Impact of Core Dielectric Properties on the Localized Surface

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The Impact of Core Dielectric Properties on the Localized Surface Plasmonic Spectra of Gold-coated Magnetic Core-shell Nanoparticles Elise Chaffin, Saheel Bhana, Ryan T. O'Connor, Xiaohua Huang, and Yongmei Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp505202k • Publication Date (Web): 10 Jul 2014 Downloaded from http://pubs.acs.org on July 13, 2014

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The Journal of Physical Chemistry

The Impact of Core Dielectric Properties on the Localized Surface Plasmonic Spectra of Gold-coated Magnetic Core-shell Nanoparticles Elise Chaffin, Saheel Bhana, Ryan O’Connor, Xiaohua Huang, and Yongmei Wang∗ Department of Chemistry, The University of Memphis, Memphis, TN, 38152

∗ Author for correspondence: [email protected]

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ABSTRACT. Gold-coated iron oxide core-shell nanoparticles (IO-Au NPs) are of interest for use in numerous biomedical applications due to their unique combined magnetic-plasmonic properties. Although the effects of the core-dielectric constant on the localized surface plasmon resonance (LSPR) peak position of Au-shell particles have been previously investigated, the impact that light-absorbing core materials with complex dielectric functions have on the LSPR peak are not well established. In this study, we use extended Mie theory for multi-layer particles to examine the individual effects of the real and imaginary components of core refractive indices on Au-shell NP plasmonic peaks. We find that the imaginary component dampens the intensity of the cavity plasmon, and results in a decrease of surface plasmon coupling. For core-materials with large imaginary refractive indices, the coupled mode LSPR peak disappears, and only the anti-coupled mode remains. Our findings show that the addition of a non-absorbing polymer layer to the core-surface decreases the dampening of the cavity plasmon and increases LSPR spectral intensity. Additionally, we address apparent discrepancies in the literature regarding the effects of Au-shell thickness on LSPR peak shifts.

KEYWORDS: Mie Theory; Nanotechnology; Optics; Iron oxide; Cobalt


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INTRODUCTION. Noble metal nanoparticles (NPs) have received considerable interest during the last two decades, in fields ranging from materials science and biology to medicine, due to their unique optical, electronic and catalytic properties.1–6 In particular, gold-coated magnetic nanoparticles have shown promise in a number of applications as a result of their unique combination of plasmonic and magnetic properties. A magnetic core enables the use of Au-shell NPs in applications such as magnetic separation, magnetic field-guided drug delivery, and magnetic resonance imaging (MRI).7,8 A gold coating facilitates the surface conjugation of various biomarkers and provides the chemical inertness that make these nanoparticles excellent candidates for immunotargeting. Additionally, gold exhibits unique optical properties that stem from the surface plasmon resonance (SPR) effect. SPR is the coherent oscillation of free electrons on the surface of noble metals upon exposure to the oscillating electromagnetic fields of incident light.9–11 For noble metal nanoparticles with a total particle sizes smaller than the wavelength of incident light, the collective oscillation of the free electrons in the particle is referred to as localized surface plasmon resonance (LSPR). Because of LSPR, noble metal NPs display strong combined absorption-scattering spectra, most commonly, with absorption in the visible region.12 The increase in the optical absorption efficiencies of these particles has been observed to be ~105-106 times stronger than that of photo-absorbing organic dyes.13 As a result of their characteristic LSPR properties, Au NPs have been used in numerous applications including sensing, imaging, and hyperthermia treatment.14–18 The unique combination of magnetic and plasmonic properties makes gold-coated magnetic NPs extremely attractive for numerous biomedical applications. Hence, there have been many synthesis efforts aiming to control the size and shape of gold-coated NPs.16,17,19–33


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It has been recognized that plasmonic optical properties of nanoshells with dielectric cores and noble metal shells are tunable through the variation of core size and shell thickness.13,34–38 One such system that has been previously investigated is the silica-core gold nanoshell. A red shift of the LSPR peak as large as 300 nm has been observed when the Au-shell thickness was decreased to only a few nanometers.34 Theoretical calculations using extended Mie theory have demonstrated the existence of a universal scaling relationship between the shift of the LSPR peak and the thickness of the Au-shell with respect to the core size.39,40 According to this relationship, a Si-Au core-shell nanoshell with a 60nm total diameter and 5nm Au-shell should have an LSPR peak in the near-infrared region (λ>750nm) and a larger core with the same Au-shell thickness would give rise to an LSPR peak at an even longer wavelength. Iron oxide (IO) NPs are the most commonly used magnetic nanoparticles in biomedical applications. Iron oxides have several different chemical compositions: iron(II) oxide, known as wüstite (FeO), and the iron(II,III) oxides, known as magnetite (Fe3O4) and maghemite (γ-Fe2O3) in the oxidized form. By comparing the dielectric properties of these magnetic NPs with those of silica-core NPs, a major difference can be observed. Magnetic NPs have complex dielectric functions, i.e., the relative permittivity (εr), of iron oxides have both real and imaginary components, = ′ + ′′ ; the imaginary component accounts for the absorption of light by the material. In contrast to iron oxides, which absorb light in the visible spectral region, silica is virtually transparent to visible light and has a near-zero imaginary component. Although the effects of shell thickness on the observed plasmonic peak of silica-Au NPs have been investigated by Jain et al.,36 the impact of complex core dielectric properties on the LSPR of core-shell particles is not well understood. Previously, Levin et al. discussed how the dielectric properties of the core impact the plasmonic properties, but the investigation focused on Au-shell


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particles with only real dielectric cores. Based on their results, they have suggested that there are two predominant trends in the shifts of plasmonic peaks that are dependent on the value of the core dielectric constant and the thickness of the Au-shell. According to their theory, a core with a small dielectric constant corresponds to a blue shift of the plasmonic peak with the increase of the Au-shell thickness; whereas, a core with a large dielectric constant results in a red shift of the plasmonic peak.27 Careful examination of reported absorption spectra of gold-coated iron oxide (IO-Au) NPs reveals that there have been no spherical IO-Au NPs synthesized that express strong near-infrared (NIR) absorption properties; only anisotropic IO-Au NPs have exhibited NIR absorption. In order to achieve the synthesis of spherical IO-Au NPs with NIR absorption, Jin et al. suggested inserting a polymer layer between IO-core and Au-shell. This approach resulted in IO-Au NPs that exhibited broad, low-intensity absorption peaks that extended into the near-infrared region.17 The effects of this polymer layer on the plasmonic properties of Au-shell NPs are not well understood. These earlier studies have been the motivation for our further examination into the effects of magnetic-core dielectric properties on the plasmonic spectra of Au-shell magnetic NPs.

THEORETICAL METHODS. In this study, we have used extended Mie theory to calculate the scattering and absorption of electromagnetic plane waves by dielectric spheres as described by the Lorenz-Mie solution to the Maxwell equations.41 By requiring continuous tangential components of the electric and magnetic fields across the boundary of two dielectric media, this theory enables the solution of the scattering of electromagnetic waves by single-component, spherical particles. In 1951, Aden and Kerker extended this theory in order to study two-layered spheres; further improvements were made by others.11,42,43 A subsequent extension of the theory


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to a multi-layered sphere was introduced by Bhandari et al,44 and an efficient recursive algorithm was developed by Wu and Wang to improve computation efficiency.45 Further improvements have been applied to the algorithm of Wu et al, but are only necessary for particles having size parameters much larger than the particles of interest in our investigation.46 Using the recursive algorithm developed by Wu and Wang,45 we have developed a code that has been implemented in Mathematica® 9. In brief, according to traditional Lorenz-Mie Theory, the extinction efficiency, Qext, can be found according to equation (1),

∞

4 = 2 + 1 +

(1a)

=

, =

! "

(1b)

where Np and Nm are the complex refractive indices of the particle and medium respectively, and R is the particle radius; an and bn are coefficients found from the boundary conditions according to equation (2), # $ # − # #$ # $ & − # &$

(2a)

# $ # − # #$ = # $ & − # &$

(2b)

=

where ψn and ζn are Ricatti-Bessel functions. For a multi-layered sphere, the recursive algorithm developed by Wu and Wang allows for the consecutive determination of these parameters for each dielectric layer. The extinction efficiency is found from the coefficients, an and bn, in the external dielectric medium. Our implementation of this algorithm has been tested through the successful reproduction previously published extended Mie theory results for Au-shell NPs (data


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not shown). The refractive index values of gold and cobalt were taken from Johnson and Christy,47,48 and the refractive index values of magnetite were taken from Goossens et al.49 We have chosen to use the refractive index of iron oxide in the magnetite (Fe3O4) form in our calculations. Other iron oxides, maghemite (γ-Fe2O3) and wüstite (FeO), have similar complex refractive index functions in the wavelengths of interest, thus, would be expected to have similar LSPR spectra and were therefore not investigated in this study.50,51

RESULTS AND DISCUSSION. Figure 1 presents the LSPR spectra of 50nm diameter coreshell nanoparticles with 5nm Au-shells and 40nm diameter cores. We have included the LSPR spectra for three core materials: hollow, which has a refractive index equal to the surrounding aqueous medium, silica, and magnetite, along with the spectrum of a 50nm diameter solid gold NP. When compared with the spectrum of the 50nm solid gold NP, the LSPR peaks of the coreshell NPs are red-shifted. The peak shifts of the hollow-core and silica-core particles are similar in magnitude; however, the LSPR peak of the magnetite core particle exhibits an even larger red shift. In contrast, the LSPR peak intensity of the magnetite-core NP is significantly lower than those of the hollow-core and silica-core NPs, although we note that its peak intensity is comparable to that of the solid-Au NP. It is apparent that the dielectric properties of the core materials can significantly impact both the peak position and intensity of LSPR spectra.


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Figure 1. LSPR spectra of core-shell particles with total diameters of 50nm, 5nm Au-shells, and 40nm diameter cores with core materials of magnetite (IO), silica, water (hollow), or gold (a solid Au NP). As was alluded to earlier, the dielectric properties of each of these core materials are rather different. According to classical electromagnetic theory, the dielectric constant is related to the refractive index of the material through the equation, = √ ) , where µr is the relative permeability (µr ≅ 1 for most materials in the visible spectrum) and εr is the relative dielectric permittivity, generally referred to as the dielectric constant. As previously introduced, nontransparent materials have complex refractive indices; both gold and magnetite have non-zero imaginary refractive index components. Silica, on the other hand, is essentially non-absorbing and has only a real refractive index with a value of 1.46. In order to assess the individual effects of the real and imaginary refractive index components of the core material on the LSPR spectrum, we have computed the spectra of core-shell NPs, identical in size, but with the imaginary components of the core refractive indices set to zero and the values of the real components varied. Data is shown in Figure 2.


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Figure 2. LSPR spectra of core-shell particles with total diameters of 50nm, 5nm Au-shells, and 40nm diameter cores with refractive indices varied from 1.33 + 0i to 3 + 0i. Values for real (Re) and imaginary (Im) components of refractive indices are shown in the legend. The spectra of a solid-gold NP, an IO-Au NP, and an IO-Au NP with only the real part of the IO refractive index are shown for comparison. The LSPR peaks are consistently red shifted and the intensity of the LSPR peaks is decreased slightly as the value of the real refractive index is increased. It should be noted that the real part of the refractive index of magnetite varies with the wavelength of incident light and has a minimum of 1.56 and a maximum of 2.43 in the visible-infrared range. The LSPR peaks for the absorbing and non-absorbing magnetite-core NPs are nearly identical; however, the intensity of the absorbing magnetite-core NP is substantially lower. This data clearly demonstrates how the LSPR peak position is affected by the real component of the core refractive index, the larger the real refractive index, the larger the red shift of the LSPR peak. Although Figure 2 may appear to signify that the LSPR peak position is solely dependent on the real component of the refractive index with imaginary component impacting only the LSPR peak intensity, this is not entirely true. In the previous example, the wavelength-dependent imaginary component of magnetite’s refractive index is small, ranging from 1.2 to 0.5; hence, its


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influence on the LSPR peak position is negligible. Figure 3a presents the LSPR spectra for coreshell nanoparticles, identical to those in Figure 2, but with uniform imaginary refractive indices of one. By comparing the spectra in Figure 3a with the analogous spectra in Figure 2, it can be clearly seen that the introduction of a small imaginary component to the refractive index, results in blue shifting and significant broadening of the LSPR peak. Figure 3b provides a clear comparison between the LSPR spectra for NPs with complex core refractive indices and NPs with only real core refractive indices.

Figure 3. LSPR spectra of core-shell particles with total diameters of 50nm, 5nm Au-shells and 40nm diameter cores (A) Core refractive indices were varied from 1.33 + i to 3 + i. (B) Core refractive indices with the real components varied from 1.33 to 3 and imaginary components of either 0 (solid) or 1 (dashed).


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Previously, Prodan et al. proposed a hybridization model to explain the optical properties of core-shell nanoparticles.37,52 According to this model, when a dielectric core is introduced into a spherical gold particle, surface plasmons are formed at the inner, shell-core interface (cavity) and on the outer gold surface (sphere). This model, which is analogous to an atomic orbital hybridization model, suggests that, in core-shell nanostructures, the sphere and cavity plasmons hybridize to form a red-shifted, (lower energy) coupled plasmon mode and a blue-shifted, (higher energy) anti-coupled mode. The extent of the plasmon coupling is dependent on the particle shell-thickness, the thinner the shell, the stronger the coupling and the greater the energy difference between the two resulting plasmon modes. The spectra in Figures 2 and 3 exhibit only the red-shifted, coupled plasmon mode; the anti-coupled mode is not observable. It should be noted that the shift of the LSPR peak in Figure 2 is not due to a change in shell thickness, but rather, a difference in the values of the core dielectric constants. An increase in the real refractive index of the core material generates a lower frequency cavity plasmon; thus, a red shift of the LSPR peak is observed. This hybridization model can also be applied to explain the observed effects of the imaginary component of the core refractive index on the LSPR peak. A core material with a complex refractive index decreases the intensity of the cavity plasmon, dampening the cavity plasmon mode. One would expect the extent of coupling between the cavity and sphere plasmon modes to decrease when the imaginary component of the refractive index is not negligible. By comparing the LSPR spectra of Au-shell NPs with real dielectric cores with identical Au-shell NPs with complex dielectric cores (shown in Figure 3b), it can be seen that the LSPR peaks of the particles with complex dielectric cores are significantly broadened and slightly blue-shifted. This suggests a decrease in coupling between the cavity and surface plasmon modes.


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Although the anti-coupled mode is not typically visible in the LSPR spectra, it can be observed in certain instances when the imaginary component of the core refractive index is large. Figure 4 presents representative calculated spectra of core-shell NPs with fixed real refractive indices and imaginary refractive index components varied from 0-3, along with the spectrum of a solid-Au NP for comparison. As the imaginary component is increased, a shoulder peak appears at around 500nm. This peak corresponds to the anti-coupled mode and has a frequency that is higher than the LSPR peak of a solid-Au NP. When the imaginary refractive index has a value between 1 and 1.5, two LSPR peaks can be clearly seen, one for the coupled mode and the other for the anticoupled mode. Upon further increase of imaginary refractive index, only the anti-coupled mode remains visible.

Figure 4. LSPR spectra of core-shell particles with total diameters of 50nm, 5nm Au-shells, and 40nm diameter cores with their refractive indices varied from 3 + 0i to 3 + 3i, along with the spectrum of a solid Au NP. The spectra of the 3+0i and solid Au NPs have been reduced to 30% intensity for comparison purposes. Similar to iron oxides, cobalt metal is known to have strong magnetic properties. In fact, the saturation magnetization of cobalt is significantly higher than both magnetite and maghemite.53 Because of cobalt’s strong magnetic properties, some work has been done to synthesize gold-


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coated cobalt (Co-Au) NPs with the objective of retaining the magnetic properties of cobaltmetal NPs without retaining their toxicity.53–56 We have computationally investigated the optical properties of Co-Au core-shell NPs. However, because cobalt has a large imaginary refractive index, for Au-shell thicknesses larger than 7.5nm, only a single, low-intensity LSPR peak that blue shifts with decreasing shell thickness is visible (Figure 5); this peak corresponds to the anticoupled plasmon mode. In the earlier study of Wang et al. a single peak that blue-shifted with a decrease of plasmonic shell thickness was observed for the case of Co-Ag core-shell NPs both experimentally and computationally using extended Mie theory. They noted that a blue-shift with decreasing shell thickness distinctly contrasted the experimental results of Jain et al. for IO-Au NPs.38,57 Our results indicate that these opposing trends can be justified by noting the dominant plasmon hybridization mode is the anti-coupled mode for Co-core NPs and the coupled mode for IO-core NPs. For shell thicknesses less than 7.5nm, a small peak can be seen at around 950nm, corresponding to the coupled plasmon mode. The absence of a coupled mode peak in the LSPR spectra of Co-Au NPs with thicker shells suggests that, due to the strong damping of the cavity plasmon by the cobalt core, there is little to no coupling between the two surface plasmons. As the thickness of the Au-shell is decreased, the distance between the two plasmons is also decreased and results in increased coupling between the two plasmon modes and the appearance of a second LSPR peak in the infrared region. However, the intensity of this peak is very weak, which suggests that these particles would be of little value for certain applications, such as photothermal therapy.


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Figure 5. LSPR spectra of (A) core-shell particles with total diameters of 50nm, 5nm Au-shells, and 40nm diameter cores with core materials of magnetite (IO), silica, water (hollow), or gold (a solid Au NP); the hollow and silica core NP spectra have been reduced to 25% intensity for comparison. (B) 50nm total diameter Co-Au NPs with Au-shell thicknesses varied from 5nm to 17.5nm. The influence of the Au-shell thickness on the LSPR spectra of core-shell NPs with silica or hollow cores has been studied previously; Jain et al. have shown that as the thickness of the Aushell layer is decreased, the magnitude of the observed LSPR red-shift increases, not according to the absolute Au-shell thickness of the NP but, rather, according to the ratio of Au-shell thickness to core radius.36 The LSPR peak shift can be fitted to a universal scaling equation (3),


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*+⁄+, = 0.97 ⋅ exp−6⁄ ⁄0.18

(3)

where λ0 is the corresponding gold sphere LSPR peak maximum, λ is the LSPR peak maximum of the core-shell particle (where only the coupled hybridization mode is observed), t is the Aushell thickness and R is the core radius. Figure 6a shows the LSPR spectra for 50nm total diameter IO-Au NPs with different shell thicknesses; the LSPR peak red shifts as the thickness of the Au-shell is decreased. This trend is consistent with the extended Mie Theory results of Brullot et al. for large (140nm total diameter) IO-Au NPs of varying shell thickness and is similar to red shift observed for silica-core and hollow NPs.36,58 Additionally, we have determined a similar scaling relationship between *+⁄+, and t/R for IO-Au NPs as shown in Figure 6b. The data in Figure 6b have been fitted to the exponential decay equation (4), *+⁄+, = 0.73 ⋅ 9−6⁄ ⁄0.39.

(4)

Comparing equation (4) with the universal relationship for silica-core NPs presented by Jain et al., the equation for IO-Au NPs has a larger decay constant, consistent with LSPR peaks that are more red-shifted than those of Si-Au NPs having identical shell-thicknesses and core radii.


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Figure 6. (A) LSPR spectra of 50nm total diameter IO-Au NPs with Au-shell thicknesses ranging from 5nm to 17.5nm. The spectrum of a 50nm diameter solid Au NP has been included for comparison. (B) The fractional shift (∆λ/λ0) of the LSPR peak maximum for three sizes of IO-Au NPs with varied shell thicknesses and core-radii (t/R). The points have been fitted to an exponential decay curve of the form *+⁄+, = ⋅ 9−6⁄ ⁄ where a = 0.73 ± 0.03 and b = 0.39 ± 0.02 (R2 = 0.987) The above results suggest that, for IO-Au NPs, a blue shift of the LSPR peak will be observed with an increase in the thickness of the Au-shell layer in a similar manner as for Si-Au NPs. However, this concept may not directly apply to typical IO-Au NP synthesis. In the above discussion, the total diameters of the particles were fixed while the shell thicknesses and core radii were varied concurrently. In typical IO-Au NP synthesis, the IO-core diameter remains constant; as the Au-shell thickness increases, the total particle size increases as well. If the Aushell thickness is small with respect to the radius of the IO-core, as expected, a blue shift of the LSPR peak will be observed with an increase of the Au-shell thickness. However, as the Aushell layer continues to grow, a red-shift of the LSPR peak can occur. Figure 7a shows the LSPR spectra for a series of Au-shell NPs with 15nm diameter IO-cores and varying shell thicknesses. Initially, as the shell thickness is increased, a corresponding blue shift of the LSPR peak is


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observed, but, as the Au-shell continues to grow, the LSPR peak begins to shift to longer wavelengths. Because of this effect, both trends have been observed in IO-Au NP synthesis literature, resulting in, what appear to be, contradictory conclusions on the effect of the Au-shell thickness on the LSPR peak shift.17,21,22,24,26,30–33,59 Although the red-shift of the LSPR peak with increasing Au-shell thickness may seem to contradict the universal scaling of equation (4), when we use the LSPR peak position of solid-Au NPs with identical particle diameters, the observed peak shifts do in fact adhere to the universal scaling equation. Figure 7b shows the fractional LSPR shifts of these particles, as compared with solid AusNPs of the same diameters with the line of best fit from Figure 6b. The data points have been fit to this equation with an R-squared value of 0.990. The observed red shift of the LSPR peak with increasing shell thickness, as shown in Figure 7a, is actually due to the increase in overall particle diameter, rather than the increase in the absolute thickness of the Au-shell layer.


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Figure 7. (A) LSPR spectra of IO-Au NPs with 15nm diameter cores and Au-shells varied from 2.5nm to 30nm. The spectrum of a 50nm diameter solid Au NP has been included for comparison. (B) The fractional shifts (∆λ/λ0) of the LSPR peak maximums of the IO-Au NPs from A. The data have been fitted to the exponential decay curve from Figure 6b (R2 = 0.990). Jin et al. proposed the introduction of a polymer gap between the IO-core and Au-shell to promote a red-shift of the LSPR spectra into the NIR region.17 We have therefore examined the effects of the addition of a thin (3nm), non-absorbing polymer layer on the LSPR spectra of IOAu NPs. We have assigned the polymer layer a real refractive index of 1.55 with no imaginary component. Figure 8a compares the spectra of IO-Au NPs without polymer layers with the spectra of IO-Au NPs containing 3nm polymer gaps, but maintaining equivalent Au-shell thicknesses and overall particle diameters (i.e., the IO-core diameter was decreased to accommodate the polymer layer). In this scenario, the addition of a polymer layer results in a blue shift of the LSPR peak and a simultaneous increase in the LSPR peak intensity. Because the polymer layer does not have a complex refractive index, the presence of a polymer gap reduces the damping of the cavity plasmon by the IO-core, hence restoring the intensity of the LSPR peak. The blue shift of the LSPR peak can be attributed to the fact that the polymer has a real refractive index of 1.55, which is smaller than that of the IO-core. However, even with the


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presence of a 3nm polymer gap, the IO-core still impacts the LSPR spectrum. This can be observed by comparing the spectrum of an IO-Au NP containing a polymer layer with the spectrum of an Au-shell NP with a core composed entirely of the polymer. In the latter spectrum, the peak position is additionally blue-shifted and the intensity is further enhanced. In contrast, in Figure 8b, we have considered the effects of inserting a polymer gap in an alternate scenario. We have fixed the IO-core diameter and Au-shell thickness, and examined the effects of inserting a 3nm polymer layer. In this scenario, the insertion of the polymer layer results in an increase of the total particle size. For the NPs with the inserted polymer gap, small red shifts and substantial increases in the intensities of the LSPR peaks are apparent. These red shifts are primarily due to the net increase of the core radii (i.e. the core comprises both the IO-core and the polymer layer) upon the introduction of the polymer layer. This results in a decrease of t/R and a slight red shift of the LSPR peak. This scenario better mimics a typical experimental setup; hence, the insertion of a polymer gap works to restore the intensity of the LSPR peak and can facilitate a red shift into the NIR region. Since IO NPs smaller than ~30nm exhibit superparamagnetic properties, it is desirable to use particles with IO cores 30nm or smaller to avoid potential magnetic agglomeration. The addition of a polymer layer between the IO-core and the Au-shell, should allow for tuning to longer wavelengths through a decrease of t/R, without necessitating an increase of the IO-core size or the fabrication of a very thin Au-shell coating. Direct comparisons of calculated LSPR spectra to those of synthesized NPs, such as those presented by Jin et al.,17 are often difficult due to varying degrees of sample heterogeneity. The absorption spectra of Jin et al. consist of broad peaks extending into the NIR region; the broadness of these peaks can likely be attributed to the range of Au-shell thicknesses for each NP sample (~1-2nm, ~2-3nm, and ~4-5nm). Their reported peak maxima of ~900nm, ~760nm, and ~660nm respectively


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compare favorably with calculated peak maxima of 925nm, 785nm, and 687nm assuming 1.5nm, 2.5nm, and 4.5nm Au-shell thicknesses corresponding to the three NP samples.

Figure 8. (A) LSPR spectra of 50nm NPs with 5nm Au-shells and: a 40nm diameter IO core, a 40nm diameter polymer (1.55 + 0i) core, or a 34nm diameter IO core with a 3nm polymer layer. (B) LSPR spectra of Au-shell NPs with 30nm IO cores, varied Au-shell thicknesses, with or without 3nm polymer layers CONCLUSIONS. In this study, we have examined the impact of the core dielectric properties on the LSPR spectra of Au-shell magnetic NPs (summarized in Table 1). We have investigated the individual effects of the real and imaginary components of core refractive indices on Au-shell NP LSPR spectra. Results indicate that an increase in the real core refractive index corresponds to a shift of the coupled plasmon mode peak to longer wavelengths. The introduction of an


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imaginary core refractive index component results in a significant broadening of the LSPR peak and a small blue shift of the LSPR coupled mode. As the imaginary component is further increased, the LSPR peak corresponding to the coupled plasmon mode weakens and eventually disappears at larger values; conversely, the anti-coupled mode LSPR peak becomes apparent near 500nm. Because of the disappearance of the coupled-mode peak for larger imaginary core refractive index values, our results have predicted that the LSPR spectra of Co-Au NPs would exhibit a single, low intensity peak corresponding to the anti-coupled plasmon mode. These weak optical properties suggest that, although Co-Au NPs exhibit strong magnetic properties, they may not be advantageous for applications that require particles with specific LSPR activity, such as photothermal therapy. Table 1. Summary of dielectric property effects on the LSPR spectra of Au-shell NPs. core refractive index real

imaginary

particle dimensions total Au-shell diameter thickness

LSPR spectra

core diameter

polymer gap

primary mode

peak shift

intensity change

a

0

=c

=

=

No

coupled

red

1

=

=

=

No

coupled

red

=

0-1

=

=

=

No

coupled

blue

coupled

blue

=

=

=

No anti-coupled

red

slight

red blue blue (thin shell) red (thick shell) blue red

slight NA

Figure 5b

Figure 7

Figure 8a

0 to ~1.5 3 > ~1.5 Cobalt IO

= =

No No

anti-coupled coupled

IO

=

No

coupled

3nm = 3nm = = a b c : increasing, : decreasing, =: constant

coupled coupled

IO IO

=


b

corresponding data

with broadening with broadening with broadening

Figure 2 Figure 3a Figure 3b

Figure 4

Figure 6

Figure 8b

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We have further demonstrated that a universal scaling relationship, correlating the plasmonic peak shift to the Au-shell thickness to core radius ratio, exists for IO-Au NPs, and is comparable to the previously observed universal scaling of Si-Au NPs.36 According to this universal scaling relationship, if the total particle size is fixed, the LSPR peak for IO-Au NPs will always exhibit a blue shift with an increase of the Au-shell thickness. However, in an experimental setting, often only one size of IO-core NPs is used for Au-shell coating. As a result, it is possible to observe both red and blue shifts of the LSPR peak as the Au-shell thickness increases upon coating; the direction of the LSPR shift is dependent on both the total particle size as well as the Au-shell thickness. Therefore, the apparent directional change of the LSPR peak shift does not necessarily correlate with differences in core dielectric properties. Finally, we have investigated the impact on the LSPR spectrum of IO-Au NPs of introducing a polymer gap, as in the synthesis efforts of Jin et al.,17 on the plasmonic spectra of IO-Au NPs. The insertion of a polymer layer effectively increases the core-diameter, resulting in the decrease of t/R and a red shift of the LSPR peak. Moreover, the polymer layer enhances the intensity of the LSPR peak by shielding the absorption of the IO-core. This implies that the insertion of a polymer layer can be an effective method to prepare stronger NIR-absorbing IO-Au NPs without the synthesis of IO-Au NPs with very thin Au-shells or the use of larger IO-cores, which can be disadvantageous due to potential magnetic agglomeration. ACKNOWLEDGMENT. This work is partially supported by The University of Memphis FedEx Institute of Technology Innovation Fund and the National Science Foundation through TN-SCORE (Grant EPS1004083). We also acknowledge the use of the high performance computing facilities at The


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University of Memphis. The current work was presented at the SERMACS 2013 meeting during a special symposium honoring Prof. Mostafa El-Sayed.


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TABLE OF CONTENTS GRAPHIC


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Impact of Core Dielectric Properties on the Localized Surface

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