A Controlled Heterogeneous Nucleation for Synthesis of Uniform

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A Controlled Heterogeneous Nucleation for Synthesis of Uniform Mesoporous Silica-Coated Gold Nanorods with Tailorable Rotational Diffusion and 1-nm-Scale Size Tunability Seokyoung Yoon, Byoungsang Lee, Chansong Kim, and Jung Heon Lee Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00724 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Crystal Growth & Design

A Controlled Heterogeneous Nucleation for Synthesis of Uniform Mesoporous Silica-Coated Gold Nanorods with Tailorable Rotational Diffusion and 1-nm-Scale Size Tunability Seokyoung Yoon,a Byoungsang Lee,b Chansong Kim,b Jung Heon Leea,b,* a

SKKU Advanced Institute of Nanotechnology (SAINT), bSchool of Advanced Materials

Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, South Korea Keywords: core-shell nanoparticles, gold nanorods, mesoporous silica, shell thickness, rotational diffusion

ABSTRACT

We demonstrate a strategy for the synthesis of discrete and uniform gold nanorod (GNR)@mesoporous silica (mSiO2) core-shell nanoparticles (NPs) with finely tuned shell thickness by significantly suppressing the formation of undesired core-free NPs. We could control the thickness of the mSiO2 shell with high precision, close to one nanometer, and hence the rotational diffusion mode of NPs by simply controlling the silica precursor injection times.

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The growth of the mSiO2 shell on the GNR seeds and the formation of core-free mSiO2 NPs could be explained by two models based on the modified LaMer’s theory. Between these two, we found most suitable one for the synthesis of GNR@mSiO2 NPs with a precisely controlled shell thickness and negligible core-free NPs as the synthesis mostly occurs via heterogeneous nucleation on GNR seeds. As our results are very simple and highly reproducible, we expect this work to provide profound insights on the synthesis of a variety of heterogeneous nanostructures.

Core-shell nanoparticles (NPs) have been studied extensively owing to their various multifunctional properties, which bring new opportunities in biomedical, environmental, and catalytic applications.1-8 Thus, core-shell NPs with diverse core components and shapes, such as gold,9 silver,10 quantum dots,11 and metal oxide12 NPs, and various types of shell materials are actively researched. Among these, Stöber method-based silica (SiO2) or mesoporous silica (mSiO2) has been substantially investigated as a shell material because of its facile synthesis, high chemical and thermal stability, large surface area,13 high loading capacity,14 simple surface functionalization,15 and tunable pore size.16 Further, core-shell NPs with mSiO2 shells have been shown to have considerable potential for drug delivery,17 plasmon-enhanced fluorescence,18 imaging,19 antibiotics20 and SERS,21,22 etc. It is very important to develop a method for achieving core-shell particles with discrete, uniform, and finely tunable shell thickness without the generation of unwanted core-free NPs. Therefore, significant efforts have been directed toward the synthesis of core-shell NPs with SiO2 and mSiO2 shells in this direction. For example, Mulvaney and co-workers. found that a


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careful control of the hydrolysis and condensation kinetics and consecutive injection of silica precursor is important to prevent the formation of silica nuclei.23 Xia and co-workers. reported that the ratio between the core material and shell precursor should be optimized to inhibit the homogeneous nucleation of silica.24,25,9 Liz-Marzán and co-workers. have shown that the shell coating regime depends on the amount of the injected shell precursor.10 In particular, Matsuura and co-workers. developed a single-step coating method based on the Stöber method.11 Murphy and co-workers. demonstrated that mesoporous silica shell thickness can be adjusted by controlling the concentration of cetyltrimethylammonium bromide (CTAB).18 Further, Tracy and co-workers. recently developed a large-scale silica overcoating method to achieve tunable shell thickness by changing the amount of the injected silica precursor.26 However, the discrete and superfinely tuned thickness of mSiO2 shells with controllable deposition regime and suppression of core-free NPs formation remain unresolved tasks in the core-shell NP synthesis and the underlying mechanism is ambiguous. Here, we demonstrate a method for the synthesis of discrete and monodisperse gold nanorod (GNR)@mSiO2 NPs with 1-nanometer-scale shell thickness tunability and highly suppressed formation of core-free NPs. To our knowledge, this is the first report on the precise regulation of the rotational diffusion of anisotropic NPs resulting from superfine-tuning of the shell thickness in the fabrication of core-shell NPs and bimodal deposition regime of shell materials. In addition, we provide an explanation on the observed results based on modified LaMer theory and suggest that a controlled precursor-dependent regime is more appropriate and intuitive for achieving superfine tunability of the shell thickness with better stoichiometry.


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Figure 1. TEM images of (a) bare GNRs and (b) GNR@mSiO2 NPs. (c) Dark field TEM image of a single GNR@mSiO2 and energy-dispersive X-ray spectroscopic elemental mapping of (d) Au, (e) O, and (f) Si. (g) UV-vis spectra of the GNRs before and after mesoporous silica encapsulation. (h) Dynamic light scattering data showing two distinct peaks before and after shell coating. (i) TEM image showing discrete GNR@mSiO2 NPs with a high coating yield. Very few core-free NPs are found in this sample (25 core-free NPs out of 1,434 core-shell NPs (~1.7 %)). The core GNRs have a distinctive optical property with a strong longitudinal surface plasmon resonance (LSPR) that can be easily tuned over the visible to near infrared (NIR) range by controlling their aspect ratio (AR).27-32 We synthesized GNRs via a silver-assisted, seedmediated method.33 The core GNRs were encapsulated with mSiO2 shell by injecting shell precursor solution consecutively multiple times to a GNR solution.11 Transmission electron microscopy (TEM) images of GNRs with high shape monodispersity and GNR@mSiO2 with 12.8-nm shell thickness after mSiO2 encapsulation are shown in Figure 1a and b. Through energy


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dispersive spectroscopy, we confirmed that the synthesized NPs have a gold core and silica shell (Figure 1c–f). UV-vis spectra show longitudinal mode of the localized plasmonic resonance (LSPR) peak and the red-shift of longitudinal LSPR peak from 785 nm to 794 nm because the surrounding of the GNRs is changed from water to mSiO2 (Figure 1g). As shown in Figure 1h, both GNRs and GNR@mSiO2 show two dynamic light scattering (DLS) peaks. Left side peak can be misunderstood as tiny impurities, but in this case it is owing to the rotational diffusion arising from their anisotropic shapes.34,35 All the DLS peaks are completely separated, implying that the NPs are well-dispersed in the solvent without aggregation. After the following optimization process, we were able to obtain discrete and uniform GNR@mSiO2 with a high coating yield and monodispersity, as shown in the low-magnification TEM image (Figure 1i). It has been reported that the mSiO2 shell thickness can be controlled in two different ways. On one hand, its thickness can be controlled either by varying the amount or the concentration of the injected mSiO2 precursors, which we define as the precursor-dependent regime.11,26 On the other hand, Murphy and co-workers have reported that the concentration of CTAB and GNR@mSiO2 shell thickness are inversely related, which we define as the CTAB-dependent regime.18 Therefore, we need to determine which one of these two is more suitable for superfine-tuning of the shell thickness and suppression of core-free NPs. The proportional relationship between the shell precursor concentration and shell thickness can be intuitively understood. However, it is difficult to explain how the concentration of CTAB, which is used as a surfactant for the stabilization of GNRs, affects the shell thickness and the related mechanism is yet to be unveiled. In order to investigate this relationship, we hypothesized that an excess CTAB micelle can function as another seed and become a homogeneous nucleation site for the formation of mSiO2 NPs (see Figure S1).36,37 This is very important as it is closely related to the formation of


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undesired core-free NPs as well as non-stoichiometric synthesis of NPs. As observed in most of the seed-medicated synthesis methods, 32,38,39 as the concentration of the seed decreases, the sizes of the NPs and/or the shell thickness of the core-shell NP generally increase. To prove this relationship, we investigated the variation in the thickness of mSiO2 shells on GNR@mSiO2 NPs as well as the size of the core-free mSiO2 NPs as a function of the amount of GNR core NPs and CTAB concentration, by keeping the other parameters constant.

Figure 2. TEM images of GNR@mSiO2 NPs synthesized under the concentration of (a) GNR 0.1 nM, (b) GNR 0.4 nM, and (c) GNR 0.8 nM by varying the CTAB concentration from 0.4–1.2 mM. (d) Variation in the mesoporous silica shell thickness with different CTAB concentrations. (e) UV-vis spectra of samples corresponding to each condition. (f) DLS data corresponding to each condition. Although most samples have double peaks, the GNR0.1 nM-CTAB0.4 mM sample has a single peak, which is depicted with a dotted line. Here, GNR 0.1 nM, 0.4 nM, 0.8 nM means the concentrations of GNRs.


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As shown in Figure 2a, both the silica shell thickness and the size of the core-free silica NPs decreased as the concentration of CTAB increased under a relatively low concentration of the GNRs (GNR 0.1 nM). As the concentration of GNRs is increased (GNR 0.4 and 0.8 nM), the shell thickness decreases, but not as much as in the case of 0.1 nM GNR. That is, the extent of the shell thickness variation decreased as the concentration of GNRs was increased, as depicted in Figure 2d (see Figure S2 for low-magnification TEM images). Thus, we believe that the case of 0.1 nM GNR (Figure 2a) is an example of the CTAB-dependent regime, whereas the case of 0.8 nM GNR (Figure 2c) is that of the precursor-dependent regime. In addition, a significant difference is observed both in the size and ratio of the core-free NPs. The relative size of the core-free NPs present for each condition can be monitored through UVvis extinction cross-section spectra. According to Mie’s theory, the green area of Figure 2e represents the typical scattering wavelength region of silica NPs. Among the conditions with identical GNR concentration, only the ones with a low CTAB concentration (GNR 0.1 nMCTAB 0.4 mM and GNR 0.4 nM-CTAB 0.4 mM) show higher scattering than those of others (see the legends in red in Figure 2e). This implies that the core-free mSiO2 NPs are large enough to scatter the incident light. Meanwhile, GNR 0.1 nM-CTAB 0.4 mM sample had only a single DLS peak, owing to the lack of rotational diffusion. This suggests that the NPs synthesized at this condition are isotropic. In addition, the ratio between core-free NPs and core-shell NPs was high when the concentration of GNR was low (0.1 nM and 0.4 nM; see Table S1). This ratio decreases significantly (≤ 3%) when high concentration of GNRs was used (GNRs 0.8 nM).


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Figure 3. (a) Modified LaMer diagram for the growth kinetics of GNR@mSiO2. Schematic of the growth kinetics of GNR@mSiO2 under (c) precursor-dependent and (d) CTAB-dependent regimes.

Next, we tried to analyze the observed results based on both precursor- and CTAB-dependent regimes. Based on the modified LaMer theory, the formation of core-shell NPs can be divided into two different regimes, viz., either heterogeneous or homogeneous nucleation and growth regimes (see Figure 3a).40 The precursor concentration (Cp) has to be higher than the solubility concentration (Csol) for the nucleation and growth to occur. To induce homogeneous nucleation, the Cp should be higher than the homogeneous nucleation threshold (Figure 3a, Chom). Thus, heterogeneous nucleation will occur when Cp is maintained between Csol and Chom. The sol-gel process of silica formation from the hydrolysis and condensation of silica precursor is shown in


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Figure 3b. When the concentration of the GNR seed is high enough (GNR 0.8 nM in Figure 2), heterogeneous nucleation will occur dominantly because the silica precursors will mostly be consumed and Cp is maintained below Chom even after following injection of precursors (precursor-dependent regime; see Figure 3c). In contrast, when the concentration of GNR is low (GNR 0.1 nM in Figure 2), silica precursors are not consumed enough (CTAB-dependent regime; see Figure 3d). Thus, the Cp of the solution following injection would eventually surpass Chom, resulting in both heterogeneous mSiO2 shell growth and homogeneous mSiO2 NPs formation at the same time. Thus, GNR@mSiO2 synthesized by CTAB-dependent regime will always be accompanied by a large number of core-free NPs. This might not be preferred because most core-free NPs are undesired and must be removed through time-consuming purification processes. Meanwhile, it is notable that the formation of core-free NPs may depend on the injection interval and punctuality of precursor injection, which affects the amount of precursor in solution. To verify whether the formation of core-free NPs can be suppressed by synthesizing the GNR@mSiO2 NPs through a precursor-dependent regime, we synthesized them in the presence of a relatively large amount of GNR seeds (1.2 nM; see Figure S3). We used 20 µL of shell precursor solution for each injection as it produced smallest portion of core-free mSiO2 NPs (see Figure S6, S7). Surprisingly, the generation of core-free NP is highly suppressed even at unconventionally high CTAB concentrations (up to 50 mM) and the shell thickness of GNR@mSiO2 NPs is consistent under all conditions. This implies that most of the precursors are consumed in the formation of mSiO2 shells. Furthermore, we investigated the kinetics of mSiO2 shell growth. The reaction rate is fast at the initial stage (~9 h) but suddenly decelerates after ~12 h (Figure S4). Even after storing for six days without washing, the as-synthesized GNR@mSiO2


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NPs have the same shell thickness, which implies that the shell coating reaction was complete with the exact stoichiometric condition (Figure S5).

Figure 4. GNR@mSiO2 NPs with superfinely tuned thickness. (a) The TEM images of GNR@mSiO2 synthesized with varied precursor injection times (S0 = 0 injection, S1 = 1 injection … S16 = 16 injections, scale bar = 50 nm). (b) mSiO2 shell thickness as a function of injection times. (c) UV-vis spectra of GNR@mSiO2 NPs with various shell thicknesses. (d) Magnified LSPR peaks of GNR@mSiO2 NPs. (f) Variation in the DLS profiles of GNR@mSiO2 NPs with various shell thicknesses. Right panels show an overview of the isotropy variation of GNR@mSiO2 NPs.


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As most of the precursors are consumed in the formation of mSiO2 shells with minimal generation of core-free NPs under the precursor-dependent regime, we speculated that this provides an opportunity for superfinely controlling the thickness of mSiO2 shells. To verify this, we synthesized GNR@mSiO2 NPs at a relatively high concentration of GNR seeds of 1.2 nM and injected the precursor solution multiple times ranging from 0 to 20. As shown in Figure 4a, we could regulate the shell growth with significant precision of close to 1-nanometer-scale shell thickness tunability and a negligible number of core-free NPs. The mSiO2 shell thickness reached a plateau after 16 injections (data not shown). The thickness increment is linearly proportional to the injection time (Figure 4b). The optical property of the GNR@mSiO2 NPs also changed as the shell thickness varied. The LSPR peak initially shows a red-shift with the increase in the shell thickness, but almost saturates near the wavelength of 793 nm (Figure 4 c, d, and e). In addition, it is remarkable that the DLS spectra show both a gradual shift and decrement in the intensity of the rotational diffusion peaks as the shell thickness increases.34,35 Eventually, the left-side peaks corresponding to the rotational diffusion mode vanished (Figure 4f, S15, and S16). We postulate that the morphological anisotropy of the GNR@mSiO2 NPs decreases as the shell thickness increases; the NP shape changes from rod to ellipsoidal and then eventually to spherical (Figure 4f, right panels). We expect that self-assembly behavior of NPs might be altered by their anisotropic shape and rotational diffusion.41 Thus, we will move on to the next study to figure out how these morphological differences at nanoscale can affect hierarchical multi-level self-assembled structures.

In summary, we have shown that both homogeneous and heterogeneous nucleation occurs during the synthesis of GNR@mSiO2 NPs under the CTAB-dependent regime, whereas


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heterogeneous nucleation occurs dominantly under the precursor-dependent regime. Thus, the precursor-dependent regime is more appropriate to suppress the formation of core-free NPs with precise stoichiometry. Eventually, we could synthesize discrete and uniform GNR@mSiO2 NPs with superfine tunability of shell thickness and negligible formation of core-free NPs. According to the modified LaMer theory, both regimes can be controlled by regulating the relative concentration of the core GNRs, which affects the concentration of the remaining precursor. In other words, the precursor injection interval and punctuality of injection may also be important factors from the point of view of precursor consumption. In addition, we show that the rotational diffusion dynamics of NPs can also be controlled with high precision. We believe that this is a robust demonstration of the synthesis of uniform, multifunctional core-shell NPs with negligible core-free NPs and our results provide profound insights on the synthesis of heterogeneous nanostructures.

ASSOCIATED CONTENT Supporting Information. Materials and detailed experimental methods; the ratio between core-free NPs and core-shell NPs of samples synthesized at different conditions (Table S1); Shell thickness determining hypothesis (Figure S1); low-magnification TEM images of GNR@mSiO2 samples shown in Figure 2 (Figure S2); TEM images of GNR@mSiO2 NPs synthesized under different CTAB concentration (Figure S3); TEM images of GNR@mSiO2 NPs shell growth kinetics. (Figure S4); TEM images of as-synthesized GNR@mSiO2 NPs and 6 days old one (Figure S5); TEM images of GNR@mSiO2 NPs synthesized with different amount of shell precursor at fixed injection time


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(Figure S6); DLS data of GNR@mSiO2 NPs synthesized with different amount of shell precursor at fixed injection time (Figure S7) are included in the Supporting Information. AUTHOR INFORMATION Corresponding Author E-mail: [email protected], Tel: +82-31-290-7404, Fax: +82-502-302-1918 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT for Bio-inspired Innovation Technology Development Project (NRF-2018M3C1B7021997) and Nano Material Technology Development Program (Grant number: 2009-0082580) and grant funded by the Ministry of Education for Basic Science Research Program (Grant number: 2015R1D1A1A01058605).

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For Table of Contents Use Only

A Controlled Heterogeneous Nucleation for Synthesis of Uniform Mesoporous Silica-Coated Gold Nanorods with Tailorable Rotational Diffusion and 1-nm-Scale Size Tunability Seokyoung Yoon,a Byoungsang Lee,b Chansong Kim,b Jung Heon Leea,b,* a

SKKU Advanced Institute of Nanotechnology (SAINT), bSchool of Advanced Materials

Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, South Korea TOC

In this paper, we synthesized mesoporous silica coated gold nanorod with super-fine tunability of mesoporous silica shell thickness up to 1-nm scale by suppressing homogeneous nucleation of silica. In addition, we demonstrated rotational diffusion of nanoparticles were able to tuned


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precisely.


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A Controlled Heterogeneous Nucleation for Synthesis of Uniform

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