Results

Figure 1: TEM images of ZnO microflowers related to syntheses by Zhang et al. (2004).

From the TEM images of the ZnO synthesis it could be concluded that no autoclave is needed to produce the microflowers, which was used by Zhang et al. (2004). The purpose using highly branched systems, is an increase of adsorbing molecules on the surface. Further, the branched ZnO clusters can be anchored more simply in a biofilm, than flat or spherical nanoplatelets. The size of the microflowers was a general problem for our proof of concept. As the particles could cover most of the distance from one electrode to the other. This implies that biofilm can be used as template to stabilize the structures, however it is no crucial factor. To circumvent this issue, we changed the procedure to a direct mineralization process, where the biofilm is directly involved in the synthesis on ZnO.

Figure 2: ZnO nanoparticles grown in presence (left) and absence of curli fibers (right) on a p-doped silica wafer

The REM images (Figure 1) visualize the difference between ZnO growth in presence and absence of curli fibers on the glass surface. In presence of curli, the particles tend to grow isotropically, with fibers coated around an assembly of individual particles. The ZnO nanoclusters provide average sizes of 0.9 µm with a standard deviation of 0.2 µm, considering 50 clusters. In absence of curli fibers, needle like structures form, which are highly uniform in shape with an average length of 1.1 µm and an average width of 0.55 µm. The size and shape is likely controlled by other factors such as sugars, less complex protein structures in the biofilm matrix or salts in the medium.

Figure 3: Figure 3: ZnO needles grown in the absence of curli fibers in an autoclave at 121°C for 20 min on a p-doped silica wafer.

This result is confirmed by the autoclave mediated synthesis (Figure 2), where similar needle like structures with equal size appear. Accordingly, the capping agents of these structures have to be included in the growth medium or biofilm.

Figure 4: REM images of mineralized biofilms grown on a FTO glass slide with 500 x magnification (left) and 2000 x magnification (right)

The biofilms grown on a FTO glass slide provide more homogeneous layers in contrast to the biofilms grown on a silica wafer (Figure 3). On the bacteria a network like structure can be found, which is most likely mineralized as the layers were sintered before measuring, to kill the bacteria. Beside the larger network like structures smaller clusters can be identified at higher magnifications. The amount of ZnO for this preparation is low compared to the high amount of organic material and hence we started to increase the ZnO ratio.

Binding of heavy metals

Due to the metal binding domains on the curli fibers, the biofilms are able to bind not only zinc oxide, but also heavy metals. This might present a safety application since biofilms could scavenge toxic heavy metal ions out of wastewater.

Copper and nickel were tested first. The corresponding metal salt was dissolved in 50 ml of distilled water, yielding a concentration of 0.05 mol/l. Absorbance spectra at t = 0 were recorded. Afterwards, a biofilm grown on a glass substrate was put into a beaker and submerged in the solution so that the biofilm is just covered. The solution was stirred gently. Absorption spectra were recorded after 5, 10, 20 and 30 minutes. During this time, the metal ions were supposed to bind to the curli, leading to a decrease in metal concentration and therefore a decrease in absorbance of the solution.

Figure 1: Absorption spectra of copper (left) and nickel (right) solutions in dependence of time after exposure to biofilm glass slides.

Unfortunately, the curli could only bind very few metal ions, leading to ambiguous results. The change in concentration was too small for the UV/Vis analyzer to detect reliably.
We then tested a different approach utilizing gold ions. In the same setup, the biofilms were covered in a solution of chloroauric acid. Gold ions were reduced with ascorbic acid, leading to formation of gold nanoparticles in solution. When the glass substrates were removed the beaker, a golden-red color was observed, leading to the conclusion that gold was bound to the cysteine subunits of the curli.

Future Prospect

For further enhancement of the solar cells the mineralization process can be improved to obtain higher surface area covered with ZnO. One part of this work must be a reliable prediction of the biofilm quality as every biofilm can differ slightly in its composition and the amount of ZnO binding domains. Also the structure and size of ZnO can be adjusted more accurately. Here a specific surface pattern on bacteria which is called S-layer. The potential surface roughness provides high loading capacities for dyes or other photosensitizers, which enhances the efficiency of the biofilm solar cell drastically.

References

• Zhang, Hui; Yang, Deren; Ji, Yujie; Ma, Xiangyang; Xu, Jin; Que, Duanlin (2004): Low Temperature Synthesis of Flowerlike ZnO Nanostructures by Cetyltrimethylammonium Bromide-Assisted Hydrothermal Process. In J. Phys. Chem. B 108 (13), pp. 3955–3958. DOI: 10.1021/jp036826f.