Highly Selective Turn-On Fluorogenic Chemosensor for Robust

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Highly Selective Turn-On Fluorogenic Chemosensor for Robust Quantification of Zn(II) Based on Aggregation Induced Emission Enhancement Feature Milan Shyamal, Prativa Mazumdar, Samir Maity, Sadhan Samanta, Gobinda P. Sahoo, and Ajay Misra* Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore-721102, West Bengal, India S Supporting Information *

ABSTRACT: An incisively designed notable aggregation-induced emission enhancement (AIEE) active fluorescence probe, 1-(2hydroxynaphthylmethylene)-2-(3-methoxy-2-hydroxybenzylidene) hydrazine (L), was synthesized via straightforward reaction from inexpensive reagents. It exhibited rapid response, superb selectivity, and swift sensitivity toward Zn2+ based on its promising CHEF/AIEE feature. L not only can sense Zn2+ through sharp colorimetric and selective turn-on fluorescence responses in DMF/H2O (9:1, v/v) medium, but also can distinguish between its significant AIEE activity in high water ratio and Zn2+ triggered AIEE activity through individual emission signals. Intriguingly, the AIEE properties of L may improve its impact. The molecules of L are aggregated into ordered one-dimensional rod-shaped microcrystals that show an obvious optical waveguide effect. Job’s plot from UV−vis absorption revealed the formation of L-Zn2+ complex with 1:1 stoichiometry. When bound with Zn2+ in 1:1 mode, enhanced turn-on emission was observed via chelation enhanced fluorescence through sensor complex (L-Zn) formation and excess addition of Zn2+, a vivid enhancement of fluorescence intensity over manifold through aggregate formation was observed. The entire process takes ∼5 s, i.e., faster response time. The probe can detect Zn2+ as low as 1.1 × 10−7 M. The AIEE mechanism of L and Zn2+ triggered AIEE mechanism were well established from fluorescence anisotropy, DLS, SEM, optical fluorescence microscope, time-resolved photoluminescence, and fluorescence reversibility study by adding Zn2+ and EDTA sequentially. Furthermore, the proposed analytical system with clear AIEE mechanism demonstrates a potential outlook for the on-site practical applications. KEYWORDS: zinc sensor, fluorescence, AIEE, SEM, DLS, real time, microcrystal, on-site detection

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permissible limit, zinc can suppress the uptake of other nutritional trace metals such as copper and iron.8 Yet again the discrepancy of Zn2+ in the human body may cause several neurological disorders such as Alzheimer’s disease and diabetes.9 It is also one of the most important cations compactly attached to DNA-binding proteins and enzymes that are playing catalytic and structural roles.10 Deficiency of Zn2+ normally for children under the age of 5 years could eventually lead to diarrhea, immune dysfunction, and even death in several cases.11 In addition, free zinc exists in some tissues such as pancreas, retina, and intestines, probably functioning as signaling agents for processes such as apoptosis and neurotransmission.12,13 Biological functions and the effective storage of such free Zn2+ are poorly understood. Hence, designing a proficient Zn2+ chemosensor is very decisive. Difficulties in developing spectroscopically based Zn2+ sensors are due to its intrinsic d10 shell with poor sensitivity or selectivity and interference from other spec-

esigned synthesis of practical chemosensors with superb selectivity and swift sensitivity to transition metal ions, especially Zn2+ ion1 and also proficient light emitters in the aggregate and solid states to the development of optical and optoelectronic devices, for instance, organic light emitting devices (OLEDs) and so forth,2 is a exciting field in supramolecular chemistry due to their potential impact in recent years. Despite having many reported zinc sensors, the researchers are constantly searching for a new one having improved selectivity, sensitivity, and reliability to satisfy miscellaneous requirements. A number of fluorescence based sensors have been developed. Most of them show one or more drawbacks in terms of real applicability including synthetic difficulties, fluorescence quenching, and cross-sensitivities with other competitive metal ions. Hence, it is worthwhile to search for a practical fluorescent sensor for selective detection of zinc. Zinc is the second most abundant transition metal as well as essential ingredient to preserve life3 with 2−4 g distributed throughout the human body.4 Various studies divulge that Zn2+ has an important role in biological processes, e.g., cellular metabolism, gene transcription, brain function, and immune function.5,6 The permissible dietary intake of zinc for humans is about 8−11 mg/day.7 At concentrations higher than the © XXXX American Chemical Society

Received: April 27, 2016 Accepted: May 4, 2016

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ACS Sensors troscopically silent d10 metal ions like Cd2+ and Hg2+.10 Therefore, developing easily synthesized chemosensors with rapid and selective recognition techniques for the environmental and biological detection of Zn2+ is imperative for monitoring and controlling safe concentration levels.14 On-site applications entail technique by means of superb selectivity, swift sensitivity, consistency, and exclusively easy operation. Recently, the fluorescent chemosensors have attracted significant interest in the context of sensing of environmentally and biologically pertinent metal ions.15 It can provide several advantages over conventional methods, for instance, easy sample preparation; and rapid, convenient, and sensitive detection. Colorimetric and fluorometric methods are particularly useful for the detection of Zn2+. Up to now, several Zn2+-selective fluorescence turn-on chemosensors have been reported,16,17 wherein the mechanism of sensing is based on metal−ligand charge transfer (MLCT), chelation enhanced fluorescence (CHEF), photoinduced electron transfer (PET), and CN isomerization. In recent times, fluorescent probes displaying aggregationinduced emission enhancement (AIEE) properties have drawn tremendous attention owing to their potential use in real-world applications.18,19 In dilute solution, AIEE active probes exhibit weak emission, and in the aggregated state, they become more emissive. Essentially, AIEE results from the suppression of nonradiating decay owing to the restriction of the intramolecular rotation (RIR)20 process after the ordered selfassembly or disorderly aggregation of organic fluorophores. Based on intense emission triggered by aggregation, AIEEactive molecules have been exploited as an alternative sensing component, where analytes are aggregated with these molecules through noncovalent weak interactions, for instance, hydrogenbonding, metal coordination interactions, and electrostatic interactions. To the best of our knowledge, chemosensors for Zn2+, especially with fluorescence turn-on technique based on the aggregation-induced emission enhancement (AIEE) method, still remain elusive.21,22 Hence, the designed synthesis of the fluorescent chemosensors for Zn2+ based on AIEE assets and also fewer synthetic difficulties has received intense attention from chemists. To develop newer AIEE active sensors for various environmentally and biologically relevant analytes,23−25 we have successfully designed and synthesized a new aggregationinduced emission enhancement (AIEE) active probe, 1-(2hydroxynaphthylmethylene)-2-(3-methoxy-2-hydroxybenzylidene) hydrazine (L) (Scheme 1). By taking advantage of its novel AIEE feature (Scheme 2), we herein report a competent and convenient procedure for detecting Zn2+ ion. It is a novel fluorescent and colorimetric dual-readout assay for Zn2+ with a rapid “turn-on” response via Zn2+ triggering AIEE activity. The

Scheme 2. Schematic Representation of Sensor and Aggregation Induced Enhanced Emission (AIEE) of L

probe is also useful for on-site visual detection of Zn2+ in paper strips.

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EXPERIMENTAL SECTION

Materials and Equipment. All of the materials for synthesis were obtained commercially and used without further purification. All the solvents used were of analytical grade. Freshly prepared deionized water was used throughout the experiment. NMR spectra were recorded on a Bruker ASCEND spectrometer in CDCl3. The ESI-MS was recorded on Qtof Micro YA263 mass spectrometer. The Fourier transform infrared (FT-IR) spectra were obtained in the range of 4000−400 cm−1 using KBr pellets on a PerkinElmer Spectrum-Two FTIR spectrometer. Synthesis of 1-(2-Hydroxynaphthylmethylene)-2-(3-methoxy-2hydroxybenzylidene) hydrazine (L). 10 mmol (1.72 g) of 2-hydroxy1-naphthaldehyde was dissolved in 50.0 mL of methanol by constant stirring. To this solution, an excess of hydrazine hydrate (5.0 mL, ∼100 mmol) was added. Then the solution was stirred for 24 h to obtain pale yellow solid product which was filtered and washed thoroughly with methanol (Scheme 1, step 1). In the next step 1.0 mmol of this product obtained in step1 was dissolved in methanol with 1 mmol of ortho-vanillin, and the mixture was stirred for 10 h to obtain a deep yellow solid product which was washed several times with methanol and then dried in a desiccator. Yield: 82%. Anal. Calcd for C19H16N2O3: C, 71.24%; H, 5.03%; N, 8.74%; Found: C, 71.12%; H, 5.17%; N, 8.63%. IR (cm−1, KBr): ν(CN) 1625 cm−1. 1H NMR (400 MHz, CDCl3, δ (ppm)) δ: 12.97 (1H, s), 11.64 (1H, s), 9.64 (1H, s), 8.79 (1H, s), 8.17−6.96 (9H, ArH), 3.98 (3H, s); 13C NMR (100 MHz, CDCl3, δ (ppm)) δ: 165.15, 163.80, 161.20, 161.06, 149.67, 148.41, 135.13, 133.73, 132.65, 129.24, 128.15, 123.97, 119.99, 119.45, 119.13, 117.58, 115.15, 107.97, 56.29 ppm; HRMS: MS-ES+ (m/z): [M + H]+: Calculated: 321.34, Found: 321.33.

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RESULTS AND DISCUSSION The synthesis of chemosensor L is depicted in Scheme 1 and synthesis details are described in Experimental Section. It was fully characterized by physicochemical and spectroscopic analysis. L is soluble in common polar organic solvents and absolutely insoluble in water. The 1H NMR (Figure S1) and 13 C NMR (Figure S2) spectra were recorded to confirm the purity and the structure of the probe. The ESI−MS (Figure S3) spectrum shows the major peak at m/z 321.33 [M + H]+, which perfectly matches the calculated molecular weight of [L]+. IR (Figure S4) spectrum shows a vibration band at 1625 cm−1 which can be assigned to stretching vibrational mode of imine (−CHN−) groups in the L molecule. The vibration band at 670 cm−1 was attributed to the vibration of the carbon hydrogen bond (−C−H). Photophysical Properties. The photophysical properties of L were investigated with absorption (Figure S5) and emission (Figure S6) studies. The emission spectrum of free L shows a band with maxima located around 498 nm on excitation at 410 nm in DMF. It is quite astounding to ascertain that L shows aggregation induced emission enhancement

Scheme 1. Synthesis of 1-(2-Hydroxynaphthylmethylene)-2(3-methoxy-2-hydroxybenzylidene) hydrazine (L)

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inset photos (Figure 1) taken under UV irradiation obviously show that the solution changes from colorless to brown upon addition of 50−90% of water. This inspection suggests that L is an AIEE-active compound. Such emission changes with large red-shifts of 22 and 72 nm compared to weak emission band at 498 nm in naphthalene-based compounds have so far never been reported. This AIEE behavior is unique in the field of fluorescent probes. The quantum yields (Table S1) of L in dilute solution (Φ = 0.002) and the aggregated state (90% water content) (Φ = 0.302) indicated the emission enhancement phenomenon. The origin of such distinguishing fluorescent behavior of L can be attributed to the change of molecular packing modes and conformations in the aggregated states.26,27 On the other hand fluorophore L is largely isolated with little interaction with other neighboring L in DMF. Therefore, the single bond rotation and large amplitude vibrational modes are responsible for the dominant nonradiative decay channel leading to the reduced emission intensity. So, the restricted of intramolecular rotation (RIR)20 and large amplitude vibrational modes of L play the crucial role in the AIEE characteristics. The single bond rotations are completely locked in the aggregated state due to the multiple physical intermolecular interactions and the boosted molecular planarity strengthen the π-conjugation of L, leading to red shift in the emission maxima. On the other hand, the bluish green and yellow emission bands at 520 and 570 nm have disappeared (Figure S8) when the concentration of L is lower than 20 μM in the DMF/water (9:1, v/v) mixture, which suggests the absence of the emissive species that display brown emission. Hence, we can speculate that the emission in the brown region is accredited to the emission from J-aggregates.28 UV−vis absorption spectrum of L is shown in Figure S9. When increasing water percentage (>40%), the aggregation starts, and the absorption peak maximum at 388 nm decreased with a slight bathochromic shift. Meanwhile, a new shoulder band emerged at 410 nm with leveled off tail absorption in the region (450−600 nm), the absorbance of which increased continuously up to 90% water content which signifies the formation of new species owing to the scattering effect from the foregoing aggregates/particles in the solution.29 This clearly indicates that L molecule is AIEE active and this phenomenon was also verified by time-resolved fluorescence spectra, DLS, and SEM study. As illustrated in Figure 1, the AIEE effect increased rapidly up to 90% water content and also increasing fluorescence lifetime (Figure S10). The data (Table S1) suggest that almost ∼20 times enhancement has been observed in decay values (302 ps) for aggregated L in DMF/H2O (1:9, v/v) compared to isolated L (15 ps) in DMF/H2O (9:1, v/v). The observed high decay value is due to the suppression of the ICT process30 due to hydrogen bonding interaction of imine donor with water caused by aggregation of the L molecules; as a result, the L molecule is more emissive. The DLS (Figure S11) results show that the size of the aggregates increased with increasing water percentage from 50% to 90% as previously reported in many other AIEE systems.23 SEM (Figures 2a and S12) study also reveals that L molecules aggregate to form entangled 1D microrods at room temperature. This morphology must be responsible for the enhanced emission and the impressive color of the hydrosol. A solid state luminescence from L microstructures was observed by optical microscopy (Figure 2b). Upon UV excitation, L shows rod-shaped microparticles with yellow emission. Dazzling light emissions are observed at the

(AIEE) effect when the volume percentage of water is increased, f w ≥ 50% in the binary solvent mixture. Moreover, the nonaggregation in DMF/H2O solutions with low water fraction, f w ≤ 40% allows us to use L as a fluorescent probe to detect bivalent metal ion, Zn2+, that has acceptable solubility in aqueous medium. To distinguish the sensory and AIEE behaviors of L, we performed UV−vis/PL sensor titration of L by adding metal ions in DMF/H2O (9:1, v/v). To better understand the photophysical properties including AIEE nature of L, we performed the quantum chemical computations using the DFT/B3LYP/6-31G (d,p) level of theory. The optimized structure of L (Figure S7) reveals that it can adopt a twisted conformation which is clearly understandable from the side view of the optimized geometry of L. It also exposes that the HOMO electron densities are localized within naphthalene and imine group, whereas the LUMO electron clouds are delocalized within the imine group, phenyl and naphthalene rings. This excited electronic energy is delocalized to the freely rotating phenyl groups, which is responsible for opening up the nonradiative deactivation channels of excited L in its isolated form in solution. The calculated HOMO and LUMO energy gap is found to be 3.72 eV, which is lower than that of the calculated HOMO−LUMO gap of naphthalene group alone (4.83 eV) due to the extended conjugated structure of L. This information is helpful in understanding the nature of transition as well as AIEE character of probe L. Aggregation Induced Emission Enhancement (AIEE) Characteristics of L. L dissolves quickly in DMF but is insoluble in water. The AIEE property of L was investigated in DMF by varying water percentage and the results are given in Figure 1. It was examined that the aggregation of L started in

Figure 1. Fluorescence spectra of L (20 μM) in DMF, upon increasing volume percentages of water from 0% to 90%. Inset: Visual change of color with increasing water percentage (0−90%) of L as seen under UV lamp (λ = 365 nm).

the DMF/H2O mixtures with higher water content (>40%). In dilute DMF solution, L (20 μM) shows structured absorption spectra, and upon excitation at 410 nm, a weak fluorescence emission at 498 nm was observed. As the volume of water is increased from 0% to 40%, the fluorescence intensity of L remains unchanged. Interestingly, the fluorescence intensity increases continuously when f w ≥ 50% and the two emission bands centered at 520 and 570 nm are rapidly turned on. The C

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sharp, well-defined absorption bands at 333 and 388 nm. The band at 333 nm was assigned to π → π* transition, while the other one at 388 nm was accredited to the n to π* electronic transition and a shoulder band at 405 nm because of the presence of a naphthalene moiety in L.22 Interestingly, the spectral response of L toward Zn2+ was associated with a sharp color change from colorless to yellow of the solution which provided the opportunity for recognition of Zn2+ by the naked eye (Figure 4, inset). A titration experiment

Figure 2. (A) Scanning electron microscopy (SEM) image of L microparticles (scale bar = 2 μm). (B) Optical fluorescence microscopy image (solid state) of L with UV light excitation (scale bar = 25 μm). (C) Optical fluorescence microscopy image displaying obvious optical waveguide effect at two ends of rod-type microcrystals (scale bar = 25 μm).

ends of the microcrystals (Figure 2c), while the fluorescence intensity was comparatively weaker along the length of 1Dstructure, signifying optical waveguide effect during the light transmission process of the microcrystals. Hence, L microrods could be appropriate as potential optical waveguides due to the transparency and well-ordered 1D microstructure in the solid state. Increasing the concentrations of L increases the number of microparticles with strong yellow light emission, shown in Figure S13. Thus, the efficient luminescence in the solid state makes it potential material for use in the fabrication of photonic devices. Analytical Performance: Fluorogenic Sensing Knack of L toward Zn(II). L is feebly emissive in its good solvent, DMF. Amusingly, poorly emissive L in DMF became highly emissive in the binary solvent mixture (DMF/H2O) of higher water content, which is discussed above. These results suggest that L is an AIEE active compound. Since the water volume percentage above 40% leads to the aggregation of L (Figure 2), all the spectroscopic measurements in the presence of various metal ions are performed in DMF/H2O (9:1, v/v) medium, where the probe does not exhibit AIEE activity on its own. UV−vis Absorption Study. The UV−vis absorption spectral responses of L toward Zn2+ in DMF/H2O (9:1, v/v) were inspected to recognize the reaction mechanism at room temperature, as depicted in Figure 3. Primarily, L displayed two

Figure 4. Fluorescence spectra of L (20 μM) (λex = 410 nm) with varying concentrations of Zn2+ in DMF/H2O (9:1, v/v); insets: relative fluorescence intensity changes with respect to Zn 2+ concentration. Visual color changes of L and Zn-L under daylight and UV light illumination (λ = 365 nm).

was performed to get a quantitative idea of the interaction between Zn2+ and L. The gradual addition of Zn2+ to L rendered systematic growth of the new absorbance band at 455 nm with a simultaneous decrease of the peaks at 388 and 333 nm (Figure 3, inset a). A distinct isosbestic point was observed at 411 nm upon addition of 1 equiv of Zn2+, indicating the interconversion between the uncomplexed and complexed species. A slight deviation from the isosbestic point was observed upon addition of excess Zn2+. Therefore, in the beginning, the selective colorimetric response of L toward Zn2+ is accredited to the chelation between Zn2+ ions and L. Upon addition of Zn2+ to L, the appearance of the new absorption band at 455 nm is ascribed to coordination of Zn2+ to the deprotonated oxygen atoms and nitrogen atoms of Schiff base (L) moiety. Such an absorption band was also observed on some complex and ascribed to the ligand−metal charge transfer transition.14 To determine the binding stoichiometry of Zn2+ with L, a conventional Job’s plot (continuous variation analysis) experiment was performed from the absorption spectra between the metal mole fraction (XM) and absorption maximum changes of L in the presence of Zn2+ (Figure 3, inset b). The intersection point was obtained at the molar fraction of Zn2+ at 0.528, which indicates that L coordinates with Zn 2+ ion in a 1:1 stoichiometric manner. It was imperative to note that the absorption titration for the interaction of L with Zn2+ divulged that there is new peak at 438 nm generated along with the peak at 455 nm after addition of excess Zn2+ (>1 equiv). This new band emerging at 438 nm finally reached a plateau at 455 nm. Thus, it may be assumed that the presence of excess Zn2+

Figure 3. UV−vis spectra of L (20 μM) in the presence of varying concentration of Zn2+. Inset: (a) Absorption changes at 388 and 455 nm by varying concentrations of Zn2+. (b) Job’s plot for Zn-L complex formation at 455 nm using UV−visible titration experiment; [Zn2+]/ [Zn2+] + [L] = mole fraction where [Zn2+] and [L] are concentrations of Zn2+ and L; L:Zn = 1:1 stoichiometry (ca. 0.528). D

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chemosensor L exhibits a very small average anisotropy value ∼0.03 owing to rapid rotation of the fluorophore on its lifetime scale, whereas with the addition of 20 equiv of Zn2+, the average anisotropy value of the resulting solution was increased to 0.25 shown in Figure 5. The significant increase of the anisotropy

induces the aggregation of L in solution along with complex formation in the primary stages, which consequently activated the AIEE activity of L. Steady State Emission Studies. The emission spectra of L and fluorescence titration with Zn2+ were investigated in DMF/H2O (9:1, v/v) mixed solvents at room temperature, shown in Figure 4. Probe L (20 μM) exhibited very weak emission and emission band is centered at around 498 nm upon excitation at 410 nm with a low quantum yield (Φ = 0.002) at room temperature. This weak fluorescence is assigned to the photoinduced electron transfer (PET) caused by the electron transmission from the nitrogen atom of imino (−CN) electron-donor to the large π-conjugation system of naphthalene fluorophore. Zn2+ was able to induce colorimetric changes. Gradual addition of Zn2+ (0−20 μM) to L (20 μM) manifested in a systematic increase of the emission intensity associated with a red shift of 34 nm from 498 to 532 nm along with a manifold increase in the fluorescence intensity with fluorescence quantum yield (Φ = 0.018) indicating a Zn2+selective turn-on fluorescent signaling behavior. Intriguingly, it was observed that fluorescence titration failed to evaluate the stoichiometry (1:1) of complex as the emission intensity at 532 nm increases (Φ = 0.638) linearly upon 20 equiv addition of Zn2+ ions (Figure 4, inset). Thus, it may be assumed that the presence of excess Zn2+ catalyzes the aggregation process along with complexation in the primary stages, which subsequently initiated the AIEE activity of L. Initially, upon complexation between chelating agent L through −OH and imine (−CHN) groups to Zn2+, chelation inhibited the free rotation and induces rigidity in the molecule to produce a CHEF effect by the stable complexation with suppressing the PET, as a result increases fluorescence intensity with bathochromic shift up to addition of 1 equiv of Zn2+. However, further addition of Zn2+ from 2.0 to 20.0 equiv really triggered the aggregation of the Zn2+-L complex to enable the AIEE behavior of L with an astounding enhancement of fluorescence intensity.31 Therefore, the possible sensing mechanism based on CHEF and AIEE-activation was proposed as given in Scheme 3. Moreover, the fluorogenic sensing ability

Figure 5. Fluorescence anisotropy of L in the absence and presence of Zn2+ in DMF/H2O (9:1, v/v) solution [L] = 20 μM, [Zn2+] = 0.4 mM, λex = 410 nm.

value indicates that free ligand L is aggregated onto large particles, and this results in a reduction of the rotational diffusion rate.32 In aggregates, fluorescence intensity was significantly increased with red shift due to decreasing microenvironmental polarity of the ICT characteristic of naphthalene chromophore. DLS Measurements. Extensive DLS measurements were also performed to understand the aggregation behavior of L in the presence of excess Zn2+, shown in Figure S14. DLS studies revealed that the average size of the aggregated L in DMF/H2O (9:1, v/v) is 29 nm approximately. This also shows that L can be well-dissolved and dispersed in DMF/H2O (9:1, v/v) solution. However, upon the introduction of 1 equiv of Zn2+, sensor complexes with a size of about ∼78 nm were observed and increasing concentration of Zn2+ resulted in a significant enhancement in the average size of aggregates, which also supports the observed Zn2+-triggered AIEE activity of the probe L.22 SEM Analysis. For more in-depth investigation into the role of Zn2+-triggered AIEE activity the SEM analysis was performed which provided further evidence for the aggregate formation of the Zn-L complex in solution, shown in Figures 6A and S15. The aggregates of Zn-L are a honeycomb-like microstructure. It may be mentioned here that the present sensor, L, is much more efficient and versatile compared to others as it can sense not only Zn2+ through selective differential turn-on via sharp colorimetric and fluorescence responses, but also induces the interesting AIEE activity of the probe with their distinct morphology to influence the sensing mechanism. Reversibility. Zn2+-triggering aggregation behavior was well supported by the reversibility of Zn-L aggregate, shown in Figure 6B. Reversibility is one of the significant features to satisfy the demand of a novel chemosensor mainly for the reusability of a probe. To inspect the reversible binding of L with Zn2+, we used EDTA as a strong chelating ligand in identical reaction conditions. Addition of a proper amount of EDTA restored the fluorescence of L when it was added to the

Scheme 3. Proposed Mechanistic Pathway for Sensing Zn2+ Ions Based on Complexation and Zn2+-Triggered AIEE

of L could be observed by the naked eye for selective turn-on responses of L toward Zn2+ in DMF/H2O (9:1, v/v) was also feasible in daylight and under UV irradiation to show a bright green fluorescence in the presence of Zn2+, respectively, shown in Figure 4, inset. This assumption was established by fluorescence anisotropy, DLS measurement, SEM analysis, reversibility and TRPL measurements. Fluorescence anisotropy measurement. Aggregation phenomena were investigated by a prevailing approach, fluorescence anisotropy. It is quite surprising to find that E

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Figure 6. (A) SEM image of the aggregates obtained after addition of Zn2+ (20 equiv) to L. Scale bar: 10 μM. (B) Fluorescence spectra of L (20 μM) in the presence of 20 equiv of Zn2+ before and after treatment with excess EDTA (λex = 410 nm). (C) Time resolved fluorescence spectra of L (20 μM) and different equivalents of Zn2+ in DMF/H2O (9:1, v/v) (λex = 410 nm, λem = 532 nm); Lamp (black), L (Blue), 1 equiv (green), 10 equiv (violet), 20 equiv (red) of Zn2+, respectively.

highly fluorescent Zn2+-treated L due to demetalation of Zn2+ from the corresponding complex, which substantiates the role of Zn2+ in triggering aggregation. Furthermore, the experimental outcome indicates that the present probe L could be easily regenerated for repeated use. TRPL Measurements. The TRPL measurements were carried out further to understand the mechanism of the turn-on sensor responses of L toward Zn2+ ion as shown in Figure 6C. The fluorescence lifetime (τ) of L in DMF/H2O (9:1, v/v) is 15 ps, whereas longer average fluorescence lifetimes of 70, 647, and 874 ps were detected in the presence of 1, 10, and 20 equiv of Zn2+, respectively. All these photophysical parameters are listed in Table S1. The data suggest that there is small change in lifetime in the presence of 1 equiv of Zn2+ whereas almost ∼43 and 58 times enhancement have been observed in the presence of 10 and 20 equiv of Zn2+, respectively. Initially, chelation occured between Zn2+ ions and L, which inhibited the free rotation and induces structural rigidity in the resulting complex and tends to produce a CHEF effect by the stable complexation with suppression of the PET.33 However, further addition of Zn2+ from 1.0 to 20.0 equiv triggered the aggregation of the already formed complex to allow the AIEE behavior of L. Hence, these data suggest that with gradual addition of Zn2+ ions, Zn2+-triggering aggregation happens and the fluorescence lifetimes of such emissive species become longer. Metal Ion Competition Studies. An important feature of metal ion sensors is their ability to selectively detect a metal ion in the presence of other relevant metal ions. Therefore, we endeavored to establish the selective sensing of L toward representative alkali (K+, Na+), alkaline earth (Ca2+, Mg2+, Al3+), transition-metal ions (Zn2+, Ni2+, Cu2+, Mn2+, Co2+, Fe3+), and heavy metal ions (Hg2+, Cd2+, Ag+, Pb2+) in single and multicomponent systems. A detailed analysis of the fluorescence emission spectrum of L (20 μM) in the presence of other metal cations (100 μM) was performed in DMF/H2O (9:1, v/v) upon excitation at 410 nm. It was interesting to note that among all the tested metal ions only Zn2+ rendered significant fluorescence turn-on responses, whereas other relevant competing metal ions had no noteworthy variance in the emission spectra. This clearly demonstrates the preference of L toward Zn2+ over the others as shown in Figure 7A. It is

Figure 7. (A) Sensor L (20 μM) in DMF/H2O (9:1, v/v) (λex = 410 nm) responses toward relevant competitive metal ions (10 equiv). Inset: Visual color change observed with addition of different metal ions to L as seen under daylight (lower) and UV light (upper) (λ = 365 nm). (B) Relative fluorescence intensities of L (20 μM) in DMF/ H2O (9:1, v/v) (λex = 410 nm) with different competing metal ions (10 equiv) in the absence and presence of Zn2+. Royal blue bar: L with different metal ions. Red bar: L with Zn2+ + different relevant competing metals.

imperative to note that our chemosensor L for Zn2+ did not have any interference from the Cd2+ ion. Generally, it is difficult to distinguish Zn2+ from Cd2+ in common solution owing to Cd2+ and Zn2+ having analogous possessions and causing a strong interference.34 Therefore, the chemosensor L has demonstrated a noteworthy propensity to discriminate Zn2+ from Cd2+ in a given solution. In order to establish the specific selectivity of L toward Zn2+, we also carried out the single and dual metal competitive analysis, as shown in Figure 7B. In a single metal system, all the metal (Na+, K+, Ca2+, Mg2+, Al3+, Ag+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+) concentrations were kept as 100 μM toward L in DMF/H2O (9:1, v/v). However, for the dual-metal competitive studies (red bars), equal amounts of aqueous solutions of Zn2+ and other relevant metal ions (100 μM+100 μM) were combined. When 10.0 equiv of Zn2+ was added into the solution of L (20 μM) in the presence of 10.0 equiv of other metal ions, the fluorescence spectra exhibited a similar pattern to that with Zn2+ (λem = 532 nm, λex = 410 nm). The photographs of L with different metal ions (under UV light) verified its sensitivity by strong green fluorescence, as F

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time-dependent fluorescent analysis of L (20 μM) with different Zn2+ concentrations was started at 5 s. As shown in Figure 8 and Figure S20, the relative fluorescence intensity was

depicted in Figure 4. These results indicate that L can be applied as an ion-selective fluorescent probe to selectively recognize and distinguish Zn2+ in the presence of various environmentally and biologically relevant competing metal ions. Counterion Effect. Most of the chemosensor responses are affected by the presence of counterions. Therefore, we have carried out the sensor titration of L toward Zn2+ in the presence of excess of other biologically relevant anions (CH3COO−, OH−, NO3−, NO2−, SO42−, and Cl−). It has been found that there is no or a negligible fluorescence enhancement by these anions as shown in Figure S16. Hence, it is concluded that the Zn2+ sensor responses of L are not really influenced in the presence of different counterions. Detection Limit and Dissociation Constant. For practical purposes, the detection limit of chemosensor L for the survey of Zn2+ was also a crucial parameter. L is proficient in detecting Zn2+ through colorimetric response and the color change is visible to the naked eye. The limit of detection (LOD) was calculated by the 3σ method23 as shown in Figure S17. The emission intensity of L without Zn2+ was measured as a function of increasing concentration of L and five times the standard deviation of blank (without Zn2+) was determined. A good linear relationship between the relative fluorescence intensity and the concentration of Zn2+ was obtained to calculate the detection limit as low as 1.1 × 10−7 M, and this is well below the permissible level of Zn2+ (70 μM) in drinking water according to the United State Environmental Protection Agency (USEPA). As a result, our fluorescent probe L displayed high sensitivity toward Zn2+ ions. Furthermore, to confirm the better selectivity of L toward Zn2+, we determined the dissociation constant of the sensor complex (Zn-L). Assuming a 1:1 complex formation, the dissociation constant (Kd) was calculated on the basis of the titration curve of the sensor L with Zn2+ by nonlinear leastsquares fitting of the emission intensity at 532 nm,23 shown in Figure S18. The Kd value of Zn-L was estimated as ∼1.47 × 10−5 M (R2 = 0.996), which clearly demonstrates the potent binding affinity of L toward Zn2+ ions. pH Dependence. It is well-known that for many practical applications, it is very essential whether the sensors are active or not at the appropriate pH conditions for successful operation. Therefore, we investigated the effect of pH on the fluorescence of L in the absence and presence of Zn2+ ions. The fluorescence measurements were made in the absence and presence of a 5.0 equiv of Zn2+ in 20 μM solution of L at different pH values. As shown in Figure S19, sensor L displayed excellent sensing knack toward Zn2+, and in the absence of Zn2+ the emission intensity was almost stable in pH range 6.0−9.0. This makes it appropriate for application in physiological pH conditions. The fluorescence intensity decreases in the acidic (pH < 6) region due to protonation of phenolic hydroxyl group of L and hence decreases the chelation ability of L.35 Again, the decreasing emission intensity in the basic (pH > 9) region has been explained due to ICT which hindered the complexation.36 Hence, the emission intensity is stable over this wide range of pH (6.0−9.0) and appropriate for applications under physiological pH conditions. Time Course of Fluorescence Response. The timedependent fluorescence intensity of the DMF/H2O (9:1, v/v) solution of L to Zn2+ was evaluated. A short response time is necessary for the fluorescent sensor to monitor Zn2+ ions in practical applications. Since the minimum response time that can be calculated is inadequate in the manipulation time, the

Figure 8. Fluorescence response of Zn2+ with different concentrations (10−100 μM) at different times in DMF/H2O (9:1, v/v), (a = 10 μM, b = 20 μM, c = 30 μM, d = 40 μM, e = 50 μM, f = 80 μM, and g = 100 μM of Zn2+, respectively).

instantly enhanced up to the maximum after the addition of Zn2+ in almost no time and remains unchanged over an extended period of time, owing to the extremely fast CHEF and AIEE processes at 532 nm. All these processes reduce the intramolecular rotation, responsible for nonradiative decay, and intensify the fluorescence based on the AIEE mechanism (Scheme 3). Repeated measurements show that the emission intensity instantly reaches a maximum after the addition of Zn2+, providing a potential “‘zero-wait’” detection method for Zn2+. This performance is much faster than previously reported by another group.37 On-Site Analysis. For convenient use in an on-site visual screening analysis, we sought to demonstrate the paper-based fluorescent sensors for visual detection of Zn2+. A series of 20 μM solutions in DMF/H2O (9:1, v/v) were carefully dropped onto filter paper strips and then dried to get light blue fluorescence spots. Each spot was dropped with 0.1 mM of various metal ions and subsequently was observed under UV irradiation, as shown in Figure 9. Clearly, it was observed that

Figure 9. Photographic image of detection of various metal ions by L on paper strip tested by simple drop and dry from solutions of L and metal ions, consecutively; the sample was irradiated by UV-light (λ = 365 nm).

Zn2+ gave a green emission area within the light blue spot, despite the fact that other competing metal ions did not change the appearance of the spots. The visual detection limit for Zn2+ was found to be 0.1 mM (0.006 ppm). In order to establish the specific selectivity of L toward Zn2+ through sharp colorimetric and selective turn-on fluorescence responses in the solid state, we also evaluated the paper-based fluorescent sensors for naked eye detection of Zn2+, as shown in Figure S21. Amusingly, it was noted that L gave no emission in G

DOI: 10.1021/acssensors.6b00289 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors daylight or UV light alone, but in the presence of Zn2+, a sharp yellow and green colored “Zn” was observed by the naked eye (daylight) and UV light, respectively. These results demonstrate a convenient application of L for visual detection of Zn2+. Hence, probe L exhibited a “turn-on” fluorescence response toward Zn2+ with superb selectivity, swift sensitivity, excellent stability, rapidity, no entail for intricate instrumentation, and operational expediency.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Fax: +91 3222 275329.

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CONCLUSIONS In summary, herein we have been able to develop the synthesis and characterization of a new simple and inexpensive fluorescent probe, which exhibits interesting aggregation induced emission enhancement (AIEE) properties. It exhibits superb selectivity and swift sensitivity toward Zn2+ through a sharp colorimetric and turn-on fluorescence response over other relevant competing metal ions which is ascribed to the chelation mediated AIEE behavior of L. The AIEE mechanism of L and Zn2+ triggered AIEE mechanism were well established from the Mie effect in level-off tail absorption, fluorescence anisotropy, dynamic light scattering, scanning electron microscopy, optical fluorescence microscopy, time-resolved photoluminescence, and fluorescence reversibility. It is also noteworthy that the choice of the probe L allows rapid detection of Zn2+ in terms of real time, as here the time response (∼5 s) is faster compared to that of the reported systems. More importantly, sensor complex (Zn-L) is wellsuited for applications under physiological pH conditions. This method also allows a simple and effective way for on-site visual detection of Zn2+ in paper strips, suggesting a potential application in analytical chemistry. It is of great importance from a fundamental and practical perspect that the astonishingly high efficiency of solid-state emission with yellow color and also optical wave-guiding effect are achieved from the compact π-conjugated system of L. These characteristics make the probe a promising candidate for real-time detection of Zn2+ ions, as well as suitable for dexterous organic emitters. Basic analytical parameters of the proposed chemosensor are compared with other recently published Zn2+ -selective chemosensors in Table S2.38−44 The data shows that the presented chemosensor is superior to others with regard to the response time and also comparable with regard to other parameters. The prime feature of our probe highlights its rapid response and swift sensitivity toward Zn2+ with visual detection in a paper strip based on its promising CHEF/AIEE feature. Therefore, in real-time detection based on the promising AIEE feature, the protocol described in this article opens a new door for the design of fluorescent sensors for easy and reliable detection of metal ions in biological and environmental samples. Further analyses on the design of novel fluorescent probes for sensing metal ions with this tenet are underway in our laboratory.

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(90%), Mie effect, DLS, counterions effect, pH effect, detection limit, dissociation constant, and also additional details of a complete characterization data to establish the sensory and AIEE behavior of L (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from CSIR (ref. no. 01(2443)/10/EMR-II), New Delhi and UGC innovative research program, Vidyasagar University for carrying out this research work. M.S. thanks RGNF, UGC, New Delhi, India for providing SRF. S.M. also thanks CSIR, New Delhi, India for his fellowship. Departmental instrumental facilities from DST FIST and UGC SAP programs are gratefully acknowledged. We gratefully acknowledge the help provide by USIC, Vidyasagar University for doing spectroscopic measurements and optical microscopic measurements.

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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/acssensors.6b00289. 1 H and 13C NMR, ESI-mass, IR, UV−vis, and PL spectra, optimized geometry with HOMO and LUMO energy of L, absorption spectra of aggregated hydrosol, timeresolved fluorescence spectra of L and aggregate hydrosol H

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DOI: 10.1021/acssensors.6b00289 ACS Sens. XXXX, XXX, XXX−XXX