Strong Deaggregating Effect of a Novel Polyamino Resorcinarene

Oct 14, 2008 - These effects are likely due to the amphiphilic nature of TNMR, which has a large hydrophilic headgroup with eight amino groups and fou...
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Langmuir 2008, 24, 13161-13167

13161

Strong Deaggregating Effect of a Novel Polyamino Resorcinarene Surfactant on Gold Nanoaggregates under Microwave Irradiation Ming Shen,* Yan Sun, Ying Han, Rong Yao, and Chaoguo Yan* College of Chemistry & Chemical Engineering, Yangzhou UniVersity, Yangzhou 225002, China ReceiVed June 21, 2008. ReVised Manuscript ReceiVed August 23, 2008 Gold nanoparticles were prepared by ethylene glycol (EG) reducing gold chloride under microwave irradiation. The EG-stabilized gold colloids varied from red to blue with increasing amounts of EG, due to particle aggregation. Addition of the macrocyclic polyamine 2,8,14,20-tetranonyl-4,6,10,12,16,18,22,24-octa(1-aminoethylcarbamoyl)methoxyresorcinarene (TNMR) reversed nanoparticle aggregation under microwave irradiation and greatly improved their dispersion stability in aqueous solutions. These effects are likely due to the amphiphilic nature of TNMR, which has a large hydrophilic headgroup with eight amino groups and four hydrophobic chains. Moreover, the large and flexible hydrophilic groups containing more N and O atoms in the TNMR molecule has a strong stretching and penetrating ability in the aqueous solution, and TNMR molecules can easily form a bilayer protecting structure on the surface of gold nanoparticles, which plays a critical role in the color-change process of the EG-stabilized gold colloid.

1. Introduction Nowadays, nanostructured metal materials have been extensively studied owing to their unique physicochemical and optoelectronic properties as well as their potential application in catalysis, microelectronics, and chemical sensors.1-3 In particular, noble metal colloids or nanoparticles exhibit characteristic optical and physical properties that are quite different from those of the corresponding bulk metals, thus arousing a great deal of interest.4-6 Normally, the size, shape, and surface modification of noble metal nanoparticles are some of the most important factors that may dramatically affect their physical/chemical properties. Therefore, a large number of methods have been developed for the synthesis of noble metal nanomaterials for the past few decades involving the use of different protecting reagents, such as alkanethiols,7,8 alkylamines,9,10 polymers,11,12 and other ligands.13,14 Recently, Sastry and co-workers prepared hydrophobic gold nanoparticles capped by a fatty amine with the simplified Brust “two-phase” method7 and researched the interaction between surface-bound alkylamines and gold nanoparticles. Their experiments showed the presence of two different modes of binding * To whom correspondence should be addressed. E-mail: shenming@ yzu.edu.cn (M.S.); [email protected] (C.Y.). Fax: +86-514-87975244. (1) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) Emory, S. R.; Haskins, W. F.; Nie, S. J. Am. Chem. Soc. 1998, 120, 8009. (4) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773. (5) Templetion, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (6) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (7) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. J. Chem. Soc., Chem. Commun. 1994, 801. (8) Templetion, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (9) Chen, X. Y.; Li, J. R.; Jiang, L. Nanotechnology 2000, 11, 108. (10) Shen, M.; Du, Y. K.; Rong, H. L.; Li, J. R.; Jiang, L. Colloids Surf., A 2005, 257-258, 439. (11) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I. Langmuir 2003, 19, 4831. (12) Zhang, J. H.; Liu, H. Y.; Wang, Z. L.; Ming, N. B. AdV. Funct. Mater. 2007, 17, 3295. (13) Shen, M.; Du, Y. K.; Yang, P.; Jiang, L. J. Phys. Chem. Solids 2005, 66, 1628. (14) Huang, Y. J.; Li, D.; Li, J. H. Chem. Phys. Lett. 2004, 389, 14.

the alkylamine with the gold surface. The weakly bound component was attributed to the formation of an electrostatic complex between the protonated amine molecules and the surfacebound AuCl4-/AuCl2- ions, while the more strongly bound species was tentatively assigned to a complex of the form [AuCl(NH2R)].15,16 In the past we also reported a new synthesis method of various alkylamine-capped gold nanoparticles under microwave irradiation in a reverse micelle system.17 The result showed that the obtained hydrophobic Au nanoparticles had a high level of stability and could be preserved for a long time in the air, which indicated a strong interaction between the amino group and gold core. Wei et al. made the first report of using several resorcinarene surfactants with larger headgroups and multiple contact sites for adsorption as dispersants for gold nanoparticles and successfully obtaining resorcinarene-coated gold nanoclusters by directly bubbling aerosols of gold nanoparticles into dilute (0.1-1.2 mM) solutions of resorcinarenes in mesitylene.18 Their experiment revealed that the stabilization of Au nanoclusters by the resorcinarenes was most likely mediated by chemisorption through multiple Au-O interactions. Misra et al. described the phase transfer of gold nanoparticles from the aqueous phase into the organic phase containing C-undecylcalix[4]resorcinarene under vigorous stirring without using any other additional phase transfer reagent.19 Other reports were made by Wei and co-workers, who used sulfur-containing resorcinarene surfactants as extractants to extract gold particles from gold hydrosol and then transferred them into toluene or chloroform.20,21 The experimental results showed that the resorcinarene surfactants with sulfur-functionalized headgroups could cause the midnanometer-sized gold particles to be stably (15) Sastry, M.; Kumar, A.; Mukherjee, P. Colloids Surf., A 2001, 181, 255. (16) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (17) Shen, M.; Du, Y. K.; Hua, N. P.; Yang, P. Powder Technol. 2006, 162, 64. (18) Stavens, K. B.; Pusztay, S. V.; Zou, S. Z.; Andres, R. P.; Wei, A. Langmuir 1999, 15, 8337. (19) Misra, T. K.; Chen, T. S.; Liu, C. Y. J. Colloid Interface Sci. 2006, 297, 584. (20) Balasubramanian, R.; Kim, B.; Tripp, S. L.; Wang, X. J.; Lieberman, M.; Wei, A. Langmuir 2002, 18, 3676. (21) Kim, B.; Balasubramanian, R.; Perez-Segarra, W.; Wei, A.; Decker, B.; Mattay, J. Supramol. Chem. 2005, 17, 173.

10.1021/la8019588 CCC: $40.75  2008 American Chemical Society Published on Web 10/14/2008

13162 Langmuir, Vol. 24, No. 22, 2008 Scheme 1. Amphiphilic Molecular Structure of TNMR

dispersed in organic solvents. Moreover, the stability of the dispersion was dependent on the chemisorptive properties of the surfactant headgroups on the surface of the gold particles. Tshikhudo et al. studied the chemical properties of water-soluble gold nanoparticles coprotected by (mercaptoalkyl)oligo(ethylene glycol) and sulfur-containing calix[4]arene ligands, which not only made the totally water-insoluble recognition system amenable to the aqueous solution but also caused the calixarene to retain its molecular recognition properties in the aqueous environment.22 Our group also made the first report of preparing a gold hydrosol stabilized by a resorcinarene containing more amino headgroups through one-step synthesis.23 However, to the best of our knowledge, there is yet no literature reporting the deaggregating effect of the resorcinarene surfactant on gold nanoparticle aggregates. In the present study, we successfully synthesized a novel polyamino resorcinarene surfactant (2,8,14,20tetranonyl-4,6,10,12,16,18,22,24-octa(1-aminoethylcarbamoyl)methoxyresorcinarene, TNMR) with eight amino headgroups, finding that the TNMR surfactant not only could be used as a good ligand for gold nanoparticles, but also had a strong deaggregating effect on gold nanoaggregates under microwave irradiation.

2. Experimental Section 2.1. Chemicals. Ethylene glycol (A.R., denoted as EG), HAuCl4 · 4H2O (A.R.), NaOH (A.R.), and CTAB (A.R.) were purchased from Shanghai-Chemical Reagent Co. Doubly distilled water was used for all solution preparations and experiments. All the glassware was cleaned by soaking in aqua regia and finally washing with doubly distilled water. 2.2. Chemical Synthesis of the Polyamino Resorcinarene Surfactant TNMR. The chemical synthesis of a novel polyaminoresorcinarene surfactant (TNMR) was done according to the method reported by Yan et al.,23,24 and the amphiphilic structure of the TNMR molecule is shown in Scheme 1. The characterization of the related compounds during the synthesis process will be described in detail in the Supporting Information. 2.3. Preparation of EG-Capped Gold Nanoparticles. The water-soluble EG-capped gold nanoparticles were fabricated by ethylene glycol reducing HAuCl4 under microwave irradiation. A typical procedure for preparing gold nanocrystals was as follows. A 0.50 mL portion of 9.7 × 10-3 M HAuCl4 aqueous solution, 9.0 mL of 2.5 × 10-3 M NaOH solution, 9.5 mL of doubly distilled water, and 1.0 mL of ethylene glycol (final concentration cEG ) 0.90 M) were added to a 100 mL beaker in turn. Then the mixture was sonicated into a transparent, isotropic solution at room temperature. Next the beaker was placed at the center of a domestic 2450 MHz microwave oven (Samsung N8A78, Suzhou Samsung Electronics Co.), which was refitted by adding a stirring apparatus. After about 30 s of microwave irradiation at the maximum power output of 700 W, the solution quickly changed from light yellow to scarlet (the reaction temperature is about 75 °C), and then a gold colloid (denoted as colloid A) reduced and stabilized by EG was obtained after continuous stirring for 5 min. The as-prepared gold colloid was (22) Tshikhudo, T. R.; Demuru, D.; Wang, Z. X.; Brust, M.; Secchi, A.; Arduini, A.; Pochini, A. Angew. Chem., Int. Ed. 2005, 117, 2973. (23) Ge, Y.; Yan, C. G. J. Chem. Res. 2004, 279. (24) Shen, M.; Chen, W. F.; Sun, Y.; Yan, C. G. J. Phys. Chem. Solids 2007, 68, 2252.

Shen et al. stored at room temperature for further characterization. As the concentration of ethylene glycol (cEG) was increased to 4.5 and 9.0 M with fixing of the concentration of HAuCl4 and total volume of the solution, a purple-red colloid (denoted as colloid B) and a blue colloid (denoted as colloid C) were prepared, respectively, according to the above-mentioned method. 2.4. Preparation of TNMR-Capped Gold Nanoparticles. TNMR-stabilized gold colloids were easily fabricated by adding the polyamino resorcinarene surfactant TNMR to the freshly prepared EG-stabilized gold colloids after a short period of microwave irradiation. Typically, 10.0 mL of 5.0 × 10-5 M TNMR aqueous solution was added to 10.0 mL of freshly prepared colloid C in a 100 mL beaker, and then 100.0 µL of 2.0 M HCl solution was added for adjusting the pH value. Then the solution was mixed into a homogeneous system at room temperature, and the beaker was placed at the center of a domestic 2450 MHz microwave oven. After about 60 s of microwave irradiation at the maximum power output of 700 W, the TNMR-stabilized gold colloid was obtained by TNMR replacing ethylene glycol capped on the gold particle surface. To our surprise, the color of the gold colloid successfully changed from blue to purple-red with an increase of the microwave irradiation time. When the same experiment was done with aliquots of colloid A and colloid B instead of colloid C, the other two TNMR-stabilized gold colloids were obtained and their color underwent a corresponding change under microwave irradiation. All of the as-prepared gold colloids were kept at room temperature for further treatment and characterization. However, a pale blue gold colloid was obtained after 1 min of microwave irradiation when a similar experiment was done, in which we mixed 10.0 mL of freshly prepared colloid C with 10.0 mL of 1.0 × 10-2 M CTAB aqueous solution to replace the same volume of 5.0 × 10-5 M TNMR aqueous solution. However, when it cooled to room temperature, the obtained gold colloid almost became colorless possibly because of the loss of its stabilization. Moreover, even through the measurement of UV-vis spectroscopy, nearly no characteristic of the surface plasmon resonance absorption band of the gold nanoparticles could be observed either. 2.5. Characterization of the Samples. Ultraviolet-visible spectra were measured on a UV-2550 PC UV-vis spectrometer (Shimadzu), and all of the freshly prepared gold colloids were immediately measured without being tackled after the microwave irradiation synthesis and cooling to room temperature. The spectral background absorption was subtracted by using the UV-vis spectrum of doubly distilled water. The freshly prepared gold colloids were dropped onto 200 mesh copper grids covered by Formvar film, and then the samples were dried in the air and placed into the desiccator prior to TEM measurement. TEM observation was conducted on a TECNAI-12 instrument (Philips), operated at an accelerating voltage of 120 kV. The TNMR-stabilized gold colloid was precipitated by adding a certain amount of Na2HPO4 aqueous solution. Then the precipitate was washed several times with water and anhydrous ethanol. Next the purified TNMR-capped gold nanoparticles were redispersed into an appropriate amount of anhydrous ethanol under ultrasonic irradiation prior to X-ray diffraction measurement. X-ray powder diffraction (XRD) data were taken with a graphite monochromator and Cu KR radiation (λ ) 0.1541 nm) on a D8 Advance Superspeed powder diffractometer (Bruker), operated in the θ-2θ mode primarily in the 20-80° (2θ) range and at a step scan of 2θ ) 0.04°. The tube voltage was 80 kV, and the tube current was 200 mA. The samples were prepared as a thin film on a glass plate through evaporation of the solvent of the TNMR or TNMR-capped gold nanoparticle ethanol solution. Fourier transmission infrared spectra of the samples were collected in the transmission mode on a Nicolet 740 FT-IR spectrometer. The spectra were obtained over 100 scans from which a background spectrum (empty cell) was automatically subtracted.

3. Results and Discussion 3.1. Effect of the EG Concentration on the Size and Morphology of the Gold Nanoparticles. The quantum energy of the microwave as an electromagnetic wave is too small to

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Figure 1. UV-vis absorption spectra (a) and digital photo (b) of EG-stabilized gold colloids synthesized by microwave irradiation: (A) cEG ) 0.90 M, (B) cEG ) 4.5 M, (C) cEG ) 9.0 M.

break down chemical bonds or to influence chemical equilibrium, but polar molecules can produce orientation polarization and steering movement in the high-frequency electromagnetic field. The interaction among molecules and molecular thermal motion hinder the steering movement of the molecules and produce the molecules’ inner friction, which results in the generation of heat.25 It is well-known that ethylene glycol as a feeble reducing agent has quite a difficult time reducing Au3+ to Au0 nanoparticles in the aqueous solution, but the reducing reaction can be quickly carried out under alkaline conditiond and microwave irradiation.26-29 Here, the formation of Au nanoclusters obtained by this method can be mainly attributed to the heating effect.25 In addition, the microwave irradiation can effectively avoid the formation of colloidal particles on the wall of the glass container, which is beneficial to enhancing the monodispersity of the prepared gold nanoparticles. Figure 1 shows the UV-vis spectra (Figure 1a) and digital photo (Figure 1b) of the three gold colloids obtained by using EG as a reductant and stabilizing agent. The maximum absorption wavelengths of colloid A, colloid B, and colloid C are respectively 523, 565, and 644 nm, corresponding to line A, line B, and line C in Figure 1a, which displays the characteristic optical signature typical of gold colloids.30 This figure also shows that the surface plasmon resonance absorption bands of gold nanoparticles become broadened and the colloidal maximum absorbencies gradually decrease with an increase of the molar ratio of EG to HAuCl4. In general, size effects can be reflected from the absorption spectra, with the plasmon resonance slightly shifting to higher energies as the particle size decreases, while with an increase of the particle size the “flocculation” (aggregation or agglomeration) of nanoparticles and the formation of nonspherical particles with a higher aspect ratio can result in a red shift of the absorption spectra.31 Therefore, the above results indicate that, among the three freshly prepared gold colloids, the size of the gold nanoparticles in colloid A might be smaller and their monodispersity might be higher, while the sizes of the gold particles in colloid B and colloid C might be larger, or there are possibly some aggregates formed among the gold nanoparticles (25) Tu, W. X.; Liu, H. F. Chem. Mater. 2000, 12, 564. (26) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubokawa, M.; Tsuji, T. Chem.sEur. J. 2005, 11, 440. (27) Bawn, C. E. H.; Hobin, T. P.; Raphael, L. Proc. R. Soc. London, Ser. A 1956, 237, 313. (28) Fievet, F.; Lagier, J. P.; Blin, B.; Beaudoin, B.; Figlarz, M. Solid State Ionics 1989, 32/33, 198. (29) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (30) Aslan, K.; Perez-Luna, V. H. Langmuir 2002, 18, 6059. (31) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.; Cao, D. L.; Bridges, F.; Buuren, A. V. J. Phys. Chem. B 2002, 106, 7005.

at the higher EG concentration. The digital photo of the three gold colloids in Figure 1b shows clearly that the colors of the three freshly prepared gold colloids change from red to purplered and then to blue with increasing amount of the reductant EG, which corresponds to their ultraviolet-visible spectra. By observing the stability of the three colloids at room temperature, we find that colloid A gradually loses its stability within several hours, resulting in the gold nanoparticles aggregating and nearly all of them precipitating onto the bottom of the container. For colloid B, the gold nanoparticles precipitate out of the system about 24 h later, while colloid C can still keep a relatively higher stability for more than a week. It is generally believed that the deeper the colors of the gold colloids, the more easily the gold nanoparticles will aggregate and precipitate. However, the above experiments show a quite different result, which actually has a good reproducibility. This surprising and seemly abnormal phenomenon may be tentatively explained as follows. The gold nanoparticles with a smaller size can be synthesized by EG reducing AuCl4- ions through fast heating of microwave irradiation with the absence of other stabilizing agents, in which EG is not only a reductant but also a stabilizing agent. However, for a system with a smaller amount of EG such as colloid A, gold particles with a nanosize can easily aggregate to decrease their surface Gibbs free energy and then precipitate down due to the lack of ethylene glycol and the weak stabilizing function of EG for gold nanoparticles. When the concentration of EG in the solution increases to 5 times more than that of colloid A (i.e., colloid B), the viscosity increases and the microenvironment of AuCl4- ions also changes a great deal. Under microwave irradiation, the amount of reducing agent has a minor influence on the nucleation rate, but the increase of the EG aqueous solution viscosity is likely beneficial to “the short-range aggregation” among gold nanoparticles or the formation of nonspheric nanoparticles, which causes the UV-vis absorption band to broaden and take on a red shift (see line B in Figure 1a). At the same time, the increase of the solution viscosity and the weak stabilizing effect of EG on the gold nanoparticles relatively enhance the stability of the gold colloid. When the EG concentration rises to 10 times more than that of colloid A (i.e., colloid C), the viscosity and the short-range aggregation among the gold nanoparticles further increase, which leads to the UV-vis absorption band further broadening and undergoing a red shift. Although the short-range aggregation of the gold nanoparticles increases with the EG concentration, the stability of the colloid becomes strengthened because of the high viscosity of the colloidal

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Figure 2. TEM micrographs and histogram of Au nanoparticles synthesized by microwave irradiation under different EG concentrations. The scale bars are 50 nm. Key: (a) cEG ) 0.90 M (d ) 16.7 ( 1.7 nm), (b) cEG ) 4.5 M, (c) cEG ) 9.0 M.

Figure 3. Evolution of the UV-vis absorption spectra (a) and digital photo (b) of the TNMR-stabilized gold colloid with the microwave irradiation time.

system and the stabilizing effect of more ethylene glycol molecules on the gold nanoaggregates. Figure 2 illustrates the TEM images and histograms of the size distribution of EG-capped gold nanoparticles obtained by microwave irradiation at different EG volumes. When cEG ) 0.90 M, the shape of the as-prepared Au nanoparticles is nearly spherical. The average diameter and the standard deviation of the particles are determined to be 16.7 ( 1.7 nm from this histogram (see Figure 2a), which shows that gold nanoparticles with a higher monodispersibility can be easily obtained through microwave irradiation synthesis using EG as a reductant and stabilizing agent at cEG ) 0.90 M. When cEG ) 4.5 M, the asprepared gold nanoparticles mostly take on a nonspherical morphology or irregular shapes, and simultaneously, an obvious aggregation among the particles appears with the increase of the EG concentration. The average diameter of the particles in Figure 2b is determined to be ca. 26 nm. What is more interesting is that a lot of small “flowerlike” nanoaggregates can be clearly observed in the TEM photo (see Figure 2c) when the concentration

of EG is increased to 9.0 M. These results can be tentatively explained as follows: the nucleation of gold nanoparticles is faster under microwave irradiation, while the surface Gibbs free energy of the newly formed particles is much higher. It seems that the decrease of the particle’s surface energy is accomplished through the formation of flowerlike aggregates in the relatively higher viscous system; meanwhile, the stabilizing effect of more EG molecules on flowerlike aggregates enhances the stability of the gold nanoparticle assemblies. These assemblies with a diameter of ca. 70 nm cause the maximum absorption wavelength to take on a red shift and the absorption band to obviously broaden. However, the size of the individual gold nanoparticle in the aggregate is estimated only to be about 20 nm (see Figure 2c), which is more or less consistent with the size of the gold particles in colloid A. 3.2. Strong Deaggregating Effect and Protecting Function of TNMR on Gold Nanoaggregates. Figure 3 shows the optical absorption spectra (Figure 3a) and the digital photo (Figure 3b) of the new gold colloidal system formed from colloid C and 5.0

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Figure 4. Evolution of λmax (a) and Amax (b) of the gold colloid with the microwave irradiation time.

× 10-5 M TNMR aqueous solution with the same volume after different irradiation times. After the novel protecting agent TNMR is added to colloid C, it can be clearly observed that UV-vis absorption bands gradually become narrow and the maximum absorption peaks obviously shift to the higher energy direction (see Figure 3a). These results indicate that, after the new colloid is treated with microwave irradiation, the maximum absorption wavelength (λmax) of the gold colloid changes from 644 to 544 nm with the evolution of the irradiation time (line a in Figure 4), but the maximum absorbance (Amax) of the solution correspondingly increases from 0.1840 to 0.3453 (line b in Figure 4), which is consistent with the color-changing process of the new gold colloid from blue to pink given by the digital photo in Figure 3b. This phenomenon suggests that the novel surfactant TNMR molecules have replaced EG molecules adsorbed on the surface of the gold nanoparticles. In addition, TNMR molecules not only can duly and validly protect gold nanoparticles but also have a strong deaggregating function for the flowerlike gold nanoassemblies during the process of microwave irradiation. The test of the new colloidal stability shows that the TNMRstabilized gold colloid can remain stable for six months without the appearance any obvious aggregates or precipitates of gold nanoparticles. Figure 5 shows the TEM photograph and histogram of the size distribution of TNMR-stabilized gold nanoparticles after the addition of TNMR and microwave irradiation for 1 min. As can be seen in the TEM photograph, the color change of colloid C induced by TNMR is actually due to the process of the morphology change of gold nanoparticles from flowerlike aggregates to individual separated particles. The average diameter and standard deviation of TNMR-capped gold nanoparticles determined from Figure 5 are 21.9 ( 2.6 nm, which is in agreement with the rough estimation from the TEM photo of flowerlike aggregates (see Figure 2c). In addition, the morphology of these gold nanoparticles in the figure is mostly nonspherical. In fact, the color change of the new gold colloid from blue to pink can also be obtained by using the conventional heating method, but the required time is longer and the effect of the color change is worse, compared with the experiment under microwave irradiation. If the new colloidal system is vigorously stirred at room temperature, it may take more than two days to make the gold colloid change color. The color change of the new gold colloid obtained from the series of experiments done above after addition of the polyamino resorcinarene surfactant TNMR can be tentatively explained as follows (see Figure 6).

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First, TNMR is a novel amphiphilic molecule with a large hydrophilic group containing eight amino headgroups and four hydrophobic chains linked to a large calix aromatic ring. The EG-capped gold nanoparticles synthesized by microwave irradiation take on obvious aggregates like “flowers” under the higher EG concentration. After addition of the novel polyamino resorcinarene surfactant TNMR, the amino headgroups of the TNMR molecule can easily produce chemisorption on the surface of the gold nanoparticle by replacing the hydroxyl of EG.24 Second, the eight flexible hydrophilic chains of a TNMR molecule have a large expanding space and a strong penetrability among gold particles of the flowerlike aggregate, and the intense molecular thermal motion under microwave irradiation speeds up the deaggregation of TNMR molecules for gold particle aggregates. Meanwhile, the timely and effective protection of TNMR for the separated gold nanoparticles prevents particles from fusion and growth during the deaggregating process. Finally, the rest of the TNMR molecules with a special amphiphilic structure can easily form a double protecting layer on the surface of the gold nanoparticle with hydrophilic groups outside and hydrophobic groups inside. Such a hydrophilic bilayer protecting structure formed by TNMR molecules on the dispersed gold nanoparticles can cause the TNMR-protected gold nanoparticles to exist stably in water. Nevertheless, the strong deaggregating effect of the TNMR surfactant on gold nanoparticle aggregates and the exact mechanism of stable protection of the TNMR molecule for gold nanoparticles are still unclear and need further work. When the same experiment was conducted with CTAB aqueous solution instead of TNMR solution, another phenomenon of color change appeared; i.e., the aggregation among gold nanoparticles was so strong with the presence of CTAB that no characteristics of the surface plasmon resonance band of the gold nanoparticles could be observed in the UV-vis spectrum. Figure 7a shows the TEM image of CTAB-capped gold nanocrystals, a networked structure formed among gold particles, which is similar to the gold networked nanostructure reported earlier.13 When 10.0 mL of 2.0 × 10-3 M PVP aqueous solution was added to colloid C to perform a similar experiment, the color of the new colloid basically remained blue as before. The morphology of the aggregates formed by gold nanoparticles was similar to that of EG-capped gold nanoaggregates (see Figure 7b), which indicated that the water-soluble PVP molecule could serve as a very good dispersant for gold nanoparticles, but it lacked the deaggregation capability for gold nanoaggregates because of its long molecule chain. From what has been mentioned above, we may tentatively draw the conclusion that the novel polyamino resorcinarene surfactant TNMR with a special molecular structure plays a decisive role in the color change process of colloid C, and the microwave irradiation speeds up this procedure and promotes the protecting function of TNMR molecules for gold nanoparticles. 3.3. FT-IR and XRD Measurement of TNMR-Protected Gold Nanoparticles. The protonated amino headgroups of the TNMR surfactant under acidic conditions lead to TNMRprotected gold nanoparticles possessing positive charge,24 which strengthens the stability of the gold colloid. After addition of an appropriate amount of electrolyte containing a high-valence negative ion such as Na2HPO4 to the gold colloid, TNMRprotected gold nanoparticles can be easily precipitated from the colloidal system and then purified. Figure 8 shows FT-IR spectra of the purified and dried TNMR and TNMR-coated Au nanocrystal powders (corresponding to parts a and b, respectively, of Figure 8). A comparison of the two FT-IR spectra not only

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Figure 5. TEM photograph and histogram of the size distribution of TNMR-stabilized gold nanoparticles under microwave irradiation.

Figure 6. Schematic diagram of the formation and morphology change of gold nanoaggregates induced by the surfactant TNMR.

Figure 7. TEM images of the effect of various stabilizing agents on the morphology of flowerlike aggregates: (a) CTAB (scale bar 50 nm), (b) PVP (scale bar 200 nm).

supports the presence of TNMR molecules on gold nanoparticles but also reveals the nature of the interaction of TNMR molecules with the gold nanoparticles.32 The features of the pure TNMR spectrum in the region of 3000-2800 cm-1 are similar to those of the spectrum of the surface-bound TNMR molecules on gold nanoparticles. The characteristic CH2 and CH3 symmetric and antisymmetric vibration peaks can be clearly observed in Figures 8 and 9, which indicates that TNMR molecules are associated with gold nanoclusters. However, in the case of surface-bound molecules, the peak positions shift slightly to lower frequencies (4-5 cm-1).33 The CH2 symmetric and antisymmetric stretching vibration peaks of pure TNMR lie at 2857 and 2927 cm-1, (32) Sau, T. K.; Murphy, C. J. Langmuir 2005, 21, 2923. (33) Kung, K. H. S.; Hayes, K. F. Langmuir 1993, 9, 263.

Figure 8. FT-IR spectra of TNMR-stabilized gold nanoparticles (a) and pure TNMR (b).

respectively, whereas those of TNMR-coated gold nanoparticles appear at 2852 and 2923 cm-1, respectively. In addition, the peak at 1441 cm-1 is attributed to the CH3 antisymmetric modes in the pure TNMR, while for TNMR-capped gold nanoparticles, the peak also shifts lower to 1438 cm-1 (about 3 cm-1) (Figure 9a2,b2). The peak at 722 cm-1 corresponds to the rocking mode of the methylene chain for pure TNMR. When TNMR coats the surface of gold nanoparticles, the peak shifts lower to 718 cm-1 (Figure 9a3,b3).34,35 It is well-known that the symmetric and antisymmetic stretching vibrations of CH2 can be used as a sensitive indicator of the ordering of alkyl chains, while the

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Figure 9. Slight structures of FT-IR spectra of TNMR-stabilized gold nanoparticles (a series) and pure TNMR (b series).

increase of frequencies and band widths is associated with the increasing incidence of gauche defects and disorder.36,37 The shift of the vibrations to lower frequency suggests that alkyl chains have more ordered structures and experience a more hydrophobic environment after they coat gold nanoparticles.32,33 Since the first-layer TNMR molecules give rise to unfavorable interaction by placing the hydrophobic tails of the surfactants toward the water environment, a double-layer arrangement with a second layer of TNMR molecules pointing their headgroups toward the water environment can be invoked (see Figure 6), as proposed by others.34,38 Figure 10 shows X-ray diffraction measurements for pure TNMR powder and TNMR-capped gold nanoparticles (samples A-C) obtained by TNMR replacing the stabilizing agent EG of colloids A-C, respectively. As TNMR molecules cannot form a crystal structure after ethanol evaporation, pure TNMR powder displays a typical noncrystal diffraction in Figure 10a. For the XRD patterns of TNMR-capped gold nanoparticles (samples A-C), the observed diffraction peaks are those expected for fcc gold on the basis of the bulk lattice constants and are broadened in contrast to the bulk due to the finite size of the nanoparticles (corresponding to parts b-d, respectively, of Figure 10). The mean diameters of these particles are consistent with the results of TEM measurements.

4. Conclusions Gold nanoparticles were fabricated by EG reducing AuCl4under microwave irradiation. The EG-stabilized gold colloids (34) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (35) Cheng, W.; Dong, S. J.; Wang, E. K. Langmuir 2003, 19, 9434. (36) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (37) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (38) Scheuing, D. R. Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991.

Figure 10. XRD patterns of pure TNMR powder and TNMR-capped gold nanoparticles obtained by replacing EG: (a) pure TNMR, (b) sample A, (c) sample B, (d) sample C.

varied from red to blue with an increase of the EG concentration, due to particle aggregation. The obtained blue colloid formed by the aggregation of gold nanoparticles had a relatively high stability, which might be attributed to the high viscosity of the gold colloid. Addition of the macrocyclic polyamino resorcinarene surfactant TNMR could reverse nanoparticle aggregation under microwave irradiation and greatly improved their dispersion stability in aqueous solutions. These effects were likely due to the amphiphilic nature of TNMR, which had a large hydrophilic headgroup with eight amino groups and four hydrophobic chains. The results showed that TNMR not only could become a good ligand for gold nanoparticles but also had a strong deaggregating effect on gold nanoparticle aggregates. The large and flexible hydrophilic groups containing more N and O atoms in the TNMR molecule had a strong stretching and penetrating ability in the aqueous solution, which played a critical role in the color-changing process. The experiment also demonstrated that the strong deaggregating effect of TNMR on gold nanoaggregates could be attributed to the chemisorption of the amino headgroups of the TNMR molecule and the formation of a stable bilayer protecting structure of TNMR molecules on the surface of the gold nanoparticles in the aqueous solution. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20773105 and 20672091). Supporting Information Available: Synthesis and characterization of TNMR. This material is available free of charge via the Internet at http://pubs.acs.org. LA8019588