In biosensors and biofuel cells, it is often desirable to accelerate the electron transfer rate between the enzyme and electrode surface to improve the performance of the devices (sensitivity or power output). To this end, in this review, we focus on three important strategies available to improve the performance of enzyme-modified electrodes: the use of protein engineering, designer polymers, and the introduction of nanomaterials. Engineering the protein or proteins that constitute the biocatalytic elements allow tuning their stability, activity, and specificity. It can also allow changing the enzyme immobilization efficiency (adsorption vs. covalent immobilization, for example). If direct electron transfer is not favorable, it may be possible to introduce polymers in the system that mediate the electron transfer to or from the electrode surface. Significant advances have recently been made on the design of polymers to modify electrodes, including molecularly imprinted polymers and responsive polymers. A third element that can be incorporated into electrodes is nanoparticles. These nanomaterials can act as scaffolds to immobilize the biocatalytic elements through adsorption or chemical reaction with functional groups, increasing the surface area and the robustness of the electrode. A wealth of nanomaterials is being tested as part of novel enzyme-modified electrode designs, including graphene, carbon nanotubes, metallic nanoparticles, silicas, and metal–organic frameworks. Some of these can also be designed as nanowires to enable or shorten the direct electron transfer from distal active sites in the enzymes. In addition to these strategies, we also highlight selected applications of enzyme-modified electrodes, including glucose biosensing, self-powered biosensors, and self-charging biosupercapacitors. We conclude the review with a reflection on novel approaches, applications, and challenges that we foresee can impact how to design the enzyme-modified electrodes of tomorrow.

Biofuel cells allow for constructing sensors that leverage the specificity of enzymes without the need for an external power source. In this work, we design a self-powered glucose sensor based on a biofuel cell. The redox enzymes glucose dehydrogenase (NAD-GDH), glucose oxidase (GOx), and horseradish peroxidase (HRP) were immobilized as biocatalysts on the electrodes, which were previously engineered using carbon nanostructures, including multi-wall carbon nanotubes (MWCNTs) and reduced graphene oxide (rGO). Additional polymers were also introduced to improve biocatalyst immobilization. The reported design offers three main advantages: (i) by using glucose as the substrate for the both anode and cathode, a more compact and robust design is enabled, (ii) the system operates under air-saturating conditions, with no need for gas purge, and (iii) the combination of carbon nanostructures and a multi-enzyme cascade maximizes the sensitivity of the biosensor. Our design allows the reliable detection of glucose in the range of 0.1–7.0 mM, which is perfectly suited for common biofluids and industrial food samples.

We propose the very first “Nernstian biosupercapacitor”, a biodevice based on only one redox polymer: poly(vinyl imidazole‐co‐allylamine)[Os(bpy)₂Cl], and two biocatalysts. At the bioanode PQQ‐dependent glucose dehydrogenase reduces the Os³⁺ moieties at the polymer to Os²⁺ shifting the Nernst potential of the Os³⁺/Os²⁺ redox couple to negative values. Concomitantly, at the biocathode the reduction of O₂ by means of bilirubin oxidase embedded in the same redox polymer leads to the oxidation of Os²⁺ to Os³⁺ shifting the Nernst potential to higher values. Despite the use of just one redox polymer an open circuit voltage of more than 0.45 V was obtained during charging and the charge is stored in the redox polymer at both the bioanode and the biocathode. By connecting both electrodes via a predefined resistor a high power density is obtained for a short time exceeding the steady state power of a corresponding biofuel cell by a factor of 8.