Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 569–574
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective and sensitive receptor for Fe3+ probing Umesh Fegade a,b, Ajnesh Singh c, G. Krishna Chaitanya d, Narinder Singh c, Sanjay Attarde b, Anil Kuwar a,⇑ a
School of Chemical Sciences, North Maharashtra University, Jalgaon 425001, MS, India School of Environmental and Earth Sciences, North Maharashtra University, Jalgaon 425001, MS, India c Department of Chemistry, Indian Institute of Technology, Ropar, Rupnagar 140001, Punjab, India d School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431 606, MS, 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
A new fluorescent urea and thiol
based chemosensor was developed and synthesized. Chemosensor showed excellent selectivity for Fe3+ ions over other interfering ions. 3+ Chemosensor can detect Fe up to 0.1 lM in semi-aqueous medium (DMSO/H2O, 8:2, v/v). Experimental results have been supported by DFT calculations.
a r t i c l e
i n f o
Article history: Received 28 August 2013 Received in revised form 29 October 2013 Accepted 3 November 2013 Available online 11 November 2013 Keywords: 1,10 -(4-Methylbenzene-1,3-diyl)bis[3-(2sulfanylphenyl)urea] Fluorescence receptor Binding constant Metal ions Fe3+ sensor
a b s t r a c t A new fluorescent receptor 1,10 -(4-methylbenzene-1,3-diyl)bis[3-(2-sulfanylphenyl)urea] (1) has been designed and synthesized. The receptor showed excellent selectivity for Fe3+ in DMSO/H2O (8:2, v/v) solvent system over other commonly coexistent metal ions. The binding constant (Ka) of receptor with Fe3+ was calculated to be 11,250 M1, 12,970 M1 and 12,970 M1 using Benesi–Hildebrand, Scatchard and Connor plot, respectively. The experimental results have been further supported by the detailed DFT calculations. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Development of synthetic chemosensor for selective sensing of metal ions in semi-aqueous medium is receiving augmented attention of chemists working in supramolecular chemistry. Design and synthesis of chemosensors for metal ion are mainly based on monitoring the changes in photo-physical properties of receptor in ⇑ Corresponding author. Tel.: +91 257 2257432; fax: +91 257 225740. E-mail address: [email protected] (A. Kuwar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.007
presence of a guest. The design of a chemosensor should abide to some basic requirements such as exhibiting a qualitative response to a particular analyte and quantitative determination for a wide concentration range of analyte [1–7]. In recent year, number of chemosensors has been synthesized for the alkali and alkaline earth metals However, there are relatively few examples of designed chemosensors for the heavy metals of first transition series which are the abundant elements in earth crust and also in human body [8,9]. These metal ions play crucial role in chemistry, biology, environment and medicine
U. Fegade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 569–574
[10]. However, cations can be toxic if the level exceeds cellular needs. Iron is one such metal which is associated with number of biological processes. Most of the iron present in biological systems is associated with enzymes, specialized transport and storage proteins (haemoglobin, the red pigment in the erythrocytes and ferretin). It plays a vital role in oxygen transfer processes in DNA and RNA synthesis. The deficiency of iron can cause anaemia, hemochromatosis, liver damage, diabetes, Parkinson’s disease and cancer. Accumulation of iron and subsequent over production of H2O2 in tissues is the main reason for oxidative stress and neurodegenerative diseases [11–16]. This paper reports the synthesis of receptor 1 and its use as sensor for Fe3+ ions in presence of various metal ions (Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+) in DMSO/H2O (8:2, v/v). In presence of Fe3+ ion, fluorescence quenching was observed and binding constant (Ka) of receptor with Fe3+ was calculated using Benesi–Hildebrand, Scatchard and Connor plots. The sensing ability of chemosensor for Fe3+ with ‘‘fluorescence off’’ mechanism in semi aqueous phase make it interesting candidate for its application in area of biology, environmental and material chemistry.
Experimental All commercial grade chemicals and solvents were used without further purification. 1H and 13C NMR spectra were recorded on a Varian NMR mercury System 300 spectrometer operating at 300 MHz for 1H NMR and 75 MHz for 13C NMR in DMSO-d6 respectively. The fluorescence and UV–visible spectra were recorded in DMSO/H2O (8:2, v/v) with Fluoromax-4 spectrofluorometer and Shimadzu UV-24500 at room temperature.
Synthesis of 1,1’-(4-methylbenzene-1,3-diyl)bis[3-(2sulfanylphenyl)urea] A mixture of 2,4-diisocyanato-1-methylbenzene (0.174 g, 1 mmol) and 2-aminothiophenol (0.250 g, 2 mmol) was refluxed in acetone with constant stirring for 1 h (Scheme 1). The white coloured precipitated product obtained was filtered and purified by recrystallization from DMF. White solid, Yield 76%, mp P 250 °C, Soluble in DMSO, 1H NMR (300 MHz, DMSO-d6, ppm) d = 2.39 (s, 3H, Ar-CH3), 7.24–7.29 (d, J = 14.7, 2H, Ar-H), 7.81–8.01 (m, 8H, Ar-H), 8.18 (s, 1H, Ar-H), 9.75 (s, 4H, NH amide); 13C NMR (75 MHz, DMSO-d6, ppm) d = 14.2, 113.7, 115.9, 116.2, 121.0, 123.2, 126.2, 129.8, 129.2, 132.5, 135.2, 149.9, 154.7. FT IR (KBr pellet, cm1) t = 740, 868, 1026, 1528, 1642, 2918, 3252. ESI-MS m/z = 423.82, calc. for C21H20N4O2S2 = 424.10.
Results and discussion Recognition studies Response of receptor 1 towards various metal ions was monitored in DMSO/H2O (8:2, v/v) solvent system using fluorescence spectroscopy. Fig. 1 illustrates the changes in emission profile of 1 in DMSO/H2O (8:2, v/v) upon addition of various metal salts. Interestingly, addition of Fe3+ to host solution resulted significant quenching in broad emission band centered at 350 nm in case of host solution (kex 289 nm). Other metal ions have not produced any significant changes in emission profile as depicted in Fig. 1. It is believed that fluorescence intensity of receptor was quenched due to interactions of different binding sites with Fe3+. Chemosensor 1 have two urea and two thiol groups which plays an important role in the binding of Fe3+. The binding pattern of 1 describes the importance of thiol and urea groups in Fe+3 binding. The thiol group is frequently considered a good binding site and plays an important role in binding of transition metal ions such as Fe+3 metal ion. Therefore, it appears that the core functionality required for 1 to efficiently bind with Fe+3 upon excitation are urea and thiol groups. The quenching of fluorescence intensity at 350 nm on addition of Fe3+ was due to cooperative influence of size compatibility and paramagnetic nature of the analyte which result in chelation enhanced quenching (CHEQ). In such metal complexes forbidden intersystem crossing (isc) become faster due to the presence of a paramagnetic atom (the metal ion) in the proximity of the fluorophore which result in fluorescence quenching. Moreover
1.4×106
Fluorescence Intensity (cps)
570
1.2×106 1.0×105 Host 5
8×10
3+
6×10
2×105 0 300
350
400
NCO
SH
450
Wavelength (nm) Fig. 1. Emission profile of receptor 1 (70 lM, kex = 289 nm) upon the addition of a particular metal salt (700 lM) in DMSO/H2O (8:2, v/v). Inset: Photograph of chemosensor 1 and 1.Fe3+ under the effect of UV light (365 nm).
Acetone, reflux 1h
OCN
2+
Fe3+
HN
2
2+
4×105
NH
NH2 +
2+
Cr , Mn , Fe ,Co , 2+ 2+ 2+ 2+ Ni , Cu ,Zn , Cd , 2+ 2+ 3+ Hg , Pb and Bi
5
HN
O
O
SH
HS
1 Scheme 1. Synthetic procedure for synthesis of receptor 1.
NH
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1.4×106
Fluorescence Intensity (cps)
0.8
0.6
Io-I/Io
0.4
0.2
0
Bi(III)
Pb(II)
Hg(II)
Zn(II)
Cd(II)
Cu(II)
Ni(II)
Co(II)
Fe(II)
Mn(II)
Cr(III)
Fe(III)
-0.4
Host
-0.2
1.2×106 1.0×105 8×105 6×105 4×105 2×105 0 300
350
400
450
Wavelength (nm)
Metal Ions Fig. 2a. Fluorescence ratiometric response (I0 I/I0) of receptor 1 (70 lM) upon the addition of a particular metal salts (700 lM) in DMSO/H2O (8:2, v/v).
the sensor is selective for Fe3+ over Fe2+ which may be due to the incompatible coordination sphere of sensor for Fe2+ in terms of size, shape and steric factors [8,17]. Fluorescence ratiometric response of receptor 1 and interference toward the studied metal ions is shown in Figs. 2a and 2b. The results showed that the receptor 1 is highly selective towards Fe3+ ions as compared to the other metal ions such as transition-metal and heavy-metal ions (Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+) with which it have shown weak quenching of fluorescence intensity. To gain more insight, the titration of receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) with Fe3+ was performed (Fig. 3). The successive increased amounts (0–10 equiv.) of Fe3+ to receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) resulted in a continuous decrease in emission band at 350 nm (kex = 289 nm). Normalized response of fluorescence signal to changing Fe+3 concentrations in the DMSO/H2O (80:20, v/v) solution shown in Fig. 4. The detection limit for Fe3+ was calculated to be as low as 0.1 lM. The stoichiometry was determined by the continuous variation method (Job plot) [18]. Fig. 5 shows the Job plot between [HG] and {[H]/([H] + [G])} where the total concentration of host
Competing ion Competing ion + Fe(III)
Fluorescence Intensity (cps)
1.4×106
Fig. 3. Changes in fluorescence spectra (kex = 289 nm) of receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) upon addition of Fe3+ from (0–10 equiv.).
Fig. 4. Normalized response of fluorescence signal to changing Fe3+ concentrations in DMSO/H2O (8:2, v/v) (chemosensor 1; 70 lM and Fe3+ salt; 0–10 equiv.).
and guest was constant and molar fraction of host was continuously varied. The results indicated the formation of a 1:1 (host:guest) complex. Using the equation: [G]tot = a/2K11(1 a)2[H]tot + a[H]tot/ 2, where [G]tot is total concentration of guest, [H]tot is the total concentration of host, a = (I I0)/(Ii I0) with I being the fluorescence intensity at a particular Fe3+ concentration while I0 and Ii are the intensities at zero and infinite Fe3+ concentrations, respectively. The association constant (Ka) was calculated by three different methods i.e. Benesi–Hildebrand [19] (Eq. (1)), Scatchard [20] (Eq. (2)) and Connor plot [21] (Eq. (3)).
1.2×106
1=F F 0 ¼ 1=ðF 1 F 0 ÞK½G þ 1=ðF 1 F 0 Þ
ð1Þ
1.0×105
F F 0 =½G ¼ ðF 1 F 0 ÞK ðF F 0 ÞK
ð2Þ
1 F=F 0 =½F ¼ KðF=F 0 Þ aK
ð3Þ
8×105
The plots obtained by applying above equations are illustrated in Figs. 6–8, respectively. Where F0 is the fluorescence intensity in the absence of Fe3+, F is the fluorescence intensity in presence of Fe3+ at particular concentration and [G] is the concentration of guest. The Ka value obtained from the Benesi–Hildebrand, Scatchard and Connor plot were found to be 11,250 M1, 12,970 M1 and 12,970 M1, respectively.
6×105 4×105 2×105
Bi(III)
Pb(II)
Hg(II)
Cd(II)
Zn(II)
Cu(II)
Ni(II)
Co(II)
Fe(II)
Mn(II)
Cr(III)
Fe(III)
0
Metal Ions Fig. 2b. Interference studies; Fluorescence intensity of receptor 1 (70 lM) containing 700 lM Fe3+ and other metal ions (700 lM).
The Stern–Volmer quenching constant The quenching can be mathematically expressed by the Stern– Volmer Eq. (1), which allows for calculating quenching constants [22].
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4.0 3.5
F/F0
3.0 2.5 2.0 1.5 0 0
3×10-5
6×10-5
9×10-5
1.2×10-4
1.5×10-4
Q (M)
Fig. 5. Job plot showing the 1:1 stoichiometry of the host and guest.
Fig. 9. Stern–Volmer plots for titrations of receptor 1 with different concentrations of Fe3+ metal salt.
0
1/F-FO
-2×104
-4×104
-6×104 2×104
0
4×104
6×104
1/G Fig. 6. Benesi–Hildebrand Plot (adjusted equation: 1/F F0 = 8E12x 9E08 1/ [G], R = 0.9947), Ka = 11,250 M1. Fig. 10. Changes in absorbance spectrum of receptor 1 (70 lM) upon the addition of Fe3+ metal ion (700 lM) in DMSO/H2O (8:2, v/v).
-3×1010
F 0 =F ¼¼ 1 þ kq s0 ½Q ¼ 1 þ K sv ½Q
where F0 and F are the fluorescence intensities in the absence and presence of the quencher, kq is the bimolecular quenching constant, s0 is the lifetime of the fluorescence in the absence of the quencher [Q] is the concentration of the quencher, and Ksv is the Stern–Volmer quenching constant. In the presence of a quencher (Lns), the fluorescence intensity is reduced from F0 to F. The ratio (F0/F) is directly proportional to the quencher concentration [Q]. Evidently:
F-F0/G
-2×1010
-7×1010
-1.2×1011 -7×106
-5×106
-3×106
-1×106
F-F0 Fig. 7. Scatchard plot (adjusted R = 0.9876), Ka = 12,970 M1.
equation:
F F0/[G] = 12,970x 1E+11,
1-(F-F0)/[G]
K sv ¼ kqs0
ð5Þ
F 0 =F ¼ 1 þ K sv ½Q
ð6Þ
According to Eq. (6), a plot of F0/F versus [Q] shows a linear graph with an intercept of 1 and a slope of Ksv. A typical plot of F0/F versus Fe3+ concentration is shown in Fig. 9. According to Eq. (4), a plot of F0/F versus [Q] shows a linear graph (Fig. 9) with an intercept of 1 and a slope of Ksv indicating that fluorescence quenching is dynamic in nature in the linear part. In order to elucidate the static or dynamic quenching without measurement of fluorescence lifetime, the absorption spectra were measured carefully to distinguish static and dynamic quenching. Dynamic quenching only affects the excited states of the fluorophores, and thus no change in the absorption spectra is predicted. It was found that the measured spectra were not influenced by the metal ions in the presence of metal ions.
1.2×1011
8×1010
4×1010
0
Naked Eye detection under UV irradiation
-7×106
-5×106
-3×106
-1×106
F-F0 Fig. 8. Connor Ka = 12,970 M1.
ð4Þ
plot
(adjusted
equation:
Y = 12,765 + 1E+11,
R = 0.9896),
In addition to selective recognition of Fe3+, receptor 1 give a remarkable fluorescent colour change which can be visualized under UV irradiation as shown in inset of Fig. 1. Visual detection studies of chemosensor 1 (70 lM) were performed with metal ion in
U. Fegade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 569–574
573
Fig. 11. The optimized structure of: (A) receptor 1, (B) 1.Fe3+ complex calculated by using B3LYP/6-31G basis set on Gaussian 09 program; red, blue, yellow and white spheres refer to O, N, S and H atoms respectively and dotted line represent H-bonds). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 An optimized bond angles, dihedral angles and energy calculated at B3LYP/6-31G level. 3+
Receptor 1 bond
Receptor 1 bond length
Receptor 1.Fe bond
C1–O1 C2–O2 O1–H1 O1–H3 O2–H3 S1–H1 S2–H2 HOMO– LUMO gap
1.224 Å 1.221 Å 2.036 Å 2.339 Å 2.253 Å 1.353 Å 1.348 Å 4.91 eV
C1–O1 C2–O2 Fe–O1 Fe–O2 Fe–S1 Fe–S2 – HOMO–LUMO gap
Receptor 1.Fe length
3+
bond
1.234 Å 1.242 Å 2.075 Å 2.040 Å 2.153 Å 2.101 Å – 1.52 eV
form of their nitrate salts i.e. Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+. The receptor 1 in DMSO/H2O (8:2, v/ v) solvent system appears fluorescent at room temperature under UV irradiation. On addition of metal ions (10.0 equivalents) to receptor 1, only Fe3+ showed fluorescent quenched. In general, the metal ions such as Co2+, Cu2+ and Ni2+ interfere in Fe3+ detection but in present study no significant colour change in solution of receptor 1 was observed with these commonly interfering metal ions.
UV–Vis absorption spectral studies The absorption spectral properties of receptor 1 were studied in DMSO/H2O (80:20, v/v) upon addition of metal ions and the appreciable change in absorption profile was observed for Fe3+ (Fig. 10). The absorption profile of receptor showed two bands at 245 and 289 nm and when Fe3+ was added to receptor 1, increase in absorbance at 245 and 289 nm was observed. The little shift is due to the receptor 1 (ligand) to metal charge transfer and it appears that the core functionality required for 1 to efficiently bind Fe3+ upon excitation are AC@O and a ASH groups. Proposed binding mode and mechanism of sensing The mechanism of Fe3+ recognition with receptor 1 was investigated by DFT calculations using Becke’s three parameterized Lee– Yang–Parr (B3LYP) exchange functional with 6-31G basis sets, on Gaussian-09 programs [23–25]. The calculations were mainly performed to explain the photo-physical properties of receptor 1 and its complex with Fe3. The optimized structure of receptor 1 showed that it has non-planar arrangement of atoms (Fig. 11). The formation of complex between 1 and Fe3+ lead to stabilize the system as confirmed from comparing the value of energies of 1 and 1.Fe3+ complex (Table 1). It was observed that 1.Fe3+ has more symmetry instead of receptor 1. Fig. 12 shows that HUMO, LUMO values of
HOMO= -5.55 eV
LUMO= -0.64eV
HOMO= -2.07 eV
LUMO= -0.55eV
Fig. 12. HOMO and LUMO energies calculated at B3LYP/6-31G(d) level of receptor 1 and receptor 1.Fe3+ complex.
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the receptor 1 and 1.Fe3+ complex from which we can see that the energy gap in 1.Fe3+ complex is less than the receptor 1 which also support for the symmetry. 4. Conclusion In conclusion, we have designed and synthesized a new fluorescence receptor 1 for selective detection of Fe3+ in semi aqueous medium (DMSO/H2O, 8:2, v/v). The Fe3+ metal ion induced significant fluorescence quenching and colour change in the receptor 1. The chemosensor 1 can detect Fe3+ selective in presence of other commonly interfering metal and can detect Fe3+ up to 0.1 lM. The 1:1 Stoichiometry of the host guest complex was established from Job plot and the binding constant (Ka) obtained from Benesi–Hildebrand, Scatchard and Connor plot were found to be 11,250 M1, 12,970 M1 and 12,970 M1, respectively. Theoretical calculations carried out at the B3LYP/6-31G formalism show that upon metal binding, charge delocalization increases significantly leading to higher receptor activity in the metal binding. Further studies on similar systems are in progress in our laboratory. References [1] (a) S. Kubik, Chem. Soc. Rev. 39 (2010) 3648–3663; (b) J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, Chem. Soc. Rev. 40 (2011) 3483– 3495; (c) H.T. Ngo, X. Liu, K.A. Jolliffe, Chem. Soc. Rev. 41 (2012) 4928–4965. [2] (a) H. Jung, N. Singh, D.O. Jang, Tetrahedron Lett. 49 (2008) 2960–2964; (b) B. Hu, P. Lu, Y. Wang, New J. Chem. 37 (2013) 1645–1653; (c) Y. Jeong, J. Yoon, Inorg. Chim. Acta 381 (2012) 2–14. [3] M. Kumar, J.N. Babu, V. Bhalla, J. Incl. Phenom. Macrocycl. Chem. 66 (2010) 139–145. [4] U. Fegade, S. Attarde, A. Kuwar, Chem. Phys. Lett. 584 (2013) 165–171. [5] U. Fegade, H. Sharma, K. Tayade, S. Attarde, N. Singh, A. Kuwar, Org. Biomol. Chem. 11 (2013) 6824–6828.
[6] U. Fegade, H. Sharma, S. Attarde, N. Singh, A. Kuwar, J. Fluoresc. (2013), http:// dx.doi.org/10.1007/s10895-013-1297-4. [7] U. Fegade, J. Marek, R. Patil, S. Attarde, A. Kuwar, J. Luminesc. 146 (2014) 234– 238. [8] Y. Wang, E. Duran, D. Nacionales, A. Valencia, C. Wostenberg, E. Marinez, Tetrahedron Lett. 49 (2008) 6410–6412. [9] (a) P.K. Chung, S.R. Liu, H.F. Wang, S.P. Wu, J. Fluoresc. 23 (2013) 1139–1145; (b) L. Fu, J. Mei, J.-T. Zhang, Y. Liu, F.-L. Jiang, Luminescence 28 (2013) 602– 606; (c) L. Huang, F. Hou, J. Cheng, P. Xi, F. Chen, D. Bai, Z. Zeng, Org. Biomol. Chem. 10 (2012) 9634–9638; (d) S. Sen, S. Sarkar, B. Chattopadhyay, A. Moirangthem, A. Basu, K. Dhara, P. Chattopadhyay, Analyst 137 (2012) 3335–3342; (e) S.-R. Liu, S.-P. Wu, Sens. Actuators B 171–172 (2012) 1110–1116; (f) N. Singh, N. Kaur, J. Dunn, M. Mackay, J.F. Callan, Tetrahedron Lett. 50 (2009) 953–956; (g) J. Mao, L. Wang, W. Dou, X. Tang, Y. Yan, W. Liu, Org. Lett. 9 (2007) 4567– 4570; (h) J.P. Sumner, R. Kopelman, Analyst 130 (2005) 528–533; (i) G.E. Tumambac, C.M. Rosencrance, C. Wolf, Tetrahedron 60 (2004) 11293– 11297. [10] M.C. Linder, M. Hazegh-Azam, Am. J. Clin. Nutr. 63 (1996) 797S–811S. [11] R. Uauy, M. Olivares, M. Gonzalez, Am. J. Clin. Nutr. 67 (1998) 952S–959S. [12] J.D. Lee, Concise Inorganic Chemistry, fifth ed., Wiley India Pvt. Ltd., India, 2008. [13] S.J. Lippard, J.M. Berg, Principles of Bioorganic Chemistry, University Science, Mill Valley, USA, 1994. [14] W. Kaim, B. Schwederski, Bioinorganic Chemistry, Wiley, Chichester, 1995. [15] A. Jacobs, Blood 50 (1977) 433–439. [16] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516. [17] V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, Acc. Chem. Res. 34 (2001) 488–493. [18] P. Job, Ann. Chim. 9 (1928) 113–203. [19] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [20] G. Scatchard, Ann. New York Acad. Sci. 51 (1949) 660–672. [21] K.A. Connors, The Measurements of Molecular Complex Stability, Wiley, New York, 1987. [22] O. Stern, M. Volmer, Phys. Z. 20 (1919) 183–188. [23] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [24] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [25] N. Singh, N. Kaur, R.C. Mulrooney, J.F. Callan, Tetrahedron Lett. 49 (2008) 6690–6692.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective and sensitive receptor for Fe3+ probing Umesh Fegade a,b, Ajnesh Singh c, G. Krishna Chaitanya d, Narinder Singh c, Sanjay Attarde b, Anil Kuwar a,⇑ a
School of Chemical Sciences, North Maharashtra University, Jalgaon 425001, MS, India School of Environmental and Earth Sciences, North Maharashtra University, Jalgaon 425001, MS, India c Department of Chemistry, Indian Institute of Technology, Ropar, Rupnagar 140001, Punjab, India d School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431 606, MS, 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
A new fluorescent urea and thiol
based chemosensor was developed and synthesized. Chemosensor showed excellent selectivity for Fe3+ ions over other interfering ions. 3+ Chemosensor can detect Fe up to 0.1 lM in semi-aqueous medium (DMSO/H2O, 8:2, v/v). Experimental results have been supported by DFT calculations.
a r t i c l e
i n f o
Article history: Received 28 August 2013 Received in revised form 29 October 2013 Accepted 3 November 2013 Available online 11 November 2013 Keywords: 1,10 -(4-Methylbenzene-1,3-diyl)bis[3-(2sulfanylphenyl)urea] Fluorescence receptor Binding constant Metal ions Fe3+ sensor
a b s t r a c t A new fluorescent receptor 1,10 -(4-methylbenzene-1,3-diyl)bis[3-(2-sulfanylphenyl)urea] (1) has been designed and synthesized. The receptor showed excellent selectivity for Fe3+ in DMSO/H2O (8:2, v/v) solvent system over other commonly coexistent metal ions. The binding constant (Ka) of receptor with Fe3+ was calculated to be 11,250 M1, 12,970 M1 and 12,970 M1 using Benesi–Hildebrand, Scatchard and Connor plot, respectively. The experimental results have been further supported by the detailed DFT calculations. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Development of synthetic chemosensor for selective sensing of metal ions in semi-aqueous medium is receiving augmented attention of chemists working in supramolecular chemistry. Design and synthesis of chemosensors for metal ion are mainly based on monitoring the changes in photo-physical properties of receptor in ⇑ Corresponding author. Tel.: +91 257 2257432; fax: +91 257 225740. E-mail address: [email protected] (A. Kuwar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.007
presence of a guest. The design of a chemosensor should abide to some basic requirements such as exhibiting a qualitative response to a particular analyte and quantitative determination for a wide concentration range of analyte [1–7]. In recent year, number of chemosensors has been synthesized for the alkali and alkaline earth metals However, there are relatively few examples of designed chemosensors for the heavy metals of first transition series which are the abundant elements in earth crust and also in human body [8,9]. These metal ions play crucial role in chemistry, biology, environment and medicine
U. Fegade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 569–574
[10]. However, cations can be toxic if the level exceeds cellular needs. Iron is one such metal which is associated with number of biological processes. Most of the iron present in biological systems is associated with enzymes, specialized transport and storage proteins (haemoglobin, the red pigment in the erythrocytes and ferretin). It plays a vital role in oxygen transfer processes in DNA and RNA synthesis. The deficiency of iron can cause anaemia, hemochromatosis, liver damage, diabetes, Parkinson’s disease and cancer. Accumulation of iron and subsequent over production of H2O2 in tissues is the main reason for oxidative stress and neurodegenerative diseases [11–16]. This paper reports the synthesis of receptor 1 and its use as sensor for Fe3+ ions in presence of various metal ions (Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+) in DMSO/H2O (8:2, v/v). In presence of Fe3+ ion, fluorescence quenching was observed and binding constant (Ka) of receptor with Fe3+ was calculated using Benesi–Hildebrand, Scatchard and Connor plots. The sensing ability of chemosensor for Fe3+ with ‘‘fluorescence off’’ mechanism in semi aqueous phase make it interesting candidate for its application in area of biology, environmental and material chemistry.
Experimental All commercial grade chemicals and solvents were used without further purification. 1H and 13C NMR spectra were recorded on a Varian NMR mercury System 300 spectrometer operating at 300 MHz for 1H NMR and 75 MHz for 13C NMR in DMSO-d6 respectively. The fluorescence and UV–visible spectra were recorded in DMSO/H2O (8:2, v/v) with Fluoromax-4 spectrofluorometer and Shimadzu UV-24500 at room temperature.
Synthesis of 1,1’-(4-methylbenzene-1,3-diyl)bis[3-(2sulfanylphenyl)urea] A mixture of 2,4-diisocyanato-1-methylbenzene (0.174 g, 1 mmol) and 2-aminothiophenol (0.250 g, 2 mmol) was refluxed in acetone with constant stirring for 1 h (Scheme 1). The white coloured precipitated product obtained was filtered and purified by recrystallization from DMF. White solid, Yield 76%, mp P 250 °C, Soluble in DMSO, 1H NMR (300 MHz, DMSO-d6, ppm) d = 2.39 (s, 3H, Ar-CH3), 7.24–7.29 (d, J = 14.7, 2H, Ar-H), 7.81–8.01 (m, 8H, Ar-H), 8.18 (s, 1H, Ar-H), 9.75 (s, 4H, NH amide); 13C NMR (75 MHz, DMSO-d6, ppm) d = 14.2, 113.7, 115.9, 116.2, 121.0, 123.2, 126.2, 129.8, 129.2, 132.5, 135.2, 149.9, 154.7. FT IR (KBr pellet, cm1) t = 740, 868, 1026, 1528, 1642, 2918, 3252. ESI-MS m/z = 423.82, calc. for C21H20N4O2S2 = 424.10.
Results and discussion Recognition studies Response of receptor 1 towards various metal ions was monitored in DMSO/H2O (8:2, v/v) solvent system using fluorescence spectroscopy. Fig. 1 illustrates the changes in emission profile of 1 in DMSO/H2O (8:2, v/v) upon addition of various metal salts. Interestingly, addition of Fe3+ to host solution resulted significant quenching in broad emission band centered at 350 nm in case of host solution (kex 289 nm). Other metal ions have not produced any significant changes in emission profile as depicted in Fig. 1. It is believed that fluorescence intensity of receptor was quenched due to interactions of different binding sites with Fe3+. Chemosensor 1 have two urea and two thiol groups which plays an important role in the binding of Fe3+. The binding pattern of 1 describes the importance of thiol and urea groups in Fe+3 binding. The thiol group is frequently considered a good binding site and plays an important role in binding of transition metal ions such as Fe+3 metal ion. Therefore, it appears that the core functionality required for 1 to efficiently bind with Fe+3 upon excitation are urea and thiol groups. The quenching of fluorescence intensity at 350 nm on addition of Fe3+ was due to cooperative influence of size compatibility and paramagnetic nature of the analyte which result in chelation enhanced quenching (CHEQ). In such metal complexes forbidden intersystem crossing (isc) become faster due to the presence of a paramagnetic atom (the metal ion) in the proximity of the fluorophore which result in fluorescence quenching. Moreover
1.4×106
Fluorescence Intensity (cps)
570
1.2×106 1.0×105 Host 5
8×10
3+
6×10
2×105 0 300
350
400
NCO
SH
450
Wavelength (nm) Fig. 1. Emission profile of receptor 1 (70 lM, kex = 289 nm) upon the addition of a particular metal salt (700 lM) in DMSO/H2O (8:2, v/v). Inset: Photograph of chemosensor 1 and 1.Fe3+ under the effect of UV light (365 nm).
Acetone, reflux 1h
OCN
2+
Fe3+
HN
2
2+
4×105
NH
NH2 +
2+
Cr , Mn , Fe ,Co , 2+ 2+ 2+ 2+ Ni , Cu ,Zn , Cd , 2+ 2+ 3+ Hg , Pb and Bi
5
HN
O
O
SH
HS
1 Scheme 1. Synthetic procedure for synthesis of receptor 1.
NH
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1.4×106
Fluorescence Intensity (cps)
0.8
0.6
Io-I/Io
0.4
0.2
0
Bi(III)
Pb(II)
Hg(II)
Zn(II)
Cd(II)
Cu(II)
Ni(II)
Co(II)
Fe(II)
Mn(II)
Cr(III)
Fe(III)
-0.4
Host
-0.2
1.2×106 1.0×105 8×105 6×105 4×105 2×105 0 300
350
400
450
Wavelength (nm)
Metal Ions Fig. 2a. Fluorescence ratiometric response (I0 I/I0) of receptor 1 (70 lM) upon the addition of a particular metal salts (700 lM) in DMSO/H2O (8:2, v/v).
the sensor is selective for Fe3+ over Fe2+ which may be due to the incompatible coordination sphere of sensor for Fe2+ in terms of size, shape and steric factors [8,17]. Fluorescence ratiometric response of receptor 1 and interference toward the studied metal ions is shown in Figs. 2a and 2b. The results showed that the receptor 1 is highly selective towards Fe3+ ions as compared to the other metal ions such as transition-metal and heavy-metal ions (Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+) with which it have shown weak quenching of fluorescence intensity. To gain more insight, the titration of receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) with Fe3+ was performed (Fig. 3). The successive increased amounts (0–10 equiv.) of Fe3+ to receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) resulted in a continuous decrease in emission band at 350 nm (kex = 289 nm). Normalized response of fluorescence signal to changing Fe+3 concentrations in the DMSO/H2O (80:20, v/v) solution shown in Fig. 4. The detection limit for Fe3+ was calculated to be as low as 0.1 lM. The stoichiometry was determined by the continuous variation method (Job plot) [18]. Fig. 5 shows the Job plot between [HG] and {[H]/([H] + [G])} where the total concentration of host
Competing ion Competing ion + Fe(III)
Fluorescence Intensity (cps)
1.4×106
Fig. 3. Changes in fluorescence spectra (kex = 289 nm) of receptor 1 (70 lM) in DMSO/H2O (8:2, v/v) upon addition of Fe3+ from (0–10 equiv.).
Fig. 4. Normalized response of fluorescence signal to changing Fe3+ concentrations in DMSO/H2O (8:2, v/v) (chemosensor 1; 70 lM and Fe3+ salt; 0–10 equiv.).
and guest was constant and molar fraction of host was continuously varied. The results indicated the formation of a 1:1 (host:guest) complex. Using the equation: [G]tot = a/2K11(1 a)2[H]tot + a[H]tot/ 2, where [G]tot is total concentration of guest, [H]tot is the total concentration of host, a = (I I0)/(Ii I0) with I being the fluorescence intensity at a particular Fe3+ concentration while I0 and Ii are the intensities at zero and infinite Fe3+ concentrations, respectively. The association constant (Ka) was calculated by three different methods i.e. Benesi–Hildebrand [19] (Eq. (1)), Scatchard [20] (Eq. (2)) and Connor plot [21] (Eq. (3)).
1.2×106
1=F F 0 ¼ 1=ðF 1 F 0 ÞK½G þ 1=ðF 1 F 0 Þ
ð1Þ
1.0×105
F F 0 =½G ¼ ðF 1 F 0 ÞK ðF F 0 ÞK
ð2Þ
1 F=F 0 =½F ¼ KðF=F 0 Þ aK
ð3Þ
8×105
The plots obtained by applying above equations are illustrated in Figs. 6–8, respectively. Where F0 is the fluorescence intensity in the absence of Fe3+, F is the fluorescence intensity in presence of Fe3+ at particular concentration and [G] is the concentration of guest. The Ka value obtained from the Benesi–Hildebrand, Scatchard and Connor plot were found to be 11,250 M1, 12,970 M1 and 12,970 M1, respectively.
6×105 4×105 2×105
Bi(III)
Pb(II)
Hg(II)
Cd(II)
Zn(II)
Cu(II)
Ni(II)
Co(II)
Fe(II)
Mn(II)
Cr(III)
Fe(III)
0
Metal Ions Fig. 2b. Interference studies; Fluorescence intensity of receptor 1 (70 lM) containing 700 lM Fe3+ and other metal ions (700 lM).
The Stern–Volmer quenching constant The quenching can be mathematically expressed by the Stern– Volmer Eq. (1), which allows for calculating quenching constants [22].
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4.0 3.5
F/F0
3.0 2.5 2.0 1.5 0 0
3×10-5
6×10-5
9×10-5
1.2×10-4
1.5×10-4
Q (M)
Fig. 5. Job plot showing the 1:1 stoichiometry of the host and guest.
Fig. 9. Stern–Volmer plots for titrations of receptor 1 with different concentrations of Fe3+ metal salt.
0
1/F-FO
-2×104
-4×104
-6×104 2×104
0
4×104
6×104
1/G Fig. 6. Benesi–Hildebrand Plot (adjusted equation: 1/F F0 = 8E12x 9E08 1/ [G], R = 0.9947), Ka = 11,250 M1. Fig. 10. Changes in absorbance spectrum of receptor 1 (70 lM) upon the addition of Fe3+ metal ion (700 lM) in DMSO/H2O (8:2, v/v).
-3×1010
F 0 =F ¼¼ 1 þ kq s0 ½Q ¼ 1 þ K sv ½Q
where F0 and F are the fluorescence intensities in the absence and presence of the quencher, kq is the bimolecular quenching constant, s0 is the lifetime of the fluorescence in the absence of the quencher [Q] is the concentration of the quencher, and Ksv is the Stern–Volmer quenching constant. In the presence of a quencher (Lns), the fluorescence intensity is reduced from F0 to F. The ratio (F0/F) is directly proportional to the quencher concentration [Q]. Evidently:
F-F0/G
-2×1010
-7×1010
-1.2×1011 -7×106
-5×106
-3×106
-1×106
F-F0 Fig. 7. Scatchard plot (adjusted R = 0.9876), Ka = 12,970 M1.
equation:
F F0/[G] = 12,970x 1E+11,
1-(F-F0)/[G]
K sv ¼ kqs0
ð5Þ
F 0 =F ¼ 1 þ K sv ½Q
ð6Þ
According to Eq. (6), a plot of F0/F versus [Q] shows a linear graph with an intercept of 1 and a slope of Ksv. A typical plot of F0/F versus Fe3+ concentration is shown in Fig. 9. According to Eq. (4), a plot of F0/F versus [Q] shows a linear graph (Fig. 9) with an intercept of 1 and a slope of Ksv indicating that fluorescence quenching is dynamic in nature in the linear part. In order to elucidate the static or dynamic quenching without measurement of fluorescence lifetime, the absorption spectra were measured carefully to distinguish static and dynamic quenching. Dynamic quenching only affects the excited states of the fluorophores, and thus no change in the absorption spectra is predicted. It was found that the measured spectra were not influenced by the metal ions in the presence of metal ions.
1.2×1011
8×1010
4×1010
0
Naked Eye detection under UV irradiation
-7×106
-5×106
-3×106
-1×106
F-F0 Fig. 8. Connor Ka = 12,970 M1.
ð4Þ
plot
(adjusted
equation:
Y = 12,765 + 1E+11,
R = 0.9896),
In addition to selective recognition of Fe3+, receptor 1 give a remarkable fluorescent colour change which can be visualized under UV irradiation as shown in inset of Fig. 1. Visual detection studies of chemosensor 1 (70 lM) were performed with metal ion in
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573
Fig. 11. The optimized structure of: (A) receptor 1, (B) 1.Fe3+ complex calculated by using B3LYP/6-31G basis set on Gaussian 09 program; red, blue, yellow and white spheres refer to O, N, S and H atoms respectively and dotted line represent H-bonds). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 An optimized bond angles, dihedral angles and energy calculated at B3LYP/6-31G level. 3+
Receptor 1 bond
Receptor 1 bond length
Receptor 1.Fe bond
C1–O1 C2–O2 O1–H1 O1–H3 O2–H3 S1–H1 S2–H2 HOMO– LUMO gap
1.224 Å 1.221 Å 2.036 Å 2.339 Å 2.253 Å 1.353 Å 1.348 Å 4.91 eV
C1–O1 C2–O2 Fe–O1 Fe–O2 Fe–S1 Fe–S2 – HOMO–LUMO gap
Receptor 1.Fe length
3+
bond
1.234 Å 1.242 Å 2.075 Å 2.040 Å 2.153 Å 2.101 Å – 1.52 eV
form of their nitrate salts i.e. Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Bi3+. The receptor 1 in DMSO/H2O (8:2, v/ v) solvent system appears fluorescent at room temperature under UV irradiation. On addition of metal ions (10.0 equivalents) to receptor 1, only Fe3+ showed fluorescent quenched. In general, the metal ions such as Co2+, Cu2+ and Ni2+ interfere in Fe3+ detection but in present study no significant colour change in solution of receptor 1 was observed with these commonly interfering metal ions.
UV–Vis absorption spectral studies The absorption spectral properties of receptor 1 were studied in DMSO/H2O (80:20, v/v) upon addition of metal ions and the appreciable change in absorption profile was observed for Fe3+ (Fig. 10). The absorption profile of receptor showed two bands at 245 and 289 nm and when Fe3+ was added to receptor 1, increase in absorbance at 245 and 289 nm was observed. The little shift is due to the receptor 1 (ligand) to metal charge transfer and it appears that the core functionality required for 1 to efficiently bind Fe3+ upon excitation are AC@O and a ASH groups. Proposed binding mode and mechanism of sensing The mechanism of Fe3+ recognition with receptor 1 was investigated by DFT calculations using Becke’s three parameterized Lee– Yang–Parr (B3LYP) exchange functional with 6-31G basis sets, on Gaussian-09 programs [23–25]. The calculations were mainly performed to explain the photo-physical properties of receptor 1 and its complex with Fe3. The optimized structure of receptor 1 showed that it has non-planar arrangement of atoms (Fig. 11). The formation of complex between 1 and Fe3+ lead to stabilize the system as confirmed from comparing the value of energies of 1 and 1.Fe3+ complex (Table 1). It was observed that 1.Fe3+ has more symmetry instead of receptor 1. Fig. 12 shows that HUMO, LUMO values of
HOMO= -5.55 eV
LUMO= -0.64eV
HOMO= -2.07 eV
LUMO= -0.55eV
Fig. 12. HOMO and LUMO energies calculated at B3LYP/6-31G(d) level of receptor 1 and receptor 1.Fe3+ complex.
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the receptor 1 and 1.Fe3+ complex from which we can see that the energy gap in 1.Fe3+ complex is less than the receptor 1 which also support for the symmetry. 4. Conclusion In conclusion, we have designed and synthesized a new fluorescence receptor 1 for selective detection of Fe3+ in semi aqueous medium (DMSO/H2O, 8:2, v/v). The Fe3+ metal ion induced significant fluorescence quenching and colour change in the receptor 1. The chemosensor 1 can detect Fe3+ selective in presence of other commonly interfering metal and can detect Fe3+ up to 0.1 lM. The 1:1 Stoichiometry of the host guest complex was established from Job plot and the binding constant (Ka) obtained from Benesi–Hildebrand, Scatchard and Connor plot were found to be 11,250 M1, 12,970 M1 and 12,970 M1, respectively. Theoretical calculations carried out at the B3LYP/6-31G formalism show that upon metal binding, charge delocalization increases significantly leading to higher receptor activity in the metal binding. Further studies on similar systems are in progress in our laboratory. References [1] (a) S. Kubik, Chem. Soc. Rev. 39 (2010) 3648–3663; (b) J. Wu, W. Liu, J. Ge, H. Zhang, P. Wang, Chem. Soc. Rev. 40 (2011) 3483– 3495; (c) H.T. Ngo, X. Liu, K.A. Jolliffe, Chem. Soc. Rev. 41 (2012) 4928–4965. [2] (a) H. Jung, N. Singh, D.O. Jang, Tetrahedron Lett. 49 (2008) 2960–2964; (b) B. Hu, P. Lu, Y. Wang, New J. Chem. 37 (2013) 1645–1653; (c) Y. Jeong, J. Yoon, Inorg. Chim. Acta 381 (2012) 2–14. [3] M. Kumar, J.N. Babu, V. Bhalla, J. Incl. Phenom. Macrocycl. Chem. 66 (2010) 139–145. [4] U. Fegade, S. Attarde, A. Kuwar, Chem. Phys. Lett. 584 (2013) 165–171. [5] U. Fegade, H. Sharma, K. Tayade, S. Attarde, N. Singh, A. Kuwar, Org. Biomol. Chem. 11 (2013) 6824–6828.
[6] U. Fegade, H. Sharma, S. Attarde, N. Singh, A. Kuwar, J. Fluoresc. (2013), http:// dx.doi.org/10.1007/s10895-013-1297-4. [7] U. Fegade, J. Marek, R. Patil, S. Attarde, A. Kuwar, J. Luminesc. 146 (2014) 234– 238. [8] Y. Wang, E. Duran, D. Nacionales, A. Valencia, C. Wostenberg, E. Marinez, Tetrahedron Lett. 49 (2008) 6410–6412. [9] (a) P.K. Chung, S.R. Liu, H.F. Wang, S.P. Wu, J. Fluoresc. 23 (2013) 1139–1145; (b) L. Fu, J. Mei, J.-T. Zhang, Y. Liu, F.-L. Jiang, Luminescence 28 (2013) 602– 606; (c) L. Huang, F. Hou, J. Cheng, P. Xi, F. Chen, D. Bai, Z. Zeng, Org. Biomol. Chem. 10 (2012) 9634–9638; (d) S. Sen, S. Sarkar, B. Chattopadhyay, A. Moirangthem, A. Basu, K. Dhara, P. Chattopadhyay, Analyst 137 (2012) 3335–3342; (e) S.-R. Liu, S.-P. Wu, Sens. Actuators B 171–172 (2012) 1110–1116; (f) N. Singh, N. Kaur, J. Dunn, M. Mackay, J.F. Callan, Tetrahedron Lett. 50 (2009) 953–956; (g) J. Mao, L. Wang, W. Dou, X. Tang, Y. Yan, W. Liu, Org. Lett. 9 (2007) 4567– 4570; (h) J.P. Sumner, R. Kopelman, Analyst 130 (2005) 528–533; (i) G.E. Tumambac, C.M. Rosencrance, C. Wolf, Tetrahedron 60 (2004) 11293– 11297. [10] M.C. Linder, M. Hazegh-Azam, Am. J. Clin. Nutr. 63 (1996) 797S–811S. [11] R. Uauy, M. Olivares, M. Gonzalez, Am. J. Clin. Nutr. 67 (1998) 952S–959S. [12] J.D. Lee, Concise Inorganic Chemistry, fifth ed., Wiley India Pvt. Ltd., India, 2008. [13] S.J. Lippard, J.M. Berg, Principles of Bioorganic Chemistry, University Science, Mill Valley, USA, 1994. [14] W. Kaim, B. Schwederski, Bioinorganic Chemistry, Wiley, Chichester, 1995. [15] A. Jacobs, Blood 50 (1977) 433–439. [16] P.D. Beer, P.A. Gale, Angew. Chem. Int. Ed. 40 (2001) 486–516. [17] V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, Acc. Chem. Res. 34 (2001) 488–493. [18] P. Job, Ann. Chim. 9 (1928) 113–203. [19] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707. [20] G. Scatchard, Ann. New York Acad. Sci. 51 (1949) 660–672. [21] K.A. Connors, The Measurements of Molecular Complex Stability, Wiley, New York, 1987. [22] O. Stern, M. Volmer, Phys. Z. 20 (1919) 183–188. [23] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [24] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785–789. [25] N. Singh, N. Kaur, R.C. Mulrooney, J.F. Callan, Tetrahedron Lett. 49 (2008) 6690–6692.
