RSC Advances

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Department of Chemistry, Madurai Kamaraj

E-mail: dcmku123@gmail.com

† Electronic supplementary informa10.1039/c3ra43843k

Cite this: DOI: 10.1039/c3ra43843k

Received 23rd July 2013Accepted 23rd August 2013

DOI: 10.1039/c3ra43843k

www.rsc.org/advances

This journal is ª The Royal Society of

A ratiometric fluorescent sensor for selectiverecognition of Al3+ ions based on a simplebenzimidazole platform†

Dharmaraj Jeyanthi, Murugan Iniya, Karuppiah Krishnaveniand Duraisamy Chellappa*

An efficient new ratiometric chemosensor, 6-imidazo-2-yl-5,6-dihydro-benzo[4,5]imidazo[1,2-c]-

quinazoline (PB), which enables selective sensing of Al3+ in aqueous DMSO was synthesized by a facile

one step condensation reaction of imidazole-2-carboxaldehyde with 2-(2-aminophenyl)-1H-

benzimidazole. The probe PB was characterized by elemental analysis, 1H and 13C NMR, ESI-MS and IR

spectral techniques. Upon addition of Al3+, the fluorescent probe PB induces ratiometric responses in

both absorption and fluorescence spectra on the basis of a charge transfer mechanism. The receptor PB

serves for highly selective, sensitive and ratiometric detection of Al3+ without the interference of

biologically and environmentally relevant cations. The detection limit is found to be 5.3 � 10�7 M. The

Job's plot analysis indicates the binding stoichiometry of the complex to be 1 : 1 (host/guest).

Introduction

Aluminium, being the most abundant metallic element in theEarth's crust (approximately 8%),1 is extensively used inindustry. It has been used for food preservation, syntheticpaper2 and rubber manufacture, besides its utility in foodadditives and cookware. Nevertheless, aluminium compoundsnot only hamper the physiological and biochemical processes ofplants,3–5 but also are deadly to sh, algae, bacteria and otherspecies in aquatic eco-systems.6 Apart from this, Al3+ ions exertseveral neurotoxic effects and contribute to diseases like Alz-heimer's disease, idiopathic Parkinson's disease,7 dementiaand myopathy. Glucose intolerance, osteomalacia, osteopo-rosis,8,9 cardiac arrest, kidney failure,10 encephalopathy,11

smoking-related diseases and pulmonary brosis are some ofthe adverse health effects associated with the accumulation ofthis metal in the human body. The World Health Organization(WHO) estimated that the provisional tolerable weekly intake ofaluminium (PTWI) for healthy individuals is 7 mg per kg bodyweight.12 Therefore, it is crucial to control the concentration ofAl3+ ions in the environment to maintain good human health.

Many optical methods such as atomic absorption spectrom-etry, atomic emission spectrometry, inductively coupled plasmaspectrometry and electrochemical methods13 are available fordetection of metal ions. Among these methods, because oftheir excellent sensitivity, selectivity, versatility and low cost

University, Madurai, Tamil Nadu, India.

tion (ESI) available. See DOI:

Chemistry 2013

instrumentation,14 uorescent chemosensors for cations andanions assume prominence in the areas of chemistry, biology,medicine and the environment. Mechanisms like Forster reso-nance energy transfer (FRET),15,16 intramolecular charge transfer(ICT),16–18 photo induced electron transfer (PET),16,19 electronicenergy transfer (EET) and excimer and exciplex formations havebeen used to explain uorescence sensing. The weak coordina-tion ability and lack of spectroscopic properties make uores-cence sensing of Al3+ ions difficult relative to sensing of othermetal ions.20 Consequently only a few uorescence probes havebeen reported for the selective sensing of Al3+ ions.21–23

It has been shown that the uorophore 2-(2-aminophenyl)-1H-benzimidazole when coupled with aldehydes like 2-pyridinecarboxaldehyde and 2-thiophene carboxaldehyde acts as asensor for Cr3+ and Fe3+ and Fe2+, respectively.24,25 These resultsencouraged us to examine the sensing properties of the sameuorophore with aldehydes appended to aza heterocycles.Ratiometric sensing is very attractive because the ratio betweenthe intensities of absorption or emission at two wavelengthsminimizes the error arising from the physical and chemicaluctuations in the sample as well as local environmentaleffects.26 Furthermore, ratiometric sensing facilitates rapidvisual sensing with high sensitivity,27 if the chemosensoraccompanies a color change.

Thus, in the present work, we have developed an efficientratiometric chemosensor, 6-imidazo-2-yl-5,6-dihydro-benzo[4,5]-imidazo[1,2-c]quinazoline, for the selective sensing of Al3+ inDMSO–H2O (1 : 9, v/v) solution based on an internal chargetransfer mechanism. The chemosensor was synthesized by asimple and straight-forward method with high yield.

RSC Adv.

Scheme 1 Synthesis of the receptor PB.

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Experimental sectionMaterials and methods

All the chemicals were purchased from Sigma-Aldrich and usedas received unless otherwise mentioned. Metal chloride saltsobtained from Sigma-Aldrich were used as a source for metalions. The solvents used were of spectroscopic grade. The FT-IRspectrum of the probe was recorded on a FT-IR spectropho-tometer (8400S Shimadzu) in the range of 4000–400 cm�1.UV-visible absorption spectra were recorded on a JASCO V-550spectrometer. All uorescence measurements were made on anF-4500 Hitachi uorescence spectrophotometer with a slit widthof 5 nm for both excitation and emission. The NMR spectra wererecorded on a Bruker (Avance) instrument operating at 300 MHzin DMSO-d6 with tetramethylsilane as an internal reference.Chemical shis were expressed in ppm and coupling constants(J) in Hz. Electrospray ionization mass spectrometry (ESI-MS)analysis was performed in the positive ion mode on a liquidchromatography-ion trap mass spectrometer (LCQ eet,Thermo Fisher Instruments Limited, US). The stock solution ofthe probe (10 mM) was prepared in DMSO. The solutions ofmetal ions were prepared from their chloride salts. Metal ionstock solutions were prepared in deionized water keeping theconcentration range from 0 to 10 mM. Doubly distilled deionizedwater was used throughout. All titrations were carried out atroom temperature. For the uorescence spectra, the excitationwavelength was kept at 350 nm.

General synthetic procedure for 6-imidazo-2-yl-5,6-dihydro-benzo[4,5]imidazo[1,2-c]quinazoline (PB)

An ethanolic solution of imidazole-2-carboxaldehyde (1 g,10 mmol) was added dropwise to a solution of 2-(2-amino-phenyl)-1H-benzimidazole (2.18 g, 10 mmol) in ethanol at roomtemperature. The reaction mixture was heated to reux for 3 h.Aer the reaction was completed, the reaction mixture wascooled to room temperature. The precipitated compound wasltered and washed with ethanol. Finally, the product was driedto obtain a white colored powder in 75% yield. 1H NMR (300MHz, DMSO-d6, TMS): d (ppm)¼ 12.5 (s, 1H), 7.94 (d, 1H, J¼ 7.8Hz), 7.63 (d, 1H, J ¼ 7.8 Hz) 6.83–7.54 (m, 9H) and 6.64 (d, 1HJ¼ 8.1 Hz). 13C NMR (75MHz, DMSO-d6, TMS): d (ppm)¼ 147.5,145.4, 144.3, 143.9, 133.4, 131.9, 125.1, 122.6, 122.5, 119.0,118.9, 115.3, 112.5, 110.5, 64.1. IR (KBr, cm�1): n(N–H), 3361;n(C–H), 3033; n(C]N), 1616; n(M–N), 559. Anal. calcd forC17H13N5: C 71.08, H 4.53, N 24.39%; found: C 71.11, H 4.49, N24.42%; MS (ESI): 288 (M + H)+.

Fig. 1 UV-visible spectral changes of PB (10 mM) upon the addition of variousmetal ions (10 mM) at 25 �C in DMSO–H2O (1 : 9, v/v, phosphate buffer, 100 mM,pH ¼ 7.54).

Results and discussion

We synthesized the target compound PB by a one step facilereaction between imidazole-2-carboxaldehyde and 2-(2-amino-phenyl)-1H-benzimidazole (uorescent moiety) in ethanol(Scheme 1) in 75% yield aer recrystallization from ethanol. Fora better understanding of the chemosensor, different spectro-scopic studies like 1H and 13C NMR, ESI-MS, FT-IR andelemental analysis were performed (Fig. S1–S5, see ESI†).

RSC Adv.

The binding affinities of the receptor PB towards variousmetal ions including Al3+ were examined by UV-visible absorp-tion and uorescence titrations in DMSO–H2O (1 : 9, v/v) solu-tion. The absorption spectrum of free PB exhibits an intenseband positioned at 354 nm due to the imine linkage28 (Fig. 1). Atthe same time, the addition of Al3+ ions leads to the appearanceof a new band at 409 nm. The probe PB undergoes a solventassisted 1,5-sigmatropic shi (ring opening) leading to the insitu formation of PBI during complex formation with Al3+ ions29

(Scheme 2).Addition of a series of biologically/environmentally relevant

metal ions such as Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Hg2+, Pb2+ and Fe3+ by using their chloridesalts to PB with the equimolar concentration resulted in negli-gible variation in the absorption spectrum of PB. These resultsreect the suitability of PB for successful quantication of Al3+

even in the presence of other representative metal ions. In otherwords, the receptor PB can be used as an efficient, selectivechemosensor for Al3+ ions. To elaborate the distinctive changein the absorption, we further titrated PB with an increasingconcentration of Al3+ ions (0–10 mM). The results showed astepwise decrease in the absorbance at 354 nm. Concurrently anew prominent band with high intensity appeared at 409 nmwith a denite isosbestic point at 380 nm (Fig. S6†). This is

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Scheme 2 Proposed mechanism for the interaction between PBI and Al3+ ions.

Fig. 2 Changes in the fluorescence spectra of PB (10 mM) upon addition ofvarious concentrations of Al3+ (0–1 � 10�5 M) in 100 mM phosphate buffer(DMSO–H2O, 1 : 9) of pH 7.54 at 25 �C. The excitation wavelength was 350 nm.

Fig. 3 Plot of the ratio of fluorescence intensities at 472 and 415 nm as afunction of Al3+ concentration.

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indicative of the complete chemical interaction between Al3+

and in situ formed PBI. At this stage, there was a color changeof the solution to bluish green, which is conspicuous to thenaked eye.

Further, the uorosensing behavior of the receptor PB(10 mM) was surveyed with the addition of biologically/envi-ronmentally important metal ions (Li+, Na+, K+, Ca2+, Mg2+,Cu2+, Co2+, Ni2+, Mn2+, Zn2+, Cd2+, Hg2+, Pb2+, Fe3+ and Al3+) inmixed solvent media (DMSO–H2O ¼ 1 : 9). The uorescenceemission maximum of the receptor PB was positioned at415 nm when excited at 350 nm with a quantum yield of 4f ¼0.34 at room temperature in DMSO. Interestingly, the bindingof an Al3+ ion led to the unique modulation remarkably thanother metal ions, with a quantum yield of 4f ¼ 0.53 (ESI†). Onaddition of Al3+ (0–10 mM) the receptor PB exhibited a prom-inent uorescence band at 472 nm with a red shi of about57 nm caused by charge transfer due to the greater chelatingability of PBI (Scheme 2). But the uorescence prole of thereceptor PB remained unperturbed in the presence of miscel-laneous ions (1 � 10�5 M) (Fig. S7†).

Upon the incremental addition of Al3+ ions (0–10 mM), agradual drop in the uorescence intensity of the band at 415 nmwith concomitant enhancement in the uorescence intensity ofthe other band at 472 nm was observed. This results in theformation of a well-dened isoemissive point around 450 nm(Fig. 2). The emission at 472 nm is likely due to the coordinationof PBI (in situ formed) with Al3+ ions. This behavior in theuorescence spectrum gives an interesting opportunity for thepotential ratiometric determination of the analytes bycomparing the ratio of the intensities of the two bands as afunction of analyte concentration. This method is preferableover single wavelength analysis because the system is free fromthe errors associated with receptor concentration, photobleaching, environmental effects and so on.30,31 The uniquenessof the receptor towards Al3+ reveals that the receptor PB can beused as a highly selective and ratiometric uorescent chemo-sensor for Al3+ ions over other relevant metal ions.

The association constant of the in situ formed PBI for Al3+

ions, when evaluated from the uorescence titration data,turned out to be 1.6 � 104 M�1 (ESI†). It is noteworthy tomention that the detection limit of PBI for Al3+ ions wasmeasured by plotting the ratiometric uorescence intensityratio [I472/I415] against Al

3+ concentration and was calculated to

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be 5.3 � 10�7 M (ESI†) with a high correlation coefficient value(R2 ¼ 0.9982) (Fig. 3).

The selective utility of the receptor PB as a uorescent che-mosensor towards Al3+ ions was further ascertained by per-forming competition experiments in the presence of othermetal ions in a DMSO–H2O system (Fig. 4). No interference wasobserved from competing metal ions thus revealing the strongbinding ability of the receptor with Al3+. From this evidence, weconclude that the receptor PB can be used as a selective uo-rescent chemosensor for Al3+ ions in the presence of all testedmetal ions.

To determine the binding stoichiometry between PBI andAl3+ in the complex, a Job's plot32 was constructed from theemission prole by maintaining the sum of the concentration ofAl3+ and the receptor constant and varying the mole fraction of

RSC Adv.

Fig. 4 A bar diagram showing the variation in the fluorescence responses of PB(10 mM) towards Al3+ (10 mM) upon addition of various metal ions (10 mM) at472 nm (lex ¼ 350 nm).

Fig. 6 1H NMR titrations of PB with Al3+ in DMSO-d6. From bottom to top: PB,PB with 0.5 equiv. Al3+, PB with 1 equiv. Al3+.

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Al3+ from 0.1 to 0.9 (Fig. 5). An emission maximum wasobserved when the mole fraction reached 0.5, reecting a 1 : 1binding stoichiometry for the newly formed PBI–Al3+ complex.

In order to strengthen the proposed composition of thecomplex between PB and Al3+ from photophysical studies, 1HNMR titrations were conducted in DMSO-d6 with the gradualaddition of Al3+ (0, 0.5, 1 equiv.). The change in the chemicalshi of the protons of the probe upon the addition of Al3+ isdepicted in Fig. 6. Compared to just the receptor, in the pres-ence of Al3+ all the aromatic protons, including those of theimidazole & phenyl benzimidazole groups, showed a downeldshi. On the other hand, with the gradual addition of Al3+, the –NH proton signal begins to shi towards the upeld region andeventually disappears with the addition of 1 equivalent of Al3+.This clearly suggests –N–Al3+ coordination via deprotonation.

Fig. 5 Job's plot for the interaction of PBI with Al3+ with a maxima at 0.5showing a 1 : 1 binding stoichiometry. The fluorescence intensity at 472 nm wasplotted against various mole fractions of Al3+.

RSC Adv.

Addition of more than 1 equiv. of Al3+ does not cause anappreciable shi in the position of the signals. This ndingconrms the 1 : 1 binding association of PB and Al3+.

Moreover, the hexa coordination of the PBI–Al3+ complex wascorroborated by mass spectrometric detection. In the presentstudy, the ligand showed a m/z at 288 which corresponds to[M + H]+, but the PBI–Al3+ showed a m/z at 432. This can beexplained as below. The probe PBI loses two NH protons andbinds with an aluminium ion in a tridentate fashion. In addi-tion, the metal may be co-ordinated by one water and twomethanol molecules, one of which coordinates in the deproto-nated form. Thus, the formed neutral aluminium complex

Fig. 7 Optimized structures of PB (a), PBI (b) and PBI–Al3+ (c), respectively. Theblue, yellow and pink spheres refer to H, C and N atoms, respectively.

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Table 1 HOMO–LUMO energy gaps calculated by the B3LYP/6-31G method

PB PBI PBI–Al3+

HOMO energy (eV) �5.4048 �5.6859 �4.4001LUMO energy (eV) �1.0234 �1.4243 �1.9105Energy gap (eV) 4.3813 4.2616 1.9930

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forms an adduct with K+. This potassium adduct is observed atm/z ¼ 432. The binding of Al3+ with the receptor was alsoconrmed by FT-IR spectral studies. The stretching frequency ofn(N–H), observed at 3361 cm�1 in PB, was absent in PBI–Al3+

due to the coordination of –N� (benzimidazole) to Al3+ viadeprotonation. The binding of the imine-N to Al3+ was observedby the existence of a 54 cm�1 shi for the –CH group in PBI–Al3+

compared to PB.To gain further insight into the reversible behaviour of the

receptor PB towards Al3+ ions, the effect of the addition of EDTAwas studied (Fig. S8†). This feature is highly desired for prac-tical applications. Upon the addition of EDTA to the complexthe emission spectrum was the same as that of PB in theabsence of Al3+ ions. This may be ascribed to the removal of Al3+

from the PBI–Al3+ complexes by EDTA.To investigate the proposed binding mode and the observed

red shi in the uorescence prole in the presence of Al3+,density functional theory calculations were performed at theBecke's three parameterized Lee–Yang–Parr (B3LYP/6-31G) andLANL2DZ(d) levels with the Gaussian 03 program package. Theresulting optimized geometries of PB, PBI and PBI–Al3+ areshown in Fig. 7. It is obvious that the Al3+ ion was bound by animine N-atom and two N-atoms from the imidazole and benz-imidazole moieties, respectively, which conrms the 1 : 1binding of Al3+ with the receptor. To further recognize theelectronic behavior, time-dependent density functional theory(TDDFT) calculations were achieved at the optimized geome-tries. The highest occupied molecular orbital (HOMO) andlowest unoccupied molecular orbital (LUMO) energies and theHOMO–LUMO energy gap are shown in Table 1. The HOMO andLUMO plots of PB, PBI and PBI–Al3+ are represented in Fig. S9.†In the PBI–Al3+ complex, the LUMO is mainly located on Al3+.Consequently the electronic transitions to the LUMO from theHOMO and other inner orbitals like HOMO�1 and HOMO�2correspond to charge transfer. A lowering of the HOMO–LUMOenergy gap was also observed in the presence of Al3+. Thuscharge transfer may be responsible for the observed emissionband at 472 nm with a bathochromic shi.

Conclusion

In summary, a differentially selective and highly sensitiveuorescent chemosensor for the detection of Al3+ in aqueousDMSO was successfully prepared by a facile one-step process.The receptor PB shows a ratiometric uorescent responsetowards Al3+ ions with good tolerance of other competing metalions. The 1 : 1 binding stoichiometry of the PBI–Al3+ complexwas conrmed by the Job's plot analysis and ESI-MS. Thereversibility of the receptor was assessed by the addition of

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EDTA. From these features, we conclude that the receptor PBhas the ability to serve as a ratiometric uorescent chemosensorfor highly toxic Al3+ ions in biological systems.

Acknowledgements

The authors D.J., M.I. and K.K., are grateful to UGC-BSR for theresearch fellowship and also thankful to DST-IRHPA, FIST andPURSE for funding and instrumental facilities.

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