Cholesteric film of Cu(II)-doped cellulose nanocrystals for colorimetric sensing of ammonia gas.

With the increasing demand of environmental monitoring for toxic and odorous ammonia gas it is desired to develop specific green, cost-effective and in situ passive colorimetric alternatives to current complex instrumentations. In this work, we designed an ammonia gas sensor based on cholesteric liquid crystal films of copper(II)-doped cellulose nanocrystals (CNCCu(II)) whose structure, optical and sensing properties were investigated. The hybrid films using the low doping Cu(II) as a color-tuning agent inherited the chiral nematic signature and optical activity of CNCs, suggesting a strong chelation between copper ions and negatively charged CNCs. The sensing performance illustrates that the CNCCu(II)125 film was sensitive to ammonia gas which could merge into nematic layers of CNCs and trigger-sensed to copper ions chelated on CNCs, consequently arousing a red-shift of reflective wavelength as well as an effective colorimetric transition. Such a hybrid film is anticipated to boost a new gas sensing regime for fast and effective on-site qualitative investigations.