A new signal-responsive interface with switchable/tunable redox properties based on a pH-responding polymer brush was studied. Poly(4-vinyl pyridine), P4VP, functionalized with Os-complex redox units was grafted to an indium tin oxide (ITO) conductive support in the form of a polymer brush. The modified electrode surface was responsive to the changes of the pH value of the electrolyte solution: at acidic pH ) 4.0 the redoxpolymer film demonstrated the reversible electrochemical process, E°) 0.29 V (vs Ag/AgCl), while at neutral pH > 6, the polymer was not electrochemically active. The reversible transformation between the active and the inactive state originated from the structural changes of the polymer support. The protonation of the pyridine units of the polymer backbone at the acidic pH resulted in the swelling of the polymer brush allowing quasidiffusional translocation of the flexible polymer chains, thus providing direct contact of the polymer-bound redox units and the conducting electrode support. The uncharged polymer formed at the neutral pH values existed in the shrunk state, when the mobility of the polymer chains was restricted and the polymer-bound redox units were not electrically accessible from the conducting support, thus resulting in the nonactive state of the modified electrode. The reversible changes of the electrochemical activity of the modified electrode and the respective structural changes of the polymer-brush were characterized in details by electrochemistry, AFM, and ellipsometry. The stepwise changes of the pH value between 3.0 and 7.0 resulted in the reversible switching on and off of the electrode redox activity, respectively. The redox activity of the modified electrode was also tunable upon precise titration of the electrolyte solution between pH 3.0 and 7.0 demonstrating a titration-like curve for the amount of the redox-active group because of the smooth transition between the swollen and the shrunk states. The primary electrochemical activity of the modified electrode was coupled with a biocatalytic oxidation of glucose in the presence of soluble glucose oxidase (GOx), thus allowing reversible activation of the bioelectrocatalytic process. The modified electrode with the pH-controlled switchable/tunable redox activity was proposed as a "smart" interface for a new generation of electrochemical biosensors and biofuel cells with a signal-controlled activity.

High-energy-density, green, safe batteries are highly desirable for meeting the rapidly growing needs of portable electronics. The incomplete oxidation of sugars mediated by one or a few enzymes in enzymatic fuel cells suffers from low energy densities and slow reaction rates. Here we show that nearly 24 electrons per glucose unit of maltodextrin can be produced through a synthetic catabolic pathway that comprises 13 enzymes in an air-breathing enzymatic fuel cell. This enzymatic fuel cell is based on non-immobilized enzymes that exhibit a maximum power output of 0.8 mW cm À 2 and a maximum current density of 6 mA cm À 2 , which are far higher than the values for systems based on immobilized enzymes. Enzymatic fuel cells containing a 15% (wt/v) maltodextrin solution have an energy-storage density of 596 Ah kg À 1 , which is one order of magnitude higher than that of lithium-ion batteries. Sugar-powered biobatteries could serve as next-generation green power sources, particularly for portable electronics.

The logic network composed of three enzymes (alcohol dehydrogenase, glucose dehydrogenase, and glucose oxidase) operating in concert as four concatenated logic gates (AND/OR), was designed to process four different chemical input signals (NADH, acetaldehyde, glucose, and oxygen). The cascade of biochemical reactions culminated in pH changes controlled by the pattern of the applied biochemical input signals. The "successful" set of inputs produced gluconic acid as the final product and yielded an acidic medium, lowering the pH of a solution from its initial value of pH 6-7 to the final value of ca. 4. The whole set of the input signal combinations included 16 variants resulting in different output signals. Those that corresponded to the logic output 1, according to the Boolean logic encoded in the logic circuitry, resulted in the acidic medium. The pH changes produced in situ were coupled with a pH-sensitive polymer-brush-functionalized electrode, resulting in the interface switching from the OFF state, when the electrochemical reactions are inhibited, to the ON state, when the interface is electrochemically active. Soluble [Fe(CN)(6)](3-/4-) was used as an external redox probe to analyze the state of the interface and to follow the changes produced in situ by the enzyme logic network, depending on the pattern of the applied biochemical signals. The chemical signals processed by the enzyme logic system and transduced by the sensing interface were read out by electrochemical means (cyclic voltammetry and Faradaic impedance spectroscopy). This readout step features a "sigmoid" processing of the signals that provides "filtering" and significantly suppresses errors. Coupling between signal-processing enzyme logic networks and electronic transducers will allow future "smart" bioelectronic devices to respond to immediate physiological changes and provide autonomous signaling/actuation depending on the concentration patterns of the physiological markers.

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An enzyme-based biofuel cell with a pH-switchable oxygen electrode, controlled by enzyme logic operations processing in situ biochemical input signals, has been developed. Two Boolean logic gates (AND/OR) were assembled from enzyme systems to process biochemical signals and to convert them logically into pH-changes of the solution. The cathode used in the biofuel cell was modified with a polymer-brush functionalized with Os-complex redox species operating as relay units to mediate electron transport between the conductive support and soluble laccase biocatalyzing oxygen reduction. The electrochemical activity of the modified electrode was switchable by alteration of the solution pH value. The electrode was electrochemically mute at pH > 5.5, and it was activated for the bioelectrocatalytic oxygen reduction at pH < 4.5. The sharp transition between the inactive and active states was used to control the electrode activity by external enzymatic systems operating as logic switches in the system. The enzyme logic systems were decreasing the pH value upon appropriate combinations of the biochemical signals corresponding to the AND/OR Boolean logic. Then the pH-switchable electrode was activated for the oxygen reduction, and the entire biofuel cell was switched ON. The biofuel cell was also switched OFF by another biochemical signal which resets the pH value to the original neutral value. The present biofuel cell is the first prototype of a future implantable biofuel cell controlled by complex biochemical reactions to deliver power on-demand responding in a logical way to the physiological needs.

A switchable bioelectrocatalytic system for glucose oxidation controlled by external biochemical signals exemplifies interfacing between bioelectronic and biochemical ensembles.

Biomarkers characteristic of liver injury, alanine transaminase and lactate dehydrogenase, were processed by an enzyme-based system functioning as a logic AND gate. The NAD+ output signal produced by the system upon its activation in the presence of both biomarkers was then biocatalytically converted to a decrease in pH. The acidic pH value biocatalytically produced by the system as a response to the biomarkers triggered the restructuring of a polymer-modified electrode interface. This allowed a soluble redox species to approach the electrode surface, thus switching the electrochemical reaction ON. The redox transformations activated by the biochemical signals resulted in an amplification of signals. This system represents the first example of an integrated sensing-actuating chemical device with the implemented AND Boolean logic for processing natural biomarkers at their physiologically relevant concentrations.

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine) (P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cations across the thin film. An ITO glass electrode was modified with the polymer brush and used to study the switchable permeability of the mixed brush triggered by changes in pH of the aqueous environment in the presence of two soluble redox probes: [Fe(CN)(6)](4-) and [Ru(NH(3))(6)](3+). The responsive behavior of the brush was also investigated using the in situ ellipsometric measurements of the brush swelling, examination of the brush morphology with atomic force microscopy (AFM), and contact angle measurements of the brush samples extracted from aqueous solutions at different pH values. The mixed brush demonstrated a bipolar permselective behavior. At pH<3 the positively charged P2VP chains enabled the electrochemical process for the negatively charged redox probe, [Fe(CN)(6)](4-), while the redox process for the positively charged redox probe was effectively inhibited. On the contrary, at pH>6 a reversible redox process for the positively charged redox probe, [Ru(NH(3))(6)](3+), was observed, while the redox process for the negatively charged redox species, [Fe(CN)(6)](4-), was fully inhibited. Stepwise changing the pH value and recording cyclic voltammograms for the intermediate states of the polymer brush allowed electrochemical observation of the brush transition from the positively charged state, permeable for the negatively charged species, to the negatively charged state, permeable for the positively charged species. The data of ellipsometric, AFM and contact angle measurements are in accord with the electrochemical study. The discovered properties of the brush could be used for the development of 'smart' sensors and drug delivery systems, for example, a smart drug delivery capsule which could release negatively charged molecules of drugs in acidic conditions, while positively charged molecules of drugs will be released in neutral conditions.