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The plasmon resonance properties of gold nanoparticles (AuNPs) in liquid ionic medium with different concentration was investigated using a UV-visible/NIR spectrophotometer. The optical absorption spectra of AuNPs suspended in NaCl solutions in the range of 400 – 800 nm were measured; specifically, 10 nm bare AuNPs separately suspended in NaCl solution with molar concentrations of 0.01M and 1M resulted in different shapes and widths in the plasmon resonance peak. The optical spectral resonances were analyzed using the Mie theory, Lorenz-Drude size-dependent dielectric functions and effective medium theory. Analysis of the spectral features by means of theoretical models showed that the damping of the plasmon resonance of AuNPs in ionic medium is size dependent. The experimental results indicated that the effect of chemical interface damping is more prominent compared with the inherent size effects of AuNPs.
© 2017 Optical Society of America
1. Introduction
The environmental influence on bare nanoparticles (NPs) has attracted much interest due to their promising potential in nano-bio. interactions [1], antimicrobial agents [2,3] and clinical applications [4–7]. It is well known that spherical gold nanoparticles (AuNPs) have strong localized surface plasmon oscillation properties [8, 9]. The tunable absorption at different wavelength such as 530 nm or near infrared 650-900 nm is used to kill cancer cells [10]. As is known, the surface plasmon oscillation peak of AuNPs strongly depends on the refractive index and the dielectric properties of the surrounding media which is characterized by the dielectric constant εm. The refractive index of the media (nm) is related to the dielectric constant via nm = εm1/2. A number of experimental [9, 11–17] and theoretical [14, 16–18] studies have been conducted to clarify the refractive index and the dielectric sensitivities of AuNPs. Despite studies of refractive index of Alkanethiol-capped and dielectric constant of surfactant coated AuNPs [16, 18], there has been limited information on the influence of the isotropic ionic liquid media (i.e., NaCl liquid) and chemical interface damping on refractive index of bare AuNPs [16]. Most studies have only focused on the ionic media effects on AuNP hydrodynamic size and absorption spectra. There has been no detailed investigation of the role of hydrodynamic size on the refractive index. Using the Mie theory, one can predict the peak position of plasmon resonance. However, the application of the Mie theory predicting the shape and width of the peak in ionic environment (i.e., NaCl liquid) is problematic due to chemical interface damping (CID) [19–21]. CID depends on the chemical properties of the energy transfer and the interface between NP clusters and surroundings media by charge transfer reaction [22]. The presence of adsorbate material or matrix can persuade electronic states on the surface of nanoparticles that rise to CID. The effect of CID on the plasmon resonance can be observed on the linewidth of the absorption plasmon resonance. The classical Mie theory is based on classical electrodynamics, particularly solid particles that are mostly homogeneous spherical in non- absorbent surrounding medium. Therefore, the electromagnetic interaction with other macroscopic particles in surrounding medium does not consider [23]. In addition, the skin depth in Au metals is 20 −30 nm [24,25]. For Au particles with a size of 10 nm, which is less than the range of skin depth the Au particles do not behave as a bulk for an electromagnetic wave [23].
This paper focuses on the investigation of the refractive index and plasmon resonance of AuNPs in an ionic surrounding media (i.e., NaCl liquid). Attention was paid to the size effects of AuNPs on the linewidth, the wavelength of plasmon resonance, and the damping and broadening of localized plasmon resonance of AuNPs in different NaCl concentration (0.01 M and 1 M). Two dielectric function models were employed to explain the observation of plasmon resonance. First, the Lorenz-Drude size dependency model was used to describe the dielectric function response of AuNPs in the visible spectral range, i.e. from 400 nm to 800 nm. Second, the effective medium model was used to interpret the dielectric function response of the AuNPs embedded into the NaCl liquid. With the aid of the effective medium theory, the plasmon resonance will be discussed in relation to the change in volume fraction of AuNPs (between 1% and 30%). Finally, the effect of CID on the plasmon resonance was investigated in relation to the refractive index responses of the AuNPs under ionic environment. The lowering and broadening of localized plasmon resonances of ligand free AuNPs was modeled theoretically and studied experimentally.
2. Experimental details
The AuNPs (brand Cytodiagnostic) with 10 nm particle size were purchased from Sciencewerke Co. (Singapore). Transmission electron microscopy (TEM) measurement was carried out using “Phillips CM12 with Docu Version 3.2 image analysis” to verify the sample size. The size distribution of the TEM image and the hydrodynamic size of Dynamic Light Scattering (DLS) for 10 nm AuNPs are shown in Fig. 1. Non-functionalized standard or reactant-free gold nanoparticles were used to optimize the high efficiency of the surrounding media. The 10 nm AuNPs were suspended in 0.1 mM phosphate buffered saline (PBS) and primary plasmon resonance was 520 nm. Our AuNPs were suspended in different concentration molarity 0.01 M and 1 M of NaCl solution. Sodium chloride (NaCl) is an ionic compound with Molar mass 58.44 g/mol (brand sigma-aldrich) and was used for different concentrations of the salt solution. An ATAGO digital refractometer RX-5000α was used to measure the refractive index of salt solutions. The refractive index for 0.01M and 1M NaCl solutions are 1.3331 and 1.3428 respectively. The hydrodynamic size distribution was monitored by Zetasizer (nanoZs) after nanoparticles were suspended in the salt solution. The absorption spectra in the wavelength range of 400 – 800 nm were measured using a UV-visible/NIR spectrophotometer (Shimadzu UV/VIS/NIR UV3600).
3. Theoretical models
The dielectric properties of AuNPs are related to the interaction between their free electrons with photon. Interband transitions, which occur in the bound electrons in an atom, and can change the form of dielectric function for intraband transitions. This effect manifests itself in the form of Mie bandwidth. In 1908, Gustav Mie solved Maxwell’s equations for a homogeneous sphere [26]. The dielectric function of metals is frequency-dependent and is denoted by ε (ω), where ε (ω) = εr (ω) + iεi (ω). Here, εr (ω) and εi (ω) are the real and imaginary parts of the dielectric function, respectively. According to the Mie theory, the position of the plasmon resonance peak is controlled by the real part of the dielectric function. In our case, the particle size was 10 nm which is much smaller than wavelength (400 ~800 nm). As a result, the particle behaves as a dipole, which contributes greatly to the dielectric function response [27]. The condition for the strong maximisation of the dipole resonance frequency to occur is εr (ω) = −2εm (ω) [28]. The broadening effect of the plasmon resonance is described by the resonance linewidth [known as the full width at half maximum value (FWHM)]. The FWHM is controlled by the imaginary part of the dielectric function, which occurs through the damping and dephasing process. In the case of metals (Au), the imaginary and real parts of dielectric function are influenced by size-dependent broadening and the frequency position of plasmon resonance, respectively [22].
When colloidal AuNPs are transferred into inorganic media (i.e., NaCl liquid), the overall optical response is changed. The changes of optical response correlate with the salt concentration in both solvent and optical properties of NPs. To quantify the change in optical properties, one may simulate the shape of the surface plasmon resonance band based on the Mie theory taking into account the effect of particle size. Generally, the mean free path of the conduction band electrons of metallic NPs is smaller than the bulk metal. This causes the surface plasmon absorption band to broaden. To apply Mie’s theory, a key step is to calculate the dielectric function of the materials. The correction for the size effect on the dielectric constant was proposed by Kreibig [29]. The size-dependent dielectric function for small particles ε (λ, r) [11] is:
where ω is the angular frequency of incident light and r is the particle radius. The first term and the third term in Eq. (1), i.e., ε bulk and ε free(r) are, respectively, the dielectric function of bulk and the dielectric function, taking into account the effects of particle size and interband. The second term is the dielectric function corresponding to the free electron contribution, which is described by the Drude model,The constant ε (∞) accounts for the high frequency dielectric constant that is not expressed in the visible range. The ε (∞) in the case of gold was 9.5 [28]. The plasma frequency ωp = (ne2 /ε0meff) for bulk gold was 9.03 eV [18, 22]. γbulk, n, meff are, respectively, the bulk relaxation frequency of the conduction electrons, electron density and effective electron mass. The classical theory of free electrons in metals explains how damping is caused by the scattering of electrons due to other electrons, phonons, impurities or lattice defects, and is given by the relationship γ0 = νF / l∞, where l∞ is the mean free path of the electrons in bulk metal and νF is the Fermi velocity (for Au νF = 1.4 × 108 cm/s). If the nanoparticle size becomes much smaller compared with the l∞ of the metal, the interactions between the electrons in the conduction band and the particle surface become considerable. This extra collision results in the decrease of the effective mean free path l and the increase in damping. In addition, all quantities such as n, meff, γ develop size dependence when the particle size becomes very small. Hence, a size-dependent term (AνF) /r is added for electron surface interactions [11,22] which is given by
where A describes information regarding the scattering processes and has taken 1.4 to account for the AuNPs [18, 22, 30]. The local changes that could influence the optical properties of the NP such as atomic distance and electron density near to the surface of particles may also affect ε (ω) [22]. Therefore, the size dependent dielectric function is [18, 22, 30] modelled byIndeed, ε (ω) influences the Mie plasmon resonance, generating complex spectra. The Mie theory provides a tool to calculate the optical properties of different- sized spherical particles [26]. The optical properties of homogeneous spheres are quantified by calculating absorption efficiency, scattering efficiency, total extinction efficiency (Qabs, Qsca and Qext) and the wavelength of their optical resonance λmax [26, 31]. In these calculations, it is necessary to provide some essential input parameters, including radius r of the nanospheres (for r = 2.5, 5, 10, 40 and 50 nm), the refractive indices of the surrounding medium (nm = 1.33) and the complex optical refractive indices ns (for metal AuNPs). The values of the complex dielectric function of gold at different wavelength have been established by Johnson and Christy for bulk [24] and corrected for NPs size [32, 33].
A study on the tunability of the optical refractive index of AuNPs dispersions has demonstrated that the refractive index depends on the volume fraction of AuNPs in the range of visible and near-infrared [18]. The effective medium theory (EMT) determines the optical response of both the nano-sized metallic particles, as well as the matrix material. Therefore, the complex optical refractive index of a solution of AuNPs can be calculated using effective medium theory. However, there are two limitations in EMT. First, it is assumed that scattering by NPs, which are smaller compared to the incident wavelengths, can be neglected. On the other hand, scattering for spherical metal NPs (even small) are not negligible. A second restricting assumption is that the particles must be well-dispersed, meaning that electromagnetic interactions between particles were omitted [23], which was not true. This electromagnetic interaction is taken into account as an average self-consistent field. In addition, the NPs concentrations must be below the percolation threshold concentration. The applicable EMT that overcomes the limitation assumption of EMT and considers the shape of NPs was represented by Maxwell Garnett in 1904. This equation is represented by Eq. (5) is for spherical NPs in which r << λ [23, 34].
4. Results and discussion
Figure 2 shows the calculated results of the Drude-Lorentz dielectric constant as a function of wavelength for five different gold nanosphere sizes (r = 2.5 nm, 5 nm, 10 nm, 40 nm and 50 nm). The Drude-Lorentz parameters for gold are taken from Rakic [30]. Figures 2(a) and 2(b) demonstrate the real and imaginary part of the dielectric constant as a function of wavelength and the results were calculated using Eq. (4). Figure 2(c) shows the total extinction efficiency Qext as a function of wavelength for these five differently-sized gold nanoparticles. In Fig. 2 (a), the flat spectra of n between 500 nm and 550 nm results in a broad absorption peak in Fig. 2(c). On the other hand, the steeper n yields the narrower absorption resonance, while there is a small red-shift in the plasmon resonance as the particle size increases. However, the resonance is dampened at larger wavelengths, where the extinction coefficient k of each particle size is growing in this limit. As can be seen in Fig. 2(b), k becomes sufficiently large for r = 2.5 nm (plasmon resonance completely damped). In Fig. 2(b), the k values below 1.0 correspond to high peak absorption. The k value has a significant effect on plasmon dephasing and the damping absorption process. The surrounding molecules/atoms induce an effective decay and a great size dependency of γ (r) in dielectric function which influences the plasmon resonance, as shown in Fig. 2(c). For γ (r) of the collective electron resonances, it was assumed nanoparticles have free surfaces. The impression of the nonionic surrounded environment has lesser importance.
Fig. 2 (a) Real part (n) and (b) imaginary part (k) of refractive indices of AuNPs in different size by Eq. (4); (c) Mie extinction spectra shows the effect of size depended dielectric function in water nm = 1.33
The effective dielectric function εeff depends on the dielectric function of the metallic components ε, the surrounding medium εm and the volume fraction of the metal f [23, 28]. Transmittance, reflectance and absorbance can be calculated from the dielectric function εeff of the effective medium. The main assumption of EMT is that the whole area is located in an equivalent mean field in which the electric field, due to the shapes of the spheres, is neglected. These spheres shapes reason to the polarizability of the spheres to be different in the directions along, as well as normal to the surface [35]. Figure 3 shows the calculated real and imaginary part of the refractive index by using the effective medium theory. The dips in position for n and the maximum position of peak at n and k for a variety of volume fraction AuNPs in two different salt concentrations (0.01M and 1M NaCl) are summarized in Table 1. From Fig. 3 and Table 1, it was noted that the calculated real part and imaginary part of the refractive index depend on the concentration of AuNPs and that the NaCl concentration is not effective as much as the AuNPs volume fraction. In the 30% volume fraction of gold, the n dips “labeled as *” are almost 500 nm for both NaCl concentrations, as shown in Fig. 3(a) and (c). The absorption peak is almost 515 nm for 10 nm primary gold nanoparticle. As a consequence, these dips area for n showing the position of the plasmon resonance frequency without considering the CID effect. The small difference between plasmon resonance for primary gold and the dips increased from interacting particles with the surrounding media of NPs. By reducing the volume fraction of AuNPs, this dispersion becomes weaker due to the scattering light arising from AuNPs more than transparent media. The red shift appears by increasing the volume fraction. This shift is shown in Table 1. Since k depends on the concentration of AuNPs, the influence of the volume fraction on the refractive index of AuNPs can be noted. A variation in the real (neff) and imaginary part (keff) of the refractive index follows harmonic oscillation behavior. This harmonic behavior is observed in volume fractions up to 30%, while deviations were observed for higher volume fractions. This deviation occurs because of the inter particle distance of AuNPs. When the average distance is shorter than 10 nm, surface plasmon coupling effects arise. Due to the coupling effect, a broadening and red shift in the plasmon frequency is to be expected. However, there is no difference for the calculated results between 0.01M and 1M NaCl (see Table 1). In other words, the results are mainly influenced by the volume fraction, but not by the concentration of the salt solution.
Fig. 3 Calculated real and imaginary parts of the refractive index by using the effective medium theory and Eq. (5) for different concentration of salt. (a) real and (b) imaginary part for 0.01M NaCl; (c) real and (d) imaginary part of refractive index for 1M NaCl. The dips position for n and the maximum position of peak at n and k for variety of volume fraction of 0.01M NaCl and 1M NaCl are summarized in Table 1.
Table 1. The Dips at real epsilon and maximum peak of real and imaginary part of epsilon
In order to better understand the influence of ionic surrounding media on the refractive index of AuNPs and compared theory and experiment, the absorption results are presented here. The experimental absorption peaks are shown in Fig. 4 against wavelength for five different volume fractions of AuNPs in 0.01M and 1M NaCl solutions. The plasmon resonance dampens and broadens significantly after the AuNPs into ionic solution is transferred. The variation in AuNPs volume fractions shows that NPs scatter light more strongly than transparent particles. Figure 4 also shows that if the interfaces are influenced by high electrolyte concentrations such as 1M NaCl, CID increases. This absorption appears because of repulsive forces rising from the surface charge of AuNPs. It is essential that this energy barrier is overcome for the single NP to interact. After the addition of a small amount of salt solution (for instance NaCl) in the colloid, this energy barrier is very strong and allows the interaction among NPs. The strong concentration of the NaCl solution resulted in a decreasing energy barrier and caused the AuNPs to interact and aggregate. The hydrodynamic size (nm) of aggregated AuNPs are shown in Fig. 4. In this work the aggregation indicators are the plasmon resonance peak FWHM and the maximum peak position. The results from Fig. 4 are summarized in Table 2. Table 2 confirms that a variation in volume fraction will be affect the plasmon resonance position and the value of absorption spectrum in different concentrations of salt. The FWHM value increased by increasing the salt concentration and the volume fraction because of CID.
Fig. 4 Experimental absorption in different volume fractions of 10 nm gold nanoparticles in 0.01M (a) and 1M (b) NaCl. In both figures the primary plasmon resonance of 10 nm gold are shown. The impact of CID on lowering and broadening of plasmon resonance clearly appear in experimental data. The hydrodynamic size (nm) of aggregated gold nanoparticles is shown inside the parenthesis.
Table 2. The variation in volume fraction will be affect the plasmon resonance position
Figure 5 shows that the structure of NPs changes after transferring the particles to a 1M NaCl and 0.01M NaCl solutions. In Fig. 5 the effect of these solutions is shown in the hydrodynamic size distribution of NPs by using the dynamic light scattering for 30% of 10 nm gold in 1M NaCl, 0.01M NaCl and can be compared with primary gold in Fig. 1 (c). The aggregation of AuNPs induces a phenomenon which is called surface plasmon coupling. This changes at maximum adsorption peak from a lower to a longer wavelength. Therefore, the color of the solution is changed from red to violet. As a consequence, these results show the damping and lowering of plasmon resonance is changed by increasing the volume fraction of NPs. Based on results of hydrodynamic size distribution and two controlled parameters (FWHM and Area) in Table 2 corroborate the AuNPs are aggregated.
Fig. 5 The size distribution of 30% of 10nm gold by using the dynamic light scattering. (a) 0.01M NaCl (b) 1M NaCl
Figure 6 shows the change in the hydrodynamic size versus the volume fraction of AuNPs in two salt concentrations. The hydrodynamic size of AuNPs increased rapidly in 1M more than 0.01M of NaCl solution which shows the NPs aggregation.
Fig. 6 The hydrodynamic size of AuNPs versus Volume fraction of two salt concentration (0.01M and 1M NaCl solution).
In this work, the real and imaginary part of the refractive index of AuNPs were calculated from experimental absorption spectra (Fig. 4) by using the Kramer Kroning simulation developed by Lucarini et al [17, 36]. The results in Fig. 7 demonstrate that increase in the volume fraction of AuNPs in 1 M NaCl causes the value of n decrease, which is completely opposite for 0.01 M NaCl. The increment in the refractive index of surrounding media (0.01M and 1M NaCl) decreases the Coulombic restoring force on the displaced electron cloud thereby diminishing the plasmon resonance. However, as explained above, for the Lorenz-Drude theory, an increase in the volume fraction shows the maximum peak of n shifts to the near-infrared region. The comparison between Fig. 7 (a) and (c) shows the steeper n spectra (500 – 700 nm) cause narrower absorption resonance. The broadening effect of plasmon resonance controlled by the imaginary part (Fig. 7 (b) and (d)) at the range of (500 – 600 nm).
Fig. 7 Real and imaginary part of the refraction index calculated from experimental absorption spectra by Kramer Kroning simulation.
The comparison between the red shift of real (n) and imaginary part (k) or broadening of refractive index in two different salt concentration for volume fractions of AuNPs shows in Fig. 8. The data shows linear fit.
Fig. 8 Comparison of red shift of real part (n) and imaginary part (k) of the refractive index for different volume fraction of 10 nm AuNPs in 1 M and 0.01 M NaCl solutions.
5. Conclusion
In summary, the theoretical results of this investigation show that the real and imaginary part of AuNPs refractive index (without including influence of CID) at visible wavelength depends on the size and volume fraction of AuNPs. However, the experimental investigations of the impact of CID on plasmon resonance and refractive index for bare AuNPs are made clear by lowering and broadening plasmon resonance peaks. On the other hand, the impact of CID is expected to be stronger than the effect of the refractive index of surrounding media (NaCl) at the interfaces of noble metals that are embedded in aqueous electrolyte solvent (high iconicity). This is confirmed by the experimental results in Fig. 4. The results of this study indicate that the refractive index of AuNPs shifts to the near-infrared region and is influenced by every high concentration of ionic medium (NaCl solution) due to CID. The perturbation of the density of the conduction electrons in nanoparticles could induce plasmon resonance shifts. After the NPs are embedded into the high-dielectric liquid media or adsorbate material, the material which interacts with the photon changes in the refractive index. The composite medium has a dielectric function response according to the effective medium theory. In other words, local changes in the optical properties of metals such as electron density near to the surface of particles affect the dielectric function response. This study propose that it might be advantageous to concentrate on the CID of AuNPs suspended in ionic media in SPR sensing applications.
Funding
The authors would like to thank Universiti Sains Malaysia for funding this research work through its Transdisciplinary Research Grant Scheme (203.PFIZIK.6763003).
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