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2009, 113, 8537–8540 Published on Web 04/22/2009
Hydrotropic Solubilization of Gold Nanoparticles Functionalized with Proto-Alkylthioporphyrazines Sandra Ristori,§ Giampaolo Ricciardi,#,* Daniela Pietrangeli,# Angela Rosa,# and Alessandro Feis§ Dipartimento di Chimica, UniVersita` di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy, and Dipartimento di Chimica, UniVersita` della Basilicata, Via N. Sauro, 85, 85100 Potenza, Italy ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: April 10, 2009
Hydrotropic anchoring of a prototype porphyrazine bearing eight ethylthio chains as peripheral substituents was shown to take place on gold nanoparticles (GNPs) by UV-vis, dynamic light scattering (DLS), Raman, resonance Raman (RR), and surface enhanced resonance Raman spectroscopy (SERRS) experiments. Density functional theory (DFT) calculations were used as a valuable help to make the hypothesis that ligation to the gold surface occurred through the concurrent contribution of the peripheral sulfur atoms and the π system of the macrocycle. The so functionalized GNPs proved to be stable in water solution for long time, thus providing a suitable chemical and structural basis to build up hierarchical structures with possible applications in technological and biomedical fields. Gold nanoparticles (GNPs) are attractive candidates for a wide variety of applications.1 This mainly stems from their peculiar optical and electronic properties,2-4 as well as from their chemical inertness, which is especially suitable for biomedical purposes.5,6 Porphyrin-like macrocycles also possess some of the optoelectronic properties shown by GNPs, due to extended π systems.7 The importance of boosting the nanoelectronic characteristics of GNPs by ligation with π conjugated compounds has been recently recognized,8,9 and it is likely to add new potentialities to this very active field of research. Moreover, medical applications such as photothermal therapy (PTT), currently under trial with either of these systems separately,10,11 could benefit from the synergic effect brought about by the joint use of gold nanoparticles and tetrapyrrolic macrocycles. Finally, the remarkable capability of azaporphyrins to carry multiple peripheral functionalities is an issue to be taken into account when using tetrapyrrolic scaffolds to functionalize gold nanoparticles.12-14 Generally, thioethers are not able to bind to gold surfaces as strongly as thiols,15 but the use of polythioethers16 or of polydentate thioethers17 can overcome this drawback, resulting in steady attachment. On the other hand, thioethers represent a good alternative to thiols, which are relatively unstable toward oxidation and light damaging. Here we describe a strategy to anchor a prototypic azaporphyrin macrocycle, i.e., the (ethylthio)porphyrazine (H2OESPz) reported in Scheme 1a, to the surface of GNPs. The porphyrazine employed in this work possesses four pairs of vicinal thioether groups, which form a tetra-bidentate coordination site, possibly cooperating with the π-system of the macrocycle to bind GNPs. Although the strength of the interaction between the azaporphyrin π-system and flat gold * Corresponding author. E-mail:
[email protected]. § Universita` di Firenze. # Universita` della Basilicata.
10.1021/jp902191f CCC: $40.75
SCHEME 1: Molecular Structure of the 2,3,7,8,12,13,17,18-Octakis(ethylthio)-5,10,15,20-(21H, 23H) Porphyrazine (H2OESPz) (a) and Its Dianion in the Form of Disodium Salt (Na2OESPz) (b)
surfaces is still a matter of a considerable debate,18 it is expected that such interaction can be enhanced on the “rough” surface of GNPs, and especially in the presence of a polar environment, in response to the need of minimizing macrocycle/solvent contacts. Actually, to circumvent the inherent insolubility of the macrocycle in watershydrogen-bonding interactions between aza-bridges and water molecules do not guarantee the efficient solvation7,19sand to allow an effective approach of the macrocycle to the water dispersed GNPs, H2OESPz was preliminarily converted into the more hydrophilic dianion in the form of disodium salt, Na2OESPz (Scheme 1b). Conversion was readily achieved, on account of the electron-deficient character of the thioporphyrazine ring,20 through reaction of the free base with NaOH in methanol solution. The obtained dianionic species promptly ligated to gold nanoparticles. Further pH adjustment to neutrality did not detach H2OESPz from GNPs, hence affording a firmly functionalized colloid. Ligation of H2OESPz to gold nanoparticles was demonstrated by surface-enhanced resonance Raman spectroscopy (SERRS). 2009 American Chemical Society
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Letters TABLE 1: Mean Diameter and Size Distribution of 95% Particles in Each Population Calculated by Laplace Inversion of the Scattered Light Intensity systems
〈D〉
95%
GNPs (as prepared) GNPs + NaOH/MeOH (pH ) 13)
39 nm 34 nm (35%) 130 nm (65%) 62 nm (21%)
34-44 nm 25-48 nm 110-150 nm 54-70 nm
200 nm (79%) 48 nm (35%) 45 nm (37%)
160-240 nm 40-56 nm 36-55 nm
180 nm (63%)
160-200 nm
GNPs + NaOH/MeOH (pH ) 13) + H2OESPz GNPs + NaOH/MeOH (pH ) 7) GNPs + NaOH/MeOH (pH ) 7) + H2OESPz
Figure 1. (A) Absorption spectra of GNPs aqueous dispersions before (dashed line) and immediately after (continuous line, pH 13) the addition of 17% v/v 0.1 M NaOH in methanol containing Na2OESPz (2 × 10-6 M final concentration). Absorption spectra (continuous line, pH 7) of the same dispersion, where the pH was immediately adjusted to 7 with citric acid (6.6 mM final concentration). (B) Absorption spectra of the dispersions containing Na2OESPz at pH 13 and pH 7, 24 days after the preparation. (C) Absorption spectra of H2OESPz 5 × 10-5 M in CHCl3. The optical path length was 1 cm (A, B) or 1 mm (C).
To the best of our knowledge this is the first time that SERRS has been used to investigate π conjugated macrocycles bound to gold nanoparticles in water solution. A few SERRS studies have been performed on immobilized Au-macrocycle systems.21-23 UV-visible spectroscopy and dynamic light scattering (DLS) measurements indicated that adhesion of the porphyrazine function to the gold surface was accompanied by only moderate particle aggregation. This was also confirmed by TEM images. Indeed, the functionalized GNPs did not coalesce and remained stable for at least 2-3 months at room temperature. As seen in Figure 1A, the absorption spectrum of the nanoparticle solution
showed a plasmon peak in the visible region at 525 nm. Upon addition of the Na2OESPz solution, the porphyrazine Q-band (see Figure 1C for a porphyrazine spectrum in CHCl3) showed up as a broad hump in the red tail of the GNPs plasmon peak, at ca. 720 nm. Over time the optical spectrum was redistributed between two bands: one at approximately 525 nm due to noninteracting nanoparticles1 and a second broad band near 670 nm (Figure 1B) due to red-shifted absorptions associated with GNPs aggregation,1,24-26 with a possible contribution from the Q-band of the macrocycle. The spectral changes occurred on a time scale of days (see Figure S1a-d in the Supporting Information). When the pH was lowered to 7 by means of citric acid, the absorption changed even more slowly, reaching equilibrium after 3 weeks. SERRS and DLS measurements were performed on the same samples as in Figure 1. The distributions of size obtained by light scattering are summarized in Table 1, and they are well in line with the above results obtained from absorption spectra. In particular, the monomodal population of particles observed after preparation (spontaneous pH of the GNPs solution was 5.5) split into two distinct size distributions, one of which showed marked particle aggregation. TEM images were in line with this overall description of the system. As an example, two images of plain and H2OESPz loaded gold nanoparticles are shown in Figure 2. Figure 3 compares the ordinary resonance Raman (RR) spectra of H2OESPz in CHCl3 solution to those of the nanoparticles dispersion plus porphyrazine at pH 7, demonstrating the SERRS effect. The spectra in CHCl3 solution and in the presence of GNPs were largely similar when the excitation wavelength was 568.2 nm, with only small intensity changes and frequency shifts for most bands.
Figure 2. TEM images of plain GNPS (a) and GNPS plus Na2OESPz 2 × 10-6 M in water solution with 17% v/v 0.1 M NaOH/ methanol (b) taken with Zeiss EM 912 Omega microscope after casting a drop of solution on a copper grid and letting the solvent evaporate. The acceleration voltage was 120 kV. In both samples the starting solution had pH ) 13. The bar scale is 100 nm. The moderate clustering induced by porphyrazine uptake was the source of the observed SERRS effect, as explained in the text.
Letters
Figure 3. (Top) resonance Raman spectra of 10-5 M H2OESPz in CHCl3 solution with 514.5 and 568.2 nm excitation wavelength, 8 and 10 min accumulation time, respectively. The intensity was normalized to that of the CHCl3 band at 1216 cm-1. (Bottom) surface-enhanced resonance Raman spectra of GNPs at pH 7, same sample as in Figure 1B, with 514.5 and 568.2 nm excitation wavelength, 12 and 15 min accumulation time, respectively. The intensity was normalized to that of the methanol band at 1018 cm-1. “p” marks laser plasma lines; “s” marks solvent bands. No background was subtracted from the spectra. The laser power at the sample was 50 mW at 514.5 nm and 60 mW at 568.2 nm. The spectral resolution was 6 cm-1 at 514.5 nm and 5 cm-1 at 568.2 nm.
The most remarkable difference between the two samples was the enhancement pattern when the excitation wavelength varied from 568.2 to 514.5 nm. Both wavelengths are resonant, or nearly resonant, with electronic transitions of the porphyrazine macrocycle (see the arrows in Figure 1). This led only to modest
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8539 intensity redistribution in the spectra obtained with the two wavelengths for the H2OESPz/CHCl3 solution. A completely different enhancement occurred in the presence of nanoparticles. In the spectrum with 514.5 nm excitation, only bands due to the solvent or to citrate appeared. Conversely, an intense spectrum of the porphyrazine chromophore was obtained upon 568.2 nm excitation. This was clear evidence that the Raman spectrum of this sample derived its intensity mainly from the plasmon resonance band at longer wavelengths (Figure 1), and that the spectrum originated from porphyrazine molecules interacting with GNPs. In fact, further enhancement was obtained by exciting the SERRS spectrum at 647.1 nm (Figure 4). On the basis of the TEM micrographs shown in Figure 2, we propose that the enhancement is due to Au nanoparticles aggregates, since it has been reported that these nanostructures are by far more efficient in the Raman enhancement than isolated Au nanoparticles.27 At 647.1 nm excitation wavelength, a comparison between H2OESPz in CHCl3 solution and on GNPs was prevented by the intense fluorescence observed in the CHCl3 solution spectra. On the contrary, fluorescence was completely quenched in the SERRS spectra. A straightforward explanation for the differences between the spectra at pH 7 and those at pH 13 is that the anionic form Na2OESPz is present at alkaline pH. On the other hand, the SERRS spectrum is largely different from the Raman spectrum of Na2OESPz in CHCl3 solution (cf. Figure S2 in the Supporting Information). Another possibility is that the properties of the nanoparticle surface are substantially altered at alkaline pH and the interaction of H2OESPz is consequently changed. To our knowledge, a complete vibrational assignment is not available for the investigated porphyrazine, and it is beyond the scope of the present work. To gain insight in the nature and frequency of the normal modes underlying the salient Raman features of H2OESPz, we turned to density functional theory (DFT) calculations. To alleviate the theoretical effort, (methylthio)porphyrazine (H2OMSPz) was considered instead of H2OESPz. Several conformations corresponding to different orientations of the methylthio groups with respect to the porphyrazine plane were theoretically explored for this model system at the DFT/BP86/ TZVP level (see Supporting Information for calculations details and ref 28 for a thorough DFT study of the influence of methylthio substituents on the conformational behavior of the porphyrin ring in a nickel(II) porphyrinate complex). The nearly degenerate C2h and C2V conformations (the latter is only ∼0.2 kJ/mol less stable
Figure 4. Surface-enhanced resonance Raman spectra of gold nanoparticles, same samples as in Figure 1B, with 647.1 nm excitation wavelength, 6 min accumulation time. No background has been subtracted from the spectra. The laser power at the sample was 90 mW. The spectral resolution was 3.5 cm-1.
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than the former), both characterized by a substantially planar porphyrazine core and a uu-uu-dd-dd (u ) up, d ) down) and uu-dd-uu-dd orientation of the vicinal methylthio groups, were found to be, among the investigated ones, the preferred conformations in the “gas phase” (see Figure S5 in the Supporting Information). Raman frequencies and intensities calculated for either of the C2h and C2V conformations reproduced equally well the main Raman spectral features of H2OESPz (compare the computed Raman spectra of Figures S6 and S7 with the offresonance, Fourier-transform solid-state Raman spectrum displayed in Figures S3 and S4 (Supporting Information)), hence suggesting that the conformational behavior has a minor impact on the experimental Raman spectrum of this molecule. The most intense bands in all the Raman spectra (RR and SERRS, and solid-state FT-Raman, see Supporting Information) were observed at 730, 1290, and 1545 cm-1. According to our DFT results for the C2V conformation, the band doublet at 730/749 cm-1, which shifts to 741/753 cm-1 at pH 13, had to be assigned to a symmetric pyrrole breathing normal vibrational mode coupled to Cβ-S stretching and expansion/contraction modes of the inner cyclic polyene ring. A not very different picture came from the analysis of the C2h conformation normal modes. The large intensity of this band fitted in with the underlying mode involving the nuclei belonging to the π conjugated system. The very intense bands at 1290 and 1545 cm-1 were assigned to totally symmetric deformation modes of the porphyrazine macrocycle. Indeed, in the energy regime of the 1290 cm-1 band, DFT calculations located a very prominent band at ca. 1270 cm-1 associated to the 32A1 and 31Ag normal vibrational modes in the C2V and C2h conformation, respectively. These modes involved symmetric deformation of the pyrrole moieties. In turn, the band at 1545 cm-1 arose from the symmetric contraction/expansion of the pyrrole rings coupled to the Nb-CR stretching, as described by the 43A1 and 42Ag normal vibrational modes in the two investigated conformations. In sum, all of the above results consistently indicate that hydrotropic anchoring of H2OESPz to GNPs takes place. This is somewhat at variance with recent studies according to which macrocycles without peripheral alkanethiols (so is H2OESPz) should not adsorb, in principle, on gold electrode surfaces.18 A plausible explanation for this discrepancy relies in the different nature of GNP and gold electrode surfaces, as well as in the active role played by the highly polar environment in “pushing” the thioporphyrazine moieties against the GNPs surface. We propose that, in the present case, anchoring occurs through the concurrent contribution of the peripheral sulfur atoms and the extended π system of the macrocycle lying flat on the surface. That even a minority amount of porphyrazine molecules adopt an edge-on orientation with respect to the surface of the gold clusters cannot be excluded on the basis of our results. However, such an event seems to be quite unlikely, in view of the hydrophobic character of the azaporphyrin ring. Given the well-known tendency of porphyrin-like molecules to pile up and build stable columnar stacks, a molecular arrangement with the macrocycle lying prone on the gold cluster surface could provide a suitable structural basis for building hierarchical structures with possible applications in technological and biomedical fields.29 Acknowledgment. The Consorzio per lo Sviluppo dei Sistemi a Grande Interfase (CSGI) and the Italian Ministero dell’ Istruzione, dell’ Universita` e della Ricerca (MIUR) (PRIN2007, 2007XWBRR4_002), are acknowledged for financial support. We express our gratitude to G. Ruani and C. Taliani (Institute
Letters for Nanostructured Materials, CNR Bologna, Italy) for the FTRaman spectrum of solid H2OESPz. Dr. Dayang Wang and Prof. Helmuth Moehwald of the Max Planck Institute of Colloids and Interfaces (Potsdam, Germany) are gratefully acknowledged for reading this manuscript and for valuable discussions. Supporting Information Available: Experimental and computational details, time-dependent absorption spectra of bare and functionalized GNPs in water solution (Figure S1), resonance Raman spectra of 10-5 M Na2OESPz in CHCl3 solution (Figure S2), Fourier-transform Raman spectrum of solid H2OESPz with 1064 nm wavelength excitation (Figures S3 and S4), schematic representation of the H2OMSPz conformers considered in the DFT calculations (Figure S5), Raman spectra computed in the gas phase for the most stable conformers of H2OMSPz (Figures S6 and S7), and Cartesian coordinates of the optimized structures of the H2OMSPz conformers. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 227, 1078. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (4) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (5) Faulk, W. P.; Taylor, G. M. Immunochemistry 1971, 8, 1081. (6) Paciotti, G. F.; Myer, L.; Weinreich, D.; Goia, D.; Pavel, N.; McLaughlin, R. E.; Tamarkin, L. Drug DeliVery 2004, 11, 169. (7) Donzello, M. P.; Ercolani, C.; Stuzhin, P. A. Coord. Chem. ReV. 2006, 250, 1530. (8) Zhang, L.; Li, X.; Mu, J. Colloid Surf. A: Physicochem. Eng. Aspects 2007, 302, 219. (9) Kanehara, M.; Takahashi, H.; Teranishi, T. Angew. Chem., Int. Ed. Engl. 2008, 47, 307. (10) Huang, X.; El-Sayed, H. I.; Quian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 2115. (11) Sternberg, E. D.; Dolphin, D.; Bruckner, C. Tetrahedron 1998, 54, 4151. (12) Sibert, J. W.; Lange, S. J.; Stern, C. L.; Barrett, A. G. M.; Hoffman, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 2020. (13) Velazquez, C. S.; Broderick, W. E.; Sabat, M.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1990, 112, 7408. (14) Mani, N. S.; Beall, L. S.; Miller, T.; Anderson, O. P.; Hope, H.; Parkin, S. R.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Chem. Soc., Chem. Commun. 1994, 18, 2095. (15) Shelley, E. J.; Ryan, D.; Johnson, S. R.; Couillard, M.; Fitzmaurice, D.; Nellist, P. D.; Chen, Y.; E., P. R.; Preece, J. A. Langmuir 2002, 18, 1791. (16) Li, X.-M.; de Jong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2001, 11, 1919. (17) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovitch, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958. (18) Sun, P.; Zong, H.; Salaita, K.; Ketter, J. B.; Barrett, A. G. M.; Hoffman, B. M.; Mirkin, C. A. J. Phys. Chem. B 2006, 110, 18151. (19) Petit, L.; Quartarolo, A.; Adamo, C.; Riusso, N. J. Phys. Chem. B 2006, 110, 2398. (20) Donzello, M. P.; Ou, Z.; Monacelli, F.; Ricciardi, G.; Rizzoli, C.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2004, 43, 8626. (21) Zaruba, K.; Matejka, P.; Volf, R.; LVolka, K.; Kral, V.; Sessler, J. L. Langmuir 2002, 18, 6896. (22) Zhang, Z.; Imae, T.; Sato, H.; Watanabe, A.; Ozaki, Y. Langmuir 2001, 17, 4564. (23) Souto, J.; Aroca, R.; DeSaja, J. A. J. Raman Spectrosc. 1994, 25, 435. (24) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557. (25) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (26) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (27) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443. (28) Rosa, A.; Ricciardi, G.; Baerends, E. J.; Zimin, M.; Rodgers, M. A. J.; Matsumoto, S.; Ono, N. Inorg. Chem. 2005, 44, 6609. (29) Westerlund, F.; Biornholm, T. Curr. Opin. Colloid Interface Sci. 2009, 14, 126.
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