Disadvantages, however, include limitations for operating at high temperatures, signal drift over time, limited selectivity or sensitivity as well as high power consumption. For these reasons, the use of novel materials such as innovative nanostructures, in place of metal-oxides, is being widely investigated in chemiresistors and transistor based device structures. Improvements have been seen for selectivity but operation at high temperature is still an open issue [7-9]. In the following, a brief review of both optical and electronic NOx sensors is reported.2.?Optical SensorsInfrared laser absorption spectroscopy is a powerful tool for sensitive and selective trace gas detection for mechatronic applications. Up to now sensitivities in the range of the parts per million/trillion by volume (ppmv/pptv) have been demonstrated .
In the absence of optical saturation and particulate-related scattering, the intensity of light I(x) propagating in a homogeneous gas of sample length L is described by:I(��)=I0(��)exp[?L��(��)c](2)where I0(��) represents the initial optical intensity, I(��) the intensity of the radiation after it passes a gas sample of length L where the species to be measured is present at the concentration c, ��(��) is the absorption cross-section.In the mid-infrared region of the electromagnetic spectrum (2�C14 ��m) most molecular species exhibit a unique spectral signature, i.e. a characteristic series of fundamental absorption lines due to transitions between rotational-vibrational states, characterized by very large cross-sections.
Hence, mid-infrared spectroscopy is in principle the best choice for the qualitative and quantitative measurements of molecular species in air. Very recently, the development of mid-ir detection techniques have received a significant boost from the invention and development of efficient mid-infrared semiconductor laser sources which promise to substitute optical methods based on the study of overtones and combination of lines falling in the near-infrared spectral region where the absorption cross sections drop by orders of magnitudes.Among the absorption techniques, direct absorption spectroscopy  and cavity enhancement approaches  take advantage of long optical path length absorption in multi-pass cells and high finesse optical cavities, respectively.
However, in spite of the high sensitivities, these techniques need sophisticated and cumbersome equipments not suitable in mechatronic applications which require compact and transportable sensors.These limitations can be overcome by fully exploiting the advantages offered by photoacoustic (PA) spectroscopy, which is one of the most Drug_discovery effective tools for exhaust gas detection, due to the high sensitivity (parts per billion, ppbv, detection limits), compact set-up, fast response time and portability.