Recent biological research has demonstrated that redox is an independent biological-signaling modality. [1-8] This redox signaling modality is best understood in host-pathogen immune interactions where an oxidative burst generates a set of reactive species (e.g., reactive oxygen species) that can sometimes be transduced into second messengers (i.e., reactive electrophiles) [4] and ultimately alters biological function through post-translational protein modification (e.g., conversion of protein cysteine residues into disulfide crosslinks). [6] The functions and the coding of information in this redox signaling modality [1] are distinctly different from the information coded in genes or in the action potentials of the ionic electrical modality. Since this redox modality involves the "flow" of electrons through oxidation-reduction reactions, it is accessible to appropriate electrochemical measurement, and this provides new opportunities for biodevice communication. [9-13] Redox has similarities to the ionic electrical modality, however electrons are the charged species moving in the redox modality. Interestingly, while water is a conductor of ionic currents, it can be considered an "insulator" for the flow of electrons since free electrons do not normally exist in aqueous

and electron-transfer, and then controlling these mechanisms for user-defined purposes. For energy applications (e.g., capacitors and batteries), the focus is on storing large quantities of electrons and controlling their discharge. [1][2][3] For information processing applications (e.g., electronics), [4,5] materials are desired that can gate and rectify electron-flow. [6,7] Other applications focus on materials that offer state-dependent properties (e.g., optoelectronics) [8] or memory (e.g., memristors). [9] With the growing emphasis on safety and sustainability, and the expanding interest in life-science applications, there is an increasing interest in the development of electronic materials that function in aqueous systems. [10] In aqueous systems, electron-transfer is constrained by the solvent and typically occurs through at least two distinctly different mechanisms each of which favors different types of materials. [11][12][13] One electron-transfer mechanism is a metal-like conductivity. [14,15] Carbon-based nanomaterials have been especially important for conferring such conductivity [14,16] with benefits that include enhanced double layer charge storage for energy applications [17][18][19][20][21] and electrocatalytic properties for sensing applications. [22][23][24] A second electron-transfer mechanism involves reduction-oxidation (redox) reactions. Redox polymers offer such properties and have been used in applications that include energy Electronic materials that allow the controlled flow of electrons in aqueous media are required for emerging applications that require biocompatibility, safety, and/or sustainability. Here, a composite hydrogel film composed of graphene and catechol is electrofabricated, and that this composite offers synergistic properties is reported. Graphene confers metal-like conductivity and enables charge-storage through an electrical double layer mechanism. Catechol confers redox-activity and enables charge-storage through a redox mechanism. Importantly, there are two functional populations of catechols: conducting-catechols (presumably in intimate contact with graphene) allow direct electron-transfer; and non-conducting-catechols (presumably physically separated from graphene) require diffusible mediators to enable electrontransfer. Using a variety of spectroelectrochemical measurements, that the capacity of the composite for charge-storage increases in proportion to the extent by which the catechol-groups can undergo redox-state switching is demonstrated. To illustrate the broad relevance of this work, how the redoxstate switching can be related to both the charge storage of energy materials and the memory of molecular electronic materials is discussed. The authors believe this work is significant because it demonstrates that: conducting and redox-active components enable distinctly different mechanisms for chargestorage and electron-transfer; these components act synergistically; and mediators provide unique opportunities to extend the capabilities of electronic materials.

A catechol‐based molecular memory film is reported by Xiaowen Shi, Gregory F. Payne, and co‐workers in article number 2000452. It is easy to fabricate, employs a simple 2‐state redox mechanism, and is conveniently read by orthogonal optical and electrical measurements. The redox memory states are stable for hours and can be repeatedly switched electrochemically. Importantly, this memory can be switched through biological mechanisms which extends catechols as molecular circuit elements for redox‐linked bioelectronics.

Catechol-based materials possess diverse properties that are especially well-suitable for redox-based bioelectronics. Previous top-down, systems-level property measurements have shown that catechol-polysaccharide films (e.g., catechol-chitosan films) are redox-active and allow electrons...