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Spacer Layer Screening Effect: A Novel Fluorescent Film Sensor for Organic Copper(II) Salts Fengting Lu¨, Lining Gao, Liping Ding, Linling Jiang, and Yu Fang* Key Laboratory for Macromolecular Science of Shaanxi ProVince, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, People’s Republic of China ReceiVed October 25, 2005. In Final Form: NoVember 5, 2005 A fluorescent film sensor was prepared by chemical assembly of pyrene on a glass plate surface via a long flexible spacer. It was found that the film is highly selective for some organic Cu2+ salts, such as copper acetate and copper propionate. The presence of inorganic Cu2+ salts and other metal(II) acetates, including Ni2+, Co2+, Pb2+, Cd2+, Zn2+, etc., had little effect upon the sensing behavior of the film for copper acetate or copper propionate. The observation was explained by employing a proposed “two-dimensional solution” model. The quenching by copper acetate of the emission of the film is static in nature due to complexation of the spacers to the metal ions. Furthermore, the response of the film sensor to copper acetate is fully reversible. To the best of our knowledge, this film sensor may be the first one that can differentiate greasy copper salts from inorganic copper salts.
Introduction A chemosensor typically consists of three components, which are a recognition part that binds the target analyte, a readout part that signals binding, and a linker that connects the recognition part and the readout part. In the case of metal ion sensors, the recognition part is usually a metal-chelating structure designed to bind the target ion selectively and the readout part is very often a fluorophore due to its high sensitivity.1,2 The complexation of the metal ion results in a variation of the position and/or intensity of the emission of the fluorophore. Recently, studies of the design and preparation of fluorescent sensors for the detection of Cu2+ have attracted increasing attention.3-7 This is because Cu2+ is a significant environmental pollutant and an essential trace element in biological systems. It is also a wellknown paramagnetic ion with an empty d shell and can strongly quench the fluorescence of a fluorophore near it via electron or energy transfer.8,9 On the basis of its quenching properties, a large number of molecular sensors for Cu2+ in aqueous or polar organic solvents have been reported.3-6 In contrast, the inves* To whom correspondence should be addressed. Phone: 86-29-85310081. Fax: 86-29-85307534-8221. E-mail:
[email protected]. (1) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-1566. (2) (a) Chemosensors of Ion and Molecule Recognition; Desvergne, J. P., Czarnik, A. W., Eds.; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997. (b) Haugland, R. P. Handbook of Fluorescent Probes and Research Chemicals, 6th ed.; Molecular Probes, Inc.: Eugene, OR, 1996. (3) (a) Kra¨mer, R. Angew. Chem., Int. Ed. 1998, 37, 772-773. (b) Singh, A.; Yao, Q.; Tong, L.; Still, W. C.; Sames, D. Tetrahedron Lett. 2000, 42, 96019605. (c) Grandini, P.; Mancin, F.; Tecilla, P.; Scrimin, P.; Tonellato, U. Angew. Chem., Int. Ed. 1999, 38, 3061-3064. (d) Berton, M.; Mancin, F.; Stocchero, G.; Tecilla, P.; Tonellato, U. Langmuir 2001, 17, 7521-7528. (e) Beltramello, M.; Gatos, M.; Mancin, F.; Tecilla, P.; Tonellato, U. Tetrahedron Lett. 2001, 42, 9143-9146. (f) Klein, G.; Kaufmann, D.; Schurch, S.; Reymond, J. L. Chem. Commun. 2001, 561-562. (4) (a) Zheng, Y.; Gatta´s-Asfura, K. M.; Konka, V.; Leblanc, R. M. Chem. Commun. 2002, 2350-2351. (b) Zheng, Y.; Huo, Q.; Kele, P.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. Org. Lett. 2001, 3, 3277-3280. (5) Brunner, J.; Kraemer, R. J. Am. Chem. Soc. 2004, 126, 13626-13627. (6) Royzen, M.; Dai, Z. H.; Canary, J. W. J. Am. Chem. Soc. 2005, 127, 1612-1613. (7) Xu, Z.; Xiao, Y.; Qian, X.; Cui, J.; Cui, D. Org. Lett. 2005, 7, 889-892. (8) (a) Wu, Q.; Anslyn, E. V. J. Am. Chem. Soc. 2004, 126, 14682-14683. (b) Gunnlaugsson, T.; Leonard, J. P.; Murray, N. S. Org. Lett. 2004, 6, 15571560. (c) Zeng, H. H.; Thompson, R. B.; Maliwal, B. P.; Fones, G. R.; Moffett, J. W.; Fierke, C. A. Anal. Chem. 2003, 75, 6807-6812. (9) Rurack, K. Spectrochim. Acta, A 2001, 57, 2161-2195.
tigation of film sensors is rare.10,11 To the best of our knowledge, however, all Cu2+ sensors reported have no selectivity for the counteranions of the metal ion. Moreover, in terms of practicability, a film sensor possesses more favorable properties than a molecular sensor used in solution. For example, film sensors have no contamination to the target systems, they can easily be made into devices, and they can be reused as well. However, molecular interaction on a surface exhibits conspicuous divergence from that in solution, and thereby, it should be given adequate attention. Chemical immobilization of small molecules onto a substrate surface is one of the important approaches to prepare monomolecular assemblies and provides a convenient way to produce surfaces with specific chemical functionalities that allow the precise tuning of surface properties.12 On the basis of this methodology, a series of fluorescent film sensors for various analytes, including nitrite,13 nitromethane,14 dicarboxylic acids,15 etc., in the aqueous phase, for the composition of the mixtures of ethanol and water,16 and for the purity of water17 have been prepared in our laboratory. It is to be noted that most of the sensing films reported by our group have been designed with a short or even no spacer, and the sensing principles of the films are based on the hydrophobic aggregation of the sensing element, pyrene, and its dependence on the presence of analytes. For the (10) (a) Zheng, Y.; Gatta´s-Asfura, K. M.; Li, C. Q.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. J. Phys. Chem. B 2003, 107, 483-488. (b) Zheng, Y.; Orbulescu, J.; Ji, X.; Andreopoulos, F. M.; Pham, S. M.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 2680-2686. (11) (a) Bronson, R. T.; Michaelis, D. J.; Lamb, R. D.; Husseini, G. A.; Farnsworth, P. B.; Linford, M. R.; Izatt, R. M.; Bradshaw, J. S.; Savage, P. B. Org. Lett. 2005, 7, 1105-1108. (b) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2005, 109, 4076-4083. (12) (a) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (b) Crego-Calama, M.; Reinhoudt, D. N. AdV. Mater. 2001, 13, 1171-1174. (13) Wang, H.; Fang, Y.; Cui, Y.; Hu, D.; Gao, G. Mater. Chem. Phys. 2002, 77, 185-191. (14) Wang, H.; Fang, Y.; Ding, L.; Gao, L.; Hu, D. Thin Solid Films 2003, 440, 255-260. (15) (a) Gao, L.; Fang, Y.; Wen, X.; Li, Y.; Hu, D. J. Phys. Chem. B 2004, 108, 1207-1213. (b) Gao, L.; Fang, Y.; Lu¨, F.; Ding, L. Sci. China, Ser. B 2004, 47, 240-250. (c) Gao, L.; Fang, Y.; Lu¨, F.; Cao, M.; Ding, L. Appl. Surf. Sci., in press. (d) Lu¨, F.; Fang, Y.; Gao, L.; Ding, L.; Jiang, L. J. Photochem. Photobiol., A 2005, 175, 207-213. (16) Ding, L.; Fang, Y.; Jiang, L.; Gao, L.; Yin, X. Thin Solid Films 2005, 478, 318-325. (17) Fang, Y.; Ning, G.; Hu, D.; Lu, J. J. Photochem. Photobiol., A 2000, 135, 141-145.
10.1021/la052866u CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005
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effective sensing of dicarboxylic acids, however, the sensing element, pyrene, must be bonded on a glass substrate surface via a relatively long, flexible spacer with imino groups in it.15 It was found that the length of the spacer and the number of imino groups in the spacer have a great effect upon the sensing performance of the film for the analytes. It might be interesting to imagine that an increase in the length of the relatively hydrophobic spacer may locate the molecules of the sensing element in a hydrophobic microenvironment provided the loading density of the spacer is adequately high and thereby protect it from contact of routine hydrophilic quenchers such as inorganic copper salts. Thus, selective sensing of organic copper salts from inorganic copper salts would be possible, which no one, to the best of our knowledge, has dealt with before. This is because complexation of Cu2+ by the imino groups in the spacer and the hydrophobic property of the anions in organic copper salts are favorable for the contact of Cu2+ with the fluorophore, pyrene. In contrast, it would be very difficult for Cu2+ in inorganic copper salts to come into contact with the sensing element, pyrene. On the basis of the above considerations and previous work in our group, we present here a film sensor, for which an even longer spacer has been introduced, for copper acetate and/or copper propionate. The film sensor shows high selectivity for organic copper salts over inorganic ones and other competing metal salts. The details are described in this paper. Experimental Section Reagents. Pyrenesulfonyl chloride (PSC) was synthesized by adopting a literature method.18 1,3-Diaminopropane (DAP; Fluka, 99%) and 3-glycidoxypropyltrimethoxysilane (GPTS; Acros, 97%) were used directly without further purification. All the solvents were of analytical grade and were used after further purification according to methods in the literature. The solutions of metal ions were prepared from Cu(Ac)2‚H2O, Cu(NO3)2‚3H2O, CuCl2, CuSO4, Pb(Ac)2‚3H2O, Zn(Ac)2‚2H2O, Cd(Ac)2, Ni(Ac)2‚4H2O, and Co(Ac)2‚3H2O and were dissolved in doubly distilled water. Ethylenediaminetetraacetic acid disodium salt (EDTA), sodium acetate, sodium propionate, and trichloroacetic acid were of analytical grade and were used without further purification. Instrumentation. The reflection-absorption infrared spectroscopy measurements of the films were conducted on a Nicolet Nexus 670 FTIR spectrometer, and their Raman spectra were recorded by using a Nicolet Almega laser scattering Raman spectrometer. The advancing contact angles of the films were measured by using a JY-82 contact angle goniometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCA PHI5400 (PerkinElmer) photoelectron spectrometer using a monochromatic Mg KR X-ray source. UV-vis absorption spectroscopy measurements were carried out on a Perkin-Elmer Lambda 950 UV-vis spectrometer. Fluorescence measurements were performed at room temperature on a time-correlated single-photon-counting Edinburgh Instruments FLS 920 fluorescence spectrometer. Film Preparation and Characterization. A glass slide was functionalized with a self-assembled monolayer of GPTS. Onto this epoxide-terminated monolayer were sequentially covalently attached a binding group and pyrene. The details of the preparation are similar to those reported before15a (Supporting Information, Figure S1). Each layer was characterized by advancing contact angle and FTIR, Raman, and XPS spectroscopy measurements, confirming the introduction of the components (cf. the Supporting Information, Table S1, Figures S4-S6). UV-vis spectroscopy was employed to evaluate the density of pyrene groups according to ref 19. The absorption at 350 nm was used to estimate the surface density of (18) Ezzell, S. A.; McCormic, C. L. In Water-soluble Polymers; Shalaby, S. W., McCormic, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; Chapter 8. (19) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 2229-2230.
Figure 1. (a) Chemical structure of the functional film. (b) Excitation and emission spectra of the film sensor in an aqueous medium. the group (F) from the Beer-Lambert law (F ) A-1), using the experimentally determined absorption coefficient of pyrenesulfonyl chloride in CH2Cl2 ( ) 13207 M-1‚cm-1). The resulting density is 4.2 pyrene groups/100 Å2, which is about 72% of the theoretical value (Supporting Information). The chemical structure and representative excitation and emission spectra of the film sensor are shown in Figure 1. It can be observed that each of the emission spectra is composed of two sharp peaks and one broad band, which may be attributed to the monomer emission and excimer emission of the fluorophore, respectively. It may be interesting to mention that, unlike the emission of pyrene in solution, the monomer emission of pyrene in the immobilized state lacks fine structures, upon which the “pyrene scale” 20,21 is based. The differences in the profile of the monomer emission may have originated from the different motion behaviors of pyrene in the two states. In the immobilized state, the orientation, rotation, and other motions of pyrene must be relatively restricted when compared with those in the solution state. Similar observations have been reported in the literature.11b,22,23 The stability of pyrene on the plate surface was examined by monitoring the fluorescence emission of the systems and the medium, within which the plate had been immersed, as a function of time at 380 nm with 350 nm as the excitation wavelength. It was found that the emission intensities and emission profiles of the film, the control, and the medium hardly changed with time, indicating that leaking of pyrene (20) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 20392044. (21) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560-2565. (22) Ulman, A. An Introduction to Ultrathin Organic films: From LangmuirBlodgett Films of Self-Assemblies; Academic Press: New York, 1991. (23) Fang, Y. Fluorescence Techniques in Colloid and Polymer Science; Shaanxi Normal University Press: Xi’an, China, 2002.
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Scheme 1. Schematic Demonstration of the Two-Dimensional Solution Model To Explain the Selective Quenching of the Emission of the Film by Copper Acetate
from the plate surface was negligible and should not affect the followup measurements.
Results and Discussion Sensing Properties and Mechanism Studies. As mentioned above, Cu2+ is one of the best-known quenchers which quenches, with high efficiency, the fluorescence emission of most fluorophores in aqueous solution.3-10,24 As for the film described in the present work, however, the quenching strongly depends on the chemical nature of the counteranions of Cu2+. Figure 2 depicts the plots of I0/I against the concentrations of various Cu2+ salts of different counteranions, where I0 and I stand for the fluorescence intensity of the film in the absence and presence of Cu2+ salts, respectively. It can be seen that, among the Cu2+ salts tested, only copper acetate quenched the fluorescence emission of the film significantly. The presence of Cu(NO3)2, CuCl2, or CuSO4 had little effect upon the emission of the film as evidenced by their weak association with the film (cf. the Supporting Information, Table S2, which gives the dissociation constants of the complexes of all tested copper salts with the spacer layer). The inset in Figure 2 shows the emission spectra of the film in the presence of different concentrations of copper acetate. The effects of counteranions on the sensing behavior of the film for Cu2+ may be understood by considering the possible microenvironment of pyrene moieties on the substrate surface. As shown in Figure 1a, pyrene was chemically attached to the substrate surface through a long flexible spacer. When immersed in a polar solvent, the less polar spacers would prefer to adopt relatively compact conformations. Therefore, it is not difficult to imagine that the spacer layer should be less hydrophilic than the bulk solution and less rigid than the substrate. Therefore, it may form an intermediate phase between the bulk solution and the solid substrate. Compared with the width and the length of the film, the thickness of the intermediate phase is so small that the phase can be nicknamed a “two-dimensional solution”. It is to be expected that the fluorophore moieties should be located within the “solution” due to their hydrophobic nature. On the basis of this model, the observations that copper acetate quenched the emission of the film in aqueous solution more efficiently than other Cu2+ salts with inorganic counteranions can be rationalized. Compared with inorganic anions, acetate is more compatible with the spacer layer, and thereby, Cu2+ has more chance to enter the layer due to charge balance and possible binding of the imino groups and quenches the emission of the fluorophore in the layer. The structure of the spacer layer and the possible quenching process of copper acetate of the emission of the film are schematically shown in Scheme 1. The explanation (24) (a) Perochon, E.; Tocanne, J. F. Chem. Phys. Lipids 1991, 58, 7-17. (b) Steiner, R. F.; Kirby, E. P. J. Phys. Chem. 1969, 73, 4130-4135.
was further supported by the results from XPS measurement. Figure 3 compares the XPS spectra of the film treated with copper acetate (a) and that of the blank or that treated with Cu(NO3)2 (b). Clearly, spectrum a is characterized by the signals of Cu 2p1/2 and Cu 2p3/2, and in contrast, spectrum b has no corresponding signals, indicating that the film treated with copper acetate contains Cu2+. Considering the fact that the method used for the treatment of the blank films, in which the films were immersed in an aqueous solution of copper acetate or copper nitrate (4 mM) for 20 min, and then rinsed thoroughly with plenty of water before XPS measurement, it was clear that the physically adsorbed copper ions should have been washed out, and the remaining copper ions were those in a binding state. This supports the tentative conclusion that copper acetate entered the spacer layer.
Figure 2. I0/I against the concentration of different copper salts. Inset: Fluorescence emission spectra of the film in the presence of different concentrations of copper acetate (from top to bottom, 0, 1.6, 2.4, 3.2, 4.8, 6.4, and 8 mM) in an aqueous medium.
Figure 3. XPS spectra of the film treated with Cu(Ac)2 (a) and the blank or that treated with Cu(NO3)2 (b) in the Cu (2p) region.
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Figure 4. Effects of sodium acetate, sodium propionate, and sodium trichloroacetate upon the sensing properties of the film for Cu(NO3)2 (A, 0.2 M Cu(NO3)2, 100 µL; B, 0.2 M NaAc; C, 0.2 M CH3CH2COONa; D, 0.2 M CCl3COONa; E, H2O; for the systems without Cu(NO3)2, water was used instead of the copper salt).
Figure 5. Response of the film sensor to various acetate salts (λex/ λem ) 350/500 nm).
To further confirm the effect of acetate on the quenching efficiency of Cu2+ of the emission of the film, a specially designed experiment was conducted in which sodium acetate was intentionally introduced into a system of Cu(NO3)2. As reported earlier, the presence of Cu(NO3)2 in water had little effect upon the emission of the film. Introduction of sodium acetate, however, enhanced the quenching significantly (cf. Figure 4). A similar result was obtained by using sodium propionate instead of sodium acetate (cf. Figure 4). Considering the results obtained from systems containing both sodium acetate and Cu(NO3)2, both sodium propionate and Cu(NO3)2, and only sodium acetate (cf. Figure 4), it can be concluded that (1) Cu2+ is the only quenching species, (2) acetate functions as a promoter to enhance Cu2+ entering the spacer layer, and (3) it is the two-dimensional solution that screens the contact of Cu2+ salts with inorganic counteranions with the fluorophore and thereby brings the film the abnormal selectivity for organic copper salts. These conclusions were further supported by the results from studies of the sodium trichloroacetate effect upon the response of the film to Cu(NO3)2. As expected, the anion behaves like a half inorganic-half organic anion, as that shown in Figure 4. The conclusion that Cu2+ is the real quenching species is also supported by comparative studies by using other metal acetates, including Pb2+, Co2+, Cd2+, Zn2+, Ni2+, etc. It was found that the quenching efficiencies are predominately connected to the nature of the metal ions, and again, only copper acetate showed a significant quenching effect on the emission of the film (cf. Figure 5).
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Figure 6. I0/I vs the concentration of Cu(NO3)2 in methanol and in water (λex/λem ) 350/500 nm).
Figure 7. Quenching plots of the fluorescence intensity and fluorescence lifetime of the film in an aqueous medium at room temperature.
Properties of the Spacer Layer. To gain further insights into the nature of the spacer layer, a titration experiment in methanol was also performed. The result is shown in Figure 6. It is not surprising that, in this case, the quenching efficiency of Cu(NO3)2 is greatly increased if compared with that in the aqueous phase, in support of the hypothesis that the spacer layer is relatively hydrophobic. The result also suggests that the spacer layer screening effect could be altered by varying the solvent. Quenching Mechanism. Considering the imino groups existing within the spacer layer, the quenching by copper acetate of the emission of the film should be static in nature due to possible complexation of the spacer to the quencher, Cu2+. It is known that static quenching is characterized by a constant fluorescence lifetime, τ; that is, the value is independent of the quencher concentration.25 For dynamic quenching, however, τ0/τ ) I0/I, where τ0 and τ stand for the fluorescence lifetimes of the fluorophore in the absence and presence of the quencher, respectively. Fluorescence lifetime measurements demonstrated that introduction of copper acetate into the solution had little effect upon the fluorescence lifetime of the film (cf. Figure 7), indicating that the quenching is static in nature. This result also indicates that the driving force for Cu2+ to enter the spacer layer is not only the effect of organic counteranions but also the complexation of the imino groups existing within the spacers to the metal ions. Moreover, it is likely that the complexation limited the diffusion of the quencher, Cu2+, and thereby, the quenching is static in nature. (25) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum Publishers: New York, 1999; Chapter 8.
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Conclusion
Figure 8. Reversibility of the film sensor for copper acetate.
Reversibility of the Quenching Process. To test the reversibility of the film sensor for Cu2+, the film was alternatively exposed to a solution of copper acetate and pure water, and the corresponding fluorescence emission was measured. After each measurement of the salt solution, the film was washed with an EDTA solution and pure water several times. It was found that the emission of the film could be fully restored (cf. Figure 8). In contrast, the emission could not be restored when the film was washed with just pure water even if the washing was repeated more than 10 times, in support of the statement that the quencher, Cu2+, was not simply dissolved in the layer, but complexed by the layer.
In the present work, a novel selective fluorescent film sensor for organic copper salts, especially copper acetate and copper propionate, was designed and studied. It was demonstrated that the uncommon selectivity originated from the screening effect of the spacer layer on inorganic ions. The quenching by copper acetate of the emission of the film is static in nature due to complexation of the spacers to the metal ions. To the best of our knowledge, the film sensor may be the first one that can differentiate greasy copper salts from inorganic copper salts. As a chemical sensor, not only is the selectivity of the film for organic copper salts good, but also the response of it to the salts is fully reversible. The sensitivity, however, of the film to the salts is limited. Further work on a new film sensor for organic copper salts based upon the same principles are in progress in our group to improve the sensitivity of the sensor. Acknowledgment. We thank the National Natural Science Foundation of China (Grants 20373039; 20543002), the Doctoral Program Foundation of the Ministry of Education of China (Grant 20040718001), and the Ministry of Science and Technology of China (Grant 2003BA310A05) for financial support. Supporting Information Available: Synthesis of the films, characterization data, calculation of the density of pyrene groups, and dissociation constants of the tested copper salts. This material is available free of charge via the Internet at http://pubs.acs.org. LA052866U
