Inspiration

The limitation of fossil fuels such as oil, coal and gas intensities the need to find different „materials“ to gain energy for a constantly rising population in the world. The energy can be provided by nature using wind, water, plants and the sun. Solar energy conversion can be a crucial factor as sun is present every day in almost every region in the world. Thereby an efficient conversion of solar energy is as important as the expenses for the solar cells. To reduce the price less expensive materials as well as the costs for production can be considered. Commercially available silicon solar cells provide a good solar energy conversion rate in combination with moderate costs. To reduce the costs for the raw materials an inspiration from photosynthesis can be drawn. Here a chlorophyll molecule is excited to inject its electron into a redox cascade. The same principle can be chosen for solar cells applying dyes which can transfer electrons to a transparent semiconductor. These semiconductors can be either ZnO and TiO2, which are produced in a large scale for the application in e.g. tooth paste or sun screen.

To reduce the production costs a large part can be covered by living cells, which work autonomous. In particular biofilms can be used since they provide the possibility to integrate metals inside its texture and can be mineralized. Hence the transparent semiconductor can be deposited by adding the initial salts to the bacteria solution. The process of mineralization can be done either during the growth of the biofilm or after the growth. The dyes for the solar cells can also be provided by E.Coli, which was demonstrated by the iGEM of Darmstadt in 2014 (link von deren homepage). The only technical process is the deposition of electrolyte and the sealing of the whole cell, which prevents this type of solar 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

Inhalt Biosolar

Grätzel Cell

Setup of a Dye Sensitized Solar Cell (DSSC)

A dye sensitized solar cell does not require expensive material or complex working conditions. It can be basically built of tooth paste or sun screen in combination with a dye obtained from fruits or tea. The starting material is a glass slide which is coated with a transparent conducting material. The commonly used coating materials are Indium Tin Oxide (ITO) or Fluorine doped Tin Oxide (FTO). On the conducting slide the transparent semiconductors ZnO or TiO2 can be deposited, which serve as electron transporting layer. This layer can be soaked with the dye. Functional groups of the dye molecules direct and anchor them on the surface of the semiconductor. On this layer an electrolyte containing Iodine and Iodide providing the electrons to flow in the cell. The cell is completed with another glass slide coated with conducting traditional materials such as graphite or platinum.

Mechanism of a DSSC

Starting with the irradiation of the complete cell the electrons in the organic dye is excited to a higher level, which is called LUMO (lowest unoccupied molecular orbital). If the LUMO level is energetically high enough the electron can be transferred to the conduction band of the transparent semiconductor. The semiconductor, commonly made of ZnO or TiO2, can then induce the electron to the electrode. The missing electron of the dye can be restored by the electrolyte and the electrolyte can reobtain its electron from the cathode. There the cycle closes and a current is flowing continuously during light irradiation.