Reversible Gating of Plasmonic Coupling for Optical Signal

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Reversible Gating of Plasmonic Coupling for Optical Signal Amplification Christopher G Khoury, Andrew M. Fales, and Tuan Vo-Dinh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04623 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on July 2, 2016

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ACS Applied Materials & Interfaces

Reversible Gating of Plasmonic Coupling for Optical Signal Amplification Christopher G. Khoury1,2, Andrew M. Fales1,2, and Tuan Vo-Dinh1,2,3,* 1

Fitzpatrick Institute for Photonics, 2Department of Biomedical Engineering, 3Department of Chemistry, Duke University, Durham, NC 27708, USA * To whom correspondence should be addressed. Email: [email protected] KEYWORDS: plasmonic coupling, SERS, magnetic, nanoparticle, reversible gating

Abstract Amplification of optical signals is useful for a wide variety of applications ranging from data signal transmission to chemical sensing and biomedical diagnostics. One such application in chemical sensing is surface-enhanced Raman scattering (SERS), an important technique for increasing the Raman signal using the plasmonic effect of enhanced electromagnetic fields associated with metallic nanostructures. One of the most important limitations of SERS-based amplification is the difficulty to reproducibly control the SERS signal. Here we describe the design and implementation of a unique hybrid system capable of producing reversible gating of plasmonic coupling for Raman signal amplification. The hybrid system is composed of two subsystems: 1) colloidal magneto-plasmonic nanoparticles for SERS enhancement; 2) a micromagnet substrate with an externally applied magnetic field to modulate the colloidal nanoparticles. For this proof-of-concept demonstration, the nanoparticles were labeled with a Raman-active dye, and it was shown that the detected SERS signal could be reproducibly modulated by controlling the externally applied magnetic field. The developed system provides a simple, robust, inexpensive, and reusable device for SERS signal modulation. These properties will open-up new possibilities for optical signal amplification and gating, as well for highthroughput, reproducible SERS detection. Introduction The modulation and gating of plasmonic coupling is of great interest for its potential use in many areas, ranging from energy, photoemission, and sensing applications.1-3 Most current technologies that utilize the tunablility of plasmonic devices do so in a static state. Once the device is produced, its optical properties are fixed, potentially limiting performance. There have been reports of reversible, regenerable substrates,4-5 though they are not actively controlled and require washing steps. If one could modulate the plasmonic properties of a device in real time, it can be optimized for the specific experimental conditions in use. This also brings about the ability to gate the optical signal amplification, which could be utilized for lasing and/or optical computing applications. Plasmonic materials are also heavily used in sensing applications, such as metal-enhanced fluorescence and surface-enhanced Raman scattering, where achieving reversible temporal control over the signal amplification process would lead to more consistent and reproducible results. Plasmonics refers to the study and application of enhanced electromagnetic properties of metallic nanostructures. According to classical electromagnetic theory, when a metallic nanostructured surface is irradiated by an incident electromagnetic field (e.g., a laser beam), conduction electrons are displaced into frequency oscillations at a resonance that is inherent to the plasmonic

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system. These oscillating electrons, referred to as “surface plasmons,” produce a secondary electric field, which adds to the incident field. Surface plasmons have been associated with important practical applications in surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS) and surface-enhanced luminescence (also referred to as metal-enhanced luminescence). Surface-enhanced Raman scattering (SERS) is a sensitive method for the spectroscopic detection of multiple species in mixtures due to intrinsically narrow Raman peaks.6 This technique often involves SERS-active probes consisting of gold or silver nanoparticles (NPs) that host a Raman molecule for spectroscopic signature and an organic or silica layer for surface protection and functionalization. The large electromagnetic (EM) enhancements generated in the vicinity of plasmonics-active nanosurfaces has enabled Raman scattering, an intrinsically weak scattering phenomenon, to become an important detection methodology that provides narrow spectral resolution and a wealth of molecular information for detection and analysis; the possibility for single molecular detection using SERS has also been demonstrated.78 Our group has been involved in the development and application of various SERS plasmonic platforms ranging from colloidal nanostars, to nanopost arrays, nanowires and nanochips,9-13 as well as their theoretical characterization using numerical and analytical techniques.14 Research thus far has concluded that the widespread use of SERS-based sensing has been limited by the difficulty to reliably quantify the detected SERS signal, particularly when dealing with colloidal NPs. The problem stems from the lack of control over critical factors that dramatically affect the obtained SERS signal, such as degree of NP aggregation, NP size homogeneity and spatial distribution of analyte molecules on the NP surfaces. Among these, aggregation contributes the greatest to enhancing the detected SERS signal, but unfortunately remains the most challenging variable to properly control. Indeed, the EM activity is particularly intense at “hot spots,” which are regions within clusters of NPs where plasmonic coupling greatly enhances the local E-field, the generated SERS signal is no longer linearly proportional to the number of Raman molecules in the probe volume, preventing reproducible signal quantification. Until now, reliable SERS quantification remains a serious problem in SERS-based detection schemes. Recently, a number of research groups have implemented diverse techniques to address this issue, and have been able to demonstrate quantitative SERS-based analyses.15-19 Systems that are devised to controllably aggregate nanoparticles and thereby increase the detected SERS signal in a reproducible manner can be categorized into two divisions: (a) passive systems, which are not influenced by external stimuli to induce aggregation, and (b) active systems, that manipulate external stimuli to trigger NP aggregation. Passive systems can be categorized into either colloid-based systems (CSs) or wafer-based systems (WSs). CSs generate strong SERS enhancements by creating “hotspots,” via the collapsing of nanoparticles in solution either in a random fashion (e.g. NaCl) or in a more controlled manner by self-assembled NP networks (e.g. complimentary ssDNA).20-22 The former enhances SERS but in a non-reproducible manner because it produces NP clusters of largely varying sizes and conformations, while the latter yield reproducible SERS signals at the expense of increased NP functionalization complexity. WSs are more resistant to confirmation changes in biofluids and achieve high and reproducible SERS enhancements by engineering their nanostructured surface to exhibit a high density of electromagnetic hotspots. Such substrates were introduced in the early stages of SERS, and are still finding widespread interest to this


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day.11, 23-29 However, these substrates are ‘static,’ single-use platforms since the Raman molecules adhere to the substrates’ metallic surface. Another class of passive SERS substrates, high refractive index (HRI) substrates, have recently been demonstrated theoretically30 and experimentally.31 These non-plasmonic HRI substrates provide a novel way of performing SERS measurements with low optical losses and negligible heating within the hot spots and surrounding environment. In an attempt to improve on passive systems for inducing reproducible NP aggregation to yield quantitative SERS signals, the development of novel, active systems has grown into an attractive field of SERS research. Active systems employ external structures or forces, such as tapering channels, capillary forces or magnetic/optical forces, to induce aggregation such that uniformly distributed hotspots are created. One such platform consists of a pinched microfluidicnanochannel junction that traps NPs but is single-use, complex to fabricate and limited for high throughput screening.32-33 The high-throughput problem was more recently addressed by an optofluidic SERS compact disc platform,34 and yet other active systems manipulated external stimuli, such as optical tweezers35 or electrokinetic effects36-37 to create strong forces to control NP aggregation,into small finite volumes. Unfortunately, each of these devices suffered from either complicated fabrication or reusability limitations or complex manipulation protocols, or even a combination of these issues. In this work we describe the design and implementation of a unique hybrid system for reversible optical amplification and gating that combines a colloidal system, comprised of Raman-active plasmonic magnetic NPs, and a substrate-based system, consisting of a micromagnet array. These sub-systems are then coupled by an externally applied magnetic field, Bext, to produce a simple, robust, inexpensive, reusable and quantifiable SERS device that has the potential for seamless integration with microfluidics. Results and Discussion The concept of optical modulation and gating using the plasmonic coupling effect of nanoparticles



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Distance (nm)

Figure 1. Distance-dependent electromagnetic field enhancement between two nanoparticles, modeled after the experimentally synthesized particles. The top panels show the EM field distribution between the particles, while the plot shows the EM enhancement at a line drawn through the center of the top panels (each panel is 50 nm in width). The gaps are (a) 10 nm, (b) 6 nm, and (c) 5 nm.

The approach of plasmonic coupling modulation and gating with nanoparticles was theoretically investigated using the finite element method in COMSOL Multiphysics. A two-dimensional model of the synthesized nanoparticles was created. A 5 nm thick gold shell surrounds a 10 nm diameter iron core, which is then surrounded by a 2.5 nm thick silver shell. This model was placed in a dimer configuration with various distances between the two particles. The presence of the substrate was not considered in these calculations. Although the substrate could have an impact on the spectral position of the plasmon resonance,38-39 we assume that the particles interrogated in this study are free in solution, not in contact with the Teflon surface of the chip or the glass coverslip. In addition, this theoretical study is only intended to show the trend in EM enhancement as particle separation is decreased, not to accurately predict the experimentally measured enhancement. A 633 nm x-polarized plane wave travelling in the y-direction was used to excite the nanoparticle pairs. As shown in Figure 1, as the gap between the particles is increased, there is a marked decrease in EM field enhancement. By plotting the |Ex / E0|4 of the different gap distances, we can estimate the relative SERS enhancement. From 10 nm to 5 nm gap distances, there is about a 5-fold increase in the predicted SERS enhancement.

Plasmonic Coupling for Raman Signal Enhancement The proposed hybrid system for optical control and gating consists of the integration of colloidal system and a substrate-based system into a novel SERS platform (Figure 2(a)). The presented SERS platform is interrogated on a confocal Raman microscope as shown in Figure 2. The laser spot is focused through a 40x objective into the gap between two micromagnets, and the SERS signal is collected in the epi-direction through the same objective while varying the magnitude of the external magnetic field. Further detail about the experimental setup can be found in Figure S3.


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Laser Polarization

Figure 2: (a) Illustration of the novel hybrid system for the enhanced and reproducible detection of SERS, which is achieved by the integration of the colloidal system (SERS nanoprobes) and a substratebased system (micromagnet array). (b) Schematic of the manipulation of the local field gradient by cobalt micromagnets with the application of an applied magnetic field Hext . Depending on the direction of Hext , the local field will either be amplified (large green arrow), diminished (small green arrow) or unchanged, which results in the formation of H-field minima (Hmin) and maxima (Hmax) in space. The MNPs concentrate at Hmax, where H is also strongest.

The colloidal system consists of gold/silver-coated plasmonic MNPs, MNPs@Au@Ag, and are synthesized with stringent parameters to ensure their integration with, and manipulation by, the substrate-based system. The latter system comprised a layer of thin cobalt micromagnets deposited on a quartz wafer; with the application of an external magnetic field, these micromagnets create intense local magnetic field gradients that concentrate MNPs in their vicinity. The platform involves the coupling of these two systems, such that, although they remain physically isolated from each other by the Teflon coating, they are able to magnetically interact with the onset of an external H-field: this H-field concentrates the Raman-active plasmonic MNPs between micromagnet junctions, where they are probed to yield an increased SERS signal that is quantifiable in space and reproducible in time. The design and analysis of each -system is presented below.

Raman-active Plasmonic Magnetic Nanoparticles (MNPs) It is hypothesized that a reproducible SERS signal can be obtained by reversibly concentrating plasmonic MNPs. This requires the NPs should be small in size, homogeneous in distribution and stable in solution. In general, NP size and stability are intricately related: MNPs that exceed ~20nm in size start developing multiple domains and lose their superparamagnetic behavior,40 which translates to an increased inter-particle magnetic interaction that renders the MNP solution prone to aggregation; this, in turn, hinders their controlled manipulation. Thus, a compromise exists between very small and unresponsive MNPs, or large and aggregation-prone MNPs. Although numerous methods for synthesizing gold-coated iron oxide nanoparticles, MNP@Au, exist in the literature,41-51 only a limited number abide by the strict criteria imposed by this platform, with many requiring complex syntheses or yielding unstable plasmonic MNPs. This study modified an alternative protocol proposed by Pham et al.42 to produce MNPs@Au that are small, homogeneous and stable in solution, Figure 3(a,b). The MNPs@Au@Ag possessed an absorption spectrum that is only slightly red-shifted compared to Au NPs, which is expected



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given their large shell thickness to diameter ratio (~50%). Moreover, the FWHM is wider for MNPs@Au than for Au NPs, which is also consistent with properties associated with core-shell structures.


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Figure 3: (a) TEM of organically-synthesized 10-12nm Fe2O3 NPs (MNPs) (b) TEM of stable and monodisperse 20-30nm gold-coated MNPs (MNPs@Au), following reduction of gold salt by citrate; the inset depicts their absorption spectrum relative to that of Au NPs synthesized without MNP seeds. (c) Synthesis and SERS characterization of MNPs@Au@Ag of increasing Ag shell thickness, showing absorption spectra during the iterative growth of a silver shell on MNPs@Au. The inset presents a baseline-corrected SERS spectrum from MNPs@Au@Ag synthesized with 100µL of AgNO3, with the addition of 200nM of DTDC; the 1132cm-1 Raman peak of interest is highlighted.

To obtain a detectable SERS signal from these NPs while ensuring they remained small, stable (non-aggregated), and therefore maneuverable by an external H-field, their plasmonic response was intensified by the growth of a thin silver shell on the gold surface, Figure 3(c). The plasmon band behavior blue-shifted with increasing silver shell thickness, consistent with the previous study.52 Importantly, a thin silver shell prevented the NPs acquiring a high extinction coefficient, which allowed for their effective optical probing. Finally, since the efficient manipulation of MNPs by an applied magnetic field requires a high volume-fraction (VF) of particles (as per the term M − 〈 〉 in equation S1), the MNPs@Au@Ag were functionalized with a short SH-PEG ligand to ensure they remained stable as a concentrated solution. Figure S3 conveys the experimental setup employed throughout the study. The use of large cross-sectioned, foot-long iron-cored solenoids minimized fringing effects and ensured a uniform magnetic field across the substrate. Maintaining an inter-solenoid separation of 2cm and utilizing a ±5A current generator provided uniform magnetic fields up to ±500G. An inverted Raman microscope excited the sample perpendicular to the substrate plane with a 633nm He-Ne laser. The ability of the Teflon-coated micromagnets to concentrate MNPs was first characterized by manipulating generic ferrofluid (EMG 705 by Ferrotec) as a function of applied magnetic field and micromagnet shape, Figure S4(a). A 5µL volume of 0.4% VF ferrofluid solution (10nm nominal size) was sandwiched between a coverslip and the micromagnet substrate and sealed with oil to prevent evaporation. Bright field images were captured as a magnetic field, Bext, ranging from 0G to 200G was applied across the substrate. For rectangular micromagnets, the concentration of ferrofluid in the micromagnet junctions increases with increasing Bext, as previously documented by Erb et al.53 This behavior is not corroborated by bow-tie micromagnets, however, which concentrated the MNPs more strongly in the junctions for



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Bext=0G, but dissipated the MNPs with increasing Bext. This is consistent with simulation data, which shows an increased magnetic field gradient along the 45-degree edge of the bow-ties when Bext increases (Figure S2(b)). This validation test also demonstrated that the hydrophobic Teflon coating prevented surface contamination, therefore allowing device reusability. Indeed, even when concentrated at magnetic hotspots, the MNP solution was easily washed off the substrate by simple rinsing, and no MNP residue was observed after drying.

Hybrid system for Plasmonic Coupling Modulation The capabilities of the hybrid SERS substrate were demonstrated by replacing the bare MNPs (ferrofluid) with the Raman-active MNP@Au@Ag. The detected SERS signal as a function of VF is investigated in Figure S4(b). Bow-tie micromagnets were employed for their stronger concentrating efficiency. Five aliquots of MNPs@Au@Ag of varying VFs were sandwiched between the micromagnet substrate and a coverslip, which were then sealed with oil. The NPs spread over a 2x2cm area, which translates to a solution layer thickness of ~16µm. Each measurement point was repeated at a different micromagnet junction for a total of five measurements per VF. The results are analyzed by introducing the term signal increase, SI, which is defined as the ratio of the SERS signal detected in the micromagnet junction, SERSjunction, to that detected in bulk solution away from the junction, SERSbulk. Temporal gating of SERS signal Other than to increase SERS detection sensitivity, the other purpose of using this hybrid platform is to provide a robust method for quantifying the detected SERS signal. Signal reproducibility (or quantification) comprises of spatial reproducibility (identical SERS signal detected at different positions on the substrate), and of temporal reproducibility (identical SERS signal detected from repeated probing at the same point in space). Spatial reproducibility results are presented in the supplementary information (Figure S5).

Temporal reproducibility: magnetic-field modulation of SERS A SERS system is considered reliable if it generates the same SERS signal under the identical experimental conditions. The following study investigated whether this hybrid system is predictable and consistent in time; this was achieved by applying a cyclical magnetic field and continually interrogating the same micromagnet junction where the NPs were being concentrated. Specifically, the questions of interest are: 1) is the colloidal system stable enough to be manipulated by an intense and oscillating magnetic field? 2) Do the micromagnets reproducibly concentrate the NPs in the same location and to the same extent? 3) Does the hybrid system boast a rapid response time? 4) Is the detected SERS signal consistent as the NPs are cyclically concentrated and released in the micromagnet junctions? The experimental setup in Figure S3 allowed for the simultaneous control of the applied magnetic field Bext and Raman probing of the MNPs@Au@Ag being concentrated in the micromagnet junctions. Large bowtie micromagnets were used in conjunction with a high VF NP solution (100x). A 5µL drop of NP solution was sandwiched and carefully sealed between the substrate and a thin coverslip, through which the 633nm HeNe laser was focused. Immersion oil


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minimized evaporation, which ensured a controlled environment. A brightfield image and a SERS spectrum were obtained for each applied magnetic field Bext. By varying Bext from -50G to 500G, three magnetic field cycles were obtained, Figure 4(a).

Figure 4. Magnetic field modulation of SERS signal between two micromagnets: (a) SERS data as a function of a periodic magnetic field Bext. The first image depicts bright-field microscope snapshots of 70x75µm bowtie micromagnets showing one complete magnetic field cycle, where the NPs are concentrated and relaxed in the micromagnet junction. The vectorized Bext is depicted as green arrows, and the induced magnetization in the micromagnets is illustrated with red arrows. The overlaid laser cross-hair conveys where the laser excitation occurred. The left graph presents the measured SERS signal as a function of Bext for three cycles of Bext from -50G to 500G and the right graph co-registers Bext and a hysteresis loop schematic (b) Chronological sequence of SERS spectra as a function of Bext. (c) SERS intensity averaged at each Bext, over 3 cycles, with corresponding standard deviation error bars.

Figure 4(a) depicts the correlation between detected SERS intensity in the micromagnet junctions and a sinusoidally varying magnetic field, Bext. The first row of bright-field images depicts a single Bext cycle, varying from a minimum of -50G to a maximum of 500G. This -50G offset was required to demagnetize the micromagnets following their magnetization by large positive fields; indeed, even after removal of Bext, the 70nm-thick micromagnets sustained a strong, in-plane remnant field. This “memory” resulted in a distinct difference between Bext=0G on the rising edge of the sine wave and Bext=0G on the falling edge of the sine wave; these were differentiated by assigning the notation +0G to the former, and -0G to the latter.



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The two co-registered graphs below the bright-field images convey the measured SERS signal intensity at each Bext, both plotted as a function of arbitrary time. For each Bext, the 1132cm-1 DTDC peak shown in Figure 3(b) was isolated and plotted. In agreement with aforementioned results, a positive correlation between Bext and detected SERS signal was observed. Interestingly, this SERS signal increased non-monotonically: a saturation point appeared to have been reached by Bext=250G, beyond which further increase in Bext did not increase the detected SERS signal. Thus, when the external field Bext was cycled between -50G and 500G in a sinusoidal manner, the SERS signal was characterized by an on/off behavior, which is understood by referring to the micromagnet hysteresis loop presented to the right of the graph in Figure 4(a). When the cobalt micromagnets are magnetized by an external magnetic field Bext, they retain a magnetization which depends |Bext|. The magnetic memory of the micromagnet system prevented a drop in the detected SERS signal, following removal of Bext, which was visually confirmed by the NPs remaining concentrated in the micromagnet junction. In order to deplete this region of NPs, Bext was reversed to a value corresponding to the coercive field of cobalt micromagnets (~50G); this depleted the NPs between the junction and in the laser focal spot, which then triggered a drop the SERS signal to a level just above background. This single cycle was repeated twice at the same micromagnet junction to confirm SERS signal reproducibility and system robustness of the hybrid platform. Again, the sinusoidal variation of the applied H-field triggered the repetitive concentrating and re-dispersal of plasmonic MNPs in the micromagnet junction, which resulted in the oscillatory nature of the detected SERS signal. This platform is therefore time-stable, and moreover, this result also constitutes the first demonstration of the reversible magnetic-modulation of plasmonic nanoprobes for SERS signal quantification. Interestingly, the SERS cycle peaks were observed to follow a decreasing trend over time. This was most likely due to progressive photobleaching of the resonant DTDC Raman dye incurred by repeated probing of the low irradiance laser in the same spot and on the same subset of NPs. This degrading SERS signal also contributed to the larger error bars conveyed in Figure 4(c). The use of a non-resonant Raman dye would increase the robustness of the system in terms of SERS signal quantification. Finally, it is noteworthy this device also boasts a fast temporal response to the magnetic stimulus: the NPs were successfully concentrated and re-dispersed at micromagnet junctions, in a cyclical fashion, with externally applied magnetic fields of amplitude ±500G oscillating at 0.02Hz, 0.1Hz and 0.5Hz. Above 0.5Hz, the micromagnets were magnetized and demagnetized too fast to perceive any concentrating and redispersing of the NPs.

Conclusion A simple and reusable hybrid platform that allows the reproducible and sensitive detection of SERS modulation from a small probe volume was designed and characterized. The colloidal subsystem comprised of small, monodisperse plasmonic NPs that were tagged with DTDC and stabilized with a layer of PEG; the substrate-based subsystem consisted of an array of uniquelyshaped, Teflon-coated cobalt micromagnets, the junctions of which generated magnetic hotspots that concentrated the magnetic SERS nanoprobes by the application of an external magnetic


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field, Bext. The platform enabled the reproducible formation of nanoparticle pellets in the micromagnet junctions, producing quantifiable SERS signal modulation both in space and over time. This led to the first demonstration of a reusable platform for the reversible plasmonic modulation of SERS, which was achieved simply by controlling the magnitude of the externally applied magnetic field. In addition to this hybrid platform having a small footprint, containing no moving parts, having a rapid response time and being simple to fabricate, its virtually flat topology and hydrophobic surface makes it multiply reusable and would allow for its seamless integration with microfluidics, either continuous or digital, to produce a novel diagnostic platform for the high throughput detection.

Acknowledgements C.G.K. acknowledges A. Madison and B.-N. Hsu from the Fair group for their help with substrate fabrication, as well as Y. Yang, L. Gao, and B. Yellen for use of their electromagnet setup and insightful discussions about magnetic field manipulation. This work was sponsored by the Duke University Faculty Exploratory Research Funds. Supporting Information. Experimental procedures and detailed characterization of the substrate as described in the main text. References 1. Atwater, H. A.; Polman, A., Plasmonics for Improved Photovoltaic Devices. Nat Mater 2010, 9 (3), 205-213. 2. Ding, T.; Sigle, D.; Zhang, L.; Mertens, J.; de Nijs, B.; Baumberg, J., Controllable Tuning Plasmonic Coupling with Nanoscale Oxidation. ACS Nano 2015, 9 (6), 6110-6118. 3. Zheng, Y.; Soeriyadi, A. H.; Rosa, L.; Ng, S. H.; Bach, U.; Justin Gooding, J., Reversible Gating of Smart Plasmonic Molecular Traps Using Thermoresponsive Polymers for SingleMolecule Detection. Nat Commun 2015, 6. 4. Zheng, J.; Jiao, A.; Yang, R.; Li, H.; Li, J.; Shi, M.; Ma, C.; Jiang, Y.; Deng, L.; Tan, W., Fabricating a Reversible and Regenerable Raman-Active Substrate with a BiomoleculeControlled DNA Nanomachine. J. Am. Chem. Soc. 2012, 134 (49), 19957-19960. 5. Kim, N. H.; Lee, S. J.; Moskovits, M., Reversible Tuning of Sers Hot Spots with Aptamers. Adv. Mater. 2011, 23 (36), 4152-4156. 6. Doering, W. E.; Nie, S., Spectroscopic Tags Using Dye-Embedded Nanoparticles and Surface-Enhanced Raman Scattering. Anal. Chem. 2003, 75 (22), 6171-6176. 7. Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S., Surface-Enhanced Raman Scattering and Biophysics. J. Phys.: Condens. Matter 2002, 14 (18), R597. 8. Nie, S.; Emory, S. R., Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering. Science 1997, 275 (5303), 1102-1106. 9. Khoury, C. G.; Vo-Dinh, T., Gold Nanostars for Surface-Enhanced Raman Scattering: Synthesis, Characterization and Optimization. J. Phys. Chem. C 2008, 112 (48), 18849-18859. 10. Vo-Dinh, T., Surface-Enhanced Raman Spectroscopy Using Metallic Nanostructures1. TrAC, Trends Anal. Chem. 1998, 17 (8–9), 557-582. 11. Vo-Dinh, T.; Hiromoto, M. Y. K.; Begun, G. M.; Moody, R. L., Surface-Enhanced Raman Spectrometry for Trace Organic Analysis. Anal. Chem. 1984, 56 (9), 1667-1670.



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Reversible Gating of Plasmonic Coupling for Optical Signal

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