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

Abstract We describe CO 2 -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 (SiO 2 ) and zirconium (ZrO 2 ) 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, NO 2 and SO 2 ) were also looked into. The sensing films responded to CO 2 concentrations in the gas phase between 0.25% and 40% CO 2 (v/v) for NAF–TONOH, 4.1% and 30% CO 2 for NBB–TONOH and 0.6% and 40% CO 2 for CMG–TONOH, with detection limits of 0.25%, 4.1% and 0.6% CO 2 (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.