Anti-reflective and anti-bacterial properties of biomimetic nanostructures

Abstract

In this thesis artificial nanostructured surfaces inspired by the moth eye were developed on both inorganic and organic substrates. Two properties, i.e. anti-reflective (AR) and anti-bacterial (AB) were studied in detail. On inorganic fused silica (Suprasil®) substrates, nanopillar arrays were fabricated by combining block copolymer micellar lithography (BCML) and reactive ion etching (RIE) techniques. The nanopillar arrays were fabricated on a large area and the parameters of the pillars were controlled. The substrates were used as molds to create nanostructures in organic substrates using two methods: replica molding and nanoimprinting. The first method transferred the pillar structure into a polyurethane substrate creating nanoholes. However, it was shown that this method was limited due to the low aspect ratio and difficulties in mold removal. Using nanoimprinting methods instead solved these problems. Both nanohole and nanopillar structures were homogeneously imprinted in a large area of the intermediate polymer stamp (IPS®) and polymethylmethacrylate (PMMA) materials. The AR properties of both organic and inorganic substrates were characterized using optical spectrometry. On Suprasil® surfaces, the transmittance was increased over a wide wavelength range of 200-1000 nm, with a maximum of 99.5% transmission per interface. Nanoimprinted IPS® and PMMA also depicted highly improved transmittance, with an increase from 91.5% to 95% with a single-sided nanohole array on IPS® and from 91.5% to 97.5% with a double-sided nanopillar array on PMMA. Excellent AR performance was achieved to a high incident angle of 60°, which significantly outperformed traditional thin-film AR coatings. A theoretical model was also set up matching the experimental results very well. The AB properties of the moth eye inspired structures were investigated on the nanostructured Suprasil®. The surface coverage of Staphylococcus sciuri (S. sciuri) bacteria was statistically analyzed by optical microscopy and the attachment sites between the bacteria and the nanostructures were observed by scanning electron microscopy (SEM). Although the surface coverage showed no significant difference between the nanostructured and planar surfaces, SEM images clearly revealed a different interaction of the bacteria and the nanostructures compared to plain surfaces. Nanofibers most likely fimbriae connecting the bacteria and the nanopillar tips were observed. Therefore, it was shown that the bacterium is able to sense the nano-scale features and respond with cell morphological alterations.