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Article Cite This: ACS Omega 2018, 3, 10306−10316
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Design of a Pyrene Scaffold Multifunctional Material: Real-Time Turn-On Chemosensor for Nitric Oxide, AIEE Behavior, and Detection of TNP Explosive Abu Saleh Musha Islam, Mihir Sasmal, Debjani Maiti, Ananya Dutta, Bibhutibhushan Show, and Mahammad Ali* Department of Chemistry, Jadavpur University, 188 Raja S.C. Mallick Road, Kolkata 700032, India
ACS Omega 2018.3:10306-10316. Downloaded from pubs.acs.org by 5.189.202.221 on 08/31/18. For personal use only.
S Supporting Information *
ABSTRACT: A dual-emission pyrene-based new fluorescent probe (N-(4nitro-phenyl)-N′-pyren-1-ylmethyl-ene-ethane-1,2-diamine (PyDA-NP)) displays green fluorescence for nitric oxide (NO) sensing, whereas it exhibits blue emission in the aggregated state. The mechanism of nitric oxide (NO/NO+) sensing is based on N-nitrosation of aromatic secondary amine, which was not interfered by reactive oxygen species and reactive nitrogen species. The aggregation-induced enhancement of emission (AIEE) behaviors of the PyDA-NP could be attributed to the restriction of intramolecular rotation and vibration, resulting in rigidity enhancement of the molecules. The AIEE behavior of the probe was well established from fluorescence, dynamic light scattering, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, optical fluorescence microscopy, and time-resolved photoluminescence studies. In a H2O/ CH3CN binary mixture (8:2 v/v), the probe showed maximum aggregation with extensive (833-fold) increases in fluorescence intensity and high quantum yield (0.79). The aggregated state of the probe was further applied for the detection of nitroexplosives. It displayed efficient sensing of 2,4,6-trinitrophenol (TNP), corroborating mainly the charge-transfer process from pyrene to a highly electron-deficient TNP moiety. Furthermore, for the on-site practical application of the proposed analytical system, a contact-mode analysis was performed.
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INTRODUCTION The role of nitric oxide (NO), a redox molecule, as a key mediator of immunity has recently garnered renewed interest and appreciation. It plays key roles in mammalian biology such as (a) in signal transductions in the central nervous system, (b) in inflammatory response, (c) in modulations of ion channels, and (d) in cardiovascular homeostasis.1−4 It is also an integral part of the immune system. As a result, monitoring of the concentration of NO in a biological system is crucial. The quantification of trace amount of NO in a biological system is very difficult as it is highly reactive and rapidly scavenged and persists in a wide range of physiologically relevant concentrations.5 Again, nitrosonium ion (NO+), an oxidized form of nitric oxide, is the key species in the process of nitrosation reactions, which most likely occur through electrophilic attack by NO/N2O3 on a nucleophile center such as thiols/amines to generate S-nitrosothiols or N-nitrosamines.6,7 The van der Waals-like attractive forces between molecules are responsible for self-association in solution or at the solid− liquid interface, which exhibit distinct changes in the absorption/emission bands as compared to those of the monomeric species. The spectral shifts tell us about the various aggregation patterns. The molecular exciton coupling theory, i.e., coupling of transition moments of the constituent probe © 2018 American Chemical Society
molecules, successfully explains the occurrence of bathrochromically shifted J-bands and hypsochromically shifted H-bands of the aggregates. The aggregation of a fluorophore (FL) is a natural process; however, it may play both a positive and a negative role in the enhancement of the emission of a fluorophore. The aggregation-induced emission (AIE) was first coined in 2001 to indicate luminogen aggregation with enhancing its emission.8,9 Contrary to the conventional emitters, the AIEgens are weakly emissive or nonemissive in dilute solutions but emit efficiently in the aggregated or solid states, which may arise due to restriction of intramolecular motions like intramolecular rotation (RIR) and intramolecular vibration (RIV).10−13 Attracted by this intriguing phenomenon, a large number of AIEgens with diversified architectures and versatile properties have been generated and applied in organic light-emitting devices, chemo- and biosensors, biological imaging, etc.14−17 Detection of explosives is of extreme importance due to the intimidation for human security as a result of illegal transport, terrorist activities, and forensic investigations.18−21 TrinitroReceived: June 9, 2018 Accepted: August 14, 2018 Published: August 31, 2018 10306
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phenol (TNP) was the first highly explosive nitrated organic compound that is widely considered suitable to withstand the shock of firing in conventional artillery. Apart from its highly explosive nature, TNP also causes severe irritation, dizziness, nausea, skin allergy, and damage of kidney and liver. It is also an important basic compound for fungicides, analytical reagents, fireworks, staining agents, and germicides and also used in glasses, matches, and leathers.22 The commercial production of TNP or its uses may cause release of TNP in the environment, leading to the contamination of aquatic systems and thus also of drinking water resources. TNP gets converted into picramic acid (2-amino-4,6-dinitrophenol) during the mammalian metabolic processes, which shows more mutagenic activity than TNP itself.23 Therefore, quick and specific recognition of TNP is very important. For the detection of explosives, various methods are available, such as gas chromatography, ion-mobility spectrometry, Raman spectroscopy, and fluorescence spectroscopy.19 Among these different techniques, the fluorescence-quenching-based technique is a favored one for molecular sensing24−34 because it is highly sensitive, specific, fast, simple, and cost-effective. In this research endeavor, we have designed a smart molecular probe which exhibits water-mediated aggregationinduced enhancement of emission (AIEE) (λem = 467 nm), which on further treatment with trinitrophenol (TNP) exhibits selective fluorescence quenching over other nitrated compounds, indicating the sensing of TNP in the blue zone. In addition, it also displays selective sensing of nitric oxide (NO) and NO+ in the (λem = 523 nm) green zone by the intramolecular charge-transfer (ICT) mechanism. A number of fluorescent probes for selective sensing of NO have been exploited till date using different fluorophores (FLs). The mechanism of NO sensing is based on (a) selective reaction of NO (Scheme 1) with o-phenylenediamine35−37 to form
Scheme 2. Synthesis of the Probe
etc. in aqueous solutions. Here, for the first time, we have designed and synthesized a pyrene derivative for sensing of nitric oxide.
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EXPERIMENTAL SECTION Materials and Methods. 1-Pyrenecarboxaldehyde, 4nitrochlorobenzene, ethylenediamine (Sigma-Aldrich), absolute ethanol, salts of cations like Ca2+, Al3+, Co2+, Cd2+, Cr3+, Fe3+, Cu2+, K+, Mn2+, Mg2+, Na+, Ni2+, Zn2+, and Pb2+ and different reactive molecules/ions like •OH, H2O2, NO3−, O2−, ClO−, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radical, NO2−, ONOO−, ascorbic acid (AA), dehydroascorbic acid (DHA), and DEA-NONOate as well as 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (DNP), 4-nitrophenol (4-NP), pcholoronitrobenzene (Cl-NB), m-dinitrobenzene (DNB), 3,5dinitrobenzoic acid (DNBA), and 4-nitrobenzoic acid (4NBA) were obtained from commercial suppliers and SigmaAldrich, which were used without further purification. The reagent-grade solvents like CH3CN (Merck, India), MeOH, and tetrahydrofuran were dried before use. Physical Measurements. A PerkinElmer RX I Fourier transform infrared (FT-IR) spectrophotometer was used to record Fourier transform infrared (FT-IR) spectra in the range 4000−400 cm−1 in solid KBr disks. To record UV−vis spectra, an Agilent 8453 diode-array spectrophotometer was used, whereas fluorescence spectra were recorded on a PTI (model QM-40) spectrofluorimeter. 1H and 13C NMR data were recorded on a Bruker 300 MHz spectrometer using trimethylsilane as an internal standard in dimethyl sulfoxide (DMSO)-d6. To obtain the electrospray ionization mass spectrometry (ESI-MS+) (m/z) spectra of the amine (L1), probe (N-(4-Nitro-phenyl)-N′-pyren-1-ylmethyl-ene-ethane1,2-diamine (PyDA-NP)), and the reaction product (PyDANP−NO), an high-resolution mass spectrometry (HRMS) spectrometer (model no: QTOF Micro YA263) was used. Time-correlated single-photon counting (TCSPC) measurements were carried out on a Hamamatsu MCP photomultiplier (R3809) and analyzed by IBH DAS6 software embedded with a nanosecond diode laser (IBH Nanoled-07) in an IBH fluorocube apparatus. Solution Preparation for UV−Vis Absorption and Fluorescence Studies. For UV−vis as well as fluorescence titrations, a 1.0 × 10−3 M stock solution of PyDA-NP was prepared in CH3CN and 1.0 × 10−2 M stock solution of NOBF4 was prepared in dry CH3CN. Similarly, 1.74 × 10−3 M stock solution of NO was prepared by a previously reported method46 in deoxygenated deionized water. The •OH, ONOO−, and HNO solutions were prepared by reported methods.43 The 1.0 × 10−3 M stock solutions of TNP and other nitroaromatics were prepared in 8:2 (v/v) water and CH3CN. The solutions of different anions and metal cations were prepared in H2O. The probe PyDA-NP (10 μM) was taken in 2.5 mL of CH3CN with 5% water in a cuvette, to
Scheme 1. Different Strategies for the Detection of NO
triazole, (b) diazo ring formation,38,39 (c) oxidative deamination,40−42 and (d) reaction of NO with thiosemicarbazide,43 leading to the formation of oxadiazole. All of these lead to the enhancement of emission. Here, we have developed a new pyrene-based probe (Scheme 2) for the selective detection of NO via the N-nitrosation44,45 reaction with a huge increase (66-fold) in fluorescence intensity (FI). Pyrene is a very important moiety to make dyes or dye precursors, and its derivatives are widely used as fluorescent sensors for lanthanide ions, adenosine triphosphate, G-quadruplex DNA, 10307
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(d, 1H, −ArH), 9.39 (s, 1H, −azomethine) (Figure S3). 13C NMR (in DMSO-d6) (δ, ppm): 161.98, 155.16, 136.11, 132.76, 131.23, 130.55, 129.73, 128.85, 127.84, 127.01, 126.81, 126.69, 126.53, 126.36, 125.39, 124.43, 124.19, 123.43, 112.42, 111.53, 60.51, 43.86 (Figure S4). ESI-MS+ (m/z): 394.1458 (PyDA-NP + H+) (Figure S5). FT-IR: 3363 cm−1 (−NH), 1594 cm−1 (−CN) (Figure S6).
which NO+ or NO was added incrementally from 0 to 80 μM, and UV−vis and fluorescence spectra were recorded. Calculation of Limit of Detection (LOD). The detection limit (LOD) of NO and NO+ has been calculated by the 3σ method. LOD = 3 × Sd /S
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From the plot of FI versus [PyDA-NP], standard deviation (Sd) of the intercept was obtained, and slope S was obtained from the fluorescence titration of PyDA-NP with NO/NO+ and taking the linear part of FI versus [NO/NO+] plots. Quantum Yield (Φ) Calculation. Fluorescence quantum yields (Φ) were calculated by the equation Φsmaple =
ODstd × A sample ODsample × A std
RESULTS AND DISCUSSION Optical Sensing Properties. The photophysical properties of PyDA-NP were investigated with absorption and fluorescence studies. The free PyDA-NP shows a weak emission at 430 nm on excitation at 390 nm in CH3CN. It is quite astounding to ascertain that PyDA-NP shows an aggregation-induced emission enhancement (AIEE) effect when the volume percentage of water is increased up to 80% in the binary solvent mixture (H2O/CH3CN). Moreover, another quite surprising finding is that PyDA-NP exhibits highly selective and sensitive recognition toward NO in CH3CN containing 5% water. Spectral Response of PyDA-NP to NO. The UV−vis spectrum of sensor PyDA-NP was recorded in 5% water in CH3CN, which displayed well-defined bands at 242, 276, 284, 342, 360, 370, and 389 nm. On gradual addition of NO+, there is a decrease in absorbance at 242, 276, and 284 nm along with the development of new bands at 243, 309, and 448 nm, with isosbestic points at 237, 253, 289, and 322 nm (Figure 1). This
× Φstd
where Asample and Astd are the areas of fluorescence spectral curves of the sample and standard, respectively, at the excitation wavelength and ODsample and ODstd are the respective optical densities. The standard aqueous acidic quinine sulfate solution (Φstd = 0.54) was used for measuring the quantum yields of PyDA-NP, PyDA-NP + NO+, and different ratios of binary mixture (v/v) H2O/CH3CN systems. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS), and X-ray Diffraction (XRD) Analyses. The crystallinity of the deposited materials on mica foils by the drop-cast method was examined by the X-ray diffraction (XRD) technique using a Bruker D8 ADVANCE X-ray diffractometer with a Bragg−Brentano goniometer geometry and Cu Kα X-radiation source (λ = 1.5418 Å). Morphological analyses were carried out by field emission scanning electron microscopy (FESEM) (JEOL, JSM-6700F) and TEM (JEOL JEM 2100F) with an acceleration voltage of 200 kV. For TEM analysis, the samples were prepared in 10−3 M range, drop-cast on a copper grid, and then allowed to vacuum-dry overnight. The extent of aggregation of Py-DANP has been studied with a DLS (Particle Size Analyzer-Zetasizer 1000 HS, U.K.) experiment. Syntheses of N1-(4-Nitro-phenyl)-ethane-1,2-diamine (L1). 1-Chloro-4-nitrobenzene (3.14 g, 20 mmol) and 10 mL of ethane-1,2-diamine were dissolved in 30 mL of acetonitrile. CuI (0.38 g, 2 mmol) was added to act as a catalyst. The reaction mixture was stirred for 6 h under reflux, then cooled down to room temperature, and poured into cold saturated brine solution. The precipitate thus formed was filtered and dried to afford 2.3 g (12.7 mmol, 63%) of yellow solid, which was used without further purification. 1H NMR (in CDCl3) (δ, ppm): 1.35 (s, 2H, NH2); 3.02 (m, 2H, CH2); 3.27 (m, 2H, CH2); 5.14 (s, 1H, NH); 6.55 (d, 2H); 8.08 (d, 2H) ESI-MS+ for ([L1 + H])+ is 182.0930 (Figures S1 and S2). Syntheses of N-(4-Nitro-phenyl)-N′-pyren-1-ylmethyl-eneethane-1,2-diamine (PyDA-NP). L1 (0.543 g, 3 mmol) was dissolved in 25 mL of EtOH. To this solution, 1pyrenecarboxaldehyde in 5 mL of ethanol (0.690 g, 3 mmol) was added dropwise and refluxed for 5 h, whereupon the yellow precipitate formed was filtered and washed with ethanol. It was further recrystallized from ethanol to obtain a pure yellow solid of PyDA-NP. (Yield, 78.34%.) 1H NMR (in DMSO-d6) (δ, ppm): 3.63 (d, 2H, −CH2), 4.00 (s, 2H, −CH2), 6.77 (d, 2H, −ArH), 7.48 (s, 1H, −NH), 8.00 (d, 2H, −ArH), 8.07−8.36 (m, 7H, −ArH), 8.55 (d, 1H, −ArH), 9.05
Figure 1. Absorption titration of PyDA-NP (5 μM) with NO+ (0−50 μM) in 5% water in CH3CN. Inset: plot of absorbance vs [NO+].
indicates an electrophilic substitution of H+ by NO+ at the NH nitrogen atom, which is confirmed by FT-IR, HRMS, and 1H NMR studies (vide infra). A plot of absorbance (at 448 nm) versus [NO+] gives a nonlinear curve of decreasing slope, which yields Kd = (6.47 ± 0.51) × 10−6 M by a nonlinear computer-fit program (Figure 1, inset). In 5% water−CH3CN, the PyDA-NP probe is highly sensitive and selective toward NO+. A fluorescence titration was carried out with a fixed concentration of PyDA-NP (10 μM) with variable concentrations of NO+ (0−80.0 μM) at 25 °C in 5% water−CH3CN. It was observed that on gradual addition of NO+ a new emission band at ∼523 nm was developed on excitation at 390 nm (Figure 2). A FI versus [NO+] plot gives a nonlinear curve, which was fitted by a nonlinear computer-fit program to give Kd = (16.8 ± 1.3) × 10−6 M. It is interesting to note that there is a reasonable agreement between the Kd values obtained from two different spectroscopic titrations. We also performed the titration of PyDA-NP with a standard saturated stock solution of NO (1.74 × 10−3 M) in 10308
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Scheme 3. Mechanism of NO/NO+ Ion Detection
corresponding to the −NH stretching frequency (Figure S9). However, for the product formed after the reaction between PyDA-NP and NO or NO+, the −NH stretching frequency vanishes and a new absorption band appears at 1411 cm−1 corresponding to the −NO stretching frequency of nitrosamine47 (Figure S9b). Again, the 1H NMR study reveals that −NH peaks of PyDA-NP at 7.48 ppm vanish to base line on reaction with NO+, indicating the fact that NO+ attacks on the −NH group to give the −N−NO moiety (Figure S7). The observation of ESI-MS+ peak at 486.1884 (PyDA-NP + NO + Na+ + CH3CN) clearly suggested the formation of the −N−NO group (Figure S8). To account for the observed sensing of NO or NO+ ion by PyDA-NP, an intramolecular charge-transfer (ICT) mechanism (Scheme 3) was invoked. Due to the formation of −N−NO, an electron-withdrawing moiety, as well as the presence of an electron-withdrawing group −NO2 on the benzene ring, leads to an enhanced ICT emission as indicated by a large red shift of 93 nm (peak shifted from 430 to 523 nm). Selectivity. Exclusivity in the detection of analyte is imperative for a real-time sensor system. Thus, the selectivity of the probe (10 μM) toward NO/NO+ is confirmed by reacting with various reactive oxygen species (ROS) and reactive nitrogen species (RNS) such as •OH, H2O2, O2−, NO3−, ONOO−, NO2−, DEA-NONOate, NO, NO+, TEMPO radical, ascorbic acid (AA), and dehydroascorbic acid (DHA) in 70 μM concentration. As shown in Figure 4, the fluorescence behavior of PyDA-NP showed almost negligible changes after the addition of ROS/RNS, except for DEANONOate, NO, and NO+. The detection of NO is not interfered by various inorganic anions (Figure S10) and cations also (Figure S11). Quantum Yield (Φ) and Detection Limit (LOD). Using quinine sulfate as a standard, the quantum yield (Φ) of PyDANP−NO becomes 0.107, whereas for the free ligand, Φ = 0.00118. To calculate the LOD of NO and NO+, the 3σ method was adopted, and it was found to be 2.13/0.307 pM,
Figure 2. (a) Fluorescence titration of PyDA-NP (10 μM) with the increase in concentration of NO+ from 0 to 80 μM at 25 °C in 5% water−CH3CN. (b) Plot of FI vs [NO+]. (c) UV-exposed photograph.
pure water. It is interesting to observe that a new emission band appears at ∼523 nm with the increase in the concentration of NO (0−14.0) μM (Figure 3a) with the probe in 5% aqueous CH3CN. However, when the NO concentration is increased beyond 14 μM, the emission peak at ∼523 nm decreases gradually and a new emission band at ∼445 nm builds up with a bathochromic shift. This interesting photophysical property is attributed to the fact that on increasing the concentration of NO up to 14 μM (6% water content), the intramolecular charge-transfer (ICT) mechanism (Scheme 3) is operative, showing the emission band at ∼523 nm, but on further increase in the concentration of NO beyond 14 μM, the water content of the resulting solution gradually increases (16% or more water content). As a result, aggregation-induced enhanced emission (AIEE) of PyDA-NP dominates over the ICT mechanism and thus an emission band at ∼445 nm builds up (vide infra). As AIEE is more effective than the ICT process at a comparatively higher water content, the emission peak at 523 nm is decreased and the peak at 445 nm builds up gradually beyond 14 μM NO (Figure 3b). Mechanism of NO Sensing. The PyDA-NP probe reacts with NO in the presence of O2 through the formation of intermediate N2O3, which acts as a source of NO+ (N2O3 → NO+ + NO2−) to give N-nitroso amine.43 On the other hand, NO+ (NO+BF4−) in CH3CN also directly reacts with PyDANP to give deep yellow N-nitrosamine. The pure product thus obtained was analyzed by the 1H NMR (Figure S7), HRMS (Figure S8), and FT-IR studies (Figure S9). From the FT-IR study, PyDA-NP showed the absorption band at 3363 cm−1
Figure 3. (a) Fluorescence titration of PyDA-NP (10 μM) with variable concentration of NO (0−14 μM) at 25 °C in 5% water−CH3CN. (b) Plot of FI vs [NO] at 523 nm. (c) Fluorescence titration of PyDA-NP (10 μM) with variable concentration of NO (14−192 μM). (d) Plot of FI vs [NO] at λem = 445 and 523 nm. 10309
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Figure 6. (a) Fluorescence titration of PyDA-NP (10 μM) with an increase in the % of water in CH3CN. (b) Plot of FI vs [% of water]. (c) Image of aggregation-induced emission of PyDA-NP under UVirradiation (λ = 365 nm).
Figure 4. Bar plot for fluorescence signals of PyDA-NP at 523 nm (λex = 390 nm) toward different biological anions in 5% water−CH3CN. PyDA-NP = 10 μM, Xn− = 80 μM.
water % shows a steady increase in FI with the increase in water %, reaches a maximum at 80% water (Figure 6b), and then decreases on further increase in water %. This indicates that aggregation maximizes at 80% water content; however, on a further increase in water content, the aggregates get ruptured. Time Course of Fluorescence Response. To gain deep insight into fluorescence enhancement, time-dependent studies have been performed on the nanoaggregates of PyDA-NP. In the presence of 40% water content in CH 3CN, the fluorescence intensity becomes almost saturated after ∼15 min. This indicates that the aggregation process is rather fast (Figure 7).
respectively (Figure S12), which seems to be the lowest LOD of the so far reported probes. From all of the results presented above, it is clear that PyDA-NP is an ideal chemosensor for the detection of NO+ or NO. Aggregation-Induced Emission. It is well known that a fluorophore may undergo aggregation naturally. It will be exciting when aggregation leads to an increase in emission of a fluorophore by a positive role instead of a negative.48 During the solvent selection process for sensory property, we found that our probe PyDA-NP is freely soluble in CH3CN. UV−vis absorption of PyDA-NP in CH3CN solution shows prominent peaks at 343, 360, 370, and 388 nm, which may be originated from the π−π* transition of the monomeric form of pyrene chromophore. Upon incremental addition of the water fraction (f w) from 0 to 80% to PyDA-NP in CH3CN, the UV peak at 358 nm is decreased and the peak at 388 nm is red-shifted toward the isosbestic point at 396 nm (Figure 5).
Figure 7. (a) Time-dependent fluorescence spectral changes of PyDA-NP (10 μM) in 40% water content CH3CN (λex = 390 nm) (each measurement performed at an equal span of 50 s). (b) Relative changes in fluorescence intensity of PyDA-NP, as a function of time (0−17 min). Figure 5. Absorption titration of PyDA-NP (10 μM) with the increase in % of water in CH3CN.
FESEM, TEM, and DLS Studies. To establish a nanolevel aggregation of the ligand (PyDA-NP) during AIEE studies, FESEM, TEM, DLS, and XRD studies were performed. Structural and morphological changes with respect to (w.r.t.) water dilutions from 0 to 100% were noticed. Also, an unusual change was observed during treatment with NO+. The surface morphology of PyDA-NP after different treatments has been analyzed by FESEM. Here, Figure 8 reveal that (a) the ligand in pure CH3CN (0% water) shows trigonal crystallites with a grain size of ∼90 nm, (b) the ligand in the presence of 80% water shows cubical crystallites with a grain size of ∼65 nm, and also, (c) the ligand in the presence of NO+ in 5% water− CH3CN shows spherical crystallites with a grain size of ∼70 nm at room temperature; the corresponding magnified crystallites are shown in the inset of the Figure 8. The changes in their surface morphologies (in the same scale bars i.e., 3
From the fluorescence spectra, with the increase in % of water content in CH3CN, a gradual enhancement of emission intensity with 20 nm red shift from 447 to 467 nm (Figure 6a) was observed, reaching a maximum at 80% of water content. This bathochromic shift in the emission band arises due to Jtype aggregation through a face-to-face packing between the monomers.49 This clearly indicates that the PyDA-NP molecule is AIEE-active. This aggregation-induced enhanced emission of PyDA-NP could be due to high rigidity of the probe in the aggregated state causing restriction of RIR and/or RIV. Again, a noticeable color change is observed from colorless to cyan under UV light illumination upon addition of different % of water (Figure 6c). A plot of FI as a function of 10310
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Figure 8. FESEM images of (a) PyDA-NP in CH3CN (b), PyDA-NP in 80% water−CH3CN, and (c) PyDA-NP + NO+ (80 μM) in 5% water− CH3CN. In the inset, corresponding magnified crystallites are shown.
Figure 9. TEM images of (a) PyDA-NP in CH3CN, (b) PyDA-NP in 80% water−CH3CN, (c) PyDA-NP + NO+ (80 μM) in 5% water−CH3CN; (d−f) the corresponding atomic fringes of three different samples are encircled.
μm) were responsible for the enhanced emission, which directly influence the impressive coloration behavior of the hydrosol, supported with optical studies (vide infra). From the TEM analysis (Figure 9), the scattered nanocrystals of Schiff base (a) PyDA-NP in CH3CN at a 100 nm scale bar indicates that the small crystallites are well dispersed. On the other hand, (b) PyDA-NP at 80% (water−CH3CN) shows the smaller crystallites to form distorted cubical grains and (c) PyDA-NP + NO+ (80 μM) in 5% (water−CH3CN) denotes spherical grains (Figure 9) having 10−15 nm crystallites size in 20 nm scale bars, respectively. The corresponding lattice fringes of three distinct samples PyDA-NP in CH3CN, PyDA-NP in 80% water−CH3CN, and PyDA-NP + NO+ (80 μM) in 5% water− CH3CN are encircled in yellow in Figure 9 and are shown in (d) 100 nm, (e) 5 nm, and (f) 5 nm scale bars, respectively. The density of lattice fringes and their orientations are completely different from each other due to their own space filling nature exerted by the incumbent interactions. From TEM analyses, it has been noticed that the shape of the small crystallites is almost the same as the surface topography in different conditions. It was also suggested that the aggregationdriven growth is one of the reasons for emission enhancement. The agglomeration tendency of PyDA-NP in 5% water−
CH3CN on NO+ charging is shown to be the predominating reason for the ICT process of sensing. Particle size determination of the prepared PyDA-NP hydrosol was performed by dynamic light scattering (DLS) measurements with successively increasing dilution of the stock solution by deionized water. The average diameters of the particles ∼38, ∼171, ∼455, and ∼341 nm have been observed after each dilution with 0, 30, 80, and 100% water content in binary (water−CH3CN) mixtures followed by ultrasonication for 15 min, respectively (Figure 10). With the increase in water fractions from 80 to 100% the average diameter of the particle starts decreasing from ∼455 to ∼341 nm, which can be attributed to the change in the nature of agglomerated structure; the solubility or dispersion of the agglomerated adduct of the pyrene system is much more affected in 100% water medium. It is to be expected that the maximum number of excimers reside together at 80% water fraction. The number of fluorogenic groups is progressively decreasing with the increase of dilution (80−100%); therefore, grain distributions are also affected, which directly influence the AIEE behavior. The change in the agglomerated structure in aqueous medium with water dilution indicates their high surface reactivities, with the adjacent moieties thereby showing the bulky nature of the 10311
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Figure 10. Dynamic light scattering (DLS) results showing the variation in particle size diameter with increasing % of water content in CH3CN solution: (a) 0% water, (b) 30% water, (c) 80% water, and (d) 100% water.
angle in radian), was applied to determine the crystallite size of the deposited films by the drop-cast method on different cleaned mica foils. The values of the dislocation density (δ), which gives the number of defects in the materials, were calculated from the average values of the crystallite size D by the relationship51 δ = 1/D2 and found to be significantly low, on the order of 10−4/nm2. A comparison between the FWHM, crystallite size (D), and dislocation density (δ) at different peaks of hybrid PyDA-NP films under different reaction conditions is shown in Table S1. The Debye−Scherrer grain size and dislocation density clearly indicate that the bare ligand PyDA-NP and its dilution in a binary mixture (water− CH3CN) in the range of 30−80%, with NO+ in 5% water, are altogether different from 100% water-treated PyDA-NP. Thus, X-ray analysis also supports the change in crystallinity nature w.r.t. dilution which may directly influence the emission process of optical properties. Optical Fluorescence Microscopy Studies. The aggregation of PyDA-NP in the presence of water can be well established with the help of solid-state luminescence studies by optical microscopy. Moreover, upon UV excitation, PyDA-NP does not show any emission, but with 80% water content in CH3CN, the PyDA-NP shows strong green emission. Again, in a 5% water−CH3CN binary mixture, the solution of PyDA-NP (10 μM) and NO+ (80 μM) displays orange emission (Figure 12). This is a beautiful illustration of an optical waveguide effect operating in a light transmission process within the nanocrystals. The responsive behavior toward light depends upon the number of excitons in different wavelengths of UV radiation. Thus, the luminescence behavior in the solid state is attributed to the fabrication of organic light-emitting diode as a suitable material.12,48 TCSPC Studies. The time-resolved single-photon counting (TCSPC) measurements were used to reveal the nature of different light-emitting species. This measurement is more robust than the measurement of fluorescence intensity because it does not depend on the excitation intensity or the
particle. Therefore, we assume that with dilution the surface topology as well as their reactive sites altogether changes, which influences the emission process. Again, the size of the aggregates is reduced with an increase in water content, resulting in a dramatic decrease in the emission intensity examined through different optical and structural analyses. XRD Analysis. From XRD analysis, it has been confirmed that the size of the crystallites gradually increases because of water dilutions effects. The diffraction patterns are almost the same, but for the PyDA-NP in 100% water, the intensity of the peaks gradually decreases (Figure S13), indicating the fact that agglomeration is no longer taking place. It has been noticed that peak broadening takes place with an increase in the % of water dilution and maximum peak broadening has been observed at 80% water dilution due to the changes in chemical environments, as evidenced from their magnification as well as lower same scale bar fitting images (Figure 11). The Debye−Scherrer equation,50 D = 0.9 × λ/β cos θ (where D is the crystallite size, λ = 1.5418 Å, β is the full width at half-maximum (FWHM), and θ is the half of the diffraction
Figure 11. X-ray diffraction patterns of overly images of (a) PyDANP (10 μM) in CH3CN; PyDA-NP (10 μM) in 30, 80, and 100% water−CH3CN; and NO+ (80 μM) in 5% water−CH3CN. 10312
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Figure 12. Optical fluorescence microscopy images (solid state) of (a) PyDA-NP (10 μM) in CH3CN, (b) PyDA-NP (10 μM) in 80% water− CH3CN, and (c) PyDA-NP + NO+ (80 μM) in 5 % water−CH3CN with UV light excitation (scale bar = 50 μm).
concentration of the fluorophore. With the increase in % of water in the binary mixture, the lifetime of PyDA-NP also increases (Table S2). As shown in Figures 13 and S14, the fluorescence decay profile of PyDA-NP with 0, 30, 80, and 100% water−CH3CN solution is well fitted with biexponential decay.
zoic acid (DNBA), and 4-nitrobenzoic acid (4-NBA). Among the different aromatic nitroexplosives, only TNP showed selective reduction of the fluorescence intensity of the aggregated (excimer) fluorophore up to 97% (Figure 14). We also performed overselectivity for the detection of TNP (Figure S16), where the fluorescence intensity is fully quenched only in the presence of TNP.
Figure 13. Time-resolved fluorescence decay of PyDA-NP in CH3CN and in the presence of 30 and 80% water content binary mixture (water−CH3CN) solution at room temperature.
As revealed previously, the J-type aggregation of PyDA-NP has been proposed from the bathochromic shift of absorption and emission bands. AIEE behavior of PyDA-NP was further investigated by measuring the fluorescence quantum yield (Φ) as a function of f w (0−100%), shown in Figure S15 and Table S2. The fluorescence quantum yield of PyDA-NP (Φ = 0.00118) in pure CH3CN is also enhanced 833-fold (Φ = 0.79) in the presence of 80% water content in CH3CN. A further increase in water % decreases Φ, for the same reason as discussed earlier. All of these results are consistent with the photophysical properties of reported J-type aggregates.52−54 We have also calculated the radiative (kr) and nonradiative (knr) rate constants of PyDA-NP at different water % using the following equations
Figure 14. Quenching efficiency (%) of the excimer fluorescence (at λem = 467 nm with λex = 390 nm) of PyDA-NP (10 μM) in 8:2 H2O/ CH3CN upon addition of different nitroexplosives.
It was observed that on addition of TNP (0−250 μM) to the solution of PyDA-NP in 8:2 H2O/CH3CN (Figure 15) the fluorescence intensity gradually decreases. To determine the quenching constant of PyDA-NP by TNP, the fluorescence intensity ratio (I0/I) was plotted against the concentration of TNP, yielding a nonlinear curve (Figure 16), and was solved by adopting an exponential quenching Stern−Volmer (SV) equation, I0/I = A ek[Q] + B.55 From this equation, the calculated quenching constant becomes (1.4 ± 0.02) × 104 M−1. The value of quenching constant is high enough to indicate the superquenching ability of TNP toward PyDA-NP. The nonlinear nature of the Stern−Volmer (SV) plot of TNP (Figure 16) can be ascribed to the self-absorption or an energy transfer process. It is pointed out here that the linear deviation of the SV plot indicates the combination of static and dynamic quenching or that the extent of quenching is high at a higher concentration of TNP. The fluorescence lifetime measurement of the sensor in the presence and absence of the quencher (TNP) is the best way to understand the
k r = ϕf /τ and τ −1 = k r + k nr
where Φf is the quantum yield and τ is the lifetime. This clearly showed that kr increases significantly with an increase in the % of water, whereas knr decreases to some extent. All of these photophysical parameters are listed in Table S2. The fascinating AIEE effects of the probe, PyDA-NP, lead us to explore its potential applications as a chemosensor for nitroaromatics such as 2,4-dinitrophenol (DNP), 2,4,6trinitrophenol (TNP), 4-nitrophenol (4-NP), m-dinitrobenzene (DNB), p-choloronitrobenzene (Cl-NB), 3,5-dinitroben10313
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from colorless to light yellow (a → c) visible to the naked eye (Figure 17). The instantaneous cyan blue fluorescence quenching (b → d) was observed under a UV lamp (365 nm).
Figure 15. (a) Fluorescence titration of PyDA-NP (10 μM) in 8:2 H2O/CH3CN with increasing concentration of TNP (0−240 μM). (b) Plot of FI vs [TNP]. (c) (i) Aggregation of PyDA-NP in 80% water content and (ii) addition of TNP (240 μM) in the aggregated state (under naked eye). (d) (i) PyDA-NP in CH3CN; aggregation of PyDA-NP (ii) in 80% water content and (iii) upon addition of TNP in the aggregated state under a UV lamp (365 nm).
Figure 17. Detection of TNP by the test strip method under (a, c) day light and (b, d) UV lamp (365 nm).
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CONCLUSIONS In conclusion, we have designed and synthesized a pyrenebased fluorescent probe, which displays dual emission; green fluorescence emission (∼523 nm) for nitric oxide due to the ICT process and aggregation-induced enhancement emission (AIEE) in the blue region (∼467 nm) for the formation of a pyrene excimer complex. The high selectivity and sensitivity of the novel probe PyDA-NP toward nitric oxide could be achieved by the N-nitrosation mechanism. The AIEE property of the novel probe was well established from UV−vis, fluorescence, DLS, SEM, TEM, XRD, optical fluorescence microscopy, and time-resolved photoluminescence studies. In the H2O/CH3CN binary mixture (8:2 v/v), the probe showed maximum aggregation with extensive (833-fold) increases in fluorescence intensity and high quantum yield (Φ = 0.79). The fascinating AIEE effects of PyDA-NP lead us to explore its potential applications as a chemosensor for the selective recognition of trinitrophenol (TNP). In addition, we have also performed contact-mode analysis for the detection of TNP, which is specifically used in the field of analytical and forensic sciences. Therefore, we strongly believe that this non-dyebased PyDA-NP molecule is a superior multifunctional material, which shows selective sensing of NO (λem = 523 nm), AIEE behavior (λem = 476 nm), and selective detection of nitroexplosive TNP.
Figure 16. Plot of I0/I against the concentration of TNP.
involvement of either dynamic or static mechanism in the quenching process. The static quenching arises due to the binding of the sensor with the quencher in the ground state, forming a nonfluorescent complex in the “dark state”, and the unbound molecules exhibit their inherent lifetimes. However, for dynamic quenching, the fluorescence lifetime of the sensor decreases via diffusion-controlled collisions between the excited sensor and the quencher. As shown in Figure S17, the fluorescence lifetime of PyDA-NP in 80% water content (water−CH3CN) remains unchanged in the presence of different concentrations (80 and 240 μM) of TNP (Table S3), which shows that fluorescence quenching of the aggregated state occurs by TNP through a static mechanism. Thus, the only reason for the upward deviation of the SV plot (Figure 16) is the superquenching ability of TNP at a higher concentration. We also studied the UV−vis absorption spectrum of the PyDA-NP (10 μM) in the AIEE state (80% water) in the presence of TNP. Interestingly, on addition of TNP (250 μM), a new absorption band (a shoulder) appears at 430 nm, which may arise due to the strong interaction between the electrondeficient TNP and π-electron-rich aromatic fluorophores, leading to the formation of a charge-transfer (CT) complex56,57 (Figure S18). The spectral changes are also accompanied with a drastic color change from almost colorless to light yellow (i → ii) (Figure 15c), visible to the naked eyes. Thus, the sensing of TNP is associated with the intercalation of the nitroaromatic compounds between two pyrene rings held together by π−π stacking interactions in the aggregated state, leading to a CT process from pyrene to the analyte (TNP). In addition to solution phase detection, we have also performed contact-mode analysis of TNP, which is particularly useful in the field of analytical and forensic sciences. Therefore, we have prepared test paper strips by dip-coating a solution of PyDA-NP (10 μM) in 80% water in CH3CN onto a Whatman 40 filter paper. After that, the paper strip was dipped in 1 mM solution of TNP, which showed an immediate color change
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01294. Fluorescence and characterization data of compounds PyDA-NP and PyDA-NP-NO: 1H NMR, 13C NMR, IR, HRMS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Abu Saleh Musha Islam: 0000-0002-2529-8662 Mahammad Ali: 0000-0003-0756-0468 10314
DOI: 10.1021/acsomega.8b01294 ACS Omega 2018, 3, 10306−10316
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from DST (ref no. 809(Sanc)/ST/P/S&T/ 4G-9/2104) West Bengal and CSIR (ref no. 01(2896)/17/ EMR-II), New Delhi, India, is gratefully acknowledged.
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