(599ag) In Situ Measurement of Biofilm Thickness in Bioelectrochemical Systems Using a Microelectrode
Biofilm is a core part of a bioelectrochemical system. Monitoring the biofilm formation provides a route to investigate the growth mechanism of electrochemically active bacteria (EAB) under various situations. The biofilm thickness variation during the biofilm formation is commonly ex situ detected with a confocal laser scanning microscopy (CLSM) in lab studies. CLSM measurement usually involves irreversible damage to the biofilm during the pretreatment process (dyeing the bacteria in the biofilm). In this work, an in situ non-invasive detection method for biofilm thickness measurement is introduced. EAB has a mediated electron transfer pathway secrets redox species to shuttle electrons between bacteria and electron acceptors during their metabolic processes and these redox species can be detected via electrochemical oxidation reactions on electrodes. Due to the redox species diffusion, a diffusion layer is existed between the biofilm and bulk solution. Therefore, when a microelectrode is set at a potential positive enough to trigger the oxidation of redox species and is approaching the biofilm, a current jump can be observed owing to the redox species concentration gradient in the diffusion layer. Based on the correlation between the oxidation current and the concentration gradient in diffusion layer, the microelectrode tip can be located in the diffusion layer. To avoid the tip touching and damaging the biofilm, a current threshold is set. Once the oxidation current reaches the threshold, the microelectrode stops approaching. Since the diffusion layer is thin compared to the biofilm thickness in a fully stirred reactor (usually less than 20 µm), the microelectrode tip position where it stopped may be approximately taken as the top point of the biofilm. Together with the position of the substrate obtained before the inoculation via an approaching curve, the biofilm thickness can be determined. In our experiment, we adopted a typical EAB, Shewanella oneidensis MR1 as the model bacteria, which could secret riboflavin as the redox specie. A Pt microelectrode with a 10 um tip was employed to monitor the biofilm formation. To ensure the accuracy of this method, the thickness of the biofilm detected with the microelectrode was compared to the thickness obtained with a CLSM. The results indicated that the errors were around 5% to 10%. After obtaining the accuracy, this method was employed to investigate the effects of some environmental factors on the biofilm formation. The results indicated oxygen, a more positive working electrode potential and higher sodium lactate concentration could increase the final biofilm thickness, while the riboflavin concentration only increased the initial biofilm formation rate but had a negligible effect on the final biofilm thickness.