Intravascular (IV) catheters are essential devices in the hospital that are used to monitor a patient’s blood and for administering drugs or nutrients. However, IV catheters are also prone to blood clotting at the point of insertion and infection by formation of robust bacterial biofilms on their surface. Nitric oxide (NO) is ideally suited to counteract both of these problems, due to its ability to inhibit platelet activation/aggregation and its antimicrobial properties. One way to equip catheters with NO releasing properties is by electrocatalytic nitrite reduction to NO by copper complexes in a multilumen configuration. In this work, we systematically investigate six closely related Cu­(II) BMPA- and BEPA-carboxylate complexes (BMPA = bis­(2-methylpyridyl)­amine); BEPA = bis­(2-ethylpyridyl)­amine), using carboxylate groups of different chain lengths. The corresponding Cu­(II) complexes were characterized using UV–vis, EPR spectroscopy, and X-ray crystallography. Using detailed cyclic voltammetry (CV) and bulk electrocatalyic studies (with real-time NO quantification), in aqueous media, pH 7.4, we are able to derive clear reactivity relations between the ligand structures of the complexes, their Faradaic efficiencies for NO generation, their turnover frequencies (TOFs), and their redox potentials. Our results show that the complex [Cu­(BEPA-Bu)]­(OAc) is the best catalyst with a high Faradaic efficiency over large nitrite concentration ranges and the expected best tolerance to oxygen levels. For this species, the more positive reduction potential suppresses NO disproportionation, which is a major Achilles heel of the (faster) catalysts with the more negative reduction potentials.

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Ratiometric sensors are self-referencing constructs that are functional in cells and tissues, and the read-out is independent of sensor concentration. One strategy for ratiometric sensing is to utilize two-color emission, where one component possesses analyte-dependent emission and the other is independent of analyte concentration, serving as an internal standard. In this way, the intensity ratio of the two components is a quantitative measure of the analyte. In this study, protein-based ratiometric oxygen sensors are prepared using the heme nitric oxide/oxygen-binding protein (H-NOX) from the thermophilic bacterium Caldanaerobacter subterraneus. The native heme cofactor is replaced with a Pd(II) or Pt(II) porphyrin as the oxygen-responsive phosphor. Mutagenesis is performed to incorporate a cysteine residue on the protein surface for thiol/maleimide coupling of the oxygen-insensitive dye, which serves as a Forster resonance energy transfer (FRET) donor for the porphyrin. While both Pd(II)-and Pt(II)-based sensors are responsive over biologically relevant ranges, the Pd sensor exhibits greater sensitivity at lower oxygen concentrations. Together, these sensors represent a new class of protein-based ratiometric oxygen sensors, and the modular platform allows the oxygen sensitivity to be tailored for a specific application. This proof-of-principle study has identified the key considerations and optimal methodologies to develop and subsequently refine protein-based ratiometric oxygen sensors.