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Langmuir 2007, 23, 8632-8636
Self-Organized Fluorescent Nanosensors for Ratiometric Pb2+ Detection Maria Arduini,† Fabrizio Mancin,*,† Paolo Tecilla,‡ and Umberto Tonellato† Dipartimento di Scienze Chimiche and CNR-ITM, UniVersita` di PadoVa, Via Marzolo 1, I -35131 PadoVa, and Dipartimento di Scienze Chimiche, UniVersita` di Trieste, Via Giorgieri 1, I-34127 Trieste, Italy ReceiVed April 4, 2007 Silica nanoparticles (60 nm diameter) doped with fluorescent dyes and functionalized on the surface with thiol groups have been proved to be efficient fluorescent chemosensors for Pb2+ ions. The particles can detect a 1 µM metal ion concentration with a good selectivity, suffering only interference from Cu2+ ions. Analyte binding sites are provided by the simple grafting of the thiol groups on the nanoparticles. Once bound to the particles surface, the Pb2+ ions quench the emission of the reporting dyes embedded. Sensor performances can be improved by taking advantage of the ease of production of multishell silica particles. On one hand, signaling units can be concentrated in the external shells, allowing a closer interaction with the surface-bound analyte. On the other, a second dye can be buried in the particle core, far enough from the surface to be unaffected by the Pb2+ ions, thus producing a reference signal. In this way, a ratiometric system is easily prepared by simple self-organization of the particle components.
Introduction Self-organization on templates is a new and attractive strategy for the easy preparation, modification, and optimization of fluorescence chemosensors.1 In this approach, fluorescent dyes and receptors are synthesized separately and then assembled on the surface of a proper template. In the assembly, the two sensor components are close enough to actively interact and convert the analyte recognition by the receptor units into a variation of the emission properties of the dyes. The synthetic work needed to prepare the system, which is often the most demanding task in the development of new chemosensors, is reduced to the minimum. In fact, it is only necessary to prepare simple subunits, while their organization and, in some cases, even the formation of the substrate binding sites2 are granted by the grafting to the template. Moreover, modification or optimization of the sensor simply requires the variation of the component ratio or the substitution of a single component.3 To date, different templates have been proposed to direct the assembly of the sensor components, ranging from surfactant aggregates4 to quartz surfaces2b,5 and nanoparticles.6 Among these, silica nanoparticles are particularly attractive for their peculiar features: they are easy to * To whom correspondence should be addressed. E-mail: fabrizio.
[email protected]. † Universita ` di Padova. ‡ Universita ` di Trieste. (1) (a) Mancin, F.; Rampazzo, E.; Tecilla, P.; Tonellato, U. Chem.sEur. J. 2006, 12, 1844-1854. (b) Arduini, M.; Rampazzo, E.; Mancin, F.; Tecilla, P.; Tonellato U. Inorg. Chim. Acta 2007, 360, 721-727. (2) (a) Arduini, M.; Marcuz, S.; Montolli, M.; Rampazzo, E.; Mancin, F.; Gross, S.; Armelao, L.; Tecilla, P.; Tonellato, U. Langmuir 2005, 21, 93149321. (b) Crego-Calama, M.; Reinhoudt, D. N. AdV. Mater. 2001, 13, 11711174. (3) (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. (4) (a) Diaz-Fernandez, Y.; Foti, F.; Mangano, C.; Pallavicini, P.; Patroni, S.; Perez-Gramatges, A.; Rodriguez-Calvo, S. Chem.sEur. J. 2006, 12, 921-930. (b) Diaz-Fernandez, Y.; Perez-Gramatges, A.; Amendola, V.; Foti, F.; Mangano, C.; Pallavicini, P.; Patroni, S. Chem. Commun. 2004, 1650-1651. (c) DiazFernandez, Y.; Perez-Gramatges, A.; Rodriguez-Calvo, S.;Mangano, C.; Pallavicini, P. Chem. Phys. Lett. 2004, 398, 245-249. (d) Zheng, Y.; Orbulescu, J.; Ji, X.; Andreopulos, F. M.; Pham, S. M.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 2680-2686. (e) Berton, M.; Mancin, F.; Stocchero, G.; Tecilla, P.; Tonellato U. Langmuir 2001, 17, 7521-7528. (f) Grandini, P.; Mancin, F.; Tecilla, P.; Scrimin, P.; Tonellato, U. Angew. Chem., Int. Ed. 1999, 38, 3061-3064.
prepare, transparent to light, and biocompatible. Moreover, they can be doped or chemically modified with organic molecules and engineered into compartments performing different functions.7 Independently from the template used, almost all the examples reported of template-based self-organized chemosensors deal with the sensing of Cu2+, which is somehow an “easy-to-catch” metal ion.1 In fact, Cu2+ is a powerful fluorescence quencher, and being at the top of the Irving-Williams series, it is selectively recognized by almost any ligand. The development of selforganized chemosensors able to detect more interesting and challenging species is therefore essential to prove the real usefulness of such systems. The ability of thiols and thiol-functionalized silica materials to sequestrate transition-metal ions is well-known.8 In this paper, we report a silica nanoparticle-based chemosensor capable of reporting the presence of Pb2+ ions. In our system, several sensor featuressworking scheme, binding sites, sensitivity, and ratiometric behaviorstake full advantage of the self-organization and structural arrangement of the particles, giving a clear example of the flexibility and broad applicability of the template-based self-organized approach. Experimental Section General Procedures. Solvents and commercially available reagents were used as received. Elemental analyses were performed by the Laboratorio di Microanalisi of the Dipartimento di Scienze Chimiche of the Universita` di Padova. The solutions used in the spectrophotometric measurements and titrations were prepared using (5) Basabe-Desmonts, L.; Beld, J.; Zimmerman, R. S.; Hernando, J.; Mela, P.; Garcı´a Parajo´, M. F.; van Hulst, N. F.; van den Berg, A.; Reinhoudt, D. N.; Crego-Calama, M. J. Am. Chem. Soc. 2004, 126, 7293-7299. (6) (a) Me´allet-Renault, R.; Pansu, R.; Amigoni-Gerbierb, S.; Larpent, C. Chem. Commun. 2004, 2344-2345. (b) Ga´ttas-Asfura, K. M.; Leblanc, R. M. Chem. Commun. 2003, 2684-2685. (c) Chen, Y.; Rosenzweig, Z. Anal. Chem. 2002, 74, 5132-5138. (7) (a) Burns, A.; Owb, H.; Wiesner, U. Chem. Soc. ReV. 2006, 35, 10281042. (b) Kim, S.; Pudavar, H. B.; Prasad, P. N. Chem. Commun. 2006, 20712073. (c) Santra, S.; Xu, J.; Wang, K.; Tan, W. J. Nanosci. Nanotechnol. 2004, 4, 590-599. (d) Prasad, P. N. Curr. Opin. Solid State Mater. Sci. 2004, 8, 11-19. (e) Van Blaaderen, A.; Vrij, A. Langmuir 1992, 8, 2921-2931. (8) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923-926.
10.1021/la700971n CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007
Fluorescence Nanosensors deionized water (R > 18 MΩ), obtained with a Milli-Q (Millipore) purification system. UV-vis absorption measurements were performed on a Perkin-Elmer Lambda 45 spectrophotometer equipped with a thermostated cell holder (1 cm quartz cells). Fluorescence spectra were recorded on a Perkin-Elmer LS-55 spectrometer equipped with a thermostated cell holder (1 cm quartz cells). Transmission electron microscopy (TEM) experiments were performed at the CSPA of the Universita` di Trieste. TEM images of the particles were obtained with a Philips EM 208 transmission electron microscope operating at 100 keV. Samples for TEM were prepared by spreading a drop of nanoparticle solution in EtOH (∼1 mg/mL) onto standard carbon-coated copper grids (200 mesh). Dimensional analysis of nanoparticles from TEM images was performed by using the Image J software.9 Dynamic light scattering (DLS) measurements were obtained with a Particle Sizing Systems Nicomp model 370 correlator equipped with a thermostated cell holder and a Spectra Physics series 2016 Ar laser operating at 488 nm. Hydrodynamic particle diameters were obtained from cumulant fits of the autocorrelation functions at 90° scattering angle. Dyes 1 and 2 (Chart 1) were synthesized as previously reported.3a Tetraethoxysilane (TEOS) and (mercaptopropyl)triethoxysilane (MPS) were Aldrich products used as received. Spectrophotometric Titrations. The total molar concentration of dye 1 in the nanoparticle ethanol suspensions resulting from the synthetic procedures was determined from their absorbance (measured on highly diluted samples to minimize light scattering from nanoparticles) using the values of the corresponding model in ethanol at 330 nm (4650 ( 30 M-1 cm-1).2a The corresponding weight on volume nanoparticle concentrations were determined by solvent evaporation under vacuum. Pb(NO3)2, Cu(NO3)2, Zn(NO3)2, Ni(NO3)2, CoCl2, HgCl2, CaCl2, and MgCl2 were analytical grade products. Metal ion stock solutions were titrated against EDTA following standard procedures. The buffer 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES; Aldrich) was used as received. The desired amounts of ethanol nanoparticle suspensions were transferred in fluorescence quartz cells, and the appropriate amounts of buffer, water, and ethanol were added to reach a final volume of 2 mL. Small volumes (up to 50 µL) of concentrated metal ion solutions were then added, and the fluorescence spectra were recorded after each addition. Titrations with Pb2+ were performed on a freshly prepared nanoparticle solution and 30 days after the purification with identical results. Ellman’s Assay.10 The nanoparticle suspensions (50 µL) in ethanol were added to a UV quartz cell containing 2.0 mL of 0.05 M buffer solution (sodium phosphate, pH 7.0). A 10 µL portion of a 3.0 mM solution of 5,5′-dithiobis(2-nitrobenzoate)sEllman’s reagentsin ethanol was then added, and the cell was incubated for 90 min (or until no further changes in the UV-vis spectrum were detected). The concentration of thiol groups is obtained by the absorbance at 412 nm ( ) 13700 cm-1 M-1). Dye-Doped Silica Nanoparticles np0. A solution of dye 1 (4.0 mg, 0.010 mmol) in ethanol (20 mL) was prepared in a thermostated vessel. TEOS (400 µL, 1.78 mmol) and ammonia (1.0 mL, 22% water solution) were subsequently added, and the reaction mixture was vigorously stirred at 25 °C for 16 h. The resulting nanoparticle suspension was filtered through a 0.45 µm filter membrane, diluted to 70 mL with ethanol, and transferred into a 75 mL Amicon ultrafiltration cell equipped with a 10 kDa regenerated cellulose membrane. The mixture was concentrated and rediluted, under a pressure of 4 bar, until the UV-vis spectrum of the filtrate showed the absence of 1 absorption. The retentate solution was finally filtered through a 0.22 µm filter membrane. Dye-Doped Silica Nanoparticles np1. A solution of dye 1 (6.0 mg, 0.015 mmol) in ethanol (20 mL) was prepared in a thermostated vessel. TEOS (400 µL, 1.78 mmol) and ammonia (1.0 mL, 22% (9) Rasband, W. Image J 1.32j; Research Services Branch, National Institute of Mental Health: Bethesda, MD (http://rsb.info.nih.gov). (10) Whitesides, G. M.; Lilburn, J. E.; Szajewski, R. J. Org. Chem. 1977, 42, 332-338.
Langmuir, Vol. 23, No. 16, 2007 8633 water solution) were subsequently added, and the reaction mixture was vigorously stirred at 25 °C for 16 h. After this time, (mercaptopropyl)triethoxysilane (MPS; 60 µL, 0.32 mmol) and TEOS (30 µL, 0.13 mmol) were added to the suspension. The stirred reaction mixture was kept at room temperature for about 1 h and then heated at 90 °C for 3 h. The warm resulting nanoparticle suspension was filtered through a 0.45 µm filter membrane, diluted to 70 mL with ethanol, and transferred into a 75 mL Amicon ultrafiltration cell equipped with a 10 kDa regenerated cellulose membrane. The mixture was concentrated and rediluted, under a pressure of 4 bar, until the UV-vis spectrum of the filtrate showed the absence of 1 absorption. The retentate solution was finally filtered through a 0.22 µm filter membrane. Dye-doped Core-Shell Silica Nanoparticles np2. TEOS (200 µL, 0.89 mmol) and ammonia (0.9 mL, 22% water solution) were added to 20 mL of ethanol in a 25 °C thermostated vessel. The reaction mixture was vigorously stirred for 4 h, and then a second portion of TEOS (200 µL, 0.89 mmol) and a second portion of ammonia (0.9 mL, 22% water solution) were added. The reaction mixture was stirred for another 4 h (DLS analysis yielded a diameter of 52 ( 13 nm for the resulting silica cores). Fluorescent dye 1 (4.0 mg, 0.010 mmol), TEOS (200 µL, 0.89 mmol), and ammonia (0.3 mL, 22% water solution) were subsequently added, and the mixture was stirred at 25 °C for an additional 16 h (DLS analysis yielded a diameter of 60 ( 11 nm for the resulting core-shell silica particles). After this time, MPS (60 µL, 0.32 mmol) and TEOS (30 µL, 0.13 mmol) were added to the suspension. The stirred reaction mixture was kept at room temperature for about 1 h and then heated at 90 °C for 3 h. The warm resulting nanoparticle suspension was filtered through a 0.45 µm filter membrane, diluted to 70 mL with ethanol, and transferred into a 75 mL Amicon ultrafiltration cell equipped with a 10 kDa regenerated cellulose membrane. The mixture was concentrated and rediluted, under a pressure of 4 bar, until the UVvis spectrum of the filtrate showed the absence of 1 absorption. The retentate solution was finally filtered through a 0.22 µm filter membrane. Dye-Doped Multishell Silica Nanoparticles np3. Fluorescent dye 2 (3 mg, 0.007 mmol) was dissolved in 20 mL of ethanol in a 25 °C thermostated vessel. The solution was stirred for 30 min, and then ammonia (0.9 mL, 22% water solution) was added. After 1 h, TEOS (200 µL, 0.89 mmol) was added, and the reaction mixture was vigorously stirred for 4 h (DLS analysis yielded a diameter of 39 ( 23 nm for the resulting particles). The resulting nanoparticle suspension was filtered through a 0.45 µm filter membrane. A second portion of TEOS (200 µL, 0.89 mmol) and a second portion of ammonia (1.4 mL, 22% water solution) were added, and the reaction mixture was stirred for another 4 h (DLS analysis yielded a diameter of 54 ( 17 nm for the resulting core-shell silica particles). After this time, fluorescent dye 1 (4.0 mg, 0.010 mmol), a third portion of TEOS (200 µL, 0.89 mmol), and a third portion of ammonia (1.4 mL, 22% water solution) were added, and the mixture was vigorously stirred for 16 h at 25 °C (DLS analysis yielded a diameter of 60 ( 4 nm for the resulting multishell particles). MPS (60 µL, 0.32 mmol) and TEOS (30 µL, 0.13 mmol) were added to the suspension. The stirred reaction mixture was kept at room temperature for about 1 h and then heated at 90 °C for 3 h. The warm resulting suspension was filtered through a 0.45 µm filter membrane, diluted to 70 mL with ethanol, and transferred into a 75 mL Amicon ultrafiltration cell equipped with a 10 kDa regenerated cellulose membrane. The mixture was concentrated and rediluted, under a pressure of 4 bar, until the UV-vis spectrum of the filtrate showed the absence of 1 and 2 absorption. The retentate solution was finally filtered through a 0.22 µm filter membrane.
Results and Discussion Silica nanoparticles np1 (51 ( 19 nm diameter, Chart 1), doped with dansyl dye 1 and surface coated with MPS (Chart 1), were prepared following the procedure of Sto¨ber11 with the (11) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69.
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Table 1. Nanoparticle Diameters (D) of Dye-Doped Silica Nanoparticles As Obtained by TEM and DLS, Nanoparticle Concentrations of the Samples (C), Amount of Thiol ([SH]), Determined by Ellman Assay, Total Sulfur ([S]), and Elemental Analysis elemental analysis np
DTEM, nm
DDLS, nm
C,a mg/mL
[SH]b, M
[S],c M
C
H
N
S
0 1
51 ( 19
31 ( 11 67 ( 22
2.78 0.89
0 9.9 × 10-4
4.76 8.10
2.01 1.97
0.30 0.18
0.32 3.74
2
59 ( 11
57 ( 4
1.62
2.0 × 10-3
7.68
1.87
0.14
4.05
3
61 ( 11
62 ( 2
1.70
0 7.9 × 10-5 (3.9 × 10-5) 1.8 × 10-4 (1.0 × 10-4) 2.1 × 10-4 (1.1 × 10-4)
2.2 × 10-3
7.16
2.45
0.22
4.28
a Determined by solvent evaporation of known amounts of particle solutions resulting from ultrafiltration. b Determined by the Ellmann assay immediately after the purification of the nanoparticles. Results of the repetition of the assays 30 days after the purification of the nanoparticles are reported in parenthesis. c Determined by elemental analysis. The sulfur amount due to MPS units is calculated by subtracting the contribution of dye 1 (proportional to the N amount) from the total amount of sulfur determined with elemental analysis.
Chart 1. Fluorescent Dyes, Schematic Representation of the Nanoparticle Structures, and Corresponding TEM Imagesa
Figure 1. Spectrofluorimetric titration of 0.08 mg/mL solutions of np0 (O), np1 (b), and np2 (0) with Pb(NO3)2 in 10% ethanol/ water. Conditions: [1] ) 0.9 × 10-5 M, HEPES buffer, 0.01 M, pH 7, 25 °C, λexc ) 292 nm, λem ) 500 nm.
subsequent modifications of van Blaaderen.7e,12 The np1 nanoparticles are soluble in polar solvents (ethanol, water), and their absorption and emission spectra are typical of the dansyl fluorophore (Supporting Information). The position of the emission maximum at 500 nm indicates that the dye molecules embedded in the particles experience a moderately polar environment and water is effectively excluded from the particle core.13 The external MPS layer was investigated by subjecting the particles both to elemental analysis, to determine the total sulfur content, and to the Ellman assay,10 which measures the amount of free thiol groups. The results obtained are reported in Table 1 together with the particle diameters obtained via dynamic light scattering (DLS) and TEM experiments. Comparison of the results obtained with the two assays indicates that only 15% of the total sulfur is present as free and accessible thiol groups, while the remaining sulfur atoms are either buried in the silica structure or in the form of disulfides. Formation of disulfide bonds is supported by the almost 50% decrease of free thiol groups observed after 30 days of storage (Table 1). Similar results have been obtained for all the other MPS-coated particles reported in this paper. Addition of Pb2+ to a water solution of the particles buffered at pH 7 produces a substantial quenching of the fluorescence emission (Figure 1). Surface thiols play a key role in the recognition of the analyte: a much higher metal ion concentration is needed to obtain a similar effect using dansyl-doped nano(12) Van Blaaderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1-18. (13) Montalti, M.; Prodi, L.; Zaccheroni, N.; Battistini, G.; Marcuz, S.; Mancin, F.; Rampazzo, E.; Tonellato, U. Langmuir 2006, 22, 5877-5881.
a Key: left, np1; center, np2; right, np3. The scale bar is given in the TEM image of np3.
particles prepared without MPS (np0). However, the extent of the quenching in saturation conditions is quite poor, and the residual fluorescence emission of the nanoparticles is about 60% of the initial value. This behavior can be easily rationalized, as previously reported for similar systems, considering the inaccessibility of the particle core to the Pb2+ ions: as a result, dyes located in the inner part of the particles are not affected by the presence of the metal ion.2,13 To lower the residual emission and to increase the sensitivity of the sensor, core-shell nanoparticles np2 (Chart 1) were prepared: a 5 nm thick shell doped with the dansyl dye 1 was grown over presynthesized 50 nm diameter “pure” silica cores, and the resulting particles were finally surface functionalized with MPS. The final diameter of np2 was found to be 59 ( 11 nm (Table 1). The effect of addition of several divalent metal cations to a pH 7 water solution of np2 is reported in Figures 1 and 2. Inspection of the titration profiles of Figure 1 reveals that the system is very effective for the detection of Pb2+ ions, and the core-shell structure produces a remarkable performance improvement. The fluorescence emission is quenched down to a plateau of about 30% of the initial value. This indicates that almost all the dansyl molecules embedded in np2, unlike those in np1, are close enough to the particle surface to be affected by the binding of the Pb2+ ions. The sensitivity is remarkable: if the amount of Pb2+ needed to decrease the initial fluorescence
Fluorescence Nanosensors
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Figure 2. Spectrofluorimetric titration of np2 (0.08 mg/mL) with different metal ions in 10% ethanol/water: (O) Pb(NO3)2, (b) Cu(NO3)2, (9) HgCl2, (]) Ni(NO3)2, (0) CoCl2; (2) CaCl2, (4) MgCl2, ([) Zn(NO3)2. Conditions: [1] ) 0.9 × 10-5 M, HEPES buffer, 0.01 M, pH 7, 25 °C, λexc ) 292 nm, λem ) 500 nm.
Figure 3. Spectrofluorimetric titration of np3 (0.08 mg/mL, [1] ) 0.9 × 10-5 M) with Pb(NO3)2 in 10% ethanol/water. Conditions: HEPES buffer, 0.01 M, pH 7, 25 °C, λexc ) 264 nm. Inset: intensity ratio of the emission at 390 and 500 nm as a function of the concentration of Pb2+.
emission by 10% is taken as a reference detection limit, a metal ion concentration down to 1.1 × 10-6 M can be measured. Formation of additional disulfide bridges after sample aging does not seem to affect the sensor performance: titration experiments with Pb2+ on aged samples produced results identical to those obtained with freshly prepared nanoparticles. The selectivity toward other divalent metal cations has also been tested (Figure 2). As expected, Cu2+ produces effects very similar to those of Pb2+ and is a serious interferent. On the other hand, no effect is observed upon the addition of Zn2+, Ca2+, and Mg2+, while Hg2+,14 Ni2+, and Co2+ produce much smaller quenching than Pb2+. The metal ion concentrations needed to reduce the initial emission by 10% are 0.3 × 10-6, 4.3 × 10-6, 10.6 × 10-6, and 12.6 × 10-6 M, respectively, for Cu2+, Co2+, Hg2+, and Ni2+. Moreover, titration with Pb2+ of a solution containing np2 and all the above metal ions (with the exception of Cu2+), each at a 50 µM concentration, results in a quenching profile very similar to that obtained in the absence of any interferent up to 5.0 × 10-6 M (Supporting Information). A substantial advantage of nanoparticle-based chemosensors, as demonstrated by Kopelman in the case of PEBBLEs,15 is the possibility to convert a simple on-off or off-on chemosensor into a more valuable ratiometric one by the embedding of a second, substrate-insensitive, dye within the particles. In the case of the self-organized systems described here, however, addition of a second fluorescent dye will result only in the quenching of the emissions of both dyes, since there can be very little difference in the interaction between the substrate and the two dyes. This has been demonstrated by preparing nanoparticles np5 in which dyes 1 and 2 (Chart 1) are homogeneously dispersed inside the nanoparticles. Titration with Pb2+ of np5 results in the undifferentiated quenching of the nanoparticle emission, and no ratiometric behavior can be observed (Supporting Information). A solution of this problem may be found in shell structuration of the silica particles as proposed recently by Wiesner and coworkers to segregate a pH-insensitive dye from a pH fluorescent indicator,16 although in such an example shell structuration is not essential for the ratiometric behavior.17 On the contrary, in
our case, such an approach appears to be essential, as the segregation of the reference dye into an inner compartment should prevent its quenching by Pb2+ ions. Multishell particles np3 (Chart 1) doped with both dyes 1 and 2 were hence prepared. The methoxynapthalene dye 2 was first embedded into 40 nm silica particles, and a 7 nm insulating silica layer was subsequently grown on such cores followed by a second 3 nm layer containing dye 1. The resulting particles were finally surface functionalized with MPS. In this way, the sensing dye 1 is close to the thiol-modified surface and the reference dye 2 is buried deep inside the particle and further protected by an “empty” silica shell. The final diameter of np3 is 61 ( 11 nm. The ratiometric behavior of np3 is shown in Figure 3: upon excitation at 264 nm a dual emission is observed due to the contribution of dyes 1 (500 nm) and 2 (390 nm); when Pb2+ is added, the band at 500 nm is substantially quenched while the one at 390 nm is marginally affected. Ratiometric calibration is shown in the inset: a 50% increase of the intensity ratio is reached in saturation conditions, and a micromolar concentration of Pb2+ can be evaluated.
(14) In the case of Hg2+, precipitation occurs at metal ion concentrations greater than 1 × 10-5 M. (15) (a) Buck, S. M.; Koo, Y. E. L.; Park, E.; Xu, H.; Brasuel, M.; Philbert, M. A.; Kopelman, R. Curr. Opin. Chem. Biol. 2004, 8, 540-546. (b) Lu, J.; Rosenzweig, Z. Fresenius J. Anal. Chem. 2000, 366, 569-575. (16) Burns, A.; Sengupta, P.; Zedayko, V.; Baird, B.; Wiesner, U. Small 2006, 2, 723-726.
Conclusions Lead is the most toxic heavy metal, and it is particularly dangerous for its effects on children. This has stimulated considerable research efforts toward the preparation of fluoroionophores for Pb2+ determination.18 In many cases, such chemosensors are complex structures that require delicate synthetic procedures. In this study we have shown how it is possible to prepare a Pb2+ sensor with a simple, 1-day procedure exploiting the self-organization of the sensor components on a silica nanoparticle template. Commercially available components (dye 1, TEOS, MPS) only need to be added in the appropriate (17) In fact, other nanoparticle sensors based on similar rhodamine/ fluorescein reference dye/pH fluorescent indicator couples simply embedded in the nanoparticles perform ratiometric pH sensing. See, for example: (a) Sun, H.; Scharff-Poulsen, A. M.; Gu, H.; Almdal, K. Chem. Mater. 2006, 18, 33813384. (b) Buck, S. M.; Xu, H.; Brasuel, M.; Philbert, M. A.; Kopelman, R. Talanta 2004, 63, 41-59. (18) Recent examples: (a) He, Q.; Miller, E. W.; Wong, A. P.; Chang, C. J. J. Am. Chem Soc. 2006, 128, 9316-9317. (b) Ma, L.-J.; Liu, Y.-F.; Wu, Y. Chem. Commun. 2006, 2702-2704. (c) Kwon, J. Y.; Jang, Y. J.; Lee, Y. J.; Kim, K. M.; Seo, M. S.; Nam, W.; Yoon, J. J. Am. Chem. Soc. 2006, 127, 1010710111. (d) Kavallieratos, K.; Rosenberg, J. M.; Chen, W.-Z.; Ren, T. J. Am. Chem. Soc. 2005, 127, 6514-6515. (e) Swearingen, C. B.; Wernette, D. P.; Cropek, D. M.; Lu, Y.; Sweedler, J. V.; Bohn, P. W. Anal. Chem. 2005, 77, 442-448.
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order to the reaction vessel, and the final product is purified by ultrafiltration. Preparation of core-shell particles is almost as easy: as a consequence the sensor can be designed to perform more complex functions, such as the ratiometric sensing of the substrate, where each nanocompartment of the particle plays a different role. Selectivity is the main drawback of the system reported here. However, it is still remarkable that the simple assembly of a layer of thiol groups on a surface results in the formation of binding sites with an enhanced affinity toward certain metal ions compared with others. In future developments, selectivity could be improved by the use of binding groups different from MPS and more specific for Pb2+. On the other hand, sensors capable of detecting several substrates with different affinities can be valuable in the emerging area of array sensors. Work is in progress in our laboratory.
Arduini et al.
Acknowledgment. We thank Mr. Claudio Gamboz and Prof. Maria Rosa Soranzo (CSPA, University of Trieste) for their kind help with the TEM analysis. Financial support for this research has been partly provided by the Ministry of Education, University and Research (MIUR Contracts 2006034123 and 2006039071), and by University of Padova (University Research Project CPDA034893).
Supporting Information Available: UV-vis and fluorescence spectra of dye-doped nanoparticles, fluorescence titration and selectivity test, and size distribution analysis of the nanoparticles. This material is available free of charge via the Internet at http://pubs. acs.org. LA700971N
