Simple synthesis of gold-decorated silica nanoparticles by in situ precipitation method with new plasmonic properties

We describe a simple method for the preparation of gold-decorated silica (SiO2) nanoparticles (NPs) by the in situ precipitation method using simple BH4− ions reduction as a procedure, where BH4− ions are adsorbed onto PEI-functionalized SiO2 NPs for stabilizing and reducing gold ions onto PEI-SiO2 surface in water under ambient conditions. The result was 3-nm gold nanoshell NPs attached to SiO2 core (~ 75 nm) with a surface plasmon resonance (SPR) at ~ 680 nm. SPR band is associated with Au NP aggregates that arise from strong interparticle interaction. This is an alternative to the gold-seeding methods and the use of anionic gold species for the obtention of gold-decorated SiO2 NPs with an important red-shift in UV–Vis absorption and with potential applications in biosensors and photothermal therapy.


Introduction
Core-shell engineered nanostructures are good candidates for the building of new materials with controlled properties of importance in several technological applications [1]. In particular, gold nanoparticles (AuNPs) in hollow gold-shell configuration exhibit unique physical-chemical characteristics, low toxicity, and optical properties due to AuNPs. Furthermore, the synthesis of gold nanoshells NPs with various dimensions and shapes presents us the possibility of obtaining metallic nanoshells with SPR band in the near-infrared ranges. These are of considerable interest by a wide range of possible applications in fluorescent sensors [2], biosensors [3], photothermal therapy [4,5], biomedical imaging [6], or drug carriers for cancer treatment [7]. The surface features directly affect its optical response which is key to these applications and especially in photothermal therapy which converts light into local heat (hyperthermia) that can kill cancer cells [8].
In fact, AuNPs can absorb or emit different wavelengths with responses ranging from the visible to the infrared region due to the in-phase resonant oscillation of free conduction band electrons by interacting with the electromagnetic fields, generate the surface plasmon resonance (SPR) [9,10], very useful for different applications. The position, intensity, and shape of the SPR band depend on the size and size distribution of AuNPs, morphology, shape or geometry, the inter-particle distances, the polarization of the incident light [11], the dielectric constant of the surrounding medium around the NPs, and interparticle interaction [10][11][12][13] which are important parameters to define their plasmonic properties. Therefore, we provide an insight into the sensitivity of SPR toward the above parameters in the following few reports. The position of absorption maximum is also affected by decreasing the particle size. However, as a modification to the Mie theory for small metal particles, the dielectric function is assumed to become intrinsic size effects (are mainly governed by their size, shape, composition, crystallinity, and structure) [10] dependent rendering a size-dependent absorption cross-section, and the underlying relaxation mechanisms, that does not distinguish the dephasing of the collective electron motion and are directly related to the width of the plasmon resonance [13]. Solid spherical Au NPs with low polydispersity and small diameters (2-5 nm) are well known to exhibit a single SPR band between 510 and 570 nm [14,15]. Otherwise, when the particle size increases, the absorption band exhibits a red-shift and a band broadening that can upsurge mainly from radiative losses (the mode cannot be trapped in the particle and coupled to freely propagating light) [13]. Moreover, that the is decrease of the interparticle distance in a system of AuNPs leads to a much stronger electron-phonon coupling [10] and the absorption tends to infrared radiation [14] that has been related by the interparticle spacing decrease, which leads to a collective plasmon oscillation of the aggregated system and results in strong dipole-dipole interaction, leading to larger redshifts of the plasmon mode [10,11,16] while for longer spacers the dipolar interaction is very weak or nonexistent and only the plasmon absorbance band is observed in the visible region [12,17].
In general, the specific geometry of metallic particles usually induces the SPR to exhibit two distinctive bands corresponding to the transverse and longitudinal modes [18]. Au NPs (diameter of 10 nm) showed the presence of a transverse mode at 520 nm, that is shifted to lower wavelengths concerning the plasmon band of spherical particles, albeit by a much smaller amount than the extended plasmon shift, while the extended plasmon (the broad near-IR absorption band) is due to the closely spaced NPs in the aggregate of the gold core NPs [19] or nanoparticle aggregates that arises from strong interparticle interaction [10]. On the other hand, the strong longitudinal band in the near-infrared region [18] is strongly dependent on the aspect ratio [11,20] or modifying their core and shell molar ratios, and metallic shell thickness [21,22].
Particularly, in a system formed by a dielectric kernel shelled with small gold crystals, and Au shell thickness increases, a shift of SPR band to the near-infrared regime can be observed [1,23]. This is often associated with a large dielectric permittivity of the core or can be understood in terms of a plasmon hybridization [24]. Moreover, the plasmon bandwidth becomes broader with a thickness decrease or with an incomplete gold shell formation [1,9,14]. Therefore, this type of core-shell structure has gained considerable attention in therapeutic and diagnostic applications that require near-IR absorption, due to the possibility of SPR band tuning by changing the Au shell characteristics [25]. A study demonstrated that by increasing the gold shell thickness, a significant reduction in the applications with a light response is obtained [26].
A common approach for the fabrication of these gold core-shell nanostructures can involve the use of silica (SiO 2 ) as a core material mainly due to its physical and chemical properties [27,28], its chemical inertness, and transparency [11]. The assembling normally starts with the synthesis of SiO 2 cores by the Stöber method, followed by its polymer-modified surface such as amine groups to their decorates with a solution of small gold colloid with 1-2 nm in diameter is used as nucleation sites for the reduction and growth of increased coverage of gold on the NPs surface [22] or by the in situ-precipitation method, which is an alternative method that can directly crystallize metallic gold on silica from gold ions solutions (instead AuNPs seeds) [29]. Also, gold ions can be reduced using the leaf extracts (at room temperature) resulting in a 'monodisperse medium' of NPs and biocompatible [30]. Gold NPs synthesized by the citrate reduction method show that by decreasing the molar ratio between citrate and gold during synthesis, the size of the particles is increased, as well as their polydisperse and being less spherical [31]. Gold NPs were stabilized by linking thiol on the surface and the presence of amine/ammonium ending groups, with NaBH 4 as the reducing agent [32]. Other synthetic strategies can be used to control the Au shell size assembled into an aggregate structure. Hollow Au nanostructures can be formed by using relatively smaller silver nanospheres as sacrificial templates and ions as the galvanic replacement reagent [33].
The in situ-precipitation method offers the advantage of fewer steps in the synthesis process and minimizing the formation of diversiform or the nucleation of gold crystals out from the silica surface. The formation of this gold shell is directly affected by the attraction strength between the SiO 2 core surface and the gold shell species in the solution, thus its packing density and architecture can be adjusted by modulating the surface potential on the core and shell particles through stringent control of key parameters, e. g. the concentration of Au 3+ and its mass ratio to the SiO 2 support and reductant, ionic strength, temperature, time, and pH of the reaction. Particularly, the hydrogen ionic potential of the medium can finely alter the ionization of the species in a solution and/or moieties attached to the surface of gold and SiO 2 NPs to optimize the adsorption and subsequent metallization of gold ions on the silica surface. The ionization of species adsorbed on silica surface can also improve the dispersion of SiO 2 NPs, avoiding the formation of multicore structures and allowing the obtention of more homogeneous shells during gold capping [29].
For this purpose, amine-terminated stabilizing agents such as polyethyleneimine (PEI)-which becomes highly positively charged depending on the pH-have been utilized to functionalize SiO 2 NPs before gold coating [25]. In fact, the amine-functionalized silica surface facilitates the adsorption of anionic gold species (such as AuCl 4 − ) to its further reduction and Au shell growth [29]. However gold cations (e.g. Au 3+ ) are expected to be poorly adsorbed on positively charged PEI-coated SiO 2 NPs to form an Au shell by in situ reductions. Thus, for overcoming this constraint and to obtain gold-shelled SiO 2 NPs from PEI-coated cores and using gold (III) acetate as Au 3+ source, we propose a simple route of synthesis where a reductant anion (BH 4 − ) is prior adsorbed on SiO 2 NPs charged positively, decreasing substantially the superficial charge of silica and allowing the adsorption of Au 3+ for a further reduction of these ions form a gold shell by the in situ-precipitation method. This is an alternative to the gold-seeding methods and the use of anionic gold species for the core-shell gold/ silica NPs obtention with an important red-shift in UV-Vis absorption.

Synthesis of the PEI-modified silica NPs core
The silica NPs were synthesized following a standard Stöber method that includes hydrolysis and subsequent condensation of tetraethyl orthosilicate (TEOS) in basic conditions [34]. In a typical synthesis, 50 mL absolute ethanol and 3 mL ammonium hydroxide (25% wt in water) were stirred for 5 min. Thus, 1.5 mL of TEOS solution was dropwise added and the mixture was maintained under magnetic stirring for 12 h at room temperature. The resulting precipitate was collected by centrifugation at 3000 rpm for 7 min and washed with ethanol 4 times and finally transferred to an aqueous medium. Then, 0.28 g of PEI (previously dissolved in 50 mL deionized water) was added to the SiO 2 dispersion (8.13 mg/mL) and the mixture was stirred at room temperature for 48 h. The obtained precipitate was washed three times with water to remove the non-attached PEI and the functionalized silica NPs (PEI-SiO 2 ) were redispersed in deionized water.

Synthesis of gold-decorated SiO 2 NPs
An amount of metallic gold was deposed on PEI-SiO 2 surface for obtaining Au nanoshell onto SiO 2 system by adapting a method reported in the literature [35]. For this purpose, it is initiated by preparation of 1.0 mg of sodium borohydride (NaBH 4 ) was dissolved in 40 mL water, after adding 1 mL of PEI-SiO 2 NPs suspension (concentration of ~ 8.13 mg/mL). The mixture is kept stirred for 15 min at pH = 7.5. Then, 5 mg of gold (III) acetate (dissolved in 5 mL water that is stirred until the Au ions have been dispersed) was added to the mixture, and the mixture was stirred for 10 min at 500 rpm at room temperature. After that, the Au@PEI-SiO 2 NPs were washed with water several times to remove the excess reactants and finally were redispersed in deionized water and the pH was adjusted to 7.

Characterization methods
The morphology and size distribution of NPs were characterized by Transmission electron microscopy (TEM) images that were collected with a JEOL JEM 1010 microscope operating at 120 kV. Samples were prepared by placing one drop of a dilute suspension of NPs in the water on a carbon-coated copper grid and air-dried at room temperature. The average particle size and distribution were evaluated by measuring 200 particles. The data were fitted with a lognormal distribution. The X-ray diffraction (XRD) patterns of the samples were obtained using a Rigaku Miniflex 600 diffractometer operating at 30 mA and 40 kV from 20 to 80° (2θ value) using Cu-Kα radiation (0.15418 nm). The samples were prepared by placing a drop of concentrated NPs suspension on a zero-background diffraction silicon sample holder. XRD patterns were analyzed using Rietveld structure refinement. X-Ray photo electron spectroscopy (XPS) was performed using a SPECS Sage HR 100 spectrometer equipped with a hemispherical electron analyzer and monochromatic Al Kα radiation operated of 1486.6 eV energy at 350 W and 10 -9 mbar. The data were analyzed using the Casas Software (Version 5.921) and peak deconvolution was performed using a 70% Gaussian and 30% Lorentzian line shape with a Shirley nonlinear sigmoidtype baseline. The XPS results were corrected using the C 1s peak (284.6 eV). All measurements were performed on freshly prepared samples to guarantee the reproducibility of the results. The surface charge of samples was measured in a Malvern Zetasizer Nano-ZS90 (Malvern Instruments Ltd., U.K.) at room temperature and samples were dispersed in water at 10 mM of KCl. The pH of aqueous solutions was adjusted by adding 1 mol/L HCl or 1 mol/L NaOH. To evaluate the optical properties of samples, the measurements were carried out using a JASCO spectrometer (UV-1700) at room temperature. The samples were diluted in 1 mL water in a standard quartz cuvette used to quantify the light that is absorbed and scattered of the sample.

Results and discussion
In order to optimize the silica NPs functionalization with PEI and to improve gold deposition, the pH of dispersions was controlled during synthesis given modifying the NPs surface charge. The surface charge of the NPs was investigated using zeta potential (ζ) measurements. Thus, firstly, the modification of SiO 2 surface by PEI to obtain the PEImodified SiO 2 NPs, and the pH was fixed in ~ 7.4, where the ζ value of bare silica was − 37.5 mV. The silica isoelectric point was close to pH 2, above which the surface was negatively charged due to the ionized moieties at the SiO 2 surface [36]. After introducing PEI, the ζ values increased to + 27.6 mV (PEI-coated SiO 2 ), indicating a successful functionalization of the SiO 2 surface with the polymer. At this pH, PEI amine groups are substantially protonated (-NH 3 + ) and the electrostatic interaction of these moieties with the negatively charged surface of SiO 2 favors the adsorption of PEI. Moreover, the unbound solution-oriented amine groups of adsorbed PEI gave rise to the positive charges on the PEI-SiO 2 surface (see Fig. 1).
In this work, to reduce gold onto PEI-SiO 2 surface, our strategy was firstly adding the reducing agent (BH 4 − ion) instead of the ionic Au 3+ , as is normally employed in this type of synthesis (with seeds of gold particles of a few nanometers in diameter are prepared [22] and their growth starts in a solution containing a reducing agent and the seed suspension [20] or Au 3+ ions to the formation of a metal shell [21]). In this case, borohydride anions are electrostatically driven to the positively charged PEI-SiO 2 surface and the surface charge decreases significantly as schematized in Fig. 1. Then, an excess of Au 3+ can be added to be more easily attracted and reduced onto the PEI-SiO 2 surface which was rich in BH 4 − ions. Figure 2 shows TEM images with detailed morphology of gold shelled NPs and the histogram of the size distribution of SiO 2 NPs before and after gold coating. The analysis of these histograms indicates the bare SiO 2 core has ~ 75 nm and, after gold capping, the average diameter of the nanostructures increased to ~ 81 nm, resulting in an Au nanoshell thickness of ~ 6 nm. Besides, a close inspection of the TEM image indicates that the Au nanoshell is inhomogeneous and randomly deposited − /Au 3+ ratio utilized during gold deposition, which can result in small AuNPs forming inhomogeneous aggregates [37] while Chen et al. [38] reported that the formation of Au NPs required a high concentration of NaBH 4 that might result in a quick reduction of Au 3+ , obtaining the formation of small-sized gold NPs. Other researchers reported that the aggregation of Au NPs has been attributed to adding excess NaBH 4 [16], suggesting the borohydride favors the formation of a large number of cores and unleashes a disordered growth process leading to polydispersity of size [39]. The increase in the amount of NaBH 4 also increases the Au NPs size [40]. All this indicates the formation of Au nanoshell NPs depends on the ratio of the BH 4 − /Au 3+ species. Au shells with similar aspects composed by randomly assembled NPs forming irregular nanochains and/or nanoflower-like crystal structures have been also reported in the literature [41,42].
In Fig. 3, the main crystalline phase observed is corresponding to the gold cubic structure with space group Fm3m (225) (JCPDS card no. 98-0000230) and a lattice parameter a = 4.187 ± 0.002 Å, determined from Rietveld refinement. Moreover, the broad peak around ~ 19° could be assigned to the amorphous SiO 2 phase [43]. The crystallite size of the gold nanoshell can be estimated from the Scherrer equation, expressed as D = K ∕Γ ⋅ cosθ hk , where is the incident radiation wavelength, Γ is the diffraction line broadening, θ hkl is Bragg´s angle. For spherical hollow NPs the Scherrer constant can be written as , where R (D = 2R) and t are the particle radius and shell thickness, respectively [44]. Using this formalism and considering R = 81 nm (from TEM analysis) the gold thickness was estimated in ~ 3 nm in good agreement with the Au nanoshell thickness from TEM measurements.
The particle's surface composition was studied by X-ray Photoelectron Spectroscopy (XPS). Figure 4 shows the survey spectra of PEI-SiO 2 and Au@PEI-SiO 2 . The peaks at binding energies (BEs) at ~ 103 eV and ~ 534 eV, corresponding to Si 2p and O 1s, are typical of amorphous silica [45]. Moreover, the peaks at BEs at ~ 285 eV and ~ 399.3 eV, corresponding to C 1s and N 1s, indicate a successful functionalization of SiO 2 surface with PEI molecules. The amine groups of N 1s can be bonds with oxygen [46,47] and it disappears when protonated amine with gold. After Au coating, peaks intensity corresponding to Si 2 s, Si 2p, O 1s, and C 1s cores decreases while for N 1s core, the peak practically disappears, a new peak corresponding to Au 4f appeared. The Si 2p spectrum was fitted with two distinct components at 101.8 eV and 102.9 eV, which could be attributed to Si-N and Si-O-Si bonds respectively, further confirming the presence of NH 2 groups onto SiO 2 [48]. After Au coating the Si 2p decreased and was fitted with Si-N and Si-OH with BEs of 101.7 eV and 103.2 eV, respectively, this latter was formed due to the electronegativity of silicon atoms with the hydrogen atom [49]. Curves fitting reveal the O 1s peaks at 531.2 and 532.5 eV resulting from Si-O/C-O and Si-O-Si, respectively [47]. After coating  [49,50]. Besides, the weak signal of Si after Au coating could be due to the small thickness of Au nanoshell and/or the presence of exposed areas of PEI-SiO 2 surface which were not completely covered with Au. Doublet deconvolution for the high-resolution Au 4f spectrum (see inset) shows two peaks at 83.4 and 87.1 eV corresponding to Au 4f 7/2 and Au 4f 5/2 . The 4f doublet splitting (~ 3.7 eV) indicates Au exits only in its metallic state [51,52]. The C 1s spectrum was fitted with three components corresponding to C-C/C-H that could be attributed to residual carbon impurities and in part to the C-C of PEI at 284.6 eV, to  [46,47,53], and verify the immobilization of PEI on the SiO2 surface. The highest BE at 288.2 eV is mainly due to the amide group (C = O) and the carboxyl group (O = C-OH) [54] that could be due to an interaction between PEI radicals and water molecules [53]. After coating, C-C/C-H bond persists and the C-N/C-O [55] bond at 286.2 eV, while the variation of C = O/O = C-OH bond was insignificant at 288.5 eV [56]. The presence of C-O/C = O plays a role in Au binding, this mechanism is the same as that suggested in the previous studies. [57]. The residual C 1s signal can be attributed to a possible carbon surface contamination from nondetached PEI as well as from unhydrolyzed TEOS on these exposed areas. Figure 5 shows SiO 2 NPs typical UV-Vis spectra before and after gold-coating. As expected, the PEI-SiO 2 NPs did not present any noticeable absorption peak in the investigated wavelength range, whereas Au@PEI-SiO 2 NPs exhibited an absorption band at ~ 680 nm due to surface plasmon resonance (SPR) of gold [57]. A spectrum of spherical AuNPs (diameter ~ 6 nm) obtained from the same procedure described in the synthesis process, but without adding PEI-SiO 2 NPs, is also shown in Fig. 5. This spectrum of spherical solid AuNPs showed a narrower SPR band between ~ 520 and 531 nm, which is characteristic of individual gold NPs [12,57,58] depending on the polydispersity and size of AuNPs [18].
Besides, the broad aspect of this band and an important red-shift at ~ 680 nm observed in sample Au@PEI-SiO 2 can be associated with the presence of a gold nanoshell forming of small AuNPs aggregates of different sizes, structures, and orientations, or densely packed structures so that the single particles are electronically coupled to each other [10], resulting in interparticle collective interactions due to the decrease of interparticle space among AuNPs [58][59][60] that contribute the total plasmon band absorption. Bellino et al. [17] reported the absorbance decay is the result of the aggregation of NPs are depending on the number and density of NPs in the aggregates wherein the particles have geometrical arrangements (shapes, sizes) [10]. Meanwhile, Rudoy et al. [21] reported a broad absorption peak for a thickness of 4 nm gold due to the formation of a discontinuous Au shell and its structure is similar to that of an island film. This can be corroborated by the results of TEM which indicate the presence of a nanoshell of inhomogeneous and/or non-uniform AuNPs aggregates [10] structures onto the SiO 2 core surface. XRD studies show that the tiny particles (3 nm size) that form part of the packed structures of aggregates.
The presence of small amounts or particle aggregates typically shows strong absorbance at longer wavelengths due to the reduced mean free path of the electrons on the long-wavelength region. From this viewpoint, mean free path corrected Mie theory affords a quantitative description of the absorbance of Au NPs can be determined directly from UV-Vis spectra for the calculation of particle diameter (with an average deviation of ∼18% for the calculation of the particle sizes) is given by [61]: d = exp 3.55 A spr A 450 − 3.11 , where A spr is the surface plasma resonance peak, and A 450 is the absorbance at 450 nm. Using the above expression, the calculated diameter was 9.5 ± 1.7 nm. The calculated diameter of the surface plasmon peak helps to acquire a clear insight into particle aggregate size that is a large number of agglomerated atoms [10] that are modeled as being spherical NPs, which is supported by the results of TEM and XRD analysis that indicate the interparticle distance was reduced and NPs start to coalesce and aggregate, with the extent of aggregates, and inhomogeneities in NPs shape and size confirmed the results observed by UV-Vis absorption.
The simple method of synthesis utilized in this workbased on in situ-precipitation-allows the obtention of gold-decorated silica NPs with an important red-shift in SPR wavelength when compared with other elaboration methods, mainly based on gold seeding on silica NPs, which involves more steps for synthesis and less red-shift efficiency. For instance, Daware et. al prepared spherical silica NPs with an average particle size of 200 nm with AuNPs (~ 8 nm) anchored onto SiO 2 surface. Using a multi-step procedure, SiO 2 was synthesized using a standard Stöber method, and, separately, AuNPs were obtained by the reduction of chloroauric with trisodium citrate and thus were functionalized with polyvinyl pyrrolidone (PVP). The self-assembly of Au and SiO 2 NPs resulted in a system with an SPR peak at 517 nm [57]. Lee et al. prepared a core-shell structure, with an 18 nm magnetic core and SiO 2 shell with a thickness of 40 nm. On silica shell surface, ca. 5.5 nm Au NPs were immobilized acting as the plasmonic substrate. In this case, PVP functionalized magnetite NPs were coated with SiO 2 using an adapted Stöber method. The silica surface was modified with thiol groups using 3-mercaptopropyltriethoxysilane (MPTS). Thus, Au NPs immobilized silica-coated MNPS were selectively collected from the unbounded excess Au NPs by applying a magnetic force. The resulting gold-coated system showed a SPR peak at 530 nm [12]. Abdollahi et al. [14] functionalized SiO 2 NPs (~ 100 nm) with amine groups using 3-aminopropyltriethoxysilane. Thus citrate-functionalized 4.5 nm-sized AuNPs, separately obtained by reduction of HAuCl 4 with NaBH 4 and trisodium citrate, were attached to the SiO 2 surface. This system showed a SPR peak at 520 nm.

Conclusions
In this study, we have succeeded Au-decorated onto SiO 2 surface at room temperature through in-situ precipitation/reduction of Au 3+ ions onto PEI-functionalized silica NPs rich in reducing BH 4 − ions have proven to be a very simple methodology proposed with appreciable optical properties. This protocol produces a ~ 6-nm thin inhomogeneous aggregates and a randomly deposited layer of AuNPs onto SiO 2 surface with an important redshift at ~ 670 nm. The plasmon coupling in small AuNPs aggregates, or different orientations of packed assem-blies´ structures, with decreased neighboring spacing and strong interparticles interaction, caused a large-shift and a significant dip-up of the Plasmon band, in contrast to that of well-dispersed particles. This could be an alternative to the traditional seeding methods of gold deposition for the obtention of core-shell multifunctional nanoplatforms of importance in potential photothermal treatments and diagnostic applications.

Compliance with ethical standards
Conflicts of interest The authors declare that they have no conflict of interest.
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