Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle

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Light-Tunable Plasmonic Nanoarchitectures using Gold Nanoparticle – Azobenzene-Containing Cationic Surfactant Complexes Liudmila Lysyakova, Nino Lomadze, Dieter Neher, Ksenia Maximova, Andrei V. Kabashin, and Svetlana A Santer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511232g • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 22, 2015

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The Journal of Physical Chemistry

Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle – Azobenzene-Containing Cationic Surfactant Complexes Liudmila Lysyakova,1,2 Nino Lomadze,1 Dieter Neher,2 Ksenia Maximova,3 Andrei V. Kabashin,3 Svetlana Santer1* 1

Department of Experimental Physics, Institute of Physics and Astronomy, University of

Potsdam, 14476 Potsdam, Germany 2

Department of Soft Matter Physics, Institute of Physics and Astronomy, University of Potsdam,

14476 Potsdam, Germany 3

Aix Marseille University, CNRS, LP3 UMR 7341, Campus de Luminy - Case 917, 13288,

Marseille Cedex 9, France [*] Prof. Svetlana Santer Department for Experimental physics, Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht Str. 24/25 14476 Potsdam, Germany e-mail: [email protected]

KEYWORDS: gold nanoparticles, azobenzene containing cationic surfactants , surface plasmons, plasmonic nanoarchitectures

ACS Paragon Plus Environment

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Abstract When arranged in proper nano-aggregate architecture, gold nanoparticles can offer controllable plasmon-related absorption/scattering yielding to distinct color effects, which critically depend on relative orientation and distance between nanoparticle constituents. Here, we report on the implementation of novel plasmonic nanoarchitectures based on gold nanoparticle – azobenzenemodified cationic surfactant complexes, which can exhibit light-tunable plasmonic response. The formation of such complexes becomes possible due to the employment of strongly negatively charged bare gold nanoparticles (~ 10 nm diameter) prepared by methods of laser ablation in deionized water. Driven by electrostatic interactions, the cationic surfactants attach and form a shell around negatively charged nano-particles resulting in neutralization of the particle charge or even overcompensation beyond which the nano-particles become positively charged. At low and high surfactant concentrations Au nano-particles are negatively or positively charged, respectively, and are represented by single specimens due to electric repulsion effects having absorption peaks around 523nm to 527nm, while at intermediate concentrations the Au nanoparticles become neutral forming nano-100 nm cluster-like aggregates that is accompanied by the appearance of an additional absorption peak at λ>600nm and a visible change in color of the solution from red to blue. Due to the presence of the photosensitive azobenzene unit in the surfactant tail that undergoes trans- to cis-isomerization under irradiation with UV light, we then demonstrate a light controlled nanoclustering of nanoparticles, yielding to a switch of plasmonic absorption band and a related change in solution color. The formed hybrid architectures with light-controlled plasmonic response can be important for a variety of tasks, including biomedical, SERS, data transmission and storage applications.


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Introduction Cationic surfactants containing photosensitive azobenzene group have attracted considerable interest due to their versatile applications in the field of light controlled shape and functionality of soft nano-objects.1,2,3,4,5 Most of these surfactants consist of a charged head group, a hydrophobic chain and a photosensitive azobenzene group. The azobenzene undergoes photo-isomerization from the more stable trans-conformation to the cis-state during UV irradiation. The lifetime of the cis-isomer in the dark ranges from few milliseconds to several hours depending on the ring substituent.6 The back-isomerization from the cis- to the transconformation can be accelerated by irradiation with visible light. For symmetrically substituted azobenzenes, this photo-isomerization is accompanied by a strong change in dipole moment of the azobenzene group from 0 to more than 3 Debye. Thus, with the azobenzene groups integrated in the hydrophobic tail of the surfactant, its hydrophobicity can be altered by irradiation with UV light.7 In the cis-conformation, the molecule carries a charge and a dipole connected through the hydrophobic spacer, while in the trans- state its dipole moment is close to zero, so that it can be considered as a conventional cationic surfactant. This in turn results in photo-controlled solubility or interactions of the surfactant with other substances.7 Such light-controlled properties of azobenzene-modified cationic surfactants can be used for structural manipulations (reversible or irreversible) with many important molecules and compounds. One of prominent examples is related to hydrogel microparticles.8,9 When dispersed in water the negatively charged hydrogels are in a swollen state having the hydrodynamic radius of about 600nm, while in the presence of azobenzene-containing cationic surfactants the particles shrink down to a radius of 300nm as the electrostatic repulsion within the soft particle is screened by the diffusing surfactant molecules. Under UV irradiation the surfactant molecules


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undergo a photo-isomerization-based transformation to the more hydrophilic cis-state and unbind from the hydrogel leading to particle swelling,10 while the irradiation of this complex by blue light leads to the reversed effect as the surfactant molecules become hydrophobic and bind again to the microgel. Another important example involves light-controlled compaction and decompaction of DNA molecules for the development of non-viral gene vectors.11 When conventional cationic surfactants interact with DNA molecule at certain concentration, typically when more than 90% of the charges along the DNA are screened by the surfactants, the DNA undergoes a transition to a compact globular state, which facilitates their cellular uptake. The latter effect was first demonstrated using azobenzene-containing trimethylammonium bromide (azoTAB) surfactant,12,13 and further improved by utilizing surfactants with longer hydrophobic spacer connecting azobenzene group and charged head.14,15,16,17 The extension of the azobenzene-based manipulation strategy toward novel organic-inorganic nano-architectures looks as a next promising step, which could take advantage of light-controlled properties of newly emerging inorganic nanostructures.18 Here, metal nanostructures are of particular interest as these systems can support oscillations of free electrons (surface plasmons), yielding

to

a

number

of

interesting

optical

effects,

including

strong

resonant

absorption/scattering at a particular wavelength and related coloration phenomena,19,20 guiding and imaging capabilities beyond the diffraction limit,21,22 strong field enhancement for Surface Enhanced Raman Spectroscopy (SERS),23,24 ultrasensitive response of plasmon parameters refractive index of the environment and related biosensing applications.25,26,27 In particular, when excited over gold nanoparticles, surface plasmons cause the appearance of a sharp absorption band in visible range, whose position strongly depends on the size of nanoparticles or refractive index of the environment.20 Furthermore, the position of the plasmonic optical signature appears


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to be critically dependent on the distance between gold nanoparticles. In particular, this position can be strongly modified by the formation of nanoparticle associates (dimers, trimers, tetramers etc.) with fixed nanoscale distance between its constituents.18,28 Thus, a proper engineering of nanoparticle aggregation architectures with controlled distance between plasmonic constituents could provide a controlled optical response, which is important for a variety of tasks, including biomedical, SERS, data transmission and storage applications.29 For instance, a crucial effect of the interparticle gap on SERS enhancement factor in plasmonic nanoantennas represented by gold nanoparticle dimers and trimers was reported in [30]. The employment of photosensitive azobenzene-containing cationic surfactants seems cunning to implement such plasmonic architectures, but the approach based on electrostatic interactions of cationic agents looks hardly possible with conventional nanoparticles prepared by chemical reduction routes as these nanoparticles are typically neutral or weakly charged.31,32 In addition, these nanoparticles are usually covered by ligands that complicate further chemical functionalization strategies. Gold nanoparticles prepared by laser ablation in water present an essentially novel object, which can have quite different properties compared to chemically synthesized counterparts. In this method, a pulsed laser radiation is used to ablate material from a solid gold target or preliminarily prepared micro/nano colloids in aqueous solutions, yielding the formation of nanoparticles.33 Such “physical” synthesis route is capable of providing a series of unique properties of formed nanoparticles, including: (i) pure, uncontaminated surface as a result of NPs synthesis in pure environment (deionized water);

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(ii) different reactivity35 and surface

chemistry36 compared to conventional colloidal nanomaterials; (iii) strong negative charging of NPs,36, 37 which contributes to a strong electrostatic stabilization of nanoparticle solutions even in


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the absence of any protecting ligand. The latter property looks especially important for electrostatic interactions based on charge compensation. In this paper, we profit from a strong negative charge and purity of laser-synthesized gold nanoparticles and design complex Au nanoparticles – azobenzene-modified cationic surfactant architectures, which can provide light-mediated structural modifications yielding controlled plasmonic response.

Experimental Section The Azobenzene containing trimethylammonium bromide surfactant was synthesized as described elsewhere.38 The chemical structure of the azobenzene containing surfactant in transconformation is shown in Figure 1d. The surfactant was dissolved in water (MilliQ) and stored in darkness at room temperature. The CMC of the surfactant in pure water is 0.5 mM at room temperature.7 The initial solution was diluted to appropriate concentrations for the preparation of solutions of gold nano-particles and surfactants.

Isomerization of surfactants in aqueous solutions Azobenzene unit incorporated into the tail of the surfactant undergoes trans-cis isomerization under irradiation with appropriate wavelength. The isomerization behavior of the surfactant in aqueous solution was previously investigated using UV-visible spectroscopy and reported in [1]. The characteristic absorption maximum of the surfactant in the initial dark state is located at 353 nm, resulting from π−π* transitions of the azobenzene unit in the transconformation (Figure 1b). The same azobenzenes in the cis-conformation exhibit absorption bands with maxima at 313 nm (π−π* transition) and at 437 nm (ν−π∗transition). Back transition to the trans-


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conformation can be either spontaneous in the dark due to thermal relaxation or triggered by visible blue light. The band with maximum at about 240 nm corresponds to the absorption of the π−conjugated benzene rings. The lifetime of the cis- isomer in water at room temperature in darkness is about 20 hours as estimated from periodically recording UV-Vis absorption spectra over time.

Synthesis of Au particles by laser ablation in aqueous solutions Bare gold nanoparticles were prepared by methods of femtosecond laser ablation in aqueous solutions.36,37,39 Briefly, a gold disc placed on the bottom of a glass vessel filled with deionized water is illuminated by focused radiation from Yb:KGW laser (wavelength 1025 nm, pulse length 480 fs, energy per pulse 95 µJ, repetition rate 1 kHz). The intensity of laser radiation is selected to ablate the gold target and the gold nanoclusters are released into the liquid ambience forming a colloidal nanoparticle solution. The platform with the cuvette is constantly moved to avoid the ablation from the same area on the target. The ablation process leads to a visible red coloration of the aqueous solutions several seconds after the beginning of the illumination onset. Nanoparticle solutions prepared by laser ablation in deionized water look deep red. In some cases, the methods of femtosecond laser fragmentation40,41 were additionally used in order to homogenize size distribution of nanoparticles. As it was shown in [36], the surface of nanoparticles prepared by laser ablation is partially oxidized with atomic percentages estimated as 88.7% for Au0, 6.6% for Au+, and 4.7% for Au3+. In addition, for relatively high pH (pH > 5) these nanoparticles are strongly negatively charged, which conditions high stability of nanoparticles solutions in the absence of any protective ligands.36 Under our experimental conditions pH is slightly basic in solutions with surfactants.


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The colloids just produced were stored in darkness at 4°C. Before mixing with the surfactant the gold nano-particles were sonicated in order to destroy possible aggregates. Concentrations of gold are given in moles pro liter. The extinction coefficient was experimentally determined to be 3000 L·mol-1·cm-1. The size distribution of the nano-particles calculated from the TEM measurement is shown in Figure 1c. Sample preparation. To prepare complexes between gold nano-particles and azobenzene containing cationic surfactants, water solutions of both components at certain concentrations were mixed at room temperature and stored for 30 minutes before measurements. Data on the complexation process of the surfactants with gold particles over time are available in SI-Figure1 of the Supporting information. For scanning electron microscopy studies, gold-surfactant systems were dropped onto nontreated silicon (111) substrate of p-type. The sample was dried with nitrogen after 10 minutes of exposure to air. For the preparation of probes for the TEM, a droplet of the suspension was placed on carbon coated copper grid dried in air. Methods. UV-Vis spectra were obtained using a Cary 5000 UV-Vis-NIR spectrophotometer (Varian Inc.) in quartz cuvettes of 1, 2, 5 and 10 mm optical pathway. The Zeta-potential was measured with Zetasizer Nano ZS (Malvern Instruments Ltd.) at scattering angle of 173°. Electron micrographs were collected using transmission electron microscope JEM-1011 (JEOL Ltd., Japan) and SEM (Ultra Plus, Zeiss, Germany) The lamps VL-4.L (Vilber Lourmat) (365 nm) and LED Spot Luxeon Royal Blue, P453E-PR09 (Conrad Electronic) (453 nm) were used for irradiation of the samples. Irradiation lasts typically 10 minutes to ensure a steady state of the surfactant molecules.1


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All measurements were performed in a yellow light laboratory in order to avoid any uncontrollable photo-isomerization of the surfactants.

Results and Discussion Nano-aggregation in gold-surfactant complexes Gold nano-particles of 10 nm in diameter show a characteristic absorption peak at λSP=523nm originating from the excitation of surface plasmons (Figure 1a). When gold nano-particles are dispersed in water at the concentration of 0.25 mM the solution has pink color (Figure 1a). The characteristic absorption peaks of the trans- and the cis-isomers of the azobenzene containing surfactants do not overlap with λSP (Figure 1b), allowing to track the change in spectral position and intensity of all three peaks: the trans- isomer (λtrans=353 nm), the cis-isomer ( λ1cis=313 nm and λ2cis=437 nm) of the surfactants, and the gold nano-particles at λSP=523 nm when nanoparticles and surfactant are mixed at different concentrations. In the following we shall designate the number of the photosensitive surfactant molecules per gold atom at the surface of a nano-particle as the surfactant loading σ:

σ =

C Azo R C ⋅ = Azo ⋅ 5.4 C Au 3d Au C Au

where CAu and CAzo are the molar concentrations of gold atoms and surfactants, R is the radius of a nano-particle (R = 5±1 nm), and dAu ≈ 0.28 nm is radius of a gold atom. In this calculation we do not account for crystalline structure of nano-particles, lattice constant and the percentage of oxidized gold atoms on the surface that are responsible for a negative surface charge in


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surfactant solution.36 However, the inaccuracy in the above approximation is much smaller than that originating from the size distribution of the nano-particles.

Figure 1. (a) UV-Vis absorption spectra of 10nm AuNPs at CAu=0.5mM (black curve), 0.25mM (blue curve) and 0.125mM (red curve). (b) UV-Vis absorption spectra of azobenzene containing surfactant at CAzo=0.045mM in trans- (black curve) and cis- (red curve) conformation. The insets show pink color of gold colloid and yellow color of the surfactant solution. (c) TEM micrograph of the gold nano-particles, with the superimposed histogram displaying the size distribution of the particles. (d) Chemical structure of the photosensitive surfactant in its trans-conformation and schematic representation of a complex between a negatively charged nano-particle and a cationic surfactant forming a double layer with charged groups pointing outwards.

To prepare complexes between negatively charged gold nano-particles and the cationic azobenzene containing surfactant, aqueous solutions were prepared for each species with


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constant concentration of nano-particles and varying concentration of the surfactant. When mixed, we achieve dispersions with different surfactant concentration while that of the gold particles is constant (CAu=0.2 mM). The absorption spectra of the gold-surfactant solutions are shown in Figure 2. With increasing σ, i.e. surfactant concentration, significant changes in the absorption spectra of the complex occur. At extremely low concentrations of the azobenzene surfactant (σ=0.002, black curve in Figure 2), the plasmon peak of the gold nano-particles levels off at 523 nm. At higher σ values, a shift towards 527 nm develops. It is attributed to the change in the refractive index of the medium near the gold particle.

Figure 2. UV-Vis absorption spectra of complexes between 10nm gold nano-particles and azobenzene containing surfactant at different concentrations (σ values are quoted in the legend). The spectra are represented as normalized to the intensity of gold plasmon peak at low CAzo (λ=523 nm).

At σ between 0.002 and 0.5, the height of the plasmon peak at 527 nm decreases and a second peak at a larger wavelength (λ>600 nm) gradually grows with increasing σ. For instance, at σ=0.15, the second peak is located at λ =700nm and its intensity is larger than that of primary


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peak at λ=527 nm (blue curve in Figure 2). For even higher surfactant concentrations, σ>0.5, the second absorption peak disappears (red curve in Figure 2). Figure 3a shows the red shift of the plasmon peak in the UV-absorption spectra of gold nanoparticles as a function of σ. Three regions can be distinguished by the value of σ. At σ5), no shift in the position of the gold plasmon wavelength (λSP) was found. The zeta-potential remained positive. This implies that at σUV>0.6 the complex is still stable with the surfactant molecules forming a shell around single gold nano-particles. Also the irradiation of the complexes in region I up to a σ value of about 0.01 does not alter the position of the plasmon absorption peak of the gold nano-particles as well as the value of Zeta-potential. The discussion above is based on the assumption that preferentially trans-isomers of cationic azobenzene containing surfactants interact with gold nano-particles. In order to understand to


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which extend the cis-isomer influences the complex formation we exposed the surfactant molecules to UV before mixing them with gold nano-particles. The complex formation was performed under constant UV illumination in order to compensate for any re-isomerization. Then, the UV light was switched off and relaxation to the trans- state was followed by recording the absorption spectra as a function of time. The corresponding data for the gold-surfactant system with σabs=0.7 are shown in Figure 6.

Figure 6. UV-Vis absorption spectra of gold nano-particles/surfactant complex as a function of time. The sample was prepared by mixing the cis-isomer of the surfactant with the gold nanoparticle suspension. The initial spectrum (black curve) has characteristic absorption peaks at λcis=437 nm, corresponding to the cis-isomer of the surfactant, λSP=527 nm characterizing the gold particles. The inset where the time dependence of optical density at 376 nm is shown demonstrates a fast transition of the surfactant back to the trans-state in the presence of gold nano-particles.

Immediately after sample preparation (black curve in Figure 6), one can see two characteristic absorption peaks related to the cis-isomer of the surfactant at λcis=437 nm, and the gold nanoparticle absorption peak at λSP=527 nm. With time the shape of the spectrum changes, now showing a characteristic maximum at λtrans=353nm, i.e. the thermal relaxation of the cis-isomer


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back into trans- occurs. The inset in Figure 6 shows the time dependency of optical density at 376 nm (D376), which is the wavelength of minimal absorption of cis-isomer of the surfactant.1 From the exponential fitting of D376(t), the lifetime of the cis-isomer of the photosensitive surfactant in gold-surfactant system is estimated (Table 1). The lifetime of the cis-isomer in the presence of gold nano-particles is found to be much shorter than in water (τ≈20 hours). It was possible to fit the decay by a single exponential only at relatively low values of σ in region III (close to σabs=0.5) The surfactant isomerization in region I is not detectable due to the low concentration (see SI-Figure 4 in the Supporting Information). Three modes of relaxation were identified (see Table 1), assigned to the fast re-isomerisation of the first surfactant layer attached to the gold nano-particle surface (τ1), the relaxation of molecules in the outer layer of the complex in region III (τ2), and the slow relaxation of the free surfactant molecules (τ3). For instance at σabs=0.7, the lifetime τ1≈ 2 minutes was found for the cis-isomers directly bound to the surface of the gold nano-particles. At the same time a mode is detected with the longer lifetime of τ2 ≈10 minutes. This time could be attributed to the surfactants forming a shell around a nano-particle. A simple argument that the second relaxation time is not just a result of the fitting procedure is supported by the measurements of the absorption kinetics of the complex between the gold nano-particles and surfactants in region III prepared by mixing the surfactant in trans-state. When the complex has formed it is irradiated with UV-light, and the isomerization from cis- to trans- occurs also within around 10 minutes. At σabs>5, that is, an excess concentration of free surfactants, the long relaxation time τ3 of free surfactant molecules is comparable with the lifetime of the cis-isomer in aqueous solution.


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The analysis of the relaxation kinetics indicates that when surfactant molecules in the cis-state are mixed with the gold nano-particles, the back isomerisation is significantly accelerated. Surfactant molecules in the cis-state undergo collisions with nano-particles resulting in the adsorption of the first layer of the surfactant on a gold surface. The attachment could proceed not only through electrostatic interactions between negatively charged nano-particles and cationic head group of the surfactant, but also through direct adsorption of the benzyl ring on the gold surface.45 Once attached to a gold particle, the cis-form relaxes rapidly back to trans, assisted by electron coupling to the surface of the gold nano-particle. Thus, the NPs are decorated with a shell of surfactants in trans-conformation within a few minutes. Our results are in good agreement with the recently reported catalytic activity of gold nano-particles on cis-trans isomerization.46 Depending on the ring substitution, two opposite effects were shown: the faster conversion to the trans- isomer was observed for the azobenzenes with the electron donating moieties, while a slowing of the cis-trans isomerization in the present of gold nano-particles was reported for symmetrical substitution on the aryl rings of azobenzene.

Table 1. The life time of the cis isomer of photosensitive surfactant in gold nanoparticle solutions at different charge ratios σ.

Life time, min

Region II 0.01600nm and confirmed by SEM and TEM measurements. In this second region the single gold nano-particles and the gold nanoclusters coexist. At even higher surfactant concentration (region III), the gold nano-particles are now positively charged due to the formation of a stabilizing shell consisting of surfactant molecules, with cationic groups pointing outwards. Additionally we show that the lifetime of the cis-isomer of the azobenzene containing surfactants is significantly changed when it is attached to the gold nano-particles . Upon interaction of the cis-isomers with gold nano-particles the thermal relaxation time to the trans-state is less than 2 minutes, while more than 20 hours is needed for the same process without nano-particles. This indicates that the surfactant molecules attached to the gold nano-particles are preferentially in the all-trans conformation. Finally we show how plasmonic properties of the gold nanoparticles may be controlled optically. It is, for instance, possible to shift the complex from the region III of single, positively charged nano-particles to the region II of gold nano-clusters easily by short-time irradiation of the complex with UV light. Under irradiation the trans- turns into the cis-isomer, which desorbs out of the stabilizing shell yielding in nano- aggregation into spherical clusters. This process, however, cannot be controlled reversibly. Once the cluster-like aggregates are formed it is not possible to break them apart.


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Acknowledgments. We thank for the financial support Volkswagen Foundation and Helmholtz Graduate School for Macromolecular Bioscience (Germany). We are grateful to Dr. C. Prietzel (the group of Prof. Dr. Joachim Koetz, University of Potsdam) for TEM measurements.

Supporting Information Available: UV-Vis absorption spectra, time dependence of gold plasmon wavelength and optical density at 600nm of gold nano-particles interacting with azobenzene containing surfactant in transconformation at σ=0.01; UV-Vis absorption spectra of gold nano-particles in water and NaCl aqueous solutions; TEM micrograph of the gold clusters of ~100nm in diameter generated after UV irradiation; and UV-Vis absorption spectra of gold nano-particles interacting with azobenzene containing surfactant in cis-conformation at σabs=0.1. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents


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References

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Light-Tunable Plasmonic Nanoarchitectures Using Gold Nanoparticle

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