Synthesis of Ag@Silica Nanoparticles by Assisted Laser Ablation
© González-Castillo et al. 2015
Received: 25 May 2015
Accepted: 5 October 2015
Published: 13 October 2015
This paper reports the synthesis of silver nanoparticles coated with porous silica (Ag@Silica NPs) using an assisted laser ablation method. This method is a chemical synthesis where one of the reagents (the reducer agent) is introduced in nanometer form by laser ablation of a solid target submerged in an aqueous solution. In a first step, a silicon wafer immersed in water solution was laser ablated for several minutes. Subsequently, an AgNO3 aliquot was added to the aqueous solution. The redox reaction between the silver ions and ablation products leads to a colloidal suspension of core-shell Ag@Silica NPs. The influence of the laser pulse energy, laser wavelength, ablation time, and Ag+ concentration on the size and optical properties of the Ag@Silica NPs was investigated. Furthermore, the colloidal suspensions were studied by UV–VIS-NIR spectroscopy, X-Ray diffraction, and high-resolution transmission electron microscopy (HRTEM).
Nowadays applications of metallic nanoparticles (NPs) in different areas of science and technology are increasing. In recent years, researchers have focused their efforts to synthesize these materials and to study their optical properties, which strongly depend on the size and shape of the NPs . An important part of these studies has been focused on the surface plasmon resonance (SPR), which consists in a resonance coupling between the electromagnetic field frequency and the free electrons oscillation frequency on the metal NPs. When resonance conditions are matched, a strong absorption of the electromagnetic field energy is brought about by metallic NPs . Thanks to these properties, Ag NPs have been widely used as biological labeling  and as antibacterial agents [4–7], among others [8, 9]. Different physical and chemical methods have been used to obtain NPs [10–15], among them is the laser ablation in liquid [16, 17].
Here, by using a new variant of laser ablation technique (assisted laser ablation) core-shell Ag@Silica NPs were synthesized [18–20]. Among other advantages, the silica coating provides inertness [4, 21, 22] and high dispersibility of the NPs [23, 24] which has enabled their use in numerous applications [25, 26]. In these previous reports, the silver salt was added before the target ablation, which led to the continuous production of silver NPs during the laser ablation process. This fact has the important disadvantage of the extinction of the laser radiation produced by the silver NPs. This effect provokes a continuous reduction of the laser intensity at the target surface, which in turn prevents the obtaining of higher NPs concentrations. In this paper, the silver salt is introduced immediately after the irradiation of the silicon target, producing also Ag@Silica NPs. Furthermore this paper is aimed to get a deeper insight in the influence of the growing parameters on the morphological, optical and structural properties of the NPs.
Optical and structural properties of the Ag@Silica NPs were studied as functions of the laser pulse energy (from 5 to 50 mJ), ablation time (from 1 to 25 min), [AgNO3] (from 5 × 10−5 to 5 × 10−4 M), and laser wavelength (1064 and 532 nm). The resulting colloids were examined by high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM3010 300 kV with a thermionic electrons source. Structural properties of the samples were also studied by X-Ray diffraction (XRD) using a Bruker XRD D8 Advance diffractometer in Bragg Brentano configuration for powders and a sweeping from 20° to 80°. The samples for the XRD study were prepared by drop casting of the Ag@Silica NPs colloids on a hot glass plate (50 °C), in order to evaporate the H2O content and to form a film thick enough to be measured by XRD. The absorption spectra of the colloidal suspensions were measured using a Perkin-Elmer Lambda 9 UV–VIS spectrophotometer and a quartz cuvette with 10 mm of path length.
Results and Discussion
Influence of the Laser Energy on the NPs Properties
For this study, an ablation time of 20 min, a wavelength of 532 nm, and a [AgNO3] of 1.25 × 10−4 M were used. Every colloid was obtained using a constant laser pulse energy, which took values between 5 and 50 mJ.
The diffractogram also reveals the presence of crystalline silicon with peaks located at 2θ = 27.8°, 46.2°, and 54.8°. The peak located at 2θ = 27.8° was used to estimate the size of the crystalline silicon present in the sample, D Si = (46 ± 1) nm. This suggests that silicon particulates were ejected from the target by the shock waves produced during the ablation process. The presence of silicon crystals in the solution is also confirmed by the dispersion tail observed in the absorbance spectrum (Fig. 2a) between 500 and 800 nm. The error of the crystallite size was estimated by error propagation in the Scherrer formula, using the standard errors for FWHM and peak position obtained during the fitting process.
The TEM images also reveal that there is no significant dependence of the nanoparticles size on the laser pulse energy. These results agree with results obtained by the absorbance measurements.
Influence of the Ablation Time on the NPs Properties
Influence of the [AgNO3] on the NPs Properties
Influence of the Laser Wavelength on the NPs Properties
In order to evaluate the influence of the laser wavelength on the synthesis process of Ag NPs, the experiment reported in the previous paragraph was also performed using a longer laser wavelength (λ = 1064 nm).
Similar to the results obtained at shorter wavelength, the diffractogram of the Fig. 9a exhibits peaks located at 2θ = 27.9°, 46.8°, and 56.6° indicating the presence of crystalline silicon in the suspension, which must be ejected from the target by the shock waves produced during the ablation process. Using the Scherrer formula and the peak located 2θ = 27.9°, it was possible to estimate the size of the silicon crystallites present in the sample, D Si = (28 ± 1) nm.
Silver NPs capped with silica (Ag@Silica NPs) were synthesized by means of the pulsed laser ablation of a Si solid target immersed in H2O and the subsequent addition of a [AgNO3]. Absorption spectra reveal the strong surface plasmon resonance of Ag NPs. The influence of the laser energy, laser wavelength, ablation time, and [Ag+] on the size and optical properties of the silver nanoparticles was investigated. Furthermore, the colloidal suspensions were studied by transmittance in the UV–VIS-NIR, X-Ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The results reveal that the NPs concentration in these colloids can be controlled by the [Ag+], the laser pulse energy, the laser ablation time, or the laser wavelength. The silica shell coating on Ag NPs should reduce the release of silver ions, thus improving the colloidal stability and facilitating its use in biomedical applications.
The authors gratefully acknowledge the Mexican founding agency CONACyT under the projects CB183728 and CB 176705, also the Instituto Politécnico Nacional under the projects SIP20130216, SIP20141409, and SIP 20151327, for financial support. We also acknowledge the Laboratorio de Microscopia Eletrônica from LNLS, Campinas, Brazil, for HRTEM measurements and for the training course of a student for TEM operation. We extend additional thanks to the Brasilian founding agencies FACEPE and CNPq for financial support.
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