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A novel probe fo

Department of Chemistry, Indian Institute

Uttarakhand, India. E-mail: ghoshfcy@iitr.

1332 273560; Tel: +91 1332 2755470

† Electronic supplementary information (E1, supplementary spectra and graphs. See

Cite this: RSC Adv., 2014, 4, 48516

Received 28th July 2014Accepted 18th September 2014

DOI: 10.1039/c4ra07731h

www.rsc.org/advances

48516 | RSC Adv., 2014, 4, 48516–485

r selective colorimetric sensing ofFe(II) and Fe(III) and specific fluorometric sensing ofFe(III): DFT calculation and logic gate application†

Kaushik Ghosh* and Sweety Rathi

A novel fluorescent probe 1 (2-((2-(naphthalen-1-ylamino)ethylimino)methyl)phenol) has been synthesized

and characterized by various spectroscopic methods. Probe 1was found to be highly selective for iron over

testedmetal ions. The naked eye detection of iron is useful for the discrimination of the +2 and +3 oxidation

state while fluorescence studies conclude selective and specific sensitivity towards Fe(III).

1. Introduction

The design and synthesis of probes or chemosensors for thedetection of metal ions are important areas of chemicalresearch.1 Iron is one of the most essential elements present inthe biosystem and exhibits several crucial roles in differentenzymatic and other biological activities;2 however, this metalion could exhibit detrimental effects alone or in a combinedstate causing diseases like hemochromatosis, cancer etc.3

Hence detection of the presence of iron and its concentration aswell as localizations are extremely important for the treatmentof such diseases. The common oxidation states of iron are Fe(II)and Fe(III) which are important for ordinary and related chem-istry.4 Naked eye detection for metal ions is important forqualitative identication as well as quantitative detection.Hence simple and easy to use colorimetric probes are highlydemanding for metal ion sensing.5 In this regard, uorescentchemosensors gained considerable attention in the recent yearsdue to simplicity, sensitivity and easy handling.6 Differentcolorimetric as well as uorimetric probes have been reportedfor iron sensing.7 We have recently reported a simple probe foruorimetric detection of Fe(III).8 However, to the best of ourknowledge, there is no report of a naphthyl-based probe whichcould detect iron by colorimetric as well as uorometric way.9

Tedious methods, complicated structures as well as interfer-ence from several metals such as Cu(II) and Cr(III) were alsoobserved in uorescence probes for Fe(III).10 In this report wehave synthesized a simple but novel probe 2-((2-(naphthalen-1-ylamino)ethylimino)methyl)phenol (1) for the colorimetricdetection of Fe(II) and Fe(III). We have investigated the sameprobe for the uorometric detection of Fe(III). The results will be

of Technology Roorkee, Roorkee 247667,

ernet.in; ghoshfcy@gmail.com; Fax: +91

SI) available: Characterization of probeDOI: 10.1039/c4ra07731h

21

discussed in the light of theoretical calculation via DFT calcu-lations. Possible application to logic gates will also bescrutinized.

2. Experimental details2.1 Materials

N-(1-Naphthyl)-ethylenediaminedihydrochloride purchased fromThomas Baker, India. Salicylaldehyde was purchased from SRL,Mumbai, India. All metal salts used in the synthesis werepurchased from commercial source and used directly (withoutany purication). 1H-NMR and 13C-NMR spectra of 1 in CDCl3were recorded using Bruker AVANCE, 500.13 MHz spectrometer;GC-MS spectrometry was recorded with Perkin Elmer. The UV-visspectra were obtained by using Evolution 600, Thermo ScienticUV-visible spectrophotometer. Emission spectra were obtainedfrom RF-5301PC with a 3 cm standard quartz cell.

2.2 UV-vis and uorescence measurements andcomputational details

Stock solution of 1 (1 mM) and chloride salt (10 mM) of metalsZn(II), Sn(II), Ni(II), Mn(II), Mg(II), Hg(II), Fe(II), Fe(III), Cu(II), Co(II),Cd(II), Ca(II) and Ba(II) were prepared in methanol for metalsensing. All uorescence spectra of 1 were recorded with the slitwidth 3/3. Fluorescence quantum yield (QY) was calculated bycomparative method using 2-aminophenol solution in 0.1 NH2SO4 as the standard.11 DFT calculations for 1 and 1–Fe(III) wasperformed at the B3LYP level using LANL2DZ basis set for Fecenter and 6-31G(d) basis set for non metal atoms.

2.3 Synthesis and characterization of 2-((2-(naphthalen-1-ylamino)ethylimino)methyl)phenol 1

N-(1-Naphthyl)-ethylenediaminedihydrochloride salt was dis-solved in water and then neutralized using a saturated solution ofKOH. Free amine was extracted with the help of dichloromethaneand dried over Na2SO4. A solution of salicylaldehyde (122 mg,

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1.00 mmol) in 5 mL of methanol was added to (186 mg,1.00 mmol) of N-(1-naphthyl)-ethylenediamine in 10 mL meth-anol with continuous stirring. Aer reuxing of few hours,reaction was cooled to room temperature. An oily compoundobtained aer evaporation of solvent. GC-MS (MeOH, m/z):290 M+. Selected IR data (KBr, nmax/cm

�1): 3428,nN–H, 1631,nC]Nimine. UV-visible [CH3OH, lmax/nm (3/M�1 cm�1)]: 213(65 555), 247 (24 695), 325 (9730), 400 (1185). 1H NMR (CDCl3,d/ppm, 500 MHz): 3.251 (t, 2H), 3.318 (dd, 2H), 4.586 (s, 1H),6.702 (d, J ¼ 7.5, 1H), 7.198 (d, J ¼ 7.5, 1H), 7.267 (d, J ¼ 7.5 Hz,1H), 7.270–7.451 (m, 5H), 7.742 (d, J ¼ 8.5, 1H), 7.799 (d, J ¼ 8,1H), 8.005 (d, J ¼ 8, 1H), 8.345 (s, 1H), 13.260 (s, 1H). 13C NMR(CDCl3, d/ppm, 500 MHz): 44.41, 58.12, 104.7, 117, 117.8, 118.73,118.78, 119.82, 123.58, 124.97, 125.89, 126.54, 128.73, 131.53,132.53, 134.42, 142.69 161.09, 166.7. Anal. calcd for C18H17N3: C,78.52; H, 6.22; N, 15.26, found: C, 76.88; H, 6.46; N, 14.45.

3. Results and discussions3.1 Characterization of 1

Fluorescent probe 1 has been synthesized by condensation ofN-(1-naphthyl)-ethylenediaminedihydrochloride and salicy-laldehyde and was characterized using spectral studies. Struc-ture of probe 1 was authenticated by various spectroscopictechniques like IR, UV-vis, 1H-NMR, 13C-NMR spectral studiesand GC-MS (data have been deposited in the ESI Fig. S1–S5†).

3.2 Naked eye detection

Unique color changes for iron metal ions with probe 1 wereobserved during investigation with other metal ions. Among therepresentative metal ions only Fe(II) (yellow-brown) and Fe(III)(purple) showed color changes that can easily be observed via“naked eye”(Fig. 1). Several uorescent probes have beenreported for the visible detection of iron but there is no reporton naked eye detection for discrimination of Fe(II) and Fe(III)using naphthyl-based probe.

3.3 UV-vis absorption studies

Absorption studies of probe 1 have been investigated usingmethanol as a solvent. UV vis spectrum in methanol exhibitedtypical naphthalene absorption band at 320 nm due to chargetransfer transition between amine and naphthyl group.12 Probe1 exhibited a band around 402 nm in visible region and slightlyyellow in color. Addition of Fe(II) into the methanolic solutiongave rise to yellow-brown color and showed an intense

Fig. 1 Images showing naked eye visible changes of probe 1 in thepresence of representative metal ions (10 equiv.).

This journal is © The Royal Society of Chemistry 2014

absorption band in the visible region around 536 nm. Additionof Fe(III) into the methanolic solution gave rise to purple colorand showed an intense absorption band in the visible regionaround 590 nm (Fig. 2). The coloration was due to a CT band inthe visible region.13 Insolubility of 1 in water prompted us tostudy this experiment in water–methanol mixed solvent media.In mixed aqueous media the band responsible for color gener-ation completely disappeared. Changes in pH in mixed aqueousmedia were examined (Fig. S6 in ESI†) and in this case also wedid not observe the band responsible for the color (Fig. S7 inESI†). Hence the naked eye detection was not possible onmoving from methanol to methanol–water solvent media aswell as for pH variation. The titration experiments with Fe(II)and Fe(III) ions were performed and results are deposited in theESI (Fig. S8–S9†).

3.4 Fluorescence studies

Further, we have examined photophysical properties of probe 1in methanol. Probe 1was found to be uorescent at lexc 320 nm.The wavelength of emission was around 420 nm. Probe 1showedmaximum emission intensity within 5min and remainsconstant for more than half an hour (Fig. S10†). Clearly, themaximum excitation and emission are at 320 nm and 420 nm,respectively, showing a large Stokes shi (Fig. S11†). Addition ofmetal ions Zn(II), Sn(II), Ni(II), Mn(II), Mg(II), Hg(II), Fe(II), Cu(II),Co(II), Cd(II), Ca(II) and Ba(II) did not cause any appreciablechange in emission intensity of probe 1. On the other hand,introduction of Fe(III) gave rise to quenching in emissionintensity with the red shi of 13 nm which further kept ondecreasing on further addition of Fe(III) (Fig. 3(A) and (B)).

Due to large value of stokes shi, uorescence resonanceenergy transfer (FRET) is not a favorable pathway for thequenching mechanism (Fig. S11†). Hence the electron transfercould be a probable mechanism for the quenching as Fe(III) is astrong Lewis acid which can accept electron easily.14

Fig. 2 UV-vis spectral change of 1 (20 mM) in methanol on addition of10 equivalent of Zn(II), Sn(II), Ni(II), Mn(II), Mg(II), Hg(II), Fe(II), Fe(III), Cu(II),Co(II), Cd(II), Ca(II) and Ba(II).

RSC Adv., 2014, 4, 48516–48521 | 48517

Fig. 3 (A) Fluorescent emission spectra of 1 (20 mM) in methanol with10 equivalent of Zn(II), Sn(II), Ni(II), Mn(II), Mg(II), Hg(II), Fe(II), Fe(III), Cu(II),Co(II), Cd(II), Ca(II) and Ba(II). (B) Fluorescent emission titration spectraof 1 (20 mM) in the presence of varying concentration of Fe(III) in MeOHat lex 320 nm (inset – change in emission intensity with number ofequivalents of Fe(III).

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Similar to absorption spectral studies the uorescenceproperties of probe 1 were also investigated in mixed aqueousmedia as well as in different pH of the solutions (Fig. S12–S13†).The data obtained in mixed aqueous media were different tothat of our results in methanol. Comparison of Fig. 3(A) andS12† clearly indicated the retention of quenching of Fe(III),however selectivity of iron sensing diminished (Fig. S14†). Wewould like to mention here that in Fig. 3(A) we foundenhancement of emission intensity as compare to probe for allmetal ions excluding Fe(III). On the other hand, we foundenhancement as well as quenching of emission intensity fordifferent metal ions (Fig. S14†). The Stern–Volmer plot,measurement of life time s ¼ 7.94 ns (probe only), 8.09 ns(probe + Fe(III)) and changes in absorption spectra clearlyindicate the static nature of quenching.15 (Fig. 4(A) and (B)).

Quantum yield values were calculated using 2 aminopyridineas standard. The values for relative quantum yield for probe aswell as for probe + Fe(III) were found to be 0.03 and 0.005.11

Fig. 5 Selectivity of metal ions at wavelength 565 nm in a solutionhaving probe 1 + metal ions (black bar) and 1 + metal ions + Fe(III) (redbar) observed using UV-vis absorption studies.

3.5 Selectivity studies

Competitive binding studies were also performed to study theinterference of the other metal ions Zn(II), Sn(II), Ni(II), Mn(II),Mg(II), Hg(II), Fe(II), Cu(II), Co(II), Cd(II), Ca(II) and Ba(II) in thedetection of Fe(III) using spectrophotometry as well as spectro-uorometry. Selectivity of probe is very crucial as the interfer-ence of metal can affect its sensitivity. For the competitive

Fig. 4 (A) Fluorescence decay profile of 1 in the presence and absenceof Fe(III) in methanol. (B) Stern–Volmer plots for titrations of 1 withdifferent concentrations of Fe(III) in methanol.

48518 | RSC Adv., 2014, 4, 48516–48521

binding studies equal concentration of Fe(III) and other metalswere taken in methanolic solution. Anti-interference experi-ment showed that among the served metal ions no metal ioninterfere with the Fe(III). Fig. 5 and 6 clearly indicates that Fe(III)acts as an excellent sensor even in presence of other metal ions.To investigate the effect of metal ions on the selectivity of Fe(III)ion mixed metal ion studies were performed using UV-vis aswell as uorescence spectral studies.

During absorption studies wavelength 565 nm was chosenfor the mixed metal ion sensing. Bar diagram clearly representsthe effect of Fe(III) on uorescence of probe 1 (Fig. 5). Rest of themetal ions did not show any band around 565 nm but onaddition of Fe(III) band arises at 565 nm only. During UV-visstudies only Cu(II) interfere with the band formation.

Fluorescence studies performed for the selectivity Fe(III)showed quenching without interference of any other metal ion(Fig. 6). Hence probe 1 can be used highly selective as well asspecic uorimetric sensor for Fe(III).

3.6 Binding stoichiometry of probe 1 and Fe(III)

The binding stoichiometry of 1 was obtained from the Job's plotmeasurement on the basis of uorescence.16 Emission intensity

Fig. 6 Selectivity of metal ions at wavelength 421 nm in a solutionhaving probe 1 + metal ions (black bar) and 1 + metal ions + Fe(III) (redbar) observed using fluorescence spectral studies.

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was plotted against mole fraction of Fe(III) at 420 nm. Themaxima clearly expressed 1 : 1 binding stoichiometry of 1 withFe(III) (Fig. 7). Binding ratio of probe and Fe(III) ion was alsocalculated using Benesi–Hildebrand plot which supports the1 : 1 stoichiometry obtained from Job's plot. Associationconstant was found to be 2.884 � 103 M�1 (Fig. 8).17 We havealso examined electrospray ionization-mass spectral (ESI-MS)studies on the probe in presence of Fe(III) ion. Resultsobtained from this experiments supported our observationsdescribed above and were deposited in the ESI (Fig. S15(A) and(B)†).

Limit of detection for Fe(III) was also calculated for a linearrange of 10–60 mM and found to be 20.85 mM (Fig. S16†).

3.7 Reversibility of probe 1

For better insight into the mechanism of Fe(III) sensing uo-rescence titration were conducted in presence of EDTA. EDTAhas a strong tendency to chelate Fe(III) and formation of Fe–

Fig. 7 Job's plot of 1 and Fe(III) in methanol, total concentration of 1and Fe(III) were maintained 100 mM and observed at 420 nm.

Fig. 8 B–H plot for binding of Fe(III) with the probe 1. Associationconstant was found to be 2.884� 103. A good linear fit of the B–H plotsupported the 1 : 1 binding stoichiometry.

This journal is © The Royal Society of Chemistry 2014

EDTA complex stops the electronic interaction of probe andFe(III) ion. Hence instead of the uorescence quenching weobserved enhancement of uorescence in presence of EDTA.Further addition of Fe(III) ion to the solution afforded uores-cence quenching. Titration of probe 1 with EDTA suggested thatbinding of Fe(III) with probe to some extent found to bereversible as well as supports the electron transfer pathway forquenching of the probe (Fig. S17†). Hence probe 1 can beconsidered as a reversible sensor for Fe(III).7c

3.8 DFT calculation

In order to understand the mechanism of such processes, weperformed theoretical calculations. DFT calculations for 1 and1–Fe(III) complex was performed at the B3LYP level usingLANL2DZ basis set for Fe center and 6-31G(d) basis set for nonmetal atoms.18 The highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) of 1 and1–Fe(III) complexes has been depicted in Fig. 9. The energy gapsbetween HOMO and LUMO in the probe 1 and 1–Fe(III) complexwere 0.10468 eV and 0.0102 eV respectively. Energy gap betweenHOMO and LUMO in complex decreases in 1–Fe(III) shows afavorable complexation according to proposed coordination.The optimized conguration showed suitable binding of Fe(III)with tridentate probe having NNO donor.19

3.9 Logic gate application

Logic gates have been attracted the attention of researchers dueto their increasing application in molecular switches andmolecular keypad devices.20

Here we have chosen Fe(III) and EDTA used for reversibilityexperiment as input.21 Truth table and logic gate have beenshown in Fig. 10. Emission intensity at 100 has been taken as athreshold value at wavelength 420 nm. Emission intensity abovethreshold gives a state OUT ¼ 0 and below OUT ¼ 1.

Fig. 9 Energy diagrams of HOMO and LUMO orbitals of probe 1 and1–Fe(III) complex calculated at the DFT level using a B3LYP/6-31+G(d,p) basis set.

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Fig. 10 Change in emission spectra 1 upon chemical inputs of Fe(III)IN1, EDTA IN2. Truth table indicates and logic functions.

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4. Conclusions

In this report a simple and novel probe 1 has been synthesizedvia a simple one step synthetic procedure and resultantcompound was characterized by various spectroscopic tech-niques. Photo-physical properties of 1 have been investigated tostudy the sensing of metal ions in methanolic solution. Probe 1selectively detected iron in both +2 and +3 oxidation statesgiving rise to yellow-brown and purple color respectively.Although there is interference of copper during absorptionspectral studies the probe was highly sensitive and selectivetowards Fe(III) during uorimetric detection because of uo-roscence quenching of the probe. Life time measurement,Stern–Volmer plot and UV-vis spectral studies indicated staticnature of quenching. Binding stoichiometry was found to be1 : 1 for Fe(III) and probe 1. DFT calculation provided that thedecrease in the energy gap between HOMO and LUMO isprobably responsible for the quenching of uorescence. Wehave performed UV-vis spectra and uorescence spectralstudies in mixed (water and methanol) solvents. UV-vis experi-ments clearly showed the disappearance of bands near 500 nmwhich were responsible for the color. Modication of the probefor experiments in aqueous media and their biological appli-cations are under progress.

Acknowledgements

KG is thankful to CSIR, India for nancial assistance no.01(2720)/13/EMR-II dated 17-APRIL-2013. KG is also thankful toDST-SERB, India for nancial assistance no. SR/S1/IC-47/2012dated 21-OCT-2013. SR is thankful to UGC for fellowship. We arethankful to DST-FIST program for providing us ESI-MS facility inour department. We are thankful to Vishal for his help.

Notes and references

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