1. Introduction

The synthesis of nanostructured materials with useful and tuneable properties is central to development in nanoscale science and technology. Nanometer scale metal particles exhibit optical, electronic, chemical and magnetic properties of great aesthetic, technological and intellectual value.1 Characteristically, silver and gold nanoparticles, exhibit a strong adsorption band in the visible region and this is indeed a small particle effect, since they are absent in the individual atoms as well as in the bulk.[2,3] The frequency of the LSPR is strongly dependent of different properties of the nanoparticles.4-8 The optical and electrokinetic properties of metal nanoparticles are strongly influenced by their composition, size, shape, and surrounding environment, such as, the proximity of other particles. The silica layer on the metal nanoparticles is important,[9,10] because it will separate metal spheres, avoiding touch with one another and can also make metal particles are more stable in air. More importantly, the existence of an outer silica coating could provides us with an opportunity to tailor its electronic, or optical properties to obtain 3D metallodielectric structures with a variety of enhanced functionalities.[11] The assembly of metallic nanoparticles presents interesting optical properties such as, single molecule detection using surface-enhanced Raman scattering (SERS)[5,12-29] and nanoscale optical devices.[30,31]

Colloidal solutions of noble metal show characteristic colours that have received considerable attention from researchers. The interesting colours observed in colloidal solutions have led to extensive study of their optical spectroscopic properties in an effort to correlate their behaviour under different microenvironmental conditions. [32-47] The reason for the present excitement in Ag and Ag-Au alloy nanoparticles research is due to the new mode of preparation,[48-50] their stability, morphology, size, composition and its role in nanoscience and future nanotechnology. This Ag@SiO₂ nanoparticles produced by assisted laser ablation (ALA) method are structured by a silver core and a porous silica shell,[49-52] which in turn, consists of the accumulation of small SiO₂ nanoparticles (1-2 nm).[49,50]

The aim of this paper is highlight the discussion in relation to the synthesis and stabilization phenomenon of the Au-Ag@SiO₂ alloy nanoparticles. The stabilization phenomenon has been accounted in the light of the DLVO theory and experimental aspects of the optical properties will be discussed.

2. Experimental Section

Materials. The reagents were purchased from, AgNO₃ (silver nitrate 99%, Paureac), KAuCl₄ (potassium tetrachloroaurate (III) 98%, Sigma-Aldrich) and Na₂CO₃ (sodium carbonate 99%, Fluka). All these chemicals were used as received without further purification or treatments and their solutions prepared in de-ionized milliQ water.

Synthesis Procedure. Nearly monodispersed in diameter size Ag@SiO₂ nanoparticles were produced by ALA method, using a third-harmonic (355 nm) Q-Switch Nd:YAG laser irradiation of a Silicon target in a aqueous solution of silver.[48-50] The ALA is a simple method for the fast (2 to 3 min), scalable synthesis of inert colloidal metal-silica nanoparticles in stable colloids. The method is based on the laser ablation of a solid target submerged in an aqueous solution of the metal salts, whose reduction will give rise to nanoparticles. In addition, ablation parameters, target materials, and metal salts can be combined and controlled to influence the size, morphology and composition of nanoparticles.[48-50]

Experimental Design. Step 1: To synthesis Ag@SiO₂ nanoparticles was used a concentration 1.25x10^-4 M of AgNO₃. After synthetised Ag@SiO₂ nanoparticles, these were filtered to eliminate bigger particles, using a NALGENE^® filter with Polietersulfona (PES) membrane with 0.2 µm pore size. Step 2: later on, 8 mL of Ag@SiO₂ nanoparticles suspension were distributed in each recipients (6 in totals) and added to them several [KAuCl₄] from 0.1 to 0.2x10^-4 M, with step of 0.02. This process is carried out maintaining a continuous agitation of the sample and a slow adding of [KAuCl₄]. Step 3: finally, a [Na₂CO₃] of 0.5 mM were added to the Au-Ag@SiO₂ alloy nanoparticles to get a pH stabilization at 8-8.5. The electrokinetic and spectroscopic characterization for Ag@SiO₂ and Au-Ag@SiO₂ alloy nanoparticles was performed measuring the absorbance, size and ζ-potential as a function of the time and [KAuCl₄].

Diameter Size Measurements. Small samples were transferred onto a copper mesh grid covered with a carbon film and let to dry. A transmission electron microscope (JEOL, mod. JEM-1010, 100 kV accelerating voltage) with a digital MegaView III camera and a ¨Analysis¨ software for the image acquisition, was employed to take the electron micrographs of the resultant Ag@SiO₂ and Au-Ag@SiO₂ alloy nanoparticles. From these electron micrographs and using the ImageJ 1.40g software (National Institute of Health, USA) the diameter size were measured.

Spectroscopic Measurements. Nanoparticle sample solutions were transferred into a quartz cell 10 mm width and their absorption spectrum were recorded using a UV-VIS Recording Spectrophotometer (UV-250 1PC) from Shimadzu Corporation, Japan.

Electrophoretic Mobility Measurements. A commercial device known as ZetaSizer NanoZS Zen3600 (Malvern Instruments Ltd., UK) was used to measure the electrophoretic mobilities (μ_e) of each nanoparticle sample solution. The electrophoretic mobility measurements were made using the M3-PALS technique, with a folded capillary cell (DTS1060). NanoZS is based on the back scattering data detection (173° scattering angle) and uses 4 mW He-Ne laser (633 nm). The ζ-potential values were calculated from the electrophoretic mobility measurements using the Henry´s approximation. Three measurements of each nanoparticle sample solution were made at 25 ± 0.1 ℃ to get average values.

3. Results and Discussion

The colour change caused, by reduction of gold salts, was found dependent on their concentration in solution. The pure Ag@SiO₂ solution turned to light yellow; in contrast, increasing the [KAuCl₄] the Au-Ag@SiO₂ solution turned yellowish red.

Figure 1. Picture of aqueous suspensions of Ag@SiO2 nanoparticles (above). Sample 1, immediately obtained by ALA method; sample 1a, plus [Na2CO3] of 0.5 mM added 5 hour later; sample 2, plus [KAuCl4] of 0.1x10-4 M; sample 3, plus [KAuCl4] of 0.12x10-4 M; sample 4, plus [KAuCl4] of 0.2x10-4 M. To the samples 2-4 the [Na2CO3] was added five hour later the addition of [KAuCl4]. TEM images for the samples 1-4 (bellow), the inset show the nanoscale size distribution particles

The histograms (inset TEM images) provide the size distribution of these nanoscale Ag@SiO₂ and Au-Ag@SiO₂ alloy particles. Their mean diameters decrease from 10.38 to 9.02 nm in relation to increasing [KAuCl₄], but the monodispersity is more accented for the sample with high [KAuCl₄]. This decrease in size should be due to the increase of the chemical reaction that take place between the Ag@SiO₂ nanoparticle and the [KAuCl₄]. For larger diameter of the nanoparticles, the surface charge distribution should be higher, which induces a higher concentration of ions in the vicinity of nanoparticles. This fact weighs the chemical reaction in these nanoparticles over, emphasizing the monodispersity.

Effect of Na₂CO₃ Solution on the Physical Stability of the Au-Ag@SiO₂ Alloy Nanoparticles. The study of the experimental stability of these systems will be based on the use of the classical DLVO theory that have been employed in colloid science to study particle-particle interactions, coagulation, sedimentation filtration and the behaviour of electrolyte solutions.[53-56] This theory is based on the idea that pair wise interaction forces, which arise from the interplay of attractive van der Waals forces (F_attr) and repulsive Coulomb forces (F_rep) screened by Debye-Hückel ion clouds. Then, the dispersed colloid is stable for F_rep >> F_attr, leading with no aggregation phenomenon.

The classical DLVO theory states that the total interaction potential between two nanoparticles () can be expressed as the sum of electrostatic repulsion (U_elec) and the van der Waals attraction (U_vdw),[53,54]

(1)

Depending on the particle size and the double layer thickness, the electrostatic repulsion potential (U_elec) between two colloidal particles of radii ¨a¨ (in general a₁ and a₂) can be expressed in the following form,[55]

(2)

where ε_r is the permittivity of the medium, Ψ_o the potential at the particle surface which can be estimated from the ζ-potential measurements,[57-61] κ the inverse Debye length and Δ the thickness of the Stern layer.

Assuming the particles to be spherical and the surface potential and the background ionic strength to be constant, the van der Waals attraction potential (U_vdw) between the two particles can be calculated as,[54-56,58]

(3)

where r is the distance between two particles (r = h+2a, here h is the approaching surfaces).[62] The van der Waals interaction is the dominant attraction and is dependent on the particle radii ¨a¨, the centre-to-centre separation distance ¨h¨ and the Hamaker constant ¨A¨, which plays an important role in the description of attraction energy between the particles.[63]

In order to calculate the electrostatic repulsion potential (U_elec), the study of the ζ-potential behaviour for Ag@SiO₂ and Au-Ag@SiO₂ alloy nanoparticles, from their electrophoretic mobilities measurements, is necessary. Immediately after obtained the Ag@SiO₂ nanoparticles (sample 1) their electrophoretic mobility were measured and from it the ζ-potential calculated using the Henry´s approximation, where f(κa) was calculated for κa ≤ 1.[64] After that, the [KAuCl₄] required were added (samples 2-4) proceeding to measure their electrophoretic mobility and from it the ζ-potential calculated.

The changes in the ζ-potential values referred to these samples can be observed from Figure 2. The electrophoretic mobility increases negatively with [KAuCl₄], from 2.34x10^-8 m²V^-1s^-1 (sample 1) to 3.40- 3.64x10^-8 m²V^-1s^-1 (samples 2, 3) respectively. For the sample 4, the increment in the electrophoretic mobility was less than the two samples before (3.35x10^-8 m²V^-1s^-1).

Figure 2. ζ-potential of the Ag@SiO2 and Au-Ag@SiO2 alloys nanoparticles as a function of the time (A-5 hours; B-24 hours; C-48 hours; D-120 hours; E-144 hours). The standard error plotted is related to the electrophoretic mobility as the experimental measurement

Five hours later, [Na₂CO₃] of 0.5 mM was added to Ag@SiO₂ nanoparticles (sample 1a) and Au-Ag@SiO₂ alloy nanoparticles (samples 2, 3, 4). This provokes an additional negative increasing in the electrophoretic mobility again, being greater in sample 1a; this effect could be a consequence of a higher accumulation of silica on the surface of nanoparticles and/or of the pH increase from 5 to 8.5.[65,66] The electrophoretic mobility values are 3x10^-8 m²V^-1s^-1 (sample 1a) and 4.21, 4.02 and 4.07x10^-8 m²V^-1s^-1 (samples 2, 3, 4), respectively. Past the 24, 48, 120 and 144 hours the electrophoretic mobility, in all cases, decreases negatively and spread to a plateau of approximately the same value (samples 2, 3, 4), in which should oscillate.

In the nanoscale Ag@SiO₂ and Au-Ag@SiO₂ systems, ζ-potential is used to estimate the electrokinetic properties and colloidal stability of the particles. The Ag@SiO₂ nanoparticles were obtained by ALA method,[48-50] after that several [KAuCl₄] were added in order to get Au-Ag@SiO₂ alloy nanoparticles. The interparticle interaction potentials between two particles can be calculated as function of their separation from Eqs. 1-3.

Table 1. Input parameters used for DLVO calculation and the stability

The changes in the interaction potential of the Au-Ag@SiO₂ alloy nanoparticles in function of the time are shown Figure 4. Before addition of the [KAuCl₄] (samples 1), the Ag@SiO₂ nanoparticles are stable since the energy barrier is high enough to prevent aggregation. From the previous measurements are inferred that the introduction of the Au³⁺ and the consequent partial oxidation of the silver and the incorporation of Au to the nanoparticles could originate a gold fine layer on the Ag nanoparticles surface.

KAuCl₄ + 3Ag → 3Ag^- + Au +K⁺ + 4Cl^-

In the same way, the metallic nanoparticles diameter diminishes and in turn the thickness of the silica layer could increase. Additionally, it is known that nanoparticles of gold and silver alloys present a higher electron density on its surface which nanoparticles of Au or Ag separately. This fact increment the catalytic activity of bimetallic Au-Ag nanoparticles, in special when Au element is mainly located near the surface.[68-70] Therefore, the addition of Au will cause an increment in the module of the ζ-potential. With the time elapse, the atoms of Au that conform the superficial layer could spread toward the interior of the nanoparticles obtaining a radial distribution with more homogeneous composition. This effect, in turn, would tend to diminish lightly the ζ-potential, stabilizing at a given value.

Figure 3. Change in the interparticle interaction potential (net energy barrier ) for Au-Ag@SiO2 alloy nanoparticles as a function of the time (A-5 hours [Na2CO3]; B-24 hours; C-48 hours; D-120 hours; E-144 hours). The inset shows the ζ-potential changes as in Figure 2

Upon the time 5 hour, after adding Na₂CO₃, the energy barrier increases significantly for all samples. As time elapses, the energy barrier decreases in the same way for all the samples, reaching approximately the same value of .

Optical Response of the Au-Ag@SiO₂ Alloys Nanoparticles in Na₂CO₃ Solution. The absorbance measurements of Ag@SiO₂ and Au-Ag@SiO₂ alloy nanoparticles systems are provided in Figure 4. The absorbance spectra (inset) display that the maximum wavelength (λ_max) for Ag@SiO₂ system is red shifted (sample 1A-E) showing a characteristic peak at 403 nm. As was commented, the Na₂CO₃ addition could cause a higher accumulation of silica on the surface of nanoparticles which provoke a red shift of λ_max.[71,72]

Figure 4. UV-VIS absorption spectra for Au-Ag@SiO2 alloy nanoparticles as a function of the time (A-5 hours; B-24 hours; C-48 hours; D-120 hours; E-144 hours). The inset shows the UV-VIS absorption spectra of the Ag@SiO2 nanoparticles

It is of highlighting that with the increment of the [KAuCl₄] the appearance of a wide absorption band displaced toward the red is observed with high clarity. Parallel, the main absorption band diminishes displacing equally toward the red. Core-shell Au-Ag@SiO₂ nanoparticles exhibit two characteristic absorbance bands. One of them is characteristic of the Ag-core nanoparticles. The other band, red shifted is attributed to the Au-shell.[73-79] Therefore, from the absorption spectra measured are inferred that just after placing [KAuCl₄] should be formed an Au-shell with passing time, the λ_max starts to diminish in the follow range: samples 2 (434.0-424.5 nm), sample 3 (450.5-442.0 nm) and sample 4 (465.0-458.5 nm). Parallel, the principal absorption band increases and the second band decreases to not be noticeable. This effect could be caused to the diffusion of the Au layer into the nanoparticle. Thus, our data suggest that the co-reduced solution consists of alloy nanoparticles and not a mixture of Ag@SiO₂ and Au nanoparticles.

Figure 5. Wavelength corresponding to the maximum absorbance for varying the [KAuCl4] in the Figure 4

Figure 5 shows the linear fashion increase of λ_max (at 144 hours) with [KAuCl₄]. From λ_max obtained for each sample can be estimated the respective Au concentration at the nanoparticles. For the samples 2, 3 and 4 (424.5, 442.0 and 458.5 nm), the Au/Ag relation estimates must be 20/80, 30/70 and 45/55 respectively.[80,81] But, according to the amount of gold introduced to each sample (2, 3 and 4) and assuming that each Au³⁺ ion replaces three silver atoms, the Au/Ag relation calculated should be 10/90, 12/88 and 14/86, respectively. Thus, the Au atoms should not be homogeneously distributed in the nanoparticle, localized mainly in the gold-rich shell.

In short, upon adding [KAuCl₄] to the colloidal solution of Ag@SiO₂, the ions Au³⁺ should react with the silver core of the nanoparticle, oxidizing the Ag. In this way, a Au shell on the nanoparticle would be structured, as show the Figure 6. This effect is reflected in the appearance of a wide absorption band displaced toward the red (Figure 4), which is characteristic of Au shell.

Figure 6. Schematic representation for the synthesis process of Au-Ag@SiO2 alloy nanoparticles

Upon lapsing 144 hours the atoms of Au that conform the shell should spread toward the interior of the nanoparticles, focusing mainly on the external atomic monolayers of the nanoparticle. In other words, the nanoparticles should be formed by a Ag core or (Ag-Au) with lower concentration of gold atoms and an shell with high Au content (Figure 6).

4. Conclusions

The synthesis of nanostructured materials with useful and tuneable properties is central to development in nanoscale science and technology. The understanding of the electronic absorption and dynamics in individual nanoparticles is essential before assembling them into devices, which is essentially the future goal of the use of nanostructured systems. For that reason, electrokinetic and spectroscopic characterization should be a first step before the application of these nanoscale Ag@SiO₂ and Au-Ag@SiO₂ alloy particles. In this work has been to understand more clearly the composition, structure and electronic properties of Ag@SiO₂ and Au-Ag@SiO₂ alloy nanoparticles. On the other hand, is established that the addition of KAuCl₄ and consequently formation of Au-Ag@SiO₂ results in greater stability of the colloidal solution. Additionally, it is demonstrated that the introduction of Na₂CO₃ causes a further increase in colloidal stability.

ACKNOWLEDGEMENTS

This research work was funded by grants as Invited Researcher from the University of Valencia, Spain. It is also a part of the activities for the Laboratory for Material and Optoelectronic Devices appointed by the Institute of Materials Science and for the Biomembranes Group appointed by the Institute of Molecular Science. To the Ministry of Science and Innovation (Juan de la Cierva Program), Spain.