In this work we report research carried out on the manufacturing of an optical sensor based on polyetherimide (PEI) and Ag nanoparticles (NPs) for food packaging and food freshness control applications. PEI/ Ag films were fabricated and tested as optical sensors by exposing them to 0.01 M and 0.005 M of 2-mercaptoethanol (2-ME). Thiol groups of 2-ME react on the surface of the sample and reduce the silver ions to Ag NPs, which lead to the change of color. The real-time measurement of meat freshness was performed for salmon, chicken, Turkey and beef. The colorimetric and UV-vis absorbance responses were measured over duration of 80 hours. The PEI/Ag sensor shows a good response to the released gases by meat. Moreover, salmon, chicken and Turkey are shown to produce color changes of PEI/Ag films after 20 hours of exposure, and beef after 60 hours of exposure. Therefore, this nano-sensor has a potential to serve as an indicator of food spoilage in the packaging.