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Exploration of the growth process of ultrathin silica shells on the surface of gold nanorods by the localized surface plasmon resonance
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 045704 (http://iopscience.iop.org/0957-4484/25/4/045704) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 045704 (7pp)
doi:10.1088/0957-4484/25/4/045704
Exploration of the growth process of ultrathin silica shells on the surface of gold nanorods by the localized surface plasmon resonance Chong Li1 , Yujie Li1 , Yunyang Ling, Yangwei Lai, Chuanliu Wu and Yibing Zhao Department of Chemistry, College of Chemistry and Chemical Engineering and the MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, Xiamen University, Xiamen, 361005, People’s Republic of China E-mail: [email protected] and [email protected] Received 15 August 2013, revised 11 November 2013 Accepted for publication 18 November 2013 Published 6 January 2014 Abstract
Ultrathin silica coating (UTSC) has emerged as an effective way to improve the compatibility and stability of nanoparticles without attenuating their intrinsic optical properties. Exploration strategies to probe the growth process of ultrathin silica shells on the surface of nanoparticles would represent a valuable innovation that would benefit the development of ultrathin silica coated nanoparticles and their relevant applications. In this work, we report a unique, very effective and straightforward strategy for probing the growth of ultrathin silica shells on the surface of gold nanorods (Au NRs), which exploits the localized surface plasmon resonance (LSPR) as a reporting signal. The thickness of the ultrathin silica shells on the surface of Au NRs can be quantitatively measured and predicted in the range of 0.5–3.5 nm. It is demonstrated that the LSPR shift accurately reflects the real-time change in the thickness of the ultrathin silica shells on Au NRs during the growth process. By using the developed strategy, we further analyze the growth of UTSC on the surface of Au NRs via feeding of Na2 SiO3 in a stepwise manner. The responsiveness analysis of LSPR also provides important insight into the shielding effect of UTSC on the surface of Au NRs that is not accessible with conventional strategies. This LSPR-based strategy permits exploration of the surface-mediated sol–gel reactions of silica from a new point of view. Keywords: ultrathin silica shell, surface coating, gold nanorod, surface plasmon, nanoparticle S Online supplementary data available from stacks.iop.org/Nano/25/045704/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
Surface coating of nanoparticles with silica shells could dramatically improve their colloidal properties and/or functions due to the unique advantages of silica in stability, inertness, porosity, processability, biocompatibility, and optical transparency [5–8]. For example, quantum dots that are coated with silica shells have been demonstrated to be not only more stable and brighter, but also less toxic to living
Silica as a surface coating material has greatly promoted the development of the use of inorganic nanoparticles in many areas including spectroscopy, biology, and catalysis [1–4]. 1 These authors contributed equally.
0957-4484/14/045704+07$33.00
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c 2014 IOP Publishing Ltd Printed in the UK
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acid (99%, TGA), ethanol (EtOH), and diethylene glycol (99%, DEG) were purchased from Sigma-Aldrich. Millipore ultrapure water (18.2 M cm) was used throughout the experiment. Instrumentation. A Hitachi UV 3900 spectrophotometer was used to record the extinction spectra. Particle characterization was performed using a Tecnai F-30 electron microscope (300 kV) and a JEM-1300 (100 kV). Samples for TEM measurements were prepared by placing a drop of the colloidal solution on a carbon coated copper grid. Synthesis of Au NRs. Au NRs were synthesized in aqueous solution by a typical seed-mediated, CTAB surfactantdirected procedure. The seed and growth solutions were prepared as described below. Preparation of the seed solution. 2.5 ml of CTAB solution (0.20 M) was mixed with 2.5 ml of HAuCl4 solution (0.6 mM) by stirring, to which 0.30 ml of freshly prepared, ice-cold NaBH4 (0.01 M) was added, leading to the formation of a brownish-yellow solution. The obtained solution was stirred for another 2 min at 27–30 ◦ C and used as the seed solution. Preparation of the growth solution. HAuCl4 (1.5 ml, 50 mM) and AgNO3 (0.275 ml, 50 mM) aqueous solutions were added to 100 ml of 0.10 M CTAB aqueous solution at 27–30 ◦ C. After several minutes of stirring, 1.25 ml of 0.08 M ascorbic acid in water was added slowly, upon which the growth solution changed from dark yellow to colorless. 0.20 ml of the seed solution was then added to the growth solution at 27–30 ◦ C. The color of the solution gradually changed within 10–20 min. After reacting for 10 h at 27–30 ◦ C, the resulting solution was centrifuged at 15 000 rpm for 15 min to remove excess CTAB surfactant. The obtained Au NRs were collected and redispersed in 70 ml of ultrapure water by sonication for further use. Au NRs of varying size could be synthesized by changing the concentration of AgNO3 in the growth solution. Synthesis of ultrathin silica shell coated Au NRs. The surface of the CTAB capped Au NRs was first coated with MTS by ligand exchange. MTS was used as a surface modifier to form a siloxane linkage on the surface of the Au NRs so that Na2 SiO3 could deposit and form an ultrathin silica coating. Typically, 50 µl of 10 mM MTS in EtOH was added to 3 ml of the prepared Au NRs. The ligand exchange reaction proceeded for 3 h under stirring. After that, different amounts (60, 45, 30, 20, 10, 6 µl) of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was added and left to react with the Au NRs for two days to achieve a homogeneous ultrathin silica coating on the surface of the Au NRs. Finally, the reaction solution was centrifuged at 8000 rpm for 20 min to remove excess MTS, Na2 SiO3 , and CTAB. During the reaction process, all LSPR extinction spectra were recorded by a UV–vis spectrophotometer. Synthesis of silica shell coated Au NRs via stepwise feeding. 500 µl of 10 mM MTS in EtOH was added to 30 ml of the prepared Au NRs under stirring. After 3 h of ligand exchange, 100 µl of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was added six times under vigorous stirring at a 24 h interval (final concentration of Na2 SiO3 : 0.88 mM). The reaction was allowed to proceed for 145 h.
systems than the pristine particles [9–12]. In addition, the biocompatibility and stability of metallic nanoparticles can be greatly improved through silica coating, which is extremely useful for their applications in biological imaging [13–16]. In many applications, the thickness of the silica shell is a key parameter that governs the performance of a core–shell hybrid material. Recently, ultrathin silica coating (UTSC) has emerged as an effective way to improve the compatibility and stability of nanoparticles without significantly attenuating their intrinsic properties in optics [17, 18]. This is of practical importance for the successful use of plasmonic nanoparticles in either sensing or surface-related studies such as surface enhanced Raman scattering (SERS) [19]. Although UTSC is very favorable and promising, it is still a challenge to prepare ultrafine core–shell nanostructures with well-controlled and ultrathin shell thicknesses at least partly due to the lack of an understanding of the imperceptible growth process of UTSC. Thus, exploration strategies to probe the growth process of ultrathin silica shells on the surface of nanoparticles would represent a valuable innovation that would greatly benefit the development of ultrathin silica coated nanoparticles and their relevant applications. Techniques that are widely used to characterize the formation of silica shells include electron microscopy and atomic force microscopy. Despite the very powerful aspects of these techniques, these forms of ex situ characterization are virtually impotent to probe the in situ growth process of UTSC on the surface of nanoparticles. Recent progress in real-time transmission electron microscopy (TEM) imaging has enabled direct observation of colloidal nanocrystal growth in solution [20, 21], but whether these latest techniques are capable of monitoring UTSC still remains unclear. The use of other approaches for this purpose such as dynamic light scattering and high-sensitivity flow cytometry would be hampered by their intrinsic lower resolution [22, 23]. In this work, we report a unique, effective and straightforward strategy for probing the growth process of ultrathin silica shells on the surface of gold nanorods (Au NRs), a strategy which exploits the intrinsic localized surface plasmon resonance (LSPR) of Au NRs as a reporting signal [24, 25]. The LSPR of metallic nanoparticles is highly sensitive to the refractive index of their surface environment [26, 27]. This is the fundamental basis of using LSPR for routine optical sensing of molecules [15, 28] and most recently for monitoring the monolayer growth of inorganic anions on gold surfaces [29]. The strategy used here to probe silica growth relies on the relationship of silica deposition on Au NRs with their surface refractive index and LSPR evolution. 2. Experimental section
Reagents. Cetyltrimethylammonium bromide (99%, CTAB), sodium borohydride (98%, NaBH4 ), chloroauric acid (HAuCl4 ), silver nitrate (99%, AgNO3 ), ascorbic acid (99%), (3-mercaptopropyl)trimethoxysilane (95%, MTS), sodium silicate (27 wt%, Na2 SiO3 ), triethylamine (99.5%), thioglycolic 2
Nanotechnology 25 (2014) 045704
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Figure 1. (a) TEM image of Au NRs coated with ultrathin silica shells. In this case, a relatively high concentration of Na2 SiO3 (0.88 mM)
was used for the preparation of the ultrathin silica shell coated Au NRs, which leads to an observable layer of silica shell surrounding the nanorods. The inserted dashed lines denote the interfacial distance between two adjacent Au NRs. More TEM images are given in figure S1 (available at stacks.iop.org/Nano/25/045704/mmedia). (b) Histogram of shell thickness distribution which reflects an effective thickness of ∼0.5 nm (determined indirectly from the interfacial distance between two adjacent Au NRs) for the pristine CTAB coated Au NRs without silica coating (black), obtained from measurements of >50 particles; histogram of silica shell thickness distribution obtained from measurements in a direct (green) or indirect manner (red), from measurements of >50 particles. The inserted data points show the mean shell thickness ± sd.
Finally, the reaction solution was centrifuged at 8000 rpm for 20 min to remove excess MTS, Na2 SiO3 , and CTAB. The reaction process was monitored by measuring the LSPR extinction spectra. This experiment was repeated three times. For comparison, synthesis of silica shells via a single injection of 600 µl of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was also performed (final concentration of Na2 SiO3 : 0.88 mM).
practical if the UTSC is too thin to be observed by TEM. By using this approach, an effective shell thickness of 0.5 nm (arising from the surface cetyltrimethylammonium bromide (CTAB) layer) for the pristine Au NRs without silica coating was obtained, which is significantly thinner than that of the silica coated Au NRs, further validating the above approach. The smaller interfacial distance between pristine Au NRs probably arises from the steric hindrance of CTAB molecules on the surface of the Au NRs. However, it is very difficult to determine the exact thickness of the CTAB monolayer on the surface of the Au NRs directly by existing technologies. In a previous report, the thickness of the surface CTAB bilayer was determined to be 3.2 ± 0.2 nm [30], a value which is significantly larger than 0.5 nm, reflecting very likely that the CTAB layer on the surface of the Au NRs is not very compact, while most free CTAB molecules in the solution are removed to satisfy the condition required for the growth of surface silica coating. To examine whether the LSPR of Au NRs is quantitatively relevant to the thickness of UTSC, shells of different thicknesses from 0.8 to 2.6 nm were prepared on the surface of Au NRs by tuning the total amount of Na2 SiO3 in solution (0.088–0.88 mM) and by using a stepwise feeding method. The LSPR of the Au NRs was monitored by a spectrophotometer (figure 2(a)), which showed a gradual shift to longer wavelength as the thickness of the silica shell increased. It is noteworthy that the LSPR spectral line width remains nearly unchanged after surface silica coating, thus unambiguously ruling out the contribution of agglomeration of Au NRs to the observed LSPR shifts [31]. To further minimize any other possible influencing factors such as different aqueous milieu, the wavelength shift of the LSPR was quantitatively extracted by comparing the LSPR peaks of silica coated Au NRs to those of corresponding Au NRs with Na2 SiO3 added immediately. Considering the relatively low concentration (50 particles).
Au NRs can be calculated. The obtained values for m, 290 nm per refractive index unit (RIU), and ld , 7.1 nm, are comparable to the values obtained in our experiment and reported in the literature [33, 34]. Interestingly, we also found that the effective thickness of the silica shell (0.5 nm) for the pristine Au NRs calculated from the fitted curve (at LSPR = 0) was in close agreement with the value obtained by TEM. These findings provide strong evidence that this quantitative model can accurately predict the formation of ultrathin silica shells on the surface of Au NRs and that the silica coating is the dominant factor responsible for the observed LSPR shift. The LSPR thus represents a unique, sensitive and straightforward approach, by which the UTSC on the surface of Au NRs can be probed in real time. The kinetics of growth of ultrathin silica shells on Au NRs was quantitatively analyzed by plotting the shell thickness derived from the LSPR shift as a function of the reaction time (figure 3(a) and figures S2–3 available at stacks. iop.org/Nano/25/045704/mmedia). Assuming that the number of reaction sites for silica deposition on the surface of the Au NRs remains unchanged during the course of silica coating, the rate constant of UTSC growth (or silica deposition) can thus be extracted from the nonlinear regression of the curves in figure 3(a) by using a pseudo-first-order rate equation (see the section ‘the growth kinetics of ultrathin silica shell on Au NRs’ in the supporting information for details and figure S4 available at stacks.iop.org/Nano/25/045704/mmedia). This would be reasonable if it is considered that the increase in the thickness of the UTSC is negligible as compared to the size of the Au NRs. Deviations from the nonlinear regression become more pronounced when the reaction time is longer, reflecting greater complexity not taken into consideration in the reaction model, which includes the extensive formation of
process. In addition, it is worth mentioning that the band position as well as the band width could vary when the shape and/or size distributions of the prepared Au NRs change; however, the size distribution shown in figure S1 (available at stacks.iop.org/Nano/25/045704/mmedia) indicates that these factors were not varied after silica coating. Considering the difficulty in directly measuring the thickness of the silica shell in the range of 0.5–1.5 nm on the surface of Au NRs, the thickness of the UTSC was obtained by measuring indirectly the interfacial distance between two adjacent Au NRs. While the silica coating (1.5–3 nm) can be clearly observed, the thickness of the UTSC was obtained through both direct and indirect approaches. No significant difference is seen between the results obtained in the direct and indirect manners shown in figure 2(b), further validating the reliability of the proposed indirect approach for the measurement of ultrathin silica shell thickness. In figure 2(b), the thickness of the UTSC is plotted versus the LSPR shift, which should be well fitted by the quantitative model that was developed to describe the correlation between the LSPR shift and the effective thickness of the adsorbate layer (see the supporting information available at stacks.iop.org/Nano/25/ 045704/mmedia, the section ‘the relationship between LSPR shift and the thickness of silica shell’) [32]. Although the correlation of our data is not very high (R2 = 0.93) due to, at least partly, the difficulty in accurately measuring the thickness of the UTSC on the surface of Au NRs, to the best of our knowledge, this is the first reported case of successfully probing ultrathin silica shells of from less than 1 nm to several nanometers on the surfaces of nanoparticles by optical detection. From the fitted curve in figure 2(b), both the LSPR sensitivity of the Au NRs to local changes in the refractive index (m) and the electromagnetic field decay length (ld ) of the 4
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Figure 3. (a) Kinetic plots of the change of silica shell thickness as a function of time (1: 0.88 mM, 2: 0.66 mM, 3: 0.44 mM, 4: 0.34 mM,
5: 0.22 mM). (b) Effect of the concentration of Na2 SiO3 on the rate of silica deposition on the surface of Au NRs.
silica aggregates and the elusory variation of the number of reaction sites for silica deposition. In figure 3(b), we found that the rate constant of UTSC growth on the surface of Au NRs was significantly increased as the concentration of Na2 SiO3 increased from 0.22 to 0.88 mM (table S1 available at stacks.iop.org/Nano/25/045704/mmedia). This was likely caused by the rise of pH from ∼7 to ∼9 associated with the addition of more Na2 SiO3 [35]. In addition, we observed that the trend of increase in the rate constant of silica growth reached a plateau at higher concentrations of Na2 SiO3 . At higher concentrations, the rapid hydrolysis of Na2 SiO3 led to extensive formation of silica aggregates in the reaction mixtures, which was probably the dominant factor responsible for the plateau observed in UTSC growth (figure 3(b)). This surmise was further confirmed by TEM (extensive silica aggregates were observed in figure S5 available at stacks.iop. org/Nano/25/045704/mmedia). Interestingly, it appears that the formation of silica aggregates has very little effect on either the uniformity of the ultrathin silica shells or the colloidal stability of the Au NRs. In some cases, a direct contact or adhesion between the Au NRs and silica aggregates may lead to a response of the LSPR of the Au NRs. However this response, even if it exists, was negligible as compared to the effect of the direct silica coating on the surface of the Au NRs. The LSPR of the Au NRs was further used to analyze the growth process of the UTSC on the surface of the Au NRs via feeding of Na2 SiO3 in a stepwise manner. Figure 4 illustrates a stepwise trajectory of the thickness of the silica shell as a function of reaction time when Na2 SiO3 was fed via six steps to the reaction mixture. For comparison, the data obtained from a single injection of Na2 SiO3 are also shown in figure 4. The shell thickness was quantitatively derived from the LSPR shift (figures S6–7 available at stacks.iop.org/ Nano/25/045704/mmedia) by using the correlation equation obtained in figure 2(b). It is interesting to find that the ultimate thickness of the silica shell on the surface of the Au NRs obtained via stepwise feeding is obviously larger than that
Figure 4. Kinetic plots of shell thickness as a function of time via
feeding of Na2 SiO3 in a stepwise or single injection manner. For either stepwise feeding or single feeding of Na2 SiO3 , the total concentration of Na2 SiO3 in the aqueous solution is exactly the same, i.e., 0.88 mM.
obtained via a single injection, although the total amount of Na2 SiO3 added is exactly the same. This finding thus implies that feeding of Na2 SiO3 in a stepwise manner reduces very likely the propensity of rapid hydrolysis of Na2 SiO3 at high concentration to form silica aggregates, which leads to more Na2 SiO3 hydrolyzed on the surface of the Au NRs instead. This also suggests that continuous and slow feeding of Na2 SiO3 may further diminish the extensive formation of silica aggregates and result in more uniform coating of ultrathin silica shells on the surfaces of the nanoparticles. Furthermore, the shell thickness shot up after the second addition while it rose only moderately after the others. The steeper increase in shell thickness after the second treatment is very likely ascribed to a sudden change in pH from mildly acidic (pH ∼ 5–6) to mildly alkaline (pH ∼ 7–8). The smaller increase of shell thickness after the third treatment is probably 5
Nanotechnology 25 (2014) 045704
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Figure 5. (a) Effect of the thickness of the ultrathin silica shell (from top to bottom: 0.67, 1.38, 1.61, 1.84 nm) on the LSPR responsiveness
of the Au NRs to thioglycolic acid (TGA, 100 mM). (b) Effect of the thickness of the ultrathin silica shell on the LSPR sensitivity of the Au NRs to local change in the refractive index.
of the data shown in figure 2(b) with the proposed quantitative model. We further examined the effect of the thickness of the UTSC on the LSPR sensitivity of the Au NRs to local change in the refractive index [15, 28], which determines the capability of enhancing the electromagnetic field on the surface of silica coated Au NRs. To determine the LSPR sensitivity of the Au NRs to local changes in the refractive index, the extinction spectra of Au NRs in water–diethylene glycol (DEG) liquid mixtures of varying volume ratios (table S2 available at stacks.iop.org/Nano/25/045704/mmedia) were recorded. The sensitivity can then be obtained through linear regression analysis of the shift of the LSPR band as a function of the solvent refractive index calculated via the Lorentz–Lorenz equation (see the supporting information and figure S9 available at stacks.iop.org/Nano/25/045704/ mmedia). As shown in figure 5(b), it is not surprising that the LSPR sensitivity gradually reduced as the thickness of the silica shell increased. This result was in good agreement with the previous phenomenon that the degree of LSPR shift was relatively slow at the latter stage while the growth of the UTSC continued steadily (figure 4 and figures S6–7 available at stacks.iop.org/Nano/25/045704/mmedia), to some extent due to the decrease in the LSPR sensitivity of the Au NRs. The data shown in figure 5(b) can be fitted with a quantitative equation which describes the relationship between the LSPR sensitivity of the Au NRs and the silica shell thickness (see the supporting information available at stacks.iop.org/Nano/25/045704/mmedia for details). This relationship between LSPR sensitivity and shell thickness may offer an alternative manner of probing UTSC growth. For either LSPR sensing or applications based on surface enhanced Raman scattering [19], it is important to consider that an ultrathin silica shell provides many benefits, but it causes an unwelcome decrease in the effect of the surface plasmonic resonance.
caused by the competition of silica self-nucleation at higher concentration of Na2 SiO3 . One of the most attractive properties of UTSC is that it can effectively shield the pristine surfaces of nanoparticles from direct contact with their surrounding environment, which greatly alleviates concerns regarding surface corrosion or photo-oxidation and distortion of signals that are indirectly probed [19]. Thus, we then probed and compared the optical responsiveness of the surface of the Au NRs after coating with different thicknesses of ultrathin silica shells by examining the response of the LSPR to thioglycolic acid (TGA), a thiol molecule that possesses high affinity to the gold surface [15, 28]. Figure 5(a) shows that the LSPR response of the Au NRs upon incubation with TGA diminished significantly as the thickness of silica shell increased from 0.7 to 1.8 nm and eventually vanished. A control experiment on the CTAB coated Au NRs demonstrated that incubation with TGA leads to a remarkable LSPR response (figure S8 available at stacks.iop.org/Nano/25/045704/mmedia). These findings imply that the metallic surface of the Au NRs would become inaccessible to small molecules, even if the Au NRs were coated with ultrathin silica shells of around 2 nm. This may also indicate an ‘island’-like growth mechanism [36] for silica deposition, a mechanism that occurs prevalently in the growth of metallic shells on the surfaces of nanoparticles [37], and most recently has been observed in surface-mediated monolayer self-assembly of inorganic anions [29]. To the best of our knowledge, very little information is available in the literature regarding the exact thickness of UTSC that should be sufficient for ‘shielding’ of nanoparticles from their environment. The responsiveness of the LSPR thus provides important insight into the shielding effect of UTSC that is not accessible with conventional analysis. In addition, it is worth mentioning that the ‘island’-like growth mechanism points out a lack of consideration of the deposition density and microstructure of the surface silica shell in this work. These issues should influence the measurements of the thickness of the UTSC and the quantitative relationship between the shell thickness and the LSPR shift, an effect which should also, to some extent, lead to relatively lower correlation (R2 = 0.93)
4. Conclusion
In summary, we have developed a unique, very effective and straightforward strategy for probing the growth process of 6
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ultrathin silica shells on the surface of Au NRs by using the LSPR. The spectral shift of the LSPR quantitatively and accurately reflects the real-time change in the thickness of the ultrathin silica shells on the Au NRs during the growth process. The kinetic analysis of the growth of the UTSC indicates a quasi-first-order deposition process on the surface of the Au NRs, a process that should be, to some extent, applicable to other material surfaces. Our results also stress the necessity of considering the Na2 SiO3 concentration-dependent formation of silica aggregates and the effect of shell thickness on the intrinsic properties of the coated material. This LSPR-based strategy thus represents an innovative and important advance in methodology, which should permit the exploration of surface-mediated sol–gel reactions of silica from a new point of view. In addition, the latest progress in LSPR studies makes the real-time detection of a single plasmonic nanoparticle technically feasible [38], which offers the possibility to directly probe the growth process of the UTSC on single Au NRs. In this work, the experimental study and main conclusions were only obtained from one nanorod with plasmon resonance near 800 nm. However, it is well known that the LSPR sensitivity in terms of plasmonic shift per RIU depends on the plasmon resonance wavelength [39, 40]. Therefore, any calibration of silica shell thickness versus plasmonic shift cannot be a universal one but will depend on the nanorod dimensions. This circumstance, to some extent, decreases the universality and practical importance of the LSPR approach for the estimation of the UTSC on the surface of Au NRs.
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Acknowledgments
We would like to acknowledge the financial support of the National Natural Science Foundation of China (21075101), the National Basic Research Program of China (2011CB910403), the Planned Science and Technology Project of Xiamen, China (3502z20113006), the Specialized Research Fund for the Doctoral Program of Higher Education of China (200803840007), and NFFTBS (No. J1310024). Author contributions C Li, Y Li, C Wu and Y Zhao conceived the study, designed the experiments and analyzed the results. C Li, Y Li, Y Lai and Y Ling performed the experiments. The manuscript was written through contributions of all authors. C Li and Y Li contributed equally. References [1] Joo S H, Park J Y, Tsung C K, Yamada Y, Yang P and Somorjai G A 2008 Nature Mater. 8 126–31 [2] Cauda V, Schlossbauer A, Kecht J, Z¨urner A and Bein T 2009 J. Am. Chem. Soc. 131 11361–70 [3] Malinge J, Allain C, Brosseau A and Audebert P 2012 Angew. Chem. Int. Edn 51 8534–7 [4] Guerrero-Mart´ınez A, P´erez-Juste J and Liz-Marz´an L M 2010 Adv. Mater. 22 1182–95 [5] Slowing I I, Trewyn B G, Giri S and Lin V Y 2007 Adv. Funct. Mater. 17 1225–36
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Exploration of the growth process of ultrathin silica shells on the surface of gold nanorods by the localized surface plasmon resonance
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 045704 (http://iopscience.iop.org/0957-4484/25/4/045704) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.151.40.2 This content was downloaded on 10/01/2014 at 07:08
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 045704 (7pp)
doi:10.1088/0957-4484/25/4/045704
Exploration of the growth process of ultrathin silica shells on the surface of gold nanorods by the localized surface plasmon resonance Chong Li1 , Yujie Li1 , Yunyang Ling, Yangwei Lai, Chuanliu Wu and Yibing Zhao Department of Chemistry, College of Chemistry and Chemical Engineering and the MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, Xiamen University, Xiamen, 361005, People’s Republic of China E-mail: [email protected] and [email protected] Received 15 August 2013, revised 11 November 2013 Accepted for publication 18 November 2013 Published 6 January 2014 Abstract
Ultrathin silica coating (UTSC) has emerged as an effective way to improve the compatibility and stability of nanoparticles without attenuating their intrinsic optical properties. Exploration strategies to probe the growth process of ultrathin silica shells on the surface of nanoparticles would represent a valuable innovation that would benefit the development of ultrathin silica coated nanoparticles and their relevant applications. In this work, we report a unique, very effective and straightforward strategy for probing the growth of ultrathin silica shells on the surface of gold nanorods (Au NRs), which exploits the localized surface plasmon resonance (LSPR) as a reporting signal. The thickness of the ultrathin silica shells on the surface of Au NRs can be quantitatively measured and predicted in the range of 0.5–3.5 nm. It is demonstrated that the LSPR shift accurately reflects the real-time change in the thickness of the ultrathin silica shells on Au NRs during the growth process. By using the developed strategy, we further analyze the growth of UTSC on the surface of Au NRs via feeding of Na2 SiO3 in a stepwise manner. The responsiveness analysis of LSPR also provides important insight into the shielding effect of UTSC on the surface of Au NRs that is not accessible with conventional strategies. This LSPR-based strategy permits exploration of the surface-mediated sol–gel reactions of silica from a new point of view. Keywords: ultrathin silica shell, surface coating, gold nanorod, surface plasmon, nanoparticle S Online supplementary data available from stacks.iop.org/Nano/25/045704/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
Surface coating of nanoparticles with silica shells could dramatically improve their colloidal properties and/or functions due to the unique advantages of silica in stability, inertness, porosity, processability, biocompatibility, and optical transparency [5–8]. For example, quantum dots that are coated with silica shells have been demonstrated to be not only more stable and brighter, but also less toxic to living
Silica as a surface coating material has greatly promoted the development of the use of inorganic nanoparticles in many areas including spectroscopy, biology, and catalysis [1–4]. 1 These authors contributed equally.
0957-4484/14/045704+07$33.00
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acid (99%, TGA), ethanol (EtOH), and diethylene glycol (99%, DEG) were purchased from Sigma-Aldrich. Millipore ultrapure water (18.2 M cm) was used throughout the experiment. Instrumentation. A Hitachi UV 3900 spectrophotometer was used to record the extinction spectra. Particle characterization was performed using a Tecnai F-30 electron microscope (300 kV) and a JEM-1300 (100 kV). Samples for TEM measurements were prepared by placing a drop of the colloidal solution on a carbon coated copper grid. Synthesis of Au NRs. Au NRs were synthesized in aqueous solution by a typical seed-mediated, CTAB surfactantdirected procedure. The seed and growth solutions were prepared as described below. Preparation of the seed solution. 2.5 ml of CTAB solution (0.20 M) was mixed with 2.5 ml of HAuCl4 solution (0.6 mM) by stirring, to which 0.30 ml of freshly prepared, ice-cold NaBH4 (0.01 M) was added, leading to the formation of a brownish-yellow solution. The obtained solution was stirred for another 2 min at 27–30 ◦ C and used as the seed solution. Preparation of the growth solution. HAuCl4 (1.5 ml, 50 mM) and AgNO3 (0.275 ml, 50 mM) aqueous solutions were added to 100 ml of 0.10 M CTAB aqueous solution at 27–30 ◦ C. After several minutes of stirring, 1.25 ml of 0.08 M ascorbic acid in water was added slowly, upon which the growth solution changed from dark yellow to colorless. 0.20 ml of the seed solution was then added to the growth solution at 27–30 ◦ C. The color of the solution gradually changed within 10–20 min. After reacting for 10 h at 27–30 ◦ C, the resulting solution was centrifuged at 15 000 rpm for 15 min to remove excess CTAB surfactant. The obtained Au NRs were collected and redispersed in 70 ml of ultrapure water by sonication for further use. Au NRs of varying size could be synthesized by changing the concentration of AgNO3 in the growth solution. Synthesis of ultrathin silica shell coated Au NRs. The surface of the CTAB capped Au NRs was first coated with MTS by ligand exchange. MTS was used as a surface modifier to form a siloxane linkage on the surface of the Au NRs so that Na2 SiO3 could deposit and form an ultrathin silica coating. Typically, 50 µl of 10 mM MTS in EtOH was added to 3 ml of the prepared Au NRs. The ligand exchange reaction proceeded for 3 h under stirring. After that, different amounts (60, 45, 30, 20, 10, 6 µl) of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was added and left to react with the Au NRs for two days to achieve a homogeneous ultrathin silica coating on the surface of the Au NRs. Finally, the reaction solution was centrifuged at 8000 rpm for 20 min to remove excess MTS, Na2 SiO3 , and CTAB. During the reaction process, all LSPR extinction spectra were recorded by a UV–vis spectrophotometer. Synthesis of silica shell coated Au NRs via stepwise feeding. 500 µl of 10 mM MTS in EtOH was added to 30 ml of the prepared Au NRs under stirring. After 3 h of ligand exchange, 100 µl of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was added six times under vigorous stirring at a 24 h interval (final concentration of Na2 SiO3 : 0.88 mM). The reaction was allowed to proceed for 145 h.
systems than the pristine particles [9–12]. In addition, the biocompatibility and stability of metallic nanoparticles can be greatly improved through silica coating, which is extremely useful for their applications in biological imaging [13–16]. In many applications, the thickness of the silica shell is a key parameter that governs the performance of a core–shell hybrid material. Recently, ultrathin silica coating (UTSC) has emerged as an effective way to improve the compatibility and stability of nanoparticles without significantly attenuating their intrinsic properties in optics [17, 18]. This is of practical importance for the successful use of plasmonic nanoparticles in either sensing or surface-related studies such as surface enhanced Raman scattering (SERS) [19]. Although UTSC is very favorable and promising, it is still a challenge to prepare ultrafine core–shell nanostructures with well-controlled and ultrathin shell thicknesses at least partly due to the lack of an understanding of the imperceptible growth process of UTSC. Thus, exploration strategies to probe the growth process of ultrathin silica shells on the surface of nanoparticles would represent a valuable innovation that would greatly benefit the development of ultrathin silica coated nanoparticles and their relevant applications. Techniques that are widely used to characterize the formation of silica shells include electron microscopy and atomic force microscopy. Despite the very powerful aspects of these techniques, these forms of ex situ characterization are virtually impotent to probe the in situ growth process of UTSC on the surface of nanoparticles. Recent progress in real-time transmission electron microscopy (TEM) imaging has enabled direct observation of colloidal nanocrystal growth in solution [20, 21], but whether these latest techniques are capable of monitoring UTSC still remains unclear. The use of other approaches for this purpose such as dynamic light scattering and high-sensitivity flow cytometry would be hampered by their intrinsic lower resolution [22, 23]. In this work, we report a unique, effective and straightforward strategy for probing the growth process of ultrathin silica shells on the surface of gold nanorods (Au NRs), a strategy which exploits the intrinsic localized surface plasmon resonance (LSPR) of Au NRs as a reporting signal [24, 25]. The LSPR of metallic nanoparticles is highly sensitive to the refractive index of their surface environment [26, 27]. This is the fundamental basis of using LSPR for routine optical sensing of molecules [15, 28] and most recently for monitoring the monolayer growth of inorganic anions on gold surfaces [29]. The strategy used here to probe silica growth relies on the relationship of silica deposition on Au NRs with their surface refractive index and LSPR evolution. 2. Experimental section
Reagents. Cetyltrimethylammonium bromide (99%, CTAB), sodium borohydride (98%, NaBH4 ), chloroauric acid (HAuCl4 ), silver nitrate (99%, AgNO3 ), ascorbic acid (99%), (3-mercaptopropyl)trimethoxysilane (95%, MTS), sodium silicate (27 wt%, Na2 SiO3 ), triethylamine (99.5%), thioglycolic 2
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Figure 1. (a) TEM image of Au NRs coated with ultrathin silica shells. In this case, a relatively high concentration of Na2 SiO3 (0.88 mM)
was used for the preparation of the ultrathin silica shell coated Au NRs, which leads to an observable layer of silica shell surrounding the nanorods. The inserted dashed lines denote the interfacial distance between two adjacent Au NRs. More TEM images are given in figure S1 (available at stacks.iop.org/Nano/25/045704/mmedia). (b) Histogram of shell thickness distribution which reflects an effective thickness of ∼0.5 nm (determined indirectly from the interfacial distance between two adjacent Au NRs) for the pristine CTAB coated Au NRs without silica coating (black), obtained from measurements of >50 particles; histogram of silica shell thickness distribution obtained from measurements in a direct (green) or indirect manner (red), from measurements of >50 particles. The inserted data points show the mean shell thickness ± sd.
Finally, the reaction solution was centrifuged at 8000 rpm for 20 min to remove excess MTS, Na2 SiO3 , and CTAB. The reaction process was monitored by measuring the LSPR extinction spectra. This experiment was repeated three times. For comparison, synthesis of silica shells via a single injection of 600 µl of freshly prepared aqueous Na2 SiO3 solution (0.54 wt%) was also performed (final concentration of Na2 SiO3 : 0.88 mM).
practical if the UTSC is too thin to be observed by TEM. By using this approach, an effective shell thickness of 0.5 nm (arising from the surface cetyltrimethylammonium bromide (CTAB) layer) for the pristine Au NRs without silica coating was obtained, which is significantly thinner than that of the silica coated Au NRs, further validating the above approach. The smaller interfacial distance between pristine Au NRs probably arises from the steric hindrance of CTAB molecules on the surface of the Au NRs. However, it is very difficult to determine the exact thickness of the CTAB monolayer on the surface of the Au NRs directly by existing technologies. In a previous report, the thickness of the surface CTAB bilayer was determined to be 3.2 ± 0.2 nm [30], a value which is significantly larger than 0.5 nm, reflecting very likely that the CTAB layer on the surface of the Au NRs is not very compact, while most free CTAB molecules in the solution are removed to satisfy the condition required for the growth of surface silica coating. To examine whether the LSPR of Au NRs is quantitatively relevant to the thickness of UTSC, shells of different thicknesses from 0.8 to 2.6 nm were prepared on the surface of Au NRs by tuning the total amount of Na2 SiO3 in solution (0.088–0.88 mM) and by using a stepwise feeding method. The LSPR of the Au NRs was monitored by a spectrophotometer (figure 2(a)), which showed a gradual shift to longer wavelength as the thickness of the silica shell increased. It is noteworthy that the LSPR spectral line width remains nearly unchanged after surface silica coating, thus unambiguously ruling out the contribution of agglomeration of Au NRs to the observed LSPR shifts [31]. To further minimize any other possible influencing factors such as different aqueous milieu, the wavelength shift of the LSPR was quantitatively extracted by comparing the LSPR peaks of silica coated Au NRs to those of corresponding Au NRs with Na2 SiO3 added immediately. Considering the relatively low concentration (50 particles).
Au NRs can be calculated. The obtained values for m, 290 nm per refractive index unit (RIU), and ld , 7.1 nm, are comparable to the values obtained in our experiment and reported in the literature [33, 34]. Interestingly, we also found that the effective thickness of the silica shell (0.5 nm) for the pristine Au NRs calculated from the fitted curve (at LSPR = 0) was in close agreement with the value obtained by TEM. These findings provide strong evidence that this quantitative model can accurately predict the formation of ultrathin silica shells on the surface of Au NRs and that the silica coating is the dominant factor responsible for the observed LSPR shift. The LSPR thus represents a unique, sensitive and straightforward approach, by which the UTSC on the surface of Au NRs can be probed in real time. The kinetics of growth of ultrathin silica shells on Au NRs was quantitatively analyzed by plotting the shell thickness derived from the LSPR shift as a function of the reaction time (figure 3(a) and figures S2–3 available at stacks. iop.org/Nano/25/045704/mmedia). Assuming that the number of reaction sites for silica deposition on the surface of the Au NRs remains unchanged during the course of silica coating, the rate constant of UTSC growth (or silica deposition) can thus be extracted from the nonlinear regression of the curves in figure 3(a) by using a pseudo-first-order rate equation (see the section ‘the growth kinetics of ultrathin silica shell on Au NRs’ in the supporting information for details and figure S4 available at stacks.iop.org/Nano/25/045704/mmedia). This would be reasonable if it is considered that the increase in the thickness of the UTSC is negligible as compared to the size of the Au NRs. Deviations from the nonlinear regression become more pronounced when the reaction time is longer, reflecting greater complexity not taken into consideration in the reaction model, which includes the extensive formation of
process. In addition, it is worth mentioning that the band position as well as the band width could vary when the shape and/or size distributions of the prepared Au NRs change; however, the size distribution shown in figure S1 (available at stacks.iop.org/Nano/25/045704/mmedia) indicates that these factors were not varied after silica coating. Considering the difficulty in directly measuring the thickness of the silica shell in the range of 0.5–1.5 nm on the surface of Au NRs, the thickness of the UTSC was obtained by measuring indirectly the interfacial distance between two adjacent Au NRs. While the silica coating (1.5–3 nm) can be clearly observed, the thickness of the UTSC was obtained through both direct and indirect approaches. No significant difference is seen between the results obtained in the direct and indirect manners shown in figure 2(b), further validating the reliability of the proposed indirect approach for the measurement of ultrathin silica shell thickness. In figure 2(b), the thickness of the UTSC is plotted versus the LSPR shift, which should be well fitted by the quantitative model that was developed to describe the correlation between the LSPR shift and the effective thickness of the adsorbate layer (see the supporting information available at stacks.iop.org/Nano/25/ 045704/mmedia, the section ‘the relationship between LSPR shift and the thickness of silica shell’) [32]. Although the correlation of our data is not very high (R2 = 0.93) due to, at least partly, the difficulty in accurately measuring the thickness of the UTSC on the surface of Au NRs, to the best of our knowledge, this is the first reported case of successfully probing ultrathin silica shells of from less than 1 nm to several nanometers on the surfaces of nanoparticles by optical detection. From the fitted curve in figure 2(b), both the LSPR sensitivity of the Au NRs to local changes in the refractive index (m) and the electromagnetic field decay length (ld ) of the 4
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Figure 3. (a) Kinetic plots of the change of silica shell thickness as a function of time (1: 0.88 mM, 2: 0.66 mM, 3: 0.44 mM, 4: 0.34 mM,
5: 0.22 mM). (b) Effect of the concentration of Na2 SiO3 on the rate of silica deposition on the surface of Au NRs.
silica aggregates and the elusory variation of the number of reaction sites for silica deposition. In figure 3(b), we found that the rate constant of UTSC growth on the surface of Au NRs was significantly increased as the concentration of Na2 SiO3 increased from 0.22 to 0.88 mM (table S1 available at stacks.iop.org/Nano/25/045704/mmedia). This was likely caused by the rise of pH from ∼7 to ∼9 associated with the addition of more Na2 SiO3 [35]. In addition, we observed that the trend of increase in the rate constant of silica growth reached a plateau at higher concentrations of Na2 SiO3 . At higher concentrations, the rapid hydrolysis of Na2 SiO3 led to extensive formation of silica aggregates in the reaction mixtures, which was probably the dominant factor responsible for the plateau observed in UTSC growth (figure 3(b)). This surmise was further confirmed by TEM (extensive silica aggregates were observed in figure S5 available at stacks.iop. org/Nano/25/045704/mmedia). Interestingly, it appears that the formation of silica aggregates has very little effect on either the uniformity of the ultrathin silica shells or the colloidal stability of the Au NRs. In some cases, a direct contact or adhesion between the Au NRs and silica aggregates may lead to a response of the LSPR of the Au NRs. However this response, even if it exists, was negligible as compared to the effect of the direct silica coating on the surface of the Au NRs. The LSPR of the Au NRs was further used to analyze the growth process of the UTSC on the surface of the Au NRs via feeding of Na2 SiO3 in a stepwise manner. Figure 4 illustrates a stepwise trajectory of the thickness of the silica shell as a function of reaction time when Na2 SiO3 was fed via six steps to the reaction mixture. For comparison, the data obtained from a single injection of Na2 SiO3 are also shown in figure 4. The shell thickness was quantitatively derived from the LSPR shift (figures S6–7 available at stacks.iop.org/ Nano/25/045704/mmedia) by using the correlation equation obtained in figure 2(b). It is interesting to find that the ultimate thickness of the silica shell on the surface of the Au NRs obtained via stepwise feeding is obviously larger than that
Figure 4. Kinetic plots of shell thickness as a function of time via
feeding of Na2 SiO3 in a stepwise or single injection manner. For either stepwise feeding or single feeding of Na2 SiO3 , the total concentration of Na2 SiO3 in the aqueous solution is exactly the same, i.e., 0.88 mM.
obtained via a single injection, although the total amount of Na2 SiO3 added is exactly the same. This finding thus implies that feeding of Na2 SiO3 in a stepwise manner reduces very likely the propensity of rapid hydrolysis of Na2 SiO3 at high concentration to form silica aggregates, which leads to more Na2 SiO3 hydrolyzed on the surface of the Au NRs instead. This also suggests that continuous and slow feeding of Na2 SiO3 may further diminish the extensive formation of silica aggregates and result in more uniform coating of ultrathin silica shells on the surfaces of the nanoparticles. Furthermore, the shell thickness shot up after the second addition while it rose only moderately after the others. The steeper increase in shell thickness after the second treatment is very likely ascribed to a sudden change in pH from mildly acidic (pH ∼ 5–6) to mildly alkaline (pH ∼ 7–8). The smaller increase of shell thickness after the third treatment is probably 5
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Figure 5. (a) Effect of the thickness of the ultrathin silica shell (from top to bottom: 0.67, 1.38, 1.61, 1.84 nm) on the LSPR responsiveness
of the Au NRs to thioglycolic acid (TGA, 100 mM). (b) Effect of the thickness of the ultrathin silica shell on the LSPR sensitivity of the Au NRs to local change in the refractive index.
of the data shown in figure 2(b) with the proposed quantitative model. We further examined the effect of the thickness of the UTSC on the LSPR sensitivity of the Au NRs to local change in the refractive index [15, 28], which determines the capability of enhancing the electromagnetic field on the surface of silica coated Au NRs. To determine the LSPR sensitivity of the Au NRs to local changes in the refractive index, the extinction spectra of Au NRs in water–diethylene glycol (DEG) liquid mixtures of varying volume ratios (table S2 available at stacks.iop.org/Nano/25/045704/mmedia) were recorded. The sensitivity can then be obtained through linear regression analysis of the shift of the LSPR band as a function of the solvent refractive index calculated via the Lorentz–Lorenz equation (see the supporting information and figure S9 available at stacks.iop.org/Nano/25/045704/ mmedia). As shown in figure 5(b), it is not surprising that the LSPR sensitivity gradually reduced as the thickness of the silica shell increased. This result was in good agreement with the previous phenomenon that the degree of LSPR shift was relatively slow at the latter stage while the growth of the UTSC continued steadily (figure 4 and figures S6–7 available at stacks.iop.org/Nano/25/045704/mmedia), to some extent due to the decrease in the LSPR sensitivity of the Au NRs. The data shown in figure 5(b) can be fitted with a quantitative equation which describes the relationship between the LSPR sensitivity of the Au NRs and the silica shell thickness (see the supporting information available at stacks.iop.org/Nano/25/045704/mmedia for details). This relationship between LSPR sensitivity and shell thickness may offer an alternative manner of probing UTSC growth. For either LSPR sensing or applications based on surface enhanced Raman scattering [19], it is important to consider that an ultrathin silica shell provides many benefits, but it causes an unwelcome decrease in the effect of the surface plasmonic resonance.
caused by the competition of silica self-nucleation at higher concentration of Na2 SiO3 . One of the most attractive properties of UTSC is that it can effectively shield the pristine surfaces of nanoparticles from direct contact with their surrounding environment, which greatly alleviates concerns regarding surface corrosion or photo-oxidation and distortion of signals that are indirectly probed [19]. Thus, we then probed and compared the optical responsiveness of the surface of the Au NRs after coating with different thicknesses of ultrathin silica shells by examining the response of the LSPR to thioglycolic acid (TGA), a thiol molecule that possesses high affinity to the gold surface [15, 28]. Figure 5(a) shows that the LSPR response of the Au NRs upon incubation with TGA diminished significantly as the thickness of silica shell increased from 0.7 to 1.8 nm and eventually vanished. A control experiment on the CTAB coated Au NRs demonstrated that incubation with TGA leads to a remarkable LSPR response (figure S8 available at stacks.iop.org/Nano/25/045704/mmedia). These findings imply that the metallic surface of the Au NRs would become inaccessible to small molecules, even if the Au NRs were coated with ultrathin silica shells of around 2 nm. This may also indicate an ‘island’-like growth mechanism [36] for silica deposition, a mechanism that occurs prevalently in the growth of metallic shells on the surfaces of nanoparticles [37], and most recently has been observed in surface-mediated monolayer self-assembly of inorganic anions [29]. To the best of our knowledge, very little information is available in the literature regarding the exact thickness of UTSC that should be sufficient for ‘shielding’ of nanoparticles from their environment. The responsiveness of the LSPR thus provides important insight into the shielding effect of UTSC that is not accessible with conventional analysis. In addition, it is worth mentioning that the ‘island’-like growth mechanism points out a lack of consideration of the deposition density and microstructure of the surface silica shell in this work. These issues should influence the measurements of the thickness of the UTSC and the quantitative relationship between the shell thickness and the LSPR shift, an effect which should also, to some extent, lead to relatively lower correlation (R2 = 0.93)
4. Conclusion
In summary, we have developed a unique, very effective and straightforward strategy for probing the growth process of 6
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ultrathin silica shells on the surface of Au NRs by using the LSPR. The spectral shift of the LSPR quantitatively and accurately reflects the real-time change in the thickness of the ultrathin silica shells on the Au NRs during the growth process. The kinetic analysis of the growth of the UTSC indicates a quasi-first-order deposition process on the surface of the Au NRs, a process that should be, to some extent, applicable to other material surfaces. Our results also stress the necessity of considering the Na2 SiO3 concentration-dependent formation of silica aggregates and the effect of shell thickness on the intrinsic properties of the coated material. This LSPR-based strategy thus represents an innovative and important advance in methodology, which should permit the exploration of surface-mediated sol–gel reactions of silica from a new point of view. In addition, the latest progress in LSPR studies makes the real-time detection of a single plasmonic nanoparticle technically feasible [38], which offers the possibility to directly probe the growth process of the UTSC on single Au NRs. In this work, the experimental study and main conclusions were only obtained from one nanorod with plasmon resonance near 800 nm. However, it is well known that the LSPR sensitivity in terms of plasmonic shift per RIU depends on the plasmon resonance wavelength [39, 40]. Therefore, any calibration of silica shell thickness versus plasmonic shift cannot be a universal one but will depend on the nanorod dimensions. This circumstance, to some extent, decreases the universality and practical importance of the LSPR approach for the estimation of the UTSC on the surface of Au NRs.
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Acknowledgments
We would like to acknowledge the financial support of the National Natural Science Foundation of China (21075101), the National Basic Research Program of China (2011CB910403), the Planned Science and Technology Project of Xiamen, China (3502z20113006), the Specialized Research Fund for the Doctoral Program of Higher Education of China (200803840007), and NFFTBS (No. J1310024). Author contributions C Li, Y Li, C Wu and Y Zhao conceived the study, designed the experiments and analyzed the results. C Li, Y Li, Y Lai and Y Ling performed the experiments. The manuscript was written through contributions of all authors. C Li and Y Li contributed equally. References [1] Joo S H, Park J Y, Tsung C K, Yamada Y, Yang P and Somorjai G A 2008 Nature Mater. 8 126–31 [2] Cauda V, Schlossbauer A, Kecht J, Z¨urner A and Bein T 2009 J. Am. Chem. Soc. 131 11361–70 [3] Malinge J, Allain C, Brosseau A and Audebert P 2012 Angew. Chem. Int. Edn 51 8534–7 [4] Guerrero-Mart´ınez A, P´erez-Juste J and Liz-Marz´an L M 2010 Adv. Mater. 22 1182–95 [5] Slowing I I, Trewyn B G, Giri S and Lin V Y 2007 Adv. Funct. Mater. 17 1225–36
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