Optical CO2-sensing layers for clinical application based on pH-sensitive indicators incorporated into nanoscopic metal-oxide supports

We describe CO2-selective films designed for clinical application based on the co-incorporation of different phenol dyes, α-naphtholphthalein (NAF), naphthol blue black (NBB) and calmagite (CMG), with a phase-transfer agent, tetraoctylammonium hydroxide (TONOH), into metal-oxide nanoporous matrices. The metal-oxide coatings consist of aluminum (AlOOH), silicon (SiO2) and zirconium (ZrO2) oxides. The chemical reaction principle and preparation of these new sensing layers are described in detail. We have investigated the effect of the type and concentration of the pH-indicator and TONOH together with that of the composition and morphological characteristics of the nanostructured material. The influence of external parameters such as flow-rate, humidity and interfering gases (NO, CO, NO2 and SO2) were also looked into. The sensing films responded to CO2 concentrations in the gas phase between 0.25% and 40% CO2 (v/v) for NAF–TONOH, 4.1% and 30% CO2 for NBB–TONOH and 0.6% and 40% CO2 for CMG–TONOH, with detection limits of 0.25%, 4.1% and 0.6% CO2 (v/v) for NAF–TONOH, NBB–TONOH and CMG–TONOH, respectively. The nanoporous membranes provided higher dispersion and accessibility of the dye, and thus a quick response time, lower probability of aggregation, the possibility of sterilization by gamma radiation, insulation from reactive chemicals and higher ambient stability.