# Air Quality Effects on Human Health and Approaches for Its Assessment through Microfluidic Chips
## 4 Emerging On-Chip Technologies for the Air Quality Measurements  
On-chip technologies enable the creation of fast, precise, simple, portable, low-cost gas sensing platforms with wide application areas, including air quality monitoring [127; 128]. The most widely known electrochemical sensing principles, based on charge generation or conductivity changes, are already available on the market and implemented as on-chip products [129]. At the same time, gas sensors based on MEMS technology are superior to other sensor types regarding selectivity, decreased power dissipation,lower operating temperature, and quicker response. Moreover, some of the \"pure\" MEMS sensor elements, such as bridges, cantilevers, membranes, etc., can be fabricated by the well-established complementary metal-oxide-semiconductor (CMOS) technology [2]. This provides unmatched opportunities regarding technology advances and price lowering for the integration of the sensing chips with microelectronics, and even allows for the co-fabrication of electronic circuitry and sensor architecture. Using MEMS structures for air quality sensing means that gravimetry needs to be employed in one way or another [130]. Gravimetric on-chip detection can primarily be used to sense comparatively larger particles in the gaseous environment: dust, microparticles, nanoparticles, and aerosols and liquid microdroplets trapped within the mechanical structure. Also, recent developments demonstrate the suitability of gravimetric principles for highly selective sensing of low molecular weight gases, such as CO\({}_{2}\)[131; 132]. MEMS gas sensors deliver the promise of higher sensitivity, as they are more selective in distinguishing between various gases. Their advantages are demonstrated by reports of a reached sensitivity of 4 Hz/ppm for CO\({}_{2}\) by using capacitive ultrasound transducers (CMUT) type sensor [132]. More convenient metal oxide electrochemical sensors have a detection limit in the range of hundreds of ppm for CO\({}_{2}\), and for this, require elevated temperatures [133; 134]. Gravimetric sensing of gas molecules requires the development of a functional layer, which selectively binds or adsorbs gas molecules. A primary example of a gravimetric detection principle is quartz crystal microbalance, which provides mass detection by analyzing changes in the resonance frequency of a crystal induced by the presence of certain molecules on the surface of the crystal [135; 136]. The common readout solution for this sensor type is to connect the crystal as an electromechanical resonator to an oscillator circuit. This provides the possibility for efficient, low noise, and compact detection [130]. Besides, of classical implementation of the gas sensing MEMS, which are based on piezoelectric materials, there is no requirement for piezoelectric properties of materials for the resonance MEMS, based on cantilevers [137; 138] or membranes [131; 132; 139] that allow for a capacitive readout. Moreover, some of the CMUTs feature vacuum-backed membrane structures and therefore are advantageous because of potentially higher working frequencies and higher resonance quality compared to other MEMS structures, which do not have vacuum-backing. Better resonance quality provides sharper frequency pointerand makes lower frequency shifts detectable, while higher oscillation frequency is important, because the absolute frequency shift is greater at higher resonance frequency for the same added mass [131], so the sensitivity potential is greater. Nevertheless, the need for materials that specifically bind the molecules of specific gases to the surface slows down the development of gravimetric sensors. For example, there are only a few candidates to establish the functional layers for CO\({}_{2}\) sensing [131; 132], and only limited progress regarding MEMS-based gravimetric sensing of other hazardous gases, such as NOx, SO\({}_{2}\), or VOCs.