Introduction

Rapid development of novel electrochemical systems was achieved in the 1970–80s when a new concept of chemically modified electrodes was introduced [1,2]. Application of organic chemistry methods to the functionalization of electrode surfaces [3] and later pioneering of novel self-assembly methods [4–6] fostered the development of numerous modified electrodes with properties unusual for bare conducting surfaces. While attempts were made to harness enhanced catalytic properties of electrodes and their selective responses to different redox species [1–6], the modified electrodes rapidly became important components of various electroanalytical systems [7] and fuel cells [8]. Novel bioelectrochemical systems [9,10], particularly used in biosensors [11,12] and biofuel cells [13,14], have emerged upon introduction of modified electrodes with bioelectrocatalytic properties. To continue this remarkable success, electrodes functionalized with various signal-responsive materials (including molecular [15], supramolecular [16], and polymeric species [17]) attached to electrode surfaces as monolayers or thin films were pioneered to allow switchable/tunable properties of the functional interfaces controlled by external signals [18].

Over the past two decades, sustained advances in chemical modification of the electrodes have given us a large variety of electrodes, switchable by various physical and/or chemical signals between electrochemically active and inactive states [15–18]. However, very few of them were used in biofuel cells [19,20]. Different mechanisms were involved in the transition of the electrode interfaces between the active and inactive states depending on the properties of the modified surfaces and the nature of the applied signals. The activity of the switchable electrodes was usually controlled by physical signals (optical [21–23], electrical [24,25] or magnetic [26–29]) which failed to provide direct communication between the electrodes and their biochemical environment in biofuel cells. Switchable electrodes controlled by biochemical rather than physical signals are needed to design a biofuel cell adjustable to its biochemical environment according to the presence or absence of biochemical substances. A new approach became possible when a polymer-modified electrode switchable between ON/OFF states by pH values [30] was coupled to biochemical reactions generating pH changes in situ [31]. This allowed transduction of biochemical input signals (e.g. glucose concentration) to the pH changes governing the electrochemical activity of the switchable electrode [32].