Reactive & Functional Polymers 68 (2008) 1144–1151
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Luminescent nanocomposites containing CdS nanoparticles dispersed into vinyl alcohol based polymers Andrea Pucci a,*, Massimiliano Boccia a, Fernando Galembeck b, Carlos Alberto de Paula Leite b, Nicola Tirelli c,*, Giacomo Ruggeri a,d,e a
Department of Chemistry and Industrial Chemistry, University of Pisa, Via Risorgimento 35, I-56126 Pisa, Italy Institute of Chemistry, Universidade Estadual de Campinas, P.O. Box 6154, CEP 13084-862, SP, Brazil c School of Pharmacy, University of Manchester, Stopford building, Oxford Road, M13 9PT Manchester, United Kingdom d INSTM, Pisa Research Unit, Via Risorgimento 35, 56126 Pisa, Italy e PolyLab-CNR, c/o DCCI, University of Pisa, via Risorgimento 35, I-56126 Pisa, Italy b
a r t i c l e
i n f o
Article history: Received 14 December 2007 Received in revised form 17 March 2008 Accepted 31 March 2008 Available online 7 April 2008
Keywords: Cadmium sulphide nanoparticles Polymer nanocomposites Luminescence (nano-)dispersion Optical responsiveness
a b s t r a c t Size-controlled cadmium sulphide nanoparticles (CdS) stabilized by mercaptoethanol layers were prepared in solution and successively dispersed into different poly(vinyl alcohol)-based polymer matrices. The absorption and the emission features of the CdS nanocomposites were found to differ from those of colloidals dispersions and to be mainly affected by the particle (loose) aggregation in phase-separated, particle-rich regions of the materials. It was also found that the optical behaviour of the nanocomposites can be modulated through uniaxial orientation, which is presumed to partially destroy the aggregations. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Size, morphology and aggregation of inorganic nanoparticles dramatically influence their optical and electronic properties, therefore methods to control these variables offer ideal means for modulating the physical properties of such materials [1–5]. For example, only clusters of noble metals and not the corresponding macroscopic materials, such as smooth surfaces or powders, assume a real colour due to the absorption of visible light at the surface plasmon resonance (SPR) frequency. Also colloidal oxide or sulfide (II–VI groups) nanocrystals made of a few hundreds up to a few thousands of atoms (quantum dots, QDs) are receiving considerable attention, due to their appealing proper-
* Corresponding authors. E-mail addresses:
[email protected] (A. Pucci),
[email protected] (N. Tirelli). 1381-5148/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.03.007
ties derived from the zero-dimensional quantum confined characteristics [6–10]; in particular attention has been focused on the size-dependence of absorption and emission features, and, more generally, of opto-electronic properties and charge-transfer phenomena [11]. In this area, however, control of size alone may not be enough and one must take into account that surface chemical derivatization plays a major role too, e.g. because of the presence of surface defect sites, or because of the control over surface interactions leading to agglomeration. The combined control over particle size and distribution, surface properties and aggregation behaviour can then open new applications in optics, electronics, catalysis and biology [12–15]. In this area, much effort has been devoted to the fabrication of nanocomposites containing nanostructured and crystalline inorganic semiconductors dispersed in polymeric matrices, which confer them with superior processability and possibly prevent agglomeration of the
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nanomaterials (kinetic stability) [16–24]. For example, nanocomposites containing highly dispersed QDs have been used to develop optically functional materials in order to confer or enhance the photoconducibility of host polymers or to modify their refractive index [25,26]. In this work, we report the preparation and the optical properties of new luminescent nanocomposites based on the dispersion of cadmium sulphide (CdS) nanoparticles within different vinyl alcohol-containing polymer matrices. CdS nanoparticles are inorganic semiconductors, widely used for light-emitting diodes (LED) and as quantum dots [15]; they combine interesting opto-electronic properties, simplicity of preparation, possibility to tailor their surface chemistry and therefore to provide them with miscibility in a dispersing matrix. Indeed, the dispersion of individual nanostructured inorganic semiconducting particles within a mechanically supporting polymer host would be an easy method to prepare new, advanced but low cost optical materials. We have prepared nano-sized CdS in solution at room temperature controlling the size of the aggregates through the use of a surface-capping agent, mercaptoethanol, which at the same time provides surface functionality [27,28]. We have then investigated the dispersability of the OH-covered nanoparticles in polar and protic polymer matrices. In particular, we have compared poly(vinyl alcohol) (PVA) with a poly(ethylene-co-vinyl alcohol) (EVAl) copolymer (with 0.44 ethylene molar fraction), whose partially hydrophobic character can reduce some of the wellknown drawbacks of PVA films, i.e., low thermal stability and moisture sensitivity. We here discuss the influence of the nature of the polymer matrix on the size, morphology and optical properties of the CdS nanoparticles, and how the optical behaviour of these materials may be influenced by the nature of the polymer, aggregation behaviour and mechanical stressinduced deformation.
2. Experimental 2.1. Materials Cadmium chloride (CdCl2 2.5H2O, >99.5%) was purchased from Carlo Erba (Italy) and was used as received. All the other chemicals were purchased from Aldrich and were used without further purification. For the preparation of CdS nanoparticles, deionised and distilled water was used. 2.2. Sample nomenclature Two polymer matrices were used: Poly(vinyl alcohol) (PVA, 99+% hydrolyzed, Mw = 146,000–186,000, supplied by Aldrich), Poly(ethylene-co-vinyl alcohol) with 44% by mol of ethylene content (EVAl44, 60.7% of non-hydrolyzed vinyl acetate units, melt index (210 °C, ASTM D 1238) = 3.5 g/ 10 min, density (25 °C) = 1.14 g/mL, supplied by Aldrich). Samples were named by polymer, nanoparticle, concentration and draw ratio, e.g. EVAl44CdS_0.5_2.
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2.3. CdS nanoparticles synthesis CdS nanoparticles were synthesized with mercaptoethanol as a capping stabilizing layer following a modified kinetic trapping method [27]. Briefly, 2.28 g (10 mmol) of CdCl2 2.5H2O was dissolved in 800 mL of deionised water previously degassed by passing N2 under vigorous stirring. After dissolution, 30 mL of degassed water containing 100 mmol of 2-mercaptoethanol was added and the pH was raised by the addition of diluted NaOH solution to pH 6.8. Then, 50 mL of a 0.2 M solution of Na2S 9H2O was added dropwise in the dark to the cadmium solution with rapid stirring and the pH incremented further to the final value of 8.20. The reaction mixture was stirred for 2 h under nitrogen atmosphere in the dark and the solution was then concentrated to 50 mL and purified by dialysis (SpectrumLabs, Cellulose Ester, 10 mL, MWCO: 500) against water. After purification the CdS particles were separated by size-selective precipitation: 200 mL of THF was added to the solution and a solid yellow precipitate was recovered after centrifugation and freeze-drying. Thermogravimetric analysis performed on the dry powder indicated a product composition of 70% CdS and 30% organic thiol (onset: 258 °C, under nitrogen atmosphere). 2.4. Nanocomposite preparation The typical procedure for the preparation of CdS/polymer nanocomposites is reported as follows: 0.335 g of the polymer (PVA or EVAl) was dissolved in 20 mL of solvent (respectively deionised water for PVA, dimethylsulfoxide for EVAl matrix) under stirring at 110 °C. After cooling to room temperature the desired amount of CdS was added and dissolved under gentle stirring. The resultant yellow dispersions were cast into polytetrafluoroethylene (PTFE) Petri dish and kept in the dark during solvent evaporation (2 days under hood at room temperature for PVA and at 45 °C for EVAl solutions). Oriented composites were obtained by uniaxial tensile drawing of the polymer matrix on thermostatically controlled hot stage at 110 °C for PVA mixtures and 90 °C for EVAl films. The draw ratio, defined as the ratio between the final and the initial length of the sample, respectively, was determined by measuring the displacement of inkmarks printed onto the films before stretching. 2.5. Physico-chemical characterization FT-IR spectra were recorded with a Perkin–Elmer Spectrum One spectrometer as dispersions in KBr. X-ray diffraction (XRD) patterns were obtained in BraggBrentano geometry with a Siemens D500 KRISTALLOFLEX 810 (CT: 1.0 s; SS: 0.050 dg and Cu Ka, k = 1.541 Å) diffractometer. Data were acquired at room temperature. Curvefitting error estimates for the peak widths were calculated using the Origin data analysis software. Thermogravimetric scans were obtained with a Perkin– Elmer TGA-7 under nitrogen flux, at a scan rate of 20 °C/ min. Bright field transmission electron microscopy (TEM) pictures were obtained on polymer composites by using
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a Carl Zeiss CEM-902 transmission electron microscope, equipped with a Castaing–Henry–Ottensmeyer energy filter spectrometer within the column. Other experimental details regarding TEM and sample preparation are presented elsewhere [29]. Particle analysis was performed using the public domain Image Tool 3.00 version image analyzer program developed at the University of Texas Health Science Center in San Antonio and is available on Internet at http://ddsdx.uthscsa.edu/dig/itdesc.html. UV–Vis absorption spectra of the polymer films were recorded under isotropic conditions with a Perkin–Elmer Lambda 650. Steady-state fluorescence spectra of the polymer nanocomposites were acquired at room temperature under isotropic excitation with the help of a Perkin–Elmer Luminescence spectrometer LS55 controlled by FL Winlab software, fitted with motor-driven linear polarizers and equipped with the Front Surface Accessory: i.e., the position of the sample was adjusted in the direction of the excitation beam in such a way that the optical axis of excitation and emission crossed in the film plane. The film roughness was diminished using ultra-pure silicon oil (poly(methylphenylsiloxane), 710Ò fluid, Aldrich) to reduce surface scattering between the polymeric films and the quartz slides used to keep them planar. Origin 7.5, software by Microcal OriginÒ, was used in the analysis of the XRD and spectroscopic data. Digital images were obtained by using a Canon PowerShot Pro1 camera exposing the films under a Camag UVCabinet II equipped with Sylvania 8W long-range lamps (366 nm).
3. Results and discussion 3.1. Characterization of CdS nanoparticles Yellow coloured, readily water-dispersable CdS nanoparticles were produced in water at slightly basic pH. The control of the initial pH (with the literature values ranging from 6.5 to 10) is well known to be a crucial factor for obtaining size-controlled CdS sols [27]. In thermogravimetric analysis the nanoparticles exhibited a thermally-induced weight loss due to desorption of organic materials starting at 258 °C; this accounts for roughly 30% of the total weight, indicating therefore that there are slightly more sulphur atoms from mercaptoethanol (66% of the total S atoms) than from sulphide ions. This is not unexpected, since the nanoparticles are very small and therefore exhibit a large surface area, which corresponds to an even larger number of cadmium ligand sites (cadmium is tetracoordinated) available there. The high temperature necessary for observing this desorption/degradation ensures that the conditions later used during nanocomposite preparation and drawing experiments (T < 110 °C) would not harm the chemical integrity of the nanoparticles. The X-ray diffractogram of the CdS nanoparticles in powder form is reported in Fig. 1. Three diffraction peaks can be found at about 27°, 44° and 51° and attributed to the (1 1 1), (2 2 0) and (3 1 1)
Fig. 1. XRD pattern of CdS nanoparticles capped by mercaptoethanol.
planes of cubic CdS phase [22,30,31]. Compared with bulk CdS, the diffraction peaks of the nanoparticles appeared broadened due to the reduced particle size and surface defects. Actually, according to the Scherrer’s equation [29] 0.9 k/D(2h) cos h on the (1 1 1) peak XRD pattern suggested an average crystallite size range of 1.5–2.4 nm. Bright-field TEM of the CdS nanoparticles from colloidal dispersions (Fig. 2) seems to indicate them to be present as objects with a (irregular) spherical shape – possibly Hbonded aggregates – but also possibly as individual nanometer-sized nanoparticles (grey spots on white background). CdS nanoparticles within the aggregates appeared to exhibit a monomodal distribution in size with an average diameter range of 2.33 ± 0.84 nm, which is in a good agreement with the XRD measurements. Fig. 3 shows the absorption and luminescence behaviour of CdS nanoparticles dispersed in dimethylsulfoxide (DMSO). A strong absorption peak at 346 nm is assigned to the optical transition of the first excitonic state of the CdS nanoparticles and its rather narrow shape is an evidence of the very small size of the dispersed particles [17,32]. The mean size of the CdS nanoparticles in DMSO can be calculated from the onset absorption (or absorption edge) of the UV–Vis spectrum (in this case pointed at ke = 385 nm) and it generally represents the large-size end of the size distribution [17,19,33]. In our case one obtains 2.3 nm, which is in a good agreement with the values from XRD and TEM; however, since this measurement provides a limit value, the real average dimensions may be smaller than that. CdS nanoparticles typically show two characteristic emission bands: one at 470 nm, attributed to recombinations from the excitonic state in the crystalline interior, and one at higher wavelengths at about 540 nm, assigned to hole–electron recombinations at surface traps [34]. In our case (Fig. 3), a strong emission band appeared at 470 nm with a small shoulder above 500 nm and may be attributed to recombinations from the excitonic state only.
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Fig. 2. Bright-field transmission electron micrograph (A) and particle size distribution (B) of CdS nanoparticles.
Fig. 3. UV–Vis and emission spectra of a dilute DMSO dispersion (1 wt.%) of CdS nanoparticles, showing the absorption edge (ke) at 385 nm. For the emission spectrum, kexc. = 390 nm.
3.2. Preparation and structural characterization of CdS/ polymer nanocomposites Composites were prepared by dispersing the CdS nanoparticles in a 1.6 wt.% polymer solution, in water for poly(vinyl alcohol) and in DMSO for poly(ethylene-co-vinyl alcohol), which were then cast as a film. All TEM measurements performed on the cross-sections of the films showed that the particles tend to accumulate close to the film surface exposed to air during casting (Fig. 4). This would suggest that during solvent evaporation a liquid–liquid phase separation in a polymer-rich and a nanoparticle-rich component can occur, similar to what often happens for dispersions of low MW compounds in polymers [35] and despite the presence of OH groups on both nanoparticles and polymers; the interactions between these groups are evidently not sufficient for ensuring good miscibility of the two systems. An extensive analysis of TEM images taken on different film portions of both PVA and EVAl nanocomposites shows that the nanoparticle size distribution has a main peak (that can be easily fit with a Gaussian curve. See Supplementary material) flanked by a more or less conspicuous component at larger sizes.
Fig. 4. Bright-field transmission electron micrograph of the cross-section of a PVACdS_1 composite film.
In PVA nanocomposites (Fig. 5A) the average size of the dispersed objects is roughly analogous to that observed by drying the nanoparticle dispersions in DMSO, although the average value is shifted to somewhat lower values (1.7 nm compared to 2.0 nm); on the other hand, EVAl matrices seem to disperse distinctly smaller particles (Fig. 5B), with an average dimension close to 1 nm and with little presence of large aggregates. This behaviour is not due to a better dispersion of the CdS nanoparticles in EVAl; on the contrary, the separation between particle-rich and particle-poor regions in EVAl is even more evident there than in PVA (pictures on the right in Fig. 5). In EVAl dispersions we face therefore the apparent oxymoron of having smaller nanoparticles in regions where they are more densely present. We can, however, hypothesize that (a) the OH-covered CdS nanoparticles also aggregate in the absence of a matrix: there is no reason why they should remain
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Fig. 5. Particle size distributions (left) and bright-field transmission electron micrographs (right) for CdS nanoparticle dispersions in PVA (A) and EVAl (B) as a function of concentration and compared to the original nanoparticles (in grey). TEM micrographs are referred to polymer films containing the 0.5 wt.% of CdS nanoparticles.
separated during solvent removal; (b) in PVA a similar aggregation takes place, but it may be mediated by the ‘‘bridging” polymer chains that interact with surface groups of different nanoparticles; we suppose that because (c) in the EVAl matrix the polymer chains, due the presence of apolar sections, could bridge less efficiently, allowing therefore for a larger distance between the nanoparticles and therefore permitting to recognize individual objects. In such a condition it would seem counterintuitive that nanoparticles coated with EVAl chain and less strongly associated cannot dispersed better in the matrix: however, phase separation can originate not only because of poor surface interactions, but possibly also because of the overall polarity of the nanoparticles, which have a considerable energetic gain in forming a polar (high dielectric constant) particle-rich phase rather than being dispersed in a low dielectric constant matrix. The question arising at this point is whether the optical properties of the nanocomposites will be more influenced by the small-distance physical separation between the individual nanoparticles (proximity at a 1 nm scale (concentration in the particle-rich phase,
which seems to be PVA > EVAl). In the first case, EVAl nanocomposites should have a behaviour intermediate between PVA samples and DMSO dispersion; in the second case, PVA nanocomposites should be mid-way between EVAl ones and DMSO dispersion. 3.3. Optical characterization of CdS/polymer nanocomposites The absorption band of CdS is indeed considerably affected by the dispersion of the nanoparticles in polymer matrices (Fig. 6). It is apparent that the band appearing at 350 nm for samples in DMSO dispersion is considerably red-shifted for both PVA and EVAl dispersions; the extent of the shift does not depend on the concentration. Indeed, since this phenomenon is caused by a quantum-confined effect [18], it depends on the aggregation of the nanoparticles, which TEM has shown not to be influenced by their concentration. PVA has produced a shift of 35 nm, while EVAl has produced a shift of 50 nm, whereas the absorption edge (ke) moves from 385 nm (CdS in DMSO) to 428 nm for PVACdS_0.5 and 453 nm for EVAl44CdS_0.5. Additionally, EVAl composites showed a considerably higher increase
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Fig. 7. Comparison between the fluorescence spectra of PVA and EVAl nanocomposites containing 2 wt.% of CdS particles with pure CdS sols (kexc. = 390 nm).
Fig. 6. Comparison between the UV–Vis spectra (after removing the scattering background through an exponential fitting; the original spectra are in the inserts) of PVA (a) and EVAl (b) nanocomposites containing different concentrations of CdS particles with a 1 wt.% CdS dispersion in water.
of the difference between absorption edge and absorption peak values, i.e., 68 nm for EVAl and 43 nm for PVA. Utilizing the generally accepted correlation between ke and the average CdS size, the diameter of the dispersed particles in PVA and EVAl can be estimated to be 2.9 and 3.6 nm, respectively. The discrepancy with the values obtained from TEM pictures is, however, only apparent: indeed the EVAl dispersions show more dense large aggregates (although smaller isolated nanoparticles) and therefore the optical behaviour may be ascribed mainly to this large-scale aggregation rather to the clustering of isolated nanoparticles. The fluorescence spectra of PVA and EVAl44 nanocomposites containing the 2 wt.% of CdS particles are compared in Fig. 7 with the emission behaviour of the CdS dispersion in DMSO. It is noteworthy that, differently from DMSO or water dispersions, where degradation [33,36,37] or dynamic
equilibrium [38] between free or bound capping ligands leads to a clear modification of the emission features of the CdS nanoparticles (usually a quenching of the luminescence intensity), the PVA- or EVAl-nanocomposites showed optical stability exceeding 6 months, a phenomenon that may be ascribed to the much slower dynamics in the polymer matrices. The emission spectra of the two nanocomposites are substantially analogous, with a 50 nm red-shift with respect to the emission of the particles in DMSO, conferring to PVA and EVAl films a typical orange-red colour1 when excited with a long-range UV radiation (Fig. 8). This shift, which confines CdS emission in the region >470 nm, does not appear to have a strong dependence either on the nanoparticle concentration or on the nature of the matrix. It seems reasonable to ascribe it to autoabsorption of the emitted radiation, due to the increase in the nanoparticle concentration following phase segregation. However, it cannot be excluded that the presence of bound polymer chains on the nanoparticle surface may increase the emission from the interface (the band at 470 nm is often attributed to luminescence from excitonic states in the nanocrystal interior and that at about 400 nm to recombinations of excitons and/or shallowly trapped electron hole pairs [22,39]). These hypotheses can be verified by reducing the extent of phase segregation without drastically changing the degree of interactions with the polymer chains, e.g. via high temperature drawing of the material. Indeed the uniaxial orientation of PVA and EVAl44 nanocomposites (respectively performed at 110 °C and 90 °C and at a draw ratio of 2 and 4) caused a substantial change in the optical properties: (a) a blue-shift is observed in the absorption peaks, with the absorption edges being shifted to 10 (PVA) and 25 nm (EVAl) with respect to the pristine nanocomposites (inset in Fig. 9); (b) analogously, the emis-
1 For interpretation of color in Fig. 8, the reader is referred to the web version of this article.
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Fig. 8. Digital images of EVAl44CdS_2 film under visible light (a) and under excitation with a long-range UV lamp (b, k = 366 nm).
Fig. 9. Normalised emission (kexc. = 390 nm) and UV–Vis absorption (inset) spectra of EVAl44CdS_2 film before and after uniaxial orientation at draw ratio of 2 and 4, respectively.
sion (EVAl shown in Fig. 9) resulted increasingly similar to that of the CdS particles dispersed in DMSO with increasing drawing ratio (=increasing forced dispersion in the matrix, as indicated by the position of the absorption edge). In particular, luminescent experiments performed by varying the excitation wavelength did not produce any significant change of the emission of both oriented and unoriented films (i.e., emission maximum and band shape, see Fig. S3 in the Supplementary material section). This aggregation-dependent shift of the emission spectrum has been observed beforehand [20,21,31,34]. No definitive explanation has been provided but it was speculated that the luminescence of nanocrystals is assigned as trap-induced electron–hole recombination and/or recombination from an excitonic state. However, it seems that the nature of the emitting states can change as the size and the aggregation extent of the clusters change. It is noteworthy that uniaxial orientation could leave nanoparticle aggregates oriented along the drawing directions; in the present case, however, we record a complete
lack of linear dichroism (see Supplementary material), which indicates that no anisotropic structure was formed. 4. Conclusions Very small (Ø 2.3 nm) luminescent CdS nanoparticles, capped with a mercaptoethanol stabilizing layer, were efficiently prepared in water under controlled pH conditions and dispersed by solution-casting into vinyl alcohol containing polymer matrices, i.e., poly(vinyl alcohol) (PVA) and poly(ethylene)-co-(vinyl alcohol) with 44% by mol of ethylene content (EVAl44). The nanoparticles clearly phase-separate, but our results seem to indicate that the interactions between the hydroxyl groups of the host matrix and mercaptoethanol residues allow the significant presence of polymer chains in the particle-rich phase, which may influence the nanoparticle short-distance separation. Additionally, we have shown that the features of the phase segregation seem to be the main factors in regulating the optical properties of the nanocomposites and,
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additionally, that these features can be controlled e.g. through mechanical orientation. Acknowledgements The authors wish to thank Prof. Francesco Ciardelli (DCCI, Pisa) for the very helpful discussions. Financial support by MIUR-FIRB 2003 D.D.2186 grant RBNE03R78E is kindly acknowledged. A.P. acknowledges the Royal Society of Chemistry for the journals grant for international authors (application no.: 07 04 584). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.reactfunctpolym.2008.03.007. References [1] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, SpringerVerlag, Berlin, 1995. [2] K.J. Klabunde, Nanoscale Materials in Chemistry, Wiley Interscience, New York, 2001. [3] K.L. Kelly, E. Coronado, L.L. Zhao, C. Schatz George, J. Phys. Chem. B 107 (2003) 668–677. [4] H.H. Huang, X.P. Ni, G.L. Loy, C.H. Chew, K.L. Tan, F.C. Loh, J.F. Deng, G.Q. Xu, Langmuir 12 (1996) 909–912. [5] J.P. Wilcoxon, B.L. Abrams, Chem. Soc. Rev. 35 (2006) 1162–1194. [6] H. Weller, Adv. Mater. 5 (1993) 88–95. [7] C.J. Murphy, Anal. Chem. 74 (2002) 520A–526A. [8] H. Sakurai, Organomet. News (2004) 100. [9] I. Willner, B. Willner, Pure Appl. Chem. 74 (2002) 1773–1783. [10] T. Murakata, Y. Higashi, N. Yasui, T. Higuchi, S. Sato, J. Chem. Eng. Jpn. 35 (2002) 1270–1276. [11] P.D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 35 (2006) 1195– 1208. [12] T. Trindade, P. O’Brien, N.L. Pickett, Chem. Mater. 13 (2001) 3843– 3858. [13] G. Schmid, B. Corain, Eur. J. Inorg. Chem. (2003) 3081–3098. [14] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307 (2005) 538–544.
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