Monodisperse Colloidal Gold Nanorings: Synthesis and Utility for

Jul 1, 2014 - (40) High quality and monodisperse Co NPs were obtained and subsequently used for template-guided synthesis of Au NRs. ... Note that the...
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Monodisperse Colloidal Gold Nanorings: Synthesis and Utility for Surface-Enhanced Raman Scattering Yue Hu,† Tsengming Chou,† Hongjun Wang,*,‡ and Henry Du*,† †

Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States S Supporting Information *

ABSTRACT: Highly monodisperse colloidal Au nanorings (Au NRs) were synthesized via galvanic replacement reaction of 1D chain assembly of Co nanoparticles (Co NPs) in HAuCl4. The Au NRs exhibited tunable localized surface plasmon resonance (LSPR) wavelengths ranging from 651 to 760 nm as the aspect ratio increased from 3.8 ± 0.5 to 7.1 ± 1.8, consistent with calculations using the finite-difference time domain (FDTD) method. The FDTD calculations indicated intense field enhancement within the cavity of well-separated Au NRs as well as the presence of hot spots between closely situated Au NRs. The Au NRs are individually active for surface-enhanced Raman scattering (SERS). We show that the steric hindrance of densely packed poly(vinylpyrrolidone) (PVP) stabilizer at the particle junctions of the 1D chain assembly of Co NPs to AuCl4− diffusion is of critical importance for Au NR formation. We further demonstrate that whether an analyte experiences the hot-spot effect or not depends strongly on its ability to enter the junction points and thus its affinity to PVP on the surface of Au NRs in our system of study. on-wire lithography,33 anodized aluminum oxide (AAO) growth template,34 colloidal lithography,31,35 nanospheretemplated wet-chemical synthesis,36 and self-assembly of metallic particles.37 While serving important purposes in demonstrating the promise of nanorings and the suitability for targeted use, these synthesis approaches not only involve more sophisticated process steps but also lead to nanorings anchored to solid support with limited feasibility for postfabrication surface modification and subsequent utilization. Clearly, there is a compelling need to develop a cost-effective and straightforward approach to fabricate free-standing, monodisperse nanorings in stable colloidal suspensions to greatly expand the field of investigation and application of the nanoring structures. Here, we report a novel one-pot method to directly synthesize colloidal Au NRs with high monodispersity in aqueous solution using a galvanic replacement reaction. Cobalt nanoparticles (Co NPs) coated with poly(vinylpyrrolidone) (PVP) were used as sacrificial template and galvanically replaced with Au to form Au NRs. By varying the size of Co NPs and the concentration of Au salt, the dimensions of Au NRs can be adjusted, leading to tunable LSPR. This capability allows for systematically tailoring the plasmonic properties of Au NRs. We identify the critical process parameters leading to the formation of Au NRs and reveal the corresponding mechanism. In addition, we combine the finite-difference

1. INTRODUCTION Localized surface plasmon resonance (LSPR) arises from collective oscillation of electrons in the conduction band of nanostructured metal upon light irradiation. The resultant electromagnetic field enhancement is a key enabling feature for a multitude of applications, ranging from sensing and imaging to cancer therapy.1−7 It is well established that LSPR strongly depends on the size, shape, composition, interparticle spacing, and dielectric environment.8,9 For example, Au nanostructures in the form of nanoshells,10−12 nanorices,13 nanorods,14,15 and nanocages16−18 have exhibited plasmon resonance wavelengths spanning across the visible and near-infrared (NIR) range,19 affording significant opportunities for spectroscopic20−22 and biological exploitations.23−27 These nanostructures are produced mainly via wet chemistry, leading to stable colloidal solutions readily available for further explorations. Compared to the solid nanoparticles with limited shift in LSPR through variation in particle diameter,28 nanorings are of particular interest because they allow significant tunability of plasmon resonance wavelengths associated with variable longitudinal and transverses axes.29 The LSPR tuning can be achieved by keeping the outer diameter fixed while changing the inner diameter, or the aspect ratio, of the nanorings. This feature is especially attractive for cellular uptake in biological applications where transport of large particles through cellular membranes will be difficult.30 Meanwhile, a large surface-tovolume ratio of nanorings reduces radiation damping and further yields intense local electromagnetic field enhancements in surface-enhanced Raman scattering (SERS).31 To date, nanoring structures have been fabricated via nanolithography,32 © 2014 American Chemical Society

Received: April 29, 2014 Revised: June 30, 2014 Published: July 1, 2014 16011

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Figure 1. (a−d) TEM images of Au NRs at various tilt angles: 0°, 15°, 30°, and 45°. (e) TEM images of Au NRs of a large area. (f, g) HRTEM images of Au NRs with fringe spacing of 0.14 nm indexed to {220} reflection of the face centered cubic (fcc). (h) EDX spectrum of Au NRs. (i) Elemental mapping of Au NRs.

measured with a transmission electron microscope (TEM, Philips CM20), a high-resolution transmission electron microscope (HRTEM, JEOL 2100F), energy-dispersed X-ray spectrometry (EDX, Philips CM20), and a UV−vis absorption spectroscope (Synergy HT multidetection microplate reader, BioTek Instruments, Inc. Winooski, VT). All the images were analyzed using Image-J 1.46 software (NIH). In order to examine Co NPs and Au NRs using TEM, HRTEM, and EDX, these nanostructures were immobilized onto copper grids (Electron Microscopy Sciences, Hatfield, PA). We designed an in situ sampling protocol, in which poly(allylamine hydrochloride) (PAH)-modified TEM grids were used to capture the nanostructures via electrostatic interaction during the synthesis process.38 Briefly, the grids coated with 300-mesh holey carbon were incubated with an aqueous solution of poly(allylamine hydrochloride) (PAH, 2 mg/mL) at pH 9 for 20 min to functionalize the surface. Upon rinsing with Milli-Q water to remove any free or loosely bound PAH, the grids were quickly dipped into the colloid solution to capture a small droplet, and the water was immediately drained with filter paper. The nanostructrues were immobilized onto the PAH-functionalized grid surface through the electrostatic interaction with PAH as well as via the binding affinity between nanostructures and amino groups of PAH. FDTD Simulation. For the simulation of plasmonic properties of gold nanorings, Lumerical FDTD solution of version 8.6.3 was used. The desired particle size and shape in 3D were built using the Cartesian coordinate system. Boundary conditions and perfectly matched layer (PML) were defined in the draw mode. The dielectric function of Au NRs is taken from measurements by Johnson and Christy.39 The surrounding medium was set to be air, the refractive index of which is 1.0. A simulation area of 10003 nm3 surrounded by 12 perfectly matched layers (PMLs) proved to be sufficient. The mesh gird

time domain (FDTD) modeling and SERS experiments to study the plasmonic behavior of these nanostructures and reveal the presence of concentrated near-field electromagnetic field inside the individual Au NR cavity and the development of hot spots between neighboring Au NRs in close proximity.

2. EXPERIMENTAL DETAILS Materials. Cobalt chloride hexahydrate (CoCl2·6H2O 99.99%), sodium borohydride (NaBH4 99%), gold(III) chloride solution (30 wt % of HAuCl4 in diluted HCl), poly(vinylpyrrolidone) (PVP, Mw = 2500, 10 000, or 55 000), poly(allylamine hydrochloride) (PAH, Mw = 15 000), trans-1,2bis(4-pyridyl)ethylene (BPE), and Rhodamine 123 (R123) were purchased from Sigma-Aldrich (St. Louis, MO). Sodium citrate trihydrate was procured from Fisher Scientific (Pittsburgh, PA). Milli-Q ultrapure water (no less than 18.2 MΩ) was used for the synthesis experiments. Synthesis of Au NRs. 100 μL of 0.4 M CoCl2·6H2O mixed with 400 μL of 0.1 M sodium citrate trihydrate was added to 100 mL of 18.2 MΩ Milli-Q water in a 500 mL three-necked round-bottom flask. The solution was deaerated by vacuuming for 40 min under continuous argon flow. Then, 1 mL of 0.1 M freshly prepared NaBH4 and 200 μL of 1 wt % PVP (Mw = 2500) were injected into the solution at the same time under vigorous stirring. The color of the solution turned from pale pink into brown in seconds, indicating the formation of Co NPs. The solution was further vigorously stirred for 40 min under argon flow protection until complete hydrolysis of the borohydride. Subsequently, 150 μL of 0.1 M HAuCl4 was added into the cobalt solution dropwise and allowed stirring for another 30 min. Upon completion of the reaction, the solution was exposed to lab ambient to oxidize the nonreacted cobalt. Characterization of Au NRs. The size and distribution, crystal structure, and LSPR of the resultant Au NRs were 16012

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Scheme 1. Schematic Illustration of the Possible Mechanism To Form Au NRs Using Co NPs as Sacrificial Templates under Continuous External Stirring

NaBH4 as reducer and sodium citrate and PVP as stabilizers under continuous stirring.40 High quality and monodisperse Co NPs were obtained and subsequently used for template-guided synthesis of Au NRs. Aqueous HAuCl4 solution was injected gently to initiate the galvanic replacement of sacrificial Co NPs at room temperature. Au NRs with high monodispersity were obtained upon completion of the reaction. The replacement reaction was robust since there exists a large differential standard reduction potential between AuCl4−/Au (0.935 V vs SHE) and Co2+/Co (−0.277 V vs SHE), which facilitated rapid exchange between Co and AuCl4− with the latter being reduced into Au0.41 Transmission electron microscopy (TEM) images in Figure 1a−d show that the as-fabricated Au NRs with outer diameter of 39.7 ± 2.3 nm, wall thickness of 9.5 ± 1.6 nm, and sidewall height of 15.6 ± 3.2 nm at different tilting angles. The highresolution transmission electron microscopy (HRTEM) images (Figure 1f,g) reveal a lattice spacing of 0.14 nm, indexing to {220} planes of face-centered-cubic (fcc) gold according to the JCPDS No. 65-2870. Energy-dispersive X-ray (EDX) analysis performed involving a large number of Au NRs suggests that Au is the only constituent element in the nanorings (Figure 1h). This observation is consistent with the element mapping of individual Au NRs (Figure 1i), indicating effective removal of Co during galvanic replacement. Discussion of the Formation Mechanism of Colloidal Au NRs. The spatial organization of sacrificial Co NPs assembly was found to be one of the key factors in determining the final Au nanostructures. Scheme 1 schematically illustrates the correlation between Co NPs assembly and resultant Au nanostructures. Co NPs with magnetic dipole (N and S) field exhibit 1D chain-like assembly along the external force line under continuous stirring,42 as shown in Figure 2a. In contrast, Co NPs exist as well-dispersed, discrete nanoparticles in a stable colloidal solution in the absence of stirring, as

size is 0.5 nm. For the ring dimers, the quartz substrate is considered. The dimension of quartz substrate is 110 nm × 110 nm × 16 nm. Surface-Enhanced Raman Measurement. In all experiments Au NRs (Au NR I in Figure 4) were deposited on thin quartz substrate (Electron Microscopy Sciences, Hatfield, PA) to achieve SERS-active substrate for measurements of trans-1,2bis(4-pyridyl)ethylene (BPE) and Rhodamine 123 (R123). The quartz surface was first modifed with a monolayer of positively charged PAH. Immobilization was achieved by electrostatic attraction between PAH and slightly negatively charged Au NRs (ζ-potential of −5 mV in Milli-Q water) as well as by the binding affinity of Au to amino groups in PAH. The absorption spectra of SERS samples were measured by a PerkinElmer UV/ vis spectrometer Lambda 40, Waltham, MA. SERS measurements were conducted at 785 nm excitation wavelength using custom-built Raman spectrometer.28 Laser power is ∼25 mW. Integration time is 20 s. The SERS data were processed using Origin 8.5 software. Analytical SERS enhancement factors (EFs) were calculated using EF = (ISERS/CSERS)/(INR/CNR), where ISERS and INR are the integrated intensities of a characteristic band from SERS and from normal Raman and CSERS and CNR are concentrations of analytes used in SERS and normal Raman experiments, respectively. Both ISERS and INR were collected under the same conditions, and the loading volume of the analyte solution was the same. Intensities of the BPE peak (∼1619 cm−1) and R123 peak (∼1209 cm−1) were used to calculate the EFs.

3. RESULTS AND DISCUSSION Monodisperse Colloidal Au NRs. In order to facilely fabricate colloidal Au NRs, we first synthesized the colloidal magnetic Co NPs as a sacrificial template. This provides a robust way to yield Au NRs directly in the aqueous solution. Briefly, highly monodisperse Co NPs were obtained using 16013

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Co NPs located at the very ends of the chain-like structures, which further confirms the illustrated mechanism in Scheme 1. The molecular weight of PVP is another key factor affecting the formation of Au NRs. Two additional PVPs with higher molecular weight, Mw 10 000 and Mw 55 000, were chosen as a control, following the same synthesis protocol as Mw 2500. Interestingly, the resultant Au NRs displayed a chain-like arrangement (Figure 3c−f), different from those of Mw 2500 (Figure 3a,b). Meanwhile, the average ring height of Au NRs in the chains seems to decrease with the increase of Mw of PVP (Figure 3d,f). This correlation further suggests that the density of PVP in the particle contact regions plays a crucial role in determing the final geometry of Au NRs. The increase of PVP Mw, that is the length of PVP chain, can increase the entanglement45 and the extensive bridging of adsorbed PVP molecules on neighboring Au NRs, resulting in the formation of permanently chained 1D assembly of Au NRs. Molecular weight of PVP can thus be used to control Au NR monodispersity and assembly. LSPR Measurements and FDTD Simulation of Au NRs. The synthesized colloidal Au NRs exhibit size-dependent LSPR with highly tunable wavelength from visible to NIR regions. Au NRs with three different geometries in Table 1, Figure 4, and Figure S2 in the Supporting Information showed markedly different LSPR characteristics. The main LSPR wavelength exhibited a strong dependence on the outer diameter/shell thickness aspect ratio (Figure 4a). As the aspect ratio increases from 3.8 ± 0.5 (Au NR III) to 7.1 ± 1.8 (Au NR I), the LSPR wavelength is progressively red-shifted from 651 nm (Au NR III) to 760 nm (Au NR I). The heights of the three Au NRs vary from 15.4 ± 4.2 nm (Au NR I) to 9.2 ± 1.8 nm (Au NR III) as shown in Figure S2 in the Supporting Information, depending on the size of the Co NPs and dictated by the aforementioned synthesis mechanism. Besides a prominent band, a lower intensity shoulder is also present in the UV−vis spectrum of various Au NRs. To obtain more quantitative insights into the LSPR behavior of Au NRs, we performed FDTD calculations of the Au NRs.

Figure 2. (a) TEM images of chain-like Co nanoparticles and the final products of Au NRs. (b) TEM images of discrete Co nanoparticles and the final products of Au NSs.

demonstrated by the sample harvested from the solution terminating stirring for 10 min (Figure 2b). This results from the steric hindrance of surface-adsorbed stabilizer of PVP, which prevents the Co NPs dipole−dipole interactions.43,44 Importantly, Au NRs were only obtained when Co NPs appeared in 1D chain-like assembly. In the case of individually dispersed Co NPs, gold nanoshells (Au NSs) were formed instead (Figure 2b). Clearly, the state of Co NPs assembly in the colloidal solution during the galvanic reaction plays a critical role in determining the final shape of Au nanostructures. Galvanic replacement was immediately initiated upon the addition of HAuCl4 into the colloidal solution containing chainlike Co NPs under continuous stirring. Note that the density of PVP in the contact regions of the 1D linked Co NPs is up to twice of that away from these sites, making the contact regions of Co NPs kinetically less accessible by AuCl4− than the center portion for preferential galvanic replacement. Upon completion of the galvanic reaction, the remaining Co caps at the contact region as well as the remnant Co cores are simply oxidized and precipitated out in the aqueous solution, leaving individually dispersed Au NRs. A very small fraction of bowl-like nanostructures was also found in the synthesized Au NRs (Figure S1 in the Supporting Information), coming from the

Figure 3. (a, b) TEM images of Au NRs obtained with PVP (Mw = 2500). (c, d) TEM images of Au NRs obtained with PVP (Mw = 10 000). (e, f) TEM images of Au NRs obtained with PVP (Mw = 55 000). 16014

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Table 1. Outer Diameter, Shell Thickness, Height, Aspect Ratio (Outer Diameter-to-Shell Thickness Ratio), Experimental SPR Position, and FDTD Simulation SPR Peak Position of Three Different Au NRs Au NRs

outer diam (nm)

shell thickness (nm)

height (nm)

aspect ratio

exp SPR (nm)

FDTD SPR (nm)

Au NR I Au NR II Au NR III

41.6 ± 1.7 33.0 ± 1.2 25.4 ± 1.1

6.2 ± 1.4 5.9 ± 1.0 6.6 ± 0.5

15.4 ± 4.2 14.1 ± 2.6 9.2 ± 1.8

7.1 ± 1.8 5.8 ± 1.2 3.8 ± 0.5

760 682 651

750 683 657

Figure 4. (a) Experimental and (b) simulated absorption spectra of three different Au NRs in Table 1. TEM images and electric field intensity distributions in the cross section and middle planes of Au NRs: (c) Au NR I, (d) Au NR II, and (e) Au NR III. The red point/arrow and white arrow are the propagation direction (k vector) and the polarization (E field vector), respectively. The scale bar is 20 nm. The color bar represents the value of |E|/|E0|. The coordinate directions are as indicated.

colloidal solution of predominantly Au NRs mixed with a small quantity of Au nanobowls, in agreement with the TEM results and in support of the mechanism of Au NR formation. We have further used FDTD to calculate the electromagnetic enhancement of Au NRs. Figure 4c−e shows the horizontal and vertical cross-sectional views of the calculated near-field electromagnetic distribution of Au NRs with different aspect ratios at 785 nm excitation. The maximum enhancement (|E|2/| E0|2), located at the upper and lower rim of the rings denoted by the point monior, increased from ∼450 to ∼1270 as the aspect ratio varied from 5 to 8 (Figure 4c−e). The calculated fields are lower than the true surface field, as reported elsewhere.46 Meanwhile, a relatively uniform intense field enhancement is found inside the ring cavity rather than the outside of the ring, which rapidly decays from the ring wall. These resluts are qualitatively consistent with previously DDA results in the gold nanorings.31 Also, Au NRs with a larger surface-to-volume ratio can further reduce the retardation effects.47 Combined with the tunability of LSPR through different aspect ratios, the high field enhancement suggests that the inner surface and the cavity of Au NRs itself can be utilized to benefit field-enhanced phenomena such as SERS-based sensing and detection. SERS Measurements. The Au NRs were further assessed for their utility in SERS. We chose two different analytes or their mixture, trans-1,2-bis(4-pyridyl)ethylene (BPE) and Rhodamine 123 (R123), in our studies. BPE has a high affinity with the PVP while R123 preferentially binds with citrate ion via the electostatic interation due to the ζ-potential of Au NRs

The outer diameters of Au NR I, Au NR II, and Au NR III for the simulation are 40, 30, and 25 nm, respectively, with the same shell thickness of 5 nm. The heights of Au NR I, Au NR II, and Au NR III are 15, 14, and 9 nm, respectively, close to the experimental values. The peak positions of FDTD calculated absorption spectra (Figure 4b) show excellent agreement with the corresponding main LSPR wavelengths in Figure 4a (also see Table 1). As the height of Au NRs in the same group is not exactly the same, we carried out additional calculations to ascertain the effect of changing heights on LSPR for the three types of Au NRs. As shown in Figure S2 in the Supporting Information, the effect on the resonance wavelength is relatively small for the range of the height value investigated. Such variations likely contributed to the broadening of the experimental UV−vis spectra. Considering the presence of a small fraction of bowl-like Au nanostructures, we further carried out calculations for the bowl structure using the same dimensions as the Au NRs (Table 2 and Figure S3 in the Supporting Information). The calculated peak positions are also consistent with those shoulder bands in the experimental UV− vis spectra. Taken together, both LSPR measurement and FDTD simulation suggest that our synthesis method leads to a Table 2. Geometry and Characteristics of Three Au Bowls Au bowls

outer diam (nm)

inner diam (nm)

FDTD SPR (nm)

Au bowl I Au bowl II Au bowl III

40 30 25

30 20 15

625 581 557 16015

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Figure 5. (a) SERS spectra of 2 ppm BPE−R123 mixture (upper spectrum), 2 ppm BPE (middle spectrum), and 2 ppm R123 (lower spectrum) obtained using Au NRs with coverage density ∼75 particles/μm2. Spectra were acquired at 785 nm (25 mW, 20 s). The characteristic bands for BPE at ∼1619 and ∼1652 cm−1 correspond to δ(C−N), ν(C−C), δ(C−H), and ν(CN). The characteristic bands for R123 at ∼1209 and ∼1609 cm−1 correspond to ν(C−O−C) and external phenyl ring stretching. (b) SERS intensities of each BPE/R123 characteristic band as a function of Au NRs coverage density for single analyte measurements. (c) SERS intensity area of BPE band at 1348 cm−1 and R123 band at 1372 cm−1 as a function of Au NRs coverage density for dual-analyte measurements. (d) Measured absorptions spectra of SERS samples with different Au NRs coverage density (CD). FDTD calculations of the absorption spectra (e) and electrical field intensity distribution (f) in the cross section and middle planes of two Au NRs located on the quartz substrate with interspacing distance of 2, 5, 10, and 20 nm, respectively. The red point/arrow and white arrow are the propagation direction (k vector) and the polarization (E field vector), respectively. The scale bar is 20 nm. The color bar represents the value of |E|/| E0|. The coordinate directions are as indicated.

(−5 mV in Milli-Q water).48 Figure 5a shows the typical SERS spectra of BPE, R123, and BPE−R123 mixture absorbed onto Au NRs. We evaluated the dependence of SERS intensity on the coverage density of Au NRs in each case. BPE is more sensitive as measured than R123 (Figure 5b). The enhancement factor (EF) of BPE reaches 8.85 × 106, which is ∼1 order higher than R123. Importantly, both BPE and R123 followed a linear intensity−coverage correlation up to ∼60 Au NRs/μm2 (Figure S4a−d in the Supporting Information). The trend rapidly diverged for BPE beyond this coverage, exhibiting an exponential dependence, while R123 continued its linear correspondence. The observed linear intensity−coverage correlation strongly indicates that Au NRs are individually SERS-active. The exponential increase in the intensity of BPE beyond ∼60 Au NRs/μm2 results from the formation of dimers and trimers of Au NRs when the coverage density increases. This experimental finding is consistent with our FDTD field calculations of dimers of Au NRs (Figure 5f), where significant field enhancement occurs in the junction. Figure 5f presents examples of electric field profiles corresponding to different interspacing distances of two Au NRs at a wavelength of 785 nm. The coupling mode of ring dimers is typical dipole−dipole mode since the polarization direction of the incident light is parallel to the ring dimer axis.49 The electric field inside individual Au NR rapidly decayed when the distance decrease from 20 to 2 nm, while the hot spot in the junction exhibit a maximum electric field intensity distribution with an exponential increase. Also, the measured absorption spectra of SERS samples with different surface coverage densities show a slightly red-shift with the inducing spaces (Figure 5d). The experimental observations are in good agreement with the

simulations, the plasmon resonance peaks of which show a redshift with decreasing the inter distance (Figure 5e). This can be explained by the dipolar interaction model. The plasmon frequency will be decreased when two dipolar plasmonic particles experience stronger attraction with reduced separation.50,51 However, R123 seemed to be insensitive to the hot spot effect of the Au NR clusters. To further probe the divergence, a solution mixture of BPE and R123 was used for the similar SERS measurements. To avoid the overlap of characteristic bands (Figure 5a), for comparison, we focused our analysis on two distinct bands, ∼1348 cm−1 for BPE and ∼1372 cm−1 for R123, and conducted peak deconvolution (Figure S4e in the Supporting Information) to resolve the spectral details as shown in Figure 5c. BPE and R123 from the mixture also followed a similar trend as in the single analyte measurements. We extended these measurements and analyses to a higher coverage ranging from ∼90 to ∼130 Au NRs/μm2, with the same general outcome (Figure S5 in the Supporting Information). The plausible interpretation is that BPE has stronger affinity with PVP than R123 at neutral pH,52 and the dense PVP at the junction, therefore, further hinders the adsorption of R123 onto Au NRs. The far more sensitive SERS response to BPE than R123 was also attributed to the strong affinity of BPE to PVP, which effectively brings BPE closer to the Au NRs surface for electromagnetic as well as chemical enhancement.

4. CONCLUSIONS In conclusion, through controlled 1D assembly of Co NP chains as template and via galvanic replacement reaction, we 16016

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have achieved the synthesis of free-standing, stable, monodisperse Au NRs colloidal solutions. The geometrical dimension of Au NRs can be tailored by changing the size of Co NPs and the concentration of gold salt, leading to tunable LSPR with intense electromagnetic field enhancements. FDTD calculations show that a relatively uniform intense field distributes inside the discrete individual Au NRs but hot spots form when the Au NRs are brought to close proximity. SERS measurements reveal that the Au NRs are individually SERS-active, with analyte-dependent Raman intensity−Au NR coverage correlation. Compared to other types of Au nanostructures, the easy access to inside the Au NRs with relatively uniform field intensity enhancement potentially opens many new opportunities to explore and exploit Au nanoplasmonics for sensing, imaging, and photomedicine. Our work has led to the development of a novel and effective synthesis strategy for free-standing Au NRs, an important category of the fascinating Au nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional figures noted in the text, including TEM images of bowl-like Au nanostructures, FDTD calculations of Au NRs with different heights, TEM images of three types Au NRs, FDTD calculations of Au nanobowls, SEM images of different immobilized Au NRs substrates for SERS measurements, peak deconvolution analysis on the SERS spectra, and Raman intensity of characteristic bands as a function of Au NR coverage density using the mixture of analytes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.D.). *E-mail [email protected] (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Center for Functional Nanomaterials at Brookhaven National Laboratory for the use of their highresolution transmission electron microscope (HRTEM, JEOL 2100F). Y.H. is financially supported by a teaching assistantship from Stevens Institute of Technology.

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REFERENCES

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