Deep Eutectic Solvents for Ammonia Capture and Detection: Molecular Design, Absorption Mechanisms, and Analytical Applications: A Brief Review

Ammonia (NH3) is a valuable resource for agriculture and a promising carbon-neutral hydrogen carrier, but current industrial emissions result in excessive environmental/health impacts. Popular approaches to removal—such as acid/water scrubbing and ionic liquids—are limited due to solvent volatility, costly regeneration, secondary salt wastes, or costly synthesis. Deep eutectic solvents (DESs) represent a sustainable and task-specific alternative, characterized by low vapor pressure and the capacity for straightforward, low-cost chemical customization. This review presents a digest of literature synthesis combining molecular design strategies, thermodynamic/atomistic mechanisms of absorption, process-scale modeling, and sensing applications for NH3, together with a classification of key DES synthesis protocols, including protic, multiacid/weak acid, azole-based, non-halide, supramolecular host–guest, and metal-coordinated systems. These synthesis strategies are designed to optimize absorption capacity, selectivity for NH3/CO2, transfer efficiency, and ease of regeneration. Molecular dynamics simulations and spectroscopies have been examined to elucidate the overall mechanism of absorption for a two-phase system (specific H-bonding, followed by weak physical dissolution), which dominates within DESs using specific atomistic interactions (hydroxyl-, amino-, or ammonium-residue) and van der Waals forces. Emerging sensing methods of DES have been briefly investigated (microextraction techniques: AALLME, DLLME; mobile phone colorimetry; and chemoresistive sensors). This work highlights the need for closer alignment between the atomistic models and the design of the sensor.