Real-Time Sensing of Hydrogen Peroxide by ITO/MWCNT/Horseradish Peroxidase Enzyme Electrode

The accurate and sensitive determination of H2O2 is very important in many cases because it is a product of reactions catalysed by several oxidase enzymes in living cells and it is essential in environmental and pharmaceutical analyses. The fabrication of enzyme protein activity based biosensors is a very promising way for this purpose because the function of biological molecules is very specific, sensitive, and selective. Horseradish peroxidase (HRP) is the most commonly used enzyme for H2O2 detection because it can oxidize hydrogen atoms and, for example, xenobiotics in the presence of H2O2. In order to define the limit of detection (LOD) of H2O2 we made calibrations with guaiacol and amplex red (AR), which are hydrogen donors of HRP. The accumulation of the reaction products, tetraguaiacol, and resorufin, respectively, then can be easily detected by absorption or emission (fluorescence) spectroscopy. In our experiments an enzyme electrode was fabricated from ITO (indium tin oxide), functionalized multiwalled carbon nanotubes (f-MWCNTs), and HRP. Although the enzyme activity was smaller by about two orders of magnitude when the enzyme was bound to the f-MWCNTs (ca. 10−2 M H2O2/(M HRP·sec) compared to ca. 2 M H2O2/(M HRP·sec) and 5 M H2O2/(M HRP·sec) with AR and guaiacol in buffer solution), LOD of the H2O2 decomposition was about 6 pM H2O2/sec and 10 pM H2O2/sec in the case of AR and guaiacol, respectively