Transparent, Antibiofouling Window Obtained with Surface Nanostructuring

Abstract

Biofouling is one of the key factors which limits the long-term performance of seawater sensors. Common measures to hinder biofouling include toxic paints, mechanical cleaning and UV radiation. All of these measures have various limitations. A very attractive solution would be to prevent biofilm formation by changing the surface structure of the sensor. This idea has been implemented successfully in various settings, but little work has been done on structuring optically transparent materials, which are often needed in sensor applications. In order to achieve good antibiofouling properties and efficient optical transparency, the structuring must be on the nanoscale. Here, we investigate a transparent, antibiofouling surface obtained by patterning a semihexagonal nanohole structure on borosilicate glass. The nanoholes are approximately 50 nm in diameter and 200 nm deep, and the interparticle distance is 135 nm, allowing the structure to be optically transparent. The antibiofouling properties of the surface were tested by exposing the substrates to the microalgae Phaeodactylum tricornutum for four different time intervals. This species was chosen because it is common in the Norwegian coastal waters. The tests were compared with unstructured borosilicate glass substrates. The experiments show that the nanostructured surface exhibits excellent antibiofouling properties. We attribute this effect to the relative size between the structure and the biofouling microorganism. Specifically, the small dimensions of the nanoholes, compared to the biofouling microorganism, make it more difficult for the microalgae to attach. However, lubrication of the substrates with FC-70 perfluorocarbon resulted in contamination at a rate comparable to the reference substrate, possibly due to the chemical attractiveness of the alkane chains in FC-70 for the microalgae.

Figure 1

Scanning electron microscopy image of diatom microalgae Phaeodactylum tricornutum from B58 strain in the fusiform. Cultured from NORCE, Bergen, Norway.

Figure 2

(A) Scanning electron microscopy image of the surface morphology of a nanostructured glass. (B) Optical images of a 10 μL super distilled water droplet on a reference glass (top) and nanostructured glass (bottom) and the corresponding optical contact angles.

Figure 3

Schematic of the antibiofouling experiment. The substrates (ref: reference substrate; nh: nanostructured substrate) are placed in a Petri dish filled with the microalgae stock and stored under UV growing lights for either 1 day, 7 days, 21 days or 180 days. (A) After a specific amount of time, the substrates are gently washed in freshly filtered seawater for 15 s by stirring so that the loose microalgae are removed, and only the sticking microalgae are left on the surface to evaluate further. (B) The substrates are placed on a microscope slide for optical analysis. (C) The images obtained are further used in statistical analysis using ImageJ software.

Figure 4

Reference substrate before (left) and after (right) washing for 15 s in 60 mL of freshly filtered seawater after submersion in the algal stock for 21 days.

Figure 5

Adhesion of microalgae Phaeodactylum tricornutum to the test substrates after the washing step. Top row: reference substrate. Bottom row: nanostructured substrate. Each column shows the corresponding substrates after a certain amount of time: (a,e) after 1 day, (b,f) after 7 days, (c,g) after 21 days and (d,h) after 180 days.

Figure 6

Percentage biofouled area for the different time intervals (1-180 days). Blue: reference substrate. Orange: nanostructured substrate. The error bars represent the mean ± standard error of three independent experiments’ mean (s.e.). No error bar is depicted for Day 1, as the s.e. is 0%. For 180 days, only one nanostructured and one reference substrate were tested. Detailed data can be found in the Supporting Information.