Guest Controlled Assembly of Gold Nanoparticles Coated with Calix[4

Jul 26, 2010 - Gold nanoparticles protected with thiolate Calix[4]arenes hosts were synthesized through an exchange reaction in toluene, starting from...
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J. Phys. Chem. C 2010, 114, 13601–13607

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Guest Controlled Assembly of Gold Nanoparticles Coated with Calix[4]arene Hosts Flavio Ciesa,†,‡ Anton Plech,*,‡ Cristina Mattioli,† Luca Pescatori,† Arturo Arduini,† Andrea Pochini,† Francesca Rossi,§ and Andrea Secchi*,† Dipartimento di Chimica Organica ed Industriale and Unita` INSTM Sez.4 - UdR di Parma, UniVersita` degli Studi di Parma, Via G. P. Usberti 17/A, I-43124 Parma, Italy, Fachbereich Physik der UniVersita¨t Konstanz, UniVersita¨tsstrasse 10, D-78457 Konstanz, Germany, and IMEM-CNR, Viale Usberti 37/A, I-43124 Parma, Italy ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: July 2, 2010

Gold nanoparticles protected with thiolate Calix[4]arenes hosts were synthesized through an exchange reaction in toluene, starting from tetraoctyl ammonium bromide stabilized gold nanoparticles having a mean core size of ∼6 nm. In low polar solvents, these new Calix[4]arene-coated nanoparticles are able to self-assemble through supramolecular interactions with dialkyl dipyridinium-based guests (2-3). The guest-induced selfassembly process between nanoparticles has been studied using UV-vis spectroscopy, dynamic light scattering, and TEM measurements. The size and the solubility of the aggregates strongly depend on the length and rigidity of the bifunctional guest used as “supramolecular linker” between the nanoparticles. In particular, the long and flexible guest 2 gives rise to superaggregates of nanoparticles that remain soluble in common low polar solvents. 1. Introduction Gold nanoparticles (AuNPs) are hybrid organic-inorganic compounds characterized by a thick organic layer self-assembled on the surface of a core of gold atoms having a nanometric size.1 A very attractive topological property of AuNPs is the possibility to insert, in the organic layer, a discrete number of “effectors” disposed in a radial three-dimensional arrangement. Upon the application of external stimuli, the effectors could promote an extensive networking of nanoparticles.2 When the effectors are molecular receptors, the networking process could be triggered through the recognition of proper guests and, being driven by non-covalent interactions, be in principle reversible. This latter approach could be based on the principles and methods of supramolecular chemistry3 for the development of nanoscale devices, sensors, switches, and nanostructured materials endowed with tunable properties.4 The idea to self-assemble gold nanoparticles through molecular recognition processes has been documented in the literature by authors who showed the feasibility of these processes in solutions.5 To the best of our knowledge, however, the possibility to exploit the cavity of calix[n]arenes6 for assembly and networking processes of AuNPs remains unexplored. A few years ago, we demonstrated that Calix[4]arene-based receptors can be used as an “active” protecting layer for AuNPs.7,8 In particular, we have studied the ability of Calix[4]rene-protected AuNPs to act as multivalent hosts toward organic ion pairs in low polar media.7a These binding properties have also been transferred to aqueous media using water-soluble AuNPs covalently coated with Calix[4]arene hosts which were able to * To whom correspondence should be addressed. E-mail: andrea.secchi@ unipr.it; [email protected]. † Dipartimento di Chimica Organica ed Industriale (F.C.’s present address: Centro per la Sperimentazione Agraria e Forestale Laimburg, Laimburg 6, I-39040 Ora, Italy). Tel: +39 0521 905409. Fax: +39 0521 905472. ‡ Fachbereich Physik der Universita¨t Konstanz (A.P.’s present address: ISS, Forschungs-zentrum Karlsruhe, Tel: +49 7247 8665. Fax: +49 7247 6287). § IMEM-CNR.

Figure 1. Structural formulas of the lower rim thiolate Calix[4]arene host (1) used for the stabilization of the AuNPs and of the monofunctional and bifunctional pyridinium-based organic salts used for the networking of the nanoparticles.

self-assemble onto flat gold surfaces covalently coated with N-alkyl pyridinium cationic guests.7b In this work, we present a study in which AuNPs having a mean core size of ∼6 nm coated with the thiolate Calix[4]arene derivative 1 (see Figure 1) are able to self-assemble through supramolecular interactions with bifunctional guests based on the dialkyl bipyridinium salts 2-3 (see Figure 1) to yield aggregates whose sizes and solubility can be controlled by the length and rigidity of the bifunctional guest used as linker between nanoparticles. 2. Experimental Section 2.1. General. Acetonitrile was freshly distilled under nitrogen and stored over molecular sieves for at least 3 h prior to use. 1H and 13C NMR spectra were recorded on instruments operating at 300 and 75 MHz, respectively. Melting points are uncorrected. Compounds 19 and 57a were synthesized according to reported procedures. Toluene solutions of gold nanoparticles characterized by a mean core of 6 ( 1 nm (see Supporting Information) and stabilized with tetraoctyl ammonium bromide (NP(TOABr)) were prepared following the procedure reported by Brust and Schiffrin.10 2.2. Synthesis. 2.2.1. Calix[4]arene-Protected Gold Nanoparticles. NP(1) 130 mg (0.16 mmol) of Calix[4]arene 1 were added to a 30 mL solution of NP(TOABr) in toluene and the

10.1021/jp105143p  2010 American Chemical Society Published on Web 07/26/2010

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resulting mixture was stirred at room temperature. After 7 days, 40 mL of abs. ethanol were added. The resulting solution was centrifuged at 5000 rpm for 15 min. The surnatant was eliminated and the precipitate taken up with 40 mL of toluene. The resulting solution was then centrifuged at 1500 rpm for 10 min. The red-ruby surnatant solution was collected and diluted with 40 mL of abs. ethanol and centrifuged again at 10 000 rpm for 20 min. After the removal of the surnatant, a red ruby solution (in toluene: λmax ) 528 nm) of NP(1) nanoparticles was then prepared taking up the resulting dark precipitate with 40 mL of toluene. Elemental analysis: C, 14.26; H, 1.51; S, 1.13. IR (KBr, cm-1): 3432 (w), 2923 (s), 2852 (s), 1464 (w). 13 C NMR (CDCl3): δ (ppm) 153.3, 151.9, 133.5, 128.8, 128.3, 125.2, 118.9, 77.1, 59.2, 31.6, 31.4, 30.1, 30.0, 29.9, 29.7, 29.1, 29.0, 27.2, 27.0, 26.4, 22.6, 22.2, 14.0. 2.2.2. General Procedure for the Synthesis of Bifunctional Guests (2) and (3). A solution of pyridine (0.6 g, 7.6 mmol) and the diiodide (3 mmol) in 50 mL of acetonitrile was refluxed at 90 °C in a sealed glass autoclave. After 2 days, the mixture was cooled at room temperature and diluted with 200 mL of ethyl acetate. The pale yellow solid that precipitates from the solution was collected by suction filtration and washed with few portions of ethyl acetate (3 × 10 mL). 2.2.3. 1,1′-(Decane-1,10-diyl)Dipyridinium Iodide (2). 1.5 g (yield: 93%). M.p.: 172.5-173.5 °C; 1H NMR (CD3OD, 300 MHz): δ (ppm) 9.04 (4H, d, J ) 5.2 Hz), 8.60 (2H, t, J ) 7.8 Hz), 8.13 (4H, bt), 4.66 (4H, t, J ) 7.8 Hz), 2.1-2.0 (4H, m), 1.4-1.2 (12H, m); 13C (CD3OD, 75 MHz) δ (ppm) 147.2, 146.2, 129.8, 63.4, 32.7, 30.5, 30.2, 27.4; MS-ESI(+) m/z: 425.1 [M-I]+. Elemental Analysis for C20H30N2I2, calculated: C, 43.50; H, 5.48; N, 5.07; observed: C, 43.12; H, 5.61, N, 5.12. 2.2.4. 1,1′-(Pentane-1,5-diyl)Dipyridinium Iodide (3). 1.4 g (yield: 95%). M.p.: 144.5-145.3 °C; 1H NMR (CD3OD, 300 MHz): δ (ppm) 9.11 (4H, d, J ) 5.7 Hz), 8.62 (2H, t, J ) 7.8 Hz), 8.14 (4H, bt), 4.73 (4H, t, J ) 7.6 Hz), 2.2-2.1 (4H, m), 1.5-1.4 (2H, m); 13C (CD3OD, 75 MHz) δ (ppm) 147.3, 146.4, 129.9, 62.8, 32.0, 24.0; MS-ESI(+) m/z: 355.0 [M-I]+. Elemental Analysis for C15H20N2I2, calculated: C, 37.37; H, 4.18; N, 5.81; observed: C, 37.21; H, 4.22, N, 5.73. 2.2.5. Synthesis of N-Octylpyridinium Iodide (4). A solution of pyridine (2.4 g, 30 mol) and 1-iodooctane (2.4 g, 10 mmol) in 100 mL of acetonitrile was refluxed at 90 °C in a sealed glass autoclave. After 2 days, the mixture was cooled at room temperature and the solvent evaporated to dryness under reduced pressure. The sticky residue was taken up with 100 mL of ethyl acetate. After cooling the solution at -20 °C overnight, 2.7 g (86%) of pure 4 were collected by suction filtration as a low melting and very hygroscopic yellow solid. 1H NMR (CDCl3, 300 MHz): δ (ppm) 9.33 (2H, d, J ) 5.4 Hz), 8.53 (1H, t, J ) 7.8 Hz), 8.13 (2H, bt), 4.96 (2H, t, J ) 7.5 Hz), 2.1-2.0 (2H, m), 1.4-1.2 (10H, m), 0.86 (3H, t, J ) 6.4 Hz); 13C (D2O, 75 MHz) δ (ppm) 149.0, 147.6, 131.6, 65.2, 34.6, 34.2, 31.8, 31.7, 28.8, 25.5, 17.0; MS-ESI(+) m/z: 192.1 [M-I]+. Elemental Analysis for C13H22NI, calculated: C, 48.91; H, 6.95; N, 4.39; observed: C, 48.65; H, 7.15, N, 4.28. 2.3. TEM Measurements. Measurements were carried out at CNR-IMEM (Parma) on a Jeol 2200FS field-emission microscope. The clusters size distribution was determined with the ImageJ software11 by statistical analysis of more than 300 clusters taken from at least three images for each sample. 2.4. UV-vis and DLS Aggregation Experiments. The guest-induced aggregation experiments were carried out by titrating chloroform solution of nanoparticles NP(1) with small aliquots of a chloroform solution containing guest of different

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Figure 2. Schematic representation of the synthesis of the Calix[4]areneprotected AuNPs NP(1) starting from tetraoctyl ammonium-stabilized ones NP(TOABr).

type (2-5). Typically, a UV-vis assembly experiment was carried out in 1 cm path cuvette by adding increasing amounts of the guest solution (2-4: c ) 10-3 M, 5: 10-5 M) to a 1 mL solution of the Calix[4]arene-protected nanoparticles NP(1) obtained dissolving 10 mg of the solid nanoparticles in 100 mL of chloroform. The variation of the maximum of the SPB of the nanoparticles was monitored upon each guest addition (up to 15 additions were accomplished corresponding to 200 µL of added guest solution). The DLS measurements were taken with a home-built correlator setup with a 1.2 mW HeNe laser as light source, a scattering geometry at 90° in 2Θ and s polarization. The detector was a single mode fiber coupled avalanche Geiger module (SensL) with a time resolution of 60 ns. The single photon signal was correlated with a 480 ns resolution correlator (correlator. com) and read by a PC. Due to the high particle density the contrast in the autocorrelation function was limited to about 0.15 for the aggregates, which still allowed determining the correlation time with good precision within 2-5 min. The nanoparticles suspension was placed in a standard 4 sided cuvette with 10 mm path length. 3. Results and Discussion 3.1. Synthesis and Characterization of the Calix[4]areneCoated Nanoparticles. Samples of AuNPs protected with the Calix[4]arene 1 (NP(1)) were prepared through an exchange reaction12 (see Figure 2) starting from a toluene solution of freshly prepared ∼6 nm AuNPs stabilized with tetraoctyl ammonium bromide10 (NP(TOABr)). Thiolate Calix[4]arene 1 was chosen as stabilizer because it can be anchored on the gold surface of the nanoparticles through its two long ω-thiolate alkyl (C11) chains promoting the formation of stable Au NPs.7a This anchoring mode has the advantage of maintaining the receptor binding site oriented toward the bulk and thus retaining its potential recognition properties.13 The TEM images collected from the NP(1) sample were analyzed with ImageJ,11 in order to determine the core size distribution of the exchanged nanoparticles. The distribution diagram and the superimposed Gaussian distribution depicted in Figure 3 revealed that the insertion of 1 on the gold surface did not substantially affect the size of the Au NPs. The mean core diameter dTEM of these NPs is still centered at 6 nm. The two samples of nanoparticles were also analyzed using UV-vis spectroscopy. This technique mainly detects the characteristic surface plasmon band (SPB). In toluene solution, NP(1) and NP(TOABr) are both endowed with a SPB centered at λ ≈ 526 nm. In chloroform solution, the maximum of this band is slightly red-shifted to λ ≈ 530 nm, but a second band appears in the UV part of the spectrum at λ ≈ 270 nm, which

Guest Controlled Assembly of Gold Nanoparticles

Figure 3. (a) TEM image and (b) core size distribution of nanoparticles NP(1).

stems from to the absorption of the Calix[4]arene phenolic nuclei of 1 (see the Supporting Information). The presence of this band confirmed that several units of 1 were inserted as stabilizing layer on the surface of the AuNPs. The elemental analysis carried out on sample NP(1) gave us further evidence of the attainment of the exchange reaction. The organic fraction was ∼18% with a sulfur content of 1.13% (see the Experimental Section). No traces of nitrogen content were found, thus indicating that the NP(1) sample was not contaminated with TOABr. The calculated S/Au ratio of 0.09 obtained from the elemental analysis is in agreement with the achievement of ∼6 nm AuNPs coated with 250-300 Calix[4]arene units.14 3.2. Guest-Induced Aggregation Experiments. The ability of the Calix[4]arene-protected nanoparticles to give rise to a superlattice of nanoparticles through the complexation of suitable bifunctional guests was evaluated in chloroform solution using UV-vis spectroscopy as investigation tool. With this technique it is indeed possible to monitor the maximum of the SPB of the AuNPs, which is sensitive to the size of the nanoparticles as well as to nanoparticle-nanoparticle electromagnetic coupling.15 As bifunctional guests we initially employed the dipyridinium diiodides 2 and 3 depicted in Figure 1. These organic salts were synthesized in high yields (>90%) by reaction of pyridine in refluxing acetonitrile with the corresponding alkyl diodide (see the Experimental Section). They experience good solubility in low polar solvents such as chloroform and toluene and present, as common structural feature, two pyridinium units linked through a flexible alkyl

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13603 chain of 10 and 5 methylene groups in 2 and 3, respectively. The affinity of this type of guest for the aromatic cavity of preorganized Calix[4]arene derivatives has been already documented.9,16 In a typical UV-vis titration experiment, aliquots of a 10-3 M solution of the guest were added to a 1 mL of a 0.1% w/v solution of the nanoparticles. The position of the nanoparticles’ SPB was monitored after each guest addition. As indicated by the collection spectra depicted in Figure 4, parts a and b, the addition of both guests determines a red shifting of the SPB maximum by approximately 30-40 nm, accompanied by peak broadening and extinction reduction. However, a careful comparison of the two plots evidences that the nature of the linker affects the course of the changes seen at the SPB during the titrations. In particular, the stepwise addition of 30 µL of solution of linker 2 first induced a smooth reduction of the SPB extinction, originally centered at λ ) 530 nm, with no appreciable red-shifting. After this titration point has been reached, the further addition of the linker progressively shifts the maximum of the SPB with the formation of a recognizable isosbestic point at ∼565 nm. The linker addition also determined a marked extinction decrease of the plasmon peak now centered at λ ) 570 with broadening of the band of a factor ∼1.8 (see Figure 4a). In contrast to 2, the experiment carried out using the short and less flexible bifunctional guest 3 as the linker showed a more abrupt decrease of the SPB extinction in the initial stages of the titration (see Figure 4b). Also in this case the progressive red-shifting of the SPB becomes apparent after the addition of at least 30-40 µL of the linker solution. It is worth observing that, different from 2, the extinction corresponding to the new maximum centered at 560 nm slightly increases for addition of the guest solution from 60 to 80 µL. Most importantly, from this titration point and afterward, the intensity of the SPB centered at λ ≈ 560 nm is progressively reduced and blue-shifted at λ ≈ 545 nm, as a second broad band at longer wavelength (λ > 700 nm) starts to develop (see Figure 4b). At the end of the titration (up to 200 µL of guest solution), when the amount of 3 in solution is higher than 0.2 µmol, the low energy band is still developing and the color of the solution has changed from the original ruby-red to darkblue. To confirm that these aggregation processes were effectively driven by the complexation of the supramolecular linkers by means of Calix[4]arene cavities resident on the surface of different nanoparticles, we devised two complementary titration experiments. In the first one, a chloroform solution of the nanoparticles NP(1) was titrated with a 10-3 M solution of N-octyl pyridinium iodide (4) (see Figure 1). This salt represents, in fact, an analogue of linkers 2 and 3 lacking of a pyridinium unit. As a consequence, it can be potentially complexed by the Calix[4]arene receptors present on the nanoparticles surfaces but it cannot promote their aggregation. The collection of spectra obtained from such titration has been depicted in Figure 4c: as expected the guest addition caused a reduction of the SPB extinction, mainly due to dilution effects, but no detectable shift at longer wavelengths of its maximum was observed. The second titration experiment was instead accomplished by adding a 10-3 M chloroform solution of 3 to a 0.1% w/v chloroform solution of ∼6 nm AuNPs protected with n-dodecanthiol.17 It is easily foreseen that these nanoparticles, lacking on their surfaces of the Calix[4]arene receptors, cannot aggregate upon the action of the supramolecular linkers. Indeed, the collection of spectra depicted in Figure 4d shows that the addition of 3

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Figure 4. Collection of UV-vis spectra recorded in CHCl3 solution during the titrations of nanoparticles NP(1) with guests (a) 2, (b) 3, (c) 4, and (d) of ∼6 nm n-dodecanthiol-protected AuNPs with guest 3. In each collection of spectra, the optical absorption of the free titrant has been depicted with a dashed line.

does not induce any red-shift of the SPB, whose intensity reduction it is reasonably ascribed to a dilution effects. It is also worth mentioning that the aggregation induced by the supramolecular linkers 2 and 3 is a reversible process. Indeed the SPB of the nanoparticles NP(1), originally centered at λ ) 530 nm, can be restored after the titration through treatment of the titrated solution with a polar solvent such as DMSO (see SI). In order to explain the different aggregation behavior experienced by 2 and 3 we considered that, as seen in other studies, the red-shifting and the broadening of SPB can be associated with an increasing size of the nanoparticles aggregates.18,19 The formation of a new broad low energy band at λ > 700 nm observed exclusively upon the addition of the short and less flexible linker 3 (cf. Figure 4, parts a and b) can be reasonably ascribed either to the formation of very compact aggregates, endowed with a lesser interparticle separation than those formed by 2.20,21 In order to tentatively describe the structure of these aggregates we devised a TEM experiments. The solution of the nanoparticles NP(1) treated with 3 was slowly evaporated on the TEM grid. After ultra high vacuum treatment, the resulting powder was submitted to TEM measurements. In Figure 5 the TEM images have been depicted, taken at different magnification, of the nanoparticles NP(1) before and after their aggregation with 3. Figure 5c clearly shows as the bifunctional guest promotes the formation of a tridimensional array of nanoparticles, in which several layers of nanoparticles

Figure 5. TEM images of nanoparticles NP(1) (a) before and (b),(c) after their aggregation with 3 (different magnification), and (d) after their aggregation with 2.

are superimposed. It is also interesting to note as, on the TEM grid, the aggregate assumes a “snake-like” shape (see Figure 5b). In a few cases, the close proximity of the particles nuclei

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determined the formation of “melted” larger nanoparticles having diameters ranging from 10 to 40 nm. The same experiment carried out using 2 yielded similar results (see Figure 5d), although the aggregates formed with this longer linker seems having more globular domains (see also SI). After having evaluated the aggregation properties of the flexible guests 2 and 3, we studied the aggregation behavior of more rigid and directional bifunctional guest: the N,N′-dimethylviologen ditosylate (5), that is a known redox active compound whose ability to coordinate simultaneously the aromatic cavity of two Calix[4]arene compounds similar to 1 has been recently observed.9 After having determined an initial red-shifting of the SPB from 530 to 550 nm (see SI), the stepwise addition of a very diluted solution (10-5 M) of this linker unfortunately determined the precipitation from the solution of a black solid material. Unlike 2 and 3, this aggregation process was found to be irreversible. Any treatment of the heterogeneous mixture with very polar solvents such as DMF and DMSO did not restore the dispersed nanoparticles. The collapsing of the colloid was reasonably ascribed to the high structural rigidity and directionality of 5 that generates a very tight 3D-network of selfassembled nanoparticles. 3.3. Dynamic Light Scattering Studies The results obtained through the UV-vis titration suggest that the length of the alkyl chain present in 2 and 3 substantially affects the assembly process of the nanoparticles. More insight into the growth and the size of the aggregates of nanoparticles was obtained through Dynamic Light Scattering (DLS) measurements.22 Dynamic light scattering probes the decay of correlation (coherence) of the scattered light from the solutes due to diffusional motion, which is directly related to their size and the properties of the solvent (refractive index and viscosity). The exponential decay rate Γ of the intensity autocorrelation /2 is proportional to the diffusion coefficient D as Γ ) q2D with the scattering vector q ) 4π/λsin(2Θ/2) and the scattering angle 2Θ. The hydrodynamic diameter (dh) of the aggregates of nanoparticles present in solution is calculated with the formula:

dh )

2 32 · π · nliq · kT

3 · η · λ2

· sin(Θ)*τ

where nliq is the refractive index of solvent, kT are the Boltzmann constant and the temperature, η is the dynamic viscosity of solvent, λ is the wavelength of light, Θ is the scattering angle, and τ is the correlation time. Figure 6a depicts the correlation functions and the fit with the generic function exp(-t/τ) during the titration of the nanoparticles NP(1) with a 5 × 10-6 M chloroform solution of the short bifunctional guest 3. For concentration of 3 lower than 10-6 M the light scattering yield remains weak which limits the precision of the correlation function determination (see black squares in Figure 6a). At this concentration, the calculated hydrodynamic diameter of the nanoparticles remains in the range of 10-20 nm (see inset of Figure 6a).23 Aggregates of nanoparticles having dh of 700-800 nm start to form abruptly in solution when the linker reaches a concentration of ∼0.7-0.8 × 10-6 M (see inset of Figure 6a). It should also be observed that in this guest concentration range small species having a 10-20 nm size are coexistent in solution with the larger aggregates (see filled gray circles in the inset of Figure 6a). For guest concentrations higher than 5 × 10-6 M, the only species

Figure 6. Dynamic Light Scattering titrations of NP(1) with (a) 3 and (b) 2. The black squares, white circles, and black triangles represent the observed correlation function derived from light scattering for increasing amount of guest added (the solid lines mark the monobiexponential fits of the correlation decay, respectively). The inset shows the dependence of the calculated hydrodynamic diameter of the aggregates on guest addition. Filled gray dots mark bimodal distributions.

diffusing in solution are aggregates with dh of ∼1 µm. It is interesting to observe that, unlike that from 3, the formation of the aggregates with the longer linker 2 occurs at higher concentration of the salt (1.5-2 × 10-6 M) and also that the size of these aggregates increases smoothly, assuming a stepwise course, and it never exceeds 600 nm (see inset of Figure 6d). To explain the different aggregation behavior induced by 2, 3, and 5, we tentatively devised the simplified binding model described in Figure 7. The interaction of nanoparticles NP(1)x, where x is the average number of Calix[4]arene sites present on each nanoparticle, with an amount y of bifunctional linker G-G may follow two different and competitive pathways: (a) the partial “saturation” of the receptor sites present on the NPs through the formation of intramolecular complexes in which the two pyridinium moieties of G-G species are complexed by two nearby Calix[4]arene cavities present on the same nanoparticle, or (b) an intermolecular complexation where the G-G species are bound by Calix[4]arene cavities present on different nanoparticles, thus creating an effective network of NPs. The addition of an excess of G-G to the intra- and intermolecular adducts might originate, respectively, a soluble superlattice of nanoparticles (path c) or a tight and low soluble 3D-network of nanoparticles that collapses from the solution, eventually (path d). By assuming that any single complexation event will be equally favored and independent from the others, some considerations can be done. First, when the amount of linker G-G (y) presents in solution is less than the overall number of Calix[4]arene sites (nx) present on the nanoparticles, the formation of the intramolecular adduct is reasonably more favored for entropic reasons then the 3D-networking process.

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Ciesa et al. that the aggregation process can be modulated by the nature of the bifunctional supramolecular linker. In particular, the long and flexible 2 gives rise to superaggregates of nanoparticles that maintain solubility properties in common low polar solvents. The competition between intra- and intermolecular binding should have a strong impact on the topology of the network, which should be the subject of further investigation. Studies aimed to render these aggregation processes reversible through external (electro)chemical stimuli are in due course in our laboratories. Acknowledgment. This research was partly supported by the University of Parma. We thank the Centro Interdipartimentale di Misure “G. Casnati” of the University of Parma for Mass and NMR measurements. The partial support by SFB 513 in Konstanz is acknowledged.

Figure 7. Simplified binding model representing the initial stages of the guest-induced aggregation process of NP(1) with the flexible linker 2 and the more rigid and directional linkers 3 or 5. The addition of 2 may give rise to intramolecular adducts (a) which then evolve in a soluble superlattice of nanoparticles (c). The addition of the more rigid linkers should induce the formation of a more compact 3D-network of nanoparticles that collapses from the solution, eventually.

Second, the 3D-networking process may instead occur with any of the linkers, regardless their length and structural flexibility. Third, the formation of the intramolecular adducts is reasonably allowed only with the flexible bis-pyridinium linkers 2 and 3. The optical and DLS experiments have demonstrated that the growth of the aggregates formed with 2 and 3 is likely regulated by the competition of the two complexation paths (a) and (b). The formation of the intramolecular adducts should be more significant with the longer and flexible 2. As a result, the aggregates formed with 2 are less packed and more solvent accessible (dh ) 0.5-0.6 µm) than those promoted by the shorter 3 (dh ) 0.8-1 µm). The noteworthy stepwise dependence of SPB position (UV) and size of the aggregates (DLS) on the concentration of the supramolecular linker can be explained considering that such processes are limited by the diffusion of the species in solution. Indeed, the formation of the intramolecular adduct is a fast process since a single G-G diffuses quickly toward a recognition site present on the surface of a nanoparticle, while the formation of the intermolecular adduct, that is the networking process, is a slow process because it derives from the encounter of two slowing diffusing nanoparticles. The observation that the aggregates do not collapse from the chloroform solution even when a large excess of 2 is added support this hypothesis. The irreversible nanoparticles precipitation always observed upon addition of the ditosylate 5 may be reasonably explained considering that this type of rigid linker yields very stable and directional intermolecular bindings with no possibilities to form intramolecular bridges. This results in a considerable loss of entropy of the growing superlattice that precipitates from the solution, eventually. 4. Conclusions The guest-induced aggregation process between nanoparticles protected with the thiolate Calix[4]arene 1 has been tackled and studied using UV-vis and DLS techniques. It has been shown

Supporting Information Available: TEM images and core size distribution diagrams of NP(TOABr) and NP(1) Au nanoparticles; UV-vis spectra of NP(TOABr) and NP(1) Au nanoparticles in chloroform and toluene solution; UV-vis titration in chloroform solution of NP(1) with linker 5 and solvent-switched UV-vis aggregation experiment of NP(1) with linker 3. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (b) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (c) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36, and references therein. (2) See, for example: (a) Lim, S. I.; Zhong, C.-J. Acc. Chem. Res. 2009, 42, 798–808. (b) Westerlund, F.; Bjørnholm, T. Curr. Opin. Colloid Interface Sci. 2009, 14, 126–134. (c) Prasad, B. L. V.; Sorensen, C. M.; Klabunde, K. J. Chem. Soc. ReV. 2008, 7, 1871–1883, and references therein. (3) Core Concepts in Supramolecular Chemistry and Nanochemistry; Steed, J. W., Turner, D. R., Wallace, K. J., Eds.; John Wiley & Sons: Chichester, 2007. (4) See, for example: (a) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. ReV. 2010, 110, 389–485. (b) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25, 13840–13851. (c) Descalzo, A. B.; Martı´nez-Ma´n˜ez, R.; Sanceno´n, F.; Hoffmann, K.; Rurack, K. Angew. Chem., Int. Ed. 2006, 45, 5924–5948. (d) Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem.sEur. J. 2004, 10, 5570–5579, and references therein. (5) (a) Kinge, S. S.; Crego-Calama, M.; Reinhoudt, D. N. Langmuir 2007, 23, 8772–8777. (b) Liu, Z.; Jiang, M. J. Mater. Chem. 2007, 17, 4249–4254. (c) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R. L.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304–4305. (6) Gutsche, C. D. Calixarenes: An Introduction (Monographs in Supramolecular Chemistry), 2nd ed.; RSC Publishing: Cambridge, UK, 2008. (7) (a) Arduini, A.; Demuru, D.; Pochini, A.; Secchi, A. Chem. Commun. 2005, 645–647. (b) Tshikhudo, T. R.; Demuru, D.; Wang, Z. X.; Brust, M.; Secchi, A.; Arduini, A.; Pochini, A. Angew. Chem., Int. Ed. 2005, 44, 2913–2916. (8) For other Calix[4]arene-protected AuNPs, see: (a) Ha, J.-M.; Solovyov, A.; Katz, A. Langmuir 2009, 25, 10548–10553. (b) Huc, V.; Pelzer, K. J. Colloid Interface Sci. 2008, 318, 1–4. (9) Pescatori, L.; Arduini, A.; Pochini, A.; Secchi, A.; Massera, C.; Ugozzoli, F. Org. Biomol. Chem. 2009, 7, 3698–3708. (10) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425–5429. (11) ImageJ, Image Processing and Analysis in JaVa, by Rasband, W., National Institute of Health, USA, see: http://rsb.info.nih.gov/ij/index.html. (12) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 63782–3789. (13) The partial functionalization of the phenolic groups of Calix[4]arene derivatives increases the preorganization of the macrocycle skeleton and strongly enhances their recognition properties in low polar solvents towards neutral and charged species; see ref 9 and references therein. (14) The S/Au ratio of ∼0.09 also accounts for AuNPs having a 25% of surface coverage and with a “footprint” for each thiolate alkyl chain of 1 ranging between 0.25 and 0.3 nm2; for the determination of the “footprint”

Guest Controlled Assembly of Gold Nanoparticles of thiolate ligands. See, for example: Fabris, L.; Antonello, S.; Armelao, L.; Donkers, R. L.; Polo, F.; Toniolo, C.; Maran, F. J. Am. Chem. Soc. 2006, 128, 326–336. (15) (a) Jain, P. K.; Huang, W.; El-Sayed, M. A. Nano Lett. 2007, 7, 2080–2088. (b) Schmitt, J.; Ma¨chtle, P.; Eck, D.; Mo¨hwald, H.; Helm, C. A. Langmuir 1999, 15, 3256–3266. (c) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters, Springer: Berlin, 1995. (16) Arduini, A.; Pochini, A.; Secchi, A. Eur. J. Org. Chem. 2000, 2325– 2334. (17) The AuNPS protected with n-dodecanthiol were prepared through a ligand exchange reaction starting from the batch of NP(TOABr) nanoparticles used for the preparation of NP(1). (18) See, for example: (a) Yan, H.; Xiong, C.; Yuan, H.; Zeng, Z.; Ling, L. J. Phys. Chem. C 2009, 113, 17326–17331. (b) Njoki, P. N.; Lim, I. I. S.; Mott, D.; Park, H.-Y.; Khan, B.; Mishra, S.; Sujakumar, R.; Luo, J.; Zhong, C.-J. J. Phys. Chem. C 2007, 111, 14664–14669. (c) Kinge, S. S.; CregoCalama, M.; Reinhoudt, D. N. Langmuir 2007, 23, 8772–8777. (d) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A. J. Phys. Chem. B 2006, 110, 15700–15707. (19) Electrodynamics calculations have been carried out to simulate the shape and the peak position of the SPB of AuNPs aggregates in water, see: Lazarides, A. A.; Schatz, G. C. J. Phys. Chem. B 2000, 104, 460–467.

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13607 (20) Simple molecular mechanics (MMFF force field) calculations carried out on 2:1 complex formed between two Calix[4]arenes units of 1 and one of 3 show that the maximum distance present between sulfur atoms is within the 5 nm range, thus comparable with the diameter determined through the TEM measurements. Arduini A., Pochini A., Secchi A., unpublished results. (21) The presence of broad low energy band at λ > 700 nm has been also interpreted as a longitudinal excitation of the plasmon resonance typical for example of nanorods and nanowires; see, for example: (a) Hussain, I.; Brust, M.; Barauskas, J.; Cooper, A. I. Langmuir 2009, 25, 1934–1939. (b) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209–217. (c) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409–435. (22) Chu, B. Laser Light Scattering: Basic Principles and Practice, 2nd ed.; Dover Publications: 2007. (23) Molecular mechanics simulations show as the thickness of the organic shell around the core of the nanoparticles NP(1) is ∼2 nm. The hydrodynamic diameter range (dh ) 10-20 nm) calculated from the DLS measurements is thus in agreement with the mean core diameter (dTEM ≈ 6nm, see Figure 2) determined through the TEM measurement.

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