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A novel system for Fe3+ ion detection based on fluorescence resonance energy transfer
Sensors and Actuators B 221 (2015) 136–147
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel system for Fe3+ ion detection based on fluorescence resonance energy transfer Ebru Bozkurt a,∗ , Mustafa Arık b , Yavuz Onganer b,∗∗ a b
Program of Occupational Health and Safety, Erzurum Vocational Training School, Atatürk University, 25240 Erzurum, Turkey Department of Chemistry, Faculty of Sciences, Atatürk University, 25240 Erzurum, Turkey
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online 27 June 2015 Keywords: Coumarin 120 Fluorescein Fluorescence resonance energy transfer (FRET) Detection of Fe3+ ions
a b s t r a c t A new sensor system was developed for metal-ion sensing based on fluorescence resonance energy transfer (FRET). FRET process was carried out between coumarin (C120) as the energy donor and fluorescein (FL) as the energy acceptor. The concentration most suitable for FRET efficiency was determined to be 0.2 M for C120 and 20 M for FL. The effects of various metal ions such as Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ on the energy transfer between the C120 and FL were explored. It was observed that the presence of Fe3+ ions inhibited the energy transfer between the two molecules while the other metal ions did not affect. This observation shows that the C120–FL system could be selectively detected for Fe3+ ions. The detection limit and binding constant of Fe3+ were determined as 2.54 M and 4.33 × 104 M−1 , respectively. The C120–FL system has a good potential for a sensor application in analytical systems due to its high selectivity for Fe3+ . © 2015 Elsevier B.V. All rights reserved.
1. Introduction Fe3+ ion plays an important role in many biological processes, given that it is a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain and facilitates the oxygen transport capacity of haemoglobin [1]. Iron deficiency can lead to anemia in the body. However, an excess of Fe3+ ions causes some types of cancers and deterioration of functions of organs such as heart, pancreas and lungs [2]. Thus, a simple, easy and fast determination of Fe3+ ions in a liquid media has been accorded signification attention. There are several methods for the detection of Fe3+ , such as atomic absorption spectroscopy, colorimetry, mass spectrometry and electrochemical and fluorescence spectroscopic analysis. The fluorescence techniques are generally used due to their ease of application, high sensitivity and efficiency [3]. There are many photo-physical processes such as photo-induced electron or charge transfer, fluorescence resonance energy transfer (FRET) and excimer formation, which are
Abbreviations: C120, Coumarin 120; FL, Fluorescein; FRET, Fluorescence resonance energy transfer; MeOH, Methanol; EDTA„ Ethylene diamine tetra acetic acid; UV–vis, ultraviolet–visible. ∗ Corresponding author. Tel.: +90 442 231 2667; fax: +90 442 231 2503. ∗∗ Corresponding author. Tel.: +90 442 231 4446; fax: +90 442 236 0948. E-mail addresses: [email protected] (E. Bozkurt), [email protected] (Y. Onganer). http://dx.doi.org/10.1016/j.snb.2015.06.097 0925-4005/© 2015 Elsevier B.V. All rights reserved.
responsible for the photo-physical changes resulting from the binding of metal ions with fluorophores. FRET, with its promising selectivity and sensitivity, is the most remarkable of these processes [4–7]. FRET is a physicochemical process based on the intermolecular energy transfer from a molecule (donor) to another molecule (acceptor) as non-radiative [8]. Professor Theodor Förster has proposed an excellent theory that determine the mechanism of FRET [9]. According to the theory, the rate of energy transfer depends on the extent of spectral overlap between the donor emission and the acceptor absorption spectra, the quantum yield of the donor, the relative orientation of the donor and acceptor transition dipoles, and the distance between the transition dipoles of the donor and acceptor [8]. Coumarin 120 (C120) and fluorescein (FL) are chosen as the respective donor and acceptor dye molecules for FRET application. Coumarin 120 is a compound of 7-amino coumarin derivative. When various functional groups are attached to the 7-position of the coumarin ring, they exhibit intense fluorescence in the blue–green region along with very high quantum yield [10]. So, they are used in various fields such as for the determination of polarity in microenvironments [11,12], the investigation of photo-induced electron transfer dynamics [13], and the nucleobase-specific quenching of fluorescent dyes [14]. Fluorescein is a dye xanthene derivative and its derivatives have been widely applied in many fields including biology and medicine [15–17]. There are fluoresceins of cationic, anionic, neutral and
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Fig. 1. Molecular structure of (a) coumarin 120 and (b) fluorescein.
dianionic forms in aqueous solutions [18]. Its dianionic form is widely used because it has a large extinction coefficient and high fluorescence quantum yield [19]. In this study, we investigated the effect of metal ions on FRET between C120 (donor) and FL (acceptor) in aqueous solution by using both colorimetric and fluorometric methods. The metal ions at various loadings were added to this system, and its energy transfer parameters were calculated. The results revealed that the energy transfer between C120 and FL was prevented by Fe3+ ions and this FRET system was a new iron ion sensor. In this regard, the energy transfer was analyzed in detail, and the binding rate of Fe3+ was determined by using Benesi–Hildebrand analysis and Job’s method. Also, the selectivity and sensitivity of the sensor system based on FRET was proved with real sample tests. The novel system, which could detect ferric ions in low detection limits, was developed.
2. Experimental 2.1. Materials 3 ,6 -Dihydroxyspiro[2-benzofuran-3,9 -xanthene]-1-one sodium salt (fluorescein, FL), 7-amino 4-methyl coumarin
(coumarin 120, C120) and methanol (MeOH) were obtained from Sigma. Stock solutions of C120 and FL of 1.0 × 10−3 M were prepared in MeOH. A certain amount of fresh probe samples in aqueous solution was obtained from this stock solution by evaporating the solvent. For all measurements, the concentrations of C120 and FL were 0.2 M and 20 M, respectively. The molecular structures of C120 and FL were given in Fig. 1. All the experiments were performed at room temperature. NaNO3 , LiCl, AgNO3 , ZnCl2 ·H2 O, Pb(CH3 COO)2 , MnCl2 , HgCl2 , CuCl2 , CdI2 , FeCl3 ·6H2 O, CrCl3 and AlCl3 (as the metal ion sources) and ethylene diamine tetra acetic acid (EDTA) were obtained from Sigma. 2.2. Apparatus The UV–vis absorption and fluorescence spectra of the samples were recorded with Perkin Elmer Lambda 35 UV/VIS spectrophotometer and Shimadzu RF-5301PC spectrofluorophotometer, respectively. For the steady-state fluorescence measurements, sample solutions were excited at 350 nm and their fluorescence intensities were recorded. Fluorescence lifetime measurements were carried out with a LaserStrobe model TM3 spectrofluorophotometer from Photon Technology International (PTI). The excitation source combined a pulsed nitrogen laser/tunable dye
Fig. 2. Fluorescence spectra of C120, FL and C120–FL in pure water.
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Fig. 3. (a) Fluorescence spectra of C120–FL in the absence and presence of 20 M metal ions in water (exc = 350 nm). (b) Photographs of C120–FL in the presence of metal ions under daylight (c) under UV.
laser. The decay curves of C120 dye (ex = 366 nm) were collected over 200 channels using a nonlinear time scale in which the time increment increased according to arithmetic progression. The fluorescence decays were analyzed with lifetime-distribution analysis software from the instrument supplying company. The goodness of fits was assessed by the value of 2 and weighed residuals
[20]. The Fluorescence quantum yields of donor molecules were calculated through the Parker-Rees equation:
∅s = ∅r
D n2 1 − 10−ODr s s Dr
n2r
1 − 10−ODs
Fig. 4. Absorption spectra of C120–FL in the absence and presence of 20 M metal ions in water.
(1)
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Fig. 5. Fluorescence spectra of C120–FL with the increasing concentration of Fe3+ (exc = 350 nm).
where D is the integrated area under the corrected fluorescence spectrum, n is the refractive index of the solution, and OD is the optical density at the excitation wavelength (ex = 350 nm). The subscripts s and r refer to the sample and reference solutions, respectively [21]. Quinine sulfate in 0.5 M H2 SO4 solution was used as the reference. The fluorescence quantum yield of quinine sulfate was 0.54 in 0.5 M H2 SO4 solution [22].
where D is the lifetime of the donor in the absence of an acceptor, r is the distance between donor and acceptor and R0 is the Förster distance. R0 is a distance by which energy transfer efficiency is 50%. This distance can be calculated through the following equation:
R0 = 0.211 2 −4 QD J ()
1/6
(3)
2.3. Calculation of FRET parameters According to Dr. Förster’s theory, the rate of FRET (kET ) is given as follows: kET =
1 D
R 6 0
r
(2)
where QD is the quantum yield of the donor in the absence of an acceptor, is the refractive index of the medium, 2 is the orientation factor. This factor describes the relative orientation in the space of transition dipoles of the donor and acceptor, and is usually assumed to be equal to 2/3, whereby J () is the spectral overlapping integral. This integral represents the degree of spectral overlap
Fig. 6. Absorption spectra of C120–FL with the increasing concentration of Fe3+ .
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Fig. 7. Change fluorescence intensity of C120 with the increasing concentration of Fe3+ .
between the emission spectrum of the donor and the absorption spectrum of acceptor, and it is calculated as follows:
J () =
0 ∞
donor, in the absence (FD ) and presence (FDA ) of the acceptor based on the following equation [23]: E =1−
FD ()εA ()4 d
0 ∞
FD ()d
FDA FD
(5)
(4) 2.4. The sensing of metal ion
where FD () is the normalized fluorescence intensity of the donor in the absence of an acceptor and εA () is the extinction coefficient of the acceptor. Energy transfer efficiency, as one of the important parameters, was calculated using the relative fluorescence intensity of the
The 1.0 × 10−2 M stock solutions of all metal ions (Li+ , Na+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were prepared in pure water. Then, 10 l of metal ion was added to 5 ml solution containing 0.2 M C120 and 20 M FL at room temperature. The spectroscopic changes in energy transfer between C120 and FL were recorded with the absorption and fluorescence Ag+ ,
Fig. 8. Job’s plot of C120–FL with Fe3+ in water.
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Fig. 9. Benesi–Hildebrand plot based on a 1:1 association stoichiometry between C120–FL and Fe3+ .
measurements. Fe3+ solution of different concentrations (1, 2, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 140, 160, 180, 200, 300 and 400 M) was added to the donor-acceptor solution. The absorption and fluorescence measurements were taken for each
solution containing Fe3+ ions. The detection limit for Fe3+ was determined with the fluorescence data. For this purpose, the 3s/k equation was used. Where s is the standard deviation of blank, k is the slope of the fit line in the fluorescence titration experiment.
Fig. 10. (a) Fluorescence spectra of C120–FL in the presence of 20 M Fe3+ ion and EDTA in water (exc = 350 nm). (b) Photographs of C120–FL in the presence of 20 M Fe3+ ion and EDTA under daylight (c) under UV.
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Fig. 11. (a) Absorption spectra (b) fluorescence spectra of C120–FL containing other metal ions in the absence and presence of 20 M Fe3+ in water (exc = 350 nm).
Moreover, the following Benesi–Hildebrand equation was used to calculate the binding constant:
were added. The mixture was diluted to 5 ml with distilled water and the fluorescence spectra were recorded [24,25].
1 1 1 = + F − F0 Fmax − F0 KS (Fmax − F0 ) [M + ]n
3. Result and discussion
(6)
where F0 , F, and Fmax are the fluorescent intensity of the donor in the absence of a metal ion, at a certain concentration of metal ion and at a complete interaction concentration of metal ion, respectively. [M+ ] is the concentration of Fe3+ , and n is the binding stoichiometry for dye and metal ion [24].
2.5. Testing real sample Tap water was used for real sample tests. After filtration, 1.5 ml of tap water was put into 5 ml volumetric flask. Then, 1 ml stock solution of the C120–FL and 3 (5 and 7) l stock solutions of Fe3+
3.1. The effect of metal ions on energy transfer In the study, C120 and FL were used as the energy donor and acceptor, respectively. The spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor was determined (Fig. S1). Then, the fluorescence spectra of C120, FL and the C120–FL were recorded at 350 nm. Fig. 2 showed that the fluorescence spectrum of the C120–FL exhibited two peaks at 432 and 506 nm. While the fluorescence peak at 432 nm was weaker than that of pure C120, the fluorescence peak at 506 nm was stronger than that of pure FL at the same excitation wavelength. This observation confirmed that the energy transfer between the
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Fig. 12. Metal ion selectivity profiles of C120–FL in the presence of various metal ions in water. Red bars represent the fluorescence intensity of C120–FL in the presence of 20 M of metal ion. Blue bars represent the fluorescence intensity in the presence of various metal ions after the addition of Fe3+ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
two molecules was occurred [4]. After the presence of the energy transfer between the two molecules was determined, the effect of metal ions on the energy transfer was examined by using the fluorescence titration method [26]. Different metal ions (Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were added to the C120–FL system, and their fluorescence spectra were recorded. As shown in Fig. 3a, Fe3+ ions prevented the energy transfer between the two molecules while the other metal ions did not present a significant change in energy transfer. Fig. 3b and c showed photographs of the C120–FL in the presence of different metal ions under daylight and UV, respectively. It was observed that the color of the solution was changed in the presence of Fe3+ ions, but the other metal ions did not induced any significant change in the color of the solution. The energy transfer parameters in the presence of all
metal ions excluding Fe3+ were calculated by using Eqs. (2)–(5). It was observed that the metal ions increased the efficiency of energy transfer (Table S1). As seen from Fig. 3b and c, colorimetric detection of Fe3+ could be also made with the C120–FL system. The UV–vis absorbance response of the C120–FL FRET system against various metal ions at different concentration was plotted (Fig. 4). As seen from Fig. 4, while the absorbance spectrum of the C120–FL system was not changed in the presence of other metal ions, significant changes occurred in the absorption spectra in the presence of Fe3+ [1]. The maximum absorption intensity of the donor increased and its maximum absorption wavelength was blue-shifted. Moreover, the intensity of the absorption peak of the acceptor decreased and shifted to red region. It was determined that the C120–FL system showed also colorimetric selectivity with the changes in the absorption spectrum when Fe3+ ions was existed [27]. 3.2. Detection of Fe3+ with the energy transfer
Scheme 1. The proposed mechanism of interaction between C120–FL and Fe3+ .
Since the effect of iron ions on the C120–FL was observed, Fe3+ ions at different concentrations (0–400 M) were added to the C120–FL for the detection of Fe3+ ions. As shown in Fig. 5, while the fluorescence peaks of FL at 506 nm disappeared, the fluorescence intensity of C120 at 432 nm decreased with the increase of Fe3+ concentration. Accordingly, when Fe3+ ions were added to the solution, it was observed that the fluorescence emission of C120 was substantially quenched and the fluorescence property of FL disappeared. The emission quenching rates of C120, FL and the C120–FL were compared under the same Fe3+ ions (Fig. S2). It was observed that the emission quenching rates of the C120–FL at 432 nm and 506 nm were higher than those of C120 and FL. This result indicates that the C120–FL is a system that can detect the highly efficient Fe3+ ions [26]. Moreover, the UV–vis. absorption response of the C120–FL to Fe3+ ions at different concentrations was measured (Fig. 6). While the absorption intensity of donor gradually increased, the absorption peak of acceptor showed bathochromic shift with increasing Fe3+ ion concentration [28,29]. According to the fluorescence measurement results, a plot of Fe3+ ion concentration versus the emission intensity of C120 showed a linear relationship (Fig. 7). The detection limit of Fe3+
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Fig. 13. (a) Fluorescence spectra of C120 (b) fluorescence spectra of FL with the increasing concentration of Fe3+ .
was determined to be 2.54 M using the 3s/k equation. Compared to recent studies regarding this value, it has been reached a very low detection limit of Fe3+ ion [24,30,31]. Moreover, according to the U.S. Environmental Protection Agency (EPA), the maximum Fe3+ levels in drinking water must be ∼5.357 M [32]. In this study, it was found that the limit of detection for Fe3+ is lower than the value permitted by the EPA. The stoichiometry of interaction between the C120–FL and Fe3+ was determined by Job’s plot analysis based on the change in the intensity of fluorescence [33]. The results indicated the interaction of a 1:1 between the C120–FL and Fe3+ (Fig. 8). According to the fluorescence titration and Job’s plot experiments, the binding constant of Fe3+ was calculated as 4.33 × 104 M−1 through the Benesi–Hildebrand equation (Fig. 9). EDTA was added to the C120–FL and Fe3+ solution in order to confirm the effect of Fe3+ ions on the energy transfer. When EDTA was added to the solution, the energy transfer between C120 and FL returned to the absence of Fe3+ ions (Fig. 10a). As seen from
Fig. 10b and c, the photographs of the C120–FL showed that the system was returned to its original state in the presence of EDTA. This indicates that C120–FL fluorescence sensor system detecting Fe3+ ion is reversible. The selectivity of the C120-FL systems was investigated for Fe3+ in the presence of other metal ions. As shown in Fig. 11, the presence of other metal ions did not affect the change in Fe3+ in regard to the energy transfer between C120 and FL. Fig. 12 showed the changes in the intensity of the emission peak at 432 nm. The peak intensity of the solution containing Fe3+ ions was not significantly affected by the presence of other metal ions. These results indicated that the selectivity of the C120–FL systems for Fe3+ ions cannot be subjected to interference by other metal ions. Consequently, it was concluded that this system is highly suitable for the detection of Fe3+ ions [34]. The interaction between the sensor system and Fe3+ ions was observed with the UV–vis. absorbance and fluorescence measurements. For this purpose, the absorbance and fluorescence
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Fig. 14. FTIR spectra of (a) C120 and Fe3+ (b) FL and Fe3+ .
measurements were recorded for C120–Fe3+ and FL–Fe3+ . As shown in Fig. 13, while the fluorescence intensity of C120 quenched, the fluorescence property of FL was disappeared in the presence of Fe3+ ions. According to the absorbance measurements, when Fe3+ ions were added to C120 and FL solutions, the absorption intensity of C120 gradually increased and shifted to blue region with increasing Fe3+ concentration. Moreover, the absorption peak of the acceptor showed bathochromic shift with increasing Fe3+ concentration (Fig. S3). Herein, the color change and weak absorbance of FL was shown due to a weak complex between FL and Fe3+ [35]. Moreover, the mechanism of interaction between the C120–FL and Fe3+ was determined through FTIR study. The FTIR spectra of C120, FL and Fe3+ were given in Fig. 14. The FTIR spectrum of C120
exhibited band at 1606 cm−1 (stretching vibration C O), and the FTIR spectrum of FL exhibited bands at 1266 cm−1 (C O stretching vibration of COOH groups) and 1620 cm−1 (stretching vibration C O). When Fe3+ ions were added to the solution, the characteristic stretching vibration C O peak of C120 and the characteristic stretching vibration C O peaks of FL disappeared, while the intensity and C O stretching vibration of COOH groups shifted to higher wavenumbers (Fig. 14a and b) [32]. This indicated that Fe3+ ions electrostatically interact with both molecules. As a result of this interaction, while the fluorescence emission of C120 was quenched, the fluorescence property of FL was lost. However, when EDTA was added to the solution containing Fe3+ ions, the stretching vibration peaks of the molecule returned to the absence of Fe3+
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Table 1 Determination of Fe3+ contents in tap water samples (n = 3). Samples
Fe3+ added (M)
Fe3+ found (M)
Recovery (%)
RSD (%)
1 2 3
6.0 10.0 14.0
5.89 9.90 14.37
98.2 99.0 102.6
6.8 7.1 7.5
ions (Fig. S4). According to the spectroscopic data, the proposed mechanism of interaction between the C120–FL and Fe3+ ions was representatively shown in Scheme 1. As shown in the proposed mechanism, Fe3+ ions have interacted with oxygen atom on the coumarin ring and oxygen atoms on the carboxyl and carbonyl groups of fluorescein [36]. 3.3. Real sample analysis To show the practical application of the new sensor system, the trap water samples were used. The stock solution of Fe3+ ions at various concentrations was added to the trap water samples. The obtained results were given in Table 1. It was found that the detected concentration of Fe3+ ions was close to that of the added Fe3+ ions. The relative standard deviation (RSD) of three measurements was less than 10%. Recovery was between 70.5% and 85.9%. Consequently, these results indicated that the C120–FL sensor system is quite satisfactory analytical applications [24,37]. 4. Conclusion In the present study, we developed a new system based on FRET in order to detect Fe3+ ions in aqueous solution. C120 and FL were chosen as the donor and the acceptor dye molecules, respectively. Different metal ions (Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were added to the C120–FL FRET system. It was determined that Fe3+ ions were the only ion that prevented the energy transfer between C120 and FL. The mechanism of interaction between the donor and the acceptor molecules and Fe3+ ions was determined through spectroscopic techniques. The interaction stoichiometry of the C120–FL FRET system with Fe3+ ions was found to be 1:1. The C120–FL system has good potential for sensor applications in analytical system, given the high selectivity for Fe3+ . Moreover, C120–FL sensor system, which has multiple selectivity (both colorimetric and fluorometric), is more simple, non-toxic and economical system compared to the other iron sensor systems. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.06.097 References [1] L. Wang, G. Fang, D. Cao, A novel phenol-based BODIPY chemosensor for selective detection Fe3+ with colorimetric and fluorometric dual-mode, Sens. Actuators, B: Chem. 207 (2015) 849–857. [2] X. Bao, X. Cao, X. Nie, Y. Xu, W. Guo, B. Zhou, L. Zhang, H. Liao, T. Pang, A new selective fluorescent chemical sensor for Fe3+ based on rhodamine B and a 1,4,7,10-tetraoxa-13-azacyclopentadecane conjugate and its imaging in living cells, Sens. Actuators, B: Chem. 208 (2015) 54–66. [3] Z. Chen, D. Lu, G. Zhang, J. Yang, C. Dong, S. Shuang, Glutathione capped silver nanoclusters-based fluorescent probe for highly sensitive detection of Fe3+ , Sens. Actuators, B: Chem. 202 (2014) 631–637. [4] H.-L. Lee, N. Dhenadhayalan, K.-C. Lin, Metal ion induced fluorescence resonance energy transfer between crown ether functionalized quantum dots and rhodamine B: selectivity of K+ ion, RSC Adv. 5 (2015) 4926–4933. [5] J. Song, F.-Y. Wu, Y.-Q. Wan, L.-H. Ma, Ultrasensitive turn-on fluorescent detection of trace thiocyanate based on fluorescence resonance energy transfer, Talanta 132 (2015) 619–624.
[6] P. Xie, F. Guo, R. Xia, Y. Wang, D. Yao, G. Yang, L. Xie, A rhodamine-dansyl conjugate as a FRET based sensor for Fe3+ in the red spectral region, J. Lumin. 145 (2014) 849–854. [7] N. Wanichacheva, O. Hanmeng, S. Kraithong, K. Sukrat, Dual optical Hg2+ -selective sensing through FRET system of fluorescein and rhodamine B fluorophores, J. Photochem. Photobiol., A: Chem. 278 (2014) 75–81. [8] S. Chatterjee, S. Nandi, S.C. Bhattacharya, Fluorescence resonance energy transfer from Fluorescein to Safranine T in solutions and in micellar medium, J. Photochem. Photobiol., A: Chem. 173 (2005) 221–227. [9] D. Seth, D. Chakrabarty, A. Chakraborty, N. Sarkar, Study of energy transfer from 7-amino coumarin donors to rhodamine 6G acceptor in non-aqueous reverse micelles, Chem. Phys. Lett. 401 (2005) 546–552. [10] S. Nad, H. Pal, Unusual photophysical properties of coumarin-151, J. Phys. Chem. A 105 (2001) 1097–1106. [11] A. Satpati, S. Senthilkumar, M. Kumbhakar, S. Nath, D.K. Maity, H. Pal, Investigations of the solvent polarity effect on the photophysical properties of coumarin-7 dye, Photochem. Photobiol. 81 (2005) 270–278. [12] H. Pal, S. Nad, M. Kumbhakar, Photophysical properties of coumarin-120: unusual behavior in nonpolar solvents, J. Chem. Phys. 119 (2003) 443–452. [13] Y. Nagasawa, A.P. Yartsev, K. Tominaga, A.E. Johnson, K. Yoshihara, Substituent effects on intermolecular electron-transfer—coumarins in electron-donating solvents, J. Am. Chem. Soc. 115 (1993) 7922–7923. [14] C.A.M. Seidel, A. Schulz, M.H.M. Sauer, Nucleobase-specific quenching of fluorescent dyes. 1. Nucleobase one-electron redox potentials and their correlation with static and dynamic quenching efficiencies, J. Phys. Chem. 100 (1996) 5541–5553. [15] R.F. Murphy, S. Powers, C.R. Cantor, Endosome Ph measured in single cells by dual fluorescence flow-cytometry—rapid acidification of insulin to Ph-6, J. Cell Biol. 98 (1984) 1757–1762. [16] N. Klonis, A.H.A. Clayton, E.W. Voss, W.H. Sawyer, Spectral properties of fluorescein in solvent–water mixtures: applications as a probe of hydrogen bonding environments in biological systems, Photochem. Photobiol. 67 (1998) 500–510. [17] H. Yao, R.A. Jockusch, Fluorescence and electronic action spectroscopy of mass-selected gas-phase fluorescein. 2 ,7 -Dichlorofluorescein, and 2 ,7 -difluorofluorescein ions, J. Phys. Chem. A 117 (2013) 1351–1359. [18] R. Sjöback, J. Nygren, M. Kubista, Absorption and fluorescence properties of fluorescein, Spectrochim. Acta, A: Mol. Biomol. Spectrosc. 51 (1995) L7–L21. [19] B. Aydin, M. Acar, M. Arik, Y. Onganer, The fluorescence resonance energy transfer between dye compounds in micellar media, Dyes Pigm. 81 (2009) 156–160. [20] E. Bozkurt, M. Acar, K. Meral, M. Arik, Y. Onganer, Photoinduced interactions between coumarin 151 and colloidal CdS nanoparticles in aqueous suspension, J. Photochem. Photobiol., A: Chem. 236 (2012) 41–47. [21] M. Toprak, B.M. Aydin, M. Arik, Y. Onganer, Fluorescence quenching of fluorescein by merocyanine 540 in liposomes, J. Lumin. 131 (2011) 2286–2289. [22] K. Rurack, M. Spieles, Fluorescence quantum yields of a series of red and near-infrared dyes emitting at 600–1000 nm, Anal. Chem. 83 (2011) 1232–1242. [23] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science+Business Media, LLC, USA, 2006. [24] Z. Zhang, S. Lu, C. Sha, D. Xu, A single thiourea-appended 1,8-naphthalimide chemosensor for three heavy metal ions: Fe3+ , Pb2+ , and Hg2+ , Sens. Actuators, B: Chem. 208 (2015) 258–266. [25] J. Mao, Q. He, W. Liu, An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution, Talanta 80 (2010) 2093–2098. [26] S. Hu, Q. Zhao, Q. Chang, J. Yang, J. Liu, Enhanced performance of Fe3+ detection via fluorescence resonance energy transfer between carbon quantum dots and rhodamine B, RSC Adv. 4 (2014) 41069–41075. [27] R. Cegielski, M. Niedbalska, H. Manikowski, Interactions of stilbazolium merocyanine with transition metal ions, Dyes Pigm. 50 (2001) 35–39. [28] M.H. Lee, H.J. Kim, S. Yoon, N. Park, J.S. Kim, Metal ion induced FRET OFF–ON in tren/dansyl-appended rhodamine, Org. Lett. 10 (2008) 213–216. [29] S. Goswami, D. Sen, N.K. Das, A new highly selective, ratiometric and colorimetric fluorescence sensor for Cu2+ with a remarkable red shift in absorption and emission spectra based on internal charge transfer, Org. Lett. 12 (2010) 856–859. [30] T. Raj, P. Saluja, N. Singh, A new class of pyrene based multifunctional chemosensors for differential sensing of metals in different media: selective recognition of Zn2+ in organic and Fe3+ in aqueous medium, Sens. Actuators, B: Chem. 206 (2015) 98–106. ˜ [31] O. García-Beltrán, B. Cassels, C. Pérez, N. Mena, M. Núnez, N. Martínez, P. Pavez, M. Aliaga, Coumarin-based fluorescent probes for dual recognition of copper(II) and iron(III) ions and their application in bio-imaging, Sensors 14 (2014) 1358. [32] Y. Zhang, G. Wang, J. Zhang, Study on a highly selective fluorescent chemosensor for Fe3+ based on 1,3,4-oxadiazole and phosphonic acid, Sens. Actuators, B: Chem. 200 (2014) 259–268. [33] J. Labuta, J.P. Hill, S. Ishihara, L. Hanykova, K. Ariga, Chiral sensing by nonchiral tetrapyrroles, Acc. Chem. Res. 48 (3) (2015) 521–529. [34] U. Fegade, J. Marek, R. Patil, S. Attarde, A. Kuwar, A selective fluorescent receptor for the determination of nickel(II) in semi-aqueous media, J. Lumin. 146 (2014) 234–238.
E. Bozkurt et al. / Sensors and Actuators B 221 (2015) 136–147 [35] L. Liu, A. Wang, G. Wang, J. Li, Y. Zhou, A naphthopyran-rhodamine based fluorescent and colorimetric chemosensor for recognition of common trivalent metal ions and Cu2+ ions, Sens. Actuators, B: Chem. 215 (2015) 388–395. [36] C. Wang, Y. Liu, J. Cheng, J. Song, Y. Zhao, Y. Ye, Efficient FRET-based fluorescent ratiometric chemosensors for Fe3+ and its application in living cells, J. Lumin. 157 (2015) 143–148. [37] Z. Li, Y. Wang, Y. Ni, S. Kokot, A rapid and label-free dual detection of Hg(II) and cysteine with the use of fluorescence switching of graphene quantum dots, Sens. Actuators, B: Chem. 207 (Part A) (2015) 490–497.
Biographies Ebru Bozkurt received her Bachelor’s degree in chemistry in 2004, master degree in physical chemistry in 2007 and her Ph.D. degree in Physical Chemistry from Atatürk University, Turkey, in 2013. She is interested in photophysics of dye molecules, fluorescence resonance energy transfers, surfactants, reverse micelles,
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colloidal semiconductors, nanoparticles, sensors. Now, she is an Assistant Professor at Program of Occupational Health and Safety, Erzurum Vocational Training School, Atatürk University, Turkey. Mustafa Arık received his Bachelor’s degree in chemistry in 1996, Master’s degree in physical chemistry in 1999 and his Ph.D. degree in Physical Chemistry from Atatürk University, Turkey, in 2003. He is interested in dye molecules, fluorescence resonance energy transfer, colloidal semiconductors, photophysics and photodynamic of molecules. Now, he is an Associate Professor at Department of Chemistry, Faculty of Sciences, Atatürk University, Turkey. Yavuz Onganer received his Bachelor’s degree in chemistry education in 1985, Master’s degree in physical chemistry in 1987 from Atatürk University, Turkey, and his Ph.D. degree in Physical Chemistry from Texas Tech University, USA, in 1993. He is interested in molecular aggregates, fluorescence resonance energy transfer, Langmuir–Blodgett thin films, colloidal semiconductors, photophysical and photodynamic of molecules. Now, he is a Professor at the Department of Chemistry, Faculty of Sciences, Atatürk University, Turkey.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A novel system for Fe3+ ion detection based on fluorescence resonance energy transfer Ebru Bozkurt a,∗ , Mustafa Arık b , Yavuz Onganer b,∗∗ a b
Program of Occupational Health and Safety, Erzurum Vocational Training School, Atatürk University, 25240 Erzurum, Turkey Department of Chemistry, Faculty of Sciences, Atatürk University, 25240 Erzurum, Turkey
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online 27 June 2015 Keywords: Coumarin 120 Fluorescein Fluorescence resonance energy transfer (FRET) Detection of Fe3+ ions
a b s t r a c t A new sensor system was developed for metal-ion sensing based on fluorescence resonance energy transfer (FRET). FRET process was carried out between coumarin (C120) as the energy donor and fluorescein (FL) as the energy acceptor. The concentration most suitable for FRET efficiency was determined to be 0.2 M for C120 and 20 M for FL. The effects of various metal ions such as Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ on the energy transfer between the C120 and FL were explored. It was observed that the presence of Fe3+ ions inhibited the energy transfer between the two molecules while the other metal ions did not affect. This observation shows that the C120–FL system could be selectively detected for Fe3+ ions. The detection limit and binding constant of Fe3+ were determined as 2.54 M and 4.33 × 104 M−1 , respectively. The C120–FL system has a good potential for a sensor application in analytical systems due to its high selectivity for Fe3+ . © 2015 Elsevier B.V. All rights reserved.
1. Introduction Fe3+ ion plays an important role in many biological processes, given that it is a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain and facilitates the oxygen transport capacity of haemoglobin [1]. Iron deficiency can lead to anemia in the body. However, an excess of Fe3+ ions causes some types of cancers and deterioration of functions of organs such as heart, pancreas and lungs [2]. Thus, a simple, easy and fast determination of Fe3+ ions in a liquid media has been accorded signification attention. There are several methods for the detection of Fe3+ , such as atomic absorption spectroscopy, colorimetry, mass spectrometry and electrochemical and fluorescence spectroscopic analysis. The fluorescence techniques are generally used due to their ease of application, high sensitivity and efficiency [3]. There are many photo-physical processes such as photo-induced electron or charge transfer, fluorescence resonance energy transfer (FRET) and excimer formation, which are
Abbreviations: C120, Coumarin 120; FL, Fluorescein; FRET, Fluorescence resonance energy transfer; MeOH, Methanol; EDTA„ Ethylene diamine tetra acetic acid; UV–vis, ultraviolet–visible. ∗ Corresponding author. Tel.: +90 442 231 2667; fax: +90 442 231 2503. ∗∗ Corresponding author. Tel.: +90 442 231 4446; fax: +90 442 236 0948. E-mail addresses: [email protected] (E. Bozkurt), [email protected] (Y. Onganer). http://dx.doi.org/10.1016/j.snb.2015.06.097 0925-4005/© 2015 Elsevier B.V. All rights reserved.
responsible for the photo-physical changes resulting from the binding of metal ions with fluorophores. FRET, with its promising selectivity and sensitivity, is the most remarkable of these processes [4–7]. FRET is a physicochemical process based on the intermolecular energy transfer from a molecule (donor) to another molecule (acceptor) as non-radiative [8]. Professor Theodor Förster has proposed an excellent theory that determine the mechanism of FRET [9]. According to the theory, the rate of energy transfer depends on the extent of spectral overlap between the donor emission and the acceptor absorption spectra, the quantum yield of the donor, the relative orientation of the donor and acceptor transition dipoles, and the distance between the transition dipoles of the donor and acceptor [8]. Coumarin 120 (C120) and fluorescein (FL) are chosen as the respective donor and acceptor dye molecules for FRET application. Coumarin 120 is a compound of 7-amino coumarin derivative. When various functional groups are attached to the 7-position of the coumarin ring, they exhibit intense fluorescence in the blue–green region along with very high quantum yield [10]. So, they are used in various fields such as for the determination of polarity in microenvironments [11,12], the investigation of photo-induced electron transfer dynamics [13], and the nucleobase-specific quenching of fluorescent dyes [14]. Fluorescein is a dye xanthene derivative and its derivatives have been widely applied in many fields including biology and medicine [15–17]. There are fluoresceins of cationic, anionic, neutral and
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Fig. 1. Molecular structure of (a) coumarin 120 and (b) fluorescein.
dianionic forms in aqueous solutions [18]. Its dianionic form is widely used because it has a large extinction coefficient and high fluorescence quantum yield [19]. In this study, we investigated the effect of metal ions on FRET between C120 (donor) and FL (acceptor) in aqueous solution by using both colorimetric and fluorometric methods. The metal ions at various loadings were added to this system, and its energy transfer parameters were calculated. The results revealed that the energy transfer between C120 and FL was prevented by Fe3+ ions and this FRET system was a new iron ion sensor. In this regard, the energy transfer was analyzed in detail, and the binding rate of Fe3+ was determined by using Benesi–Hildebrand analysis and Job’s method. Also, the selectivity and sensitivity of the sensor system based on FRET was proved with real sample tests. The novel system, which could detect ferric ions in low detection limits, was developed.
2. Experimental 2.1. Materials 3 ,6 -Dihydroxyspiro[2-benzofuran-3,9 -xanthene]-1-one sodium salt (fluorescein, FL), 7-amino 4-methyl coumarin
(coumarin 120, C120) and methanol (MeOH) were obtained from Sigma. Stock solutions of C120 and FL of 1.0 × 10−3 M were prepared in MeOH. A certain amount of fresh probe samples in aqueous solution was obtained from this stock solution by evaporating the solvent. For all measurements, the concentrations of C120 and FL were 0.2 M and 20 M, respectively. The molecular structures of C120 and FL were given in Fig. 1. All the experiments were performed at room temperature. NaNO3 , LiCl, AgNO3 , ZnCl2 ·H2 O, Pb(CH3 COO)2 , MnCl2 , HgCl2 , CuCl2 , CdI2 , FeCl3 ·6H2 O, CrCl3 and AlCl3 (as the metal ion sources) and ethylene diamine tetra acetic acid (EDTA) were obtained from Sigma. 2.2. Apparatus The UV–vis absorption and fluorescence spectra of the samples were recorded with Perkin Elmer Lambda 35 UV/VIS spectrophotometer and Shimadzu RF-5301PC spectrofluorophotometer, respectively. For the steady-state fluorescence measurements, sample solutions were excited at 350 nm and their fluorescence intensities were recorded. Fluorescence lifetime measurements were carried out with a LaserStrobe model TM3 spectrofluorophotometer from Photon Technology International (PTI). The excitation source combined a pulsed nitrogen laser/tunable dye
Fig. 2. Fluorescence spectra of C120, FL and C120–FL in pure water.
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Fig. 3. (a) Fluorescence spectra of C120–FL in the absence and presence of 20 M metal ions in water (exc = 350 nm). (b) Photographs of C120–FL in the presence of metal ions under daylight (c) under UV.
laser. The decay curves of C120 dye (ex = 366 nm) were collected over 200 channels using a nonlinear time scale in which the time increment increased according to arithmetic progression. The fluorescence decays were analyzed with lifetime-distribution analysis software from the instrument supplying company. The goodness of fits was assessed by the value of 2 and weighed residuals
[20]. The Fluorescence quantum yields of donor molecules were calculated through the Parker-Rees equation:
∅s = ∅r
D n2 1 − 10−ODr s s Dr
n2r
1 − 10−ODs
Fig. 4. Absorption spectra of C120–FL in the absence and presence of 20 M metal ions in water.
(1)
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Fig. 5. Fluorescence spectra of C120–FL with the increasing concentration of Fe3+ (exc = 350 nm).
where D is the integrated area under the corrected fluorescence spectrum, n is the refractive index of the solution, and OD is the optical density at the excitation wavelength (ex = 350 nm). The subscripts s and r refer to the sample and reference solutions, respectively [21]. Quinine sulfate in 0.5 M H2 SO4 solution was used as the reference. The fluorescence quantum yield of quinine sulfate was 0.54 in 0.5 M H2 SO4 solution [22].
where D is the lifetime of the donor in the absence of an acceptor, r is the distance between donor and acceptor and R0 is the Förster distance. R0 is a distance by which energy transfer efficiency is 50%. This distance can be calculated through the following equation:
R0 = 0.211 2 −4 QD J ()
1/6
(3)
2.3. Calculation of FRET parameters According to Dr. Förster’s theory, the rate of FRET (kET ) is given as follows: kET =
1 D
R 6 0
r
(2)
where QD is the quantum yield of the donor in the absence of an acceptor, is the refractive index of the medium, 2 is the orientation factor. This factor describes the relative orientation in the space of transition dipoles of the donor and acceptor, and is usually assumed to be equal to 2/3, whereby J () is the spectral overlapping integral. This integral represents the degree of spectral overlap
Fig. 6. Absorption spectra of C120–FL with the increasing concentration of Fe3+ .
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Fig. 7. Change fluorescence intensity of C120 with the increasing concentration of Fe3+ .
between the emission spectrum of the donor and the absorption spectrum of acceptor, and it is calculated as follows:
J () =
0 ∞
donor, in the absence (FD ) and presence (FDA ) of the acceptor based on the following equation [23]: E =1−
FD ()εA ()4 d
0 ∞
FD ()d
FDA FD
(5)
(4) 2.4. The sensing of metal ion
where FD () is the normalized fluorescence intensity of the donor in the absence of an acceptor and εA () is the extinction coefficient of the acceptor. Energy transfer efficiency, as one of the important parameters, was calculated using the relative fluorescence intensity of the
The 1.0 × 10−2 M stock solutions of all metal ions (Li+ , Na+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were prepared in pure water. Then, 10 l of metal ion was added to 5 ml solution containing 0.2 M C120 and 20 M FL at room temperature. The spectroscopic changes in energy transfer between C120 and FL were recorded with the absorption and fluorescence Ag+ ,
Fig. 8. Job’s plot of C120–FL with Fe3+ in water.
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Fig. 9. Benesi–Hildebrand plot based on a 1:1 association stoichiometry between C120–FL and Fe3+ .
measurements. Fe3+ solution of different concentrations (1, 2, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 140, 160, 180, 200, 300 and 400 M) was added to the donor-acceptor solution. The absorption and fluorescence measurements were taken for each
solution containing Fe3+ ions. The detection limit for Fe3+ was determined with the fluorescence data. For this purpose, the 3s/k equation was used. Where s is the standard deviation of blank, k is the slope of the fit line in the fluorescence titration experiment.
Fig. 10. (a) Fluorescence spectra of C120–FL in the presence of 20 M Fe3+ ion and EDTA in water (exc = 350 nm). (b) Photographs of C120–FL in the presence of 20 M Fe3+ ion and EDTA under daylight (c) under UV.
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Fig. 11. (a) Absorption spectra (b) fluorescence spectra of C120–FL containing other metal ions in the absence and presence of 20 M Fe3+ in water (exc = 350 nm).
Moreover, the following Benesi–Hildebrand equation was used to calculate the binding constant:
were added. The mixture was diluted to 5 ml with distilled water and the fluorescence spectra were recorded [24,25].
1 1 1 = + F − F0 Fmax − F0 KS (Fmax − F0 ) [M + ]n
3. Result and discussion
(6)
where F0 , F, and Fmax are the fluorescent intensity of the donor in the absence of a metal ion, at a certain concentration of metal ion and at a complete interaction concentration of metal ion, respectively. [M+ ] is the concentration of Fe3+ , and n is the binding stoichiometry for dye and metal ion [24].
2.5. Testing real sample Tap water was used for real sample tests. After filtration, 1.5 ml of tap water was put into 5 ml volumetric flask. Then, 1 ml stock solution of the C120–FL and 3 (5 and 7) l stock solutions of Fe3+
3.1. The effect of metal ions on energy transfer In the study, C120 and FL were used as the energy donor and acceptor, respectively. The spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor was determined (Fig. S1). Then, the fluorescence spectra of C120, FL and the C120–FL were recorded at 350 nm. Fig. 2 showed that the fluorescence spectrum of the C120–FL exhibited two peaks at 432 and 506 nm. While the fluorescence peak at 432 nm was weaker than that of pure C120, the fluorescence peak at 506 nm was stronger than that of pure FL at the same excitation wavelength. This observation confirmed that the energy transfer between the
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Fig. 12. Metal ion selectivity profiles of C120–FL in the presence of various metal ions in water. Red bars represent the fluorescence intensity of C120–FL in the presence of 20 M of metal ion. Blue bars represent the fluorescence intensity in the presence of various metal ions after the addition of Fe3+ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
two molecules was occurred [4]. After the presence of the energy transfer between the two molecules was determined, the effect of metal ions on the energy transfer was examined by using the fluorescence titration method [26]. Different metal ions (Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were added to the C120–FL system, and their fluorescence spectra were recorded. As shown in Fig. 3a, Fe3+ ions prevented the energy transfer between the two molecules while the other metal ions did not present a significant change in energy transfer. Fig. 3b and c showed photographs of the C120–FL in the presence of different metal ions under daylight and UV, respectively. It was observed that the color of the solution was changed in the presence of Fe3+ ions, but the other metal ions did not induced any significant change in the color of the solution. The energy transfer parameters in the presence of all
metal ions excluding Fe3+ were calculated by using Eqs. (2)–(5). It was observed that the metal ions increased the efficiency of energy transfer (Table S1). As seen from Fig. 3b and c, colorimetric detection of Fe3+ could be also made with the C120–FL system. The UV–vis absorbance response of the C120–FL FRET system against various metal ions at different concentration was plotted (Fig. 4). As seen from Fig. 4, while the absorbance spectrum of the C120–FL system was not changed in the presence of other metal ions, significant changes occurred in the absorption spectra in the presence of Fe3+ [1]. The maximum absorption intensity of the donor increased and its maximum absorption wavelength was blue-shifted. Moreover, the intensity of the absorption peak of the acceptor decreased and shifted to red region. It was determined that the C120–FL system showed also colorimetric selectivity with the changes in the absorption spectrum when Fe3+ ions was existed [27]. 3.2. Detection of Fe3+ with the energy transfer
Scheme 1. The proposed mechanism of interaction between C120–FL and Fe3+ .
Since the effect of iron ions on the C120–FL was observed, Fe3+ ions at different concentrations (0–400 M) were added to the C120–FL for the detection of Fe3+ ions. As shown in Fig. 5, while the fluorescence peaks of FL at 506 nm disappeared, the fluorescence intensity of C120 at 432 nm decreased with the increase of Fe3+ concentration. Accordingly, when Fe3+ ions were added to the solution, it was observed that the fluorescence emission of C120 was substantially quenched and the fluorescence property of FL disappeared. The emission quenching rates of C120, FL and the C120–FL were compared under the same Fe3+ ions (Fig. S2). It was observed that the emission quenching rates of the C120–FL at 432 nm and 506 nm were higher than those of C120 and FL. This result indicates that the C120–FL is a system that can detect the highly efficient Fe3+ ions [26]. Moreover, the UV–vis. absorption response of the C120–FL to Fe3+ ions at different concentrations was measured (Fig. 6). While the absorption intensity of donor gradually increased, the absorption peak of acceptor showed bathochromic shift with increasing Fe3+ ion concentration [28,29]. According to the fluorescence measurement results, a plot of Fe3+ ion concentration versus the emission intensity of C120 showed a linear relationship (Fig. 7). The detection limit of Fe3+
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Fig. 13. (a) Fluorescence spectra of C120 (b) fluorescence spectra of FL with the increasing concentration of Fe3+ .
was determined to be 2.54 M using the 3s/k equation. Compared to recent studies regarding this value, it has been reached a very low detection limit of Fe3+ ion [24,30,31]. Moreover, according to the U.S. Environmental Protection Agency (EPA), the maximum Fe3+ levels in drinking water must be ∼5.357 M [32]. In this study, it was found that the limit of detection for Fe3+ is lower than the value permitted by the EPA. The stoichiometry of interaction between the C120–FL and Fe3+ was determined by Job’s plot analysis based on the change in the intensity of fluorescence [33]. The results indicated the interaction of a 1:1 between the C120–FL and Fe3+ (Fig. 8). According to the fluorescence titration and Job’s plot experiments, the binding constant of Fe3+ was calculated as 4.33 × 104 M−1 through the Benesi–Hildebrand equation (Fig. 9). EDTA was added to the C120–FL and Fe3+ solution in order to confirm the effect of Fe3+ ions on the energy transfer. When EDTA was added to the solution, the energy transfer between C120 and FL returned to the absence of Fe3+ ions (Fig. 10a). As seen from
Fig. 10b and c, the photographs of the C120–FL showed that the system was returned to its original state in the presence of EDTA. This indicates that C120–FL fluorescence sensor system detecting Fe3+ ion is reversible. The selectivity of the C120-FL systems was investigated for Fe3+ in the presence of other metal ions. As shown in Fig. 11, the presence of other metal ions did not affect the change in Fe3+ in regard to the energy transfer between C120 and FL. Fig. 12 showed the changes in the intensity of the emission peak at 432 nm. The peak intensity of the solution containing Fe3+ ions was not significantly affected by the presence of other metal ions. These results indicated that the selectivity of the C120–FL systems for Fe3+ ions cannot be subjected to interference by other metal ions. Consequently, it was concluded that this system is highly suitable for the detection of Fe3+ ions [34]. The interaction between the sensor system and Fe3+ ions was observed with the UV–vis. absorbance and fluorescence measurements. For this purpose, the absorbance and fluorescence
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Fig. 14. FTIR spectra of (a) C120 and Fe3+ (b) FL and Fe3+ .
measurements were recorded for C120–Fe3+ and FL–Fe3+ . As shown in Fig. 13, while the fluorescence intensity of C120 quenched, the fluorescence property of FL was disappeared in the presence of Fe3+ ions. According to the absorbance measurements, when Fe3+ ions were added to C120 and FL solutions, the absorption intensity of C120 gradually increased and shifted to blue region with increasing Fe3+ concentration. Moreover, the absorption peak of the acceptor showed bathochromic shift with increasing Fe3+ concentration (Fig. S3). Herein, the color change and weak absorbance of FL was shown due to a weak complex between FL and Fe3+ [35]. Moreover, the mechanism of interaction between the C120–FL and Fe3+ was determined through FTIR study. The FTIR spectra of C120, FL and Fe3+ were given in Fig. 14. The FTIR spectrum of C120
exhibited band at 1606 cm−1 (stretching vibration C O), and the FTIR spectrum of FL exhibited bands at 1266 cm−1 (C O stretching vibration of COOH groups) and 1620 cm−1 (stretching vibration C O). When Fe3+ ions were added to the solution, the characteristic stretching vibration C O peak of C120 and the characteristic stretching vibration C O peaks of FL disappeared, while the intensity and C O stretching vibration of COOH groups shifted to higher wavenumbers (Fig. 14a and b) [32]. This indicated that Fe3+ ions electrostatically interact with both molecules. As a result of this interaction, while the fluorescence emission of C120 was quenched, the fluorescence property of FL was lost. However, when EDTA was added to the solution containing Fe3+ ions, the stretching vibration peaks of the molecule returned to the absence of Fe3+
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Table 1 Determination of Fe3+ contents in tap water samples (n = 3). Samples
Fe3+ added (M)
Fe3+ found (M)
Recovery (%)
RSD (%)
1 2 3
6.0 10.0 14.0
5.89 9.90 14.37
98.2 99.0 102.6
6.8 7.1 7.5
ions (Fig. S4). According to the spectroscopic data, the proposed mechanism of interaction between the C120–FL and Fe3+ ions was representatively shown in Scheme 1. As shown in the proposed mechanism, Fe3+ ions have interacted with oxygen atom on the coumarin ring and oxygen atoms on the carboxyl and carbonyl groups of fluorescein [36]. 3.3. Real sample analysis To show the practical application of the new sensor system, the trap water samples were used. The stock solution of Fe3+ ions at various concentrations was added to the trap water samples. The obtained results were given in Table 1. It was found that the detected concentration of Fe3+ ions was close to that of the added Fe3+ ions. The relative standard deviation (RSD) of three measurements was less than 10%. Recovery was between 70.5% and 85.9%. Consequently, these results indicated that the C120–FL sensor system is quite satisfactory analytical applications [24,37]. 4. Conclusion In the present study, we developed a new system based on FRET in order to detect Fe3+ ions in aqueous solution. C120 and FL were chosen as the donor and the acceptor dye molecules, respectively. Different metal ions (Li+ , Na+ , Ag+ , Zn2+ , Pb2+ , Mn2+ , Hg2+ , Cu2+ , Cd2+ , Fe3+ , Cr3+ and Al3+ ) were added to the C120–FL FRET system. It was determined that Fe3+ ions were the only ion that prevented the energy transfer between C120 and FL. The mechanism of interaction between the donor and the acceptor molecules and Fe3+ ions was determined through spectroscopic techniques. The interaction stoichiometry of the C120–FL FRET system with Fe3+ ions was found to be 1:1. The C120–FL system has good potential for sensor applications in analytical system, given the high selectivity for Fe3+ . Moreover, C120–FL sensor system, which has multiple selectivity (both colorimetric and fluorometric), is more simple, non-toxic and economical system compared to the other iron sensor systems. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.06.097 References [1] L. Wang, G. Fang, D. Cao, A novel phenol-based BODIPY chemosensor for selective detection Fe3+ with colorimetric and fluorometric dual-mode, Sens. Actuators, B: Chem. 207 (2015) 849–857. [2] X. Bao, X. Cao, X. Nie, Y. Xu, W. Guo, B. Zhou, L. Zhang, H. Liao, T. Pang, A new selective fluorescent chemical sensor for Fe3+ based on rhodamine B and a 1,4,7,10-tetraoxa-13-azacyclopentadecane conjugate and its imaging in living cells, Sens. Actuators, B: Chem. 208 (2015) 54–66. [3] Z. Chen, D. Lu, G. Zhang, J. Yang, C. Dong, S. Shuang, Glutathione capped silver nanoclusters-based fluorescent probe for highly sensitive detection of Fe3+ , Sens. Actuators, B: Chem. 202 (2014) 631–637. [4] H.-L. Lee, N. Dhenadhayalan, K.-C. Lin, Metal ion induced fluorescence resonance energy transfer between crown ether functionalized quantum dots and rhodamine B: selectivity of K+ ion, RSC Adv. 5 (2015) 4926–4933. [5] J. Song, F.-Y. Wu, Y.-Q. Wan, L.-H. Ma, Ultrasensitive turn-on fluorescent detection of trace thiocyanate based on fluorescence resonance energy transfer, Talanta 132 (2015) 619–624.
[6] P. Xie, F. Guo, R. Xia, Y. Wang, D. Yao, G. Yang, L. Xie, A rhodamine-dansyl conjugate as a FRET based sensor for Fe3+ in the red spectral region, J. Lumin. 145 (2014) 849–854. [7] N. Wanichacheva, O. Hanmeng, S. Kraithong, K. Sukrat, Dual optical Hg2+ -selective sensing through FRET system of fluorescein and rhodamine B fluorophores, J. Photochem. Photobiol., A: Chem. 278 (2014) 75–81. [8] S. Chatterjee, S. Nandi, S.C. Bhattacharya, Fluorescence resonance energy transfer from Fluorescein to Safranine T in solutions and in micellar medium, J. Photochem. Photobiol., A: Chem. 173 (2005) 221–227. [9] D. Seth, D. Chakrabarty, A. Chakraborty, N. Sarkar, Study of energy transfer from 7-amino coumarin donors to rhodamine 6G acceptor in non-aqueous reverse micelles, Chem. Phys. Lett. 401 (2005) 546–552. [10] S. Nad, H. Pal, Unusual photophysical properties of coumarin-151, J. Phys. Chem. A 105 (2001) 1097–1106. [11] A. Satpati, S. Senthilkumar, M. Kumbhakar, S. Nath, D.K. Maity, H. Pal, Investigations of the solvent polarity effect on the photophysical properties of coumarin-7 dye, Photochem. Photobiol. 81 (2005) 270–278. [12] H. Pal, S. Nad, M. Kumbhakar, Photophysical properties of coumarin-120: unusual behavior in nonpolar solvents, J. Chem. Phys. 119 (2003) 443–452. [13] Y. Nagasawa, A.P. Yartsev, K. Tominaga, A.E. Johnson, K. Yoshihara, Substituent effects on intermolecular electron-transfer—coumarins in electron-donating solvents, J. Am. Chem. Soc. 115 (1993) 7922–7923. [14] C.A.M. Seidel, A. Schulz, M.H.M. Sauer, Nucleobase-specific quenching of fluorescent dyes. 1. Nucleobase one-electron redox potentials and their correlation with static and dynamic quenching efficiencies, J. Phys. Chem. 100 (1996) 5541–5553. [15] R.F. Murphy, S. Powers, C.R. Cantor, Endosome Ph measured in single cells by dual fluorescence flow-cytometry—rapid acidification of insulin to Ph-6, J. Cell Biol. 98 (1984) 1757–1762. [16] N. Klonis, A.H.A. Clayton, E.W. Voss, W.H. Sawyer, Spectral properties of fluorescein in solvent–water mixtures: applications as a probe of hydrogen bonding environments in biological systems, Photochem. Photobiol. 67 (1998) 500–510. [17] H. Yao, R.A. Jockusch, Fluorescence and electronic action spectroscopy of mass-selected gas-phase fluorescein. 2 ,7 -Dichlorofluorescein, and 2 ,7 -difluorofluorescein ions, J. Phys. Chem. A 117 (2013) 1351–1359. [18] R. Sjöback, J. Nygren, M. Kubista, Absorption and fluorescence properties of fluorescein, Spectrochim. Acta, A: Mol. Biomol. Spectrosc. 51 (1995) L7–L21. [19] B. Aydin, M. Acar, M. Arik, Y. Onganer, The fluorescence resonance energy transfer between dye compounds in micellar media, Dyes Pigm. 81 (2009) 156–160. [20] E. Bozkurt, M. Acar, K. Meral, M. Arik, Y. Onganer, Photoinduced interactions between coumarin 151 and colloidal CdS nanoparticles in aqueous suspension, J. Photochem. Photobiol., A: Chem. 236 (2012) 41–47. [21] M. Toprak, B.M. Aydin, M. Arik, Y. Onganer, Fluorescence quenching of fluorescein by merocyanine 540 in liposomes, J. Lumin. 131 (2011) 2286–2289. [22] K. Rurack, M. Spieles, Fluorescence quantum yields of a series of red and near-infrared dyes emitting at 600–1000 nm, Anal. Chem. 83 (2011) 1232–1242. [23] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science+Business Media, LLC, USA, 2006. [24] Z. Zhang, S. Lu, C. Sha, D. Xu, A single thiourea-appended 1,8-naphthalimide chemosensor for three heavy metal ions: Fe3+ , Pb2+ , and Hg2+ , Sens. Actuators, B: Chem. 208 (2015) 258–266. [25] J. Mao, Q. He, W. Liu, An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution, Talanta 80 (2010) 2093–2098. [26] S. Hu, Q. Zhao, Q. Chang, J. Yang, J. Liu, Enhanced performance of Fe3+ detection via fluorescence resonance energy transfer between carbon quantum dots and rhodamine B, RSC Adv. 4 (2014) 41069–41075. [27] R. Cegielski, M. Niedbalska, H. Manikowski, Interactions of stilbazolium merocyanine with transition metal ions, Dyes Pigm. 50 (2001) 35–39. [28] M.H. Lee, H.J. Kim, S. Yoon, N. Park, J.S. Kim, Metal ion induced FRET OFF–ON in tren/dansyl-appended rhodamine, Org. Lett. 10 (2008) 213–216. [29] S. Goswami, D. Sen, N.K. Das, A new highly selective, ratiometric and colorimetric fluorescence sensor for Cu2+ with a remarkable red shift in absorption and emission spectra based on internal charge transfer, Org. Lett. 12 (2010) 856–859. [30] T. Raj, P. Saluja, N. Singh, A new class of pyrene based multifunctional chemosensors for differential sensing of metals in different media: selective recognition of Zn2+ in organic and Fe3+ in aqueous medium, Sens. Actuators, B: Chem. 206 (2015) 98–106. ˜ [31] O. García-Beltrán, B. Cassels, C. Pérez, N. Mena, M. Núnez, N. Martínez, P. Pavez, M. Aliaga, Coumarin-based fluorescent probes for dual recognition of copper(II) and iron(III) ions and their application in bio-imaging, Sensors 14 (2014) 1358. [32] Y. Zhang, G. Wang, J. Zhang, Study on a highly selective fluorescent chemosensor for Fe3+ based on 1,3,4-oxadiazole and phosphonic acid, Sens. Actuators, B: Chem. 200 (2014) 259–268. [33] J. Labuta, J.P. Hill, S. Ishihara, L. Hanykova, K. Ariga, Chiral sensing by nonchiral tetrapyrroles, Acc. Chem. Res. 48 (3) (2015) 521–529. [34] U. Fegade, J. Marek, R. Patil, S. Attarde, A. Kuwar, A selective fluorescent receptor for the determination of nickel(II) in semi-aqueous media, J. Lumin. 146 (2014) 234–238.
E. Bozkurt et al. / Sensors and Actuators B 221 (2015) 136–147 [35] L. Liu, A. Wang, G. Wang, J. Li, Y. Zhou, A naphthopyran-rhodamine based fluorescent and colorimetric chemosensor for recognition of common trivalent metal ions and Cu2+ ions, Sens. Actuators, B: Chem. 215 (2015) 388–395. [36] C. Wang, Y. Liu, J. Cheng, J. Song, Y. Zhao, Y. Ye, Efficient FRET-based fluorescent ratiometric chemosensors for Fe3+ and its application in living cells, J. Lumin. 157 (2015) 143–148. [37] Z. Li, Y. Wang, Y. Ni, S. Kokot, A rapid and label-free dual detection of Hg(II) and cysteine with the use of fluorescence switching of graphene quantum dots, Sens. Actuators, B: Chem. 207 (Part A) (2015) 490–497.
Biographies Ebru Bozkurt received her Bachelor’s degree in chemistry in 2004, master degree in physical chemistry in 2007 and her Ph.D. degree in Physical Chemistry from Atatürk University, Turkey, in 2013. She is interested in photophysics of dye molecules, fluorescence resonance energy transfers, surfactants, reverse micelles,
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colloidal semiconductors, nanoparticles, sensors. Now, she is an Assistant Professor at Program of Occupational Health and Safety, Erzurum Vocational Training School, Atatürk University, Turkey. Mustafa Arık received his Bachelor’s degree in chemistry in 1996, Master’s degree in physical chemistry in 1999 and his Ph.D. degree in Physical Chemistry from Atatürk University, Turkey, in 2003. He is interested in dye molecules, fluorescence resonance energy transfer, colloidal semiconductors, photophysics and photodynamic of molecules. Now, he is an Associate Professor at Department of Chemistry, Faculty of Sciences, Atatürk University, Turkey. Yavuz Onganer received his Bachelor’s degree in chemistry education in 1985, Master’s degree in physical chemistry in 1987 from Atatürk University, Turkey, and his Ph.D. degree in Physical Chemistry from Texas Tech University, USA, in 1993. He is interested in molecular aggregates, fluorescence resonance energy transfer, Langmuir–Blodgett thin films, colloidal semiconductors, photophysical and photodynamic of molecules. Now, he is a Professor at the Department of Chemistry, Faculty of Sciences, Atatürk University, Turkey.