- Email: [email protected]
“Test kit” for detection of biologically important anions: A salicylidene-hydrazine based Schiff base
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 314–318
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
‘‘Test kit’’ for detection of biologically important anions: A salicylidene-hydrazine based Schiff base Sasanka Dalapati a, Md Akhtarul Alam b,⇑, Sankar Jana a, Saswati Karmakar c, Nikhil Guchhait a,⇑ a
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India Department of Chemistry, Aliah University, DN-41 & 47, Sector-V, Saltlake, Kolkata 700 091, India c Department of Chemistry, Sree Chaitanya College, Habra, North 24 Parganas, West Bengal, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" ‘‘Test kit’’ was prepared for the
detection of biologically important anions. " ‘‘Test kit’’ can selectively detect F and AcO ions. " Color change of the ‘‘test kit’’ in presence of anions were detected by naked-eye. " Predicted complexation behavior from theoretical structural optimization corroborates with the experimental results.
a r t i c l e
i n f o
Article history: Received 13 August 2012 Received in revised form 15 October 2012 Accepted 23 October 2012 Available online 1 November 2012 Keywords: Test kit Schiff base Colorimetric Aqueous–acetonitrile DFT calculation Anion detection
a b s t r a c t Test paper coated with Schiff base [(N,N/-bis(5-nitro-salicylidene)hydrazine] receptor 1 (host) can selectively detect fluoride and acetate ions (guest) by developing yellow color which can be detected by naked-eye both in aqueous–acetonitrile solution and in solid supported test kit. UV–vis spectral analysis shows that the absorption peaks at 288 and 345 nm of receptor 1 gradually decrease its initial intensity and new red shifted absorption bands at 397 nm and 455 nm gradually appear upon addition of increas ing amount of F and AcO ions over several tested anions such as H2 PO 4 , Cl , Br , I , NO3 , NO2 , HSO4 , HSO3 , and ClO4 in aqueous–acetonitrile solvent. The colorimetric test results and UV–vis spectral analysis are in well agreement with 1H NMR titration results in d6-DMSO solvent. The receptor 1 forms 1:2 stable complexes with F and AcO ions. However, similar kind of observation obtained from UV–vis titrations in presence of AcOH corresponds to 1:1 complexation ratio indicating the formation of H-bonding interaction between the receptor and anions (F and AcO ions). So, the observed 1:2 complexation ratio can only be explained on the basis of deprotonation (1 eqv.) and H-bonding (1 eqv.) interactions [1]. The ratiometric analysis of host–guest complexes corroborates well with the proposed theoretical model optimization at Density Functional Theory (DFT) level. Ó 2012 Elsevier B.V. All rights reserved.
Introduction
⇑ Corresponding authors. Tel.: +91 33 23508386; fax: +91 33 23519755 (N. Guchhait). E-mail addresses: [email protected] (M.A. Alam), [email protected] (N. Guchhait). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.10.038
There are a few examples of test kits which are reported in the current chemical literature for the detection of ions (cation or anion). Importantly, the colorimetric detection of ions by a cheap method is gaining faster attention in the recent development of chemosensors design [2–6]. It is found that small quantities of ions
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S. Dalapati et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 314–318
play an important role both in the environmental and biological systems [7–12]. However, excess amount of ions may cause of several serious diseases as well as immune system disruptions [13– 15]. Because of significant importance, detection of ions with the help of easily synthesized receptor and minimal instrumental assistance be always welcomed for the purpose of practical applications. Recently, Li and co-workers reported and well established a receptor (as our receptor 1, Scheme 1) without ANO2 signaling unit capable for selective sensing of F ion in DMSO solvent [16]. Due to the problem of practical applicability of their receptor, here we report nitro substituted Schiff-base receptor 1 by considering three important factors: (i) the nitro group (ANO2) having electron withdrawing effect is expected to enhance the acidity of the phenolic AOH group and as a consequence the hydrogen donor efficiency of the receptor 1 is also increased, (ii) the stronger acidity of phenolic AOH group may enhance its sensitivity and thus useful in aqueous environment, (iii) UV–vis and 1H NMR absorption properties of the chromogenic nitro-phenyl moiety may be altered by receptor–anions interaction and thus promote the colorimetric and spectral sensing recognition events [17,18]. The merit of this designing principle is undoubtedly reflected in our experimental and theoretical results. Receptor 1 selectively recognizes F and AcO anions by naked-eye color change, both in aqueous–acetonitrile solution as well as in solid supported ‘‘test kit’’. UV–vis and 1H NMR titration results can precisely explain both the deprotonation and H-bonding phenomena. Interestingly, UV–vis spectral analysis reveals that 1 (host) in absence of AcOH form 1:2 complexes with fluoride and acetate ion (guest), respectively. But, in presence of AcOH it forms 1:1 H-bonded complex. So, receptor involves both deprotonation and H-bonding interactions in the absence of acid. Furthermore, the proposed binding models of the receptor-anions complexes have been optimized at Density Functional Theory (DFT) level, which corroborates well with the experimental findings.
Experimental Reagents All reagents and solvents were used as received from commercial sources without further purification. Tetrabutylammonium (Bu4 Nþ ) salts of F , AcO , H2 PO 4 , Cl , Br , I , NO3 , NO2 , HSO4 , HSO , and ClO anions and d -DMSO were purchased from Sig6 4 3 ma–Aldrich chemical company. Spectroscopic grade solvents were used for the spectroscopic measurements. Apparatus Electronic absorption spectra have been measured by Hitachi UV–vis (Model U-3501) spectrophotometer. IR spectrum (KBr pellet, 4000–400 cm1) has been recorded on a Parkin Elmer (model 883) infrared spectrophotometer. 1H NMR spectra have been measured by Bruker, Avance 300 spectrometer using d6-DMSO solvent with tetramethylsilane (TMS) as internal standards. Methods used Due to poor solubility of 1 in aqueous–acetonitrile media, the mother solution of the receptor was prepared with solvent mixture (H2O:CH3CN:DMSO = 4:95:1 v/v). This stock solution has been used for all experimental consequence (except for 1H NMR titration). UV–vis titration data were used for the determination of association constants (K) and stoichiometries of the host–guest com-
NO2 + H2N-NH2 CHO
OH
MeOH 25º C
O2N
N
N
NO2
HO
OH
1 Scheme 1. Synthesis of compound 1.
plexes. The binding or association constants (K) have been determined using Benesi–Hildebrand (B–H) relation [19,20]. The ground state optimization at DFT level has been carried out in vacuum with B3LYP-hybride functional and 6-311++G(dp) basis set using Gaussian 03 suit [21]. Synthesis of receptor 1 (N,N/-bis(5-nitro-salicylidene)hydrazine) For the synthesis of receptor 1 (Scheme 1), a methanolic solution (10 ml) of 5-nitro-salicylaldehyde (2.00 gm, 11.9 mmol) was added dropwise to a methanolic solution (5 ml) of hydrazine monohydrate (0.19 ml, 5.83 mmol) with constant stirring at 0 °C. The reaction mixture was stirred for 1 h. at room temperature. A light yellow color solid was precipitated. Filtered off the reaction mixture and washed with cold methanol. The solid (receptor 1) was dried under vacuum to obtain the product in the pure form [22,23]. Yield 82% (1.62 gm, 4.9 mmol). 1H NMR in d6-DMSO, 300 MHz, d (ppm): 7.12 (d, J = 9 Hz, 2H), 8.23 (dd, J = 9 Hz and 2.7 Hz, 2H), 8.66 (d, J = 2.7 Hz, 2H), 9.05 (s, 2H). IR (KBr): 3089 (AOH), 1632 (CH@N), 1574 (C@C), 1234–1486 (ANO2), 969–1099 (NAN) cm1. Result and discussion Visual color change Visual color change of receptor 1 (1.0 105 M) has been investigated in aqueous–acetonitrile solvent. Upon addition of 1–2 equivalents of F/AcO ion in absence of AcOH, the colorless solution of 1 changes to an intense yellow color, which is detectable by naked-eye (Fig. S1). This indicates deprotonation/H-bonding interaction between the receptor and anions. But, in presence of 10 equivalents AcOH, addition of 1–2 equivalent F/AcO ion the colorless solution of 1 changes to light yellow which is also detectable by naked-eye indicating only H-bonding interaction between the receptor and anions. In contrast, H2 PO 4 ion did not exhibit considerable naked-eye color change within the same concentration range as that of F/AcO ion. However, at higher concentration of H2 PO 4 ion a faint yellow color is observed (Fig. S1) in presence of AcOH indicating H-bonding interaction only. Other ions, such as Cl , Br , I , NO 3 , NO2 , HSO4 , HSO3 and ClO4 did not exhibit any naked-eye detectable color change which means the absence of such interactions with these ions. UV–vis spectroscopic titration The anion recognition properties of receptor 1 have also been investigated by monitoring UV–vis spectral change upon addition of different anions in aqueous–acetonitrile solvent. Receptor 1 (1.0 105 M) exhibits strong absorption bands at 288 and 345 nm (Fig. 1, blue color1). Upon addition of increasing amount of F ion, the absorbance at 288 and 345 nm gradually decrease and new red shifted absorption bands at 397 nm and 455 nm 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. UV–vis titration spectra of receptor 1 (1.0 105 M) upon addition of F ion (0–4 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:2 complex formed between 1 and F ion (inset).
gradually appear (Fig. 1, red/orange color, detection limit 5 lM). A distinct isosbestic point at 365 nm is observed during the titration process indicating the existence of equilibrium between the complexed and uncomplexed form of the host (1). The stoichiometry of host–guest interaction has been determined from UV–vis spectral titration with the help of Benesi–Hildebrand (B–H) relation. As shown in Fig. 1 (inset), the B–H plot of 1/[A A0] vs. 1/[F]2 provides a straight line indicating 1:2 complexation. In presence of 10 equivalents AcOH, titration with the continuous addition of fluoride anion, the change in the absorption spectra is almost similar (Fig. 2. detection limit 12 lM) with that in absence of AcOH, but it involves with 1:1 host–guest interaction (B–H plot of 1/[A A0] vs. 1/[F] shows a straight line, Fig. 2 (inset)) with association constant (K) 3.26 104 M1. Similar type of UV–vis spectral changes have been observed in case of AcO (Fig. 3) ion and complex is formed with 1:2 ratio (1:OAc) in absence of AcOH (detection limit 7 lM) and with 1:1 ratio in presence of AcOH (Fig. 4, detection limit 15 lM) with association constant 8.22 103 M1. So, it is noteworthy to mention here that in absence of AcOH, host–guest interaction involves with deprotonation (1 eqv.) and H-bonding (1 eqv.) phenomena, where as in presence of AcOH it involves with only H-bonding interaction (1 eqv.). In contrast, H2 PO 4 ion exhibits only a tiny spectral change even in the presence of 100 times more concentration
Fig. 3. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of AcO ion (0–4 equiv.) in aqueous–acetonitrile solvent and (B–H) plot for 1:2 complex formed between 1 and AcO ion (inset).
Fig. 4. UV–vis titration spectra of receptor 1 (1.5 105 M, in presence of 10 equivalents AcOH) upon addition of OAc ion (0–2 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:1 complex formed between 1 and OAc ion (inset).
Fig. 5. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of H2 PO 4 ion (0–400 equiv.) in aqueous–acetonitrile solvent. Fig. 2. UV–vis titration spectra of receptor 1 (1.5 105 M, in presence of 10 equivalents AcOH) upon addition of F ion (0–2 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:1 complex formed between 1 and F ion (inset).
(Fig. 5). Presence of AcOH is immaterial in case of H2 PO 4 ion, i.e. both (with and without the presence of AcOH) reveal similar results with the addition of H2 PO 4 ion, indicating host–guest H-bonding interac-
S. Dalapati et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 314–318
317
ger complexes; F > AcO H2 PO 4 > Cl , Br , I , NO3 , NO2 , HSO4 , HSO3 , ClO4 [24]. Fluoride anion forms stronger H-bonding complex due its higher basicity.
NMR titration experiments
Fig. 6. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of different anions (F and OAc = 4 equiv., others anions = 400 equiv.) in aqueous– acetonitrile solvent.
tion only. Other anions, such as Cl , Br , I , NO 3 , NO2 , HSO4 , HSO3 , and CIO did not exhibit any notable spectral or color change, indi4 cating the absence of host–guest interactions (Fig. 6). In general, Schiff-bases have a tendency to undergo phenolimine and keto-amine tautomerization equilibrium. In the present case, keto-amine tautomer of 1 is facilitated by the electron withdrawing nitro (ANO2) group present at the para position with respect to the phenolic AOH group, as a result the acidity of phenolic AOH group increases and thereby enhances deprotonation/H-bonding interaction with highly basic fluoride and acetate ions. The deprotonation/H-bonding interaction increases the electron density on the phenolic ‘‘O’’ atom (donor) and a charge transfer from electron rich O atom to electron deficient ANO2 group (acceptor) is facilitated, resulting the appearance of the yellow color [24,25]. The selectivity and sensitivity of receptor 1 toward the anions can be rationalized on the basis of their basicity as well as their structural representations. As the basicity of F and AcO ions are higher with respect to the rest of the anions tested, they are bound to form stronger complexes with receptor 1 and responsible for remarkable color changes as well as UV–vis spectral changes. On the other hand, H2 PO 4 ion with lower basicity and inability of binding with similar fashion due to its tetrahedral geometry, forms weaker complex. Therefore, dihydrogenphosphate anion hardly induces color change and UV–vis spectral changes. However, the color discrimination in case of F and AcO ions did not recognize visually. But, UV–vis spectral changes of 1 in presence of various anions (Fig. 6) clearly discriminate their interaction behaviors. The detail spectral analysis (Fig. 6) and association constants of the host–guest complexation follow the trend as an order of stron-
Furthermore, to investigate the nature of host–guest interaction in the highest polar organized media, 1H NMR titration experiments were performed in d6-DMSO (partial 1H NMR, Figs. S2–S3). The receptor 1 exhibits a broad signal (d) at 12.19 ppm corresponds to the AOH proton in absence of F ion. Upon addition of 0.5 equiv. of F ion, peak of the AOH proton disappears and the aromatic protons Hc and Hd are shifted to upfield from 8.22 to 8.14 and 7.12 to 6.95 ppm, respectively, (Fig. S2) indicating the possibilities of deprotonation as well as H-bonding interactions. The stronger host–guest interaction between 1 and F ion increases the electron density in the aromatic ring which is responsible for the upfield shift of aromatic proton. On further addition of more than two equivalents of fluoride anion, all the aromatic peaks were shifted to upfield, which might be due to the complete deprotonation of phenolic AOH proton. Under similar experimental condition upon titration of receptor 1 with acetate ion, similar type of chemical shift was observed indicating invariable host–guest interactions (Fig. S3). Other competitive anions (Cl, Br, I, NO 3 , NO2 , HSO4 , 1 HSO , and CIO ) hardly induce H NMR chemical shift of receptor 3 4 1 indicating the absence of such interactions. Test kit preparation and sensing behaviors of receptor 1 On the basis of host–guest interaction pattern, concluded from colorimetric, UV–vis and 1H NMR changes; we have shifted our interest about the potential practical applications of 1 as F/AcO anion sensor [9]. To explore this possibility, we have prepared a test paper (Whatman-40) coated with Schiff base (test kit) by dropping aqueous–acetonitrile solution of 1 (1 104 M) and then dried it in air. For the detection of anions, solutions of anions (1 104 M) were dropped onto the test paper (test kit) and then dried it in air. Interestingly, bright yellow color appears on that positions where F and AcO ions were actually dropped (Fig. S4). Other anions, such as H2 PO 4 and Cl exhibit only a faint yellow color at higher concentration, but Br, I, NO 3 , NO2 , HSO , HSO , and ClO ions hardly induce any detectable color 4 3 4 changes. The above experiment describes the potential application of sensor 1 as a test kit for the detection of fluoride and acetate ions. Simple theoretical modeling and optimization The most stable conformers of receptor 1 and the nature of binding interaction with anions have also been predicted with
Fig. 7. Optimized geometry of 1 at DFT level with B3LYP-hybride functional and 6-311++G(dp) basis set.
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Table 1 Some useful theoretical parameters (DFT level with B3LYP functional and 6-311++G(dp) basis set) of receptor 1 and after complexation with anions (A is F and AcO anions). Numbering of atoms is presented in Fig. 7. Objects
H1 N1
H1–O1
C@O1
H2 N2
H2–O2
C@O2
O1 A
O2 A
1 1F 1AcO
1.772 2.640 2.765
0.989 1.610 1.628
1.333 1.255 1.254
1.772 2.640 2.765
0.989 1.610 1.628
1.333 1.255 1.254
– 2.539 2.620
– 2.539 2.620
the help of theoretical calculations. The optimized global minimum of receptor 1 shows that two nitro-salicylidene moieties are situated in s-trans conformation (Fig. 7) and each phenolic AOH group is intramolecularly H-bonded with imine nitrogen (distances H1 N1 and H2 N2 are 1.772 Å and 1.772 Å respectively, Table 1). These kind of structural orientation of the host (1) favors to interact with incoming guest (e.g. fluoride and acetate ion) through opposite direction and thus form 1:2 host–guest complexes (Figs. S5–S6). Importantly, optimized structure of 1F/1AcO complex shows that both ions are preferably deprotonated the highly acidic phenolic AOH protons (O1 F = O2 F = 2.539 Å, O1 AcO = O1 AcO = 2.620 Å, Table 1, Figs. 7 and S5–S6). However, in both complexes (1F and 1AcO) the O1–H1 and O2–H2 band distances are remarkably higher compared to the bare receptor (1, Table 1). And side by side the keto-bond (C–O1 and C–O2 bonds) distances are decreased (get more double bond character, Table 1). These bond variations clearly indicate that host–guest complex formation promotes the receptor 1 to undergo phenolimine to keto-amine tautomerization process. Conclusions In summary, we have demonstrated an easily prepared Schiff base 1 [N,N/-bis(5-nitro-salicylidene)hydrazine] coated test paper (test kit) that can selectively detect F and AcO ions without the help of any spectroscopic instrument by simply developing naked-eye detectable bright yellow color in aqueous–acetonitrile medium. The formation of host–guest complex facilitates the charge transfer process between phenolic oxygen and the electron withdrawing nitro group, namely: conversion of phenol-imine to keto-amine tautomer. In the presence of AcOH, 1:1 host–guest complexation occurs (detection limit belongs to 12–15 lM), indicating H-bonding complex formation. On the other hand, absence of AcOH, 1:2 host–guest complexation occurs (detection limit belongs to 5–7 lM) indicating deprotonation as well as H-bonding interactions. Bathochromic spectral shift in the UV–vis titration and chemical shift in 1H NMR titration strongly admit the host– guest interactions. Furthermore, predicted stoichiometries of host–guest complexes based on Density Functional Theory (DFT) level optimization corroborates well with the experimental findings. This easy-to-prepared and cheap test kits would always be advantageous to detect F and AcO ions in economically poor and undeveloped regions in the world.
Acknowledgments This work is supported by grants from DST, India (Project No. SR/S1/PC/26/2008) and CSIR, India (Project No. 01(2161)07/EMRII) to NG and UGC, India (Project No. PSW-194/11-12 (ERO)) to SK. SD and SJ would like to acknowledge UGC for Fellowship. MAA thank to the VC of AU for giving him permission to work at CU. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.10.038. References [1] F. Zapata, A. Caballero, A. Espinosa, A. Tárraga, P. Molina, J. Org. Chem. 73 (2008) 4034–4044. [2] D. Saravanakumar, S. Devaraj, S. Iyyampillai, K. Mohandoss, M. Kandaswamy, Tetrahedron Lett. 49 (2008) 127–132. [3] X.-F. Shang, Spectrochim. Acta Part: A 72 (2009) 1117–1121. [4] S. Devaraj, D. Saravanakumar, M. Kandaswamy, Tetrahedron Lett. 48 (2007) 3077–3081. [5] V. Bhalla, R. Tejpal, M. Kumar, Tetrahedron 67 (2011) 1266–1271. [6] P. Das, A.K. Mandal, M.K. Kesharwani, E. Suresh, B. Ganguly, A. Das, Chem. Commun. 47 (2011) 7398–7400. [7] R. Sheng, P. Wang, Y. Gao, Y. Wu, W. Liu, J. Ma, H. Li, S. Wu, Org. Lett. 10 (2008) 5015. [8] Y. Fu, H. Li, W. Hu, Eur. J. Org. Chem. (2007) 2459–2463. [9] Z. Lin, Y. Zhao, C. Duan, B. Zhang, Z. Bai, Dalton. Trans. 11 (2006) 3678–3684. [10] R.M. Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476. [11] P.A. Gale, R. Quesada, Coord. Chem. Rev. 250 (2006) 3219–3244. [12] F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609–1646. [13] C.D. Geddes, Meas. Sci. Technol. 12 (2001) R53–R88. [14] M. Keerekoper, Endocrinol. Metab. Clin. North Am. 27 (1998) 441–452. [15] S. Dalapati, S. Jana, N. Guchhait, Chem. Lett. 40 (2011) 279–281. [16] Q. Li, Y. Guo, J. Xu, S. Shao, Sens. Actuators B 158 (2011) 427–431. [17] H. Miyaji, J.L. Sessler, Angew. Chem. Int. Ed. 40 (2001) 154–157. [18] M. Boiocchi, L.D. Boca, D.E. Gomez, L. Fabbrizzi, M. Licchelli, E. Monzani, J. Am. Chem. Soc. 126 (2004) 16507–16514. [19] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [20] I.D. Kuntz, F.P. Gasparro, M.D. Johnston, R.P. Taylor, J. Am. Chem. Soc. 90 (1968) 4778–4781. [21] M.J. Frisch, Gaussian 03 Revision B 03, Gaussian Inc., Pittsburgh (PA), 2003. [22] S. Dalapati, M.A. Alam, S. Jana, N. Guchhait, J. Fluorine Chem. 132 (2011) 536– 540. [23] S. Dalapati, B.K. Paul, S. Jana, N. Guchhait, Sens. Actuators, B 157 (2011) 615– 620. [24] Y.M. Hijji, B. Barare, A.P. Kennedy, R. Butcher, Sens. Actuators, B 136 (2009) 297–302. [25] X. Bao, J. Yu, Y. Zhou, Sens. Actuators, B 140 (2009) 467–472.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
‘‘Test kit’’ for detection of biologically important anions: A salicylidene-hydrazine based Schiff base Sasanka Dalapati a, Md Akhtarul Alam b,⇑, Sankar Jana a, Saswati Karmakar c, Nikhil Guchhait a,⇑ a
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India Department of Chemistry, Aliah University, DN-41 & 47, Sector-V, Saltlake, Kolkata 700 091, India c Department of Chemistry, Sree Chaitanya College, Habra, North 24 Parganas, West Bengal, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" ‘‘Test kit’’ was prepared for the
detection of biologically important anions. " ‘‘Test kit’’ can selectively detect F and AcO ions. " Color change of the ‘‘test kit’’ in presence of anions were detected by naked-eye. " Predicted complexation behavior from theoretical structural optimization corroborates with the experimental results.
a r t i c l e
i n f o
Article history: Received 13 August 2012 Received in revised form 15 October 2012 Accepted 23 October 2012 Available online 1 November 2012 Keywords: Test kit Schiff base Colorimetric Aqueous–acetonitrile DFT calculation Anion detection
a b s t r a c t Test paper coated with Schiff base [(N,N/-bis(5-nitro-salicylidene)hydrazine] receptor 1 (host) can selectively detect fluoride and acetate ions (guest) by developing yellow color which can be detected by naked-eye both in aqueous–acetonitrile solution and in solid supported test kit. UV–vis spectral analysis shows that the absorption peaks at 288 and 345 nm of receptor 1 gradually decrease its initial intensity and new red shifted absorption bands at 397 nm and 455 nm gradually appear upon addition of increas ing amount of F and AcO ions over several tested anions such as H2 PO 4 , Cl , Br , I , NO3 , NO2 , HSO4 , HSO3 , and ClO4 in aqueous–acetonitrile solvent. The colorimetric test results and UV–vis spectral analysis are in well agreement with 1H NMR titration results in d6-DMSO solvent. The receptor 1 forms 1:2 stable complexes with F and AcO ions. However, similar kind of observation obtained from UV–vis titrations in presence of AcOH corresponds to 1:1 complexation ratio indicating the formation of H-bonding interaction between the receptor and anions (F and AcO ions). So, the observed 1:2 complexation ratio can only be explained on the basis of deprotonation (1 eqv.) and H-bonding (1 eqv.) interactions [1]. The ratiometric analysis of host–guest complexes corroborates well with the proposed theoretical model optimization at Density Functional Theory (DFT) level. Ó 2012 Elsevier B.V. All rights reserved.
Introduction
⇑ Corresponding authors. Tel.: +91 33 23508386; fax: +91 33 23519755 (N. Guchhait). E-mail addresses: [email protected] (M.A. Alam), [email protected] (N. Guchhait). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.10.038
There are a few examples of test kits which are reported in the current chemical literature for the detection of ions (cation or anion). Importantly, the colorimetric detection of ions by a cheap method is gaining faster attention in the recent development of chemosensors design [2–6]. It is found that small quantities of ions
315
S. Dalapati et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 314–318
play an important role both in the environmental and biological systems [7–12]. However, excess amount of ions may cause of several serious diseases as well as immune system disruptions [13– 15]. Because of significant importance, detection of ions with the help of easily synthesized receptor and minimal instrumental assistance be always welcomed for the purpose of practical applications. Recently, Li and co-workers reported and well established a receptor (as our receptor 1, Scheme 1) without ANO2 signaling unit capable for selective sensing of F ion in DMSO solvent [16]. Due to the problem of practical applicability of their receptor, here we report nitro substituted Schiff-base receptor 1 by considering three important factors: (i) the nitro group (ANO2) having electron withdrawing effect is expected to enhance the acidity of the phenolic AOH group and as a consequence the hydrogen donor efficiency of the receptor 1 is also increased, (ii) the stronger acidity of phenolic AOH group may enhance its sensitivity and thus useful in aqueous environment, (iii) UV–vis and 1H NMR absorption properties of the chromogenic nitro-phenyl moiety may be altered by receptor–anions interaction and thus promote the colorimetric and spectral sensing recognition events [17,18]. The merit of this designing principle is undoubtedly reflected in our experimental and theoretical results. Receptor 1 selectively recognizes F and AcO anions by naked-eye color change, both in aqueous–acetonitrile solution as well as in solid supported ‘‘test kit’’. UV–vis and 1H NMR titration results can precisely explain both the deprotonation and H-bonding phenomena. Interestingly, UV–vis spectral analysis reveals that 1 (host) in absence of AcOH form 1:2 complexes with fluoride and acetate ion (guest), respectively. But, in presence of AcOH it forms 1:1 H-bonded complex. So, receptor involves both deprotonation and H-bonding interactions in the absence of acid. Furthermore, the proposed binding models of the receptor-anions complexes have been optimized at Density Functional Theory (DFT) level, which corroborates well with the experimental findings.
Experimental Reagents All reagents and solvents were used as received from commercial sources without further purification. Tetrabutylammonium (Bu4 Nþ ) salts of F , AcO , H2 PO 4 , Cl , Br , I , NO3 , NO2 , HSO4 , HSO , and ClO anions and d -DMSO were purchased from Sig6 4 3 ma–Aldrich chemical company. Spectroscopic grade solvents were used for the spectroscopic measurements. Apparatus Electronic absorption spectra have been measured by Hitachi UV–vis (Model U-3501) spectrophotometer. IR spectrum (KBr pellet, 4000–400 cm1) has been recorded on a Parkin Elmer (model 883) infrared spectrophotometer. 1H NMR spectra have been measured by Bruker, Avance 300 spectrometer using d6-DMSO solvent with tetramethylsilane (TMS) as internal standards. Methods used Due to poor solubility of 1 in aqueous–acetonitrile media, the mother solution of the receptor was prepared with solvent mixture (H2O:CH3CN:DMSO = 4:95:1 v/v). This stock solution has been used for all experimental consequence (except for 1H NMR titration). UV–vis titration data were used for the determination of association constants (K) and stoichiometries of the host–guest com-
NO2 + H2N-NH2 CHO
OH
MeOH 25º C
O2N
N
N
NO2
HO
OH
1 Scheme 1. Synthesis of compound 1.
plexes. The binding or association constants (K) have been determined using Benesi–Hildebrand (B–H) relation [19,20]. The ground state optimization at DFT level has been carried out in vacuum with B3LYP-hybride functional and 6-311++G(dp) basis set using Gaussian 03 suit [21]. Synthesis of receptor 1 (N,N/-bis(5-nitro-salicylidene)hydrazine) For the synthesis of receptor 1 (Scheme 1), a methanolic solution (10 ml) of 5-nitro-salicylaldehyde (2.00 gm, 11.9 mmol) was added dropwise to a methanolic solution (5 ml) of hydrazine monohydrate (0.19 ml, 5.83 mmol) with constant stirring at 0 °C. The reaction mixture was stirred for 1 h. at room temperature. A light yellow color solid was precipitated. Filtered off the reaction mixture and washed with cold methanol. The solid (receptor 1) was dried under vacuum to obtain the product in the pure form [22,23]. Yield 82% (1.62 gm, 4.9 mmol). 1H NMR in d6-DMSO, 300 MHz, d (ppm): 7.12 (d, J = 9 Hz, 2H), 8.23 (dd, J = 9 Hz and 2.7 Hz, 2H), 8.66 (d, J = 2.7 Hz, 2H), 9.05 (s, 2H). IR (KBr): 3089 (AOH), 1632 (CH@N), 1574 (C@C), 1234–1486 (ANO2), 969–1099 (NAN) cm1. Result and discussion Visual color change Visual color change of receptor 1 (1.0 105 M) has been investigated in aqueous–acetonitrile solvent. Upon addition of 1–2 equivalents of F/AcO ion in absence of AcOH, the colorless solution of 1 changes to an intense yellow color, which is detectable by naked-eye (Fig. S1). This indicates deprotonation/H-bonding interaction between the receptor and anions. But, in presence of 10 equivalents AcOH, addition of 1–2 equivalent F/AcO ion the colorless solution of 1 changes to light yellow which is also detectable by naked-eye indicating only H-bonding interaction between the receptor and anions. In contrast, H2 PO 4 ion did not exhibit considerable naked-eye color change within the same concentration range as that of F/AcO ion. However, at higher concentration of H2 PO 4 ion a faint yellow color is observed (Fig. S1) in presence of AcOH indicating H-bonding interaction only. Other ions, such as Cl , Br , I , NO 3 , NO2 , HSO4 , HSO3 and ClO4 did not exhibit any naked-eye detectable color change which means the absence of such interactions with these ions. UV–vis spectroscopic titration The anion recognition properties of receptor 1 have also been investigated by monitoring UV–vis spectral change upon addition of different anions in aqueous–acetonitrile solvent. Receptor 1 (1.0 105 M) exhibits strong absorption bands at 288 and 345 nm (Fig. 1, blue color1). Upon addition of increasing amount of F ion, the absorbance at 288 and 345 nm gradually decrease and new red shifted absorption bands at 397 nm and 455 nm 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. UV–vis titration spectra of receptor 1 (1.0 105 M) upon addition of F ion (0–4 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:2 complex formed between 1 and F ion (inset).
gradually appear (Fig. 1, red/orange color, detection limit 5 lM). A distinct isosbestic point at 365 nm is observed during the titration process indicating the existence of equilibrium between the complexed and uncomplexed form of the host (1). The stoichiometry of host–guest interaction has been determined from UV–vis spectral titration with the help of Benesi–Hildebrand (B–H) relation. As shown in Fig. 1 (inset), the B–H plot of 1/[A A0] vs. 1/[F]2 provides a straight line indicating 1:2 complexation. In presence of 10 equivalents AcOH, titration with the continuous addition of fluoride anion, the change in the absorption spectra is almost similar (Fig. 2. detection limit 12 lM) with that in absence of AcOH, but it involves with 1:1 host–guest interaction (B–H plot of 1/[A A0] vs. 1/[F] shows a straight line, Fig. 2 (inset)) with association constant (K) 3.26 104 M1. Similar type of UV–vis spectral changes have been observed in case of AcO (Fig. 3) ion and complex is formed with 1:2 ratio (1:OAc) in absence of AcOH (detection limit 7 lM) and with 1:1 ratio in presence of AcOH (Fig. 4, detection limit 15 lM) with association constant 8.22 103 M1. So, it is noteworthy to mention here that in absence of AcOH, host–guest interaction involves with deprotonation (1 eqv.) and H-bonding (1 eqv.) phenomena, where as in presence of AcOH it involves with only H-bonding interaction (1 eqv.). In contrast, H2 PO 4 ion exhibits only a tiny spectral change even in the presence of 100 times more concentration
Fig. 3. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of AcO ion (0–4 equiv.) in aqueous–acetonitrile solvent and (B–H) plot for 1:2 complex formed between 1 and AcO ion (inset).
Fig. 4. UV–vis titration spectra of receptor 1 (1.5 105 M, in presence of 10 equivalents AcOH) upon addition of OAc ion (0–2 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:1 complex formed between 1 and OAc ion (inset).
Fig. 5. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of H2 PO 4 ion (0–400 equiv.) in aqueous–acetonitrile solvent. Fig. 2. UV–vis titration spectra of receptor 1 (1.5 105 M, in presence of 10 equivalents AcOH) upon addition of F ion (0–2 equiv.) in aqueous–acetonitrile solvent and Benesi–Hildebrand (B–H) plot for 1:1 complex formed between 1 and F ion (inset).
(Fig. 5). Presence of AcOH is immaterial in case of H2 PO 4 ion, i.e. both (with and without the presence of AcOH) reveal similar results with the addition of H2 PO 4 ion, indicating host–guest H-bonding interac-
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ger complexes; F > AcO H2 PO 4 > Cl , Br , I , NO3 , NO2 , HSO4 , HSO3 , ClO4 [24]. Fluoride anion forms stronger H-bonding complex due its higher basicity.
NMR titration experiments
Fig. 6. UV–vis spectral changes of receptor 1 (1.0 105 M) upon addition of different anions (F and OAc = 4 equiv., others anions = 400 equiv.) in aqueous– acetonitrile solvent.
tion only. Other anions, such as Cl , Br , I , NO 3 , NO2 , HSO4 , HSO3 , and CIO did not exhibit any notable spectral or color change, indi4 cating the absence of host–guest interactions (Fig. 6). In general, Schiff-bases have a tendency to undergo phenolimine and keto-amine tautomerization equilibrium. In the present case, keto-amine tautomer of 1 is facilitated by the electron withdrawing nitro (ANO2) group present at the para position with respect to the phenolic AOH group, as a result the acidity of phenolic AOH group increases and thereby enhances deprotonation/H-bonding interaction with highly basic fluoride and acetate ions. The deprotonation/H-bonding interaction increases the electron density on the phenolic ‘‘O’’ atom (donor) and a charge transfer from electron rich O atom to electron deficient ANO2 group (acceptor) is facilitated, resulting the appearance of the yellow color [24,25]. The selectivity and sensitivity of receptor 1 toward the anions can be rationalized on the basis of their basicity as well as their structural representations. As the basicity of F and AcO ions are higher with respect to the rest of the anions tested, they are bound to form stronger complexes with receptor 1 and responsible for remarkable color changes as well as UV–vis spectral changes. On the other hand, H2 PO 4 ion with lower basicity and inability of binding with similar fashion due to its tetrahedral geometry, forms weaker complex. Therefore, dihydrogenphosphate anion hardly induces color change and UV–vis spectral changes. However, the color discrimination in case of F and AcO ions did not recognize visually. But, UV–vis spectral changes of 1 in presence of various anions (Fig. 6) clearly discriminate their interaction behaviors. The detail spectral analysis (Fig. 6) and association constants of the host–guest complexation follow the trend as an order of stron-
Furthermore, to investigate the nature of host–guest interaction in the highest polar organized media, 1H NMR titration experiments were performed in d6-DMSO (partial 1H NMR, Figs. S2–S3). The receptor 1 exhibits a broad signal (d) at 12.19 ppm corresponds to the AOH proton in absence of F ion. Upon addition of 0.5 equiv. of F ion, peak of the AOH proton disappears and the aromatic protons Hc and Hd are shifted to upfield from 8.22 to 8.14 and 7.12 to 6.95 ppm, respectively, (Fig. S2) indicating the possibilities of deprotonation as well as H-bonding interactions. The stronger host–guest interaction between 1 and F ion increases the electron density in the aromatic ring which is responsible for the upfield shift of aromatic proton. On further addition of more than two equivalents of fluoride anion, all the aromatic peaks were shifted to upfield, which might be due to the complete deprotonation of phenolic AOH proton. Under similar experimental condition upon titration of receptor 1 with acetate ion, similar type of chemical shift was observed indicating invariable host–guest interactions (Fig. S3). Other competitive anions (Cl, Br, I, NO 3 , NO2 , HSO4 , 1 HSO , and CIO ) hardly induce H NMR chemical shift of receptor 3 4 1 indicating the absence of such interactions. Test kit preparation and sensing behaviors of receptor 1 On the basis of host–guest interaction pattern, concluded from colorimetric, UV–vis and 1H NMR changes; we have shifted our interest about the potential practical applications of 1 as F/AcO anion sensor [9]. To explore this possibility, we have prepared a test paper (Whatman-40) coated with Schiff base (test kit) by dropping aqueous–acetonitrile solution of 1 (1 104 M) and then dried it in air. For the detection of anions, solutions of anions (1 104 M) were dropped onto the test paper (test kit) and then dried it in air. Interestingly, bright yellow color appears on that positions where F and AcO ions were actually dropped (Fig. S4). Other anions, such as H2 PO 4 and Cl exhibit only a faint yellow color at higher concentration, but Br, I, NO 3 , NO2 , HSO , HSO , and ClO ions hardly induce any detectable color 4 3 4 changes. The above experiment describes the potential application of sensor 1 as a test kit for the detection of fluoride and acetate ions. Simple theoretical modeling and optimization The most stable conformers of receptor 1 and the nature of binding interaction with anions have also been predicted with
Fig. 7. Optimized geometry of 1 at DFT level with B3LYP-hybride functional and 6-311++G(dp) basis set.
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Table 1 Some useful theoretical parameters (DFT level with B3LYP functional and 6-311++G(dp) basis set) of receptor 1 and after complexation with anions (A is F and AcO anions). Numbering of atoms is presented in Fig. 7. Objects
H1 N1
H1–O1
C@O1
H2 N2
H2–O2
C@O2
O1 A
O2 A
1 1F 1AcO
1.772 2.640 2.765
0.989 1.610 1.628
1.333 1.255 1.254
1.772 2.640 2.765
0.989 1.610 1.628
1.333 1.255 1.254
– 2.539 2.620
– 2.539 2.620
the help of theoretical calculations. The optimized global minimum of receptor 1 shows that two nitro-salicylidene moieties are situated in s-trans conformation (Fig. 7) and each phenolic AOH group is intramolecularly H-bonded with imine nitrogen (distances H1 N1 and H2 N2 are 1.772 Å and 1.772 Å respectively, Table 1). These kind of structural orientation of the host (1) favors to interact with incoming guest (e.g. fluoride and acetate ion) through opposite direction and thus form 1:2 host–guest complexes (Figs. S5–S6). Importantly, optimized structure of 1F/1AcO complex shows that both ions are preferably deprotonated the highly acidic phenolic AOH protons (O1 F = O2 F = 2.539 Å, O1 AcO = O1 AcO = 2.620 Å, Table 1, Figs. 7 and S5–S6). However, in both complexes (1F and 1AcO) the O1–H1 and O2–H2 band distances are remarkably higher compared to the bare receptor (1, Table 1). And side by side the keto-bond (C–O1 and C–O2 bonds) distances are decreased (get more double bond character, Table 1). These bond variations clearly indicate that host–guest complex formation promotes the receptor 1 to undergo phenolimine to keto-amine tautomerization process. Conclusions In summary, we have demonstrated an easily prepared Schiff base 1 [N,N/-bis(5-nitro-salicylidene)hydrazine] coated test paper (test kit) that can selectively detect F and AcO ions without the help of any spectroscopic instrument by simply developing naked-eye detectable bright yellow color in aqueous–acetonitrile medium. The formation of host–guest complex facilitates the charge transfer process between phenolic oxygen and the electron withdrawing nitro group, namely: conversion of phenol-imine to keto-amine tautomer. In the presence of AcOH, 1:1 host–guest complexation occurs (detection limit belongs to 12–15 lM), indicating H-bonding complex formation. On the other hand, absence of AcOH, 1:2 host–guest complexation occurs (detection limit belongs to 5–7 lM) indicating deprotonation as well as H-bonding interactions. Bathochromic spectral shift in the UV–vis titration and chemical shift in 1H NMR titration strongly admit the host– guest interactions. Furthermore, predicted stoichiometries of host–guest complexes based on Density Functional Theory (DFT) level optimization corroborates well with the experimental findings. This easy-to-prepared and cheap test kits would always be advantageous to detect F and AcO ions in economically poor and undeveloped regions in the world.
Acknowledgments This work is supported by grants from DST, India (Project No. SR/S1/PC/26/2008) and CSIR, India (Project No. 01(2161)07/EMRII) to NG and UGC, India (Project No. PSW-194/11-12 (ERO)) to SK. SD and SJ would like to acknowledge UGC for Fellowship. MAA thank to the VC of AU for giving him permission to work at CU. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.10.038. References [1] F. Zapata, A. Caballero, A. Espinosa, A. Tárraga, P. Molina, J. Org. Chem. 73 (2008) 4034–4044. [2] D. Saravanakumar, S. Devaraj, S. Iyyampillai, K. Mohandoss, M. Kandaswamy, Tetrahedron Lett. 49 (2008) 127–132. [3] X.-F. Shang, Spectrochim. Acta Part: A 72 (2009) 1117–1121. [4] S. Devaraj, D. Saravanakumar, M. Kandaswamy, Tetrahedron Lett. 48 (2007) 3077–3081. [5] V. Bhalla, R. Tejpal, M. Kumar, Tetrahedron 67 (2011) 1266–1271. [6] P. Das, A.K. Mandal, M.K. Kesharwani, E. Suresh, B. Ganguly, A. Das, Chem. Commun. 47 (2011) 7398–7400. [7] R. Sheng, P. Wang, Y. Gao, Y. Wu, W. Liu, J. Ma, H. Li, S. Wu, Org. Lett. 10 (2008) 5015. [8] Y. Fu, H. Li, W. Hu, Eur. J. Org. Chem. (2007) 2459–2463. [9] Z. Lin, Y. Zhao, C. Duan, B. Zhang, Z. Bai, Dalton. Trans. 11 (2006) 3678–3684. [10] R.M. Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476. [11] P.A. Gale, R. Quesada, Coord. Chem. Rev. 250 (2006) 3219–3244. [12] F.P. Schmidtchen, M. Berger, Chem. Rev. 97 (1997) 1609–1646. [13] C.D. Geddes, Meas. Sci. Technol. 12 (2001) R53–R88. [14] M. Keerekoper, Endocrinol. Metab. Clin. North Am. 27 (1998) 441–452. [15] S. Dalapati, S. Jana, N. Guchhait, Chem. Lett. 40 (2011) 279–281. [16] Q. Li, Y. Guo, J. Xu, S. Shao, Sens. Actuators B 158 (2011) 427–431. [17] H. Miyaji, J.L. Sessler, Angew. Chem. Int. Ed. 40 (2001) 154–157. [18] M. Boiocchi, L.D. Boca, D.E. Gomez, L. Fabbrizzi, M. Licchelli, E. Monzani, J. Am. Chem. Soc. 126 (2004) 16507–16514. [19] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [20] I.D. Kuntz, F.P. Gasparro, M.D. Johnston, R.P. Taylor, J. Am. Chem. Soc. 90 (1968) 4778–4781. [21] M.J. Frisch, Gaussian 03 Revision B 03, Gaussian Inc., Pittsburgh (PA), 2003. [22] S. Dalapati, M.A. Alam, S. Jana, N. Guchhait, J. Fluorine Chem. 132 (2011) 536– 540. [23] S. Dalapati, B.K. Paul, S. Jana, N. Guchhait, Sens. Actuators, B 157 (2011) 615– 620. [24] Y.M. Hijji, B. Barare, A.P. Kennedy, R. Butcher, Sens. Actuators, B 136 (2009) 297–302. [25] X. Bao, J. Yu, Y. Zhou, Sens. Actuators, B 140 (2009) 467–472.