Laser-Generated Bimetallic Ag-Au and Ag-Cu Core ...

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nanoshell plasmon modes, are investigated for simultaneous sensing in two widely separated wavelength regions. A sequen- tial laser ablation method is used ...

Plasmonics DOI 10.1007/s11468-014-9854-5

Laser-Generated Bimetallic Ag-Au and Ag-Cu Core-Shell Nanoparticles for Refractive Index Sensing M. P. Navas & R. K. Soni

Received: 13 June 2014 / Accepted: 1 December 2014 # Springer Science+Business Media New York 2014

Abstract Localized surface plasmon resonance (LSPR) wavelength of Ag, Au, and Cu nanoparticles (NPs) falls in visible region and is highly sensitive to size, shape, and surrounding medium. Refractive index sensitivity (RIS) and figure-of-merit (FOM) of Ag, Au, and Cu are analyzed for different particle sizes using the quasi-static Mie theory. The simulation results reveal that RIS and FOM of Ag NPs are higher than Au and Cu NPs. Bimetallic Ag-Au and Ag-Cu core-shell NPs exhibit two resonance peaks, corresponding to hybridization of core and nanoshell plasmon modes, are investigated for simultaneous sensing in two widely separated wavelength regions. A sequential laser ablation method is used to generate bimetallic Ag-Au and Ag-Cu core-shell NPs in liquid medium, and their LSPR peak shift and broadening are monitored in different refractive index liquids. Laser-generated Ag-Au NPs with Au shell of 1– 2 nm show optimum RIS and FOM in lower-wavelength Ag plasmon channel. The Au shell not only improves the chemical stability of Ag NPs but also increases the index sensitivity at an optimum thickness. Further, in higher-wavelength Au plasmon channel, both RIS and FOM increase with shell thickness, but their values are lower than those in Ag plasmon channel. Keywords Refractive index sensing . Pulsed laser ablation in liquid . Ag nanoparticles . Bimetallic core-shell nanoparticles . Plasmon hybridization

Introduction Metal particles exhibit strong dipolar excitation in the form of localized surface plasmon resonances (LSPR), in which free M. P. Navas : R. K. Soni (*) Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India e-mail: [email protected]

electron cloud motion confined at metal-dielectric interface is set in resonance by optical radiation at a particular wavelength [1]. It is well known that this resonance oscillation is a unique characteristic of metal which is currently used in a wide range of applications in bio-sensing, biomarker, photonic crystals, and nanophotonics [1–9]. The LSPR frequency of metal nanoparticles (NPs) strongly depends on size, shape, morphology, and surrounding medium. For instance, a small variation in the refractive index of the surrounding medium results in a measurable shift in the LSPR absorption frequency. This unique characteristic of plasmon resonance in metal nanoparticles forms the basis of sensitive refractive index sensors [10–13]. Recently, it has been shown by FDTD simulations that aluminum (Al) is an excellent material for refractive index nanosensor with large wavelength tunability from deep-UV to near-IR region [14]. Other metals like Pt, Pd, and Ni have also shown good plasmonic response [15–17], though noble metals (Au, Ag, and Cu) dominate the scientific research due to their unique optical properties [18–20]. Colloidal solution of metal nanoparticles exhibits strong absorption in the visible region and displays very intense color, while other metals show weak and broad band in the UV region. Because of relatively easier surface chemistry, the possibility of attaching molecules, biocompatibility, and stability, coinage metals are preferred in applications like bio-sensing [8, 9] and drug delivery [21]. Spherical silver, gold, and copper nanoparticles have LSPR absorption band in blue, green, and red regions, respectively [22, 23] which can be tuned by varying sizes and shapes [24, 25]. The quest for an optimal refractive index sensor motivated researchers to different nanometallic geometries [12, 26]. Generally, it is believed that anisotropic geometry exhibits enhanced sensing capability than spherical nanoparticles. But difficulty in fabrication of uniform, stable, and chemically free anisotropic structures like nanorods, nanotriangles, nanocubes, nanopyramids, and nanostars has somewhat

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limited their practical applications. Currently, much focus has been directed towards understanding refractive index sensing properties of bimetallic and trimetallic nanoparticles [27]. In an earlier work, we have shown that Ag-Au core-shell structure has better refractive index sensitivity (RIS) and stability than single metallic sensors [28]. Bimetallic alloys, core-shell, onion-like structures are expected to provide greater flexibility and a wide range of wavelengths for index sensing. A variety of wet chemical methods has been employed for the synthesis of single and bimetallic nanoparticles in recent years. A chemical-free method to synthesize pure and stable nanoparticles for sensing and biological application is still in an early stage. Laser ablation of metal target in liquid medium is very promising in this aspect and extensively used for the generation of metals, semiconductors, alloys, and oxide nanoparticles in liquid [29–32]. The laser-ablated NPs have many advantages over chemically reduced nanoparticles as they are chemically pure, stable, and easily size controlled. As no chemical is used in laser ablation, obtained nanoparticles show high chemical purity, electrical charge, and, as a result, electrostatical stability. The size of the laser-generated NPs is directly related to the fluence and wavelength of the laser beam and is controlled by changing fluence/wavelength [33]. Presently, laser ablation method is extensively applied for the generation of multimetallic nanoparticles [34, 35]. Sequential ablation of one metal in colloidal solution of other metal and simultaneous ablation of two metals are generally employed for the generation of bimetallic nanoparticles [36, 37]. In this study, we experimentally and theoretically investigate refractive index sensing properties of monometallic Ag, Au, and Cu NPs and bimetallic Ag-Au and Ag-Cu core-shell NPs prepared by laser ablation. Numerical simulations based on the quasi-static Mie theory have been carried out for RIS and figure-of-merit (FOM) using monometallic and bimetallic NPs. Sequential ablation of Au or Cu in freshly prepared colloidal solution of Ag nanoparticles has been used to generate bimetallic Ag-Au and Ag-Cu nanoparticles. Core-shell nanoparticles exhibit two LSPR peaks corresponding to core and shell metals with a small shift from the LSPR of individual nanoparticle due to plasmonic interaction [37]. Core-shell nanoparticles offer additional advantages of refractive index sensing through hybridized plasmon modes in two widely separated wavelength regions simultaneously.

150-mm focal length. The height of water above the target was kept at 10 mm with a total volume of 8 ml. The distance between the lens and the target was adjusted to obtain a spot size of 0.35 mm on the target. In order to obtain NPs of different sizes, laser fluence was varied in the range of 1.8– 10.7 J/cm2. For the preparation of bimetallic Ag-Au core-shell and Ag-Cu core-shell NPs, a sequential two-step ablation method, shown schematically in Fig. 1, was used. In this method, first, Ag NP colloidal solution was prepared by laser ablation of Ag target in pure water, followed by ablation of Au or Cu target in freshly prepared Ag NP colloidal solution. The Ag NPs act as seed and form core for ablated hot Au or Cu species. The ablation duration was 5–20 min for Au and 5– 40 min for Cu to obtain core-shell morphology with varying shell thickness. Optical absorption spectra of freshly prepared NPs were measured using a UV–vis spectrophotometer (Perkin-Elmer Lambda35) having a 0.5-nm spectral resolution. Prepared NPs were tested for refractive index sensitivity towards water, ethanol, and THF. Sensing parameters viz. RIS and FOM were calculated from quasi-static Mie theory simulations of LSPR absorption of spherical monometallic Ag, Au, and Cu NPs and bimetallic Ag-Au and Ag-Cu core-shell NPs of different sizes and shell thickness and compared with experimental results.

Results and Discussion In metal nanoparticles, the plasmon resonance is localized on the surface, which can be directly excited by visible light. The quasi-static Mie theory provides a convenient method to calculate the extinction spectra of spherical metal nanoparticle.

Experimental For the preparation of monometallic nanoparticles, a Qswitched Nd-YAG laser operating at a wavelength of 532 nm, pulse width of 5 ns, and repetition rate of 10 Hz was used. The laser beam was directed on the pure (99.9 %) metal target (Ag, Au, and Cu) immersed in deionized water in a glass vessel using a high-reflectivity mirror and a lens of

Fig. 1 Schematic illustration of monometallic and bimetallic nanoparticle generation by pulsed laser ablation in liquid. Inset shows formation of monometallic nanoparticles and bimetallic core-shell nanoparticles by single-step and two-step sequential laser ablation, respectively

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For a particle size much smaller than the wavelength of optical radiation (2R≪λ), the extinction coefficient is [38] 3=2

C ext ¼

24π2 R3 εm εi λ ðεr þ 2εm Þ2 þ ε2i

ð1Þ

where εm is the dielectric constant of the surrounding medium, R is the radius of NP, and ε=εr +iεi is a frequency-dependent complex dielectric function of the metal NP. Figure 2a shows the calculated absorption spectra of a single spherical Ag, Au, and Cu NP of size 20 nm in air (refractive index=1) and water (refractive index=1.33). The LSPR peak of Ag, Au, and Cu appears at 361, 497, and 582 nm, respectively, in air and shifted to 400, 520, and 610 nm in water. The LSPR wavelength tunability of these metal NPs with size and surrounding medium provides a unique advantage for their application in plasmonic nanosensors. Among these metals, Ag NP exhibits a strong LSPR absorption in the short-wavelength region with minimum line broadening. However, in visible wavelengths,

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Size (nm) Fig. 2 a Calculated absorption spectra of spherical Ag, Au, and Cu NPs with a size of 20 nm in air (dash line) and water (solid line). b The LSPR peak shift of Ag, Au, and Cu nanoparticles in air (dash line) and water (solid line) with size. The shift is calculated with respect to a 5-nm particle

Au shows superior plasmonic characteristics than Cu. When interband transition energy in metal is near the plasmon resonance energy, the LSPR peak shows a significant damping and asymmetric line broadening. In Ag, the interband transitions fall in deep-UV region, therefore do not interfere with LSPR peak; however, in the case of Au and Cu, interband transitions strongly interfere and alter the LSPR line shape. Further, a large LSPR broadening can be seen in Cu due to its higher imaginary dielectric constant than Ag and Au. Mie theory simulations predict that LSPR peak redshifts with increasing NP size. The calculated rate of peak shift with size (δλ/δr) is 1.67, 0.96, and 0.67, for Ag, Au, and Cu NPs, respectively, clearly suggesting that Ag NP provides a larger size-induced shift than the other two metals. The higher rate of shift is advantageous for higher sensitivity of LSPR-based nanosensors. The size-dependent shift in LSPR peak wavelength in air and water is plotted in Fig. 2b. In large refractive index medium, a relatively smaller energy is required to collectively excite the surface electrons; therefore, plasmon resonance exhibits characteristic shift towards lower energy in highly polarizable medium. The LSPR peak of metal NPs generally redshifts with increase in refractive index of the surrounding medium and exploited for index sensing with metal nanoparticles. The calculated RIS from LSPR peak shift per unit change in refractive index for Ag, Au, and Cu NPs is shown in Fig. 3a. It is observed that RIS increases from 153 to 265 nm/refractive index unit (RIU) for Ag NP, 128 to 233 nm/RIU for Au NP, and 117 to 212 nm/RIU for Cu NP for sizes 5 to 50 nm. From Fig. 3a, it can be seen that the LSPR line broadening (full width at half maximum (FWHM)) of Ag NP decreases from 49 to 34 nm with increases in size from 5 to 30 nm, reaches a minimum at size 30 nm, and then increases again to 37 nm at size 50 nm. Similarly, for Au and Cu, the minimum FWHM is obtained at sizes 45 and >50 nm, respectively. For size 40 nm due to increased FWHM. The optimum FOM is obtained for 40-nm Ag NP.

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Size (nm) Fig. 3 a Size-dependent RIS (solid lines) and FWHM (dash lines) of Ag, Au, and Cu nanoparticles. b FOM of Ag, Au, and Cu nanoparticles

A comparison of FOM of three metals clearly indicates that for index sensitivity, Ag is a superior metal than Au and Cu. We therefore synthesized Ag NPs by laser ablation in water to experimentally study their refractive index sensing properties. The average size of the NPs was controlled by varying laser fluences during ablation. Figure 4a shows absorption spectra of Ag NPs with varying fluences. The LSPR peak wavelength was observed at 396.9, 398.2, 401.8, 404.9, and 404.2 nm for fluences of 1.72, 1.79, 4.18, 7.15, and 10.73 J/cm2, respectively. The average particle size determined from absorption spectra was 8, 16, 22, 28, and 26 nm, with 10 % error, at different fluences [40]. Figure 4b shows a TEM image of Ag nanoparticles generated with laser fluence of 1.79 J/cm2. From the TEM image analysis, the calculated average size of nearly spherical nanoparticles is 17 nm, which is in good agreement with Mie theory calculation. The particle size distribution was fitted with a Gaussian function, 3 2   ! 6 202:29847 7 ðR−R0 Þ 2 7 6 rffiffiffiffi 5exp −2 y ¼ 2:58197 þ 4 ð2Þ 14:63165 π 14:63165 2 where y is the abundance of the nanoparticles, R0 is the average particle radius, and R is the particle radius. The

Fig. 4 a Absorption spectra of laser-generated Ag nanoparticles in water. The laser fluence was varied from 1.8 to 10.7 J/cm2. b TEM image of laser-generated Ag nanoparticles at a fluence of 1.8 J/cm2. Inset shows calculated particle size distribution

Gaussian size distribution with fitting parameters from TEM image analysis was included in the theoretical simulation, and index sensing parameters RIS and FOM were compared with experimental results for laser-generated NPs, as shown in Fig. 5. The average size of all generated NPs was below 35 nm, smaller than the optimum size of Ag NPs for maximum sensitivity. The measured RIS of nanoparticles increases with size, in qualitative agreement with simulation results, but their values are lower than theoretical values due to size and shape anisotropy of generated particles, as shown in Fig. 5a. Similarly, FOM was also calculated with Gaussian size distribution. The calculated FOM shows a maximum at 30 nm as shown in Fig. 5b. It can be seen that measured FOM of the generated particles are lower than theoretical values for small particles due to additional broadening caused by nonspherical shape and surface morphology. From the above results, it is clear that high RIS and FOM make Ag a superior metal for index sensing. On the other hand, Au NPs are superior in chemical stability,

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Size (nm) Fig. 5 a Comparison of calculated RIS including Gaussian size distribution and experimentally measured RIS of Ag nanoparticles of average sizes of 16, 22, and 28 nm. b Comparison of calculated FOM including Gaussian size distribution and measured FOM of Ag nanoparticles of average sizes 5–50 nm

biocompatibility, and functionalization of a variety of chemical and biological molecules. Bimetallic combination of these metals in the form of core-shell can tune LSPR in the visible region by varying the core size and shell thickness. Moreover, coating of chemically stable metal over a less stable core metal provides long-term stability to core-shell nanoparticles. Using the extended Mie theory, we have calculated absorption spectra of bimetallic Ag-Au and Ag-Cu core-shell nanoparticles. For core-shell structure, the modified polarizability α is α ¼ 4πR3

ðεs −εm Þðεc þ 2εs Þ þ f ðεc −εs Þðεm þ εs Þ ðεs −2εm Þðεc þ 2εs Þ þ f ð2εs −2εm Þðεc −εs Þ

ð3Þ

Here, f is the ratio of core to the total radius; εc and εs are the frequency-dependent dielectric constant of core and shell, respectively; and εm is the dielectric constant of the surrounding medium.

Figure 6 shows simulated absorption cross section of AgAu core-shell NPs in water with a core size of 16 nm and shell thickness of 1 to 4 nm. With increasing Au shell thickness, the LSPR peak of spherical Ag nanoparticle at 397.6 nm shifts to 407.5 nm. The Au coating on Ag significantly dampens and broadens the plasmonic band of the Ag core. On the other hand, the peak absorption around 510 nm increases with Au shell thickness. The bimetallic core-shell NPs excite two plasmon resonance modes which are sensitive to the surrounding medium and can be exploited for refractive index sensing in two different wavelength regions [27]. The plasmon mode frequencies of a complex nanostructure are explained in terms of interaction between the plasmon resonances of its elementary components [41]. In Ag-Au core-shell nanoparticles, geometry-dependent plasmon response is due to an interaction between three linearly independent dipolar plasmons [37, 42, 43] Firstly, Au nanoshell is seen as an interaction between the essentially fixed-frequency plasmon response of a nanosphere and that of a nanocavity. qffiffiffiffiffiffiffi 1 The sphere plasmon frequency is ωs ¼ ωB 2lþ1 cavity plasqffiffiffiffiffiffiffi lþ1 mon frequency is ωc ¼ ωB 2lþ1 where ωB is the bulk plasmon frequency and l is the spherical harmonics. Because of the finite shell thickness the cavity plasmons interact with each other; the strength of interaction depends on the shell thickness. This interaction results in splitting of the plasmon modes into lower energy symmetric ω-(bonding) and higher energy anti-symmetric ω+ (anti-bonding) plasmon modes. The mode frequency is, " # rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  r 2lþ1 ω2B 1 1 ω ¼ ð4Þ 1 þ 4l ðl þ 1Þ 2l þ 1 R 2 where r and R are the inner and outer radius of the shell. The hybridization of Au shell into bonding and antibonding 1.0

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Wavelength (nm) Fig. 6 Calculated absorption spectra of bimetallic Ag-Au core-shell nanoparticles in water using the Mie theory. The Au shell thickness varies at 1–4 nm, and core size is fixed at 16 nm

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plasmons is schematically depicted in Fig. 7 along with shell thickness-dependent energy levels diagram. The absorption spectrum of Au nanoshell exhibits two plasmon bands; the short wavelength band arises due to the asymmetric coupling of the inner and outer surface charges and the longer wavelength band arises due to symmetric coupling. Further, the inner surface plasmon coupling can be modulated by varying shell thickness. The thinner shell enhances the coupling strength which reflects in redshift of the symmetric plasmon band and a weaker asymmetric plasmon band. In widely studied nanosphere-in-a-nanoshell structures, it has been observed that the plasmon coupling in the metal nanoshell is greatly altered by the presence of metal nanosphere. It has been reported that in the absorption spectrum of Au-Ag core-shell structure, there are two absorption bands which correspond to the outer surface of silver and interface between gold core and silver shell, respectively [44], and attributed to antisymmetric or symmetric coupling between

the symmetric plasmon modes of the silver nanoshell and the gold nanosphere plasmon mode [45]. The plasmon hybridization theory predicts three dipolar plasmon resonances for the Au-Ag core-shell nanoparticles: antibonding of the Ag shell ω+, bonding of Au-Ag core-shell ω−−, and antibonding of AuAg core-shell ω−+. These hybridized plasmon resonances show shift with shell thickness. However, due to very small dipole moments, resulting from opposite dipoles of core and shell, the low-energy resonance modes are weak. In Ag-Au core-shell nanoparticle, interaction of Ag sphere plasmon ωs-Ag with bonding and antibonding plasmons of Au shell, respectively, is depicted Fig. 7a. The Au shell antibonding plasmon ω+ weakly interacts with Ag sphere plasmon ωsnon Ag to form nonbonding mode ω+ , leading to plasmon band around 500 nm. The Au nanoshell ω− mode couples with the Ag sphere to form an antibonding mode ω−+ which results in a plasmon band around 400 nm. On increasing Au shell thickness, the interaction between plasmon modes of nanoshell in the first hybridization becomes weaker; therefore, the low energy mode ω− blueshifts. This will also cause blueshift of the hybridized mode ω−+ at small thickness (2 nm) as shown in Fig. 7b. The antibonding mode of the Au nanoshell, on the other hand, redshifts with increasing shell thickness, reaching close to sphere plasmon ωs-Au. As shown in Fig. 8, with increasing laser ablation time, the Au shell thickness increases, and as a result, the strong plasmon band slightly redshifts from 397.5 to 406.9 nm and very small redshift for weak plasmon band around 514 nm, in agreement with calculated results. Weak plasmon band corresponding to ω−− mode could not be detected due to very small dipole moment. The magnitude of plasmon band shift observed in experiments, however, is different as we have not considered a dielectric constant of the surrounding medium water and charge transfer at the coreshell interface in our calculations. 1.0

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Pure Ag [email protected] 5 min [email protected] 10 min [email protected] 20 min

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Fig. 7 a Plasmon hybridization diagram of Ag-Au core-shell nanoparticle. b Calculated plasmon energies of the Ag core, first hybridized Au nanoshell modes (ω+) and (ω−) and second hybridized modes (ω++) and (ω−−) as a function of Au shell thickness

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Wavelength (nm) Fig. 8 Absorption spectra of laser-generated Ag-Au core-shell nanoparticles in water. The ablation time of Au in Ag colloidal solution varies from 0 to 20 min

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Fig. 9 TEM image of Ag-Au core nanoparticles generated by ablating gold in Ag colloidal solution for 20 min. Inset shows calculated particle size distribution

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We used the sequential ablation method shown schematically in Fig. 1 to generate Ag-Au NPs in water. In this method, a pure Au target was ablated in a freshly prepared Ag NP colloidal solution by laser ablation in water. The LSPR peak of Ag NP shifts from 397.5 to 406.9 nm after the sequential ablation of Au, and a second peak appears around 510 nm confirming formation of a core-shell structure as shown in Fig. 8. The calculated size was 16 nm from the LSPR peak of Ag NPs and used as a core size in our numerical simulations. A different ablation time was used to prepare four samples, labeled as AgAu-0, AgAu-1, AgAu-2, and AgAu-3 for time of 0, 5, 10, and 20 min, with different Au shell thickness. The Ag LSPR peak redshifts with increasing ablation time and appears at 397.5, 400.5, 402.1, and 406.9 nm for 0, 5, 10, and 20 min, respectively. With more Au content resulting from thicker shell, the peak around 510 nm becomes more prominent in core-shell particles. The lower-wavelength (higher energy) band around 400 nm is assigned to ω−+ hybridized plasmon mode, and higher wavelength (lower energy) around 510 nm is assigned to nonbonding ω+ plasmon modes, which are in good agreement with the hybridization theory and Mie theory simulations for the Ag-Au NPs depicted in Fig. 6. A comparison of Mie theory simulations with observed absorption spectra of core-shell nanoparticles gives Au shell thickness of 0.8, 1.1, and 3.9 nm for samples AgAu-1, AgAu-2, and AgAu-3, respectively. Figure 9 shows the TEM image of Ag-Au core-shell nanoparticles (AgAu-3) prepared by ablation of Au in Ag colloidal solution for 20 min. From TEM image analysis, we obtained a 4-nm thickness of Au shell which is very close to thickness of 3.9 nm calculated from the absorption measurement. Figure 10a shows absorption spectra of Ag-Cu NPs generated by a sequential ablation in water. On increasing ablation time of Cu in Ag colloidal solution, the LSPR peak of Ag shifts towards longer wavelength with reduced intensity. Also, an increasing in absorption in the wavelength region 450 to 700 nm can be seen. The Mie theory simulations of absorption spectra for Ag-Cu NP with fixed silver core of 20 nm and shell of 0 to 5 nm are shown in Fig. 10b. Though results are

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consistent with experimental observations, the simulated LSPR peaks are generally narrow because simulations were done for single particle, whereas experimentally generated

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Fig. 11 TEM image of Ag-Cu nanoparticles generated by ablation of Cu plate in Ag colloidal solution for 40 min

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particles have significant size dispersion. Further, it should be noticed that the plasmonic response of Ag is significantly larger than Cu; therefore, LSPR peak of Cu in Ag-Cu coreshell NPs is weak and appears only as featureless absorption in higher-wavelength region. A closer look at Ag peak reveals that with increasing ablation time of Cu in Ag colloidal solution, the Ag peak weakly redshifts; the LSPR peak around 400 nm shifted to 409 nm, along with increase in broadening from 65 to 81 nm for ablation of Cu for 40 min. Similar features are also seen in Mie theory simulations of Ag-Cu with increase in shell thickness and confirm formation of an Ag-Cu core-shell structure. Figure 11 shows the TEM image of Ag-Cu prepared by ablation of Cu for 40 min. The TEM image shows that, unlike Au, Cu surface coating on Ag core is highly nonuniform due to a significant lattice mismatch between Ag and Cu. The refractive index sensing properties of Ag-Au core-shell nanoparticles was studied for interacting core and shell plasmon ω−+ mode (Ag Channel) and ω+ mode (Au channel) in different surrounding media. Here, for the Ag channel, we found that RIS increases from 144 to 199.5 nm/RIU (~56 nm/ RIU) with an increase in Au shell thickness to 10 nm, as shown in Fig. 12a. Along with the increase in RIS, the FWHM of LSPR peak increases from 25 to 145 nm (~120 nm). Large broadening of Ag channel can adversely affect FOM of the Ag channel. As shown in Fig. 12b, the FOM decreases continuously with increasing Au shell thickness. The strength of Au channel increases with shell thickness, leading to two important features. Firstly, the line broadening decreases with increase in shell thickness up to 4 nm; after that, it starts to increase with particle size due to retardation effect. Secondly, RIS increases consistently with Au shell thickness. As a result, the quality of the sensor improves

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significantly. The FOM corresponding to Au channel also increases with increase in shell thickness, but for thicker shell, it reduces marginally due to increase in FWHM. Even though FOM of the Au channel increases with shell thickness, its value is much lower than that in the Ag channel. In the Ag channel, FOM decreases from 5.5 to 1.5, whereas in the Au channel, FOM increases from 0 to 1.1 with shell thickness from 0 to 5 nm. For Ag-Cu core-shell nanoparticles, the peak corresponding to Cu is broad and nearly absent due to the strong plasmonic damping in Cu; thus, Cu channel is not suitable for sensing purpose. However, for Ag channel, RIS is found to increase with increase in shell thickness from 0 to 5 nm accompanied with increase in FWHM and reduction in FOM. Compared with Ag-Au nanoparticles, the FOM for Ag channel of Ag-Cu nanoparticles is lower as shown in Fig. 12b. From the above analysis, it is clear that Ag-Au is a better refractive index sensor than Ag-Cu with additional advantage of two wavelength channels for sensing. In Ag channel, maximum RIS is obtained for AgAu-2 (shell thickness ~1.1 nm) and minimum for pure Ag nanoparticle AgAu-0.

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The FOM of Ag channel is expected to decrease consistently with increase in Au shell thickness; however, maximum FOM was obtained for AgAu-0 and AgAu-2 as shown in Fig. 13. For the Au channel, the RIS is 29.4 and 44.1, broadening is 46.7 and 41 nm, and FOM is 0.63 and 1.07 for samples AgAu2 and AgAu-3, respectively.

Conclusions The size-dependent refractive index sensitivity of spherical Ag, Au, and Cu nanoparticles was calculated using the Mie theory. The simulation results reveal that RIS and FOM of Ag nanoparticles were higher than Au and Cu nanoparticles. Also, RIS and FOM were calculated for bimetallic Ag-Au and Ag-Cu core-shell nanoparticles exhibiting two LSPR peaks corresponding to hybridization of core and nanoshell plasmons. A sequential laser ablation method was used to generate bimetallic Ag-Au and Ag-Cu core-shell nanoparticles in water and the LSPR peak shift and broadening were monitored in different refractive index liquids. For Ag-Au nanoparticles with Au shell of 1–2 nm, optimum RIS and FOM in lower-wavelength Ag plasmon channel was observed. Bimetallic Ag-Au nanoparticles showed better sensing properties than Ag-Cu nanoparticles. The Au shell improves the chemical stability of Ag NPs and provides higher index sensitivity at optimum thickness. Further, in higherwavelength Au plasmon channel, both RIS and FOM increase with shell thickness, but their values were lower than those in the Ag plasmon channel.

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