Preparation of a natural dye doped Ormosil coating for the detection of formaldehyde in the optical gas sensor

A novel Ormosil sensing coating for the optical detection of formaldehyde was reported in which the natural dye extracted from rose was used. The rose anthocyanin had the flavylium form at the acidic condition and the absorbance at 506 nm (deep rosy) reached its apex at pH 1.5. The energy-minimized geometric configuration of anthocyanin molecule in the flavylium form indicated a nearly planar geometry and its HOMO orbital showed the enlargement of conjugation system. The natural dye was used as a colorimetric indicator to the reaction between formaldehyde and hydroxylamine sulfate in the Ormosil coating, which was prepared from methyltriethoxysilane (MTES) and polydimethylsiloxane (PDMS) through the sol–gel process. The surface morphology of the Ormosil sensing coating was studied by the atomic force microscopy and the nanoscale rill-like folds were explained by the aggregation–hybridization mechanism. The optical absorbance of the Ormosil sensing coating at 506 nm was found linear with the concentration of formaldehyde and the sampling time for measurement was optimized as 5 min. The Ormosil sensing coating could be used as a low-cost and disposable testing strip in a prototypical optical gas sensor, which was then applied in determination of formaldehyde in the indoor air.