Deaggregation of Gold Nanoparticles Induced

May 11, 2005 - We show in this paper how the aggregation of gold colloids ..... the SP band of the gold nanoparticles upon addition of the 1/2 thiols ...
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Langmuir 2005, 21, 5537-5541

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Reversible Aggregation/Deaggregation of Gold Nanoparticles Induced by a Cleavable Dithiol Linker Cristian Guarise,† Lucia Pasquato,*,‡ and Paolo Scrimin*,† University of Padova, Department of Chemical Sciences and ITM-CNR, Padova Section, Via Marzolo, 1-35131 Padova, Italy, and University of Trieste, Department of Chemical Sciences, Via Giorgieri, 1-34127 Trieste, Italy Received December 3, 2004. In Final Form: January 24, 2005 Aqueous solutions of Au colloids (12 ( 4 nm size) when treated with a blend of mono- and dithiols aggregate forming stable clusters, as evidenced by the shift of their surface plasmon (SP) band from 512 to ca. 600 nm. The presence of carboxylate ester functions on the dithiol allows its cleavage by addition of a cleaving agent, such as hydrazine. The cleavage process results in the breaking down of the clusters of nanoparticles and the shift of the SP band back to lower wavelengths. Further addition of dithiol causes the formation of the clusters again. The aggregation/deaggregation process may be monitored visually by following the color change from pink-red to purple and vice versa in the forward and backward steps, respectively.

Introduction There is an increasing interest in nanoparticles and gold nanoparticles in particular because of the many fields in which they find applications, ranging from catalysis, medicine, and electronics, just to mention a few. This paper deals with the optical properties of gold nanoparticles as a means to obtain information on processes occurring in the solution where they are dissolved. This is possible because when the dimensions of these systems are close to 5 nm (or larger) they present an absorption band at 510-520 nm due to the excitation of the surface plasmon (SP)1 that shifts to longer wavelengths upon cluster formation. The progress of the aggregation can be followed with the naked eye as the solution changes from pink-red to purple-blue. The possibility to control nanoparticle aggregation has been exploited in many areas, in particular, for chemical sensing.2 One of the more interesting application is probably the detection of DNA oligomers where incredible sensibilities have been obtained.2,3,4 A number of examples of irreversible5 aggregation have been reported. Reversible aggregation6 is more difficult to obtain because the aggregated nanoparticles have the tendency to collapse to larger, insoluble materials. They can be made * Authors to whom correspondence should be addressed. † University of Padova. ‡ University of Trieste. (1) For a review on plasmonic nanomaterials, see: Wei, A. In Nanoparticles. Building Blocks for Nanotecnology; Rotello, V., Ed.; Kluver Academic/Plenum Publishers: New York, 2003; pp 173-200. (2) For recent reviews, see: (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (b) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849-1862. (c) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346. (d) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128-4158. (3) Recent reviews: Schultz, D. A. Curr. Opin. Biotechnol. 2003, 13, 13-22. Pasquato, L.; Pengo, P.; Scrimin, P. Nanoparticles. Building Blocks for Nanotecnology; Rotello, V., Ed.; Kluver Academic/Plenum Publishers: New York, 2003; pp 251-282. (4) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (b) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757-1760. (c) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609-611. (5) (a) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-799. (b) Chen, S. Adv. Mater. 2000, 12, 186-189. (c) Mayya, K. S.; Patil, V.; Sastry, M. Bull. Chem. Soc. Jpn. 2000, 73, 1757-1761. (d) Sharma, J.; Chaki, N. K.; Mandale, A. B.; Pasricha, R.; Vijayamohanan, K. J. Colloid Interface Sci. 2004, 272, 145-152.

more stable by covering the gold core with an appropriate protective monolayer (monolayer-protected gold nanoclusters, Au-MPCs) typically constituted by organic thiols because of the strong interaction of this functional group with the gold surface of the nanoparticle. Hydrocarbonlike thiols provide MPCs soluble in apolar solvents,1a,1c,7 while thiols functionalized with polyether8 or other polar functional groups9 may be used as capping agents in aqueous solution. Results and Discussion We show in this paper how the aggregation of gold colloids induced by the addition of a dithiol can be reversibly controlled by chemically cleaving a labile function present in the dithiol itself. The concept is represented in a pictorial way in Figure 1. Thus, in (a), the colloidal nanoparticles aggregate because of the addition of the dithiol, while in (b), the cleavage of the dithiol results in the deaggregation of the colloids. Further addition of the dithiol induces their aggregation again in (c). This constitutes the first example of deaggregation of a cluster of gold nanoparticles by cleavage of a covalent bond. To prevent the irreversible precipitation of the aggregated nanoparticles, they had to be covered by a protective monolayer, and for this purpose, the dithiol employed to induce the aggregation process was used in (6) (a) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682-6688. (b) de la Fuente, J. M.; Barrientos, A. G.; Rojas, T. C.; Rojo, J.; Can˜ada, J.; Fernandez, A.; Penades, S. Angew. Chem., Int. Ed. 2001, 40, 2258-2261. (c) Otsuka, H.; Akiyama, Y.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2001, 123, 8226-8230. (d) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958-4959. (7) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475-481. (8) Pengo, P.; Polizzi, S.; Battagliarin, M.; Pasquato, L.; Scrimin, P. J. Mater. Chem. 2003, 13, 2471-2478. (9) (a) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 12696-12697. (b) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (c) Gittins, D. I.; Caruso, F.; ChemPhysChem 2002, 1, 110-113. (d) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M. Chem. Commun. 2002, 2294-2295. (e) Foos, E. E.; Snow, A. W.; Twigg, M. E.; Ancona, M. G. Chem. Mater. 2002, 14, 2401-2408. (f) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19, 3545-3549.

10.1021/la0470232 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005

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Figure 1. Pictorial representation of the aggregation/deaggregation processes occurring to the gold nanoparticles upon addition of the thiol blend (a), hydrazine (b) and thiol 2 again (c). The same processes (a-c) lead to the changes of the maximum of the SP band reported in Figure 5. Scheme 1. Structure of Thiols 1 and 2 and Synthetic Procedure Followed for the Obtainment of the Latter.

conjunction with monothiol 1, previously described by us for the preparation of monolayer-protected gold nanoparticles soluble in aqueous solution.8 The structure of dithiol 2 comprises two 16-mercaptohexadecanoic acid groups connected by esterification with triethylene glycol. The two ester moieties constitute the cleavable units. The synthetic protocol followed for its synthesis is reported in Scheme 1. For our experiments, we used a commercial aqueous solutions of “naked” gold colloids of 10 nm nominal size.10 Their stability in the aqueous solution is guaranteed by the presence of 0.04% trisodium citrate. Dynamic light scattering measurements of different batches of these colloids revealed a rather broad size distribution that varies from sample to sample (average 12 ( 4 nm). This difference in size had no relevance in the outcome of the experiments reported below. We first determined the minimal concentration of thiols required to cover the gold surface and prevent precipitation of the gold colloids when they aggregate. Solutions of “naked” colloids are rather sensitive to the ionic strength of the medium, and when this is increased (for instance, by addition of a concentrated solution of NaCl), the pink color turns quickly blue because of the formation of aggregates. Upon standing, this color fades and a dark

precipitate eventually forms.11 Thus, the absorbance of “naked” colloids in the 600 nm region immediately increases when they are treated with concentrated NaCl. In contrast, thiol-passivated colloids do not aggregate and their solution remains pink after this treatment. Figure 2 reports the absorbance of the colloids at 600 nm upon treatment with concentrated NaCl ([NaCl]final ) 0.09 M) as a function of the concentration of the thiol blend added. The profile may be divided into two regions: below and above the 1.2 × 10-5 M thiol concentration. Below this concentration, there is too little thiol added and the nanoparticles aggregate (intense blue color develops by treatment with NaCl). Solutions of these aggregates are unstable, and a precipitate is formed after several minutes. By increasing the concentration of thiols added, the aggregation progressively decreases and, at [thiols]total) 1.2 × 10-5 M, no aggregation is observed (i.e., the solution remains pink, and no band develops at 600 nm). Since, however, the thiol blend contains dithiol 2 (15:1 1/2 blend), upon further increasing the concentration of thiols, the absorbance at 600 nm increases again due to the aggregation process induced by dithiol 2, as will be discussed in detail below. At variance with the aggregates formed in the absence or with too little added thiols, they remain stable due to the presence of the passivating

(10) Throughout the text, we will refer to these colloids as 10 nm nanoparticles, although their size is slightly larger than the nominal one, as determined from the light scattering experiments.

(11) For a similar approach, see: Le´vy, R.; Thanh, N. T. K.; Doty, R. C.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J.; Brust, M.; Fernig, D. G. J. Am. Chem. Soc. 2004, 126, 10076-10084.

Reversible Aggregation/Deaggregation of Gold Nanoparticles

Figure 2. Effect of the addition of NaCl ([NaCl] ) 0.09 M) to the gold colloids solution previously treated with different concentrations of thiols (15:1 thiol 1/2 blend).

Figure 3. Change of the absorbance in the 400-700 nm region for Au colloids upon addition of a 1/2 thiol blend ([thiols]total ) 3.0 × 10-5 M, [2]/[1] blend)1:15, curves were taken at 2 min intervals). The dotted line represents the final absorbance profile reached after several hours.

Figure 4. Changes of the apparent hydrodynamic diameter of the nanoparticles upon addition of the thiol 1/dithiol 2 blend. Conditions are the same as in Figure 3.

monolayer and no precipitate is formed. From the experiments reported in Figure 2, the minimum concentration of thiols preventing the formation of unstable aggregates proved to be 1.5 × 10-5 M. Because of this, all the experiments described below have been performed above this critical concentration. We have already anticipated (see above) that colloids exposed to dithiol 2 aggregate. The kinetics of this process are reported in Figure 3 where the change of absorbance in the 400-700 nm region for colloids treated with a blend of thiols 1 and 2 ([thiols]total ) 3.0 × 10-5 M) is reported. Following the thiol addition, the maximum of the SP band starts to shift from 514 nm to a longer wavelength. The process, associated with the formation of clusters of nanoparticles, as independently confirmed by dynamic light scattering experiments (see Figure 4), could be conveniently monitored by following the increase of absorbance at 600 nm.12

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Figure 5. Increase of the absorbance at 600 nm (left abscissa) and corresponding change of the maximum of the SP band (right abscissa) of 10 nm gold nanoparticles in water after 20 min of exposure to a blend of thiols 1 and 2 ([thiols]total ) 1.5 × 10-4 M) as a function of the mole fraction of 2.

Figure 6. (a) Time course of the change of the maximum of the SP band of the gold nanoparticles upon addition of the 1/2 thiols blend ([thiols]total ) 3.0 × 10-5 M, [2]/[1] blend)1:15); (b) change induced by addition of 1.5 × 10-2 M NH2NH2 (arrow 1) and (c) uncleaved thiol 2 ([2] ) 1.5 × 10-5 M, arrow 2). Note the change of the time scale.

Figure 5 reports the rate of aggregation as a function of the thiol blend composition. For practical purposes, we have reported the increase of absorbance at 600 nm after the arbitrary time of 20 min. The sigmoidal profile indicates that the process is highly cooperative, and when the mole fraction of 2 is >0.35, it becomes extremely fast. This can be explained by considering that, when the number of dithiols covering a nanoparticle increases, the probability of interaction with another nanoparticle also increases in a nonlinear fashion, and this results in a faster aggregation process. The fact that we are following the formation of a cluster and not the growth of a single nanoparticle is confirmed by the change of the position but not of the intensity of the SP band during the kinetic process (see Figure 3). Recent simulations13 indicate that such a shift without a decrease of the extinction coefficient is associated with a clustering of the nanoparticles, while when this latter decreases, the nanoparticles grow in size. By adding to the aggregates obtained by treating the gold colloids with a thiol blend ([thiols]total ) 3.0 × 10-5 M, [2]/[1] blend ) 1:15) a 1.5 × 10-2 M aqueous hydrazine solution, the maximum of the SP band shifted back to shorter wavelengths (see Figure 6b), although the rate of the process was much slower than that leading to (12) As pointed out by a reviewer, the absorption spectra indicate the coexistence of aggregated and isolated nanoparticles. This could appear to be in contradiction with the dynamic light scattering results. We note that the light scattering data report average diameters and do not rule out the coexistence of aggregated and isolated nanoparticles because a multimodal analysis has not been carried out. (13) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640-4650.

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Conclusion In conclusion, we have shown how the aggregation of gold nanoparticles can be reversibly controlled by addition of a blend of cleavable- and colloid-stabilizing thiols. The unprecedented chemically induced deaggregation process allows the control of nanoparticle communication. Although hydrazine is not a particularly effective cleaving agent, it is conceivable that by the tuning of the appropriate reactant and functional group present in the labile dithiol this protocol can be applied to the sensing of very specific and, most important, effective cleaving agents such as, for instance, biological catalysts.16 Figure 7. Increase of the absorbance at 600 nm (left abscissa) and corresponding change of the maximum of the SP band (right abscissa) of 10 nm gold nanoparticles in water after 240 min of exposure to a blend of thiols 1 and 2 at different time of incubation with hydrazine. (Conditions: incubation solution, [thiols]total ) 1.5 × 10-4 M, [hydrazine] ) 1.5 × 10-2 M; diluted 1:10 for measure; thiol composition, [2]/[1] )1:1.5). Note that because of the lower concentration of dithiol with respect to Figure 5 the elapsed time before determining the change of absorbance is longer.

aggregation (Figure 6a). Hydrazine cleaves the ester bonds of dithiol 2 leading to the corresponding hydrazide14 and thus breaks the tethers that keep the cluster together. The deaggregation process kinetics parallel that of the cleavage of the ester bond by hydrazine. Obviously, if dithiol 2 is treated with hydrazine before its addition to the nanoparticles, the extent of cluster formation is reduced as a function of the amount of ester tether cleaved. Thus, when we treated a thiol blend ([thiols]total ) 1.5 × 10-4 M, [2]/[1] ) 1:1.5) with hydrazine (1.5 × 10-2 M) and withdrew at successive time intervals aliquots of the solution and added them to a nanoparticle solution (final thiols concentration was 1.6 × 10-5 M), we observed a decrease of the amount of cluster formation connected with the proceeding of the cleavage process. Figure 7 reports the absorbance at 600 nm and the position of the maximum of the SP band for nanoparticles treated with the above solution of thiols as a function of the incubation time with hydrazine. Once again, the shape of the curve is sigmoidal as in the case of Figure 5 in accord with the cooperativity of the process. Below a critical dithiol concentration, the formation of the aggregate becomes practically negligible. We were also successful in inducing aggregation of deaggregated particles after the hydrazine-induced cleavage. In fact, by further adding an excess of 2 ([2] ) 1.5 × 10-5 M) to the nanoparticles treated with hydrazine, we observed again the shift of the SP band to longer wavelengths (Figure 6c). This time the aggregation process was slower than the previous one when the blend of thiols was added to “naked” thiols (Figure 6a) but, obviously, faster than the cleavage due to the hydrazine present. Functionalization of the Au surface of the nanoparticle now requires the exchange of thiol 2 with the thiols already covering it (thiol 1 and cleaved thiol 2). This process is known to take several hours,15 contrary to the very fast one occurring with the native, “naked” colloids. Thus the aggregation/deaggregation/aggregation protocol anticipated in Figure 1 is effectively demonstrated by the experiments reported in Figure 6. (14) The cleavage reaction was also followed by withdrawing aliquots of the reaction mixture and detecting the formation of the hydrazide cleavage products by HPLC-ESI-MS. (15) Montalti, M.; Prodi, L.; Zaccheroni, N.; Baxter, R.; Teobaldi, G.; Zerbetto, F. Langmuir 2003, 19, 5172-5174.

Experimental Section General Information. NMR Spectra were recorded on Bruker AC 200, AC 250, or Avance 400 spectrometers at 25 °C. For 1H NMR, data are reported as follows chemical shift in ppm, from the chemical shifts of the residual protons in the deuterated solvents, on the δ scale, integration, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; sp, septet; br, broad; dsp, doublet of septets), coupling constants (Hz), and assignment. For 13C NMR, the signals of the solvent were used as reference. UV-vis spectra were recorded at 25 °C on a Perkin-Elmer Lambda 16 spectrophotometer equipped with a thermostated cell holder. Light scattering experiments were performed with a Spectra Physics 2016 argon laser operating at 488 nm, interfaced with a Nicomp 370 model Particle Sizing autocorrelator. The experiments were carried out at 25 °C. Synthesis of 16-Ethoxythiocarbonylsulfanyl-hexadecanoic Acid. 16-Bromoesadecanoic acid, (100 mg, 0.298 mmol) was dissolved in pure acetone. To this solution were added 67 mg (0.418 mmol) of potassium dithiocarbonate O-ethyl ester salt dissolved in the same solvent. The final volume of the reaction mixture was about 16 mL. The mixture was stirred overnight at room temperature under a nitrogen atmosphere. The workup procedure was the following: the acetone was evaporated completely, and the residue was taken up in diethyl ether/water. Drops of 1 N HCl (until strong acidic reaction on litmus paper) and NaCl solution (to break the emulsion) were added to the aqueous phase. Ether extractions of the aqueous phase were repeated four times (4 × 15 mL), the organic layer was dried over MgSO4, and the solvent was evaporated. 16((Ethoxy(thiocarbonyl))thio)hexadecanoic acid (107 mg) was obtained in 96% yield. 1H NMR (250 MHz, CDCl3) δ: 1.2-1.8 (m, 29 H, CH2, CH3); 2.35 (t, 2H, J ) 7.3, CH2-COOH); 3.15 (t, 2H, J ) 10.7, CH2-S); 4.85 (q, 2H, J ) 7.5, CH2-OCS). Synthesis of Dithiocarbonic Acid S-(15-Chlorocarbonylpentadecyl) Ester O-ethyl Ester. To a solution of 16-((ethoxy(thiocarbonyl))thio)hexadecanoic acid (536 mg, 1.42 mmol) in 5 mL of dichloromethane, thionyl chloride (2 mL, 27.5 mmol) was added. The reaction mixture was stirred at room temperature under nitrogen for 2 h. The solvent was removed with the aid of a rotary evaporator and, eventually, with a vacuum pump. The acyl chloride was obtained in quantitative yield. 1H NMR (250 MHz, CDCl3) δ: 1.2-1.8 (m, 29 H, CH2, CH3); 2.85 (t, 2H, J ) 7.25, CH2-COCl); 3.15 (t, 2H, J ) 10.7, CH2-S); 4.65 (q, 2H, J ) 7.5, CH2-OCS). Synthesis of 16-Ethoxythiocarbonylsulfanyl-hexadecanoic Acid 2-{2-[2-(16-Ethoxythiocarbonylsulfanyl-hexadecanoyloxy)-ethoxy]-ethoxy}-ethyl Ester. To a solution of triethylene glycol (99 mg, 0.66 mmol) in 3 mL of dry dichloromethane, a solution of dithiocarbonic acid S-(15-chlorocarbonylpentadecyl) ester O-ethyl ester (60 mg, 1.42 mmol) in 2 mL of dry dichloromethane and 0.5 mL (3.1 mmol) of dry diisoproylethylamine was added. The mixture was stirred at room temperature under a nitrogen atmosphere for 18 h. The reaction mixture was diluted with 10 mL of dichloromethane and extracted with KHSO4 10% (2 × 15 mL) and with a saturated NaCl solution (1 × 15 mL). The organic layers were collected and dried over Na2SO4. After the solvent was removed, the crude product was purified by flash chromatography (eluent, (16) Zhao, M.; Josephson, L.; Tang, Y.; Weissleder, R. Angew. Chem., Int. Ed. 2003, 42, 1375-1378.

Reversible Aggregation/Deaggregation of Gold Nanoparticles ethyl acetate/petroleum ether 1:3 v/v). Three hundred one milligrams of product were obtained. Yield 53%. 1H NMR (250 MHz, CDCl3) δ: 1.2-1.8 (m, 58 H, CH2, CH3); 2.3 (t, 4 H, J ) 7.5, CH2-COO); 3.15 (t, 4H, J ) 10.7, CH2-S); 3.65 (m, 8H, CH2-O); 4.2 (t, 4 H, J ) 5, CH2-OOC); 4.65 (q, 4H, J ) 7.5, CH2-OCS). 13C NMR (62.9 MHz, CDCl3) δ: 13.71, 24.78, 28.24, 28.58, 28.78, 29.01, 29.16, 29.35, 33.85, 34.07, 35.79, 63.16, 69.14, 69.57, 70.44, 173.66, 187.46. ESI-TOF: [M + H]+ ) 867.5157. Synthesis of 16-Mercapto-hexadecanoic Acid 2-{2-[2-(16Mercapto-hexadecanoyloxy)-ethoxy]-ethoxy}-ethyl Ester (2). In 5 mL of dry dichloromethane, 21.2 mg (0.024 mmol) of the previous compound was added. To the mixture, 0.1 mL of 1,2diaminoethane was added. The reaction mixture was stirred for 3 h under a nitrogen atmosphere at room temperature. The reaction mixture was diluted with 4 mL of dichloromethane and extracted with 10% KHSO4 (2 × 8 mL) and with a saturated NaCl solution (2 × 8 mL). The organic layers were collected and

Langmuir, Vol. 21, No. 12, 2005 5541 dried over Na2SO4. After the solvent was removed, the crude product was purified over a preparative TLC plate (20 cm × 20 cm × 1 mm) using as eluent a mixture of chloroform (99.5%) and ethanol (0.5%) in a nitrogen atmosphere (drybox). The silica with the compound was recovered, extracted with chloroform/ ethanol (97:3), filtered, and the solvent evaporated. Product 2 was obtained in 60% yield. 1H NMR (250 MHz, CD3OD) δ: 1.2-1.8 (m, 52 H, CH2); 2.3 (t, 4 H, J ) 7.5, CH2-COO); 2.51 (m, 4H, CH2-SH); 3.65 (m, 8H, CH2-O); 4.2 (t, 4 H, J ) 5, CH2-OOC). ESI-MS: [M + Na]+ ) 713.2.

Acknowledgment. We thank MIUR (Contract No. 2002031238) for financial support and Dr. Paolo Pengo for discussions. LA0470232