(323a) A Paper Indicator for Triple-Modality Sensing of Nitrite Based on Colorimetric Assay, Raman Spectroscopy, and Electron Paramagnetic Resonance Spectroscopy

Nitrite exists ubiquitously in environments, foods and biological fluids, and plays important roles in various biological, physiological, and food preservation processes. Sensing nitrite is of critical importance for understanding and controlling these processes. Many methods are available for detecting nitrite via diverse modalities such as colorimetric assay, Raman spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy, among which the paper indicator-based colorimetric assay is featured by ease of use, commercial availability, and low cost. However, this method is prone to interference by deep-colored pigments that exist in certain analyte liquids such as the whole blood because the paper can be stained by the pigments as a result of direct contact between the paper and the liquid. This limitation can be overcome if the paper indicator allows not only the colorimetric assay but also other modalities for sensing nitrite. Moreover, by acquiring data from different sensing modalities, chance of false positive and false negative detection can be reduced.

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 a 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. PAS is a polymer with aromatic primary amine as side functional group, which can be converted to aryldiazonium salt in the presence of nitrous acid. Aryldiazonium salt can react with 2-naphthol to produce azo compounds of deep red color. This type of reaction underlies the widely used Griess colorimetric assay. Moreover, 2-naphthol reacts with nitrous acid to generate nitrosonaphthol that is a nitroso compound, and nitroso compounds can react with nitric oxide (NO) produced from an acidified nitrite solution to generate free radical products with EPR signals. In addition, aryldiazonium salt can introduce defects in single-walled carbon nanotubes, which can be detected readily by Raman spectroscopy. This method is simple and inexpensive, and promises to have wider applicability than the existing paper indicators.