Article pubs.acs.org/JPCC
Hollow Gold Nanoprism as Highly Efficient “Single” Nanotransducer for Surface-Enhanced Raman Scattering Applications Bidhan Hazra,†,∥ Kamalika Das,†,∥ Sudeshna Das Chakraborty,§ Mrigank Singh Verma,† Mayanlambam Manolata Devi,‡ Nirmal Kumar Katiyar,‡ Krishanu Biswas,‡ Dulal Senapati,§ and Manabendra Chandra*,† †
Department of Chemistry and ‡Department of Material Science and Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh 208016, India § Chemical Science Division, Saha Institute of Nuclear Physics, Kolkata 700064, India ABSTRACT: In this Article, we report the optical properties of anisotropic hollow nanostructure, hollow gold nanoprism (HGN, an equilateral triangular gold nanoprism with a circular cavity) and its application as a highly active “nonaggregated” surface-enhanced Raman scattering (SERS) substrate. Small colloidal HGNs are prepared using sacrificial galvanic replacement method. Our study reveals that HGNs are highly stable nanostructure in solution and robust against salt or ligand induced aggregation. We find that the LSPR spectra of the HGNs can be tuned by controlling their aspect ratios. Excellent SERS were detected from the Jaggregates of pseudoisocyanine dye, adsorbed on the HGN surface. Interestingly, strong SERS is obtained without the need of any aggregation of the nanoparticles. Numerical simulations reveal that a single HGN can not only harvest large local electromagnetic fields but also offer large sensing volume due to efficient plasmon hybridization, due to the presence of the cavity. This large sensing volume is a great advantage, and it is the key factor behind the observation of SERS from nonaggregated HGNs. A careful optimization of the plasmon-hybridization mediated field enhancement, for example, by controlling the cavity positions through the use of lithographic techniques, can enhance the SERS signal even further. All of the observations and analyses made in this work clearly suggest that HGNs will be an excellent choice in SERS-based chemical and biological sensing applications. Moreover, the cavity of an HGN has the potential for being used as nanoreactor, in the future.
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INTRODUCTION Over the past few years, the world has witnessed the emergence of plasmon-supporting metal nanostructures as attractive signal transducers in optical chemo- and biosensors.1−5 The transducer effects in these nanoscale plasmonic structures originate from the excitation of their localized surface plasmon resonances (LSPRs), which produces large local electric fields at structure-specific frequencies allowing them to function as electromagnetic antennas. The local electric field associated with LSPR decays rapidly into the surroundings, providing an excellent transducer effect in which changes in the local refractive index are converted into a frequency shift of the plasmon mode.6 Also, light amplification by a nanoparticle transducer can be used to facilitate solar-to-electric energy conversion,7,8 mediate nonlinear optical (NLO) processes,9−15 and enhance molecular spectroscopy methods.16,17 One of the most important applications of electric-field amplification by the plasmonic nanoparticle transducers at optical frequencies is the enhancement of Raman scattering signal (known as surfaceenhanced Raman scattering or, SERS) from the surfaceadsorbed molecules. SERS is one of the most powerful molecular-level sensing techniques that is ultrasensitive and has high specificity.16−20 © XXXX American Chemical Society
SERS has been successfully applied to the study of a variety of chemical and biological systems.16,19,20 Although nanoparticlebased SERS platforms hold promise for chemically specific and ultrasensitive detection, including localized intracellular sensing, the reproducibility of the SERS-based sensors is limited by need of aggregation of the metal nanoparticles. Because single nanoparticles generally do not yield large enough E-field enhancement, aggregation is utilized in SERS studies. Aggregation produces highly localized electric fields at the interparticle junctions that act as SERS “hotspots”.21 Molecules placed in these hotspots generate highly enhanced Raman signals. Therefore, SERS-active substrates are prepared using salt- or ligand-induced aggregation of the plasmonic nanoparticles.21 However, nanoparticle aggregation is a random process and difficult to control. Therefore, aggregates with a variety of shapes and sizes are produced and that lead to inconsistent local surface chemistry, plasmon resonances, and plasmonic E-field distributions. This inevitably affects the Received: September 19, 2016 Revised: October 15, 2016 Published: October 25, 2016 A
DOI: 10.1021/acs.jpcc.6b09467 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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replacement and growth process. In the second step, 0.49 g of CTAB was taken in 45 mL of water, and then 2 mL of 10−2 M AgNO3 was added dropwise into this CTAB solution under stirring condition. Next, 300 μL of 10−2 M HAuCl4 was added drop by drop. The color of the solution turns yellow-brown. This was followed by dropwise addition of 320 μL of 10−1 M of ascorbic acid that changes the color of the solution from yellow-brown to colorless. To this colorless solution, another 300 μL of 10−2 M HAuCl4 and 320 μL of 10−1 M of ascorbic acid (AA) were added in a quick succession. This was followed by an immediate addition of an aliquot of the seed solution. Amount of seed was varied from 250 μL to 2.5 mL. Variation in the amount of seed solution led to the formation of various HGNs with different edge lengths and cavity diameters. All of the synthesis work was carried out at the ambient room temperature of 18−20 °C. Characterization. All of the HGN samples were aged for 7 days before characterizing them or carrying out any experiment with them. Extinction spectra of the Ag seed and all of the HGNs in aqueous solution were recorded using either a singlechannel fiber-optic spectrophotometer (Ava-spec 3648, Avantes) or a dual-channel UV−vis−NIR spectrophotometer (Jasco V670). Morphology of the particles was characterized using a “FEI Tecnai G2 12 Twin” transmission electron microscope, operated at 200 kV acceleration voltage. Particlesize analysis was done using ImageJ software. Raman Scattering Experiments. Surface-enhanced Raman scattering (SERS) experiments were performed using a confocal Raman optical microscope (Olympus BX51) equipped with a triple grating imaging spectrometer (Princeton Instrument Acton Spectra Pro SP-2500). Samples for SERS studies were prepared as follows. A 0.4 mL aliquot of 1 mM pseudoisocyanine (PIC) iodide dye was added to 1.6 mL of HGN and mixed well. This solution was kept in the dark and undisturbed for 72 h. Addition of PIC dye did not lead to any aggregation of the HGNs. This is clearly evident from the fact that we did not observe any shift in the LSPR spectra upon the addition of the dye. After aging for 72 h, the HGN−PIC mixture was centrifuged at 10 000 rpm, and the sediment was washed with water to remove any excess PIC dye that was not actually adsorbed onto the gold surface. This procedure was repeated twice, and finally the sediment was redispersed in water. A couple of microliters of an extremely diluted portion of this HGN−dye composite solution was then drop casted onto a cover-glass and dried under nitrogen. A CW He−Ne laser (632.8, 18 mW, AIRIX Corp.) was employed for the Raman excitation. In a separate set of experiments, we recorded the Raman spectra of the HGN−dye composites (same as that used in the confocal Raman microscopy measurements) in solution phase using an excitation wavelength of 532 nm. The absorption spectra of all of the experimentally studied HGNs were simulated numerically using the finite difference time domain (FDTD) technique. Three-dimensional FDTD simulations were done using the commercially available software package “FDTD solutions v8.12” (Lumerical). All of the simulations were done on single nanostructures. An environmental refractive index of 1.33 was used to mimic the aqueous environment. A cubic mesh of 1 nm was used for the simulation of absorption spectra, while a meshing of 0.25 nm × 0.25 nm × 0.5 nm was employed for calculating the electric field distribution. The refractive indices (both real and imaginary) were taken from the literature.32The dispersive refractive indices of gold were fitted in the spectral range 300−
reproducibility of the resulting SERS substrates and their performances. To improve the reliability of SERS probes, it is therefore necessary to develop nanoparticles that are SERS-active even under nonaggregated condition. Certain nanostructures like gold nanoshells,22 gold nanocrescent moons,23 and gold splitring resonator24 have been recently developed that could generate sufficient SERS signal from individual particles due to their tunable LSPR, and their ability to strongly localize as well as amplify surface electromagnetic fields. However, the relatively large size of these nanostructures will ultimately limit their applicability in certain applications such as localized intracellular sensing. To push the size limit of sensing, as required by systems biology, even smaller probes will be required. Driven by this need, a new class of smaller plasmonic nanospheres that are hollow inside have been developed, and their SERS activities under nonaggregated condition have been demonstrated.25−28 Controllable spectral tunability due to plasmon hybridization is the key advantage of these hollow nanospheres. While spectral tunability is one aspect, the other and probably the most important factor controlling the efficiency of a nanostructure as a single SERS substrate is its ability to localize and enhance the electromagnetic field. This latter property is superior in particles having anisotropic shape.29,30 Therefore, it is expected that a particle that is hollow as well as anisotropic in shape can be an extremely good choice as single-particle level SERS substrate. One such anisotropic hollow nanostructure is hollow gold nanoprism. A hollow gold nanoprism (HGN) is basically an equilateral triangular gold nanoprism with a circular cavity. Synthesis of HGN has recently been reported.31 However, the stability, the LSPR behavior, electromagnetic field localization and enhancing efficiency, and most importantly the SERS performance of these HGNs have not been investigated experimentally. In this Article, we have synthesized several HGNs with smaller dimensions and various aspect ratios and systematically investigated their stability, spectral tunability, electromagnetic field enhancement efficiencies, and their SERS performances at nonaggregated condition. Our experimental results have been complemented by numerical simulations based on finite difference time domain technique. Our study reveals that HGNs are a highly stable nanostructure in solution that can be used as nonaggregated nanotransducers in SERS applications due to their controllably tunable LSPRs and their ability to harvest extremely large plasmonic surface fields and larger sensing volume.
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MATERIALS AND METHODS Synthesis of HGNs. Hollow gold nanoprisms (HGN) were synthesized following a protocol originally developed by Senapati et al.31 The HGN synthesis process involves two steps. In the first step, silver nanoseeds of 4−5 nm are prepared. In the second step, HGN is prepared via sacrificial galvanic replacement method. Details of the synthetic procedure can be found elsewhere.31 A brief description of the stepwise synthesis is presented here. In the first step, 0.5 mL of 10−2 M AgNO3 was added dropwise into 20 mL of Milli-Q (18 MΩ cm) water, kept under constant stirring. Next, 200 μL of 2.5 × 10−2 M trisodium citrate (TSC) was added gradually to this AgNO3 solution followed by a dropwise addition of 60 μL of 10−1 M freshly prepared ice cold NaBH4, and the stirring was stopped within 10 s. The color of the solution gradually became transparent yellow. The solution was kept undisturbed in the dark for 4 h before using it as the seed for the sacrificial galvanic B
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analyzing the TEM images for all of the HGNs, are listed in Table 1. It can be seen from Figure 2 that the side lengths (tipto-tip distance) and the dimension of the cavities of the HGNs can be varied by controlling the amount of the seed solution. The size of the cavity increases with increasing seed concentration. On the other hand, the side length of the HGNs shares more or less an inverse relationship with seed concentration. Overall, the aspect ratio33 (AR; AR = L*R/ T[R−r], where L is the side length, r is the cavity radius, and R is the shortest distance between the cavity surface and the sides of the prism) of the HGNs increase with increasing seed concentration. The TEM analysis also reveals that the corners (tips) of many of the HGN structures are not sharp, rather rounded (or snipped) to some extent. In fact, perfect triangular prism shapes with sharp tips are found to be much less in number as compared to the nanoprisms with rounded (or snipped) tips. Tunable Optical Properties of HGNs. We recorded the optical absorption spectra of the synthesized HGNs after aging them for about a week. Under normal laboratory conditions, all of the HGNs were found to be stable for several weeks as their absorption spectra did not show any change. The ensemble averaged optical absorption spectra of the HGNs are presented in Figure 3a. The absorption spectrum of the Ag-seed is also provided as the inset in Figure 3a. Spectral positions of peaks observed for all of the HGNs are summarized in Table 2. The LSPR spectral position seen for isolated HGNs depends directly on the aspect ratio and is therefore frequency-tunable by controlling the aspect ratio. The larger is the aspect ratio, the more red-shifted is the LSPR maxima. In our recent theoretical work, we have shown that the LSPR spectral position of an HGN depends on the side length, cavity diameter, and most importantly on the distance between the edge and the cavity surface.33 The last factor dictates the strength of coupling between the cavity plasmon and the outer surface plasmon.33 This type of coupling is known as plasmon hybridization.12,33−37 The shorter is the intersurface distance, the stronger is the plasmon hybrization and the more red-shifted is the LSPR spectrum. Therefore, the LSPR spectrum monotonically red-shifts when the cavity diameter is increased for HGNs with fixed side lengths (or side length is decreased for HGNs with fixed cavity diameters). In the present case, the HGNs synthesized by adding different amount of Ag seeds have various combinations of side lengths and cavity diameters. To understand the exact dependence of the LSPR on the structural parameters of the HGNs, we numerically simulated the LSPR spectra of the synthesized HGNs. The dimensional parameters (including corner rounding) of the HGNs for the FDTD simulation were taken from the TEM analyses. The simulated absorption spectra of the HGNs are shown in Figure 3b. The
1200 nm by a six-coefficient model. The multi-coefficient model fitting was done by enforcing passivity with an imaginary weight of 1 and tolerance of 0.1. The simulation configuration is shown in Figure 1.
Figure 1. Schematic of the optical simulation configuration is shown. A single hollow gold nanoprism is placed in an aqueous environment, in the xy plane. The incident photon propagates along the z-direction. The polarization of light is kept fixed in the y-direction.
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RESULTS AND DISCUSSION Six different HGNs (HGN-I−HGN-VI) were synthesized using Ag seed nanoparticles as sacrificial template. Transmission electron micrographs of all of the synthesized HGNs are shown in Figure 2. All of the dimensional statistics, obtained by
Figure 2. Transmission electron micrographs of six HGNs, synthesized using the sacrificial galvanic replacement method. (I) HGN I, (II) HGN II, (III) HGN III, (IV) HGN IV, (V) HGN V, (VI) HGN VI. Each scale bar corresponds to 20 nm.
Table 1. Structural Parameters: Edge Lengths, Cavity Diameters, and Aspect Ratios Obtained by Statistical Analysis of the TEM Images of All HGNs sample HGN HGN HGN HGN HGN HGN a
I II III IV V VI
edge length (L, nm) 30.6 38.3 45.2 51.4 59.0 73.8
± ± ± ± ± ±
1.7 1.8 1.6 3.0 5.0 7.0
diameter (2r, nm) 5.7 5.4 5.7 5.2 3.9 3.2
± ± ± ± ± ±
0.4 0.8 0.4 1.0 1.0 0.5
(R−r)a 8.6 12.7 15.1 16.4 20.1 23.3
± ± ± ± ± ±
1.6 1.6 1.2 2.0 2.8 2.2
aspect ratio [AR = L*R/T[R−r]] 2.5 3.0 3.4 3.8 4.0 4.9
± ± ± ± ± ±
0.1 0.2 0.1 0.3 0.4 0.4
R−r = Shortest distance between cavity surface and rectangular surface of the prism. C
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Figure 3. (a) Normalized absorption spectra of the synthesized HGNs are shown. The spectrum for HGN-II has not been shown to have better clarity. Wide tunability of the spectra is quite obvious. Absorption spectrum of silver nanoseed is also provided as inset. (b) Simulated normalized absorption spectra of all six HGNs are shown. Qualitative agreement between the experimental and theoretical spectra is quite good. (c) Aspect ratio dependency of the LSPR peak positions for HGNs is shown. Aspect ratio is defined as L*R/T[R−r], where L is side length, r is cavity radius, T is the thickness, and R is the shortest distance between the center of the cavity (coinciding with the center of mass) and side wall. The experimentally observed trend matches very well with the trend obtained numerically.
significantly. Another important fact to note from Figure 3a and b is that the simulated line widths are systematically lower than the experimental line widths. We attribute this lowering of line width to the fact that in the simulations, only one isolated HGN was used and the dimension used to simulate the LSPR spectra of HGNs was the average dimension obtained from the TEM analyses. However, the actual ensemble of the colloids has finite size distribution, which leads to inhomogeneous broadening of the LSPR. Moreover, the synthesized HGNs contain various degrees of defects, including corner rounding or snipping, which too contribute to the overall inhomogeneous broadening of the LSPR line widths in the experiment than in the simulations. Stability of HGNs against Aggregation. We investigated the stability of the synthesized HGNs under harsher conditions to confirm the suitability of these nanoprisms in future applications that demand robustness against aggregation. The limited stability of wet-chemically synthesized nanoparticles unlike the nanostructures prepared via top-down methods (e.g., electron beam lithography) can be problematic, leading to the aggregation of the nanoparticles to an extent that inhibits their application. Most metal nanoparticles are vulnerable to aggregation when they are subjected to high ionic strength, induced by the addition of salt.11 Also, addition of other molecules into the nanoparticle dispersion leads to aggregation.12,38 To investigate their stability, we treated the asprepared HGN solutions with different concentrations of aqueous NaCl solution, and the corresponding LSPR responses
Table 2. Aspect Ratio Dependence of the Experimental and Simulated LSPR Maxima of All of the HGNs
sample HGN HGN HGN HGN HGN HGN
I II III IV V VI
aspect ratio [AR = L*R/T[R−r]]
experimental LSPR maxima (eV)
simulated LSPR maxima (eV)
± ± ± ± ± ±
2.18 2.17 2.14 2.08 2.05 1.91
2.10 2.06 2.04 2.00 1.95 1.83
2.5 3.0 3.4 3.8 4.0 4.9
0.1 0.2 0.1 0.3 0.4 0.4
relative peak positions of the simulated LSPR spectra of the HGNs match qualitatively well with the experimentally measured ones. Also, the aspect ratio dependency of the LSPR spectra obtained through simulation matches quite well with that determined through experiments (Figure 3c). Here, we note that the simulated LSPR spectra of all of the HGNs are slightly red-shifted as compared to the experimentally determined ones. This slight difference in the exact peak positions of the experimental and the simulated spectra is most probably due to a slight difference in the actual dielectric constants of the surrounding medium in the actual sample and that used in the simulation. Also, a smaller (5 nm) are obvious from the TEM image. It is obvious that HGN I did not aggregate upon the addition of either NaCl or EDT, even at high concentration. (c) LSPR spectra of redispersion of centrifuged HGN I in water, methanol, and ethanol. The alcoholic redispersion leads to aggregation of the HGNs, while the aqueous one does not. D
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Figure 5. (a) Normalized absorption spectra of as-prepared HGN VI (cyan), aqueous solution of ∼10−4 M PIC (red), and HGN VI in the presence of ∼10−4 M PIC dye (black). (b) Normalized absorption spectrum of the PIC-adsorbed HGN VI (spectrum shown in (a)) after centrifugation, washing, and redispersing in water (black); spectrum of HGN VI (cyan) is also shown for comparison. The spectra (black curve) show no sign of aggregation of HGN upon PIC addition/adsorption. The dip at 2.12 eV is due to an interaction between the surface plasmon of the HGN and the excitonic transition of the PIC J-band.
bidentate ligands. We believe that this excellent stability of HGNs in such harsh environments is due to an extremely robust capping of the HGN surfaces by CTAB. To probe if this speculation is justified or not, that is, whether or not a robust surface-capping by CTAB is solely responsible for the stability of HGNs, we performed another set of control experiments. The hypothesis of our control experiment is as follows. If CTAB capped HGNs are centrifuged, washed, and redispersed in a suitable solvent, which dissolves CTAB so well (far better than water) that the solvent−CTAB interaction is favored over the Au−CTAB interaction, then the CTAB molecules will be desorbed from HGN surface to the bulk solvent. In that case, the surface of HGNs will no longer remain protected, and, therefore, the nanoprisms will rapidly aggregate. On the other hand, if the washed HGNs are redispersed back in water again, then no aggregation should take place. However, it is important to note here that using solution phase solubility is not sufficient to account for ligand−surface interaction because the nanoparticle/environment interface is complex and cannot be described completely by solution phase measurement alone. Nevertheless, through the proposed control experiments, we may at least get some idea about the role of surface capping in protecting the HGNs against aggregation. It is known that CTAB is infinitely soluble in alcohols such as methanol or ethanol (please note that CTAB has comparatively much lower solubility in water than methanol; see Merck Index, 12th ed., no. 2068). We precipitated and separated the HGNs from their aqueous solutions by low-speed centrifugation and then redispersed them in methanol, ethanol, and also in water. The LSPR spectra were recorded for these three different solutions immediately. Redispersion of the centrifuged HGNs back into water shows absolutely no change in their LSPR, which means that (i) no aggregation occurred due to centrifugation and (ii) CTAB layers were intact on the surface of HGNs. On the contrary, redispersion of the HGNs in the alcohols led to aggregation, as it is evident from the LSPR spectra (Figure 4c). Here, we note that in a separate set of experiments, we added alcohols directly into the as-prepared aqueous HGN solution. It is apparent from the spectral red shift that the nanoprisms started aggregating when the alcohol content became high. Thus, the result of this experiment also leads to the same conclusion as the results presented in Figure 4c. All of these results suggest that the surface capping of
were studied using absorption spectroscopy. Plasmon supporting metal nanoparticles, when aggregated, tend to have broader LSPR bands with the LSPR maxima shifted to lower frequencies. Figure 4a depicts the behavior of LSPR as a function of NaCl concentration for one of the hollow gold nanoprisms, HGN-I. From Figure 4a, we see that neither any shift in the LSPR maxima nor any appreciable spectral linebroadening is evident even after addition of >0.3 M NaCl to the HGN-I solution (please note that the lowering of the absorbance with increasing salt concentration is purely a dilution effect). This observation indicates that the HGNs did not undergo any aggregation. However, it is worth mentioning here that in a previous work with hollow gold nanospheres, we observed red shift/blue shift/no shift of LSPR spectra depending on the interfacial structure, when aggregated.37 Therefore, to be sure whether the observation of no shift in LSPR is truly because of nonaggregation of HGNs, we studied the morphologies of the NaCl treated HGNs using TEM. One of the images is shown as an inset in Figure 4a. The HGNs appear to be isolated in the TEM images, and wide interparticle separations are clearly visible. This observation is different from our previous experience with hollow gold nanospheres, which formed small (dimer, trimer, etc.) aggregates through surfacecontact and surface-necking.37Therefore, our result unequivocally suggests that there was no salt-induced aggregation of HGNs, unlike most other metal nanoparticles; for example, citrate capped solid gold nanoparticles are known to aggregate at a much lower salt concentration (a few mM only).11 This excellent stability of HGNs in salt solutions of high ionic strengths is most likely due to an extremely nice packing of a CTAB double-layer all around each and every nanoprism. Apart from NaCl, we studied the stability of HGNs in the presence of an α,ω-dithiol, ethanedithiol(EDT), as well. α,ω-Dithiols or such bidentate ligands are well-known to induce aggregation in gold nanoparticle solution at quite low concentrations.12,38,39 We treated the HGN solutions with different concentrations of EDT and followed up the response by using absorption spectroscopy. The results for experiments carried out on HGNI are presented in Figure 4b. It is clear from Figure 4b that HGNs survived any kind of aggregation, which is otherwise experienced by many nanoparticles. In summary, the HGNs turn out to be extremely robust against aggregation that could possibly be induced either by increased ionic strength or by E
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The Journal of Physical Chemistry C HGNs by CTAB is extremely robust in aqueous medium and this robust capping is the origin of the high stability of the hollow gold nanoprisms against aggregation. Surface-Enhanced Raman Scattering from Isolated HGNs. We used 1,1′-diethyl-2,2′-cyanine (pseudoisocyanine or commonly known as PIC) iodide as the test molecule for investigating the ability of the HGNs to perform as single nanoparticle SERS probe. PIC dyes are a classic example of a family of dye molecules that form J-aggregates in aqueous media.40−44 Formation of J-aggregates enhances the relative intensities of a few vibrational Raman modes, for example, a doublet near 1630 cm−1 and a triplet near 1360 cm−1, as well as a band near low frequency region at 605 cm−1.45−47 These modes involve the vibrations of phenyl or pyridyl moieties in the quinolone macrocycle. Because π−π interactions between the aromatic rings play a dominant role in holding the aggregate together, it is expected that the vibration of the aromatic groups will be amplified through induced electronic distribution changes in adjacent aromatic groups, leading to increased polarizability and in turn enhanced Raman scattering. PIC is known to form J-aggregate on roughened metal surfaces, for example, silver electrode, and thereby its SERS spectrum can be recorded.48 In our experiment, we added PIC dye into the as-prepared HGN solution to reach a final concentration of 100 μM. At this concentration of PIC, no J-aggregate was formed in solution as verified from the spectrum of aqueous PIC solution having the same concentration (red curve in Figure 5a). Also, addition of PIC did not lead to any aggregation of the HGNs as evident from the spectrum of the as-prepared PIC−HGN VI mixture (Figure 5a). The absorption spectrum of the as-prepared HGN−PIC mixture pretty much resembles the addition of the individual absorption spectra of monomeric PIC and HGN VI except for an appearance of a small dip at ∼2.12 eV. Figure 5b shows the spectrum of the centrifuged and washed PIC−HGN VI composite. An LSPR spectrum of as-prepared HGN is also provided in Figure 5b for comparison purpose. A close look at the spectrum of HGN−PIC composite reveals that there is no signature of any residual monomer after washing. However, a sharp dip at ∼2.12 eV is now clearly visible. The position of the observed dip corresponds to the absorption maxima (excitonic resonance maxima) of the J aggregate of PIC dye.49 The origin of the observed dip in the spectra is due to an interaction between the localized surface plasmons of the HGN and the excitons of the PIC-aggregates.49−51 Appearance of this spectral dip (at the excitonic resonance frequency) even after centrifugation and washing clearly indicates that PIC molecules are adsorbed onto the HGN surface, and, moreover, the PIC molecules adsorb onto the HGN surface in such an ordered fashion that exactly resembles their J-aggregate structure. It is to be noted here that these J-aggregate adsorbed HGNs are not aggregated, as was already evident from Figure 5b. Thus, we could prepare the nonaggregated HGNs with dye molecules adsorbed on their surfaces. We drop-casted a couple of microliters of extremely diluted PIC−HGN composite solution on a glass coverslip and used it for confocal Raman microscopy measurements after drying under nitrogen. Figure 6a shows the normalized spectrum obtained from confocal Raman spectroscopy measurement for PIC−HGN VI composite, while Figure 6b shows the same obtained for PIC−HGN I. Both of the measurements result in clear observation of SERS spectra of the adsorbed PIC dye molecules. Spectral positions and relative intensities of the SERS peaks reconfirm the formation of J-
Figure 6. Surface-enhanced Raman scattering (SERS) spectra of PIC J-aggregates adsorbed onto the surface of (a) HGN VI and (b) HGN I. The spectra were acquired at the single particle level (on glass surface) using a confocal Raman microscope using 632.8 nm excitation. Part (c) shows the solution phase, ensemble-averaged SERS spectra of PIC J-aggregates adsorbed on HGN I; a 532 nm excitation was used in this case. A magnified region (600−1200 cm−1) of the same spectrum is shown as an inset.
aggregate on the HGN surface by the PIC molecules. It is important to note here that the actual peak intensities in the unnormalized SERS spectra (not shown) of PIC−HGN VI are much higher than those observed for PIC−HGN I. This is quite expected as the LSPR maximum of HGN VI is in resonance with the Raman excitation frequency. We also recorded ensemble averaged, solution phase Raman spectra of the same PIC−HGN composite solutions that were used for preparing sample for confocal Raman measurements (using 532 nm excitation). We did the solution phase measurements because we wanted to see if the nonaggregated HGNs were capable of showing SERS in solution also. The solution phase Raman spectra for PIC−HGN I composite are shown in Figure 6c. Distinct surface-enhanced Raman peaks are evident in this measurement also. The similarity in the spectral patterns is apparent when one compares Figure 6b and c. The most important and interesting point to note from these observations is that the resulting spectra are from nonaggregated HGNs, either in the solid or in the solution phase. This means that the HGNs are capable of providing sufficient SERS intensity from individual particles. The possible explanation for such observation is that the individual HGNs must be capable of hosting extremely large local electromagnetic field when they are photoexcited. To probe into this possible explanation, we numerically simulated the optical excitation induced enhancements of local electric fields for the HGNs studied here. The local electric field (E-field) distributions for two different HGNs (I and VI) are shown in Figure 7a and b, respectively. The E-field distributions correspond to the respective LSPR maxima for the two HGNs. It is clear from Figure 7 that both of the HGNs strongly enhance the excitation E-field. The electric F
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Figure 7. Electric-field distributions calculated for (a) HGN I and (b) HGN VI, at their respective LSPR maxima. Large electric field enhancement within the cavity can be clearly seen.
field enhancement values
It is important to mention here that some other anisotropic nanostructures may offer near field enhancement comparable to that produced by the HGNs. However, for such anisotropic particles, maximum field is concentrated at a very small region, particularly at the tips. Molecules will show SERS only when they are adsorbed onto those tiny hotspots in those particles. HGN has the advantage over these nanostructures because apart from the prism-tips, the inner cavity also supports a strong electric field, resulting in a tremendous increase in the overall sensing volume. So, although certain nanoparticles may generate sufficient electric field enhancement but for single particle-level sensing (be it RI-sensing or Raman-sensing) applications, HGN or such hollow nanostructures are extremely well suited due to their larger sensing volume. Moreover, due to the large near field enhancement, the cavity of an HGN can be used as a nanoreactor in applications like photocatalysis.
| E| 4 |E 0 | 4
( ) observed for the HGNs range
from 105 to 106. Interestingly, we can see from the E-field distribution plots that apart from the outer surfaces and tips of the nanoprisms, the inner cavity also supports a very large Efield. So, the overall E-field strength in the near-field of HGNs is very high. The observation of such high local E-field is primarily due to the fact that HGN is a plasmonically hybrid nanostructure, very similar to a plasmonic nanoparticle dimer (most primitive nanoparticle aggregate).36,52 In a plasmonic nanoparticle dimer, the outer surface plasmon modes of two nanoparticles capacitively couple to generate a huge amount of electric field, and it is this enhanced E-field that leads to SERS from molecules placed in the interparticle junction.36,52An HGN has two different surfaces and hence two different plasmon modes. Therefore, HGN too can have very strong plasmonic E-fields due to the coupling between their interior and exterior surface plasmons.33 The coupling between these two plasmon modes in an HGN generates large electric fields, which are expected to greatly enhance the Raman scattering from the molecules, adsorbed on its surface. In other words, a single HGN can act like a plasmonic nanoparticle dimer in terms of the performance toward enhancing the Raman scattering from the adsorbed molecules, and this is what is also reflected in our experimental results. Now, for such a hollow nanostructure, the extent of hybridization is governed by several factors like the distance between surfaces that support certain plasmon modes. Thus, in case of HGNs, the size and position of the cavity play crucial roles in dictating the LSPR position. For an HGN where the center of the cavity is positioned at the center of mass of the equilateral triangular prism, as in the present case, an increase in the cavity diameter leads to stronger hybridization. In our recent theoretical work, we have shown that a decrease in tip to cavity−surface distance (by shifting the cavity away from the center of mass toward the tip of the nanoprism) results in a favorable interaction between the plasmon modes supported by the outer surface and cavity surface giving rise to hybridized plasmon mode of lower energy, that is, a red-shifted LSPR peak.33 Therefore, an ability to control the cavity size and position can make the HGNs even better SERS probes. Unfortunately, the existing chemical route is not capable of controlling these parameters at will. However, advanced top-down methods like E-beam lithography can surely do that, and we are currently trying out this possibility in our laboratory.
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CONCLUSIONS In this study, we have investigated the optical properties of hollow gold nanoprisms with a special emphasis on their SERS performances. Wet-chemically synthesized HGNs are found to be extremely stable and quite robust against aggregation. We attribute this stability to an extremely robust capping of the HGNs by CTAB molecules. The LSPR peak positions of the HGNs are structure-specific and highly tunable. This spectral tunability, which is observed because of efficient plasmon hybridization between the inner cavity-plasmon and outer surface plasmon modes, is a linear function of the aspect ratios of the HGNs. Raman excitation of pseudoisocyanine-adsorbed HGN yielded excellent SERS signal from the molecular Jaggregates. Interestingly, strong SERS was observed without the need of any aggregation of the nanoparticles; that is, SERS was observed from isolated or single HGNs. FDTD simulations reveal that a single HGN can localize and tremendously enhance the excitation induced electromagnetic fields due to efficient plasmon hybridization, due to the presence of the cavity. Moreover, the overall sensing volume is also very high for the HGNs. The large electromagnetic field along with a large sensing volume plays the key role behind the observation of SERS from nonaggregated HGNs. Thus, the controllable tunability of the LSPR and the large sensing volume further enhance the applicability of the HGNs as Raman SERS probes. Overall, it turns out from this comprehensive study that HGNs are extremely well suited as nanotransducers in SERS-based chemical and biological sensing applications, especially those G
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requiring nonaggregated nanostructures with small size and high stability.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
B.H. and K.D. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Council of Scientific and Industrial Research, Government of India (Sanction no. 01(2824)/15/EMR-II, Project no. CSIR/CHM/2015220). We thank Dr. Anindita Gayen for many helpful discussions.
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ABBREVIATIONS HGN, hollow gold nanoprism; EDT, ethanedithiol; FDTD, finite-difference time-domain; SERS, surface-enhanced Raman scattering; SERRS, surface-enhanced resonance Raman scattering; LSPR, localized surface plasmon resonance
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