A quasi-reagentless point-of-care test for nitrite and unaffected by oxygen and cyanide

Bilirubin oxidase as a single enzymatic oxygen scavenger for the development of reductase-based biosensors in the open air and its application on a nitrite biosensor

The commercialization of amperometric or voltammetric biosensors that operate at potentials lower than -0.2 V vs SHE has been hindered by the need for anoxic working conditions due to the interference of molecular oxygen, whose electrochemical reduction can potentially mask other redox processes and generate reactive oxygen species (ROS). A deoxygenation step must be thus integrated into the analytical process. To this end, several (bio)chemical oxygen scavenging systems have been proposed, such as the bi-enzyme system, glucose oxidase/catalase. Still, a few issues persist owing to enzyme impurities and the formation of oxygen reactive species. Here in, we propose a new mono-enzymatic oxygen scavenging system composed of a multicopper oxidase as a single biocatalytic oxygen reducer. As a model, we used bilirubin oxidase (BOD), which catalyzes the direct reduction of oxygen to water in the presence of an electron donor substrate, without releasing hydrogen peroxide. Both the direct electron transfer and mediated electrochemical approach using different co-substrates were screened for the ability to promote the enzymatic reduction of oxygen. An optimal combination of BOD with sodium ascorbate proved to be quick (5 min) and effective. It was subsequently employed, as a proof-of-concept, in a voltammetric biosensor based on a multiheme cytochrome c nitrite reductase, which performs the reduction of nitrite to ammonia at potentials below -0.3 V vs SHE. The nitrite biosensor performed well under ambient air, with no need for a second enzyme to account for the build-up of oxygen reactive intermediaries.

A novel, rapidly preparable and easily maintainable biocathode electrochemical biosensor for the continuous and stable detection of nitrite in water

Nitrite is a highly toxic and pathogenic pollutant that is widely distributed in various nitrogenous wastewaters. Therefore, there is an urgent need for fast and stable nitrite detection to avoid water pollution and protect human health. In this study, we developed a novel rapidly preparable and easily maintainable biocathode electrochemical biosensor (BEB) using nitrite-reducing bacteria as the detectors to realize continuous nitrite monitoring in wastewater. The preparation of the biocathode was shortened by the polarity inversion method to less than 6 d. The BEB could detect nitrite solution samples in the range of 0.1- 16.0 mg NO₂^--N L^-1 within 1.7 min. The BEB was also successfully used to detect nitrite in real wastewater with a relative error < 4.0% and a relative standard deviation < 5.8%. In addition, the BEB could be easily maintained by an operation mode of microbial fuel cells and stably detected nitrite for at least 150 tests. Our study provided a feasible and convenient way to develop electrochemical biosensors based on the biocathode for continuous and stable monitoring of pollutants in wastewater.

Interfacing the enzyme multiheme cytochrome c nitrite reductase with pencil lead electrodes: Towards a disposable biosensor for cyanide surveillance in the environment

The present study reports a novel voltammetric biosensor for cyanide based on its inhibitory effect on cytochrome c nitrite reductase (ccNiR). Interestingly, the earlier development of a point-of-care test for nitrite based on the direct electrochemistry of ccNiR has shown that the cyanide inhibition depends on the type of carbon material employed as transducer (Monteiro et al., 2019). In this work, commercial graphite pencil leads were employed in the construction of both working and pseudo-reference electrodes, with ccNiR being simply drop casted onto the former. In this way, we produced a functional and fully integrated voltammetric biosensor for nitrite quantification that also allows to observe a decrease in the catalytic current due to cyanide addition. Under turnover conditions, the biosensor showed a linear response with the logarithm of cyanide concentration in the 5-76 μM (cyclic voltammetry) and 1-40 μM (square-wave voltammetry) ranges, with a sensitivity of 20-25% ln [cyanide μM]^-1 and a detection limit of 0.86-4.4 μM. The application of the pencil lead as a putative pseudo-reference was very promising, since the potentials profile matched those observed with a true reference electrode (Ag/AgCl). Overall, the direct electron transfer between ccNiR and a pencil lead electrode was demonstrated for the first time, with cyanide-induced inhibition being easily monitored, paving the way for the employment of these low-cost bioelectrodes as cyanide probes for on-site surveillance of aquatic environments.

Electrochemical Trimethylamine N-Oxide Biosensor with Enzyme-Based Oxygen-Scavenging Membrane for Long-Term Operation under Ambient Air

An amperometric trimethylamine N-oxide (TMAO) biosensor is reported, where TMAO reductase (TorA) and glucose oxidase (GOD) and catalase (Cat) were immobilized on the electrode surface, enabling measurements of mediated enzymatic TMAO reduction at low potential under ambient air conditions. The oxygen anti-interference membrane composed of GOD, Cat and polyvinyl alcohol (PVA) hydrogel, together with glucose concentration, was optimized until the O₂ reduction current of a Clark-type electrode was completely suppressed for at least 3 h. For the preparation of the TMAO biosensor, Escherichia coli TorA was purified under anaerobic conditions and immobilized on the surface of a carbon electrode and covered by the optimized O₂ scavenging membrane. The TMAO sensor operates at a potential of -0.8 V vs. Ag/AgCl (1 M KCl), where the reduction of methylviologen (MV) is recorded. The sensor signal depends linearly on TMAO concentrations between 2 µM and 15 mM, with a sensitivity of 2.75 ± 1.7 µA/mM. The developed biosensor is characterized by a response time of about 33 s and an operational stability over 3 weeks. Furthermore, measurements of TMAO concentration were performed in 10% human serum, where the lowest detectable concentration is of 10 µM TMAO.