# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 5 Sensing Technologies
### Optic  
It is well known that specific gases absorb different wavelengths, and every gas has its peculiar absorption property to different wavelengths; the wavelengths and the target gases are listed in the High-Resolution Transmission Molecular Absorption Database (HITRAN) [136]. Many techniques explore these optical characteristics to measure gas concentrations in the environment, and other rely on adding materials that react with gases in the presence of light, reflecting, absorbing or shifting wavelength from the emitted beam to fiber optics, in other words, these materials react with the target gas in the presence of determined wavelengths, emitting a different wavelength, absorbing or reflecting the emitted light. The materials used to produce these effects are normally nanomaterials, employing polymers or metals [28; 97].  
The fluorescence-quenching sensors are normally composed by a matrix of nanomaterials, permissible and sensitive to the target gas, mainly composed of polymers and in some cases metals, attached to the tip of a fiber optic, with a 50:50 Y-type optical coupler. On the one tip of the coupler, a light source is attached, and an optical spectrometer is installed on the other. The polymer matrix, with the target gas and adequate wavelength and energy, will react and emit a different wavelength, where the intensity of the light is proportional to the gas concentration. The response time is variable, being approximately a few seconds, depending on the chosen materials, intensity of the light source and target gas concentrations [28; 94; 97; 115]. Figure 6 illustrates this technique of sensing gases.  
Absorption spectroscopy techniques, such as tunable diode laser absorption spectroscopy (TDLAS) consists in the emission of a modulated wavelength at a determined frequency and amplitude trough a fixed length with the presence of the gas; part of the light is absorbed, and then the light beam is detected by a photo-diode; on the TDLAS technique, the gas is sensed by the analysis of the harmonics of the signal, making use of the Beer-Lambert law [28; 31; 98]. [PERSON] et al. [137] discussed integration between the optical spectrometry and electrochemistry to sense NH\({}_{3}\), without any calibration. [PERSON] et al. [138] presented another sensor, using ultraviolet (UV) absorption spectroscopy with a calcium fluorite (CaF\({}_{2}\)) window to analyze breath moisture in parts per billion levels.  
Optical time domain reflectometry (OTDR) is a common technique used in optical networks, and had been studied to sense chemicals, using the reflected light signal as a measurement parameter. The signal is attenuated and scattered by the optical fiber, depending on the length and specifications of the fiber; part of the scattered signal returns to the source, being able to measure the level of the signal and compare with the expected returning signal in normal operational conditions, with the absence of the target gas [116].  
Infrared (IR)-based gas sensors use the target gas absorption of IR light to determine which gas is present on the environment, and it is possible to determine its concentration by the level of attenuation on the signal. The light beam is emitted by a diode and received by a photo-detector, analyzed by a micro-controller unit (MCU) and displayed on a screen or shared with other devices [28]. CO\({}_{2}\) sensors based on IR light have been developed over the past 4 years, researchers focused on meliorate characteristics of these sensors, whether it was the response time, sensitivity, accuracy and selectivity of the devices [2; 78; 139; 140; 141]. Figure 7 shows a representation of an optical gas sensor, coupled with a duct.  
Figure 6: The fluorescent-quenching effect gas detector—this measures the target gas by evaluating the reflected wavelength from the sensing material on the presence of the gases.