Antibacterial Property of Si Nanopillar Array Fabricated Using Metal Assisted Etching; Mimic a Cicada Wing
Authors: Ito Takeshi , Nakade Kazuki , Asai Naoto , Shimizu Tomohiro , Shingubara Shoso 
Published: February 2020

Many living organisms including plants, insects and animals acquire functional nano and micro structures along the course of evolution. Many researchers study their functions caused by their structures and try to mimic them artificially. These research categories are called “bio-mimetics” or “bio-mimicry” and so on. In this study, we focused on a cicada wing which has superhydrophobic surface with antireflection to the light. In addition, antibacterial property was found in the cicada wing in 2012 [1-2]. The wing has nanopillars. The height, width and pitch of them are in sub micrometer size. We mimicked nanopillars on the cicada (Cryptotympana facialis) wing by using bottom up process such as a metal assisted etching. In addition, we show their antibacterial property in this report.
SEM image of the cicada wing is shown in Fig. 1, and showed that circular truncated cones were spread all over the wing. As the results of measuring their structures, the width (w), pitch (p), and height (h) were 148 nm, 196 nm, 229 nm, respectively. Si nano pillars were fabricated on Si (100) wafer. The fabrication process of nanopillar array on Si was written below. After the Si wafer was cleaned with piranha solution (H2O2:H2SO4=1:3) for 30 minutes, the wafer was dipped in an ammonia solution (NH4OH:H2O=1:3) for 1 hour to be hydrophilicity. (1) Polystyrene (PS) beads were coated on the Si wafer using a spincoater to have mono layer of PS beads. (2) The diameter of PS beads was decreased using O2 plasma. (3) Au thin layer was deposited on the Si wafer coated with PS beads by the DC sputtering method. (4) Si under the Au layer was etched selectively due to the metal assisted etching [3-4]. (5) Au layer was removed chemically using aqua regalis. (6) Finally, PS beads were removed using O2 plasma as written above. SEM image of fabricated sample was shown in Fig. 2. Nanopillars were arrayed with regularity. W, p and h of the fabricated nanopillar were 150 nm, 200 nm and 200 nm, respectively. These data was the same as the dimension of the cicada wing discussed above.
A protocol based on JIS (Japan Industrial Standards) Z 2801 was applied to evaluate the antibacterial property of samples which had nanostructures. E. coli (NBRC3972) was served as a bacteria assay. The concentration of viable bacteria was 3.9×104 CFU/mL at the beginning of the test. The results of antibacterial property test were shown in Table 1 with sample dimension (h, w, p). In general, it can be said that sample has antibacterial activity when survival rate (SR) is lower than 0.01. Here, SR was calculated as coliform number after the test divided by that before the test. Sample A, B and C showed high antibacterial property because the concentration of viable bacteria was lower than 1 CFU/mL. The fabricated samples showed obvious antibacterial properties. Antibacterial property dependent with the dimension of the nanostructures will be reported in near future. Our proposed process was has advantages for industrial usage because of low cost and low environmental load based on wet process.

Figure 1