Inspiration

The limitation of fossil fuels such as oil, coal and gas intensifies the need to find different sources to provide energy for a constantly rising world population. Renewable energy can be supplied by natural agents such as wind, water, plants or the sun. The conversion of solar energy in particular is a crucial issue as the sun presents an inexhaustible and easily accessible energy source for most inhabited regions of the Earth. Thus, optimizing the balance between efficient conversion of solar energy and the affordability and ease-of-manufacturing of solar cells is an important task for the future. In this regard, lower production costs will benefit manufacturer, customer, and the environment alike.

Commercially available silicon solar cells provide a decent solar energy conversion rate in combination with moderate costs. As with most technologies, these factors may be improved by imitating natural processes, in this case photosynthesis. Upon absorption of a photon, a chlorophyll molecule is excited and donates its high energy electron into a redox cascade. This principle can be applied to solar cells by adding dyes that transfer electrons to a transparent semiconductor. Possible semiconductors are zinc oxide (ZnO) and titanium dioxide (TiO₂), which are both produced in large quantities as ingredients of tooth paste, sun screen etc.

To reduce the production costs, a large area of the solar cell can be covered by autonomously working, living bacteria. Especially biofilms provide a promising approach because they can integrate metals into their structure and may be mineralized. Hence, the transparent semiconductor can be deposited by adding the initial salts to the bacteria solution. Mineralization may be performed either during the growth of the biofilm or after its growth. The electron donating dyes can also be provided by Escherichia coli, which was demonstrated by the iGEM team from Darmstadt in 2014. The only technical process is the deposition of the electrolyte and the sealing of the complete solar cell, which prevents the cell from drying out.

Biofilm

According to a IUPAC recommendation, a biofilm is an... “Aggregate of microorganisms in which cells that are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) adhere to each other and/or to a surface. [...] A biofilm is a system that can be adapted internally to environmental conditions by its inhabitants. […] The self-produced matrix of EPS, which is also referred to as slime, is a polymeric conglomeration generally composed of extracellular biopolymers in various structural forms.” (Vert et al., 2012).

Figure 1: Steps of formation and maturation of a biofilm (Vlamakis et al., 2013).

Bacteria form the three-dimensional structures shown in Figure 1 to survive in the face of environmental stress. To assemble these aggregates, the bacteria have to specialize themselves to attach to the surface and to communicate with other microorganisms. In the process, they will lose their flagella, produce proteins for quorum sensing and induce expression of extracellular polymeric substances usually called slime.

The importance of curli fibers for our project

Curli fibers, or simply curli, are thin, extracellular, proteinaceous structures produced by E.coli and other bacteria. Next to influencing community behavior and host cell adhesion, these amyloid fibers play a role in surface contacts and cell aggregation and mediate the formation of biofilms (Barnhart and Chapman, 2006).

Curli-related proteins are the products of two operons containing seven genes in total: csgBAC and csgDEFG. Of the seven proteins, CsgD is the transcriptional regulator and CsgE/F are responsible for the processing of CsgA. CsgC mediates the secretion of CsgA through the translocator CsgG. CsgB serves as the origin of nucleation and anchors CsgA, which makes up the majority of curli fibers, to the outer membrane (Nguyen et al., 2014; Hobley et al., 2015). The production of curli is demonstrated in Figure 2.

Figure 2: Biosynthetic pathway and formation of curli fibers from CsgA subunits (Hobley et al., 2015)

Up to 40% of the total biofilm volume can be occupied by curli (Nguyen et al., 2014). Since curli consist mostly of CsgA monomers that interact with each other and possibly with other substances in the biofilm, modifying these monomers provides a simple way to change the properties of the whole biofilm.

Grätzel Cell

Setup of a Dye Sensitized Solar Cell

A dye sensitized solar cell (DSSC) does not require expensive material or complex working conditions. It can be literally built out of tooth paste or sun screen combined with a dye obtained from fruits or tea. The starting layer is a glass slide coated with a transparent conducting material. Commonly used coating materials are indium tin oxide (ITO) or fluorine doped tin oxide (FTO). The transparent semiconductors ZnO or TiO₂ can be deposited on the conducting slide and serve as the electron transporting layer, which is then soaked with a dye. Functional groups of the dye molecules direct and anchor them on the surface of the semiconductor. An electrolyte containing iodine and iodide is added onto this layer to provide electrons and facilitate current flow. The cell is completed with another glass slide coated with traditional conducting materials such as graphite or platinum.

Mechanism of a DSSC

Upon irradiation of the solar cell, the electrons in the organic dye are excited to a higher level, called the lowest unoccupied molecular orbital (LUMO). If the LUMO level is energetically high enough, the electron can be transferred to the conduction band of the transparent semiconductor and from there continue to the anode. The missing electron of the dye is restored by the electrolyte and the electrolyte regains its electron from the cathode. This results in a continuous current flow for the duration of the irradiation.

Parts

References

• Barnhart, M. M., & Chapman, M. R. (2006). Curli biogenesis and function. Annual review of microbiology, 60, 131. doi: 10.1146/annurev.micro.60.080805.142106


• Hobley, L., Harkins, C., MacPhee, C. E., & Stanley-Wall, N. R. (2015). Giving structure to the biofilm matrix: an overview of individual strategies and emerging common themes. FEMS microbiology reviews, 39(5), 649-669. doi: 10.1093/femsre/fuv015


• Nguyen, P. Q., Botyanszki, Z., Tay, P. K. R., & Joshi, N. S. (2014). Programmable biofilm-based materials from engineered curli nanofibres. Nature communications, 5. doi: 10.1038/ncomms5945


• Vert, M., Hellwich, K. H., Hess, M., Hodge, P., Kubisa, P., Rinaudo, M., & Schué, F. (2012). Terminology for biorelated polymers and applications (IUPAC Recommendations 2012). Pure and Applied Chemistry, 84(2), 377-410. doi: 10.1351/PAC-REC-10-12-04


• Vlamakis, H., Chai, Y., Beauregard, P., Losick, R., & Kolter, R. (2013). Sticking together: building a biofilm the Bacillus subtilis way. Nature Reviews Microbiology, 11(3), 157-168. doi: 10.1038/nrmicro2960