Accepted Manuscript A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan Kaku Dutta, Ramesh C. Deka, Diganta Kumar Das PII: DOI: Reference:
S1386-1425(13)01521-7 http://dx.doi.org/10.1016/j.saa.2013.12.090 SAA 11459
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
2 September 2013 12 November 2013 15 December 2013
Please cite this article as: K. Dutta, R.C. Deka, D.K. Das, A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2013.12.090
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A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan Kaku Dutta. Ramesh C. Dekaa. Diganta Kumar Das* Department of Chemistry, Gauhati University, 781014, Assam, India, e-mail [email protected] a
Department of Chemistry, Tezpur University, Napaam, Tezpur – 784 028, Assam, India.
Address for corresponding author:
Diganta Kumar Das Department of Chemistry, Gauhati University, Guwahati-781014, Assam, India Email: [email protected] Tel: +91-0361-2570535 Fax: +91-0361-2700311
ABSTRACT Single molecule acting as both fluorescent and electrochemical sensor for Zn2+ ion is rare. The product (L) obtained on condensation between benzil and L-tryptophan has been characterized by H NMR, ESIMS and FT-IR spectroscopy. L in 1:1 (v/v) CH3OH:H2O solution shows fluorescence emission in the range 300 nm to 600 nm with max at 350 nm when is excited with 295 nm. Zn2+ ion could induce a ten fold enhancement in fluorescent intensity of L. Fluorescence and UV/visible spectral data analysis shows that the binding ratio between Zn2+ ion and L is 1:1 with log= 4.55. Binding of Zn2+ ion disrupts the photoinduced electron transfer (PET) process in L and causes the fluorescence intensity enhancement. When cyclic voltammogram is recorded for L in 1:1 (v/v) CH3OH:H2O using glassy carbon (GC) electrode, two quasi reversible redox couples at redox potential values -0.630 ± 0.005 V and -1.007 ± 0.005 V are obtained (Ag-AgCl as reference, Scan rate 0.1 Vs-1). Interaction with Zn2+ makes the first redox couple irreversible while the second couple undergoes a 0.089 V positive shift in redox potential. Metal ions - Cd2+, Cu2+, Co2+, Hg2+, Ag+, Ni2+, Fe2+, Mn2+, Mg2+, Ca2+and Pb2+, individually or together, has no effect on the fluorescent as well as electrochemical property of L. DFT calculations showed that Zn2+ ion binds to L to form a stable complex. The detection limit for both fluorescence as well as electrochemical detection was 10-6 M. Keywords L-tryptophan. Fluorescence. Photoinduced electron transfer. Zn2+. Sensor. Cyclic voltammetry.
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1. Introduction Zinc is an essential element in all biological systems [1]. Among many functions - Zinc regulates gene expression [2], is involved in cellular apoptosis [3], is constituent of RNA and DNA polymerases [4]. Zinc also helps immune system, endocrine system and gastroenterological systems [5]. Deficiency in Zn2+ or disorder in Zn metabolism is believed to lead neurological problems and diseases such as Alzheimer’s, amyotrophic lateral sclerosis, Parkinson’s disease and epilepsy [6]. Fluorescent sensors to detect metal ions have gained considerable interest because of their simplicity, high sensitivity and real-time detection [7-10]. A number of fluorescent sensors are known for zinc. A novel pyrazoline derivative was synthesized starting from a chalcone and 3-chloro-6hydrazinylpyridazine and found to determine Zn2+ ion with high selectivity and a low detection limit in CH3CN:EtOH (90/10, v/v) [11]. New bis(pyrrol-2-yl-methyleneamine) ligands were also reported to act as selective sensor for Zn2+ [12]. NNO-di(quinoline-2-methylene) -1,2-phenylenediimine was reported to exhibit high selectivity toward Zn2+ over other metal ions including Cd2+ due to the formation of a 1:1 metal:ligand complex [13]. A ratiometric fluorescent sensor for zinc ion based on covalently immobilized derivative of benzoxazole is also known [14]. Fluorescent sensors for Zn2+ ion are also known based on 2-1minobenzamide [15] and 2-(2′-hydroxy-3′-naphthyl)benzoxazole chromophore and di(2-picolyl)amine [16], We recently reported fluorescent Zn2+ ion sensors based on ferrocene derivative [17] and macrocyclic ligands having N,N,N,N coordination sites [18]. A novel fluorescent sensor which can selectively detect Zn2+ ion in aqueous media is reported [19]. Off-on fluorescent chemosensor based on diketopyrrolopyrrole is reported which has been applied for imaging Zn2+ ion in living cell [20]. BINOL-based ratiometric fluorescent sensor for Zn2+ ion with fluorescence “off-on” mode in aqueous medium has been reported [21]. Ratiometric fluorescent Zn2+ ion sensor of carboxamidoquinoline with 2-chloro- N-(quinol-8-yl)-acetamide as a receptor demonstrating a 15 fold enhancement in fluorescence intensity has been synthesized [22]. Turn-on fluorescent sensor based on a triphenylamine-aminophenol conjugate has been synthesized for Zn2+ ion sensing in CH3CN [23]. Pyridine–pyridone scaffold based fluorescence “on-off” densor for Zn2+ ion is also known [24]. Bis(8-carboxamidoquinoline) dangled binaphthol derivatized fluorescent sensor for “off-on” mode detection of Zn2+ ion is reported recently [25]. Carbazole incorporated thiazole based Zn2+ ion selective fluorescent sensor by fluorescent “on” mode is reported [26]. Reports on electrochemical sensing of Zn2+ ion is seemingly rare. Reports are available for Zn2+ ion determination based on adsorptive stripping voltammetry [27,28] and cyclic voltammetry [29,30] techniques. Electrochemical sensors for Zn2+ ion estimation in water and brain are reported [31,32]. One single molecule capable of selectively sensing Zn2+ ion both by fluorescence and electrochemistry is probably not reported.
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Here we report that the Schiff base obtained by condensation between benzyl and tryptophan act as a fluorescent as well as voltammetric sensor for Zn2+ ion which is free from interferences of metal ions Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+. Density function theory (DFT) is increasingly being recognized as an important tool to determine structural and electronic properties of metal complexes. We performed DFT calculations on the zinc complex to confirm the stability of the complex.
2. Experimental 2.1 Chemicals and experimental techniques All the chemicals were from Merck. The metal salts (except AgNO3 and PbNO3) were sulphates and recrystallized from water (Millipore). Dichloromethane (99.9%), triethylamine (99.9%), methanol (99.9%, HPLC grade), DMSO (99.9%, HPLC grade), benzil and l-tryptophan (Merck, Germany) were reagent grades and used as received. Alumina powder (99.9 %) was used as a polishing reagent (CHI, USA). Metal salt solutions (10−6 M) were prepared in millipore water. 10-2 M standard stock of 1:1 (v/v) CH3OH:H2O solution of L was prepared. The UV/Visible spectra were recorded on a Shimadzu UV/Vis -1800 spectrophotometer. Fluorescence spectra were recorded in a Hitachi 2500 spectrophotometer using quartz cuvette at room temperature. 1H-NMR spectra were recorded using a Bruker Ultrashield 300 MHz NMR spectrometer. Chemical shifts were expressed in ppm (in DMSO-d6, with TMS as internal standard) and coupling constants (J) in Hz. FTIR data were measured as KBr pallet, using a Perkin Elmer spectrophotometer (RX1). Electrochemical experiments were carried out at room temperature using CHI 600B Electrochemical Analyzer (USA) in a conventional three electrode system with Ag/AgCl (3M NaCl) as the reference electrode, a platinum wire as the counter electrode and a glassy carbon disc (GC) as working electrode. 0.1 M tetrabutylammonium perchlorate (TBAP) solution was used as supporting electrolyte. Nitrogen gas was purged through the electrolytic solution (3 mL) for at least 5 minutes to remove any dissolved oxygen before every experiment. Nitrogen atmosphere was maintained over the electrolytic solution during each experiment. Prior to every experiment the GC electrode was cleaned as reported [33].
2.2 Synthesis and characterisation of L 204 mg (1 mmol) of L-tryptophan was taken in 30 mL dichloromethane (DCM) in a beaker. To this solution 1 mL (5 mmol) of triethylamine was added and stirred till a clear solution was obtained. 210 mg (1 mmol) of benzil was added to this solution and stirred overnight and a pale yellow precipitate was obtained (L). The solvent was evaporated in a rotary evaporator and the product was washed with 3
2% HCl water solution to remove the excess triethylamine as hydrochloride salt. Product obtained upon solvent removal was purified by SiO2 column using ethylacetate/hexane (98:2) as an eluent. Thin layer chromatography showed Rf = 0.61. (Yield, 59%; melting point, 200 ºC; solubility, DMSO). The UV/Visible spectra of L in 1:1 (v/v) CH3OH:H2O solution showed λmax at 260 nm. 1H NMR of L showed δH values at 3.19 (merged with DMSO water) and 2.5 (s) ppm (-CH2-); at 4.2 (s) ppm (CH-, 1 α C=N), at 7.48 and 7.5 ppm (-CH- of benzene ring), at 7.6 ppm (-CH-. 1 β C=N), at 7.7 for (CH-, 1 β C=N and 1α C=O and vice versa) at 7.84 for (-CH-, 1 β C=O). FTIR spectra showed peaks at 1709 cm-1 (νC=N); 1606 cm-1 (νC=O, carboxylate ion); 1528 cm-1 (νNH deformation) and 3431 cm-1 (νindole NH), no broad band at 3040 cm-1 indicates absence of NH4+. ESI-MS showed molecular ion peak at 398. The detailed fragmentation patterns with peak assignments have been provided in supplementary information.
3. Results and discussion 3.1 Fluorescence studies of interaction between L and Zn2+ ion The fluorescence spectra of L (10-2 M) in 1:1 (v/v) CH3OH:H2O solution was recorded by varying the excitation wavelength. Highest fluorescence intensity with max at 350 nm was observed for emission range 300 nm to 600 nm when excited with 295 nm with slit width 5 nm both for excitation and emission. The fluorescence intensity remained unaffected for over a period of 1 week after the preparation of the solution. Hence, the excitation was kept at 295 nm for all the experiments. We studied the effect of pH in the range 2.0 to 12.0 on the fluorescence spectra of L. Fig 1 shows the plot of fluorescence intensity versus pH which confirms that the intensity is independent of pH till the value 10.0 beyond which it increases. All our experiments have been performed at pH 7.0. The quantum yield of L was found to be 0.10. The fluorescence intensity of L in 1:1 (v/v) CH3OH:H2O solution was measured at different added concentration of a number of metal ions – Na+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+, Hg2+ and Pb2+. A steady and smooth enhancement in fluorescent intensity was observed only for Zn2+ ion that saturates at 1.0 equivalent (Fig. 2). The overall enhancement in intensity was found to be 10 fold to the intensity when no Zn2+ ion was present. The plot of I/Io (I is the intensity at a given concentration of Zn2+ ion, Io is the intensity at zero Zn2+ ion concentration) found to increase linearly with Zn2+ ion concentration (Fig.2, inset). The enhancement in fluorescence intensity of L on interaction with Zn2+ ion may be attributed to the reversal of the photoelectron transfer of electrons from oxygen to indole rings upon Zn2+ binding (Scheme 1). The quantum yield of L in presence of Zn2+ ion was calculated to be 0.14 which is 0.04 higher than that of pure L which further confirms interaction between L and Zn2+ ion. 4
Complex formation between L and Zn2+
Structure of L
Scheme 1 Fig 3 compares the effect of Zn2+ ion and a number of other metal ions on the fluorescent intensity of L through a bar diagram. From the figure it is clear that effect of Zn2+ ion on the fluorescent intensity of L is quite distinct from the rest of the metal ions. Titrations carried out with metal ions Ca2+, Mg2+, Mn2+, Pb2+ and Cd2+ registered a very small increase in fluorescence intensity. Titrations carried out with metal ions Fe2+, Ni2+ and Cu2+ ions exhibited small quenching while moderate quenching was observed for Hg2+ ion (Fig. 3). Addition of Co2+ ion showed almost no change in fluorescence intensity of L.
3.2 Determination of binding constant by fluorescence and UV/Visible spectroscopy The binding constant and the stoichiometry of binding between L and Zn2+ ion was calculated as reported [34] by plotting log [(I-Io)/(Imax -I)] against log[Zn2+]. Fig. 4 shows this plot which is linear and a least squares fitting of data yielded the slope as 0.9874 ± 0.02 (R2 =0.998) indicating a 1:1 binding between L and Zn2+. The binding constant (β) was calculated to be log β = 4.55. In order to confirm the binding and stoichiometry of Zn2+ ion with the ligand, absorbtion spectra of L were recorded in 1:1 (v/v) CH3OH:H2O at different added concentration of Zn2+ ion (Fig 5A). The absorbance at max value 262 nm was found to decrease with increasing Zn2+ ion concentration. An isobestic point was observed at 228 nm indicating a transition between free species and complex species [35]. The plot of log[(Ao-As)/(As-A)] versus log [Zn2+], for the absorbance at max value 262 nm, was found to be linear (Fig. 5B). The least squares fitting of data showed that the slope was 1.005 ± 0.002 (R2 = 0.99) indicating 1:1 stoichiometric binding between L and Zn2+ which is in agreement with that obtained from fluorescence data. The binding constant was calculated and found to be (log β = 3.9) which is close to that obtained from fluorescence intensity plot.
3.3 Interference by other metal ions To further investigate the interfering effect of other metal ions on fluorescent detection of Zn2+ ion, a mixed solution of metal ions - Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+
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was prepared in 1:1 (v/v) CH3OH:H2O containing L (10-2 M), so that concentration of each metal ion in the solution was 10-6 M. The anions provided by these metal salts in the solution were Cl-, SO42-, NO3and CH3COO-. Fig. 6 shows the effect of Zn2+ ion on the fluorescence intensity of L when the solution has the mixture of cations and anions. Fluorescent titration of this solution against Zn2+ ion imparted 9.5 fold enhancement in fluorescent intensity, equivalent to the enhancement when solution had no other ions. This clearly confirms that L can specifically detect Zn2+ ion in presence of a number of other metal ions as well as anions. 3.4 DFT calculation of interaction between L and Zn2+ ion For theoretical calculation studies, the initial structure of the Zn2+ complex was generated from the available experimental data. The complex was fully optimized using BLYP functional and DNP basis sets as implemented in the program DMol3 [36]. In order to confirm the stability of the complex we performed vibrational frequencies calculations at the optimized structure with the same level of theory. The optimized geometry of the zinc complex along with Zn-O and Zn-N distances is shown in Fig. 7. The optimised geometry is in accordance with that proposed in Scheme 1. The coordination environment is ONO involving one O each from carbonyl and acetate origin and the immine N. The calculation clearly shows that the imidazole N is not involved in bonding. The theoretically calculated Zn-O and Zn-N bond distances are in agreement with available experimental results. In the vibrational frequency calculations, no imaginary frequency was found for the complex suggesting that the optimized complex represents a stable structure (local minima) in the potential energy surface. 3.5 Electrochemical study of interaction between L and Zn2+ ion Fig. 8 shows the cyclic voltammogram of L (0.02 mM) in 1:1 (v/v) CH3OH:H2O at different scan rates on GC electrode. Two redox couples were observed with redox potential values -0.630 ± 0.005 V (E = 0.100 V) and -1.007 ± 0.005 V (E = 0.180 V) at scan rate 0.1 Vs-1 versus Ag-AgCl as reference. It is reported that benzil in acetonitrile show quasi reversible cyclic voltammogram with redox potential value -1.002 V versus Ag-AgCl reference electrode [37]. Therefore the redox couple obtained at -1.007 ± 0.005 V must originate at the benzil part of L. The plot of cathodic and anodic currents for both the redox couples against square root of scan rate was found to be linear indicating a reversible redox process. Inset, Fig. 8 shows the current versus square root of scan rate plot for the -1.007 ± 0.005 V couple. The effect of Zn2+ ion on cyclic voltammogram of L has been shown in Fig. 9A. The solid line curve is for the sensor when no Zn2+ ion was added and dotted line curve is when Zn2+ ion was present in the solution. Gradual addition of aqueous solution of Zn2+ ion into the electrolytic medium shifted the cyclic voltammogram of -1.007 ± 0.005 V couple in positive direction till the redox potential became 0.918 V ± 0.005 V at Zn2+ ion concentration 0.061 mM and remained constant thereafter. Thus a 6
+ 0.089 V shift was observed due to interaction with Zn2+ ion. The oxidation peak current of the other couple was found to decrease with Zn2+ ion and finally became irreversible. The dependence of redox potential of L versus Zn2+ ion concentration for the -1.007 ± 0.005 V couple has been shown in Fig. 9B. Cyclic voltammogram of L was also recorded in presence of Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+ ions. No significant effect in the cyclic voltammogram of L was observed when these ions were added into the electrolytic medium singly or together.
3.6 Analytical applications We have tested the ability of L in determination of Zn2+ ion concentration in multimineral tablet. Multimineral tablet (Supra Cal, MMC Health Care, H.P. India) containing 280 mg Ca2+, 100 mg Mg2+, 200 IU vitamin D3 and 4 mg Zn2+ per tablet was purchased from chemist’s shop. One tablet was dissolved in 50 mL phosphate buffer solution (PBS) (pH 7.0) and a standard solution of Zn2+ ion was prepared by dissolving 0.4 g ZnSO4 in 50 mL PBS. The fluorescent intensity of 0.1 mM solution of L was measured, separately by adding standard Zn2+ ion solution as well as the tablet solution so that Zn2+ ion concentration in solution was 0.025 mM, 0.05 mM, 0.075 mM, 0.1 mM, 0.125 mM and 0.150 mM. Fig 10 compares the relative increase in intensity (that is the difference in intensity at a given concentration of Zn2+ from the intensity at zero Zn2+ ion concentration divided by the intensity at zero Zn2+ ion concentration) at the same Zn2+ ion concentration for the standard and tablet solution and found to be in excellent agreement. We have tested the ability of L in determination of Zn2+ ion concentration in multimineral tablet. Multimineral tablet (Supra Cal, MMC Health Care, H.P. India) containing 280 mg Ca2+, 100 mg Mg2+, 200 IU vitamin D3 and 4 mg Zn2+ per tablet was purchased from chemist’s shop. One tablet was dissolved in 50 mL millipore water (pH 7) and a standard solution of Zn2+ ion was prepared by dissolving 0.4 g ZnSO4 in 50 mL millipore water (pH 7). The fluorescent intensity of 0.1 mM solution of L was measured, separately by adding standard Zn2+ ion solution as well as the tablet solution so that Zn2+ ion concentration in solution was 0.025 mM, 0.05 mM, 0.075 mM, 0.1 mM, 0.125 mM and 0.150 mM. Fig. 10 compares the relative increase in intensity (that is the difference in intensity at a given concentration of Zn2+ from the intensity at zero Zn2+ ion concentration divided by the intensity at zero Zn2+ ion concentration) at the same Zn2+ ion concentration for the standard and tablet solution and found to be in excellent agreement.
5. Conclusion In summary, we have shown that the prepared compound L acts as a fluorescent sensor for Zn2+ ion by “switch on” mode. L and also act as electrochemical sensor for Zn2+ ion. A host of metal ions Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+ has no interfering effect on fluorescent determination of Zn2+. This group of metal ions do not interfere the electrochemical 7
interaction between L and Zn2+ also. The structural parameters of the complex calculated using DFT are in very good agreement with the available experimental values. The absence of imaginary vibrational frequency in the calculated spectra confirms that zinc ion binds to the ligand to form a stable structuact.
Acknowledgement Department of Science & Technology, New Delhi (FIST program) and University Grants Commission, New Delhi (SAP program) are thanked for financial help.
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Figure caption:
Fig. 1 Fluorescence response of 1:1 (v/v) CH3OH:H2O solution of L at different pH values from 2.0 to 12.0. Fig. 2 Changes in fluorescence spectra of L as a function of added Zn2+ ion concentration in 1:1 (v/v) CH3OH:H2O solution. Inset: Plot of I/Io as a function of Zn2+ ion concentration. Fig. 3 Bar diagram showing the effect of metal ions Fe2+, Cu2+, Cd2+, Co2+, Pb2+, Hg2+, Ag+, Ca2+, Mn2+, Ni2+ and Mg2+ ions (1 equivalent of various metal ions in 1:1 CH3OH:H2O) on the fluorescence intensity of L. Fig. 4 Plot of log [Zn2+] as a function of log {(Imax -I)/(I - Io)} for L against Zn2+ in 1:1 CH3OH:H2O. Fig. 5 [A] The absorbtion spectrum of L in 1:1 CH3OH:H2O solution at different added concentration of Zn2+. [B] Plot of log(Ao-As/As-A) as a function of Zn2+ ion concentration. The plot shows a slope of 1.1 1 (R2 = 0.9978) indicating 1:1 complexation between Zn2+ and L. Fig. 6 Change in fluoresence intensity of L as a function of Zn2+ ion (0 - 0.40x10-4 M) in presence of 10-6 M each of Na2(CH3COO), CaCl2, MgSO4, MnSO4, FeSO4, CoNO3, NiSO4, CuSO4, CdSO4, PbNO3, HgCl2 and AgNO3. Fig. 7 [A] Optimized structure of the zinc complex. [B] HOMO and LUMO orbitals of the optimized zinc complex.
Fig. 8 Cyclic voltammogram of L in 1:1(v/v) CH3OH:H2O at various scan rates using GC as working electrode and Ag-AgCl as reference. Inset: Plot of anodic and cathodic currents as a function of square root of scan rate. Fig. 9 [A] Cyclic voltammetric response of L in 1:1 (v/v) CH3OH:H2O at Zn2+ ion concentration 0 mM (dotted line) and 0.061 mM (continuous line) at scan rate 0.1 Vs-1. Working electrode GC; Reference electrode Ag-AgCl). [B] Plot of redox potential versus concentration of Zn2+ ion.
Fig. 10 Plot of ∆I/I0 of L at different Zn+2 ion concentration for supracal tablets solution ( ) and standard Zn+2 ion solution (
)
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• • •
New molecule from condensation of tryptophan and salcylaldehyde synthesisied. The molecule acts as fluorescent and electrochemical sensor for Zn2+ ion. The interaction between the sensor and Zn2+ ion is proved by DFT calculations.
S1386-1425(13)01521-7 http://dx.doi.org/10.1016/j.saa.2013.12.090 SAA 11459
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
2 September 2013 12 November 2013 15 December 2013
Please cite this article as: K. Dutta, R.C. Deka, D.K. Das, A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2013.12.090
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new fluorescent and electrochemical Zn2+ ion sensor based on Schiff base derived from benzil and L-tryptophan Kaku Dutta. Ramesh C. Dekaa. Diganta Kumar Das* Department of Chemistry, Gauhati University, 781014, Assam, India, e-mail [email protected] a
Department of Chemistry, Tezpur University, Napaam, Tezpur – 784 028, Assam, India.
Address for corresponding author:
Diganta Kumar Das Department of Chemistry, Gauhati University, Guwahati-781014, Assam, India Email: [email protected] Tel: +91-0361-2570535 Fax: +91-0361-2700311
ABSTRACT Single molecule acting as both fluorescent and electrochemical sensor for Zn2+ ion is rare. The product (L) obtained on condensation between benzil and L-tryptophan has been characterized by H NMR, ESIMS and FT-IR spectroscopy. L in 1:1 (v/v) CH3OH:H2O solution shows fluorescence emission in the range 300 nm to 600 nm with max at 350 nm when is excited with 295 nm. Zn2+ ion could induce a ten fold enhancement in fluorescent intensity of L. Fluorescence and UV/visible spectral data analysis shows that the binding ratio between Zn2+ ion and L is 1:1 with log= 4.55. Binding of Zn2+ ion disrupts the photoinduced electron transfer (PET) process in L and causes the fluorescence intensity enhancement. When cyclic voltammogram is recorded for L in 1:1 (v/v) CH3OH:H2O using glassy carbon (GC) electrode, two quasi reversible redox couples at redox potential values -0.630 ± 0.005 V and -1.007 ± 0.005 V are obtained (Ag-AgCl as reference, Scan rate 0.1 Vs-1). Interaction with Zn2+ makes the first redox couple irreversible while the second couple undergoes a 0.089 V positive shift in redox potential. Metal ions - Cd2+, Cu2+, Co2+, Hg2+, Ag+, Ni2+, Fe2+, Mn2+, Mg2+, Ca2+and Pb2+, individually or together, has no effect on the fluorescent as well as electrochemical property of L. DFT calculations showed that Zn2+ ion binds to L to form a stable complex. The detection limit for both fluorescence as well as electrochemical detection was 10-6 M. Keywords L-tryptophan. Fluorescence. Photoinduced electron transfer. Zn2+. Sensor. Cyclic voltammetry.
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1. Introduction Zinc is an essential element in all biological systems [1]. Among many functions - Zinc regulates gene expression [2], is involved in cellular apoptosis [3], is constituent of RNA and DNA polymerases [4]. Zinc also helps immune system, endocrine system and gastroenterological systems [5]. Deficiency in Zn2+ or disorder in Zn metabolism is believed to lead neurological problems and diseases such as Alzheimer’s, amyotrophic lateral sclerosis, Parkinson’s disease and epilepsy [6]. Fluorescent sensors to detect metal ions have gained considerable interest because of their simplicity, high sensitivity and real-time detection [7-10]. A number of fluorescent sensors are known for zinc. A novel pyrazoline derivative was synthesized starting from a chalcone and 3-chloro-6hydrazinylpyridazine and found to determine Zn2+ ion with high selectivity and a low detection limit in CH3CN:EtOH (90/10, v/v) [11]. New bis(pyrrol-2-yl-methyleneamine) ligands were also reported to act as selective sensor for Zn2+ [12]. NNO-di(quinoline-2-methylene) -1,2-phenylenediimine was reported to exhibit high selectivity toward Zn2+ over other metal ions including Cd2+ due to the formation of a 1:1 metal:ligand complex [13]. A ratiometric fluorescent sensor for zinc ion based on covalently immobilized derivative of benzoxazole is also known [14]. Fluorescent sensors for Zn2+ ion are also known based on 2-1minobenzamide [15] and 2-(2′-hydroxy-3′-naphthyl)benzoxazole chromophore and di(2-picolyl)amine [16], We recently reported fluorescent Zn2+ ion sensors based on ferrocene derivative [17] and macrocyclic ligands having N,N,N,N coordination sites [18]. A novel fluorescent sensor which can selectively detect Zn2+ ion in aqueous media is reported [19]. Off-on fluorescent chemosensor based on diketopyrrolopyrrole is reported which has been applied for imaging Zn2+ ion in living cell [20]. BINOL-based ratiometric fluorescent sensor for Zn2+ ion with fluorescence “off-on” mode in aqueous medium has been reported [21]. Ratiometric fluorescent Zn2+ ion sensor of carboxamidoquinoline with 2-chloro- N-(quinol-8-yl)-acetamide as a receptor demonstrating a 15 fold enhancement in fluorescence intensity has been synthesized [22]. Turn-on fluorescent sensor based on a triphenylamine-aminophenol conjugate has been synthesized for Zn2+ ion sensing in CH3CN [23]. Pyridine–pyridone scaffold based fluorescence “on-off” densor for Zn2+ ion is also known [24]. Bis(8-carboxamidoquinoline) dangled binaphthol derivatized fluorescent sensor for “off-on” mode detection of Zn2+ ion is reported recently [25]. Carbazole incorporated thiazole based Zn2+ ion selective fluorescent sensor by fluorescent “on” mode is reported [26]. Reports on electrochemical sensing of Zn2+ ion is seemingly rare. Reports are available for Zn2+ ion determination based on adsorptive stripping voltammetry [27,28] and cyclic voltammetry [29,30] techniques. Electrochemical sensors for Zn2+ ion estimation in water and brain are reported [31,32]. One single molecule capable of selectively sensing Zn2+ ion both by fluorescence and electrochemistry is probably not reported.
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Here we report that the Schiff base obtained by condensation between benzyl and tryptophan act as a fluorescent as well as voltammetric sensor for Zn2+ ion which is free from interferences of metal ions Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+. Density function theory (DFT) is increasingly being recognized as an important tool to determine structural and electronic properties of metal complexes. We performed DFT calculations on the zinc complex to confirm the stability of the complex.
2. Experimental 2.1 Chemicals and experimental techniques All the chemicals were from Merck. The metal salts (except AgNO3 and PbNO3) were sulphates and recrystallized from water (Millipore). Dichloromethane (99.9%), triethylamine (99.9%), methanol (99.9%, HPLC grade), DMSO (99.9%, HPLC grade), benzil and l-tryptophan (Merck, Germany) were reagent grades and used as received. Alumina powder (99.9 %) was used as a polishing reagent (CHI, USA). Metal salt solutions (10−6 M) were prepared in millipore water. 10-2 M standard stock of 1:1 (v/v) CH3OH:H2O solution of L was prepared. The UV/Visible spectra were recorded on a Shimadzu UV/Vis -1800 spectrophotometer. Fluorescence spectra were recorded in a Hitachi 2500 spectrophotometer using quartz cuvette at room temperature. 1H-NMR spectra were recorded using a Bruker Ultrashield 300 MHz NMR spectrometer. Chemical shifts were expressed in ppm (in DMSO-d6, with TMS as internal standard) and coupling constants (J) in Hz. FTIR data were measured as KBr pallet, using a Perkin Elmer spectrophotometer (RX1). Electrochemical experiments were carried out at room temperature using CHI 600B Electrochemical Analyzer (USA) in a conventional three electrode system with Ag/AgCl (3M NaCl) as the reference electrode, a platinum wire as the counter electrode and a glassy carbon disc (GC) as working electrode. 0.1 M tetrabutylammonium perchlorate (TBAP) solution was used as supporting electrolyte. Nitrogen gas was purged through the electrolytic solution (3 mL) for at least 5 minutes to remove any dissolved oxygen before every experiment. Nitrogen atmosphere was maintained over the electrolytic solution during each experiment. Prior to every experiment the GC electrode was cleaned as reported [33].
2.2 Synthesis and characterisation of L 204 mg (1 mmol) of L-tryptophan was taken in 30 mL dichloromethane (DCM) in a beaker. To this solution 1 mL (5 mmol) of triethylamine was added and stirred till a clear solution was obtained. 210 mg (1 mmol) of benzil was added to this solution and stirred overnight and a pale yellow precipitate was obtained (L). The solvent was evaporated in a rotary evaporator and the product was washed with 3
2% HCl water solution to remove the excess triethylamine as hydrochloride salt. Product obtained upon solvent removal was purified by SiO2 column using ethylacetate/hexane (98:2) as an eluent. Thin layer chromatography showed Rf = 0.61. (Yield, 59%; melting point, 200 ºC; solubility, DMSO). The UV/Visible spectra of L in 1:1 (v/v) CH3OH:H2O solution showed λmax at 260 nm. 1H NMR of L showed δH values at 3.19 (merged with DMSO water) and 2.5 (s) ppm (-CH2-); at 4.2 (s) ppm (CH-, 1 α C=N), at 7.48 and 7.5 ppm (-CH- of benzene ring), at 7.6 ppm (-CH-. 1 β C=N), at 7.7 for (CH-, 1 β C=N and 1α C=O and vice versa) at 7.84 for (-CH-, 1 β C=O). FTIR spectra showed peaks at 1709 cm-1 (νC=N); 1606 cm-1 (νC=O, carboxylate ion); 1528 cm-1 (νNH deformation) and 3431 cm-1 (νindole NH), no broad band at 3040 cm-1 indicates absence of NH4+. ESI-MS showed molecular ion peak at 398. The detailed fragmentation patterns with peak assignments have been provided in supplementary information.
3. Results and discussion 3.1 Fluorescence studies of interaction between L and Zn2+ ion The fluorescence spectra of L (10-2 M) in 1:1 (v/v) CH3OH:H2O solution was recorded by varying the excitation wavelength. Highest fluorescence intensity with max at 350 nm was observed for emission range 300 nm to 600 nm when excited with 295 nm with slit width 5 nm both for excitation and emission. The fluorescence intensity remained unaffected for over a period of 1 week after the preparation of the solution. Hence, the excitation was kept at 295 nm for all the experiments. We studied the effect of pH in the range 2.0 to 12.0 on the fluorescence spectra of L. Fig 1 shows the plot of fluorescence intensity versus pH which confirms that the intensity is independent of pH till the value 10.0 beyond which it increases. All our experiments have been performed at pH 7.0. The quantum yield of L was found to be 0.10. The fluorescence intensity of L in 1:1 (v/v) CH3OH:H2O solution was measured at different added concentration of a number of metal ions – Na+, Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+, Hg2+ and Pb2+. A steady and smooth enhancement in fluorescent intensity was observed only for Zn2+ ion that saturates at 1.0 equivalent (Fig. 2). The overall enhancement in intensity was found to be 10 fold to the intensity when no Zn2+ ion was present. The plot of I/Io (I is the intensity at a given concentration of Zn2+ ion, Io is the intensity at zero Zn2+ ion concentration) found to increase linearly with Zn2+ ion concentration (Fig.2, inset). The enhancement in fluorescence intensity of L on interaction with Zn2+ ion may be attributed to the reversal of the photoelectron transfer of electrons from oxygen to indole rings upon Zn2+ binding (Scheme 1). The quantum yield of L in presence of Zn2+ ion was calculated to be 0.14 which is 0.04 higher than that of pure L which further confirms interaction between L and Zn2+ ion. 4
Complex formation between L and Zn2+
Structure of L
Scheme 1 Fig 3 compares the effect of Zn2+ ion and a number of other metal ions on the fluorescent intensity of L through a bar diagram. From the figure it is clear that effect of Zn2+ ion on the fluorescent intensity of L is quite distinct from the rest of the metal ions. Titrations carried out with metal ions Ca2+, Mg2+, Mn2+, Pb2+ and Cd2+ registered a very small increase in fluorescence intensity. Titrations carried out with metal ions Fe2+, Ni2+ and Cu2+ ions exhibited small quenching while moderate quenching was observed for Hg2+ ion (Fig. 3). Addition of Co2+ ion showed almost no change in fluorescence intensity of L.
3.2 Determination of binding constant by fluorescence and UV/Visible spectroscopy The binding constant and the stoichiometry of binding between L and Zn2+ ion was calculated as reported [34] by plotting log [(I-Io)/(Imax -I)] against log[Zn2+]. Fig. 4 shows this plot which is linear and a least squares fitting of data yielded the slope as 0.9874 ± 0.02 (R2 =0.998) indicating a 1:1 binding between L and Zn2+. The binding constant (β) was calculated to be log β = 4.55. In order to confirm the binding and stoichiometry of Zn2+ ion with the ligand, absorbtion spectra of L were recorded in 1:1 (v/v) CH3OH:H2O at different added concentration of Zn2+ ion (Fig 5A). The absorbance at max value 262 nm was found to decrease with increasing Zn2+ ion concentration. An isobestic point was observed at 228 nm indicating a transition between free species and complex species [35]. The plot of log[(Ao-As)/(As-A)] versus log [Zn2+], for the absorbance at max value 262 nm, was found to be linear (Fig. 5B). The least squares fitting of data showed that the slope was 1.005 ± 0.002 (R2 = 0.99) indicating 1:1 stoichiometric binding between L and Zn2+ which is in agreement with that obtained from fluorescence data. The binding constant was calculated and found to be (log β = 3.9) which is close to that obtained from fluorescence intensity plot.
3.3 Interference by other metal ions To further investigate the interfering effect of other metal ions on fluorescent detection of Zn2+ ion, a mixed solution of metal ions - Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+
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was prepared in 1:1 (v/v) CH3OH:H2O containing L (10-2 M), so that concentration of each metal ion in the solution was 10-6 M. The anions provided by these metal salts in the solution were Cl-, SO42-, NO3and CH3COO-. Fig. 6 shows the effect of Zn2+ ion on the fluorescence intensity of L when the solution has the mixture of cations and anions. Fluorescent titration of this solution against Zn2+ ion imparted 9.5 fold enhancement in fluorescent intensity, equivalent to the enhancement when solution had no other ions. This clearly confirms that L can specifically detect Zn2+ ion in presence of a number of other metal ions as well as anions. 3.4 DFT calculation of interaction between L and Zn2+ ion For theoretical calculation studies, the initial structure of the Zn2+ complex was generated from the available experimental data. The complex was fully optimized using BLYP functional and DNP basis sets as implemented in the program DMol3 [36]. In order to confirm the stability of the complex we performed vibrational frequencies calculations at the optimized structure with the same level of theory. The optimized geometry of the zinc complex along with Zn-O and Zn-N distances is shown in Fig. 7. The optimised geometry is in accordance with that proposed in Scheme 1. The coordination environment is ONO involving one O each from carbonyl and acetate origin and the immine N. The calculation clearly shows that the imidazole N is not involved in bonding. The theoretically calculated Zn-O and Zn-N bond distances are in agreement with available experimental results. In the vibrational frequency calculations, no imaginary frequency was found for the complex suggesting that the optimized complex represents a stable structure (local minima) in the potential energy surface. 3.5 Electrochemical study of interaction between L and Zn2+ ion Fig. 8 shows the cyclic voltammogram of L (0.02 mM) in 1:1 (v/v) CH3OH:H2O at different scan rates on GC electrode. Two redox couples were observed with redox potential values -0.630 ± 0.005 V (E = 0.100 V) and -1.007 ± 0.005 V (E = 0.180 V) at scan rate 0.1 Vs-1 versus Ag-AgCl as reference. It is reported that benzil in acetonitrile show quasi reversible cyclic voltammogram with redox potential value -1.002 V versus Ag-AgCl reference electrode [37]. Therefore the redox couple obtained at -1.007 ± 0.005 V must originate at the benzil part of L. The plot of cathodic and anodic currents for both the redox couples against square root of scan rate was found to be linear indicating a reversible redox process. Inset, Fig. 8 shows the current versus square root of scan rate plot for the -1.007 ± 0.005 V couple. The effect of Zn2+ ion on cyclic voltammogram of L has been shown in Fig. 9A. The solid line curve is for the sensor when no Zn2+ ion was added and dotted line curve is when Zn2+ ion was present in the solution. Gradual addition of aqueous solution of Zn2+ ion into the electrolytic medium shifted the cyclic voltammogram of -1.007 ± 0.005 V couple in positive direction till the redox potential became 0.918 V ± 0.005 V at Zn2+ ion concentration 0.061 mM and remained constant thereafter. Thus a 6
+ 0.089 V shift was observed due to interaction with Zn2+ ion. The oxidation peak current of the other couple was found to decrease with Zn2+ ion and finally became irreversible. The dependence of redox potential of L versus Zn2+ ion concentration for the -1.007 ± 0.005 V couple has been shown in Fig. 9B. Cyclic voltammogram of L was also recorded in presence of Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+ ions. No significant effect in the cyclic voltammogram of L was observed when these ions were added into the electrolytic medium singly or together.
3.6 Analytical applications We have tested the ability of L in determination of Zn2+ ion concentration in multimineral tablet. Multimineral tablet (Supra Cal, MMC Health Care, H.P. India) containing 280 mg Ca2+, 100 mg Mg2+, 200 IU vitamin D3 and 4 mg Zn2+ per tablet was purchased from chemist’s shop. One tablet was dissolved in 50 mL phosphate buffer solution (PBS) (pH 7.0) and a standard solution of Zn2+ ion was prepared by dissolving 0.4 g ZnSO4 in 50 mL PBS. The fluorescent intensity of 0.1 mM solution of L was measured, separately by adding standard Zn2+ ion solution as well as the tablet solution so that Zn2+ ion concentration in solution was 0.025 mM, 0.05 mM, 0.075 mM, 0.1 mM, 0.125 mM and 0.150 mM. Fig 10 compares the relative increase in intensity (that is the difference in intensity at a given concentration of Zn2+ from the intensity at zero Zn2+ ion concentration divided by the intensity at zero Zn2+ ion concentration) at the same Zn2+ ion concentration for the standard and tablet solution and found to be in excellent agreement. We have tested the ability of L in determination of Zn2+ ion concentration in multimineral tablet. Multimineral tablet (Supra Cal, MMC Health Care, H.P. India) containing 280 mg Ca2+, 100 mg Mg2+, 200 IU vitamin D3 and 4 mg Zn2+ per tablet was purchased from chemist’s shop. One tablet was dissolved in 50 mL millipore water (pH 7) and a standard solution of Zn2+ ion was prepared by dissolving 0.4 g ZnSO4 in 50 mL millipore water (pH 7). The fluorescent intensity of 0.1 mM solution of L was measured, separately by adding standard Zn2+ ion solution as well as the tablet solution so that Zn2+ ion concentration in solution was 0.025 mM, 0.05 mM, 0.075 mM, 0.1 mM, 0.125 mM and 0.150 mM. Fig. 10 compares the relative increase in intensity (that is the difference in intensity at a given concentration of Zn2+ from the intensity at zero Zn2+ ion concentration divided by the intensity at zero Zn2+ ion concentration) at the same Zn2+ ion concentration for the standard and tablet solution and found to be in excellent agreement.
5. Conclusion In summary, we have shown that the prepared compound L acts as a fluorescent sensor for Zn2+ ion by “switch on” mode. L and also act as electrochemical sensor for Zn2+ ion. A host of metal ions Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Hg2+ and Pb2+ has no interfering effect on fluorescent determination of Zn2+. This group of metal ions do not interfere the electrochemical 7
interaction between L and Zn2+ also. The structural parameters of the complex calculated using DFT are in very good agreement with the available experimental values. The absence of imaginary vibrational frequency in the calculated spectra confirms that zinc ion binds to the ligand to form a stable structuact.
Acknowledgement Department of Science & Technology, New Delhi (FIST program) and University Grants Commission, New Delhi (SAP program) are thanked for financial help.
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Figure caption:
Fig. 1 Fluorescence response of 1:1 (v/v) CH3OH:H2O solution of L at different pH values from 2.0 to 12.0. Fig. 2 Changes in fluorescence spectra of L as a function of added Zn2+ ion concentration in 1:1 (v/v) CH3OH:H2O solution. Inset: Plot of I/Io as a function of Zn2+ ion concentration. Fig. 3 Bar diagram showing the effect of metal ions Fe2+, Cu2+, Cd2+, Co2+, Pb2+, Hg2+, Ag+, Ca2+, Mn2+, Ni2+ and Mg2+ ions (1 equivalent of various metal ions in 1:1 CH3OH:H2O) on the fluorescence intensity of L. Fig. 4 Plot of log [Zn2+] as a function of log {(Imax -I)/(I - Io)} for L against Zn2+ in 1:1 CH3OH:H2O. Fig. 5 [A] The absorbtion spectrum of L in 1:1 CH3OH:H2O solution at different added concentration of Zn2+. [B] Plot of log(Ao-As/As-A) as a function of Zn2+ ion concentration. The plot shows a slope of 1.1 1 (R2 = 0.9978) indicating 1:1 complexation between Zn2+ and L. Fig. 6 Change in fluoresence intensity of L as a function of Zn2+ ion (0 - 0.40x10-4 M) in presence of 10-6 M each of Na2(CH3COO), CaCl2, MgSO4, MnSO4, FeSO4, CoNO3, NiSO4, CuSO4, CdSO4, PbNO3, HgCl2 and AgNO3. Fig. 7 [A] Optimized structure of the zinc complex. [B] HOMO and LUMO orbitals of the optimized zinc complex.
Fig. 8 Cyclic voltammogram of L in 1:1(v/v) CH3OH:H2O at various scan rates using GC as working electrode and Ag-AgCl as reference. Inset: Plot of anodic and cathodic currents as a function of square root of scan rate. Fig. 9 [A] Cyclic voltammetric response of L in 1:1 (v/v) CH3OH:H2O at Zn2+ ion concentration 0 mM (dotted line) and 0.061 mM (continuous line) at scan rate 0.1 Vs-1. Working electrode GC; Reference electrode Ag-AgCl). [B] Plot of redox potential versus concentration of Zn2+ ion.
Fig. 10 Plot of ∆I/I0 of L at different Zn+2 ion concentration for supracal tablets solution ( ) and standard Zn+2 ion solution (
)
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• • •
New molecule from condensation of tryptophan and salcylaldehyde synthesisied. The molecule acts as fluorescent and electrochemical sensor for Zn2+ ion. The interaction between the sensor and Zn2+ ion is proved by DFT calculations.
