Pt-Decorated MWCNTs-Ionic Liquid Composite-Based Hydrogen Peroxide Sensor to Study Microbial Metabolism Using Scanning Electrochemical Microscopy

Hydrogen peroxide (H₂O₂) is a highly relevant metabolite in many biological processes, including the oral microbiome. To study this metabolite, we developed a 25 μm diameter, highly sensitive, nonenzymatic H₂O₂ sensor with a detection limit of 250 nM and a broad linear range of 250 nM to 7 mM. The sensor used the synergistic activity of the catalytically active Pt nanoparticles on a high surface area multiwalled carbon nanotube and conducting ionic liquid matrix to achieve high sensitivity (2.4 ± 0.24 mA cm^-2 mM^-1) for H₂O₂ oxidation. The unique composite allowed us to miniaturize the sensor and couple it with a Pt electrode (25 μm diameter each) for use as a dual scanning electrochemical microscopy probe. We could detect 65 ± 10 μM H₂O₂ produced by Streptococcus gordonii (Sg) in a simulated biofilm at 50 μm above its surface in the presence of 1 mM glucose and artificial saliva solution (pH 7.2 at 37 °C). Because of its high stability and low detection limit, the sensor showed a promising chemical image of H₂O₂ produced by Sg biofilms. We were also able to detect 30 μM H₂O₂ at 50 μm above the biofilm in the presence of the H₂O₂-decomposing salivary lactoperoxidase and thiocyanate, which would not otherwise be possible using an existing H₂O₂ assay. Thus, this sensor can potentially find applications in the study of other important biological processes in a complex matrix where circumstances demand a low detection limit in a compact space.