Upconverting nanoparticle based optical sensor for carbon dioxide

We demonstrate a novel optical sensor for carbon dioxide in concentrations between 0 and 3%. The sensing scheme is based on the optical interrogation of a 12-μm polystyrene (PS) film containing upconverting nanoparticles (UCNPs; 40–100 nm in size) of the type NaYF4:Yb,Er, and the longwave absorption pH probe bromothymol blue (BTB) in its anionic (blue) form. PS is chosen as a matrix because it displays permeation selectivity for CO2 and rejects protons. The color of BTB in the PS matrix depends on the partial pressure of CO2 gas. The UCNPs are photoexcited with a 980-nm laser diode to give a green (542 nm) and a red (657 nm) emission whose intensity is screened off (depending on whether BTB is present in its blue or yellow form) due to an inner filter effect. The luminescence intensities of the UCNPs at 542 nm and 657 nm increase with increasing concentration of CO2. The pH probe BTB (a sulfonate) is used in the form of a lipophilic ion pair with the tetrabutylammonium cation (TBA). The strong base tetraoctylammonium hydroxide is added to the system and acts as a base to convert BTB in its phenoxide (blue) form, but also creates a buffer system. This is the first optical sensor for CO2 that is based on the use of UCNPs. Its response time is ∼10 s on switching from pure argon gas to 1% CO2 in argon, the recovery time of the sensing film is ∼180 s, and the detection limit is 0.11% of CO2.