Silica-coated gold nanoshells : Surface chemistry, optical properties and stability

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Introduction
Gold nanostructures come in various shapes and sizes, and their optical properties due to the localized surface plasmon resonance (LSPR) can vary significantly depending on their morphology.Common types of gold nanostructures are nanospheres [1], nanorods [2][3][4][5], nanowires [6], nanocages [7], and, finally, nanoshells [8,9].Gold nanoshells (NSs) consist of a spherical layer of gold around a core, usually silica.Their important advantage, which distinguishes them from other nanoparticles, is their spherical shape, enabling them to undergo a well-controlled and uniform surface functionalization.Furthermore, their LSPR can be spectrally tuned from the ultraviolet (UV) up to the near-infrared (NIR) by controlling the thickness of the gold layer.The NIR range is of particular importance, as it is the region in which human tissue exhibits low autofluorescence and low absorption, allowing the NIR radiation to penetrate deep into the tissue.Besides, NSs are very efficient in converting the energy of incident photons into thermal energy [10].This property, combined with their low toxicity [11], makes them applicable in biomedicine, including cancer therapy [12,13].
Proper surface modification of NSs, involving compounds with protruding functional groups, is essential for their subsequent utilization.These groups facilitate the attachment of additional chemical compounds to the exposed sites on the surface of NSs.Compounds containing thiol groups readily bind to pristine gold surfaces, creating strong gold-sulfur covalent bonds [8].This mechanism is employed to attach other types of chemical species, such as polymers.Among these, polyethylene glycol (PEG) terminated with a thiol group is commonly used.The distance between the gold surface and the polymer's end can be precisely controlled by the number of polymer units (mers).Binding to gold surface can also occur via amine functional groups, in which the free electron pairs of nitrogen can bind to gold atoms [14].This approach allows proteins to directly attach to the surface of NSs.For instance, it is possible to utilize human serum albumin (HSA) as a spacer layer between the nanoparticle surface and a fluorescent dye [15].Furthermore, the LSPR exhibits high sensitivity to the refractive index of the surrounding medium [16], making it suitable for biosensing applications.Notably, by functionalizing NSs with molecular recognition agents, analytes of interest can be sequestered near the nanoparticle surface, causing a change in the local refractive index and, subsequently, a spectral shift in the LSPR wavelength [17].
Silica is a commonly utilized material for surface modification of nanoparticles.Its widespread adoption can be attributed to its rapid preparation, ease of further modification, transparency, as well as its chemical inertness, and biocompatibility [18,19].Additionally, silica-coated nanoparticles often exhibit increased resistance to photodegradation in comparison to their uncoated counterparts.This resistance allows samples to be stored in the presence of light without causing alterations to their absorption spectra.The silica layer has demonstrated remarkable stability when exposed to high temperatures, up to 550 • C, protecting the encapsulated nanostructures [20].The coating of nanoparticles with silica can also substantially enhance their stability under acidic conditions.The thickness of the silica layer inversely affects the impact of the solution on the nanostructures, with thicker layers exhibiting greater resilience [21].Finally, the silica layer deposited on NSs can serve as a spacer to enable fluorescence enhancement [22][23][24][25][26] by preventing the fluorophore molecules from direct contact with the metal surface that could lead to emission quenching.
Here, we investigate colloidal NSs composed of 120 nm diameter silica spheres covered by a gold layer with thickness varying in the range 8-26 nm.Our experimental results reveal the correlation between the thickness of the gold layer and the spectral position of the LSPR extinction band.We also investigate the effect of an additional silica layer on the extinction spectra and the stability of the colloidal system, as well as the spectral broadening of the LSPR band caused by the clustering of NSs.We supplement our experimental results with numerical simulations, wherein we focus on the absorption and scattering contributions in the extinction spectra, which is relevant for applications such as photothermal therapy.Furthermore, the simulations of NS dimers and trimers reveal the origin of spectral broadening observed in the experiments.Our findings can have direct applications in the development of colloidal systems for biosensing, surface-enhanced spectroscopy, and controllable emission enhancement.They also contribute to the fundamental knowledge on properties of nanosystems.

Synthesis
All chemicals were purchased from Sigma-Aldrich except for 120 nm diameter silica nanoparticles which were bought from nanoComposix.The synthesis of NSs was based on the method described by the Halas group [27].In the first step of the process, a 1% solution of tetrachloroauric(III) acid in deionized water was prepared.This solution was   stored in the dark before use.Next, a suspension of colloidal THPC gold was prepared from mixing: tetrakis(hydroxymethyl)phosphonium chloride (80% in H 2 O), 1 M sodium hydroxide solution, and aged 1% tetrachloroauric(III) acid solution.This solution was stored in the fridge before use.To prepare the solution for plating, 50 mg of potassium carbonate was dissolved in 200 ml of deionized water.Then 3 ml of aged gold chloride solution was added.The 120 nm silica nanospheres were coated with 5-10 layers of (3-aminopropyl)triethoxysilane (APTES 99.99%): 7 ml of the silica nanospheres solution was diluted with deionized water and centrifuged twice at 3000 rcf for 25 min and then suspended in ethanol.The obtained solution was transferred to a plastic bottle, injected with 400 μl of APTES, and left stirring overnight.The solution was centrifuged twice and finally suspended in ethanol.Next, a solution of gold seeds (1.5 nm-3 nm) on APTES-coated silica nanospheres was prepared: colloidal THPC gold was placed in a glass container, 500 μl of 1 M sodium chloride solution, and 4 ml of PC were added.It was stirred overnight, then centrifuged twice at 1500 rcf for 20 min, and finally suspended in water.
The synthesis of gold nanoshells was conducted as follows: 3 ml of plating solution was introduced into a plastic cuvette, and then an appropriate volume of seed solution was added.To this mixture, 15 μl of formaldehyde was quickly added and the resulting mixture was shaken intensively for 2 min.To achieve the intended concentration of gold nanoshells, a scaling-up process was carried out: the plating solution, the seed solution, and the formaldehyde were mixed in appropriate proportions.The solution was stirred for 10 min and then centrifuged twice for 20 min at 350 rcf, finally suspended in water.The solution was stored in a fridge.
In a glass vessel, 3 ml of the scaled-up gold nanoshells solution was added to 4 ml of deionized water and mixed thoroughly.Ethanol, ammonia water, and an appropriate amount (2-6 μL) of tetraethoxysilicate were added dropwise.The whole mixture was left at slow mixing overnight.The suspension was subjected to two centrifugations for 20 min and resuspended in 10 ml of ethanol.The solution was stored in the fridge.

Characterization methods
The Malvern Zetasizer Ultra Red instrument was employed for measuring the hydrodynamic size of nanoparticles within colloidal solutions, as well as determining the zeta potential through the application of electrophoretic Dynamic Light Scattering (DLS) methodology.This was accomplished using the Smoluchowsky model.
The sizes of NSs were characterized using an FEI Tecnai G2 20 X-TWIN Transmission Electron Microscope.The solutions were deposited onto TEM grids, subjected to drying, and subsequently measured.The absorption spectra of the solutions were determined using a Cary spectrophotometer.

Numerical simulations
The numerical simulations were performed using commercial finiteelement software COMSOL Multiphysics (Wave Optics module) by solving the full-wave scattering problem in the frequency domain.The model geometry consisted of concentric spheres, with a silica core of 120 nm in diameter, surrounded by a gold layer (various thicknesses) and embedded in a uniform environment with a refractive index equal to that of water.The more complex geometries of the nanoshell dimers and trimers are presented in Fig. 9.The additional layer of silica was not considered.The optical constants of silica (core), gold (shell), and water (surrounding medium) were taken from the refractiveindex.infodatabase, following Refs.[28][29][30].The outermost spherical layer of the model was set to a perfectly matched layer with the scattering boundary condition set to absorb spherical waves.The background field was defined as a linearly polarized plane wave.The maximum size of the mesh elements in the gold layer was set to either 10 nm or half of the

Table 1
Summary of the NS properties, including: the thickness of the gold layer (R Au ), the thickness of the silica layer covering the NSs (R SiO2 ), hydrodynamic diameter measured by DLS and reported as the number average (D h ), the position of the extinction peak (λ max ), the shift in the maximum extinction band in the case of silica layer coverage (Δλ), surface zeta potential (Zeta pot.), and the concentration of TEOS used in the reaction mixture to prepare the silica coating (C TEOS ).Fig. 5. Representative TEM images of NSs with silica coatings; a) NSs@Au_8.5nm@SiO 2 _12.5 nm (NSs with a gold layer thickness equal 8.5 nm and a silica layer of thickness 12.5 nm), b) NSs@Au_12.5nm@SiO 2 _20 nm, c) NSs@Au_16.5nm@SiO 2 _19 nm, d) NSs@Au_11 nm@SiO 2 _5.5 nm, e) NSs@Au_7.5nm@SiO 2 _14 nm, f) NSs with a gold layer thickness equal 24.5 nm, accompanied by clusters of unevenly attached silica and free silica in the form of spherical nanoparticles.
shell thickness, depending on which was smaller, to 20 nm in the silica core, and to 1/10 of the free-space wavelength in other parts of the model.The scattered power was calculated by integrating the radial component of the Poynting vector of the scattered field over a boundary enclosing the nanostructure, while the absorption was obtained by similarly integrating the radial Poynting vector component of the total field.The extinction was obtained by summing the absorption and scattering.

Results
The NSs with various gold thicknesses were synthesized by varying the SEED-to-gold solution ratio.Subsequently, the extinction spectra were measured, and the size distributions were determined based on the TEM images.The dependence of the spectral position of the LSPR extinction peak on the gold layer thickness is presented in Fig. 1.For NSs with a gold layer thickness of approximately 25 nm, the peak of the extinction band is centered around 700 nm.As the gold thickness decreases, the band gradually shifts towards longer wavelengths, eventually reaching 930 nm for NSs with a thin and non-continuous gold layer.The dependence of LSPR spectral position on shell thickness is a wellknown property of NSs, which has been attributed to hybridization between the shell's inner and outer surface plasmon modes [31].As the shell becomes thinner, the hybridization becomes stronger, causing a red shift of the symmetrically coupled hybrid LSPR mode, which is responsible for the main dipolar LSPR band observed in the experiments.This mechanism underlies the unique tunability of the NSs resonant optical response that makes them of great interest for applications Fig. 6.TEM images of NSs@Au_17 nm@SiO2_19 nm at various magnifications, illustrating the formation of NS clusters composed of two or three NSs, which are later encapsulated by a silica layer.The clustering of NSs during the synthesis stage directly impacts the properties of the extinction spectrum and its broadening (see Fig. 4c).ranging from photothermal therapy [32] to nonlinear optics [9].
The variation of the gold thickness was confirmed using TEM.Fig. 2 shows examples of the TEM images of NSs from a monodisperse solution, captured in both bright and dark fields.The spectral shift of the extinction band due to various gold thicknesses is linked to different colors of the colloidal solutions, as exemplified in Fig. 3. On the left side of the image, solutions of NSs with a thicker gold shell, where a smaller quantity of SEED was used during synthesis, exhibit a pink color.Moving to the right, the gold layer thickness decreases proportionally, as a greater amount of the SEED nanoparticles was introduced into the reaction solution, giving rise to a blue-green color.
Next, we coated the NSs with an additional silica layer, and we explored its effect on the spectral position of the extinction band.The extinction spectra for NSs with gold layer thicknesses of 12.5 nm, 16.5 nm, and 17 nm are presented in Fig. 4a, b, and 4c, respectively.Each of these figures shows the measured extinction spectra for various thicknesses of the additional silica layer.A summary of the properties of the silica-coated (as well as uncoated) NSs is provided in Table 1.The parameters of the NSs are encoded in the abbreviated labels.For example, "NSs@Au_12.5 nm" denotes NSs with a 12.5 nm gold layer thickness, without an additional silica layer, while "NSs@Au_12.5 nm@SiO2_8 nm" represents NSs with a 12.5 nm gold layer thickness and an 8 nm silica layer.Table 1 presents information about the gold layer thickness (R Au ), the silica layer thickness covering the NSs (R SiO2 ), the hydrodynamic diameter measured by the DLS method and reported as the number average (D h ), the position of the extinction peak (λ max ), the shift of the extinction peak due to silica layer coverage (Δλ), surface zeta potential (Zeta pot.), and the concentration of TEOS employed in the reaction mixture for the preparation of the silica coating (C TEOS ).
We have also observed the formation of NS clusters, depending on the quality of the SEED particles and the choice of the synthesis parameters.The clustering of NSs causes the broadening of the extinction spectrum (see Fig. 4c).This effect is significant even in highly monodisperse solutions, as exemplified by NSs@Au_17 nm, where the gold layer thickness is 17.0 ± 3.3 nm (see Table 1).However, by selecting appropriate synthesis parameters, we can obtain colloidal solutions of separated and monodisperse NSs characterized by a narrow extinction band (see Fig. 4a and b), which can subsequently be coated with a silica layer (see Fig. 5).The NS clusters can be coated with a silica layer as well, as demonstrated in Fig. 6.The formation of the silica coating is influenced by various parameters, including solution pH, the amount of TEOS, stirring speed, and centrifugation rate [33][34][35].In our study, we observed that the extinction peak of the NSs with a thin gold layer undergoes a red shift with an increasing concentration of TEOS in the reaction mixture (see Fig. 7).However, when the TEOS concentration reaches 3.33 × 10 − 4 and 4.00 × 10 − 4 mol/dm 3 , the samples exhibit a sudden increase in hydrodynamic diameter (see Table 1), primarily due to the formation of additional silica structures as by-products (see Fig. 5f).These structures are substantial in size and are often attached to the NSs, rendering standard centrifugation insufficient for their effective separation.Frequently, at the highest employed TEOS concentration, the silica layer thickness no longer increases, but instead, suspended silica byproducts are formed in the solution (see Fig. 7d).Notably, we also observed that the extinction spectra of the solution containing clustered NSs (NSs@Au_17 nm) are highly sensitive to the thickness of the silica layer, determined by the TEOS concentration (see Fig. 4c).The extinction increases significantly in the range 900-1100 nm with an increasing silica layer thickness.
In the case of NSs with the thickest silica layer (NSs@Au_25.5nm),we have found that the addition of TEOS induces a red shift in the extinction band, yet further increase of the silica layer thickness no longer results in substantial changes in the spectrum.One may conclude that the samples with a thinner gold layer are better suited for utilization in sensors reliant on the LSPR shifts caused by the refractive index variations on their surface.
Additionally, zeta potential measurements provided valuable insights into the stability of the NSs with and without the silica layer (see Tables 1 and 2).In general, NSs demonstrated remarkable stability during the experiments; a brief ultrasonication for just a few minutes effectively disperses NSs that had been stored over several months.We found that the additional silica layer enhances their stability, as evidenced by the increase in the absolute zeta potential values.These highly stable silica-coated NSs hold great promise for advancing research in colloidal nanoparticle-based sensor applications.Extinction spectra obtained by numerical simulations (see Fig. 8) are in good agreement with the experimental results.In particular, two LSPR peaks are clearly visible, corresponding to the electric dipole (in the NIR range) and electric quadrupole (in the visible range).The contribution of the scattering increases with increasing shell thickness, which strongly affects the dipole peak, leading to its spectral broadening through the increased radiative damping.On the other hand, the contribution of absorption remains significant at the quadrupole resonance, as it is significantly less radiating than the dipole.High absorption in plasmonic nanoparticles is of interest for photothermal therapy, in which the energy of incident light is converted into heat.Taking into account the presented results, NSs with thinner shells appear more suitable for this application.
To explain the origin of the spectral broadening observed for NS clusters in Fig. 4c, we simulated the optical response of NS dimers and trimers.The results are presented in Fig. 9, together with the corresponding 3D schemes of the studied geometries and the incident light configurations.In the case of trimers, both linear (Fig. 9e) and compact arrangements (Fig. 9i) have been considered.Due to the anisotropic shape of the clusters, the spectra are strongly dependent on the incident light polarization (illustrated as red arrows) and propagation direction (magenta arrows).For example, a strongly red-shifted collective dipole resonance of the dimer (around 1350 nm, see Fig. 9b) and the linear trimer (around 1800 nm, see Fig. 9f) is only excited by the incident light with polarization parallel to the long axis of the cluster.On the other hand, for the incident light propagating along the z-axis (see Fig. 9j), the compact trimer is optically isotropic in the xy-plane due to its three-fold rotational symmetry, but its spectrum changes significantly if the light is incident from the side (e.g., propagating along x, see Fig. 9k and l).
A colloidal solution is an ensemble of NS clusters with random orientations.Therefore, the experimentally measured extinction spectra would correspond to an average of the simulated spectra.Strong variations among these spectra are the cause of the apparent spectral broadening of the measured extinction bands.Furthermore, NS trimers obtained by chemical synthesis may contain many intermediate geometries between the linear and the compact trimer (see Fig. 6), yielding the collective dipole peaks in the broad range from around 1250 nm (compact trimer, Fig. 9j) to around 1800 nm (linear trimer, Fig. 9f).Despite the dominant contribution of the scattering, the contribution of absorption in these peaks remains significant.This means that the light could be efficiently converted into heat within a macroscopic volume of a sufficiently dense colloidal solution, in which the secondary scattered light continues to interact with the neighboring nanostructures and hence can be absorbed by them.This could make the NS clusters suitable for applications that require efficient conversion of light into heat over a broad spectral range, e.g., in solar energy harvesting.

Conclusions
In this work, we have investigated the properties of gold nanoshells, in which the gold layer and the additional silica layer had various thicknesses.The gold layer thickness was controlled by using different SEED-to-gold solution ratios during the synthesis.We found that the main LSPR peak in the extinction spectrum undergoes a red shift with decreasing shell thickness.The addition of the silica layer has been found to further influence the extinction spectra, however, this effect turned out to saturate at some point.Based on the effect of the silica coating, we noticed that the NSs with thinner gold layers are more sensitive to the refractive index changes in the surrounding environment, making them more suitable for sensing applications.By measuring the zeta potential, we have found that the additional silica layer improves the stability of the colloidal system.Furthermore, we observed the formation of the clustered NSs, depending on the synthesis parameters, especially on the quality of the SEED solution.The parameters were subsequently optimized towards obtaining colloidal solutions with monodisperse and separated NSs exhibiting narrow extinction peaks.The experimental results were supplemented by numerical simulations, showing a good agreement between the theoretical and experimental extinction spectra.NSs with a thin gold layer have demonstrated the highest absorption contribution relative to the scattering, making them better suited for applications such as photothermal therapy.Furthermore, the extinction spectra calculated for NS dimers and trimers have shown strong dependence on the incident light polarization and propagation direction, providing the explanation for the experimentally observed spectral broadening in the samples containing NS clusters.Our results are of potential interest for the design of advanced colloidal systems for applications in biosensing, surface-enhanced spectroscopy, light-to-heat conversion, and plasmon-enhanced fluorescence.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Dependence of the spectral position of the extinction peak on the thickness of the gold shell covering a silica sphere (diameter 120 nm).

Fig. 2 .
Fig. 2. Examples of TEM images of monodisperse NSs, captured in the bright field (left) and in the dark field (right).

Fig. 3 .
Fig. 3. Photograph of colloidal solutions of NSs.The gold layer thickness systematically decreases from left to right, changing the transmitted color from pink to blue-green.

Fig. 4 .
Fig. 4. Normalized extinction spectra for NSs with a gold shell thickness of a) 12.5 ± 3.0 nm, b) 16.5 ± 3.0 nm, c) 17.0 ± 3.3 nm, covered with layers of silica of various thicknesses.The parameters of the NSs are encoded in the legends, e.g., NSs@Au_12.5nm@SiO 2 _8 nm corresponds to NSs with a gold layer thickness of 12.5 nm and a silica layer thickness of 8 nm.

Fig. 8 .
Fig. 8. Calculated spectral dependence of the extinction (blue line), scattering (red line) and absorption (black like) of NSs with a) 8 nm shell thickness, b) 15 nm shell thickness, c) 23 nm shell thickness and d) 30 nm shell thickness.

Fig. 9 .
Fig.9.Numerical simulations for the NS dimers (a-d), linear trimers (e-h) and compact trimers (i-l).Panels a, e and i illustrate the assumed geometries, with the silica cores shown in light-blue color and the gold shells shown in yellow color.Panels b-d, f-h and j-l show the calculated spectra for various configurations of the incident light polarization (indicated by red arrows; b, f, jparallel to the x-axis, c, d, g, h, j, kparallel to the y-axis, lparallel to the z-axis) and direction of incidence (indicated by magenta arrows; b, c, f, g, jparallel to the z-axis, d, h, k, lparallel to the x-axis).

Table 2
Results presenting the stability of NSs over time.'Zeta pot.' represents surface zeta potential values, and 'Δt' represents the number of days that have passed between synthesis and measurement.