# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Commercial Biosensors
#### 4.1.3 Gas Sensors  
These sensors concern the quantitative measurement of gas concentration, mainly oxygen and carbon dioxide, due to their big effect on the metabolism of packaged vegetables and hence their quality. For carbon dioxide detection, optical-based gas sensors are a promising technology in food packaging applications. Commonly used types rely on fluorescence with the use of CO\({}_{2}\)-sensitive dyes or on colorimetric change as result of using pH sensitive dyes [118]. Dry solid CO\({}_{2}\) sensor is an opto-chemical sensor that uses fluorescent dyes that change their luminescence with the carbon dioxide level. An example on this type that can be used in vegetable packaging is in ref. [119] which is based on the use of pH-sensitive fluorescent dyes 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) incorporated within plastic films. As the CO\({}_{2}\) concentration increases, the fluorescence of the dye decreases. The response time for this dye is less than 2 min, and its recovery time of less than 40 min [119]. The advantages of HPTS plastic film CO\({}_{2}\) sensor are: its stability in various environments such as water and acidic solutions, high sensitivity (%CO\({}_{2}\) = 0.29%),  
Figure 11: **(a)** Optical top view for Janus droplets shows their transparency. **(b)** Part of Janus partials forming carbohydrate-based interactions is capable of binding to ConA which is like antigens of _E. Coli._ **(c)** Binding with ConA shows how Janus droplets cluster causing optical changes. Figure 12: Schematic of immunomagnetic separation (IMS) with the use of nanoparticles (NPs) and antibodies. and its shelf life can extend to more than 6 months if kept in a dark storage environment [119]. The limitation lies in difficulties of finding suitable fluorescent material for food applications and better sensitivity is required to cover the measuring range of 0-100% [118]. Another example on this dry carbon dioxide sensor type was tested on ready-to-eat packaged salads in [118] that is based on Forster Resonance Energy Transfer (FRET). FRET mechanism relies on energy transfer from donor fluorophore to acceptor chromophore [120]. Here, phosphorescent donor dye Pt-porphyrin (PtTFPP) will emit energy at 650 nm to be absorbed by pH indicator chromophore (\(\alpha\)-naphtholphthalein NP) acceptor. The mixture of both complexes will result in films which have varying optical responses with carbon dioxide concentration. The concentration of carbon dioxide affects the FRET. It is highest kin no CO\({}_{2}\) and decreases as the CO\({}_{2}\) increases. For this sensor, the energy transfer will have optical responses from blue to colorless when the CO\({}_{2}\) is in the range of 0-10%, which makes this sensor suitable with modified atmosphere vegetable packages that have the same range of CO\({}_{2}\)[118]. The response time of this sensor is 1 min with a recovery time of less than 4 min. Its sensitivity is constant for 21 days at 4 \({}^{\circ}\)C which is considered suitable for vegetable packaging applications [107; 118; 121]. The safety of this sensor was tested and it showed no traces of dyes migrating to the packaged food, which is a big advantage for commercial uses. The limitation of this sensor is its temperature sensitivity, deterioration of its performance if carbon dioxide or oxygen concentrations are higher than 10%, susceptibility to signal drift, and cross-sensitivity with O\({}_{2}\). To overcome the cross-sensitivity for this sensor type, the oxygen concentration is measured to compensate for its effect, thus, the sensor can be used to evaluate the concentration of both CO\({}_{2}\) and O\({}_{2}\) at the same time [118]. A sol-gel based optical carbon dioxide sensor that uses the pH indicator 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilized on a hydrophobic modified silica matrix forming membranes, was developed in [114]. The working principle of this sensor depends on the luminance intensity being quenched by CO\({}_{2}\) that leads to its fluorescence.