Flat microliter membrane-based microbial fuel cell as ‘‘on-line sticker sensor” for self-supported in situ monitoring of wastewater shocks Zhiheng Xu a , Bingchuan Liu a , Qiuchen Dong b , Yu Lei b , Yan Li a , Jian Ren b , Jeffrey McCutcheon b , Baikun Li a,⇑ a Department of Civil & Environmental Engineering, University of Connecticut, Storrs, CT 06269, United States b Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, United States highlights  Flat microliter MMFC was developed as ‘‘on line sticker sensor”.  Hydrophilic and microporous membrane shortened the acclimation duration.  MMFC sensors responded well to Cr 6+ and Ni 2+ shocks in wastewater.  OCP of MMFC sensors well reflected shock types and concentrations.  The voltage of the MMFC was clearly correlated with shock concentrations. graphical abstract article info Article history: Received 9 July 2015 Received in revised form 19 August 2015 Accepted 21 August 2015 Available online 29 August 2015 Keywords: Microbial fuel cell Membrane On line sticker biosensor Wastewater shock Open circuit potential (OCP) abstract Novel flat membrane-based microbial fuel cell (MMFC) sensors were developed by compacting two filter membranes coated with carbon ink. High micro-porosity and hydrophilicity of membranes offered the distinct advantages of short acclimation period (couple hours), simple compact configuration with micro- liter size, and high sensitivity and stability. MMFC sensors were examined at two toxic shocks (chromium and nickel) in a batch-mode test chamber, and rapidly responded to shock types and concentrations. The variation of voltage output was correlated with open circuit potential (OCP). Filter membranes facilitated bacterial attachment and shortened acclimation. The MMFC sensors showed good reusability and recov- ered several days after toxic shocks. The robustness of MMFC sensors was validated through 1-month tests. The stability of sensor signals was examined with coefficient of variance (CV) statistical analysis. The flat microliter MMFC has a great potential as ‘‘on-line sticker sensor” for real time in situ monitoring of wastewater quality. Published by Elsevier Ltd. 1. Introduction Wastewater treatment plants (WWTPs) have been designed and operated based on the average concentrations of contaminants. Short-term and long-term shocks (e.g. organic compounds and heavy metals) in wastewater disturb the stability of WWTPs. Real-time wastewater shock sensors are critical to provide effec- tive precaution strategies and minimize shock impacts. Diverse biosensors (e.g. biological oxygen demand, ammonia, and heavy metal) have been developed to monitor the contaminants in wastewater. However, the biosensor performance is directly depended on the coated enzymes and microorganisms, which have inherent problems, such as short lifetime (Okochi et al., 2004; http://dx.doi.org/10.1016/j.biortech.2015.08.081 0960-8524/Published by Elsevier Ltd. ⇑ Corresponding author. Tel.: +1 860 486 2339. E-mail address: baikun@engr.uconn.edu (B. Li). Bioresource Technology 197 (2015) 244–251 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech