Tsz Kin Tam

Biofuel Cell Controlled by Enzyme Logic Systems

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.

Biochemically controlled bioelectrocatalytic interface.

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

Enzyme logic gates for the digital analysis of physiological level upon injury.

A biocomputing system composed of a combination of AND/IDENTITY logic gates based on the concerted operation of three enzymes: lactate oxidase, horseradish peroxidase and glucose dehydrogenase was designed to process biochemical information related to pathophysiological conditions originating from various injuries. Three biochemical markers: lactate, norepinephrine and glucose were applied as input signals to activate the enzyme logic system. Physiologically normal concentrations of the markers were selected as logic 0 values of the input signals, while their abnormally increased concentrations, indicative of various injury conditions were defined as logic 1 input. Biochemical processing of different patterns of the biomarkers resulted in the formation of norepiquinone and NADH defined as the output signals. Optical and electrochemical means were used to follow the formation of the output signals for eight different combinations of three input signals. The enzymatically processed biochemical information presented in the form of a logic truth table allowed distinguishing the difference between normal physiological conditions, pathophysiological conditions corresponding to traumatic brain injury and hemorrhagic shock, and abnormal situations (not corresponding to injury). The developed system represents a biocomputing logic system applied for the analysis of biomedical conditions related to various injuries. We anticipate that such biochemical logic gates will facilitate decision-making in connection to an integrated therapeutic feedback-loop system and hence will revolutionize the monitoring and treatment of injured civilians and soldiers.

Responsive Interface Switchable by Logically Processed Physiological Signals: Toward “Smart” Actuators for Signal Amplification and Drug Delivery

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.

Biofuel Cell Logically Controlled by Antigen-Antibody Recognition: Towards Immune-Regulated Bioelectronic Devices

A switchable biofuel cell logically controlled by immune signals was developed as a model prototype for future adaptive implantable bioelectronic devices regulated by immune reactions. The cell demonstrated NOR Boolean logic operation in situ controlled by antibody signals.

Switchable electrode controlled by Boolean logic gates using enzymes as input signals

Application of Boolean logic operations performed by enzymes to control electrochemical systems is presented. Indium-tin oxide (ITO) electrodes with the surface modified with poly-4-vinyl pyridine (P4VP) brush were synthesized and used as switchable electrochemical systems. The switch ON and OFF of the electrode activity were achieved by pH changes generated in situ by biocatalytic reactions in the presence of enzymes used as input signals. Two logic gates operating as AND/OR Boolean functions were designed using invertase and glucose oxidase or esterase and glucose oxidase as input signals, respectively. The electrode surface coated with a shrunk P4VP polymer at neutral pH values was not electrochemically active because of the blocking effect of the polymer film. The positive outputs of the logic operations yielded a pH drop to acidic conditions, resulting in the protonation and swelling of the P4VP polymer allowing penetration of a soluble redox probe to the conducting support, thus switching the electrode activity ON. The electrode interface was reset to the initial OFF state, with the inhibited electrochemical reaction, upon in situ pH increase generated by another enzymatic reaction in the presence of urease. Logically processed biochemical inputs of various enzymes allowed reversible activation-inactivation of the electrochemical reaction.

Electrochemical Nanotransistor from Mixed‐Polymer Brushes

Functional interfaces with switchable properties controlled by external physical signals (light, electrical potential, magnetic field) or chemical inputs (presence or absence of chemicals, variation of their concentrations) have received considerable attention in the last two decades. Their applications for controlled surface wettability, thin-films permeability, catalysis, and particularly for assembling modified electrodes, used in switchable sensors, fuel cells, and memory units, resulted in novel materials, devices, and systems. Recent integration of signal-responsive materials, functionalized switchable interfaces and biocomputing systems processing biochemical information resulted in the novel concept of biochemically controlled ‘‘smart’’ materials with built-in Boolean logic. Stimuli-responsive thin polymer films and particularly polyelectrolyte brushes bound at interfaces allowed efficient transduction of biochemically processed inputs into electronic output signals using the polymer thin-film transition between shrunken and swollen states changing the electrochemical activity. In most of the studied systems, including switchable electrode surfaces, membranes and nanostructured assemblies, the changes of their interfacial properties were controlled by altering the bulk solution pH value introduced into the systems or generated by in situ biochemical processes. Very few of these systems operated upon local pH changes generated at the interfaces without alteration of the solution properties. Various applications of the switchable interfaces for sizeand charge-selective separation, microfluidic devices, and in the future implantable biomedical devices require their functioning without changing their physiological environment, thus using only local signals resulting in the interfacial changes. Many applications are in fact limited by the nature of the external signal that can interfere with other functions of the device. Electrical potential, magnetic field, or nearinfrared and visible light are the most-demanded external signals to tune and switch the properties of stimuli-responsive materials. Only a few examples of changes in stimuli-responsive materials upon application of electrical potential are reported. Polyelectrolyte brushes with electroactive counterions were recently used as an effective platform for surfaces with electrochemically switchable wetting properties. The electroactuation of microcantilevers coated on one side with cationic polyelectrolyte brushes represent an exciting example of a miniaturized electromechanical device based on stimuliresponsive polymer brushes, where the response is triggered by an electrical potential applied to the functionalized electrode. This Communication demonstrates the possibility to reversibly switch the electrochemical activity of a polymerbrush-functionalized electrode between OFF/ON states by applying electrical signals which alternate only local, interfacial pH values. The fabricated device mimics the performance of switching electronic devices such as transistors. It operates in aqueous solutions and could be used in buffered biological environments. This device is only 25 nm thick in Z-direction and could be as small as the size of the grafted polymer coil (ca. 10 nm) in the XY-plain. The indium tin oxide (ITO) single-side coated conducting glass-electrode surface was modified with a mixed-polymer brush composed of two different polyelectrolytes poly(2-vinyl pyridine) (P2VP, molecular weight 56000 g mol ) and polyacrylic acid (PAA, molecular weight 100500 gmol ) covalently bound to the primary silane layer, Scheme 1. The thickness, composition, and structure of the P2VP/PAA mixed-polymer brush were studied by atomic force microscopy (AFM), ellipsometry, and contact angle measurements, revealing the P2VP/PAA ratio of ca. 1:3 (w/w) and the polymer film thickness of 24 2 nm in dry condition. The polymers can be protonated or deprotonated depending on the pH value of the electrolyte solution resulting in thin films with different charges, thus providing access to the electrode for ionic species charged opposite to the polymer and insulating the surface against the species carrying the same electrical charge. At the isoelectric point, when the charges on one of the polymers are compensated by the charges on another and the polymers form a polyelectrolyte complex, the neutral polymer film creates a hydrophobic barrier for the penetration of all ionic species to the electrode conducting support. The pH-switchable electrochemical activity of the P2VP/PAA-mixed-brush-modified electrode for selective reactions of anionic and cationic redox species ([Fe(CN)6] 4 and [Ru(NH3)6] 3þ, respectively) at different pH values was studied in details and reported elsewhere. Applying cyclic voltammetry to follow the transition of the