Altering the substituents of salicylic acid to improve Berthelot reaction for ultrasensitive colorimetric detection of ammonium and atmospheric ammonia.

The Berthelot reaction is a classic method for detection of ammonium (NH) and atmospheric ammonia (NH) by using salicylic acid (SA) as the chromogenic substrate. However, there lacks a method for improving the activity of the Berthelot reaction to enhance the analytical performance for detection of NH and NH. Here, five SA analogues with electron-withdrawing groups (-F) and electron-donating groups (-CH and -OCH) at different positions of the aromatic ring have been chosen as the alternative to SA for Berthelot reaction. Among these analogues, 4-methoxysalicylic acid (4-OCH-SA) shows the best colorimetric response and color change at a NH concentration of 30 μM, and the sensitivity of 4-OCH-SA-based colorimetric assay for NH increases 1.75-fold compared with that of SA-based colorimetric method. This enhancement effect is attributed to the strong electron-donating property of 4-OCH group, activating the two-step electrophilic aromatic substitution reaction in the Berthelot reaction. Additionally, visual and sensitive detection of NH is realized, along with a low limit of detection down to 0.037 ppm. Furthermore, we demonstrate that this assay is reliable and practical for detection of NH and NH in real water and air samples with good accuracy.