A colorimetric and absorption ratiometric anion sensor based on indole & hydrazide binding units

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 78–84

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A colorimetric and absorption ratiometric anion sensor based on indole & hydrazide binding units Linbo Zou, Boren Yan, Dingwu Pan, Zan Tan, Xiaoping Bao ⇑ State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Center for Research and Development of Fine Chemicals, Guizhou University, Guiyang 550025, People’s Republic of China

g r a p h i c a l a b s t r a c t

 An effective anion sensor L bearing

0.40

0.6

0.35 0.30

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0.25 0.20 0.15

Absorbance

indole & hydrazide units was synthesized.  Sensor L displayed the colorimetric and absorption ratiometric responses towards F, AcO and H2PO4 in DMSO.  Sensor L underwent NH deprotonation event upon addition of excessive F in DMSO-d6.

A 456 /A 298

h i g h l i g h t s

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[F ](µM)

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a r t i c l e

i n f o

Article history: Received 29 November 2014 Received in revised form 13 March 2015 Accepted 27 March 2015 Available online 3 April 2015 Keywords: Colorimetric sensor Absorption ratiometry Indole Hydrazide NH

a b s t r a c t A colorimetric and absorption ratiometric anion sensor (L) based on indole and hydrazide binding units was designed and synthesized, and its recognition & sensing properties towards different anions were studied by naked-eye observations, UV–vis and 1H NMR titration spectra. Sensor L could selectively recognize biologically important F, AcO and H2PO 4 in DMSO over other anions, along with a significant change in its color and absorption spectrum, resulting from the formation of corresponding 1:2 (L/F) and 1 1:1 (L/AcO and L/H2PO 4 ) complexes. The H NMR titration experiments proved that sensor L experienced deprotonation of NH fragment and produced [HF2] species, whereas a stable H-bonding complex was formed in the presence of AcO and H2PO 4. Ó 2015 Elsevier B.V. All rights reserved.

Introduction More and more attention has been focused on the development of anion receptors or sensors due to key roles of anions in many chemical and biological processes [1–5]. Among common anions, fluoride, acetate and dihydrogenphosphate ions received the most concerns because of their special physiological & biochemical functions in the human body. For example, fluoride was proved to be associated with dental care, osteoporosis and environmental pollution [6]. Acetate ion in the human protein components works as a ‘‘molecular switch’’ in diverse cellular processes, ⇑ Corresponding author. Tel.: +86 851 8292170; fax: +86 851 3622211. E-mail address: [email protected] (X. Bao). http://dx.doi.org/10.1016/j.saa.2015.03.109 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

such as cell division, DNA inheritance and cell aging [7]. Dihydrogenphosphate and its analogues play key roles in signal transduction and energy storage of biological system [8]. Compared with fluorescent and electrochemical sensors, colorimetric sensors had some obvious advantages, such as its convenience, low cost and good visualization [9]. On the other hand, ratiometric sensors could provide more reliable quantitative information since they utilized the ratio of absorption/emission intensity at two different wavelengths as a function of analyte concentration, relative to conventional sensors only depending on a single wavelength for quantitative analysis [10–12]. Most of colorimetric anion sensors were composed of two parts, that is, binding unit and chromophore unit. The extensively-used binding units include amide, urea, thiourea, pyrrole, indole and

L. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 78–84

sulfonamide. And recently, a few anion sensors based on hydrazide NH unit were reported, which displayed remarkable differences in their recognition behaviors in spite of small variations in their chemical structures (having nearly the same binding unit and chromophore moiety for these sensors). For example, Shi et al. [13] reported diacylhydrazine-based colorimetric receptors for selective sensing of F and AcO (from colorless to yellow) over other anions (including H2PO 4 ) in acetonitrile solution, with the formation of a 1:1 stoichiometric complex between the receptors and the anions. Chen and co-workers [14] developed an organogelator containing a hydrazide unit, and its chloroform gel was transferred into a red solution after treatment with F, AcO and H2PO 4 , due to the disruption of intermolecular H-bonding interactions among receptor molecules via deprotonation of hydrazide NH. Trivedi et al. [15] synthesized a benzohydrazide-based anion receptor with one p-nitrophenyl chromophore, which demonstrated obvious color and spectral changes in DMSO solution only in the presence of F and AcO (excluding H2PO 4 ). Very recently, Bai and co-workers [16] prepared a p-nitrobenzohydrazide derivative and it formed 1:1 H-bonded complexes  with AcO and H2PO 4 in DMSO. For F , an initial H-bonding interaction was followed by deprotonation of NH. The addition of F, AcO and H2PO 4 into DMSO solution of the receptor induced a remarkable color change from colorless to red. To our best knowledge, there was no example of integrating hydrazide NH and other NH units into a single anion receptor, which afforded colorimetric & absorption ratiometric responses upon addition of anions. Herein, we developed an effective anion sensor L (Scheme 1) with indole & hydrazide NHs as binding units and p-nitrobenzene moiety as a chromophore. Different with the previously reported hydrazide-based anion sensors [13–16], L demonstrated a good selectivity towards F (1:2 complex), AcO and H2PO 4 (1:1 complex) over other anions in DMSO, along with a marked color change and absorption ratiometric responses during the recognition process. Experimental General All chemicals were purchased from commercial suppliers and used without further purification. Dry CH2Cl2 and TEA were obtained by refluxing over CaH2 and distillation before use. All the anions were used as their tetra-n-butylammonium salts. Melting points were determined on a XT-4 binocular microscope (Beijing Tech Instrument Co., China). IR spectra were recorded on a Shimadzu IR Prestige-21 spectrometer in KBr disk. 1H and 13C NMR spectra were measured on a JEOL-ECX 500 NMR spectrometer at room temperature using TMS as an internal standard. Mass spectra were recorded on an Agilent LC/MSD Trap VL. Elemental analysis was performed on an Elementar Vario-III CHN analyzer. UV–vis spectra were recorded on a TU-1900 spectrophotometer (Beijing Pgeneral Instrument Co., China). Synthesis of sensor L Indole-2-acylhydrazine [17] (100 mg, 0.57 mmol) and p-nitrobenzoyl chloride (106 mg, 0.57 mmol) were dissolved in dry CH2Cl2 (8 mL) in the presence of dry TEA (0.25 mL), and the O

resulting mixture was stirred at room temperature for 18 h. The formed precipitate was filtered off and washed with water, giving L as a light-yellow solid in 76% yield (140 mg). Mp > 250 °C. 1H NMR (500 MHz, DMSO-d6): d 11.76 (s, 1H, H1), 10.91 (s, 1H, H2), 10.65 (s, 1H, H3), 8.39 (d, J = 9.0 Hz, 2H, H4), 8.17 (d, J = 9.0 Hz, 2H, H5), 7.67 (d, J = 8.70 Hz, 1H, H6), 7.46 (d, J = 8.30 Hz, 1H, H7), 7.29 (s, 1H, H8), 7.23 (t, J = 8.20 Hz, 1H, H9), 7.08 (t, J = 7.50 Hz, 1H, H10); 13C NMR (125 MHz, DMSO-d6): d 164.6, 160.9, 149.6, 138.2, 136.8, 129.5, 129.2, 127.1, 124.0, 121.9, 120.1, 112.6, 103.8; IR (KBr, cm1): 3323 (NAH), 1633 (C@O), 1524 (NO2), 1346 (NO2); MS-ESI: 323.1 ([MH]); Anal. Calcd for C16H12N4O4: C, 59.26; H, 3.73; N, 17.28. Found: C, 59.13; H, 3.89; N, 17.55. UV–vis and 1H NMR titration experiments Stock solution of sensor L being studied was prepared in DMSO with the final concentration of 20 lM. To a 3.0 mL of the above solution was added DMSO solution of each anion (15 mM) using a Hamilton syringe. After each of addition, the UV–vis spectrum was recorded. All the titration experiments were carried out on a Pgeneral TU-1900 spectrometer at 298 K. To a 0.5 mL of DMSO-d6 solution of L (3.0 mM) was added DMSO-d6 solution of each anion (60 mM). After each of addition, the 1H NMR spectrum was measured. Results and discussion UV–vis studies Initially, sensor L was examined for its colorimetric sensing capabilities in DMSO upon addition of different anions. As shown in Fig. 1, a significant color change from colorless to yellow was observed in the presence of F, AcO and H2PO 4 . In contrast, no perceptible color change happened after addition of other anions,    such as Cl, Br, I, ClO 4 , N3 , NO3 and HSO4 . Subsequently, the selectivity of L for various anions was further confirmed by UV– vis spectroscopy. From Fig. 2, addition of F, AcO or H2PO 4 anions to L induced the appearance of a new absorption peak around 456 nm (responsible for the vivid color change), concomitant with an evident decrease in the original peak at 296 nm. Upon treatment with other seven anions, very slight or no spectral change was found for L. Encouraged by the above phenomena, the binding interactions of sensor L with F, AcO and H2PO 4 were investigated in detail through UV–vis titration spectra. As shown in Fig. 3, upon addition of increasing amounts of F to DMSO solution of L, the absorbance at 296 nm was gradually decreased and a new absorption peak at 456 nm progressively evolved. A distinct isosbestic point was detected at 312 nm, indicating that only two absorbing species coexisted in the titration mixture [18]. The plot of 1/(A–A0) displayed an excellent linear relationship with 1/[F]2 according to the Benesi–Hildebrand equation (Fig. 4), proving that L bound F in a 1:2 stoichiometry with a binding constant of (1.15 ± 0.15)  109 M2. Significantly, there was a good linear relationship between the ratio of the absorbance (A456/A296) in sensor L and the concentration of F ranging from 0 to 55 lM, therefore allowing quantitative

COCl

NHNH2

7

8

6

N H1

9

dry CH 2Cl2 N H

79

Et3N, r.t., 18 h

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O O

NO 2

Scheme 1. Synthesis of sensor L.

HN NH 2

3

L

5 4

NO 2

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      Fig. 1. Color changes of sensor L (50 lM) in DMSO upon addition of 8.0 equiv of different anions (from left to right: free L, F, AcO, H2PO 4 , Cl , Br , I , HSO4 , NO3 , N3 and ClO 4 ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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+F

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+AcO _ +H2PO4

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y=51.5231*x+5.9353 r=0.9965

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Sensor L, +Cl , +Br , +I , +NO3 , +ClO4 , -

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Fig. 2. UV–vis changes of sensor L (50 lM) in DMSO upon addition of 8.0 equiv of different anions.

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1/[F ]

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Fig. 4. Benesi–Hildebrand plot (k = 456 nm) of sensor L and F.

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y=0.00707*x+0.00498 r=0.9935

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Wavelength(nm) Fig. 3. UV–vis spectral changes of sensor L (20 lM) in DMSO upon addition of 0– 9.25 equiv of F.

detection of F by absorption ratiometric method (Fig. 5). The detection limit (LOD) of L for F was calculated to be 6.3 lM, through the formula (LOD = 3r/k, r and k represent standard deviation and slope of linear regression curve, respectively) [19]. Similar spectral changes were observed when addition of AcO or H2PO 4 to DMSO solution of L. However, the best fitting of

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[F ] ( M) Fig. 5. Plot of the absorbance ratio at 456 and 296 nm (A456/A296) as a function of F concentration.

titration isotherm was achieved on assuming the formation of a 1:1 complex between L and these two anions (Fig. 6 and 7), giving the binding constant of (3.44 ± 0.40)  104 M1 and (7.38 ± 0.38)  103 M1 for AcO and H2PO respectively. 4, Furthermore, the detection limit of sensor L for AcO and H2PO 4 was 12.0 and 18.7 lM (Fig. 8 and 9), respectively.

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Wavelength(nm) Fig. 6. UV–vis spectral changes of sensor L (20 lM) in DMSO upon addition of 0– 11.0 equiv of AcO. The inset was non-linear curve fitting of the absorbance at 456 nm against the added AcO according to a 1:1 binding ratio.

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[H2PO4 ] ( M) Fig. 9. Plot of the absorbance ratio at 456 and 296 nm (A456/A296) of L as a function of H2PO4 concentration.

Determination of association constants (Ka) 0.6

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When assuming a 1:2 binding stoichiometry between L and F, the binding constant (Ka) could be determined by the Benesi– Hildebrand equation [20,21] as follows:

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" # 1 1 1 ¼ þ1 A  A0 A1  A0 K a ½F 20

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Wavelength(nm) Fig. 7. UV–vis spectral changes of sensor L (20 lM) in DMSO upon addition of 0– 22.5 equiv of H2PO 4 . The inset was non-linear curve fitting of the absorbance at 456 nm against the added H2PO 4 according to a 1:1 binding ratio.

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y=0.00449*x+0.04788 r=0.9907

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A0 is the absorbance of L without F, A is the absorbance of L after treatment of F, A1 is the absorbance of L upon addition of excessive F, [F]0 is the concentration of added F. A good linear relationship was obtained by plotting 1/(A  A0) against 1/[F]2 (as shown in Fig. 4), indicating the formation of 1:2 (L/F) complex. The binding constant (Ka) was determined from the ratio of intercept/slope. In a supramolecular system with the formation of 1:1 binding complex, the binding constant can be calculated using the following equation [22]:  h i1=2  2 A ¼ A0 þ ðAlim  X 0 Þ=2C 0 ðC H þ C G þ 1=K a Þ  ðC H þ C G þ 1=K a Þ  4C H C G

where A is the absorbance of the whole system; A0 is the absorbance of free receptor; CH and CG are the corresponding concentration of receptor compound and guest anion; Ka is the binding constant. The binding constants and correlation coefficients (R) were obtained from the nonlinear curve fitting of A versus CG/CH. The obtained correlation coefficients (R = 0.9950 and 0.9987 for AcO and H2PO 4 , respectively) were both larger than 0.99, proving the formation of 1:1 binding complex between L and AcO/H2PO 4 [23–28]. The spectrometric data were processed based on the Benesi– Hildebrand equation and nonlinear curve fitting method for 1:2 (L/F) and 1:1 (L/AcO and L/H2PO 4 ) binding models, respectively, by using the Origin 6.0 program.

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[AcO ] ( M) Fig. 8. Plot of the absorbance ratio at 456 and 296 nm (A456/A296) of L as a function of AcO concentration.

To disclose the nature of interactions between sensor L and anions, 1H NMR titration studies were carried out in DMSO-d6. The chemical shift changes of L upon addition of 1.0 equiv of different anions were displayed in Fig. 10, from which we could find: (1) Only the presence of F, AcO and H2PO 4 caused an obvious change in the chemical shifts of NH and aromatic CH protons,

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Fig. 10. Partial 1H NMR spectra of L (3.0 mM in DMSO-d6) upon addition of 1.0 equiv of various anions.

Fig. 11. Partial 1H NMR spectra of L (3.0 mM) in DMSO-d6 upon addition of different equivalents of F.

showing effective interactions between L and these three anions;   (2) the addition of HSO 4 , NO3 and N3 induced a certain broadening of hydrazide NH signals and no detectable change for indole NH and aromatic CH protons was found, indicating very weak interactions between L and these three anions; (3) the addition of Cl, Br, I and ClO 4 to L did not produce any discernible change in all of the proton chemical shifts, suggesting no interactions between L and these four anions. The observed high affinity and selectivity of L for F, AcO and H2PO 4 over other anions were rationalized due to the following reasons: (1) Strong competitiveness of DMSO solvent disfavored the binding of L with these seven anions; (2) The above three anions had much larger basicities (namely, better H-bonding accepting capabilities) than those of the remaining anions.

Subsequently, the binding interactions between L and F, AcO 1 and H2PO 4 were investigated in detail through H NMR titration spectra. As shown in Fig. 11, addition of 0.48 equiv of F caused a slight upfield shift (0.03 ppm) for H1, concomitant with the disappearance of H2 & H3 signals and the appearance of a new broad signal at d = 10.66 ppm, providing direct evidence for the formation of NAH  F H-bonds [14]. While gradually increasing the concentration of F up to 2.0 equiv, the signal of hydrazide NH completely disappeared and indole NH was found to experience an upfield shift of 0.12 ppm in this circumstance. Interestingly, a new triplet peak at 16.08 ppm (assigned to HF 2 species [29–31]) developed after addition of 3.55 equiv of F, indicative of the appearance of NH deprotonation in the context of 1H NMR titration experiments. During the stage of H-bonding interactions ([F]/

L. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 78–84

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Fig. 12. Partial 1H NMR spectra of L (3.0 mM) in DMSO-d6 upon addition of different equivalents of AcO.

Fig. 13. Partial 1H NMR spectra of L (3.0 mM) in DMSO-d6 upon addition of different equivalents of H2PO 4.

[L] = 0–2.6), all the aromatic protons underwent an upfield shift to a different extent, with the most prominent protons for H8 and H4 (0.49 and 0.17 ppm, respectively). Upon addition of AcO, the chemical shift changes of L (Fig. 12) were similar to those of F, except for no event of NH deprotonation. Different with F and AcO, a slight downfield shift of indole NH signal in L was found while gradually increasing the concentration of H2PO 4 added (Fig. 13). Moreover, the variation trend of hydrazide NH was the same as that of AcO.

Based on the obtained UV–vis and 1H NMR titration results, a   possible binding model between L and H2PO 4 /AcO /F in DMSO was proposed in Scheme 2. The formation of such H-bonding interactions (or the appearance of NH deprotonation upon addition of excessive F) modulated electronic properties of the chromophore, resulting in absorption spectral and color changes due to a new charge-transfer interaction between anion-bound amide moiety and the electron-deficient p-nitrophenyl chromophore.

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L. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 78–84 O N H

A

O

O

N NH H L

N H NO2

O

O

N N H H A

NO 2

+FO N H

F

N NO 2

H O

A O

N H

N H

N H

N H

A = AcO-; H2 PO4 -

O

+FN H NO 2

O

N H

N H

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F

O N N H H O

N H NO 2

e ex

O N H

s

ve si

F-

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F

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O

NO 2

F H F

  Scheme 2. A possible binding model between L and H2PO 4 /AcO /F in DMSO.

Conclusions In conclusion, we synthesized an effective colorimetric and absorption ratiometric anion sensor L based on indole and hydrazide NHs. Sensor L displayed a good selectivity for F, AcO and 1 H2PO 4 over other anions, through UV–vis and H NMR titration experiments. 1H NMR titration experiments proved that sensor L underwent NH deprotonation event upon interaction with excessive F. Acknowledgements We gratefully thank the National Natural Science Foundation of China (No. 21161005) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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