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An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution
Talanta 80 (2010) 2093–2098
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Talanta journal homepage: www.elsevier.com/locate/talanta
An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution Jie Mao a,b,∗ , Qun He b,∗∗ , Weisheng Liu b a b
Department of Chemistry, Anhui Science and Technology University, Bengbu, Anhui 233100, PR China College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 31 October 2009 Accepted 3 November 2009 Available online 13 November 2009 Keywords: Iron(III) Probe Fluorescence enhancement Rhodamine
a b s t r a c t An “off–on” rhodamine-based fluorescence probe for the selective signaling of Fe(III) has been designed exploiting the guest-induced structure transform mechanism. This system shows a sharp Fe(III)-selective fluorescence enhancement response in 100% aqueous system under physiological pH value and possesses high selectivity against the background of environmentally and biologically relevant metal ions including Al(III), Cd(II), Fe(II), Co(II), Cu(II), Ni(II), Zn(II), Mg(II), Ba(II), Pb(II), Na(I), and K(I). Under optimum conditions, the fluorescence intensity enhancement of this system is linearly proportional to Fe(III) concentration from 6.0 × 10−8 to 7.2 × 10−6 mol L−1 with a detection limit of 1.4 × 10−8 mol L−1 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of fluorescent chemical devices is a promising field [1–3]. An important area within this field is the design of fluorescent probes for various metal ions [4–8]. Ions play a fundamental role in a wide range of chemical and biological processes, and numerous efforts have been made to the development of effective fluorescent probes. Sensors based on ion-induced changes in fluorescence appear to be particularly attractive due to their simplicity, high sensitivity and instantaneous response [1–8]. The trivalent form of iron is an essential element in man. It provides the oxygen-carrying capacity of heme and acts as a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain. On the other hand, iron deficiency chlorosis is a widespread agricultural problem that affects the development and decreases the yield of many susceptible crops growing on calcareous and alkaline soils [9]. Thus, there is an urgent need to develop chemical sensors that are capable of detecting the presence of iron ions in environmental and biological samples at physiological pH value. Considerable efforts have been devoted to developing fluorescent probes for various metal ions over the last few decades [10–13]. To date, some Fe(III)-selective fluorescent probes have
∗ Corresponding author at: Department of Chemistry, Anhui Science and Technology University, 222 Tianshui Road, Bengbu, Anhui 233100, PR China. Tel.: +86 550 6732036. ∗∗ Corresponding author. Tel.: +86 931 8912422. E-mail addresses: [email protected] (J. Mao), [email protected] (Q. He). 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.11.013
been achieved [14–17], since paramagnetic Fe(III) is described as one of the most efficient fluorescence quenchers among the transition metal ions, the signal transduction occurrence via chelation enhanced fluorescence (CHEF) with these inherent quenching metal ions is a challenging task. The rhodamine framework is an ideal mode to construct CHEF OFF–ON fluorescent probes due to its particular structural property, i.e. it undergoes equilibrium between non-fluorescent spirocyclic (“off” signal) and strongly fluorescent ring-open (“on” signal) forms and the two forms always behave with completely different fluorescent properties [18]. Heretofore, some studies on rhodamine-based probes have been reported [13,19–24]. We synthesized a rhodamine-based probe (RC) able to recognize and determine Fe(III) at biological pH value in aqueous solution, depending on a fluorescence “off–on” mode. Furthermore, the real samples contain lots of impurities, which influence the determination. Hence, an appropriate sample pretreatment method is necessary before the determination. Solid-phase extraction (SPE) techniques have recently been among the most popular separation methods for the enrichment of metal ions prior to their determination. The basic principle of SPE is the concentration and purification of analytes from solution by sorption on a solid sorbent [25]. This has several advantages over other techniques, namely conservation of species, good enrichment factors, and ease of automation [25,26]. Accordingly, the choice of the solid sorbent is the most crucial factor. According to previous research, activated carbon-bound ethylenediamin (AC-EDA) showed an excellent adsorption capacity toward Fe(III). And, the application of AC-EDA for the separation, preconcentration and
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elution of trace Fe(III) from real samples was performed with satisfactory results [27]. In this study, firstly, we applied AC-EDA to pretreat aqueous samples for the purification and Fe(III) enrichment. Secondly, based on the high selective response to Fe(III) of the fluorescent probe RC, a fluorescence method was establish to determine Fe(III) in aqueous solution with satisfactory result. 2. Experimental 2.1. Apparatus Fluorescence spectra measurements were performed on an F4500 spectrofluorimeter (Hitachi, Japan). The absorption spectra were observed by a UV-Cary100 spectrophotometer (Varian, Australia). Infrared spectra (4000–400 cm−1 ) in KBr were recorded on a Nicolet NEXUS 670 FT-IR spectrometer, Nicolet (Madison, WI, USA). A VarioEL element analyzer (Elementar Analysensysteme, Hanau, Germany) was used for elemental analysis. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals, Beijing, China) was used in the column process. A PTFE (polytetrafluoroethylene) column (50 mm × 9.0 mm i.d., Tianjin Jinteng Instrument Factory, Tianjin, China) was used. A pH-10C digital pH meter was utilized to measure the pH values of aqueous solutions. 1 H and 13 C NMR spectra were taken on a Varian mercury-300 spectrometer with TMS as an internal standard and CDCl3 as solvent. HRMS were determined on a Bruker Daltonics APEXII 47e FT-ICR spectrometer. 2.2. Reagents and chemicals All reagents were of analytical grade or the best grade commercially available, and were put into use without further purification. Activated carbon (AC, gas chromatographic grade, 40–60 mesh) (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China), ethylenediamine (EDA) (Dong Feng Biochemical Reagent Factory, Shanghai, China), Rhodamine 6G (Shanghai Chemical Reagent Company, Shanghai, China), diethylenetriamine (Tianjin The Second Reagent Factory, Tianjin, China), and N,N dicyclohexylcarbodiimide (DCC) (Sinopharm Chemical Reagent Co. Ltds., Shanghai, China) were used in this work. The solutions of metal ions were prepared from their nitrate salts (The First Reagent Factory, Shanghai, China). HEPES buffer solutions (0.2 mol L−1 , pH = 7.2) were prepared in water. Deionized water was used as solvents throughout. 2.3. Synthesis procedure Activated carbon powder was first purified with 10% (v/v) hydrochloric acid solution for 24 h so as to remove the metal ions and other impurities sorbed on it. Then 10 g of purified activated carbon was suspended in 300 mL of 32.5% (v/v) nitric acid solution under stirring and heating for 5 h at 60 ◦ C. The mixture was filtered and washed with deionized water to neutral and dried under vacuum at 80 ◦ C for 8 h. The product was carboxylic derivative of activated carbon (AC-COOH). For the synthesis of activated carbon-bound ethylenediamin (AC-EDA), a 5.0 g amount of AC-COOH was suspended in 150 mL of ethylenediamine under stirring and heating, then 5.0 g of N,N dicyclohexylcarbodiimide (DCC) was added into the suspension and refluxed at 120 ◦ C for 48 h [28]. The product (AC-EDA) was filtered off, washed with ethanol and dried under vacuum at 80 ◦ C for 8 h. The synthetic pathways of AC-EDA is illustrated in Fig. 1. The modified activated carbon was confirmed by IR analysis. Comparison of the IR spectrum of AC-COOH with activated carbon, a new bang (1710.17 cm−1 ) appeared in AC-COOH due to C O of
Fig. 1. synthetic pathways of AC-EDA.
the carboxylic acid group, which indicated the carboxylic derivative of activated carbon was prepared successfully. When AC-COOH was modified by EDA, several new peaks appeared in the spectrum. According to the literature [29,30], the new peaks can be assigned as follows: the peak at 1658.17 cm−1 is due to C O , the peak at 1528.56 cm−1 is caused by C–N and ıN–H . The bands around 3379.73 cm−1 can be assigned to N–H . Elemental analysis indicated 59.10% carbon, 7.818% nitrogen and 2.904% hydrogen in AC-EDA. It could be calculated that 1 g activated carbon contained 0.154 g EDA. Consequently, the above experimental results suggest that activated carbon is successfully modified by ethylenediamine. The compound (RC) was prepared according to a minor modification of the literature procedure [21]. Rhodamine 6G (958 mg, 2 mmol) was dissolved in 20 mL of hot methanol, followed by the addition of diethylenetriamine (0.67 mL, 10 mmol). The reaction mixture was refluxed for 10 h till the fluorescence of the solution was disappeared. After cooling to the room temperature, the solvent was evaporated in vacuo. CH2 Cl2 (100 mL) and water (200 mL) were added, and the organic layer was separated, washed twice with water and dried over anhydrous sodium sulfate. After filtration of sodium sulfate, the solvent was removed under reduced pressure. Then the resulting pink solid was purified by column chromatography (CH2 Cl2 :CH3 OH = 5:1, v/v) to give 148 mg of RC (pink solid) in 14.8% yield. 1 H NMR (300 MHz, CDCl3 ): ı 7.90 (t, 1H), 7.43 (t, 2H), 7.01 (t, 1H), 6.32 (s, 2H), 6.22 (s, 2H), 3.52 (t, 2H), 3.20 (t, 4H), 2.54 (t, 2H), 2.31 (m, 4H), 1.85 (s, 6H), 1.36 (t, 6H) ppm; 13 C NMR (75 MHz, CDCl ) ı 168.2, 153.3, 151.4, 147.1, 132.1, 130.7, 3 127.9, 127.7, 123.4, 122.3, 117.4, 105.6, 96.1, 64.7, 51.3, 47.3, 41.3, 39.9, 38.0, 16.4, 14.3 ppm; HRMS (ESI) m/z obsd 500.3025 ([M+H]+ calcd 500.3020 for C30 H37 N5 O2 ). 2.4. Sample pretreatment procedure A total of 50 mg adsorbent AC-EDA was packed in the PTFE column (50 mm × 9.0 mm i.d.) plugged with a small portion of glass wool at both ends. Before use, HCl (0.5 mol L−1 ) solution and doubly distilled deionized water were successively passed through the microcolumn in order to equilibrate, clean and neutralize it. Portions of aqueous standard or sample solutions containing metal ions were prepared. Each solution was passed through the column at a flow rate of 4.0 mL min−1 controlled by a peristaltic pump. Afterwards, the metal ions retained on column were eluted with a 3 mL of 0.5 mol L−1 HCl solution. The elution was collected for the fluorescence determination procedure. 2.5. Fluorescence determination procedure The elution was adjust to pH 7.0 by NaOH solution and transferred into a 20.0 mL colorimetric tube. The solutions were added in the following order: 2.0 mL 0.2 mol L−1 HEPES buffer solutions (pH 7.2), 2.0 mL 1.0 × 10−4 mol L−1 RC solution. The mixture was diluted to 20.0 mL with water, and was allowed to stand for 5 min at 25 ◦ C before a fluorescence measurement was made. The intensity of the fluorescence was measured at 552 nm (ex = 500 nm) in a 1 cm quartz cell with a slit width of 5 nm for both excitation and emission.
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Fig. 2. Synthetic pathways of RC.
3. Results and discussion 3.1. Optical characteristics of the probe RC As depicted in Fig. 2, compounds RC was facilely synthesized from the reaction of rhodamine 6G with triethylenetetriamine. Although RC is a derivative of rhodamine 6G, it form nearly colorless solutions in HEPES aqueous buffer solution (pH = 7.2), indicating that the spirocyclic forms exist predominantly. The characteristic peak near 64.7 ppm (9-carbon) in the 13 C NMR spectra of RC also supports this consideration [31]. When no metal ion was added to the solution of RC, almost no fluorescence signal in the range from 520 to 600 nm could be observed at pH 7.2 in water (Fig. 3 bottom line), whereas a significant enhancement of the characteristic fluorescence of rhodamine 6G emerged soon after Fe(III) was injected into the HEPES solution of RC. Besides, the fluorescence intensity at 552 nm enhanced as the Fe(III) concentration increased. Furthermore, the enhanced extents were in good proportion to the Fe(III) concentrations in a certain range, and other physiologically important cations, even if their concentrations were 50 times higher than that of Fe(III), and only Fe(III) ion can effectively enhance the fluorescence of RC. It was interesting that the enhanced extents of the fluorescence were in good proportion to the concentrations of Fe(III), indicating that an assay of Fe(III) in this way to be good practice. And, the distinct discrimination between Fe (III) and other ion made it possible for the RC to be used for the analysis of Fe (III) in the presence of other ions in aqueous system.
Fig. 3. Fluorescence spectra of RC (1.0 × 10−5 mol L−1 ) with the addition of increasing concentrations of Fe(III) in HEPES buffer solution (pH = 7.2) with an excitation at 500 nm.
Fig. 4. Absorption spectra of RC (a) and RC with Fe(III) (b) [RC, 1.0 × 10−5 mol L−1 ; Fe(III), 2.0 × 10−5 mol L−1 ].
It is showed that RC had no rhodamine framework typical absorbance from 400 to 600 nm (Fig. 4a), which was in the visual range. For that reason, the aqueous solution of RC was colorless. However, with addition of Fe(III) the rhodamine typical absorbance at 527 nm appeared (Fig. 4b), which indicated rhodamine particular structure formed. Meanwhile, the job’s plot indicated that a 1:2 stoichiometry for the binding mode of Fe(III) and RC (Fig. 5), and the binding constant was 6.43 × 103 mol−1 . It has been reported that many rhodamine derivatives are colorless and non-fluorescent substances due to their stable ‘spirolactam form’ [13,19,20]. In our study, RC is also colorless and non-fluorescent in aqueous solution for the same reason. If the ‘spirolactam form’ is opened, the fluorescence could be recovered due to substantially different spectral characteristics of spirocyclic and ring-open forms of rhodamine framework. The fluorescence spectra of RC in aqueous solution in the absence and presence of Fe(III) are shown in Fig. 3. It can be seen that RC displays almost no obvious spectral characteristics in either its absorbance or emission spectrum from 400 to 600 nm. However, a significant enhancement of fluorescence with an absorbance maximum at 527 nm and an emission maximum at 552 nm was observed when Fe(III) was added. We speculated that the spiro-
Fig. 5. Job’s plot according to the method for continuous variations, indicating the 1:2 stoichiometry (Fe(III)/RC) [the total concentration of RC and Fe(III) is 2.0 × 10−5 mol L−1 ].
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Fig. 6. Mechanism for the fluorescence enhancement of RC upon the addition of Fe(III).
lactam form was opened upon the addition of Fe(III) to RC, and a highly delocalized -conjugated structure of RC was formed (Fig. 6) [13,19]. Thus, a significant enhancement of absorbance and fluorescence occurred. With this special coordinate mode, Fe(III) was able to enhance the fluorescence of RC, though the other ions failed, indicating the coordinate moiety of RC match perfect with Fe(III) instead of other ions. 3.2. Optimization of the analytical procedures 3.2.1. Effect of RC concentration The concentration of RC influenced the Fe(III) determination. If the concentration of RC was too high, the sensitivity for the detection of Fe(III) would be decreased. On the contrary, the linear range would become narrow if the concentration was too low. For a balance between the sensitivity and the linear range, the optimum concentration of RC for the Fe(III) determination was 1.0 × 10−5 mol L−1 . 3.2.2. Effect of the pH The effect of pH on the fluorescence intensity of RC in the system is shown in Fig. 7. It can be seen that the fluorescence kept constant tiny intensity between pH 5.5 and 8.5 in water. However, the fluorescence intensity increased sharply when pH value below 5.0 and became even more acidic. Finally, a physiological pH value 7.2 was chosen for the fluorescence intensity determination.
ability, all the fluorescence measurements were conducted under 25 ◦ C. 3.2.4. Effect of time Under the optimum conditions, the effect of time on the Fe(III) determination was studied at room temperature. The fluorescence intensity changed soon after Fe(III) added into RC aqueous solution and remained stable from 1 min to 10 h. After all reagents had been added they were left standing for 5 min ahead of a measurement. In this study, 5 min was set as the standard for all fluorescence intensity measurements. 3.3. Tolerance of foreign substances The effect of substances, including common metal ions was examined for interference. The results are summarized in Fig. 8 and Table 1. It is showed in Fig. 8 that other ions but Fe(III) failed to arouse fluorescence response of RC system. And, Table 1 shows that the influence of metal ions in biological systems, such as K+ , Ca2+ , Na+ , and Zn2+ though with much higher concentration compared with Fe(III), was tiny to the RC-Fe system. Consequently, it is clear that other ions’ interference is slight to this system.
3.2.3. Effect of temperature Under a temperature series, the fluorescence intensity of RC almost kept constant. For that reason, we could know that this system possessed stability to temperature for Fe(III) determination in aqueous solution. But considering the convenience and maneuver-
Fig. 7. Evaluation of the fluorescence intensity (at 552 nm) of RC (1.0 × 10−5 mol L−1 ) under different pH values with an excitation at 500 nm.
Fig. 8. Fluorescence spectra response of RC (1.0 × 10−5 mol L−1 ) under the optimum conditions with different ions (4.0 × 10−6 mol L−1 ) with an excitation at 500 nm. Other ions including K(I), Ca(II), Na(I), Mg(II), Zn(II), Pb(II), Co(II), Ba(II), Cu(II), Al(III), Cd(II), and Ni(II).
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Table 1 Interference of potentially interfering ions. Co-existing substance
Co-existing concentration (×10−6 mol L−1 )
Change in intensity of fluorescence (%)
K(I) Ca(II) Na(I) Mg(II) Zn(II) Pb(II) Co(II) Ba(II) Cu(II) Al(III) Cd(II) Ni(II) Fe(II)
100 100 100 100 100 100 100 100 50 50 50 50 50
−1.95 4.31 −0.40 1.78 −2.41 −4.15 2.47 −3.30 3.63 4.32 −2.1 3.37 4.83
Note: Determination conducted under the optimum conditions, samples contain 1.0 × 10−6 mol L−1 Fe(III) and other ions. Table 2 Analytical parameters for the determination of Fe(III). Sample
Linear range (×10−8 mol L−1 )
Linear regression equation (c, ng mL−1 )
Detection limit (3, mol L−1 )
Correlation coefficient
Fe(III)
6.0–720
F = 1.67 × 107 c + 3.25
1.4 × 10−8
0.998
Table 3 Results of the sample analysis. Real samples
Fe(III) (×10−8 mol L−1 ) a
Recovery (%) b
c
d
Found
Added
Sum result (n = 5)
Tap water
75.4 125.2 170.9
73.8
78.5
0 50.0 100.0
95–103 94–102 95–104
Waste water
0 200.0 400.0
187.3 384.4 572.5
181.4
189.3
94–105 95–106 94–105
a b c d
Found
Detected by ICP-AES method. Standard Fe(III) solution addition. Detected by the method of this paper. The result found by the method of this paper.
3.4. Analytical characteristics Under the optimum conditions, the relationship between F [the fluorescence intensity enhancement of RC upon the addition of varying concentrations of Fe(III)] and the concentration of Fe(III) was obtained. The analytical parameters are given in Table 2, which demonstrates a linear relationship between F and the concentrations of Fe(III) over a wide range, and the limit of detection reaching the 1.4 × 10−8 mol L−1 . 3.5. Analytical applications The present method was applied to determine real samples, tap water and industrial waster water (the samples were supplied by gansu center for disease prevention and control). Table 3 shows the analysis results which were very close to those detected by ICP-AES method, which were 78.5 × 10−8 and 189.3 × 10−8 mol L−1 in the determination of samples, respectively (the result was supplied by Analytical Chemistry Institute of Lanzhou University). Therefore, the present method could be applied for the analysis of Fe(III) in aqueous system. 4. Conclusions In conclusion, we have developed a novel fluorescent probe RC for Fe(III) based on the enhancement of fluorescence intensity of RC after adding Fe(III). Under the optimum conditions, the relative fluorescence intensity increase was linearly proportional to the
concentration of Fe(III) in a wide range, and the limit of detection was 14 nmol L−1 . The probe RC had remarkably high selectivity and sensitivity. Moreover, the analytical results of real samples were satisfactory. Fluorescence enhance mechanism is presumably due to the chelation of Fe(III) with the oxygen atoms of the amide groups of RC results in the formation of the open-ring form, which leads to the enhancement of fluorescence signals of RC. Acknowledgements This study was supported by the research fund of Anhui Science and Technology University and the NSFC (Grants 20431010, 20621091 and J0630962). References [1] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997, pp. 1–276. [2] U.S. Spichiger-Keller, Chemical Sensors and Biosensors for Medical and Biological Applications, Wiley–VCH, Weinheim, Germany, 1998, pp. 1–413. [3] D.A. Leigh, M.A.F. Morales, E.M. Perez, J.K.Y. Wong, C.G. Saiz, A.M.Z. Slawin, A.J. Carmichael, D.M. Haddleton, A.M. Brouwer, W.J. Buma, W.H. Wurpel, S. Leon, F. Zerbetto, Angew. Chem. Int. Ed. 44 (2005) 3062. [4] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515. [5] B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3. [6] K. Rurack, Spectrochim. Acta 57A (2001) 2161. [7] K. Rurack, U. Resch-Genger, Chem. Soc. Rev. (2002) 116. [8] V. Amendola, L. Fabbrizzi, F. Forti, M. Licchelli, C. Mangano, P. Pallavicini, A. Poggi, D. Sacchi, A. Taglieti, Coord. Chem. Rev. 250 (2006) 273. [9] Y. Chen, P. Barak, Adv. Agron. 35 (1982) 217. [10] X.B. Zhang, J. Peng, C.L. He, G.L. Shen, R.Q. Yu, Anal. Chim. Acta 567 (2006) 189.
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Talanta journal homepage: www.elsevier.com/locate/talanta
An rhodamine-based fluorescence probe for iron(III) ion determination in aqueous solution Jie Mao a,b,∗ , Qun He b,∗∗ , Weisheng Liu b a b
Department of Chemistry, Anhui Science and Technology University, Bengbu, Anhui 233100, PR China College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 31 October 2009 Accepted 3 November 2009 Available online 13 November 2009 Keywords: Iron(III) Probe Fluorescence enhancement Rhodamine
a b s t r a c t An “off–on” rhodamine-based fluorescence probe for the selective signaling of Fe(III) has been designed exploiting the guest-induced structure transform mechanism. This system shows a sharp Fe(III)-selective fluorescence enhancement response in 100% aqueous system under physiological pH value and possesses high selectivity against the background of environmentally and biologically relevant metal ions including Al(III), Cd(II), Fe(II), Co(II), Cu(II), Ni(II), Zn(II), Mg(II), Ba(II), Pb(II), Na(I), and K(I). Under optimum conditions, the fluorescence intensity enhancement of this system is linearly proportional to Fe(III) concentration from 6.0 × 10−8 to 7.2 × 10−6 mol L−1 with a detection limit of 1.4 × 10−8 mol L−1 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction The development of fluorescent chemical devices is a promising field [1–3]. An important area within this field is the design of fluorescent probes for various metal ions [4–8]. Ions play a fundamental role in a wide range of chemical and biological processes, and numerous efforts have been made to the development of effective fluorescent probes. Sensors based on ion-induced changes in fluorescence appear to be particularly attractive due to their simplicity, high sensitivity and instantaneous response [1–8]. The trivalent form of iron is an essential element in man. It provides the oxygen-carrying capacity of heme and acts as a cofactor in many enzymatic reactions involved in the mitochondrial respiratory chain. On the other hand, iron deficiency chlorosis is a widespread agricultural problem that affects the development and decreases the yield of many susceptible crops growing on calcareous and alkaline soils [9]. Thus, there is an urgent need to develop chemical sensors that are capable of detecting the presence of iron ions in environmental and biological samples at physiological pH value. Considerable efforts have been devoted to developing fluorescent probes for various metal ions over the last few decades [10–13]. To date, some Fe(III)-selective fluorescent probes have
∗ Corresponding author at: Department of Chemistry, Anhui Science and Technology University, 222 Tianshui Road, Bengbu, Anhui 233100, PR China. Tel.: +86 550 6732036. ∗∗ Corresponding author. Tel.: +86 931 8912422. E-mail addresses: [email protected] (J. Mao), [email protected] (Q. He). 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.11.013
been achieved [14–17], since paramagnetic Fe(III) is described as one of the most efficient fluorescence quenchers among the transition metal ions, the signal transduction occurrence via chelation enhanced fluorescence (CHEF) with these inherent quenching metal ions is a challenging task. The rhodamine framework is an ideal mode to construct CHEF OFF–ON fluorescent probes due to its particular structural property, i.e. it undergoes equilibrium between non-fluorescent spirocyclic (“off” signal) and strongly fluorescent ring-open (“on” signal) forms and the two forms always behave with completely different fluorescent properties [18]. Heretofore, some studies on rhodamine-based probes have been reported [13,19–24]. We synthesized a rhodamine-based probe (RC) able to recognize and determine Fe(III) at biological pH value in aqueous solution, depending on a fluorescence “off–on” mode. Furthermore, the real samples contain lots of impurities, which influence the determination. Hence, an appropriate sample pretreatment method is necessary before the determination. Solid-phase extraction (SPE) techniques have recently been among the most popular separation methods for the enrichment of metal ions prior to their determination. The basic principle of SPE is the concentration and purification of analytes from solution by sorption on a solid sorbent [25]. This has several advantages over other techniques, namely conservation of species, good enrichment factors, and ease of automation [25,26]. Accordingly, the choice of the solid sorbent is the most crucial factor. According to previous research, activated carbon-bound ethylenediamin (AC-EDA) showed an excellent adsorption capacity toward Fe(III). And, the application of AC-EDA for the separation, preconcentration and
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elution of trace Fe(III) from real samples was performed with satisfactory results [27]. In this study, firstly, we applied AC-EDA to pretreat aqueous samples for the purification and Fe(III) enrichment. Secondly, based on the high selective response to Fe(III) of the fluorescent probe RC, a fluorescence method was establish to determine Fe(III) in aqueous solution with satisfactory result. 2. Experimental 2.1. Apparatus Fluorescence spectra measurements were performed on an F4500 spectrofluorimeter (Hitachi, Japan). The absorption spectra were observed by a UV-Cary100 spectrophotometer (Varian, Australia). Infrared spectra (4000–400 cm−1 ) in KBr were recorded on a Nicolet NEXUS 670 FT-IR spectrometer, Nicolet (Madison, WI, USA). A VarioEL element analyzer (Elementar Analysensysteme, Hanau, Germany) was used for elemental analysis. An YL-110 peristaltic pump (General Research Institute for Non-ferrous Metals, Beijing, China) was used in the column process. A PTFE (polytetrafluoroethylene) column (50 mm × 9.0 mm i.d., Tianjin Jinteng Instrument Factory, Tianjin, China) was used. A pH-10C digital pH meter was utilized to measure the pH values of aqueous solutions. 1 H and 13 C NMR spectra were taken on a Varian mercury-300 spectrometer with TMS as an internal standard and CDCl3 as solvent. HRMS were determined on a Bruker Daltonics APEXII 47e FT-ICR spectrometer. 2.2. Reagents and chemicals All reagents were of analytical grade or the best grade commercially available, and were put into use without further purification. Activated carbon (AC, gas chromatographic grade, 40–60 mesh) (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China), ethylenediamine (EDA) (Dong Feng Biochemical Reagent Factory, Shanghai, China), Rhodamine 6G (Shanghai Chemical Reagent Company, Shanghai, China), diethylenetriamine (Tianjin The Second Reagent Factory, Tianjin, China), and N,N dicyclohexylcarbodiimide (DCC) (Sinopharm Chemical Reagent Co. Ltds., Shanghai, China) were used in this work. The solutions of metal ions were prepared from their nitrate salts (The First Reagent Factory, Shanghai, China). HEPES buffer solutions (0.2 mol L−1 , pH = 7.2) were prepared in water. Deionized water was used as solvents throughout. 2.3. Synthesis procedure Activated carbon powder was first purified with 10% (v/v) hydrochloric acid solution for 24 h so as to remove the metal ions and other impurities sorbed on it. Then 10 g of purified activated carbon was suspended in 300 mL of 32.5% (v/v) nitric acid solution under stirring and heating for 5 h at 60 ◦ C. The mixture was filtered and washed with deionized water to neutral and dried under vacuum at 80 ◦ C for 8 h. The product was carboxylic derivative of activated carbon (AC-COOH). For the synthesis of activated carbon-bound ethylenediamin (AC-EDA), a 5.0 g amount of AC-COOH was suspended in 150 mL of ethylenediamine under stirring and heating, then 5.0 g of N,N dicyclohexylcarbodiimide (DCC) was added into the suspension and refluxed at 120 ◦ C for 48 h [28]. The product (AC-EDA) was filtered off, washed with ethanol and dried under vacuum at 80 ◦ C for 8 h. The synthetic pathways of AC-EDA is illustrated in Fig. 1. The modified activated carbon was confirmed by IR analysis. Comparison of the IR spectrum of AC-COOH with activated carbon, a new bang (1710.17 cm−1 ) appeared in AC-COOH due to C O of
Fig. 1. synthetic pathways of AC-EDA.
the carboxylic acid group, which indicated the carboxylic derivative of activated carbon was prepared successfully. When AC-COOH was modified by EDA, several new peaks appeared in the spectrum. According to the literature [29,30], the new peaks can be assigned as follows: the peak at 1658.17 cm−1 is due to C O , the peak at 1528.56 cm−1 is caused by C–N and ıN–H . The bands around 3379.73 cm−1 can be assigned to N–H . Elemental analysis indicated 59.10% carbon, 7.818% nitrogen and 2.904% hydrogen in AC-EDA. It could be calculated that 1 g activated carbon contained 0.154 g EDA. Consequently, the above experimental results suggest that activated carbon is successfully modified by ethylenediamine. The compound (RC) was prepared according to a minor modification of the literature procedure [21]. Rhodamine 6G (958 mg, 2 mmol) was dissolved in 20 mL of hot methanol, followed by the addition of diethylenetriamine (0.67 mL, 10 mmol). The reaction mixture was refluxed for 10 h till the fluorescence of the solution was disappeared. After cooling to the room temperature, the solvent was evaporated in vacuo. CH2 Cl2 (100 mL) and water (200 mL) were added, and the organic layer was separated, washed twice with water and dried over anhydrous sodium sulfate. After filtration of sodium sulfate, the solvent was removed under reduced pressure. Then the resulting pink solid was purified by column chromatography (CH2 Cl2 :CH3 OH = 5:1, v/v) to give 148 mg of RC (pink solid) in 14.8% yield. 1 H NMR (300 MHz, CDCl3 ): ı 7.90 (t, 1H), 7.43 (t, 2H), 7.01 (t, 1H), 6.32 (s, 2H), 6.22 (s, 2H), 3.52 (t, 2H), 3.20 (t, 4H), 2.54 (t, 2H), 2.31 (m, 4H), 1.85 (s, 6H), 1.36 (t, 6H) ppm; 13 C NMR (75 MHz, CDCl ) ı 168.2, 153.3, 151.4, 147.1, 132.1, 130.7, 3 127.9, 127.7, 123.4, 122.3, 117.4, 105.6, 96.1, 64.7, 51.3, 47.3, 41.3, 39.9, 38.0, 16.4, 14.3 ppm; HRMS (ESI) m/z obsd 500.3025 ([M+H]+ calcd 500.3020 for C30 H37 N5 O2 ). 2.4. Sample pretreatment procedure A total of 50 mg adsorbent AC-EDA was packed in the PTFE column (50 mm × 9.0 mm i.d.) plugged with a small portion of glass wool at both ends. Before use, HCl (0.5 mol L−1 ) solution and doubly distilled deionized water were successively passed through the microcolumn in order to equilibrate, clean and neutralize it. Portions of aqueous standard or sample solutions containing metal ions were prepared. Each solution was passed through the column at a flow rate of 4.0 mL min−1 controlled by a peristaltic pump. Afterwards, the metal ions retained on column were eluted with a 3 mL of 0.5 mol L−1 HCl solution. The elution was collected for the fluorescence determination procedure. 2.5. Fluorescence determination procedure The elution was adjust to pH 7.0 by NaOH solution and transferred into a 20.0 mL colorimetric tube. The solutions were added in the following order: 2.0 mL 0.2 mol L−1 HEPES buffer solutions (pH 7.2), 2.0 mL 1.0 × 10−4 mol L−1 RC solution. The mixture was diluted to 20.0 mL with water, and was allowed to stand for 5 min at 25 ◦ C before a fluorescence measurement was made. The intensity of the fluorescence was measured at 552 nm (ex = 500 nm) in a 1 cm quartz cell with a slit width of 5 nm for both excitation and emission.
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Fig. 2. Synthetic pathways of RC.
3. Results and discussion 3.1. Optical characteristics of the probe RC As depicted in Fig. 2, compounds RC was facilely synthesized from the reaction of rhodamine 6G with triethylenetetriamine. Although RC is a derivative of rhodamine 6G, it form nearly colorless solutions in HEPES aqueous buffer solution (pH = 7.2), indicating that the spirocyclic forms exist predominantly. The characteristic peak near 64.7 ppm (9-carbon) in the 13 C NMR spectra of RC also supports this consideration [31]. When no metal ion was added to the solution of RC, almost no fluorescence signal in the range from 520 to 600 nm could be observed at pH 7.2 in water (Fig. 3 bottom line), whereas a significant enhancement of the characteristic fluorescence of rhodamine 6G emerged soon after Fe(III) was injected into the HEPES solution of RC. Besides, the fluorescence intensity at 552 nm enhanced as the Fe(III) concentration increased. Furthermore, the enhanced extents were in good proportion to the Fe(III) concentrations in a certain range, and other physiologically important cations, even if their concentrations were 50 times higher than that of Fe(III), and only Fe(III) ion can effectively enhance the fluorescence of RC. It was interesting that the enhanced extents of the fluorescence were in good proportion to the concentrations of Fe(III), indicating that an assay of Fe(III) in this way to be good practice. And, the distinct discrimination between Fe (III) and other ion made it possible for the RC to be used for the analysis of Fe (III) in the presence of other ions in aqueous system.
Fig. 3. Fluorescence spectra of RC (1.0 × 10−5 mol L−1 ) with the addition of increasing concentrations of Fe(III) in HEPES buffer solution (pH = 7.2) with an excitation at 500 nm.
Fig. 4. Absorption spectra of RC (a) and RC with Fe(III) (b) [RC, 1.0 × 10−5 mol L−1 ; Fe(III), 2.0 × 10−5 mol L−1 ].
It is showed that RC had no rhodamine framework typical absorbance from 400 to 600 nm (Fig. 4a), which was in the visual range. For that reason, the aqueous solution of RC was colorless. However, with addition of Fe(III) the rhodamine typical absorbance at 527 nm appeared (Fig. 4b), which indicated rhodamine particular structure formed. Meanwhile, the job’s plot indicated that a 1:2 stoichiometry for the binding mode of Fe(III) and RC (Fig. 5), and the binding constant was 6.43 × 103 mol−1 . It has been reported that many rhodamine derivatives are colorless and non-fluorescent substances due to their stable ‘spirolactam form’ [13,19,20]. In our study, RC is also colorless and non-fluorescent in aqueous solution for the same reason. If the ‘spirolactam form’ is opened, the fluorescence could be recovered due to substantially different spectral characteristics of spirocyclic and ring-open forms of rhodamine framework. The fluorescence spectra of RC in aqueous solution in the absence and presence of Fe(III) are shown in Fig. 3. It can be seen that RC displays almost no obvious spectral characteristics in either its absorbance or emission spectrum from 400 to 600 nm. However, a significant enhancement of fluorescence with an absorbance maximum at 527 nm and an emission maximum at 552 nm was observed when Fe(III) was added. We speculated that the spiro-
Fig. 5. Job’s plot according to the method for continuous variations, indicating the 1:2 stoichiometry (Fe(III)/RC) [the total concentration of RC and Fe(III) is 2.0 × 10−5 mol L−1 ].
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Fig. 6. Mechanism for the fluorescence enhancement of RC upon the addition of Fe(III).
lactam form was opened upon the addition of Fe(III) to RC, and a highly delocalized -conjugated structure of RC was formed (Fig. 6) [13,19]. Thus, a significant enhancement of absorbance and fluorescence occurred. With this special coordinate mode, Fe(III) was able to enhance the fluorescence of RC, though the other ions failed, indicating the coordinate moiety of RC match perfect with Fe(III) instead of other ions. 3.2. Optimization of the analytical procedures 3.2.1. Effect of RC concentration The concentration of RC influenced the Fe(III) determination. If the concentration of RC was too high, the sensitivity for the detection of Fe(III) would be decreased. On the contrary, the linear range would become narrow if the concentration was too low. For a balance between the sensitivity and the linear range, the optimum concentration of RC for the Fe(III) determination was 1.0 × 10−5 mol L−1 . 3.2.2. Effect of the pH The effect of pH on the fluorescence intensity of RC in the system is shown in Fig. 7. It can be seen that the fluorescence kept constant tiny intensity between pH 5.5 and 8.5 in water. However, the fluorescence intensity increased sharply when pH value below 5.0 and became even more acidic. Finally, a physiological pH value 7.2 was chosen for the fluorescence intensity determination.
ability, all the fluorescence measurements were conducted under 25 ◦ C. 3.2.4. Effect of time Under the optimum conditions, the effect of time on the Fe(III) determination was studied at room temperature. The fluorescence intensity changed soon after Fe(III) added into RC aqueous solution and remained stable from 1 min to 10 h. After all reagents had been added they were left standing for 5 min ahead of a measurement. In this study, 5 min was set as the standard for all fluorescence intensity measurements. 3.3. Tolerance of foreign substances The effect of substances, including common metal ions was examined for interference. The results are summarized in Fig. 8 and Table 1. It is showed in Fig. 8 that other ions but Fe(III) failed to arouse fluorescence response of RC system. And, Table 1 shows that the influence of metal ions in biological systems, such as K+ , Ca2+ , Na+ , and Zn2+ though with much higher concentration compared with Fe(III), was tiny to the RC-Fe system. Consequently, it is clear that other ions’ interference is slight to this system.
3.2.3. Effect of temperature Under a temperature series, the fluorescence intensity of RC almost kept constant. For that reason, we could know that this system possessed stability to temperature for Fe(III) determination in aqueous solution. But considering the convenience and maneuver-
Fig. 7. Evaluation of the fluorescence intensity (at 552 nm) of RC (1.0 × 10−5 mol L−1 ) under different pH values with an excitation at 500 nm.
Fig. 8. Fluorescence spectra response of RC (1.0 × 10−5 mol L−1 ) under the optimum conditions with different ions (4.0 × 10−6 mol L−1 ) with an excitation at 500 nm. Other ions including K(I), Ca(II), Na(I), Mg(II), Zn(II), Pb(II), Co(II), Ba(II), Cu(II), Al(III), Cd(II), and Ni(II).
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Table 1 Interference of potentially interfering ions. Co-existing substance
Co-existing concentration (×10−6 mol L−1 )
Change in intensity of fluorescence (%)
K(I) Ca(II) Na(I) Mg(II) Zn(II) Pb(II) Co(II) Ba(II) Cu(II) Al(III) Cd(II) Ni(II) Fe(II)
100 100 100 100 100 100 100 100 50 50 50 50 50
−1.95 4.31 −0.40 1.78 −2.41 −4.15 2.47 −3.30 3.63 4.32 −2.1 3.37 4.83
Note: Determination conducted under the optimum conditions, samples contain 1.0 × 10−6 mol L−1 Fe(III) and other ions. Table 2 Analytical parameters for the determination of Fe(III). Sample
Linear range (×10−8 mol L−1 )
Linear regression equation (c, ng mL−1 )
Detection limit (3, mol L−1 )
Correlation coefficient
Fe(III)
6.0–720
F = 1.67 × 107 c + 3.25
1.4 × 10−8
0.998
Table 3 Results of the sample analysis. Real samples
Fe(III) (×10−8 mol L−1 ) a
Recovery (%) b
c
d
Found
Added
Sum result (n = 5)
Tap water
75.4 125.2 170.9
73.8
78.5
0 50.0 100.0
95–103 94–102 95–104
Waste water
0 200.0 400.0
187.3 384.4 572.5
181.4
189.3
94–105 95–106 94–105
a b c d
Found
Detected by ICP-AES method. Standard Fe(III) solution addition. Detected by the method of this paper. The result found by the method of this paper.
3.4. Analytical characteristics Under the optimum conditions, the relationship between F [the fluorescence intensity enhancement of RC upon the addition of varying concentrations of Fe(III)] and the concentration of Fe(III) was obtained. The analytical parameters are given in Table 2, which demonstrates a linear relationship between F and the concentrations of Fe(III) over a wide range, and the limit of detection reaching the 1.4 × 10−8 mol L−1 . 3.5. Analytical applications The present method was applied to determine real samples, tap water and industrial waster water (the samples were supplied by gansu center for disease prevention and control). Table 3 shows the analysis results which were very close to those detected by ICP-AES method, which were 78.5 × 10−8 and 189.3 × 10−8 mol L−1 in the determination of samples, respectively (the result was supplied by Analytical Chemistry Institute of Lanzhou University). Therefore, the present method could be applied for the analysis of Fe(III) in aqueous system. 4. Conclusions In conclusion, we have developed a novel fluorescent probe RC for Fe(III) based on the enhancement of fluorescence intensity of RC after adding Fe(III). Under the optimum conditions, the relative fluorescence intensity increase was linearly proportional to the
concentration of Fe(III) in a wide range, and the limit of detection was 14 nmol L−1 . The probe RC had remarkably high selectivity and sensitivity. Moreover, the analytical results of real samples were satisfactory. Fluorescence enhance mechanism is presumably due to the chelation of Fe(III) with the oxygen atoms of the amide groups of RC results in the formation of the open-ring form, which leads to the enhancement of fluorescence signals of RC. Acknowledgements This study was supported by the research fund of Anhui Science and Technology University and the NSFC (Grants 20431010, 20621091 and J0630962). References [1] J.P. Desvergne, A.W. Czarnik, Chemosensors of Ion and Molecule Recognition, Kluwer, Dordrecht, 1997, pp. 1–276. [2] U.S. Spichiger-Keller, Chemical Sensors and Biosensors for Medical and Biological Applications, Wiley–VCH, Weinheim, Germany, 1998, pp. 1–413. [3] D.A. Leigh, M.A.F. Morales, E.M. Perez, J.K.Y. Wong, C.G. Saiz, A.M.Z. Slawin, A.J. Carmichael, D.M. Haddleton, A.M. Brouwer, W.J. Buma, W.H. Wurpel, S. Leon, F. Zerbetto, Angew. Chem. Int. Ed. 44 (2005) 3062. [4] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515. [5] B. Valeur, I. Leray, Coord. Chem. Rev. 205 (2000) 3. [6] K. Rurack, Spectrochim. Acta 57A (2001) 2161. [7] K. Rurack, U. Resch-Genger, Chem. Soc. Rev. (2002) 116. [8] V. Amendola, L. Fabbrizzi, F. Forti, M. Licchelli, C. Mangano, P. Pallavicini, A. Poggi, D. Sacchi, A. Taglieti, Coord. Chem. Rev. 250 (2006) 273. [9] Y. Chen, P. Barak, Adv. Agron. 35 (1982) 217. [10] X.B. Zhang, J. Peng, C.L. He, G.L. Shen, R.Q. Yu, Anal. Chim. Acta 567 (2006) 189.
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