There is also an optical spectroscopic sensor based on reflectivity for CO\({}_{2}\)[7] which measures concentrations of between 0.8-44.6%.  
In 2016, two sensors were designed for the detection of CO\({}_{2}\). The first one measures concentrations in the range 0-500 ppm with a sensitivity of 6 \(\times\) 10\({}^{-9}\) RIU/ppm and an uncertainty of 20 ppm. It uses the refractometry principle and has the lowest uncertainty of the market [8]. The second one uses a micro-cantilever structure made of ZnO to detect the CO\({}_{2}\). This sensor is based on the former. The foundation is a crystalline structure of ZnO nanorods. The response to the gas is produced when the resonance frequency of the microcantilever vibration is increased. It takes approximately 3 min for the gas desorption. This sensor only measures the presence/no presence of CO\({}_{2}\)[9].  
As for planar waveguide optical sensors, there is one made of planar waveguides with a cladding made of extract of Alstonia Scholaris leaves. It takes 4-5 s for the measurement and needs a period of about 5 s to measure again. The range of measurement is 0-62,500 ppm and it has a lifetime of 25-30 days. It is not able to measure with humidity above 70% [10].  
Another sensor is made of an ellipsoidal planar waveguide with which it is possible study the dependence of the concentration and temperature. It has been demonstrated that an increase of 80 \({}^{\circ}\)C produces a standard deviation of 5%. This sensor has a range of 0-3000 ppm [11].  
An optical fiber sensor has been developed for detecting concentrations of CO\({}_{2}\) in oceans. It requires a stabilization time of 5 min and has a sensitivity of 0.2371 RIU\({}^{-1}\)[12].  
Another optical fiber sensor is based on fluorescence. It works in the mid-infrared range and considers a minimum of CO\({}_{2}\) in 400 ppm (atmospheric average). The range is 500-3000 ppm and the resolution is 5% [13].  
The next sensor is based on the absorption of transmitted light for an optical fiber. The response time varies between 20 s and 100 s, depending on the thickness of the layer [14].  
Based on the principle of the Fabry-Perot interferometer (with optical fiber), a CO\({}_{2}\) sensor was constructed to measure concentrations of 0-700 ppm [15].  
There is another optical fiber sensor which measures a range of 0-75% of CO\({}_{2}\) and has a dynamic response of 6.1 min and 8 min when the up time is 2.12 min and the down time is 2.95 min [16].  
In 2009, the first sensor plasmon resonance (SPR) was introduced and it operated in the mid-infrared range to detect CO\({}_{2}\). It is based on the model of Kretschmann, with a glass prism of CaF\({}_{2}\) and a layer of Ti and another of Au. The sensitivity is so far the highest of all the CO\({}_{2}\) sensors, 10\({}^{-5}\) RIU\({}^{-1}\)[17].  
Another SPR with a mixture of polymer is able to trap CO\({}_{2}\) in a reversible way. It works with a wavelength of 640 nm and measures CO\({}_{2}\) dissolved in synthetic air. The limit of concentration is 10 ppm [18].  
In 2015, [PERSON] et al. manufactured a gas sensor with the Kretschmann configuration, modifying the hemiprism, and after the layer of Au, they used a ZnO layer in a cylindrical way where the CO\({}_{2}\) circulates. This is a sensor of presence; it is not able to measure quantities [19].  
It is interesting to mention some plasmonic optical fiber sensors as well, since they are currently attracting a large amount of research interest for the detection of gases [20,21].  
Table 1 summarizes the key characteristics of the studied sensors so that they can be compared easily and quickly. The number in brackets corresponds to the reference number of the paper.  
In view of all the limitations of the sensors described above, we propose a CO\({}_{2}\) sensor able to detect very high, high, normal, low and very low concentrations which could work both indoors and outdoors without any modification of the structure, wavelength, etc.