Nano-photonic chemical sensor using rare-earth upconversion phosphors

The objective of the paper was to demonstrate feasibility of a chemical (ammonia) sensor using dye-doped polymer nanocomposite with upconversion phosphor nano-particles. The micro-crystalline powder of upconversion rare-earth phosphor NaYF4:Yb3+, Er3+ was synthesized using a simple wet process followed up by baking in open air. The powder was reduced into nano-colloid with 100-nm nano-particles using the ball milling process. The nano-colloid was added to the solution of polymer poly(methyl methacrylate) known as PMMA. Additionally, a pH indicator dye (Phenol Red or Bromothymol Blue) was dissolved in polymer solution. The dye-doped polymer nanocomposite films were deposited on substrates using the dipping process followed by baking in order to evaporate the solvent. The deposited nano-photonic sensor film had bright green upconversion fluorescence with a spectral peak at 540 nm attributed to the nano-photonic rare-earth phosphor pumped with a 980 nm infrared diode laser. The spectrum of green emission matched the absorption band of the indicator dye exposed to ammonia. When the film was exposed to ammonia, it demonstrated an optical response in the form of the drop of intensity of green radiation measured with a silicon photodiode power meter. The sensitivity of the developed chemical sensor was close to 0.4% ammonia in air, and the response time was close to 5 minutes.