Altering the substituents of salicylic acid to improve Berthelot reaction for ultrasensitive colorimetric detection of ammonium and atmospheric ammonia
2021
Yu, Haili | Zhang, Guihua | Cai, Yanhua | Dong, Faqin
The Berthelot reaction is a classic method for detection of ammonium (NH₄⁺) and atmospheric ammonia (NH₃) 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 NH₄⁺ and NH₃. Here, five SA analogues with electron-withdrawing groups (-F) and electron-donating groups (-CH₃ and -OCH₃) 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-OCH₃-SA) shows the best colorimetric response and color change at a NH₄⁺ concentration of 30 μM, and the sensitivity of 4-OCH₃-SA-based colorimetric assay for NH₄⁺ 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-OCH₃ group, activating the two-step electrophilic aromatic substitution reaction in the Berthelot reaction. Additionally, visual and sensitive detection of NH₃ 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 NH₄⁺ and NH₃ in real water and air samples with good accuracy.
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