para-Mercaptobenzoic acid-modified silver nanoparticles as sensing media for the detection of ammonia in air based on infrared surface enhancement effect.

To utilize the large signals provided by surface-enhanced infrared absorption (SEIRA) measurements for chemical sensing, a new sensing scheme was proposed and demonstrated for detection of ammonia in air samples. To increase the SEIRA effect, a sensing phase composed of multi-layers of silver nanoparticles (AgNPs) was prepared using a chemically controlled electroless deposition method. para-Mercaptobenzoic acid (pMBA) served as the controlling agent in formation of AgNPs, a surface modification agent of AgNPs for sensing, and a stabilizer to protect the AgNPs from coagulation and oxidation. The sensing approach utilized the interaction between pMBA and ammonia, which involves the formation of carboxylate-ammonium complex. After interaction, the enhanced IR absorption bands of pMBA on AgNPs were significantly changed and able to provide quantitative information on the ammonia concentrations. To optimize the conditions for preparing sensing elements, parameters used to form multi-layers of AgNPs were systematically varied and their corresponding sensitivities in detection of ammonia were recorded. The results indicate that AgNPs with diameters in the range of 100 nm provided the best performance in terms of detecting ammonia via the SEIRA effect. Also, the analytical signal generally increased as the number of layers of AgNPs increased, but was limited to certain layers, depending on the reaction conditions used in preparation of AgNPs. The sensing elements were found to be highly selective to ammonia and the detection limit approached 150 ppb with a linear range up to 25 ppm.