Elsevier

Materials Characterization

Volume 140, June 2018, Pages 320-332
Materials Characterization

A simple and “green” technique to synthesize long-term stability colloidal Ag nanoparticles: Fs laser ablation in a biocompatible aqueous medium

https://doi.org/10.1016/j.matchar.2018.04.021Get rights and content

Abstract

A comparative study of spectral characteristics, size distribution, composition, morphology and long-term stability of colloidal Ag NPs synthesized by ultrashort pulse laser ablation of a solid target and by chemical salt reduction, both in Trisodium Citrate (TSC) aqueous solution was carried out. Several techniques were independently used to characterize optical, structural and compositional properties of the colloidal samples. Both synthesis routes yield bare core Ag and Ag@Ag2O core@shell NPs, with size distributions of roughly similar size centered at about 1.2 nm radius. Stability analysis of samples was conducted for several weeks and after one-year of fabrication, analyzing the characteristics of the plasmon resonance in the optical extinction spectra and independently by zeta potential measurements. For laser ablation colloids, plasmon peak reaches redshift saturation regime at the second week, while salt reduction colloids seem to reach saturation at times beyond one year. After one year, colloids synthesized by UPLA still show a clear single plasmon resonance in their optical spectra together with higher negative zeta potential values, compatible with very good stability characteristics and no signs of agglomeration. A suitable selection of laser pulse energy and citrate concentration may be used for tuning plasmon resonance peak position and FWHM for specific applications. The biocompatibility properties and good stability of the colloids generated by “green” UPLA may boost its use as long-term antimicrobial additive in films, antifungal paints and antibacterial composites.

Introduction

The interest in the synthesis of metallic nanomaterials had a significant growth in the last years, due to their peculiar physical and chemical properties that span a wide range of applications in different fields of science and technology [[1], [2], [3], [4], [5], [6]]. In particular, silver nanoparticles (Ag NPs) have attractive scientific interest owing to their extensive prospects in biosensors [7,8], antimicrobial and antiviral agents [[9], [10], [11], [12], [13]]. It has been recently reported the successful use of 10 nm sized Ag NPs as additives in water-borne paints for antifungal activity, since indoor-grown molds contribute to deterioration of human health [14].

The ability to control size, shape, functionalization and stability of Ag NPs is essential for expanding their possible applicability. For this goal, typical methods are based on chemical reduction of silver salts in solution, commonly used for providing good size control and resulting in final spherical shape. However, this approach leaves chemical residuals in the final colloidal suspension, which may be toxic for certain applications, thus adding an extra difficulty in sample purification.

Pulsed laser ablation of solid targets in liquids has become an alternative method for overcoming the mentioned drawback. Mafuné et al. [15] employed different concentrations of sodium dodecyl sulfate (SDS) to stabilize 10 nm average diameter NPs fabricated by ns laser ablation with different energies. They found a high stability when the NPs surface was coated with double SDS layers. Bae et al. [16] synthesized 5–50 nm silver NPs colloids also by ns laser ablation in NaCl aqueous solutions with different concentrations, obtaining a reduction of the average particle size. However, they conclude that addition of NaCl produced less stable colloids than in neat water during their 50 days experiments. Grade et al. [17] used a 10-ps pulse laser at 1064 nm with a 110 μJ pulse energy for obtaining Ag, Au and Ag-Au alloys for studying the biological behavior and antibacterial and cytotoxic effects on bacteria and cells for different ratios of gold and silver within the particles were tested. Additionally, the influence of two commonly used stabilizers (albumin and citrate) on colloid aggregation and the bioactivity of all synthesized materials were examined.

In this work, Ag colloidal suspensions synthesized by Ultrashort (fs) pulse laser ablation (UPLA) of a solid Ag target using two pulse energies and different mM Trisodium Citrate (TSC) concentrations were characterized. Size distribution, structure and spectral characteristics of the obtained Ag NPs were analyzed by means of optical absorption spectroscopy (OAS). Shape was studied with Atomic Force Microscopy (AFM) while structure and morphology by Transmission Electron Microscopy (TEM) techniques. Composition and possible interactions of NPs with adjacent stabilizer molecules was studied using micro-Raman spectroscopy. The influence of TSC concentration on plasmonic characteristics and size distribution of Ag NPs synthesized by UPLA was also studied.

Stability properties of the generated colloids were analyzed along one year after sample fabrication, recording the evolution of experimental absorption spectra together with zeta potential measurements. Long-term stability results are compared with those for salt reduction chemical route synthesis using TSC as stabilizer. The results may shed light on possible applications of Ag colloids in biocompatible antibacterial compounds.

Section snippets

Ag NPs Synthesized by UPLA and Silver Salt Reduction Methods

Silver colloidal suspensions were generated using a 120 fs pulse width Ti:Sapphire chirped pulse amplification system (Spectra Physics), centered at 800 nm wavelength, 1 kHz repetition rate and a maximum output pulse energy of 1 mJ. A solid silver disk (99.99%) placed at the bottom of a beaker filled with 4 mL of milli-Q water or different TSC aqueous solutions was used as ablation target (Fig. 1(a)).

Ablation was performed using pulse energies of 100 μJ and 500 μJ focused by a 5 cm focal length

Spectral Analysis: Composition and Size Distribution

Fig. 2 shows normalized experimental absorption spectra of Ag colloids fabricated by UPLA with 100 μJ and 500 μJ pulse energy in 1 mM TSC solution. A spectrum corresponding to silver salt reduction synthesis for the same TSC concentration is also shown. Curves of theoretical fits using Mie theory for metal spherical NPs [19], taking into account the experimental bulk dielectric function given by Johnson and Christy [20] together with the corrective terms for free and bound electron

Conclusions

Colloidal suspensions of Ag NPs were synthesized via ultrashort pulse laser ablation and chemical salt reduction in different mM TSC concentrations. Spectral characteristics, size distribution, structure, composition and morphology of colloidal Ag NPs were analyzed by AFM, TEM, optical spectroscopy and zeta potential.

Fitting of the absorption spectra using Mie theory with appropriate size-dependent metal dielectric function resulted in the presence of bare core Ag and Ag@Ag2O core@shell NPs,

Acknowledgements

This work was Granted by PIP 0280 and PIP 0720 of CONICET, MINCyT-PME 2006-00018, AMPCyT-PICT 585/2014, 11/I197 (Facultad de Ingeniería) of Universidad Nacional de La Plata, Argentina, and 11/X680 (Facultad de Ciencias Exactas) of Un1iversidad Nacional de La Plata, Argentina. We thank C2NANO-Brazilian Nanotechnology National Laboratory (LNNano) at Centro Nacional de Pesquisa em Energia e Materiais (CNPEM)/MCT (#19927 and 18425) and Research Proposal TEM-16976 for the use of TEM. AFM was carried

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