Characterization of thio compounds for a surface-controlled electroless deposition method in the preparation of silver nanoparticles on germanium for surface-enhanced infrared absorption measurements

Electroless reduction of silver chloride precipitates for the preparation of highly sensitive substrates for surface-enhanced infrared absorption (SEIRA) measurements

To prepare silver nanoparticles (AgNPs) on infrared-transmitting crystal for surface-enhanced infrared absorption (SEIRA) measurements, a new strategy is proposed and demonstrated using electroless reduction of preformed silver chloride (AgCl) particles. Silver chloride precipitates were formed using an additive of polyvinyl pyrrolidone (PVP) to vary the size and shape of the precipitates. After settling on germanium substrates, the preformed particles of AgCl were reduced electrolessly and spontaneously coagulated to AgNPs. The resulting AgNPs showed a multilayer structure, but the AgNPs were isolated, as shown by the lack of absorption-band distortion in the SEIRA measurements. Hence, the sensitivity and analyte-loading capacity for SEIRA measurements are improved significantly. To optimize the chemical deposition and electroless reduction method, we examined several parameters, including the concentrations of reagents during AgCl precipitation and the reaction time required in the deposition-reduction steps. We used para-nitrobenzoic acid (pNBA) to probe the intensity of the SEIRA effect for the prepared substrates. To better correlate the SEIRA performances with each variable, we examined the prepared substrates using a scanning electron microscope and SEIRA. The results indicate that two major morphologies of AgNPs are observed: nanoparticles and nanorods. The distributions of nanorods we observed were related to the procedures used to prepare the substrates. Based on SEIRA signals, we observed enhancement factors approaching three orders of magnitude compared to conventional transmission measurement. Also, based on the morphologies, the large signals were mainly caused by the formation of multilayers of non-percolated AgNPs.

Surfactant-assisted electroless deposition of silver nanoparticles on Ge crystal for ultra-sensitive detection by surface-enhanced infrared absorption spectroscopy

Fabrication of highly sensitive substrates for SEIRA measurements via assistance of surfactant to electrolessly deposit silver nanoparticles on Ge crystal.

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.