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An easily accessible internal charge transfer chemosensor exhibiting dual colorimetric and luminescence switch on responses for targeting Cu2+
Sensors and Actuators B 150 (2010) 574–578
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
An easily accessible internal charge transfer chemosensor exhibiting dual colorimetric and luminescence switch on responses for targeting Cu2+ Sabir H. Mashraqui ∗ , Mukesh Chandiramani, Rupesh Betkar, Sushil Ghorpade Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E Kalina, Mumbai 400098, Maharashtra, India
a r t i c l e
i n f o
Article history: Received 14 June 2010 Received in revised form 23 August 2010 Accepted 25 August 2010 Available online 24 September 2010 Keywords: Chemosensor Michler’s ketone benzhydrazide Internal charge transfer UV–visible Fluorescence Cu2+ selectivity
a b s t r a c t A new internal charge transfer probe, MIZON was synthesized by condensing Michler’s ketone with 4(N,N -dimethylamino)benzhydrazide. The spectrophotometric and fluorimetric titrations with metal ions of biological and environmental importance were performed in H2 O–CH3 CN (1:1 v/v) system buffered by Tris–HCl, pH = 7.4 ± 0.1. The probe, MIZON was found to interact selectively with Cu2+ , inducing remarkably high absorbance red shift by 240 nm and 11-fold enhancement in the emission intensity. In contrast, several other metal ions, viz. Li+ , Na+ , K+ , Ca2+ , Mg2+ , Ba2+ , Pb2+ , Co2+ , Cd2+ , Zn2+ , Ni2+ and Hg2+ , even at substantially higher concentration than Cu2+ , did not exert significant perturbation in the photophysical profile of the probe. The detection limit of Cu2+ , calculated from the more sensitive fluorescence data was found to be 5.5 × 10−7 M. Potential, therefore is clearly available in MIZON to detect Cu2+ in micromolar range via dual visible color change from yellow to green and fluorescence switch-on response. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The design of optical chemosensors for the selective detection of heavy metal ions of biological and environmental interest is currently attracting interest of chemists, biologists and environmentalists [1–4]. The optical probes typically feature a covalently linked photoresponsive unit with a selective receptor such that the analyte binding translates into recognizable optical transductions, thereby facilitating the detection of specific targets. Of the various photophysical mechanisms known for designing the signalling devices, the photoinduced electron transfer (PET) is by far the most widely used protocol, which delivers either fluorescence on–off or off–on response, depending upon the structures of the chemosensors and the identity of the cations [3,5]. Though, not as extensively investigated, the push–pull chemoreceptors are attracting increasing attention for their capacity to deliver colorimetric as well as fluorimetric signalling responses [6,7]. Copper is essential to all life forms [8] and plays role in many important biological functions, which include haemoglobin synthesis, nerve functions, oxygen binding, and electron transfer processes [9,10]. Copper is also an environmental pollutant and its overdose beyond physiological limits can cause gastrointestinal problems, renal dysfunction as well as serious health disorders such as Alzheimer, Parkinson, Menkes and Wilson diseases [11–13].
∗ Corresponding author. Tel.: +91 22 26526091; fax: +91 22 26528547. E-mail address: sh [email protected] (S.H. Mashraqui). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.08.037
Consequently, strong interest persists in designing selective and sensitive Cu2+ chemosensors for biological and environmental monitoring. A majority of the optical Cu2+ sensors exhibits less desirable fluorescence turn off responses due to the quenching effect of the paramagnetic species [14–22]. Even though, Cu2+ fluorescence turn-on type sensors are steadily increasing in numbers [23–32], however cross-affinities, particularly with Pb2+ , Hg2+ and Ni2+ ions, aqueous incompatibility or short wavelength excitation limit their practical applications. Consequently, there is room to improve the binding characteristics of Cu2+ chemosensors and, given its quenching character, it is especially attractive to design probes offering dual color modulation as well as fluorescence amplification upon Cu2+ binding. An attractive feature of photoreceptors with donor–acceptor character is the possibility of synthetically tuning the transition energy of their internal charge transfer (ICT) states to allow colorimetric ‘naked eye’ response [33]. Hydrazones derived from heterocyclic chromophores are well-known spectrophotometric reagents for the detection of transition metal ions. For examples, Lucifer yellow carbohydrazide is known to bind Cu2+ selectively [34] and recently Jiang and coworkers reported a lariate 4-(N,N -dimethylamino)benzhydrazide as an efficient fluorescent charge transfer probe for Cu2+ [35]. In context to our ongoing interest in the ICT-based metal ion sensors [36–40], we now report the synthesis and metal ion binding studies of a new hydrazone derivative, 4-dimethylamino-benzoic acid[bis(4-dimethylamino-phenyl)-methylene]-hydrazide, designated as MIZON. The chemosensor, MIZON belongs to the donor–acceptor
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Scheme 1. Synthesis and the proposed interaction of MIZON with metal ions, Mn+ .
module, featuring azomethine and an amide carbonyl as the bidentate metal chelating ligand. As our photophysical studies reveal, the probe has potential to be utilized as a highly Cu2+ selective colorimetric as well as fluorescent turn on chemosensor in H2 O–CH3 CN aqueous buffered solution at pH 7.4.
C26 H31 N5 O: C, 72.72; H, 7.22; N, 16.31. Found: C, 72.93; H, 7.54; N, 16.09. 3. Results and discussion 3.1. Synthesis and proposed metal ion interaction of MIZON
2. Experimental 2.1. Reagents and instrumentation Reagents and solvents used were purchased from S.D. Fine Chemicals or Sigma–Aldrich, India. IR was recorded using PerkinElmer FT-IR spectrometer and pellets were made by using KBr. 1 H NMR and 13 C NMR spectra were recorded on BRUKER make, model Avance II 300 of 300 MHz. UV–vis spectra were recorded using Shimadzu UV–vis recording spectrophotometer, model no. UV-2401PC. Fluorescence studies were carried out using Shimadzu spectrofluorometer, model no. RF-5301PC. The slit width was set at 3 nm for both excitation and emission and the PMT detector voltage was 700 V. 2.2. Synthesis of 4-dimethylamino-benzoic acid[-bis-(4-dimethylamino-phenyl)-methylene]-hydrazide, MIZON Michler’s ketone 1 (0.268 g, 1 mmol) and 4-(N,N dimethylamino) benzoic hydrazide 2 (0.179 g, 1 mmol) were refluxed in 20 ml of dichloromethane containing 1 ml of BF3 –etherate for 12 h. The reaction mixture was concentrated under reduced pressure and the crude oily product was treated with saturated aqueous NaHCO3 . Greenish-yellow solid obtained was filtered, washed with water and air dried. Purification by column chromatography using SiO2 (eluant CHCl3 ) allowed isolation of the desired product, MIZON as pale yellow crystalline solid in 68% yield (0.292 g) mp 234–236 ◦ C; IR (KBr; cm−1 ): 3359, 1669, 1607, 1543, 1359, 1230, 1063, 820, 757; 1 H NMR (300 MHz, DMSO-d6 ): ı 2.94 (s, 12H), 2.99 (s, 6H), 6.65 (m, 4H), 6.85 (d, 2H, J = 8.7 Hz), 7.14 (d, 2H, J = 9.0 Hz), 7.31 (d, 3H, J = 8.7 Hz), 7.65 (d, 1H, J = 9.0 Hz), 9.35 (s, 1H); 13 C NMR (300 MHz, DMSO-d6 ): ı 168.91, 152.93, 129.38, 128.13, 119.09, 117.43, 117.25, 114.45, 112.38, 110.78, 38.97, 38.8; ESI MS: m/z: 430.33 [M]+ ; Anal. Calcd. for
The synthesis of MIZON was carried out in single step as depicted in Scheme 1. The condensation of Michler’s ketone 1 with 4-(N,N -dimethylamino)benzoic hydrazide 2 was effected in dichloromethane in presence of BF3 –etherate under reflux for 12 h. The probe, MIZON was isolated as a yellow crystalline solid in 68% yield after extractive work-up and SiO2 column purification. The chemosensor, MIZON belongs to the donor–acceptor module, with strong electron donating, dimethylanilino chromophores placed on either ends of the azomethine and the amide carbonyl groups, together constituting a bidentate metal chelating site. As illustrated in Scheme 1, metal ion binding at the hydrazone chelate would boost its acceptor character, thus inducing enhanced charge donation from the donor, anilino groups. Furthermore, in analogy to the reported metal ion-induced deprotonation of amide bearing chemosensors [41–43], we also anticipated the deprotonation of the hydrazone–NH of MIZON upon interaction with strongly coordinating metal ion(s). These phenomena might entice highly discriminating absorbance and emission modulations to form the basis for selective metal ion detection. 3.2. UV–vis evaluation of metal binding interaction The photophysical sensitivity of MIZON towards physiological and environmental relevant metal ions, Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ , Co2+ and Cu2+ as their perchlorate salts was examined under the optimized solvent condition in H2 O–CH3 CN (1:1 v/v) buffered by 10 mM Tris–HCl to maintain a pH of 7.4 ± 0.1. The UV–vis spectrum of MIZON (2.8 × 10−5 M) displayed absorption maxima at 316 and 364 nm with molar extinction coefficients of 2.89 × 104 M−1 cm−1 and 3.39 × 104 M−1 cm−1 , respectively. These maxima can be attributed to the local and charge transfer (CT) transitions, respectively. Consistent with its CT nature, the lower energy maximum shifted to the red in solvents of increasing polarity, going from 346 nm in cyclohexane to 365 nm in
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Fig. 1. Absorption spectra of MIZON (2.8 × 10−5 M) without and with the Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ , Co2+ (1.41 × 10−2 M each) and Cu2+ (5.67 × 10−4 M) in H2 O–ACN (1:1 v/v) at pH = 7.4 ± 0.1.
methanol (see supporting information). As shown in Fig. 1, except for a slight decrease in the absorbance, the UV–vis profile of the probe remained essentially invariant to the added Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ up to 500 equiv, implying none or very poor affinities of these metal ions towards the probe. However, addition of just 20 equiv of Cu2+ to the probe’s solution (2.83 × 10−5 M) gave rise to a new, highly red shifted maximum at 604 nm at the expense of the original CT band at 364 nm. Additionally, the local transition at 316 nm was hypsochromically shifted to 311 nm with a slight increase in its absorbance. The spectrophotometric titration of MIZON (2.8 × 10−5 M) with Cu2+ is depicted in Fig. 2 and the inset shows the plot of changes in 364 and 604 nm maxima as a function of increasing concentrations of Cu2+ . At 1:20 ligand/Cu2+ mole ratio, the original CT band of the probe is largely replaced by a 604 nm absorption maximum, i.e. a 240 nm red shift. The observation of isosbestic points at 326 and 412 nm suggests the formation of a well-defined MIZON–Cu2+ complex, for which 1:1 complexation stoichiometry was established from the Job’s method of continuous variation (see supporting information). The high red shift of 240 nm in the CT band implies a very strong interaction between MIZON and Cu2+ , which can be
Fig. 3. Ratiometric response of MIZON (2.835 × 10−5 M) towards various metal ions (5.67 × 10−4 M).
attributed to increased charge transfer interaction from the donor, dimethylanilino chromophores, placed in direct conjugation to the chelating hydrazone chromophore, acting as an acceptor. The IR spectrum of MIZON displayed bands at 3359 and 1669 cm−1 , which can be assigned to the stretching of NH and the hydrazone carbonyl functions, respectively. However, in the MIZON–Cu2+ complex, the IR band at 3359 cm−1 is absent, while the 1669 cm−1 band is shifted to lower frequency at 1603 cm−1 . These results support the NH deprotonation and participation of the hydrazone carbonyl in the MIZON-Cu2+ complexation (see supporting information). As shown in the supporting information, Figure 7, no perceptible color variations were noticeable upon adding Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ . By contrast, the initial yellow solution of the probe instantly turned to dark green upon exposure to Cu2+ . This distinctive color change is useful in permitting the naked eye detection of Cu2+ . In addition to the colorimetric response, the spectrophotometric signalling can also be employed for the ratiometric analysis of Cu2+ . Fig. 3 shows the plot of A604 /A364 ratios obtained by adding 5.67 × 10−4 M (ca. 20 equivalents) each of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Cu2+ , Ni2+ and Co2+ into a solution of MIZON at 2.85 × 10−5 M. With the exception of Cu2+ , no significant influence is observed in the A604 /A364 ratios with other metal ions. This experiment clearly validates the practical utility of MIZON as a selective ratiometric Cu2+ sensor in the presence of different metal ions. 3.3. Metal ions binding studies by fluorescence
Fig. 2. Spectrophotometric titration of MIZON (2.83 × 10−5 M) with Cu2+ (0–5.67 × 10−4 M). Inset: Absorbance plot of MIZON against increasing amount of Cu2+ at max 364 and 604 nm.
Excitation of the probe at the Cu2+ -induced absorbance at 604 nm produced no detectable emission spectrum. This is likely due to the internal charge transfer nature of this excitation, which is known to be predominantly non-radiative in character [40,44,35]. However, excitation of the probe at 364 nm exhibited a weakly emissive band at 429 nm with a quantum yield, ˚f of 0.0025, measured with respect to anthracene (˚f = 0.27). The weak emission of MIZON may in part to the electron transfer from the nitrogen lone pair of the C N group to the excited state, i.e. PET quenching and in part to the C N dynamic isomerization [45,46]. As shown in Fig. 4, the fluorimetric titration of the probe in the buffered H2 O–CH3 CN (1:1 v/v) system with increasing Cu2+ resulted in a linear increase in the emission intensity of the 429 nm maximum. At the saturating concentration of 1.2 × 10−4 M of Cu2+ , the intensity of emission maximum peaked, leading to ca. 11-fold enhancement (˚f raising to 0.026) with respect to that of the free probe at 429 nm. Consistent with the spectrophotometric results discussed above, the emission
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Fig. 4. Fluorometric titration (ex = 364 nm) of MIZON (1.0 × 10−6 M) with Cu2+ (0–1.2 × 10−4 M) in H2 O–ACN (1:1 v/v) at pH 7.4.
profile of the probe was also essentially insensitive to the presence of 500 equiv. of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ . This observation indicates weak or total absence of binding interaction of these metal ions even in the excited states. The pH dependent studies revealed essentially no change either in absorbance or the emission behaviour of the probe in the pH range 5.5–11.8. However, upon lowering the pH from 4.5 to 1.5, the probe’s absorption band at 364 nm progressively decreased in intensity, while a new red shifted band at 460 nm emerged concurrently. On the other hand, examination of the fluorescence profile in the pH range 4.5–1.5 revealed increasing quenching of the fluorescence, with a maximum of 80% quenching being observed at pH 1.5 (see supporting information). However, it may be noted that we have performed all our metal ion binding studies at the physiologically relevant, pH 7.4 at which the probe’s optical behaviour is essentially insensitive. The fluorescence switch-on signalling observed in the present case may be due to the chelation-induced blocking of the PET process and the conformational confinement of the C N isomerization [24,45–47]. These processes, together may contribute to suppressing the non-radiative channels, thereby enhancing the emission output of MIZON upon Cu2+ complexation. 3.4. Selective binding of Cu2+ The validity of the selective binding of MIZON towards Cu2+ was established by performing a competitive fluorescence spectral measurement in the presence a matrix of ions. As shown in Fig. 5, addition of 5.0 × 10−4 M each of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ to the solution of the probe (1.0 × 10−6 M) barely affected the fluorescence intensity of the free probe. Subsequently, upon adding Cu2+ (1.2 × 10−4 M) to the matrix solution resulted in the fluorescence intensity enhancement by ca. 10-fold, a value similar to that recorded with Cu2+ alone. This experiment clearly underscores significantly strong complexing ability of MIZON towards Cu2+ even in the presence of relatively higher concentrations other added metal ions. The apparent stability constants (log Ks), determined using the nonlinear curve fitting of the spectrophotometric and fluorimetric titrations gave values of 4.12 and 4.14, respectively. This observation suggests that the binding interaction is of equal magnitude both in the ground and the excited states. The log Ks for other metal ions could not be reliably calculated due to insignificant optical perturbations. The detection limit of Cu2+ , calculated from the
Fig. 5. Fluorescence spectrum of MIZON (1 × 10−6 M), MIZON + matrix of metal ions other than Cu2+ (5.0 × 10−4 M each) and MIZON + matirx + Cu2+ (1.2 × 10−4 M) in H2 O–ACN (1: 1 v/v) solution at pH = 7.4, ex = 364 nm.
more sensitive fluorescence data was found to be 5.5 × 10−7 M (see supporting information). 4. Conclusion In summary, we have designed a new ICT-based probe, MIZON that can selectively signal the presence of Cu2+ by means of dual, naked eye visualization as well as by the fluorescence ‘off–on’ signalling response in buffered H2 O–CH3 CN system. The excitation wavelength lies in the visible region and the probe can measure Cu2+ down to micromolar concentrations. Noteworthily, no significant optical interferences arise from the alkali metal ions, the coordinatively competing and biologically coexisting Mg2+ , Ca2+ , Zn2+ and Co2+ as well as toxic Cd2+ , Ni2+ , Pb2+ and Hg2+ even in concentrations significantly higher than Cu2+ . These attributes augur well for the applications of the probe in monitoring Cu2+ concentrations in real samples down to micromolar range. Acknowledgements The authors express thanks to B.R.N.S., Government of India for generous research funding and M.C. and R.B. thank the U.G.C. for fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2010.08.037. References [1] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997. [2] B.T. Nguyen, E.V. Anslyn, Indicator–displacement assays, Coord. Chem. Rev. 250 (2006) 3118–3127. [3] A.P. De Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Signaling recognition events with fluorescent sensors and switches, Chem. Rev. 97 (1997) 1515–1566. [4] L. Prodi, F. Bolletta, M. Montalti, N. Zaccheroni, Luminescent chemosensors for transition metal ions, Coord. Chem. Rev. 205 (2000) 59–83. [5] R. Martinez-Manez, F. Sancenon, Fluorogenic and chromogenic chemosensors and reagents for anions, Chem. Rev. 103 (2003) 4419–4476. [6] B. Valeur, Molecular Fluorescence. Principles and Applications, Wiley-VCH, Weinheim, 2002.
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Biographies Sabir H. Mashraqui is a Professor of Chemistry at the Department of Chemistry, University of Mumbai, India. He received his PhD degree from I.I.T., Bombay in 1978 and carried out post-doctoral work at Brandies University, U.S.A. (1980–82) and University of Groningen, the Netherlands (1983–85). His current interests are in the areas of cations/anion chemsensors, organic nonlinear optics and synthesis of novel cyclophanes. Mukesh Chandiramani received his MSc degree from the University of Mumbai in the year 2004. He worked as a research assistant at the Sophisticated Analytical Instrument Facility at I.I.T., Bombay (2004–06). Currently, he is doing Ph.D. under the supervision of Prof. Sabir H. Mashraqui. Rupesh Betkar received his MSc degree from the University of Mumbai in the year 2006. Currently, he is doing PhD under the supervision of Prof. Sabir H. Mashraqui. Sushil Ghorpade received his MSc degree from the University of Mumbai in the year 2008. Currently, he is doing PhD under the supervision of Prof. Sabir H. Mashraqui.
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An easily accessible internal charge transfer chemosensor exhibiting dual colorimetric and luminescence switch on responses for targeting Cu2+ Sabir H. Mashraqui ∗ , Mukesh Chandiramani, Rupesh Betkar, Sushil Ghorpade Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E Kalina, Mumbai 400098, Maharashtra, India
a r t i c l e
i n f o
Article history: Received 14 June 2010 Received in revised form 23 August 2010 Accepted 25 August 2010 Available online 24 September 2010 Keywords: Chemosensor Michler’s ketone benzhydrazide Internal charge transfer UV–visible Fluorescence Cu2+ selectivity
a b s t r a c t A new internal charge transfer probe, MIZON was synthesized by condensing Michler’s ketone with 4(N,N -dimethylamino)benzhydrazide. The spectrophotometric and fluorimetric titrations with metal ions of biological and environmental importance were performed in H2 O–CH3 CN (1:1 v/v) system buffered by Tris–HCl, pH = 7.4 ± 0.1. The probe, MIZON was found to interact selectively with Cu2+ , inducing remarkably high absorbance red shift by 240 nm and 11-fold enhancement in the emission intensity. In contrast, several other metal ions, viz. Li+ , Na+ , K+ , Ca2+ , Mg2+ , Ba2+ , Pb2+ , Co2+ , Cd2+ , Zn2+ , Ni2+ and Hg2+ , even at substantially higher concentration than Cu2+ , did not exert significant perturbation in the photophysical profile of the probe. The detection limit of Cu2+ , calculated from the more sensitive fluorescence data was found to be 5.5 × 10−7 M. Potential, therefore is clearly available in MIZON to detect Cu2+ in micromolar range via dual visible color change from yellow to green and fluorescence switch-on response. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The design of optical chemosensors for the selective detection of heavy metal ions of biological and environmental interest is currently attracting interest of chemists, biologists and environmentalists [1–4]. The optical probes typically feature a covalently linked photoresponsive unit with a selective receptor such that the analyte binding translates into recognizable optical transductions, thereby facilitating the detection of specific targets. Of the various photophysical mechanisms known for designing the signalling devices, the photoinduced electron transfer (PET) is by far the most widely used protocol, which delivers either fluorescence on–off or off–on response, depending upon the structures of the chemosensors and the identity of the cations [3,5]. Though, not as extensively investigated, the push–pull chemoreceptors are attracting increasing attention for their capacity to deliver colorimetric as well as fluorimetric signalling responses [6,7]. Copper is essential to all life forms [8] and plays role in many important biological functions, which include haemoglobin synthesis, nerve functions, oxygen binding, and electron transfer processes [9,10]. Copper is also an environmental pollutant and its overdose beyond physiological limits can cause gastrointestinal problems, renal dysfunction as well as serious health disorders such as Alzheimer, Parkinson, Menkes and Wilson diseases [11–13].
∗ Corresponding author. Tel.: +91 22 26526091; fax: +91 22 26528547. E-mail address: sh [email protected] (S.H. Mashraqui). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.08.037
Consequently, strong interest persists in designing selective and sensitive Cu2+ chemosensors for biological and environmental monitoring. A majority of the optical Cu2+ sensors exhibits less desirable fluorescence turn off responses due to the quenching effect of the paramagnetic species [14–22]. Even though, Cu2+ fluorescence turn-on type sensors are steadily increasing in numbers [23–32], however cross-affinities, particularly with Pb2+ , Hg2+ and Ni2+ ions, aqueous incompatibility or short wavelength excitation limit their practical applications. Consequently, there is room to improve the binding characteristics of Cu2+ chemosensors and, given its quenching character, it is especially attractive to design probes offering dual color modulation as well as fluorescence amplification upon Cu2+ binding. An attractive feature of photoreceptors with donor–acceptor character is the possibility of synthetically tuning the transition energy of their internal charge transfer (ICT) states to allow colorimetric ‘naked eye’ response [33]. Hydrazones derived from heterocyclic chromophores are well-known spectrophotometric reagents for the detection of transition metal ions. For examples, Lucifer yellow carbohydrazide is known to bind Cu2+ selectively [34] and recently Jiang and coworkers reported a lariate 4-(N,N -dimethylamino)benzhydrazide as an efficient fluorescent charge transfer probe for Cu2+ [35]. In context to our ongoing interest in the ICT-based metal ion sensors [36–40], we now report the synthesis and metal ion binding studies of a new hydrazone derivative, 4-dimethylamino-benzoic acid[bis(4-dimethylamino-phenyl)-methylene]-hydrazide, designated as MIZON. The chemosensor, MIZON belongs to the donor–acceptor
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Scheme 1. Synthesis and the proposed interaction of MIZON with metal ions, Mn+ .
module, featuring azomethine and an amide carbonyl as the bidentate metal chelating ligand. As our photophysical studies reveal, the probe has potential to be utilized as a highly Cu2+ selective colorimetric as well as fluorescent turn on chemosensor in H2 O–CH3 CN aqueous buffered solution at pH 7.4.
C26 H31 N5 O: C, 72.72; H, 7.22; N, 16.31. Found: C, 72.93; H, 7.54; N, 16.09. 3. Results and discussion 3.1. Synthesis and proposed metal ion interaction of MIZON
2. Experimental 2.1. Reagents and instrumentation Reagents and solvents used were purchased from S.D. Fine Chemicals or Sigma–Aldrich, India. IR was recorded using PerkinElmer FT-IR spectrometer and pellets were made by using KBr. 1 H NMR and 13 C NMR spectra were recorded on BRUKER make, model Avance II 300 of 300 MHz. UV–vis spectra were recorded using Shimadzu UV–vis recording spectrophotometer, model no. UV-2401PC. Fluorescence studies were carried out using Shimadzu spectrofluorometer, model no. RF-5301PC. The slit width was set at 3 nm for both excitation and emission and the PMT detector voltage was 700 V. 2.2. Synthesis of 4-dimethylamino-benzoic acid[-bis-(4-dimethylamino-phenyl)-methylene]-hydrazide, MIZON Michler’s ketone 1 (0.268 g, 1 mmol) and 4-(N,N dimethylamino) benzoic hydrazide 2 (0.179 g, 1 mmol) were refluxed in 20 ml of dichloromethane containing 1 ml of BF3 –etherate for 12 h. The reaction mixture was concentrated under reduced pressure and the crude oily product was treated with saturated aqueous NaHCO3 . Greenish-yellow solid obtained was filtered, washed with water and air dried. Purification by column chromatography using SiO2 (eluant CHCl3 ) allowed isolation of the desired product, MIZON as pale yellow crystalline solid in 68% yield (0.292 g) mp 234–236 ◦ C; IR (KBr; cm−1 ): 3359, 1669, 1607, 1543, 1359, 1230, 1063, 820, 757; 1 H NMR (300 MHz, DMSO-d6 ): ı 2.94 (s, 12H), 2.99 (s, 6H), 6.65 (m, 4H), 6.85 (d, 2H, J = 8.7 Hz), 7.14 (d, 2H, J = 9.0 Hz), 7.31 (d, 3H, J = 8.7 Hz), 7.65 (d, 1H, J = 9.0 Hz), 9.35 (s, 1H); 13 C NMR (300 MHz, DMSO-d6 ): ı 168.91, 152.93, 129.38, 128.13, 119.09, 117.43, 117.25, 114.45, 112.38, 110.78, 38.97, 38.8; ESI MS: m/z: 430.33 [M]+ ; Anal. Calcd. for
The synthesis of MIZON was carried out in single step as depicted in Scheme 1. The condensation of Michler’s ketone 1 with 4-(N,N -dimethylamino)benzoic hydrazide 2 was effected in dichloromethane in presence of BF3 –etherate under reflux for 12 h. The probe, MIZON was isolated as a yellow crystalline solid in 68% yield after extractive work-up and SiO2 column purification. The chemosensor, MIZON belongs to the donor–acceptor module, with strong electron donating, dimethylanilino chromophores placed on either ends of the azomethine and the amide carbonyl groups, together constituting a bidentate metal chelating site. As illustrated in Scheme 1, metal ion binding at the hydrazone chelate would boost its acceptor character, thus inducing enhanced charge donation from the donor, anilino groups. Furthermore, in analogy to the reported metal ion-induced deprotonation of amide bearing chemosensors [41–43], we also anticipated the deprotonation of the hydrazone–NH of MIZON upon interaction with strongly coordinating metal ion(s). These phenomena might entice highly discriminating absorbance and emission modulations to form the basis for selective metal ion detection. 3.2. UV–vis evaluation of metal binding interaction The photophysical sensitivity of MIZON towards physiological and environmental relevant metal ions, Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ , Co2+ and Cu2+ as their perchlorate salts was examined under the optimized solvent condition in H2 O–CH3 CN (1:1 v/v) buffered by 10 mM Tris–HCl to maintain a pH of 7.4 ± 0.1. The UV–vis spectrum of MIZON (2.8 × 10−5 M) displayed absorption maxima at 316 and 364 nm with molar extinction coefficients of 2.89 × 104 M−1 cm−1 and 3.39 × 104 M−1 cm−1 , respectively. These maxima can be attributed to the local and charge transfer (CT) transitions, respectively. Consistent with its CT nature, the lower energy maximum shifted to the red in solvents of increasing polarity, going from 346 nm in cyclohexane to 365 nm in
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Fig. 1. Absorption spectra of MIZON (2.8 × 10−5 M) without and with the Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ , Co2+ (1.41 × 10−2 M each) and Cu2+ (5.67 × 10−4 M) in H2 O–ACN (1:1 v/v) at pH = 7.4 ± 0.1.
methanol (see supporting information). As shown in Fig. 1, except for a slight decrease in the absorbance, the UV–vis profile of the probe remained essentially invariant to the added Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ up to 500 equiv, implying none or very poor affinities of these metal ions towards the probe. However, addition of just 20 equiv of Cu2+ to the probe’s solution (2.83 × 10−5 M) gave rise to a new, highly red shifted maximum at 604 nm at the expense of the original CT band at 364 nm. Additionally, the local transition at 316 nm was hypsochromically shifted to 311 nm with a slight increase in its absorbance. The spectrophotometric titration of MIZON (2.8 × 10−5 M) with Cu2+ is depicted in Fig. 2 and the inset shows the plot of changes in 364 and 604 nm maxima as a function of increasing concentrations of Cu2+ . At 1:20 ligand/Cu2+ mole ratio, the original CT band of the probe is largely replaced by a 604 nm absorption maximum, i.e. a 240 nm red shift. The observation of isosbestic points at 326 and 412 nm suggests the formation of a well-defined MIZON–Cu2+ complex, for which 1:1 complexation stoichiometry was established from the Job’s method of continuous variation (see supporting information). The high red shift of 240 nm in the CT band implies a very strong interaction between MIZON and Cu2+ , which can be
Fig. 3. Ratiometric response of MIZON (2.835 × 10−5 M) towards various metal ions (5.67 × 10−4 M).
attributed to increased charge transfer interaction from the donor, dimethylanilino chromophores, placed in direct conjugation to the chelating hydrazone chromophore, acting as an acceptor. The IR spectrum of MIZON displayed bands at 3359 and 1669 cm−1 , which can be assigned to the stretching of NH and the hydrazone carbonyl functions, respectively. However, in the MIZON–Cu2+ complex, the IR band at 3359 cm−1 is absent, while the 1669 cm−1 band is shifted to lower frequency at 1603 cm−1 . These results support the NH deprotonation and participation of the hydrazone carbonyl in the MIZON-Cu2+ complexation (see supporting information). As shown in the supporting information, Figure 7, no perceptible color variations were noticeable upon adding Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ . By contrast, the initial yellow solution of the probe instantly turned to dark green upon exposure to Cu2+ . This distinctive color change is useful in permitting the naked eye detection of Cu2+ . In addition to the colorimetric response, the spectrophotometric signalling can also be employed for the ratiometric analysis of Cu2+ . Fig. 3 shows the plot of A604 /A364 ratios obtained by adding 5.67 × 10−4 M (ca. 20 equivalents) each of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Cu2+ , Ni2+ and Co2+ into a solution of MIZON at 2.85 × 10−5 M. With the exception of Cu2+ , no significant influence is observed in the A604 /A364 ratios with other metal ions. This experiment clearly validates the practical utility of MIZON as a selective ratiometric Cu2+ sensor in the presence of different metal ions. 3.3. Metal ions binding studies by fluorescence
Fig. 2. Spectrophotometric titration of MIZON (2.83 × 10−5 M) with Cu2+ (0–5.67 × 10−4 M). Inset: Absorbance plot of MIZON against increasing amount of Cu2+ at max 364 and 604 nm.
Excitation of the probe at the Cu2+ -induced absorbance at 604 nm produced no detectable emission spectrum. This is likely due to the internal charge transfer nature of this excitation, which is known to be predominantly non-radiative in character [40,44,35]. However, excitation of the probe at 364 nm exhibited a weakly emissive band at 429 nm with a quantum yield, ˚f of 0.0025, measured with respect to anthracene (˚f = 0.27). The weak emission of MIZON may in part to the electron transfer from the nitrogen lone pair of the C N group to the excited state, i.e. PET quenching and in part to the C N dynamic isomerization [45,46]. As shown in Fig. 4, the fluorimetric titration of the probe in the buffered H2 O–CH3 CN (1:1 v/v) system with increasing Cu2+ resulted in a linear increase in the emission intensity of the 429 nm maximum. At the saturating concentration of 1.2 × 10−4 M of Cu2+ , the intensity of emission maximum peaked, leading to ca. 11-fold enhancement (˚f raising to 0.026) with respect to that of the free probe at 429 nm. Consistent with the spectrophotometric results discussed above, the emission
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Fig. 4. Fluorometric titration (ex = 364 nm) of MIZON (1.0 × 10−6 M) with Cu2+ (0–1.2 × 10−4 M) in H2 O–ACN (1:1 v/v) at pH 7.4.
profile of the probe was also essentially insensitive to the presence of 500 equiv. of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ . This observation indicates weak or total absence of binding interaction of these metal ions even in the excited states. The pH dependent studies revealed essentially no change either in absorbance or the emission behaviour of the probe in the pH range 5.5–11.8. However, upon lowering the pH from 4.5 to 1.5, the probe’s absorption band at 364 nm progressively decreased in intensity, while a new red shifted band at 460 nm emerged concurrently. On the other hand, examination of the fluorescence profile in the pH range 4.5–1.5 revealed increasing quenching of the fluorescence, with a maximum of 80% quenching being observed at pH 1.5 (see supporting information). However, it may be noted that we have performed all our metal ion binding studies at the physiologically relevant, pH 7.4 at which the probe’s optical behaviour is essentially insensitive. The fluorescence switch-on signalling observed in the present case may be due to the chelation-induced blocking of the PET process and the conformational confinement of the C N isomerization [24,45–47]. These processes, together may contribute to suppressing the non-radiative channels, thereby enhancing the emission output of MIZON upon Cu2+ complexation. 3.4. Selective binding of Cu2+ The validity of the selective binding of MIZON towards Cu2+ was established by performing a competitive fluorescence spectral measurement in the presence a matrix of ions. As shown in Fig. 5, addition of 5.0 × 10−4 M each of Li+ , Na+ , K+ , Ba2+ , Ca2+ , Mg2+ , Pb2+ , Hg2+ , Cd2+ , Zn2+ , Ni2+ and Co2+ to the solution of the probe (1.0 × 10−6 M) barely affected the fluorescence intensity of the free probe. Subsequently, upon adding Cu2+ (1.2 × 10−4 M) to the matrix solution resulted in the fluorescence intensity enhancement by ca. 10-fold, a value similar to that recorded with Cu2+ alone. This experiment clearly underscores significantly strong complexing ability of MIZON towards Cu2+ even in the presence of relatively higher concentrations other added metal ions. The apparent stability constants (log Ks), determined using the nonlinear curve fitting of the spectrophotometric and fluorimetric titrations gave values of 4.12 and 4.14, respectively. This observation suggests that the binding interaction is of equal magnitude both in the ground and the excited states. The log Ks for other metal ions could not be reliably calculated due to insignificant optical perturbations. The detection limit of Cu2+ , calculated from the
Fig. 5. Fluorescence spectrum of MIZON (1 × 10−6 M), MIZON + matrix of metal ions other than Cu2+ (5.0 × 10−4 M each) and MIZON + matirx + Cu2+ (1.2 × 10−4 M) in H2 O–ACN (1: 1 v/v) solution at pH = 7.4, ex = 364 nm.
more sensitive fluorescence data was found to be 5.5 × 10−7 M (see supporting information). 4. Conclusion In summary, we have designed a new ICT-based probe, MIZON that can selectively signal the presence of Cu2+ by means of dual, naked eye visualization as well as by the fluorescence ‘off–on’ signalling response in buffered H2 O–CH3 CN system. The excitation wavelength lies in the visible region and the probe can measure Cu2+ down to micromolar concentrations. Noteworthily, no significant optical interferences arise from the alkali metal ions, the coordinatively competing and biologically coexisting Mg2+ , Ca2+ , Zn2+ and Co2+ as well as toxic Cd2+ , Ni2+ , Pb2+ and Hg2+ even in concentrations significantly higher than Cu2+ . These attributes augur well for the applications of the probe in monitoring Cu2+ concentrations in real samples down to micromolar range. Acknowledgements The authors express thanks to B.R.N.S., Government of India for generous research funding and M.C. and R.B. thank the U.G.C. for fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2010.08.037. References [1] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997. [2] B.T. Nguyen, E.V. Anslyn, Indicator–displacement assays, Coord. Chem. Rev. 250 (2006) 3118–3127. [3] A.P. De Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Signaling recognition events with fluorescent sensors and switches, Chem. Rev. 97 (1997) 1515–1566. [4] L. Prodi, F. Bolletta, M. Montalti, N. Zaccheroni, Luminescent chemosensors for transition metal ions, Coord. Chem. Rev. 205 (2000) 59–83. [5] R. Martinez-Manez, F. Sancenon, Fluorogenic and chromogenic chemosensors and reagents for anions, Chem. Rev. 103 (2003) 4419–4476. [6] B. Valeur, Molecular Fluorescence. Principles and Applications, Wiley-VCH, Weinheim, 2002.
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Biographies Sabir H. Mashraqui is a Professor of Chemistry at the Department of Chemistry, University of Mumbai, India. He received his PhD degree from I.I.T., Bombay in 1978 and carried out post-doctoral work at Brandies University, U.S.A. (1980–82) and University of Groningen, the Netherlands (1983–85). His current interests are in the areas of cations/anion chemsensors, organic nonlinear optics and synthesis of novel cyclophanes. Mukesh Chandiramani received his MSc degree from the University of Mumbai in the year 2004. He worked as a research assistant at the Sophisticated Analytical Instrument Facility at I.I.T., Bombay (2004–06). Currently, he is doing Ph.D. under the supervision of Prof. Sabir H. Mashraqui. Rupesh Betkar received his MSc degree from the University of Mumbai in the year 2006. Currently, he is doing PhD under the supervision of Prof. Sabir H. Mashraqui. Sushil Ghorpade received his MSc degree from the University of Mumbai in the year 2008. Currently, he is doing PhD under the supervision of Prof. Sabir H. Mashraqui.