
Abstract
Recently, with the growing interest in nanoscience and nanotechnology, nanoparticles have attracted increasing attention due to their unique properties and their wide range of applications. More recently, the synthesis of noble metal nanoparticles has been intensively studied by researchers and used in the biomedical field due to their antibacterial activities. In view of that, the main objective of the present work is to synthesize and evaluate metal nanoparticles via pulsed laser ablation of metal targets in deionized water and use them as antibacterial activity. Accordingly, silver and gold nanoparticles were synthesized by irradiating silver and gold target by pulsed laser ablation with laser parameters (1064 nm wavelength,10 Hz frequency) in deionized water and then examined for antibacterial activity. Colloidal solutions of silver and gold nanoparticles were prepared with different pulsed laser energies (620, 740, 880, and 1000 mJ) and different laser pulses (360, 750, 880, and 100.1500) of wavelength 1064 nm and frequency 10 Hz.
To determine their structures, morphologies, optical, and infrared spectra, the synthesized (Ag NPs) and (Au NPs) were characterized by using various high-throughput analytical techniques such as (UV-Vis) spectroscopy, transmission electron microgram (TEM), Fourier transform infrared (FTIR) spectra, and Zeta potential.
UV-Vis spectroscopy of synthesized nanoparticles showed the inverse relationship between the size and the band gap energy of the prepared nanoparticles. All the results indicated the fact that Ag NPs particle size ablated at high laser energy is bigger than that obtained from low laser energy. However, high laser energy yielded a higher Ag NPs concentration and a lower energy gap than that of low laser energy. Therefore, the obtained Ag NPs for the sample synthesized at 880 mJ were more characterized and defined as an optimized sample in this study.
Absorption spectrum around 400 nm for all samples indicated the production of spherical Ag nanoparticles in the solution, where surface plasmon resonance SPR depended on the shape and size of the suspended nanoparticles in the deionized water. However, an inverse relationship was observed between the size and the band gap energy of the prepared nanoparticles. We attribute this to the effect of quantum confinement, where an decrease in particle size leads to discrete in the electronic levels and resulting bandgap spacing.
About the synthesized Au NPs, the UV-visible spectrum exhibits many peaks, approximately between 525 and 530 nm, indicating the formation of AuNP; An increase in the intensity of the absorption peak correlates with a higher concentration of AuNPs in the solution. Additionally, the enhancement of absorption peak strength with an increasing number of pulses correlates with a larger concentration of Au NPs in the solution, as seen by the greater concentration of Au NPs at 1500 pulses compared to the lower pulse preparations.
Transmission electron micrograms (TEM) and EDX have also been performed to determine the morphology, shape, size distribution, and elemental composition of the Ag and Au nanoparticles. The TEM micrograph clearly displays nearly spherical and uniformly distributed nanoparticles. Almost all, Ag NPs are spherical in shape with some clusters and an increase in particle size with increasing laser energy to 880 mJ, while at higher energy a cloud of particles formed as new targets above the target surface, which decreased the laser ablation efficiency. The biggest Ag NPs size obtained from TEM of the sample prepared at 880 mJ is consistent with the optical result. A narrow particle size distribution of Ag NPs was observed at an ultra-small size range ranging between (2 to 6) nm, with a few large particles up to (7) nm being observed. The majority of Ag nanoparticle size was observed to be in the small range, and the average particle diameter of the synthesized Ag nanoparticles was obtained approximately to be (3) nm. We investigated the Fourier transform infrared spectroscopic (FTIR) spectrum of synthesized nanoparticles to identify the presence of functional groups in biomolecules responsible for the bio reduction of Ag+ and capping/stabilization. For all samples, the overall results obtained from FTIR spectra confirm the formation of Ag NPs inside the deionized water.
Zeta potential of synthesized nanoparticles showed the determined value of zeta potential was found to be (-35.21) mV, suggesting that the Ag NPs particles repel each other because they are negatively charged in dispersed medium and that prevents aggregation. However, the negative value of the obtained zeta potential in deionized water is due to the adsorption of hydroxide anions from the medium. The high negative value of zeta potential confirms excellent long-term stability, good colloidal nature, high dispersity, and resist agglomeration of Ag nanoparticles because of negative-negative repulsion. Noteworthy, it is well known that zeta potential with a value greater than (-34) mV indicates good stability of Ag NPs.
The results showed that the properties of synthesized Ag NPs and Au NPs depend much more on the laser parameters. We can use laser energy to control the properties of the prepared nanoparticles. A uniform distribution of spherical ultra-small Ag NPs with an average size of (3) nm and Au NPs with an average size of (14) nm was obtained and suspended in the deionized water, which is the most effective size for antibacterial activity. However, the result indicated that the ablated nanoparticles are stable for 4 months in deionized water.
Afterward, the antibacterial activity of the colloidal solution of the optimized synthesized (Ag NPs) and (Au NPs) against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) bacteria was examined by using the well diffusion method.
It was found that the prepared (Ag NPs) exhibited a strong activity against E. coli and S. aureus bacteria growth. which might be because Ag ions are highly toxic to microorganisms. The average zones of inhibition of (Ag NPs) were found to be about (26) mm for E. coli and (32) mm for S. aureus bacteria.
Conversely, the observed results of the investigation revealed that the (Au NPs) did not show any antibacterial effect against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) bacteria, which might be attributed to the presence of thick layers of peptidoglycans present on the cell membrane.