 mg, 0.6 mmol) in1,4-dioxane (5 mL) . The mixture reacted at 85°C for 14 h under N2 atmosphere.1,4-dioxane was removed under reduced pressure to get a viscous residue, which was purified by column chromatography (petroleum ether/EtOAc, 5/1, v/v) to afford compound 2c (155 mg, 91 %) as a yellow solid. 1H NMR (400 MHz, CDCl3), δ: 8.06 (s, 1H, NH), 7.95 (m, 2H, Ar-H), 7.43 (dd, J1=2.4 Hz, J2=8.8 Hz, 1H, Ar-H), 7.24 (d, J=8.4 Hz, 1H, Ar-H), 6.85 (s, 1H, Ar-H), 2.71 (s, 3H, CH3), 1.54 (s, 9H, CH3). Synthesis of compound 2c. 2-Methyl-6-aminoquinoline (500 mg, 3.2 mmol) and n-BuBr (433 mg, 3.2 mmol) in DMF (15 mL) was stirred at 80°C for 6 h and then cooled to room temperature. Water (40 mL) was added to quench the reaction. Then, the mixture was extracted with EtOAc (100 mL) for
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three times. The organic layer was washed with water (50 mL) for three times and then dried with anhydrous MgSO4. The mixture was concentrated to dryness under reduced pressure. The residue was purified by column chromatography (petroleum ether/EtOAc, 6/1, v/v) to afford compound 2c (400 mg, 59%). 1H NMR (400 MHz, CDCl3), δ: 7.82 (d, J=8.4 Hz, 1H, Ar-H), 7.80 (d, J=9.2 Hz, 1H, Ar-H), 7.15 (d, J=8.4 Hz, 1H, Ar-H), 7.05 (dd, J1=2.8 Hz, J2=8.8 Hz, 1H, Ar-H), 7.67 (d, J=2.8 Hz, 1H, Ar-H), 3.20(t, J=7.2 Hz, 2H, CH2), 2.66 (s, 3H, CH3), 1.71-1.63 (m, 2H, CH2), 1.52-1.43 (m, 2H, CH2), 0.99 (t, J=7.6 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3), δ: 154.2, 145.8, 142.4, 134.2, 129.2, 128.2, 122.2, 121.2, 103.0, 43.7, 31.4, 24.8, 20.4, 13.9; TOFMS (ESI) calcd for C14H18N2: 215.1543([M+H]+), found: 215.1541. Synthesis of compound 2d.25 1H NMR (400 MHz, CDCl3), δ: 7.87 (m, 2H, Ar-H), 7.33 (dd, J1=2.8 Hz, J2=9.2 Hz, 1H, ArH), 7.16 (d, J=8.4 Hz, 1H, Ar-H), 6.79 (d, J=2.8 Hz, 1H, ArH), 3.04 (s, 6H, CH3), 2.67 (s, 3H, CH3). Synthesis of compound 3. (Boc)2O (128 mg, 0.6 mmol) was dissolved in 1,4-dioxane (2 mL), and then added to the solution of 2c (50 mg, 0.23 mmol) in 1,4-dioxane (5 mL). The mixture reacted at 85°C for 14 h under N2 atmosphere. 1,4dioxane was removed under reduced pressure to get a viscous residue, which was purified by column chromatography (petroleum ether/EtOAc, 8/1, v/v) to afford compound 3 (50 mg, 69%) as a yellow oily liquid. 1H NMR (400 MHz, CDCl3), δ: 8.15 (d, J=6.8 Hz, 2H, Ar-H), 7.63 (d, J=10.4 Hz, 2H, Ar-H), 7.36 (d, J=8.4 Hz, 1H, Ar-H), 3.74 (t, J=7.6 Hz, 2H, CH2), 2.85 (s, 3H, CH3), 1.60-1.52 (m, 2H, CH2), 1.45 (s, 9H, CH3), 1.37-1.25 (m, 2H, CH2), 0.90 (t, J= 7.6 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3), δ: 158.8, 154.7, 145.9, 140.0, 136.1, 129.9, 128.8, 126.5, 123.9, 122.2, 80.4, 49.9, 30.7, 28.3, 25.2, 19.9, 13.8; TOFMS (ESI) calcd for C19H26N2O2: 315.2067 ([M+H]+), found: 315.2065. Preparation of sample solutions for spectral measurements. The bulk solution of a sensor in DMSO is diluted to the given concentration with acetonitrile. Final sample solution is the acetonitrile solution containing 1% DMSO. All solvents used in measurements are spectroscopic pure and used without further purification. Measurement of detection limits. The calculation of detection limit was based on the fluorescence titration. As an example, the fluorescence intensity (I475) change of the sensor 3 was fitted linearly with the increasing concentrations of DCP over a range of 0−4.5 µM. From the plot, the slope (k) was obtained to be 21.987 µM-1 (R2 = 0.984). Then standard deviation (σ) was obtain to be 0.0571 by blank measurement. The detection limit of 3 toward DCP was calculated to be 8 nM in terms of the formula (3σ/k). Measurements of pKa. The measurements of pKa for the protonated pyridine-N atom were performed through recording UV/Vis absorption spectra of a sensor in aqueous solutions at different pH. which were three buffers at a pH range of 4−9, involving 0.1 M critric acid−0.1 M disodium hydrogen phosphate buffer for pH 4–5.5, 0.1 M Na2HPO4−0.1 M NaH2PO4 for 6–8 and 0.1 M K2CO3−0.1 M NaHCO3 buffer for pH > 8. Water for sample preparation was purified with a Millipore water system. All pH values of solutions were further measured with an MQK-PHS-3C pH meter. Preparation of test strip with sensor 3. Polyethylene oxide (1.5 g, Mw = 1000,000 Dalton) was slowly added to a
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solution of 3 (3 mg) in DCM (40 mL). The mixture was stirred mildly until a viscous and uniform mixture was formed. After standing for a while to get bubbles disappeared, the mixture was poured into a glass plate (φ =10 cm) and kept in a dry atmosphere for 24 hours. Detection of DCP vapor with the membrane with 3. The experiment was carried out in a fume hood. The saturated vapor pressure of DCP is 0.1 mmHg at 25°C so that the maximum concentration that can be detected is 130 ppm.24 Various amounts of DCP vapor were obtained by preparing different concentrations of DCP solutions in DSMO. Six concentrations of DCP solutions were prepared, 0, 20 %, 40 %, 60 %, 80 % and 100 %. The detection was performed in centrifuge tubes. The sample solution was placed in the bottom of the tube, and test stripes were placed at upside of tubes and rapidly close the lid of the tube. After two minutes, fluorescence images of these tubes were taken under 365 nm light.
1. These data show that a stronger donor results in a stronger ICT process in both sensors and sensing products. Related to sensors, fluorescence maxima of sensing products exhibit large red-shifts (87-110 nm). The fluorescence quantum efficiencies (Фf) of six sensors were measured with quinine sulfate in 0.1 M H2SO4 (Фf = 0.546)26 as a reference, listed in Table 1, and display an increase trend with the donating ability of the amine group. Table 1. Properties of fluorescent sensors for DCP Sensors
λa/λ λba
Fa/Fbb
pKa
kobs /s-1
Фf
LOD /nM
2a
453/366
0.66
5.4
1.44
0.08
37
2b
460/370
1.25
5.5
1.70
0.10
21
3
476/375
3.41
5.7
2.34
0.11
8
■ RESULTS AND DISCUSSION
1
535/425
0.04
6.3
3.00
0.40
592
Design and synthesis of the sensors. The 6aminoquinoline was chosen as the fluorophore due to its ICT character and its good photo-stability. The nitrogen atom of the pyridine moiety served as not only electron acceptor, but also the active site for the sensing reaction. As a donor, the donating ability of the 6-amine group alters with the Nsubstituent, which was selected in terms of its electron supply and demand. For example, alkyl is an electron-donating group, and t-butyloxycarbonyl (Boc) and acetyl (Ac) are electronwithdrawing groups. Thus, the order of the donating ability is NHAc < NHBoc < N(n-Bu)Boc < NH2 < NHn-Bu < NMe2. With 2-methyl-6-aminoquinoline as a starting material, six 6-aminoquinolines were synthesized and the synthetic route of was illustrated in Scheme 3. Using acid anlydrides or halohydrocarbons as reactants, the amine is acylated or alkylated to afford 2a22 and 2b23 or 2c and 2d25 in good yields (59-91%), respectively. In the presence of (Boc)2O, 2c is acylated to form 3 in the yield of 69%. All new compounds were fully characterized by 1H NMR, 13C NMR, and high-resolution mass spectroscopy (HRMS).
2c
545/435
0.03
6.5
4.63
0.33
172
2d
560/450
0.01
6.7
9.15
0.37
408
Scheme 3. Synthesis of the sensorsa
a
Fluorescence peaks of before (λb) and after (λa) addition of 20 equiv. DCP. b The ratio of intensities at the peak after (λa) to before (λb) addition of 4.5 equiv. DCP. After that, we monitored the response of these sensors to different concentrations of DCP in the range of 0-4.5 µM by UV/Vis absorption and fluorescence spectroscopies (Figure S2 and Figure 1). As shown in Figure 1, fluorescence spectra of 10 µM sensor solutions upon additions of different concentrations of DCP solutions exhibit a gradual change in the response mode from the ratiometic to the turn-off. In fact, fluorescence response for all six sensors display the ratiometric mode, and the appeared long-wavelength peak increase gradually from 2d (very weak), 2c, 1 to 3, and then decrease from 3, 2b to 2a. The ratios of fluorescence intensities at the new peak (Fa) to the initial peak (Fb) upon addition of 4.5 equiv. DCP from Figure 1 display above changes, and the values (λa/λb) are listed in Table 1. 300
a N 1
1
R
b
n-Bu
λex = 320 nm
2a:AcNH
N 2a: 1 R =H, 2R = Ac 2b: 1R =H, 2R = Boc 2c: 1 R =H, 2R = n-Bu 2d: 1R = 2 R = CH 3
400
Boc N N 3
2b: BocNH
300
Intensity(a.u.)
H2 N
R N
Intensity(a.u.)
2
200 λex = 320 nm
100
200
100
0
0
a
(a) (Ac)2O, pyridine, r.t., 3 h for 2a; (Boc)2O, 1,4-dioxane, 85°C, 14 h for 2b; n-BuBr, DMF, 80°C, 6 h for 2c; NaH, CH3I, DMF, 40 °C, 12 h for 2d. (b) 2c, (Boc)2O, 1,4-dioxane, 85 °C, 14 h.
350
400
450
500
550
400
450
500
600
1:NH2
3:n-BuNBoc 750
λex = 320 nm
Intensity(a.u.)
600
550
Wavelength /nm
1000
800
Spectral response and detection of DCP in solutions. With six sensors in hand, we measured their UV/Vis absorption and fluorescence spectra before and after addition of DCP shown in Figure S1 as Supporting Information. The absorption and fluorescence spectra of six sensors (10 µM) in CH3CN solutions display a little red-shift in the increased order of the donating ability of their 6-amine groups. After addition of 20 equiv. DCP, the absorption and fluorescence spectra exhibit a larger red-shift related to those of sensors. The values of the corresponding fluorescence peaks (λa/λb) were listed in Table
350
600
Wavelength /nm
Intensity(a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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400
λex = 340 nm
500
250
200
0
0 350
400
450
500
Wavelength /nm
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550
600
350
400
450
500
Wavelength /nm
550
600
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λex = 360 nm
2c:n-BuNH
80
40
80
4 3
9
2d:Me2N
λex = 380 nm
Intensity(a.u.)
Intensity(a.u.)
Boc 5 N
120
n-Pr
7
n-Pr
Boc 5' N 7' 8'
5' 7'
8'
4' 3'
9'
N
8
4'
DCP/D 2O
N D
+
DO
O P O O
Cl
9'
3'
e d
40
c 0
0
350
400
450
500
550
600
350
400
Wavelength /nm
450
500
550
600
b
Wavelength /nm
4
Figure 1. Fluorescence spectra of the sensors (10 µM) upon gradual addition of a solution of DCP (0–4.5 equiv) in CH3CN containing 1% DMSO (λex=320 nm).
As an example, the plot of the ratio (F476/F377) of fluorescence intensities at 476 nm and 377 nm vs the concentration of DCP was obtained based on data from 3 of Figure 1. Through fitting a straight line, the limit of detection (LOD) of 3 toward DCP was calculated to be 8 nM in terms of the formula (3σ/k). Similarly, the LODs of other sensors were also obtained and listed in Table 1. The plots of the fluorescence intensities at the peaks vs. the concentration of DCP were provided in Fig. S3 as Supporting Information (SI). LODs of three sensors (1, 2c and 2d) with a turn-off response are higher than those of 2a, 2b and 3. The sensing mechanism. To investigate the effects of the electron-donating ability of the amine group on the nucleophilicity of the pyridine-N atom with increasing, we recorded time-dependent fluorescence intensity at initial peak for all sensor before and after the addition of 20 equiv. DCP, shown in Figure 2a. Based on these data, the pseudo-first-order kinetic plots can be obtained in terms of the plot of –ln(F0/F) vs. time (t), shown in Figure 2b. Thereby, the observed rate constant (kobs) can be obtained from the slope of the fitting straight line, listed in Table 1. Absolutely, the nucleophilicity of the N atom of pyridine agrees with the donating ability of the 6amine group. 3
b
a
2a 2b 3 1 2c 2d
0.9
2 −ln(F/F0)
Normalized Intensity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.6
2a 2b 3 1 2c 2d
0.3
1
0
0.0 0
200
400
Time /s
600
800
0
200
400
600
800
Time /s
Figure 2. (a) Time-dependent normalized fluorescence intensities of the sensors (10 µM) upon the addition of 20 equiv. DCP in CH3CN containing 1% DMSO. (b) Plots of –ln(F0/F) vs. time.
Moreover, the nucleophilicity of the pyridine can be estimated with its basicity or the pKa value of the protonated pyridine. The pH titration experiments were performed by recording UV/Vis absorption spectra of a sensor in different pH aqueous solutions (Figure S4). The pKa values of the protonated pyridine moiety for six quinoline derivatives were obtained, and listed in Table 1. These values of pKa (5.4−6.7) fit also the order of the nucleophilicity.
8 8.0
5
3
7
9
a 7.5
3.5 ppm
Figure 3. Partial 1H NMR spectra of 3 in CD3CN (0.1% D2O) before (a) and after increasing additions of DCP (b, c, d and e). 1 H NMR titration of the sensing reaction of 3 to DCP provided another support for the sensing mechanism. The experiment was performed in the solvent mixture of CD3CN with 0.1 % D2O (Figure 3). Upon stepwise additions of DCP (from b to e in Figure 3), the chemical shifts of the protons (3, 4, 5, 7 and 8) at quinoline unit move to the low field gradually, while no significant change for the methylene (9) at n-Bu. The observation shows clearly that the pyridine-N atom of 3 is protonated gradually upon stepwise additions of DCP. In addition, a control experiment shows that trace water in the solution is necessary for the sensing reaction. The response of the sensor 3 to DCP was observed with a dryness acetonitrile and in common acetonitrile without further processing as solvents respectively (Figure S5). As shown in Figure S5, no remarkable variation in the dryness solvent and a dramatic change for non-processing solvent were observed in both absorption and fluorescence spectra of the sensor 3 to 20 equiv. DCP. Above results fully confirm that the ratiometric fluorimetric detection of DCP is achieved via the catalytic hydrolysis of DCP/protonation of quinoline sensors. Selectively. Three organophosphorus compounds, diethyl cyanophosphonate (DCNP), dimethyl methylphosphonate (DMMP) and triethyl phosphate (TEP), and glacial acetic acid (HAc), as potential interferents were employed for the selectivity experiments. As shown in Figure 4a, the fluorescence maximum of the sensor 3 solution red-shifts from 377 nm to 476 nm, and the ratio of intensities at 476 nm to 377 nm [(F476/F377)a/(F476/F377)b] is higher than 12000 folds only for 20 equiv. DCP, and no obvious change for the color of solutions of all analytes. The selectivity can be observed by the naked eyes under irradiation from a hand-held UV lamp (365 nm) (Figure 4b). The solution emits strong cyan fluorescence of the sensor 3 in the presence of DCP solution, and blank and other analytes (DCNP, HAc, DMMP and TEP) display weak blue fluorescence (no significant difference).
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ACS Sensors creased to zero within 100 s and fluorescence intensity at 480 nm increased to maximum, and the fluorescent color changes from blue to cyan, shown in Fig. 6b. The membranes with 3 exhibit excellent photostability for them before and after exposure to DCP vapor. These show that the test strip can achieve a fast and effective detection of DCP vapor.
900
(a) DCP Intensity(a.u.)
600
blank and other analytes 300
1.2
350
400
450
500
550
600
Wavelength /nm
Figure 4. (a) Fluorescence spectra of the probe 3 (10 µM) before (1) and after addition of 20 equiv. of DCP(2), 100 equiv. of other analytes (3: DCNP, 4: HAc, 5: DMMP and 6: TEP) and. (b) Fluorescence images of above solutions.
Similarly, the selectivity of other sensors was also checked from their response to DCP and other interferents, shown in Figure S6 and Figure S7. Figure S6 displays clearly the changes in fluorescence maxima and their intensities (Fa/Fb) before and after the sensing reaction with DCP. These changes ascribe to different extents of ICT process, which depends on both the donating ability of 6-amine group and the accepting ability of the (protonated) pyridine unit. The photographs in Figure S7 provide a visual exhibition. With increasing the donating ability of 6-amine group, or acceptor from the pyridine to the protonated pyridine, the fluorescence color and the brightness reveal regular changes: the red-shift color and first increase and then decrease in the brightness. The relationship between the ICT character and the fluorescence efficiency of sensors and the sensing products can be illustrated in Figure 5. With increasing degrees of ICT, the fluorescence efficiency of a D-A molecule first increases, then decreases and finally near to non-fluorescent in the twisted ICT (TICT) state. In our previous paper, this relationship was clearly demonstrated.27 The related fluorescence efficiency between a sensor and its sensing product decides the fluorescence response mode, which is one important factor of the sensitivity of a chemosensor. In general, a turn-on fluorescent sensor would give a higher sensitivity over other modes such as ratiometric or turn-off mode. Ratiometric
2aH+ 2bH+ 3H+ 2c 2d 1
turn-on
3 2b 2a
Φf
turn-off
1H+ 2cH+ 2dH+ TICT state
Φf ~ 0
ICT Character D
A
D
AH +
Figure 5. Relationship between the ICT character and the fluorescence efficiency of sensors and the sensing products as well as the response mode.
Detection of DCP vapor with the test strips with 3. Finally, the detection of DCP in the gas phase is our goal. For this purpose, the test strip with sensor 3 was fabricated by dissolving sensor 3 and polyethylene oxide in DCM to form a membrane after the solvent volatilizing. As shown in Figure 6a, the emission maximum red-shifted from 377 to 480 nm upon exposure to DCP vapor. Fluorescence intensity at 377 nm de-
200
b
Exposed to DCP vapor
a 150 Intensity(a.u.)
0
Normalized Intensity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0.8
0.4
@377 nm 100
50 @480 nm 0
0.0 350
400 450 500 Wavelength /nm
550
600
0
2
4 6 Time /min
8
10
Figure 6. Fluorescence spectra (a), time-dependent intensity (b) and photos (inset) of the testing membrane with 3 before and after exposure to vapor of DCP (50 µL in liquid).
Next, the test strips were exposed to various amounts of DCP vapor for 2 min in centrifuge tubes (Figure 7). Originally, the membrane emitted purple fluorescence in the absence of DCP vapor. With increasing concentration of DCP solutions, leading to volatilize more DCP vapor, the fluorescence color of the membranes changes from blue to cyan gradually. The test strip with 3 can achieve a semiquantitative detection of DCP vapor.
Figure 7. Fluorescent response of 3 based on polyethylene oxide membrane upon exposure to various amounts of DCP vapor.
Similarly, the selectivity in gas phase was investigated through detecting the vapors of DCP and interferents (TEP, DCNP, DMMP, HAc and concentrated hydrochloric acid (HCl)) with the test membrane. As shown in Figure 8, the membrane emits cyan fluorescence in DCP vapor, and bluegreen fluorescence for the membrane exposed to HAc vapor. A similar response to HCl vapor was observed at the beginning, following to darken quickly. This could be the result of accumulation of HCl vapor on the test strip. In solution, no fluorescence response of sensors to HAc may ascribe to its weak acidity. On the basis of the sensing mechanism, the pyriding-based sensor shouldn’t discriminate between DCP and a strong acid. However, the acid-induced response is diffusioncontrol and faster than that promoted by DCP. Except HAc and HCl, no fluorescence response was observed for other analytes. These observations shows that the test strip with the sensor 3 is promising to detect selectively nerve agents in gas phase.
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We are grateful for financial support from the National Natural Science Foundation of China (Grant No. 21272224) and Anhui Provincial Natural Science Foundation (Grant No. 1708085MB33).
REFERENCES
Figure 8. Fluorescent response of 3 based on polyethylene oxide membrane upon exposure to various vapors of analytes (50 µL): 1, blank, 2, DCP, 3, TEP, 4, DCNP, 5, DMMP, 6, HAc and 7, HCl.
■ CONCLUSION In summary, we have prepared six 6-aminoquinolines with different N-substituents as chemosensors for a nerve-agent simulant DCP. The donating ability of the amine group depends on N-substituents, thereby, giving varying degrees of the ICT for both a sensor and its protonated form. With enhancing ICT process, the sensors and their protonated states exhibit the corresponding photophysical property including red-shift of both absorption and fluorescence maxima, and fluorescence efficiencies first increase and then decrease. Correspondingly, fluorescence response of the sensors to DCP exhibits different modes. These observations would gain some new insights in molecular design of fluorescent sensors based on the ICT strategy. Meanwhile, the sensing mechanism of catalytic hydrolysis of DCP was demonstrated. Among these sensors the sensor 3 to DCP displays ratiometric fluorescence response and a low limit of detection (8 nM). Furthermore, a facile testing strip with 3 has been fabricated with polyethylene oxide for real-time selective monitoring of DCP vapor.
ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.? Photophysical properties of sensors and their sensing products, UV/Vis absorption and fluorescence spectra of the sensors with various amounts of DCP, Measurements of pKa values for sensors, The effect of trace water on the sensing reaction, Experiments for the selectivity of sensors, and NMR spectra of new compounds (PDF)
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected]. fax: +86 551 63601592; tel: +86 551 63607992.
ORCID Qin-Hua Song: 0000-0001-6501-1382 Notes The authors declare no competing financial interest. Funding Sources National Natural Science Foundation of China (Grant No. 21272224), and Anhui Provincial Natural Science Foundation (Grant No. 1708085MB33).
ACKNOWLEDGMENT
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SYNOPSIS TOC
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