# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Commercial Biosensors
#### 4.1.3 Gas Sensors
The fluorescence intensity lifetime is then converted and measured in phase domain with the use of the dual luminophore referencing (DLR) method [121; 122; 123; 124]. In this technique, the fluorescent dye is co-immobilized with an inert referencing luminophore such as ruthenium complex that has a long lifetime and high absorption rate of blue to green light. In phase domain, the HPTS dye fluorescence intensity signal will have a zero-phase angle due to its short lifetime while the reference luminophore will have a phase shift. The total measured signal will represent the superposition of both signals leading to a different combined measured phase. Thus, the change in carbon dioxide concentration will lead to change in the amplitude of fluorescence intensity signals of both luminophores. This amplitude change will correspond to a phase change in the total measured signal which is a function of carbon dioxide in the sample gas [124]. This sensor has a resolution better than 1%, a limit of detection of 0.08%, and a minimized cross-sensitivity with oxygen of 0.6% [112; 125]. It is stable for more than seven months, which makes it suitable for vegetable packaging applications [107; 124]. However, this sensor suffers from oxygen sensitivity, as the commonly used reference luminophore (ruthenium complex) is affected by oxygen, and from temperature sensitivity. Therefore, the need of a correction protocol is essential to guarantee accurate measurements of gas concentrations [121; 122; 123; 124]. A nondispersive infrared (NDIR) sensor can be used to monitor the storage environment of a package or can be integrated as part of RFID tags. This type depends on the selective light absorption of gases at different wavelengths due to their different quantum energies. For carbon dioxide, it absorbs at 2.7, 4.3, and 15 \(\upmu\)m, the light passes through two tubes, one acting as the reference with nitrogen which does not absorb light, and the other having the gas to be measured. The ratio of the attenuation between the tubes is proportional to carbon dioxide concentration. The advantages of these sensors are their accurate reading, immunity to temperature, and humidity, but they are relatively more expensive compared to thermal resistors-based carbon dioxide sensors [126; 127]. Another type of sensing is based on thermal resistors whose resistance changes according to the thermal conductivity of carbon dioxide passing through it. The thermal resistors are two arms of a Wheatstone bridge that is shown in Figure 13. This type relies on the variation of the conductivity with the concentration of carbon dioxide, but it is also affected by temperature and humidity. The idea here is to measure the resistance of the thermal resistor in the CO\({}_{2}\) atmosphere by comparing its value to an identical thermistor in a reference chamber with sealed atmosphere under same temperature and humidity conditions. An increase in the concentration of the gas causes the molecular weight to be heavier compared to the dry air, shifting the output to positive, and the opposite occurs when there is less carbon dioxide in the air. The linear response of this sensor made it very suitable for gas detection up to 100% [13; 128]. However, the drawback of this type is in the dependence on other factors such as humidity and temperature that makes the sensor reading unreliable unless these conditions are the same and constant in both chambers [13; 128]. Many other carbon dioxide sensors were developed based on other techniques such as emulsions of iconic liquids at room temperature [129], and the polymer hydrogel-based sensor [130]. However, an important concern with these sensors that are used to quantify carbon dioxide will be the possibility of migration of the sensor materials to the food. Monitoring oxygen inside the package is important because an adequate amount of O\({}_{2}\) is needed to maintain vegetables freshness. Therefore, a lot of mechanisms were developed to measure oxygen content inside a package.