Synthesis of gold Nanoshells through Improved Seed-Mediated

Feb 10, 2016 - poor locations.15. The great potentials described above confirm the key roles of gold nanoshells in shaping the future of nanotechnolog...
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Synthesis of gold Nanoshells through Improved Seed-Mediated Growth Approach: Brust-like, in Situ Seed Formation Yongping Gao, Jinlou Gu, Liang Li, Wenru Zhao, and Yongsheng Li* †

Lab of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: Gold nanoshells have shown great potentials in various fields. However, the widely used seed-mediated growth method based on a silica template for gold nanoshells is a complex and time-consuming procedure. In this work, mercaptosilica was first used as a template to synthesize gold nanoshells through improved seed-mediated growth method. It is verified that gold seeds were formed and attached onto the mercaptosilica nanospheres through Brust-like, in situ process, which makes this method extremely time-saving and easy to manipulate. Importantly, the key factors affecting the in situ process were demonstrated, allowing fine control on the synthesis in a highly reproducible manner. The as-synthesized nanoshells are monodisperse with well-defined morphology and tunable near-IR plasmon resonance. Furthermore, other metal nanoparticles such as Pt and Pd could be grafted onto the surface of mercaptosilica nanospheres through the same Brustlike, in situ process. These provide new insights into seed attachment, and the improved seed-mediated growth approach based on Brust-like, in situ seed formation will take an important step forward toward the widespread application of gold nanoshells.

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INTRODUCTION Gold nanoshells, composed of a dielectric core and a concentric Au layer, have been extensively investigated due to their tunable plasmonic properties.1 By appropriately adjusting the core-toshell ratio, the plasmon resonance effect can be tuned from visible to the near-infrared (near-IR) biological transparency window,2 allowing their applications for both in vivo diagnosis and therapy.3−7 Specifically, gold nanoshells have presented promising potentials for contrast enhancement in fluorescencebased bioimaging8 and surface-enhanced Raman spectroscopy (SERS)9 by precisely controlling the distance between the nanoparticle and the fluorophore. Furthermore, these nanoparticles have been used as DNA vectors,10,11 in which incident light can release DNA bound to their surface, presenting great potentials in light-triggered delivery systems for gene therapy. Furthermore, various diagnostic and therapeutic functions have been incorporated into gold nanoshells.12−14 Very recently, silica−gold nanoshells have been demonstrated to be efficient nanogenerators of steam when illuminated with sunlight, which can find important compact solar applications such as sterilization of water and surgical instruments in resourcepoor locations.15 The great potentials described above confirm the key roles of gold nanoshells in shaping the future of nanotechnology. To this end, seed-mediated growth process has been widely employed for synthesizing gold nanoshells over the past 2 decades,16−20 which is a complex and time-consuming © XXXX American Chemical Society

procedure involving many steps related to surface functionalization and the preparation of colloidal gold seeds. In addition, some disadvantages such as the possible agglomeration and unreliable electrostatic attraction between -NH2 groups and gold seeds also impede the subsequent overgrowth and the utilization of gold nanoshells.21 Recently, Watanabe et al.22 have demonstrated a flow synthesis of gold nanoshells by applying a nonsegmented single-phase microreactor to the gold seeding and shell growth. This flow process allows one-step, in situ seed attachment without the formation of unattached free gold seeds, greatly decreasing the synthesis time. In fact, a facile seed-mediated growth approach based on the one-pot, in situ reduction procedure has been previously developed to fabricate multifunctional gold nanoshells.23,24 Gold seeds were believed to be attached via Au−S bonding, but the underlying mechanism on the in situ formation of gold seeds and key factors affecting the in situ process are not clear yet, which is important for designing and fabricating gold nanoshells with well-controlled morphology, uniformity, and high reproducibility. On the other hand, Brust methods in one25 and two26 phases are widely used to prepare thiolate-protected Au nanoparticles. These Au nanoparticles are clusters with diameters below 2 nm, and the Brust process is based on Received: November 26, 2015 Revised: February 9, 2016

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DOI: 10.1021/acs.langmuir.5b04344 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Schematic Illustration for the Fabrication of MSS@Au Nanoshells

Renishaw inVia Raman microscope configured with a 785 nm excitation laser line. Triplicate measurements with integration times of 10 s were performed directly on the gold nanoshells suspension in a 1 cm path length quartz cuvette. Laser power at the sample was measured to be 20 mW. Synthesis of MSS@Au Nanoshells. First, MSS were prepared as previously reported.28 In a typical procedure, 53 μL of MPTMS and 500 μL of ammonia (28%) were added to 14.5 mL of pure water, and the mixture was shaken for 1 min using an IKA MS3 minishaker at a speed of 2500 rpm. The mixture was incubated at room temperature for another 6 h to obtain MSS. The MSS suspension was then centrifuged at 8000 rpm for 18 min. Most of the supernatant was removed with just 1.2 mL left, and the MSS precipitate was redispersed in the remaining supernatant. Then, the aforementioned suspension was mixed with 70 mL of 0.5 mM HAuCl4 solution adjusted by 0.01 M NaOH solution to pH = 9.0. After 15 min of stirring, 4 mL of 0.01 M NaBH4 was added to reduce Au precursor. The resulting red-brown suspension was left stirring for another 1−2 h and then centrifuged at 7000 rpm for 18 min. The precipitate was redispersed in 3 mL of water to obtain MSS-Au. In the final gold shell growth step, 10 mg of potassium carbonate (K2CO3) and 1.5 mL of 0.01 M HAuCl4 solution were added to 40 mL of water and the solution was aged for 10 min. To this vigorously stirred solution, 200 μL of MSS-Au suspension was added. Then, a 500 μL (78.8 mM) aliquot of L-ascorbic acid was injected to form the final MSS@Au nanoshells. For MSS@Au of different sizes, various sizes of MSS were obtained via controlling the amount of MPTMS followed by the aforementioned gold seed grafting and seed-mediated growth process. For MSSRB@Au, 50 μL of rhodamine B (1 mg/mL in ethanol) was added into 14.5 mLof water together with 53 μL of MPTMS and 500 μL of ammonia (28%), and the mixture was shaken for 1 min using an IKA MS3 minishaker at a speed of 2500 rpm. The mixture was incubated at room temperature for another 6 h to obtain MSSRB. Then, through the preceding standard procedure of Synthesis of MSS@Au Nanoshells, MSSRB@Au nanoshells were obtained. To investigate the effects of thiol molecule (M-SH) concentration over seed size and seed attachment, different amounts of supernatant (i.e., different amounts of M-SH) remained with MSS, followed by the preceding seed attachment procedure in Synthesis of MSS@Au Nanoshells to obtain different kinds of MSS-Au. For postattachment process, MSS suspension was centrifuged to separate MSS from supernatant and MSS was washed two times. Then, 1.2 mL of supernatant was mixed with 70 mL of 0.5 mM HAuCl4 solution adjusted by 0.01 M NaOH solution to pH = 9.0. After 15 min of stirring, 4 mL of 0.01 M NaBH4 was added to reduce Au precursor. The resulting red-brown suspensions were mixed with the aforementioned MSS. After 2 h of stirring, the resulting suspension was centrifuged for TEM characterization. Ellman’s Reaction. The free -SH groups of MSS and M-SH in MSS suspension were determined by Ellman’s reaction, according to the literature. Briefly, the obtained MSS were washed three times by centrifugation and re-dispersed in 15 mL of water. A 1 mL aliquot of MSS suspension and 20 μL of Ellman’s reagent (2 mg/mL) were then added to 2 mL of phosphate buffer (pH = 7.8) to react for 3 min at room temperature. Then the mixture was centrifuged, and the absorbance of the supernatant was measured at a wavelength of 412 nm by a UV−vis spectrophotometer. The amount of thiol moiety was calculated from the corresponding standard curve elaborated between 0.01 and 0.1 mM L-cysteine (r2 = 0.9991). A 20 μL aliquot of

Au−S bonding, which seems highly related to the in situ seed formation via Au−S bonding. Knowledge of the structure and chemistry of Au clusters by Brust method over the past 2 decades27 may help us understand the underlying mechanism on the in situ seed formation. Herein, gold nanoshells with different dimensions were first fabricated on mercaptosilica nanospheres (denoted as MSS) through the improved seed-mediated growth method based on the in situ seed formation. On the basis of these, the mechanism of seed formation is postulated that gold seeds were in situ formed onto MSS through a process similar to the Brust method (denoted as a Brust-like process). Compared with the widely used method based on silica template developed by Oldenburg et al.,16 this process is especially time-saving and simple to conduct, free from complicated surface modification, aforehand preparation of gold seeds, and frequent phase transferring and repeated washing. Most importantly, it is demonstrated that the molar ratio of thiol molecules to gold precursor definitely determines the Brust-like seeds attachment, facilitating the design and synthesis of gold nanoshells with higher dispersivity and reproducibility. It further allows us to produce high-quality metal nanoparticles (such as Pt or Pd) grafted mercaptosilica nanospheres through the same in situ process. Finally, owing to this improved method, SERS inside gold nanoshells was achieved by facilely incorporating reporter molecules into mercaptosilica nanopheres.

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EXPERIMENTAL SECTION

Chemicals. Sodium borohydride (NaBH4, 96%) and L-ascorbic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH, ≥96%), ammonium hydroxide aqueous solution (28%), and potassium carbonate anhydrous (K2CO3) were obtained from ShangHaiLingFeng Chemical Reagent Co., Ltd. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4·3H2O), (3mercaptopropyl)trimethoxysilane (MPTMS) and rhodamine B were purchased from Sigma-Aldrich. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O) and potassium tetrachloropalladate (K2PdCl4, Pd ≥ 32.6%) were obtained from Aladdin. Ultrapure water (18.2 MΩ·cm) was used in all experiments. All chemicals were used as received without further purification. Characterization. Morphology and structure were observed on a JEM 2100F transmission electron microscope (TEM). The shell thickness was determined by measuring the thickness of the shell with a stronger image contrast using the TEM software DigitalMicrograph. UV−vis−near-IR spectra were recorded on a UV-3600 Shimadzu spectroscope. Size distributions of the samples and ζ potentials were measured using a Nicomp TM 380 ZLS ζ potential/particle sizer (PSS Nicomp particle size system, Orlando, FL, USA). XPS data were collected on a K-Alpha X-ray photoelectron spectrometer (Thermo Fisher, Waltham, MA, USA). The X-ray source was a monochromated Al Kα line, and the pass energy of analyzer for a high-resolution spectrum was fixed at 50 eV. The binding energies were referenced to 284.6 eV (C 1s peak for C−C bonds). XPS samples were re-dispersed in 0.5 mL of water after three times of washing and then dripped onto a piece of aluminum foil with a much higher concentration. Samples were tested before being dried. SERS spectra were obtained on a B

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Langmuir supernatant obtained after centrifuging MSS suspension and 150 μL of Ellman’s reagent were then added to 3 mL of phosphate buffer to react for 3 min to obtain the free -SH concentration of M-SH. Synthesis of MSS-Pt and MSS-Pd. MSS-Pt were prepared via the same in situ reduction method except that H2PtCl6 solution was used. For MSS-Pd, MSS suspension was mixed with 30 mL of 1 mM K2PdCl4 solution adjusted by ammonia to pH = 9.0 and 4 mL of 0.01 M NaBH4 was added immediately to form MSS-Pd.

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RESULTS AND DISCUSSION The fabrication process of gold nanoshells through improved seed-mediated growth method is depicted in Scheme 1. With the rapid hydrolysis and condensation of the single silica source of MPTMS in alkaline medium, MSS with abundant thiol groups on the outer surface were first synthesized. Then, according to the in situ reduction procedure23,24 with significant modification, small Au nanoparticles with a diameter of about 2 nm (termed as “Au seeds”) were in situ formed onto the outer surface of the as-synthesized MSS. Finally, gold nanoshells were formed via the continuous growth of the attached gold seeds. As displayed in Figure 1, large amounts of uniform and highly dispersed gold seeds are well-distributed on the outer Figure 2. TEM images of MSS@Au nanoshells with different diameters (a) 200, (b) 300, and (c) 400 nm. Insets of a−c are the magnified TEM images. (d) UV−vis−near-IR spectra for gold nanoshells with varied size of 200, 300, and 400 nm, corresponding with TEM images in a−c.

smaller with respect to the light wavelength.30,31 With broadening plasmon band overlapping favorably with a major portion of the solar spectrum, MSS@Au nanoshells presented here have great potentials in water desalination and purification as well as sterilization.15 Obviously, the step of in situ seed formation plays the vital role in this improved seed-mediated growth approach, which is straightforward and endows great stability and dispersivity of the gold nanoshells. In order to achieve high reproducibility and versatility, it is undoubtedly necessary to understand the underlying mechanism involved in the in situ formation process. Based on the previous report,28 it is known that numerous monomers and oligomers of mercaptopropylsilicate containing free -SH groups (denoted as M-SH) that exist in the reaction suspension (Figure S2) with the formation of MSS. The amount of M-SH molecules in the suspension was calculated to be 14 mM in this experiment according to the Ellman’s reaction (the details are shown in the Experimental Section). However, the amount of free -SH groups on the outer surface of MSS is about 0.062 mM (Figure S2). With the coexistence of M-SH and Au precursor, Au seed formation is analogous to the classic preparation of gold clusters first developed by Brust et al.25,26 Therefore, it is deduced that the in situ formation of Au seeds on the outer surface of MSS is through the Brust-like process in one phase. As illustrated in Scheme 2, it begins with the addition of NaOH into HAuCl4 solution to adjust the pH value to 9.0. Notably, different from the Brust method, the process here was conducted in alkaline conditions to ensure the colloidal stability of MSS, where Au(III) precursor exists in species of Au(OH)xCly−.32 Then, the Au precursor, i.e., Au(OH)xCly−, is reduced by M-SH (Scheme 2a), producing Au(I)-SM polymers.33 This step occurs in the suspension and on the outer surface of the MSS, simultaneously (Scheme 2b). Finally, the excess precursor and the Au(I)-SM polymers react

Figure 1. TEM images (a, b) of MSS-Au with different magnifications.

surface of the MSS. Especially, Au seeds grafted MSS (designated as MSS-Au) maintain the spherical morphology and high dispersivity of MSS. In Figure 2a, it is observed that the as-synthesized MSS@Au nanoshells possess well-defined core−shell nanostructure, uniformity, and high dispersivity with average diameter of 200 nm and shell thickness of ∼22 nm. More importantly, gold nanoshells with different diameters (200, 300, and 400 nm; Supporting Infomation Figure S1) and the same shell thickness, as shown in Figure 2b,c, were successfully fabricated by simply tuning the diameters of MSS, demonstrating the tunable structure of gold nanoshell and the versatility of the synthetic approach. It is well-known that the plasmon resonance band of gold nanoshells can be tailored from visible to the near-IR region by adjusting the ratio of the shell thickness to the core diameter. The smaller this ratio, the more red-shifted is the plasmon band of the nanoshell.16 As shown in Figure 2d, the extinction band was found to shift to the near-IR region for MSS@Au of 200 nm, located at 700 nm. Interestingly, upon increasing the diameter to 300 and 400 nm with the same shell thickness, UV−vis−near-IR spectra exhibit a red shift accompanied by a substantial broadening of the band peak, spanning the near-IR region from 750 to 1300 nm. This result is consistent with the literature report for gold nanoshells.29 The broadening band probably stems from the phase retardation with higher order multipole resonance modes excited and participating in the plasmon hybridization as the nanoshell size is no longer much C

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Langmuir

reduced by NaBH4. In addition, no free -SH groups were detected after Au(I)-SM polymers formed on the outer surface of MSS (Figure S3) as assumed. The ζ potential is less negative to −15 mV from −41 mV for MSS (Figure S4). Upon reduction by NaBH4, ζ potential returns to −31 mV, demonstrating the stability of the formed MSS-Au. In order to further verify the Brust-like, in situ seed formation mechanism, a postattachment process was carried out (Figure 4a; the details are shown in the Experimental Section). Au(III)

Scheme 2. (a) Possible Reactions Involved in the Brust-like Formation of Au Seeds. (b) Schematic Representation of the Proposed Mechanism of the Brust-like, in Situ Formation of Au Seeds on the Outer Surface of MSS

with NaBH4 to form Au seeds, both on the outer surface of the MSS and in the suspension. Upon centrifugation, MSS-Au could be easily separated from the suspension. Obviously, it is the Au(I)-SM polymers produced on the surface of the MSS (designated as MSS-[Au(I)-SM]n) that act as initiation sites and induce the in situ formation of Au seeds, and the assynthesized seeds in water phase are water-soluble and highly dispersed, which endows MSS-Au with great stability. To demonstrate the proposed mechanism, the elemental states of Au on the outer surface of MSS before and after reduction were further investigated in detail using an X-ray photoelectron spectrometer. Before the addition of NaBH4, the 4f signal of Au at 84.6 eV corresponding to Au(I) (Figure 3a), based on the report of gold(I)−thiolate complexes,34 was detected. As the sample was tested by XPS after repeated washing, it is suggested that Au(I)-SM polymers exist on the outer surface. After reduction, the signal at 84.0 eV corresponding to metallic Au(0) was measured, demonstrating the formation of Au seed nanoparticles (Figure 3b). Meanwhile, the presence of the signal of Au(I) confirms the resistance of Au(I)-SM toward reduction, indicating that the assynthesized Au seeds possess Au@Au(I)-SM core−shell nanostructures as reported.33 To our surprise, additional peaks corresponding to Au(III)35 were also observed at 86.0 eV after reduction, which contributes about 70% of the overall signal. It remains unclear why Au(III) was not completely

Figure 4. (a) Schematic illustration of the postattachment process. TEM images of MSS-Au prepared by (b) in situ formation process and (c) postattachment process. The inset digital photograph shows the color of the as-obtained colloidal suspensions, corresponding to TEM images in b and c.

Figure 3. Au 4f XPS spectra for (a) MSS-[Au(I)-SM]n and (b) MSS-Au showing elemental states of Au. D

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Figure 5. TEM images of MSS-Au prepared with varied molar ratios of M-SH to Au(III) precursor, (a) 5, (b) 0.85, (c and d) 0.3, (e) 0.09, and (f) 0. Images below a and b are magnified images of the red rectangles in a and b.

precursor along with the identical amount of M-SH was initially reduced by NaBH4 to form Au seeds. With the formation of red-brown suspension containing gold seeds, it is suggested that the process of seed formation is similar to the one-phase Brust method. After incubation with MSS and centrifugation, redispersed colloid suspension for postattachment process remains colorless, in contrast to the red-brown for in situ formation process (Figure 4b). The TEM image shown in Figure 4c further indicates that Au seeds were unable to be attached onto the surface of MSS with such a postattachment process. These prove that Au seeds were formed and attached onto the MSS through Brust-like, in situ procedure. Besides, it is known that the size of gold nanoparticles synthesized by one- or two-phase Brust method largely depends on the ratio of thiolate to Au(III).36 Thus, it is anticipated that the molar ratio of M-SH to Au(III) precursor plays a significant role in the seed size and its attachment onto the surface of the MSS. During the preparation process, the amount of M-SH in the MSS suspension added into the Au(III) precursor solution was adjusted through centrifugation instead of uncontrollable dialysis against water.23,24 When the molar ratio is 5, only a few gold clusters with diameter of sub-1-nm were found to be grafted onto the outer surface, as shown in Figure 5a. Upon decreasing the ratio to 0.85, large numbers of sub-2-nm-sized Au seeds were found to be attached, on the outer surface of MSS (Figure 5b). However, Au seeds were not well distributed with some sites uncovered by Au seeds. With further decreasing the ratio to 0.3, highly dense and well-distributed coverage of Au seeds with diameter of about 2 nm were achieved (Figure 5c,d), which are capable for the successful growth of nanoshells in high quality. As the ratio was extremely decreased to 0.09 or even to 0, rapid growth and aggregation of Au seeds were observed. As shown in Figure 6e, some seeds were found to be sparsely distributed on the surface for the ratio of 0.09, while few seeds could be attached without M-SH added (Figure 5f). In addition, as the seeds obtained with the molar ratio of 5, 0.85, and 0.3 are too small (