A pocket-sized 3D-printed attenuated total reflection-infrared filtometer combined with functionalized silica films for nitrate sensing in water.

• Small, cost-effective mid-IR sensor for nitrate in aqueous solutions based on a 3D-printed ATR-IR filtometer. • ATR crystals were coated mesoporous silica functionalized with trimethylammonium groups. • Enrichment factors of up 1600 and 1.2 mg L−1 limits of detection were achieved. Strong anion-exchange materials carry a positive charge that allows them to trap and concentrate anions while releasing other anions. Here, we introduced a quaternary ammonium anion exchange group into mesoporous silica films coated on attenuated total reflection (ATR) crystals to enrich nitrate from aqueous phase in the volume probed by the evanescent field. The ion-exchange and enrichment capabilities of the films were characterized using FTIR spectroscopy. Thereby fast analyte enrichment and full sensor recovery were observed. In addition, high enrichment factors of up to 1600 were achieved. After characterization, these coated ATR crystals were used in a dedicated ATR-IR filtometer comprising a Fabry-Pérot filter detector unit and a miniaturized thermal emitter with a overall dimensions of only 80 mm × 120 mm × 70 mm. The filter covered the spectral region between 1250–1800 cm−1 allowing for recording IR spectra of nitrate enriched into the mesoporous silica film. The sensor was calibrated using the Langmuir adsorption model as calibration function. From this a limit of detection of 1.2 mg L−1 was derived for the ATR-IR filtometer. This emphasizes the high potential of functionalized mesoporous silica films combined with low-cost filtometers for portable water sensors. [ABSTRACT FROM AUTHOR]