Surface Enhanced Raman Spectroscopy of a Au@Au Core–Shell

We have synthesized a novel Au@Au core–shell structure containing a spiky shell and characterized the surface enhanced Raman spectroscopy (SERS) of ...
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Surface Enhanced Raman Spectroscopy of a Au@Au Core−Shell Structure Containing a Spiky Shell Debrina Jana,†,§ Zohre Gorunmez,‡,§ Jie He,† Ian Bruzas,† Thomas Beck,‡,† and Laura Sagle*,† †

Department of Chemistry, College of Arts and Sciences, University of Cincinnati, 301 West Clifton Court, Cincinnati, Ohio 45221-0172, United States ‡ Department of Physics, College of Arts and Sciences, University of Cincinnati, 301 West Clifton Court, Cincinnati, Ohio 45221-0172, United States S Supporting Information *

ABSTRACT: We have synthesized a novel Au@Au core−shell structure containing a spiky shell and characterized the surface enhanced Raman spectroscopy (SERS) of such a structure. The experimental and calculated SERS intensities for probe molecules residing between the core and shell are considerably higher in this structure containing a spiky shell, when compared to the smooth shell counterpart. Moreover, the SERS intensities for probe molecules residing on the outside of the spiky core−shell structure are comparable to those measured for the dye residing between the core and shell and similar to that of a gold nanostar. Finite-difference time-domain calculations in combination with hybridization theory are able to predict the extinction spectral features and show that the field enhancements are considerably larger than the smooth core− shell structure throughout the visible and infrared regions of the spectrum. This spiky Au@Au core−shell system shows excellent promise as a next generation SERS nanotag, in which encapsulated probe molecules yield increased SERS signal and the increased surface area of the shell allows for a greater degree of functionalization.

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INTRODUCTION Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of probing single molecules with unprecedented detail for processes ranging from biological sensing and chemical transformation to optical waveguides and electronics.1−3 Particularly for sensing and imaging applications, SERS offers an attractive alternative to fluorescence in that excitation can be achieved with a wide range of colors, including the more transparent infrared regions, and the narrow bandwidth of the peaks allow for a greater degree of multiplexing.4−6 Indeed, plasmonic Si@Au core−shell structures, in which dye is embedded in the silica layer, are widely used as multiplexed biological imaging agents and are commonly referred to as SERS “nanotags”.7,8 Moreover, core−shell plasmonic structures have found tremendous use in catalysis, optics, and chemical and biological sensing.9−14 Plasmonic core−shell structures offer the potential to exhibit high field enhancement and superior reproducibility when compared to other types of SERS substrates, since the region of enhanced electromagnetic field or “hot spot” in between the core and shell can be facedly tuned. In addition, the amount and location of probe molecules can be controlled through covalent coupling to the matrix residing between the core and shell. One recent study used DNA to both tether dye molecules and act as a reproducible spacer between the core and shell.15 This study generated core−shell structures containing 1 nm gaps between © 2016 American Chemical Society

the core and shell where the dye molecules reside. While the SERS enhancement factors observed were greater than 108, within the single molecule regime, most solution phase methods of producing core−shell structures make it difficult to produce such small gaps between the core and shell. Thus, most plasmonic core−shell structures with dye embedded between the core and shell show significantly lower SERS enhancement factors, in the range of 106, due to larger gap sizes of 3−6 nm.16,17 Core−shell structures that are facedly synthesized and exhibit greater SERS enhancement would be useful as an improved SERS nanotag. Interestingly, the plasmonic core−shell structures synthesized in previous reports all contain either smooth or satellite (aggregated) shells. Both experimental and computational work on gold nanostars show outstanding SERS enhancement due to the sharp tips on the outer surface.18−20 However, it remains unclear how far in the interior of the structure these field enhancements at the tip penetrate. A structure containing a starlike shell, exhibiting greater SERS enhancement for dye embedded between the core and shell, should prove useful as Special Issue: Richard P. Van Duyne Festschrift Received: February 29, 2016 Revised: April 3, 2016 Published: April 5, 2016 20814

DOI: 10.1021/acs.jpcc.6b02135 J. Phys. Chem. C 2016, 120, 20814−20821

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aqueous solution was added to 5 mL of Rose Bengal-labeled PAH (see section above) solution dropwise under vigorous magnetic stirring. For the samples with dye on the outside, just an 8 mg/ mL solution of PAH was added in this step. This solution was kept stirring for ∼4 h in the dark. After that, the solution was centrifuged at 14 000 rpm for 10 min and after removing the supernatant, the precipitate was redispersed in 5 mL of doubly distilled water. This washing step was repeated 2−3 more times to get rid of any excess PAH or Rose Bengal and the final redispersed solution was used in the next step which will be termed as Au-PAH-RB in the text afterward. This step allows for the attachment of both PAH and the Rose Bengal dye to the gold nanoparticle. In the next step, another polyelectrolyte, PAA, is attached to the first layer of PAH by adding 5 mL of a solution containing Au-PAH-Rose Bengal dropwise to 5 mL aqueous solution containing 20 mg PAA with vigorous stirring. After stirring the resultant solution mixture in the dark for another 4 h, centrifugation at 14 000 rpm for 10 min was done. Like the previous step, washing was performed multiple times and the final redispersion volume was maintained at 5 mL. The layer-bylayer assembly of alternatively charged polyelectrolytes was then repeated 6−7 times until a desired thickness of ∼3 nm was achieved. It was found that more than 6−7 polyelectrolyte layers were hard to achieve due to loss of material in the washing steps. This Au nanoparticle having layers of polyelectrolyte on it will be mentioned as “Au core” in the text hereafter. Formation of Smooth Gold Shell on Au Core (Au CoreSmooth Shell). For the formation of a smooth shell on the Au core, 1 mL aqueous solution of 0.1 mM PLH was added dropwise to 1 mL of Au core solution prepared in the previous step under vigorous stirring and kept stirring for 4 h. The resulting solution was centrifuged and washed a few times to get rid of excess PLH and then finally redispersed in 1 mL of doubly distilled H2O. Next, 30 μL of 10 mM HAuCl4 aqueous solution was added dropwise to the PLH-coated Au core particles under vigorous stirring. Finally after 1 h, 50 μL of 10 mM aqueous ascorbic acid solution was added to the previous solution and kept stirring for another 12 h. After centrifuging the particles at 14 000 rpm for 10 min and washing a couple of times, Au core−smooth shell nanoparticles were ready to use. Formation of Spiky Gold Shell on the Au Core (Au CoreSpiky Shell). For the formation of a spiky shell on the Au core, we have followed and modified the reaction procedure described by Brenda et al. which reports the formation of a spiky gold shell on polystyrene beads.23 In the two-step synthesis procedure, first the Au core was decorated by small Ag seeds. Briefly, 100 μL of 0.01 M Ag(NH3)2+ (prepared by mixing 1 mL of 0.01 M AgNO3 and 20 μL of 1 M NH4OH) was added to 1 mL of Au core solution with stirring. After 30 min of stirring, the solution was centrifuged at 14 000 rpm for 30 min to get rid of unadsorbed silver ions. Washing was repeated one more time and the precipitate was dispersed in 500 μL of doubly distilled water. Next, 100 μL of 0.01 M aqueous solution of NaBH4 was added to reduce the adsorbed silver ions to silver nanoparticles on the surface of the Au core. After 6 h of aging, the solution was centrifuged at 14 000 rpm for 30 min and the precipitate was redispersed in 500 μL of doubly distilled water. In the second step of the synthesis, a growth solution was prepared comprised of aqueous solutions of CTAB (10 mL, 0.1M), HAuCl4 (421 μL, 0.01M), AgNO3 (64 μL, 0.01M), and ascorbic acid (67 μL, 0.1 M). Ten microliters of the Ag seed decorated Au core solution was added to the growth solution with mild shaking. The color of the solution changed to blue within 15 min and the solution was

an improved SERS nanotag. In addition, the increased surface area of the shell will allow for improved functionalization with biological targeting and/or therapeutic agents for theranostic applications. Herein, we have synthesized a novel Au@Au core−shell structure containing a spiky, nanostar-like outer shell. The SERS signal for probe molecules residing both in between the core and shell and absorbed to the outer surface were measured and compared to that of a smooth core−shell structure of similar dimensions. It was found that the SERS enhancement in the region between the core and shell was significantly higher for the structure containing the spiky shell. In addition, when dye was placed on the outside of the spiky core−shell structure, SERS enhancement was similar to that observed with a gold nanostar of comparable diameter, and also considerably larger than the smooth shell counterpart. Finite-difference time-domain (FDTD) calculations show the hybridization model used to describe plasmonic “nanomatryoshka” structures can also be used to describe these spiky core−shell structures.21 The FDTD calculations support higher SERS signal for probe molecules both outside and in between the core and shell for the spiky shell structures, when compared to the smooth core−shell structures. In addition, the calculations indicate higher field enhancements for the spiky structures should be observed throughout the visible and infrared regions of the spectrum. The higher SERS signal and larger surface area should make these structures attractive for biological imaging applications.

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EXPERIMENTAL METHODS Materials. All glassware was cleaned with aqua regia (3:1 volume ratio of concentrated hydrochloric acid and nitric acid) and rinsed thoroughly with deionized water prior to synthesis. Citrate functionalized Au nanoparticles of diameter 60 nm were purchased from BBI International. All the polyelectrolytes used in this work viz. poly(allylamine hydrochloride) (PAH, M.W. 15000), poly(acrylic acid) (PAA, M.W. 1800), and poly(Lhistidine) (PLH, M.W. 5000) were purchased from SigmaAldrich. Rose Bengal dye, Cetyltrimethyammonium bromide (CTAB), Tetrachloroauric acid (HAuCl4.3H2O), silver nitrate (AgNO 3 ), sodium borohydride (NaBH 4 ), 1-ethyl-3-[3(dimethylamino)propyl]carbodiimide hydrochloride (EDC), and N-hydroxysuccinimide (NHS) were also obtained from Sigma-Aldrich, whereas dimethylformamide (DMF) was purchased from Fisher Scientific. All reagents and solvents were obtained commercially and used without further purification. Doubly distilled water (18.2Ω) was used throughout the synthesis. Labeling the Polyelectrolyte with Rose Bengal Dye. First, Rose Bengal dye was conjugated with PAH.17 Briefly, for 10 mL of 0.1 mM Rose Bengal solution, 1.9 mg EDC and 1.2 mg NHS were added and stirred for 15 min. Then 40 mg PAH was added to the above mixture and kept stirring overnight in the dark. The concentrations above of PAH and Rose Bengal were chosen particularly to ensure binding of only a small fraction of amino groups on PAH to the Rose Bengal dye. Synthesis of Layer by Layer Assembly of Polymers on Au Nanoparticles. First, the citrate functionalized Au nanoparticles were centrifuged once to wash off excess citrate present. After discarding the supernatant, the precipitate was dissolved in doubly distilled water. The layer-by-layer assembly method was used with modifications to conjugate the dye onto the Au nanoparticles and also to increase the thickness of the polyelectrolyte layer.17,22 First, 5 mL of the Au nanoparticle 20815

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Figure 1. Schematic of how the smooth and spiky core−shell structures were constructed. First, a layer-by-layer deposition of oppositely charged polymers is carried out on a 60 nm gold colloid (core). Rose Bengal dye is conjugated to the first polymer layer. These dye-conjugated cores were then used to make both the smooth and spiky core−shell structures. The smooth core−shell structure is created in one step with the addition of silver nitrate and hydroquinone. The spiky core−shell structure involves the deposition of silver seed particles onto the polymer-coated core, followed by the growth of the spiky shell.

kept undisturbed for another 2 h. All the synthesis described herein were performed in room temperature. Synthesis of Gold Nanostars. Synthesis of gold nanostars were carried out following the method reported by the LizMarzán group with slight modifications.24 Briefly, in the first step, small gold colloids of 16 nm in diameter were synthesized according to the standard sodium citrate reduction method.25,26 In the second step, these gold colloids were functionalized by polyvinylpyrrolidone (PVP) by mixing the colloidal solution with PVP (MW = 10 000) (Sigma-Aldrich) under stirring for 24 h at room temperature.27 PVP functionalized nanoparticles were then transferred to ethanol by centrifugation and subsequent removal of supernatant. In the final step, 82 μL of aqueous 50 mM HAuCl4 (Sigma-Aldrich) solution was mixed with 15 mL of 10 mM PVP (MW = 29 000) (Sigma-Aldrich) solution in N,Ndimethylformamide followed by the rapid addition of 16 nm gold seeds (43 μL) prepared in the second step. Color change of the solution from pink to colorless and finally to blue within 15 min indicated the formation of gold nanostars in the solution. Addition of Dye to the Outside of the Spiky Core−Shell and Nanostar Particles. Rose Bengal dye was adsorbed to the surface of both of the gold nanostars and spiky core−shell structures. To both spiky core−shell and nanostar solutions containing very similar number of particles, as measured by dynamic light scattering, 5 mL of these particles were incubated with a 0.1 mM aqueous solution of Rose Bengal dye for at least 24 h. The solutions of particles and dye were measured as is. LSPR Measurements. All LSPR measurements were carried out using liquid transmission cell and a USB2000+VIS-NIR (Ocean Optics, Inc.) configured with fiber optic cables and a cuvette holder. The data were collected using the SpectraSuite

software provided by Ocean Optics and each spectrum was constructed by averaging 100 scans each collected with 4 ms integration time. Frequency values for the individual plasmon resonances were obtained by fitting the peaks of interest to a Gaussian function using the Origin 9.0 software (OriginLab, Inc.). SERS Measurements. For surface enhanced Raman spectroscopic studies, a drop of each dispersion was transferred into a small borosilicate glass capillary tube (Kimble Chase) having 1 mm inner diameter. It was placed on the stage of a Renishaw inVia Raman microscope equipped with a Renishaw MS20 nanopositioning stage. The laser intensity of the samples was 1.07 mW and 1.02 mW, respectively, from 785 nm diode laser and 633 nm helium−neon laser. Measurements were acquired with a 50× objective, exposure time of 10 s, and each spectrum was constructed from an average of 1 to 5 scans with a grating of 1200 mm−1. Between different Raman sessions, the 520.7 cm−1 peak of a silicon wafer was used to calibrate the spectrograph. All SERS spectra reported with standard deviations were constructed from measuring at least three different samples. TEM Measurements. Transmission electron microscopy (TEM) studies were carried out using an FEI CM-20 at 200 keV. Samples were prepared on ultrathin carbon type-A, 400 mesh Cu grids coated with Formvar (Ted Pella). Samples were first diluted in doubly distilled water and then micropipetted dropwise onto the grid surface. Excess nanoparticle solution was subsequently wicked away. FDTD Calculations. Three-dimensional FDTD calculations were carried out using the commercial software Lumerical FDTD Solutions (Lumerical Solutions, Inc., http://www. lumerical.com/tcad-products/fdtd/). The geometry of each 20816

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The Journal of Physical Chemistry C individual structure was defined from the experimental TEM images. The core radius, r, and the polymer shell thickness, d, were fixed to 30 and 3 nm, respectively, for all two studied structures. The outer shell of the spiky structure contained a 3 nm thick gold shell and 18 spikes distributed on the outer shell surface. The spikes were built by using the truncated cones provided by the software. Each spike has the height (tip-to-tip) in the range of ∼9−27 nm. The outer shell of the smooth structure, on the other hand, had a 12 nm thick, continuous gold shell. The tip-to-tip diameter of the entire spiky core−shell structure and gold nanostars (used for comparison) were approximately 114 nm while the diameter of the smooth structure was 90 nm. The refractive index (RI) of water was used as the external medium for all the core−shell structures, which was assumed to be 1.35 for our wavelength range of interest of 400−1900 nm. The RI of gold was assigned from tabulated Johnson and Christy data in the wavelength range of interest.28 For the polymer coating the gold core, it was assumed that the optical properties did not change with wavelength and that it was a nondispersive material with RI of 1.42. In order to include interband transitions of bound electrons of the gold metal, a corrected Drude model in the Lumerical FDTD Solutions software was used. The model used to fit the Johnson and Christy data over the wavelength range of interest has a two critical point (2CP) correction to the Drude model which becomes the Drude+2CP dispersive model.29 With this dispersive model, free-electron and interband transitions of gold metal were taken into consideration. The near field enhancement comparisons were done for both excitation wavelengths at 633 and 785 nm, whereas the extinction spectrum was computed over the broad wavelength range of 400−1900 nm (∼0.65−3.01 eV). Simulation boxes for each calculation have been nonuniformly meshed with the gridding of 0.5 nm to 2.5 nm (inside to out) to capture as much detail as possible without exceeding a memory size of 24 GB. The field profiles shown in Figures 3 and 4 are cross sectional views of the xz-plane in the center of the structures. The extinctions and enhancement factors (|E/E0|4) were averaged for two different polarizations (x and y directions) of the incident field by taking the values of all the grid points in the region of interest. When comparing the fields residing between the core and shell, the region of interest was defined as 3 nm beyond the core and before the shell. The region of interest for field enhancements on the outside of the structures was defined as the region in between an inner sphere of radius of 39 nm and outer sphere of radius 60 nm. After summing the |E/E0|4 values for these various structures, they were then divided by the volumes with which these grid points reside to correct for slight differences in the size of the regions of interest.

Figure 2. Experimental absorption spectra for the core (black), core with smooth shell (red), and core with spiky shell (blue). Pictures of the aqueous solutions containing the smooth core−shell (red) and spiky core−shell (blue) structures (a). Calculated extinction spectra for all structures: core (black), core with smooth shell (red), and core with spiky shell (blue) (b).

silver nitrate, tetrachloroauric acid, and ascorbic acid. Evidence for both smooth and spiky shell formation was first acquired by TEM, shown in Figure 1. TEM images show the smooth core− shell structures appear larger than the 60 nm core and to have a shell made up of similar material as the core. Likewise, the spiky core−shell structures consist of a 60 nm core, with a shell composed of multiple spiky features, somewhat like that of a gold nanostar. The next characterization of the core−shell structures was carried out using Vis-NIR spectroscopy to measure the localized surface plasmon modes, see Figure 2. Both the smooth and spiky core−shell structures contained peaks in the visible and infrared region. In order to better understand these spectral features we have compared the experimental spectra with spectra calculated using a combination of FDTD and hybridization theory. This method has been applied to several types of plasmonic structures, such as star-shaped gold nanoparticles,30 layered gold nanoshells referred to as “nanomatryoshkas”,21 and two-dimensional Au@ Au ring-disk structures.31 As shown in Figure 2b, the calculated extinction spectra agree nicely with experimental data acquired for bulk solutions of the two core−shell structures. For the smooth core−shell structure, the observed LSPR peaks are 582 and 1359 nm, while the peaks were calculated to be 572 and 1296 nm. Likewise for the spiky core−shell structure, the experimental LSPR peaks are observed at ∼770 and 1750 nm, and the calculated peaks at 742 and 1749 nm. The good agreement between experimental and calculated extinction spectra indicates that the hybridization model can be applied to explain the

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RESULTS AND DISCUSSION The core−shell structures containing smooth and spiky shells were synthesized from the same 60 nm gold core, which was sequentially coated in polymers of alternating charge in a layerby-layer deposition method, similar to other reports in the literature.22 The probe molecules were coupled using carbamide chemistry to the first polymer layer containing polyallylamine. After each polymer deposition, the particles were centrifuged, washed, and resuspended for the next deposition. Once 6−8 layers of polymer were formed on the outer surface of the core, a smooth shell was grown using aqueous HAuCl4 and ascorbic acid. The spiky shell was grown using a two-step procedure which involved first putting small silver nanoparticle seeds onto the core, then growing the spiky shell through addition of CTAB, 20817

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Figure 3. Diagrams illustrating how the plasmon modes of the core and shells hybridize to form the core−shell structures (a). The calculated energies for the hybridization, shown in blue, are the same as those depicted in the spectra shown in Figure 2b. Diagrams showing charge distributions for each mode are depicted above the blue lines. Calculated extinction spectra for the core, shell, and hybridized structures are shown in (b) with the smooth core−shell structure on top and the spiky core−shell structure on the bottom. The dashed lines in the spectra indicate how the modes hybridize to produce the observed spectra of the core−shell species. The cross-sectional views of calculated electric field intensities, E/E0, for the smooth and spiky core−shell structures at the corresponding plasmon modes of 572 and 1296 nm (top) and 742 and 1749 nm (bottom) are shown in (c). The geometry of the calculated smooth and spiky core−shell structures in both three and two dimensions are illustrated in the first row (c). The incident light direction and polarization direction are in the positive z and x directions, respectively. The “bonding” resonances are depicted on the bottom of each diagram and correspond to the field diagrams of higher wavelength values, on the right. The “antibonding” resonances are depicted on the top of each diagram and correspond to the field diagrams of lower wavelength values, on the left.

modes.21,33,34 Both effects are expected to be more pronounced for the spiky core−shell structure, due to both increased structural asymmetry and increased coupling with these multipole resonances. The calculated cross-sectional electric field intensities for the structures at the resonances corresponding to the two hybridization modes, shown in Figure 3a, are shown in Figure 3c. The increased overall field intensities for the spiky core−shell structure is most likely due to the coupling of a larger number of modes, particularly for the tips, and the higher field intensities at the tips. For the smooth core−shell system, the field enhancements between the core and shell are most prominent for the lower energy “bonding” mode. In contrast, the spiky core−shell system shows comparably high field enhancements in the region between the core and shell at both hybridized modes. In fact, as shown in Figure S1, the spiky core− shell system shows considerably greater field enhancement in the region between the core and shell, when compared to the smooth shell counterpart, throughout the visible and infrared regions of the spectrum. Further studies are currently underway, which include changing the features of the spiky shell and the spacing between the core and the shell, to better understand the key structural factors giving rise to the hybridization of modes in the spiky core−shell system. Next, the SERS spectra of Rose Bengal dye molecules residing between the core and shell for both the smooth and spiky systems were compared, see Figure 4. Indeed, both experimental and computational data show the spiky core−shell structures at both 633 and 785 nm excitation wavelengths exhibit increased

plasmon resonances observed for the spiky core−shell system.21,32 A further understanding of how the core and shell hybridize for both structures is illustrated in Figure 3. The hybridization diagrams, shown in Figure 3a and b, depict how the plasmon resonances of the core and shell combine to form lower energy and higher energy hybridized modes. Depicted above each hybridized mode in Figure 3a is the calculated charge distribution amplitude for that mode. For the smooth core−shell structure, the charge distributions indicate that both modes are dipolar, with the lower energy mode being bonding, and the higher energy mode antibonding. These hybridized modes observed for the smooth core−shell structure are similar to those reported for the two-dimensional disk-ring structures.31 For the spiky core− shell structure, while the lower energy hybridized mode has dipolar, bonding character much like the smooth core−shell system, the higher energy hybridized mode clearly includes multipole components, as evidenced by the charge distribution and the increased asymmetry of the hybridization diagram itself. The presence of multipole resonances are most prevalent in the hybridized spectra, where they are calculated to be at 600, 660, and 965 nm, see Figure 3b. These multipolar resonances have also been reported for the gold nanostar system and most likely arise from coupling between the spikes of the shell.30 The asymmetry of the hybridization for these systems when compared to molecular orbital theory has been shown to arise from two phenomena: phase retardation due to nonuniform field distributions on the particles, and coupling with higher order 20818

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Figure 4. Experimental SERS spectra for the core containing polymer-conjugated dye (black), smooth core−shell structure (red), and the spiky core− shell structure (blue), at both 633 and 785 nm excitation wavelengths (a). The cross-sectional views of calculated electric field intensities, E/E0, for the smooth and spiky core−shell structures at an excitation wavelength of 633 nm (top) and 785 nm (bottom). The geometry of the calculated smooth and spiky core−shell structures in both three and two dimension is illustrated in the first row (b). The incident light direction and polarization direction are in the positive z and x directions, respectively. Calculated SERS enhancement factors of each structure at the two excitation wavelengths of 633 and 785 nm (c). The values shown in this graph were integrated over the volume which makes up the region between the core and shell, where the dye resides, see the description of this normalization procedure in the Experimental Methods section.

Figure 5. Experimental SERS spectra for dye molecules adsorbed to the outer surface of a gold nanostar (red), spiky core−shell structure (blue), and between the core and shell of the spiky core−shell structure (black) with 785 nm excitation (a). Calculated SERS enhancement factors of each structure at the two excitation wavelengths of 633 and 785 nm (b). The values shown in this graph were integrated over the volumes which make up the regions where the dye resides, see description in the Experimental Methods section and Figure S3. Cross sectional views of the electric field intensities for the nanostar and spiky core−shell structures at 785 nm excitation are shown in Figure S6.

SERS enhancement compared to the smooth structure. The experimental data obtained with 633 nm excitation showed an

increase in SERS signal of roughly 2-fold. This difference is expected to be fairly small, since neither the spiky nor the smooth 20819

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CONCLUSIONS We have synthesized a novel core−shell plasmonic structure composed of a spherical gold core and spiky gold shell. When comparing this spiky core−shell Au@Au structure to a smooth shell structure, the SERS signal is significantly higher for probe molecules placed both in between the core and shell and on the outer surface of the shell throughout the visible and infrared regions of the spectrum. This increased SERS intensity for the spiky core−shell system is due to increased field enhancements at the tips as well as the coupling of a larger number of modes. These spiky core−shell structures are expected to be advantageous for the next generation of SERS nanotags, which offer the ability to combine biological imaging and therapeutics, since the spiky shell allows for increased SERS signal, greater functionalization and biomolecule targeting, and improved photothermal properties.

systems have plasmon bands that overlap with the 633 nm excitation wavelength to a greater extent. However, with 785 nm excitation (which is closer to the LSPR peak observed in the visible region of the spectrum for the spiky structure) a larger difference of ∼8 fold was obtained when comparing the spiky to the smooth structure. The differences in observed SERS intensity were verified by measuring multiple batches of core−shell structures, as well as directly measuring the amount of dye coupled to each structure and correcting for any differences, see Figures S2 and S4 of the Supporting Information section. The calculated SERS intensity values (|E/E0|4) also agree with these data, as shown in Figure 4c. Based on the calculations, the predicted difference between the smooth and spiky systems is larger with 785 nm excitation compared to 633 nm excitation, indicating a ∼10-fold increase for the spiky system at 785 nm, and only a ∼4-fold increase at 633 nm. Additionally, the e-field diagrams show larger field enhancements in the region between the core and shell for the spiky system when compared to the smooth shell system at both 633 and 785 nm. It is also important to note that both the smooth and spiky core−shell structures have substantially larger field enhancements and SERS signal than from the core alone, roughly 10−20 times at 633 nm and almost 100 times at 785 nm. Thus, these structures offer large improvements over most currently used SERS nanotags, which are composed of a gold core and silica or polymer shell.35 We then sought to compare the SERS activity of probe molecules in contact with the outer tips of the spiky core−shell structure with probe molecules residing in between the core and shell. Since the shell of the spiky structure is similar to that of a gold nanostar, we also investigated how the SERS signal would compare to that of the nanostar. Gold nanostars have been shown to be exceptional SERS substrates, capable of yielding high SERS enhancement without the need for aggregation.18 The experimental SERS data obtained for these two species with 785 nm excitation are shown in Figure 5a and appear to be similar in intensity to those of the nanostar. In all cases, the amount of dye residing on the inside and outside of the spiky core−shell structure, and the outside of the gold nanostar, was determined by dissolving the gold nanostructures in excess NaCN and measuring the fluorescence of the dye released into solution, see Figure S4. It was found that the dye adsorbed on the outside of the structures was ∼3.7 times greater; thus the spectra of encapsulated dye shown in Figure 5a were multiplied by this factor. Once again, the calculations agree nicely with experimental results, which also indicate only small differences between the spiky core−shell structure and the nanostar with 785 nm excitation. Interestingly, placing the probe molecule between the core and shell versus outside the structure for multiple batches of the spiky core−shell system (see Figure S5) does not lead too much decrease in the SERS signal. This configuration would be most desirable for a SERS nanotag to avoid detachment of the probe molecule and interference from solution components. However, for SERS nanotags to be utilized in both biological imaging and targeted drug delivery, the outside shell must also be functionalized. The spiky core−shell structures possess a clear advantage in this respect, since the increased surface area of the outer shell will allow for a greater degree of functionalization with targeting species and/or therapeutic agents. Moreover, it has been shown that heat generation, particularly at the tips of nanostar-like structures is considerably higher than that of a smooth structure, which should aid in photothermal therapeutic applications.36

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02135. Additional characterization data on the spiky core−shell and nanostar structures as well as results from control experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 1 513 556 1034; Fax: 1 513 556 9239; E-mail: saglela@uc. edu. Author Contributions

§ It should be noted that both D.J. and Z.G. have contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by University of Cincinnati start-up funds and by support from the Ministry of National Education of Turkey for Z.G. The authors gratefully thank Yan Zhao and Prof. Peng Zhang for helpful discussions and procedures on the layerby-layer deposition method used to make the core−shell structures. The authors also gratefully acknowledge Prof. Jeff McMahon for helpful discussions and guidance on the FDTD calculations. Lastly, we thank Dr. Anthony Stender and Prof. Emilie Ringe for taking UV−vis measurements on some of our samples.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b02135 J. Phys. Chem. C 2016, 120, 20814−20821

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DOI: 10.1021/acs.jpcc.6b02135 J. Phys. Chem. C 2016, 120, 20814−20821