iGEM Erlangen

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 absorbtion 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 E. 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

A biofilm is a system that can be adapted internally to environmental conditions by its inhabitants. Note 2: The self-produced matrix of extracellular polymeric substance, which is also referred to as slime, is a polymeric conglomeration generally composed of extracellular biopolymers in various structural forms.

Bacteria form these described aggregates in three-dimensional structures, to survive in the face of environmental stresses. To build these aggregates, the bacteria have to specialize themselves to attach to the surface and to communicate with other microorganisms: They will lose their flagella and will produce proteins for quorum sensing and induce expression of extracellular polymeric substances often called slime.

Enteric bacteria such as Escherichia coli and Salmonella spp. express proteinaceous extracellular fibers called curli that are involved in surface and cell-cell contacts that promote community behavior and host colonization. Between the basal biofilm and the outer biofilm there are pores, canals and corridors to transport a lot of substances like nanoparticles through the “slime”.

Curli and the importance for our project

Enteric bacteria such as Escherichia coli and Salmonella spp. express proteinaceous extracellular fibers called curli that are involved in surface and cell-cell contacts that promote community behavior and host colonization. (paper auf studon) CsgA monomers are the major part of the Curli fibers and are secreted in the extracellular environment by E.coli W3110 itself. Being extracellular it has the opportunity to interact with various substances in a biofilm. Some bacterial strains are producing an extracellular matrix called biofilm, which is protecting them from environmental impacts. This matrix is composed of proteins, polysaccharides, lipids and nucleic acids. One of the main structural components in Escherichia coli biofilms are curli fibers, with a diameter of 4-7 nanometer that can made up to 10-40% of the whole biofilm.(Nguyen et.al) These fibers are amyloid structures, which are anchored on the bacterial cell surface and are assembled of 13 kDa CsgA proteins. For the production of these fibers the curli-system consists of two operons, containing seven genes: csgBAC and csgDEFG. The self-assembly and nucleation of CsgA on the cell surface is mediated by CsgB. CsgC/G are responsible for the secretion and CsgE/F for producing of CsgA. CsgD is the transcriptional regulator of this system. The following figure shows the Curli-producing process. For our project it was important, that the curli fibers are promoting the growth of biofilms and to interact with special nanoparticles.

After investigating old iGEM Team projects and other articles, we decided to ask our friends from the iGEM Team Marburg 2015 if they can send us their biobrick plasmid pPickUp (BBa_K1650047) and their E.coli strain W3110. pPickUp has the advantage having a SpyTag, which has strong similarities to some ZnO-Nanoparticle binding proteins which are already published. If the SpyTag can interact with our nanoparticles we do neither need an extra tag, which are already published nor an adaptor protein with interacts with the scaffold protein CsgA and the nanoparticles – killing two birds with one stone

Biofilm Solarcell

Under Construction

Grätzel Cell

Setup of a Dye Sensitized Solar Cell (DSSC)

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