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“Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F− and CN– ions
Journal Pre-proofs “Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F- and CN- ions Priya Alreja, Navneet Kaur PII: DOI: Reference:
S1387-7003(19)30806-8 https://doi.org/10.1016/j.inoche.2019.107600 INOCHE 107600
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Inorganic Chemistry Communications
Received Date: Revised Date: Accepted Date:
6 August 2019 18 September 2019 10 October 2019
Please cite this article as: P. Alreja, N. Kaur, “Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F- and CN- ions, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/j.inoche.2019.107600
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© 2019 Published by Elsevier B.V.
“Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F- and CN- ions Priya Alreja,a and Navneet Kaur*b a
School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, 147004, India b
Department of Chemistry, Panjab University, Chandigarh 160014, India
Corresponding author. Tel.: +91 172 2534430; fax: +91 172 2545074; e-mail: [email protected]; [email protected] Abstract: A simple, novel colorimetric and absorption ratiometric chemosensor 1 based on 1, 10-phenanthroline and naphthalene platforms has been synthesized and demonstrated. The photophysical response of chemosensor 1 was investigated by colorimetric, UV-vis and 1H NMR titration method. 1 behaved as colorimetric sensor for F- and CN- and exhibited naked eye-detectable color changes based on hydrogen bonding interactions followed by deprotonation. Moreover, 1 displayed an absorption ratiometric response for both F- and CN- ions (1:1 complex) in the recognition event. The recognition mechanism has been supported by DFT computational calculations and 1H NMR studies. Significantly, the detection limits of chemosensor 1 towards F- and CN- have been evaluated to be 2.8 × 10-7 M and 5.8 × 10-7 M, which are lower than the maximum values of F- (1.5 mg/L) and CN- (1.9 µM) ions permitted by WHO. Chemosensor 1 can be conveniently and effectively used as test kit for sensing of fluoride and cyanide ions in the solid state without any additional equipment. The color change was more intense for F- ions in comparison to CN- ions both in the solution and solid state which well corroborated to the higher binding constant and lower limit of detection value of F- ion. Keywords: Colorimetric chemosensor; Ratiometric absorption; Test kit; Anion Recognition; Schiff base The immense interest in designing and developing tailor made chemosensors for anions is enormously growing in the field of supramolecular chemistry [1-2]. The special attention has been focused on the development of host molecules that can recognize and sense anion species via visible and optical responses [3-4]. This may possibly be due to major roles played by various anions in the field of chemical industry, biochemistry and environmental science [5-6]. The colorimetric anion sensors offer a promising method for both the qualitative and quantitative detection of anions that are medically and environmentally relevant. The colorimetric detection excludes the need of sophisticated instrumentation, does not require any pretreatment of sample and manual expertise and offers good visualization [7-8]. The ratiometric sensors, then again, could furnish additional reliable quantitative information since they make
use of the ratio of absorption/emission intensity at two different wavelengths as a function of analyte concentration. On the other hand, conventional sensors mostly utilize a single wavelength for quantitative analysis [9-10]. Amongst various anions, fluoride and cyanide ions have been intensively studied owing to their important roles in human lives and environment considering both negative and positive effects. Although the biological importance of fluoride is well recognized due to its major role in dental care and treatment of osteoporosis but its high doses at the same time are hazardous [11]. Cyanide has well established role as an important raw material both in the various industrial processes employed for the production of organic chemicals and for the synthesis of vital polymers like nitrils, nylon and acrylic plastics. However it is one of the major toxic and environmental inorganic pollutants [12-13]. Therefore, substantial efforts have been devoted to the development of novel methods for the detection of fluoride and cyanide in the past decade. Being the most electronegative and reactive, fluoride exhibits appreciable tendency to form strong hydrogen bonds (non-covalent interaction). Thus, various chemosensors containing acidic NH and OH groups or possessing proper functionalization in the framework that results in enhancement of –OH and –NH proton’s acidity have been developed to detect fluoride [14-15]. Meanwhile, the detection of cyanide has been achieved through various approaches such as hydrogen bonding [16-17], chemodosimeters [18-19] and displacement approaches [20-21]. Schiff base molecules with OH/SH groups, in recent times have been exploited to sense anions as they owe easy synthesis with high purity, structural modification, biodegradability, solubility in common solvents and pronounced photophysical properties [22-23]. The introduction of naphthalene group, which as chromophore and fluorophore, makes direct visual recognition possible [24]. The extended πconjugation enhances the intramolecular charge transfer (ICT) which is expected to be highly sensitive towards external perturbations. It is an excellent fluorophore due to presence of both donor (–OH moiety in 2-position) as well as acceptor (aldehyde group or its derivative as imine groups in 1-position) sites [25]. In correlation to our ongoing research in supramolecular chemistry, in this report, we introduce a novel, colorimetric and absorption ratiometric chemosensor 1 based on 2-hydroxy Schiff base. This colorimetric chemosensor displayed a visually notable color change from colorless to yellow with F- and CN- ions only, the color being more intense with F- ions. Only two literature reports are available on the colorimetric analyte sensing using phenanthroline based Schiff bases [26-27]. Moreover, in most of earlier published literature reports on Schiff bases, the LOD values for sensed anions lie in the range of μM [28-31]; while the detection limits of the chemosensor 1 reported in this manuscript lies in the range of 100 nm towards F- and CN-. Interestingly, 1 can selectively detect F- and CN- ions in solid state also as test kits by coating a test paper (Whatman-40) with acetonitrile solution of 1 have been prepared.
Additionally, DFT computational calculations and 1H NMR studies have been carried out to complement the experimental findings. The chemosensor 1 has been synthesized by the condensation of 5-amino-1, 10-phenanthroline and 2-hydroxy-1-naphthaldehyde in absolute ethanol (Scheme-1). 1 H NMR, 13C NMR and ESI-MS spectra of 1 have been shown in Figs. S1–S3.
N
NH 2 (i)
N
(ii) CHO (i)
HO
N
N OH
N 1
(ii) Abs. ethanol , Ref lux
Scheme 1. Synthesis of chemosensor 1
The anion binding characteristics of chemosensor 1 were evaluated in CH3CN via colorimetric and UVVis absorption spectra. 100 equiv. of various anions such as F-, Cl-, Br-, I-, AcO-, H2PO4-, CN- and HSO4were added as their tetrabutylammonium salts. At first, the chemosensing characteristics of 1 with said anions were detected visually. As depicted in Fig. 1, the colorless solution of 1 turned yellow with F- and CN- only, the intensity of color change being more upon F- addition. With AcO- ions, just a tinge of yellow color appeared upon addition to solution of 1, but it faded away after 10 minutes of addition.
Fig. 1. Colorimetric response of 1 (20 µM) in CH3CN upon addition of 100 equiv. of various anions.
Without anions, 1 (20 μM) displayed absorption bands at 320 and 377 nm in addition to a sharp high energy band at 270 nm. In 2-Hydroxy Schiff bases, the UV-vis bands in the range 320-400 nm are result of ICT transition within the whole Schiff base molecule, indicating intramolecular hydrogen bond between the –OH group and the –HC=N (azomethine group) [32]. As seen from UV-vis spectra of 1 (Fig. 2a), addition of only F-, CN- and AcO- could induce a clear red shift of 86 nm. Fig. 2b displays the bar
representation of absorption changes of 1 at 320 and 377 nm with all the added anions. The changes were negligible at these two wavelengths with all the added anions except F-, CN- and AcO-.
Fig. 2. (a) UV-vis spectral responses of 1 (20 μM) with 100 equiv. of various added anions; (b) Bar representation of absorption changes of 1 with added anions at 320 and 377 nm.
To get detailed binding parameters, UV-vis titrations of 1 were performed with the sensed anions in CH3CN. With just the introduction of F- and CN- ions to 20 μM solution of 1, the intensity of 377 nm absorption started decreasing with simultaneous appearance of new absorption band at 463 nm (Fig. 3). With progressive additions, the newly emerged band at 463 nm grew in intensity accompanied by decrease in intensity of 377 nm band. Emergence of new band in the visible range changed the colorless solution of 1 to yellow; the intensity of color change being more for F- ions than CN- ions (Fig. 1).
Fig. 3. Family of UV-vis spectra recorded during the course of titration of 1 (20 µM) with (a) F- ions and (b) CNions; Inset: Plot of absorbances at 463 and 377 nm versus concentration of F - and CN- ions
The isosbestic points at 408 and 415 nm, respectively, in F- and CN- titrations indicated the clean conversion throughout the titration processes. The absorbance values attained the saturation when 8.5 and 10 equiv. of F- and CN- ions have been added, respectively. Job’s plot analysis revealed stoichiometric ratio of 1:1 between 1.A- (A- = F-, CN-) (Fig. S4). In view of that, the association constants of 1 with
sensed anions were evaluated based on UV-vis titration using Benesi-Hildebrand equation given as below [33]. 1 1 1 = + Amax-Ao A-Ao [Amax-Ao] K [C]
Here, Ao, A, and Amax is the absorbance of free 1, measured with F-/CN- and measured with excess amount of F-/CN- at 463 nm, respectively. K is the association constant and [C] is the concentration of F/CN- ions added. A linear plot of 1/(A-A0) versus 1/[ F-/CN-] indicated 1:1 stoichiometry of 1 with F-/CNions . The values of 6.4 × 104 M-1 and 5.3 ×104 M-1 have been respectively calculated for F- and CN- ions. Furthermore, the ratio of the absorbance (A463/A377) also exhibited a good linear relationship against the concentration of F- and CN- ions (Fig. S5). Hence quantitative detection of sensed anions was feasible using an absorption ratiometric method. The limits of detection (LOD) values have been reasonably estimated to be 2.8 × 10-7 M and 5.8 × 10-7 M for F- and CN- ions, respectively, using the relation [34]: LOD = 3s–1 where = standard deviation of response and s = slope of the calibration curve Linear regression graph of titration was used to calculate standard deviation and slope of linear response. Importantly, the calculated LOD values are lower than WHO limits of detection, both for F- (1.5 mg/l) and CN- ions (1.9 µM) [35-36]. The color changes are most probably due to the deprotonation of –OH group of 1 on addition of Fand CN- ions, which could further enhance π-delocalization. This resulted in lowering of the energy of the ππ* transition and therefore, accounts for the appearance of the new absorption band at longer wavelength (463 nm) accompanied by visual color changes. AcO- ions showed similar spectral behavior with emergence of new band at 463 nm but contrastingly new band did not produce any color change as only small increase was observed in absorbance at longer wavelength than that of F- and CN- ions; Moreover, the absorbance at 337 nm remained unperturbed till the end of the titration (Fig. S6a). AcO- ions also exhibited 1:1 binding ratio with 1, as indicated by Job’s plot (Fig. S6b). The mechanism of OH deprotonation has been well supported by the spectral changes of 1 with strong base tetrabutylammonium hydroxide that clearly corroborated to the changes with the sensed anions (Fig S7). Reversibility of a chemosensor is an important factor in order to determine whether it can bind to analyte repeatedly or not. The reversibility (Fig. S8) and reusability (Fig. S9) of the sensor has been evaluated with alternate additions of sensed anions and TFA (trifluoroacetic acid). The addition of about 1
equiv. of TFA completely reversed the absorption changes of 1+F-/CN- solutions indicating the release of 1. The selectivity of chemosensor 1 was endorsed towards F- and CN- ions in the presence of various interfering ions (Cl-, Br-, I-, AcO-, H2PO4-, and HSO4-) and also in each other’s presence, by using UV-vis absorption spectroscopy. 100 equiv. of aforementioned ions were added to 1.F- (20 µM of 1 with 6 added equiv. of F-) and 1.CN- (20 µM of 1 with 8 added equiv. of CN-) acetonitrile solution. The bar graphs (Figs. S10) clearly represent the non-interference of various anions in the recognition process except that F- and CN- do interfere with each other. The sensing behavior of chemosensor 1 identified from the UV-vis titrations with F- and CNions was supported by performing 1H NMR titration experiments in the absence and presence of sensed anions (Fig 4), in DMSO-d6 at room temperature.
Fig. 4. 1H–NMR titration of 1 (4 × 10-2 M) with incremental addition of (a) TBAF and (b) TBACN in DMSO-d6.
The peak at 15.36 was assigned to phenolic hydroxyl group, which appeared at a downfield due to formation of intramolecular hydrogen bonding with imine nitrogen. With the addition of 0.02 equiv. of fluoride ion, the signal at 15.36 declined in intensity, which can be attributed to hydrogen bonding interactions between chemosensor 1 and F- ions. The signal however, vanished completely when 0.2 equiv. of fluoride have been added. The 1H NMR titration of 1 in DMSO was even performed with the excess addition of tetrabutylammonium fluoride (6 equiv.) but the peak corresponding to HF2- did not appear (Fig S11). This may be due to the reason that HF2- is not stable in highly polar solvent like DMSO [37-38]. The 1H NMR titration of 1 with CN- ions also exhibited the similar behavior as that shown by Fions. As the UV-vis titrations were performed in CH3CN, so the 1H NMR titrations of 1 with F- and CN-
were also performed in CD3CN (Fig S12 and S13), where similar broadening and then disappearance of – OH signal has been observed. As job’s plot revealed the formation of 1:1 stoichiometry between chemosensor 1 and sensed anions (F- and CN-), computational studies were done to infer the structural features of these complexes and to further ascertain the sensing mechanism. Both the chemosensor 1 and the 1:1 species were optimized by DFT method and in order to further understand the interaction of sensed ions with the chemosensor 1, TD-DFT calculations were carried out. The structural optimization of the chemosensor 1 and its 1· F-/ CN- complexes was done using the Gaussian 09 package adopting the exchange correlation function B3LYP and the basis sets 6-31G (d,p) [39]. The optimized structures of the chemosensor 1 and its 1· F-/ CN complexes are shown in Fig. S8. The energy comparison diagram (Fig. 5) depicted that on association of 1 with F- and CN- ions, the calculated energy gap of HOMO and LUMO decreased with respect to free chemosensor indicating the formation of stable complexes. The computational calculations are also in good agreement with the visual observations and experimental data that the color change is deeper, hence interaction is little stronger with F- ions in comparison to CN- ions.
Fig. 5. DFT computed energy level diagrams of HOMO and LUMO of (a) 1 (b) 1 with F- (c) 1 with CN-
Moreover, as revealed in Figure 4, H of OH was finally abstracted by sensed ions supporting the 1
H–NMR results of hydrogen bonding interactions followed by deprotonation. Furthermore, the TD-DFT
computed H-F distance in HF (0.96 Å) and C-H distance in HCN (1.116 Å), after abstraction are in good harmony with the literature values of 0.91 and 1.079 Å respectively [36]. Also, the absorption wavelengths calculated using TD-DFT computational calculations are in close agreement with the experimental values (Fig. S9). On the basis of all the above findings following sensing mechanism can be proposed and justified:
N
N
N OH
N
1
Hydrogen bonding assisted deprotonation A- = F-, CN-
N O
N
H A-
1.A
Scheme 2. Proposed binding mode of chemosensor 1 with anions in solution.
In view of visual and optical observations, it was meaningful to investigate the practical application of the chemosensor 1 as anion sensor. Primarily, test strips were prepared via a simplified manner by immersing a test paper (Whatman-40) in acetonitrile solution of 1 (10-3 M) for nearly 10 min and then drying it in air for some time [40]. This was done to coat the test paper with chemosensor 1. For the purpose of anion recognition, the acetonitrile solution of all the tested anions (F-, Cl-, Br-, I-, AcO-, H2PO4-, CN- and HSO4-, 10-3 M) were added on the test paper in the appearance of round spots. Interestingly as expected, only the spot containing F- and CN- ions turned yellow, the color being more intense with F- ions in comparison to CN- ions (Fig. 6). No other anion exhibited visual color changes, supporting 1 as an effective chemosensor for F- and CN- ions both in the solution and solid state.
Fig. 6. Color change of test paper containing 1 (10-3 M) in presence of various anions
To sum up, we have developed a new and simple colorimetric chemosensor for the sensing of anions in acetonitrile. The color of the chemosensor 1 exhibited pronounced color changes (colorless to yellow) on introduction of F- and CN- ions only. The study of both 1H NMR spectroscopy and TD-DFT computational calculations indicate and well synchronize with each other to support complexation in 1:1 stoichiometry, via hydrogen bonding interactions in the beginning followed by deprotonation of OH group. Attractively, paper strips could be successfully prepared and used for the easy detection of F- ions with naked eye. For better potential applicability, further studies are underway in our laboratory to improve water solubility and sensitivity.
Acknowledgements The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the NMR and Mass spectra and are grateful to DST (Grant no. SR/FT/CS–36/2011) and DST PURSE-II (Grant no. 48/RPC) for the financial assistance. Supplementary data Supplementary data associated with this article can be found, in the online version. References
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N
N N
N 1
N
OH
N O H A-
F-, CN-
Highlights:
A novel Schiff base chemosensor based on 1, 10-phenanthroline and 2-hydroxynapthaldehyde Schiff base has been designed and synthesized.
The chemosensor displays pronounced color changes (colorless to yellow) with F- and CN- ions only.
In addition, chemosensor demonstrates as an absorption ratiometric response in the sensing process.
Chemosensor coated paper strips serve as an easy test kit for the naked eye detection of Fand CN- ions.
The authors declare no conflicts of interests.
S1387-7003(19)30806-8 https://doi.org/10.1016/j.inoche.2019.107600 INOCHE 107600
To appear in:
Inorganic Chemistry Communications
Received Date: Revised Date: Accepted Date:
6 August 2019 18 September 2019 10 October 2019
Please cite this article as: P. Alreja, N. Kaur, “Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F- and CN- ions, Inorganic Chemistry Communications (2019), doi: https://doi.org/10.1016/j.inoche.2019.107600
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
“Test kit” of chromogenic and ratiometric 1,10-phenanthroline based chemosensor for the recognition of F- and CN- ions Priya Alreja,a and Navneet Kaur*b a
School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, 147004, India b
Department of Chemistry, Panjab University, Chandigarh 160014, India
Corresponding author. Tel.: +91 172 2534430; fax: +91 172 2545074; e-mail: [email protected]; [email protected] Abstract: A simple, novel colorimetric and absorption ratiometric chemosensor 1 based on 1, 10-phenanthroline and naphthalene platforms has been synthesized and demonstrated. The photophysical response of chemosensor 1 was investigated by colorimetric, UV-vis and 1H NMR titration method. 1 behaved as colorimetric sensor for F- and CN- and exhibited naked eye-detectable color changes based on hydrogen bonding interactions followed by deprotonation. Moreover, 1 displayed an absorption ratiometric response for both F- and CN- ions (1:1 complex) in the recognition event. The recognition mechanism has been supported by DFT computational calculations and 1H NMR studies. Significantly, the detection limits of chemosensor 1 towards F- and CN- have been evaluated to be 2.8 × 10-7 M and 5.8 × 10-7 M, which are lower than the maximum values of F- (1.5 mg/L) and CN- (1.9 µM) ions permitted by WHO. Chemosensor 1 can be conveniently and effectively used as test kit for sensing of fluoride and cyanide ions in the solid state without any additional equipment. The color change was more intense for F- ions in comparison to CN- ions both in the solution and solid state which well corroborated to the higher binding constant and lower limit of detection value of F- ion. Keywords: Colorimetric chemosensor; Ratiometric absorption; Test kit; Anion Recognition; Schiff base The immense interest in designing and developing tailor made chemosensors for anions is enormously growing in the field of supramolecular chemistry [1-2]. The special attention has been focused on the development of host molecules that can recognize and sense anion species via visible and optical responses [3-4]. This may possibly be due to major roles played by various anions in the field of chemical industry, biochemistry and environmental science [5-6]. The colorimetric anion sensors offer a promising method for both the qualitative and quantitative detection of anions that are medically and environmentally relevant. The colorimetric detection excludes the need of sophisticated instrumentation, does not require any pretreatment of sample and manual expertise and offers good visualization [7-8]. The ratiometric sensors, then again, could furnish additional reliable quantitative information since they make
use of the ratio of absorption/emission intensity at two different wavelengths as a function of analyte concentration. On the other hand, conventional sensors mostly utilize a single wavelength for quantitative analysis [9-10]. Amongst various anions, fluoride and cyanide ions have been intensively studied owing to their important roles in human lives and environment considering both negative and positive effects. Although the biological importance of fluoride is well recognized due to its major role in dental care and treatment of osteoporosis but its high doses at the same time are hazardous [11]. Cyanide has well established role as an important raw material both in the various industrial processes employed for the production of organic chemicals and for the synthesis of vital polymers like nitrils, nylon and acrylic plastics. However it is one of the major toxic and environmental inorganic pollutants [12-13]. Therefore, substantial efforts have been devoted to the development of novel methods for the detection of fluoride and cyanide in the past decade. Being the most electronegative and reactive, fluoride exhibits appreciable tendency to form strong hydrogen bonds (non-covalent interaction). Thus, various chemosensors containing acidic NH and OH groups or possessing proper functionalization in the framework that results in enhancement of –OH and –NH proton’s acidity have been developed to detect fluoride [14-15]. Meanwhile, the detection of cyanide has been achieved through various approaches such as hydrogen bonding [16-17], chemodosimeters [18-19] and displacement approaches [20-21]. Schiff base molecules with OH/SH groups, in recent times have been exploited to sense anions as they owe easy synthesis with high purity, structural modification, biodegradability, solubility in common solvents and pronounced photophysical properties [22-23]. The introduction of naphthalene group, which as chromophore and fluorophore, makes direct visual recognition possible [24]. The extended πconjugation enhances the intramolecular charge transfer (ICT) which is expected to be highly sensitive towards external perturbations. It is an excellent fluorophore due to presence of both donor (–OH moiety in 2-position) as well as acceptor (aldehyde group or its derivative as imine groups in 1-position) sites [25]. In correlation to our ongoing research in supramolecular chemistry, in this report, we introduce a novel, colorimetric and absorption ratiometric chemosensor 1 based on 2-hydroxy Schiff base. This colorimetric chemosensor displayed a visually notable color change from colorless to yellow with F- and CN- ions only, the color being more intense with F- ions. Only two literature reports are available on the colorimetric analyte sensing using phenanthroline based Schiff bases [26-27]. Moreover, in most of earlier published literature reports on Schiff bases, the LOD values for sensed anions lie in the range of μM [28-31]; while the detection limits of the chemosensor 1 reported in this manuscript lies in the range of 100 nm towards F- and CN-. Interestingly, 1 can selectively detect F- and CN- ions in solid state also as test kits by coating a test paper (Whatman-40) with acetonitrile solution of 1 have been prepared.
Additionally, DFT computational calculations and 1H NMR studies have been carried out to complement the experimental findings. The chemosensor 1 has been synthesized by the condensation of 5-amino-1, 10-phenanthroline and 2-hydroxy-1-naphthaldehyde in absolute ethanol (Scheme-1). 1 H NMR, 13C NMR and ESI-MS spectra of 1 have been shown in Figs. S1–S3.
N
NH 2 (i)
N
(ii) CHO (i)
HO
N
N OH
N 1
(ii) Abs. ethanol , Ref lux
Scheme 1. Synthesis of chemosensor 1
The anion binding characteristics of chemosensor 1 were evaluated in CH3CN via colorimetric and UVVis absorption spectra. 100 equiv. of various anions such as F-, Cl-, Br-, I-, AcO-, H2PO4-, CN- and HSO4were added as their tetrabutylammonium salts. At first, the chemosensing characteristics of 1 with said anions were detected visually. As depicted in Fig. 1, the colorless solution of 1 turned yellow with F- and CN- only, the intensity of color change being more upon F- addition. With AcO- ions, just a tinge of yellow color appeared upon addition to solution of 1, but it faded away after 10 minutes of addition.
Fig. 1. Colorimetric response of 1 (20 µM) in CH3CN upon addition of 100 equiv. of various anions.
Without anions, 1 (20 μM) displayed absorption bands at 320 and 377 nm in addition to a sharp high energy band at 270 nm. In 2-Hydroxy Schiff bases, the UV-vis bands in the range 320-400 nm are result of ICT transition within the whole Schiff base molecule, indicating intramolecular hydrogen bond between the –OH group and the –HC=N (azomethine group) [32]. As seen from UV-vis spectra of 1 (Fig. 2a), addition of only F-, CN- and AcO- could induce a clear red shift of 86 nm. Fig. 2b displays the bar
representation of absorption changes of 1 at 320 and 377 nm with all the added anions. The changes were negligible at these two wavelengths with all the added anions except F-, CN- and AcO-.
Fig. 2. (a) UV-vis spectral responses of 1 (20 μM) with 100 equiv. of various added anions; (b) Bar representation of absorption changes of 1 with added anions at 320 and 377 nm.
To get detailed binding parameters, UV-vis titrations of 1 were performed with the sensed anions in CH3CN. With just the introduction of F- and CN- ions to 20 μM solution of 1, the intensity of 377 nm absorption started decreasing with simultaneous appearance of new absorption band at 463 nm (Fig. 3). With progressive additions, the newly emerged band at 463 nm grew in intensity accompanied by decrease in intensity of 377 nm band. Emergence of new band in the visible range changed the colorless solution of 1 to yellow; the intensity of color change being more for F- ions than CN- ions (Fig. 1).
Fig. 3. Family of UV-vis spectra recorded during the course of titration of 1 (20 µM) with (a) F- ions and (b) CNions; Inset: Plot of absorbances at 463 and 377 nm versus concentration of F - and CN- ions
The isosbestic points at 408 and 415 nm, respectively, in F- and CN- titrations indicated the clean conversion throughout the titration processes. The absorbance values attained the saturation when 8.5 and 10 equiv. of F- and CN- ions have been added, respectively. Job’s plot analysis revealed stoichiometric ratio of 1:1 between 1.A- (A- = F-, CN-) (Fig. S4). In view of that, the association constants of 1 with
sensed anions were evaluated based on UV-vis titration using Benesi-Hildebrand equation given as below [33]. 1 1 1 = + Amax-Ao A-Ao [Amax-Ao] K [C]
Here, Ao, A, and Amax is the absorbance of free 1, measured with F-/CN- and measured with excess amount of F-/CN- at 463 nm, respectively. K is the association constant and [C] is the concentration of F/CN- ions added. A linear plot of 1/(A-A0) versus 1/[ F-/CN-] indicated 1:1 stoichiometry of 1 with F-/CNions . The values of 6.4 × 104 M-1 and 5.3 ×104 M-1 have been respectively calculated for F- and CN- ions. Furthermore, the ratio of the absorbance (A463/A377) also exhibited a good linear relationship against the concentration of F- and CN- ions (Fig. S5). Hence quantitative detection of sensed anions was feasible using an absorption ratiometric method. The limits of detection (LOD) values have been reasonably estimated to be 2.8 × 10-7 M and 5.8 × 10-7 M for F- and CN- ions, respectively, using the relation [34]: LOD = 3s–1 where = standard deviation of response and s = slope of the calibration curve Linear regression graph of titration was used to calculate standard deviation and slope of linear response. Importantly, the calculated LOD values are lower than WHO limits of detection, both for F- (1.5 mg/l) and CN- ions (1.9 µM) [35-36]. The color changes are most probably due to the deprotonation of –OH group of 1 on addition of Fand CN- ions, which could further enhance π-delocalization. This resulted in lowering of the energy of the ππ* transition and therefore, accounts for the appearance of the new absorption band at longer wavelength (463 nm) accompanied by visual color changes. AcO- ions showed similar spectral behavior with emergence of new band at 463 nm but contrastingly new band did not produce any color change as only small increase was observed in absorbance at longer wavelength than that of F- and CN- ions; Moreover, the absorbance at 337 nm remained unperturbed till the end of the titration (Fig. S6a). AcO- ions also exhibited 1:1 binding ratio with 1, as indicated by Job’s plot (Fig. S6b). The mechanism of OH deprotonation has been well supported by the spectral changes of 1 with strong base tetrabutylammonium hydroxide that clearly corroborated to the changes with the sensed anions (Fig S7). Reversibility of a chemosensor is an important factor in order to determine whether it can bind to analyte repeatedly or not. The reversibility (Fig. S8) and reusability (Fig. S9) of the sensor has been evaluated with alternate additions of sensed anions and TFA (trifluoroacetic acid). The addition of about 1
equiv. of TFA completely reversed the absorption changes of 1+F-/CN- solutions indicating the release of 1. The selectivity of chemosensor 1 was endorsed towards F- and CN- ions in the presence of various interfering ions (Cl-, Br-, I-, AcO-, H2PO4-, and HSO4-) and also in each other’s presence, by using UV-vis absorption spectroscopy. 100 equiv. of aforementioned ions were added to 1.F- (20 µM of 1 with 6 added equiv. of F-) and 1.CN- (20 µM of 1 with 8 added equiv. of CN-) acetonitrile solution. The bar graphs (Figs. S10) clearly represent the non-interference of various anions in the recognition process except that F- and CN- do interfere with each other. The sensing behavior of chemosensor 1 identified from the UV-vis titrations with F- and CNions was supported by performing 1H NMR titration experiments in the absence and presence of sensed anions (Fig 4), in DMSO-d6 at room temperature.
Fig. 4. 1H–NMR titration of 1 (4 × 10-2 M) with incremental addition of (a) TBAF and (b) TBACN in DMSO-d6.
The peak at 15.36 was assigned to phenolic hydroxyl group, which appeared at a downfield due to formation of intramolecular hydrogen bonding with imine nitrogen. With the addition of 0.02 equiv. of fluoride ion, the signal at 15.36 declined in intensity, which can be attributed to hydrogen bonding interactions between chemosensor 1 and F- ions. The signal however, vanished completely when 0.2 equiv. of fluoride have been added. The 1H NMR titration of 1 in DMSO was even performed with the excess addition of tetrabutylammonium fluoride (6 equiv.) but the peak corresponding to HF2- did not appear (Fig S11). This may be due to the reason that HF2- is not stable in highly polar solvent like DMSO [37-38]. The 1H NMR titration of 1 with CN- ions also exhibited the similar behavior as that shown by Fions. As the UV-vis titrations were performed in CH3CN, so the 1H NMR titrations of 1 with F- and CN-
were also performed in CD3CN (Fig S12 and S13), where similar broadening and then disappearance of – OH signal has been observed. As job’s plot revealed the formation of 1:1 stoichiometry between chemosensor 1 and sensed anions (F- and CN-), computational studies were done to infer the structural features of these complexes and to further ascertain the sensing mechanism. Both the chemosensor 1 and the 1:1 species were optimized by DFT method and in order to further understand the interaction of sensed ions with the chemosensor 1, TD-DFT calculations were carried out. The structural optimization of the chemosensor 1 and its 1· F-/ CN- complexes was done using the Gaussian 09 package adopting the exchange correlation function B3LYP and the basis sets 6-31G (d,p) [39]. The optimized structures of the chemosensor 1 and its 1· F-/ CN complexes are shown in Fig. S8. The energy comparison diagram (Fig. 5) depicted that on association of 1 with F- and CN- ions, the calculated energy gap of HOMO and LUMO decreased with respect to free chemosensor indicating the formation of stable complexes. The computational calculations are also in good agreement with the visual observations and experimental data that the color change is deeper, hence interaction is little stronger with F- ions in comparison to CN- ions.
Fig. 5. DFT computed energy level diagrams of HOMO and LUMO of (a) 1 (b) 1 with F- (c) 1 with CN-
Moreover, as revealed in Figure 4, H of OH was finally abstracted by sensed ions supporting the 1
H–NMR results of hydrogen bonding interactions followed by deprotonation. Furthermore, the TD-DFT
computed H-F distance in HF (0.96 Å) and C-H distance in HCN (1.116 Å), after abstraction are in good harmony with the literature values of 0.91 and 1.079 Å respectively [36]. Also, the absorption wavelengths calculated using TD-DFT computational calculations are in close agreement with the experimental values (Fig. S9). On the basis of all the above findings following sensing mechanism can be proposed and justified:
N
N
N OH
N
1
Hydrogen bonding assisted deprotonation A- = F-, CN-
N O
N
H A-
1.A
Scheme 2. Proposed binding mode of chemosensor 1 with anions in solution.
In view of visual and optical observations, it was meaningful to investigate the practical application of the chemosensor 1 as anion sensor. Primarily, test strips were prepared via a simplified manner by immersing a test paper (Whatman-40) in acetonitrile solution of 1 (10-3 M) for nearly 10 min and then drying it in air for some time [40]. This was done to coat the test paper with chemosensor 1. For the purpose of anion recognition, the acetonitrile solution of all the tested anions (F-, Cl-, Br-, I-, AcO-, H2PO4-, CN- and HSO4-, 10-3 M) were added on the test paper in the appearance of round spots. Interestingly as expected, only the spot containing F- and CN- ions turned yellow, the color being more intense with F- ions in comparison to CN- ions (Fig. 6). No other anion exhibited visual color changes, supporting 1 as an effective chemosensor for F- and CN- ions both in the solution and solid state.
Fig. 6. Color change of test paper containing 1 (10-3 M) in presence of various anions
To sum up, we have developed a new and simple colorimetric chemosensor for the sensing of anions in acetonitrile. The color of the chemosensor 1 exhibited pronounced color changes (colorless to yellow) on introduction of F- and CN- ions only. The study of both 1H NMR spectroscopy and TD-DFT computational calculations indicate and well synchronize with each other to support complexation in 1:1 stoichiometry, via hydrogen bonding interactions in the beginning followed by deprotonation of OH group. Attractively, paper strips could be successfully prepared and used for the easy detection of F- ions with naked eye. For better potential applicability, further studies are underway in our laboratory to improve water solubility and sensitivity.
Acknowledgements The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the NMR and Mass spectra and are grateful to DST (Grant no. SR/FT/CS–36/2011) and DST PURSE-II (Grant no. 48/RPC) for the financial assistance. Supplementary data Supplementary data associated with this article can be found, in the online version. References
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N
N N
N 1
N
OH
N O H A-
F-, CN-
Highlights:
A novel Schiff base chemosensor based on 1, 10-phenanthroline and 2-hydroxynapthaldehyde Schiff base has been designed and synthesized.
The chemosensor displays pronounced color changes (colorless to yellow) with F- and CN- ions only.
In addition, chemosensor demonstrates as an absorption ratiometric response in the sensing process.
Chemosensor coated paper strips serve as an easy test kit for the naked eye detection of Fand CN- ions.
The authors declare no conflicts of interests.