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β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 220 (2019) 117099
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β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level Aritra Das a, Shashikant U. Dighe b, Nilimesh Das a, Sanjay Batra b,⁎, Pratik Sen a,⁎ a b
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, UP, India Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Sitapur Road, Lucknow 226031, India
a r t i c l e
i n f o
Article history: Received 25 December 2018 Received in revised form 29 April 2019 Accepted 6 May 2019 Available online 17 May 2019 Keywords: Fluoride sensing Turn-on fluorescence Chemosensor β-carboline
a b s t r a c t A novel β-carboline-based chemosensor, having an acidic N\\H proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water. © 2019 Published by Elsevier B.V.
1. Introduction Fluoride ions play a pivotal role in numerous biological and environmental processes [1–5]. Intake of fluoride up to a certain level is necessary to maintain the dental health and to prevent osteoporosis [6,7]. However, because of the potential toxicity at high dose, fluorine can lead to diseases like fluorosis, osteosarcoma, catalase immune reactivity, neurotoxicity. [8–15]. There are reports even about skeletal fluorosis and crippling skeletal fluorosis due to high fluoride concentration [16]. Moreover, excess fluoride can severely affect the intelligence quotient and neurodevelopment among children [17,18]. Since drinking water is one of the major sources of fluoride intake, the World Health Organization (WHO) recommends the desired and safe level of fluoride concentration in drinking water should be around 1.5 mg/ L [19–21]. As a result, to detect and maintain safe levels of fluoride in drinking water, several sensitive fluoride detection methodologies are in place. These include ion chromatography, ion-selective electrode method, capillary electrophoresis, atomic absorption spectrometry and nuclear magnetic resonance [22–27], but, they require complicated and expensive instrumentation. Conversely, colorimetric and fluorescence-based methods, which allow quick visual detection of fluoride, are also reported [28–35,39,40]. Based on the high affinity of boron towards fluoride ion, Hudnall et al. developed a number of boron compounds for its detection [36–38]. Liu et al. too developed molecules containing boron for selective fluoride sensing [39]. Cho et al. described naphthalene urea derivative for fluoride detection [40] whereas Hinterholzinger ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Batra), [email protected] (P. Sen).
https://doi.org/10.1016/j.saa.2019.05.004 1386-1425/© 2019 Published by Elsevier B.V.
et al. reported a novel metal-organic framework for turn-on fluorimetric sensing of fluoride [41]. Yamaguchi et al. used borane and silane compounds for fluorimetric sensing of fluoride [42]. Apart from these, different carbazole, β-carboline based systems, and different Schiff bases have been also reported as potential fluoride sensors [43–52]. However, most of the techniques are either affected by poor selectivity over fluoride, interference from other anions and lower binding constant. This status of fluoride detection in water underscores the need for the development of a complementary method which is selective and quantitative for fluoride ions and is operational under aqueous conditions [1–3]. In an ongoing program related to the development of fused-βcarbolines, we serendipitously discovered that 5-(methoxycarbonyl)2-(4-(trifluoromethyl) phenyl)-2,11dihydroimidazo[1′,5′:1,2]pyrido [3,4-b]indol-4-ium chloride (MPIPIC), is capable of selective and quantitative detection of fluoride at ppb level through change in color and turn-on fluorescence. Remarkably, this chemosensor was demonstrated to estimate the fluoride ion in tap water and therefore could be used in the field without any requirement of elaborate instrumentation. Herein, we present the details of the result of this study. 2. Experimental 2.1. Materials The synthesis of 5-(Methoxycarbonyl)-2-(4-(trifluoromethyl) phenyl)-2,11-dihydroimidazo[1′,5′:1,2]pyrido[3,4-b]indol-4-ium chloride (MPIPIC) was accomplished following the method reported by us earlier [53]. Tetrabutylammonium (TBA+) salt of fluoride (F−), acetate (OAc-), chloride (Cl−), bromide (Br−), iodide (I−), nitrate (NO− 3 ),
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− perchlorate (ClO− 4 ), dihydrogen borate (H2BO3 ) was purchased from Sigma-Aldrich and used as received. HPLC grade acetonitrile purchased from Finar Chemical Ltd., India and used after distillation. The solution of different anions of TBA+ salt was prepared in acetonitrile.
2.2. Procedure for the synthesis of 5-(methoxycarbonyl)-2-(4(trifluoromethyl) phenyl)-2,11dihydroimidazo[1’,5’:1,2] pyrido[3,4-b] indol-4-ium chloride (MPIPIC) To a round bottom flask containing dry ethanol (10 mL) was added acetyl chloride (71 μL, 1.5 eq) at 0 °C under stirring. After 5 min F formaldehyde (37 μL, 1.5 eq) and 4-(trifluoromethyl)aniline (0.128 g, 1.2 eq) were added dropwise at same temperature. The reaction was allowed to proceed for 20 min after which methyl 1-(dimethoxymethyl)-9Hpyrido[3,4-b]indole-3-carboxylate (0.2 g, 1.0 eq) was added under stirring. The reaction mixture was allowed to warm to room temperature and continued for 12 h (Scheme 1). The separated solid in the reaction mixture was filtered and washed with ice cold ethanol (2 mL) to obtain the analytically pure product (98%, 0.344 g) as a white solid. Mp 210–212 °C; Rf = 0.39 (CH2Cl2: MeOH, 9:1, v/v); IR (KBr) νmax: 1223, 1598, 1754, 3410 cm−1. 1H NMR (400 MHz, DMSO d6): δ (ppm) 4.08 (s, 3H), 7.38–7.43 (m, 1H), 7.53–7.57 (m, 1H), 7.77 (d, J = 8.2 Hz, 1H), 8.18 (d, J = 8.6 Hz, 2H), 8.24 (d, J = 8.6 Hz, 2H), 8.37 (d, J = 7.9 Hz, 1H), 9.11 (s, 1H), 9.17 (d, J = 1.8 Hz, 1H), 10.81 (d, J = 1.8 Hz, 1H), 13.91 (s, 1H); 13C NMR (100 MHz, C DMSO d6): δ (ppm) = 53.6, 112.3, 113.4, 116.0, 121.2, 122.7, 123.2, 123.9, 125.6, 126.9, 127.9, 128.0, 129.1, 129.2, 139.4, 162.1 (See supplementary data for 1H NMR and 13C NMR spectra). MS (ESI+): m/z = 410.1. ESI-HR-MS calculated for C22H15F3N3O2 [M]+: 410.1111, found: 410.1115. 2.3. Methods UV–vis absorption spectra were recorded on a commercial spectrophotometer (UV-2450, Schimadzu, Japan) and fluorescence spectra were obtained using a commercial fluorimeter (Fluoromax-4, Jobin Yvon, USA). Fluorescence lifetime measurements were done by time correlated single photon counting (TCSPC) method in a commercial instrument (Lifespec II by Edinbrugh Instruments, UK). All the experiments were done at 25 °C. We have done various experiments for investigation of anion binding properties of MPIPIC with different anions (such as F−, + − − OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ) in the form of TBA salts. 3. Results and discussion In this work, we have studied the photophysical properties of MPIPIC in acetonitrile in the absence and presence of the different an− − ions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). The steady-state absorption spectrum of MPIPIC in acetonitrile displays an absorption maximum at 375 nm with molar extinction coefficient 8500 M−1 cm−1 (Fig. 1). The addition of F− (70 μM) to a solution of MPIPIC (18.9 μM) led to the formation of a new absorption band centered at 428 nm, which is 53 nm red-shifted compared to pure MPIPIC. Whereas addition of − − other anions (Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ) to the MPIPIC solution do not impose any change to the absorption spectrum of MPIPIC (see Fig. 2a). Such a major spectral shift of 53 nm found in the
Fig. 1. Absorption and emission spectra of MPIPIC in acetonitrile. For absorption measurement, 18.9 μM MPIPIC in 10 mm cuvette was used and for emission measurement, the sample was excited at 375 nm. The fluorescence quantum yield has been determined to be 0.15.
absorption spectrum of MPIPIC in the presence of F− with a distinct color change under ambient light, makes the present sensor suitable for naked-eye detection of F− (Fig. 2b). Upon exciting at 375 nm, MPIPIC showed a relatively high intense fluorescence band (Φf = 0.150) centered at 430 nm (Fig. 1). A new emission band centered at 554 nm (Φf = 0.135) was witnessed on the addition of F− in the solution of MPIPIC, which is 124 nm redshifted from the sensor. A change in color from blue to green under UV light was also observed through the naked eye (Fig. 3b). It is interesting to note that the addition other anions (Cl−, Br−, I−, NO− 3 , − H2BO− 3 and ClO4 ) to the MPIPIC solution do not foist any specific color change (Fig. 3a). It is to note that acetate (OAc−) also infer similar spectral and visual change to the MPIPIC solution, as in the case of F−. We further investigated the selectivity of MPIPIC towards F− over the other anions through a competitive experiment. Herein, we added F− in the mixture of MPIPIC and the other anions (10 equivalents that of F−) and recorded the absorption and emission spectra for each one of them. In this context we would like to mention that to check the selectivity over other ions it is necessary to add F− into the solution which is already treated with the competitor anions [54]. The absorption and emission spectra obtained were almost same as in the presence of F− alone as shown in Fig. 4a and b, respectively. This data clearly demonstrates the selectivity of MPIPIC towards F− over other common anions except OAc−. The large observed red shift in absorption (53 nm) and emission (124 nm) maximum of MPIPIC in the presence of F− and OAc− indicates a major electronic redistribution in the MPIPIC upon interaction with F− and OAc−, whereas the interaction between MPIPIC and other anions is too small to exhibit any significant change in the absorption and emission spectra. Both UV–Vis and fluorescence studies present MPIPIC as a potential colorimetric as well as turn-on fluorescence sensor for F− and also reveal OAc− as a potent competitor in the process. Fig. 5 shows the fluorescence lifetime of MPIPIC in presence of F− and OAc−. The fluorescence transients recorded at 430 nm clearly
Scheme 1. Synthesis of 5-(Methoxycarbonyl)-2-(4-(trifluoromethyl)phenyl)-2,11-dihydroimidazo[1′,5′:1,2]pyrido[3,4-b]indol-4-ium chloride (MPIPIC)
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− − Fig. 2. (a) Absorption spectra of MPIPIC (18.9 μM) in acetonitrile in presence of 70 μM of different anions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). (b) Visual color change upon addition of F−/OAc in acetonitrile solution of MPIPIC under ambient light.
reveals the difference in fluorescence lifetime in presence of F− (3 ns) and OAc− (2.3 ns). Thus while the steady state absorption and emission study fail to differentiate between the F− and OAc−, time-resolved fluorescence measurement surely can. The difference in fluorescence lifetime of MPIPIC in presence of the two different anions implies different effect on the photophysics of MPIPIC, which will be investigated in future studies.
Interference of OAc− in detection of F− and vice versa has been well-documented in the past, the primary reason being competition between the two basic anions for acidic proton [55,56]. This led us to speculate that the mechanism of complexation involves the abstraction of the acidic proton. To confirm the actual mechanism behind the foretold sensing phenomena, we performed 1 H NMR titration experiment.
− − Fig. 3. (a) Fluorescence spectra (λexe = 357 nm) of MPIPIC (18.9 μM) in acetonitrile in the presence of 70 μM of different anions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). (b) Visual color change upon addition of F−/OAc− in acetonitrile solution of MPIPIC under 365 nm UV-radiation.
Fig. 4. (a) Absorption spectra and (b) emission spectra (excited at 357 nm) of MPIPIC in the presence of F− (70 μM) along with excess of other anions including OAc−, Cl−, Br−, I−, NO− 3 , − H2BO− 3 and ClO4 (700 μM).
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Fig. 5. Fluorescence transient of MPIPIC in presence of F− and OAc− at 430 nm. 1 H NMR titration experiment was carried out in DMSO solvent in 400 MHz NMR instrument to understand the nature of interactions between MPIPIC and F− ion towards the sensing ability by gradual addition of F− in MPIPIC. From Fig. 6 it is quite clear that the 1 H NMR peak corresponding to the proton attached to the indole nitrogen of the β-carboline moiety in MPIPIC molecule disappear upon addition of F−. This result apparently indicates that the proton attached to the indole nitrogen in MPIPIC is quite acidic in nature due to the electron withdrawing effect of the –CO2 CH3 group and the positively charged nitrogen at the γ position. Because of this internal charge transfer, F− readily interacts to it and deprotonate the –NH group of MPIPIC molecule, which is responsible for the yellow color and emission property. As OAc− also capable of abstracting acidic proton it binds with MPIPIC to give similar spectral and visual effects as does F−. In this contribution, we set our goal to use the newly synthesized β-carboline based chemo-sensor, MPIPIC, as a quantitative F− sensor in drinking water. In this context, we think that the inability of MPIPIC to distinguish between F− and OAc− will not impose a big problem. To the best of our knowledge drinking water does not contain OAc− [55–59].
Fig. 7. Job's plot for determination of binding stoichiometry between F− and MPIPIC in acetonitrile.
The binding stoichiometry and binding constant were measured by titrating MPIPIC with F− in acetonitrile medium. The binding stoichiometry between MPIPIC and F− was estimated using the Job's plot method recording the change in absorbance at 450 nm with varying mole-fraction of MPIPIC and F−, keeping the total mole-fraction constant [51,52,60–62]. From the Job's plot, we deduced that F− is associated with MPIPIC in a 1:1 stoichiometric ratio (Fig. 7). On gradual addition of F− (0.00 μM - 53.32 μM), the MPIPIC absorption peak at 375 nm reduces monotonically along with the generation of the new band at 428 nm involving two isosbestic points at 357 nm and 392 nm. In the corresponding emission study, we observed an isoemissive point at 500 nm with the generation of the new emission band centered at 554 nm, while the intensity of the MPIPIC emission at 430 nm decreased monotonically. The titration data are shown in Fig. 8. As the overall binding stoichiometry between MPIPIC and F− has been found to be 1:1, the corresponding equilibrium can be written as S + F ⇌ SF with equilibrium/binding constant K, where S is the sensor molecule (MPIPIC), F is the F− and SF is the complex between MPIPIC and F−. For the ith step of the titration, the concentration of the complex
Fig. 6. 1H NMR data of MPIPIC and MPIPIC after addition of F− along with assignments of the peaks. The spectra is recorded using d6-DMSO solvent in a 400 MHz spectrometer.
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Fig. 8. Change in (a) absorption and (b) emission spectra (excited at 357 nm) of MPIPIC in acetonitrile with gradual addition of F− (0.0 μM – 53.3 μM). Solid black lines are the globally fitted lines using Eqs. 2 and 3.
can be written as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 80 )ffi u 2 1 <@ 1 u 1 ð1Þ ½SF i ¼ ½ST i þ ½ F T i þ Þ−t ½ST i þ ½ F T i þ Þ −4½ST i ½ F T i 2: K K where [ST]i and [FT]i are the total concentration of the sensor and F− in the ith step, respectively. The total absorbance of the mixture for the ith step of the titration can be written as the linear combination of the absorbance of the two other steps as A1 þ ½ST i αS8 −½SFi αS8 −αSF A8 Ai ¼ ½ST i αS1 −½SFi αS1 −αSF 1 8
ð2Þ
In the above equation, Ai is the absorbance at any wavelength for the ith step of the titration, A1 and A8 are those for the first and the last step of the titration, [SF]i is the concentration of the complex in the ith step, [ST]i and [FT]i are the total concentration of the sensor (MPIPIC) and F− in the S SF ith step and αS1, αSF 1 , α8, α8 are some global parameters. We can also show that any ith fluorescence spectrum during the titration can also be expressed in terms of the initial and the final spectra of the titration as Fi ¼ ½ST i αS1 −½SFi αS1 −αSF F1 þ ½ST i αS8 −½SFi αS8 −αSF F8 1 8
ð3Þ
where Fi is the emission spectrum of the ith step of the titration and F1 and F8 are the initial and final emission spectra of the titration, respectively. The other parameters are same as discussed earlier.
In order to determine the binding constant of the complex, we have globally fitted the absorption and emission spectra in Fig. 8a and b, respectively. Our analysis shows that the binding constant is 1.05 ± 0.1 × 105 mol−1 L. We have determined the limit of detection of F− by MPIPIC by a similar approach used by Shortreed et al. and others [63,64]. We have normalized the fluorescence intensity data of MPIPIC solution between minimum fluorescence intensity (without F− ion) and the maximum fluorescence intensity (at 53.3 μM) at each concentration. This normalized form of the change in fluorescence intensity at 554 nm, i.e. [(I − I0)/ (Imax − I0)]554 nm, as a function of F− concentration is plotted in Fig. 9, which is fitted linearly. The point at which this line cross the x-axis was taken as the limit of detection and we got the limit of detection as low as 20 ppb. This value is much less than the safe limit of F− and makes the MPIPIC an appropriate sensor for F− detection and estimation. Though MPIPIC is water-insoluble, we have been able to estimate the F− ion concentration in drinking water by this molecule. For this estimation, we have made columns of glass wool in glass droppers and have added 500 μL aqueous solution of KF of different concentrations (ca. 10 μM, 20 μM, 30 μM, 40 μM and 60 μM) in five different columns. All of these columns have been kept in an oven for 14 h so that they become properly dried. 18 mL of the MPIPIC solution in acetonitrile (18.9 μM) have been used to eluate F− from the columns as MPIPIC-F− complex. As expected, we have noticed that the color of the eluate becomes yellow indicating the formation of MPIPIC-F− complex. For each eluate from the five different columns with various F− loadings, the emission spectra have been recorded and a calibration curve has been formed by plotting the ratio of the fluorescence intensity at 554 and 430 nm against the concentration of F−added to the columns (see Fig. 10). We have performed exactly the same experiments with 500 μL of tap water in three different columns and from the recorded emission spectra of the elute we have estimated the F− concentration to be 24 ± 2 μM or 0.46 ± 0.04 mg/L. The same tap water sample has been analyzed with the Orion™ F− ion selective electrode and the concentration of F− is found to be 0.5 mg/L. 4. Conclusion
Fig. 9. Normalized intensity vs concentration of F− plot and the linear fit of some intermediate points for the determination of limit of detection.
In conclusion, in this work we have discovered a β-carbolinebased novel chemosensor, MPIPIC, containing an acidic proton attached to indole nitrogen. Fluoride (F−) uptakes this acidic proton from the sensor and changes the color of the sensor solution from colorless to yellow and a new green fluorescence band originates around 554 nm. The present sensor is selective towards F−, as incubation of other anions hardly impose any visual color change or fluorescence property. The binding constant between MPIPIC and F− is found to be 1.05 ± 0.1 × 105 mol−1 L, which is fairly high. The
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Fig. 10. Determination of concentration of F in tap water from the calibration curve.
detection limit of MPIPIC towards F− is found to be as low as 20 ppb. More importantly, we have used MPIPIC to estimate the F− concentration in tap water successfully.
[19] [20] [21] [22] [23] [24]
Acknowledgments AD acknowledges Ministry of Electronics and Information Technology, Government of India, while SUD and ND gratefully acknowledge the CSIR, New Delhi for financial support in the form of fellowship. PS thanks Visvesvaraya PhD Programme of Ministry of Electronics & Information Technology (MeitY), Government of India for providing young faculty research fellowship. Authors acknowledge SAIF Division of CDRI for providing the spectroscopic data. SB acknowledge the financial grant from DST, New Delhi (SR/ S1/OC-01/2010) for the project under which this work was carried out. This work is financially supported by Indian National Science Academy, India. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.05.004.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level Aritra Das a, Shashikant U. Dighe b, Nilimesh Das a, Sanjay Batra b,⁎, Pratik Sen a,⁎ a b
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, UP, India Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, Sector 10, Jankipuram Extension, Sitapur Road, Lucknow 226031, India
a r t i c l e
i n f o
Article history: Received 25 December 2018 Received in revised form 29 April 2019 Accepted 6 May 2019 Available online 17 May 2019 Keywords: Fluoride sensing Turn-on fluorescence Chemosensor β-carboline
a b s t r a c t A novel β-carboline-based chemosensor, having an acidic N\\H proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water. © 2019 Published by Elsevier B.V.
1. Introduction Fluoride ions play a pivotal role in numerous biological and environmental processes [1–5]. Intake of fluoride up to a certain level is necessary to maintain the dental health and to prevent osteoporosis [6,7]. However, because of the potential toxicity at high dose, fluorine can lead to diseases like fluorosis, osteosarcoma, catalase immune reactivity, neurotoxicity. [8–15]. There are reports even about skeletal fluorosis and crippling skeletal fluorosis due to high fluoride concentration [16]. Moreover, excess fluoride can severely affect the intelligence quotient and neurodevelopment among children [17,18]. Since drinking water is one of the major sources of fluoride intake, the World Health Organization (WHO) recommends the desired and safe level of fluoride concentration in drinking water should be around 1.5 mg/ L [19–21]. As a result, to detect and maintain safe levels of fluoride in drinking water, several sensitive fluoride detection methodologies are in place. These include ion chromatography, ion-selective electrode method, capillary electrophoresis, atomic absorption spectrometry and nuclear magnetic resonance [22–27], but, they require complicated and expensive instrumentation. Conversely, colorimetric and fluorescence-based methods, which allow quick visual detection of fluoride, are also reported [28–35,39,40]. Based on the high affinity of boron towards fluoride ion, Hudnall et al. developed a number of boron compounds for its detection [36–38]. Liu et al. too developed molecules containing boron for selective fluoride sensing [39]. Cho et al. described naphthalene urea derivative for fluoride detection [40] whereas Hinterholzinger ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Batra), [email protected] (P. Sen).
https://doi.org/10.1016/j.saa.2019.05.004 1386-1425/© 2019 Published by Elsevier B.V.
et al. reported a novel metal-organic framework for turn-on fluorimetric sensing of fluoride [41]. Yamaguchi et al. used borane and silane compounds for fluorimetric sensing of fluoride [42]. Apart from these, different carbazole, β-carboline based systems, and different Schiff bases have been also reported as potential fluoride sensors [43–52]. However, most of the techniques are either affected by poor selectivity over fluoride, interference from other anions and lower binding constant. This status of fluoride detection in water underscores the need for the development of a complementary method which is selective and quantitative for fluoride ions and is operational under aqueous conditions [1–3]. In an ongoing program related to the development of fused-βcarbolines, we serendipitously discovered that 5-(methoxycarbonyl)2-(4-(trifluoromethyl) phenyl)-2,11dihydroimidazo[1′,5′:1,2]pyrido [3,4-b]indol-4-ium chloride (MPIPIC), is capable of selective and quantitative detection of fluoride at ppb level through change in color and turn-on fluorescence. Remarkably, this chemosensor was demonstrated to estimate the fluoride ion in tap water and therefore could be used in the field without any requirement of elaborate instrumentation. Herein, we present the details of the result of this study. 2. Experimental 2.1. Materials The synthesis of 5-(Methoxycarbonyl)-2-(4-(trifluoromethyl) phenyl)-2,11-dihydroimidazo[1′,5′:1,2]pyrido[3,4-b]indol-4-ium chloride (MPIPIC) was accomplished following the method reported by us earlier [53]. Tetrabutylammonium (TBA+) salt of fluoride (F−), acetate (OAc-), chloride (Cl−), bromide (Br−), iodide (I−), nitrate (NO− 3 ),
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− perchlorate (ClO− 4 ), dihydrogen borate (H2BO3 ) was purchased from Sigma-Aldrich and used as received. HPLC grade acetonitrile purchased from Finar Chemical Ltd., India and used after distillation. The solution of different anions of TBA+ salt was prepared in acetonitrile.
2.2. Procedure for the synthesis of 5-(methoxycarbonyl)-2-(4(trifluoromethyl) phenyl)-2,11dihydroimidazo[1’,5’:1,2] pyrido[3,4-b] indol-4-ium chloride (MPIPIC) To a round bottom flask containing dry ethanol (10 mL) was added acetyl chloride (71 μL, 1.5 eq) at 0 °C under stirring. After 5 min F formaldehyde (37 μL, 1.5 eq) and 4-(trifluoromethyl)aniline (0.128 g, 1.2 eq) were added dropwise at same temperature. The reaction was allowed to proceed for 20 min after which methyl 1-(dimethoxymethyl)-9Hpyrido[3,4-b]indole-3-carboxylate (0.2 g, 1.0 eq) was added under stirring. The reaction mixture was allowed to warm to room temperature and continued for 12 h (Scheme 1). The separated solid in the reaction mixture was filtered and washed with ice cold ethanol (2 mL) to obtain the analytically pure product (98%, 0.344 g) as a white solid. Mp 210–212 °C; Rf = 0.39 (CH2Cl2: MeOH, 9:1, v/v); IR (KBr) νmax: 1223, 1598, 1754, 3410 cm−1. 1H NMR (400 MHz, DMSO d6): δ (ppm) 4.08 (s, 3H), 7.38–7.43 (m, 1H), 7.53–7.57 (m, 1H), 7.77 (d, J = 8.2 Hz, 1H), 8.18 (d, J = 8.6 Hz, 2H), 8.24 (d, J = 8.6 Hz, 2H), 8.37 (d, J = 7.9 Hz, 1H), 9.11 (s, 1H), 9.17 (d, J = 1.8 Hz, 1H), 10.81 (d, J = 1.8 Hz, 1H), 13.91 (s, 1H); 13C NMR (100 MHz, C DMSO d6): δ (ppm) = 53.6, 112.3, 113.4, 116.0, 121.2, 122.7, 123.2, 123.9, 125.6, 126.9, 127.9, 128.0, 129.1, 129.2, 139.4, 162.1 (See supplementary data for 1H NMR and 13C NMR spectra). MS (ESI+): m/z = 410.1. ESI-HR-MS calculated for C22H15F3N3O2 [M]+: 410.1111, found: 410.1115. 2.3. Methods UV–vis absorption spectra were recorded on a commercial spectrophotometer (UV-2450, Schimadzu, Japan) and fluorescence spectra were obtained using a commercial fluorimeter (Fluoromax-4, Jobin Yvon, USA). Fluorescence lifetime measurements were done by time correlated single photon counting (TCSPC) method in a commercial instrument (Lifespec II by Edinbrugh Instruments, UK). All the experiments were done at 25 °C. We have done various experiments for investigation of anion binding properties of MPIPIC with different anions (such as F−, + − − OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ) in the form of TBA salts. 3. Results and discussion In this work, we have studied the photophysical properties of MPIPIC in acetonitrile in the absence and presence of the different an− − ions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). The steady-state absorption spectrum of MPIPIC in acetonitrile displays an absorption maximum at 375 nm with molar extinction coefficient 8500 M−1 cm−1 (Fig. 1). The addition of F− (70 μM) to a solution of MPIPIC (18.9 μM) led to the formation of a new absorption band centered at 428 nm, which is 53 nm red-shifted compared to pure MPIPIC. Whereas addition of − − other anions (Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ) to the MPIPIC solution do not impose any change to the absorption spectrum of MPIPIC (see Fig. 2a). Such a major spectral shift of 53 nm found in the
Fig. 1. Absorption and emission spectra of MPIPIC in acetonitrile. For absorption measurement, 18.9 μM MPIPIC in 10 mm cuvette was used and for emission measurement, the sample was excited at 375 nm. The fluorescence quantum yield has been determined to be 0.15.
absorption spectrum of MPIPIC in the presence of F− with a distinct color change under ambient light, makes the present sensor suitable for naked-eye detection of F− (Fig. 2b). Upon exciting at 375 nm, MPIPIC showed a relatively high intense fluorescence band (Φf = 0.150) centered at 430 nm (Fig. 1). A new emission band centered at 554 nm (Φf = 0.135) was witnessed on the addition of F− in the solution of MPIPIC, which is 124 nm redshifted from the sensor. A change in color from blue to green under UV light was also observed through the naked eye (Fig. 3b). It is interesting to note that the addition other anions (Cl−, Br−, I−, NO− 3 , − H2BO− 3 and ClO4 ) to the MPIPIC solution do not foist any specific color change (Fig. 3a). It is to note that acetate (OAc−) also infer similar spectral and visual change to the MPIPIC solution, as in the case of F−. We further investigated the selectivity of MPIPIC towards F− over the other anions through a competitive experiment. Herein, we added F− in the mixture of MPIPIC and the other anions (10 equivalents that of F−) and recorded the absorption and emission spectra for each one of them. In this context we would like to mention that to check the selectivity over other ions it is necessary to add F− into the solution which is already treated with the competitor anions [54]. The absorption and emission spectra obtained were almost same as in the presence of F− alone as shown in Fig. 4a and b, respectively. This data clearly demonstrates the selectivity of MPIPIC towards F− over other common anions except OAc−. The large observed red shift in absorption (53 nm) and emission (124 nm) maximum of MPIPIC in the presence of F− and OAc− indicates a major electronic redistribution in the MPIPIC upon interaction with F− and OAc−, whereas the interaction between MPIPIC and other anions is too small to exhibit any significant change in the absorption and emission spectra. Both UV–Vis and fluorescence studies present MPIPIC as a potential colorimetric as well as turn-on fluorescence sensor for F− and also reveal OAc− as a potent competitor in the process. Fig. 5 shows the fluorescence lifetime of MPIPIC in presence of F− and OAc−. The fluorescence transients recorded at 430 nm clearly
Scheme 1. Synthesis of 5-(Methoxycarbonyl)-2-(4-(trifluoromethyl)phenyl)-2,11-dihydroimidazo[1′,5′:1,2]pyrido[3,4-b]indol-4-ium chloride (MPIPIC)
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− − Fig. 2. (a) Absorption spectra of MPIPIC (18.9 μM) in acetonitrile in presence of 70 μM of different anions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). (b) Visual color change upon addition of F−/OAc in acetonitrile solution of MPIPIC under ambient light.
reveals the difference in fluorescence lifetime in presence of F− (3 ns) and OAc− (2.3 ns). Thus while the steady state absorption and emission study fail to differentiate between the F− and OAc−, time-resolved fluorescence measurement surely can. The difference in fluorescence lifetime of MPIPIC in presence of the two different anions implies different effect on the photophysics of MPIPIC, which will be investigated in future studies.
Interference of OAc− in detection of F− and vice versa has been well-documented in the past, the primary reason being competition between the two basic anions for acidic proton [55,56]. This led us to speculate that the mechanism of complexation involves the abstraction of the acidic proton. To confirm the actual mechanism behind the foretold sensing phenomena, we performed 1 H NMR titration experiment.
− − Fig. 3. (a) Fluorescence spectra (λexe = 357 nm) of MPIPIC (18.9 μM) in acetonitrile in the presence of 70 μM of different anions (F−, OAc−, Cl−, Br−, I−, NO− 3 , H2BO3 and ClO4 ). (b) Visual color change upon addition of F−/OAc− in acetonitrile solution of MPIPIC under 365 nm UV-radiation.
Fig. 4. (a) Absorption spectra and (b) emission spectra (excited at 357 nm) of MPIPIC in the presence of F− (70 μM) along with excess of other anions including OAc−, Cl−, Br−, I−, NO− 3 , − H2BO− 3 and ClO4 (700 μM).
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Fig. 5. Fluorescence transient of MPIPIC in presence of F− and OAc− at 430 nm. 1 H NMR titration experiment was carried out in DMSO solvent in 400 MHz NMR instrument to understand the nature of interactions between MPIPIC and F− ion towards the sensing ability by gradual addition of F− in MPIPIC. From Fig. 6 it is quite clear that the 1 H NMR peak corresponding to the proton attached to the indole nitrogen of the β-carboline moiety in MPIPIC molecule disappear upon addition of F−. This result apparently indicates that the proton attached to the indole nitrogen in MPIPIC is quite acidic in nature due to the electron withdrawing effect of the –CO2 CH3 group and the positively charged nitrogen at the γ position. Because of this internal charge transfer, F− readily interacts to it and deprotonate the –NH group of MPIPIC molecule, which is responsible for the yellow color and emission property. As OAc− also capable of abstracting acidic proton it binds with MPIPIC to give similar spectral and visual effects as does F−. In this contribution, we set our goal to use the newly synthesized β-carboline based chemo-sensor, MPIPIC, as a quantitative F− sensor in drinking water. In this context, we think that the inability of MPIPIC to distinguish between F− and OAc− will not impose a big problem. To the best of our knowledge drinking water does not contain OAc− [55–59].
Fig. 7. Job's plot for determination of binding stoichiometry between F− and MPIPIC in acetonitrile.
The binding stoichiometry and binding constant were measured by titrating MPIPIC with F− in acetonitrile medium. The binding stoichiometry between MPIPIC and F− was estimated using the Job's plot method recording the change in absorbance at 450 nm with varying mole-fraction of MPIPIC and F−, keeping the total mole-fraction constant [51,52,60–62]. From the Job's plot, we deduced that F− is associated with MPIPIC in a 1:1 stoichiometric ratio (Fig. 7). On gradual addition of F− (0.00 μM - 53.32 μM), the MPIPIC absorption peak at 375 nm reduces monotonically along with the generation of the new band at 428 nm involving two isosbestic points at 357 nm and 392 nm. In the corresponding emission study, we observed an isoemissive point at 500 nm with the generation of the new emission band centered at 554 nm, while the intensity of the MPIPIC emission at 430 nm decreased monotonically. The titration data are shown in Fig. 8. As the overall binding stoichiometry between MPIPIC and F− has been found to be 1:1, the corresponding equilibrium can be written as S + F ⇌ SF with equilibrium/binding constant K, where S is the sensor molecule (MPIPIC), F is the F− and SF is the complex between MPIPIC and F−. For the ith step of the titration, the concentration of the complex
Fig. 6. 1H NMR data of MPIPIC and MPIPIC after addition of F− along with assignments of the peaks. The spectra is recorded using d6-DMSO solvent in a 400 MHz spectrometer.
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Fig. 8. Change in (a) absorption and (b) emission spectra (excited at 357 nm) of MPIPIC in acetonitrile with gradual addition of F− (0.0 μM – 53.3 μM). Solid black lines are the globally fitted lines using Eqs. 2 and 3.
can be written as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 80 )ffi u 2 1 <@ 1 u 1 ð1Þ ½SF i ¼ ½ST i þ ½ F T i þ Þ−t ½ST i þ ½ F T i þ Þ −4½ST i ½ F T i 2: K K where [ST]i and [FT]i are the total concentration of the sensor and F− in the ith step, respectively. The total absorbance of the mixture for the ith step of the titration can be written as the linear combination of the absorbance of the two other steps as A1 þ ½ST i αS8 −½SFi αS8 −αSF A8 Ai ¼ ½ST i αS1 −½SFi αS1 −αSF 1 8
ð2Þ
In the above equation, Ai is the absorbance at any wavelength for the ith step of the titration, A1 and A8 are those for the first and the last step of the titration, [SF]i is the concentration of the complex in the ith step, [ST]i and [FT]i are the total concentration of the sensor (MPIPIC) and F− in the S SF ith step and αS1, αSF 1 , α8, α8 are some global parameters. We can also show that any ith fluorescence spectrum during the titration can also be expressed in terms of the initial and the final spectra of the titration as Fi ¼ ½ST i αS1 −½SFi αS1 −αSF F1 þ ½ST i αS8 −½SFi αS8 −αSF F8 1 8
ð3Þ
where Fi is the emission spectrum of the ith step of the titration and F1 and F8 are the initial and final emission spectra of the titration, respectively. The other parameters are same as discussed earlier.
In order to determine the binding constant of the complex, we have globally fitted the absorption and emission spectra in Fig. 8a and b, respectively. Our analysis shows that the binding constant is 1.05 ± 0.1 × 105 mol−1 L. We have determined the limit of detection of F− by MPIPIC by a similar approach used by Shortreed et al. and others [63,64]. We have normalized the fluorescence intensity data of MPIPIC solution between minimum fluorescence intensity (without F− ion) and the maximum fluorescence intensity (at 53.3 μM) at each concentration. This normalized form of the change in fluorescence intensity at 554 nm, i.e. [(I − I0)/ (Imax − I0)]554 nm, as a function of F− concentration is plotted in Fig. 9, which is fitted linearly. The point at which this line cross the x-axis was taken as the limit of detection and we got the limit of detection as low as 20 ppb. This value is much less than the safe limit of F− and makes the MPIPIC an appropriate sensor for F− detection and estimation. Though MPIPIC is water-insoluble, we have been able to estimate the F− ion concentration in drinking water by this molecule. For this estimation, we have made columns of glass wool in glass droppers and have added 500 μL aqueous solution of KF of different concentrations (ca. 10 μM, 20 μM, 30 μM, 40 μM and 60 μM) in five different columns. All of these columns have been kept in an oven for 14 h so that they become properly dried. 18 mL of the MPIPIC solution in acetonitrile (18.9 μM) have been used to eluate F− from the columns as MPIPIC-F− complex. As expected, we have noticed that the color of the eluate becomes yellow indicating the formation of MPIPIC-F− complex. For each eluate from the five different columns with various F− loadings, the emission spectra have been recorded and a calibration curve has been formed by plotting the ratio of the fluorescence intensity at 554 and 430 nm against the concentration of F−added to the columns (see Fig. 10). We have performed exactly the same experiments with 500 μL of tap water in three different columns and from the recorded emission spectra of the elute we have estimated the F− concentration to be 24 ± 2 μM or 0.46 ± 0.04 mg/L. The same tap water sample has been analyzed with the Orion™ F− ion selective electrode and the concentration of F− is found to be 0.5 mg/L. 4. Conclusion
Fig. 9. Normalized intensity vs concentration of F− plot and the linear fit of some intermediate points for the determination of limit of detection.
In conclusion, in this work we have discovered a β-carbolinebased novel chemosensor, MPIPIC, containing an acidic proton attached to indole nitrogen. Fluoride (F−) uptakes this acidic proton from the sensor and changes the color of the sensor solution from colorless to yellow and a new green fluorescence band originates around 554 nm. The present sensor is selective towards F−, as incubation of other anions hardly impose any visual color change or fluorescence property. The binding constant between MPIPIC and F− is found to be 1.05 ± 0.1 × 105 mol−1 L, which is fairly high. The
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Fig. 10. Determination of concentration of F in tap water from the calibration curve.
detection limit of MPIPIC towards F− is found to be as low as 20 ppb. More importantly, we have used MPIPIC to estimate the F− concentration in tap water successfully.
[19] [20] [21] [22] [23] [24]
Acknowledgments AD acknowledges Ministry of Electronics and Information Technology, Government of India, while SUD and ND gratefully acknowledge the CSIR, New Delhi for financial support in the form of fellowship. PS thanks Visvesvaraya PhD Programme of Ministry of Electronics & Information Technology (MeitY), Government of India for providing young faculty research fellowship. Authors acknowledge SAIF Division of CDRI for providing the spectroscopic data. SB acknowledge the financial grant from DST, New Delhi (SR/ S1/OC-01/2010) for the project under which this work was carried out. This work is financially supported by Indian National Science Academy, India. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.05.004.
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