The last few decades have witnessed a steady increase of the social and political awareness for the need of monitoring and controlling numerous agricultural, environmental and industrial activities. Consequently, the most important governmental agencies have promulgated rules and directives to restrict the level of many chemical compounds in waters, foodstuff and industrial products. Worldwide legislation is thus driving the development of novel and highly efficient analytical tools. Clinical diagnostics and research in biomedical sciences are also demanding optimal sensors tailored for the specific needs of real-time measurements either in vitro or in vivo [1].

The fast-growing biosensor technology is one of the most active R&D domains of Analytical Sciences focused on the challenge of taking analytical chemistry to the field. Electrochemical biosensors based on redox enzymes, in particular, are highly appealing due to their usual quick response, high selectivity and sensitivity. In addition, they are cost-effective and easy to fabricate in portable dimensions. However, the implementation of these biosensorial systems may face several obstacles. Once chosen the enzyme candidate for the specific recognition of a target analyte, it should be efficiently immobilized atop the electrode surface, minimizing protein leakage and denaturation. Also, the immobilization matrix should work as a barrier against fouling and interfering species and, more importantly, should guarantee an efficient electronic communication between the redox sites of the protein and the transducing platform. Therefore, surface modification of electrodes plays a critical role on the construction of enzyme based electrochemical biosensors [1].

This communication aims to provide an overview of the many different strategies lately proposed to overcome the setbacks above mentioned. The pros and cons of some of these approaches will be illustrated through the case study of nitrite biosensors based on redox enzymes with catalytic activity for this analyte. In fact, our group has proposed a variety of routes for the construction of nitrite biosensors using the fast and robust cytochrome c nitrite reductase from Desulfovibrio desulfuricans, which catalyzes the reduction of NO₂^- to NH₄⁺. Fully integrated bioelectrodes including synthetic redox mediators combined with a variety of immobilization were initially proposed [2-4]. More advanced strategies operating through direct electron transfer [5] and exploiting nanostructured materials [6,7] were subsequently reported. Very recently, we have also proposed a novel biosensor based on cyt. cd₁ nitrite reductase (converts NO₂^- into NO), which was successfully co-entrapped with its physiological redox partner, the electroactive cyt. c₅₅₂ [8], indicating that the cooperative use of enzymes and their physiological redox partners could become a new trend in the design of electrochemical biosensors.

References: [1] M.G. Almeida et al., Sensors  2010, 10, 11530; [2] M.G. Almeida et al. Biosens. Bioelectron. 2007, 22, 2485; [3] S. Silva et al. Electrochem. Commun. 2004, 6, 404; [4] H. Chen et al., Electrochem. Commun. 2007, 9, 2241; [5] Silveira et al. Biosens. Bioelectron. 2010, 25, 2026; [6] C. Silveira et al. Electroanalysis 2010, 22, 2973; [7] C. Silveira et al., in preparation; [8] A.S. Serra et al.  Anal. Chim. Acta, 2011, 693, 41-46.

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