Langmuir 2008, 24, 8387-8392
8387
Amplified Fluorescence Response of Chemosensors Grafted onto Silica Nanoparticles Sara Bonacchi,† Enrico Rampazzo,† Marco Montalti,*,† Luca Prodi,† Nelsi Zaccheroni,† Fabrizio Mancin,‡ and Pierluigi Teolato‡ Department of Chemistry “G. Ciamician”, UniVersity of Bologna, Via Selmi 2, I-40126 Bologna, Italy, and Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy ReceiVed March 10, 2008 In conventional fluorescent chemosensors, the recognition of the target by the receptor unit affects the fluorescence properties of a single covalently coupled fluorescent moiety. Here we show for the first time that when a suitable TSQ derivative is densely grafted onto the surface of preformed silica nanoparticles electronic interactions between the individual chemosensor units enable the free units to recognize the state of the surrounding complexed ones. As a result, the fluorescence transduction is not limited to the local site where binding occurs, but it involves a wider region of the fluorophore network that is able to transfer its excitation energy to the complexed units. Such behavior leads to an amplification of the fluorescence signal. What we report here is the first example of amplification in the case an off-on chemosensor due to its organization onto the surface of silica nanoparticles. We also describe a simple general model to approach amplification in multifluorophoric systems based on the localization of the excited states, which is valid for assemblies such as the supramolecular ones where molecular interactions are weak and do not significantly perturb the individual electronic states. The introduction of an amplification factor f in particular allows for a simple quantitative estimation of the amplification effects.
Introduction Together with specificity, increased sensitivity is the main goal in the design of new families of chemosensors.1,2 The supramolecular approach offers an efficient strategy for the modular control of both properties by locating the signaling and receptor functions in dedicated sites of the architecture and controlling electronic communication between the components. In the simplest devices, the complexation process occurring at the level of a single receptor site modifies the fluorescence signal of only the dye molecule that is directly linked to it. However, in more complex systems it is possible to control the deactivation path of the excited states of several fluorophores simultaneously by changing the status of a single binding site. In these structures, a much larger variation in the fluorescence signal is expected, as demonstrated by the complete quenching of a dendrimer bearing 64 fluorescent units upon the complexation of a single cobalt ion.3 Although these types of supramolecular structures are interesting, the complexity of their synthesis may be discouraging, especially when low-cost final materials are required. Nanoparticles modified with organic molecules have been shown to * Corresponding author. E-mail:
[email protected]. † University of Bologna. ‡ University of Padova. (1) See, for example, (a) Fluorescent Chemosensors of Ion and Molecule Recognition; Desvergne, J.-P., Czarnik, A. W., Eds.; NATO-ASI Series; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996. (b) Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A. W., Ed.; American Chemical Society: Washington, DC, 1992. (c) Fabbrizzi, L. Coord. Chem. ReV. 2000, 205, 1–232. (d) 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. (e) Prodi, L. New J. Chem. 2005, 29, 20–31. (2) For recent examples, see. (a) Yuichiro, K.; Yasuteru, U.; Suguru, K.; Hirotatsu, K.; Tetsuo, N. J. Am. Chem. Soc. 2007, 129, 10324–10325. (b) Juewen, L.; Brown, A. K.; Meng, X.; Cropek, D. M.; Istok, J. D.; Watson, D. B.; Lu, Y. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2056–2061. (c) Li-Juan, F.; Wayne, J. E., Jr J. Am. Chem. Soc. 2006, 128, 6784–6785. (d) Feng, H.; Yuhui, B.; Zhigang, Y.; Thomas, M. F.; Jianzhang, Z.; Xiaojun, P.; Jiangli, F.; Yunkou, W.; Shiguo, S. Chemistry 2007, 13, 2880–2892. (e) Shengju, O.; Zhihua, L.; Chunying, D.; Haitao, Z.; Zhiping, B. Chem. Commun. 2006, 4392, 4. (3) Vogtle, F.; Gestermann, S.; Kauffmann, C.; Ceroni, P.; Vicinelli, V.; Balzani, V. J. Am. Chem. Soc. 2000, 122, 10398–10404.
be an intriguing alternative in this context.4 These nanostructures can host tens of thousands of functional moieties, can be easily prepared under mild conditions, and, if properly designed, can act as efficient nanosensors. In previous papers, we demonstrated that silica nanoparticles are not merely a scaffold or container in which a large number of functional moieties can be collected and segregated from the environment but rather a suitable architecture in which the different components, once autoassembled, are able to communicate with each other5,6 to perform complex functions no differently from the way in which dendrimers or other complicated supramolecular systems do. In this article, we show how the self-assembly of a large number of units of a modified chemosensor on the surface of preformed silica nanoparticles can be exploited to prepare nanosensors for metal ions with an amplified response to complexation. To show evidence of and clearly explain the characteristic response of structures such as dendrimers, nanoparticles, and conjugated polymers to complexation, we extended the concept of signal amplification to cases in which luminescence quenching takes place as already proposed in the literature by other authors.5c Of course, this is appropriate when considering the relative variation of the signal but obviously does not apply when the signal intensity is taken into account by itself. Besides the purely terminological aspects, from an applicative point of view the amplification of (4) (a) Wang, L.; Wang, K. M.; Santra, S.; Zhao, X. J.; Hilliard, L. R.; Smith, J. E.; Wu, J. R.; Tan, W. H. Anal. Chem. 2006, 78, 646–654. (b) Mancin, F.; Rampazzo, E.; Tecilla, P.; Tonellato, U. Chem.sEur. J. 2006, 12, 1844–1854. (c) Burns, A.; Ow, H.; Wiesner, U. Chem. Soc. ReV. 2006, 35, 1028–1042. (d) Wang, L.; Tan, W. H. Nano Lett. 2006, 6, 84–88. (e) Buck, S. M.; Koo, Y. E. L.; Park, E.; Xu, H.; Philbert, M. A.; Brasuel, M. A.; Kopelman, R. Curr. Opin. Chem. Biol. 2004, 8, 540–546. (f) Van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921–2931. (5) (a) Montalti, M.; Prodi, L.; Zaccheroni, N.; Falini, G. J. Am. Chem. Soc. 2002, 124, 13540–13546. (b) Montalti, M.; Prodi, L.; Zacheroni, N.; Zattoni, A.; Reschiglian, P.; Falini, G. Langmuir 2004, 20, 2989–2991. (c) Montalti, M.; Prodi, L.; Zaccheroni, N. J. Mater. Chem. 2005, 15, 2810–2814. (d) Rampazzo, E.; Bonacchi, S.; Montalti, M.; Prodi, L.; Zaccheroni, N. J. Am. Chem. Soc. 2007, 129, 14251–14256. (6) (a) Rampazzo, E.; Brasola, E.; Marcuz, S.; Mancin, F.; Tecilla, P.; Tonellato, U. J. Mater. Chem. 2005, 15, 2687–2696. (b) Brasola, E.; Mancin, F.; Rampazzo, E.; Tecilla, P.; Tonellato, U. Chem. Commun. 2003, 3026–3027.
10.1021/la800753f CCC: $40.75 2008 American Chemical Society Published on Web 06/20/2008
8388 Langmuir, Vol. 24, No. 15, 2008 Scheme 1. Chemical Formula of the Fluorescent Silane 1H and the Reference Compound 2H (Left) and Schematic Representation of Silica Nanoparticles Covered with Silane 1H (N, Right)
Bonacchi et al.
on the self-organization of 1H molecules on the surface of preformed silica nanoparticles yields nanostructures where intermolecular interactions are much more efficient, representing a major step forward in chemosensor design.
Experimental Section
the fluorescence signal is clearly much more attractive than quenching but unfortunately much more difficult to achieve. In this article, we present a proof-of-principle system that demonstrates how, on the basis of an amplification scheme inspired by the one proposed by Swager for conjugated polymers,7 it is possible to design nanoparticles bearing luminescent units on their surface that show an amplified off-on response to metal ion complexation. In this context, we emphasize that our approach to the issue of amplification is based on a supramolecular philosophy and assume that the electronic states are still localized on the molecular components even in organized systems. From this point of view, our approach differs from the one proposed for conjugated polymer for which an excitonic model is surely more suitable. Ligand 1H contains a 6-methoxy-8-p-toluensulfonamidequinoline (TSQ) and was selected because of its well-known ability to coordinate zinc ions selectively.8 This complexation process takes place together with deprotonation of the sulfonamidic group and leads to a red shift of the fluorescence band and a strong increase in its intensity. The organization of TSQ units in a multichromophoric network (Scheme 1) therefore appears to be suitable for achieving an amplification of the response to zinc complexation. According to Scheme 2, the complexed units may in fact behave as strongly luminescent energy traps toward which the excitation energy collected by the whole system is conveyed. As a consequence, not only would the fluorescence of the moieties directly involved in the coordination be switched on but the surrounding uncomplexed units would also generate an intense fluorescence signal by transferring their excitation energy to the luminescent complexes. Of course a mechanism of this kind requires very efficient electronic communication between the donor and acceptor components and hence a suitable average distance between the TSQ moieties. In a previous paper,9 we reported that a condition of this kind could not be achieved in silica nanoparticles doped with ligand 1H because these materials behave as sensors for zinc ions but do not showing an amplified response. Here we show that a different strategy based (7) (a) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 12389–12390. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537–2574. (c) Levitsky, I. A.; Krivoshlykov, S. G.; Grate, J. W. J. Phys. Chem. B 2001, 105, 8468–8473. (d) Me´allet-Renault, R.; Pansu, R.; Amigoni-Gerbierb, S.; Larpent, C. Chem. Commun. 2004, 2344–2345. (8) (a) Fahrni, C. J.; O’Halloran, T. V. J. Am. Chem. Soc. 1999, 121, 11448– 11458. (b) Jiang, P.; Guo, Z. Coord. Chem. ReV. 2004, 248, 205–229. (c) Kikuchi, K.; Komatsu, K.; Nagano, T. Curr. Opin. Chem. Biol. 2004, 8, 182–191. (d) Xue, G. P.; Bradshaw, J. S.; Dalley, N. K.; Savage, P. B.; Izatt, R. M.; Prodi, L.; Montalti, M.; Zaccheroni, N. Tetrahedron 2002, 58, 4809–4815. (9) Teolato, P.; Rampazzo, E.; Arduini, M.; Mancin, F.; Tecilla, P.; Tonellato, U. Chem.sEur. J. 2007, 8, 2238–2245.
All reagents were purchased from Aldrich and were used without further purification. Spectroscopic-grade DMSO was purchased from Merck. Compounds 1H and 2H were synthesized according to ref 9. Ultrapure water was obtained using a Milli-Q system by Millipore. Synthesis of the Nanoparticles. The grafting of 1H onto silica nanoparticles was carried out in acid conditions. Silane precursor 1H (4.5 mg, 7.6 µmol) was dissolved in 1.5 mL of ethanol. Acetic acid (1.0 mL), silica nanoparticles Ludox AS-30 (0.1 mL), and Milli-Q grade water (1.0 mL) were then added. The resulting mixture was stirred at 80 °C for 48 h. Surface-modified nanoparticles were isolated by centrifugation (10 000 rpm for 30 min) after the addition of 7.0 mL of acetonitrile. To completely eliminate the unreacted silane, the white solid was washed with 7.0 mL of acetonitrile, 7.0 mL of aqueous NaHCO3 (pH 8), and finally with 7.0 mL of water (MilliQ). Centrifugation was repeated after each step. Finally, the resulting nanoparticles were dispersed in 7.0 mL of DMSO. TEM Images. For TEM investigations, a drop of nanoparticles in DMSO solution was transferred onto holey carbon foils supported on conventional copper microgrids. A Philips CM 100 transmission electron microscope operating at 80 kV was used. Photophysical Measurements. The fluorescence spectra were recorded using an Edinburgh FLS920 spectrophotometer equipped with a Hamamatsu R928P photomultiplier. The same instrument equipped with a PCS900 PC card was used for the time-correlated single photon counting (TCSPC) experiments. All of the photophysical measurements were performed in aerated DMSO solutions. Stability Constants. Stability constants were determined by fitting the absorption spectra recorded during the titration of nanoparticles N or reference compound 2H in DMSO with increasing amounts of Zn2+, Cd2+, Co2+, and Cu2+ (perchlorate salts). Solutions with the same concentration of TSQ molecules (1 × 10-5 M) were used, and the solutions of metal ions were 1 × 10-3 M. Global analysis program SPECFIT was used.10 The affinity of 2H for Cd2+, Co2+, and Cu2+ is very low, and only limiting values for the association constants could be obtained in these cases.
Definition of the Amplification Factor When a dilute solution (namely, one having an absorbance A
