Photo-electrochemical communication between thylakoid membranes and electrodes for harnessing sunlight

Abstract

Development of environmentally friendly and cost-efficient energy sources has been a challenge for modern society since long ago. Sunlight is an infinite and costless source of energy on Earth, so it would be an enormous advantage to efficiently harvest and transform it into electrical energy used in our everyday life. In nature the process, which converts light energy into chemical energy, is called photosynthesis. In green plants photosynthesis takes place in chloroplasts, which contain network of thylakoid membranes – sites of sunlight energy harnessing. The main feature of this process is high-energy electrons produced as a result of photoexcitation of thylakoid membrane components, photosystem I (PSI) and photosystem II (PSII), by visible light of certain wavelength. A lot of efforts have been devoted to employ electrons, generated by photosynthetic membranes and chlorplasts as well as by isolated PSI and PSII, for energy production. However, isolated reaction centers (RC-s) imply a complicated extraction procedure, are compatible only with precious metal electrodes and suffer from instability, chloroplasts in their turn significantly restrict communication between photosynthetic components of thylakoid membranes and electrodes. Therefore thylakoid membranes, which allow stability of the photosynthetic components, are relatively easy to immobilize and extract, were considered as the best option to work with. This project can be viewed as a small puzzle-piece in worldwide research on the development of alternative energy sources, as well as an extensive continuation of earlier investigations made in the field of photosynthetic electricity. In the present study electrochemical communication between thylakoid membranes, isolated from spinach, and graphite electrodes was investigated to improve the photocurrent generation. For this a number of parameters were considered, e.g. different mediators for efficient electron transfer, various strengths and pHs of phosphate buffer saline (PBS). The maximum photocurrent generation (95.78 µA cm-2) was obtained while two mediators were used together, e.g. an osmium redox polymer and p-benzoquinone, at 50 mM PBS pH7. Os-polymeric mediators, immobilized onto the electrode, represent a novel approach, which allows save costs and environment because a lower amount of mediator is needed, as well as provide a robust and simple system. The photocurrent origin was confirmed by using known herbicides, such as DCMU (3-(3,4-dichlorophenyl)-1,1-dimethyl urea) and CCCP (carbonyl cyanide 3-chlorophenylhydrazone), and resulted in ca 90 % inhibition compared to a non-inhibited system. To enhance the experimental stability cross-linkers PEGDGE (poly(ethylene glycol) diglycidyl ether), PEI (polyethylenimine) and glutaraldehyde were used. Although the stability was increased by using glutaraldehyde, e.g. current remained stable for 4500 s, the photocurrent density fell drastically. To minimize the photo-oxidative damage of thylakoid membranes superoxide dismutase (SoD) and catalase (Cat) were used, that resulted in a little increase of the photocurrent. Open circuit potential measurements for non-mediated and mediated systems, of importance for a future biofuel cell construction, resulted in 0.120 V and 0.180 V respectively.

Popular Abstract

Development of environmentally friendly and cost-efficient energy sources has been a challenge for modern society since long ago. Sunlight is an infinite and costless source of energy on the Earth, so it would be an enormous advantage to efficiently harvest and transform it into electrical energy used in our everyday life. This study is a tiny part of global research on solar fuels, the main idea of which is to harness light energy by using biological material and transform it into electrical power.
In nature the process of light energy transformation into chemical energy is called photosynthesis, which takes place in thylakoid membranes of chloroplasts, which are components of green plants leaf cells. It uses sunlight to convert CO2 and H2O into molecular O2, which we all breathe, and sugars, which are energy source for plant growth as well as for animals.
Since long ago researchers noticed, that energy derived from photosynthesis has a potential to be employed in sustainable sunlight conversion. A driving force for the whole process is the generation of high energy electrons, which are kicked out from receptor molecules by light of a certain wavelength. These electrons participate in several transfer reactions, and it is certainly challenging to employ them for energy production. In the modern world electrical energy is needed for heating, driving cars, running household and industrial machines etc. At the same time there is a continuous need for the development of environmentally friendly and cost-efficient energy sources, because natural resources, such as coal, gas and oil, are not infinite.
This study focused on the imitation of photosynthesis for energy production. As a biological material for the experiments thylakoid membranes, isolated from spinach leaves, were used. By illuminating the electrodes modified with thylakoid membranes, with light of a certain intensity, remarkable current densities were detected. This was done in the presence of substances called mediators, which help transfer electrons to the electrode. Various experimental conditions, such as electrolyte strength and pH, were also tested.
Thylakoid membranes consist of several components, which are involved in electron generation and transfer reactions. A part of this project was to find out which component is the main source of photocurrent by means of photosynthetic inhibitors – known herbicides. This study once again confirmed that the main source of electrons is a protein complex, which has the name “photosystem II”.
Unfortunately, the biological material suffers from a short active life time under illumination, which means degradation and decrease of current with time. This study also attempted to keep the obtained photosynthetic electricity stable, with the help of cross-linking materials. As a result stability was achieved, but not the same current output.
As a consequence of strong illumination highly reactive oxygen species (ROS), damaging the protein environment of thylakoid membranes, are produced. Some experiments were made to neutralize them by using certain enzymes. As a result higher currents were generated proving that ROS are one of the main sources of photodamage.
This project can be considered as a small puzzle-piece in the worldwide investigation on development of alternative energy sources. The results obtained are noteworthy, and give challenge for future studies, especially with a focus on generation of stable photocurrent.

• author: Aleksejeva, Olga
• supervisor: Kamrul Hasan
• organization: Department of Chemistry
• course: KEMR16 20141
• year: 2014
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: analytisk kemi, analytical chemistry
• language: English
• id: 4780433
• date added to LUP: 2014-11-18 15:41:34
• date last changed: 2014-11-18 15:41:34