Growth Mechanism of Flowerlike Gold ... - ACS Publications

Sep 26, 2008 - Time-dependent surface plasmon resonance (SPR) absorption spectroscopy and the resonance Rayleigh scattering (RRS) technique were ...
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J. Phys. Chem. C 2008, 112, 16348–16353

Growth Mechanism of Flowerlike Gold Nanostructures: Surface Plasmon Resonance (SPR) and Resonance Rayleigh Scattering (RRS) Approaches to Growth Monitoring Wei Wang, Xuan Yang, and Hua Cui* Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, Peoples Republic of China ReceiVed: June 5, 2008; ReVised Manuscript ReceiVed: August 8, 2008

A facile method is proposed for the room-temperature synthesis of flowerlike gold nanostructures (AuNFs) with a size of 50-115 nm by reducing HAuCl4 with ascorbic acid (AA) in the presence of chitosan. It was found that the concentration of chitosan controlled the size, while that of AA influenced the morphology of as-prepared AuNFs. With higher concentration of AA flowerlike nanostructures were produced, whereas with lower concentration of AA quasi-spherical nanoparticles were formed. Time-dependent surface plasmon resonance (SPR) absorption spectroscopy and the resonance Rayleigh scattering (RRS) technique were used to monitor the growth processes. According to the temporal evolutions of SPR maximum absorption wavelength and RRS intensity, a second growth mechanism is proposed to explain the effect of AA concentration on the morphology and the effect of chitosan concentration on the size of obtained gold nanostructures. In order to determine whether or not the present method is suitable for synthesis of flowerlike gold nanostructures by use of other reductants in the presence of chitosan, four other conventional reductants, including gallic acid (GA), oxalate acid (OA), tartaric acid (TA), and sodium citrate (Cit), instead of AA were examined. The intrinsic reason for the different performances of these reductants was further investigated, and the results also supported the proposed second growth mechanism. 1. Introduction Fundamental research and practical applications of gold nanomaterials are now becoming attractive subjects due to their promising optical properties, catalysis, and biocompatibility.1-4 Because their chemical and physical properties are highly morphology and size dependent, structure-directing agents were introduced in the controllable synthesis of various anisotropic gold nanomaterials by inducing the preferential growth along some specific directions.5 Hydrophilic polymers are one kind of effective structure-directing agents via their abundant active groups and flexible molecular conformations.6-10 By using various structure-directing polymers such as PVP,6-8 PEG,9 and PVA,10 many kinds of anisotropic gold nanostructures, such as nanorods, nanoflowers, and nanoplates, have been synthesized and applied in many fields. Chitosan is one kind of deacetylated derivatives of chitin. By virtue of its polycationic, biocompatible, and film-forming properties, chitosan has been widely used in drug delivery, the food industry, and biosensors11 as well as in the synthesis of isotropic gold nanospheres by acting as reductant and/or stabilizer for several years.12-17 However, its new role as a structure-directing agent in the synthesis of anisotropic gold nanostructures has been rarely reported until recently.18-20 In two recent reports, 2-dimensional (2-D) gold nanosheets18 and chainlike gold nanoaggregates19 were prepared by reducing HAuCl4 with chitosan solely. Moreover, 3-D flowerlike gold nanostructures (AuNFs) were obtained in our group by reducing HAuCl4 with chitosan in the presence of a coreductant, luminol.20 However, specific temperature control was requested in all cases. Besides, luminol was not a conventional reductant, which limited the accessibility of this method for the synthesis and applications of AuNFs. Therefore, in the present work, more * To whom correspondence should be addressed. Phone: +86-5513606645. Fax: +86-551-3600730. E-mail: [email protected].

effort is made to synthesize AuNFs using several conventional reductants instead of luminol to reduce HAuCl4 in the presence of chitosan under room-temperature conditions. The effect of selected reductant on the morphology of obtained AuNFs was further investigated. Though the heterogeneous preparation and growth mechanism of flowerlike gold nanostructures on the surface of solid substrates via chemical or electrochemical methods have been studied by several groups,21-25 the homogeneous synthesis of gold nanoflowers was relatively complex and exhibited quite different behaviors,6,9,26-32 and the growth mechanism of such 3-D nanostructures was still not comprehensively understood. In our previous work, a second growth mechanism was proposed to explain the chitosan-assisted formation of AuNFs under the homogeneous condition.20 However, it was principally based on the structure characterization of AuNFs after synthesis was completed. The proposed mechanism would be more convictive if direct kinetic information about the growth process is provided. Time-dependent spectroscopic investigation is an important and effective approach to monitor the growth process and explore the corresponding growth mechanism. The commonly used techniques included surface plasmon resonance (SPR) absorption spectroscopy,26,27,33,34 X-ray scattering,35,36 and transmission electron microscopy (TEM).36,37 However, expensive instruments were requested in both X-ray scattering and TEM analysis, and it was difficult to obtain continuous and comprehensive information from time-dependent TEM images. Though SPR absorption spectroscopy was often utilized to monitor the growth process of metal nanomaterials,26,27,33,34 its potential application was more or less limited by its relatively low sensitivity.38 Moreover, the spectral analysis might be rather difficult when the other compounds in the reaction system exhibited overlapping absorption in the characteristic bands of nanomaterials.39

10.1021/jp804970x CCC: $40.75  2008 American Chemical Society Published on Web 09/26/2008

Flowerlike Gold Nanostructures Resonance Rayleigh scattering (RRS) was another interesting optical property of nanomaterials in which Rayleigh scattering was performed near the maximum SPR absorption wavelength.40 The linear Rayleigh scattering spectrum could be recorded on a conventional fluorometer by simply setting the synchronous mode (∆λ ) 0). RRS was first proposed to detect the extended aggregates of chromophores40 and the interactions between chromophores and many substances such as polysaccharide,41 proteins,42 and inorganic ions.43 Since metal nanomaterials were found to exhibit strong RRS signal and the RRS spectrum was also morphology and size dependent,38,44-46 it has now been an effective technique in the quantitative determination of DNA,38,44,45 amino acid,47 and dye39 by measuring the change on RRS signal induced by the interactions between nanomaterials and analytes. However, there were few reports on applications of time-dependent RRS signal to monitor the growth process of nanomaterials. This technique was believed to be helpful for offering useful kinetics information during the growth process of nanomaterials and providing new insights into the investigation on the growth mechanism because RRS signal was more sensitive than SPR absorption method,38,45 and it exhibited better selectivity to external interferences.41 Therefore, both SPR and RRS techniques were utilized in the present work to monitor the growth process of gold nanoflowers, which provided novel evidence to verify the previously proposed second growth mechanism. 2. Experimental Section Chemicals and Solutions. A HAuCl4 stock solution (2‰ HAuCl4, w/w) was prepared by dissolving 1.0 g of HAuCl4 · 4H2O (Shanghai Reagent, China) in 412 mL of purified water and stored at 4 °C. Chitosan (Mv ) 1000 kDa, degree of deacetylation ) 90%) was purchased from Shanghai Reagent (China) and used as received. Solutions of 0.02 mol/L ascorbic acid (AA), gallic acid (GA), oxalate acid (OA), tartaric acid (TA), and sodium citrate (Cit) were freshly prepared by dissolving corresponding compounds in water. All other reagents were of analytical grade. Ultrapure water was prepared by a Millipore Milli-Q system and used throughout. Synthesis of Gold Nanoflowers. All glassware used in the synthesis was cleaned in a bath of freshly prepared 3:1 (v/v) HNO3-HCl and rinsed thoroughly prior to use. Gold colloids were prepared via reduction of HAuCl4 by selected reductant in the presence of chitosan. In a typical procedure with AA as a reductant, 50 mg of chitosan was dissolved in 10 mL of acetic acid solution (HAc, 2.5% v/v). After adding 1 mL of freshly prepared 0.02 mol/L AA solution, the mixture was diluted to 24 mL. While stirring vigorously, 1.25 mL of HAuCl4 stock solution was added rapidly. The solution was continuously stirred for 10 min, during which time a rapid color change from pale yellow to dark blue was observed. The weight of chitosan or the volume of reductant was varied to synthesize gold nanomaterials of different morphologies and sizes. To avoid degradation of chitosan in HAc solution during storage, chitosan powder was freshly dissolved prior to synthesis. All syntheses were performed at room temperature (about 20 °C). Characterization and Growth Monitoring of Gold Nanoflowers. As-synthesized colloidal solutions were subsequently characterized by a UV-vis absorption spectrophotometer (Agilent UV-8453, US) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010, Japan). Because the grating system of the UV-8453 spectrophotometer was fixed after the light passed through the sample cell, obvious photo-oxidation was observed in kinetics experiments due to continuous irradia-

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Figure 1. UV-vis absorption spectra of AuNFs synthesized by varying the volume of AA solution (A) or chitosan (B). Curves 1-5: 0, 0.5, 0.6, 1.0, and 2.0 mL AA solution (0.02 mol/L) was used in the presence of 50 mg of chitosan, respectively. Curves 6-9: 100, 50, 25, and 12.5 mg of chitosan was used in the presence of 1.0 mL of AA, respectively. The maximum absorption wavelength of curves 1-9 were 530, 549, 573, 586, 593, 575, 586, 623, and 661 nm, respectively.

tion by ultraviolet light. Therefore, a high-pass filter (cutoff wavelength ) 400 nm) was fixed between the light source and sample cell to avoid the photo-oxidation effect. For the scanning electron microscopy (SEM) analysis of AuNFs, the colloids were centrifuged, redispersed in water to avoid the influence of chitosan film, subsequently dropped on a piece of quartz glass and dried under an infrared lamp, and finally measured by SEM (JEOL JSM-6700F, Japan). The resonance Rayleigh scattering measurements were performed on a fluorometer (Hitachi F-7000, Japan) by setting a synchronous mode (∆λ ) 0). Electrochemical experiments were performed by a CHI760B electrochemical working station (Chenhua Inc., China). A glassy carbon electrode served as the working electrode in the present work. 3. Results and Discussion 3.1. Synthesis of Gold Nanoflowers Using Ascorbic Acid as a Reductant. It was reported that flowerlike gold nanostructures could be obtained by reducing HAuCl4 with luminol in the presence of chitosan.20 However, luminol was not a conventional reductant for chemical synthesis. Herein, a commonly used reductant, ascorbic acid (AA), was utilized instead of luminol for synthesis of AuNFs. When HAuCl4 was added to the mixture solution of AA and chitosan at room temperature, a rapid color change from pale yellow to purple or dark blue, depending on the amount of AA, was observed in less than 3 min, implying formation of gold nanostructures. This point was subsequently confirmed by the HRTEM observations. Figures 1 and 2 display the UV-vis absorption spectra and corresponding HRTEM images of as-prepared AuNFs. In the absence of AA, chitosan could slowly reduce HAuCl4 to form red-colored colloids in 48 h, which exhibited a maximum absorption wavelength (λmax) at 530 nm (curve 1 in Figure 1). However, in the presence of AA, the reaction process was greatly facilitated and it took only 3 min to produce purple- or bluecolored colloids. Curves 2-5 in Figure 1A show the SPR absorption spectra of obtained colloids by gradually increasing the concentration of AA. The corresponding HRTEM images are shown in Figure 2A-D, respectively. Figure 3 shows a typical SEM image of as-prepared AuNFs, which confirmed that the flowerlike nanostructures consisted of many smallersized nanodots. According to HRTEM images in Figure 2A-D, a gradual shape evolution from quasi-spheres to multipods to flowerlike nanostructures was clearly observed with increasing concentration of AA, which was accompanied by a red shift of

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Figure 2. HRTEM images of AuNFs. Conditions for synthesizing AuNFs in images (A-F) were the same as those of curves 2, 3, 4, 5, 8, and 9 in Figure 1, respectively. Gold nanostructures in images G and H were obtained by adding 1.0 mL of GA and OA as reductants in the presence of 50 mg of chitosan, respectively. Calculated diameters were 60 ( 10 (A), 52 ( 6 (B), 50 ( 8 (C), 51 ( 10 (D), 70 ( 11 (E), 115 ( 15 (F), 43 ( 6 (G), and 98 ( 10 (H) nm by accounting about 40 units.

SCHEME 1: Schematic Illustration of Reductant-Dependent Second-Growth Process and the Corresponding Spectroscopic Monitoring

Figure 3. Typical SEM image of as-prepared AuNFs.

λmax from 530 to 593 nm as shown in Figure 1A. On the other hand, the effect of chitosan concentration was also investigated since chitosan played an important role in the synthesis of gold nanostructures. Curves 6-9 in Figure 1B show the SPR absorption spectra of prepared colloids by decreasing the amount of chitosan. The corresponding HRTEM images of curves 7-9 are shown in Figure 2C, 2E, and 2F, respectively. It was found that the size of AuNFs greatly increased from 50 to 115 nm with decreasing amount of chitosan, and the AuNFs of larger size exhibited longer λmax as shown in Figure 1B. According to the results above, one can summarize that the concentration of AA influenced the morphology, whereas that of chitosan controlled the size of as-prepared nanostructures. Blue-colored colloids containing gold nanoflowers were obtained at a high concentration of AA, resulting in longer λmax than purple-colored colloids containing quasi-spherical nanoparticles prepared with a lower concentration of AA. Since the AuNFs were primarily stabilized by chitosan, less chitosan led to largersized AuNFs due to insufficient protection. These results were well consistent with our previous studies, in which similar trends were observed and a second growth mechanism was proposed from both experimental and electromagnetic simulation aspects.20 The similar phenomenon in both cases also indicated

the previously proposed second growth mechanism might also be suitable for formation of AuNFs in the present work, though different reductants were used and the heating source was removed. 3.2. Growth Monitoring by SPR and RRS. The previously proposed second growth mechanism is shown in Scheme 1. Lots of smaller-sized nanodots were produced by reducing HAuCl4 with a proper reductant and stabilized by chitosan (stage I). Such nanodots subsequently led to formation of initial nanoaggregates due to the interactions between surface-bound flexible chitosan molecules and vicinal nanodots (stage II). Thereafter, a secondgrowth process happened on the surface of such nanoaggregates to form stable flowerlike nanostructures until the gold source was exhausted (stage III). Recently, Zhu proposed a similar twostage mechanism for the chitosan-assisted formation of chainlike gold nanoaggregates in which linear nanoaggregates were selfassembled from initially produced nanoparticles due to the insufficient surface protection.19 Though initial nanodots and nanoaggregates were involved in both mechanisms, the second growth stage was a particular and important step for formation of AuNFs in the present mechanism. However, this step was principally based on the HRTEM results in our previous work,

Flowerlike Gold Nanostructures

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Figure 5. Linear Rayleigh scattering spectra of pure water (curve 1), 0.2% chitosan in 1% HAc solution (curve 2), as-prepared Colloid I (curve 3), and Colloid II (curve 4). The inset displays the temporal evolutions of RRS intensity at 580 nm during the synthesis of Colloid I and II.

Figure 4. Time-dependent SPR absorption spectra during the synthesis of Colloid I (A) and Colloid II (B). Twelve selected spectra were recorded before and at 3, 5, 10, 15, 20, 25, 30, 40, 60, 100, and 150 s after addition of HAuCl4. The insets display the corresponding temporal evolution of maximum SPR absorption wavelength (λmax) at a time interval of 0.5 s. The solid lines in both insets were curve fitted from the discrete experimental data.

and no direct kinetics information was provided to support this second growth process. Fortunately, the room-temperature reaction in the present work offered us an opportunity for further investigation of the growth mechanism by virtue of real-time spectroscopic monitoring to the growth process. UV-vis absorption spectroscopy was first utilized to characterize the growth process of gold nanomaterials since gold nanostructures exhibit morphology- and size-dependent SPR absorption bands in the visible region. Figure 4 displays the time-dependent SPR absorption spectra of two reaction processes using different concentrations of AA. Addition of 1.0 and 0.5 mL of AA led to formation of blue-colored colloids containing nanoflowers (Figure 2C) and purple-colored colloids containing quasi-spherical nanoparticles (Figure 2A), respectively. For the convenience of expression, the two products are exclusively called Colloid I (blue) and Colloid II (purple) in the present paper. The SPR absorption spectra were measured at a time interval of 0.5 s in both cases. Though 401 spectra were obtained in 200 s, only 12 typical ones at different moments are shown in Figure 4. The insets show the temporal evolutions of λmax at a time interval of 0.5 s. As shown in the inset of Figure 4B, there is an obvious turning point in the temporal evolution of λmax during the synthesis of Colloid II in which λmax is first red shifted to 558 nm in 25 s and subsequently blue shifted to 549 nm and maintained at 549 nm. This was an unusual phenomenon because a continuous red shift was generally observed during the growth process of nanostructures in most cases,27 like the

curve profile in the inset of Figure 4A for the synthesis of Colloid I. It was interesting that quite different profiles of SPR absorption spectra were observed by simply changing the concentration of AA, and such a difference could be reasonably explained according to the proposed second growth mechanism as shown in Scheme 1. The emergence and initial red shift of the SPR band was caused by formation of primary nanodots and their subsequent aggregations. If a high concentration of AA was utilized, the gold source was greatly consumed in this stage and subsequent growth on the surface of gold nanoaggregates was inhibited, resulting in formation of stable flowerlike gold nanostructures. Therefore, a continuous red shift of λmax was observed in Figure 4A for Colloid I. However, if the concentration of AA was not high enough to exhaust the gold source at this stage, notable second growth would then happen on the surface of nanoaggregates, resulting in formation of quasispherical nanoparticles with relatively larger size. This process would lead to the reversed blue shift of λmax as shown in Figure 4B for Colloid II because flowerlike gold nanostructures exhibited larger λmax according to the results above (section 3.1). Because various metal nanomaterials also exhibited morphology- and size-dependent Rayleigh scattering signals, the RRS technique was further used to monitor the above-mentioned growth processes of Colloids I and II. To the best of our knowledge, the present work was the first attempt to use RRS as a monitoring tool to investigate the kinetics and mechanism during the homogeneous growth of nanomaterials. The linear Rayleigh scattering spectra of ultrapure water, 0.2% chitosan solution (dissolved in 1% HAc), Colloid I, and Colloid II are shown as curves 1-4 in Figure 5, respectively. The Rayleigh scattering intensity of chitosan solution was 7 times stronger than that of ultrapure water because of the considerable hydrodynamic radius of chitosan macromolecules. In the presence of nanomaterials, the light scattering intensity was further increased by one time. Moreover, a novel Rayleigh scattering peak around 580 nm appeared as shown in curve 4 for Colloid II. This peak was ascribed to be resonance Rayleigh scattering because it was located around the maximum SPR absorption wavelength of Colloid II (549 nm).39,40 However, this RRS peak disappeared in Colloid I, which was blue colored and exhibited longer SPR absorption (λmax ) 583 nm) than that of Colloid II. A similar disappearance of this RRS peak was recently reported by Li, which was also accompanied by the red shift of the SPR band.45 This phenomenon might be caused by the lower Rayleigh scattering efficiency at longer wavelength since it is

16352 J. Phys. Chem. C, Vol. 112, No. 42, 2008 well known that the Rayleigh scattering intensity is inversely proportional to the quartic of the incident light wavelength. Therefore, the RRS intensity of metal nanomaterials were affected by two factors. Larger SPR absorbance and lower SPR maximum absorption wavelength led to stronger RRS intensity. The difference in RRS spectral profiles of Colloids I and II was subsequently utilized to monitor these two growth processes. If the RRS intensity at 580 nm was continuously recorded at a time interval of 0.2 s by setting a single-wavelength mode, temporal evolutions of RRS intensities were observed during the synthesis of Colloids I and II as shown in the inset of Figure 5. A first increase followed by a decrease in RRS intensity was observed in the synthesis of Colloid I. However, the RRS intensity continuously increased during the synthesis of Colloid II. SPR absorbance rapidly increased in the initial stage during the synthesis of both Colloids I and II, leading to the great increase in RRS intensity. If λmax increases to 583 nm (for Colloid I), RRS intensity would be inhibited due to the weaker scattering at longer wavelength. This effect led to a decrease in RRS intensity (Colloid I in the inset of Figure 5). As for Colloid II, λmax reversely decreased from 558 to 549 nm and SPR absorbance kept increasing slowly at the same time. Therefore, a continuous increase in RRS intensity was observed during the synthesis of Colloid II (Colloid II in the inset of Figure 5). Moreover, a turning point during the continuous increase was clearly observed, suggesting the better sensitivity of the RRS techniquecomparedwithconventionalSPRabsorptionspectroscopy. On the basis of the discussion above, the growth processes and the corresponding time-dependent SPR and RRS are summarized as follows (Scheme 1). After addition of HAuCl4, formation of primary nanodots and their subsequent aggregation led to an increase in both λmax of SPR absorption spectra and RRS intensity at 580 nm. The different second growth processes then happened on the surface of nanoaggregates, depending on the concentration of reductant. If a relatively low concentration of AA was used, the residual HAuCl4 would cause notable second growth and form purple-colored colloids containing quasi-spherical gold nanoparticles of larger size (Colloid II). In this case, a blue shift of λmax and a continuous increase in RRS intensity were observed. However, if the concentration of AA was high enough to almost exhaust HAuCl4 in the initial stages, blue-colored colloids containing flowerlike nanostructures were obtained during the slight second growth (Colloid I), which led to a continuous red shift of λmax and a decrease in RRS intensity. Though either SPR or RRS technique supported the above discussion independently, the RRS method demonstrated higher sensitivity for the kinetics analysis. Taking the growth monitoring to Colloid II as an example, a slight red shift (10-20 nm) followed by a blue shift (9 nm) of λmax was recorded by the SPR absorption spectroscopy. However, for the same growth process, the RRS intensity was enhanced by 5 times, showing its higher sensitivity and better resolution. 3.3. Reductant Effect. Since AA only acted as a reductant to produce primary nanodots in stage I, it was realized that similar flowerlike gold nanostructures might also be obtained if the other reductants were utilized instead of AA. Therefore, four kinds of conventional reductants, including gallic acid (GA), oxalate acid (OA), tartaric acid (TA), and sodium citrate (Cit), were examined to investigate the accessibility of the present method for the synthesis of AuNFs. When other conditions remained identical, with GA or OA color changes from pale yellow to purple similar to that with AA were observed. However, no obvious color change was observed in 6 h with TA or Cit. Figure 6A and 6B shows the absorption spectra of

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Figure 6. UV-vis absorption spectra of AuNFs synthesized by varying the volume of GA (A) or OA (B) solution. Curves 1-4: 0.2, 0.5, 1.0, and 2.0 mL of GA solution was used in the presence of 50 mg of chitosan, respectively. Curves 5-7: 1.0, 1.5, and 2.0 mL of OA solution was used in the presence of 50 mg of chitosan, respectively. The maximum absorption wavelengths of curves 1-7 were 525, 541, 558, 562, 573, 592, and 621 nm, respectively.

Figure 7. Normal pulse voltammograms of 1 mmol/L AA, GA, OA, TA, and Cit in 1% HAc solution on a glassy carbon electrode.

prepared colloids by varying the concentration of GA and OA, respectively. A similar red shift of λmax was observed in both cases with increasing concentration of reductants, which is consistent with the results in Figure 1A. However, there were still two obvious differences in these systems. First, the reaction rates were quite different when the same concentrations of reductants were used. When 1.0 mL of GA, AA, or OA was added, it approximately took 50, 100, or 7200 s to complete the synthesis by monitoring the SPR absorbance, respectively. The corresponding HRTEM images of obtained colloids using GA, AA, and OA as reductants are shown in Figure 2G, 2C, and 2H, respectively. The results exhibited the second difference in the reductant-dependent morphology. Flowerlike nanostructures were observed in the presence of AA (image C) and GA (image G), while quasi-spherical nanoparticles of larger size were obtained in the presence of OA (image H). According to the experimental results, by selecting different reductants one can realize that three kinds of performances were observed among these five compounds. GA and AA produced similar AuNFs in a shorter reaction time of less than 200 s, while OA generated larger quasi-spherical nanoparticles in 2 h. However, TA and Cit could not lead to any observable change in at least 6 h. It was thus assumed that the reducing abilities of these compounds might be the intrinsic reason for their different performances. The electrochemical measurements subsequently supported this point. Figure 7 shows the normal pulse voltammograms (NPV) of all five reductants in 1% HAc solution. Five compounds were separated into three classes

Flowerlike Gold Nanostructures according to the NPV behavior, which well matched the results in chemical synthesis. For type I reductants such as AA and GA, the stronger reducing ability facilitated formation of primary nanodots and consumption of gold source, leading to formation of AuNFs. However, the oxidation potential of type III reductants such as TA and Cit was too high to effectively induce formation of primary nanodots at room temperature. As for type II reductants like OA, though they could produce nanodots at the first stage, the reaction rate was too slow to effectively consume HAuCl4. Therefore, the notable second growth would lead to formation of larger quasi-spherical nanoparticles instead of AuNFs. 4. Conclusion A general and facile routine for the synthesis of flowerlike gold nanostructures at room temperature was proposed by reducing HAuCl4 with several reductants in the presence of chitosan. The relationships between the concentrations of both reductant and chitosan and the morphology of as-prepared AuNFs were established on the basis of SPR absorption spectra and HRTEM observations. By taking five conventional organic acids (salt) as model compounds, three types of reductants were found to exhibit different but gradual influences on the morphology of as-prepared gold nanostructures. The reducing ability was considered to be the intrinsic reason for this reductant effect according to the proposed second growth mechanism. Moreover, time-dependent SPR absorption spectroscopy and the RRS technique were used to support the second growth mechanism. The RRS technique offered novel kinetics information to the growth process using a conventional fluorometer, which was experimentally validated to be more sensitive and powerful than the commonly used SPR method. The present investigation on the Rayleigh scattering techniques provided novel insights and spectroscopic tools into growth monitoring of nonmetal nanomaterials such as SiO2 and polymer nanospheres, which do not exhibit any characteristic SPR peaks in the UV-vis region. Therefore, the present work was of significance to the spectroscopic monitoring to the growth process and further understanding the polymer-assisted formation of 2-D and 3-D nanostructures. Acknowledgment. Support of this research by the National Natural Science Foundation of P. R. China (Grant Nos. 20625517 and 20573101) and the Overseas Outstanding Young Scientist Program of Chinese Academy of Sciences is gratefully acknowledged. References and Notes (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Guo, S.; Wang, E. Anal. Chim. Acta 2007, 598, 181. (3) Hu, M.; Chen, J.; Li, Z. Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. ReV. 2006, 35, 1084. (4) Shan, J.; Tenhu, H. Chem. Commun. 2007, 44, 4580. (5) Yuan, H.; Ma, W.; Chen, C.; Zhao, J.; Liu, J.; Zhu, H.; Gao, X. Chem. Mater. 2007, 19, 1592. (6) Kumar, P. S.; Pastoriza-Santos, I.; Rodriguez-Gonzalez, B.; Abajo, F. J. G. d.; Liz-Marzan, L. M. Nanotechnology 2008, 19, 015606. (7) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833.

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