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 4 + ) and atmospheric ammonia (NH 3 ) 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 4 + and NH 3 . Here, five SA analogues with electron-withdrawing groups (-F) and electron-donating groups (-CH 3 and -OCH 3 ) 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 3 -SA) shows the best colorimetric response and color change at a NH 4 + concentration of 30 μM, and the sensitivity of 4-OCH 3 -SA-based colorimetric assay for NH 4 + 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 3 group, activating the two-step electrophilic aromatic substitution reaction in the Berthelot reaction. Additionally, visual and sensitive detection of NH 3 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 4 + and NH 3 in real water and air samples with good accuracy.