# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Sensors
#### 4.1.2 Biosensors  
Biosensors are analytical devices that detect, record, and convert biological responses into easily measured signals such as electrical or optical signals. This category of sensors consists of two main elements: bio-receptors, which recognize the target analyte, and transducers, which convert the target recognition into a measurable signal. These bio-receptors can be further classified into antibodies, enzymes, cells, DNA, biomimetic, or phage, while the transducer can be electrical, optical, chemical, magnetic, or micromechanical [31, 84]. In general, the first two types of these bio-receptors are widely used in the food packaging industry and have a promising potential in vegetable packaging for contamination assessment. In this section, the focus will be mainly on antibody-based biosensors. Antibody-based bio-receptors rely on the concept of \"lock and key\" fit with the antigens. Antigens are proteins found on the surface of the pathogen and can also be synthesized to bind with a large set of pathogens. Antibodies are made up from light and heavy chains of polypeptide and have two regions: a variable region which change its structure to fit the antigen, and a constant region with constant structure [31]. As biosensors, antibodies are generally immobilized on the surface of the detector, so that, with their unique property of binding to the antigen and recognizing the molecular structure, they will sense the presence of pathogens, as shown in Figure 8a [31]. The process of antibodies attachment to labels such as isotopes, fluorophores, or enzymes for the purpose of detection, is termed antibody labeling. The interactions between antigen and antigen-specific antibodies are converted to digital signals that can be read out with the use of transducers that are electrochemical, magnetic, optical, or mass-based. Figure 8b illustrates the functional blocks of antibody-based biosensors. The sensitivities of these sensors depend on both the sensitivity of the transducer and the quality of antibodies that are used. In this section, an overview of different types of antibody-based biosensors will be briefly discussed [31, 80]. The use of optical systems as a part of biosensors has been extensively investigated. These biosensors use light-based interactions to measure the biochemical reactions. One of the common methods for freshness detection of foodborne pathogens is surface plasmon resonance (SPR) [80]. Figure 8: (**a**) Antigen and antigen-specific antibody “Lock and Key” sensing. (**b**) Functional block of antibody-based biosensors. Figure 7: Map prediction for different samples with increasing number of _E. coli_ from left to right, the blue is the control sample, and the left color is the target color expected. This opto-electronic phenomenon is based on the energy transfer to electrons in a metal surface as from visible or near infrared monochromatic light through a prism into surface plasmon resonance to be detected by photodiodes. The photon energy transferred to electrons will free them and lead to the generation of electromagnetic waves (surface plasmons) that resonate and absorb light; thus, the reflectivity will be minimum at this specific angle [80]. This angle is a function of refractive index which depends on the mass of immobilized antibodies. The reflected beam will shift to longer wavelength due to the biomolecular interaction changing the refractive index [80]. Hence, measuring this change will qualitatively detect the presence of these analytes without the need for enriching and culturing. Figure 9a summarizes the working principle of this biosensor. The advantages of this SPR technology are their high sensitivity and selectivity and they can be made in the form of compact chips. On the other hand, the use of prism is not always suitable for chip-based sensors. Moreover, there is a limit in the number of simultaneous measurements that can be performed. To overcome these issues, the use of multiple channels sensors was proposed to allow for parallel measurements.