A paper indicator for triple-modality sensing of nitrite based on colorimetric assay, Raman spectroscopy, and electron paramagnetic resonance spectroscopy

Paper indicators based on colorimetric assays are widely used for nitrite detection, but their application to liquids with strong colours is restricted. We report a novel paper indicator that allows for sensing nitrite by colorimetric assay, Raman spectroscopy, and electron paramagnetic resonance spectroscopy with non-overlapping signal wavelength ranges through non-contact means. The paper indicator was prepared by impregnating poly(4-aminostyrene), 2-naphthol and single-walled carbon nanotubes in a regular filter paper. All three ingredients were essential to realize the triple-modality sensing. This method is simple and inexpensive, and promises to have wider applicability than the existing paper indicators.