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Development of a highly selective cell-permeable ratiometric fluorescentchemosensor for oxorhenium(V) ion†

Supriti Sen,a Titas Mukherjee,a Sandipan Sarkar,*a Subhra Kanti Mukhopadhyayb

and Pabitra Chattopadhyay*a

Received 16th July 2011, Accepted 16th August 2011

DOI: 10.1039/c1an15609h

A novel 6-(2-pyridinyl)-5,6-dihydrobenzimidazo[1,2-c]quinazoline (HL) serves as a first-time highly

selective and sensitive ratiometric fluorescent chemosensor probe for oxorhenium (ReO(V)) ion in

acetonitrile : water ¼ 9 : 1 (v/v) at 25 �C. The decrease in fluorescence at 410 nm and increase in

fluorescence at 478 nm with an isoemissive point at 444 nm in the presence of ReO(V) ion is accounted

for by the formation of mononuclear [ReOL2Cl] complex, characterized by physico-chemical and

spectroscopic tools. The fluorescence quantum yield of the chemosensor (HL) was only 0.198 at 410

nm, and it increased more than 3-fold in the presence of 2 equiv. of the ReO(V) ion at 478 nm.

Interestingly, the introduction of other metal ions and relevant anions caused the fluorescence intensity

at 478 nm to be either unchanged or weakened. The fluorescence-response fits a Hill coefficient of 2.088

indicates the formation of a 1 : 2 stoichiometry for the L-ReO(V) complex. In the concentration

range of 0–20 mMof oxorhenium(V) species calibration graph was linear with correlation coefficient (R)

of 0.99994 and the calibration sensitivity was found to be 4.0 � 10�7 M. The cellular image in the

confocal microscope clearly indicated the presence of ReO(V) in Candida albicans cells using this

chemosensor (HL).

Introduction

Development of practical selective fluorescent chemosensors for

the detection of important species, such as certain transition and

non-transition metal ions and anions is of current interest

because of a variety of biological and environmental problems

which cause serious problems for human health and ecology.1–5

The detection of metal ions in environmental or biological

systems by fluorometric methods has gained tremendous atten-

tion in recent years. Fluorescence methods have several advan-

tages over other techniques, including ease of detection,

sensitivity, and tenability. Numerous literature reports have

appeared that explore fluorescence sensing of main-group and

transition-metal ions. In this context, ratiometric responses are

more attractive because the ratio between the two emission

intensities can be used to measure the analyte concentration and

sensor molecule concentration, providing a built-in correction

for environmental effects, such as photo-bleaching, the envi-

ronment around the sensor molecule (pH, polarity, temperature,

aDepartment of Chemistry, The University of Burdwan, Golapbag,Burdwan, 713104, India. E-mail: pabitracc@yahoo.com; sandipanbu@yahoo.co.in; Tel: +91-9434473741bDepartment of Microbiology, The University of Burdwan, Golapbag,Burdwan, 713104, India

† Electronic supplementary information (ESI) available. CCDCreference number 651980. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/c1an15609h

This journal is ª The Royal Society of Chemistry 2011

and so forth), and stability under illumination.6–9 As a conse-

quence, the development of fluorescent metal ion and anion10–12

sensors is a topic of current impetus.13–16

Nowadays, coordination chemistry of rhenium17,18 continues

to be a topic of intensive research, mainly due to the variety of

applications of rhenium complexes, ranging from medicinal

chemistry19–24 and catalysis25–27 to photophysics28,29 and materials

chemistry.30 The main type of rhenium complexes labeled with

b-emitting isotopes 186Re/188Re used in nuclear medicine for

systematic radiotherapy of abnormal tissues or in the treatment

of cancer31,32 is oxorhenium(V);33 and though rhenium is not

a biometal, as an element in the periodic group next to that of

molybdenum and tungsten, transfer reactions involving Re¼O

are of value as potential models.34 But the method for the

determination of rhenium is rare35,36 and specifically the detec-

tion technique for oxorhenium(V) ion is still unexplored. To our

best knowledge, ratiometric amplified chemosensor for the

detection of ReO(V) ion is not established.

Herein, we report a study of ratiometric amplified chemo-

sensor for selective detection of ReO(V) ion using nitrogenous

heterocyclic organic compound (HL). The fluorescent property

of the organic moiety in acetonitrile : water (9 : 1) solvent shows

that the emission intensity at 410 nm (appearing due to LH)

decreases gradually with the generation of a new peak at 478 nm

on addition of ReO(V) ion due to the formation of mononuclear

[ReO(L)2Cl] complexes. The very negligible change of this

emission intensity in the presence of other alkali, alkaline earth

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and transition metal ions has also been recorded. Fluorescence

intensity has been measured at different pH and in different

solvents. Job’s plot analysis and Hill coefficient plot support the

formation of 1 : 2 oxorhenium complex. Formation of 1 : 2

complex of ReO(V) with organic moiety in the solution is also

proved by the isolation of [ReO(L)2Cl] in solid state followed by

characterization by elemental analysis, mass and 1HNMR

spectroscopy.

Experimental

Materials and physical measurements

All of the solvents were of analytical grade. The elemental

analyses (C, H and N) were performed on a Perkin Elmer 2400

CHN elemental analyzer. Electronic absorption spectra were

recorded on a JASCO UV-Vis/NIR spectrophotometer model

V-570. IR spectra were recorded using JASCO FT-IR spec-

trometer model 460 plus preparing KBr disk. 1HNMR spectra

were recorded on a Bruker AC300 spectrometer using TMS as an

internal standard in appropriate dueterated solvents. Electro-

spray ionization (ESI) mass spectra were recorded on a Qtof

Micro YA263 mass spectrometer. Molar conductance (LM) was

measured in a Systronics conductivity meter 304 model using

�10�3 mol L�1 solutions in methanol. Atomic absorption spec-

trometric estimation of rhenium in the complex was made with

a GBC Avanta 908BT instrument. The measurement of pH was

done with the help of a digital pH meter, Systronics, Model 335.

The fluorescence spectra of the titration of oxorhenium ion with

organic moiety were obtained at an emission wavelength of 478

nm in the Fluorimeter. Time resolved experiment was done using

HORIBA JOBIN Yvon single photon counting set up. Fluo-

rescence lifetimes were determined from time-resolved intensity

decay by the method of time-correlated single-photon counting

using IBM Decay Analysis Software for Windows v6.1.98 and

a nano LED at 372 nm as the light source.

X-ray crystal structure analysis of HL was carried out by

collecting the diffraction data using Mo-Ka (l ¼ 0.71073 �A)

radiation at 293 K. The crystals were positioned at 70 mm from

the image plate and 95 frames were measured at 2� intervals witha counting time of 2 min. Data analysis was carried out with the

XDS program.37 The structures were solved using direct methods

with the SHELXS97 program.38 The non-hydrogen atoms were

refined with anisotropic thermal parameters. The hydrogen

atoms bonded to carbon were included in geometric positions

and given thermal parameters equivalent to 1.2 times those of the

atom to which they were attached. The hydrogen atoms attached

to the water molecules were located in difference Fourier maps

and refined with distance constraints. An empirical absorption

correction was carried out on 1 using the DIFABS program.39

Refinement on all four structures was carried out with a full

matrix least squares method against F2 using SHELXL97.

The luminescence property of the sensor was investigated in

water : acetonitrile(1 : 9, v/v) solvent. pH study was done in 100

mM HEPES buffer solution by adjusting pH with HCl or

NaOH. In vivo study was performed at biological pH �7.4 with

100 mM HEPES buffer solution. The stock solutions (�10�2 M)

for the selectivity study of the probe (HL) towards different

metal ions were prepared taking nitrate salts of aluminium(III),

4840 | Analyst, 2011, 136, 4839–4845

copper(II), chromium(III), lead(II), cadmium(II), silver(I); chloride

salts of nickel(II), cobalt(II), mercury(II), potassium(I), calcium

(II), manganese(II); perchlorate salts of sodium, iron (II/III), zinc

(II); Na2WO4$2H2O; Na2MO4$2H2O and NaReO4 in water :

acetonitrile (1 : 9, v/v) solvent. In this selectivity study the

amount of these metal ions was a hundred times greater than that

of the probe used. Fluorescence titration was performed with

Bu4N[ReOCl4] in water : acetonitrile (1 : 9, v/v) solvent varying

the metal concentration 0 to 100 mMand the probe concentration

was 17 mM.

Preparation of [6-(2-pyridinyl)-5,6-dihydrobenzimidazo[1,2-c]

quinazoline] (HL)

An ethanolic solution of 2-(2-aminophenyl)benzimidazole, (2.09

g, 10.0 mmol) was added to pyridine-2-carboxylaldehyde (1.07 g,

10.0 mmol) in ethanol (25 mL) at room temperature. Then this

mixture was allowed to reflux for 4 h. The white colored crys-

talline precipitate of the compound (HL) was obtained from the

yellow colored solution through slow evaporation of the solvent.

C19H14N4: Anal. Found: C, 76.56; H, 4.75;N, 18.49; Calc.: C,

76.48; H, 4.73; N, 18.78. m.p. 231 � 1 �C, EI-MS: [M + H]+, m/z,

299.34; IR (KBr, cm�1): nC¼N, 1477;.1H NMR (d, ppm in dmso-

d6): 8.437 (d, 1H, j ¼ 3.9); 7.906 (d, 1H, j ¼ 7.2); 7.768–7.697 (m,

2H); 7.631 (d, 1H, j ¼ 7.2); 7.351–7.096 (m, 7H); 6.853–6.769 (m,

2H); Yield: 90%.

Preparation of the oxorhenium(V) complex ([ReO(L)2Cl])

Methanolic solution of Bu4N[ReOCl4] (1.0 mmol, 587.0 mg) was

added into the (2.0 mmol, 596.0 mg) methanolic solution of

ligand at stirring condition. The mixture was stirred for another 4

h to complete the complexation. Then the resulting solutions

were kept aside at room temperature. After a few days, deep

green crystalline complex were collected by filtration followed by

washing with water, methanol and dried in vacuo.

[ReO(L)2Cl] (1): ReOC38H26ClN8: Anal. Found: C, 54.78; H,

3.11; N, 13.38; Re, 22.21; Calc.: C, 54.84; H, 3.14; N, 13.46; Re,

22.37. IR (cm�1): nC¼N, 1474; nRe¼O, 957. Conductance (Lo,

ohm�1 cm2 mol�1) in methanol: 45; ESI-MS in methanol: [M +

Na]+, m/z, 857.6668 (obsd. with 8% abundance) (Calc.: m/z,

857.32; where M ¼ [ReO(L)2Cl].1H NMR (d, ppm in CD3OD):

8.208–8.164 (m, 2H); 7.735 (d, 1H, j ¼ 8.2); 7.504 (t, 1H, j ¼ 7.7);

7.371–7.020 (m, 7H); 6.805 (d, 1H, j ¼ 8.2); 6.636 (t, 1H, j ¼ 7.9).

Yield: 70–75%.

Results and discussion

Synthesis and characterization

Organic moiety was synthesized by the reaction of 2-(2-amino-

phenyl)benzimidazole with pyridine-2-carboxylaldehyde in

ethanol solvent (Scheme 1). Single crystals were obtained from

the methanolic solution of this organic compound. The physico-

chemical and spectroscopic tools along with the structural

characterization by single crystal X-ray crystallography confirm

the structure of the organic moiety. The solid state structure of

HL has already been reported in the literature,40 and for that

reason we are not describing the structure here. The ORTEP

This journal is ª The Royal Society of Chemistry 2011

Scheme 1 Synthetic strategy of the compounds (HL and [ReO(L)2Cl]).

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view of HL is illustrated in Fig. s1 together with the numbering

scheme.†

To establish the fact of the formation of the oxorhenium(V)

complex, the solid state [ReO(L)2Cl] complex was obtained

from the reaction of one molar Bu4N[ReOCl4] with two molar

of the organic moiety in the methanol medium in stirring

condition. The complex is soluble in acetonitrile and methanol.

The conductivity measurement of the complex in methanol at

300 K suggests that the complex exists as nonelectrolytes in

solution state. The ESI mass spectrum of the complex in

methanol shows a peak at m/z 857.66 with 8% abundance,

which can be assigned to [M + Na]+ (calculated value at m/z,

857.32) where M ¼ [ReO(L)2Cl]. The IR stretching frequency

centred at 957 cm�1 assignable to nRe¼O confirm the presence of

oxorhenium(V) ion in the complex.41 The 1HNMR spectrum

obtained in CD3OD confirm the presence of the HL bound to

oxorhenium(V) complex. One hydrogen is less than the corre-

sponding HL which indicates that here this organic entity (HL)

behaves as bidentate monobasic ligand. All these data confirm

the composition of the oxorhenium complex with a formula of

[ReO(L)2Cl].

Fig. 1 Excitation and emission spectra of HL (10 mM) in water : aceto-

nitrile (1 : 9, v/v).

This journal is ª The Royal Society of Chemistry 2011

Spectral characteristics

Organic moiety (HL) shows emission spectrum at 410 nm in

water : acetonitrile (1 : 9) solvent mixture excited at 376 nm

considering the absorption at 350 nm (Fig. 1). Fluorescence

quantum yields (F) were estimated by integrating the area under

the fluorescence curves with the equation:

fsample ¼ ODstandard�Asample

ODsample�Astandard

�fstandard (1)

where A is the area under the fluorescence spectral curve andOD

is the optical density of the compound at the excitation wave-

length. The standard used for the measurement of fluorescence

quantum yield was anthracene (F ¼ 0.29 in ethanol).

The emission intensities of the organic molecule in the presence

of various concentrations of oxorhenium(V) ion were measured.

Addition of various concentrations of oxorhenium(V) solution,

fluorescence intensity of organic moiety gradually decreased and

a new peak appeared at 478 nm showing a isoemissive point at

444 nm (Fig. 2). Ratiometric signaling of fluorescence output at

two different wavelengths plotted as a function of concentration

of ReO(V) (Fig. 3) indicates that the fluorescence intensity ratio

of wave length 478 nm and 410 nm (I478/I410) gradually increases

with increase of the concentration of ReO(V) ion and after

reaching the saturation binding level, I478/I410 value is constant.

The fluorescence quantum yield has been calculated in the

absence and presence of oxorhenium(V) ion and from this

measurement it is clear that the fluorescence quantum yield

increases more than three times the amount gained in the absence

of oxorhenium(V) ion (Table 1).

From Job’s plot analysis it is revealed that maximum emission

shows at 1 : 2 ratio (ReO : L) (Fig. 4). Hill coefficient plot also

supports the formation of 1 : 2 complex (Fig. 5). These data indi-

cate that the complex species in solution should form1 : 2 complex

withReO(V) clear from themass spectrumandNMR.The binding

constant value was determined from the emission intensity data

following the modified Benesi–Hildebrand equation:42,43

1/(Fx � F0) ¼ 1/(Fmax � F0) + (1/K[C]1/2)(1/(Fmax � F0),

Fig. 2 Emission spectra of 17 mM of HL in the presence of 0, 2, 4, 6, 8,

10, 12, 14, 16, 18, 20, 22, 30, 37, 55, 75, 100 mM of ReO(V) in water :

acetonitrile (1 : 9, v/v) at room temperature. Inset: fluorescence

enhancement versus concentration of ReO(V).

Analyst, 2011, 136, 4839–4845 | 4841

Fig. 3 Ratiometric signaling of fluorescence output at two different

wavelengths is plotted as a function of [ReO(V)] taking [HL] ¼ 17 mM.

Table 1 Fluorescence quantum yield (Ff) and life time (sf in ns) of thecorresponding singlet excited states

Ff sf (ns) kr(108s�1) knr (10

9 s�1) c2

HL 0.1985 0.23 8.63 3.48 1.031HL + ReO(V) 0.6230 0.22 28.19 1.7 1.094

Fig. 4 Job’s plot analysis showing maximum emission at 2 : 1 ratio

[HL : ReO(V)].

Fig. 5 Fluorescence intensity at 478 (Fx) of L versus increasing

concentration of log[ReO(V)]. The concentration of L was 500 mM. The

fluorescence response fits to a Hill coefficient of 2 (�2.088). It is consis-

tent with the formation of a 1 : 2 (ReO(V) : L) stoichiometry for the

resulting oxorhenium(V) complex.

Fig. 6 Binding constant (K) value of 0.18 � 104 M�1/2 determined from

the intercept/slope of the plots.

Fig. 7 Time-resolved fluorescence decay of HL (10 mM) in the absence

and presence of added ReO(V) (20 mM) (at lex ¼ 376 nm) in water :

acetonitrile (1 : 9, v/v).

Fig. 8 Change of relative fluorescence intensity profile of organic moiety

in the presence of different cations in acetonitrile : water (9 : 1, v/v) at

room temperature (lex ¼ 376 nm).

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where F0, Fx, and FN are the emission intensities of organic

moiety considered in the absence of ReO(V) ion, at an interme-

diate ReO(V) concentration, and at a concentration of complete

4842 | Analyst, 2011, 136, 4839–4845

interaction, respectively, and where K is the association constant

and [C] is the ReO(V) concentration. K value (0.18 � 104 M�1/2)

was calculated from the intercept/slope using the plot of (Fmax �F0)/(Fx � F0) against [C]

�1/2 (Fig. 6). The fluorescence average

lifetime measurement of organic moiety in the presence and

absence of ReO(V) ion in the water : acetonitrile (1 : 9) medium

are 0.22 ns and 0.23 ns respectively (Fig. 7). According to the

This journal is ª The Royal Society of Chemistry 2011

Fig. 9 Change of relative fluorescence intensity of organic moiety in the

presence of ReO(V) and ReO4� ion in acetonitrile : water (9 : 1, v/v) at

room temperature (lex ¼ 376 nm).

Fig. 10 Fluorescence response of organic moiety (17 mM) of HL in

absence (black) and in presence of ReO(V) 200 mM (red) ion at different

pH in HEPES buffer.

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equations: s�1 ¼ kr + knr and kr ¼ Ff/s,44 the radiative rate

constant kr and total nonradiative rate constant knr of organic

moiety, L, and oxorheniun complex with L are listed in Table 1.

The data suggest that knr has just slightly changed but the factor

that induces fluorescent enhancement is mainly ascribed to the

increase of kr.

Selectivity

The fluorescent response of organic moiety towards the different

metal ions was investigated with 100 times concentration of

Fig. 11 Fluorescence response to pH of organic moiety (17 mM) in HEP

This journal is ª The Royal Society of Chemistry 2011

alkali (Na+, K+), alkaline earth (Mg2+, Ca2+), and transition-

metal ions (Ni2+, Zn2+, Cd2+, Co2+, Cu2+, Fe2+/3+, Cr3+, Hg2+) and

Pb2+, Ag+, ReO(V) (Fig. 8). It reveals that organic moiety has

excellent selectivity to ReO(V) ion in comparison to other

cations. Ability of recognization of ReO(V) ion by the organic

moiety was further investigated in the presence of other back-

ground metal ions. Fig. 8shows that the ReO(V) ion response to

the sensor is little affected by the presence of alkali and alkaline

earth metal ions. In the presence of ReO4� ion, the organic

moiety does not show any emission at 478 nm (Fig. 9). Therefore,

organic moiety is highly selective towards the ReO(V) ion and it

may also be used for the detection of ReO(V) in a mixture of ReO

(V) and ReO4� ion.

Effect of pH

The fluorescence intensity of organic moiety was measured at

various pH values adjusting the pH using HEPES buffer in

presence and absence of ReO(V) ion (Fig. 10). In the absence of

ReO(V) ion, organic moiety exhibited fluorescence of weak

intensity at lower than ca. pH 5.0 and showed pH independency

over the pH range 5.0 to 10.0. But in the presence of ReO(V), the

fluorescence intensity gradually increased upto ca. pH 6.0

showing a plateau in the region of pH 6.0 to 10.0. It indicates that

the fluorescence intensity almost does not vary in the pH range of

6.0–10.0. But fluorescence intensity of the organic moiety in the

presence of ReO(V) ion is higher than that in the absence of ReO

(V) ion. This is due to the formation of [ReO(L)2Cl] complex

through the deprotonation of the nitrogen atom of the N–H

group of the pyridine ring42–44 (pKa ¼ 4.16) (Fig. 11) of HL.

Analytical figure of merit

The sensitivity of the sensor towards the oxorhenium(V) ion has

been checked. Calibration graph was linear over the concentra-

tion range of 0–20 mM of oxorhenium(V) species having the

calibration sensitivity of 4.0 � 10�7 with correlation coefficient

(R) of 0.99994 (viz. Fig. s2†).

Cell study

The intracelluar ReO(V) imaging behavior of HL on Candida

albicans cells was studied at biological pH�7.4 using HEPES

buffer (Fig. 12) with an inverted fluorescence microscope (Leica

DM 1000 LED), digital compact camera (Leica DFC 420C), and

an image processor (Leica Application Suite v 3.3.0). Candida

ES buffer at room temperature; (a) pKa ¼ 4.1638 (b) pKa ¼ 4.1319.

Analyst, 2011, 136, 4839–4845 | 4843

Fig. 12 Imaging of HL with ReO(V) incubated Candida cells. (a) Only Candida cells; (b) Candida cells 20 h incubation with ReO(V) salt; (c) Candida 20

h incubation with HL; (d) Candida 20 h incubation with ReO(V) salt + HL; (e) Candida 44 h incubation with ReO(V) salt + HL; (f) Incubation of

Candida with HL for 44 h and then ReO(V) was added; (g) Incubation of candida with ReO(V) salt for 44 h and then HL was added; (h) Incubation of

Candida with ReO(V) salt along with some other ions (Cd(II), Hg(II), MO42� and WO4

2�) for 24 h and then HL was added.

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cells were grown in sterile YEG medium containing rhenium

(10�3 concentration). Control cells were grown in the same way

without adding rhenium in the medium. After incubation for

a varying period of time (20 and 44 h), actively growing cell

cultures were centrifuged at 3000 rpm for ten minutes in an

ependorf table top cooling centrifuge at 4 �C. Cell pellets werethen washed once by normal saline and stored at �20 �C. Beforeobservation cells were treated with the HL and mounted on clean

glass slides and observed under high power magnification of

a fluorescence microscope (DM1000, Leica) using UV filter. The

Candida cells displayed very weak intracellular fluorescence

(shown in Fig. 12a). ReO(V) incubated Candida cells also fluo-

resce faintly (depicted in Fig. 12b). After adding HL to the

Candida cells at room temperature they displayed strong fluo-

rescence (viz. Fig. 12c) demonstrating that HL is cell permeable.

But when HL was introduced into the ReO(V) incubated cell

fluorescence is highly increased (viz. Fig. 12d and 12e). These

results indicate that HL is an effective intracellular ReO(V)

imaging agent with cell permeability. Cell budding of the

Candida albicans cells has not been interrupted even after incu-

bation 44 h in presence of ReO(V) (vide Fig. 12f and 12g). It

clearly indicates the non-toxic nature of HL to the eukaryotic

cells. In another study, the probe showed the excellent selectivity

towards ReO(V) ions in biological medium even in the presence

of Cd(II), Hg(II), MO42� and WO4

2� ions (vide Fig. 12h).

Conclusion

A new ratiometric fluorescent chemosensor for oxorhenium(V)

ion has been developed and it is highly selective with almost no

interference. The probe, 6-(2-pyridinyl)-5,6-dihydrobenzimidazo

[1,2-c]quinazoline] (HL) might be the first ReO(V) fluorescent

chemosensor. This phenomenon has been accounted for with the

formation of mononuclear [ReO(L)2Cl] complex which has been

characterized spectroscopically after the isolation of this coor-

dination complex. The sensor HL, containing lone electron pairs

on N exhibits high selectivity for ReO(V) ions not only in abiotic

systems but also in living cells, presumably due to the CHEF

4844 | Analyst, 2011, 136, 4839–4845

effect during the chelation of HL toward the ReO(V) ion in a 2 : 1

complex mode.

Acknowledgements

Financial assistance from the Department of Science and

Technology (DST), New Delhi, India, is gratefully acknowl-

edged. Both S. Sarkar and S. Sen wish to thank to Council of

Scientific and Industrial Research (CSIR), New Delhi, India, for

offering the fellowships. We are grateful to the honorable

reviewers for their valuable comments for the improvement of

this article.

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