| Summary: | This study investigated the surface properties of optical sensor layers prepared using sol-gel technology and their response to dissolved NH<sub>3</sub>. A glass substrate was used to fabricate the optical sensor layers. The sol-gel solution was applied to the glass substrate using three different techniques: spin coating (SC), inkjet printing (IP), and spray coating (SP). In this work, we have attempted to investigate the effects of the different techniques for producing the sensor layers and to determine their response in the presence of ammonia. The surface properties (surface free energy—SFE and surface chemical composition—XPS) and spectral properties (response to ammonia and real-time response) of the prepared optical sensor layers were characterised. The results show that the sensor layers prepared by different techniques have similar SFE and XPS values, but different responses to dissolved NH<sub>3</sub> solution and different responses in real-time measurements (exposure to fresh fish). Sensor layers prepared with a spray coating (SP) are the most responsive, the most sensitive, and have a higher response over time and the biggest colour change compared to SC and IP sensor layers.
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