Synergistic effect in a two-phase laser procedure for production of silver nanoparticles colloids applicable in ophthalmology

This paper reports on the production of Ag nanoparticles (AgNPs) in a water solution based on a two-phase pulsed laser procedure in view of therapeutic ophthalmological applications requiring AgNPs size of <= 10 nm with a narrow size distribution. Nanoparticles of this size scale are capable of penetrating the complex ocular barriers, thus ensuring effective non-invasive drug delivery to the retina. Moreover, the ocular irritation, which is currently associated to the conventional ocular drug administration routes, would be avoided. In the first phase, AgNPs larger than 20 nm were fabricated via laser ablation of a Ag target in water by irradiation with the fundamental wavelength (lambda = 1064 nm) generated by a Nd:YAG laser. During the second phase, to reduce the mean size of the as-obtained nanoparticles and properly adjust the size distribution, the water colloids were additionally irradiated with the ultraviolet harmonics (355 nm and 266 nm) of the same laser source. The effect of the key laser parameters - wavelength, fluence and laser exposure time - upon the nanoparticles morphology was studied. The most suitable post-ablation treatment of initial colloids was obtained by a consecutive irradiation with the third (lambda = 355 nm) and the fourth (lambda = 266 nm) harmonics of the fundamental laser wavelength. By using this approach, a synergistic effect between two mechanisms of light absorption by AgNPs was induced. As a result, contaminant-free colloids of AgNPs with sizes of less than 10 nm and quite a narrow size distribution with a standard deviation of 1.6 nm were fabricated. The toxic effect of the as-produced AgNPs on Gram-positive and Gram-negative bacteria and Candida albicans was explored. The most efficient action was reached against Pseudomonas aeruginosa and Escherichia coli. A potential application was proposed of the synthesized AgNPs colloidal aqueous solutions with antimicrobial action as a non-invasive method for ocular infections prevention and treatment.