An overview of hydrogel-modified electrodes for electrochemical biosensing

As an innovative sensing platform, electrochemical biosensors have been extensively developed in academic research for diverse applications: clinical diagnostics, food safety, environmental monitoring, pharmaceutical analysis, etc. These devices convey molecular recognition events to prompt and readable electrical signals for point-of-care and on-side detection of specific target analytes and contaminants with high performance and rapid response often in small sample volume. Considering the limited lifetime, activity and stability of direct immobilization biorecognition molecules, different modification strategies of electrode surface have been developed over the years using nanomaterials, self-assembled monolayers, conducting/anti-fouling/redox polymers and even composite materials of above. This is intended to introduce functional groups and increasing the effective surface area of electrode surface so as to lower the sensor detection limit. In particular, hydrogel-modified electrochemical biosensors with high sensitivity, selectivity, as well as sustained stability represent a rapidly advancing frontier in analytical biotechnology, are combining the unique properties of hydrogels and the specific recognition properties of biomolecules for target molecules with the precision of electrochemical transduction. The main strategies for hydrogel polymer surface immobilization are discussed, comparing non-covalent and covalent attachment of hydrogels. Notably, the covalent immobilization of hydrogel layers with tunable porosity for improved ion diffusion on electrode surface, are interesting since they are forming polymer interface with long-term stability and reducing non-specific interactions. Special attention is then given to the methods for biorecognition element immobilization in/on hydrogel functionalized electrodes, including physical adsorption, encapsulation, affinity binding and covalent binding. For bio-elements covalent coupling, the covalent immobilization within or onto hydrogel matrix enhanced bio-elements retention to give efficient response, prolonged sensor lifetime through providing biocompatible interface and precise orientation of biomolecules, which processes a competitive edge in terms of real water sample detection. Typical examples of hydrogels-based electrochemical biosensors for clinical diagnostics, food safety, environmental monitoring, and pharmaceutical analysis are highlighted with recent breakthroughs. Finally, current challenges are outlined providing a roadmap for next-generation hydrogel biosensor design.