Methanol detection by film strips containing CD polymer

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Due to the serious threat of methanol to human health, the convenient, quick and specific detection of methanol is of great importance. Therefore, in this study, a biomass-derived chiral nematic composite film is fabricated by the co-evaporation of cellulose nanocrystals (CNC) and water-soluble polymer of β-cyclodextrin (PCD). The freestanding iridescent CNC-PCD film shows distinguishing structural colors of red and yellow green for methanol and ethanol respectively, realizing the identification of the two homologous alcohols by naked eyes. Besides, the quick, reversible and quantitative colorimetric sensing of the CNC-PCD film to methanol is verified. As its structural color and maximum reflection wavelength redshift to the greatest extent for methanol than other alcohols, the CNC-PCD film exhibits the specificity and selectivity for methanol detection in both single and mixed solvents. Such less-consumed and easy-to-handle CNC-PCD film may be useful as a colorimetric sensor to detect or discriminate methanol in some industrial products.

Shift of the maximum reflective wavelength of the CNC-PCD2 film in (a) single and (b) mixed solvents; (c) proposed mechanism for the methanol-responsive structural color change of the CNC-PCD film.

The detection mechanism of methanol is speculated: the β-CD molecules in PCD are likely to form intermolecular hydrogen bonds, driving PCD to self-assemble compactly (Miyake et al., 2003; Ward et al., 2014). However, the added methanol competitively forms new hydrogen bond with the single bondOH of β-CD rims, so that loosen the close packing of PCD. As a result, the pitch of CNC is enlarged and the composite film exhibits methanol-responsive structural color change. The possible mechanism is schemed in Fig. 6c. It is worth mention that ethanol also caused the change of the chiral nematic structure of CNC-PCD, but because of its weaker polarity than methanol, the change degree of structural color and reflection wavelength was feeble, consequently, methanol was discriminated by CNC-PCD from ethanol. As for the other solvents with further weaker or none polarity, it is more difficult for them to form intermolecular hydrogen bonding with β-CD molecules, so as to realize the specific recognition of CNC-PCD for methanol.

Chun-yan Hu, Lan Bai, Fei Song, Yu-liang Wang, Yu-zhong Wang (2022) Cellulose nanocrystal and β-cyclodextrin chiral nematic composite films as selective sensor for methanol discrimination. Carbohydrate Polymers 296, 119929.

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