Structure, morphology and absorption characteristics of gold nanoparticles produced via PLAL method: Role of low energy X-ray dosage

Abstract

This paper reports the effects of low energy X-ray irradiation doses on the structures, morphologies and absorbance of some colloidal gold nanoparticles (AuNPs) produced in distilled water via the one-step pulse laser ablation in liquid (PLAL) method. An Nd:YAG pulse laser (wavelength of 1064 nm and fluence ranges of 0.076–7.692 J/cm²) was used to ablate the gold plate surface (acted as a target) immersed in distilled water (10 mL). The laser pulse duration was adjusted to 2.5 min (1000 pulses), 5 min (2000 pulses). Simultaneously, the colloidal suspension was irradiated with low energy X-ray. The obtained samples were characterized thoroughly using different analytical instruments. High quality, pure, surfactant-free AuNPs with well-defined morphology and broad size distribution were achieved. The recorded values of the Zeta potential of the as-synthesized AuNPs were increased from -33.1 to -41.2 mV which was mainly due to the low energy X-ray dose-mediated photo- and Auger-electrons generation plus the fragmentation of the bigger NPs into highly stable tinier species inside the colloidal suspension. The observed blue-shift in the absorbance peaks of the NPs centered at 523, 529, and 526 nm irradiated with the corresponding fluences of 0.076, 3.846, and 7.692 J/cm² was ascribed to the quantum size effects. It is established that the synergy between laser ablation and low energy X-ray does may be effective to prepare the contaminants-free AuNPs in the liquid suspension in a simple rapid and cost-effective way.

Introduction

In response to the exponential growth and ever-increasing medical demands of various inorganic and organic nanoparticles, intensive researchers have been dedicated to synthesize some highly stable nanoparticles in a rapid, easy and economical manner. Amongst all types of inorganic NPs useful for diverse applications, the AuNPs are the celebrated one because of its non-toxicity, great stability and flexibility related to the surface manipulations. In fact, the small size (below 30 nm) AuNPs due to their excellent chemical stability have widely been used for the diagnosis (as preventive agent and radio-sensitizer) and treatment (in the radiation therapy, drug delivery and photothermal therapy). Some other factors that attract the AuNPs to the researchers include their non-toxic and biocompatible nature; rapid adjustment and conjugation with the antibodies, ligands and proteins; reasonable surface functionality; shape-dependent surface plasmon resonance; and speedy physical interaction with X-rays [1], [2], [3], [4], [5], [6], [7], [8].

In recent times, sundry physical methods, chemical reduction approaches, sonochemical and green techniques have been developed to produce good quality colloidal AuNPs with desired properties suitable for varied applications [9], [10], [11], [12], [13], [14], [15]. In the past, to synthesize AuNPs via laser ablation method, solid gold target has been irradiated using laser beams at various density, power, wavelength, repetition rate, pulse duration, number of pulses and exposure time [9].

The near-infrared ablation at 1064 nm results in a bottom-up synthesis process dominated by plasma ablation. Laser absorption frees electrons, which interfere with bound electrons and then free them. If the number of collisions increases, the solvent becomes ionized, igniting a plasma. The substance circling the plasma confines it, increasing its stability and absorption ever further. As the expanding plasma is quenched by the confining material, condensation happens in NPs in varying geometries [[16], [17], [18]]. It is worth mentioning some of the advantages of the laser ablation approach such as the ease of processing; elimination of chemical reagents, surfactants and stabilizers; and potential for the production of contaminants-free (pure) samples with customized properties needed for specific applications. Recent studies indicated that the lased ablated colloidal suspension containing the NPs shows aggregation tendency due to the feasibility of bigger size fragments ejected from the target surface, leading to the ill-defied morphology and broad size distributions of the NPs obtained from the laser-produced plasma [19], [20], [21]. Sylverstre et al. [22] lowered the laser energy enabled heating of the samples and demonstrated a remarkable size reduction of the AuNPs in aqueous cyclodextrins. In another study, the mean size of the AuNPs produced in sodium-dodecyl sulfate solution by the PLAL technique was reduced below 8 nm [23]. Despite constant efforts for tailoring various characteristics of the produced colloidal AuNPs in the liquid suspension by controlling the laser processing parameters in the PLAL process an optimized approach still remains deficient.

The -fluence of a geometrical parameter which can be calculated using energy and the spot size of laser beam.

The laser ablation of solid metallic targets in water typically results in the large and widely dispersed NPs due to the post-stabilization and coalescence of the atomic species ejected from the target surface. Utilization of the surfactant in the solution was proven to be useful at it encircles the NPs at an optimal growth rate and prevents the agglomeration. However, the use of surfactants often generates some impurities in the colloidal suspension that may further increase the samples toxicity and reduce the medical applications feasibility. To avoid this problem, Tarasenko et al. [24] prepared some AuNPs by laser ablation in pure water. The results indicated the complete absence of chemical impurities in the colloidal suspension containing AuNPs. Meanwhile, Kabashin et al. [8] proposed two different approaches to eliminate the potential chemical contamination of the samples which could define the standards for the treatment of almost monodispersed AuNPs in water. The first method suggested the use of the femtosecond laser ablation for the AuNPs preparation that rapidly cools the ablation products, thereby preventing the aggregation. The second method recommended the synthesis of the AuNPs in clean water at 532 and 266 nm using the PLAL technique combined with the irradiation from the nanosecond pulsed laser. Motivated by this post-treatment of the PLAL produced AuNPs, few researchers have tried to improve the overall properties of the NPs in the colloidal suspension.

Considering the immense fundamental and applied significance of the AuNPs especially in the nanomedicine we prepared some AuNPs in distilled water using the adaptable PLAL technique. The influence of low energy X-ray irradiation dose on various characteristics of the PLAL grown colloidal AuNPs was evaluated for the first time. Systematic characterizations of the produced colloidal AuNPs revealed their high purity, good stability and biocompatibility. The obtained results were analyzed, discussed and interpreted. The proposed strategy for the production of the colloidal AuNPs in distilled water via the combined methods was shown to be useful in terms of the simplicity, rapidity and cost-effectiveness. These AuNPs may be potential for the future nanomedicine and treatments.