Optimized Nanoelectrode Geomerty for Enhanced Direct Extraction of Photosynthetic Electrons from Living Single Algal Cells
Authors: Hong Hyeonaug , Ryu WonHyoung 
Published: February 2020

Plant cells highly efficiently convert solar energy into photosynthetic electrons during photosynthesis. The photosynthetic electrons (PEs) are transported via redox reactions between molecules in the thylakoid membrane of plant cells. Most of the PEs are utilized for cell growth and small fractions are stored in a form of carbohydrates within plant cells. There have been various approaches to harvest the PEs from the photosynthetic electron transfer chain during photosynthesis. These works have focused on indirect extraction of PEs from isolated photosynthetic components such as photosystem I (PSI) or fragments of thylakoid membrane that contains most of molecules involved in photosynthetic electron transfer. However, despite many advances made from those approaches, there are still challenges such as limited stability of isolated photosynthetic molecules and lowered efficiency due to use of mediators. We previously reported the feasibility of “direct” extraction of PEs from living single algal cells using AFM-compatible nanoelectrodes (NEs). Although the result demonstrated continuous extraction of PEs from living cells at high efficiency, fabrication of the customized NEs was expensive and had poor yield. More importantly, gradual leakage from algal cells occurred after NE insertion into plant cells, resulting in limited PE extraction for only up to an hour. In this study, we aimed to study NE geometry that enabled longer direct extraction of PEs from living algal cells that our previous work without using any mediator. For this, we developed horizontally-tilted cantilever NE system by which NE insertion into living cells could be optically monitored in situ and light-triggered PE extraction could be performed. A commercial AFM tip was custom-shaped into a cantilever with a nanowire (NW) at one end using focused ion beam (FIB). The NW was coated with Au and insulated with a Si3N4 passivation layer by CVD processes. For highly-localized electrochemical functions, another FIB milling was performed to expose the embedded Au only at the tip of the NW. First, we studied how the NW size affected the long-term stability of algal cells after NW insertion. Algal cells were stable for up to about 17 hours when they were inserted by NWs thinner than 500 nm, and they even proliferated after NW insertion. The electrochemical performance of the NW electrode was confirmed with the characteristics of an ultramicro electrode by running cyclic voltammetry analysis using potassium ferricyanide (K3[Fe(CN)6], 0.01M) as a redox system and potassium nitrate (KNO3, 0.1M) as an supporting electrolyte. Light-triggered electrical currents were measured after NW insertion into algal cells with light intensity of 99 μmol·m-2s-1 and potential of 0.4V (versus Ag/AgCl) applied to the NW. Potential-dependent analysis for photocurrents was performed to identify the donor of PEs. Disappearance of the light-triggered currents upon application of photosystem inhibitors also confirmed the currents originated from photosynthesis. Finally, long-term monitoring of cell stability and extraction of PEs was demonstrated for 17 hours after NW insertion into algal cells.