Photochemical formation of chitosan-stabilized near-infrared-absorbing silver Nanoworms: A "Green" synthetic strategy and activity on Gram-negative pathogenic bacteria

Abstract

A facile, single-step, non-seeded photochemical protocol for producing a new type of anisotropic silver nanostructure, "nanoworms", with curved longer dimensions and smooth, rounded edges. The nanoworms exhibit surface plasmon resonance (SPR) absorption in the near-infrared window (NIRW) region and are stabilized using biocompatible polymer chitosan, rendering biocompatibility and amplified safety for biological utility of the composition. Both NIRW-absorbing nanoworms and visible-absorbing nanospheres herein are attained exclusively by employing green chemistry principles. Contrary to seed-mediated or polyol techniques, the protocol demonstrates the feasibility to selectively synthesize NIRW-absorbing silver nanostructures in a single step and in complete absence of any known reducing agent. The effect of irradiation, pH, and concentration of starting materials on the formation of nanoworms vs nanospheres is investigated in detail and analyzed by optical spectroscopy and electron microscopy. The dominant SPR obtained in the NIRW region of the nanoworms results from anisotropic AgNPs, as opposed to agglomeration. From TEM images, it is also very clear that a strong correlation exists between the SPR peak maximum and the size distribution of the anisotropic nanoworm structures, with SPR peak maximum exhibiting red shift with the increase in the size of the nanoworm population. Although there is significant size variation of different nanoworms of a given population, all samples exhibit remarkable stability. The nanoworms retained their NIRW-absorbing features even at physiological pH and at a constant ionic strength. The nanodispersions also retained their SPR features in King's B medium. Antipathogenic assays reveal that the anisotropic NIRW-absorbing nanoworms exhibit the highest growth inhibition compared to other spherical nanosilver and molecular silver forms on Gram-negative pathogenic bacteria, Pseudomonas syringae pv. maculicola ES4326 and P. syringae pv. tomato DC3000. These results underscore shape effects of AgNPs and suggest that nanoworms favor the adhesion to (curved) rod-shaped Gram-negative bacteria, resulting in the highest inhibition compared to isotropic AgNPs (smaller spheres), sulfa antibiotics (silver sulfadiazine), and silver ions (AgNO₃).

Keywords: Anisotropic; Antipathogenic; Colloids; Nanocolloids; Nanodispersions; Nanoparticles; Nanosilver; Near-infrared (NIR); Surface plasmon resonance (SPR).